Discovery 2. Manual — part 54

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

17-2-12 DESCRIPTION AND OPERATION

Fuel metering
For a satisfactory combustion process, precise fuel injection quantity, timing and dispersion must be ensured. If the
air:fuel mixture in the combustion chamber is not thoroughly atomized and dispersed during the combustion stroke,
some of the fuel may remain unburnt which will lead to high HC emissions.

Ignition timing
The ignition timing can be changed to minimise exhaust emissions and fuel consumption in response to changes due
to the excess air factor. As the excess air factor increases, the optimum ignition angle is advanced to compensate for
delays in flame propagation.

Exhaust Emission Control Components
The exhaust emission control components are described below:

Catalytic converter

1 Exhaust gas from manifold
2 Cleaned exhaust gas to tail pipe
3 Catalytic converter outer case

4 1st ceramic brick
5 2nd ceramic brick
6 Honeycomb structure

The catalytic converters are located in each of the front pipes from the exhaust manifolds.

The catalytic converter's housings are fabricated from stainless steel and are fully welded at all joints. Each catalytic
converter contains two elements comprising of an extruded ceramic substrate which is formed into a honeycomb of
small cells with a density of 62 cells / cm

2

. The ceramic element is coated with a special surface treatment called

'washcoat' which increases the surface area of the catalyst element by approximately 7000 times. A coating is applied
to the washcoat which contains the precious elements Platinum, Palladium and Rhodium in the following relative
concentrations: 1 Pt : 21.6 PD : 1 Rh

EMISSION CONTROL - V8

DESCRIPTION AND OPERATION 17-2-13

Catalytic converters for NAS low emission vehicles (LEVs) from 2000MY have active constituents of
palladium and rhodium only. The active constituents are 14PD: 1Rh and the palladium coating is used to
oxidise the carbon monoxide and hydrocarbons in the exhaust gas.

The metallic coating of platinum and palladium oxidize the carbon monoxide and hydrocarbons and convert them into
water (H

2

O) and carbon dioxide (CO

2

). The coating of rhodium removes the oxygen from nitrogen oxide (NO

x

) and

converts it into nitrogen (N

2

).

CAUTION: Catalytic converters contain ceramic material, which is very fragile. Avoid heavy impacts on the
converter casing.

Downstream of the catalytic converters, the exhaust front pipes merge into a single pipe terminating at a flange joint
which connects to the exhaust intermediate pipe.

WARNING: To prevent personal injury from a hot exhaust system, do not attempt to disconnect any
components until the exhaust system has cooled down.

CAUTION: Serious damage to the catalytic converter will occur if leaded fuel is used. The fuel tank filler neck
is designed to accommodate only unleaded fuel pump nozzles.

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

Heated Oxygen Sensor (HO2S)

1 Connection cable
2 Disc spring
3 Ceramic support tube
4 Protective sleeve
5 Clamp connection for heating element
6 Heating element
7 Contact element

8 Sensor housing
9 Active sensor ceramic

10 Protective tube
11 Post-catalytic converter sensor

NAS spec. only)

12 Pre-catalytic converter sensor

The heated oxygen sensor is an integral part of the exhaust emission control system and is used in conjunction with
the catalytic converters and the engine management control unit to ensure that the air:fuel mixture ratio stays around
the stoichiometric point of

λ

= 1, where the catalytic converters are most effective. Combinations of four (NAS only)

or two heated lambda sensors are used in the exhaust system dependent on market legislation.

EMISSION CONTROL - V8

17-2-14 DESCRIPTION AND OPERATION

The heated oxygen sensor is screwed into threaded mountings welded into the top of the front exhaust pipes at
suitable locations. They are used to detect the level of residual oxygen in the exhaust gas to provide an instantaneous
indication of whether combustion is complete. By positioning sensors in the stream of exhaust gases from each
separate bank of the exhaust manifold, the engine management system is better able to control the fuelling
requirements on each bank independently of the other, so allowing much closer control of the air:fuel ratio and
optimising catalytic converter efficiency.

Two pre-catalytic converter heated oxygen sensors are mounted in the front pipes for monitoring the oxygen content
of the exhaust gas. NAS models also have two additional post-catalytic converter heated oxygen sensors in the
exhaust front pipe.

CAUTION: HO2 sensors are easily damaged by dropping, over torquing, excessive heat or contamination.
Care must be taken not to damage the sensor housing or tip.

The oxygen sensors consist of a ceramic body (Galvanic cell) which is a practically pure oxygen-ion conductor made
from a mixed oxide of zirconium and yttrium. The ceramic is then coated with gas-permeable platinum, which when
heated to a sufficiently high temperature (

≥

350

°

C) generates a voltage which is proportional to the oxygen content

in the exhaust gas stream.

The heated oxygen sensor is protected by an outer tube with a restricted flow opening to prevent the sensor's
ceramics from being cooled by low temperature exhaust gases at start up. The post-catalytic sensors have improved
signal quality, but a slower response rate.

The pre-catalytic and post-catalytic converter sensors are not interchangeable, and although it is possible to mount
them in transposed positions, their harness connections are of different gender and colour. It is important not to
confuse the sensor signal pins; the signal pins are gold plated, whilst the heater supply pins are tinned,
mixing them up will cause contamination and adversely affect system performance.

Each of the heated oxygen sensors have a four pin connector with the following wiring details:

l

Sensor signal ground (grey wire – connects to engine management ECM)

l

Sensor signal (black wire – connects to engine management ECM)

l

Heater drive (white wire – connects to engine management ECM)

l

Heater supply (white wire – connects to fuse 2, underbonnet fuse box)

The ECM connector pins for exhaust emission control are listed in the following table:

ECM Connector 2 (C635) pin-out details for exhaust emission control system

The heated oxygen sensors should be treated with extreme care, since the ceramic material within them can be easily
cracked if dropped, banged or over-torqued; the sensors should be torqued to the recommended values indicated in
the repair procedures. Apply anti-seize compound to the sensor's threads when refitting.

WARNING: Some types of anti-seize compound used in service are a health hazard. Avoid skin contact.

WARNING: To prevent personal injury from a hot exhaust system, do not attempt to disconnect any
components until the exhaust system has cooled down.

CAUTION: Do not allow anti-seize compound to come into contact with tip of sensor or enter exhaust system.

NOTE: A new HO2 sensor is supplied pre-treated with anti-seize compound.

Pin Number

Function

Signal Type

Control

2-01

Post-cat sensor heater (RH) - NAS only

Output, Drive

PWM, 12 - 0V

2-07

Post-cat sensor heater (LH) - NAS only

Output, Drive

PWM, 12 - 0V

2-08

Post-cat sensor (RH) - NAS only

Ground, Signal

0V

2-09

Pre-cat sensor (LH)

Ground, Signal

0V

2-10

Pre-cat sensor (RH)

Ground, Signal

0V

2-11

Post-cat sensor (LH) - NAS only

Ground, Signal

0V

2-13

Pre-cat sensor heater (RH)

Output, Drive

PWM, 12 - 0V

2-14

Post-cat sensor (RH) - NAS only

Input, Signal

Analogue, 0 - 1V

2-15

Pre-cat sensor (LH)

Input, Signal

Analogue, 0 - 1V

2-16

Pre-cat sensor (RH)

Input, Signal

Analogue, 0 - 1V

2-17

Post-cat sensor (LH) - NAS only

Input, Signal

Analogue, 0 - 1V

2-19

Pre-cat sensor heater (LH)

Output, Drive

PWM, 12 - 0V