Abstract: An internal combustion engine is so constructed that the timing of the intake and exhaust valves can be changed during operation of the engine making it possible to select the optimum timing for the valves in accordance with the operating conditions. The timing can be changed from the exterior of the engine on the basis of factors such as the quantity and nature of pollutants in the exhaust gases, the rate of revolution of the engine and the load on the engine. Adjustment can be effected either manually or automatically.The construction of the engine is such that the cam shaft or cam shafts can be replaced with cam shafts of a different design and that the cylinder head can be replaced with a head of a different design.Adjustment of the timing of the intake and exhaust valves is effected through the use of a planetary gear train which also provides the necessary reduction in the speed of the cam shaft relative to the crank shaft.