PURPOSE: To secure high combustion efficiency with such fuel as being low in combustion speed and explosion-proofness by installing a compressor, a combustion chamber and an expander, feeding the combustion chamber with compressed air from the compressor, while inducting a combustion gas produced in the combustion chamber into the expander, and expanding this combustion gas.

CONSTITUTION: A compression piston 101 and an expansion piston 102 are reciprocated in a compression cylinder 115 and an expansion cylinder 116 with rotation of a crankshaft 106. Each of valves 107, 110 is synchronously opened or closed to the compression piston 101 and the expansion piston 102. A combustion chamber 112 is a pressure vessel with those of constant pressure resistance, heat resitance and constant volume, which is interconnected to the compression cylinder 115 via a feed valve 108 and also even to the expansion cylinder 116 via an induction valve 109. With this constitution, combustion is unnecessary to be done explosively, namely, it is continuously carried out in the combustion chamber 112 at a time till compressed air is inducted to the expansion cylinder 116 since it has been fed to the combustion chamber 112. Therefore even in such fuel as being low in combustion speed, high combustion efficiency is securable.