Emission control - K Series KV6 (EVAP Control): Description and operation

The correct operation of the catalytic converters is dependent upon close control of the oxygen content of the exhaust gas. The quantity of oxygen in the exhaust gas is monitored by the Engine Control Module (ECM) using an input from the Heated Oxygen Sensor (HO2S) upstream of the catalytic converters. The ECM also monitors the condition of the catalytic converters using an input from the HO2S downstream of the catalytic converters.

EVAP Control

The EVAP control system reduces the level of hydrocarbons released into the atmosphere by fuel vapour 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: l A two way valve. l A vapour separator. l An EVAP canister. l A purge valve. l A Diagnostic Module for Tank Leakage (DMTL) (NAS only). l An air filter (NAS only). l Interconnecting vent pipes.

The EVAP control system is connected to the Onboard Refuelling Vapour 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.

Vapour vented from the fuel tank passes through the EVAP control system to atmosphere. The EVAP canister absorbs fuel from the vapour 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 vapour 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 vapour 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.