There is disclosed an internal combustion engine. The internal combustion engine includes a cylinder unit and a piston unit. The cylinder unit is rotatably coupled to a pair of spaced-apart crankshafts, with the cylinder unit moveable along a longitudinal axis. The piston unit is continually disposed within the cylinder unit, with the piston unit rotatively coupled to a second pair of spaced-apart crankshafts. The piston is moveable along the longitudinal axis in a direction opposite the direction of the cylinder during a combustion cycle. Both the cylinder unit and the piston unit are structured to be balanced relative to the respective center-of-gravity of each unit. Each center-of-gravity is located midway between the pair of spaced-apart crankshafts to which each unit is rotatively coupled.