EVAP Control

EMISSION CONTROL - K SERIES KV6

DESCRIPTION AND OPERATION

17-3-5

In the catalytic converters the exhaust gases are passed through honeycombed ceramic elements coated with a 
special surface treatment called 'washcoat'. The washcoat increases the surface area of the ceramic elements by a 
factor of approximately 7000. On top of the washcoat is a coating containing the elements which are the active 
constituents for converting harmful emissions into inert by-products. The active constituents consist of platinum and 
rhodium. Platinum adds oxygen to the carbon monoxide and the hydrocarbons in the exhaust gases, to convert them 
into carbon dioxide and water respectively. The rhodium removes oxygen from the Nitrous Oxides (NOx) to convert 
them into nitrogen.

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. 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).

l

An air filter.

l

Interconnecting vent pipes.

The EVAP control system is connected to the Onboard Refuelling Vapour Recovery (ORVR) valve 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.

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.

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.

The DMTL periodically checks the EVAP control system and fuel tank for leaks when the ignition is switched off.

On 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 vapour 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 litres additional fuel 
to be added to the fuel tank.

The fuel tank on 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 vapour produced during refuelling and the subsequent load on the EVAP 
canister.