Introduction to On Board Diagnostics (II)

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Catalyst Efficiency Monitor: There are three types of catalysts: pellet (bead), ceramic monolith, and metal monolith. They differin the method by which they support the noble metals which convert exhaust gases to HC and NOx free gases. Three-waycatalytic converters typically contain platinum, and/or palladium, along with rhodium as catalytic materials. The term three-way refers to the ability of the converter to simultaneously oxidize HC and CO and reduce NOx. . Catalyst converters operateefficiently within a prescribed temperature range when placed at proper location in the exhaust gases’ path. Operation attemperatures which exceed the recommended maximums may cause irreversible damage to the catalyst, and components of theconverter. Since unburned fuel into the converter can cause catastrophic astrophic failure, misfire detection is a must for safe converteroperation. Misfire detection is described previously. Converter also must have an over temperature detection algorithm todetect excessive temperature in the converter. This is done by decreasing dthe A/F ratio’s lambda value to less than 1. Thisalgorithm cannot work for coastdown conditions or overrun conditions. Therefore Deceleration fuel cutoff c(DFCO) is sued tocontrol catalyst temperature during vehicle coastdown, , when the engine intake manifold pressure is drive too low to allow acomplete combustion. To prevent unburned fuel from entering the converter, the fuel injectors are shut off by the enginecontroller. Spark advance is filtered and thresholds are set to control torque reversal ”bump” while still protecting the converter.The catalyst monitor evaluates the converter efficiency as mandated by the OBD II to ensure that the catalyst is cleaning up theexhaust gases and reducing emissions from the exhaust gases. The diagnostic evaluates the oxygen storage capacity of theconverter by comparing the signal output of the post-converter oxygen sensor with the pre-converter oxygen sensor. Accordingto EPA regulations, a catalyst is regarded as malfunctioning when n the average hydrocarbon conversion efficiency falls between50 and 60%. The diagnostic system is required to detect when the hydrocarbon emission (HC) concentration of the catalyst(closest to the engine ) is more than 40 to 50% of the engine-out emission concentration. The check is performed with the vehicleoperating at between 20 and 50 miles/hr with the speed held at a reasonably steady state condition. The output signal waveform of the oxygen sensor (lambda sensor) ,at the front end of the tconverter close to the engine, oscillates between lean and richvalue of 100 millivolts and 900 millivolts due to closed-loop loop control strategy that keeps the Air/Fuel ratio at stoichiometry(lambda value equal to 1). For a converter whose oxygen storage e capacity is good, the output of the oxygen signal at the far end eof the converter should be flat, without any oscillation. This is due to the converter’s ability to store oxygen when the gas is lean(and rich in oxygen) and give up oxygen when the gas is rich (and d short of oxygen). This characteristic enables the oxidation ofhydrocarbons and the reduction of NOx in the exhaust gas simultaneously. The diagnostic consists of measuring mthe averageripple in the output signal wave form of the oxygen sensor at the e far end of the converter and comparing the ripple with asimilar oscillation at the input signal wave form of the oxygen sensor at the near end (closest to the engine) of the converter. Ifthe difference is above a value that corresponds to more than 60% 6converter efficiency then the converter efficiency isconsidered good. As a second check the catalyst temperature at the toutlet is monitored and compared to the catalysttemperature at the input to the converter. If the catalyst is functioning properly, it creates an exothermic reaction resulting in ahigher outlet catalyst temperature. But this is not always reliable. The sensitivity of the outlet gas temperature to catalystefficiency may be too low to reliably detect the difference at the t60% HC conversion efficiency level.Signal characteristics from the oxygen sensors for fresh, degraded, ed, and failed catalysts are explained in detail in a later section.The misfire detection diagnostic which is previously described is an important preventive measure that protects the converterfrom extreme temperature spike that can severely reduce converter r efficiency or even cause catalyst destruction altogether.Catalyst converter diagnostics are described in more detail in a later section.
Fundamentals of PowertrainControl strategies & OBD IIDiagnosticsOxygen Sensor & Heater Monitoring: An oxygen sensor performs best t when its operating temperature is maintained within a specific range above 260 O C. For this reason aheater is used to keep the oxygen sensor temperature at the desired value.The OBD II diagnostic requires that the heater of the oxygen sensor sor must be monitored periodically for its normal operation. The circuit continuity is checked, the voltageacross the heater is checked, the current carried by the heater element is checked ( Max. 20 A), as well as the temperature of the toxygen sensor. For added reliability, theheater is directly controlled by the the controller without any relay. If the heater is found defective on any of these accounts, the PCM sets a fault code.The PCM has a special input circuit for detecting short circuit or open circuit (break) of the sensor wiring and monitors the switching frequency (closed-loop) loop) of the controlloop.Oxygen sensor diagnostic requires the following checks: Circuit continuity ,and the bias voltage of 450 millivolts in the sensor circuit are verified. The voltage across thesensor should read 450 millivolts with the ignition key On and engine not started. If the voltage is not present a fault code (DTC) isset. During the closed loop operation of the vehicle, after the sensor attains the operating temperature (above 300 O C ), the sensor voltage should oscillate between about100 to 250 mv at the low end and 700 to 900 mv at the high end. The frequency of oscillation of this sensor voltage is between 1.25 Hz to 2.5 Hz, depending upon the fuelcontroller, fuel injection system, and vehicle operation. If the oscillation is slower than normal meaning that the oxygen sensor r is responding slowly to the A/F ratio input,then it is due to the sensor being exposed to high heat for a long period of time. This can cause a deviation in the A/F ratio from fthe optimum stoichiometry value,resulting in increased emissions. The deviation can be detected by monitoring the signal output oscillation of upstream oxygen (lambda) sensor and comparing it with thesystem operation frequency (1.25 Hz to 2.5 Hz) obtained from the e controller. A fault code is stored if the oxygen sensor at the upstream of the converter is oscillatingslower than the system frequency. A MIL is also illuminated. Additionally the controller compares the output signal (voltage) of the additional lambda sensor downstreamof the converter with the oxygen (lambda) sensor signal upstream. . Using this information the controller can detect deviations of the average value in the A/F ratio thatdetermines the system frequency. If system is operating rich and the lambda sensor indicates lean, then it is misfire problem. If system is operating lean, and the lambdasensor voltage stays near bias (450 mv) ) and engine does not go into closed loop, the sensor is having an open circuit and is defective. Slow transient response in A/F shiftcan also be caused by fuel control problem or carbon deposits or due to mild driving mode. Fuel system must be checked before deciding that oxygen sensor is faulty. If theA/F ratio is fluctuating due to excessive correction, to the pre set data map of optimum fuel required for each load and engine RPM, provided by the oxygen sensor, it is anindication of a faulty fuel system. The OBD II legal requirements s are: The diagnostic system shall monitor the output voltage, the tresponse rate, and any other parameterthat can affect emissions, and all fuel control oxygen sensors for fmalfunction.all fuel control oxygen sensors for malfunction. In case of a faulty sensor the MIL shall be illuminated and the DTC shall be stored in the computer.Oxygen sensor diagnostics are described in more detail in a later r section.
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Functionality of PowertrainControl
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