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and that, combined with a growing concern about greenhouse warming and the role of CO2 as a greenhouse gas, has once again focused attention on the efficient use of transportation fuel. Vehicle efficiency involves technological improvements in fuel economy—improving the miles per gallon of vehicles. The more efficient a vehicle is, the less fuel it burns to travel a given distance. The less fuel it burns, the lower are the amounts of CO2 emitted. These methods of emission reduction and their cost-effectiveness are evaluated in the following sections for light-duty vehicles, heavy-duty trucks, and domestic air carriers. The emphasis is on light-duty vehicles because they represent the largest and most thoroughly studied sector. Table 23.1 shows the amount of fuel used by each type of vehicle for different modes of operation. The information presented here indicates that light-duty vehicles consume the largest quantity of transportation fuel, with heavy-duty trucks second and aircraft third.

Recent Trends

The recent trend in fuel economy from 1975 to 1989 for the new U.S. passenger car fleet is presented in Figure 23.1a (Amann, 1989). Figure 23.1b shows the fuel economy index (FEI) for the period from 1930 to 1990. The FEI is an index of powertrain efficiency including weight and performance. Studies by Leone and Parkinson (1990) and by Greene (1989) indicate that the trend in the period from 1975 to 1982 was a response to increased fuel prices and fuel economy regulations. As discussed below in the ''Barriers to Implementation" section, there is some disagreement on the relative impact of fuel prices and regulations on the supply of fuel-efficient vehicles. The vehicles manufactured during that period were, on the average, 450 kg (1000 pounds) lighter within each market segment, were degraded in performance and other attributes, and incorporated various fuel-efficient technologies. Figure 23.2 indicates how consumer preferences for vehicles changed from 1972 to 1986. As shown, some consumers accepted the smaller vehicles offered to improve energy efficiency, while others resisted the change in performance and either did not buy cars or shifted to light-duty trucks and vans.

Because market conditions and fuel prices cause consumer preferences for fuel-efficient vehicles to change over time, one should distinguish between the trends in overall vehicle fuel economy and powertrain efficiency. Therefore it is important to look not only at miles per gallon but also at the FEI. The FEI is used to control for other vehicle changes, as shown in Figure 23.1b for the period from 1930 to 1990. This parameter, used to judge passenger cars for many decades, provides a better indicator of powertrain efficiency than does fuel economy alone by controlling for both weight and performance.