Abstract: An internal combustion engine comprising a cylinder head with a rotary intake and exhaust valves mounted on the head, the head being interchangeable with cylinder blocks intended for conventional poppet type valve engines. According to a basic aspect of the invention, primary and secondary combustion chambers are provided which are isolated from each other by the piston when at the top of its stroke, enabling combustion to initially take place in the primary chamber with a reduced amount of fuel. Optionally, means are further provided to vary the intake valve opening in accordance with changes in speed of the engine.