Abstract: An internal combustion engine having a fixed casing or housing defining a cavity of circular cross section, and cam mechanism within the cavity near the center thereof. A rotor rotates in the annular space between the casing and the cam, and carries flaps which are moved by the cam mechanism as the rotor rotates, to form, in conjunction with the end walls and cross members of the rotor and the interior surface of the casing, combustion chambers of varying size. At one point in the cycle of rotation, the flaps of one chamber move to a position forming a rather large intake chamber, which receives a combustible mixture.