Abstract: This application discloses an internal combustion engine comprising a hollow cylindrical body member, two pairs of vanes mounted in said body member for rotation coaxially therewith, whereby to divide the body member into four chambers, the vanes of each pair being diametrically opposite each other, and said pairs having limited rotatability with respect to each other, means permitting rotation of each pair of vanes in one direction only, a rotary power output member connected to each of said vane pairs to be driven thereby by either vane pair as it turns, and a coaxial, gearless, crankless phasing system, said body member having an inlet port for admitting an explosive air-gas mixture as each chamber reaches an angular position diametrically opposite to that at which said mixture was admitted thereto, said chamber having an exhaust port spaced angularly in front of said inlet port by a degree equal to the minimum angular extent of each of said chambers.