Abstract: An internal combustion engine of the open cycle type in which a porous, heat retaining regenerative member is placed in a cylinder head between the reciprocal piston and the intake and exhaust valve thereof. The regenerator member is heated to high temperatures during the exhaust cycle of the engine and preheats a fresh charge of air during the compression stroke of the engine so that, when the compressed air charge is mixed and ignited with a fuel charge, the engine's efficiency will be increased and the engine may operate at a lower compression ratio, thereby reducing heat loss to the engine. In one embodiment, the piston has means for releasably coupling the regenerator member therewith for movement with the piston during two of the strokes of the cycle of operation of the engine, the coupling and uncoupling being a function of the position of the piston in the cylinder.