PURPOSE: To improve combustibility of mixture, by providing a cylindrical projection concentrical with the center of the combustion chamber on the bottom surface of the combustion chamber, and thereby changing the motion of sprayed fuel into circumferential direction and enhancing utilization of air in the combustion chamber.

CONSTITUTION: A diesel engine comprises a piston 1, combustion chamber 7, fuel valve 3, cylinder head 4 and a cylinder liner 5. The height h1 of the cylindrical projection 71 is selected to be smaller than 1/3 of the depth H of a piston cavity and the radial position r1 of the projection 71 is selected to be greater than 1/3 of the redius Rc of combustion chamber 7. In such an arrangement, fuel 6 sprayed from a fuel valve 3 impinges at first onto the peripheral surface of the combustion chamber 7 and then is reflected toward the center of the combustion chamber 7. The motion of the spray fuel reflected toward the center of the combustion chamber 7 is changed into circumferential motion by the function of the cylindrical projection 71. Therefore, it is enabled to enhance utilization of air in the combustion chamber and hence to improve combustibility of mixture.