Evaporative Emissions System: Description and Operation

Evaporative System Schematic:

PURPOSE
When the engine is not running, the evaporative system collects fuel vapors that would otherwise be discharged into the atmosphere and temporarily stores them in a charcoal canister. When the engine is started, stored vapors are mixed with fresh air and directed to the intake to be burned in the engine. This system helps prevent raw hydrocarbon emissions and aids fuel economy by eliminating fuel loss through evaporation.

The system is controlled by the Powertrain Control Module (PCME). The system consists of solenoid valve (purge control), charcoal canister, two-way check valve, three-way check valve, vapor separator, the PCME, and engine sensors.

OPERATION
The PCME 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).

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

THREE-WAY CHECK VALVE
To prevent fuel tank damage, the three-way check valve vents it to atmosphere if tank pressure exceeds a specified amount.

SOLENOID VALVE (PURGE CONTROL)
This 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 PCME 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 vents the charcoal canister to the intake manifold when operating conditions are suitable.

FUEL/VAPOR VALVE
The fuel/vapor valves close off the evaporative system when inverted, preventing fuel from leaking out through the system if the vehicle is overturned.