An internal combustion engine is disclosed. The engine includes a power cylinder, a power piston reciprocating therein and being connected to a rotating output crank. A free floating double acting auxiliary piston reciprocates within an auxiliary cylinder. Passages connect the upper end of the auxiliary cylinder to a combustion chamber and a source of air. The exhaust from the combustion chamber is connected to the underside of the auxiliary piston through a valved passage. A return cylinder and a cushioning cylinder are coaxially aligned with the auxiliary cylinder at opposite ends thereof and open therein. Elongated pistons extend from the auxiliary piston and into the return cylinder and cushioning cylinder during reciprocation of the auxiliary piston. A gas transfer passage connects the return cylinder with the exhaust from the combustion chamber.