Fuel Efficiency Estimates for Future Light Duty Vehicles, Part B: Powertrain Technology and Drive Cycle Fuel Economy

This study evaluates the fuel economy implication of powertrain technologies capable of reducing light duty vehicle fuel consumption for compliance with 2025 CAFE standards. In a companion paper, a fully integrated GT-Power engine model was used to evaluate the effectiveness of one plausible series of engine technologies, including valve train improvements such as dual cam phasing and discrete variable valve lift, and engine downsizing with turbocharging and cooled EGR. In this paper, those engine efficiency/performance results are used in a vehicle drive cycle simulation to estimate the impact of engine and transmission technology improvements on light duty vehicle fuel consumption/economy over the EPA’s FTP and HWY test schedules. The model test vehicle is a midsized sedan based on the MY2012 Ford Fusion. Transmission configurations included 6 and 8-speed automatic transmissions and a continuously variable transmission (CVT), all modeled with loss maps representative of modern designs. Results showed a 23% reduction in combined fuel consumption with the adoption of engine technologies alone with a 6-speed transmission, with a corresponding increase in fuel economy from 31.8 MPG to 41.3 MPG. Increasing to 8 gears or adjusting the transmission shift logic had negligible impact on fuel consumption. An additional 2.7% reduction in fuel consumption (with equivalent gain to 42.8 MPG) was achieved by adopting a high efficiency 8-speed transmission with reduced torque losses. Combining engine and transmissions improvements with reductions in vehicle weight, rolling resistance and drag reduced fuel consumption by 35% relative to the baseline, achieving a final combined fuel economy of 48.8 MPG. Overall, the largest improvements in fuel economy came from expanding the highest efficiency region of the engine fuel consumption map to lower loads more commonly visited during drive cycle operation, while transmission efficiency changes provided a secondary improvement of less magnitude than changes to the vehicle mass and drag characteristics.