Evaporative Emissions System: Description and Operation

Fig. 69 Evaporative Emission Control Diagram:

The evaporative control system consists of a charcoal canister, a vapor purge control system, a carburetor vapor control system, and a fuel tank vapor control system. The evaporative control system minimizes the amount of fuel vapor escaping into the atmosphere.

CHARCOAL CANISTER
The charcoal canister is a container for the temporary storage of fuel vapor until is can be purged from the canister into the engine and burned. The canister is filled with activated charcoal which absorbs the fuel vapors until they are drawn in by the engine.

VAPOR PURGE CONTROL SYSTEM
The vapor purge control system determines when the stored fuel vapors are released to the engine. When the coolant temperature is above the switching value for thermovalve A,(55°C/131°F), manifold vacuum is directed to the purge control valve, which allows venturi vacuum to draw fresh air through the bottom of the canister into the engine, purging the stored fuel vapor.

CARBURETOR VAPOR CONTROL SYSTEM
The carburetor vapor control system consists of the air vent cut-off diaphragm, the vacuum holding solenoid valve, and inner and outer vents.
Fuel vapor from the float chamber is directed into the charcoal canister through the outer vent passage when the engine is not running and the thermovalve in the air vent cut-off diaphragm is above its set value, (40°C/104°F). If the engine is running, fuel vapor is directed into the air cleaner.

FUEL TANK VAPOR CONTROL SYSTEM
The fuel tank vapor control system consists of a fuel cut-off valve, liquid/vapor separator, a two-way valve, and fuel filler cap. All fuel vapor inside the fuel tank is directed to the charcoal canister through the fuel cut-off valve and the liquid/vapor separator to prevent build up of vapor in the tank which might otherwise escape when the fuel filler cap is removed.
Normal pressures in the fuel tank are regulated by the two-way valve. A secondary relief valve for pressure or vacuum is incorporated in the filler cap in the event the two-way valve fails.