4.35.1 Description
Secondary Air Injection, or Air Injection Reaction (AIR), is fitted to the vehicle to reduce exhaust emissions to meet federal emission standards. The AIR system comprises a pump, valves, associated pipes and an absolute pressure sensor.
The pump provides a supply of air into the exhaust manifolds during the cold start period of the engine. The AIR cycle lasts for up to 65 seconds. The hot unburned fuel particles leaving the combustion chamber mix with the air injected into the exhaust manifolds and immediately combust. This subsequent combustion of the unburned and partially burnt carbon monoxide (CO) and hydrocarbon (HC) particles help to reduce the emission of these pollutants from the exhaust system. The additional heat generated in the exhaust manifold also provides rapid heating of the exhaust system catalytic converters.
The AIR system is monitored by measuring the system pressure using the absolute pressure sensor at several instances during its cycle of operation. The system pressure is first measured before operation of the pump. The pump is then switched on and, with a one second delay, the switching valve is opened. After a stabilizing period, the system pressure is measured again, this time by taking the average of a one second duration reading, and normalizing for variations in battery voltage and atmospheric pressure. If the system pressure measured at this time has not raised enough with respect to the initial pressure reading, then a failure will be flagged. A second pressure measurement is made after the requirement for AIR in the exhaust system has expired, but with the pump remaining operational, i.e. the pump is left running, against a closed switching valve. Again this pressure measurement is the average of a one second duration of readings normalized for variations in battery voltage and atmospheric pressure. If the system pressure measured at this time has not risen enough or has risen too much with respect to the system pressure during normal operation a failure will be flagged. A final pressure reading is taken after the AIR system has been switched off to ensure the system shuts down.
Flow performance is calculated separately for A and B Bank to determine flow performance. The expected airflow provided by the SAI pump is derived as a function of atmospheric pressure and battery voltage. In this diagnostic the expected flow is subtracted from the measured airflow (calculated from exhaust lambda) to produce an error signal. This error signal is integrated over the run time of the diagnostic and compared to the failure threshold table to determine correct airflow or insufficient flow to a Bank.
This strategy can detect a single point of failure anywhere in the system as shown below: