Evaporative Emissions System: Description and Operation

Evaporative Emission Control System (EVAP):

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.

NORMAL DRIVING
1. Engine is at normal operating temperature (coolant temperature above 140°F / 60°C).
2. Driving in gear.
3. Accelerator depressed (idle switch off).
4. ECU is in "closed loop" operation (oxygen sensor functioning normally).

DECELERATION
1. Idle switch is "ON"
2. Coolant temperature above 60°C (140°F)
3. Engine speed is above 3500 RPM
4. Transmission is in gear
5. Intake air temperature sensor is below 75°C (167°F)

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.

PURGE CONTROL SOLENOID VALVE

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.

TWO WAY CHECK VALVE AND CHECK AND CUT VALVE

The two way check valve prevents fuel vapors stored in the charcoal canister from returning to the fuel tank.

PCV VALVE

The PCV valve is operated by intake manifold vacuum. When the engine is running at idle, the PCV valve is slightly opened and a small amount of blow-by gas is drawn into the combustion chamber. At higher speeds, the PCV valve is further opened and a larger amount of blow-by gas is drawn into the combustion chamber.