Catalyst Monitoring

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The three-way catalyst has been successfully adopted to gasoline engine to reduce pollutant emission. The engine is generally operated at the stoichiometric air:fuel ratio (A:F, about 14.7) by using a lambda sensor in order to maximize the efficiency of catalytic conversion from CO, Cm Hn , and NOx into CO2 ,H2 O, and N2. It has been reported that the catalytic activity gradually decreases under the exposure to high temperature or chemical poisons such as lead or lubricant derived phosphorus. However, the catalyst degradation is hardly detectable unless a malfunction indicator is in-stalled. Therefore, the monitoring of catalyst degrada-tion is important in order to maintain low emissions of CO, HC, and NOx . Many researchers have studied the catalyst-monitor-ing algorithm. Typically, two A:F sensors were placed before and after the catalyst and their signals were compared in order to check the changes of A:F due to the catalytic reactions. A pair of lambda sensors or wide range A:F sensors have been suggested to be installed for this purpose.

The lambda sensor measures electromotive forces between the exhaust and the air across a stabilized zirconia tube. This potentiometric sensor shows an abrupt signal change at the stoichiometric point of A:F because the concentration of oxygen in the exhaust changes extensively around this point after the equili-bration of gas phase over the Pt catalyst attached to the sensor. This feature makes it easy for one to control the engine at the stoichiometric point. The cost of lambda sensor is fairly low owing to successful mass production and the sensor stability has also been confi-rmed. On such background, the two-lambda sensor method has been proposed to detect the deterioration of catalyst. However, limitation of this method has been pointed out: When the HC conversion efficiency goes down to less than 70%, both of the sensors pro-duce the similar sensor signals in the fuel-lean or the fuel-rich regimes leading only to a small difference of EMF in between.


The wide range A:F sensor consists of an oxygen pumping cell, a sensing cell, a gas diffusion barrier, and a feedback control circuit. The last one controls the direction and amount of oxygen pumping according to the electromotive forces of the sensing cell. This sensor can detect even a small A:F change within the fuel-lean or the fuel-rich regime because its signal is almost proportional to A:F. So the catalyst monitor using the sensors of this type can detect the degradation of HC conversion efficiency down to 40%. However, the sensor has been fabricated by stacking several layers of YSZ and other materials which are very different in property and:or structure from each other. It is not easy to co-fire the laminates due to the differences in shrinkage during sintering. Furthermore, the operating algorithm is not simple, needing a complex driving circuit.

In this study, a new sensor based on the limiting current principle was designed to detect the catalyst degradation. Unlike conventional YSZ devices, the sensor utilized a layered porous composite of YSZ and Al2O3 , which could work as an oxide-ionic conductor as well as a gas diffusion barrier. The sensor was fabricated by stacking three layers of identical YSZ-Al2O3 composite with Pt electrodes being inserted in between. The stacking gave rise to an oxygen-pumping cell sandwiched between gas diffusion barriers, the limiting current of which was almost proportional to the concentration of O2 or CO in CO2 -containing atmosphere. Being easy to fabricate and simple in driving algorithm, the new sensor seems to fit well to the catalyst monitoring system. In this paper, the limiting current characteristics of the sensor were measured in CO-CO2-N2 atmosphere (fuel-rich condition) and O2-CO2-N2 (fuel-lean condition). Finally, the sensors were installed before and after the three-way catalyst in the exhaust line from a gasoline engine to confirm the applicability to the catalyst monitoring system.

References

Journal Jong-Heun Lee, Byung-Ki Kim, Kyo-Yeol Lee, Ho-In Kim, and Ki-Woo Han, "A New Catalyst Monitoring Sensor for Gasoline Engine using YSZ-Al2O3 as Solid Electrolyte and Gas Diffusion Barrier," Sensors and Actuators B, 59(1), 9-15 (1999)
Patent Jong-Heun Lee, "Sensor fitted to a vehicle exhaust", DE 19848578