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Stirling Thermal Motors, Inc., (STM) of Ann Arbor, Michigan, has been developing a general purpose Stirling engine designated the STM4-120.* The configuration of the STM4-120 was based on the Ford/Philips automotive Stirling engine which was successfully demonstrated in a 4500 pound Ford Torino in 1976. This engine, designated the 4-215, was a four-cycle, double-acting engine with a fixed angle swashplate drive and produced 175 horsepower (128 kW). During the Ford program, three obstacles to mass production were identified: the complexity of the mean pressure power control, the life and reliability of the rod seals, and the complex geometry and manufacturability of the heater heads. In the conceptual design phase of the STM4-120, effort was concentrated on addressing and overcoming these obstacles.