Evaporative Emissions System: Description and Operation

EVAP Canister (Typical):

DESCRIPTION
The Evaporative Emissions Control System (EVAP) limits fuel vapors from escaping into the atmosphere. The EVAP transfers fuel vapor from the sealed fuel tank to an activated carbon (charcoal) storage device (vapor canister). The canister will store the vapors until the engine is able to use the extra fuel vapor.

When the engine is able to use the extra fuel vapor, the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.

The fuel tank is sealed with a fuel cap that is NOT normally vented to the atmosphere. The fuel tank cap has a safety valve which allows for both pressure and vacuum relief.

OPERATION
Fuel vapors from the fuel tank are purged and flow into the vapor canister tube labeled "tank" and are absorbed by the carbon. The canister is purged when the engine is able to use the extra fuel vapor. A vacuum source is applied to the vapor canister tube labeled "purge" to draw fresh air through the top of the canister. The air mixes with the vapor and the mixture is drawn into the intake manifold to be consumed in the normal combustion process.

EVAP Canister Purge Control
The vacuum source is applied to the EVAP canister in three ways. Some applications use a ported vacuum source directly from the injection unit. Another type of control uses manifold vacuum regulated by a EVAP purge control valve (some are integral with the EVAP canister while others are remote) which is actuated by ported vacuum and acts as a check valve to keep fuel vapors which are stored in the EVAP canister from entering the intake manifold while the vehicle is at rest. A hard start condition could be caused by valve which is stuck open.

The last type of EVAP canister control is the EVAP canister purge solenoid.