The piston is formed as a disc (2) guided on a circular path (1) and with its opposing faces forming the piston faces (4,5) aligned substantially perpendicular to the rotary direction of the piston disc. The piston disc is connected rigidly and mechanically to the shaft for transferring torque by a rotating holding element (6). The circular paths of the piston disc and holding element are enclosed on all sides by a housing (8) so that the distance between the disc and surrounding inner face of the housing is restricted to a minimum but contact between disc and housing is excluded. A movable slider (9) selectively shuts off or releases the cross-section of the circular path and is controlled by gearing so that the cross- section of the circular path is only opened temporarily for the passage of the piston disc. The gas is supplied to the inside of the housing through an inlet opening (13) behind the slider and is discharged through an outlet opening (14) in front of the slider.
