Evaporative Emissions System: Description and Operation

Evaporative Emission System Control
EVAP stands for "Evaporative Control System". Vapor which evaporates from the fuel in the fuel tank is routed to and stored in the EVAP canister from where it is introduced, at a convenient time, into the combustion process by way of the canister purge (CP) valve using the negative pressure in the intake manifold.

EVAP System Control:

The System Consists Of:

Fuel Tank (1)
To avoid the fuel tank deforming due to negative pressure it has a reinforcement plate screwed to its underside.

Roll-over valve (2)
The roll-over valve is a safety valve which closes if the car tilts sideways by more than 45°, preventing fuel leaking in the event of an accident.

EVAP Canister (3)
Fuel vapor from the fuel tank passes a bed of activated charcoal in the EVAP canister. The activated charcoal binds the fuel and pure air flows out into the atmosphere through the vent at the top of the EVAP canister. 80-140 g fuel can be bound in the EVAP canister, depending on the volume of charcoal.

Canister Purge Valve (4)
The canister purge valve is located between the EVAP canister and the engine intake manifold, the valve is used to control the flow of fuel vapor from the EVAP canister to the engine. It is connected to a 12 Volts supply from the main relay and to the engine control module. The engine control module opens the valve by grounding the cable.

Control
Normally the canister purge valve is closed and only opens when the engine control module is emptying the canister. When the EVAP canister is emptied the valve is first pulsed, this pulsing increases to bring the mixture from the canister up to a value which varies according to engine load and speed. Because of the negative pressure in the inlet manifold, fresh air is sucked into the engine through the EVAP canister vent or through the EVAP canister shut-off valve. As the air passes through the EVAP canister, fuel stored in the activated charcoal is drawn into the engine and burned.