A Review of Hybrid Electric Vehicles (Hev)

Proceedings of the ASME 2014 International Mechanical Engineering Congress and Exposition IMECE2014 November 14-20, 2014, Montreal, Quebec, Canada

IMECE2014-37846

A REVIEW OF HYBRID ELECTRIC VEHICLES (HEV)

BALAGOVIND N K KARTHA Dr. VINOD KUMAR GOPAL Dept. of Mechanical Engineering Asst. Professor, Dept. of Mechanical Engineering Amrita Vishwa Vidyapeetham, Amritapuri campus, Amrita Vishwa Vidyapeetham, Amritapuri campus, Clappana P.O, Kollam, Kerala, India-690525 Clappana P.O, Kollam, Kerala, India-690525 Email: [email protected] Email: [email protected]

AKILESH NARAYANAN SIDHU SURESH VINAYAK JAYAPRAKASH Dept. of Mechanical Engineering Dept. of Mechanical Engineering Dept. of Mechanical Engineering Amrita Vishwa Vidyapeetham, Amrita Vishwa Vidyapeetham, Amrita Vishwa Vidyapeetham, Amritapuri campus, Amritapuri campus, Amritapuri campus, Clappana P.O, Kollam, Kerala, Clappana P.O, Kollam, Kerala, Clappana P.O, Kollam, Kerala, India-690525 India-690525 India-690525 Email: [email protected] Email:[email protected] Email: [email protected]

ABSTRACT the added power requirement. The motor also acts as a generator that extracts energy from the engine to recharge the This technical paper provides a literature review on battery. Hybrid Electric Vehicles and various hybrid topologies associated with it. Different kinds of hybrids are available in different parts of the world in various brand names. The main aim of this review is to identify the pros and cons of some prominent topologies that could pave a way to the proposition of new and efficient design. Results from the study indicate that hybrids have a potential to bridge the gap between pure gasoline vehicles to plug in electric vehicles leaving behind time for the technology to mature and to make improvements in the infrastructure for Eco-friendly vehicles.

INTRODUCTION

Hybrid vehicles are those that are powered by more than one source of energy. Generally, hybrids use internal combustion engines as the main source and a battery powered motor as the alternate source. The aim of the system is to reduce Fig 1: Typical Map of an Internal Combustion Engine [2] the fuel consumption by assisting the engine with the motor so that the effective load on the engine is reduced. On obtaining The reasons why the use of IC engine is inevitable are in the performance curves of an IC engine, it could be found that plenty. The main reason for the use of IC engines in the specific fuel consumption of the engine does not remain a automobiles is its quicker throttle response than any other constant and is dependent on the factors like speed, torque or source of energy such as fuel cells, external combustion engines power (Fig 1). Hybrid systems aim at maintaining the engine at or motors. It also has a longer lifetime compared to fuel cells the most fuel efficient regions, while the motor compensates for and batteries. External combustion engines, unlike internal

