Evaporative Emissions System: Description and Operation

Fig. 130 Evaporative System:

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 evaporative emission control system is controlled by the engine control unit (ECU). The system consists of a vapor separator, solenoid valve (purge control), a charcoal cannister, two-way check valve, check-and cut valve, vapor separator, the ECU, and engine sensors.

The ECU monitors operating conditions through various sensors mounted on the engine, and regulates the amount of evaporative gases drawn into the engine by alternately energizing and de-energizing the solenoid valve (duty cycle signal - "ON" time vs. "OFF" time). The ECU energizes the solenoid valve by completing its ground circuit. The system operates when the following conditions are met.

1. Engine is at normal operating temperature.
2. Driving in gear.
3. Accelerator depressed (idle switch off).
4. ECU is in "closed loop" operation (oxygen sensor functioning normally).

CHARCOAL CANNISTER


The charcoal cannister is filled with activated charcoal to condense and store gasoline vapors from the fuel tank when the engine is not running.

FUEL VAPOR SEPARATOR

The fuel/vapor separator mounted outside the fuel tank, separates fuel vapors from liquid fuel to prevent saturation of the charcoal canister. Liquid fuel is drained back to the fuel tank from the bottom of the separator. A hose connected at the top of the separator collects vapors and routes them to the canister through the check and cut valve and two-way check valve.

CHECK AND CUT VALVE

The check and cut valve 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.

SOLENOID VALVE (PURGE CONTROL)

The purge control solenoid valve is a normally closed, electrically operated vacuum solenoid valve. It gets battery power from the main relay when the ignition is turned "ON" and is energized when the ECU completes the circuit to ground. The solenoid operates on a duty cycle ("ON" time vs "OFF" time). The greater the percent duty cycle, the more vapors are purged. It is connected in-line between the PCV valve and the charcoal canister and vents the charcoal cannister to the intake manifold when operating conditions are suitable.