PURPOSE: To enable a powerfull turbulent flow to be generated in a combustion chamber by allowing the opening area of a recessed part opening on the top of a piston to be narrowed toward the inside thereof.
CONSTITUTION: In a gasoline engine, on whose intake port 12 is provided an injector 13 injecting fuel toward a combustion chamber 6, and then, on the top of a piston 1 is provided the combustion chamber 6 by way of forming a recessed part 5 opening toward the under surface side of a cylinder head 4. In addition, the opening area of the opening 5a of the recessed part 5 is narrowed toward the inside of the recessed part 5, and at the same time, in the neighborhood of a top dead point of the piston 1 is inserted an ignition plug 8, which is extended at the under surface of the cylinder head 4. And further, the timing of fuel injection from the injector 13 is set to the initial stage of an intake stroke, and at the same time, the fuel spray is directed to the inside of the recessed part 5 forming the combustion chamber 6. Hereby, the air-fuel mixture is designed to be generated practically only within the recessed part 5, and consequently, both a sure suppression of knockings and a sharp decrease in generating the unburnt HC can be achieved, while an improvement in the efficiency of combustion and a decrease in generating NOx are executed.