# Cost, Effectiveness, and Deployment of Fuel Economy Technologies for Light-Duty Vehicles(2015)

## Chapter: Appendix N: Effect of Compression Ratio of Brake Thermal Efficiency

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Page 401
Suggested Citation:"Appendix N: Effect of Compression Ratio of Brake Thermal Efficiency." National Research Council. 2015. Cost, Effectiveness, and Deployment of Fuel Economy Technologies for Light-Duty Vehicles. Washington, DC: The National Academies Press. doi: 10.17226/21744.
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Appendix N

Effect of Compression Ratio of Brake Thermal Efficiency

The effects of compression ratio on brake thermal efficiency at full load as well as part load operating conditions typical of those encountered in the CAFE test procedure are important considerations for maximizing light-duty vehicle fuel economy. Thermal efficiency of the ideal Otto cycle is given by this equation:

 Indicated Thermal Efficiency = 1 – 1/CR(1-k) (1)

Where: CR = compression ratio

k = ratio of constant pressure to con-
stant volume specific heats

Equation 1 shows that indicated thermal efficiency increases, but at a decreasing rate, as compression ratio is increased. However, mechanical efficiency decreases as compression ratio is increased due to higher loads on the pistons, rings, and bearings of the engine. Brake thermal efficiency is the product of indicated thermal efficiency and mechanical efficiency.

At full load conditions, mechanical efficiency can be relatively high. However, at part load conditions, mechanical efficiency will be significantly lower, even for relatively constant friction levels. The significant effect of load on mechanical efficiency is illustrated by the equation for mechanical efficiency:

 Mechanical Efficiency = BMEP/IMEP = BMEP/(BMEP + FMEP) (2)

Where: BMEP = brake mean effective pressure

IMEP = indicated mean effective

pressure

FMEP = friction mean effective

pressure

The effects of compression ratio on brake thermal efficiency together with indicated thermal efficiency and mechanical efficiency are shown in Figures 2.12(a) for full load conditions and Figure 2.12(b) for part load conditions. These figures provide the following insight into the effects of compression ratio on brake thermal efficiency:

• At full load, brake thermal efficiency increases, but at a decreasing rate, with increasing compression ratio, similar to indicated thermal efficiency.
• Up to 3 percent reduction in fuel consumption for naturally aspirated engines might be realized if compression ratio is increased from today’s typical level of 10:1 to approximately 12:1. Possibly greater reductions in fuel consumption might be realized for turbocharged engines capable of operating at higher boost pressures without knock so that further downsizing could be realized. Increasing gasoline octane from 91 RON of regular grade gasoline to 95 RON has been estimated to facilitate operation at a 12:1 compression ratio.

At part load, nearly insignificant improvements in brake thermal efficiency on the CAFE test cycles are expected to be obtained by increasing compression ratio beyond approximately 12:1 due to the increasingly lower mechanical efficiency.

Page 401
Suggested Citation:"Appendix N: Effect of Compression Ratio of Brake Thermal Efficiency." National Research Council. 2015. Cost, Effectiveness, and Deployment of Fuel Economy Technologies for Light-Duty Vehicles. Washington, DC: The National Academies Press. doi: 10.17226/21744.
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Next: Appendix O: Variable Compression Ratio Engines »
Cost, Effectiveness, and Deployment of Fuel Economy Technologies for Light-Duty Vehicles Get This Book
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The light-duty vehicle fleet is expected to undergo substantial technological changes over the next several decades. New powertrain designs, alternative fuels, advanced materials and significant changes to the vehicle body are being driven by increasingly stringent fuel economy and greenhouse gas emission standards. By the end of the next decade, cars and light-duty trucks will be more fuel efficient, weigh less, emit less air pollutants, have more safety features, and will be more expensive to purchase relative to current vehicles. Though the gasoline-powered spark ignition engine will continue to be the dominant powertrain configuration even through 2030, such vehicles will be equipped with advanced technologies, materials, electronics and controls, and aerodynamics. And by 2030, the deployment of alternative methods to propel and fuel vehicles and alternative modes of transportation, including autonomous vehicles, will be well underway. What are these new technologies - how will they work, and will some technologies be more effective than others?

Written to inform The United States Department of Transportation's National Highway Traffic Safety Administration (NHTSA) and Environmental Protection Agency (EPA) Corporate Average Fuel Economy (CAFE) and greenhouse gas (GHG) emission standards, this new report from the National Research Council is a technical evaluation of costs, benefits, and implementation issues of fuel reduction technologies for next-generation light-duty vehicles. Cost, Effectiveness, and Deployment of Fuel Economy Technologies for Light-Duty Vehicles estimates the cost, potential efficiency improvements, and barriers to commercial deployment of technologies that might be employed from 2020 to 2030. This report describes these promising technologies and makes recommendations for their inclusion on the list of technologies applicable for the 2017-2025 CAFE standards.

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