A novel device for controlling the compression pressure of an internal combustion engine is disclosed.The combustion chamber of each cylinder of the engine is divided into two virtual spaces, a gas exchange space and a control space. The intake and exhaust valves move in a plane substantially perpendicular to the cylinder centerline and open into the gas exchange space.The position of a preferably toroidal volume control slider determines the control space volume and subsequently, the geometrical compression ratio of the engine. At least one actuation cam bidirectionally drives said control slider, by means of a slot and captive roller arrangement.The device further comprises actuator means to rotate the cam to a predetermined angular position, as a function of engine load.Thus, the device of the invention is capable of maintaining a constant compression pressure, under varying load, by altering the geometrical compression ratio of the engine.