MASTER's THESIS Automotive Hybrid Technology
2008:217 CIV MASTER'S THESIS
Automotive Hybrid Technology Status, Function and Development Tools
Gustaf Lagunoff
Luleå University of Technology MSc Programmes in Engineering Mechanical Engineering Department of Applied Physics and Mechanical Engineering Division of Machine Elements
2008:217 CIV - ISSN: 1402-1617 - ISRN: LTU-EX--08/217--SE Abstract A diminishing oil reserve and increased environmental concern puts new demands on our vehicles. This thesis aims to identify the strengths and weaknesses of a conventional vehicle and explain the technology behind.
Alternative energy sources are introduced and together with the knowledge learnt, their potentials are discussed. Unfortunately, none of them can be found to fulfil all future demands. Instead, hybrid vehicles are identified as a solution with high potential.
Hybrid vehicles are consequently defined and the additional components are explained. The multiple energy sources of a hybrid vehicle bring increased drivetrain flexibility but also increased control complexity. With the goal to enhance the fuel economy and reduce emissions, optimum operating conditions are discussed for each drivetrain component and concrete control targets are extracted.
Due to the complexity, computer modelling and simulation are expected to be an essential tool when it comes to hybrid vehicle development and optimization. As a result, cost efficient component models are suggested and finally a number of control optimization procedures are compared.
The result of this thesis is a summary of relevant knowledge needed to reduce the development effort of hybrid vehicles. The key aspect is to understand the synergy effect of a hybrid drive train which enables the designer to approach the full potential of each component.
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Acknowledgements It is funny, even though this thesis is the end of a long journey for me; it feels like the subject itself is the beginning of something new. I am very grateful for my time at Luleå University of Technology and I still hope that every day ahead gives me the opportunity to learn something new.
I would like to thank senior teacher Elisabet Kassfeldt at LTU for her support on all levels and Professor Björn Kjellström at LTU for his feedback regarding internal combustion engines.
From Haldex I would like to thank my supervisor Andreas Lindin for his support and department director Daniel Hervén for his support and for the opportunity to work with Haldex in the first place.
Finally, many many thanks to my family for their pep ups and support throughout all of my education and a special thanks to my dear friend David Wiberg for his more or less constructive opinions.
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TABLE OF CONTENTS
1. INTRODUCTION 1
1.1 PROBLEM FRAMING 1 1.2 OBJECTIVES 2 1.3 DRIVING FORCES 3 1.4 SOURCE CRITICISM 9 1.5 MAIN RESULTS 9 1.6 OUTLINE 10
2. VEHICLE ENERGY EFFICIENCY 11
2.1 DEFINITION OF FUEL ECONOMY 11 2.2 CARBON FOOTPRINT 12 2.3 WHAT AFFECTS THE FUEL ECONOMY? 12 2.4 POSSIBLE ACTIONS AND THEIR POTENTIAL 13
3. DRIVETRAIN COMPONENTS IN A CONVENTIONAL VEHICLE 15
3.1 INTERNAL COMBUSTION ENGINE – THE PRIMARY FUEL CONVERTER 15 3.2 TRANSMISSION 35 3.3 DIFFERENTIALS 41 3.4 AUXILIARY SYSTEMS 41
4. VEHICLE PROPULSION ALTERNATIVES 42
4.1 CONVENTIONAL VEHICLES 42 4.2 ELECTRIC VEHICLES 44 4.3 FUEL CELL VEHICLES 44 4.4 HYBRIDS 45 4.5 THE CUSTOMER POINT OF VIEW 46 4.6 NOW, TOMORROW AND THE FUTURE 47
5. WHAT IS A HYBRID? 48
5.1 HYBRIDS IN HISTORY 48 5.2 HYBRID CARS 49
6. SOURCES OF HYBRID POWER AND ENERGY STORAGE 51
6.1 HUMAN 52 6.2 ENVIRONMENTAL 53 6.3 PNEUMATIC AND HYDRAULIC 55 6.4 FLYWHEEL 58 6.5 ELECTRICAL 61
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6.6 CHOOSING HYBRIDIZATION METHOD 66
7. DEGREE OF HYBRIDIZATION AND OPERATING MODES 68
7.1 MEASURING THE DEGREE OF HYBRIDIZATION 68 7.2 OPERATING MODES 71 7.3 DOH CLASSES 72 7.4 PHEV’S ELABORATED 77 7.5 CONCLUSIONS REGARDING THE DOH 77
8. DRIVETRAIN STRUCTURES 78
8.1 CONVENTIONAL 79 8.2 PARALLEL HYBRID 80 8.3 SERIES HYBRID 82 8.4 COMBINED (SPLIT) HYBRID 84 8.5 ALL WHEEL DRIVE, AWD 85
9. COMPONENTS OF A HYBRID ELECTRIC DRIVETRAIN 91
9.1 ELECTRICAL ENERGY STORAGE DEVICES 91 9.2 ELECTRICAL MACHINES 112 9.3 POWER ELECTRONICS 119 9.4 ELECTRIFIED AUXILIARY SYSTEMS 123 9.5 ADDITIONAL MECHANICAL DEVICES – POWER SPLIT 126
10. MODELLING 127
10.1 MODEL DEPTH AND FIDELITY LEVEL 127 10.2 ICE MODELLING 127 10.3 TRANSMISSION MODELS 130 10.4 VEHICLE DYNAMICS 132 10.5 VIRTUAL DRIVER AND DRIVING CYCLES 134 10.6 ELECTRICAL MACHINE MODELS 137 10.7 BATTERY MODELS 139 10.8 ULTRA CAPACITOR MODELS 141 10.9 POWER ELECTRONIC MODELS 141 10.10 AVAILABLE SIMULATION PLATFORMS 142
11. CONTROL 145
11.1 SYNERGY IS THE KEY 145 11.2 WHERE TO START 145 11.3 KEY COMPONENT CONTROL ASPECTS 146 11.4 PROCEDURES 149 11.5 REGENERATIVE BRAKING 156
12. FUTURE WORK 160
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REFERENCES 161
APPENDIX A – PUGH MATRIX 167
APPENDIX B – NOMENCLATURE 168
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1. Introduction This thesis is intended as a compilation of relevant knowledge to act as an incentive and basis for hybrid vehicle design and development. It consists of, among other things; the background and current status of hybrid vehicle technology as well as the fundamentals of important components. The text aims to be interesting and instructive to a wide range of readers and the main goal is to enlighten the potential and synergy-behavior of a hybrid vehicle drivetrain.
1.1 Problem framing If you are reading this text you have probably not missed the increased environmental concern and the lively debate regarding the increased oil price the last couple of years. On top of that, when the world’s largest oil consumer, i.e. the USA, reduces its car use by 5.4 % in August 2008 compared to the last year, it is evident that something is happening in the world of transport [Teknikens Värld].
In the end it all comes down to one thing, supply and demand. It is widely accepted that our crude oil resources will start to fall in a couple of years [EIA] while the total need for personal and freight transportation most likely will not. The result, all according to the supply and demand relationship, will keep on pushing the oil prices upwards. At the moment (2008-11-17) we are in the middle of a world-wide recession of the business cycle which in addition has hit the automotive industry especially hard. Together with a strengthened dollar, the oil price has dropped again [OPEC] but in the long term, supply and demand will rule. Extended to the next step, and as mentioned already noticed as a drop in US car use, the reaction to the increased transportation costs are strong and new alternatives are immediately sought for [Teknikens Värld].
Furthermore, as a reaction to proof of man kinds negative effect on the environment (which will be mentioned again later), an increased environmental interest has aroused worldwide [Johansson]. Due to the contribution from light and heavy vehicles, new legislation and emission targets are constituted around the world to limit our bad influence. As a result, new and tougher demands are put on vehicle manufacturers and in the end the customer. Once again ruled by supply and demand the new emission targets will, together with the diminishing fossil fuel reserve, act as a catalyst in the automotive industry and open new doors.
There are many ways to reduce the use of fossil fuel and methods available to try and reach stringent emission demands. However, none of them offers a single solution to all problems. Instead it can be expected to be a combination of a few technical enhancements and energy sources etc. but one of those doors contains hybrid vehicle technology…
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1.2 Objectives The objective of this thesis can be divided into three parts, beginning with some useful information about the powers of the automotive industry together with a brief look at concerned parts of the environmental situation. By presenting the wide picture coupled with a concrete problem framing the reader is encouraged to gain interest in the challenging future of vehicle technology.
The second part is devoted to the coherence and functionality of vehicular drivetrain components. Based on the drawbacks of a conventional vehicle and the general goals, this part tries to define, explain and compare alternative and enhanced technical solutions. From and including the latter half, the content is pervaded by the core subject, i.e. hybrid technology and hybrid electric vehicles in particular.
Part three is aimed at applying the gathered knowledge into a future design process and exemplifies how it can be done in an effort-efficient way. Computer modeling and computer assisted control optimization is identified as powerful resources and an approach is suggested.
Furthermore, some parts are directly intended to act as useful development tools related to the “Haldex eBAX” which is an internal hybrid vehicle project within the Haldex Traction AB corporation.
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1.3 Driving forces The main driving forces behind the development of hybrid vehicle technology, and hence this thesis, have already been briefly mentioned but let us take a closer look.
1.3.1 The oil price The oil price is governed by a number of geological, financial and political parameters etc. which nature is not in the scope of this thesis. Instead, a more important question is: how has the oil price changed throughout time and does it affect the automotive industry?
Figure 1 - Crude oil prices 1920- 2007 [BP]
Figure 1 visualizes the crude oil price between 1920 and 2007. Recently, past the range of the graph, the oil price peaked at a monthly average of over $130 US in July 2008 but is at the moment (2008-11-14) down to $49 US [OPEC]. By using Figure 1 to look back in time it is very clear that something happened during the 1970’s that made the oil price increase rapidly in a similar way. The Yom Kippur war and the Iranian revolution can be considered as two contributing factors but more interesting, how did this affect the automotive industry? And can it be used as an indication about what is to come?
By cross referencing the years of interest (1970-1985) when the oil price was high with average fuel economy figures based on statistical data of the US car fleet from the US Environmental Protection Agency, a very interesting phenomenon can be identified in Figure 2.
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Figure 2 - Adjusted fuel consumption by model year 1975 – 2006 [EPA Fuel Economy]
Coincident with the increase in oil price the average fuel consumption of the US vehicle fleet dropped substantially! From around 1975 to 1987 the average fuel consumption dropped with almost 40 %, which is a great deal. As always, there is probably more than one contributing factor to this powerful change but it is evident that the oil price has a great effect on the average fuel economy of the vehicle fleet.
However, as astonishing the 1975-1985 enhancements might be it is unfortunately almost as shocking to see what happens thereafter. By looking at the statistics from 1990 to 2006 the average consumption seems to be almost constant or even increasing if seen as the average of both trucks and cars. Of course, one asks the question, why? Well, by looking at Figure 1 again it can be established that the oil price was more or less the same during the end of the 20th century and the first years of the 21st century which meant that the direct economical stimulus from the oil cost had lessened.
But, does this mean that there have been no technical development and efficiency enhancements of vehicles since the end of the 80’s? No, it does not. Nevertheless it means that they have not been used to increase the fuel economy of vehicles or to reduce the use of fossil fuel. Why? – Supply and demand! By looking at Figure 3, which plots vehicle weight and performance by model year, it is obvious where the development effort was put when the oil supply and cost was not the considered as driving factors anymore, i.e. customer demand shifted.
Once again coincident with the years of the 1970’s oil crisis something drastically happens. In Figure 3 it can be noted that the average weight of the vehicle fleet drops substantially while the performance stops increasing or even decreases. Why these technical design- changes were made and what impact they have on the average fuel consumption will be discussed later on but it can be established that they had an important role.
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Figure 3 - Weight and performance by model year 1975 – 2006 [EPA Fuel Economy]
Continuing in the same way as the average fuel consumption, the weight and performance drastically changed direction after the oil crisis was over. Instead of putting development and optimization efforts on fuel economy enhancements; performance, comfort and road safety was prioritized instead. As a result, cars have gotten heavier and heavier while the performance has simultaneously increased. Basic understanding of physics directly indicates that more power is needed to make these curves reality. More power means that more fuel is consumed but as the engine efficiency has improved over the years, the final result is a more or less a constant fuel consumption which is in accordance with Figure 2.
Summarized, there is a powerful relationship between the crude oil price and the average fuel economy. The data in this comparison is based on the USA alone but to the author’s opinion it relates to the rest of the world as well. It is interesting to see how well an increase in the vehicle fuel economy interest and hence where the automotive R&D resources are put, correlates with an increasing oil price.
Based on the outlook of the future where e.g. Dr. Faith Birol, chief economist at the International Energy Agency, projects record high oil prices after the ongoing financial crisis is over [DI], we are likely to expect continued and increased commitments related to fuel economy.
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1.3.2 Environmental effects and emission targets We, i.e. the people of the world, are living above our assets. In 2007 we used over 30 % more natural resources than the planet can produce during the same amount of time. If we keep on expanding in the same pace, by 2050, two planets will be “consumed” each year [Johansson].
The result is unfortunately upsetting with climate changes to follow. Changes that have been established are amongst others:
Melted glaciers and ice-caps which disturbs thermal distributions Change in agricultural seasons on the northern hemisphere Increased average sea-temperatures which leads to increased sea-levels Spread of desert areas in e.g. Africa Death of coral reeves due to increased water temperatures Double increase of cat. 4 and 5 hurricanes the last 30 years Etc…
It is Important to realize that the environmental changes and reactions are not just unpleasant, they are sometimes devastating and very costly; hurricane Katarina is an example of a category 5 hurricane which cost the US government billions of dollars in repairs and took almost 2000 lives. Dramatic changes are therefore required and motivated by many reasons; monetary savings is one of them.
CO2 is the most prevalent Green House Gas (GHG) and around 20 % of the worlds CO2 emissions originate from transportation fuel [Johansson + EU Parliament]. In the USA for example, the transportation sector stood for 29 % of the total GHGs in 2006. These estimates of transportation GHGs do not include emissions from auxiliary lifecycle processes, such as the extraction and refining of fuel and the manufacture of vehicles, which are also significant sources of international GHG emissions [EPA GHG]. Drastic reductions in GHGs have to be seen through to limit the global warming in time and the transportation sector is obviously an important factor to consider.
Average vehicle emission targets has been constituted around the world to reduce the use of fossil fuel, reduce GHGs and put demands on the vehicle manufactures. Let’s look at some examples that concerns light vehicles (cars). The European, Japanese and Korean automotive industry (ACEA, JAMA and KAMA) has obliged to reduce the average CO2 emissions to 140g/km in newly produced cars, effective from the end of 2008 in Europe and 2009 in Japan and Korea. However, these targets will most likely not be met [EU Parliament].
Furthermore did the European Council in 2006 decide on a new strategy for sustainable development in Europe which included that the average CO2 emission level from cars should be limited to 120 g/km effective from the end of 2012. But the question is, how? If the European target of 2008 has not been met, how should the even tougher one of 2012 be managed?
In the USA, similar targets are set but instead specified as fuel economy by miles per gallons (regardless of fuel type). Nevertheless they can be translated and compared with the ones [6] stated for Europe (using a specific fuel). However, the targets are somewhat more complicated in the USA as two different ones can apply depending on what state you are in. The baseline is the Corporate Average Fuel Efficiency, or CAFE, which is effective in all states but for those which have accepted the Californian Air Resource Board, or CARB, standards; the CAFE target is superseded. Converted from mpg to l/100km and g/km for a gasoline/petrol fueled car, Table 1 compares the US targets to the European [CRS Emissions + EU Parliament].
Light vehicle emission targets CO2 [g/km] Eq. fuel cons. (gasoline) [l/100km] Europe - 2008 140 6,0 Europe - 2012 120 5,2 USA CAFÉ - 2015 153 6,6 USA CARB - 2015 132 5,7
Table 1 - European and US emission targets
By the author’s opinion, the method to reach these targets is not a single effort but instead expected to be a combination of many, one of which regards hybrid vehicle technology.
But what really affects the fuel economy and emissions of a vehicle? This topic will be elaborated in chapter 2 but some possible efforts are:
Reduced rolling resistance (reduced mass, tires, road surface, tire pressure monitoring etc.) Reduced wind resistance (improved aerodynamics, smaller vehicles etc.) In addition to bio fuel vehicles, an increased mixing of bio fuels without interference with vehicle certifications into fossil fuel (in e.g. 5-15 % Ethanol in gasoline/petrol) Promote an eco-friendly driving behavior Increased drivetrain efficiency (ICE development, Hybrid technology, Fuel cells etc.)
All of these parameters influence the fuel economy of a vehicle to some extent which will be discussed later on. However, by the opinion of e.g. the European Union, the largest enhancements will have to, and are expected to, be done in the area of drivetrain technology and optimization [EU Parliament].
1.3.3 Room of improvement and energy security The USA consumes 20 million barrels of oil every day. Approximately 60 % of those barrels are imported from abroad, i.e. 12 million. Of those imported barrels, 20 % are from the Middle East. That means that every single day the USA alone are dependent on a shipment of around 2.4 million barrels from countries in the Middle East.
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Moreover, 70 % (14 million barrels) of the total US oil consumption is represented by the transportation sector, divided into 9 million for cars/light trucks, 2 million for heavy trucks and 3 million for other types of transportation [Peng, Filipi].
In the end, this means that by improving only the light vehicle-fleet fuel economy from its current average of 21 mpg to 27.3 mpg (30 %) would reduce the total oil consumption by about 30 % (2.7 million barrels). That is enough to entirely eliminate the need for Middle Eastern oil.
Energy security, i.e. ensuring sufficient national energy resources, is not just an important question for the USA but for all countries around the world. As there is an uneven distribution of energy supplies among countries coupled with ever increasing energy demands; limited energy supply can lead to significant vulnerabilities. The USA and their dependency of Middle Eastern oil is a good example of this and how it affects the automotive industry.
FreedomCAR is a concrete example on how energy security is one of the large driving forces behind new automotive technology. The US FreedomCAR project is a part of the “Office of Energy Efficiency and Renewable Energy” program which is aimed at developing more energy efficient and environmentally friendly highway transportation technologies that will enable the U.S to use less petroleum and ensure the national energy security. FreedomCAR has sponsored a range of high-risk research projects with the goal to develop the components and infrastructure that is needed to enable a new era of personal transportation in the U.S [FreedomCAR].
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1.4 Source criticism In this type of thesis, qualitative sources are a necessity. Therefore the sources and especially those which are of great impact have to be audited and criticized. To reduce the risk of misleading information, correlation of many “smaller” sources has been prioritized over “large” ones. However, some has been of extra importance and are therefore discussed below.
Regarding the area of internal combustion engines; the basic functionality, modeling and theoretical material is almost entirely based on Heywood, Internal Combustion Engine Fundamentals [Heywood]. But, to the author’s opinion, Heywood can be considered as a standard reference of the matter and is used as source in many similar texts.
The short piece on alternative fuels is inspired by a summary conducted by the Swedish road administration, Vägverket. Vägverket can be considered as an independent organization but the theoretical and fact based parts has nonetheless been cross referenced with e.g. Heywood.
To a large extent, the evaluation of energy storage technologies relevant to hybrid vehicle propulsion has been influenced by the work of the INVESTIRE network [INVESTIRE]. The source is a multi party project, i.e. with both academic and commercial parties from multiple nationalities, organized by the European Commission and is therefore considered as reliable. But, as the project was last updated in 2005, additional sources have been used to confirm and update relevant information.
Moreover, the application-related discussion of electrical machines has been inspired by the material gathered in the International Energy Agency’s review on electrical traction machines [IEA HEV] for hybrid applications. As the originator has to be seen as a non profit- oriented organization the material can, to the authors opinion, be considered as directly unbiased.
1.5 Main results Due to the nature and objective of the thesis, the result can more or less said to be a source of current information. Consequently, a lot of the content can be considered as research and development subjects and on top of that they originate from a wide range of research areas. This has in some cases highlighted the difficulty to obtain relevant, qualitative and comprehensive information regarding the area of hybrid vehicle technology, something this thesis is considered to bridge.
Regarding the Haldex eBAX, the thesis treats the sufficient knowledgebase that is necessary to:
Understand the entirety of the environmental and oil-related challenges of the automotive industry. Understand the weaknesses and potentials of a conventional drivetrain in aspect to fuel economics and emissions Gain relevant knowledge which enables hybrid technology to be identified as an effort of great potential in the fuel economic and emission related area [9]
Be aware of relevant targets, definitions and ongoing development of hybrid technology Delimit a work and cost efficient development and optimization process of hybrid vehicle systems with regards to effort-potentials and tools etc.
1.6 Outline This chapter contains the main problem framing, thesis objectives and some of the driving forces of automotive technology and hence this text. It also treats source criticism and summarizes the main result.
Chapter 2 defines important expressions, identifies potentials and clarifies the main objective.
A technical dissertation follows in chapter 3 and is aimed at the weaknesses of a conventional drivetrain in terms of efficiency.
In chapter 4 alternative fuels and primary energy sources are discussed and the hybrid is introduced.
The following chapter 5 tries to explain what a hybrid really is, both in general and vehicular terms.
Chapter 6 exemplifies hybrid vehicles in terms of different secondary energy sources.
Chapter 7 is used to explain important parameters and useful definitions. It also elaborates plug-in hybrids.
Conventional and hybrid drivetrain structures are defined and compared in chapter 8. Additionally, all wheel drive is mentioned and the Haldex eBAX is put into context.
In chapter 9, a second technical dissertation follows, this time devoted to the additional components that can be found in a hybrid electric drivetrain and how they can be used to complement a conventional drivetrain.
Chapter 10 is aimed at using computer modeling as a development tool for hybrid vehicle technology. It briefly suggests some economical modeling-strategies of important components based on the previously explained functionality.
The last chapter, chapter 11, summarizes the potential and potential of hybrid vehicles by suggesting an approach to a control strategy.
The appendices include nomenclature and a Pugh-matrix used to suggest a suitable drivetrain layout for the Haldex eBAX.
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2. Vehicle energy efficiency What is fuel economy, what defines it and what affects the fuel consumption of a vehicle? Well, to most people it’s all about travelling from point A to point B, i.e. a given distance, using the least amount of energy as possible. In the case of a conventional vehicle powered by an internal combustion engine, the energy would be measured in volume of fuel, e.g. litres of gasoline/petrol or diesel. This type of measurement is called the “Tank-to-distance” fuel economy.
However, fuel economy can be viewed in a number of perspectives. Below is a list with three common ways of assessing fuel economy:
1) Well-to-wheel (Well-Refinery-Station-Tank-Engine-Driveline-Wheel)
2) Wheel-to-distance (Wheel work – Travelled distance)
3) Tank-to-wheel (Tank-Engine-Driveline-Wheel)
The first measurement (Well-to-wheel) considers the whole chain of events from the crude oil source to the work done by the wheels of the car. With this measurement, the energy used for refinement, transportation and storage etc. is also taken into account. The Well-to- wheel CO2 emissions is also very interesting as it measures the total CO2 contribution coupled with the use of a specific fuel and not only the part that takes place in the car. A fuel can be very efficient in the final application, i.e. the car, but consume large amounts of energy during production, transportation and storage which in the end make it inappropriate to use.
Wheel-to-distance, the second type of assessment, measures the “external fuel economy”. It answers the question; how far does the vehicle travel with the help of a specific amount of work done by the wheels? External factors are e.g. vehicle mass, aerodynamics and rolling resistance.
The last method to asses the fuel economy is the Tank-to-wheel method. It measures the efficiency of the internal vehicle systems, e.g. the engine, and answers the question; how much work is done by the wheels for a specific amount of used fuel. In this thesis, when fuel economy is referred to, this measurement is used. However, assuming constant external factors such as aerodynamics and mass etc., Tank-to-distance can be used as an equivalent.
2.1 Definition of fuel economy Depending on where you are in the world different units are used for measurements, e.g. metrics are used in Sweden, Germany and Japan etc. while for example the USA mainly uses the U.S. customary units which are derived from the British imperial units. Coupled with this is the two most common ways to measure automobile fuel economy:
1. By measuring the amount of fuel that is used per unit distance – commonly “litres per 100 kilometres” [l/100km].
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2. By measuring the travelled distance per unit of fuel used – commonly “miles per gallon” [mpg] but can also be written as “kilometres per litre” [km/l]
The first method is used by all countries in the EU and by Japan etc. The second method is mainly used in the USA where it is expressed using gallons for volume. Great Britain has used the second method also but is starting to use the first method instead. It is important to remember that a U.S. gallon is not equal to an imperial gallon which leads to . In this thesis, metric units will be used as much as possible, i.e. l/100km.
2.2 Carbon footprint
Directly related to the fuel economy and the fuel used is the carbon dioxide (CO2) emission. The CO2 emission level is measured in “grams per kilometer” [g/km]. As a product of the chemical composition of the fuel that is used, the CO2 emission level is directly proportional to the fuel consumption of the vehicle at optimum combustion. The average net CO2 emission level in kilograms relates to the fuel consumption per liter by [DEFRA]:
Petrol – 2.32 kg/liter Diesel – 2.63 kg/liter
As a result, fuel economy regulations, statistics and targets can be expressed and compared approximately both by fuel consumption figures (l/100km) and by CO2 emission levels (g/km).
2.3 What affects the fuel economy? Why does a vehicle consume fuel at all and why does it consume as much as it does, i.e. what affects the fuel economy? As well known, energy cannot disappear or be destroyed, only converted and that goes for vehicles also. When for example a car is filled with fuel, the internal reservoir, i.e. the fuel-tank, is charged with energy contained by the liquid fuel. The goal of the procedure is to use the fuel to make the vehicle move forward, and as said, travel from point A to point B by turning the wheels of the vehicle in a mechanical motion.
In other words, the goal of a conventional vehicle is to convert the energy of the fuel it has been loaded with into a mechanical motion of the vehicle. This conversion is not done in one step but in a chain of events, defined as the drivetrain of the vehicle. In the ideal world, a certain amount of energy is put in at one end of a system, i.e. the drivetrain in this case, and the same amount of energy is harnessed at the other end, preferably in another form. Unfortunately, the real world and the drivetrain of a vehicle is not ideal and even worse, not even close.
Each of these multiple steps of conversion from the chemical energy of the fuel to the mechanical energy of a moving vehicle is non-ideal, which means that energy is lost during the way, e.g. into heat. Instead of describing the basic energy transport of a vehicle by words, it is visualized in Figure 4. In the figure, the vehicle drivetrain is split into two parts, the engine and the drive line. The drive line consists of components such as a gearbox and a differential etc. Consider the values as approximates [USDE Fueleconomy].
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Figure 4 - Vehicle energy transport and conversion
2.4 Possible actions and their potential The energy loss inflicted by energy conversion in the drivetrain and the losses coupled with auxiliary systems, standby and friction braking in the car can be summarized as internal losses. Aerodynamic and rolling losses can be classified as external losses.
By taking a look at the magnitude of the external losses, which are 7 % during urban use and 18 % during highway use, it is evident that a lot of energy is lost internally. Only a small fraction of the initial energy reaches the external system at equilibrium. By those numbers, it can also be noticed that the potential for internal enhancements is greater in the urban load case. This thesis will, due to the potential, focus on methods which can minimize the internal losses of a vehicle drivetrain.
However, the external losses and their components still have to be considered if emission targets are to be met in the future. Rolling resistance as a function of vehicle mass, tire design, road surface etc. and aerodynamic resistance as a function of frontal area and coefficient of drag etc. are also crucial parameters to optimize.
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As an example, based on data from car sales in the USA during 2002, the relationship between vehicle weight and vehicle fuel economy can with the help of regression analysis be described as [Peng, Filipi]:
In Figure 5 the regression analysis is visualized and converted to metric units. It is very clear that there is a strong relationship between vehicle weight and fuel economy. This also explains the substantial reduction in vehicle weight during the 1970’s oil crisis, earlier shown in Figure 3.
N = 950 vehicles R2 = 0,87 ε = 17 %
Figure 5 - Relationship between vehicle weight and fuel economy
Furthermore, fuel economy can be extended past the definition used above by also considering to what benefit a certain distance has been covered. If the purpose is transport of people from e.g. home to work, the “fuel benefit” would be much higher if the vehicle is occupied by 2, 3 or 4 people instead of just one. Therefore ride-sharing is also a powerful tool to reduce the total use of fuel and GHG emissions.
Summarized, a lot of variables affect the fuel economy of a vehicle and future emission targets will without doubt put demands on optimization on many areas. However, as drivetrain optimization according to Figure 4 has a great potential this thesis is devoted to discuss that topic.
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3. Drivetrain components in a conventional vehicle The basics of a conventional vehicle drivetrain have not changed much from the ones used in the first cars ever made in the beginning of the 20th century. It still consists of an internal combustion engine, or ICE, coupled with a gearbox of some kind for speed range selection and finally a connection to the wheels with the help of a differential. Of course enormous improvements has been made regarding the components themselves but it is interesting to see how little has changed during more than 100 years.
Now, more than ever, we are faced with the task to lower the energy consumption of our vehicles and the main reasons are a future shortage of fuel and the negative environmental impacts. But how does the internal characteristics of the most important components in a conventional drivetrain affect the fuel consumption and to what extent? How do the components work and can their performance be improved?
The goal of the following chapter is to explain the technology behind the components of a conventional vehicle drivetrain, identify the weak points and summarize recent development to try to predict what is to come.
3.1 Internal combustion engine – the primary fuel converter An internal combustion engine is defined as a device that obtains its power from heat and pressure produced by the combustion of a fuel-and-air mixture inside a closed chamber or cylinder [Your dictionary]. The ICE was invented in the late 1800s and has had an enormous impact on the society. It is considered to be one of the most important inventions of all time and just the mere thought about how the ICE has revolutionized our transportation alternatives, with airplanes, trains, cars, motorcycles and so on is astonishing. Internal combustion engines exist in all sizes, capable of delivering power in the range from a couple of watts to magnitudes exceeding 75 MW.
Internal combustion engines are today the dominant prime mover technology in several areas such as vehicular (truck and automobile), railroad, marine, aircraft, home use and stationary industry. The majority of the ICE’s built produce power in the range of 100 kW and the largest application is road going vehicles. As of the year 2006, in the United States alone there are about 250 million motor vehicles powered by ICE’s and the number is still increasing rapidly [BTS].
The first ICE used the principle of a reciprocating piston in a cylinder, in which a piston oscillates up and down in a cylinder thus transmitting power from the engine with the help of a connecting rod between the piston and the rotating crankshaft. The components of an ICE, block, piston, valves, crankshaft, connecting rod basically remains the same today as in the late 1800s. The main difference between a modern engine and the ones built 100 years ago are the thermal efficiency and emission levels. Since the first use of ICE’s the main research has for many years been aimed at improving thermal efficiency and reducing noise and vibration. As a result of this, the thermal efficiency of a modern engine can reach levels over 50 % compared to 10 % in the beginning of the 20th century. In the 1970’s it was noted that the ever increasing number of motor vehicles started to have a negative impact on the environment, namely air quality. Since then the main development effort has instead been put on reducing the emission levels from the engines. In the last decade, lowering of [15] emission levels together with maintained efficiency, are probably the single most important factor of optimization in engine design and development [Heywood].
3.1.2 Engine cycles Internal combustion engines can be divided in classes by which combustion cycle they use during operation. There are two major alternatives, Otto and Diesel. The Otto cycle is named after Nikolaus Otto (1832-1891) who developed a four-stroke engine in 1876 and is considered to be the inventor of the modern internal combustion engine and the founder of the ICE industry. An ICE running the Otto cycle is also referred to as a SI or “Spark Ignition” engine as it demands a spark to ignite the mixture of fuel and air in the cylinder. The Diesel cycle is named after Rudolf Diesel (1858-1913) who in 1897 developed an engine design for direct injection of fuel into the combustion chamber. The Diesel cycle is also referred to as a CI or “Compression Ignition” engine as the fuel will automatically ignite while being injected into the combustion chamber. Both the Otto and Diesel cycles can respectively be operated in two- or four-stroke modes. Two-stroke engines will not be treated in this text due to their significant drawbacks regarding the efficiency and the negative environmental effects. Very large two-stroke diesels on the other hand are suitable for and common in large ships as they can reach high thermal efficiencies while producing power at a high power to weight ratio coupled with a very low rotational speed.
Otto cycle As visualized in Figure 6, the four-stroke Otto cycle has following operation sequences:
1. An intake stroke that draws a combustible mixture of fuel and air past the throttle and the intake valve into the cylinder
2. A compression stroke with the valves closed which raises the temperature of the mixture, followed by a spark that ignites the mixture toward the end of the compression stroke.
3. An expansion or power stroke as a result of the combustion of the fuel-air mixture trapped in the cylinder.
4. An exhaust stroke that pushes out the burned gases past the exhaust valve.
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Figure 6 - The four stroke Otto cycle [Wikimedia Commons]
In a SI engine the air enters through an intake manifold which distributes the air evenly to all of the cylinders of the engine, i.e. if there is more than one. The fuel, commonly gasoline, is mixed with the air using a fuel injector that squirts small droplets of fuel into the air. Before the use of fuel injection SI engines relied on carburetors but in modern engines the fuel is injected into the intake manifold, intake port or directly into the cylinder using electronically controlled injectors. This results in the cylinder filling with a homogeneous charge of the air/fuel mixture. It is important that this charge has the correct mixture of air and gasoline to ensure optimum combustion (maximum efficiency). For gasoline this is done with a relationship of 14,7:1 between air and gasoline, referred to as the stoichiometric ratio corresponding to λ=1.0. However, in some special cases, even higher efficiencies can be reached at mixture ratios slightly above 14,7:1, i.e. leaner.
When the mixture is ignited by a spark, a turbulent flame develops and propagates through the mixture, raising the cylinder temperature and pressure. The flame is extinguished when it reaches the cylinder walls. If the pressure rise is too high and fast, the compressed gases in front of the flame front will auto-ignite, causing what is known as detonation or “knock”. Knock is the limiting factor of the compression ratio of a SI engine as soon to be explained, and also the main factor limiting the thermal efficiency of the engine. After combustion the burned gases exit the cylinder via the exhaust port into the exhaust manifold which collects the exhaust gases from all of the cylinders and channels them to the exhaust, leaving the engine. In the Otto cycle, a throttle is used to control the amount of air that is inducted into the cylinders. If the throttle is closed the amount of air entering the engine will be reduced
[17] and because of that also the cylinder pressure. Furthermore, since fuel flow is metered in proportion to the air to maintain the desired air-fuel ratio, the throttle will control the power output from a SI engine [Heywood].
