Swirl flap

Swirl flaps are small butterfly valves fitted to the intake manifold just before the cylinder head intake ports of many modern automotive diesel engines, including those from BMW, Vauxhall and Alfa Romeo. The flaps are smaller than the intake runners and therefore allow air to pass around them even when "closed".

Swirl flap position is adjusted by an electrical or vacuum-activated 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 rpm, 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 certain toxic emissions and may also improve low-end power and torque.

The disadvantages of swirl flaps are mainly associated with fouling by exhaust gas recirculation, which leaves tarry deposits on the flaps and the inside of the intake manifold. Over time the flaps can begin to stick and the engine management system may report an error code if the correct flap position cannot be achieved within a few percent of the design specification. More seriously, the flaps or their mountings can fracture under the strain and parts can enter the cylinder. This almost always causes major engine damage due to the very small clearances inside a diesel engine. The BMW M47N engine manufactured between 2001 and 2004 is notorious for swirl flap failure, as the small rivets used to secure the flaps to their spindle will fall into the intake ports if the spindle fractures; in 2004 the intake manifold was redesigned to eliminate these rivets. Some owners choose to have the flaps removed as a precaution, with blanking plates fitted to maintain manifold sealing. In many cases the impact on driveability and fuel economy is negligible.