PURPOSE: To promote combustion and reduce the fuel consumption by shaping spherically or cylindrical extruded part on the circumferential wall face of a combustion chamber to which the fuel from a fuel injection nozzle is dashed for increasing the flow speed of gas on the surface of the extruded parts along the circumferential wall of the combustion chamber.

CONSTITUTION: At the top of a piston 2, a combustion chamber 8 is provided to be cylindrically concaved, and along its circumferential wall 9, spherical extruded parts 10 are provided to be extruded from the circumferential wall 9 at proper intervals. At the center of a bottom wall face 12 of a combustion chamber 8, a convex projected part 13 is provided to promote the swirling stream of gas in the combustion chamber 8. Extruded parts 10 on the circumferential wall 9 increase the flow speed of the air swirl along the circumferential wall 9 on the surface of the extruded parts 10, while the increase of the flow speed also reduces the thickness of the boundary layer on the surface of extruded parts 10 for promoting the atomization and mixing with fuel F. This construction permits to stabilize and improve the combustion in the combustion chamber 8.