Evaporative Emissions System: Description and Operation

Evaporative Emission Control System:

SYSTEM DESCRIPTION

The Evaporative Control System consists of the fuel tank and filler cap, a check/relief valve, two roll-over/float valves, a vacuum switching valve, a vent switching valve (carburetor float bowl vent), a ventilation valve, and charcoal canister with integral purge control valve. The system collects fuel vapors, that would otherwise be discharged into the atmosphere, and condenses them in a charcoal canister where they are temporarily stored. When the engine is started, the stored fuel and vapors are atomized with fresh air and directed to the intake to be burned in the engine. This system of vapor recovery helps not only to prevent raw hydrocarbon emissions, but also promotes fuel economy by eliminating fuel loss through evaporation.

Charcoal Canister:

CHARCOAL CANISTER

The charcoal canister is filled with activated charcoal, to condense and store fuel vapors as they enter the canister. The charcoal separates fuel vapors from air, allowing air to escape through the vent while containing the hydrocarbons. It has a valve built onto it to control purging of the fuel vapors. When the engine is not running, the diaphragm spring holds the valve closed, preventing fuel vapor from escaping through the intake system and air cleaner. When the engine is running at normal operating temperature, carburetor ported vacuum is applied to the diaphragm of the purge control valve. When sufficient vacuum is present (indicating the engine is operating under load at a minimum rpm) the purge control valve opens and fuel vapors released from the charcoal (mixed with fresh air) are drawn into the intake manifold and burned along with the normal air/fuel charge.

Check/Relief Valve:

CHECK/RELIEF VALVE

The check/relief valve is installed in-line to the charcoal canister. When the tank vapor pressure exceeds a specified level, the valve opens to relieve the excess pressure and let the vapors into the charcoal canister. When the vapor pressure in the tank is less than atmospheric by a specific value the valve allows ambient air from the canister to enter the tank to restore pressure equilibrium. Stated simply, the valve will open in either direction when the pressure differential is great enough. A slightly greater differential is required to relieve pressure within the tank, than is required to equalize lower than ambient pressures.

Fuel Filler Cap:

FUEL FILLER CAP

The fuel filler cap is designed with a two stage valve to prevent vapor loss. If vapor pressure within the tank is less than atmospheric the vacuum valve will open, allowing ambient air into the tank to equalize pressures. If vapor pressure in the tank exceeds a specific level, the pressure valve will open allowing the excess fuel vapors to escape into the atmosphere. The pressure valve is a safety feature and does NOT open under normal conditions.

Fuel Tank:

FUEL TANK

The fuel tank incorporates vents at the top with special valves to prevent liquid fuel from leaking through the evaporative system in case of overfilling or in the event the vehicle is rolled over in an accident.

Rollover/Float Valve:

ROLL-OVER/FLOAT VALVE

The rollover/float valves are designed to prevent fuel from leaking through the evaporative system in the event the vehicle has turned over in an accident. They also prevent fuel from saturating the evaporative system with liquid fuel in case the tank is over filled.

Vacuum Switching Valve (ECS System):

VACUUM SWITCHING VALVE (CANISTER PURGE)

A vacuum switching valve is used in the vacuum line to the purge control valve. This is a normally closed vacuum valve that opens when the ECM energizes its solenoid. During idle or cold operation the valve is closed (de-energized) preventing purging of the charcoal canister. The ECM will energize the valve, permitting purge, only under certain conditions. Those conditions are:

1. Engine must be at operating temperature.
2. Engine must run for a minimum length of time.
3. Vehicle must be operating above a specified road speed.
4. Throttle must be open greater than a specified angle.

Vent Switching Valve:

VENT SWITCHING VALVE

This is an electrically operated solenoid valve, built into the carburetor. When the engine is not running, the valve closes the float bowl vent to the air cleaner and opens the passage to the charcoal canister. When the engine is running, the valve closes the passage to the charcoal canister to block the flow of ambient air into the carburetor, and opens the float bowl vent to the air cleaner to allow fuel vapors to be drawn into the intake system for combustion.

Ventilation Valve (Operation @ WOT Shown):

VENTILATION VALVE

The ventilation valve is a vacuum operated valve that is used to improve hot fuel handling under wide open throttle conditions. When the engine is hot and the throttle is wide open, especially at high engine speeds, pressure in the float bowl is reduced due to the high velocity of air passing over the bowl vent. This may reduce the amount of fuel delivered through the main metering circuit. The ventilation valve opens the float bowl to ambient air from the charcoal canister, maintaining constant pressure in the float bowl for accurate fuel metering.