Swirl flap

A swirl flap is a small butterfly valve fitted to four-stroke internal combustion engines with at least two intake valves. It is installed inside or just before one of a cylinder's two intake ports, allowing to throttle its intake port's air flow, causing a swirl in the other intake port not fitted with a swirl flap. The swirl improves the air-fuel mixing process in direct injected engines, typically diesel engines, under low load conditions.