Abstract: An annular ring is closed on each side by a side plate secured thereto thus forming a cylindrical chamber or casing. A rotor is mounted on a shaft which is supported axially by the side plates, the rotor being within the chamber. A plurality of pistons are mounted for pivotal action around the periphery of the rotor and are in sealing engagement with the side plates. These pistons have a transverse contact line engaging the inner surface of the annular ring. This inner surface is contoured so that a compression lobe and a power lobe are formed and as the rotor rotates, fuel and air is drawn in through an intake in the wall of the ring, is compressed by one of the pistons, and then firing takes place so that expansion occurs as the piston is passing through the power lobe with the residual gases being exhausted through the exhaust port. A complete cycle occurs through 360.degree.