Evaporative Emission Control Canister: Description and Operation

Fig. 18 Evaporative Control System:

The evaporative emission control system recovers and stores fuel vapors to reduce HC emissions. The EVAP system consists of a liquid-vapor separator, charcoal cannister, purge valve, fuel storage tank, and a non-vented filler cap.

When the vehicle is stopped and the engine is OFF, fuel vapors are collected in the fuel-vapor separator. 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.

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.

When the engine is started, the ECU controls the purge valve to allow cannister purging under the following conditions:

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.
Input parameters. Coolant temperature, engine speed, engine load.
Output. Purge valve duty cycle.