Abstract: A rotary positive displacement engine includes one or more power rotors, which are acted upon by a pressurized charge of gas, such as steam, and an annular barrier rotor geared for synchronous rotation with the power rotors. The rotors rotate within intersecting cylindrical bores in the engine housing. The power rotors have cylindrical outer surfaces from which opposed vanes extend which are acted upon by the powering charge. The barrier rotor has an outer cylindrical surface, located in close proximity to the cylindrical surface of the power rotors, and ports for delivering the powering charge to the power rotors. The barrier rotor thus forms both a charge delivery mechanism and a barrier between the exhaust ports and the expanding gas powering the engine. Located within the barrier rotor is a stator which has ports in fluid communication with the ports in the barrier rotor when the respective ports are aligned.