A method of operating a four-stroke internal combustion engine of the carburator or fuel injection type, with one or several reciprocating pistons or one or several rotary pistons wherein during the intake movement of the one or the several pistons an additional gaseous or liquid or gas-liquid mixture fluid is introduced into the cylinder working space through a cylinder side wall aperture in the vicinity of the piston to establish a layer charge overlying the piston. The introduction of this fluid may be effected by suction effects caused by the piston movement. The amount of fluid supplied may be controlled in dependence upon combustion pressure and/or exhaust gas composition. The internal combustion engine includes an aperture in a cylinder wall in communication with a fluid source whereby this aperture is adapted to be obstructed or exposed by the moving piston to control the admission of the additional fluid. Between the aperture and the fluid source may be provided control valve means.