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The National Research Council (NRC) recently published a report entitled “Effectiveness and Impact of Corporate Average Fuel Economy (CAFE) Standards” intended to help U.S. policymakers in the formulation of CAFE policy. In the Report, the NRC projects fuel consumption reductions from the application of a wide range of engine, transmission, and vehicle technologies. The Report employs a simple multiplicative method to aggregate the effects of multiple technologies on fuel consumption. In this paper, a basic energy balance calculation is used to examine the NRC results against theoretical limits. Theoretical limits are calculated using measured and simulated breakdowns of system energy losses incurred during vehicle operation on EPA driving cycles. This analysis demonstrates the inherently optimistic results produced by simple aggregation methodologies. Methods for enhancing the accuracy of the technology-aggregation process are proposed.

In the twenty-first century, exhaust emission control will remain a major technical challenge especially as additional pressures for fuel and energy conservation mount. To address these needs, a wide variety of engine and powertrain options must be considered. For many reasons, the piston engine will remain the predominant engine choice in the twenty-first century, especially for conventional and/or parallel hybrid drive trains. Emissions constraints favor the conventional port fuel-injected gasoline engine with 3-way exhaust catalyst, while energy conservation favors direct-injection gasoline and diesel engines. As a result of recent technological progress from a competitive European market, diesels, and most recently, direct-injection (DI) diesels now offer driveability and performance characteristics competitive with those of gasoline engines. In addition, DI diesels offer the highest fuel efficiency.