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, purge valve and interconnecting vent pipes and hoses. The vent 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 charcoal canister via the tank venting system. The venting system consists of roll over valves and a liquid vapor separator mounted internally in the tank and then externally via a breather line. The breather line allows the fuel vapor to flow to the charcoal canister via a 'Y' piece mounted on the filler pipe.
On NAS vehicles the vapor generated in the fuel tank during refueling flows without restriction to the charcoal canister.
On all vehicles except NAS, the vapor is restricted in its path to the charcoal canister but can flow freely during the refueling operation to atmosphere, via the filler opening.
The vapor passes 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.

All NAS vehicles have a fuel tank leakage monitoring system. This is a legislative requirement. The fuel tank leakage monitoring system periodically checks the EVAP (evaporative emission) system and the fuel tank for leaks when the ignition is switched off. The fuel tank leakage monitoring system comprises the previously described components of the EVAP (evaporative emission) system with the addition of a fuel tank leakage monitoring pump and a fuel tank leakage monitoring filter.