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Discovery II

EMISSION CONTROL - V8 > DESCRIPTION AND OPERATION > Exhaust Emission Control System

Page 244
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EMISSION CONTROL - V8

DESCRIPTION AND OPERATION 17-2-11

Exhaust Emission Control System

The fuel injection system provides accurately metered quantities of fuel to the combustion chambers to ensure the 
most efficient air to fuel ratio under all operating conditions. A further improvement to combustion is made by 
measuring the oxygen content of the exhaust gases to enable the quantity of fuel injected to be varied in accordance 
with the prevailing engine operation and ambient conditions;  any unsatisfactory composition of the exhaust gas is 
then corrected by adjustments made to the fuelling by the ECM.

The main components of the exhaust emission system are two catalytic converters which are an integral part of the 
front exhaust pipe assembly. The catalytic converters are included in the system to reduce the emission to 
atmosphere of carbon monoxide (CO), oxides of nitrogen (NO

x

) and hydrocarbons (HC). The active constituents of 

the catalytic converters are platinum (Pt), palladium (PD) and rhodium (Rh). Catalytic converters for NAS low 
emission vehicles (LEVs) from 2000MY have active constituents of palladium and rhodium only. 
The correct 
functioning of the converters is dependent upon close control of the oxygen concentration in the exhaust gas entering 
the catalyst. 

The two catalytic converters are shaped differently to allow sufficient clearance between the body and transmission, 
but they remain functionally identical since they have the same volume and use the same active constituents.

The basic control loop comprises the engine (controlled system), the heated oxygen sensors (measuring elements), 
the engine management ECM (control) and the injectors and ignition (actuators). Other factors also influence the 
calculations of the ECM, such as air flow, air intake temperature and throttle position. Additionally, special driving 
conditions are compensated for, such as starting, acceleration, deceleration, overrun and full load.

The reliability of the ignition system is critical for efficient catalytic converter operation, since misfiring will lead to 
irreparable damage of the catalytic converter due to the overheating that occurs when unburned combustion gases 
are burnt inside it.

CAUTION: If the engine is misfiring, it should be shut down immediately and the cause rectified. Failure to do 
so will result in irreparable damage to the catalytic converter.

CAUTION: Ensure the exhaust system is free from leaks. Exhaust gas leaks upstream of the catalytic 
converter could cause internal damage to the catalytic converter.

CAUTION: Serious damage to the engine may occur if a lower octane number fuel than recommended is used. 
Serious damage to the catalytic converter and oxygen sensors will occur if leaded fuel is used.

Air : Fuel Ratio
The theoretical ideal air:fuel ratio to ensure complete combustion and minimise emissions in a spark-ignition engine 
is 14.7:1 and is referred to as the stoichiometric ratio.

The excess air factor is denoted by the Lambda symbol 

λ

, and is used to indicate how far the air:fuel mixture ratio 

deviates from the theoretical optimum during any particular operating condition.

l

When 

λ

 = 1, the air to fuel ratio corresponds to the theoretical optimum of 14.7:1 and is the desired condition for 

minimising emissions.

l

When 

λ

 > 1, (i.e. 

λ

 = 1.05 to 

λ

 = 1.3) there is excess air available (lean mixture) and lower fuel consumption can 

be attained at the cost of reduced performance. For mixtures above 

λ

 = 1.3, the mixture ceases to be ignitable.

l

When 

λ

 < 1, (i.e. 

λ

 = 0.85 to 

λ

 = 0.95) there is an air deficiency (rich mixture) and maximum output is available, 

but fuel economy is impaired.

The engine management system used with V8 engines operates in a narrower control range about the stoichiometric 
ideal between 

λ

 = 0.97 to 1.03 using closed-loop control techniques. When the engine is warmed up and operating 

under normal conditions, it is essential to maintain 

λ

 close to the ideal (

λ

 = 1) to ensure the effective treatment of 

exhaust gases by the three-way catalytic converters installed in the downpipes from each exhaust manifold.

Changes in the oxygen content has subsequent effects on the levels of exhaust emissions experienced. The levels 
of hydrocarbons and carbon monoxide produced around the stoichiometric ideal control range are minimised, but 
peak emission of oxides of nitrogen are experienced around the same range.

EMISSION CONTROL - V8 > DESCRIPTION AND OPERATION > Exhaust Emission Control System

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