All internal combustion engines are per definition “heat engines” which is a device that converts thermal energy to mechanical work. Heat engines run on a specific thermodynamic cycle, such as the Otto or the Diesel. These cycles can be modeled to understand how energy is transported and converted in the engine. Studying the gas cycles as models of ICE’s is useful for illustrating some of the important factors affecting engine performance. Essential to note is that gas cycle calculations treat the combustion process as an equivalent heat release. Modeling the processes as constant volume, constant pressure or finite heat release simplifies the reality since the details of physics and chemistry of combustion are not required.
Constant volume heat addition Often referred to as the Otto cycle, the constant volume heat addition cycle treats the special case of an ICE engine where the combustion takes place so rapidly that the piston does not move during the process, i.e. combustion is considered to be done at constant volume. The working fluid in the cycle is assumed to be an ideal gas with constant specific heats for calculation. Doing this makes it possible to reach the goal of this investigation, deriving a simple mathematical expression for the efficiency of the cycle.
The Otto cycle used for analysis is shown in Figure 7 and the four basic processes are:
1 to 2 – Isentropic compression 2 to 3 – Constant volume heat addition 3 to 4 – Isentropic expansion 4 to 1 – Constant volume heat rejection
Figure 7 - Constant volume heat addition - "Otto"- cycle
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Cycle work is described with the following relations:
Heat addition
Heat rejection
Compression Stroke
Expansion stroke
Where:
The compression ratio of an engine is defined as
Leading to the thermal efficiency denoted as
Inserting eq. 2.1 and 2.2 into eq. 2.6 gives
And the indicated mean effective pressure (IMEP) will then be
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Observed is that IMEP is proportional to the heat addition Qin and that the thermal efficiency of the Otto cycle only depends on the specific heat ratio and on the compression ratio.
For the Ideal Otto cycle with inputs r = 10 and γ = 1,3 the indicated efficiency is calculated to η = 0.50 which is about twice as large as for a similar engine in real life. Why? – there are a number of reasons for this difference. Summarized, the constant volume heat addition model does not take internal friction, the real combustion processes and heat losses into account and therefore the indicated efficiency is much larger compared to what can be expected in reality.
The normal range of compression ratios in modern SI engines is 9 – 12:1 which means that even thoughgh all losses not are included in the calculation, the efficiency will be constrained to a corresponding interval calculated by the Otto cycle. The compression ratio is limited by two main factors, the structural strength of the engine and the knock γ resistance. The maximum pressure, P3, scales with compression ratio as the function r . Engine head and block can only withstand a certain amount of stress before they break, thus limiting the compression ratio to retain P3 below the maximum pressure allowed by the γ-1 component strength. Furthermore the maximum temperature, T3, also scales with r and if this temperature exceeds the temperature of auto-ignition the fuel-air mixture will combust ahead of the flame front. This is, as mentioned, called engine knock and can damage the engine but most importantly it lowers the efficiency! [Heywood].
Diesel cycle The four-stroke Diesel cycle is very similar to the four-stroke Otto cycle but instead of igniting the fuel-air mixture compressed in the cylinder with the help of a spark the Diesel engine compresses air and then injects fuel directly into the combustion chamber. It is the heat of the compressed air that ignites the injected fuel. For visualization refer to Figure 6 of the Otto cycle but replace spark ignition with fuel injection.
The diesel cycle has the following sequence:
1. An intake stroke that draws inlet air past the intake valve into the cylinder
2. A compression stroke that raises the air temperature above the auto-ignition temperature of the fuel. Diesel fuel is sprayed into the cylinder near the end of the compression stroke.
3. Mixing, evaporation, ignition and combustion of the diesel fuel during the later stages of the compression stroke and the first part of the expansion stroke.
4. An exhaust stroke pushes out the burned gases past the exhaust valve.
A diesel engine is not dependent of a specific mixture ratio of the fuel-air mixture in the same way as an SI engine. The Diesel can be said to work with excess air as in its original form it does not use a throttle to regulate air-flow into the cylinder. Instead power is controlled by the amount of fuel that is injected into the cylinder. In order to auto-ignite the fuel-air mixture Diesel engines has to work at a higher compression ratio compared to the [20]
Otto, thus enabling a higher theoretical efficiency. As fuel is present in the combustion chamber during compression, a Diesel engine will not be limited by knock in the same way as the Otto. Diesel performance is in the end limited by smoke which forms if the fuel-air mixing is inadequate, i.e. too rich.
Constant pressure heat addition The constant pressure heat addition cycle is also known as the Diesel cycle as it models the special case of an internal combustion engine whose combustion is controlled so that the beginning of the expansion stroke occurs at constant pressure. The compression ratio of the Diesel engine is higher to that of an Otto engine and commonly is in the range of 15-20:1, thus reaching cylinder temperatures high enough for self-ignition of the fuel. The combustion process and its duration are controlled by the timing of fuel injection. Fuel is sprayed directly into the cylinder at high pressure ranging from around 15° BTDC (Before Top Dead Center) to 5° BTDC and the duration is a function of the engine load. As the air is already compressed when the fuel enters the combustion chamber, combustion will start in the areas of fuel spray with an air-fuel mixture ratio close to stoichiometric for the diesel fuel which is 14.6:1. Otherwise the cycle reassembles the Otto cycle.
The four basic processes of the constant pressure cycle model are visualized in Figure 8 and can be described as:
1 to 2 – Isentropic compression 2 to 3 – Constant pressure heat addition 3 to 4 – Isentropic expansion 4 to 1 – Constant volume heat rejection
Figure 8 - Constant pressure heat addition - "Diesel" - cycle
Again assuming constant specific heats, cycle work can be described as:
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Heat addition
Heat rejection
Compression Stroke
Expansion stroke
Where parameter is defined as
or in terms of heat transfer
This leads to the indicated efficiency stated as
By looking at equation 3.15 it can be noted that the term in the brackets will be greater than 1 and therefore the efficiency of the CI engine is lower than the one for a SI engine at the same compression ratio. But, as the CI engine not is limited by knock, in reality a CI engine normally has almost twice the compression ratio of the SI engine, thus retaining a higher efficiency [Heywood].
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Dual cycle In reality, a modern Diesel engine does resemble neither the constant volume nor the constant pressure model. Instead it can be described as something in between, with a certain amount of heat being added during constant volume and the rest during constant pressure. This cycle is called the dual cycle and can be used to more accurately model CI engines as Diesel’s are slower than constant volume but faster than constant pressure. The heat input distribution between the two parts, constant volume and constant pressure, can to some extent be chosen by the designer with the help of engine geometry, fuel and injection system parameters. Peak pressure is often limited by this distribution control and hence the cycle is also referred to as the “limited pressure cycle”.
The processes of the dual cycle are shown in Figure 9 and can be described as:
1 to 2 – Isentropic compression 2 to 2.5 – Constant volume heat addition 2.5 to 3 – Constant pressure heat addition 3 to 4 – Isentropic expansion 4 to 1 – Constant volume heat rejection
Figure 9 - Constant volume/Constant pressure heat addition - "Dual" - cycle
Heat addition (divided into the two parts)
Heat rejection
Compression Stroke
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Expansion stroke
Where parameter is defined as
or transformed to more natural variables of heat transfer
where
Leading to the indicated efficiency being stated as
Using α = 1 for the constant pressure model or β = 1 for the constant volume model they can be expressed as special cases of the dual cycle. The dual cycle model demands more input information from either the fractions of constant volume and constant pressure heat addition, or the maximum pressure, P3 [Heywood].
Splitting the two parts of heat addition 50/50 is a common assumption and based on that, an example calculation of the mixed cycle efficiency is performed for comparison to the Otto cycle.
Inputs:
Unknowns:
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Using eq. 3.22
Calculations:
Eq. 3.18
Assuming 50 % of fuel combustion at constant volume, eq. 3.16
Ideal gas law
and into eq. 3.24
Calculation of 50 % fuel combustion at constant pressure, analog to combustion at constant volume
and into eq. 3.23
and into eq. 3.22
The conclusion of the dual cycle analysis of a modern CI engine is that the calculated efficiency is higher than the one for the SI Otto cycle using normal input values. Often the compression ratio of CI engines climb above 16:1 (used in the example), which would have increased the indicated efficiency even more, but the drawback of very high compression CI engines is the large formation of emission due to high temperatures [Heywood].
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3.1.3 Efficiency explained Otto and Diesel engines are as said the primary alternatives when it comes to automobile energy conversion. Using these simple combustion cycle models it is shown that the theoretical maximum of the indicated efficiency are below 60 % in both cases. With normal inputs, of e.g. the compression ratio, the Ideal Otto cycle indicates around 50 % efficiency while the Ideal Diesel, analyzed with the mixed cycle model, reaches approximately 55-60 %. As mentioned earlier these models are simplifications of reality but give a good indication of what parameters are affecting the thermal efficiency of an engine.
But, the indicated efficiency calculated by these simple models does not take into account the real physics of combustion; neither does it calculate for heat loss to the cooling system during combustion. In the Otto and Diesel cycles the fuel is assumed to burn at rates which result in constant volume top dead centre combustion, or constant pressure combustion while actual engine pressure and temperature data does not match these profiles. The next step in modeling accuracy is a “Finite heat release” model which can answer questions such as the effect of spark timing and heat transfer on engine work and efficiency. These models compensate for the fact that the state-changes during compression and expansion not are isentropic and can therefore not be described with algebraic equations anymore. Instead it can be described by a Weibe function which extends outside the scope of this text. Also, combustion cycles like “Miller” and “Atkinson” which are modifications of the original Otto cycle are not investigated, but use the same principles as for the models mentioned.
Concluding the modeling of common ICE combustion processes with the used methods, the inaccuracy has to be accepted. But are inaccurate modeling of heat losses and time dependent combustion processes the total reason to why the effective efficiency of ICE’s sometimes are almost half of what the model estimates? No, it is not.
The components of ICE efficiency [Heywood]:
Combustion efficiency – from fuel to heat (estimated by the gas cycle models)
Thermal efficiency – from heat to piston force during compression and expansion
Gas exchange efficiency – gas transport in and out of the cylinder
Mechanical efficiency – movement of the mechanical components
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The mean effective pressures Modern gasoline engines have an average efficiency of about 25 – 30 % when used to power an automobile. In other words 70 – 75 % of the energy is lost as heat. There are several sources of this loss of energy into heat and they also apply to CI engines. Divided into its main consumers; pumping work, mechanical friction work and accessory work, they can be summarized and quantified as the total work of the “Friction Mean Effective Pressure” or FMEP, dimensionialized in the same way as IMEP. IMEP, the indicated mean effective pressure during combustion, is a measure of the work delivered to the piston per unit displaced volume. MEP scales out the effect of engine size and IMEP is per definition the work per unit displacement volume done by the gas during compression and expansion.
In other words, not all of the work transferred to the piston from the gases contained inside the cylinder (the indicated work) is available at the drive shaft for actual use. That portion of the work transferred which is not available for real usage is the friction work, i.e. the work of FMEP. Let’s look closer on the contributions to FMEP.
Mechanical friction mean effective pressure MFMEP is the source of the negative work, i.e. reducing the total work of the engine, done by the friction force of all moving components in an engine. The friction force, Ff, is defined as the product of the friction coefficient, f, and the normal force Fn. Furthermore the friction power Pf, is the product of the friction force Ff, and the relative velocity U.
The mechanical friction mean effective pressure consequently is said to scale as
where N is the engine speed, Vd, is the engine displacement volume, b is the cylinder bore, s is the piston stroke and nc is the number of cylinders.
The mechanical friction of ICE components depends on what friction regime it’s operating in. High speed conformal surfaces, such as the journal bearings, will operate in the full-film regime and as a result produce a low friction force. The oscillating movement of the piston- cylinder contact on the other hand will partly be operating in the boundary lubrication regime which means a high frictional force, the same goes for the sliding contacts in the valve train of the engine. The piston-cylinder contact and the valve-train are also, because of this, considered to be the main factors of MFMEP. Methods of reducing MFMEP can consist of reduced oil viscosity, roller cam followers and low friction coatings of the piston and ring assembly e.g [Heywood].
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Pumping mean effective pressure PMEP is the source of the negative work done by the losses inflicted by the pumping of air into and out of the engine. It is the sum of the pressure drops across flow restrictions during the intake and exhaust strokes. The flow restrictions are located in the intake system, intake valves, exhaust valves and exhaust system. One of the largest restrictions of a SI engine is the throttle itself. Because the power of a SI engine is controlled by throttling of air into the engine, the pumping losses will be large when the power output is low, thus in combination with a low IMEP at low load the efficiency of a SI engine is strongly dependent on the load subjected to it, performing badly at low load and best at high load. The CI engine on the other hand does not need a throttle for power control and can therefore reduce pumping losses significantly compared to the SI engine in low load conditions, on the other hand modern CI engines normally have electronically controlled air intake throttles anyway but for other reasons. The components of PMEP work scale with a combination of variables but roughly increases quadratic to an increase in the mass flow of air.
Especially for SI engines a lot of the recent development in engine technology has concerned the PMEP. First of, as of the end of the 90’s almost all SI engines utilize electronically controlled throttle bodies instead of cable operated. By the driver demanding a torque output from the engine management system, or EMS, the EMS itself can control the throttle body based on all available engine parameters, such as the rotational speed, and therefore minimize the effect of throttle body induced pumping losses. As the next step in development of SI engines, e.g. BMW, has removed the throttle body completely, instead controlling intake air flow and hence power output with the help of continuously variable intake valve lift. Thus eliminating one of the efficiency drawbacks of the SI compared to the CI engine.
Auxiliary mean effective pressure AMEP is the summed source of the remaining crankcase friction contributions such as the oil pump, water pump and the no-load friction of the alternator. But essentially for a complete system analysis the negative work done by the air conditioning compressor, power steering pump and other auxiliary devices could be added. The drawback of powering these auxiliary devices directly from the ICE is that the speed of the engine varies. All of the auxiliary devices have a varying efficiency of its own and can only reach its maximum efficiency at the specific operating point it was optimized for. Because of this it would be much better if components like the air conditioning compressor and the power steering pump could be operated at a fixed speed and only when they were needed. Common auxiliary devices and how they can be adapted to increase vehicle efficiency with the help of hybrid technology will be mentioned again later on.
Brake mean effective pressure Finally we sum the contributions of frictional work and state [Heywood]:
Leading to
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Equation 3.29 describes the relationship between the indicated work of IMEP and the available work done by BMEP at the drive shaft of the engine. The optimization goal is of course to minimize the negative effect of the FMEP and all of its contributions. This can be done in two ways: engine component design and development or by running the engine at a load corresponding to the minimal sum of the negative work of all of the frictional components. The real life definition of useful engine performance parameters used for efficiency evaluation follows.
BMEP for a 4-stroke engine based on the torque output is in the end defined as:
and in terms of power
Where:
Moving on, the brake specific fuel consumption (BSFC) is the fuel flow rate , divided by the brake power
This means that BSFC is a measure of engine efficiency, actually inversely related to the efficiency, i.e. the lower the BSFC the better the engine and higher efficiency. Ignoring the issue of assigning a value to the energy content of the fuel, the brake thermal efficiency can be written as:
Equation 3.14 indicates that BSFC is a valid measure of efficiency provided is held constant. This means that two different engines can be compared with BSFC measurements provided they are using the same fuel.
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Brake efficiency At this point, the maximum indicated efficiencies of the combustion cycles are known and the losses associated with the necessary components of a normal ICE and its auxiliary devices are mentioned. By summarizing all of these contributions the result can be translated to what more or less corresponds to the maximum possible efficiency of an engine, i.e. the ratio of the energy input of the fuel to the energy output from the engine shaft.
As an engine is its own air-supplier the power output is governed by the amount of air that the intake stroke manages to transport into the cylinder. The “Volumetric Efficiency” or ev, is a measure of this and is defined as the mass of fuel and air inducted into the cylinder divided by the mass that would occupy the displaced volume at the current density of the air in the intake manifold. Important to note is that volumetric efficiency is a mass ratio, not a volume ratio as the name suggests. The volumetric efficiency of an engine will be low at low engine speed, increase to its maximum and then drop at higher speeds. This characteristic is mostly affected by the design of the intake and exhaust manifold and the camshaft profile.
At a certain throttle position, the volumetric efficiency will reach its maximum at a specific engine speed and consequently the maximum amount of oxygen will at the same time be present in the cylinder. This corresponds to the highest possible combustion pressure and hence the IMEP, i.e. the BMEP if measured at the output shaft, has reached its maximum. BMEP will at this point be directly controlled by the throttle position of the SI engine and the fuel injection timing of the CI engine. Consequently, for the SI engine, increasing the throttle to its maximum answers to an increase in BMEP to its maximum (assuming optimal engine control). From eq. 3.30 it can be shown that this operating point also corresponds to the maximum torque delivery of the engine.
By creating a plot of BMEP, i.e. proportional to torque, as a function of engine speed, one point in this plot will have the maximum efficiency. But, what affects its position?
The engine speed of maximum efficiency is given by a number of factors but the most important one is the balance between the engine speed scaling of mechanical friction and the heat losses. Because of the latter being dependent on cylinder wall area different engine sizes will have different optimal engine speeds. For a 0.5 liter/cylinder displacement the optimum rpm will be in the range of 2-3000 rpm but for 2 liter/cylinder engines it drops to around 1200-1500 rpm. As a result, this is also the engine speeds modern automobiles respectively commercial vehicles are geared to work in during highway driving. Mechanical friction losses increase with increased speed but if the speed is dropped too much the combustion cycle will require more time and hence the heat losses will increase, therefore the thermal efficiency decreases with decreased engine speed.
The other positioning factor of the maximum efficiency load point is as mentioned, BMEP. As told, BMEP is a function of IMEP and the losses inflicted during operation. Starting from a low BMEP the efficiency tend to increase as BMEP is increased. However, when BMEP approaches the maximum in SI engines, the combustion efficiency tends to drop due to the engine controller richening the air/fuel mixture to prevent knock. On the other hand if we decrease BMEP (by throttling down) too much, the load will decrease and both the gas
[30] exchange efficiency and the mechanical efficiency will decrease. In the case of Otto engines also PMEP will increase with lower load and hence the gas exchange efficiency will decrease even further. Summarized, the operating point of maximum efficiency will be a tradeoff process between all of these variables.
A CI Diesel engine is in its original form better in all of these aspects and will therefore also have a higher efficiency. The combustion efficiency is better, the thermodynamic cycle is better (as calculated), there are no pumping losses due to throttles and combined with a higher mean pressure it increases the mechanical efficiency [Heywood].
Using a dynamometer and a fuel consumption measuring equipment a topology plot of the fuel consumption and hence the inverted efficiency can be created. Figure 10 shows an example plot derived from engine experiments performed in an engine dynamometer with a CI engine. The operating point of maximum efficiency is close to maximum BMEP, i.e. torque, at a level of 197g of diesel fuel for each kWh of produced energy at the crank.
From Figure 10 it can be observed that the area of maximum efficiency is very limited!
Figure 10 - BSFC plot of a 1.9l VAG TDI engine – [Tdiclub]
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In vehicle efficiency The maximum power of an ICE for conventional vehicle use is dimensioned by the desired maximum acceleration capacity of the vehicle. In normal road use of a vehicle, the velocity changes a lot depending on the traffic situation and the percentage of use that a vehicle spends at maximum acceleration is very small. During urban driving the velocity is low and the energy consumption of the vehicle itself will as a result, also be low. In these situations the required power output from the engine might be in the regime of 5 % of the maximum capacity. Looking at Figure 10 again it is evident that the area of maximum efficiency is very limited, and as concluded earlier, close to the point of maximum load. The sad result of this is that an ICE of a conventional vehicle spends most of its time converting energy at an operating point far from optimum!
Figure 11 - ICE load distribution of a conventional vehicle [Vinot 06]
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Figure 11 visualizes the distribution of operating points during normal driving with a manual 5-speed car in urban and highway conditions [Vinot 06]. For each level in the topology a number (in this case 600-250 g/kWh) specifies the Brake Specific Fuel Consumption, or BSFC, for specific load region. The colors in the plot visualize the stochastic distribution of all load points that were collected during the empirical test. For example, orange contains 90 % of all the load points found in the cycle.
It is evident that the engine is run during optimum conditions at a very small fraction of time. No more than <10 % in urban conditions and below approximately 15 % during highway driving. The mean ICE load increases with highway driving as the average speed increases, due to increased drag. Therefore the engine will run at a lower BSFC and consume less fuel for the given power output, i.e. efficiency is increased.
This is the most important factor regarding the low real-life average efficiency we obtain today. No matter how much effort is put into engine development to increase its efficiency, it can never reach high levels if it is used in non-optimal conditions. In a conventional drivetrain, where the engine is mechanically connected to the wheels for propulsion the vehicle energy consumption and power demand will directly affect the load of the ICE. As long as this is the case, the load point and hence the efficiency cannot be shifted into optimal conditions while conforming to the driver demand.
The solution is to decouple the power demand of the primary fuel converter from the direct power demand of the vehicle with the help of new drivetrain technology and layouts.
3.1.4 Emissions Unfortunately high engine efficiency, i.e. low fuel consumption, does not necessarily mean low pollutant emissions. Instead vehicle emissions are a function of the vehicle design, fuel type, driving style, driving cycle and engine control strategy amongst other variables.
In all ICE’s which combust fossil fuels, a number of different types of emissions will be formed. Some of them are regulated by legislation and some are not. ICE fuel consists of hydrocarbons (HC) which in ideal combustion will be converted into carbon dioxide (CO2) and water (H2O). This means that, if combustion is ideal, the CO2 emissions of a vehicle will be proportional to the fuel consumption and is often an important parameter in “green car” classifications.
Emissions regulated by law for passenger cars in Europe are hydrocarbons (HC), carbon monoxide (CO), nitrogen oxide (NOx) and particle matters (PM) [EC Automotive]. The emission formation mechanisms will not be mentioned in this thesis as the same methods for emission control in conventional vehicles, apply in theory to hybrid vehicles also.
As mentioned earlier, optimum combustion takes place close to the stoichiometric air/fuel ratio. The problem with conventional drivetrains is that the air/fuel ratio cannot be kept at stoichiometric all the time, e.g. during acceleration. This is because maximum ICE power often takes place at around 15-20 % richer mixture ratios than optimum, eg. due to knock limitations in SI engines. As a result, emission control and after-treatment is much more difficult during transient engine operation. The closer to a static load point we can get, the
[33] easier it will be to limit emissions and maximize the efficiency of e.g. the catalyst. Application of hybrid technology, which will be discussed later on, is one way to dampen the ICE transients and hence reduce emission formation.
Furthermore, it is very important to remember the governing mechanisms of pollutant formation and which variables that affects the efficiency of exhaust after-treatment systems when the control strategy of a hybrid vehicle is specified.
3.1.5 The future of ICE’s With or without hybrid technology, the development of automotive ICE’s will continue. Additionally some new methods and possibilities arise together with ICE use in hybrid drivetrains and that will hopefully work as catalyst on ICE development. Below, by the author’s opinion, the trends of automotive ICE development are listed.
Trends
Near / Now: SI - continuously variable valve lift, i.e. throttle removal to reduce PMEP. SI - Direct injection – Flexible control, decreased pumping losses and better knock control SI/CI - Downsizing with high pressure turbo charging – reduction in engine size increases efficiency potential, still maintaining a good performance with the use of high performance materials SI - Hybrid drivetrain adapted combustion cycles – Atkinson / Miller – increase in efficiency while the decrease in performance is compensated by the hybrid system. CI – Very high pressure injection system with high resolution control CI - SCR – Selective catalytic regeneration – NOx reduction with UREA CI – EGR – Exhaust gas recirculation – NOx reduction with inert gas CI – Particle filters
Far: SI/CI – Camless engines (friction reductions) / Free valves (lift independent on engine rotation) SI/CI – Variable compression, such as the Saab SVC engine SI/IC – New combustion processes – CAI (Controlled Auto Ignition) or HCCI (Homogenous Charge Compression Ignition) Pure electrical energy generators – E.g. free piston ICE’s
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3.2 Transmission A transmission, or gearbox, is the part of a motor vehicle or machine etc. that transmits power from the engine to the driven members, as the wheels, by means of belts, fluids, gears, etc [Your Dictionary]. A SI engine of an automobile rotates at a speed in the range of approximately 600-7000rpm while the wheels of the vehicle have to operate in the range of 0 to about 1800rpm. This means that the engine cannot be connected to the wheels directly or they would spin to fast and always move the car as long as the engine is running. The same goes for a CI engine, though the maximum engine speed often are limited to around 4500rpm.
The solution to the problem is to connect a gearbox between the engine and the wheels, hence enabling the driver to alter the gearing ratio between the engine and the wheels during operation. By doing this, the speed of the vehicle and the operating speed and load of the engine, can be adapted to fit the current driving situation.
There are many alternatives when it comes to gearboxes but the main differences between them is in what way they transfer the energy from the engine and how the gears are selected. The most common alternatives on the automobile market today are listed below in Table 2.
Transmission name “Clutch system” Speed altering method Shifting input Manual Manual dry lamella clutch Gear pairs Manual mechanic Robotic Comp. contr. dry lamella Gear pairs Manual “by wire” / clutch Automatic Automatic Torque converter Planetary gear set Manual “by wire” / Automatic CVT Torque converter / Variable centrifugal Continuously variable Comp. contr. wet clutch pulley Dual-clutch Comp. contr. dual wet Gear pairs Manual “by wire” / lamella clutches Automatic
Table 2 - List of common gearbox alternatives in automobiles
The automatic transmission type has been much more popular in America compared to e.g. Europe. The advantage of the automatic gearbox is the fact that the driver does not have to shift gears by his- or herself, therefore facilitating the use of the vehicle. In some countries it is possible to take a driving license that only applies to cars with automatic gearboxes for those who think manual shifting is too difficult.
All of the alternatives above use some kind of device to alter the ratio between the rotating speeds of the input shaft to the rotating speed of the output shaft. The goal is of course to lose as little energy as possible during this process but some types manage better than the others.
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3.2.1 Manual The manual gearbox is the simplest in design; it consists of a number of fixed gear-ratios, normally 5 or 6, plus the addition of a reverse gear. The gearing ratio is selected by the driver itself by moving the selector to the desired gear based on inputs from the current driving situation, e.g. engine speed. The procedure is fully manual and works in sequence by the driver disengaging the clutch by pressing the clutch pedal, thus disconnecting the engine from the rest of the drivetrain. While the clutch is disengaged the driver selects the desired gear and then smoothly engages the clutch again.
Normally the gears are positioned in an “H-pattern” with neutral in the middle, this enables the driver to jump from one gear to another without shifting constraints and e.g. skip gears. The other alternative, which is the case in motorcycles and many racing applications, is the sequential shifting mechanism. A sequential gearbox has to be shifted in sequence, up or down, and it is not possible to skip gears. The result is that the work load of the driver increases in some cases, e.g. when the vehicle is approaching an intersection and needs to shift gear from fifth to second, this has to be done with three separate commands with the sequential transmission. Which should be compared to just one (two when counting neutral) with the H-pattern gearbox, and therefore the H-pattern type fits normal vehicles better.
As mentioned, the manual gearbox disables the input torque by the driver manually disengaging the clutch. While the clutch is disengaged and shifting takes place, the engine idles and no load is submitted. During this period of time, per definition, the efficiency of the engine is 0 % as no power is transmitted and therefore it is important to minimize the shifting, clutch disengaging and clutch engaging time.
The transmission itself basically consists of an input shaft and an output shaft each equipped with a number of gears. By moving selector forks inside the transmission, pair of gears on each shaft corresponding to a given ratio can be engaged or disengaged.
The efficiency of an ideal gear-to-gear connection is approximately 99 % which is high but unfortunately a modern automotive manual transmission is not as simple as that. The losses can be divided into two main components, the mechanical losses and the spin-losses. The mechanical losses arise from the high pressure contact between the gears and scales with an increase in load. The main affecting factors are base oil, oil additives, resulting viscosity, oil level and of course gear design. The spin-losses are not load dependent, but instead speed dependent and will therefore be dominant under low load conditions [GMPT]
In Table 3, the efficiency of a modern FWD manual 6-speed transmission is stated. It is measured in a vehicle with the help of a chassis dynamometer during a MVEG-B cycle.
Input torque: 30Nm 50Nm 200Nm Gear 6 90 % 94 % 97 % Gear 5 - 94,3 % - Gear 4 - 95 % -
Table 3 - Manual transmission efficiency - at 3000rpm input [GMPT]
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Methods to increase the efficiency of manual gearboxes could for example be a revised lubrication system, decreased oil viscosity and low friction coatings of gears and bearings but there is not much room for improvement.
3.2.2 Robotic A “robotic” gearbox is basically a robotized manual gearbox controlled by a computer. The computer is used to operate solenoids or other type of actuators governing the shifting and clutch mechanisms otherwise controlled by the driver. The computer is then programmed to either listen to the will of the driver, i.e. “shifting by-wire” or shift automatically based mainly on engine rpm and driver torque demand. The positive features are the automatic shifting capability while sustaining the high efficiency of the mechanical gearbox combined with manual shifting without clutch operation. Above that, there are no advantages over the conventional manual gearbox.
The robotic transmission has not made a large impact on the market, mostly because of the need for relatively advanced auxiliary technology which comes with a high price to achieve decent shifting performance. Therefore it has been classified as a “luxury” item. Robotic gearboxes can be found in many Ferrari and Lamborghini high performance cars for example.
3.2.3 Automatic Automatic transmissions does not use parallel shafts with pairs of fixed gear ratios like the manual transmission, instead it uses a planetary gear set and clutches which alter the gear set constraints.
Any planetary gear set has three main components:
A sun gear Planet gears and the planet gears’ carrier A ring gear
Each of these three components can be the input, output or can be held stationary. Choosing which of the components does what determines the gear ratio of the gear set. The basic functionality of a planetary gear set is explained in Table 4.
Case Input Output Stationary Gear ratio A Sun (S) Planet carrier Ring (R) 1+R/S (C) B Planet carrier (C) Ring (R) Sun (S) 1/(1+S/R) C Sun (S) Ring (R) Planet carrier -R/S (C)
Table 4 - Planetary gear functionality
Additionally, locking any two of the three components together will force the whole device to a 1:1 gear ratio. Using a normal gearing for the desired application the first case in Table 4 would function as a reduction (>1:1) and the second one as an overdrive, i.e. the output speed is faster than the input speed. The last case will also be a reduction but operate in the
[37] reverse direction. By combining all of these functions with the help of clutches that can select which axle of the planetary gear set serves to which function, the basics of a 3-speed automatic gearbox are defined.
But, just like in the case of manual transmissions the automatic transmissions need a way to let the engine idle while the car comes to a stop. Manual transmissions use a clutch but an automatic transmission uses a hydrodynamic torque converter. The torque converter allows the engine to spin somewhat independently of the transmission. If the engine rotational speed is low, i.e. also the input shaft of the torque converter, a small amount of torque is passed through to the transmission itself. That’s why it only takes a very light press on the brake pedal to keep an automatic car standing still during idle. But as the engine speed is increased more and more fluid will be pumped into the torque converter, causing more torque to be transmitted to the planetary gear set. At a certain operating point the torque from the engine and the torque transmitted will equalize.
The main drawback of an automatic transmission equipped with a torque converter is the fact that the transmission will never move at exactly the same speed as the engine, there will always be some hydraulic slip in the converter. This difference in speed consumes energy and the efficiency of the transmission drops. A solution to the problem is to lock the speed of the engine to the speed of the converter when they approach the same speed. This is done in modern automatic gearboxes but there will always be some slip during acceleration. Compared to a manual transmission, a modern automatic transmission equipped with a torque-converter operates at about 5 % less efficiency.
Summarized, the only advantage of the automatic gearbox over the manual version is the automatic and often smooth gear shifting procedure. Modern automatic transmissions can often be operated in “manual mode” but the only difference is that the driver requests a gear from the transmission control unit, or TCU, instead of it calculating the optimal gear by itself. These “manual override” automatic gearboxes are often mistaken for robotic or dual clutch gearboxes. From an efficiency and fuel consumption point of view, the automatic transmission is obsolete.
3.2.4 CVT Mechanical transmissions consisting of gear pairs or planetary gears are constrained to the gear ratios chosen by the designer. The idea of the Continuously Variable transmission, or CVT, is as its name implies to offer a continuously variable gear ratio between the engine and the wheels.
There exist many types and different designs of CVT transmission but the most common one, if seen to all types of motor vehicles, is probably the “Variable-diameter pulley” CVT. Two V- belt pulleys are split perpendicular to their axis of rotation with a V-belt running in between them. The gear ratio is changed by moving the two parts of each pulley closer or further away from each other. Due to the V-shaped belt this causes the belt-pulley radius to increase or decrease. By doing this the gearing ratio between the pulleys can be changed while maintaining a proper amount of tension of the belt. The principle of a CVT transmission is shown in Figure 12.
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Figure 12 - CVT gearbox principle [Wikimedia Commons]
The main advantage of the CVT transmission is that the engine can be operated at its most efficient speed all the time, independent of the vehicle speed, as the transmission ratio is continuously variable. This can increase ICE efficiency and reduces emissions significantly in some situations but unfortunately there are some drawbacks of the CVT technology also. First off, the torque handling capability of CVT transmissions are low due to the design demand of some type of belt or chain as a connection between the two pulleys. Second off, as the CVT gearbox transfers power with the help of friction forces it requires very high contact pressures between the belt/chain and the “pulleys”, which also makes it prone to wear.
Moreover many CVT transmissions still use an efficiency limiting torque converter to limit the minimum torque to a level that doesn’t move the vehicle. CVT gearboxes have not been very popular so far and the main reason is the lack of increase in total drivetrain efficiency and the “boring” driving experience reported by the drivers *GMPT].
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3.2.5 Dual-clutch The dual-clutch transmission is the latest trend in automotive transmission technology for conventional drivetrains. The design of the dual-clutch gearbox is simple but yet ingenious. It basically consists of two manual gearboxes in parallel, i.e. for a 6-speed dual-clutch gearbox there are basically two 3-speed gearboxes in one casing, plus the reverse gear of course which is added to one of the “internal gearboxes”.
One of the transmission shafts (shaft 1) incorporates gears 1, 3, 5 and reverse while the other shaft (shaft 2) contains gears with even numbers, i.e. 2, 4 and 6. Each of the shafts has their own multi plate clutch that can be controlled individually. When a vehicle equipped with a dual-clutch transmission accelerates with first gear engaged, second gear is already pre-selected on transmission shaft 2 but without having its clutch engaged. When shifting is to be done, the Transmission Control Unit, or TCU, disengages the clutch of shaft 1 and engages the clutch of shaft 2 simultaneously, hence shifting is very fast and there will be no interruption in torque delivery. After shifting is done the TCU guesses what the next gear will be, i.e. during acceleration probably third gear, and preselects that gear on the disengaged shaft and so on.
