Abstract: A rotary engine, preferably operating on the 4-event Otto cycle, has a stator ring stationary on a base which also supports a shaft rotational about an axis. The stator ring has oppostie faces normal to the axis. A pair of rotor rings concentric with and mounted on the shaft abut the stator ring side faces. There are cylinders through the stator ring with their axes parallel to the shaft axis. Hollow pistons in the cylinders at their opposite ends bear against cam rings facing and complementary to each other and disposed in the rotor rings. The stator, pistons and cam rings define separate, variable volume chambers to which fuel mixture is supplied through ports in the stator ring and in the piston walls. Ignition devices fire the fuel mixture inside the hollow pistons. Exhaust is through ports in the pistons and stator ring. Lubrication and water cooling connections are provided to and from the stator ring.