Evaporative Emissions System: Description and Operation

Fig. 7 EVAPORATIVE CONTROL SYSTEM:

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. Fig. 7.

Fig. 65 EVAPORATIVE CONTROL SYSTEM (TYPICAL):

The evaporative control system consists of a charcoal canister (which contains activated charcoal), vacuum valve, roll-over valve and connecting hoses. Fig. 65.
The evaporative control system dampens pressure fluctuations within the gas tank resulting from changes in fuel volume and temperature. The vacuum valve controls the fuel vapors going to the intake manifold. The vacuum valve is controlled by the throttle valve upstream vacuum.
When the engine is idling, the vacuum valve is closed. Fuel vapor is absorbed by the activated charcoal in the canister. As engine speed increases, upstream vacuum also increases, opening the vacuum valve. This allows gasoline vapors to be drawn into the combustion chamber.


COMPONENTS:
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Fig. 26 EVAPORATIVE CONTROL CANISTER:

EVAPORATIVE CONTROL CANISTER (CARBON FILTER)
The canister contains active carbon which absorbs gasoline vapors. The use of alcohol-gasoline fuels will reduce the effectiveness of the charcoal in the canister. Fig. 26.

Fig. 27 ROLL-OVER VALVE:

ROLL-OVER VALVE
The roll-over valve closes when the vehicle is inclined to one side more than 45°. This prevents fuel leakage from the evaporative system hose in the event of a collision. Fig. 27.

Fig. 28 AIR VALVE:

AIR VALVE (PURGE-CONTROL VALVE)
The air valve controls the connection between the evaporative control canister and intake manifold. The valve closes when the engine is not running, that prevents the escape of gasoline vapors from the canister. A purge control vacuum signal is taken from downstream of the throttle plate. At engine idle a small purge flow occurs through the constant purge hole in the canister valve. When the throttle opens, the vacuum to the air valve allows full canister purge at a controlled rate. Fig. 28.

Fig. 29 TANK RELIEF VALVES:

TANK RELIEF VALVES
The tank relief valves are located in the fuel filler cap. Two safety valves prevent damage to the fuel system in the event of damage or blockage to the vent system and hoses. One valve opens in the event of over-pressure, the other in the event of excessive vacuum in the system. Fig. 29.