Thomas, J., "Vehicle Efficiency and Tractive Work: Rate of Change for the Past Decade and Accelerated Progress Required for U.S. Fuel Economy and CO2 Regulations," SAE Int. J. Fuels Lubr. 9(1):290-305, 2016, doi:10.4271/2016-01-0909.
A major driving force for change in light-duty vehicle design and technology is the National Highway Traffic Safety Administration (NHTSA) and the U.S. Environmental Protection Agency (EPA) joint final rules concerning Corporate Average Fuel Economy (CAFE) and greenhouse gas (GHG) emissions for model years 2017 (MY17) through 2025 (MY25) passenger cars and light trucks. The chief goal of this current study is to compare the already rapid pace of fuel economy improvement and technological change over the previous decade to the required rate of change to meet regulations over the next decade. EPA and NHTSA comparisons of the model year 2005 (MY05) US light-duty vehicle fleet to the model year 2015 (MY15) fleet shows improved fuel economy (FE) of approximately 26% using the same FE estimating method mandated for CAFE regulations. Future predictions by EPA and NHTSA concerning ensemble fleet fuel economy are examined as an indicator of required vehicle rate-of-change. A set of 40 same-model vehicle pairs for MY05 and MY15 is compared to examine changes in energy use and related technological change over the 10 year period. Powertrain improvements measured as increased vehicle efficiency, and vehicle “mass-glider” improvements measured as decreased tractive work requirements are quantified. The focus is first on conventional gasoline powertrain vehicles which currently dominate the market, with diesels and hybrids also examined due to their potential importance for CAFE compliance. Results indicate 10 years of progress for the studied vehicle set yielded reduced tractive effort of 5.6% and improved powertrain efficiency of 16.5%. Further analysis shows that this high rate of powertrain progress must increase by 90% or more in order to meet the 2025 CAFE standards. Comparison of MY15 vehicle FE values to CAFE target values is offered as well as conjecture on whether gasoline powertrains are adequate to meet regulations under reasonable assumptions.