To reduce nitrogen oxides produced in the operation of a four-stroke Diesel engine.

For operating a four-stroke Diesel engine, the arrangement is furnished with at least one exhaust hole 7 for exhausting the exhaust gas and at least one suction hole 8 for supplying the air, and water is injected into a cylinder 2 furnished internally with a piston 3 reciprocatively, whereby the exhaust hole 7 is closed in the exhaust stroke of the piston 3 so that the exhaust hole 7 is closed already before the piston 3 reaches the idling top dead center at the end of the exhaust stroke, and the water injection into the cylinder 2 is conducted when the piston 3 is in the region of the idling top dead center.