Evaporative Emissions System Function

Evaporative Emissions System Function

Depending on air pressure and ambient temperature, varying quantities of fuel vapors form above the fuel surface in the fuel tank.

The Evaporative Emission (EVAP) system prevents these Hydrocarbon (HC) emissions from entering the atmosphere.

Fuel vapors penetrate into the EVAP canister via gravity valves integrated in the fuel tank.

The activated charcoal in the canister stores these gases like a sponge.

While driving, with oxygen sensor control active (engine warm), the EVAP canister purge regulator valve, (also called the regeneration valve) is activated by the Engine Control Module (ECM) in pulses dependent on load and engine speed. The opening time is dependent on input signals.

During the cleansing process (regeneration of active charcoal), the intake manifold vacuum sucks fresh air through the air openings of the EVAP canister. Fuel vapors temporarily stored in the active carbon and fresh air are supplied in doses to combustion via the EVAP canister purge regulator valve.

When no voltage is applied (for example, open wiring circuit), the solenoid valve is closed. The EVAP canister is not purged.

Function Description of Leak Diagnosis

The EVAP canister system (including the fuel tank) is equipped with a leak diagnostic that detects leaks in the system.

The diagnosis operates by pressurizing the system and should detect leaks where the damage exceeds 1 mm in diameter.

During the diagnostic, the Leak Detection Pump (LDP) generates a positive pressure of approximately 30 mbar in the EVAP system. The pump is switched off once the pressure is reached once. When the pressure falls to below a certain figure, the pump will switch on again. On Board Diagnostic (OBD) monitors the switch intervals and stores a Diagnostic Trouble Code (DTC) in the DTC memory if the intervals are too short.