Evaporative Emissions System: Description and Operation
GENERAL INFORMATIONThe evaporative emission control system prevents fuel vapors generated in the fuel tank from escaping into the atmosphere.
Fuel vapors from the fuel tank flow through the vapor pipe/hose to be stored temporarily in the evaporative emission (EVAP) canister.
When the vehicle is in operation, fuel vapors stored in the EVAP canister flow through the chamber, the EVAP purge solenoid, purge port and intake manifold plenum to the combustion chamber.
When the engine coolant temperature is low or when the intake air quantity is small (when the engine is at idle, for example), the engine control module brings the EVAP purge solenoid into the OFF state to shut off the fuel vapor flow to the intake manifold plenum. This ensures driveability when the engine is cold or running under low load and also stabilizes the emission level.
An EVAP ventilation solenoid is provided between the EVAP canister and atmosphere to monitor for OBD-II EVAP leaks. This solenoid is normally OFF. However, it turns ON when monitoring the OBD-II EVAP leaks and shuts off the atmosphere flow to the EVAP canister. Then the fuel tank differential pressure sensor monitors the fuel vapor pressure to detect OBD-II EVAP leaks. The fuel vent valve and the leveling valve prevent fuel from being overfilled. The fuel vent valve and the leveling valve valve prevents fuel leaks just if the vehicle is rolled over in an accident.
The vent valve releases the air from the fuel tank through the evaporative emission canister into the atmosphere when the fuel tank pressure increases due to refueling, etc. The vent valve and the air filter supply the atmospheric air to the evaporative emission canister when the fuel tank pressure decreases.
System Diagram:
