<p>A projection (3) positioned in the center of a concave section of a toroidal combustion chamber (2) is divided into a section with a steep slope (3c) extending from the bottom (2b) to the head surface and a section with a gentle slope (3b). The head surface is in the form of a flattened conical trapezoid. Because of its shape, the projection makes it possible for the air which is close to the center section to flow close to the surface of a side wall of the concave section. The air which had been at the center section is added to the amount of air supplied for combustion and is mixed with the fuel in the vicinity of the surface of the side wall of the concave section. By setting the angle at which the air flows along the side wall surface (re-entrant angle) in the combustion chamber, the flow of the air close to the side wall surface is made turbulent, which improves the agitation of the fuel.</p>
