Abstract: An internal combustion design which allows for enhanced control of the fuel-air mixture and complete exhaust of combustion gases during the exhaust stroke. The internal combustion engine does not require that the combustion gas outlet valve enter into the cylinder to allow combustion gases to exit the cylinder. The valving structure includes one or more ring valves which form the upper circumferential wall of the cylinder. The ring valve or valves can either reciprocate or rotate. The engine includes a variable-length piston rod manufactured in two parts, which parts can reciprocate relative to one another along their length. A spring--either mechanical or pressure-operated--or the inertial masses of the parts, may be used between the piston rod parts to control the reciprocation of the parts relative to one another.