Abstract: A rotary engine including a piston assembly having first and second adjacent hubs. The hubs are rotatably mounted in a housing about a common axis where they are coupled to two drive shafts that are concentrically arranged about the common axis. A first and second set of pistons extend radially outwardly from the first and second hubs, respectfully. Each piston head from the second set of piston heads is circumferentially spaced from a piston head of the first set to form a fuel expansion chamber therebetween. The distance between the rotational exes of the hubs and the outer peripheral surface of the piston heads is at least three times the distance between the outer peripheral surface of the piston assembly hubs and the outer periphery of the piston heads, i.e., the radial depth of the expansion chambers.