Evaporative Emissions System: Description and Operation

Evaporative Emission Control System:

EVAPORATIVE EMISSION CONTROL SYSTEM
The Evaporative Emission Control System is designed to prevent fuel vapors from being emitted into the atmosphere from either the fuel tank or the carburetor fuel bowl. The system consists of a Pressure/Vacuum Relief Filler Cap, a Rollover Safety Check Valve, a Carburetor [1][2]Bowl Vent Valve, a Charcoal Canister, and connecting lines and hoses. Canister purging is controlled by a temperature sensitive Thermo Valve.

Bowl Vent Valve:

CARBURETOR [1][2]BOWL VENT VALVE
The carburetor must be vented in order to keep atmospheric pressure in the fuel bowl. The pressure differential is needed for precise fuel metering. Also when the engine is turned off, the fuel bowl needs to be vented to the charcoal canister in order to meet evaporative emission standards and relieve any pressure that may build up from heat expansion. A Bowl Vent Valve controlled by manifold vacuum switches the fuel bowl between an internal vent and an external vent.

When the engine is turned off, spring pressure forces the control plunger to close the internal vent passage. Fuel vapors from the carburetor bowl are then routed to the canister through the external vent passage. This helps to prevent the emission of Hydrocarbons into the atmosphere.

When the engine is started, manifold vacuum (1.2" vacuum or more) is applied causing diaphragm B to be pulled in against spring pressure (B) to seat the control valve. Once manifold vacuum exceeds 2.0" vacuum, the control plunger and the seal attached to diaphragm A are pulled in, opening the internal vent passage and closing the external vent passage. The check valve will hold vacuum on momentarily if the intake manifold vacuum should drop suddenly. After the engine is turned off and vacuum drops below 0.4", the spring force on diaphragm B will open the control valve, venting vacuum from diaphragm A. Spring pressure will force the control plunger to seal the internal vent, opening the external vent.

Charcoal Canister, Typical:

CHARCOAL CANISTER
The charcoal canister is located under the hood and is filled with activated charcoal granules, which will hold up to one-third their own weight in fuel vapors. Activated charcoal is used as a vapor trap because of its great surface area. Each gram of activated charcoal has a surface area of 1,100 square meters, or more than a quarter acre. The canister is connected to the fuel tank by a vent line. Within the vent line is a two-way check valve which opens to allow excess fuel tank pressure and vapors into the canister for storage, until the engine is started. During the canister purge cycle fresh air is drawn through the filter at the bottom of the canister, purging the charcoal of fuel vapors.

Fuel Filler Cap:

PRESSURE/VACUUM FILLER CAP
The pressure-vacuum filler cap is designed to hold a slight pressure on the fuel which occurs from normal heat expansion. The evaporation rate decreases under slightly higher pressures, so the cap helps to reduce evaporative emissions. When vacuum in the tank exceeds specifications, the vacuum relief portion of the filler cap will allow atmospheric pressure to enter the tank. If the pressure within the tank exceeds a specified value, the pressure relief valve will open to relieve excess pressure. This would occur when the pressure relief check valve within the fuel vapor line became plugged.

Check Valves:

ROLLOVER AND PRESSURE RELIEF CHECK VALVES
The rollover check valve is installed within the fuel line between the fuel tank and the carburetor. Its primary purpose is to seal off the fuel tank in the event of accidental vehicle rollover. The pressure relief check valve is located within the vapor line between the fuel tank and the charcoal canister. It is designed to vent excess pressure and fuel vapors from the tank into the charcoal canister, when the fuel tank pressure becomes positive and reaches a specified value.

Purge Control System:

PURGE CONTROL VALVE
The purge control valve is spring loaded closed (blocks canister purge). After the thermo valve closes and the throttle is past the idle position, the purge control valve will respond to port vacuum being applied to its vacuum chamber. When the valve opens, manifold vacuum is allowed to purge the charcoal canister of the fuel vapors.




THERMO VALVE
The thermo valve is threaded into the intake manifold and senses the coolant temperature of the engine. Until the engine coolant has warmed up to operating temperature, the thermo valve is open venting ported vacuum to the atmosphere. This improves driveability while the engine is still cold. As the engine temperature increases, the wax pellet within the thermo valve expands until the vent passage is blocked. Port vacuum is then applied to the purge control valve.