Evaporative Emissions System: Description and Operation

Fig. 2 Evaporative emission control system diagram:

CARBON CANISTER (SYSTEM DIAGRAM)

To prevent the loss of fuel vapor from the fuel system into the atmosphere a closed evaporative emission-control system, Fig. 2 is used to recover and burn fuel system vapors.

Typical charcoal canister:

While the engine is not in operation fuel vapors generated inside the fuel system are absorbed and stored in the carbon canister, Fig. 15. While the engine is running fuel vapors absorbed in the canister are drawn into the intake manifold through the purge-control valve, and an orifice.

Fig. 16 Typical bowl vent valve:

BOWL VENT VALVE

The bowl vent valve (BVV), Fig. 16 opens when intake manifold vacuum working on the diaphragm of the valve exceeds the pre-set value after the ignition key is turned on. Once open, the BVV is kept open by the solenoid even when intake manifold vacuum is equal to atmospheric pressure for as long as the ignition key remains on. While the valve is open vapors created in the carburetor float boal are vented to the carbon canister. When the engine is turned off the valve closes.

Fig. 17 Purge control valve.:

PURGE CONTROL VALVE

The purge control valve, Fig. 17 remains closed during idle to prevent excess HC emissions caused by fuel vapor venting into the intake manifold. When throttle port vacuum exceeds a pre-set level the purge control valve opens venting fuel vapors from the carbon canister into the intake manifold to be burned.

Fuel Filler Cap Operation:

FUEL FILLER CAP

The fuel filler cap, Fig. 41 is equipped with a relief valve to prevent the escape of fuel vapors to the atmosphere and to prevent the colapse of the fuel tank under negative pressure.

Overfill Limiter (Two Way Valve):

OVERFILL LIMITER

The overfill limiter, Fig. 21 consists of a pressure valve and a vacuum valve. The pressure valve is designed to open when the fuel tank internal pressure has increased over normal pressure and the vacuum valve opens when a vacuum has been produced in the tank.

Fuel Check Valve:

FUEL CHECK VALVE

The fuel check valve, Fig. 83 is used to prevent fuel leaks should the car suddenly roll over. This valve is connected in the fuel vapor line (between the canister and overfill limiter) and is mounted on the firewall.

The fuel check valve contains two balls. Under normal conditions, the gasoline vapor passage in the valve is opened, but if roll over occurs one of the balls closes the fuel passage preventing fuel leaks.