Evaporative Emissions System: Description and Operation

Evaporative Emissions (EVAP) system
Functional description of leak detection
Depending upon the air pressure and ambient temperature, fuel vapor will form above the level of fuel in the tank.

The Evaporative Emissions (EVAP) system prevents these HC emissions escaping to the atmosphere.

In limited quantities, fuel vapors pass through a gravity valve (which closes at an angle of 45° located at the highest point in the tank and through a pressure retention valve into the activated charcoal filter in the EVAP canister.

The activated charcoal stores these vapors like a sponge.
When the oxygen sensor control is active when driving (engine warm), the solenoid valve (-N80-, also called regeneration valve), is activated (pulsed) from engine control module depending upon load and engine speed. The opening period is dependent 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 pulse-fed for combustion.

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.

No voltage (e.g. wiring open circuit) the solenoid valve is closed and the EVAP canister will not be purged.

The vacuum pipe from the throttle valve control part 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 by-pass guarantees the crankcase breathing.

Functional description of leak detection
The Evaporative Emissions (EVAP) system (including fuel tank) is equipped with leak diagnosis capability which will detect whether the system is leaking.

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

During the diagnosis the leak detection pump -V144- builds up a pressure of approximately 30 bar in the Evaporative

Emissions (EVAP) system. The pump will switch off when the pressure is attained. When the pressure falls to below a certain figure, the pump will switch on again. The On Board Diagnostic monitors the time between the switched on periods, and if it is too short it will register a malfunction in the DTC memory.

This diagnostic operates by pressurizing the system and is capable of sensing leakage in cases where the source is at least 1 mm in diameter.

Note:
^ Hose connections are secured with either spring-type or clamp-type clips.
^ When replacing an existing clip, always use the spring-type.
^ VAS 5024 or VAG1921 pliers are recommended for installing spring clips.

Observe safety precautions, Refer to Powertrain Management, Fuel Delivery and Air Induction, Service Precautions, Vehicle Damage Warnings
Observe rules for cleanliness, Refer to Powertrain Management, Fuel Delivery and Air Induction, Service and Repair, Observe rules for cleanliness
Overview of Evaporative Emissions (EVAP) canister system, overview, Refer to Powertrain Management, Evaporative Emissions System, Service and Repair,Evaporative Emissions System, Overview