Evaporative Emissions System: Description and Operation

Fig. 79 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. 79 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. 19 Purge control valve.:

PURGE CONTROL VALVE

The purge control valve, Fig. 19 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.

Fig. 21 Relief valve fuel filler cap:

FUEL FILLER CAP

The fuel filler cap, Fig. 21 is equipped with a vacuum relief valve to prevent the escape of fuel vapor into the atmosphere as well as preventing the colapse of the fuel tank under negative pressure.

If the pressure in the fuel tank drops below -4.413 +/- 1.275 kPa (-.640 +/- .185 psi) the valve will open to relieve the 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.