Evaporative Emissions System: Description and Operation

Functional Description

Depending upon the air pressure and ambient temperature, fuel vapor will form above the level of fuel in the tank. The EVAP system prevents these hydrocarbon emissions from escaping into the atmosphere.

Fuel vapors from the highest point in the fuel tank pass through the gravity valve (which closes at an angle of 45) and through a pressure retention valve (in limited quantities) to the EVAP canister.

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

When the oxygen sensor control is active when driving (engine warm), EVAP canister purge regulator valve -N80- is activated (pulsed) from the engine control module depending upon load and engine speed. The opening period depends on the input signals.

Intake manifold vacuum draws fresh air through the vent opening on the underside of the EVAP canister during the purging procedure (regenerating the activated charcoal). The fuel vapors stored in the activated charcoal and fresh air are fed to combustion in metered quantities.

The pressure retention valve prevents fuel vapors from being drawn from the tank when the solenoid valve is open and intake manifold vacuum is present. It therefore ensures that the evacuation of the EVAP canister has priority.

When there is no electrical signal (i.e. open circuit) the solenoid valve remains closed. The EVAP canister will not be purged.

The vacuum line from the throttle valve control module to the EVAP canister has an additional bypass which is connected with the crankcase breather valve. The non-return valve integrated in the crankcase breather valve prevents the intake air gassing into the crankcase during some partial load ranges. This improves the overall crankcase breathing. The bypass guarantees the crankcase breathing.

Description of leakage diagnosis

The EVAP system, including the fuel tank, is equipped with leakage diagnosis which can detect if there is a leak in the system.

The diagnosis is based on a pressure method and indicates leaks with a diameter greater than 1 mm.

During the diagnosis, the Leak Detection Pump (LDP) -V144- develops above 30 mbar pressure in the EVAP system. Once the pressure is attained, the pump switches off. If the pressure drops below a certain value, the pump switches on again. The On Board Diagnostic (OBD) monitors the intervals at which the pump is switched on or off and if the intervals are too short, writes a fault to the Diagnostic Trouble Code (DTC) memory.