An initial automotive evaluation of a Stirling engine, accomplished by adapting a stationary power plant (the P-40 engine) designed and built by United Stirling AB (USAB) of Sweden for a vehicle installation, verified the feasibility of the kinematic Stirling engine. As a follow-on to this program, an engine designed specifically for automotive application (the Mod I) was created. The performance of the Mod I engine met or exceeded all its performance goals. Further improvements required to advance the Mod I engine technology level were addressed in the Upgraded Mod I design, which included use of nonstrategic materials, increased cycle temperatures, improved drive unit technology, and regenerator/cooler part-power optimization. This paper presents Mod I engine performance as measured on a dynamometer and in a vehicle, and traces the initial development of the Upgraded Mod I.