Vortex valves are small valves, mounted on the intake manifold just before the cylinder head build in many modern cars with diesel engines, including those from Audi, BMW, Vauxhall and Alfa Romeo. The valves are smaller than runners intake and therefore allow air to flow around them, even when "closed". The photo shows cleaned vortex valve removed from the intake manifold of BMW M47TU 2-liter diesel engine. The very top is made of stainless steel attached to the spindle of two small Torx screws. The sealing O-ring and an external lever actuation can be seen below the valve itself.
Swirl valve position is controlled by electrical or vacuum-activated servo mechanism, which is under the control of the management system of the engine. In a typical performance valves will be closed at idle, creating additional turbulence in the intake. With increasing of the engine speed, valve opens gradually until, at about 2000 rpm, they are parallel to the airflow and there is almost no resistance. Their aim is to ensure that air which enters the cylinder is enough and can provide a good fuel-air mixture to enter the cylinder even at low engine speeds. This helps to reduce some toxic emissions and can improve low-end power and torque.