A rotary power device of the swinging piston type comprises a cylindrical rotor mounted between internal and external portions of a stator. The internal portion of the stator provides intake and exhaust channels and may hold an igniter. The external portion of the stator includes a middle portion with an oval cam track. The rotor includes a multiplicity of sector-shaped compartments arranged at equal angular intervals around the inner stator. Each compartment is open at the periphery of the rotor and has an inner opening to the central bore in alignment with ports in the central internal stator. A sector-shaped piston is mounted in each compartment for pivotal motion about a pivot axis at one vertex of the compartment. Each piston includes a roller follower for engaging the oval-shaped cam track as the rotor rotates, which causes angular reciprocation of the pistons within the compartments while performing intake and exhaust cycles through inner openings. The rotary power device may be modified to serve as an internal combustion engine, a compressor, a pump, or a hydraulic motor by replacement or modification of the central internal stator.