Introduction to On Board Diagnostics (II)

Fundamentals of PowertrainControl strategies & OBD IIDiagnosticsSecondary Air System Monitoring: Secondary air system is used to improve the performance of the catalytic converter (Three way) by providing extraoxygen rich air to either the converter itself or to the exhaust manifold. The catalyst temperature must be above about 200 o C to efficiently oxidize HC andreduce NOx. . During engine warm-up when the catalytic converter is cold, HC and CO are oxidized in the exhaust manifold by routing secondary air tothe exhaust manifold in controlled quantify by the PCM. This creates extra heat to speed warm-up of the converter and EGO sensor, enabling the PCM togo into closed-loop loop mode more quickly.During open-loop control (cold converter) the converter is liable to be damaged if excessive heat is applied to it, to warm it up. This can happen ifexcessive amounts of HC and CO are oxidized in the exhaust manifold during periods of heavy loads which call for fuel enrichment, , or during severedeceleration. During start-up and such heavy loads, the secondary air is not let into exhaust manifold but directed into the air cleaner where it has noeffect on exhaust temperatures.After warm-up, during closed-loop loop operation, the secondary air is used to supply oxygen to the e second chamber of the three-way catalyst, in dual-chamber converter system. In a dual-chamber converter, the first chamber contains rhodium, palladium, , and platinum to reduce NOx and to oxidize HCand CO. The second chamber contains only platinum and palladium. The extra oxygen from the secondary air improves the converter’s s ability to oxidizeHC and CO in the second converter chamber. The control of the secondary air is done by using two solenoid valves similar to the EGR pintle valve. Onevalve switches air flow to the exhaust manifold or to the air cleaner (atmosphere). The other valve switches air flow to the exhaust manifold or to thecatalytic converter. The air routing is controlled based on engine ne coolant temperature and Air/Fuel ratio, indicated by the lambda sensor. If the control isopen-loop and if the coolant temperature is below threshold and Air/Fuel ratio is not too rich, then the air flow is directed to the exhaust manifold. Ifcoolant temperature is higher than threshold and the Air/Fuel ratio is rich (lambda < 1) then the secondary air is directed to the air cleaner which exitsto the atmosphere. If the control is closed-loop, loop, then the lambda sensor is monitored for correlated deviations when the secondary air flow is changedfrom exhaust manifold, or catalytic converter, or air cleaner, depending don coolant temperature, and lambda value. The OBD II requirement is that thesecondary air system shall have the diagnostic system monitor the e proper functioning of the secondary air delivery, and any air switching valve(solenoid).The critical parameters of the secondary air system are monitored d and if found to be out of permissible range of values, the fault code is set. The MIL isilluminated.Secondary air diagnostics are described in more detail in a later r section.