Introduction to On Board Diagnostics (II)

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.