Abstract: The engine comprises a generally toroidal cylinder containing a piston having optional permanent magnets or electromagnets disposed thereon. The piston rotates through the cylinder due to a force exerted on it by expanding vapor. A shutter valve extends across the cylinder at one point and the vapor is injected between the shutter valve and the piston thereby forcing the piston to traverse the extent of the cylinder. An exhaust port is disposed such that the piston will pass it toward the completion of one revolution. The expanding vapor is exhausted through this port after which the piston momentum forces it past the shutter valve where a fresh supply of vapor is injected and the cycle repeats itself. Sensors and valving arrangements are supplied for providing optimum timing of the vapor injection. Also coils are disposed about the toroidal surface through which the magnet carrying piston passes for the production of electrical energy.