According to Volkswagen which was the first manufacturer on the market to mass-produce dual-clutch gearboxes with the launch of their DSG in 2002, shifting takes no more than 9ms. As a result of the fast, smooth and constant torque shifting, VW states a reduction in fuel consumption of up to 10 % compared to a similar car equipped with a manual gearbox and even more compared to the much less fuel efficient torque-converter alternative of an conventional automatic. The only drawback except for higher cost and weight compared to a conventional manual gearbox is if the computer takes a wrong guess what the next gear will be. If the pre-selection is wrong, the TCU has to select a new gear when the driver demand comes which increases the shifting time to around 400 ms instead of 9 ms [VW DSG].
To the author’s opinion, the dual-clutch transmission will replace the automatic transmissions in Europe and probably in a further future, also in the rest of the world.
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3.3 Differentials The differential unit of a conventional drivetrain allows the wheels used for propulsion to rotate at different speeds. Especially during cornering, the inner and outer wheels will not follow the same path and hence not rotate at the same speed. The differential compensates for the differences in rotation rates thus preventing the drivetrain from “lockup”.
Apart from some special cases almost all differentials in the automobile industry are based on bevel gears of the hypoid type which allows for engagement of the gears in different planes. Hypoid gears are stronger, operate more quietly and can be used for higher gear ratios than standard bevel gears [GMPT]. The differential gears act as a balance of the torque distribution between the wheels. In situations where the friction coefficient of the road varies from side to side, the torque supplied to the side with better grip is limited to a value defined as twice the torque produced by the wheel on the low friction side [Bosch]. If the torque of one wheel exceeds the frictional resistance that wheel will spin and the other one will stand still. In a traction point of view this is an undesirable effect which can be prevented with a differential locking or braking mechanism, such as the Haldex eLSD for example.
The efficiency of a hypoid gear in an automobile differential is approximately 95 %. The relative low efficiency (for one set of gears) is mostly due to the sliding motion induced by the hypoid gear contact, which puts high demands on oil performance. Possible ways of increasing the efficiency can be low viscosity oil, reduced oil bath level or low friction coatings, but there is not much room for improvement here either [GMPT].
3.4 Auxiliary systems As mentioned, a modern automobile has several auxiliary systems connected to the engine. Some of them are crucial, such as the water pump used for cooling of the engine, the oil pump used for engine lubrication and the alternator used for charging of the battery. Others are there for safety or comfort increasing applications, such as power-steering and air conditioning etc. In a conventional vehicle all of these systems are powered by devices connected directly to the engine shaft by the means of belts or gears which signifies that they will be driven by the alternating speed of the engine.
In the same way as an ICE has an optimum operating speed, all of the auxiliary power devices do too. If e.g. the compressor of an air-conditioning system is powered by a source running at a preferred constant speed and shut off when not needed there is a substantial energy saving potential, growing with vehicle size and ambient temperature. Also, some of the auxiliary systems can be shut off during acceleration to increase performance and reduce consumption by load-leveling.
Optimization of auxiliary subsystems is not easy using conventional drivetrains due to the constraints of the engine speed, but, new technology such as hybrid power sources increases the possibility of improvement. The available methods will be mentioned as a part of hybrid drivetrain components.
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4. Vehicle propulsion alternatives Two main objectives for future vehicles have been identified, i.e. reduce the use of fossil fuel and reduce the amount of emissions affecting the environment. To some extent they go hand in hand but as mentioned, low fuel consumption and low emissions are not always the same thing. Let’s take a look at some alternatives and roughly how they can be used to reach the set targets.
4.1 Conventional vehicles As previously laid down, a conventional vehicle with an all-mechanical drivetrain suffers from the direct dependency between the current vehicle load and the ICE load. Therefore the total average efficiency is hard to increase without having to sacrifice performance.
However, alternative fuels can be used in the ICE to reduce emissions, minimize or erase the use of fossil fuel, and in some cases increase the efficiency of the ICE itself.
4.1.1 Ethanol and methanol Ethanol and methanol are fully renewable fuels that are can be used in ordinary SI-engines without large modifications. Ethanol can also be used in CI-engines but suffers from a low cetane-number and thus requires ignition enhancers.
Methanol has been used for a long time as replacement for petrol. It is a very toxic alcohol which has less than half the heating energy compared to petrol. This requires the fuel system to be dimensioned for much higher flows. The same goes for ethanol but the heating energy is somewhat higher (around 40 % of petrol) and the physical and chemical behaviour is more similar to petrol as well [Heywood].
Methanol is very aggressive to some metals and plastics which require many components in the fuel system to be replaced if a petrol engine is to be converted to run on methanol. Additionally, the combustion of methanol generates formic acids which can be very corrosive for the engine and exhaust system. To limit these damages, a special lubricant has to be used. Ethanol is less aggressive to the engine, however some components, e.g. rubber sealing’s, has to be replaced and at the same time some galvanic corrosion can arise [VV Fuels].
Compared to petrol, the vapour pressure for both ethanol and methanol is lower. Combined with a higher enthalpy of vaporization, cold starting is an issue. Starting a conventional engine with pure ethanol is almost impossible below +10 °C and therefore these alcohols are often mixed with 15-25 % petrol for use in normal vehicles.
The combustion temperature of these alcohols is lower than the one for petrol. NOx emissions are thus reduced but in the same time acetaldehyde and formaldehyde, which both is cancerous emissions, increase.
Common for both ethanol and methanol is increase resistance to knock compared to petrol due to a higher octane number. This enables the compression ratio to be increased and the ignition to be advanced and thus a higher efficiency can be reached [Heywood].
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Both alcohols can be produced from biomass and the CO2 emissions from combustion of bio- ethanol contribute less to global warming compared to petrol. How much on the other hand depends on which raw product and production method that was used. Straw sugar ethanol renders a low contribution while ethanol from grains contributes much more, especially if the distillation plant is heated with coal. Compared to methanol, ethanol production is much more complex which makes it more expensive. Furthermore, as production of these alcohol fuels both can consume grains that otherwise could have been used for food, they have been criticized as a long time alternative [VV Fuels].
Ethanol mixed with petrol is a very popular “bio-fuel” already. So called “Flex-fuel” vehicles are used all over the world and in e.g. Sweden, E85 has gained a large part of the market.
4.3.2 Biogas and natural gas Both biogas and natural gas consist mainly of methane. They both stay gaseous under very high pressure and are therefore most often stored in their original form (CNG, 250 bar). There are also systems which are based on liquid storage (LNG) but there is a limited use so far. Methane is an excellent fuel in SI-engines due to a high octane number. When used in modern engines at stoichiometric combustion, methane works well with ordinary three-way- catalysts and therefore obtain good emission characteristics also. However, it is very important to limit leakage of methane in the atmosphere as it is a much worse GHG gas compared to CO2 [VV Fuels].
Biogas is an excellent renewable fuel but the main drawbacks are: difficult (low energy density) storage, lack of infrastructure and a relative expensive conversion of petrol fuelled SI-engines. Natural gas suffers from the same problems as Biogas but as natural gas is a fossil fuel it cannot be used to plan a sustainable environment for the future.
4.3.3 FAME Rapeseed oil and other vegetable oils could in theory be used as fuels in CI-engines that otherwise run on diesel. But due to the high viscosity of the raw-pressed oil at low temperatures it cannot be used without refining. The long molecular chains also contribute with deposits during combustion. One method to reduce the viscosity is to “esterize” the oil with methanol to obtain fatty acid methyl ester, or FAME. If rapeseed oil is used as raw product, Rapeseed Methyl Ester (RME) is created. Other raw products, such as soybeans and palm-oil lead to Soy Methyl Ester (SME) and Palm Methyl Ester (PME).
RME has approximately 10 % less energy compared to diesel and thus there is a similar increase in fuel consumption. The difference in emissions however is relatively small between RME and ordinary diesel. Instead the advantage is its bio-mass origin which limits the environmental impact to some extent. Due to the long molecules, use of FAME at low temperatures is also problematic [VV Fuels].
4.3.4 DME Dimethyl ether is normally used to power e.g. spray cans. It contains a high amount of oxygen and because of that it is almost soot-free when used in CI-engines. DME is very energy efficient to produce from biomass and good emission levels are achievable. DME is gaseous at room temperature and has to be pressurized above ~5 bar and stored as a liquid.
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The largest drawback of DME is the need of complex injection and management technology due to bad lubrication characteristics and a very low viscosity [VV Fuels].
4.3.5 Synthetic diesel Fischer-Tropsch diesel or synthetic diesel is relatively common in some parts of the world today, e.g. South-Africa. It is made in a catalytic process from either gaseous carbon or natural gas, so called syngas, but can also be made from gasification of biomass.
Synthetic diesel is a very pure diesel without sulphur or aromatics and therefore the particle emissions are less than for standard diesel fuels while the NOx emissions are unaffected. The GHG reduction potential of synthetic diesel is good but not as good as for DME if it’s made from biomass. However, Fischer-Tropsch diesel benefits from the fact that it can be used in unmodified diesel engines and be mixed with ordinary diesel in any concentration [VV Fuels].
4.2 Electric vehicles An electric vehicle is by itself a zero emission vehicle. Due to the nature of electrical machines, the internal energy efficiency is also very good. The GHG contribution from an electric vehicle, or EV, is therefore to almost 100 % governed by the emissions of the electricity source. To a very large extent the environmental impact is therefore affected by where on the planet the vehicle is charged as different types of electrical energy generation are used in different parts of the world. Renewable energy sources, such as wind, water and solar power etc. makes an EV the ideal choice when it comes to emissions and energy efficiency.
Unfortunately, there is a drawback. EV’s have always been limited by their short range capability due to the low energy density of available battery technology. Recent battery development has increased the potential of EV’s but new applications, such as “plug-in hybrids” arise in parallel and offer even a greater level of flexibility together with many of the best features of an EV. This subject will be discussed in detail further on but EV’s will most certainly have a part of the future automotive world, especially in short-range urban transport applications.
4.3 Fuel cell vehicles A fuel cell generates electricity and consequently a fuel cell vehicle is more or less an EV where the battery has been “replaced” by a fuel cell.
Fuel cells can be powered with either hydrogen directly or indirectly with e.g. methanol. The potential of fuel cells are tremendous as the energy source is 100 % renewable and the only emission of the cell itself is pure water. Unfortunately the fuel cell technology has so far been expensive and there are difficulties, coupled with e.g. fuel storage.
At the moment, due to the drawbacks, fuel-cells are not suitable for mass production of vehicles but the technology brings hope to the future and has by the author’s opinion a great potential in the long term. Fuel cell technology and vehicle application will be discussed again in chapter 6.
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4.4 Hybrids A hybrid is a flexible vehicle that utilizes an additional energy source to enhance the behavior of e.g. a conventional vehicle. The hybrid will be defined and explained in detail from the chapter 5 and on, but let’s jump directly to the potential benefits of hybrid technology for use as comparison to other vehicle propulsion methods:
Main features Load leveling and/or intermittent engine use Engine downsizing/rightsizing Regenerative braking Increased electrical power/energy – if electrical hybrid Pre-charging (Plug In hybrids)
Possibilities Reduced fuel consumption and emission levels due to e.g. an optimized engine load Enhanced driving experience – e.g. performance and NVH Increased electrical power adds new possibilities – e.g. electrification of auxiliaries Zero emission operation (Plug In, renewable energy depletion)
On top of these new features and their possibilities which potential will be thoroughly discussed ahead, the ICE of a hybrid vehicle can of course also be run on a renewable fuel.
Without saying too much, let’s let the numbers talk by themselves by comparing the Lexus RX350 vs. the Lexus RX400h hybrid. Both cars are “identical” apart from the hybrid drivetrain in the RX400h [Lexus].
Lexus RX350 Lexus RX400h Weight [kg] 1990 2120 Acc. 0-100km/h [s] 7,8 7,6 Fuel cons. urban [l/100km] 15,7 9,1 Fuel cons. highway [l/100km] 8,5 7,6 Fuel cons. combined [l/100km] 11,2 8,1
Emissions CO2 combined [g/km] 264 192 Price [€] (Lexus Sweden) ~43000 ~48000
Table 5 - Comparison (non-hybrid/hybrid) - Lexus RX350/RX400h - [Lexus]
As seen in Table 5 the RX400h has better acceleration performance while the combined fuel consumption is over 27 % lower. And on top of that the hybrid vehicle is 130 kg heavier! By referring to eq. 2.1 this type of behavior is called the “vertical-leap effect”, where the fuel consumption is much less than predicted for the given vehicle weight [Peng, Filipi].
This simple example demonstrates the real life potential of hybrid vehicle technology and is a motivation to learn more.
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4.5 The customer point of view Renewable fuels, electric vehicles, fuel cells, hybrids; if it isn’t confusing for an engineer, it will most likely be for the customer. Therefore it is important, as always with new products, to remember what the customer seeks.
To begin with, the cost is always important. By looking at Table 5 again it is clear that the only drawback of the RX400h in the fact sheet is the increased initial cost. The price difference in Sweden as an example is 5000 € [Lexus]. With the help of the reduced combined fuel consumption the fuel cost savings are 3.875 €/100km at a fuel cost of an 1.25 € average. The payback time will consequently be 1290000 km or 6.5 years at a 20000 km/year average. No additional maintenance cost for the hybrid system is calculated for in this example because of the 8 year or 160000 km (whichever comes first) hybrid system warranty [Lexus].
Without statistics to back it up, but to the authors belief, the average Lexus RX400h buyer will not keep his or hers car for almost 7 years, which therefore makes it more costly than the RX350 alternative. However there are some more factors to consider. First off, the second hand value of the RX400h is probably higher than the RX350 but more importantly, if the RX400h is classified as an “environmentally friendly vehicle” in the country it is being used in, some additional benefits can follow.
Examples of such, often government-funded, benefits are:
Monetary contribution – new car buy Tax reductions Free road tolls Free parking Etc…
These “eco-vehicle” enticements are common and have the potential to shorten the payback time to a great extent. Nevertheless, seen in the long run, the increased cost has to be pushed down as volumes increase. The enticements will probably decrease in the future. Another government funded stimulus that is intended to reduce the average emission levels of a vehicle fleet can be re-funded recycling of old vehicles. Not only does it remove the “dirty” vehicles from the road, it also encourages new car buys.
So, if the potential customer can afford the car, what does he or she look for next? Studies by Peng and Filip have shown that if you are developing a groundbreaking product, e.g. the first mid-size production car below 3 l/100km in consumption, it is ok to push one attribute to the extreme, e.g. the body design. However, if customers have to sacrifice more than one important feature or if they encounter more than one stand out problem, e.g. less than 4 seats, lousy performance etc., they decline and choose another alternative. This is believed to be the reason why the first hybrid car that was introduced in the USA, the Honda Insight, did not sell nearly as well as the Toyota Prius which was introduced one year later. It did not help that the Insight was even more economical than the Prius. The stand-out design, small compartment and poor performance was considered a too large sacrifice [Peng, Filipi].
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4.6 Now, tomorrow and the future There is a reason why ICE’s continues to be the fuel converter of choice for vehicle propulsion. The power density of up to 90 kW and above per displacement liter makes the ICE relatively compact and powerful. At the same time, the maximum obtainable efficiency is relatively high at 35 % and above. Also, the ICE can use a fuel with high energy density which makes storage and transportation easy.
Batteries and fuel cells are at the moment 20-100 times less energy dense than ordinary ICE fuel [INVESTIRE]. As no breakthrough in battery or fuel cell technology is in the near scope, ICE based vehicles has to be adopted to meet new standards. Major forms of automotive fossil fuels, i.e. crude oil and natural gas, may be depleted in 35-50 years if no major change in behavior is taken [EIA].
To the authors opinion, in the short to mid-term range, fuel saving has to be done with a combination of vehicles that are optimized for their intended use. Hybrid vehicles offer the flexibility to widen the efficient operating range of a specific vehicle and together with e.g. advanced ICE technology and renewable fuels it is a powerful path to direct action. The technology is available and on top of that, the plug-in hybrid offers the possibility of operation with extremely low emissions for a limited range in areas where electricity is generated by renewable sources.
In the long run, personal ground transportation has to be reworked to use only renewable energy sources. If nothing drastically happens to battery technology, the authors hope is with hydrogen as the energy carrier. But for now, let’s learn about hybrids and their enormous potential, as the technology is available and makes it possible to maximize the potential of our current fuel converter of choice, the ICE.
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5. What is a hybrid? A hybrid is per definition something that has two different types of components performing essentially the same task [Your Dictionary]. In the case of hybrid vehicles, the device normally used for propulsion is accompanied by another device, also contributing with a propulsive force, but using another energy source and transport method.
The two energy sources of a hybrid vehicle can be divided into a primary and a secondary source. The primary energy source is the “original one” and is commonly refilled manually, e.g. with petrol at a petrol-station. The secondary energy source however, often transparent to the driver, is generally refilled and emptied automatically by the vehicle itself. Most often with energy that from the beginning originates from the primary source but sometimes with energy from an external system that otherwise would not have been harnessed.
5.1 Hybrids in history Before we proceed, one thing should be clear, hybrid vehicles is not a new invention! Ships, submarines and locomotives have all been using hybrid technology since the beginning of the 20th century. The first diesel motor ship was also the first diesel-electric ship, the Russian tanker Vandal was launched in 1903, hence has been using hybrid technology for over 100 years (if it still floats) [Britannica]. The hybrid drive consisted of a diesel engine connected to a generator which generated electricity, then used by an electrical motor rotating the propeller. Furthermore also steam turbine-electric propulsion has been used since the 1920s in American battleships. A modern example of hybrid ship propulsion is electric motors mounted in pods underneath the hull, called azimuth thrusters, which allows for 360° rotation of the thrust and are shown in Figure 13.
Figure 13 - An example of hybrid technology used in ships - Siemens azimuth thrusters [Wikimedia GNU]
Another large and existing application of hybrid technology is diesel-electric trains. Also developed in the 1920’s, these hybrid vehicles was initially used for moving trains around in
[48] railroad yards but soon became popular in wider type of use. The main reasons were the great simplification of transmitting power from one engine to all wheels, using multiple motors and electricity instead of complex mechanics, also reducing maintenance cost [Britannica].
5.2 Hybrid cars So what about hybrid cars, that has to be a new invention considering the none existing range of hybrids cars on the market before 1997, where the launch of the nowadays famous Toyota Prius took place. Strangely, the answer is no.
Dr Ferdinand Porsche (Porsches founder), in 1899 a young engineer at Jacob Lohner & Co, built the first hybrid car known to mankind. The Lohner-Porsche hybrid consisted of a petrol engine rotating at constant speed, driving a simple generator. The generator charged a bank of accumulators, which in turn, powered electric motors mounted in the hubs of the front wheels. The simplicity of the drivetrain rendered drive shafts, transmissions and chains etc. unnecessary, hence enabling Porsche to achieve a, for the time being, high efficiency of the vehicle.
With its first appearance on the “World exhibition of Paris”, April 14th 1900, the Lohner- Porsche, seen in Figure 14, amazed the automobile industry of that time and became the success which promoted Porsches career. The patent was sold to Emil Jellinek later on, at the time, a member of the Daimler-Motoren-Gesellschaft board and whose daughter Mercedes Jellinek, later came to name one of today’s large car-brands [Porsche-Lohner]
Figure 14 - Worlds first hybrid car - the 1899 Lohner Porsche [Wikimedia commons]
Questions that arises, is of course, what happened? Why did development stop? What happened with the hybrids? Why wait until now? [49]
The answers to those questions are of course complex but could most likely be summarized as the effects of higher vehicle costs, lack of understanding oil consumption and supply problematic, and at some stages being limited by technology. As the total oil consumption by vehicles was low (for some time partly due to the low number of vehicles) and with an “unlimited” supply of oil – why bother developing more expensive vehicles that no one would buy?
Supply and demand is often the answer too many questions and probably this one too. As the number of oil consuming vehicles increased during the 20th century, demand did too. Supply has for long not been a problem, i.e. the demand was met. At some point the demand will exceed the possibility of our oil supplies and hence demand for alternatives arise. This is where we are now or soon to be, and as always, if there is demand - supplying creates a possibility of monetary profits and is therefore prioritized. The hybrids are back!
Fortunately, as a product of hybrid vehicle component technology being of interest in other applications than cars, e.g. batteries and electrical machines, we are not at the same stage of technical development as we where when Porsche built his prototype in 1899.
Between 1910 and 1990 the development of hybrid cars known to the general were almost non-existing. In 1989 Audi presented a concept based on the 100 model, named 100 Duo which was equipped with a electric motor to power the wheels, it was followed by a second generation in 1991 [Audi]. But it would wait until 1997 and the introduction of the Toyota Prius before large scale series production of hybrid cars began. It was followed by the Honda Insight in 1999, becoming the first mass produced hybrid car to be sold outside of Japan [Honda+Toyota]
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6. Sources of hybrid power and energy storage The first classification that should be made of a hybrid vehicle is to identify the energy sources, i.e. the storage methods that are used within the vehicle. For a conventional vehicle with an all-mechanical drivetrain using an internal combustion engine (ICE) as energy converter, the energy is most often stored as liquid gasoline or diesel. The ICE converts the chemical energy of the fuel to mechanical energy which is then routed to the wheels for propulsion, hence the alternative reference, fuel or energy converter.
A hybrid vehicle consists of a primary and secondary energy source. The primary energy source fuels the primary energy converter and normally uses a liquid fuel as source and an ICE as converter. With the addition of a secondary energy source, and a secondary converter, the hybrid vehicle is constituted.
The purpose of the secondary energy source, in efficiency enhancing applications, is to eliminate or reduce the effect of, the bad characteristics of the primary energy source. This can be done in several ways, with different sources of secondary energy transport as a tool. But which one is to be preferred?
In a vehicle, volume and mass are limiting factors, and as we know, a physically large vehicle with high mass will consume more energy than a light and nimble alternative. To minimize the addition of mass and volume together with the entry of a secondary energy source, energy and power density is therefore important factors, both in respect to mass and volume.
In the same time, there is no purpose of adding another energy source and a secondary energy converter if the resulting mean efficiency is lower than the primary alternative alone, i.e. high secondary energy converter and storage method efficiency is of the essence. Summarized the most important factors of general alternative energy sources for hybrid vehicles are:
Energy to mass density Energy to volume density Power to mass density Power to volume density Secondary energy storage efficiency Secondary energy converter efficiency Cost
Hybridization can be achieved in many ways, with a number of alternatives for energy sources. The most common ones are:
Human Environmental Pneumatic and Hydraulic Flywheel
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Electrical
Let’s have a look on the idea of these alternatives. But first it should be noticed, if energy and power density is discussed in HEV related texts, hence also in this thesis, the energy and power to mass ratio is primarily considered, if not otherwise mentioned. Even though it conflicts with the normal (mass per unit volume) definition of density.
6.1 Human One of the most common applications of hybrid technology in the middle of the 20th century is a matter a fact, the “Human – ICE” hybrid. Bicycles equipped with helper motors in various locations were introduced in the early 1950s, and are since known as mopeds. An example of this type is the VéloSoleX, which simply had a rubber roller driving the front tire, enabling the operator to use the engine and their own muscle power in parallel to propel the vehicle.
The design was very simple, but effective. A more modern application is human electric hybrid vehicles whose drivetrain consists of a human being, an electrical motor/generator and an energy accumulator of some kind, e.g. a battery. Most often it has the characteristics of a bicycle but with improved acceleration and the possibility of regenerative braking. One could also keep generating energy while standing still, in traffic stops for example.
The “TWIKE”, seen in Figure 15, is an example of a human electric hybrid vehicle designed to carry two passengers and some cargo. It can be driven in electric only mode or electric and pedal power mode. The TWIKE I was developed for the 1986 World EXPO in Vancouver by a group of Swiss students, but only driven by human power. The TWIKE II, which was developed by a group of enthusiast, was converted to hybrid drive and the vehicle rights is nowadays own by a German company who still produces the TWIKE. According to TWIKE enthusiast, over 850 vehicles has been sold, mostly to Switzerland and Germany.
Figure 15 - TWIKE - A human electric hybrid vehicle [Wikimedia Commons]
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Regarding the use of human powered hybrids in the future, their main limitation is the low power output of a human being, not to mention the large sacrifice of comfort, arriving exhausted at your destination. The concept is good though, but mainly for improving the possibilities of short range urban transport, normally done with a bicycle. The human hybrid is summarized in Table 6.
Pros Cons 100 % Renewable Low comfort Low Cost Low power Low energy capacity
Table 6 - Pros and Cons of Human powered hybrids
6.2 Environmental Sunshine and wind are two enormous environmental forces with unlimited potential; however the problem is to extract them.
A solar vehicle is an electric vehicle powered by solar energy, captured by solar panels mounted on the surface of the vehicle (generally, the roof). Photovoltaic cells in the panels, convert the solar energy to electricity which is then stored in some type of accumulator, e.g. a battery. The electricity is then used to propel the vehicle with the help of an electrical motor. At the present, solar vehicles are not practical for everyday use in large scale as the power to area density of the solar panels is too low, as seen in Figure 16, making the vehicles very large if normal car performance is wanted. On the other hand, solar panels could be used as a third, non propulsive, energy source, e.g. charging the batteries of a HEV during hot days to compensate for air conditioning.
Figure 16 - Solar vehicle with maximized top area [Wikimedia commons]
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Another environmental force that possesses large quantities of energy is the wind. The problem is once again the extraction as the change in direction and strength makes it difficult. The wind is most easily harnessed at sea where there are no obstructions in the way. Large ships often use very large diesel engines as their primary energy source and quite often electricity as a secondary alternative. Hence ships are often already hybrids, making the addition of wind or solar power, create a “Tribrid vehicle”. A tribrid application example is shown in Figure 17 where large and slow seagoing vessels harness the energy of the wind.
One great thing about harnessing the force of nature though, is the fact that the energy itself is “free”.
Figure 17 - Ship harnessing wind energy [www.skysails.info]
Pros Cons 100 % Renewable Ineffective extraction Unlimited energy Solar power is spacious Low cost Low usable power
Table 7 - Pros and Cons of environmentally powered hybrids
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6.3 Pneumatic and hydraulic The principle of energy storage in a compressed gas or liquid basically consists of two components; a storage volume (accumulator) and a transformer (compressor/expander) which constitutes the link between the energy of the stored gas or liquid and the mechanical output.
A pneumatic/hydraulic energy storage system, which inserts a liquid step between the air and “shaft” generates pressure in the liquid first and then transmits the energy to the air. The purpose for this, seemingly complex system is to increase the transforming efficiency, during compression and expansion from high pressure to low pressure and vice versa. Liquid dynamics imply better energy density for both volumetric (reciprocating) and centrifugal (turbine) machines than the gaseous state (EPA Hydraulic).
However, the laws of thermo-dynamics clearly state that all of these transformation concepts will be limited in efficiency by strong heat up during compression and strong cooling during expansion. The isothermal deviation lowers the efficiency a lot, e.g. a temperature variation of 50 °C translates to about a 10 % drop in efficiency. This fact states that compressor and expander design must limit large temperature variations during operation, otherwise efficiency will drop and the energy consumption increases.
There are three main alternatives when it comes to pneumatic/hydraulic hybrid drivetrain design [Nextenergy + EPA Hydraulic + INVESTIRE].
6.3.1 Direct acting pneumatic mechanical reciprocation system Direct acting pneumatic systems has a very low efficiency due to no possibility of regeneration so far, hence it is limited by technology. Expansion (energy output) is created with the means of pressure reduction over a motor from around 200-300 bar to 30-40 bar, trying to avoid strong cooling in the reciprocating motor. Together with a standard multistage compressor (η = 50 %) complete cycle efficiency will be approximately 25 % which is low [INVESTIRE].
6.3.2 Liquid accumulator piston systems Standard hydraulic oil is used to compress or expand trapped air, nitrogen or another gas into a system integrated accumulator. The complete cycle efficiency can reach up to 70 % with standard steel bottles with a rated pressure of 200-300 bar. Due to slow changes in pressure seen over the whole cycle and the heat exchanger effect of the large area of the bottles, the heat fluctuations will be kept at a minimum. The drawbacks are large packs of accumulators and the need to have a liquid reservoir with a capacity of minimum ~60 % of the accumulator volume. The result is poor energy density with around 3 Wh/kg with a 250 bar steel accumulator. The power density on the other hand is high, as hydraulic motors can reach over 7 kW/kg. Application wise the system is better suited for high power to capacity ratios and fast transient discharge/charge cycles, such as stop and go routes [INVESTIRE].
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A “perfect” application would be stop and go delivery vans, which is also adopted by UPS and EPA in a recent joint venture. Figure 18 shows the drivetrain layout of the Eaton built van.
Functional description [EPA Hydraulic]:
The high pressure accumulator stores energy as a “battery” using hydraulic fluid to compress nitrogen gas.
The low pressure reservoir stores the low pressure fluid after it has been “used” by the pump/motor.
The rear drive pump/motor converts the pressure from the hydraulic fluid into rotating power for the wheels, and recovers braking energy which is stored in the high pressure accumulator.
The engine pump/motor pressurizes and transfers hydraulic fluid to the rear drive pump/motor and/or high pressure accumulator
A hybrid controller monitors the driver’s acceleration and braking demands and commands the hybrid system components.
Figure 18 - Hydraulic hybrid delivery truck drivetrain [EPA Hydraulic]
The EPA/Eaton/UPS hydraulic hybrid is expected to enhance the fuel economy during normal operation up to 60-70 % compared to a conventional delivery vehicle [EPA Hydraulic]. To a
[56] large extent this is possible due to the special and highly transient driving cycle these vehicles are used in. The main concept is the same as for all types of hybrids though; fuel economy is increased by the main three ways: while braking, the vehicles kinetic energy is recovered and stored, while moving, the engine operates in a more efficient load point, and when standing still or decelerating, the engine can be shut off.
6.3.3 Reciprocating interface system Categorized as under development, high efficiency compressor/expanders close to isothermal conditions thanks to heat exchangers integrated into the piston work chambers, can be an a interesting alternative for hybridization in the future. Computations have promised energy densities of around 30 Wh/kg for a 250 bar pressure using steel bottles, and even higher with CNG4 composite accumulators. At a present development stage [ITpower] the system seem to reach 65 % efficiency which is a little lower compared to the liquid piston system. However, as this air-to-oil interface system uses only air in the accumulators the energy density is much higher. Unfortunately, the use of ambient air in the system requires additional components and the projected price is about twice the one for a liquid piston system [INVESTIRE].
Summarized, the pneumatic/hydraulic hybrid lacks energy storage density but can be an interesting complement to the electrical hybrid in the future. The special features of the pneumatic/hydraulic hybrid such as the inherent advantages of pressurized systems, e.g. accurate SOC estimation and tolerance to high rate of charge/discharge make it a strong option and even now, topical in some special applications such as the delivery van.
The cost of most pneumatic/hydraulic hybrid systems are quite low as the technology itself is widely spread and used in numerous industrial applications. Also energy storage does not require any expensive raw materials. The EPA/Eaton/UPS truck is projected to add 10-15 % of base vehicle cost at high volume production [EPA Hydraulic]
Pros Cons High power density Low energy density High pump/motor eff. (dep. On ) Not as flexible and precise as el. Very eff. regeneration and reuse (>electrical) Can be noisy High ROC / ROD capability Mature tech – not sensitive to environment Exact SOC estimation Relative low cost
Table 8 - Pros and Cons of pneumatic/hydraulic hybrids
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6.4 Flywheel Storage of energy in a flywheel works by acceleration of a rotor to a very high speed and storing the energy in the system as inertial energy. An accelerating torque causes the flywheel to increase its speed and store energy, while a decelerating torque causes the flywheel to reduce its speed and regenerate energy.
Since the industrial revolution, flywheels have been used in most rotating engines and machines for very short time energy storage. One good example is the smoothing of pressure pulses in an ICE, minimizing engine speed oscillations during operation.
The amount of energy that can be stored in a flywheel is governed by two main variables, the inertia of the rotor and the rotor rotational speed.
The kinetic energy of a rotating mass can be described as [Nordling, Österman]:
where J is the moment of inertia, and ω is the angular velocity.
The moment of inertia is a function of the mass and shape of the flywheel:
where x is the distance of the differential mass dmx from the axis of rotation.
In the simple example of a flywheel with the mass m concentrated inside the edge at radius r, then the moment of inertia is given by:
Substituting eq. 6.3 into 6.1 gives:
Which shows that high angular velocity is of much larger influence than mass to achieve high stored energy.
The tensile strength of the material defines the maximum angular velocity. An example is the imaginary flywheel with all of the mass concentrated at rim radius r, the tensile stress in the rim at speed ω is:
Thus defining the maximum angular velocity ωmax for a given strength of the material σmax.
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The maximum amount of energy storage in a flywheel is then described by:
Which shows that the maximum energy that may be stored for a given mass is achieved by a flywheel made from material which combines high tensile strength with low density. Hence, to obtain a high specific energy at high speeds, composite materials are better than metal.
A material comparison for flywheel energy storage is shown in Table 9.
Material Density Stength Theoretical max. spec. energy
Steel (AISI 4340) 7800 1800 32 Alloy (AlMnMg) 2700 600 31 Titanium (TiAl6Zr5) 4500 1200 37 Glass fibre reinforced polymer 2000 1600 111 Carbon fibre reinforced polymer 1500 2400 222
Table 9 - Comparison of flywheel rotor material on energy storage capacity [INVESTIRE]
Another great feature of the kinetic energy storage with the help of flywheels is the quick charging and discharging. In the case of a flywheel-electrical storage system where a motor/generator is connected to the flywheel, used both for acceleration (energy storage) and deceleration (energy usage), the rate of charge/discharge is only limited by the electrical machine itself. Therefore it is possible to withdraw and add large amounts of energy in a far shorter time than with traditional chemical batteries.
Flywheels are not affected by temperature changes in the same way as batteries, nor do they suffer from “memory effect”. They are also environmentally friendly consisting largely of carbon materials. Another great function is the ability of exact calculation of system SOC by just measuring the rotational speed of the flywheel and calculating backwards.
The application of flywheel energy storage in hybrid vehicles can be done in different ways, some examples are:
Pure mechanical hybrid drives – ICE and flywheel linked by a CVT.
Electromechanical hybrid drives – ICE via motor/generator linked to flywheel, driving electrical machines.
