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.The technological breakthroughs which overcame these obstacles include variable piston stroke power control combined with a pressurized crankcase resulting in a hermetically sealed unit, and compliantly mounted oil scrapers. The heat exchanger portion of the STM4-120 has a typical stacked heat exchanger configuration with a connecting duct to the cylinder. These features simplify the power control and enhance performance, reliability and manufacturability.The STM4-120 comes in two versions. The heat pipe heater (condenser) in the STM4-120RH (Remote Heated) is a tube bundle brazed to small, lightweight endplates resulting in an easily fabricated assembly. By substituting an annular direct flame heat exchanger for the heat pipe condenser and designing sufficient flexibility into the duct, the drawbacks of typical direct flame heater heads can be avoided.The direct flame version of this engine is designated the STM4-120DH (Direct Heated). Its design was motivated by renewed emphasis on engine emissions and concern over the greenhouse effect which will provide incentives for new engines for applications using standard gaseous and liquid fuels including transportation niche markets.This paper presents a comparison of the two engine configurations and performance while at the same operating conditions. Descriptions of the ranges of operation and different applications for each configuration are presented and discussed.