Evaporative Emissions System: Description and Operation

EVAPORATIVE SYSTEM OPERATION

The evaporative emission system employed on this vehicle is of the charcoal canister design. This system is designed to prevent gasoline vapor emissions, from the tank or the carburetor, from escaping to the atmosphere. When the vehicle is not running, fuel in the fuel tank evaporates, and the vapors travel through vent hoses or tubes to be stored in the charcoal canister. Once the vehicle is running, these vapors are drawn into the engine and consumed with combustion gases.

The system consists of 2 Charcoal Canisters, an Electric [1][2]Bowl Vent Valve, a Pressure Vacuum Filler Cap, and a Thermal Bowl Vent Valve.

CHARCOAL CANISTERS

When the engine is not running, and fuel starts to evaporate in the carburetor, the vapors pass through the vapor hoses to the primary canister where they are held until they are drawn into the intake manifold when the engine is running. As vapors are formed in the fuel tank, they pass through the vapor hoses to the secondary canister where they are held until they are drawn into the intake manifold when the engine is running.

Any liquid fuel goes into a reservoir in the bottom of the canister to protect the carbon bed above. These vapors are absorbed into the carbon. The canister is purged when the engine is running at idle and off idle by utilizing a dual source of vacuum. Ambient air is allowed to enter the canister through the air tube in the top. The air mixes with the vapor and is drawn into the intake manifold.

ELECTRIC [1][2]BOWL VENT VALVE

The electric [1][2]bowl vent valve is located between the carburetor fuel bowl and the charcoal canister and air cleaner assembly. When the engine is not running the valve is de-energized, allowing for fuel vapors to vent from the carburetor bowl to the charcoal canister and the air cleaner assembly.

When the engine is running the valve is energized, which allows fuel vapors only to vent from the carburetor bowl to the air cleaner and not to the charcoal canister.

Pressure-Vacuum Filler Cap:

PRESSURE VACUUM FILLER CAP

The fuel tank is sealed with a specially engineered pressure/vacuum relief filler cap. The relief valves in the cap are a safety feature, and operate only to prevent excessive pressure or vacuum in the tank caused by malfunction in the system or damage to the vent lines.

THERMAL [1][2]BOWL VENT VALVE (LIGHT DUTY)

The thermal [1][2]bowl vent valve is located in between the carburetor and the charcoal canister. The purpose of this valve is to improve cold startability in cold ambient temperatures.

The thermal [1][2]bowl vent valve is designed to remain closed until a specified temperature has been reached. While this valve is closed, fuel vapors are trapped within the carburetor bowl. These vapors will condense to liquid in the bowl which provides more fuel for the next cold start. Once above the specified temperature, the thermal bowl vent valve opens allowing fuel vapor to flow from the carburetor bowl to the charcoal canister.

Vapor Containing Air Cleaner Assembly:

VAPOR CONTAINMENT AIR CLEANER ASSEMBLY (LIGHT DUTY)

The system components are: vapor containing door, door actuator, and vapor containing delay valve. This system is located on the air cleaner snorkel between the heated inlet air system door, and the air filter element.

This systems purpose is to reduce hydrocarbon (HC) emissions. It does this by closing the trap door, 40 seconds after the engine has been shut off, so that no fuel vapors can escape. When the engine is started again, the door opens about 20°, and since it is hinged at the top, it opens further once the air flow through the snorkel increases in velocity.