FUEL EVAPORIZATION CONTROL SYSTEM

The fuel evaporization control system prevents vapors from the fuel tank and carburetor from entering the atmosphere (fig. 4-54). Older, pre-emission vehicles used vented fuel tank caps. Carburetor bowls were also vented to the atmosphere. This caused a considerable amount of emissions. Modern vehicles commonly use fuel evaporization control systems to prevent this source of pollution. The major components of the fuel evapotization control systems are the sealed fuel tank cap, fuel air dome, liquid-vapor separator, roll-over valve, fuel tank vent line, charcoal canister, carburetor vent line, and the purge line.

• SEALED FUEL TANK CAP is used to keep fuel vapors from entering the atmosphere through the tank filler neck. It may contain pressure and vacuum valves that open in extreme cases of pressure or vacuum.
• When the fuel expands (from warming), tank pressure forces fuel vapors out a vent line or line at the top of the fuel tank, not out of the cap.
• FUEL AIR DOME is a hump designed into the top of the fuel tank to allow for fuel expansion. The dome normally provides about 10 percent air space to allow for fuel heating and volume increase.
• LIQUID-VAPOR SEPARATOR is frequently used to keep liquid fuel from entering the evaporation control system. It is simply a metal tank located above the main fuel tank. Liquid fuel condenses on the walls of the separator and then flows back into the fuel tank.
• ROLL-OVER VALVE is sometimes used in the vent line from the fuel tank. It keeps liquid fuel from entering the vent line after an accident where the vehicle rolled upside down. The valve contains a metal ball or plunger valve that blocks the vent line when the valve is turned over.
• FUELTANK VENTLINE carries fuel vapors up evaporization control to a charcoal canister in the engine compartment
• CHARCOAL CANISTER stores fuel vapors when the engine is NOT running. The metal or plastic canister is filled with activated charcoal granules capable of absorbing fuel vapors.
• CARBURETOR VENT LINE connects the carburetor fuel bowl with the charcoal canister. Bowl vapors flow through this line and into the canister.
• PURGE LINE is used for removing or cleaning the stored vapors out of the charcoal canister. It connects the canister and the engine intake manifold.

Basic operation of a fuel. system is as follows:

1. When the engine is running, intake manifold vacuum acts on the purge line, causing fresh air to flow through the filter at the bottom of the canister. The incoming fresh air picks up the stored fuel vapors and carries them through the purge line. The vapors enter the intake manifold and are pulled into the combustion chambers for burning.
2. When the engine is shut off, engine heat produces excess vapors. These vapors flow through the carburetor vent line and into the charcoal canister for storage. The vapors that form in the tank flow through the liquid vapor separator into the tank vent line to the charcoal canister.  The charcoal canister absorbs these fuel vapors and holds them until the engine is started