Evaporative Emissions System: Description and Operation

Crankcase And Evaporative Emission Control System (PCV & EVAP):

SYSTEM OPERATION

When the engine is not running the evaporative system collects fuel vapors that would otherwise be discharged into the atmosphere, and condenses them in a charcoal canister where they are temporarily stored. When the engine is started, the stored fuel and vapors are atomized with fresh air and directed to the intake to be burned in the engine. This system of vapor recovery helps not only to prevent raw hydrocarbon emissions, but also promotes fuel economy by eliminating fuel loss through evaporation.

The system consists of the "check and cut" valve, charcoal canister, purge control valve, and one way check valve.


CHARCOAL CANISTER
The charcoal canister is filled with activated charcoal to condense and store fuel vapors from the tank until they can be drawn into the engine to be burned.

PURGE CONTROL VALVE
The purge control valve is a vacuum operated valve. When the engine is operating at part throttle, venturi ported vacuum opens the purge control valve, which allows vapors from the charcoal canister and crankcase into the intake system to be burned with the air/fuel charge.

CHECK VALVE
The check valve operates similar to a PCV valve, except that it is not connected to manifold vacuum. The check valve vents crankcase pressure to the intake system between the air flow meter and throttle body. It opens when the case pressure is greater than atmospheric and the purge control valve is closed.

Check And Cut Valve:

CHECK AND CUT VALVE

The check and cut valve, located on the fuel tank, has three functions:

1. Vents the fuel tank to the atmosphere if the pressure in the tank exceeds approx. 0.78 - 1.0 psi (5.37 - 6.87 kPa), to prevent fuel tank damage.

2. Allows air from the atmosphere to enter the tank, if the pressure in the tank is approx. 0.14 - 0.71 psi (0.98 - 4.91 kPa) less than atmospheric, to prevent fuel tank damage.

3. Blocks fuel from leaking through the evaporative system in the event the vehicle is overturned.