1 Copyright © 2014 by ASME combustion engines, take longer time to get heated up as well. and sudden accelerations can be met by the power assist Also, the battery and electrical technology are not mature from the electric machine. Thus, the dependency on engines enough to meet factors such as affordability and serviceability can be reduced by ensuring satisfactory power delivery. that the present day engines offer [1]. 4) Idle shut off: This is a technique which is employed in all The objective of this technical paper is to provide micro hybrid cars where the engine is shut off during idling information and review about the various hybrid technologies conditions. This reduces fuel being burnt during long traffic available. Though, hybrid vehicles of different engine-motor blocks. combinations are available, none of them have completely revolutionized the automobile industry. This article provides HYBRID CONFIGURATIONS insight into the basic hybrid powertrain designs envisaging that it would pave the way to the design and development of Basically, hybrids are classified according to the way the effective configurations and algorithms to serve as an engine and motor is coupled to the drive shaft [4,5]. alternative to complete engine driven vehicles. 1) The parallel hybrid: In a parallel hybrid, the engine and GENERAL PRINCIPLES motor are individually connected to the drive shaft. The engine and the motor can separately or conjointly power the The design of hybrid systems starts with finding the vehicle, which gives the system the flexibility to run the operating states of the engine and based on these states, a engine at the desired operating point. Here the overall control system is designed to on/off engine/motor or use both efficiency of the system is high as a result of direct power simultaneously. Some of the classic examples of successful transfer from the engine to the wheels. hybrids include the Toyota Prius, Honda Insight and the Ford Fusion. Each vehicle uses their own algorithms to start/stop the engine/motor according to the power requirement. The main aim of hybrid technology is to reduce the dependency of fuel and to extract out the maximum possible energy. The term operating states of the engine refer to the condition at which the vehicle is moving. Examples are idling, hill climb, highway cruising, driving through high traffic, short trips at low speeds, strong accelerations and decelerations, over Fig 2: Parallel hybrid topology [4] speeding, overtaking etc. Ideally, the hybrid system aims at maintaining the engine at the optimal fuel efficient point so that 2) The series hybrid: In a series hybrid, there is no direct the fuel consumption during the trip is reduced. Example to connection between the wheels and the drive shaft. The illustrate the concept is as follows. energy from the engine is converted to electric energy using During the state of low power requirement, the vehicle a generator and the vehicle is powered using this energy via runs on motor alone while the engine is shut off or is used to motor. The advantage of this system is that the load on the charge the battery. In case of high power requirements, the engine could be maintained a constant so that the overall engine is also coupled to the wheels and the total power output fuel consumption could be minimized. The engine could would be the sum of motor power and engine power. There also be turned off when the battery is full. But the could be cases where the engine alone would be powering the disadvantage is that the motors will have to handle huge vehicle as in the case of highway cruising where the vehicle is power and therefore will become expensive and bulky. moving at a steady speed. Whichever operating state the vehicle is in, the algorithm aims at moving the vehicle with least possible fuel consumption. In this way, the dependency on the engine is reduced, thereby reducing the fuel consumption. The capability of the hybrid technology is assessed according to the following factors [3].

1) Utilization the optimal load points of the combustion engine. Fig 3: Series hybrid topology [4] 2) Regenerative Braking: It is the process of recovering the

energy released as heat during braking of the vehicle. 3) Complex hybrids: Such hybrid systems make use of both Though all energy cannot be recovered, if used efficiently, a series and parallel configurations according to the reduction in fuel consumption of up to 30% is possible [3]. requirement. This system makes use of a planetary gear 3) Engine Sizing: Downsizing the engine leads to reduction in system to connect the engine, motor, the generator and the fuel consumption. Additional power required for hill climbs

2 Copyright © 2014 by ASME wheels together. The motor, engine and generator will be As hybrid technology developed, vehicles came into market connected to the planet gears and the wheels will be with various names such as micro hybrid, mild hybrid, full connected to the sun gear. The whole setup will be within a hybrid and plug in hybrid [8, 9]. ring gear. Based on the control system, the motor and generator will push/pull energy to/from the wheels. Here the 1. Micro hybrids are those that has the capability to start/stop series and parallel configurations work together. However, the engine during idling. However, the system does not have this system is very complicated and requires great precision the capability to propel the vehicle with electric power. in order to achieve better fuel efficiency. 2. Mild hybrids have the capability of micro hybrids and in addition posses limited electrified propulsion capability or in other words, electric assistance to the engine. Examples include Honda Insight and Chevrolet Malibu. 3. Full hybrids are those that has full capability to propel the vehicle with electric only as in the case of the Toyota Prius and Hyundai Sonata hybrid. 4. Plug in hybrids are full hybrids with an additional capability to charge by plugging the vehicle to the external power socket. Fig 4: Complex hybrid topology [4]

