Operating principle of the absorber (VW Passat B6)

Depending on the pressure and ambient temperature, fuel vapors form to a greater or lesser extent above the surface of the fuel in the tank.

The activated carbon absorber system prevents these hydrocarbon vapors from entering the atmosphere.

Fuel vapors pass from the highest point in the tank through a pressure reducing valve, depending on the throttling, into an absorber with activated carbon.

Activated carbon absorbs these gases like a sponge.

When driving and with active lambda control (warm engine), the engine control unit opens the electromagnetic valve 1 of the activated carbon absorber, (also regenerating valve), depending on the load and the number of revolutions. The opening time depends on the incoming signals.

During the cleaning process (activated carbon regeneration) due to the vacuum in the intake manifold, clean air is sucked in through the ventilation pipe of the absorber. The fuel vapors and clean air contained in the activated carbon at a certain point in time are dosed and then burned in the engine.

The pressure reducing valve prevents fuel vapors from being sucked out of the tank when the activated carbon canister solenoid valve is open and there is a vacuum in the intake manifold. Thus, it ensures preferential emptying of the canister.

The original article is posted on the website (vwmanual)

In the absence of power supply (eg break in electrical circuit) the electromagnetic valve is closed. In this case, the absorber is not emptied.