Swirl Flap

On modern diesel and gasoline engines, swirl flaps are tiny butterfly valves found in the inlet manifold. They are intended to help manage the fuel-to-air ratio, lower emissions, and help produce more torque at low engine speeds. When the engine is running lightly, the flaps close, forcing swirling air into the combustion chamber, which improves emissions and increases torque. The flaps typically activate to a completely open position after 2000 rpm and have relatively little impact on engine output and emissions.