Abstract: A three-mode control system for controlling the flow of inlet air into a supercharged spark ignition engine comprising an air expansion and compression apparatus, an inlet manifold, and an inlet port control valve for controlling the intake of air into the expansion and compression apparatus. The inlet port control includes an inlet port valve which is open at full engine load and progressively closes when the engine load is progressively reduced while the inlet manifold pressure remains above a predetermined level. An air flow throttle valve located upstream of the expansion and compression apparatus is open at full engine load and progressively closes when the engine load is progressively reduced and the inlet manifold pressure falls below the predetermined level. A bypass duct which can optionally bypass air around the expansion and compression apparatus is closed at full engine load and progressively opens when the engine load is reduced and the inlet manifold pressure falls below the predetermined level.