A piston-type internal combustion engine is described including a variable valve timing device controlling the timing of the intake valve, a coupling between the accelerator pedal and the variable timing device for varying the timing of the intake valve in response to the movement of the accelerator pedal, and means for maintaining a substantially constant compression ratio in the cylinder notwithstanding variations in the engine output. The latter means comprises a floating piston crown which is permitted to move further inwardly into the cylinder at the end of the compression stroke when the delay in closing the intake-valve is increased, than when the delay is decreased.