The fuel tank ventilation system serves the purpose of buffering gaseous fuel components that continuously evaporate from the fuel in the fuel tank. The system basically consists of the carbon canister (AKF), the fuel tank vent valve and a line system that establishes the connection between the fuel tank, carbon canister and intake system.
The amount of hydrocarbons that accumulates from the tank system greatly depends on the fuel temperature, ambient temperature, air pressure and fuel level in the tank.
The evaporated fuel components, mainly hydrocarbons, are absorbed in the activated carbon of the carbon canister. Since the storage capacity of the carbon canister is limited it must be purged at regular intervals.
For this purpose, fresh air passes through the carbon canister as part of the controlled regeneration process. The fresh air drawn through the carbon canister is enriched with hydrocarbons according to the level of charge of the activated carbon and then fed into the engine for combustion. The fuel tank vent valve controls the volume fed to the engine.
During vehicle operation the DME controls the fuel tank vent valve at intervals dependent on load and engine speed based on a special control algorithm.
The fuel tank vent valve is closed when the ignition is switched off. This prevents fuel vapours from the AKF reaching the intake system when the engine is not running.
The operation of the fuel tank ventilation system is monitored continuously as part of on-board diagnosis. Malfunctions cause fault codes to be entered in the DME fault code memory.
The diagnostic program additionally provides the option of carrying out both the complete system test as well as the separate fuel tank vent valve test.