Abstract: Apparatus and a method for the introduction of a combustible mixture into a conventional cylinder of a conventional internal combustion engine. In addition to the conventional camshaft-controlled intake valve, an inlet check valve that is responsive to the pressure within the cylinder is provided. The inlet valve admits combustion air from the surroundings to the cylinder when the piston of the cylinder is in its suction strike, and it is closed off when the piston is in the compression, power, and exhaust strokes. Means can be provided to control the amount of air available to be admitted by the inlet valve in dependence upon the RPM of the engine. Also, a plurality of inlet valves may be provided, as well as a means for sequentially supplying air to them as a function of the engine RPM or only above a certain RPM.