Evaporative Emissions System: Description and Operation

Fig. 92 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. 92 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. 32 Bowl vent valve:

BOWL VENT VALVE

The bowl vent valve (BVV), Fig. 32 controls the venting of the carburetor float bowl.

Fig. 33 Bowl vent valve operation (engine off):

1. While the engine is not running spring force closes the inner vent passage, Fig. 33 and fuel vapors flow to the canister through through the outer vent passage.

Fig. 34 Bowl vent valve operation (vacuum > 1.2 in. Hg.):

2. When the engine is running and intake manifold vacuum reaches more than 1.2 in. Hg (4 kPa, 0.6 psi), diaphragm B, Fig. 34 is pulled to the right seating the control valve.

Fig. 35 Bowl vent valve operation (vacuum > 2.0 in. Hg.):

3. If intake manifold vacuum exceeds 2.0 in Hg (6.7 kPa, 1.0 psi), diaphragm A is pulled to the left and the inner float bowl passage is opened, Fig. 35. If intake manifold vacuum decreases temporarilly while the engine is operating the check valve will maintain 2.0 in Hg (6.7 kPa, 1.0 psi) of vacuum to hold the inner float bowl passage open.

Fig. 36 Bowl vent valve operation (vacuum < 0.4 in. Hg.):

4. When the engine is turned off intake manifold vacuum falls below 0.4 in. Hg (1.3 kPa, 0.2 psi). Diaphragm B, Fig. 36 moves to the left opening the control valve venting the vacuum on diaphragm A. The BVV then moves to the right isolating the carburetor inner vent passage from the atmosphere.

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.

Fig. 94 Thermo valve:

THERMO VALVE

A thermo valve, Fig. 94 is incorporated in the EGR system for sensing the coolant temperature at the intake manifold. The thermo valve prevents EGR flow until the engine warms up to prevent drivability problems. The thermo valve also holds the purge control valve closed while the coolant temperature is lower than a pre-set value to reduce CO and HC emissions during engine warm-up. Once the engine warms up the thermo valve allows the purge control valve to open.