Evaporative Emissions System: Description and Operation

Fig. 134 Evaporative System:

DESCRIPTION AND OPERATION
The system is controlled by the ECU and consists of a purge control solenoid valve, charcoal canister, two-way check valve, check-and-cut valve, and vapor separator.

When the engine is not running, the evaporative system collects fuel vapors that would otherwise be vented to atmosphere. Vapors are temporarily stored in a charcoal canister. When the engine is started, stored vapors are directed to the intake to be burned in the engine. The system prevents raw hydrocarbon emissions and promotes fuel economy by eliminating fuel loss through evaporation.

The ECU monitors operating conditions 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 system operates when the following conditions are met.

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


CHARCOAL CANISTER
The canister is filled with activated charcoal to condense and store fuel vapors when the engine is not running.

CHECK AND CUT VALVE (THREE WAY CHECK VALVE)
The check and cut valve has three functions:

^ To prevent fuel tank damage, vents tank to atmosphere if internal pressure exceeds approximately 0.78 - 1.0 psi (5.37 - 6.87 kPa).
^ To prevent tank damage, allows air from atmosphere to enter tank if internal pressure is approximately 0.14 - 0.71 psi (0.98 - 4.91 kPa) less than atmospheric pressure.
^ Blocks fuel from leaking through evaporative system in event of vehicle rollover.

ENGINE CONTROL UNIT
ECU monitors signals from input sensors and switches and controls purge control solenoid valve.

FUEL VAPOR VALVE
The fuel vapor valve, installed in top of fuel tank, prevents liquid fuel from entering evaporative control system and saturating the charcoal canister if fuel tank is overfilled.

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 PCV valve and charcoal canister, venting the charcoal canister when operating conditions are suitable.

TWO-WAY CHECK VALVE
Valve prevents fuel vapors stored in canister from returning to the fuel tank.