This article may be too technical for most readers to understand. (June 2012) (Learn how and when to remove this template message)
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 Audi, BMW, Vauxhall and Alfa Romeo. The flaps are smaller than the intake runners and therefore allow air to pass around them even when "closed". The photo shows a cleaned swirl flap removed from the intake manifold of a BMW M47TU 2-litre diesel engine. The flap itself is made from stainless steel and secured to a spindle by two small Torx screws. The sealing O-ring and external actuating lever can be seen below the flap itself.
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 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 in one position 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 M47TU 2-litre engine manufactured between 2001 and 2004, and the similar 3-litre engine, is notorious for swirl flap failure, as the small screws 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. 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.
In July 2012 The Sunday Times (UK) published letters from affected BMW owners indicating that BMW UK does not accept there is a widespread problem despite the reportedly high numbers of cars being scrapped as a result of total engine failure. One correspondent on The Isle of Man achieved an 80% contribution to his repair costs, perhaps because poor publicity in a small area worried the car manufacturer. In the meantime those unlucky enough to see their viable cars turned to scrap in a matter of seconds have little or no recourse to BMW UK. However some environmentalists in Europe are now beginning to voice their concerns about the issue. The environmental impact of potentially scrapping thousands of cars for the sake of a minor modification is now being raised in some countries where the defective cars have been sold.