Emission Control for Land Rover V8 Engine in Range Rover P38
EMISSION CONTROL
11
DESCRIPTION AND OPERATION
The HO
2
S sensor needs to operate at high
temperatures in order to function correctly (350
°
C
(662
°
F)). To achieve this, the sensors have integral
heater elements, controlled by a pulse width
modulated (PWM) signal from the ECM. The heater
element warms the sensor’s ceramic layer from the
inside so that the sensor is hot enough for operation.
The heater elements are supplied with current
immediately following engine start and are ready for
closed loop control within 20 to 30 seconds (longer at
cold ambient temperatures less than 0
°
C (32
°
F)).
Heating is also necessary during low load conditions
when the temperature of the exhaust gases is
insufficient to maintain the required sensor
temperatures. The maximum tip temperature is 930
°
C
(1706
°
F).
A non-functioning heater element will delay the
sensor’s readiness for closed loop control and
influences emissions. A diagnostic routine is utilised to
measure both sensor heater current and the heater
supply voltage so its resistance can be calculated.
The function is active once per drive cycle, as long as
the heater has been switched on for a pre-defined
period and the current has stabilised. The PWM duty
cycle is carefully controlled to prevent thermal shock
to cold sensors.
The heated oxygen sensors age with mileage,
causing an increase in the response time to switch
from rich to lean and lean to rich. This increase in
response time influences the closed loop control and
leads to progressively increased emissions. The
response time of the pre-catalytic converter sensors
are monitored by measuring the period of rich to lean
and lean to rich switching. The ECM monitors the
switching time, and if the threshold period is
exceeded, the fault will be detected and stored in the
ECM as a fault code (the MIL light will be illuminated
on NAS vehicles). NAS vehicle engine calibration
uses downstream sensors to compensate for aged
upstream sensors, thereby maintaining low emissions.
Diagnosis of electrical faults is continuously monitored
for both the pre-catalytic converter sensors and the
post-catalytic converter sensors (NAS only). This is
achieved by checking the signal against maximum
and minimum thresholds for open and short circuit
conditions. For NAS vehicles, if the pre- and
post-catalytic sensors are inadvertently transposed,
the lambda signals will go to maximum but opposite
extremes and the system will automatically revert to
open loop fuelling. The additional sensors for NAS
vehicles provide mandatory monitoring of catalyst
conversion efficiency and long term fuelling
adaptations.
Failure of the closed loop control of the exhaust
emission system may be attributable to one of the
failure modes indicated below:
•
Mechanical fitting and integrity of the sensor.
•
Sensor open circuit / disconnected.
•
Short circuit to vehicle supply or ground.
•
Lambda ratio outside operating band.
•
Crossed sensors.
•
Contamination from leaded fuel or other sources.
•
Change in sensor characteristic.
•
Harness damage.
•
Air leak into exhaust system (cracked pipe / weld
or loose fixings).
System failure will be indicated by the following
symptoms:
•
MIL light on (NAS only).
•
Default to open-loop fuelling for the defective
cylinder bank.
•
If sensors are crossed, engine will run normally
after initial start and then become progressively
unstable with one bank going to its maximum
rich clamp and the other bank going to its
maximum lean clamp - the system will then
revert to open-loop fuelling.
•
High CO reading.
•
Strong smell of H
2
S (rotten eggs).
•
Excessive emissions.
See FUEL SYSTEM, Description and operation.
