The military requirement for a lightweight, air-cooled compression ignition engine family having two members, specifically 10 and 20 hp versions at the rated speed of 3600 rpm, is discussed in this paper. Design and preliminary development programs for such an engine family have been completed, and specifics are related. Development of a practical combustion system for the high rated speed was found to be feasible and is demonstrated on the basis of test data. Endurance data to date indicate that the design life of 6000 h between major overhauls will be attainable. The plan for the future includes additional testing of updated prototype engines as well as further investigations in the areas of noise reduction and control of harmful exhaust emissions.