Abstract: Projecting parts of the piston and the cylinder head are formed in such a manner that two portions of the piston surface are spaced from the inner surface of the cylinder head and a third portion of the piston surface is next to the inner surface of the cylinder head when the piston is at the upper dead center point, said third portion of the surface comprising a zone which is at a distance from the inner surface of the cylinder head and forming a connecting surface between said two first portion, these latter forming, with the cylinder head, respectively, an initial combustion zone and an expansion zone for the gases originating from the combustion, while the connecting zone together with the cylinder head forms an opening for the passage of said gases from the first zone, guiding these latter along the cylinder head toward a wall of the expansion zone, the outline of which, in a plane containing the axis of the above-mentioned opening, is a line which is curved in the direction of the opening forming the me