Abstract: A hot gas engine is run at constant speed and power output controlled by the supply of working gas. Former such systems are simplified both in the number of control valves necessary and in the conduit system array by providing an engine speed controlled solenoid check valve to control a gas compressor suction stroke alternately (a) at normal speeds for free flow in both directions or (b) at higher speeds showing an excess of power to admit working gas from the engine to the compressor for pumping into a working gas reservoir. One embodiment provides hydraulic control from an oil supply with pressure dependent on engine speed of a short circuiting solenoid which upon a sudden burst of engine speed when the load is removed establishes equal gas pressure on both sides of the engine piston.