Evaporative Emissions System: Description and Operation: Overview

Evaporative Emissions

OVERVIEW
The EVAP (evaporative emission) control system reduces the level of hydrocarbons released into the atmosphere by fuel vapor venting from the fuel tank. The system comprises a charcoal canister, a purge valve and interconnecting pipes and hoses. The vent pipes are connected to the system components using quick release connectors.
On NAS market vehicles, the system is sealed and contains a Diagnostic Monitoring Tank Leakage (DML) pump to pressurize the system and check for leakage.
Fuel vapor is generated in the fuel tank and the amount of fuel vapor increases as the fuel temperature increases. Fuel vapor can flow freely to atmosphere or where fitted, to the charcoal canister via the tank vent system. The venting system comprises of a liquid vapor separator in the top of the tank which is connected via hoses and the fuel pump module to the charcoal canister. The vent hose from the fuel pump module allows hydrocarbon fuel vapor to flow to the charcoal canister.
On ROW vehicles, the vapor is restricted in its path to the charcoal canister but can flow freely during the fuel filling operation to atmosphere, via the filler opening.
On NAS vehicles, the vapor generated in the fuel tank during refueling flows without restriction to the charcoal canister.
The vapor from the fuel tank is passed from the vapor separator into the charcoal canister where it is absorbed and stored by the charcoal. Because there is a limit to the amount of vapor the canister can contain, the fuel vapor is purged from the canister when the engine is running and burned in the engine during the combustion cycle.