Evaporative Emissions System: Description and Operation

Evaporative Control System (2.8 Liter V6):

SYSTEM DESCRIPTION

The Evaporative Control System consists of the fuel tank and filler cap, a thermal vacuum valve, and charcoal canister. The system collects fuel vapors, that would otherwise be discharged into the atmosphere, and condenses them in a charcoal canister where they are temporarily stored. When the engine is started, the stored fuel and vapors are atomized with fresh air and directed to the intake to be burned in the engine. This system of vapor recovery helps not only to prevent raw hydrocarbon emissions, but also promotes fuel economy by eliminating fuel loss through evaporation.

Fuel Vapor Canister:

CHARCOAL CANISTER

The charcoal canister is filled with activated charcoal, to condense and store fuel vapors as they enter the canister. Any liquid fuel goes into a reservoir at the bottom of the canister to protect the integrity of the charcoal bed above. The canister is purged when the engine is running at operating temperature, above idle speed. Ambient air is let in through the air tube at the top, mixed with the fuel vapor and drawn into the intake system to be burned along with the normal air fuel charge.

Fuel Filler Cap:

FUEL FILLER CAP

The fuel filler cap is designed with a two stage valve to prevent vapor loss. If vapor pressure within the tank is less than atmospheric the vacuum valve will open, allowing ambient air into the tank to equalize pressures. If vapor pressure in the tank exceeds a specific level, the pressure valve will open allowing the excess fuel vapors to escape into the atmosphere. The pressure valve is a safety feature and does NOT open under normal conditions.

Fuel Tank:

FUEL TANK

The fuel tank incorporates vents at the top with special valves to prevent liquid fuel from leaking through the evaporative system in case of overfilling or in the event the vehicle is rolled over in an accident.

Rollover/Float Valve:

ROLL-OVER/FLOAT VALVE

The rollover/float valves are designed to prevent fuel from leaking through the evaporative system in the event the vehicle has turned over in an accident. They also prevent fuel from saturating the evaporative system with liquid fuel in case the tank is over filled.

Thermal Vacuum Valve:

THERMAL VACUUM VALVE (PURGE CONTROL)

A thermal vacuum valve is used in line to the purge control valve. It prevents vacuum from being applied until the engine reaches the minimum operating temperature for canister purging. The thermal vacuum valve reaches into the coolant stream to measure the coolant temperature. When cold (below 165 degrees F), the valve is closed. The valve opens only at higher temperatures, insuring proper operating conditions for effective purging of the charcoal canister and adequate combustion of the purged fuel vapors.