PURPOSE: To change the squish area to an inert gas area to dissolve the problem of end gas knocking by mounting an inert gas supply valve at the side of the cylinder wall rather than that of a usual position of a suction and an exhaust valve and feeding, as the flow of inert gas, a part of exhaust gas of which the pressure is accumulated from this valve into a cylinder.

CONSTITUTION: A cylinder head part contains a combustion chamber 4 which has an ignition plug 3 in the middle position between a suction valve 1 and an exhaust valve 2. In such constitution small-diameter valves 6 and 6' through which inert gas is to be supplied are formed in squish areas 5 and 5', respectively, which surround the combustion chamber 4. Thus inert gas is carried to these small-diameter valves 6 and 6' from a pressure accumulating chamber 7 in which the pressure of a part of burnt exhaust gas is accumulated, via passages 8 and 8'. To enable inert gas flow to be carried into a cylinder as flows 9 and 9' running along the internal wall of the cylinder, the direction 10 of a valve seat and a guide groove 11 are set up.