An internal combustion engine includes in one aspect a source of a pressurized working medium and an expander. The expander has a housing and a piston, movably mounted within and with respect to the housing, to perform one of rotation and reciprocation, each complete rotation or reciprocation defining at least a part of a cycle of the engine. The expander also includes a septum, mounted within the housing and movable with respect to the housing and the piston so as to define in conjunction therewith, over first and second angular ranges of the cycle, a working chamber that is isolated from an intake port and an exhaust port. Combustion occurs at least over the first angular range of the cycle to provide heat to the working medium and so as to increase its pressure. The working chamber over a second angular range of the cycle expands in volume while the piston receives, from the working medium as a result of its increased pressure, a force relative to the housing that causes motion of the piston relative to the housing.