Abstract: A four-stroke internal combustion engine includes a first intake manifold in communication with an intake port of a cylinder. A crankshaft chamber is in communication with a cylinder bore of the cylinder and includes an inlet port adjacent the cylinder bore. A second intake manifold is in communication with the crankshaft chamber for supplying air or gas into the cylinder bore via the inlet port during a second stage of an intake stroke of a piston. An exhaust port of the cylinder is opened during a second stage of a power stroke to perform a first-stage discharge of exhaust gas. Air or gas is supplied into the cylinder bore via the inlet port during a third stage of the power stroke to perform a second-stage discharge of the exhaust gas. Exhaust gas not discharged during the third stage of the power stroke is discharged during an exhaust stroke.