Evaporative Emissions System: Description and Operation

Evaporative Emissions Control Schematic:

EVAPORATIVE EMISSION CANISTER OPERATION

An evaporative emission control system is used to prevent emission of fuel vapor. The vapors generated in the fuel tank while driving or while the engine is not running, passes through a 2-way check valve and enters the charcoal canister where the charcoal absorbs and stores the fuel vapor. While the engine is running, the fuel vapor stored in the canister is drawn into the intake manifold together with fresh air to be burned. While the engine is not running, the fuel vapor is stored in the canister.

Two Way Check Valve:

TWO WAY CHECK VALVE

The vapors generated in the fuel tank while driving or while the engine is not running, passes through a 2-way check valve and enters the charcoal canister where the charcoal absorbs and stores the fuel vapor. It is located on the fuel tank.

The 2-way check valve is provided to keep the pressure in the fuel tank constant. When the pressure in the fuel tank becomes positive and reaches its specified value, it opens the valve to let the vapor flow into the charcoal canister. On the other hand, when the pressure in the fuel tank becomes negative and reaches its specified value, it opens the valve to let the air flow into the fuel tank.

Fuel Filler Cap Cross Section:

FUEL FILLER CAP

A ratchet tightening device on the threaded fuel filler cap reduces the chances of incorrect installation, which would prevent sealing of fuel vapors. After the gasket on the fuel filler cap makes contact with the fuel filler neck flange, a loud clicking noise is heard, indicating that the seal has been set. The cap used on this vehicle has a vacuum relief valve incorporated into it. If the pressure in the fuel tank becomes negative (vacuum), the relief valve opens to release the pressure created within the fuel tank. Only when vacuum becomes excessive within the tank does this occur. Normally, vacuum pressure is relieved by a two-way check valve which is incorporated into the fuel vapor line.

CAUTION: If the pressure in the fuel tank is not released prior to system service, the fuel in the fuel tank may be forced out through the fuel hoses due to the pressure during disconnection.