PURPOSE: To improve the combustion efficiency of the titled engine, reduce the smoke concentration within exhaust gas and improve the combustion cost by providing a coarse surface part and a smooth surface part on the side wall surface of a cavity of a piston and providing a fuel injection nozzle injecting fuel towards the smooth surface part.

CONSTITUTION: An engine has a cylinder block 1, a piston 3, a cylinder head 5 and a fuel injection nozzle 7. On the side wall surface of the cavity 4 of the piston 3, a coarse surface part 11 and a smooth surface part 12 are formed. Atomized fuel injected through the fuel injection nozzle 7 arrives at the smooth surface part 12 and is brought into contact therewith. This atomized fuel is diffused and distributed within the cavity 4 and disturbed on the coarse surface part 11 and mixing with air is promoted. In this manner, combustion is carried out efficiently and reduction of the concentration of smoke within the exhaust gas and improvement of the fuel consumption rate can be accomplished.