Evaporative Emissions System: Description and Operation

Evaporative Control System (EVAP):

COMPONENTS
The EVAP system consists of a non-vented fuel cap, vent valve (51), charcoal canister (52), two purge switchover valves (Y58/2 and Y58/3) and the routing hoses. The canister contains activated charcoal and a hose connects it to the intake manifold (at the electronic accelerator actuator) 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 70° C (158°F) and a specific intake air mass range, the LH control units (N3/2 and N3/3) apply a current to the purge switchover valves. The valves open 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.

In order to control the purge quantity, the LH control units activate the switch-over valves at a frequency of 7.5 Hz (cycles). The purge quantity is determined by the duration of each cycle.