Exhaust emission control operation (Emission control - V8)

Exhaust emission control operation

The oxygen content of the exhaust gas is monitored by heated oxygen sensors using either a four sensor (NAS only) or two sensor setup, dependent on market destination and legislative requirements. Signals from the heated oxygen sensors are input to the engine management ECM which correspond to the level of oxygen detected in the exhaust gas. From ECM analysis of the data, necessary changes to the air:fuel mixture and ignition timing can be made to bring the emission levels back within acceptable limits under all operating conditions.

Changes to the air:fuel ratio are needed when the engine is operating under particular conditions such as cold starting, idle, cruise, full throttle or altitude. In order to maintain an optimum air:fuel ratio for differing conditions, the engine management control system uses sensors to determine data which enable it to select the ideal ratio by increasing or decreasing the air to fuel ratio. Improved fuel economy can be arranged by increasing the quantity of air to fuel to create a lean mixture during part-throttle conditions, however lean running conditions are not employed on closed loop systems where the maximum is λ = 1. Improved performance can be established by supplying a higher proportion of fuel to create a rich mixture during idle and full-throttle operation. Rich running at wide open throttle (WOT) for performance and at high load conditions helps to keep the exhaust temperature down to protect the catalyst and exhaust valves.

The voltage of the heated oxygen sensors at λ = 1 is between 450 and 500 mV. The voltage decreases to 100 to 500 mV if there is an increase in oxygen content (λ > 1) indicating a lean mixture. The voltage increases to 500 to 1000 mV if there is a decrease in oxygen content (λ < 1), signifying a rich mixture.

The heated oxygen sensor needs to operate at high temperatures in order to function correctly (≥ 350° C). To achieve this the sensors are fitted with heater elements which are controlled by a pulse width modulated (PWM) signal from the engine management 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 about 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.

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 predefined 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 (200 milliseconds), 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 postcatalytic converter sensors (NAS only). This is achieved by checking the signal against maximum and minimum threshold for open and short circuit conditions. For NAS vehicles, should the pre- and post-catalytic converters be 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 the catalyst conversion efficiency and long term fuelling adaptations.

Note that some markets do not legislate for closed loop fuelling control and in this instance no heated oxygen sensors will be fitted to the exhaust system.