test results for 1994 cars were utilized to develop estimates of diesel fuel efficiency benefit over a
gasoline engine.6lTable 3-8 shows the fuel economy benefits for a diesel engine relative to an equal performance
gasoline engine on the EPA city/highway composite test, based on engine brake specific fuel
consumption data, and consultation with auto manufacturers. In practice, it is difficult to obtain a
good equal performance comparison between a diesel-and gasoline-powered vehicle, as the diesel
will typically have more torque at low speed, but is rpm limited with lower peak power relative to
the gasoline engine.
Table 3-9 is a representative sample of gasoline- and diesel-powered models of the same cars
matched for approximately equal performance. In virtually every case, the percentage
improvements in fuel economy are higher than the averages suggested by manufacturers, noted
above; in particular, the DI turbocharged diesels from VW appears extremely fuel efficient. Table
3-9 also shows that a diesel’s fuel efficiency benefit decreases with increasing speed, as a result of
its high internal fiction. Moreover, modem four-valve spark ignition engines are closing the fuel
economy difference, especially as technologies such as variable valve controls (which reduce
pumping loss) are adopted.
Prospects for the Diesel in the United States.
The potential for the diesel in the United States revolves around three issues--consumer
acceptance, fuel prices, and ability to meet future emission standards.Consumer acceptance of the diesel should improve significantly with the new generation of
engines. OTA had the opportunity to evaluate the VW DI diesel and the Mercedes four-valve
diesel, and these new engines minimize performance differences relative to their gasoline engine
counterparts in terms of power, acceleration, noise, and vibration. In fact, diesel sales in Europe
have increased significantly with the new engines despite unchanged fuel prices from 1993.The major factors behind the lack of consumer interest in the United States are supposedly the
low fuel prices and the higher price of diesel relative to gasoline. Undoubtedly, these factors do
not help diesel market penetration, but they are not the sole factors controlling diesel market
penetration. Figure 3-3 provides the diesel market penetration in Germany during a 15-year
period, and also provides VW’s explanations for the observed changes over the years.^62 As can be
seen, W believes that vehicle tax policies, perceived emission benefits, and fuel prices have all
contributed to the large oscillations in diesel sales. If W is correct, it may be possible to
implement vehicle tax policies to favor the diesel, if the United States decides that fuel
conservation is a high priority. Further, to the extent that consumer perceptions of poor
performance and unreliability have influenced U.S. diesel sales, experience with the new
generation of diesels conceivably might bolster a diesel comeback.
(^61) U.K.
Department of Transportation “New Car Fuel Consumption - The Official Figures” January 1994.
62 VW research and Development, material provided to OTA, May, 1994.