Electrochemical drives – Accumulator linked to electrical machine, using flywheel as buffer
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The main limitations of flywheel energy storage are relative high self-discharge coupled with the full cycle efficiency being highly dependent on the rate of discharge plus the speed limitation of the maximum tensile strength. Regarding energy and power density, composite rotors have achieved up to 100 Wh/kg which is much lower than the theoretical maxima for the materials used as specified in Table 9, a limitation in design so far. Power density is mainly limited by the auxiliary devices but in an electrical system figures around 1.5 kW/kg is possible. [INVESTIRE]
In 2009, FIA, the controlling organization of Formula 1, has granted use of kinetic energy recovery, or KERS. The methods of recovery are many but some teams will be using flywheel energy storage, capable of 400 KJ of energy storage and 60 kW of power during 6s at a time. The system consist of a high energy density flywheel rotating at almost 60000rpm coupled to a CVT. Total system weight is stated to be 24 kg [RCE KERS] making the power density of the system 2,5 kW/Kg.
The cost of flywheel energy storage is mainly decided by the peripheral equipment needed for harnessing and storage of energy in the flywheel and can therefore be quite high depending on the wanted level of flexibility and rate of charge/discharge limitations. The pros and cons of flywheel energy storage are summarized below in Table 10.
Pros Cons High energy density Overloading causes explosion “Unlimited” ROC/ROD High self discharge Long lifetime / No memory effect Cycle eff. highly dependent on ROC/ROD Exact SOC estimation Risk of high auxiliary system cost
Table 10 - Pros and Cons of flywheel energy hybrids
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6.5 Electrical In conventional vehicles, the engine power which is used to provide maximum acceleration is much larger than the one needed for steady speed cruising at legal velocities. This is because ICE torque is minimal at low rotational speeds due to the fact that an ICE is its own air supplier, i.e. low speed equals little air to use for combustion. An electrical motor on the other hand delivers its maximum torque at stall speed (standstill) and is therefore a very nice complement to the ICE’s torque characteristic. Another feature of the complement of electrical motor torque is the possibility of using alternatives to the Otto ICE combustion cycle, normally not suitable for vehicle use due to deficient torque delivery but in this case possible. Cycles like Atkinson and Miller are modifications of the Otto cycle and have the possibility of increased efficiency, but normally with the cost of decreased elasticity, i.e. a reduced useable operating range.
Electrical energy can be stored in many ways, electrochemical and electrostatic storage are two examples, flywheel is another but it is an indirect method which requires additional electrical machines for conversion. The most common type of electrochemical energy storage products are better known as batteries and electrostatic storage products as capacitors. The technology behind both alternatives will be treated as a part of the components of hybrid electric vehicles later on.
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6.5.1 Electrochemical - Batteries An electrochemical energy storage product, e.g. a battery is a device that converts chemical energy directly to electrical energy per definition (Your dictionary).
Batteries are used in a very wide range of applications. Since its invention in 1800 by Alessandro Volta, the battery has become a popular power source for many household and industrial applications and is today a very large industry. There are both rechargeable and non rechargeable batteries but the only ones interesting for hybrid applications are of course the rechargeable ones since there are no batteries with the capacity to power a vehicle for a lifetime, without recharging. Rechargeable batteries, also referred to as “secondary batteries”, can have their chemical reactions reversed by supplying electrical energy to the cell, hence restoring their original composition.
Since the introduction, the battery development effort has been concentrated on extending the capacity, increasing cycle lifetime, increasing calendar lifetime, minimizing the self- discharge rate and increasing the power output while maintaining temperature stability to mention some things.
The energy density, power density and all other characteristics of a battery greatly depend on what type of chemical composition it is designed with. For this comparison, the best available alternative will be used. Other parameters that are affected by chemical composition are temperature stability and the SOC estimation accuracy, all of which will be mentioned more later on.
State of the art battery technology delivers an energy density of up to 220 Wh/kg and at the same time a power density of up to 600 W/kg. If power output is of greater importance than energy capacity, a high power alternative could increase power density up to 3000W/kg with the sacrifice of lowering the energy density to around 130 Wh/kg at maximum [INVESTIRE]. The main drawback of the electrochemical design is the fact that the chemical process is time consuming, especially during charging. This limits rate of energy that could be stored with the electrochemical processes of a battery, making for example high power regenerative braking difficult. A solution to some of these problems is the electrostatic energy storage method, unfortunately with some drawbacks of its own.
Pros Cons Widely used technology Difficult SOC est. with some types Relative high energy and power density Energy/Power tradeoff Very flexible use together with el. machines Limited ROC Temperature sensitive Limited lifetime
Table 11 - Pros and Cons of electrical hybrids (battery powered)
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6.5.2 Electrostatic A capacitor is a device consisting of two or more conducting plates separated from one another by a dielectric non-conductor, as glass, mica, plastic or dry air. It is used for storing an electric charge [Your dictionary]. In the same way that two inductors can be magnetically coupled to create a transformer, two conductors can be electrostatically coupled to form a capacitor.
Capacitors are often used in electronics as short term energy storage devices and they can also be used as a frequency filter because of their electrical characteristics. The first known capacitor was invented in October 1745 by Ewald Georg von Kleist of Pomerania in Germany. A normal capacitor cannot be used as a secondary energy storage device in hybrid vehicles; the energy density is far to low. Capacitor development has lead to so called “super-“ or “ultra-capacitors” which follow the same principle as ordinary capacitors but summarized utilize “double layers” of ions and therefore gets a capacity several orders of magnitude higher than ordinary capacitors [Zorpette].
A modern ultra-capacitor has an energy density of around 3-5 Wh/kg in high power versions, combined with a power density of up to 10000 W/kg. Thus still not fulfilling the energy density requirements of a secondary energy storage device in HEV applications. But, these features make the ultra-capacitor ideal for high power, highly transient but relative low energy storage, i.e. those encountered during acceleration or regenerative braking of a HEV, making it a very good complement to the slow but energy dense electrochemical energy storage method [Zorpette].
Pros Cons Very efficient energy cycle Relative low energy density High ROC/ROD capacity Volt. change with SOC (expensive aux. equip.) Long lifetime Sensitive to overcharging Complements batteries Relative low power to vol. density Exact SOC estimation High power to mass density
Table 12 - Pros and Cons of electrical hybrids (Ultracapacitor powered)
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6.5.3 Electrochemical – Fuel cells Fuel cells are another type of electrochemical storage device but differ quite a lot from the ones already mentioned, namely batteries. A fuel cell converts the chemical energy of a fuel directly into usable energy, i.e. electricity and heat, without any type of combustion. Summarized the fuel cell produces electricity from fuel on the anode side and an oxidant on the cathode side, which react to each other in the presence of an electrolyte. Fuel is electrochemically oxidized on the cathode electrode surface and oxidant is reduced on the anode electrode surface. Ions created by these reactions can flow through the electrolyte between the two electrodes and if connected in a closed circuit, current can be generated [Larminie].
The main difference compared to a battery is that a fuel cell consumes its reactant which in turn has to be continuously refilled; a battery does not, as it stores energy in a closed chemical system. On the other hand, while the electrodes of a battery react and change during charge/discharge, the electrodes of a fuel cell are catalytic and relatively stable.
The principle used in fuel cell energy conversion was discovered by German scientist Christian Friedrich Schönbein in 1838 and published in the “Philosophical Magazine”, January 1839. The first commercial use of a fuel cell was done in the late 1950’s in a joint project by GE and NASA [Larminie].
There are many types of fuel cells and the main difference between them is what fuel and oxidant that is used. A hydrogen cell uses hydrogen as fuel and oxygen (e.g. from air) as oxidant. Other types use hydrocarbons or alcohols as fuel and air, chlorine or chlorine dioxide as oxidants.
A fuel cell can be used both for generation of electricity, supplying it with fuel, or as a storage device, supplying it with electricity. When used for energy storage the fuel cell converts the electric energy into fuel that later on could be re-converted into electricity. One type of fuel cell that is capable of this is called a Unitized Regenerative Fuel Cell, or URFC. The principle for two-way fuel cell energy conversion is realized of two different devices: a “reverse converter” that converts electricity into a fuel, i.e. as the normal choice of fuel is hydrogen it is basically an electrolyzer, and the fuel cell “stack” that converts the fuel into electricity.
In hybrid vehicle application of fuel cells to function as an energy storage system, two solutions exist. One is the mentioned URFC which both can function as an electrolyzer and a fuel cell. The alternative and “the predecessor” is also the most mature one. It consists of an external fuel production unit, e.g. an electrolyzer together with a gas storage container, connected to a fuel cell that converts the stored gasses into electrical energy. A problem, in both cases, is that the gas produced by either the fuel production unit or the URFC in generation mode, has to be stored and hydrogen storage is difficult!
Hydrogen is very energy to mass dense and is therefore a very good fuel, but the problem with hydrogen is storage. As hydrogen is gaseous over -252.9 °C storage as liquid imposes large energy losses and very well insulated containers. Another alternative is storage under very high pressure, but the containers are still large and gets quite heavy if the pressure is
[64] high. A lot of research is done on the topic and promising alternatives has been presented, such as storage of hydrogen in metal hydrides but still, the main limitation of all hydrogen based energy converters is storage.
The URFC as an example shows promising use of fuel cells as bidirectional energy storage devices and coupled to a lightweight hydrogen storage device it is shown to reach energy densities over 400 Wh/kg, which is twice the forecasted amount of known battery technology. It sounds promising but the high energy density is coupled with a very low power density at around 10-30 W/kg and for example electrode lifetime of URFC’s the technology still needs to be proven [INVESTIRE].
But, maybe the most interesting thing about fuel cells is not the use as a secondary energy storage device in a HEV but the ability of functioning as a fuel converter, hence being an alternative to the primary energy source of a HEV, the ICE. The main benefit of using a fuel cell as the primary energy converter is the increased efficiency compared to an ICE. The efficiency of a fuel cell is unfortunately dependent on the amount of power used; drawing more power means larger currents which in turn increases the losses in the fuel cell. As most of the losses in a fuel cell result in a voltage drop the fuel cell voltage is said to be almost linear to the efficiency, i.e. a higher cell voltage means a higher efficiency [Larminie]. The tank-to-wheel efficiency of a fuel cell vehicle is about 45 % at low loads and shows average values of about 36 % in driving cycles like the NEDC, compared to a normal diesel vehicle at around 22 %. But, it is also important to take into account the losses coupled with the production, transportation and storage of fuel, e.g. hydrogen. Fuel cell vehicles running on compressed hydrogen show an approximate fuelcell-to-wheel efficiency of 22 % and liquid hydrogen vehicles stop at around 17 %, thereby falling below the level of a conventional and much less expensive vehicle [Helmolt, Eberle].
Summarized, a combination of the lack in efficiency increase at the moment, difficult fuel storage, lack of fuel infra structure and high cost of fuel cell raw materials does not suggest design of hybrid vehicles with fuel cells as secondary energy storage devices or primary fuel cell powered vehicles at the moment. But, the technology still brings hope to the future. The technical aspect of the different electrical energy storage alternatives will be discussed furthermore in chapter 9.
Pros Cons High energy density Low power density High and incr. eff. of the cell itself Expensive Promising – future hope No incr. in total veh. eff. at the moment
Table 13 - Pros and Cons of fuel-cell hybrids
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6.6 Choosing hybridization method Let’s get back to the original question, what source of secondary power should be chosen in a hybrid vehicle? As always, they all have their pros and cons and in some way it’s unfortunately a question about choosing the one that is “least bad”. Starting off with energy and power densities, Figure 19 shows a plot of the power to mass density versus energy to mass density of the applicable alternatives and gives a good overview.
Figure 19 - Energy/Power density of electrical storage methods [Wikimedia Mod]
The ideal case is of course a high energy density combined with a high power density and the flywheel energy storage method seems to offers the highest combinational value of the two. But as discussed, the cycle efficiency of flywheel energy storage is highly dependent on the rate of charge/discharge due to the high rate of self discharge while slowing down. Therefore it lacks all of the abilities needed to serve as a secondary energy storage system in a HEV. The characteristics of the flywheel storage method are better suited for short term, highly transient, energy storage which is not always the case in normal driving, e.g. during highway use. On the other hand it could be a great compliment to a secondary energy source that is not capable of high energy transients, thus serving as a third energy source, e.g. ideal for regenerative braking. Another drawback of the flywheel storage method is that a mechanical system doesn’t decouple the ICE from the direct power demand of the vehicle, i.e. a flywheel-electric system is needed which still calls for an electric propulsion system. The conclusion is that for hybrid vehicle applications, the flywheel storage method is best suited for pure regenerative braking systems, such as the F1 KERS.
Moving on to the pneumatic/hydraulic hybrid the main drawback is clear, the storage method calls for large containers and a large amount of unpressurized liquid as a reservoir, [66] thus lowering the energy density a lot. On the other hand the power density is good and in the same way as flywheels and ultra-capacitors it can absorb or supply large amounts of energy in a short period of time. Because of this pneumatic/hydraulic hybrids are best suited for stop and go traffic use, combining the high roundtrip efficiency of storage and the capacity for high rates of charge and discharge.
Electrics, divided into batteries, capacitors and fuel cells each in turn have their own pros and cons. Starting with fuel cells used as a secondary energy source in a HEV, the limitation in power density, low total cycle efficiency, immature bidirectional alternatives and high cost, limits the use for now. But as mentioned, fuel cells have a very interesting future ahead of them, e.g. in fuel storage development, partly as an electrical energy storage device but mainly as a possible alternative to the ICE.
Left are batteries and ultra-capacitors, none of which combines energy density and power density that well compared to e.g. gasoline. At this point, it has to be accepted that there is no alternative currently available that suits all our needs and that is also one of reasons why hybrid electric vehicles has become the most popular choice of hybridization among the large car manufacturers. Hybridization with the help of electrical machines and electrical energy storage devices consist of components with high individual efficiency which in turn can be connected together with the help of a grid that, depending on voltage differences, supports high efficiency too. In Figure 19 it is easy to establish that conventional batteries and ultra-capacitors complement each others features well and together can support both high amounts of stored energy and produce large amounts of power while still retaining a relative low weight.
As we know, in a modern vehicle there are a lot of auxiliary devices, such as power-steering, air-conditioning and an alternator. The alternator in turn powers even more auxiliary subsystems such as electronic control units, lightning and actuators for example. With the help of a high voltage electrical system capable of high power the use of these auxiliary systems can be enhanced, i.e. obtain an increase in efficiency. Systems connected directly to the ICE mechanically can be “electrified” and through that be used more efficiently and intermittent as an example.
Furthermore, compared to the alternatives, there are no large drawbacks of an electrical hybrid. The technologies behind the components of a HEV are used in other industrial and home applications, making many of them mature while stimulating further development. In the same way, the component cost can be reduced as the volume increases and e.g. better batteries are of interest to many product fields. A HEV also flexibly supports fuel cells in the future and are well suited for plug-in use, which is “pre-charging” of the vehicle by an external power source.
In the future we will probably see all kinds of hybrid on the market. They all show strength in some area and that feature might be the most important in a certain type of “special application”. The most important thing is to continue development while still making use of what we have for now.
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During the rest of this thesis, the emphasis will be given to hybrid electric vehicles or HEV’s as it is the choice of the industry and the intended system for application of future hybrid components from Haldex Traction.
7. Degree of hybridization and operating modes Are all hybrids alike? And is there really any difference between a Honda Civic Hybrid and a Toyota Prius Hybrid? Hybrid as hybrid one can think, but unfortunately it is not that trivial.
7.1 Measuring the degree of hybridization When a HEV are to be designed, there are a number of important factors to consider. One of them is the sizing of the ICE and the electrical machine or machines. What should the total power capacity be and how should the power be divided between the two? This is defined as the “Degree of hybridization”, or DOH, and is one of the toughest design variables to specify, as well as it can consume large development efforts, e.g. with simulation. The second largest issue comes as a product of the dimensioning; how should power be managed and divided between the sources? Power management is one of the largest research areas of HEV’s and will be mentioned again later on.
The total power output of the powertrain is dimensioned by the acceleration demands of the vehicle and can easily be simulated; the hard part is to distribute the maximum power capability between the hybrid power sources. The HEV drivetrain will either be ICE dominated, EM dominated or in special cases equally distributed.
In the first case, the primary propulsive device will be the ICE which means that the EM will probably be much less powerful and utilize a relatively small energy accumulator. In the second case, the EM will be dominating and the ICE is downsized, probably to act as an intermittent energy source and charger of a large battery pack along with booster capabilities during maximum power output.
To aid classification and use as a development and control tool, the degree of hybridization should be quantified. At the present time (Oct. 2008) no industrial standard can be sourced on how this should be done but three methods are found to be used.
Classification by:
1. Secondary system power- and accumulator-capacity [Andrew + GM Dudenhofen] 2. Hybrid system functions and possible operating modes [Wikipedia + GM Dudenhofen] 3. DOH equation [Baumann]
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7.1.1 Definitions Independent on what classification method is used, the classes themselves tend to be the same but are inconsequently named. In this thesis the following classes and names will be used.
Mild hybrid Power assist hybrid Full hybrid Plug-in hybrid (essentially a capacity classification)
However, GM for example seems to refer to “Mild hybrids” as “Micro hybrids” and instead call “Power assist hybrids”, “Mild hybrids” *GM Dudenhofen+. Also, it is unclear if the same classification methods should be used without regard to the hybrid drivetrain layout, all of which will be defined in chapter 9. For parallel and power-split layouts more than one energy-path to the wheels is available which allows for distinct operating regimes, i.e. “all- electric” or “all-mechanical”. But in the case of a series hybrid drivetrain the functionality and operating mode of the hybrid system is limited by the layout itself, still being a hybrid per definition. Therefore, sometimes series hybrids are mentioned as a separate classification along with plug-ins which basically is more of a capacity classification than a measure of the degree of hybridization, namely because Plug-in hybrids (PHEVs) can be operated as a self sustaining HEVs also.
7.1.1.1 Secondary system power- and accumulator-capacity This method defines the degree of hybridization based on the power capacity of the secondary energy source alone, without mentioning the power level of the ICE, combined with the energy capacity of the electric accumulator system.
The power and energy levels are not standardized either but in this text represented as those used by companies Johnson Controls and SAFT which are found cited in texts [Andrew] and by the power levels that GM specifies for the corresponding hybridization level [GM Dudenhofen]
This classification method does not address functionality of the drivetrain or the total power capacity of the vehicle, i.e. the distribution of power resources. E.g. 10-15 kW electric power, i.e. a mild hybrid per the SAFT definition, could be a very large power addition in a small vehicle while in a very large vehicle barely be defined as a starter motor, which brings confusion to the degree classification.
This classification can also be used as a basis for component dimensioning during the development as future customers will expect a certain level of performance at each hybridization level. In the extent of this thesis, power levels for each class will correspond to the ones stated by GM.
7.1.1.2 Hybrid system functions and possible operating modes This is a very common method of HEV classification in articles by the press, public [Wikipedia] and car manufacturers [GM Dudenhofen], i.e. GM uses both hybrid system functionality and power level for classification. It is based on the expected functionality and available operating modes. In this text a summary of the mentioned functionality is used. [69]
The method precisely describes how hybrid vehicles are expected to perform but not in what way the total power output is split between the energy sources. Neither does it quantify “how good” a vehicle performs during certain tasks, e.g. its all-electric performance, which makes comparison between vehicles harder.
7.1.1.3 DOH Equation The DOH equation [Baumann] quantifies the level of domination between the two power sources of a HEV drivetrain. The DOH is measured with a number between 0-1 which represents the ratio of maximum power output of the two energy conversion devices. Applied to hybrid vehicles using the same drivetrain layout the DOH number permits a distinct classification and comparison opportunity. It is defined as:
Where ED1 is energy device 1 and ED2 is energy device 2. The DOH equation can be expanded and applied to more than two conversion devices but in the case of a HEV with 1 ICE and 1 electrical traction motor, eq. 7.1 could be written:
If eq. 7.2 is applied to a conventional vehicle (CV) or an electrical vehicle (EV), i.e. non hybrids, the result is:
and
While in the case of the hybrid drivetrain being fully balanced, i.e. the result is:
Quoting [Baumann]:
“The DOH is an important mechatronic design tool because it provides a quantitative measure of where power is flowing in a hybrid. This helps the designer decide what type of control strategy to use and what component (i.e., the ICE, EM, or both) will be targeted for control.”
As the DOH equation measures dominance, if an HV is significantly ICE dominated, i.e. DOH<0.48 biased towards the ICE side [Baumann], the equation leads to the conclusion that the variables and control of the ICE has a greater potential for reduction in energy
[70] consumption compared to the weaker electrical machine. This tells the designer to initially use the electrical machine to maximize the performance of the ICE and not the other way around.
As an example on how this could be used in the design of a hybrid vehicle; the size of the ICE is chosen first in an ICE dominated drivetrain. When the ICE has been selected, with the help of its efficiency matrix, such as the ones in Figure 11, the “distance” between the average operating point of the engine using a conventional drivetrain and its operating point of maximum efficiency can be used for the electrical machine dimensioning. That is to say the lack of power production until the ICE reaches its optimum efficiency could be absorbed by an electrical machine with the same power level. This method applies to parallel and power split hybrids, but not so much series hybrids as the generator in that case absorbs all of the engine power. Furthermore this method cannot replace simulations but gives the designer a very good “point in the right direction”. In the case of a hybrid vehicle with electrical motor domination, the idea is analog but with concentration on the electrical machine.
This method quantifies the hybridization numerically and serves as a good design tool but it does not describe or put demands on the functionality of the vehicle. A power ratio between an ICE and an electrical motor does not tell us whether e.g. it can operate in an all-electric mode or not. At the same time, most conventional vehicles would be hybrids with this classification if all of the electrical motors in the vehicle are accounted for, e.g. the starter motor. In such cases the information is miss-directive and additional information is needed.
Furthermore there exist no standard regarding the correspondence between the numerical intervals of the DOH equation to the specific hybridization classes. E.g. is equal to a mild hybrid? For an indication of intervals, reference calculation follows.
7.2 Operating modes Compared to a conventional vehicle, HEV’s can be operated in many more ways. Depending on how the energy is transferred through the drivetrain and from where it was fetched, i.e. from the fuel through the engine or from the batteries, different operating modes can be defined.
Regarding the energy stored in the electrical accumulator of an HEV, the vehicle can be operated in two main ways [NREL]:
Charge sustaining (CS) Charge depleting (CD)
It also exist a combination of these two modes which is termed blended mode or mixed- mode. The operating mode of a hybrid vehicle directly controls the discharge and charging strategy, thus affecting the size and type of accumulator used, i.e. the battery specification in HEV’s.
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7.2.1 Charge sustaining The CS mode of operation is used in all HEV’s on the market. The goal of the CS mode is to maintain the SOC of the battery inside a relative narrow band predetermined by the designer. During operation the SOC will fluctuate but the average SOC will be held constant. In this band of SOC the battery maintains a high power capacity, to aid acceleration, while still allowing for regenerative braking (fast charging) when needed and a maintained battery health.
CS operation can be controlled in numerous ways, i.e. when and how the battery is to be charged/discharged, and is a question of the energy control strategy and sometimes by the layout of the drivetrain.
7.2.2 Charge depleting CD originates as the operating mode of an EV. In an EV, energy is stored in a large battery and during the course of operation the SOC will sink continuously until it reaches its minimum SOC and the vehicle stops. To continue driving the battery has to be connected to the grid and be recharged.
In the same way, “Plug-in Hybrid Electrical Vehicles” which has the ability to store electrical energy from the grid also operate in the CD mode until the energy of its battery is depleted. But, in contrast to an EV, the PHEV does not need to be connected to the grid again to continue operating.
When the SOC of a PHEV has reached its minimum level during CD operation the vehicle cannot continue any further using only the electrical energy that has been pre-stored. By changing to CS operation the PHEV can start its ICE, charge the battery and keep on moving, i.e. what defines the difference compared to an EV.
7.2.3 Blended/mixed mode During all-electric operation in CD mode a PHEV operates with the ICE shut off. But if the vehicle power demand exceeds the power capacity of the battery the ICE can be started to assist the battery, i.e. during heavy acceleration. This is called blended or mixed mode operation.
Mixed mode operation also offers the possibility to downsize the electrical power of a PHEV while still offering decent performance. In such cases the vehicle may not manage to operate in all-electric mode (CD) other than in situations with low power demand, such as urban driving. But as the vehicle will be electrically assisted all the time, the fuel consumption can be lowered significantly.
In the opposite way, an HEV can switch to CD operation if wanted and technically possible, which can be useful in future zero-emission areas such as urban centers.
7.3 DOH Classes This part applies the three mentioned methods to classify the degree of hybridization of hybrid vehicles.
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7.3.1 Mild hybrid A mild hybrid is the first step from a conventional vehicle to become a hybrid vehicle. Its hybrid functionality is most often very limited. In some cases it is exploited by car manufacturers to market slightly modified, e.g. only with Start/Stop functionality, conventional vehicles as hybrids. Start/Stop functionality enables the vehicle to automatically shut of the ICE if the engine is idling to save fuel. Below follows the definition of a mild hybrid based on the three different methods.
Secondary system power- and accumulator-capacity
[Andrew] 0.5 - 1 kWh 10 - 15 kW
[GM Dudenhofen] 1 – 6 kW (Micro hybrid)
Hybrid system functions and possible operating modes [GM Dudehofen] As the first step of hybridization and often, in essence, a conventional vehicle with an oversized starter motor. The increased size of the starter motor renders Start/Stop functionality possible. When the vehicle stops, e.g. at a stoplight, or during braking the engine can be shut down and quickly started again when needed. To maintain auxiliary system functionality while the engine is shut down to reduce the fuel consumption during idle, the electrification of crucial auxiliary systems is added. Mild hybrids often use a relatively low voltage system, e.g. the 42 V net, as low power levels are used.
In some applications the larger starter motor can also be used for limited brake energy recovery and the battery capacity tend to be small, e.g. 3 standard lead-acid batteries in series (2005 Chevrolet Silverado Hybrid for example).
To be noticed, the latest line of mild hybrid systems, such as the GM BAS (Belt Alternator Systems) can also provide a small amount of power assist during acceleration.
A mild hybrid system can be connected with the means of belts to the engine due to the low torque levels, differencing it from power assist systems.
DOH equation [Baumann]
2006 GM Saturn Vue Green [GM] ICE power = 127 kW EM power = 5 kW
(biased to ICE)
7.3.2 Power assist “Power assist” is the second step of hybridization which puts greater demands on performance and functionality. Higher energy storage and power levels which in the end
[73] corresponds to an increase in electrical system voltage. Below follows the definition of a power-assist hybrid based on the three different methods.
Secondary system power- and accumulator-capacity
[Andrew] 1 - 3 kWh 20 - 60 kW
[GM Dudenhofen] 5 – 20 kW (Mild hybrid)
Hybrid system functions and possible operating modes [GM Dudenhofen] The power assist hybrid has the same functionality as the most advanced mild hybrids but with higher power capability. The electrical motor is most often placed between the ICE and the gearbox where it both can start the engine and supply a tractive torque to the wheels depending on e.g. clutch operation.
The first power assist hybrids on the market could not propel the vehicle using only electric drive, such as the Honda Insight equipped with the Honda IMA (Integrated Motor Assist) of the first version. Modern power assist hybrids on the other hand do, like the 2006+ Honda Civic Hybrid [Honda] which takes it closer to being a full hybrid.
The fuel saving capacity of a power assist hybrid system is higher than mild hybrid systems due to a larger power capability and increased flexibility, e.g. all electric drive at low speeds.
The electrical system of a power assist hybrid is probably in the medium voltage range, i.e. around 150 V and the battery power capacity is much larger than the ones used in mild hybrids.
DOH equation [Baumann]
1999 Honda Insight [Insightcentral] ICE power = 52 kW EM power = 10 kW
(biased to ICE)
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7.3.3 Full hybrid The final step of the hybridization process which requires full drivetrain flexibility and reasonable use in all-electric mode (Zero emission mode). Below follows the definition of a full hybrid based on the three different methods.
Secondary system power- and accumulator-capacity
[Andrew] 2 - 20 kWh 80 - 200 kW (Bus)
[GM Dudenhofen] 25 – 40+ kW (Full hybrid)
Hybrid system functions and possible operating modes [GM Dudenhofen] A full hybrid should be able to run solely on electrical power, ICE power or a combination of both with usable performance, e.g. manage urban driving. The Toyota Prius is an example of a full hybrid and its high voltage battery pack enables it to use all-electric propulsion during city driving. The voltage of the power pack is more than 250 V [Toyota]
A full hybrid should have a high flexibility of the hybrid drivetrain and support:
All electric mode Cruise mode (overproduction - power is split to road and battery) Battery charge mode (no propulsion, i.e. idling) Power boost mode (during acceleration)
DOH equation [Baumann]
2008 Toyota Prius [Toyota] ICE power = 57 kW EM power = 50 kW
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7.3.4 Plug-in hybrid Currently (October 2008) there are no PHEV’s in production but judging by the prototypes shown at recent auto-shows, it is thoughght to be the next step of hybridization and the most important short term solution to a significant reduction in use of fossil fuel. Below follows the definition of a plug-in hybrid based on the three different methods.
Secondary system power- and accumulator-capacity
[Andrew] 10 - 20 kWh 50 - 100 kW
[GM Dudenhofen] 40+ kW (Plug-in hybrid)
Hybrid system functions and possible operating modes [GM Dudenhofen] The definition of a PHEV is a full hybrid with the added functionality of pre-charging through an electrical outlet combined with an increased battery pack capacity. As a result, the all- electric (EV) mode is essential for its operation and a PHEV puts much higher demands on the all-electric performance. In theory a plug-in hybrid could be of parallel, series or power- split design but the key is the high efficiency operation in EV mode, which some designs manage better than others. The drivetrain-layouts will be discussed further in chapter 9.
A PHEV shares the characteristics of both a conventional HEV, having an electric motor and a backup ICE for power, and of an EV, also having a plug for grid connections. Therefore it is sometimes referred to as a “Range Extended Electrical Vehicle”, or REEV *GM+.
The maximum possible range during CD mode in a PHEV is defined as the All-electric Range (AER) [NREL]. The AER is commonly designated together with the class name on the form PHEV-*“AER”+. It has to be noted that In US documentation the AER is denoted in miles but in this thesis SI units are used. As an example, a PHEV with an AER of 40km would be named a PHEV-40 vehicle [NREL]
The AER is defined as: “after a complete recharge, the total distance driven before the ICE starts for the first time”. The AER will of course vary depending on driving style and driving situation and is therefore only valid for a specific reference cycle.
DOH equation [Baumann] As there are no PHEV’s currently on the market the DOH reference calculation is performed on the 2010 Chevrolet Volt preliminary specification. The Volt is also a serial PHEV which leads to the use of two electrical machines, one generator and one traction motor. In the calculation, only the power of the traction motor is used as electrical power.
2010 Chevrolet Volt [GM] ICE power = 53 kW EM power = 111 kW
[76]
As the DOH suggests, the Volt is an electrically dominated vehicle which often is the case of serial hybrids as the EM is the performance limiting motor.
7.4 PHEV’s elaborated The external, non technical, benefits of PHEVs are the possibility of a large reduction in oil use per kilometer coupled with a reduction of GHG’s. The positive environmental impact of PHEV’s depends heavily on the production method of the grid electricity. In U.S. there is an expected mix of natural gas, coal fire and a small amount of wind energy [US-EIA]. Compared to Europe where the energy is converted onto the grid with the help of far less coal and where the use of diesel cars are much larger, the positive environmental potential is even greater [EU Commission].
As for now, for all fuels evaluated other than oil, fuel use in a PHEV provides the best potential for fuel consumption reductions and GHG reductions compared to the alternatives. Also, regardless of source, when converted to electrical energy, use of that electricity in CD mode will result in more than double the service for the same amount of raw product compared to a fuel cell vehicle in the short term (~2015). Therefore, based on low GHG electricity generation, it will be a lot more rewarding to concentrate development on PHEV’s instead of FCV’s in the near future. [IEA HEV]
With regards to the driving behavior, cost, climate sensitivity and need for a wall-outlet, the target market will initially be households with plug equipped garages.
One of the questions that have to be addressed in PHEV development is minimization of engine restarts during CS mode and engine and catalyst temperature control. This is important because it affects the efficiency of the engine and most importantly the emissions.
7.5 Conclusions regarding the DOH The conclusion of this thesis is that the degree of hybridization should be classified by its functionality, i.e. possible operating modes, but in the same time quantified with the DOH equation to enable vehicle comparison and act as a design tool.
Furthermore the development and most of all the consumers would benefit from a standardized classification of hybrid vehicles. It is important to understand the difference between e.g. a mild hybrid truck as the Chevrolet Silverado which has only Start/Stop functionality and a full hybrid as the Toyota Prius. The difference in fuel saving and environmental potential is huge, yet both of them are marketed as “hybrid cars” which is confusing and misleading for the customers.
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8. Drivetrain structures
There are many ways to combine the different components of a vehicle drivetrain and the alternatives just increase with hybridization. Roughly said, with hybrid vehicles there are two main configurations, that is the parallel and the series drivetrains. Simplified, all other layouts could be described as combinations, or adaptations of these two. An important variable to consider when the drivetrain layout is selected is of course the total efficiency. However, one should know that the variation in efficiency between the different concepts is strongly dependent upon the applied load case, e.g. urban or highway driving. Some are more efficient when the load is low while others show their strength when load is high.
As we know, the key to increase the efficiency of the ICE, which is the weak point of the total drivetrain efficiency, is to decouple the engine power demand from the direct power demand of the vehicle. Hybridization enables us to do so, but to different extent depending on the layout of the drivetrain.
The choice of layout is also affected by the DOH and the component specifications as e.g. a series hybrid will completely depend on the power of the electrical motors when it comes to the performance limiting factor. Moreover the complexity of the power management strategy is greatly affected by the chosen layout, e.g. the simple flow characteristic and static operation of the series hybrid compared to the multidirectional flow and complex dynamics of a power split hybrid. During the following chapter, hybrid-electric vehicles are referred to.
[78]
8.1 Conventional
Figure 20 - Conventional drivetrain layout
The conventional drivetrain layout, as shown in Figure 20, has looked more or less the same since the birth of the modern automobile. It is a series layout with all-mechanical connections between the fuel converter and the wheels. The gearing ratio and hence the speed of the car and the working speed of the engine can be changed with the help of a gearbox, controlled either by the driver or by pre-programmed logics (automatic). As a result of this, and as mentioned before, the load of the ICE is strictly governed by the desired propulsion power of the vehicle and hence the average efficiency of the fuel conversion will be low.
