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Results of the final design phase of a free-piston hydraulic Advanced Stirling Conversion System were reported at the 1991 IECEC [1]. This project was funded by the United States Department of Energy and administered by NASA Lewis Research Center through Sandia National Laboratories. The final design met program objectives with the exception of a shortfall in nominal power output and efficiency, and an exceeded weight limit. These deficiencies were due to new bellows design data. Four Stirling convertor configurations were evaluated as part of the Technology Assessment Task, which included combinations of alternative technologies. Alternative technologies included gas and flexural bearings, moving magnet and stationary magnet linear alternators, and seven different control options. This paper describes the approach of the Technology Assessment and summarizes the conclusions and recommendations.

Rod seals, and to a lesser extent piston seals, are the primary impediments to long life and high reliability for kinematic Stirling engines. Hermetic metal bellows have been successfully demonstrated for 6.9 years and 4 × 109 cycles in a free-piston Stirling engine. A totally different radioisotope fueled free-piston Stirling engine with a flexing metal diaphragm was still operating at last report after more than 12 years and 3 × 1010 cycles. A concept for implementing long life bellows to function as rod seals and piston seals in kinematic Stirling engines has been developed and is presented for the first time in this paper.