PURPOSE: To actuate an engine with the fuel having a wide steam pressure and continuously operate it by injecting the fuel into a cylinder during the first stroke, reducing pressure of a fuel-air mixture during the second stroke, increasing pressure and igniting during the third stroke and exhausting the exhaust gas from the cylinder during the fourth stroke.

CONSTITUTION: During the first stroke in which a piston 17 moves from BDC to TDC, air is sucked into a cylinder 19 through an inlet 31, the inlet 31 is closed by movement of the piston 17, exhaust valve 21 is closed and fuel is injected into the cylinder 19. Then, during the second stroke in which the piston moves from TDC to BDC, the exhaust valve 21 is kept closed, pressure of a fuel-air mixture is reduced. During the third stroke from BDC to TDC, pressure of the fuel-air mixture is increased, and then ignited. During the following fourth stroke from TDC to BDC, the ignited mixture expands, the exhaust valve 21 and the inlet 31 are opened and the exhaust gas is exhausted from the cylinder 19.