Evaporative Emissions System: Description and Operation

Functional description of the evaporative emissions system
To some extent, depending on the air pressure and temperature, vapors form in the tank above the fuel surface.

The evaporative emissions system prevents these HC emissions from escaping into the air which we breathe.

From the highest point in the tank, the vapors travel through the gravity valve (which closes at an inclination of 45 °) and the pressure retention valve, throttled to some extent, and reach the charcoal canister.

The activated charcoal stores these gases like a sponge.

While driving under active A/F control (engine warm), the engine control module times and controls the solenoid valve (N80, also called the purge valve), depending on load and engine speed. The time that it stays open depends on the incoming signals.

During the purge cycle, (regeneration of the activated charcoal), a partial vacuum in the intake manifold sucks fresh air through the ventilation aperture on the bottom of the canister. The fresh air and the fuel vapors temporarily stored in the charcoal are fed for combustion in proper amounts.

The pressure retention valve prevents fuel vapors from being sucked from the tank when the solenoid valve is open and there is a partial vacuum in the intake manifold. It thereby grants priority to the emptying of the charcoal canister.

The solenoid valve is closed when there is no current (e.g. disconnected wire) and the charcoal canister is not emptied.

The partial vacuum from the throttle valve control module to the charcoal canister is also connected with the crankcase ventilating valve by a bypass. The check valve integrated there prevents intake air from reaching the crankcase for some partial loads. The crankcase ventilation is therefore generally improved. The bypass makes sure that the crankcase will be ventilated.

Functional description of leakage diagnosis
The evaporative emissions system (including fuel tank) is equipped with a leakage diagnosis which can detect leaks.

The diagnosis is based on the overpressure procedure and is supposed to show leaks which are larger than 1 mm in diameter.

The leak detection pump (V144) builds up an overpressure of about 30 mbars in the charcoal canister during the diagnostic process. The pump is turned off when the pressure is attained. If the pressure drops below a specified value, the pump is turned on again. The On Board Diagnosis supervises the switching intervals and enters a fault into fault memory if the intervals are too short.


NOTE:
- Hose connections are fastened with spring band clips and collar bands.
- In principle, replace collar bands with spring band clips.
- The VAG 1921 pliers are recommended for spring band clips.


Review safety measures.
Maintain cleanliness.
Overview diagram of the evaporative emissions system.