Evaporative Emissions System: Description and Operation
The evaporative control system prevents gasoline vapors from escaping into the atmosphere. Vapors from the gas tank are routed to the evaporative control canister. Carbon in the canister stores the gasoline vapors until the engine is started. The intake manifold vacuum of the running engine draws the vapors into the combustion chamber where they are burned.
Fig. 37 Evaporative Control System:
The evaporative control system consists of two expansion tanks, a vapor line orifice, carbon canister with valve, connecting hoses and the fuel filler cap, Fig. 37.
When the engine is off, the carbon canister control valve is closed. Fuel vapors are absorbed by the carbon in the canister. When the engine is running at normal or high load conditions, the vacuum in the intake manifold will open the control valve. This enables the vapors to be drawn into the air filter housing where it mixes with the fresh intake air.
COMPONENTS:
EXPANSION TANK #1 AND #2
Expansion tank #1 and #2 are used as temporary overflow containers.
When the gas tank is full and the outside temperature changes from cold to hot (or other influencing factors), the fuel expands into the expansion tanks. As the fuel level drops, the gas flows back into the tank via the hose connections.
ORIFICE
The vapor line orifice is located in the vapor line between expansion tank #1 and expansion tank #2. It limits the vapor flow to a specific amount.
CARBON CANISTER (INCORPORATES CONTROL VALVE)
The canister contains carbon which absorbs the gasoline vapors from the fuel tank. When vacuum is applied to the control valve in the canister, the carbon releases the fuel vapors to the air filter housing. The vacuum to the control valve is taken from the intake manifold. The use of alcohol-gasoline fuels will reduce the effectiveness of the carbon in the canister.
FUEL FILLER CAP
The filler cap is a non vented type to keep the fuel vapors in the gas tank system. Never replace non vented type filler cap with vented type cap.