One of the most successful hybrid vehicles in the market Development of hybrids Toyota Prius, uses a complex hybrid Powertrain termed as the Toyota Hybrid system [10]. This vehicle was launched in the The development of the first hybrid car is reported to be in market in December 1997 and is the first mass produced hybrid 1899 by the Pieper establishment of Liège, Belgium [5]. passenger vehicle in the world. Since then, it has undergone Various sources also say that there had been developments in several improvements in the engine and the powertrain. In the hybrid tricycles even before. However, the credit for designing 2004 Prius, the highly efficient THS II was introduced with an and developing the first full functional hybrid car goes to Dr. efficient gasoline engine that runs on Atkinson cycle and with a Ferdinand Porsche in 1900. Porsche’s hybrid was a series powerful permanent magnet AC synchronous motor. The 2004 hybrid where 2 water cooled combustion engines were used to Prius comprised of a 1.5l gasoline engine, a powerful motor for generate electricity to run the wheel hub motors. Two engines maximum power 50kW at 1200 RPM, a power control unit and produced 5hp together and was completely used to power the a 6.5Ah 21kW Ni-MH (Nickel Metal Hydride) battery pack. motors. The main aim of these motors was to assist the weakly This system was a combination of parallel and series hybrid powered gasoline engines. However, these cars were very costly configurations by making use of a power split device in order to and had many disadvantages. As a result, the product did not extract the advantages of both series and parallel systems [11]. last longer [6]. It has 2 motors and a generator that is coupled to the engine and By 1995, various companies took up hybrids as a the wheels by means of the power split device that comprises of challenge in order reduce the greenhouse gas emissions from the planetary gear system as discussed before. Depending upon the vehicles. Hybrids, even now, is an interim solution to the the driving conditions, the power control unit distributes the fuel crisis, leaving time for the development and power from the engine and the motors to the wheels such that commercialization of pure electric vehicles. the overall efficiency of the system is maximized. The power is transferred from the engine simultaneously while the generator extracts power to recharge the battery. The THS has also got regenerative braking where the kinetic energy is recovered during deceleration and braking. The main advantage of the system is that when the power control unit finds that that the engine is running at least efficiency, the engine is cut off and the vehicle runs on pure electrical energy leading to decrease in dependability on engine power during the course of running. A similar type of the hybrid drive train was used by BMW and Mercedes before.

Fig 5: 1901 Lohner Porsche Semper Vivus [7]

Fig 6: Toyota Prius Hybrid Drive Train [10]

In 2010, the Toyota Prius [12] was equipped with an improved hybrid drivetrain called the Toyota Hybrid Synergy drive. The system was powered by a 1.8l 73 kW, 4 cylinder Fig 7: Honda Insight Torque and Power comparison gasoline engine runs on Atkinson cycle and an electric drive 36 with and without IMA[15] HP power rating. The improved drive train showed better performance as compared to its predecessors. The hybrid power As part of the IMA, Insight employed a 10KW Permanent train resembled to its predecessor, some refinements were made Magnet Brushless DC motor coupled between the engine and to increase the overall efficiency [13]. The primary motor acts transmission in a series; so that during deceleration of the as a mechanical assist to the ICE and also as a generator to vehicle the rate of deceleration can be calculated and the motor recharge the battery during regenerative braking. The secondary could be used to regenerate the kinetic energy to charge the motor acts as a generator that extracts power from the engine to 144V 6.5Ah Nickel Metal Hydride (Ni-MH) battery. During recharge the battery. The power split system is known as an accelerations, the power requirement is calculated by the power ECVT (Electronic Continuously Variable Transmission) control unit in accordance with the position of the throttle because of its ability to shift gears and drive wheels without the position sensor and motor assist is delivered to the main drive use of clutches or any hydraulic systems. This ECVT system, shaft. The state of charge of battery is also a governing factor which is controlled by the PCU (Power Control Unit) ensures that decides the amount of current supplied to the motor for it to that the engine runs in the desired torque-speed range. They assist the engine. More the current, more will be the role of have also incorporated an improved battery cooling system. motor in powering the vehicle. However, the system as a whole is bulky. During acceleration of IMA also incorporates a powerful idle stop system in order the vehicle, this system does not favour and as a result, the to account for reduction in fuel consumption during idling. The vehicle shows reduced throttle response. Also, the generator engine is shut down during idling and during restart, the engine mostly runs by consuming the engine power and not producing is cranked to 600 RPM or more by the high torque series motor enough electrical power. and then ignited using fuel. This results in reduced fuel The 2000 model Honda insight is also one among the most consumption during restart. successful HEV ever launched. Honda Insight, a 2 seater coupe, The 2010 IMA [16] was designed with better features to aimed at reducing the fuel consumption by 50% by a variety of control the fuel consumption. The engine capacity was procedures which includes improved structural and increased to 4 cylinders and 1.3L. It incorporated continuously aerodynamic design, electronic valve adjustments through variable transmissions in order to reduce the frictional losses VTEC (Variable valve Timing and valve lift, Electronic and energy losses during manual shift. As before, IMA only Control) technology, improved catalytic converter and aimed at assistance and not complete dependence on electric Integrated Motor Assist (IMA). Insight was employed with a 3 power. The 2010 Insight had an improved control unit that cuts cylinder and 1000 cm3 IC engine, which was considered off the fuel during decelerations. During decelerations, the optimal for the vehicle with IMA. This downsizing was mainly engine rotates but without fuel. Also, the ECON mode in Insight to ensure minimal friction and other mechanical losses. As a provides the user a choice to opt for better fuel saving or not. The ECON mode is for city traffic where slow speed movement is required. In the ECON mode, the accelerator pedal is also

