Swirl flap
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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.[1]
Swirl flap position is adjusted by an electrical or vacuum-actuated servo mechanism which is under the control of the engine management system. In a typical implementation the flaps will be closed at idle speed, creating additional turbulence in the intake. As engine speed increases, the flaps are gradually opened until, at around 2,000/min, they are parallel to the airflow and present virtually no resistance. Their purpose is to ensure that the air entering the cylinder is sufficiently turbulent for good fuel-air mixing even at low engine speeds. This aids in reducing emissions and may also improve low-end torque.