PURPOSE: To substantially increase the volume of a combustion chamber in a piston cavity without changing the compression ratio for aiming at improving the combustion of an engine, by providing a projecting member to a cylinder liner within the inner periphery of the top clearance chamber at a top dead center of the piston.

CONSTITUTION: A cylinder 2 is secured therein with a ring-like projecting member 8 made of heat-resistant materials, which covers over the inner periphery of a top clearance chamber 6 formed at the top dead center of a piston. With this arrangement, air which is filled in between the top rand 9 and the liner 2 of the piston and which does not directly contribute combustion is excluded from combustion purpose, while combustion air is massed in the vicinity of the combustion chamber, and therefore, the dead volume of air is decreased so that injected fuel is effectively utilized in a short time. Accordingly, the highest burning pressure is ensured at a value which is substantially equal to the value that was used before the policy against exhaust gas was enforced, and as well the combustion after ignition may be rapidly progressed so that the emission of NOx may be remarkably limited to improve the combustion.