Abstract: Apparatus for generating hot combustion gases and steam including a piston block with a piston reciprocally mounted in a bore therethrough having solenoids and sensors for selectively introducing hydraulic fluid into the bore on the first side of the piston to move the piston. Inlets and sensors for selectively introducing air and fuel into the combustion chamber comprising a portion of the bore and to initiate combustion are also provided. The hydraulic fluid used to move the piston is introduced into the bore in a discharge chamber in the bore on the second side of the piston, vaporized and exhausted into a well through an exhaust valve. The exhaust valve is biased toward the closed position by a pneumatic cylinder which resists the force of combustion.