Abstract: A rotor has two opposite lobes which cooperate with the inner surface of a cylindrical wall and further cooperate with four equally spaced retractible radial vanes to form successive pairs of diametrically opposite expanding chambers and successive pairs of diametrically opposite contracting chambers. The pairs of diametrically opposite chambers progress step by step in the direction of rotation of the rotor. Actuating fluid is supplied continuously to the expanding chambers from space inside the rotor through ports on the trailing sides of the two lobes of the rotor and the contracting chambers are continuously exhausted to an exhaust space inside the rotor through exhaust passages from the leading sides of the two lobes.