Optimizing Internal Combustion Engine Efficiency in Hybrid Electric Vehicles

Dylan Humenik Ben Plotnick

27 April 2016

TABLE OF CONTENTS

Section Points Abstract /10 Motivation /25 Technical /25 background Future /25 development References /10 Writing/figures /5 TOTAL /100

Abstract For our final project in ME 431, we would like to investigate the topic of hybrid electric vehicles. This topic has been an active topic in the automotive community for the last couple decades, and we’re confident that it will continue to take up a significant market share of the automotive industry. Traditionally, hybrid electric vehicles use both an internal combustion engine, and an electric motor. For this project we will assume a gasoline SI engine. One class of hybrid electric vehicles utilizes the internal combustion engine as part of the powertrain to drive the wheels, while another class of hybrid electric vehicles uses the internal combustion engine solely to charge the batteries for the electric drivetrain. We’d like to investigate the ways that overall efficiency is improved with the addition of an electric motor to the powertrain. Ignoring other efficiency improving technologies like regenerative braking, our focus will solely be on the improvements and benefits that an electric motor can provide to the powertrain – all else being equal. We anticipate our inquiry to include discussions and analyses of the curves relating to efficiency (i.e. bmep, 휂푡ℎ, etc.) and how they translate into mpg. Identifying key regions of the power curve that are bad for fuel economy, and investigating how switching out internal combustion power for electric power in these regions can improve efficiency and fuel economy, is what we assume to be the heart of our project (e.g. can using battery power, that was previously generated by the internal combustion engine running at an optimal efficiency, significantly improve efficiency by eliminating the need for using the internal combustion engine at an inefficient region of its power band?). One conclusion that we may determine is which class of hybrid electric vehicle is more efficient – those that use the engine as part of the powertrain or those that use it solely as a battery charger.1–3

Motivation The hybrid sector of the automotive industry is a large and growing sector of the automotive industry, particularly in passenger vehicles. The expansion of the hybrid market is threefold: manufacturers are driven to be more efficient than their competitors, consumers are demanding more fuel efficient and environmentally friendly vehicles, and governments are incentivising and regulating fuel efficiency and emissions.4 The causes of this market trend are also numerous, and entire classes can be taught on the topic. However, the market trend can be summed up as being generally caused by a few important factors: geopolitical instability leading to volatile up- and-down oil prices leads consumers to prefer less fuel-guzzling cars5,6, fossil fuel emissions and their relation to climate change are still a factor regardless of oil price7, and increasing battery and electric drivetrain technology have made hybrids more attainable.8 It is unlikely that the industry will see a complete and rapid shift towards electric vehicles - instead, a slower market growth is expected - hybrid electrics will be an enduring fixture in the automotive industry as the market sits between fossil fuel-powered vehicles and electric vehicles.

The goal of hybrid electric vehicle design is to use the architecture, the combination of electric and gasoline powertrain elements, to reduce fuel consumption below that of the gasoline- powered equivalent. There are several hybrid electric vehicle architectures that have been developed. Parallel hybrids allow both the internal combustion engine and the electric motor deliver power to the wheels. Series hybrids use the internal combustion engine to power the batteries or directly power the electric motor. Series-parallel hybrids, also known as power-split hybrids, are able to perform both functions of series and parallel hybrids.9 Trends that emerge with these architectures include the fact that series hybrids tend to do better at lower speeds and city driving, parallel hybrids tend to do better at higher speeds and highway driving, and series- parallel hybrids tend to perform the best because they combine good performance at both lower and higher speeds. However, series-parallel hybrids tend to be more expensive.9 In addition to fuel consumption, other metrics that are affected by hybrid electric vehicle architectures include emissions, torque, and efficiency.

Following instructor feedback, we decided to specifically focus on fuel efficiency. We decided to narrow down to this single metric because many of the other metrics are related to fuel efficiency, including emissions. Fuel efficiency metric will most extensively capture the goals and demands of manufacturers, consumers, and governments. For this analysis, we will be focusing on fuel efficiency from an internal combustion engine standpoint. In reality, there are other methods that can increase the overall fuel efficiency of the entire automobile. Things like regenerative braking, idling shut-down (where the internal combustion engine shuts off during idle), and plug-in technology can add further increases in fuel efficiency. However, for the following analysis, the focus will be with the role of the internal combustion engine in improving fuel efficiency.

Technical Background Internal combustion engine efficiency varies with engine speed due to a number of factors. At low speeds, efficiency is reduced by throttling and the turbulence associated with it. The air restriction causes turbulence which reduces engine efficiency and reduces fuel efficiency. At high speeds, efficiency is reduced by pumping losses when air is reaching its maximum velocity. In addition, mechanical friction at high speeds contributes to the drop-off in efficiency. Engines range in peak efficiency, but generally the peak efficiency is attained around 75% of rated engine power or redline speed.10

One of the advantages of series hybrids is that the internal combustion engine can operate at its peak fuel efficiency in terms of engine speed and load. This means that the internal combustion engine can be optimized to power the vehicle at its most efficient spot in the power band. Particularly in series hybrid electric vehicles, the internal combustion engine can be optimized to run at its peak efficiency independent of the load, because the load falls entirely on the electric motor. The internal combustion engine and battery system can be optimized to achieve optimal performance and efficiency while transferring power to the battery.9

Figure 1 illustrates this optimal performance range of internal combustion engines. The brake mean effective pressure (bmep) is related to efficiency by its capacity to do work irrespective of volume.10 Also shown is the brake specific fuel consumption, which directly corresponds to fuel efficiency. It is shown in this plot that there is an optimal range for internal combustion engines in terms of efficiency, and that normal driving will not necessarily operate the engine there for most of the time. The advantage with hybrid vehicles is that the internal combustion can be allowed to spend most of its time (charging the battery or contributing to the powertrain) in its optimal efficiency range.

