Evaporative Emissions System: Description and Operation: Overview

Evaporative Emissions

OVERVIEW
The EVAP (evaporative emission) system reduces the level of hydrocarbons released into the atmosphere by fuel vapor venting from the fuel tank. The system comprises of an EVAP (evaporative emission) canister, an EVAP (evaporative emission) purge valve, interconnecting vapor pipes and, on NAS vehicles, a DMTL pump and filter. The vapor pipes are connected to the system components using quick release connectors.
Fuel vapor is generated by the fuel in the tank and the amount of vapor produced increases as the fuel heats up. Fuel vapor can flow freely to the EVAP (evaporative emission) canister via the tank venting system.
On NAS vehicles the vapor generated in the fuel tank during refueling flows without restriction to the EVAP (evaporative emission) canister.
On all vehicles except NAS, the vapor is restricted in its path to the EVAP (evaporative emission) canister, but can flow freely during the refueling operation to atmosphere via the fuel filler pipe opening.
The vapor passes into the EVAP (evaporative emission) 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.