On the other hand, the small amount of components in the drivetrain and the low amount of energy-conversions keeps the rest of the drivetrain efficiency at a high level, up to 90 % for a vehicle with a manual gearbox [GMPT]. As a result of this, the conventional drivetrain is most efficient at high and constant loads, similar to those found at high speed highway driving. Unfortunately, the aerodynamic losses increase quadratic to the speed.
Pros Cons High mechanical efficiency Low fuel conversion efficiency due to bad ICE load Reliable and mature Low total average efficiency Cost efficient
Table 14 - Pros and Cons of the conventional drivetrain layout
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8.2 Parallel hybrid
Figure 21 - Parallel hybrid drivetrain layout
Parallel hybrid drivetrain layouts basically consist of a conventional drivetrain with an alternative energy source added in parallel, hence the name. This means that the ICE is still connected to the wheels by a gearbox and a mechanical drivetrain, thus still constrained by fixed gearing rations. The result of this is that the load of the ICE is still directly linked to the continuously variable power demand of the vehicle, which we know has a low average and reduces the mean efficiency of the engine.
However, compared to the conventional vehicle, the load point of the ICE can to some extent be influenced with the help of the electrical machine(s). E.g. the speed of the engine can be chosen with the gearbox and the torque with the electrical machine(s). Depending on the configuration, the extent of the load point shifting could be limited by the SOC of the battery though, e.g. if SOC = 100 %, the load of the engine cannot be increased while still harnessing all of the generated energy at constant vehicle load.
If we take the electrical hybrid as an example, conversion of a conventional vehicle to a parallel hybrid could be performed just by adding one electrical machine and increasing the battery size, or by adding a separate traction battery and an electrical power converter for a higher power output. This layout is shown in Figure 21 and can be considered to be the most convenient way of converting a conventional in-production vehicle to hybrid drive. Parallel hybrids of this sort are often classified as mild hybrids and sometimes, depending on the functionality and power level, power-assist hybrids.
Parallel hybrids can be further categorized by the way the two energy sources are mechanically connected. When they are joined by an axis truly in parallel the final rotational speed of each source in the direction of this axis has to be the same and the torques provided are added together. If only one of the energy sources are desired for propulsion the other one still has to rotate at the same speed in an idling state or be disconnected by a
[80] one-way clutch or “freewheel”. In an automobile application it would be more convenient to connect the two sources through a differential gear, i.e. the torques of the sources has to be the same and the speeds will be added together, defined by the ratio of the differential. Still, there will be a drawback if the use of only one of the energy sources is desired. The non propulsive source still has to resist a large amount of torque at “stand-still” or be equipped with a one-way clutch or an automatic locking mechanism.
Principally a parallel hybrid vehicle could take on 4 different operating modes; pure electric operation, pure ICE operation, ICE operation while charging batteries and finally operation with all power sources active and delivering a propulsive force. Compared to the power-split hybrid, later defined, there is no possibility of infinite variation between each operating mode, hence the reason of the limited possibility of ICE load point shifting. A solution to the ICE load point variation would be to use a CVT gearbox, e.g. adopted by Honda in the Insight [Insightcentral]. A CVT gearbox would increase ICE efficiency but simultaneously lowers the gearbox efficiency to some extent.
Figure 22 - Parallel hybrid drivetrain with 2 electrical machines
For increased flexibility, as shown in Figure 22, another electrical machine could be added, e.g. to enable simultaneous charging of the battery during electric operation (“series- mode”), i.e. if the mechanics of the drivetrain allows. Several car manufacturers have adopted the parallel drivetrain layout to variable extent, one example is the 1999 Honda Insight which was the first series production parallel hybrid vehicle, equipped with a 50 kW ICE and a 10 kW electrical motor (Insightcentral).
Depending on the placement of the electrical machine relative to the transmission, parallel hybrids are often also classified as pre- or post-transmission hybrids. The main difference is that the electrical traction motor of a pre-transmission hybrid often can, and is, used as a starter motor for the ICE as well.
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Pros Cons Rel. good eff. (few conversions) ICE still dep. on direct veh. load Dual drive Powerful acc. Smaller ICE Limited flexibility (4 modes) EV functionality (plug-in compatible) Risk of rel. advanced control Works with a single el. machine (easy retrofit) Relatively low cost (basic version)
Table 15 - Pros and Cons of the parallel hybrid drivetrain layout
8.3 Series hybrid
Figure 23 - Series hybrid drivetrain layout
The main advantage of the series hybrid drivetrain layout is the fully independent operation of the ICE. As seen in Figure 23, in the series hybrid drivetrain there is no mechanical connection whatsoever between the ICE and the wheels, thus not constraining the ICE load point directly to the variable power demand of the vehicle. This enables the ICE to run continuously at its optimal load point, achieving maximum efficiency. If the power demand of the vehicle is lower than the power delivered by the ICE at its most efficient load point the ICE can be used intermittent, running only when the SOC of the energy accumulator drops below a preset limit. In the end, it could be described as the ICE efficiency being dependent on the low pass filtered power demand of the vehicle system.
In the same way as there is one large benefit of the series layout, there is unfortunately one large drawback as well, namely the high amount of energy conversions. As a result of this,
[82] the total drivetrain efficiency will suffer from the requirement of all energy being converted from mechanical to electricity and back, assuming a HEV vehicle.
Also, because of the in series power flow, the performance limiting component in the system will be the electrical traction motor, based on the total power delivery of the accumulators and generator being sufficient. Furthermore, in the long term, the maximum continuous power output of the system will be limited by the generators capacity, sustaining charge and supplying the traction motor with maximum power. The result of this will be the need for two powerful and therefore large and heavy electrical machines.
Another benefit of the series layout hybrid drivetrain is the ease of high efficiency operation during all-electric drive. As a matter a fact one could describe the series layout as an EV equipped with a power pack (ICE) that extends the range of the vehicle, hence the alternative name “Range-extended electrical vehicle”, or REEV [GM]. The transition from hybrid electric to all electric drive is done just by disabling the start of the ICE, thus also giving the possibility of zero-emission operation, e.g. in urban areas. This results as smooth operation in EV-mode which also makes the vehicle ideal for use as a PHEV. This has been realized by GM whom is currently in development of the GM Volt, a REEV P-HEV which is planned to be launched during 2010 [GM]
Due to the low amount of mechanical energy transports, which often are sensitive to direction changes and geometrical layout and therefore affects efficiency and size, the series drivetrain enables relatively free placement of the different drivetrain components. This enables alternative placement of, e.g. the ICE and makes use of otherwise ignored space possible [Jonasson]
Pros Cons ICE independent on direct veh. load Multi. energy conver. Lowers eff. Max. poss. ICE efficiency High cost and weight (dual el. machines) Very suitable for plug-in use (PHEV) Packaging opportunity (mini. mech.) Easy use of Fuel cells as energy conv.
Table 16 - Pros and Cons of the series hybrid drivetrain layout
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8.4 Combined (Split) hybrid
Figure 24 - Combined (split) hybrid drivetrain
In an attempt to unite the positive properties of both the parallel and the series hybrid drivetrains, a combination of the two has been gathered under the name “combined hybrid drive”. With the help of a so called “power-split device”, or PSD, it is with some constraints possible to shift between the features of the two layouts. The goal of the system is, while maintaining the high efficiency of the mechanical energy path of a parallel setup, to decouple the engine power demand from the instantaneous power demand of the vehicle, i.e. the driver. Doing this ideally, one would have the possibility to load the engine at its maximum efficiency working point, while in the same time eliminate the need for all of the energy to pass through a high amount of conversion steps, as in the case of a series hybrid. Unfortunately, some of the internal constraints of the power split device can limit the load leveling possibility to some extent, e.g. the maximum speed of the electrical machines at the selected gearing ratio. These constraints and the functionality of a power-split device are discussed in chapter 8.
The Toyota Hybrid System, or THS, was launched in 1997 with the Toyota Prius and is the most well known application of a combined hybrid, i.e. a power split hybrid. The THS was based on an electro-mechanical power split transmission developed by TRW and published in a 1971 SAE paper and therefore it could be said to be the predecessor of today’s electric power split systems [Gelb, Richardson, Wang, Berman]. During the last years there have been a number of derivatives of this design, most of them increasing in mechanical complexity. Compared to the layout example of Figure 24, the first version of the THS only used one electrical machine. Together with the launch of the 2003 Prius, a new generation of the THS was introduced and simultaneously renamed to “Hybrid Synergy Drive”, or HSD. At the same time it was also equipped with the second electrical machine. As the features of the power split hybrid enable a step-less variable gearing ratio between the engine speed and wheel speed, i.e. vehicle speed, the power split hybrid is often said to be equipped with an “Electronically-controlled Continuously Variable Transmission” or E-CVT. [84]
Compared to the serial hybrid, the split hybrid can reduce the efficiency loss due to multiple energy conversions and combine the use of both power sources simultaneously. Furthermore, compared to the parallel hybrid, the power split architecture supports both speed and torque load leveling, i.e. inside the constraints of the maximum performance of the components themselves. Summarized, the power split hybrid has a very high degree of flexibility as power can be split between a mechanical and electrical path step-less. However, as always, increased flexibility increases the degrees of freedoms and makes control and optimization much more advanced.
Pros Cons High comfort (CVT) Advanced mech. Moderate tot. eff. Flexible ICE operation (decoupling) Complex control Proven by Toyota Transmission dev. Effort Compact Single stage ver. Limited EV func. Single stage Not suitable for plug-in use
Table 17 - Pros and Cons of the power-split hybrid drivetrain layout
8.5 All wheel drive, AWD By looking at the lineups of cars from the largest manufacturers during the last years, one can notice that AWD is becoming a common option, not only for off-road purpose which was its first intent of use, but also for regular road cars. An increase in the European AWD sales is also reported during the last years [ACEA]. AWD systems add another dimension of traction in slippery (low µ) situations. It delivers not only a large increase in safety but can also be used to increase the “feel” and enhance the driving dynamics on normal surfaces. AWD systems can furthermore be divided into different classes, regarding in what way and how often they supply traction torque. One example is permanent AWD which supplies propulsion torque to all wheels regardless of the driving situation but controls the torque distribution automatically. Another example is also the most basic variant which consist of manual locking mechanisms, enabling the driver to lock the differentials of the car, i.e. constraining all of the wheels to the same speed.
The drawback of AWD is the increase in parasite drag of the additional mechanical components of the drivetrain. PTU’s, drive-shafts and hypoid differentials all have efficiencies below 100 % and will therefore increase the fuel consumption of the vehicle. A step in the right direction to cure this problem is the modern and electronically controlled “on-demand” AWD systems, such as the one developed by Haldex Traction AB. This third example of AWD systems only transfer torque to the rear wheels (in case of a FWD car) when needed, e.g. during wheel slip or under steer. However, there will always be some
[85] parasite drag in the system and the added weight tends to increase the fuel consumption slightly. However, to the extreme, an AWD system could have the opposite effect in low µ driving. If a vehicle is operated in the parts of the world with sub zero temperature winters and is often driven in urban areas, a lot of the energy is lost while the tires of the car spin during acceleration. While reducing wheel slip, the difference in fuel consumption should probably not be neglected in these cases.
AWD is, as said, not new technology and already available for conventional drivetrains. The interesting area is the adaptation to, and the new possibilities provided with, hybrid technology. As there is no need for mechanical connections between the wheels and the fuel converter in some hybrid vehicle drivetrain layouts, new solutions for AWD arise.
If we take the series layout as a first example, it has no mechanical connection between the fuel converter and the wheels at all and as mentioned earlier it could be described as an EV with a range extending addition. Conversion to all wheel drive in this case would simply consist of adding another electrical machine to the layout, turning the second pair of wheels. Of course it is possible to use only one electrical motor for traction and connect it to a mechanical drivetrain of the same functionality as a conventional vehicle, but why not make use of the added “stowing” possibilities.
Furthermore, in a conventional vehicle the ICE is mounted to the chassis of the vehicle. The power it produces is thereafter transferred to the wheels with some kind of flexible mechanical connection, e.g. constant velocity joints and differentials. Regardless of layout, in an HEV application the motors that are used for traction does not have to be mounted to the chassis, but could instead be positioned close to the wheels as a part of the non suspended components of the suspension. This would eliminate the need for a number of mechanical connections between the motor and the wheels, all of which normally would be reducing the efficiency of the vehicle. Motors of this type are often called “hub-motors”.
The drawback of hub-motors is the increase in un-sprung weight and the added inertia of the wheel setup, mostly affecting the handling characteristics of the vehicle. Another difficulty is the low rotational speed of the wheels, making the design of high efficiency electrical machines more difficult. A company called PML specializes in the development of this type of motors and delivered their solution to the “Volvo C30 Plug-In Concept” shown at the 2007 Frankfurt auto show [PML]. The drivetrain layout of the AWD series hybrid concept car is shown in Figure 25.
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Figure 25 - Volvo ReCharge Concept - AWD series hybrid – [PML]
In Europe, the dominating part of the vehicles sold during the last years is conventional FWD cars [ACEA]. There are many reasons for this but the main thing is the safe handling characteristics and compact layout of the drivetrain. Adoption of hybrid technology to vehicles is expensive and will in some cases demand development of new and tailor-made platforms. Because of this, conversion of existing platforms to hybrid drive is an interesting short term solution and if done right, you can get AWD “for free”.
Using the parallel drivetrain layout as example, it has been explained earlier that this type of drivetrain can be described as a conventional drivetrain with an alternative energy drivetrain added “on top”. If this secondary drivetrain is added to power the none-mechanically driven axle, you have a parallel hybrid car with all wheel drive! The layout of such a car is visualized in Figure 26. In the example a single traction motor is used, connected to the wheels via a differential, but of course, hub-motors could be used in this application also.
Figure 26 - Parallel hybrid with AWD [87]
Lexus, whom are owned by Toyota and uses a version of the HSD system in their cars, produces the already mentioned RX400h which is an example of a power split hybrid with AWD. The RX400h uses a power split drivetrain at the front and is equipped with an additional electrical motor at the rear, often called MGR, which connects to an ordinary differential. The AWD system is not full-time and the rear motor is activated “on demand” [Lexus]
A conventional FWD or RWD vehicle, equipped with a single secondary propulsion system at only the none mechanically driven axle, i.e. powered by an alternative (hybrid) energy source, is often referred to as an “through the road parallel”.
A “fun-to-know” fact is that the first known AWD car was actually a hybrid. Built by Ferdinand Porsche as a race car the same year he showed his first hybrid on the 1900 world exhibition [Porsche-Lohner].
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8.5.1 The Haldex eBAX The eBAX is a project within the Haldex Traction AB Corporation who normally manufacture intelligent on demand AWD couplings intended for conventional vehicles. Their new eBAX projects aims at taking advantage of the flexibility and fuel savings potential of a hybrid vehicle and combine that with their AWD know-how and advanced vehicle dynamics real- time software.
This combinatory product has the potential to hybridize a conventional vehicle and at the same time add AWD capability in one package. Due to confidentiality, no technical description or design is shown.
Recommended layout and application potential As mentioned, a couple of different alternatives exists when it comes to hybrid vehicle adapted AWD. To recommend a suitable drivetrain layout for the Haldex eBAX and at the same time measure its potential to other known alternatives, Pugh-matrix based decision was used. Five different hybrid vehicle AWD concepts were compared to a conventional drivetrain with the Haldex Limited Slip Coupling.
The 6 different alternatives were measured on the following points:
Available platforms – Number of current platforms on the market that are suitable for the specific hybrid AWD concepts (platform sales volume, available space etc.) Development effort – Estimated dev. effort to reach near concept full-potential (control complexity – traction, fuel economics and dynamics) System cost – Estimated total hybrid AWD system cost (compared to Haldex LSC) Conversion effort – Application effort of AWD concept on a conventional vehicle (no. of new components, redesign demands of crucial components etc.) Dynamics potential – Vehicle dynamics increasing potential (cornering, stability, accident avoidance – high/low μ) Traction potential – Traction enhancing potential (low μ, off-road etc) Fuel/Emissions potential – Fuel savings and emission reduction potential (Average ICE efficiency, drive line efficiency, no. energy conversion steps, re-gen potential) PHEV potential – Operating potential as PHEV (charge depleting efficiency) Mass – Increase of mass compared to conventional vehicle with the Haldex LSC
The Pugh-matrix can be found in Appendix A.
From the comparison it was concluded that the most suitable, and highest potential application for the Haldex eBAX would be a parallel hybrid based on a conventional vehicle. Conventional vehicles are increasingly equipped with Integrated Starter Generators, or ISGs, to enable Start/Stop operation in e.g. urban traffic. The increased electric power of these ISG’s which often are in the range of 5-10 kW is enough or close to enough to charge the additional battery pack needed for traction purposes. In this application the added value of the eBAX would be very high as a conventional vehicle with an ISG can be equipped with only two additional components, i.e. the Haldex eBAX and a traction battery, and thus obtain both hybrid vehicle functionality and AWD capability. If the battery size is increased and an externally connectable charger is added, it also renders Plug-In use possible. However, if
[89] simulations or empirical tests show that the continuous power of the ISG in a specific vehicle does not offer a high enough load shifting capability, it also can require upgrading depending on the goal.
The second most interesting concept would be to offer the eBAX as a high performance AWD solution to existing FWD parallel or power split hybrids. As the eBAX has the possibility to utilize the same vehicle dynamics real-time software as the LSC, the eBAX will be of interest even though the vehicle manufacturer designs the main part of the hybrid drivetrain by themselves. In such applications, no auxiliary components have to be added with the attachment of the eBAX AWD system.
Difficulties The largest difficulty of the development and marketing process for the eBAX will, to the author’s opinion, be to optimize and prove the full fuel savings and emission reducing potential of the eBAX. As the key with hybridization is to decouple the direct vehicle load from the ICE, optimization of the fuel-economy related part of the control strategy will require control of the engine management system to some extent (for torque distribution). However, in modern EMSs the driver input tend to be requested torque, therefore it could be possible to intercept that signal and reroute it via the hybrid controller which then calculates the optimum torque distribution between the two sources by modifying the signal. However, control of the ISG power electronics would be needed as well. Close cooperation with a vehicle manufacturer, i.e. a customer, would of course be ideal in this case.
Depending on how the product is to be marketed, i.e. as a AWD capable hybridization package or as an AWD add-on for hybrid vehicles, the control task put on Haldex would be very different. In the first case, as already mentioned, a hybrid controller with torque distribution software including load shifting, regen-control, SOC-calculation, SOC-sustaining etc. has to be incorporated in the Haldex control software (plus the AWD dynamics). In the other alternative however, where the product is marketed as an AWD add-on for hybrid vehicles, the hybrid torque distribution control would already be a central part of the hybrid vehicle which is developed by the vehicle manufacturer. In this case, the software development effort would be much less for Haldex as only the AWD functionality has to be incorporated in their ECU. Unfortunately, some additional problems would arise at the same time, as the AWD software has to cooperate with the hybrid controller or otherwise conflicting torque demands will certainly follow.
Summarized, the potential for the Haldex eBAX is assessed as good. The vehicle fleet that has been a target for the Haldex LSC in the past has to comply with tougher emission and fuel consumption targets to reduce the average values. These platforms, existing and future, would greatly benefit from the eBAX concept while still remaining AWD vehicles. It is hard to estimate the fuel savings potential without a complete simulation environment but to the author’s belief it would be in the regime of 10-20 %, highly dependent on the driving cycle and component specification.
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9. Components of a hybrid electric drivetrain This chapter is a technical briefing on the current and future technology behind the most important components in a HEV drivetrain.
9.1 Electrical energy storage devices The following chapter deals with the problem of energy storage in the form of electricity in hybrid electric vehicle applications and a technical description of the alternatives. The emphasis lies on batteries and ultra-capacitors, not fuel-cells as their technology still needs to be proven on many areas.
As mentioned as a part of “hybridization power sources”, the ideal energy storage system of a HEV is small but delivers large amounts of power while containing large amounts of energy. On top of that, it does it at a low cost and during the whole lifetime of the vehicle. As one could guess, the current situation is not ideal and there are some tradeoffs. The most important characteristics of a battery (and other alternatives, such as the capacitor) are as follows [INVESTIRE + NREL]:
Energy density Power density Maximum allowed charging/discharging current Lifetime – cycle and calendar life Thermal requirements Safety and monitoring problems Cost Recycling and production – environmental impact
Electric energy storage devices, e.g. batteries, have been a part of automobiles and vehicles of all kinds for a long time. They are often used as the energy source for ICE starting and energy buffer for auxiliary systems such as, microprocessors and lightning. The type of battery that has been used is almost exclusively the lead-acid type mostly due to its robustness and low price. The power demand of the electrical consumers in a conventional vehicle is very low, commonly around 1150 W [Bosch], and therefore the lead-acid battery does a job very good at the standard of 12 V (14 V charging), limiting currents to around 100 A at maximum load. The power demand of the vehicle propulsion systems on the other hand is much larger and renders the standard voltage insufficient. A normal car has at least 50 kW of tractive power which would require an impossible 4100 A at 12 V, during all electric operation.
The solution is an increase in voltage. Commercial vehicles have for long used the double voltage of 24 V but this is still not high enough. In 1994, MIT and Mercedes Benz took the initiative of tripling the system voltage of automobiles to 36 V and it was supported by Ford, GM, Delphi, Siemens and others which ended up in a standard called the “42 V Power Net”. As the system bus voltage is 3x12 V, the “42 V” name can sound somewhat odd but it referrers to three times the charging voltage of 14 V. The 42 V system was expected to be adopted by all large manufacturers but several problems arose and kept the new standard from series production. Some of them were:
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Fuses became more expensive due to the higher voltage level and the inductive fault current associated with the breaking arc. The windings of small electric motors became larger due to the higher number of winding turns and thicker isolation.
Reduced lifetime of filament lamps as the filaments themselves has to be made thinner at a lower current.
And most importantly, the currents are still too high for some interesting power levels such as hybrid drivetrains.
In the future, we will probably se multiple voltage systems used in modern vehicles. For example:
A 12 V system used for low power consumers, such as instrumentation, human interface devices, entertainment systems and lightning etc.
A 5 V system used for microprocessors used in various controllers (logic)
A 100-500 V system for the traction-devices and maybe other high power devices such as electrical suspension components.
One of the main drawbacks of a high voltage system (>75 V) compared to the 12 V system used today is the danger to people. A 12 V system is more or less harmless to humans but a high voltage system with the power capability of traction devices is lethal. Therefore the safety and shielding of a high voltage system is much more important and can under no circumstances come in contact with humans during operation or service. As a result of this, it is also very inappropriate to use the chassis as a common ground for the electrical system at a high voltage level. Instead it is better to keep the high voltage system separated from the low voltage system [IEA HEV].
9.1.1 Dependency The electrical energy accumulator of an HEV has two main performance limiting variables, namely the energy capacity and the power capacity.
The power capacity will, as the name implies, limit the power output (propulsion) and power input (regenerative braking or ICE over production). The maximum power output of the battery can, depending on drivetrain layout and degree of hybridization, limit the performance of the vehicle, e.g. acceleration and top speed. Most hybrid vehicles have the possibility to combine the power of the ICE and battery during maximum power output but the performance will still be dependent on battery power as long as the traction motor manages to convert all of the power produced to mechanical rotation.
In the same way, but in the other direction, the maximum rate of charge (power handling during charging) probably will limit the amount of regenerative braking that is possible. When a vehicle comes to stop from a high velocity, a lot of energy has to be converted from
[92] kinetic to electric. If it is all to be harnessed, at a high rate, it is also crucial that the designed battery capacity and the current SOC “has room” for the extra energy.
The energy capacity on the other hand does not affect direct acceleration performance but during high power output over a long time the capacity can be limiting. When the SOC of the battery drops below a specific level, the power output sinks and finally stops. E.g. during towing or hill climbing the performance can be severely affected if the capacity of the battery is too low.
Furthermore, in PHEV applications the capacity of the battery is even more important because of its direct impact on the AER, i.e. fuel savings. The energy capacity of a PHEV battery generally has to be much larger than the one used in HEV’s because of the CD operation mode. However, the maximum power output can still be increased by the ICE with the help of mixed mode operation.
The conclusion is that battery power and energy specifications are a crucial design parameter of a HEV and PHEV. The battery performance and specifications of a hybrid vehicle has to be evaluated with the help of reference driving cycle tests on a chassis dynamometer, but based on the DOH, vehicle mass and drivetrain layout, a good indication can be given.
9.1.2 Performance and cost goals The performance of a conventional vehicle depends heavily on what the customer expects and pays for. A sports-car is fast and expensive, while an estate-wagon is slow but has to be price worthy. The same grouping of course goes for HEV’s and PHEV’s but it is important to realize the strong connection between “vehicle performance/cost” to the “battery performance/cost”. HEV’s will use the battery as a buffer and hence a high power capacity is more important than a high energy capacity. For PHEV’s the case is the other way around as they should be able to operate on pre-stored electric energy as long as possible.
The USDE has via the USABC stated performance and cost goals of future HEV and PHEV energy storage methods. With the main target to reduce USA’s oil dependency these technology goals, which are summarized in Table 18, are set for 2015 [USABC].
Table 18 - USABC Energy storage goals
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The cost goals translate into:
-PHEV (high energy/power ratio) Cost - US$500 / kWh
-HEV (high power/energy ratio) Cost - US$293 / kWh
Furthermore the performance goals are converted to common comparison numbers.
-HEV (high power/energy ratio) Energy density – 56 Wh/kg Power density – 750 W/kg (10 s pulse)
-PHEV (high energy/power ratio) Energy density – 97 Wh/kg Power density – 317 W/kg (10 s pulse)
These performance figures will be used as reference values during the extent of this thesis and the cost goals will at the end of this chapter be compared to the projected cost by the manufacturers.
9.1.3 Batteries A battery is a device that converts chemical energy directly to electrical energy. The accumulator is built of one or more voltaic cells. Each cell consist of two half cells connected in series by a conductive electrolyte. One half cell acts as positive electrode (anode) and the other one is the negative electrode (cathode). The power of a battery comes from a reduction-oxidation (redox) reaction between the two electrodes. The reduction occurs at the cathode and oxidation at the anode. The electrodes do not touch each other but are electrically connected by a liquid or solid electrolyte.
The electromotive force, or EMF, determines each of the half-cells ability to drive electric current from the cell to an external circuit. First discovered by Volta, the net EMF of the battery is the difference of the EMFs of each half-cell. This means, if the electrodes have EMF and the net EMF of the battery is , namely the reduction potential of the half-reductions.
, the electrical potential difference across the terminals of a battery is known as the terminal voltage and measured in volts [V]. The EMF of a battery is equal to only if the battery is neither charging nor discharging. Internal resistance of the cells and the battery will result in a lower than EMF voltage during discharging and a higher than EMF voltage during charging. The ideal battery maintains its voltage at until it is exhausted but during practical use the internal resistance will increase as it is discharged, thus also the open circuit will get lower if checked now and then during discharge. Also, if resistance and voltage are plotted against each other as a function of time during discharge, the curve will not be a straight line and the shape will vary with the chemistry of the battery. This feature creates some problems of its own in some applications.
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The EMF of a battery cell depends on the chemical composition and its concentration. E.g. alkaline batteries produce approximately 1.5 V/cell whereas the high energy lithium compounds can provide cell voltages above 3 V.
There exist a number of chemical compositions used in batteries today but not all are suited for HEV’s. Most importantly they have to be rechargeable and powerful enough. The following types are considered:
Lead-Acid (PbAc) Nickel based (NiXx ..) Lithium based (LiXx..)
9.1.3.1 Lead-acid batteries Lead-acid batteries are well known as a part of a conventional vehicle where they function as starter batteries. Those kinds of batteries are designed to deliver pulsed power, i.e. large amounts of energy during a short period of time (cranking). Lead-acid batteries intended for traction use in vehicles have a different design as the variation in SOC is much larger and their task is to supply a small amount of energy (per cell) during a long time. This latter type is applied in e.g. forklifts, delivery-carts, EVs and HEVs.
The discharge process of a lead-acid battery sends electrons from the negative electrode through an external load to the positive electrode which receives them, i.e. the battery current. The charging process reverses the reactions. The lead-acid battery is built with the reactions between lead, lead oxide and sulphuric acid, which is the electrolyte. During discharge the following reactions take place [INVESTIRE]:
As seen in eq. 9.3 the concentration of the electrolyte will reduce during the discharge process. As a function of this, lead-acid battery cell voltage will drop due the electrolyte being a part of the chemical process. The EMF of a lead-acid battery is a matter a fact only dependent on the acid concentration of the cells and not the amount of lead, lead oxide or led suplphate as long as all of them are available, i.e. the cell voltage of the lead-acid battery is not constant even at low discharge currents. Furthermore low temperature also lowers the cell voltage making high friction cold starts even harder.
Lead-acid batteries tend to separate the electrolyte into oxygen and hydrogen over time due to the charging process. Conventional lead-acid batteries are vented to the atmosphere to reduce high pressure buildup in the cells during charging and therefore require refilling of distillated water to maintain a sufficient electrolyte level. Modern lead-acid batteries are sealed and valve regulated (VRLA) and can recombine these gasses instead. With the help of a porous separator in the cell that transfers the oxygen from the positive electrode to the negative electrode, recombining it with hydrogen to water, the electrolyte can be kept in [95] good condition during a long time. VRLA batteries are therefore maintenance free as they do not need refilling with water but they come at a higher cost and are more sensitive to deep discharges/surcharges.
Lead-acid batteries are a very old design to the date but still remain a popular choice when it comes to automotive energy storage. The main reason is the robust design and proven technology at a low cost. It can also manage high currents during e.g. cranking but there is a large drawback. Lead, a very high density material, is a crucial component which increases weight and therefore decreases power and energy density a lot. Additionally lead-acid batteries have to be recycled in the correct way to limit their environmental impact. The lead-acid battery is summarized in Table 19 [INVESTIRE].
Pros Cons Robust and mature Low energy density Relative high power capacity High mass due to Pb density Low cost (high volume) Consumes electrolyte Easy SOC estimation Pb is environmentally unfriendly
Table 19 - Pros and cons of Lead-Acid batteries
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9.1.3.2 Nickel based batteries All nickel based batteries use the same type of alkaline electrolyte and the same nickel oxide material as positive electrode. For negative electrode material there exist several options and the most common ones are:
Cadmium (NiCd) Metal hydride (NiMh) Zinc (NiZn)
Nickel Cadmium batteries In a NiCd battery the electrolyte is potassium hydroxide, surrounding a nickel oxide anode and a metallic cadmium cathode. The following reactions describe the discharge process [INVESTIRE]:
Nickel Cadmium batteries made a great success as a rechargeable alternative to the standard 1.5 V non-rechargeable batteries in the beginning of the 90 s. Typical NiCd cell voltage is 1.2 V and as non-rechargable 1.5 V batteries rapidly drops to this voltage level they worked without modification in numerous electrical appliances.
NiCd batteries have a flat discharge characteristic, i.e. constant voltage over a wide SOC range, combined with a good temperature tolerance. NiCd cells usually work by recombining the gases of the chemical process in a fully sealed container and are thus maintenance free. However NiCd batteries suffer from severe memory effect which reduces the capacity of the battery. Due to changes in the crystalline formation during charging NiCd crystal growth prevents the battery from discharging beyond the point it was charged from. That is why NiCd batteries not should be charged continuously in small amounts, limiting its use in HEV applications.
NiCd is believed to be an obsolete technology due to better alternatives and the environmental hazard of the cadmium. For early EV applications, NiCd batteries have been used because of their higher energy density compared to PbAc batteries but are today replaced by NiMh batteries. The NiCd battery is summarized in Table 20 [INVESTIRE].
Pros Cons High energy and power density compared to PbAc Memory effect Relatively high ROC/ROD capability High cost Cd is enviro. unfriendly
Table 20 - Pros and cons of Nickel-Cadmium batteries [97]
Nickel Metal hydride batteries Instead of cadmium as in NiCd batteries, NiMH batteries use hydrogen as the active element. It uses the same kind of potassium hydroxide electrolyte and the electrode is made of a metal hydride. The metal hydride has the possibility of absorbing and desorbing hydrogen as the battery is discharged and charged. The chemical reactions during discharge are [INVESTIRE]:
NiMH batteries have the same cell voltage as NiCd batteries (1.2 V) but offers many improvements in its design, e.g. higher energy density, less memory effect and better robustness. And in the same time it is more environmentally friendly.
Nowdays, NiMH batteries are the primary choice for rechargeable alternatives to the standard non-rechargable 1.5 V battery. The reduced memory effect combined with the increased energy density is the main reason for the large market share.
NiMH batteries are also the first type of technology used in mass produces HEV’s and Toyota has built over 1 million vehicles with NiMH traction batteries! [Prius Sales]. Unfortunately NiMH batteries are not perfect and compared to many other technologies they have a relative high self discharge rate and the coulombic efficiency is lower than the one found with NiCd and PbAc batteries, i.e. a lower efficiency of the charge/discharge cycle. The NiMH battery is summarized in Table 21 [INVESTIRE].
Pros Cons High energy density compared to NiCd High self discharge Less memory effect comp. to NiCd Low cycle effiency Relatively eniromentally friendly Tolerant to over charge and discharge Relatively low cost battery management
Table 21 - Pros and cons of Nickel Metal Hydride batteries
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Nickel Zinc batteries The cell voltage of NiZn batteries is 1.65 V and it is a third version of the alkaline Ni batteries. NiZn batteries have a very good cycle life and can be cycled to 0 % ROC. Unfortunately NiZN batteries are heavy which generates a quite low energy density and additionally it also suffers from the same problem as NiMH batteries, i.e. a high self discharge rate. The NiZn battery is summarized in Table 22 [INVESTIRE].
Pros Cons Good cycle life Low energy density High ROC capacity Bulky Deep cycle cap. to 0 % High self discharge Low cost materials Overall cheaper compared to NiCd and NiMh
Table 22 - Pros and cons of Nickel Zinc batteries
9.1.3.3 Lithium based batteries The battery technologies described until now all use high density metals as a crucial component. Lead (Pb, 11340 kg/m3) batteries are very heavy and Nickel (Ni, 8800 kg/m3) are quite heavy as well. With the goal to reduce the mass of the components and through that increase the energy and power density of the battery, lithium (Li, 530 kg/m3) is ideal. This metal is not as well known as lead and nickel but is available in the same way in the top layer of the earth’s surface. On top of its low density, lithium has the highest electrochemical potential of all metals mentioned which results in high cell voltages, from 2 V to 5 V depending on which composition of materials are chosen. A high cell voltage is a crucial part of high power and energy densities which is very important in EV and HEV applications.
