Evaporative Emissions System: Description and Operation

Evaporative Emission Control System:

EVAPORATIVE EMISSION CONTROL SYSTEM (EVAP)
The evaporative emission control system is used to prevent emission of hydrocarbons. The vapors generated in the fuel tank while driving or when the engine is at a stop enters the charcoal canister where the charcoal absorbs and stores the fuel vapor. When the throttle valve opens off idle the canister is purged by manifold vacuum opening the purge control valve which in turn draws air through the filter at the bottom of the canister. The vapors then pass through the purge control valve and into the intake manifold to be consumed in the combustion process. While the engine is running (under all throttle conditions) a small amount of the vapors are drawn into the manifold through the constant purge orifice.

Carbon Canister:

CHARCOAL CANISTER
The Charcoal Canister is a device for storing fuel vapor. It contains activated charcoal that absorbs and stores fuel vapor.

PURGE CONTROL VALVE
The purge control valve is incorporated into the top of the canister, it is opened by throttle vacuum to allow purging of the canister.

Fuel Tank Vacuum Relief Valve:

FUEL TANK VACUUM RELIEF VALVE
The fuel tank vacuum relief valve is incorporated into the fuel cap. It is designed to prevent fuel vapor from escaping into the atmosphere and to allow the introduction of air into the fuel tank to displace the fuel as it is used.

Fig. 102 Fuel Check Valve:

FUEL CHECK VALVE
The fuel check valve is designed to prevent raw fuel but allowing fuel vapors from the fuel tank to enter the canister.