Evaporative Emissions System: Description and Operation

Evaporative Emission Control:

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 escaping from the carburetor and fuel tank into the atmosphere. When the vehicle is not running, fuel in the fuel tank and carburetor 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 is made up of a canister with a charcoal filter, a tank pressure control valve, a vent switching valve, a canister purge valve, a thermal vacuum valve and connecting hoses.

Purge Control Valve Operation:

CHARCOAL CANISTER

The charcoal canister is filled with activated carbon. This carbon stores up fuel vapors coming from the carburetor and the fuel tank.

The canister also has a purge control valve. This valve serves to purge fuel vapor from the activated carbon so it can be delivered to the intake manifold. The purge control valve diaphragm is activated by carburetor vacuum. When the engine is not running, the diaphragm spring holds the purge control valve closed.

When the engine is running, carburetor ported vacuum is applied to the diaphragm. If the vacuum exceeds the specified value of the spring, the diaphragm opens the purge control valve. The fuel vapor from the activated carbon is then delivered to the intake manifold.

Tank Pressure Control Valve:

TANK PRESSURE CONTROL VALVE

When the engine is not running, fuel vaporizes inside the fuel tank. This vaporization increases the pressure inside the tank. Once the pressure raises to a pre-calibrated level, it flows through the by-pass orifice of the tank pressure control valve and is stored in the vapor canister. This valve is held closed by a calibrated spring so that fuel does not drain into the vapor canister.

Should a negative pressure develop in the fuel tank, ambient air will flow through the charcoal canister and by-pass orifice into the fuel tank.

VENT SWITCHING VALVE

The Vent Switching Valve is an electrically operated solenoid valve built into the carburetor. When the engine is not running, the valve opens to allow fuel vapor from the carburetor bowl to enter the charcoal canister. When the engine is running, the valve closes to block the flow of ambient air from going into the carburetor through the vapor canister.

Thermal Vacuum Valve:

THERMAL VACUUM VALVE (TVV)

The Thermal Vacuum Valve controls when the source vacuum is allowed to flow to either the canister purge control valve, and the EGR control valve. Vacuum is allowed to flow to any of the previously mentioned components based on the temperature of the engine. Below a certain temperature full vacuum is allowed to flow through the Thermal Vacuum Valve (TVV), after the engine reaches normal operating temperature, the TVV switches and shuts "OFF" all vacuum to any components.