4 Copyright © 2014 by ASME mapped and the engine response would be smoothened thereby motor is utilized for accelerating the vehicle and engine kicks in reducing fuel consumption. The air conditioner will also be shut only in case of additional torque requirements or low SOC of off during the idling conditions in ECON mode. the battery. In spite of the fact that Insight could bring in more Ford’s Fusion also incorporates a technology similar to efficient operating region for the IC engine, the electric power that of the THS, where, the 2.5 L engine that is run on Atkinson train is just for power assistance. The vehicle cannot run in full cycle is coupled to 2 motors via a power split device. Most of electric mode. Therefore, the fuel saved during cruising will the energy saving is done through regenerative braking, idle also be less. shut off, electric operation at low speeds and low loads and electric assist for sudden accelerations. Both the engine and motor in Fusion are very powerful and is comparable to that of Toyota Camry who shares a similar hybrid topology. General Motors also designed and launched their hybrid drive train capable of propelling the vehicle in engine mode, electric mode and mixed mode using 2 sets of planetary gear systems. Here, the motors that drive the wheels will be swapped out for fixed gears, locking the engine to the drive shaft. Unlike the Prius, it also comprises of two clutches [8].

Fig 8: Hybrid architectures [9] The 2011 Hyundai Sonata hybrid also incorporates a similar mechanism using a single motor between the engine and the 6 speed transmission. But the Sonata uses a much powerful Fig 9: GM hybrid topology [8] engine and motor as compared to the Insight. However, the motor not only assists the engine, but also has the capability to Apart from the car manufacturers, Stridsbergs Powertrain power the vehicle by itself. The motor is used to start the AB developed the Strigear power train comprising of two vehicle from rest and the engine is later restarted in the course electric motors, one simple gear box and a one way clutch of the motion. This reduces the fuel consumed in starting the assembled tightly together and replacing the original gearbox vehicle from rest. The engine will automatically be stopped unit [19]. This series-parallel hybrid system is designed such once the vehicle starts to decelerate and the battery is charged in that it can be easily integrated to almost all car models. One of the process. During cruising, the vehicle can run on the motor the motor is on the engine side and is used to maintain the or engine depending upon the SOC (State of Charge) of the engine at optimum performance and to generate electricity; battery and the cruising speed. Sonata uses Lithium polymer whereas, the other one is used to power the vehicle. Both the batteries for better charge-discharge efficiencies and life [17]. motors are separated by a one way clutch. Strigear stands for Luxury cars manufacturer Lexus also released hybrid Sequentially run, TRIple GEARbox connected engine/motor vehicles whose drive train resembled to that of the Toyota hybrid [4]. In this system, it is the one way clutch that plays the Hybrid Synergy drive. But it differed in the total output power major role of using the vehicle as a series or parallel hybrid. and in the cooling systems [18]. Lexus LS 600h employed a Once the clutch is engaged, it becomes a parallel hybrid and power split device as in Toyota Hybrid Synergy drive and once it is disengaged, it becomes a series hybrid. [20] The PCU employed two motors, one for assisting the engine and the other continuously monitors the efficiency of the series – parallel as generator to transfer power from the engine to the battery. system and decides which type to choose. The clutch is engaged Due to the energy liberated in the charge-discharge cycles of the and disengaged according to this and thus has proven to battery, the battery gets heated up and this could result in produce better results than other hybrid topologies available. decreased life of the battery. Henceforth efficient cooling systems are employed for batteries and electronics too. The

