Abstract: An internal combustion engine that includes a rotary head that has an expansion chamber and that includes intake and exhaust slots. A drum is disposed in close noncontacting proximity to the inner side of the rotary head. The drum and the rotary head are substantially rotationally symmetric about a common axis. Radially moveable plates respond to a mechanical control mechanism adapted to move each plate from a retracted position in which the plate is located entirely within the outer surface of the drum and an extended position in close proximity to an upper surface of the expansion chamber to substantially seal the expansion chamber. At least one of the plates is present between the intake slot and the exhaust slot such that the combustion gas within the expansion chamber exerts a force on the plate and the drum to produce rotational motion of the rotary head about the common axis.