PURPOSE: To effectively convert combustion energy into piston force for enhacing the output of an engine, by forming a jet port in the periphery of the inlet opening of a cavity, and as well arranging a projection adapted to project into the jet port when the piston is in the vicinity of the top dead center thereof, in the lower surface of an exhaust valve.

CONSTITUTION: A cavity 7 is formed in the top of a piston 2, having the area of its inlet opening 7a narrower than the crossectional area of the inside of the cavity 7. A jet port 8 is formed in a predetermined position in the periphery of the inlet opening 7a. A projection 9 is adapted to protrude into the jet port 8 when the piston 2 is in the vicinity of the top dead center of the piston 2. Upon the completion of compression stroke, the jet port 8 is closed by the projection 9 so that air flows into the cavity 5 only from the inlet opening 7a having the narrow area, thereby strong squish may obtained and fuel is smoothly mixed with air. During expansion stroke burnt gas is ejected from both opening 7a and jet port 8 so that combustion energy is effectively converted into force for depressing the piston 2, thereby the output of the engine is enhanced.