This review has thrown light upon the various hybrid topologies available in the market today. It also highlights the cons of these topologies for the engineers to understand and to innovate solutions for the same. There is no doubt that hybrids will pave the future for the battery electric vehicles and fuel cell vehicles. However, it is evident that it is not possible unless the cost is brought down by reducing the complexity of engineering. There also lies a possibility of designing retrofit hybrid kits that can be employed into any vehicle to convert it into a hybrid vehicle. This can be done by replacing the rear wheel with hub motors so that the battery could be recharged by regenerative braking. By this the engine can be made to run at Fig 10: Strigear hybrid topology [21] optimum RPM with the motor supplementing the power requirement. The BMW i8 is a recently developed sports vehicle that has captured public attention for its hybrid system which ACKNOWLEDGMENTS incorporates a rear wheel driven 1.5L twin turbo charged engine and a motor that drives the front wheels. The vehicle can be Acknowledgements to Dept of Mechanical Engineering for operated in two wheel and four wheel drive modes. The hybrid all the support. topology also provides the freedom for the driver to choose whether to drive in full electric, full engine or both. The vehicle REFERENCES gives an overall fuel efficiency of 2.1L/100km. As an added advantage, the i8 is equipped with a plug in facility to charge [1] C. Shen, P. Shan, T. Gao, “A Comprehensive Overview of the Li ion batteries. The i8 is equipped with a 131hp motor and Hybrid Electric Vehicles”, International Journal of Vehicular a 231 HP engine and can accelerate the vehicle from 0 to Technology, vol 2011, Article ID 571683. 100kmph in 4.4Sec. The hybrid vehicle is capable of running in electric mode only for up to 35km. The engine automatically [2] A.E. Halvaii, “Computer Aided Designing Tool For fires when the battery SOC is below the minimum required electric, Hybrid Electric and Plug-in Hybrid Electric Vehicles” , level. [22] Dissertation submitted to Office of Graduate Studies of Texas Besides those mentioned, several other Automotive A&M University for Doctor of Philosophy. Groups like Mazda, GM, Mercedez, Peugeot etc. have also released their hybrid cars. But the topologies looked similar to [3] P. Strandh, “ Combustion Engine Models for Hybrid that of those mentioned above and most of them gave similar Vehicle System Development”, thesis submitted to Dept of efficiencies. However the complexity of the hybrid drive train Heat and Power Engineering, Lund Institute of Technology. has always laid its initial and maintenance cost high compared to the cars of the same segment. [4] K.Jonasson, P. Strandh, M Alakula. “ Comaparative Study It can also be identified that none of the hybrid systems are of Generic Hybrid Topologies”. in Electric Vehicle Symposium creating significant change in fuel consumption. The extra 2001, vol (18). amount spent in buying the car is not worthy as compared to the money spent for refuelling a normal car [22]. Most of the hybrid [5] Mariam Khan,Narayan C. Kar. “Hybrid Electric Vehicles systems in the market are bulky and battery management for Sustainable Transportation: A Canadian Perspective”. in systems are creating huge issues. However, latest hybrid cars World Electric Vehicle Journal Vol. 3 - ISSN 2032-6653 are equipped with efficient Battery Management Systems inclusive of proper cooling for better life of the battery. Lithium [6] Porsche Press release New York April 20 2011. URL: polymer batteries also reduce the weight and bulk of the http://press.porsche.com/news/release.php?id=642 conventional Ni-MH batteries. The Toyota’s Hybrid Synergy Drive, Honda’s Integrated Motor Assist and Sonata’s Blue [7] Lohner Semper Vivus Review, The Telegraph, 4 May Drive Technology are creating evident fuel saving, although it is 2011. URL: http://www.telegraph.co.uk/motoring/car- not adequate as compared to the cost of the vehicle. Though manufacturers/porsche/8467153/Lohner-Semper-Vivus- BMW’s latest technology employed in i8 is found effective, it is review.html nowhere near the reach of common man. [8] H Kang, “An Analysis of Hybrid- Electric Vehicles as the car of the Future”. Thesis submitted to Dept. of Mechanical

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