Method and apparatus to improve the efficiency of internal combustion engines in which compression and combustion of an Air/Fuel mixture is carried out without a compression stroke as used in conventional internal combustion engines and expansion of the combusted mixture is performed to increase the expansion ratio resulting in an increase in the amount of work extracted. An externally compressed Air/Fuel Mixture is combusted in a variable volume combustion chamber and expanded into a conventional cylinder displacing a power piston to produce rotation of a crankshaft to drive a vehicle or other device. Separation of the variable volume combustion chamber from the conventional cylinder eliminates the conventional Otto Cycle compression stroke and provides a substantially larger expansion ratio than the compression ratio in a conventional engine.