Abstract: An internal combustion engine is provided having opposed pistons operating in axially spaced relation within common power cylinders. Opposed heads of the pistons define a space within which occurs a cycle of compression, ignition, combustion, and expansion. One piston controls exhaust ports, and the other piston controls air injection and fuel-air charge injection ports. Additional air injection ports, and associated supply passages or tubes, are provided to admit compressed air over the head of the exhaust piston to cool the exhaust piston head and also form a gas flow inhibiting region below the exhaust port, near the end of the power/exhaust stroke. Rotary valves control the in-flow of air and air-fuel charge to charging cylinders. Air injection ports have an optional improved construction to enhance air injection into the power cylinders. Throttle valves controlling flow of air into purge air charging cylinders are provided as an option.