Abstract: A double acting Diesel cycle hot gas engine comprising a plurality of engine cylinders, each having a free piston unit adapted to perform work, providing a continuous flow of compressed air to an energy consumption source and to a storage tank connected therewith. The free piston unit includes an air compression piston and a power piston, spaced apart from each other and adapted to move together within each respective engine cylinder through a connecting rod extended therebetween in response to the movement of a working medium under a substantially constant pressure acting against one side of the power piston and to the pressure and expansive power generated by the burning gases of an air fuel mixture acting on the opposite side of said power piston. Cycle control means are provided to release the potential energy accumulated within the working medium which is initially used to drive the power piston to compress the air fuel mixture and thus, initiating the operative cycle of the engine.