The most common lithium based battery is the cobalt-based, lithium-ion (Li-ion) and the version, lithium-ion polymer (LiPo, Li-Polymer). In the same way as with nickel based batteries there exists several types of chemical composition and some of the most important ones will be mentioned briefly.
An example of the chemical reactions of a lithium battery during discharge is (Lithium- managanes dioxide cell) [INVESTIRE]:
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Cobalt based lithium ion batteries Modern rechargeable lithium batteries do not use metallic lithium as a component because of its poor cycling efficiency due to corrosion reactions with the electrolyte. Instead materials acting as a matrix in which lithium atoms are inserted are used to solve this problem, thus greatly increasing cycle life. In cobalt based Li-ion batteries carbon is used for cathode and normally lithium cobalt dioxide (LiCoO2) is chosen for anode. But in high energy applications, such as HEVs, which require large stacks of cells while at the same time safety is of the essence, more complex anodes consisting of 2-3 different oxide types has been developed. The increased complexity increases the reliability of the cells and actually creates a possibility of reduction in system cost. Li-ion cell chemistry strongly depends on a so called “intercalation” mechanism which governs the insertion of lithium ions into the crystalline lattice of the host electrode, without changing its structure.
Li-Ion battery electrodes need to comply with two important features: an open crystal structure which allows insertion or extraction of lithium ions and the in the same time the ability to accept compensating electrons. Because lithium itself reacts strongly with water, the electrolyte of a Li-ion battery is a non aqueos organic salt and functions only as conductor and doesn’t take part in the chemical reaction at all. As a positive side effect, there is no formation of hydrogen or oxygen gases due to gassing of the electrolyte as in NiMH and PbAc batteries.
Li-ion cells have a very high EMF and provide a 3.7 V cell voltage at no load. That corresponds to the same voltage as 3 NiMH cells and results in a higher energy density and a smaller amount of cells in high voltage applications such as HEV’s. Also, the chemistry of Li- ion cells use very thin separators which makes it possible to have electrodes with a very large surface area relative to the cell itself, thus enabling support for high currents.
Li-ion battery technology is today the number one choice for portable electronic devices with relatively high energy consumption and the room for a bit more expensive batteries, such as laptops, cameras and mobile phones.
Li-ion cells support very high discharge rates, up to 40 C as well as good deep cycling possibilities. Li-ion cells also maintain their specified voltage down to approximately an 80 % DOD. In some cases, this is a positive feature but when it comes to SOC estimation it makes things a lot harder, as a simple voltage measurement does not give an indication of the current SOC.
Energy transfer in the other direction, i.e. charging, is also possible at high rates but the cells are sensitive to over-charging. If overcharging takes place, degradation of the electrolyte and as a result, a lowering of the capacity can take place. Compared to other battery technologies Li-ion puts higher demands on the charging circuit and monitoring equipment which can increase the price of the battery management system. In contrast to NiCd batteries intermittent charging of Li-ion batteries are preferred to prevent over charging. The Li-Ion battery is summarized in Table 23 [INVESTIRE].
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Pros Cons High energy density compared to all alt. Sensitive to overcharging High power density compared to all alt. Deep discharge can affect lifetime No memory effect Temperature sensitive High ROC / ROD capacity Difficult SOC estimation Complex and expensive management
Table 23 - Pros and cons of Lithium Ion batteries
Lithium polymer batteries Li-Polymer batteries replace the liquid electrolyte of a Li-ion battery to an electrolyte consisting of an ion conductive polymer, making it a special version of the Li-ion battery. The solid polymer electrolyte makes the battery safer and leak resistant. In the same time, the packaging is simplified as the cell design allows for more shapes, such as the popular laminated design contained in “pouches”. Figure 27 visualizes the components and design of a Li-polymer battery cell. The Li-Polymer battery is summarized in Table 24 [INVESTIRE].
Figure 27 - Lithium polymer battery cell design [www.avestor.com]
Pros Cons Same positive features as Li-Ion Same negative features as Li-Ion Leak resistant Flexible packaging
Table 24 - Pros and cons of Lithium polymer batteries
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Alternative based lithium batteries As mentioned there exist a number of different types of lithium batteries, many of them still in development. The most promising types are:
Lithium Manganese (LiMn2O4) This composition produces a higher cell voltage (3.8 to 4 V) but with a lower energy density than Li-ion batteries. Manganese batteries are safer and more environment friendly than the cobalt ones but suffers from a 20 % reduction in energy density and a limited temperature stability.
Lithium Nickel (LiNixCoyAzO2) The “LiNi” design has a higher energy density but a lower cell voltage compared to Li-ion. One drawback is that the chemistry develops heat which requires thermal management of the battery pack. There are also some safety concerns that new nickel based materials are developed to fix.
Lithium Phosphate (LiFePO4) This kind of battery uses a positive electrode consisting of a lithium transition metal phosphate which enables better thermal and chemical stability. These cells are much safer and more stable than cobalt based cells and are therefore good for automotive use. The cell voltage varies between 2 V and 5 V depending on what transition metal is chosen. Unfortunately the energy density is lower compared to the lithium phosphate composition but it offers lower cost, higher safety and a reduced environmental impact.
The alternative lithium based batteries in comparison to Li-Ion are summarized in Table 25 [INVESTIRE].
Pros Cons Safer (Managanese) Lower energy density (Managanese) Higher energy density (Nickel) Develops heat (Nickel) Much safer (Phosphate) Lower energy density (Phosphate) Lower cost (Phosphate) Better enviro. impact (Phosphate)
Table 25 - Pros and cons of alternative Lithium batteries
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9.1.3.4 Battery wear The expected lifetime of a conventional vehicle is for example in USA 16.1 years for 50 % of the fraction of passenger cars [ARB Lifetime]. During this time, no major component replacements or restorations of a conventional vehicle is expected or planned. If HEV’s should have a chance to compete with conventional vehicles and keep the non fuel-related running costs at the same level as a conventional vehicle it is important that the lifetime of the hybrid components last as long as possible.
Batteries are unfortunately the weakest point of a hybrid vehicle when it comes to lifetime and a major development challenge at the present, and in the future. The lifetime of a battery can be divided into two parts, the calendar lifetime and the cycle lifetime [Badin].
Battery lifetime components:
Calendar lifetime Cycle lifetime
Ageing reasons and effect Ageing leads to deterioration in both cycling and storage, i.e. energy/power capacity and efficiency. The ageing catalysts are different depending on what type of battery technology is used, mainly aqueos or non aqueos [Badin].
Aqeuos battery ageing mechanisms (e.g. PbAc, NiCd and NiMh):
Current collector corrosion Increased internal resistance Deterioration of positive electrode active material Decrease in capacity Electrolyte loss Increased internal resistance
Non aqueos batter ageing mechanisms (e.g. Li-ion):
Lithium oxidation by the electrolyte Increased internal resistance Deterioration of positive electrode active material Decrease in capacity Other less significant
Summarized ageing effects:
Decrease in capacity due to loss or deterioration of positive electrode active material Decrease in peak power (charge and discharge) capabilities due to an increase in internal resistance
Cycle life The average charge/discharge cycle life of an automotive battery is a function of the traveled distance but very dependent on the driving pattern and is therefore very hard to predict. During development of hybrid vehicles accelerated lifetime tests are sought for but has shown to be very difficult. Experiments have to accurately simulate the energy transfer and the associated environment. The affecting parameters are amongst others current patterns
[103] during charge/discharge, SOC window, temperature and resting phases. Example lifetimes based on hybrid vehicle warranty information are:
Honda Civic Hybrid (Panasonic NiMH) – 160 000 km [Honda] Toyota Prius Hybrid (Panasonic NiMH) – 160 000 km [Toyota]
Unfortunately there are no Li-Ion equipped hybrid cars on the market yet but as reference the designers hope to give the Chevrolet Volt a 240 000km battery warranty [Popular mechanics]
Calendar life Calendar life is another important fact to take into account when designing an HEV. Compared to commercial vehicles, privately owned cars spend a lot more of their lifetime at rest which renders the calendar life just as important as cycle life. In the case of HEV battery calendar life estimations, the battery is considered to be stored at 50 % SOC. There are some published papers regarding the calendar life of HEV batteries but the test methods are inconsequent and the material for comparison is quite varying in quality. Instead the calendar life warranty for production HEV’s are presented:
Honda Civic Hybrid (Panasonic NiMH) – 8 years [Honda] Toyota Prius Hybrid (Panasonic NiMH) – 8 years [Toyota]
And the expected calendar life warranty of the Li-Ion pack in the Chevrolet Volt is 10 years [Popular mechanics]
9.1.3.5 Battery management As already told, the cell voltage of a battery varies between approximately 1.2 -5 V per cell. To keep battery currents at an acceptable level the voltage of a full HEV traction battery pack has to be in the range of 100-400 V depending on the propulsive power of the vehicle. E.g. the battery pack of a Tesla Roadster EV delivers a maximum of 200 kW at a voltage of 374 V. That corresponds to over 530 A of current at maximum power output which is quite a lot and even higher currents put very high demands on wiring and connectors and is therefore unwanted [Tesla Motors]. To reach the needed voltage levels several cells has to be connected in series. Each cell has its own characteristics and behavior which leads to the need of a monitoring system that is capable of voltage measurements for all the individual cell segments. If this is neglected SOC variations between the cells will appear and could cause individual cells to be overcharged. Independent of the battery chemistry, all types of batteries will be damaged if overcharged but in some cases, like with the Li-ion battery, the result could be fatal. It is therefore very important to equip HEV’s with a battery management system to prevent this from happening.
Battery management – Balancing and monitoring The two most important battery types for HEV’s are the currently used NiMH type and the upcoming Li-Ion type. These two alternatives are very different when it comes to battery management. The Li-ion battery has a lower mass and volume for the same amount of energy compared to the NiMH type but has much tougher management requirements due to the risk of ignition if overcharged. To ensure the safety of an HEV equipped with large
[104] battery packs a number of parameters has to be monitored continuously, even when the vehicle is shut off. For Li-ion batteries the cell voltage remains almost constant down to around 20 % SOC which makes simple voltage measurements insufficient for a simple SOC calculation. In the NiMH case the voltage variations are larger and SOC estimation is therefore easier [Chenghui]. To ensure the safety of a large Li-ion battery the voltage of each cell has to be measured very accurately and combined with temperature and current measurements, the SOC of each cell can be calculated. This information acts as the basis for the crucial equalization of cell voltages. Because Li-ion cells, in contrast to NiMH cells, cannot be balanced with so called “trickle charging”, i.e. overcharging with low current, each cell has to be balanced individually. Additionally in large battery packs it is crucial to detect abnormal conditions, such as temperature and high current, on cell level to prevent a chain reaction.
As a result of battery management being crucial, the reliability of the management system itself becomes very important as well. A combination of hardware and software safety features is needed to maintain a good battery environment and health. Summarized, the main tasks of the battery management system are [Badin]:
Cell voltage equalization Cell abnormality detection (Temperature, current…) Battery temperature monitoring and control Control of battery circuit breaker
The cell equalization can be done in different ways but the product is either charging of a cell or discharging of a cell. One method is to remove energy by converting it to heat (shunting) but a much better way is transport of energy from overcharged cells to undercharged cells. This requires advanced cell balancing circuits and is much more expensive.
As an example of battery management complexity in EV’s and HEV’s, the Tesla Roadster uses a Li-Ion based battery pack at 374 V which means a minimum of 101 cells connected in series to reach the nominal voltage. In the Tesla’s case however, the total number of cells are 6800 which most of them are connected in parallel “strings” to form around 101 “large cells”. These 101 strings has to be balanced, monitored and thermally controlled continuously. If one of these 6800 cells malfunctions, it has to be isolated immediately to prevent “spreading”.
On top of a microprocessor controlled management system, the cells in the Tesla Roadster also have internal passive safety systems. The first one is an “internal positive temperature coefficient (PTC) current limiting device” that limits current based on cell temperature and the second system is a “current interrupt device” (CID) that mechanically disconnects a cell from the circuit in case of over pressure on the inside.
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Battery thermal management Batteries are temperature sensitive components; they have always been and will probably still be for some time. The battery temperature affects the performance in many ways, e.g. discharge power, charge acceptance and lifetime. And as battery performance, life and cost directly affects the same variables of the vehicle in total, battery thermal management is crucial.
A vehicle is designed by the manufacturer to operate in a certain temperature range. The batteries however, still a part of the vehicle, most often does not comply to the same range. Instead a much narrower range is specified by the battery manufacturer. Additionally the battery has “optimum temperature range” and if maximum performance is sought for, it is important to control the temperature of the battery within its interval, even though the vehicle itself is being operated in a wider temperature range.
In the same time, it is confirmed that an automotive battery pack consists of a large number of small cells. The thermal management system also has to limit the temperature variation between the cells themselves. Otherwise the charge and discharge characteristics of individual cells can differ with electrically unbalanced cells and decreased battery performance as the result.
Besides the main objectives of the battery thermal management system, i.e. a battery pack at an optimum average temperature and an even temperature distribution inside the battery, there are additional demands. The thermal management system has to be compact, lightweight and delivered at a low cost whilst consuming a low amount of power. If the thermal management system consumes large amounts of power it opposes the initial idea of the hybrid system, i.e. a reduction in fuel consumption [Badin].
The heat generation of a battery is strongly dependent on the chemistry used. Some types produce less heat than others. In Table 26 the heat generations of three types are listed [NREL]. The heat generation is additionally a function of the discharge or charge rate, surrounding temperature, SOC and the dependence also varies with the chemical composition. Heat generation comes from the electrochemical reactions (enthalpy changes) and the internal resistance of the cells.
Battery type Cycle W/Cell @ 0°C W/Cell @ 22-25°C W/Cell @ 40-50°C VRLA, 16.5Ah 1C, SOC 100-0 % 1,21 1,28 0,4 VRLA, 16.5Ah 5C, SOC 100-0 % 16,07 14,02 11,17 NiMH, 20Ah 1C, SOC 70-35 % - 1,19 1,11 NiMH, 20Ah 5C, SOC 70-35 % - 22,79 25,27 Li-Ion, 6Ah 1C, SOC 80-50 % 0,6 0,04 -0,18 Li-Ion, 6Ah 5C, SOC 80-50 % 12,07 3,5 1,22
Table 26 – Heat generation during battery discharge [NREL]
The discharge rate expressed in “C” refers to the energy capacity of the battery. 1 C of a 5 Ah battery is 5 A, 3 C is 15 A and so on. From Table 26 it can be seen that NiMH has the largest amount of heat generation, and the cycle efficiency is also lowest. For all types, heat generation is dependent of the discharge rate, i.e. high rate equals more heat. For similar
[106] currents the heat generation of VRLA and Li-Ion is roughly the same. Generally more heat is also generated at lower temperature due to the increase in internal resistance. Additionally it is very interesting to notice that the Li-Ion battery actually absorbs heat (endothermic) at high temperatures and low rates. The conclusion of the measurements is that NiMH battery packs will require more cooling compared to the others. The Li-Ion pack is however smaller at the same energy content which puts higher demands on the cooling system layout and design.
Heat can be transferred in many ways but when it comes to cooling a battery pack; air, liquids or phase-change are probably the best alternatives. The thermal management system also has to be able to heat the battery in case of a low ambient temperature. A vehicle has to be able to start and operate satisfactory at temperatures below freezing. In a HEV the engine excess heat can be used but the problem is that it takes time for the engine to heat up. In such cases a fuel powered heater or an external heating system can be utilized. As an EV does not have an engine it has to rely on auxiliary heating systems. During operation, excess heat from motors and power electronics can be used to heat the batteries though. There are also investigations made on self-heating battery technology but it is still in the research stage.
Temperature control by air, liquid or phase-change can all utilize the vehicles air- conditioning system for energy transport but it is important to minimize the additional energy consumption. Liquid cooling has a greater potential than air at the same flow rate but if cells are to be submerged in a liquid, it has to be dielectric. Otherwise the cells have to be mounted to some kind of heat exchanger which reduces the cooling effect. Phase change has an even larger potential but consumes more energy.
A battery most often performs better at higher temperatures but it is also coupled with a reduce lifetime, this makes thermal management a bit of a trade-off process. Summarized, battery voltage and thermal management is crucial and especially with a high number of SOC sensitive Li-ion cells or heat generating NiMH cells.
9.1.4 Capacitors In a conventional capacitor, energy is stored by removal of electrons from one metal plate and storing it on another that is situated in parallel at a very small distance away. The charge separation creates an electrical potential difference between the plates. The amount of energy that can be stored is basically a function of size and material properties of the plates which affects the number of charges stored and the dielectric breakdown limit between the plates which controls the maximum potential. To increase the maximum potential between the two plates, various materials could be placed in between which leads to higher energy densities but not close to those of a modern battery. The drawback of ordinary capacitors is the very low energy density and high self discharge rate.
However, as the development of capacitors has made tremendous steps forward, a new breed of capacitors has come in production, the so called ultra or super capacitors. Double- layer capacitors, most often referred to as ultra or super capacitors offer a number of key advantages compared to batteries and conventional capacitors. Ultra capacitors can deliver more than 100times the energy of a conventional capacitor.
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Ultra capacitors consist of two, hence “double layer”, active carbon electrodes which are immersed into an electrolyte. Separated by a membrane that allows transport of charged ions the two electrodes are prevented to get electrical contact. The electrolyte supplies and conducts the ions from one electrode to the other, if an electrical charge is applied between the two. In the double layer capacitor the “dielectric”, i.e. the membrane, is very thin (nanometers). That combined with a very large surface area of the electrodes results in extremely high capacitances while maintaining normal sizes [INVESTIRE].
However, double layer capacitors can only withstand a low voltage (around 2 to 3V). So in the same way as batteries, high voltage ultra capacitors have to be built by serial connection of many ultra capacitors.
Ultra capacitors rely on an electrostatic effect which is purely physical and compared to the chemical process of batteries, highly and quickly reversible. Charge and discharge is done via movement of ions within the electrolyte which is different to the electrochemical process of a battery, even though both of them contain an electrolyte.
Summarized, the fundamental features of the double-layer capacitor offers a number of distinct performance advantages over the battery, namely an increased power density, very high lifetime both in regards to cycles (millions or more compared to 200–1000 for most commercially available rechargeable batteries) and the calendar. As the energy storage process is based on physical reactions instead of chemical ones, the overall wear is much lower compared to batteries. The design is also very robust and resistant to environmental changes. Because of its very low internal resistance the power output but also the cycle efficiency is very high (>95 %). Because of the long lifetime and relatively harmless materials used, the environmental impact is low and the cost of ultra capacitors has dropped very much, to a cost/power level below that of batteries. The drawbacks however is still the limited energy density and the, compared to batteries, high self-discharge rate [INVESTIRE].
Regarding the energy density, existing double-layer capacitors can reach levels of 10 Wh/kg but standard cells from e.g. Maxwell Technologies which is one manufacturer are in the range of 4-6 Wh/kg. In a couple of years though, the difference between battery and ultra capacitor energy density might not be as big, there are currently some very interesting projects in development. Active charcoal that is used as an insulating barrier or “membrane” in the capacitor is not the ideal material. Research, e.g. at MIT, has been concentrated on development of new membranes with an increased usable surface area for electron storage. Another method of energy density increase would be to increase the voltage but this requires an improvement of the insulator by several orders of magnitude. The company EEStor has developed a new barium titanate based insulator which they say is capable of extremely high voltages, in the order of several thoughsands.
Possible future of capacitors:
MIT LLES – has demonstrated 30 Wh/kg, predicts to reach 60 Wh/kg [Wikipedia]
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EEStor – claims a prototype of 277 Wh/kg and future production at almost 700 Wh/kg which is far above today’s battery technology – using a voltage of 3500V [Wikipedia]
Discharge of ultra capacitors is possible down to 0V but in real life applications, approximately 50-100 % of the maximum voltage is used. This corresponds to 75 % of the energy (25-100 % SOC).
The energy of a capacitor is calculated by [Physics Handbook]:
where C is the capacitance in farads and U is the voltage which results in a very easy SOC determination during operation. A negative effect is that the varying voltage can lower the cycle efficiency due to counteraction by the auxiliary electronics that needs to operate with varying voltage levels, compared to e.g. Li-Ion batteries. As in the case of Li-Ion batteries, and the limitation of around 2.5 V per ultra capacitor cell, trickle charge cannot be used for cell balancing and requires an external circuitry for large packs. These systems tend to be integrated in the capacitor pack and move energy between the cells.
The greatest thing with ultra capacitors though, is the complementary characteristics to those of batteries. Where ultra capacitors have a gradual change of voltage with the SOC, batteries respond to current changes with voltage changes. As the internal impedance of ultra capacitors is much lower than of batteries they tend to provide the higher currents and the dynamic response while the battery absorbs low level currents from or to the ultra capacitor to equalize voltage differences. The result will be that under a defined discharge/charge cycle a parallel connected ultra capacitor will narrow the voltage range of the battery, thus increasing its life and optimize its performance [Zolot].
The use of ultra capacitors in HEV applications as a complement to batteries is summarized in Table 27.
Pros Cons Very efficient (>95 %) Low energy density (for now) High power density and ROC/ROD Voltage changes with SOC Long lifetime and robust Easy SOC determination Complements batteries with its dynamics Relatively good cost to power ratio
Table 27 - Pros and cons of ultra capacitor energy storage in HEV's
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9.1.5 Technology comparison The most important technical features of an electrical energy storage system in HEV applications are the energy and power density. High mass and a large volume will affect the performance, fuel consumption and flexibility of the vehicle which are three very important parameters. In the same time, no matter how good the performance is, if the cost is too high the vehicle won’t sell.
Depending on the degree of hybridization and area of usage an HEV or P-HEV will always have to find the best combination of energy and power when it comes to storage. As concluded, modern batteries are relatively good at storing electrical energy while modern capacitors can supply large amounts of power. The performance of the readily available alternatives in HEV electric energy storage is compared to each other in Figure 28.
Figure 28 - The performance of electrical energy storage alternatives [METI]
Clearly the choice for high energy densities is the Li-ion or Li-Polymer technology. Lithium batteries have the potential to deliver a decent power density as well but unfortunately there is always a tradeoff between the energy and power density. However, capacitors are still the choice for high power densities, reaching above 10 kW/kg coupled with the good dynamics of the technology itself.
Japan is currently the largest manufacturer of Li-Ion batteries and until now the main market has been cellular phones and portable computers (laptops). METI, Japan’s Ministry of [110]
Economy, Trade and Industry plans to spend US 1.72 $ billion on battery development over five years, starting 2008. The goals of the battery development are to reduce battery cost and increase its performance. The projected cost is presented in Table 28 [METI].
Year 2010 2015 2020 2030 Battery cost – [$/kWH] 822 247 164 41
Table 28 - Projected battery cost per stored kWh of energy [METI]
The projected cost by METI corresponds quite well to the energy storage cost goals that the USDE has set for its USABC HEV/P-HEV projects. To a reasonable extent, it can to the author’s opinion, therefore be expected to come true.
The conclusion that can be made is that current HEV and PHEV design should be made with a combination of Li-Ion/Li-Polymer batteries and when needed combined with an ultra capacitor. With regards to the cost development and performance advantage of the lithium based battery alternatives the drawbacks, e.g. increased management cost, should be accepted and solved. In low-end applications, NiMh batteries could be of interest but as lithium battery volumes will increase, the NiMh battery will be expensive and obsolete.
At a distant future there are no clear signs where the development will take us but nothing points to a major breakthrough and electric energy storage will probably always be a tradeoff between energy and power capacity. One thing is clear though, energy storage research is the, without doubt, most prioritized research area of hybrid vehicle drivetrains and will be very interesting to follow.
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9.2 Electrical machines In a HEV, the traction system will consist of at least one electrical machine. The minimum number will be found in the most basic variant of a parallel hybrid and the most in an all wheel drive, hub motor equipped series hybrid, which means a minimum of five electrical machines!
One of the main demands on an electrical machine in a vehicle used for traction purposes is a high torque density, i.e. a high torque to weight ratio. Additionally the motor should be able to deliver this maximum torque at a wide speed range or at least at the lower part of its speed range. An electrical machine has a completely different torque delivery characteristic compared to an ICE and that is also why they work so well together! An electrical machine delivers its maximum torque from zero rotational speed. It thereafter retains maximum torque for some speed range, until it reaches the so called “base speed”. Above this speed, torque starts to drop at a rate that more or less maintains a constant power during the rest of the operating speed range (ideal case). The basic characteristics of an electrical machine are shown in Figure 29.
Figure 29 - Torque and power characteristics of an electrical machine
The basics of electrical machines take advantage of the interaction between an electric current and a magnetic field. This is called the “Lorenz force” and is the basis for torque production. All electrical machines have a stator and a rotor with a gap in between but the component design can either be plates or cylinders. The most common type is the cylindrical versions and the magnetic field is either produced by a permanently magnetized material or alternatively with the use of electromagnets.
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When currents are drawn through the motor windings in a certain direction relative to the polarity of the adjacent permanent magnet (for example) and in the other direction near magnets with the reversed polarity, torque will be produced. This can be done in many ways and with many types of rotor and stator configuration, along with different magnetization methods, but the basic idea is the same.
Direct current (DC) machines move the currents in the conductors in such a way that the direction is always correct relative to the magnets when the rotor moves. The current was originally distributed with the help of a mechanical switch or “brush” but more modern variants are brushless which increases life and efficiency. The drawback is the need for an external current controller to distribute power through the separate windings of the machine. Brushless DC motors are sometimes said to operate on “multi-phase DC”.
The alternative to DC machines is the Alternate Current (AC) type which has the winding split into one separate winding for each phase, usually three. Current is fed through the windings in such a way that it always flows in front of the magnets in the desired way which produces torque.
The difference between a BLDC (Brush Less DC) and a BLAC (Brush Less AC) machine is that the phase current waveforms are more or less rectangular for the BLDC, while in a BLAC drive the phase current waveforms are essentially sinusoidal. As pure DC machines require brushes followed by the need for an extensive maintenance effort, it is not suitable for traction motor use and will therefore be left out in the continued discussion [Zhu and Howe].
As a product of the basic properties of the magnetic flux of all electrical machines, two fundamental features can be denoted [IEA HEV]:
1. The voltage required to run a machine is proportional to the speed multiplied with the magnetic flux from the magnets. 2. The torque produced is proportional to the current supplied to the machine multiplied with the magnetic flux. (Based on correct phasing of AC motors)
With reference to Figure 29 again, these features are visualized. Up to the base speed of the machine, the voltage is not a limitation and the performance is therefore determined by the maximum current, i.e. motor resistance. This means that, up to this speed limit, maximum constant torque can be accomplished. However, above the base speed, the voltage required for the same current is not met and as torque is a product of flux and current, torque will drop. When the speed of the machine increases, the flux will decrease. The machine will keep operating at points where the product of flux and speed is slightly less than the voltage as the speed increases.
Summarized, in this area (flux weakening), the torque (current) will drop and the speed (voltage) will increase at a rate producing constant power in ideal conditions. Real life
[113] electrical machine torque (up to the base speed) and power (from the base speed) tend to drop slowly as the speed increases [Zhu and Howe]
There are many types of electrical machines on the market and as usual, they all have their advantages and disadvantages. The next part will briefly explain the difference between the alternatives with regards to the use in HEV. The most important features of HEV traction motors are [Zhu and Howe]:
High torque and power density High start torque (taking off) and high power at high speeds (maximum velocity) Wide speed range and matched performance to the inverter (power electronics) High efficiency over a wide torque and speed range High short time over load capacity (boost acceleration/regenerative braking) Good reliability (lifetime) Low cost
9.2.1 Induction Machine Electrical induction machines (IM), sometimes called asynchronous machines (ASM), are a very mature and robust technology. IM’s are used in a very wide range of applications, from cassette players to large trains. The main reason why the induction machine is so popular is the simple design, which can be viewed in Figure 30. As it is not equipped with any brushes or permanent magnets the cost is relatively low. In the same time the maintenance effort is minimal due to the low number of components. The torque control of an IM is also good but more difficult compared to e.g. the PM machine. The constant power range can be increased to around 4-5 times the base speed which often is desired in vehicle applications.
Figure 30 - "Squirrel cage" induction motor [Wikimedia GNU]
The most important design parameters of an induction machine is the number of poles, number of stators and rotor slots, stator an rotor slot shape and finally the winding disposition. In comparison to machines designed for a constant supply frequency, the IM is not dependent on rotor slot shape to control the starting torque as this can be affected by the supply frequency. With a correct supply voltage and frequency, the start torque can be
[114] very close to the maximum torque which is good in vehicle applications. The maximum speed of an IM is limited by its pull-out torque but further design optimization extends outside the scope of this text.
Compared to the others, the main drawback of the IM is the somewhat difficult torque control. The difficult task is to estimate the flux wave position. For a SM (Synchronous Machine) the flux wave can easily be determined by the help of the rotor position and the currents but for the IM, the position does not provide any information since the flux rotates relative to the rotor as well. Practical solutions to the problem involve mathematical models to calculate the flux position using variables that are measurable, so called Field Oriented Control (FOC) algorithms. Unfortunately this makes IM torque control somewhat computational heavy.
Some examples of vehicles that are using IM’s are the GM EV-1 (recalled) and the Tesla Roadster. As the IM does not generate any voltage without excitation, it is very safe [Zhu and Howe]. The pros and cons of electrical induction machines are summarized below in Table 29.
Pros Cons Simple design Low power density compared to PM Robust (mech. + thermal) Relatively difficult torque control Low cost Low maintenance Flux weak. speed range extension Safe
Table 29 - Pros and cons of Induction Machines
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9.2.2 Permanently Magnetized machines A PM machine can be run on either AC or DC power but they are often classified as sinusoidal (AC) or trapezoidal (DC) back-EMF machines depending on the power characteristics. To optimize machine performance (e.g. torque density) it is best to operate a machine that generates a sinusoidal back-EMF on AC and a trapezoidal back-EMF on DC. In the same way as for IM’s, at low speed there is a optimal flux level to minimize iron and copper losses, i.e. retain maximum efficiency.
For optimal current and torque control of BLAC or BLDC machines the rotor position has to be known. The current characteristics of the BLDC do not require as high resolution as the more complex current synchronization of the BLAC drive though. BLDC drives often use hall- sensors for positioning while BLACs tend to use high resolution decoders. However, there are some methods of BL motor operation without the use of sensors, e.g. floating winding back-EMF detection. Unfortunately it is not suitable for most traction applications due to a low starting torque and low efficiency at low speed.
Without regard to which technical solution is chosen, compared to the asynchronous alternative the torque control is much simpler as the flux relates to the rotor without slip. Synchronous machines are therefore the primary choice of electrical generation in power stations which generate energy for the frequency sensitive power net.
There are a number of different mechanical designs when it comes to PM machines. The stator is usually of cylindrical shape and manufactured using a stack of laminated metal steel sheets to minimize the eddy currents induced by the rotating flux. However there are some new technical solutions on the way, such as sintered metal powder stators, to reduce the eddy currents even more and increase efficiency. The rotor can have its magnets applied either to the rotor surface or buried below the surface. A rotor with embedded magnets is shown in Figure 31 which shows an example of a PM 3-phase “DC” motor. For permanent magnets, the most popular materials are Neodymium-Boron Iron and Samarium-Cobalt. Magnet positioning and stator layout is one of the most important design parameters of a PM motor.
Figure 31 - Permanent Magnet DC motor stator and rotor [Wikimedia - Sebastian Koppehel]
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Because of the fact that the magnets are permanently magnetized combined with the permeability being close to 1, it is very difficult to demagnetize the magnets with stator current, i.e. flux weakening, compared to electrically magnetized motors. This makes it a little bit harder to increase the speed range of a PM motor but it is still very good.
Most HEV’s currently on the market use PMSM (Permanent Magnet Synchronous Motors), i.e. a BLAC, for traction motors, e.g. the Toyota Prius and the Honda Civic Hybrid. Since there are no power losses due to magnetic excitation in PM motors two of the largest benefits are the high power density and the resulting wide speed range. Additionally due to the high power density and relatively low rotor mass, the response is very good. Unfortunately the high performance materials used in the magnets are quite expensive which increases the total cost of PM motors.
Compared to the induction machines and other alternatives, the PM motors are intolerant to heat but summarized they are a very good alternative to use as traction machines in vehicle applications due to the high power ratio, good efficiency and slightly less computational heavy control.
The pros and cons of electrical PM machines are summarized below in Table 30.
Pros Cons High power and torque density Higher cost compared to IM Relatively simple torque control Low thermal robustness High efficiency Can be sensitive to heavy vibration Fast response Wide speed range
Table 30 - Pros and cons of Permanently Magnetized electrical machines
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9.2.3 Switched Reluctance Machine In contrast to PMDC/AC and induction motors, all reluctance machines are based on a very robust rotor structure without magnets or windings. Therefore it is very high speed and temperature stable. The Switched Reluctance Machine, or SRM, is one of many types of reluctance machines. The cost has a potential of being low but small rotor gap tolerances bumps the price. The peak torque capacity of a reluctance machine is also limited due to the ease of flux saturation of the stator. Additionally the power management is quite complex which can easily lead to noise, vibration and erratic torque delivery. However, the SRM still are a good choice in some vehicle traction applications due to the wide power range and the high temperature stability. A typical SRM is shown in Figure 32.
Figure 32 – Switcher reluctance machine [Wikimedia Commons]
The inductance for one phase is at its maximum when one of the stator poles is aligned with a rotor pole and at its minimum when it is in between. The rotor is rotated by running a current through the winding of a stator pole as a rotor pole gets closer and the inductance increases. For generation the stator poles are commuted instead. The most important design variable that affects performance is the ratio of aligned poles to unaligned poles (inductances).
The SRM can be operated in different modes but the most common method is to control the phase currents with PWM which enables a relatively smooth torque control up to the base speed. Above the base speed, continuously variable commutation advance is needed so the excitation and de-excitation angles to the rotor can be varied. The maximum speed of an SRM is limited by the induced back-EMF on the floating windings, however, new “two-phase overlap” versions of the SRM solves this problem.
Summarized the SRM can be a possible choice in some high speed, high temperature applications but is outrun by the PM motors in most areas. Also, as the IM has very similar characteristics but with much higher magnetic utilization, it would most likely also be a better choice in most vehicle applications.
In Table 31 below, the pros and cons of switched reluctance machines are summarized.
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Pros Cons Robust Noise Very good thermal capabilities Risk of erratic torque delivery Relatively low cost Modest peak torque density
Table 31 - Pros and cons of Reluctance machines
9.3 Power electronics Electrical energy can take many shapes and that is one of its benefits. Waveforms, voltages, phasing and so on, all of them are variables that can be changed. In an EV or an HEV there will be a lot of electrical consumers and one or more electrical storage devices. However, all of them don’t use or deliver the same type of electrical energy, which means that some type of conversion has to be done between them.