Figure 1: Performance map of bmep and bsfc versus mean piston speed for an automotive spark ignition engine.10

One final thing that would illustrate the gains in fuel efficiency that arise from combining an internal combustion engine with an electric drive system, as in hybrid electric vehicles, would be to list the fuel efficiencies of popular hybrids on the market. However, these fuel efficiency metrics also include improvements due to other systems such as regenerative braking, idling shut-down, and plug-in technology (where the true fuel efficiency gets very complicated when considering the fuel burned at the power generating station). Only mentioning that the fuel efficiencies for hybrid electric vehicles range from just a few miles per gallon above conventional vehicles to well above 50 miles per gallon. Furthermore, the range of hybrid electric architectures, energy saving/regenerative add-ons, and variety in design make it hard to pinpoint the percentage of fuel efficiency boost directly resulting from optimizing internal combustion engine performance as discussed earlier, and the percentage of fuel efficiency boost resulting from these add-ons. However, it is estimated that a significant contribution to the boost in fuel efficiency comes from this efficiency optimization of the internal combustion engine, as evidenced in the fact that some hybrids perform better during highway driving, where regenerative braking and idle shut-off are minimized.9 Overall, the fact that hybrid electric vehicles are able to run the internal combustion engine in its optimal efficiency range, while conventional engines are subjected to the entire range of engine speeds, represents a very powerful technique for reducing the total amount of fuel consumed in driving.

Figure 2: Future trends of relative fuel consumption in various automotive vehicle types.11

Future Development According to Figure 2, by the year 2030, the relative fuel consumption of hybrid-electric gasoline engines will be 17% of the fuel consumption of conventional gasoline engines from the year 2009. These trends will be dictated by advancements in several areas of Hybrid Electric Vehicle (HEV) design, including weight reduction, battery storage capabilities, and fuel improvements, among others.

Figure 3: Projected market share of automotive vehicular types9

According to Figure 3, the market share of mild and full hybrid vehicles will significantly increase by the year 2022. Mild hybrids comprise hybrid vehicles with kinetic energy recovery (regenerative braking) and Full hybrids comprise hybrid vehicles that incorporate engine assist to the battery. Conventional ICE’s will likely still retain the majority of the market share, but HEV’s will likely become far more competitive in the market. According to Michael Nikowitz, “The future of [Electric Vehicles] relies on how good the car batteries can become: how much power they can hold, and for how long”.9 The trends dictated in Figure 3 will likely heavily rely on this observation. Furthermore, the main obstacles in the way of HEV dominance in the automotive industry is the high cost of battery production and the limited infrastructure for vehicle charging stations, etc.

Figure 4: Cost and Specify Energy projections for Lithium ion batteries9

As shown in Figure 4, the specific energy storage capabilities of Lithium batteries are projected to increase over time, while the costs to produce those batteries will decrease over time. These projections greatly correlate and explain the market share predictions shown in Figure 3. The future of Hybrid vehicles looks promising, not only for the hybrid electric industry, but for the overall sustainability of the automotive industry as a whole. With the prospect of battery technologies becoming more advanced and accessible, a future where hybrid electric-gasoline engines are the standard automobile, will soon become a reality.

References 1. Shabbir, W. & Evangelou, S. A. Real-time control strategy to maximize hybrid electric vehicle powertrain efficiency. Appl. Energy 135, 512–522 (2014).

2. Gökce, K. & Ozdemir, A. An instantaneous optimization strategy based on efficiency maps for internal combustion engine/battery hybrid vehicles. Energy Convers. Manag. 81, 255–269 (2014).

3. M. Sabri, M. F., Danapalasingam, K. A. & Rahmat, M. F. A review on hybrid electric vehicles architecture and energy management strategies. Renew. Sustain. Energy Rev. 53, 1433–1442 (2016).

4. Chan, C. C. The state of the art of electric, hybrid, and fuel cell vehicles. Proc. Ieee 95, 704– 718 (2007).

5. Chen, Y., Yu, J. & Kelly, P. Does the China factor matter: What drives the surge of world crude oil prices? Soc. Sci. J. 53, 122–133 (2016).

6. Kim, J. E. Energy security and climate change: How oil endowment influences alternative vehicle innovation. Energy Policy 66, 400–410 (2014).

7. Samaras, C. & Meisterling, K. Life cycle assessment of greenhouse gas emissions from plug- in hybrid vehicles: Implications for policy. Environ. Sci. Technol. 42, 3170–3176 (2008).

8. Lofsten, H. Industrialization of hybrid electric vehicle technology: identifying critical resource dimensions. J. Technol. Transf. 41, 349–367 (2016).

9. Nikowitz, M. Advanced Hybrid and Electric Vehicles: System Optimization and Vehicle Integration. (Springer, 2016).

10. Ferguson, C. R. & Kirkpatrick, A. T. Internal Combustion Engines: Applied Thermosciences. (John Wiley & Sons, 2015).

11. Lynette W. Cheah. Cars on a Diet: The Material and Energy Impacts of Passenger Vehicle Weight Reduction in the U.S. 1–121 (2010).

Appendix

Management Report: Dylan Humenik – 8th Semester Standing Ben Plotnick – 8th Semester Standing

Meeting 1: 4/19/16 – 7pm-8pm Met to discuss Part 1 of the project, and the general focus of the party. Dylan typed up the abstract.

Meeting 2: 4/26/16 – 6:30pm-11pm Met to discuss the report after reviewing the instructor’s comments about Part 1. Began researching papers, gathering information, and developing our report. Wrote each section of the report.

Timecard: Dylan Humenik: 9-11 hours Ben Plotnick: 9-11 hours