Exhaust emission control operation (Emission control - V8) (page 2 of 2)

Failure of the closed loop control of the exhaust emission system may be attributable to one of the failure modes indicated below: l Mechanical fitting & integrity of the sensor. l Sensor open circuit / disconnected. l Short circuit to vehicle supply or ground. l Lambda ratio outside operating band. l Crossed sensors. l Contamination from leaded fuel or other sources. l Change in sensor characteristic. l Harness damage. l Air leak into exhaust system (cracked pipe / weld or loose fixings).

System failure will be indicated by the following symptoms: l MIL light on (NAS and EU-3 only). l Default to open-loop fuelling for the defective cylinder bank. l 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. l High CO reading l Strong smell of H₂S (rotten eggs) l Excessive emissions

Fuel metering When the engine is cold, additional fuel has to be provided to the air:fuel mixture to assist starting. This supplementary fuel enrichment continues until the combustion chamber has heated up sufficiently during the warm-up phase.

Under normal part-throttle operating conditions the fuel mixture is adjusted to provide minimum fuel emissions and the air:fuel mixture is held close to the optimum ratio (λ = 1). The engine management system monitors the changing engine and environmental conditions and uses the data to determine the exact fuelling requirements necessary to maintain the air:fuel ratio close to the optimum value that is needed to ensure effective exhaust emission treatment through the three-way catalytic converters.

During full-throttle operation the air:fuel mixture needs to be made rich to provide maximum torque. During acceleration, the mixture is enriched by an amount according to engine temperature, engine speed, change in throttle position and change in manifold pressure, to provide good acceleration response.

When the vehicle is braking or travelling downhill the fuel supply can be interrupted to reduce fuel consumption and eliminate exhaust emissions during this period of operation.

If the vehicle is being used at altitude, a decrease in the air density will be encountered which needs to be compensated for to prevent a rich mixture being experienced. Without compensation for altitude, there would be an increase in exhaust emissions and problems starting, poor driveability and black smoke from the exhaust pipe. For open loop systems, higher fuel consumption may also occur.