R&D. There is no final value, since the program is still in progress, and in any case it will always be difficult to determine, since the net benefits (positive and possibly negative) are ill-defined. However, the total potential environmental and security benefits are immense, and to the committee they seem well worth the cost of the program to date.

The current annual cost of PNGV-related R&D is made up roughly of the total federal and industry funding, $240 million per year plus $980 million per year, plus the $130 million supplier industry contribution, totaling $1350 million. DOE’s contribution to PNGV might be taken as the 50 percent matching with industry that was planned when PNGV was formed. However, the potential benefits of PNGV (environmental and security) are more nearly the result of the total program costs, so perhaps a better ratio for DOE’s contribution to the benefits is 130 divided by 1350, or 10 percent. Figures are not available to match DOE’s funding with the specific degrees of success and failure in the benefits matrix chart, but DOE’s funding was specifically aimed more at basic enabling research than at product development, and 10 percent might be considered a typical percentage for basic research in any major R&D effort. On the other hand, DOE’s contribution is much more than its dollar input. The government involvement in PNGV certainly served as a catalyst to accelerate industry’s R&D on fuel economy, and the expertise of the national laboratories has a value beyond dollars. On these bases the committee believes that the potential benefits of PNGV measure favorably against the expenditures of DOE since 1993.


Program Description and History

The transportation sector is the dominant user of oil in the United States, accounting for more than 60 percent of the nation’s oil demand and using more than is domestically produced. Passenger cars are the most energy-intensive subsector of the transportation sector, consuming over one-third of all transportation energy; they consumed 8743 trillion Btu out of the total 24,411 trillion Btu consumed in the transportation sector in 1997. These data are taken from the 1999 Transportation Energy Data Book, which is published annually by the Oak Ridge National Laboratory and DOE (Davis, 1999).

DOE’s Office of Transportation Technologies (OTT) worked for many years to develop Stirling engines for automotive applications. The rationale for this work included the potential for high average thermal efficiency, multifuel capability, low maintenance requirements, smooth operation, and low emissions. None of the efforts to date has resulted in the development of a commercial product in the intended use or other uses.

The first DOE Automotive Stirling Engine program was initiated in response to the energy crisis of the mid-1970s. The OPEC action spurred the examination of a wide range of alternative propulsion systems for autos. At that time, it was felt that the Stirling engine was attractive for an automotive engine because it offered high efficiency and multifuel capability, the latter point being particularly attractive because of the gasoline shortages and price volatility of the time. The Stirling engine was actually invented in 1816. In the late 1930s the Phillips Company in the Netherlands revived the engine and continued independent development for the next 20 years. In the late 1940s, General Motors started research on the engine and in 1958 signed a formal agreement with Phillips for cooperative R&D. By May 1969, GM had accumulated over 22,000 hours of operation on Stirling engines from 2 to 400 hp. Because the Stirling engine uses an external continuous combustion process, it can be designed to operate on virtually any fuel. Several automotive concepts were developed and evaluated along with the Stirling engine. The second foray into Stirling engine development came about as a result of the PNGV program.

OTT worked with Mechanical Technology Incorporated (MTI) from 1978 until 1987 to develop an automotive Stirling engine. The goals of the program included a 30 percent fuel economy improvement, low emission levels, smooth operation, and successful integration and operation in a representative U.S. automobile. At the culmination of the program, the engine was demonstrated in a 1985 Chevrolet Celebrity, meeting all the program technical goals. The Stirling engine was never put into production for a number of reasons, including commensurate improvements in Otto cycle engines, high manufacturing cost, and lack of interest from the mainstream automobile manufacturers. Subsequent to DOE’s involvement, NASA supported further development of the MTI Stirling engine for a few years but then eventually abandoned it.

From 1993 until 1998, General Motors teamed with Stirling Thermal Motors (STM) to develop and demonstrate a Stirling engine for hybrid vehicles as part of the PNGV initiative. The engine was designed to drive a generator in a series hybrid configuration. Six engines were eventually built by STM, and three were delivered to General Motors for testing. By the end of the program, the Stirling hybrid propulsion system was integrated into a 1995 Chevrolet Lumina. The Stirling hybrid vehicle failed to meet several key requirements. Specific shortcomings included lower-than-expected thermal efficiency, high heat rejection requirements, poor specific power, and excessive hydrogen leakage. The engine did meet its emission target, demonstrating half the ultralow-emission-vehicle (ULEV) standard. There are no plans for further development of the Stirling hybrid concept with GM or any other auto manufacturer. STM is working to commercialize a small Stirling-powered generator for commercial use.

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