An auxiliary power system for an internal combustion engine. The system employs a vapor or hot gas engine coupled to an internal combustion engine by means of an overruning clutch assembly. The internal combustion engine, operated conventionally, generates heat in its cooling and exhaust systems as well as in accessory equipment. The heat generated in the internal combustion engine and its accessories is used for generating vapor in one or more heat exchangers, the vapor preferably being from a liquid with a boiling point well below that of water. The generated vapor is used to drive a vapor engine. Temperature-sensitive means is employed to measure the heat generated in one or more parts of the system for controlling a linear solenoid valve for automatically controlling the internal combustion engine temperatures and thereby the amount of vapor generated. A vacuum-controlled throttle valve is also employed in conjunction with the solenoid-operated valve for automatically controlling the input of vapor to the vapor engine for controlling its idle speed and for providing maximum acceleration as needed. Safety valve means and vapor condensing means are provided at the output of the heat exchangers and steam engine to condense the generated vapor for recirculation in the system.