A high efficiency combined cycle internal combustion and steam engine includes a cylinder and a piston with an internal combustion chamber outward of the piston, a fixed cylinder cap and a steam expansion chamber inside the piston. The cylinder cap can be heated to reduce condensation of steam entering from a steam generator fired by waste combustion heat. Following exhaust, residual steam can be recompressed prior to admitting the next charge of steam. A wrist pin connected to an inner end of the piston skirt inwardly of the cylinder cap is coupled to a connecting rod secured to a crankshaft. One valve or a pair of steam inlet valves are connected to communicate in series within the cylinder cap inside the piston. The steam mass admitted is regulated to reduce fuel consumption. Coolant can be superheated in the combustion exhaust manifold.

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