Abstract: An engine comprises an output shaft journalled in a crankcase for rotation about an axis, an even number of cylinders, each having its axis parallel to the shaft axis, arranged in diametrically opposite pairs radially equidistant from the shaft axis and equally spaced-apart circumferentially, and a piston movable axially along each cylinder. A cross-head beam for each pair of pistons is connected to the respective pistons and journalled on a carrier that is received on the shaft for rotation with and movement axially along the shaft. A mechanism selectively moves the carrier axially along the shaft to vary the clearance volumes of the cylinders. The engine operates on a six-stroke cycle consisting of a four-stroke combustion cycle followed by a two-stroke steam cycle. The water injected for the steam cycle cools the engine. The intake air for the combustion cycle is supercharged and preheated before induction.