Trunk engine used for marine purposes. The piston has attached to it a trunk at the lower end of which the pitman is connected directly with the piston. The trunk works through a stuffing-box in cylinder-head. The effective area of the upper side of the piston is greatly reduced by the trunk. To equalize the power on both sides of piston, high-pressure steam has been first used on the upper side and afterward exhausted into and used expansively in the part of cylinder below.