data and comparing them with the market shares reported for domestic manufacturers in the SRI report. By and large, the estimates are in good agreement. Differences (e.g., market shares of 4-valve engines) seem to arise from the inclusion of Honda's U.S. production in the SRI data and its exclusion from EEA's domestic estimates (Honda's U.S. production is considered imported for corporate average fuel economy [CAFE] purposes).

The percentage improvement in fuel economy that can be ascribed to a given technology continues to be debated among scientists and engineers. While there has been agreement on some technologies, the committee found contention about others. Most of the arguments have to do with the definitions of technologies—the same name is often given to quite different versions of a generic technology in different sources. Some differences have to do with the details of how a technology is implemented. Most engine technologies considered, for example, can be optimized for performance or fuel economy. When optimized for performance, they do not yield as great a fuel economy benefit.

The automotive industry and the U.S. Department of Energy (DOE), together with EEA, a DOE contractor, have spent a considerable amount of time and effort attempting to resolve the debate over fuel economy potential. In meetings over nearly two years, engineers and experts from the domestic manufacturers and DOE have scrutinized definitions, assumptions, and estimation methods. This process produced revisions of several estimates and a narrowing of differences, but not complete agreement. Estimates made by nearly all the major automobile manufacturers have been compiled by Ford Motor Company (1991). Estimates for particular sets of technologies have also been developed by Berger et al. (1990) and by SRI (1991). The committee considered all these sources, which are compiled in Table B-1. It elected to base its shopping cart projections on two sets of estimates—those developed by EEA (1991a) under the sponsorship of DOE and those developed by SRI (1991) under the sponsorship of the MVMA. The SRI estimates, developed to serve as a consensus from the domestic industry, are generally similar, but not identical to the estimates provided by Ford. The EEA and SRI reports are the only sources that provide technology-specific information on both percentage fuel economy improvements and costs. The cost estimates are summarized in Table B-2.

Under the category of engine technologies in Table B-1 are included those technologies that address the thermodynamic efficiency of combustion, internal engine friction, and pumping losses, as well as energy used by essential engine accessories, such as oil pumps and alternators, and nonessential accessories, such as air-conditioners and power steering.