Evaporative Emissions System: Description and Operation

PURPOSE
The evaporative emission control system prevents the escape of fuel vapors into the environment from the storage tank. The EVAP system consists of:

^ Liquid-vapor separator (expansion tank)
^ Charcoal cannister
^ Purge valve
^ Fuel storage tank and non-vented filler cap

Liquid-Vapor Separator Location:

OPERATION

Liquid-vapor Separator
When the engine is OFF, fuel vapors are collected in the fuel-vapor separator (expansion tank). Most of the fuel vapor condenses into liquid and flows back to the fuel tank. The liquid-vapor separator also allows for expansion of fuel in a full tank where temperatures may vary as much as 80° F. This is necessary because the fuel tank has no direct vent to the atmosphere.

Charcoal Canister Location:

Charcoal Cannister
Excess fuel vapors are absorbed by the charcoal cannister. The charcoal cannister is equipped with a filter in the bottom surface to allow fresh air to draw through the cannister during purging.

Air Mass Meter, EVAP Purge Valve:

Purge Valve
When the engine is started, DME controls the purge valve allowing cannister purging based on the following inputs:

^ Coolant temperature
^ Engine speed
^ Engine load

Purging. Coolant temperature above threshold, closed loop system functioning and/or engine load above threshold.
No purging. Coolant temperature below threshold, closed loop system inactive and/or engine load below threshold.