FUEL EVAPORIZATION CONTROL
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.
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.
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.
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
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.
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
VENTLINE carries fuel vapors up evaporization control to a charcoal canister in the engine
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.
VENT LINE connects the carburetor fuel bowl with the charcoal canister. Bowl vapors flow
through this line and into the canister.
LINE is used for removing or cleaning the stored vapors out of the charcoal canister. It
connects the canister and the engine intake manifold.
operation of a fuel. system is as follows:
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.
- 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