Abstract: In accordance with this invention a vapor engine is provided having a first piston of a given diameter and a second piston having a larger diameter than the first piston. Walls are provided within which the pistons travel so as to define a first piston space and a second piston space. An apparatus are provided for connecting the first and the second piston to a power output. A rotary valve, having passageways therein for communicating with the first piston space and with the second piston space and for receiving vapor under pressure from an input source and for discharging exhaust vapor from the engine, is a part of the combination.