PURPOSE: To substantially reduce or eliminate pollutants by providing a combustion process having a combustion passage of time substantially at a constant pressure over the period of a minimum crank angle of 45° after a top dead point so as to improve performance of an internal combustion engine.

CONSTITUTION: Compressed cooling air from a cooler 26 is introduced to the compression cylinder 28 of an engine 20, here the air is cooled again and then introduced to a combustion chamber, i.e., a cylinder 32. Then, products from combustion are introduced through an exhaust manifold 34 to expansion cylinders 36 and 38, then sent to a serge chamber 40, and then exhausted through the inside of a compressor 24. Thus, since a combustion process having a combustion passage of time substantially at a constant pressure for over the period of a minimum crank angle 45° after a top dead point is provided, a compression ratio is reduced, and a combustion temperature is set substantially equal to an NOx forming temperature. Also, since energy release is controlled, pollution, both chemical and heat to the atmosphere is greatly reduced.