Abstract: The exhaust valve has a valve member which opens away from the combustion chamber of the engine cylinder and which is biased for opening by the gas pressure prevailing in the combustion chamber. The valve is kept closed by an opposed hydraulic pressure which in a holding chamber acts on a holding piston secured to the spindle of the valve member. When the valve is to be opened, the pressure in the holding chamber is relieved and one or more valve controlled flow passages are opened, through which hydraulic liquid flows from the holding chamber to a reservoir defined by a further piston secured to the valve spindle. The two pistons are preferably of substantially equal size. When the exhaust valve is to be closed a hydraulic pressure is applied to a substantially smaller closing piston which is also secured to the valve spindle.