EVAP Control: Notes
The EVAP control system reduces the level of hydrocarbons released into the atmosphere by fuel vapor venting from the fuel tank.
On NAS models, a positive pressure leak detection function is incorporated to monitor the integrity of the system. The EVAP control system comprises:
- A two way valve.
- A vapor separator.
- An EVAP canister.
- A purge valve.
- A Diagnostic Module for Tank Leakage (DMTL) (NAS only).
- An air filter (NAS only).
- Interconnecting vent pipes.
The EVAP control system is connected to the Onboard Refuelling Vapor Recovery (ORVR) valve (NAS only) and/or the roll over valves in the fuel tank. The ORVR valve and the roll over valves are float valves that allow inward and outward venting of the fuel tank, but prevent the escape of fuel into the vent pipes due to fuel slosh or if the vehicle overturns. The ORVR valve is normally closed when the fuel tank is full and normally open at all other fuel levels. The roll over valves are normally open at all fuel levels.
On all except NAS vehicles, venting occurs through the roll over valves and the two-way valve with changes of tank pressure.
On NAS vehicles, when the fuel tank is less than full, venting is unrestricted through the ORVR valve. Only when the fuel tank is full does venting occur, with changes of tank pressure, through the roll over valves and the two-way valve.
Vapor vented from the fuel tank passes through the EVAP control system to atmosphere. The EVAP canister absorbs fuel from the vapor and relatively fuel free air vents to atmosphere. Since there is a limit to the storage capacity of the EVAP canister, when the engine is running fuel is purged from the EVAP canister and burned in the engine.
On NAS vehicles, to reduce the load on the EVAP canister during refuelling, a proportion of the air expelled from the tank is recirculated through a pipe connected between the top of the vapor separator and the filler tube. The recirculation flow is induced by fuel in the filler tube flowing past a restrictor installed in the recirculation pipe connection on the filler tube. With the recirculation flow present, less fresh air enters the tank, which reduces the volume of vapor generated and fuel deposited in the EVAP canister.
On NAS vehicles, the DMTL periodically checks the EVAP control system and fuel tank for leaks when the ignition is switched off.
On non-NAS vehicles from 2002.5 model year - Modifications are introduced to increase the capacity of the fuel tank. The modification comprises a reduction in height of the internal tube of the Roll Over Valves (ROV). This modification reduces the air gap at the top of the fuel tank, therefore allowing for additional fuel to be added to the tank before the refuelling nozzle shuts off.
On NAS vehicles from 2002.5 model year - Modifications are introduced to increase the capacity of the fuel tank. The modification comprises a change to the vent line from the forward Roll Over Valve (ROV). The vent from the ROV now connects to the vent line between the two-way valve and the vapor separator. Venting from the forward ROV is no longer restricted by the two-way valve. The ROV now controls the refuelling nozzle shut-off. When the ROV closes, pressure in the tank increases, shutting off the refuelling nozzle. This modification allows up to 5 liters additional fuel to be added to the fuel tank.
The fuel tank on NAS vehicles from 2002.5 model year also incorporates a new fabric sleeve over the filler pipe inlet in the fuel tank. The sleeve reduces the amount of vapor produced during refuelling and the subsequent load on the EVAP canister.