Evaporative Emissions System: Description and Operation

Evaporative Emission Control System Schematic:

COMPONENTS
The EVAP system consists of a non-vented fuel cap, vent valve (51), charcoal canister (77), purge switchover valve (Y58/1) and the routing hoses. The canister contains activated charcoal and a hose connects it to the throttle valve via the purge switchover valve.

OPERATION
When pressure in the fuel tank reaches 30 - 50 mbar (0.435 - 0.725 psi.) the vent valve opens allowing fuel vapors to flow from the fuel tank to the charcoal canister where they are absorbed.

With the engine running at a coolant temperature above 80° C (176°F) and a specific intake air mass range, the HFM-SFI control module (N3/4) applies a current to the purge switchover valve. The valve opens and vacuum from the intake manifold reaches the purge valve. Thus, the purge valve opens allowing fuel vapor from the charcoal canister to be drawn into the intake manifold via a port in the throttle plate.

In order to control the purge quantity, the HFM-SFI control module activates the switch-over valve at a frequency of 5,10 or 20 Hz (cycles) depending on engine load and other conditions . The purge quantity is determined by the duration of each cycle.