Evaporative Emissions System: Description and Operation

GF47.30-P-3013V Fuel Evaporation Control System, Function







The fuel evaporation control system prevents fuel vapors escaping to atmosphere. This is done by storing the fuel vapors temporarily in the activated charcoal canister.

When the engine is running, the fuel vapors stored in the activated charcoal canister, are drawn off due to the manifold vacuum and combusted in the engine. The purge switchover valve carries out metering of fuel vapor from the activated charcoal canister and to do this is actuated on the ground side by the ME-SFI control unit.

Purging of the activated charcoal canister takes place at:
- coolant temperature > 40 °C
- Locking time after engine start elapsed, approx. 45 seconds
- Deceleration fuel shutoff inactive.

To prevent pulse noise caused by pulsing of the purge switchover valve, a damper reservoir is located upstream of the valve.

The idle speed control prevents changes in engine speed resulting from purging.

A richer or leaner fuel-air mixture is produced in line with the charge of the activated charcoal canister with fuel vapors.

Air admission to fuel tank
Air is admitted through the activated charcoal canister. The air flows through the ventilation connection at the activated charcoal canister (77) into the fuel tank (75).

Air release from fuel tank
The fuel tank is vented through the activated charcoal canister. In this case, the fuel vapors flow to the activated charcoal canister where they are stored, or if purging is activated, they are drawn off into the intake manifold.

Vent valve (model 171 and 230, except (USA))
On vehicles with vent valve (76), the fuel tank is ventilated outwards at pressure levels of 30 to 50 mbar, and ventilated inwards at vacuum levels of 1 to 16 mbar. In addition, this prevents overfilling the fuel tank.

(USA)
During refueling the fuel vapors are collected additionally from the fuel tank and passed on to the activated charcoal filter (ORVR = Onboard Refueling Vapor Recovery). An activated charcoal canister is used with around 3 liter capacity. The activated charcoal canister shutoff valve (Y58/4) and fuel tank pressure sensor (B4/3) are necessary for the checking the leaktightness.