Abstract: A method is proposed for operating a four stroke diesel engine, in which method water is injected into a cylinder (2) in which a reciprocable piston (3) is arranged, and which cylinder (2) has at least one outlet (7) for conducting off exhaust gases as well as at least one intake (8) for supplying air, with the outlet (7) being closed during the exhaust stroke of the piston (3) so that the outlet (7) is already closed before the piston (3) reaches the upper idle dead center at the end of the exhaust stroke, and with water being injected into the cylinder (2) when the piston (3) is located in the region of the upper idle dead center.