Abstract: An external combustion engine is disclosed of the type including two or more circular rotors rotatably supported, one of the rotors acting as a power rotor and rotating in tangential contact with the other sealing rotor. The power rotor is formed with a radially projecting piston passing into a mating recess with the sealing rotor at a corresponding circumferential location. Steam or other working fluid is admitted via an intake port located opposite one face of the power rotor to cause rotation of the power rotor by expansion of the fluid in a working chamber defined by the space behind the piston. As the power rotor continues to rotate, the working chamber passes into communication with an exhaust port preparator to another power stroke. A vacuum relief port is provided at an intermediate location which relieves any vacuum condition which develops behind the piston during part throttle operating conditions of the engine.