Modern power electronics use semiconductors in switching mode which leads to lower energy losses, smaller dimensions and a decrease in cost compared to linear regulators.
9.3.1 Motor control In a vehicle with an electrical propulsion system the energy is most likely stored or buffered in an electrochemical accumulator, i.e. a battery. Batteries store electrical energy with DC power and that is also what can be extracted from it. As recently talked about, electrical traction motors on the other hand most likely are to use AC current or multiphase DC current. For that to be possible, some kind of energy conversion device has to be placed in between the battery and the electrical machine, let’s call it the motor controller. Additionally it is required that this device can manage the power output from the machine, i.e. the power consumption from the battery; otherwise the vehicle will have no throttle and speed control.
Furthermore, a battery of an EV or an HEV has a specified voltage which more or less tends to be a function of the DOH. To maintain a high efficiency the currents in electrical traction system should be kept from very high levels and that is done by increasing the voltage of the battery with more cells. However, to increase the power density of an electrical machine, which is a very important parameter in vehicle applications, the voltage from the battery can be boosted to even higher levels when it passes the motor controller.
The most common types of semiconductors in power electronics are:
Diodes IGBT transistors MOSFET transistors
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For each application parameters such as current capacity, voltage capacity, switching frequencies, efficiency and so on has to be considered for each component choice. As the power levels increase, component optimization gets more and more important because of the increase risk of significant losses.
Common diodes, i.e. a device that flows current in one direction and blocks current in the other direction, more or less function in the same way. However, IGBT and MOSFET are two competing alternatives when it comes to transistors and they are both used in the automobile industry. Power electronics in HEV’s on the market tend to use IGBT’s, the question is why? [THSII].
Both alternatives are available for the voltage levels found in HEV’s but the difference is the switching behavior. When a MOSFET changes state from on to off the resistance is quite high and the losses are proportional to the square of the current. The IGBT on the other hand shows a similar behavior to a simple bipolar junction transistor where losses during state change is a product of the smallest on-voltage and the very low on-resistance. As a result the IGBT will have a better efficiency in high current applications and the MOSFET in low current applications, somewhere in between they are equal [Rizzoni].
Depending on the parameters of a three phase power converter (used as motor controller) the efficiency can be in the range of 90-99 %. Switching speed is an important parameter when it comes to efficiency and for low voltage applications MOSFET switching speed is higher than the IGBT. But for high voltage HEV applications, the IGBT is a better choice regarding the switching speed as well.
Semiconductors are mainly built by silicon and cooled by some kind of heat sink. As the power levels in an HEV traction system are quite high, heat dissipation can be in the range of 10-20kW for the motor controller. This requires extensive cooling, normally a separate liquid circulation system, and if the temperature of the component reaches it maximum it cannot continue to operate, which is unacceptable. With the use of optimized materials, such as silicon carbide, the maximum operating temperature is increased which enables the electronics to share cooling system with the engine. That is both an economical and space saving benefit [Rizzoni].
9.3.2 Conversion modes The basic tasks of power management components in a HEV vehicle:
DC to DC DC to AC AC to DC (AC to AC)
Where the most important ones are DC to DC (auxiliaries and other consumers) and DC to AC (traction system), i.e. use of battery energy, and of course, AC to DC (battery charging).
DC to DC conversion
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DC is the power source for all electrical systems in a conventional vehicle and is therefore still going to be used for many of the medium to low power sub-systems in a HEV. Additionally all types of microcontrollers, obviously an essential part of an HEV, use low voltage DC power around 3.3 V or 5 V.
As discussed as a part of the electrical energy storage chapter, multiple voltages are likely to exist in parallel in modern HEV’s and EV’s. If energy is to be shared between these systems, DC-DC converters have to be used.
DC-DC electrical energy conversion alters the voltage of a DC power source from one level to another. Examples of usage are:
Battery charging from low voltage DC supplies, e.g. solar panels or fuel-cells. Stabilized voltage input to motor controller from e.g. an ultra-capacitor which has very varying voltage. Low to medium power DC motor drive from the traction battery, e.g. a power steering pump. Steady power supply to logics
DC to AC (AC to DC) conversion DC to AC conversion is mainly used to power AC motors, which most often are used for propulsion. Some DC-DC and DC-AC converters are unidirectional and some are bidirectional. In the case of traction motor drive, it is important that the motor can spin in both ways as otherwise the vehicle won’t be able to reverse. In most simple DC supply cases, a unidirectional DC-DC converter is sufficient.
DC to AC conversion can be made in a number of ways of varying complexity. As mentioned, some motors work better with pure sinusoidal waveforms compared to trapezoidal waveforms but the latter requires more advanced electronics to generate. Advanced power control extends outside the scope of this text but in the big picture, PWM is used in different ways.
Examples of conversion circuits
DC - DC: Step down converter (Buck converter) – From high to low voltage Step up converter (Boost converter) – From low to high voltage Buck-Boost converter – Multi function voltage converter
DC - AC / AC – DC / DC – DC: Full bridge converter – 2 or 3 phase, e.g. unidirectional drive of dc machines
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9.3.3 Plug-in charging As previously mentioned, PHEVs have the added possibility of pre-charging through an electrical wall-supply before use. A great potential for fuel-savings and decreased environmental impact but it requires a high capacity battery charger, i.e. a high power AC to DC converter.
If the HEV electrical system is designed in a flexible way, the vehicle can utilize the same AC- DC converter as it normally uses for battery charging from the vehicles own generator. For flexible operation it should also be capable of accepting both a 1-phase and a 3-phase power source.
A normal power outlet (1-phase) in the USA delivers 15 A at 120 V (1,8 kW) while a normal European outlet delivers 10 A at 230 V (2,3 kW). This means that, with the 11.6 kWh battery goal of the USDE PHEV project, the minimum charging time is:
6,4 h charging in the USA using a 1-phase power source 5 h charging in Europe using a 1-phase power source
In other words, less than the normal time a human being spends asleep every night but still quite a long time. If 3-phase power is used instead, which in Europe means a minimum of 16 A at 400 V (6,4 kW), the charging time is reduced to:
1,8 h charging in Europe using a 3-phase power source
At this power level and energy capacity PHEV’s could be recharged to around 70-80 % during e.g. a normal lunch break which would increase the fuel saving potential of a PHEV even more. Based on an approximate energy consumption of 0.16 kWh/km this would correspond to “range input rate” of around 40 km every hour as an example.
9.3.4 Auxiliary power As a function of the very high electrical power capacity of a HEV compared to a conventional vehicle some new applications arise, e.g. operation as an auxiliary power unit.
This could be useful in many applications, e.g. increased possibility to run home appliances on the road (during traveling or for work) and use as a backup generator in the home during power failures. As the vehicle already is equipped with advanced power electronics, the system can be very flexible.
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9.4 Electrified auxiliary systems Mentioned as a part of the “components of conventional vehicle” chapter, a modern vehicle is equipped with a number of auxiliary systems that aids with functionality, comfort and safety for example. These systems tend to consume more power as extended functionality is added. The energy consumption of the auxiliary devices tends to increase with vehicle size as well.
The basic idea with the electrification of auxiliary systems that has otherwise been powered directly from the ICE with the means of e.g. belts has a pair of reasons. First off, the speed of an ICE varies with load, especially in a conventional drivetrain and it is very difficult to maintain a high efficiency of the auxiliary system through the whole speed range. Hence, if a constant speed electric machine is used as a power source instead, the efficiency of the auxiliary system itself can be increased. The second reason is the possibility of load-leveling. As concluded in the ICE chapter the load of the engine greatly affects the efficiency. With the help of electrified auxiliary systems the load of the ICE can be affected to some extent by e.g. switching systems off during heavy acceleration and switching systems on during low load cruising. Load-leveling also increases the performance of the vehicle.
The drawbacks of electrification compared to a direct connection to the ICE are the additional energy conversion steps. Therefore the efficiency increasing potential always has to be measured against the energy conversion efficiency.
9.4.1 Air conditioning In today’s modern cars an air conditioning system based on the phase-change principle is almost standard. The AC system is powered by a compressor which shifts the phase of trapped gas into liquid and when the liquid is again heated by the cabin, it evaporates in an endothermic process which moves heat from the cabin.
The power-need of the AC compressor is greatly affected by the size of the vehicle cabin but as a result the savings potential also increase with vehicle size. The real power demand of an AC system in a normal car is around 4 kW but in a large vehicle like a bus, the power demand could be in the range of 25-40 kW. However, as the losses scales almost proportional to the engine speed, the power drawn from the engine could in reality be 2-3 times more [IAE HEV]
9.4.2 Steering The steering system of an automobile has to comply with stringent demands. Most importantly it has to be reliable, exact and easy to use. Automobile steering systems can be divided into three main categories:
Passive mechanical steering– Power input from the driver only (old and light road going vehicles) Power assisted steering – Power input from driver and assist system (electric or hydraulic – on all modern vehicles) By wire steering – Control signal from driver, power from an auxiliary system (non road going / Slow vehicles – safety concern)
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All modern road-going vehicles use some kind of power assisted steering. Normally the auxiliary power source is hydraulic and fed by a pump connected to the ICE. Regardless of steering usage, the pump will always be engaged and rotate with a speed synchronized to the ICE. From an efficiency point of view this is not very good as the power demand of the steering system is close to zero when e.g. the car is moving straight ahead.
The most common type of power assisted steering is based on a rack-pinion mechanical gear which converts the rotation of the steering wheel to a translation of the steering rods. As said, normally hydraulic power is used to make steering easier but in light to medium weight vehicles an electrical motor based system can be used. Instead of controlling the power assist with a valve system like the hydraulic version, the electronic version detects torque in the steering column which then controls an electrical motor that does the work. Both of these systems are fail-safe as they also work without the power assist system online.
The electrical power steering system is much better from an efficiency point of view compared to the classic hydraulic system as the load of the ICE (indirect via electricity) can be varied depending on the driving situation. Therefore it is recommended for future HEV applications. However, electrical power steering is not as good at handling high loads, i.e. in heavy vehicles, but in such cases electrification of a hydraulic system can be done. This means that instead of powering the hydraulic pump directly from the ICE it can be powered by an easily controlled electrical motor which adds flexibility and bumps efficiency.
9.4.3 Pneumatic systems (Commercial vehicles) Large commercial vehicles, trucks and buses for example, normally use a pneumatic system for control and powering of many subsystems, e.g. brakes, suspension and doors. These pneumatic systems are in turn conventionally powered by an air-compressor driven by the ICE via a belt drive. As all other auxiliary devices powered directly from the ICE, the efficiency is greatly dependent on the varying speed of the ICE.
However, as the pneumatic system is an essential part of the vehicle safety, i.e. it’s the brake power source; the pneumatic system has to be online, controlled and refilled all the time. This also requires the compressor to run almost all the time, even when the ICE of an HEV drivetrain is shut off. The result is that the efficiency drops as large amounts of energy has to be converted from mechanical (ICE shaft) to electrical (battery/motor) and then finally to pneumatic. Unfortunately there are no alternatives if a pneumatic safety system is to be used, as intermittent engine operation is an essential part of an HEV.
Instead, a better alternative would be to use electrically powered “brake-by-wire” system as the one developed by Haldex Brake AB for commercial vehicles.
9.4.4 Other consumers Examples of other auxiliary systems that could benefit from electrification and through that, intermittent use and a fuel saving potential, could be e.g. ICE coolant pumps, fuel pumps and gearbox pumps.
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On top of that, some already electrically powered systems could benefit from the increased voltages of a HEV battery in the future with regards to size, flexibility and efficiency, e.g. entertainment systems.
9.4.5 Summarized potential Even though the power demand of each auxiliary system is very low compared to the power demand of the vehicle propulsion system itself, the sum of all components can have a significant impact on the fuel consumption, especially in certain driving situations.
The keys to minimize the impact of auxiliary systems on fuel consumption are optimized control strategies, e.g. activation during braking (regenerative use). Simulations of electrified auxiliary systems in a conventional bus have shown potential savings of up to 80 % for some subsystems (doors, suspension, parking brake and steering combined) [IAE HEV]. In a normal car, the potential might not be as big, but electrification of auxiliary systems still adds flexibility and is sometimes a must. The increased power capacity of the onboard electrical system also adds a lot of new possibilities for future high power systems.
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9.5 Additional mechanical devices – Power split As mentioned, the core component of a combined or power split hybrid is the power split device. The PSD concept is based on the same kind of planetary gear set that is used for gearing in an automatic gearbox. The automatic gearbox was discussed in chapter 3. However, by connecting the three different shafts in a smart way, the planetary gear set can be used in a somewhat alternative way.
Figure 33 – Use of a power split device in the Toyota HSD
The HSD mode of operation with only a single power split device, i.e. a single 3 shaft planetary gear set, is a good application example which also can be called an “input-split hybrid”. The PSD is connected to 3 components, the ICE, a generator often referred to as MG1 “Motor/Generator1” and finally to the motor, or MG2 “Motor/Generator2”. As visualized in Figure 33, the ICE is connected to the planetary carrier of the PSD, the generator or MG1 is connected to the sun gear. Finally the motor and by it the differential is connected to the ring gear. This configuration delivers a fixed amount of torque through the electrical path from the engine to the wheels, keeping the mechanical solution relative simple but unfortunately it comes with some drawbacks of its own.
With the Prius as a continued example, the maximum speed of the vehicle in pure electric operation will be mainly limited by the maximum speed of MG1 as it has to rotate in the opposite direction of MG2 with a speed 2.6 times larger [Olszewski ]. Another drawback is the efficiency dependency of how much power is routed via the electrical path. If the ICE load is to be held at optimum at high vehicle velocities the electrical machines has to rotate at high speeds which decreases the efficiency of the transmission alone due to the gearing ratios in the PSD being optimized for lower velocities.
To limit these problems and increase the optimized speed range of a power slit hybrid some derivates of the HSD design has been developed, for example the GHC, Global Hybrid Cooperation (formerly known as AHS, Advanced Hybrid System) , “two-mode hybrid system” and the two-stage hybrid system used in the Lexus RX-400h and the Toyota Highlander These, and some other, systems use two ore more planetary gear sets of Ravigneax type together with two clutches which controls the multiple degrees of freedoms that enables dual torque ratios [Peng, Filipi]. [126]
10. Modelling Development of automotive technology is resource demanding and especially the testing phase tend to be time consuming and costly. To increase the efficiency of the development process, computer simulations of subsystems are very powerful tools. As a second step, these subsystems can also be assembled into a complete vehicle model, which is an essential part of control strategy development, if such are needed.
However, simulations are useless without truthful models of the components themselves. If the models are inexact, the result won’t work in reality or be far from ideal. Unfortunately, component modelling is also a time-consuming process and depending on the accuracy demands, very large efforts could be needed. Summarized, the modelling of drivetrain components is basically a trade-off between model flexibility (depth) and the cost of the work effort needed to develop the model.
10.1 Model depth and fidelity level Any HEV drivetrain component can be classified as a process, e.g. a physical, chemical or thermodynamic process. That process could in turn be described as a function of other, smaller processes, and so on. The depth of a model relates to what level a component is described at. Sometimes a model with high depth is a must, but sometimes basic understanding, i.e. a shallow model, is enough.
In this chapter two types of models will be exemplified, a HiFi (High Fidelity) version and a LoFi (Low Fidelity) version. The HiFi model would describe a component using low level processes closely linked to the e.g. the basic physics. Whereas the LoFi model would describe a component using e.g. empirical data (lookup tables) which are derived from a real-life component test.
For each important component or functionality in the drivetrain, a HiFi model will be exemplified. However, the emphasis will be put on suggesting a sufficient LoFi model with a depth that fulfils its purpose in a future simulation of complete HEV drivetrains and the goal of the simulation itself, e.g. fuel consumption metering. That is to say, if a LoFi model is possible at all.
10.2 ICE modelling Engine modelling has been a prioritized research area for some years and is a priceless tool for engine and engine management development. The main reason for that is the development in microcontroller technology, i.e. the processing power, which has taken large steps forward. Nowadays, the engine control unit of a normal car has the same, or more, computing power as a desktop PC had just a couple of years ago. This increase in processing power makes it possible to replace the steady state engine models (tables) with dynamic real-time models of complete engines in the control unit.
With the use of dynamic engine models, a wider range of load cases can be calculated for and optimized, e.g. such advanced processes like emission formation and non-ideal processes like turbo-charging. The same thing goes for combustion engine modelling on a PC, it’s a choice between dynamic or static models.
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10.2.1 Theoretical or empirical The processes of an ICE can either be described as a system of equations which theoretically describes the system dynamically and generates a model that can be fitted for many different applications. The main drawback is the difficulty to describe the non ideal part of processes which takes places in reality which can make development and calibration very demanding.
The alternative is a model based completely on empirical data. The engine of interest has to be connected to a dynamometer together with all the sensor and measuring equipment that is needed. Real life performance data is then stored in lookup tables of varying dimensions for the variables of interest. Data is thereafter collected by the base model depending on the input parameters and an accurate output is returned. The drawback of this method is limited flexibility, i.e. if engine parameters or configuration are changed, new lookup tables have to be generated. It is also possible to combine both of these methods and describe the basic functions with equations and use empirical data for complex processes and load cases.
10.2.2 Theoretical approach (HiFi) A theoretical approach is based on the cycle models mentioned in chapter 3. As already discussed, net indicated work is given by:
which emphasizes the importance of correct cycle models. Using a SI engine as an example; since the efficiency of an actual engine is less than the efficiency of its equivalent Otto cycle it indicates that some thing is missing and hard to include in the model. If the PV-diagram of an Otto cycle is compared to a matched real life cycle (by temperature, pressure and gas composition) it can be seen that the compression curve is nearly exact due to the compression being nearly isentropic. However, the pressure of the actual combustion cycle does not reach the same peak pressure as the one modelled by the Otto cycle due to the fact that combustion takes place at a non-constant volume. Modelling ICE’s with simple gas cycle models ignores the facts listed below and can therefore not be classified as HiFi models [Heywood]:
Heat loss Mass loss Finite burn rates Finite blowdown rates
A HiFi ICE model would have to be based on finite heat release calculations with heat transfer included. Such models extend outside the scope of this thesis. Additionally, when it comes to theoretical modelling of combustion engines it can be done either on an average level, over one or more engine cycles which sum the state and work of all cylinders into a “Mean Value Engine Model” or “MVEM”. Or, you look at every specific cylinder which gives the possibility to resolve e.g. torque delivery with higher resolution and detect cylinder pulsations. An example of such a model is a “Cylinder-by-Cylinder Engine model” or “CCEM”.
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For most applications, such as HEV control simulations, a MVEM model is more then sufficient.
Moreover the complexity of the gas composition assumption in the cylinder can be chosen at different levels. Either the gas is considered homogenous and ideal and is then called a “Single zone model”. The alternative is to divide the cylinder into two separate zones, one with fuel and one with air. Each zone is considered homogenous and no heat transfer between the two are accounted for but it is still gets computational heavy.
Summarized, theoretical ICE modelling can almost be as advanced as one wish and is therefore mostly used for ICE development and in advanced concepts where physical prototypes are not available [Karlsson, Fredriksson].
10.2.3 Empirical approach (LoFi) The LoFi version of ICE modelling does, as mentioned, not necessarily return a less exact results but lacks in flexibility. A basic empirical ICE model will consist of a torque matrix and a fuel consumption matrix. The inputs are engine speed (constrained by e.g. the vehicle model or the generator) and engine throttle (controlled e.g. by the virtual driver via the driving cycle).
As visualized in the example of Figure 34 the inputs to the torque matrix are engine throttle and engine speed. Consequently the inputs to the fuel consumption matrix (g/s) will be engine torque and engine speed. The fuel consumption matrix can be based on a BSFC matrix with BMEP input as well, which is what normally is available from the manufacturer. The effective fuel consumption is integrated over time to accumulate the total consumption.
The output of the engine model is engine torque which is fed into the vehicle load model in a conventional vehicle or e.g. into a generator model for a series hybrid and so on.
Figure 34 - Basic LoFi engine model example
For HEV functionality evaluation and control strategy development, this kind of LoFi empirical model is recommended as it gives an accurate response to load changes at a
[129] minimum work effort. Of course, it is essential to base the model on accurate data based on the engine which will be used in the final application to obtain real absolute values. However, for relative comparisons, engine data from a similar engine (comb. cycle, size, aspiration-method etc.) is often enough.
10.2.4 Engine scaling One important “physical” optimization factor in HEV drivetrain development is engine downsizing, or “rightsizing”. Simple scaling of look-up tables in the empirical model can be used to some extent but if the scaling exceeds 10-15 % the characteristics of the engine changes. Gas exchange, combustion, heat transfer and mechanical losses does not scale well and in such cases it is better to use a HiFi code to generate new tables for static use or switch to a dynamic model [Peng, Filipi].
For another method to avoid the dependency of empirical efficiency-tables for specific ICEs during HEV simulation and optimization, a universal representation can be made. By using a parameterized model together with a smart curve-fitting algorithm, an easily scalable model can be created. This type of model is very cost efficient as a part of computer HEV optimization [Pisu, Rizzoni].
10.2.5 Emission modelling In short, modelling of emissions is very hard, independent of whether empirical data is used or dynamic models are applied. Static use of empirical data cannot account for the transients in the system which has great impact on emissions. Moreover, dynamic models have to be very advanced to accurately model formation and after-treatment of emissions. Summarized, if emissions are of interest in the simulation, static tables should be used but only for rough indications.
The only solution to accurately evaluate the effects of e.g. changes in control strategies on emissions is to use an “Engine-in-the-loop”.
10.3 Transmission models The task of a transmission is to change the gearing ratio between the engine and a load, i.e. multiply or divide torque and speed. Furthermore, as mentioned, it transfers torque with varying efficiency as a function of vehicle velocity and which gear (speed) is used plus the torque transmitted (force).
Additionally, shifting logics (automatic shifting) is incorporated in some gearboxes but in this case it is chosen to be separated as a part of vehicle control and will be mentioned later on.
10.3.1 Component based modelling (HiFi) A transmission consists of a couple of subsystems (clutch included) which will work in sequence during shifting. The hard part is to model the shifting procedure itself compared to the static functionality of a gearbox which is quite basic. The adjacent future in transmission technology will most likely be dominated by dual-clutch type transmissions and therefore modelling is suggested to use such a gearbox as reference.
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A HiFi version would model the characteristics of the clutches (time delays, friction characteristics etc.) in detail, continue with a detailed model of the gear-set and include models to describe losses in real-time (inertia, churning etc.). Finally, it would have to include a model of the management system (selectors, hydraulics, mechatronics etc.). This type of HiFi model would be very advanced and demand extensive knowledge of the transmission design, therefore it is more likely to be used e.g. for development of new transmission components or advanced transmission control.
10.3.2 Modelling of basic functionality (LoFi) For HEV control optimization and fuel efficiency simulations the basic functionality of the gearbox is enough in a model. The inputs to such a model can e.g. be engine torque (from the ICE model) and vehicle velocity (from the vehicle dynamics model). Additionally a gear selection is received from the “cyber shifter” based on a shifting strategy which is a part of transmission control. The current gear is used as input to a gear ratio table and a gear efficiency table. In the example visualized in Figure 35 the gear efficiency is only dependent on the selected gear which contradicts the information in chapter 3 [GMPT]. Hence, an efficiency matrix based on both engine torque and shaft speed, i.e. indirectly the vehicle velocity, should be used to increase the validity of the efficiency compensation.
Figure 35 - Transmission model example
Furthermore, the shifting delay which could be interesting in some cases is incorporated in the cyber shifter and not the transmission model. Modelling of torque-converter based transmissions is left out of this text because of its anticipated reduction of importance. The main difference however is that engine speed is usually used as input instead of torque to the transmission model and the pumping torque of the converter is used in the engine model together with the inertia to calculate the current engine speed [Peng, Filipi].
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10.4 Vehicle dynamics A vehicle model can be made very advanced, especially when it comes to modelling the complex behaviour of tires. Principally there are some categories; models that manage tyre slip or models that don’t. Furthermore, models that incorporates chassis dynamics and those who don’t and finally how many directions of chassis-dynamics that are considered (longitudinal, lateral and vertical).
Obviously, the more flexibility and degrees of freedoms that are added, the more complex the model gets.
10.4.1 Non linear vehicle dynamics (HiFi) An advanced vehicle model calculates multidimensional tire loads, suspension performance, weight shifting, drivetrain traction distribution and of course mass, inertia, rolling and aerodynamic losses and so on. Such a detailed traction model could be used but it is however not necessary if the focus of the HEV simulations is on fuel economy, i.e. wheel-slip is not good for the fuel economy anyhow. On the other hand, if dynamic HEV drivetrain performance simulations are to be done, the complexity needs to be increased.
10.4.2 Ideal 1-dimensional vehicle dynamics (LoFi) For basic HEV functionality and fuel consumption evaluation, a simple longitudinal vehicle model is enough. The main components are drivetrain torque, brake torque, rolling resistance, aerodynamic resistance, grade resistance (if any) and finally, the vehicle mass.
The resistances are described as:
Rolling resistance
Wind resistance (aerodynamic drag)
Grade resistance (gravity)
Where:
And:
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Remember that, while rolling and wind resistance are losses, grade resistance is not thus a negative angle of inclination changes sign on the grade contribution.
As seen in the example visualized in Figure 36, all contributions are converted to forces acting on the vehicle and summarized in a mux, i.e. the sum of the forces is calculated. With the help of Newtons second law the vehicle acceleration is finally calculated.
The vehicle speed can as a suggestion be returned to the transmission model and thereafter to the engine model as it constrains the engine speed in a conventional drivetrain.
Figure 36 - Basic vehicle dynamics model
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10.5 Virtual driver and driving cycles The most important input to a vehicle simulation has to be the desired speed, without speed, nothing moves. The desired speed is governed by the selected driving cycle. A driving cycle is a set of data points representing speed as a function of time.
There are many different driving cycles, some of them are used to simulate special load cases such as urban or highway driving, while others try to include “a little bit of all” and are used to get a wider picture of the fuel consumption of a vehicle in normal use. Such cycles has been developed as tools for vehicle certification and the most common ones are used respectively by the US government (FTP-75) and by the European Economic Community, EEC (NEDC) for vehicle emission and fuel consumption certification.
Realistic driving cycles are not just critical for the evaluation of vehicles but also an essential tool during the design phase of new HEV’s as they directly affect the validity of the control optimization.
10.5.1 NEDC A combination of the UDC (Urban Driving Cycle) and the EUDC (Extra Urban Driving Cycle) test cycle is also known as MVEG-A. The UDC part represents city driving conditions and is characterized by low velocity and low engine load. In MVEG-A the UDC is repeated 4 times and afterwards the EUDC is added to account for more aggressive, high speed driving situations. The maximum speed of the EUDC part of the cycle is 120 km/h compared to 50 km/h for the UDC.
However, from the year 2000, the 40 s idling period in the beginning of the MVEG-A cycle is removed and the NEDC, New European Driving Cycle is constituted. The NEDC cycle is also known as the MVEG-B cycle and is currently the one used for vehicle consumption and emission certification in Europe [VV Emissions + Dieselnet]. In Figure 37 the NEDC cycle is visualized.
Figure 37 - New European Driving Cycle (NEDC) - [Wikimedia GNU License] [134]
The NEDC has low acceleration rates and is therefore considered to be mild. It also has a limited chance of regeneration due to the soft and a small number of retardations.
Short facts about the NEDC [Dieselnet]:
Total distance: 11,01 km Total time: 1180 s (+10 min time off between part 2 and 3) Average speed: 40,65 km/h Top speed: 120 km/h
10.5.2 FTP-75 The FTP-75 (Federal Test Procedure) is used for emission certification of light duty vehicles in the USA. However, since the year 2000, two complementary cycles are also run; the US06 (High speed driving) and the SC03 (use of air-conditioning). This version of the FTP-75 is called the SFTP (Supplemental Federal Test Procedure).
The FTP-75 originates from the older FTP-72 cycle but adds a third phase (the last 505 s), which is identical to the first 505 s but with a hot engine. Thus the FTP-75 can be divided into three parts [EPA FTP]:
Cold start phase (0-505 s) Transient phase (505-1369 s) Hot start phase (Engine stopped 10 min, restart 0-505 s)
The FTP-75 is visualized below in Figure 38.
Figure 38 – Federal Test Procedure 75 (FTP-75) – [Wikimedia GNU License]
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The FTP-75 is considered “tougher” than the NEDC due to the increased number of accelerations and retardations but also increases the potential of regenerative braking, i.e. hybridiziation.
Short facts about the FTP-75 [EPA FTP]:
Total distance: 17,77 km Total time: 1874 s (+10 min time off between part 2 and 3) Average speed: 34,1 km/h Top speed: 91,5 km/h
10.5.3 The virtual driver As seen in the visualization of the two most common test cycles earlier, the velocity is not a continuously differentiable function which means that speed tracking cannot be done by a fitted torque function. During normal driving and when the same type of test is performed in real life, the driver of the car has to follow the requested speed of the cycle within some specified margin. Therefore it is logical to put the same demands on the virtual driver which drives the car in the simulation.
It is possible to use a feedforward controller to follow the desired speed but it will not be reusable if model parameters change too much. Instead a feedback driver should be used to increase the flexibility of the simulation environment.
A closed loop PI-controller is recommended but a constant gain feedback might not work well at low speed if there are too much nonlinearities. Also, separate gains for acceleration and deceleration could be required. The integrating part is desired to ensure a small steady state error, but as always, integration might cause problems with stability [Peng, Filipi]
In Figure 39, a simple example of a virtual driver with actual and desired speed as input, and a “throttle/brake” signal as output is visualized. In the example, positive throttle values equals’ positive torque (from the propulsion devices) and negative throttle values means negative torque (from the friction or regeneration brakes).
Figure 39 - Basic example of a cyberdriver
In a conventional vehicle the braking signal ( ) can simply be saturated as the maximum braking torque and fed into the vehicle force mux of the vehicle dynamics model. [136]
In a HEV application, the throttle value has to be fed into a “hybrid controller” which, if the throttle is positive, divides the torque between the propulsion devices, or if the torque is negative, divides the torque between the regenerative braking device and the friction brakes.
If the cyber driver was to be divided into a HiFi and LoFi model, the LoFi alternative could use pre-programmed PI(D)-controllers, e.g. such that are available in Matlab/SimuLink. While the HiFi model would be divided into its individual segments and built from scratch with a separate braking and acceleration loop for more flexible control and tuning possibilities.
10.6 Electrical machine models As with ICE’s, electrical machines can be described with the help of equations to some extent. These equations will be different depending on what type of electrical machines that are used, much due to different motion characteristics of the internal flux.
From the basic inputs of machine speed (constrained by e.g. the vehicle velocity or the ICE speed) and a desired torque, the electrical machine model has to be able to generate an output torque and calculate the electrical power needed to deliver that torque. Or, in the case of electrical energy generation, absorb a torque and calculate the power generated.
10.6.1 Theoretical approach (HiFi) A theoretical model would be divided into two parts, the torque production and the losses. Torque production is as said individual for each type of motor but are all based on electrical relations to begin with. These types of equations extend outside the scope of this thesis and are therefore not included.
In an electrical machine there are several types of losses but the most important ones are the copper losses and the iron losses. The copper losses are the result of the winding currents flowing in the windings and the iron losses are the result of the magnetic flux rotating in the core material, i.e. eddy currents. Copper losses are proportional to the squared currents, i.e. also proportional to the squared torque as the torque and currents are proportional to each other. The iron losses on the other hand are proportional to the flux and the speed squared.
All of these mathematical relationships are then to be summarized into a dynamic model of the machine characteristics and can thereafter then be used in the simulations. However, this type of models requires extensive knowledge of electrical machine theory and increases the complexity of the whole simulation-model as e.g. voltages and currents has to be included.
For HEV evaluation and optimization purposes, the LoFi alternative is often enough.
10.6.2 Empirical approach (LoFi) Very much alike the empirical ICE model, the LoFi model of an electrical machine is also based on data stored during evaluation of the performance in a “test-bench”. The torque delivery characteristics of electrical machines have already been mentioned and for some rotational speed the torque output is proportional to the current, up to the level of
[137] maximum torque. Therefore, the torque demand into the model can be expressed as a fraction of the maximum torque (0-1). Moreover, at the current speed and the desired torque, the machine operates at a given efficiency defined as:
Where , depending on whether the vehicle is operating during acceleration (consumption) or deceleration (storing). Eq. 10.6 is easily used backwards to calculate the electrical power consumption.
In the example visualized in Figure 40, the inputs to the model are motor speed and the motor torque demand (e.g. from the hybrid controller). The model utilizes two lookup tables for calculation of the output torque and the electrical power that is consumed. The “max torque” table returns the maximum possible torque at the current speed which is then used to saturate the torque demand. The delivered torque is thereafter used together with the rotational speed, i.e. the power output, as input to the 2-D “efficiency lookup” table which is then used to calculate the electrical power consumption from either a generator or an accumulator.
Figure 40 - Example of en empirical electrical machine model
Based on empirical data of good quality, this type of model is cost-efficient and very useful in basic HEV simulations. However, as with empirical ICE models, these types of models lacks flexibility and are therefore unsuited as a tool for development and optimization of the electrical machines themselves. Also, dimensioning and rightsizing is an issue as lookup- tables cannot be scaled very much [IEA HEV]. But, in the same way as mentioned for ICE’s, a
[138] parameterized model combined with a curve fitting algorithm can be used to easily support scaling and add flexibility during HEV optimization [Pisu, Rizzoni]
10.7 Battery models A battery model has to describe the losses inflicted during both charging and discharging of a battery while estimating the very much important SOC. Unfortunately, SOC estimation is dependent on many variables, such as currents, age, temperature etc. which makes it hard to obtain a high level of accuracy with computer simulations.
In white papers, numerous models are mentioned to predict the behaviour of batteries at a varying degree of accuracy. The available models differ in complexity and the amount of empirical tests that has to be done to calibrate and implement the models into simulations also varies.
According to Singh and Nallanchakravarthula battery models can be divided into two types [Singh, Nallanchakravarthula]:
1. Electrochemical battery models 2. Equivalent circuit battery models
The simplest models are based only on the basic electrochemical behaviour of a battery and will therefore be referred to as a LoFi models, and the equivalent circuit models as HiFi models. However, when it comes to battery modelling, a LoFi model cannot predict the SOC well enough in some cases. Additionally, the simplest equivalent circuit battery models, such as the “EMF-Resistance” model, cannot either.
Below follows a short description of known approaches to battery modelling.
