The solenoid valve for the carbon filter is located on the lower left at the rear of the engine compartment
- 1 - tube to the carbon filter;
- 2 - electromagnetic valve;
- 3 - electrical connection;
- 4 - pipe to the engine intake manifold.
The carbon filter (2) is located at the rear in the right front wheel well. Further visible are: the tube to the electromagnetic valve (1), the tube to the fuel tank ventilation (3).
Fuel tank ventilation is important for perfect fuel supply: air must enter the fuel tank in the same volume as the fuel is consumed, otherwise low pressure will form in the fuel tank and the fuel supply will stop.
Next, the fuel tank must be ventilated to allow the fuel to expand when heated. When adding fuel, enough air must be forced out of the fuel tank to prevent the fuel being added from spilling out of the filler neck.
Fuel tank ventilation in Audi A4
- At the top of the fuel tank, a vent tube is connected. It first goes through a connector on the filler pipe into the so-called gravity valve. It closes the tube as soon as the car tilts to the side more than 45°. This prevents fuel from spilling out through the vent tube in an accident.
- From there, the ventilation tube is not simply led out, but enters the carbon filter in the right front wheel arch. The purpose of such a device is to capture the environmentally harmful fuel vapors that come out through this tube (arising, for example, due to heating of the contents of the fuel tank). When the engine is running, under a certain load, the gases are sucked out of the carbon filter again. When this happens is determined by the electromagnetic valve controlled by the fuel injection.
- Parallel to this system, there is a high-speed ventilation, which leads the filler pipe through a compensating volume in the form of a special-purpose container in the filler shaft of the fuel tank. This ventilation path is important, above all, during refueling, when the fuel level rises quickly and fuel vapors must be displaced, but in such a way that they do not enter the atmosphere in too large quantities.
- During movement along the same path, atmospheric air enters the fuel tank in accordance with the fuel consumed, so low pressure cannot form in the fuel tank.
(A link to the original source is available on the website: AudiManual.ru)