Evaporative Emissions System: Description and Operation

Fig. 6 EVAP System Schematic:

Fig. 7 EVAP System Operation:

SYSTEM DESCRIPTION

The evaporative emission control system is designed to minimize the amount of raw fuel vapors escaping into the atmosphere. It consists of a charcoal canister, a vapor purge control system and a fuel tank vapor control system.
The charcoal canister is used as a temporary storage facility for fuel vapors until they can be purged from the canister into the engine where the vapor is burned.
The vapor purge control system consists of a vacuum control valve (VCV) and bi-metal vacuum switching valve (BVSV). The VCV controls the flow of vapors from the canister to the intake manifold. The BVSV monitors the coolant temperature and allows no vacuum to pass if the coolant is below 45°C (113°F) and lets vacuum pass at 64°C (147°F) or above to open the VCV.
The fuel tank vapor control system consists of the fuel tank, fuel tank cap and a vapor separator. The fuel tank cap has a check valve which allows air to come into the tank but no vapor to escape. This allows fuel vapor to be removed for storage in the canister. The vapor separator removes any liquid fuel from the vapor before it goes to the canister.


BI-METAL VACUUM SWITCHING VALVE

The bi-metal vacuum switching valve (BVSV) is a temperature sensative valve that controls the vacuum that is used to operate the vacuum control valve (VCV). The BVSV only lets vacuum pass when the engine is warm, this insures that the canister only purges when the engine is warm.

CHARCOAL CANISTER

The charcoal canister is used as temporary storage for fuel vapors until they can be purged from the canister into the engine where they are consumed during combustion.

VACUUM CONTROL VALVE

The Vacuum Control Valve (VCV) is opened to allow the vacuum from the purge port to remove any stored vapors from the canister and pull ambient air into the fuel tank.