10.7.1 Electrochemical battery models
Peukert equation The Peukert equation states a relationship between the discharge current and the “constant current” discharge time which decreases the current with increased time.
Battery capacity is often measured in Ah which basically is based on the definition of Peukert’s law.
where:
the capacity at 1 A discharge rate [Ah] the discharge current Peukert constant [dimensionless] is the discharge time [h]
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Summarized, the Peukert law is a good indication of performance but only considers two variables, i.e. the discharge current and time, for “SOC estimation” which is far from truthful. Therefore it is not recommended for HEV simulations [Singh, Nallanchakravarthula].
Shepherd model equation The Shepherd equation uses the EMF, internal resistance, polarization resistance, current and the integral of the current over time as inputs. The output describes the discharge behavior of a battery directly in terms of voltage. It is often used in conjunction with the Peukert equation to obtain the SOC and voltage at given power consumptions. However closer to reality, the SOC estimation still is insufficient [Singh, Nallanchakravarthula].
10.7.2 Equivalent circuit battery models
Resistance (Thévenin) battery model This type of model is the most basic equivalent circuit battery model and it consists of a voltage source (at EMF) in series with a resistor, and sometimes (Thévenin) in parallel with a capacitor and a resistor. Due to the static behavior of the components in the model the result is not very accurate. In reality the resistance, voltage and capacitance varies with thermal conditions and the SOC [Singh, Nallanchakravarthula + Peng, Filipi].
Linear electric model As a step up from the Thévenin model the linear model is built by the EMF being supplied across the consumer and a capacitor. In series with this is a network of three capacitors and three resistors that model overcharging and on top of that there is a fourth resistor in parallel to model self-discharge [Singh, Nallanchakravarthula].
RC battery model Very similar to the linear electric model, the RC model is a dynamic model where each component has its own physical meaning. However, it consists of two capacitors (Cb,Cc) and three resistors (Re,Rc,Rt). The SOC is estimated by measuring the voltage over the Cc capacitor which is shown in Figure 41 [Peng, Filipi + Johnson, Pesaran, Sack].
Figure 41 - RC battery model
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According to NREL and SAFT, the RC model is dependent on three main tests for calibration and sizing of the components in the equivalent circuit [Johnson, Pesaran, Sack]:
A residual capacity test – to determine the maximum Ah capacity An open circuit voltage test – to determine the Voc versus SOC relationship A internal resistance test – to R and C value determination
From what can be told off the battery test and evaluation methods mentioned in the texts [Johnson, Pesaran, Sack] and [Peng, Filipi], battery model calibration requires extensive work and special equipment. Therefore, premature HEV simulation projects are recommended to use pre-derived models.
However, both Johnson, Pesaran, Sack and Peng, Filipi agree that an RC battery model is sufficient and recommended for HEV simulations as both the voltage response and SOC estimation shows good validity when it is compared to empirical data.
10.8 Ultra capacitor models Capacitors can be modeled with simple resistance or RC circuits, much as the ones mentioned for batteries above, and can be referred to as LoFi. However, according to Maxwell who is a large manufacturer of double layer ultra-capacitors, the transient voltage behavior of ultra capacitors is not current/resistive ideal. Also, the voltage increase at a constant charging current varies slightly in slope and when charging stops, an energy distribution phenomenon creates voltage irregularities. Summarized, their opinion is that simple “LoFi” RC models are therefore inadequate to describe the nonlinear ultra capacitor electrode behavior. Instead, categorized as “HiFi”, complex network models have to be used [Miller, Nebrigic, Everett].
In cooperation with the Ansoft company, Maxwell has developed what they call a “nonlinear multiple time constant equivalent circuit model”. It is based on distributed RC network with three nonlinear time constants [Miller, Nebrigic, Everett]. A similar model is mentioned and recommended by Peng and Filipi [Peng, Filipi]. The functionality and details extends outside the scope of this thesis but this kind of model is summarized as recommended.
The Maxwell/Ansoft model is downloadable as a “library” for the Ansoft Simplorer software from the Ansoft website (2008-11-06).
10.9 Power electronic models Power electronics are used for voltage level and type conversions between e.g. batteries and motors. Boosting motor voltage helps to improve power density but the efficiency of the conversion is not 100 %. In simple HEV simulations, the power electronics can be modeled by a constant efficiency drop at an average value of around 95-98 % [Peng,Filipi] but if converter efficiency is to be included as a part of the power distribution strategy, it has to be dynamic.
The main components of the losses in HEV power electronics are commutation and conduction losses. Commutation losses are a product of the switching behavior of the voltage converter as they (even though in a minimal amount of time) work with high voltage
[141] drops and high currents simultaneously. Conduction losses occur in all electronic components when a current flows through them as there always are “some” resistance. However, the conversion losses depend not so much on the currents but more on the voltage ratio. With large voltage steps comes a lower efficiency but up to ratios around 1:2.5 the efficiency of DC/DC conversions tend to stay above 95 % [Alaküla].
Summarized, if large amounts of energy are to be converted in large steps, a lookup table of the ratio-to-efficiency characteristics of a converter can be utilized.
10.10 Available simulation platforms Summarized, powertrain simulation models are a very powerful vehicle development tool and in particular when it comes to HEV control. There are a number of off-the-shelf vehicle simulation platforms on the market but only some of them cover the area of hybrid technology.
The minimum feature demand of a vehicle simulation platform suitable for HEV evaluation and development, i.e. including conventional vehicles as references, are by the author’s opinion:
Validated simulation results Validated and pre-programmed component library (HEV + Conventional) Fuel economy and performance evaluation ability Easy HEV concept topology evaluation Hybrid drivetrain energy flow visualization Control and sizing optimization algorithms (DOE) Easy-to-use graphical user interface Emission estimation (considered as a plus)
And of course at a cost that makes the solution price-worthy. Below, three alternatives are shortly described.
AVL – CRUISE
Claims: Written in MATLAB, Simulink Multi-parameter sensitivity analysis of any scalar or value in characteristic or map. Achieves performance, fuel economy, emissions and drivability optimization in one integrated simulation loop. DOE plans can be imported and exported Advanced result representation (plots, diagrams etc.) HIL – Capability Co-Simulation with other AVL High Fidelity component models Co-Simulation with MATLAB/Simulink (control systems) Drive quality and vibration simulation Thermal management simulations
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Emission estimation C-interpreter available for adding component extensions or control strategies (99 I/O “ports”)
Noticed: Lacks validation results (Official website – 2008-11-05) Possible limitation in simulation environment flexibility (closed code)
Cost: Available upon request (2008-11-05)
More information: http://www.avl.com/wo/webobsession.servlet.go/encoded/YXBwPWJjbXMmcGFnZT12aWV 3Jm1hc2s9dmlldyZub2RldGl0bGVpZD02NDc0Ng_3D_3D.html
AVL – ADVISOR
Claims: Written in MATLAB, Simulink Graphical user interface Model library AWD capability (unknown functionality) Emission prediction DOE capability (sizing)
Noticed: Old software, not updated since 2004. Lacks validation results (Official website – 2008-11-05)
Cost: Available upon request (2008-11-05)
More information: http://www.avl.com/wo/webobsession.servlet.go/encoded/YXBwPWJjbXMmcGFnZT12aWV 3Jm1hc2s9dmlldyZub2RldGl0bGVpZD0zMjc0Mg_3D_3D.html
ANL – PSAT
Claims:
Forward-looking model Written in MATLAB (R14SP3), Simulink and StateFlow Ensured modularity and flexibility
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Very user-friendly graphical user interface (C#) Complete Simulink models and data sets provided Multiple-option component model libraries Designed for co-simulation environments (via e.g. MATLAB, Simulink) Real-time simulation (PSAT-PRO) HIL-capability – Rapid prototyping (PSAT-PRO) High-Fidelity component models, e.g. ICE, batteries and fuel cells, are available via separate licensing. Used by numerous automotive companies, suppliers, agencies and universities. AWD capability (unknown functionality)
Validated within 2 % for mild/power-assist hybrid vehicles (Honda Insight, Ford P2000) and 5 % for full hybrid vehicles (Toyota Prius) regarding fuel economy and battery SOC estimation on several driving cycles.
Noticed: Awarded R&D 100 Mentioned in many white papers Very good validity
Cost:
U.S organization – $ 12000 / License, up to 3 Foreign organization - $ 20000 / License, up to 3
More information: http://www.transportation.anl.gov/modeling_simulation/PSAT/index.html
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11. Control Compared to a conventional vehicle, hybrid technology adds a lot of new possibilities. These possibilities are equivalent to an increase of the degrees of freedoms in the system. Additional to the flexibility that is added, each degree of freedom still has to be constrained in some way, e.g. by assigning values to variables in the system or by declaring relationships between others. How this is done has the potential to greatly affect the functionality and performance of the system in real life.
11.1 Synergy is the key As probably learnt by now, a hybrid vehicle has a primary and secondary energy source. The whole idea of a hybrid drivetrain is to take advantage of the positive features of each source and via them try to limit the drawbacks of each other, e.g. the low average efficiency of an ICE in a conventional drivetrain or the short range of an EV. Hence, synergy is the key!
A control strategy is very dependent on what type of drivetrain layout a hybrid vehicle uses and by the components it consists of. As a result, the final strategy will be unique for each application. Nevertheless, the basic idea is the same so let’s take a look at some examples with a high level of flexibility, e.g. a power-split or parallel hybrid electric vehicle.
11.2 Where to start As mentioned in chapter 7, the DOH can be a good first-indication of potentials and how to distribute an optimization effort. But as always, it’s good to start by specifying the main goal and the key problems.
Main goal:
While meeting the power demand from the driver and assuring vehicle safety, develop a control strategy that minimizes the fuel consumption and emission levels of the vehicle.
Key problems:
Control the total amount of power that is generated by the vehicle and how it should be split between the multiple power sources.
Minimize energy loss by e.g. harnessing kinetic energy during retardation in a way that is transparent to the driver.
Maintain an adequate battery charge which complies with the power request of the driving situation while maintaining a good battery health.
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11.3 Key component control aspects The key components of an HEV in general are the ICE, the accumulator and the traction motors. A short summary on their instantaneous control targets follows.
11.3.1 The ICE Depending on the drivetrain layout, the ICE can be load shifted torque wise, speed wise or both. The main goals of ICE load control are to:
Minimize the BSFC (maximize the efficiency) independent of vehicle load Minimize emissions independent of vehicle load
Based on the suggested LoFi, empirical, ICE model from the previous chapter, the BSFC topology plot for the current engine is brought into focus. The accumulated fuel consumption for a load case is minimized by following the shortest path to the “sweet-spot”, i.e. the area of minimum BSFC, and concentrating all of the load points to that area. Based on Figure 10, a close to optimum ICE load path is exemplified in Figure 42.
Figure 42 - Optimal ICE load path example
Without consideration to engine calibration and exhaust treatment systems, when it comes to engine load levelling, the key to reach low emission levels is to avoid BMEP and speed transients and hence keep the engine load as close to static as possible.
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Other important factors to consider are:
Number of engine restarts (reduces the efficiency) Engine temperature fluctuations (reduces the efficiency) Catalyst and particle filter temperature fluctuations (increases emissions) Load limiting (speed, temperature and NVH etc.)
However, depending on the drivetrain layout, its component specification and the supervisory control idea, minimum instantaneous ICE BSFC might not always be sought after. Examples of such cases are SOC constraints, heavy acceleration and similar load cases where other important components (e.g. indicated by the DOH) might be working inefficiently. Hence, the goal is always to minimize the cost of fuel and emissions that are accumulated over the whole cycle, from point A to point B (more on that in chapter 11.4).
11.3.2 The accumulator The main idea of the electrical traction system in an HEV is to handle the peaks and sinks of the vehicle load. For the electrical accumulator, i.e. a battery or an ultra capacitor, this involves transient energy storage. The SOC of an accumulator will increase or decrease depending on the vehicle load but only within its SOC design window. Capacitors generally are not limited by a SOC window but to enhance the performance of adjacent devices it can be profiting to limit the, e.g. the minimum, SOC to some extent. Batteries however always are limited to a specified SOC window due to efficiency and lifetime reasons.
It is important to remember that the SOC window will also affect the component sizing as:
where
By flipping the picture; we know that the SOC window strongly affects the battery lifetime and efficiency, while the driving style via the control strategy controls the SOC. From those facts it is evident that the vehicle control strategy is very important to the battery lifetime.
Moreover, the SOC of the accumulator always has to be “suitable” and at a level that copes with the oncoming load case. For example the SOC cannot be too high at the beginning of regenerative braking or all of the energy cannot be harnessed. As well as it cannot be to low or the combined system power won’t be sufficient in some cases, e.g. to overtake a car in front of you.
Several factors affects the accumulator related part of a control strategy and some examples follows:
Total capacity (indirect) Current SOC
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ROC/ROD limitations Current cycle efficiency Additional health parameters, e.g. temperature Operating mode, e.g. charge sustaining, charge depleting etc. Driving style – stochastic variables Current load case characteristic, e.g. urban / highway
As SOC related control is highly dependent on the characteristics and performance of the battery cells themselves, close cooperation with the cell manufacturer is of the essence to reach optimum efficiency and lifetime.
In an interview with the magazine Popular mechanics, Bob Lutz who is GM’s vice president and product chief commented on the SOC window of the soon to come lithium equipped Chevrolet Volt. Cite [Popular mechanics]:
“The Volt will only use 50 percent, or 8 KWh, of its 16 KWh capacity. There’s a sweet spot for long battery life that includes operating temperature and the charge/discharge cycle. The Volt’s battery will operate between 30 and 80 percent of its capacity, which is sort of like never revving an engine higher than 4000 rpm.”
The Chevrolet Volt is intended as a PHEV which performance, i.e. the AER, is directly dependent on the SOC window and hence the motivation to “push the limits” is greater in the PHEV case compared to HEVs. Once again, optimum battery control needs to fit both the intended vehicle operation mode and the battery specifications.
11.3.3 The electrical machine(s) The main efficiency-affecting variables of electrical machines are as known from chapter 9 quite similar to those of ICE’s, i.e. torque and speed. However, the efficiency of electrical machines is much higher and the variations are not as big inside the normal operating range either.
Electrical machines deliver their maximum torque from a very low speed; a feature that complements ICE’s very well and shall be taken advantage off. Many hybrids on the market today are ICE-dominated and hence the initial potential, as indicated by the DOH, would be greater if the EM “supports” the ICE and not the other way around.
The main goal of the electrical machine related part of the control strategy is to:
As a function of the power split, i.e. influenced by the DOH, optimization procedure, performance target etc. – level the ICE load Obtain as high internal efficiency as possible without interference with higher priorities Comply with specifications (peak and pulsed power, currents, temperatures etc.) In special cases – assist with vehicle traction/dynamics – e.g. the Haldex eBAX
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Summarized, depending on the energy saving and efficiency enhancing potential of the components in the drivetrain, some will have to be instantaneously prioritized over others. That basic prioritization is then used as guideline in an optimization process. However, it is important to understand and remember, even though the optimum control targets for each individual component is known and specified they will most likely never be fulfilled all in the same time. Hence the real control target will be to optimize the sum of all component contributions.
11.4 Procedures Control strategies can be developed in a number of ways, with varying level of difficulty and result. This chapter is intended as a basic comparison of different power split strategies suitable for e.g. parallel and combined (power-split) hybrids. The concrete objective of HEV control is to find the sequence of optimal power splits at instant time that over a given driving cycle minimizes the fuel consumption and the emission levels. The large problem is of course the need to see “ahead of time” as torque and speed data for the whole cycle is crucial to calculate the optimum result.
This part of the thesis exemplifies three different approaches to torque distribution in multiple path hybrid vehicle drivetrains. The common goal is of course to minimize the fuel consumption and reduce the emission levels.
Control procedure (example methodology):
1. Rule based (Load Leveling Finite State Machine, LL-FSM) 2. Instantaneous optimization (Adaptive Equivalent fuel Consumption Minimization Strategy, A-ECMS) 3. Horizon optimization (Dynamic Programming, DP)
Procedure 1 and 2 [Pisu, Rizzoni] can be implemented in real-time systems while procedure 3 inherently cannot as it requires a finite driving cycle as input. It is possible though to extract a rule based procedure from the result of the dynamic programming, or, implement a stochastic driver model and use it as input to the dynamic programming procedure [Peng, Filipi]. However, procedure 3 is ideal for reference calculations as the optimum solution can be found with DP.
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11.4.1 Rule based control Rule based control is in the group of “intelligent control techniques”. Other control procedures in the same group are e.g. fuzzy logic and artificial neural networks [Pisu, Rizzzoni]. Rule based control is the simplest way to get started. Based on a prioritization, preferably from the DOH, rules are defined to control the system behavior.
Examples could be:
IF the SOC is below a certain limit charge the battery to 80 % SOC
IF the requested power is above ICE power use EM support
IF the ICE efficiency is low due to low bmep AND the load is below the EM max power activate all-electric drive ELSE increase ICE load by increasing the SOC
And so on… The rule based control is simple in concept, however, the number of rules needed to control a complex system increases dramatically with added functionality, which in the end makes it hard to overlook and adapt. Rule based control can also be said to act component centered, as the rules always prioritize one thing above the others, e.g. the ICE load in an ICE-dominated vehicle.
Additionally, tuning has to be done through trial-and-error which doesn’t enable the designer to see the full (theoretical) potential of the system, i.e. is 20 % improvement any good? On top of that the tuning is not re-usable as a tool because of the direct hardware dependency.
An example of a rule based control procedure is the Finite State Machine, or FSM [Pisu, Rizzoni]. In the Pisu, Rizzoni example the FSM uses eight state switches to shift among the possible driving situations according to event-triggered rules. The power split decision between the two machines (ICE and EM) is dependent on the values of accelerator pedal position (alpha), brake pedal position (beta), battery SOC and the requested torque.
Eight states are divided and chosen as follows:
1. Start 2. Stop 3. Cruise – Normal driving without significant accelerations 4. Hard acceleration – EM torque addition 5. Hard deceleration – Hard braking with friction brakes active immediately (safety prioritize) 6. Regeneration – Regenerative braking unless the battery is at maximum SOC 7. Recharge – Recharging of batteries when the SOC drops to a specified limit according to battery specification. 8. Reverse – use only ICE (not shown in figure)
An example layout of the FSM control procedure is visualized in Figure 43.
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Figure 43 - Finite State Machine control layout [Pisu, Rizzoni]
The pros and cons of rule based control are summarized in Table 32 [Pisu, Rizzzoni + Peng, Filipi].
Pros Cons Rapid draft High tuning effort in complex systems Works in simple systems Not reusable Low initial effort Can hide system potential Low computational effort Requires defined breakpoints (calibration heavy) Sensitive to shift points (if any gearbox)
Table 32 - Pros and cons of rule based control
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11.4.2 Instantaneous optimization There are many types of instantaneous optimization strategies, the Equivalent Consumption Minimization Strategy, or ECMS is one example. ECMS, in the same way as rule based control, is a method that optimizes the current system status by taking a “snapshot” of it. The difference is, while the rule based control is centered on prioritized components; ECMS takes a picture of the whole system instead.
The rule based FSM procedure does not explicitly try to optimize energy consumption or emissions, but instead depends on pre-calibrated rules. The ECMS however is based on an explicitly formulated cost function that describes the whole drivetrain.
But first, the optimum solution if only considering fuel economy is obviously:
Where is the fuel mass flow to the ICE. But optimization based on eq. 11.3 requires that the whole driving cycle is finished and thus it cannot be implemented in real time. Instead we have to convert eq. 11.3 into an instantaneous optimization function, which only considers the instantaneous cost.
Unfortunately, eq. 11.4 is not equivalent to eq. 11.3 as the instantaneous minimum can affect the future in a way that increases total consumption over the full cycle. As the future is not known, there is not much of an alternative in real-time situations though.
The fuel mass flow to the ICE is a great measure of that specific components cost. However, what the ECMS method does is to formulate a cost function for the equivalent fuel consumption of the whole vehicle. That function is then optimized with regards to constraints, emissions and drivability for example.
An example declaration follows:
Where is the penalty associated with the use of stored electric energy which in the end originates from an equivalent amount of fuel consumed by the ICE. The SOC is taken into account in the optimization by expressing as a function of the current SOC and the SOC window. Furthermore is the equivalent fuel consumption associated with the direct use of the electric motor and the energy it consumes.
Simplified, eq. 11.5 can be expressed as:
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Where is the vehicle velocity and is the power demand. The can be defined with varying level of complexity but eq. 11.7 tries to exemplify the basic idea:
Where is the mechanical power of the electrical machine and is the efficiency from electrical to mechanical power.
The ECMS procedure depends strongly on the definition of the equivalent cost coupled with the use of the electric motor . In order to calculate its value, an equivalence factor is used. It can be shown that the equivalence factor is related to the average powertrain efficiency over a certain window of time [Pisu, Rizzzoni]. Because of this it will vary with different driving conditions which mean that a specific value for that is suitable for one driving cycle, does not have to be near optimum for another one. A solution to this problem is A-ECMS which adds an adaptive feedback algorithm which can relate the equivalence factor to the current driving cycle.
The result of ECMS operation will be a set of 3-dimensional lookup tables. Given the vehicle speed, driver’s power demand and the current SOC, optimal engine power is indicated by the lookup tables. For parallel hybrids, assuming “perfect transmission shifts”, the vehicle speed can be dropped as a dependent variable [Peng, Filipi]
The ECMS concept is only introduced in this chapter. However, simulation results has shown that the ECMS [Peng, Filipi] and even more, the A-ECMS [Pisu, Rizzoni] delivers very good results at a feasible development effort, close to those of DP.
The pros and cons of the A-ECMS procedure are summarized in Table 33.
Pros Cons Less sensitive to trans. shift points Req. eff. maps of crucial components Single tunable parameter (autotuning) Lacks possibility of low level control Easily portable Very low computational effort Close to DP solution
Table 33 - Pros and cons of A-ECMS control
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11.4.3 Horizon optimization Dynamic Programming, DP, is an example of a horizon optimization procedure which considers the whole vehicle and the future. Dynamic programming can be used to calculate the minimum cost of the whole system but only in a none-casual way, i.e. it needs to see what lies ahead, and thus requires a finite driving cycle as input [Pisu, Rizzoni].
DP calculates the minimum cost trajectory between multiple grid points for predefined stages in a driving cycle. It always delivers the global optimum solution but only to the extent of the grid accuracy. That is to say if the chosen decision-grid is made more coarse, the solution will most likely return an answer further away from optimum. The DP procedure will only be discussed in general in this text.
First off, for each future state, which can be a single or multiple variable-space (e.g. the SOC), the cycle is described by a function of the previous state, a vector with the control signals of interest derived from the vehicle model and finally a disturbance indirectly introduced by the speed profile of the driving cycle [Pisu, Rizzoni].
The future state can e.g. be described as:
Where denotes the next state and the current state, while are the input signals from the vehicle model and is the disturbance inflicted by the target speed used as input to the vehicle model, i.e. the power demand. Figure 44 visualizes the DP process for any given state ).
Figure 44 - Dynamic programming states
A vector, for each state , with example-variables of interest could look like:
Where is the ICE torque at state k, the EM torque and the transmission gear position. In the next step, the current state cost L, has to be declared as a function of the desired optimization variables as inputs. For example:
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Where L furthermore has to be defined internally and weighted (prioritized) to describe how the optimization variables, in this case (the accumulated fuel consumption) and (the emissions) depend on the state and input variables.
Finally, the goal of the process is to minimize the complete cycle cost, J, defines as:
Or with whatever variables that are of interest. To account for the possible difference in initial and final SOC [Peng, Filipi], e.g. define as:
An additional component has to be added and weighted ( to the total cost, e.g.:
Within the constraints put on the system by e.g. component specification, SOC window etc. the cost function can at the end be solved by backward recursive fixing. The calculation will not be discussed in this text but the main drawback is that all state grid points has to be solved, which makes it very computational intensive [Peng, Filipi].
Real time application of DP As the DP problem preferably is solved with backward recursive calculations, “the future” has to be known. Therefore it cannot be applied directly in real time, RT, systems. However the knowledge learnt by DP can be extracted into rules, e.g. by generating a power split lookup table based on desired variables, which then can be used in RT.
Another, more advanced method, is to model the real time driver demand as a stochastic process. By assuming that there is an underlying stochastic Markov-process that represent the expected power demand from the driver in empirical data from known driving cycles, driver probabilities can be calculated. The stochastic data can then be fed into the DP procedure which generates an “optimal control policy” *Peng, Filipi+.
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The pros and cons of DP are summarized in Table 34 below.
Pros Cons Optimum solution possible Cannot be implemented RT directly Great use as “offline-reference” Very computational heavy in fine grids RT use possible indirectly Requires a great level of knowledge Complicated RT use
Table 34 - Pros and cons of DP control
11.5 Regenerative braking One of the key features of electric and hybrid vehicles is the possibility of regenerative braking. In a conventional vehicle, friction brakes are used to convert the kinetic energy of the vehicle to heat during retardation. This means that the kinetic energy which originates from the fuel is lost directly to heat. In transient driving cycles, e.g. urban traffic, regeneration of the otherwise lost energy offers a great potential for fuel savings. But, as brakes also are an essential part of the vehicles safety systems it is crucial that the safety is not compromised with the introduction of regenerative braking.
The main targets are therefore (high priority):
1. Sufficient and correct braking force (compliance with the requested retardation) 2. Preserved vehicle stability in all situations (correct brake force distribution) 3. Consistency (robust, fail-safe and easily operated etc.) 4. Legislation compliance (e.g. European 71/320/EEC)
When the safety can be guaranteed, the secondary targets can be dealt with. To fully harness the potential of a regenerative braking system and hence regenerate as much energy as possible it is crucial to optimize the regenerative braking control strategy.
The secondary targets are (low priority):
1. Maximize the regenerated energy and consequently minimize the use of friction braking 2. Comply with component constraints (e.g. the SOC window, EM max. gen. power. and max battery ROC) 3. Offer smooth operation with good NVH characteristics
Regenerative braking is a balance between compliance with the main targets and at the same time minimizing the use of friction braking, i.e. regenerating as much energy as possible. The regenerative braking control strategy is obviously a key component, both in regards to safety and the fuel consumption.
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11.5.1 Safety/economics trade-off Regenerative braking control strategies can be divided into two main categories, parallel and series braking strategies.
Parallel braking strategy The parallel braking strategy always makes use of both regenerative braking and friction braking simultaneously. Figure 45 visualizes the strategy from which it can be seen that an arbitrary amount of regenerative braking (dependent on the component specification etc.) is used as basis. While the total retardation demand, i.e. the sum of the both brake contributions, is met by adding friction braking on top of the regenerative braking regardless of the demand itself.
The advantages of this strategy are simple control and smooth operation because both brake-components scale linearly and there are no abrupt changes in the force distribution. However, compared to the series braking strategy, less energy will be captured because of the fact that some energy always will be lost to heat by the friction braking [Miller Regen].
Figure 45 - Parallel regenerative braking strategy
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Series braking strategy Compared to the parallel strategy, the series strategy uses solely regenerative braking up to a specified limit which can be seen in Figure 46. The limit is specified as a “pedal position” which indirectly can be translated into a certain braking force.
By only using regenerative braking up to a certain level of retardation, all of the kinetic energy (without compensation for the non-ideal efficiency) can be harnessed during mild retardations. If the retardation level is increased above the preset level, the friction brakes will activate and energy is once again lost directly into heat.
The obvious advantage over the parallel strategy is that 100 % regenerative braking is possible up to a certain level of retardation; hence more energy can be harnessed. However, the force distribution control between the two brake-components will be more advanced and there is an increased risk for jerky brake operation [Miller Regen].
Figure 46 - Series regenerative braking strategy
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Regenerative braking limits Due to the characteristics of EMs, regenerative braking will be limited in some ways. First off, the voltage of electrical machines is proportional to the rotational speed. As the speed decreases during retardation, the voltage will also. If AC machines are used, AC-DC conversion is needed but on top of that, the varying voltage of the EM will require DC-DC conversion to match the accumulator voltage.
Based on the discussion in chapter 9 the regenerative efficiency will therefore vary with speed, both in regards to the DC-DC converter efficiency and to the efficiency of the EM itself. When the speed approaches zero, so will also the voltage. Below a certain level, the input voltage to the DC-DC converter will be too low for sustained operation and the torque capability of the EM will decrease rapidly. As a result, friction brakes will still be crucial during the last part of retardation close to stand-still.
In addition, as braking with an EM is useless if the energy can’t be stored, SOC monitoring is also a crucial part of the regeneration strategy. When the SOC reaches the maximum SOC stated by the SOC window, braking action has to be shifted from regenerative braking to friction braking in a way that is transparent to the driver. If the regenerative braking simply is shut off, the safety will be compromised.
11.5.2 Stability issues Depending on the drivetrain layout and vehicle functionality, many EVs and HEVs will much likely only have regenerative braking capability on one axle, i.e. front or rear. In some cases this can strongly affect the potential of regenerative braking due to vehicle instability.
By ignoring any regenerative braking capability of an eventual ISG coupled to the front axle via the engine crank, an application with the Haldex eBAX will only have regenerative braking capability at the rear wheels. Therefore, if purely regenerative braking is wanted, braking torque can only be introduced on the rear wheels. In some situations, especially in low µ road conditions, regenerative braking can therefore cause severe vehicle instability.
For a given maneuver, the severity of the degradation is dependent upon the retardation torque and the surface friction coefficient. On high µ surfaces, simulations have shown that the introduced instability can be contained by modern ESP systems without any risk. In low µ conditions however, the stability reduction is much more severe and cannot be compensated for with ESP [Hancock, Assadian].
One way of solving the problem would be to constrain the rotating speeds of the rear wheels to those of the front wheels by the means of a mechanical clutch, like the Haldex LSC. But in the eBAX case the idea is to skip any mechanical connection between the two axles and thus the method cannot be used. Another alternative to retain stability would be to shift into friction braking at the front wheels once longitudinal slip is detected on either of the rear wheels *Hancock, Assadian+. But still, to the author’s belief, the potential of rear axle regenerative braking on low µ surfaces will drop substantially compared to high µ.
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However, it has been shown that a similar concept to the eBAX on the other hand has the potential for vehicle stability enhancements in other situations, especially during cornering combined with positive lateral acceleration [Donghyun, Sungho, Hyunsoo].
12. Future work The result of this thesis was intended as a summary of relevant knowledge that could be used to reduce the development effort of hybrid vehicles. The key aspect is to understand the synergy effect of a hybrid drive train and through that approach the full potential of each drive train component.
To the author’s hope, these targets have been met. With the gained knowledge, the next logical step would be to put the component modelling suggestions into practice. By using the suggested LoFi models, a very useable hybrid vehicle simulation environment can be created at a relatively low effort.
Depending on the application, component sizing is most likely the first step of an optimization process. In the eBAX case, the EM power is limited by size and the control targets are still uncertain but a parameterized control strategy can still be outlined. When sufficient component specifications and empirical data are collected, the A-ECMS optimization approach is recommended in real-time applications together with backwards recursive DP solution as “off-line” reference.
For Haldex it is important to establish one or two concrete applications to delimit the control targets. As well as specify to which extent the drive train components can be controlled in the final application, e.g. can engine torque be controlled? These factors are crucial to the control optimization process due to the varying constraints of different applications.
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Appendix A – Pugh matrix Power split hybrid with DT eBAXrear Series hyb. and rear eBAX with motor DT front Parallel hybrid with DT eBAXrear Parallell hybrid with DT hubrear motors Series hybrid with DT 4 hub motors Convential with DT Haldex LSC Alternatives Criteria weight Criterias based Pugh-matrix decision Decision-baseHEVAWD for layoutdrive train theHaldexfor eBAX platforms Available 3 2 4 3 2 5 4 effort Development 4 2 4 2 1 5 3 System cost 2 2 4 2 1 5 5 effort Conversion 4 3 4 3 2 5 4 potential Dynamics 3 3 4 3 4 3 4 potential Traction 3 3 3 4 5 4 3 potential Fuel/Emissions 5 3 4 4 3 0 5 potential PHEV 3 5 4 4 5 0 5 Mass Mass 3 2 4 2 1 5 3 TOTAL 141 120 103 110 119 98
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Appendix B – Nomenclature
2WD – Two Wheel Drive (FWD or RWD) 4WD – Four Wheel Drive (AWD) ACEA - European Automobile Manufacturers Association AER – All Electric Range AWD – All Wheel Drive BAS – Belt Alternator System (GM) BMEP – Brake Mean Effective Pressure BTDC – Before Top Dead Center CAFE – Corporate Average Fuel Economy CCEM – Cylinder-by-Cylinder Engine model CD – Charge Depleting operating mode CS – Charge Sustaining operating mode CVT - Continuously Variable Transmission DOH – Degree Of Hybridization ECMS – Equivalent Consumption Minimization Strategy ECU – Electronic Control Unit E-CVT - Electronically-controlled Continuously Variable Transmission EEC – European Economic Community eLSD – Electronic Limited Slip Differential eBAX – Haldex electronic all wheel drive system EM – Electrical Motor EMF – Electromotive Force EMS – Engine Management System EV – electrical vehicle FMEP – Frictional Mean Effective Pressure FOC – Field Oriented Control FWD – Front Wheel Drive GHG – Green House Gas GM – General Motors HEV – Hybrid Electric Vehicle HSD – Hybrid Synergy Drive (THSII) ICE – Internal IM – Induction Machine IMA – Integrated Motor Assist (Honda) IMEP – Indicated Mean Effective Pressure JAMA – Japanese Automobile Manufacturers Association KAMA – Korean Automobile Manufacturers Association KERS – Kinetic Energy Recovery System LSC – Limited Slip Coupling MFMEP – Mechanical Friction Mean Effective Pressure MVEM – Mean Value Engine Model NEDC – New European Driving Cycle NREL – National Renewable Energy Laboratory NVH – Noise Vibration Harshness PHEV40 – PHEVxx (where xx denotes the AER)
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PM – Permanent Magnet brushless machine PMEP – Pumping Mean Effective Pressure PMSM – Permanent Magnet Synchronous Motors PSD – Power Split Device PTU – Power takeout unit REEV – range extended electrical vehicle RME – Rapeseed Methyl Ester R&D – Research and Development ROC – Rate Of Charge ROD – Rate Of Discharge RT – Real Time RWD – Rear Wheel Drive SAFT – “SAFT” battery company SOC – State Of Charge SRM – Switched Reluctance Machine TCU – Transmission Control Unit THS – Toyota Hybrid System URFC - Unitized Regenerative Fuel Cell USABC – United States Advanced Battery Consortium USDE – United States Department of Energy
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