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Detection of NOx Storage-Reduction Catalyst Performance by Thick Film ZrO2 NOx Sensor
Published October 22, 2006 by Society of Automotive Engineers of Japan in Japan
A gasoline lean-burn engine is watched as one of measures against global warming because of an economical car that is effective for reducing atmospheric levels of carbon dioxide. Then, NOx storage-reduction catalyst has been developed for NOx reduction under excess air conditions, because three-way catalyst cannot reduce the NOx in exhaust gas under the lean air-fuel ratio condition. This catalyst system stores NOx during lean combustion and as the storage amount increases, rich-spike is generated to temporarily increase reduction component such as unburned HC. This catalyst has a mechanism to purify NOx using this reduction component and as NOx storage and reduction is repeated; NOx is reduced. Here, the best timing for rich-spike generation is right before storage amount reaches its limit. However, with the current technology, the calculation of storage amount to determine rich-spike timing is done with indirect method of estimating NOx storage amount by computing with NOx emission amount stored beforehand under each driving conditions. Therefore, the estimated value is not highly accurate and precise method of calculating NOx storage amount needs to be investigated. On the other hand, when capability of NOx storage-reduction catalyst to store NOx is decreased, not only does NOx emission amount increase, but unburned HC reduction component slips and then increase of emission gas progresses. Consequently, searching the method to detect NOx storage reduction will be a significant issue for NOx storage-reduction catalyst.
In this report, NOx sensors, which are zirconia type, were placed at the inlet side and the outlet side of the NOx storage-reduction catalyst that was installed in the exhaust system of a direct injection gasoline engine. A test vehicle with the engine was then driven on chassis dynamometer for varied driving tests and on road for actual driving tests. Using measured data such as NOx concentration signal and air-fuel ratio signal detected by the two sensors and other data, analysis was carried out. In order to grasp NOx storage capability under lean-burn driving conditions, effective index is presented and method of detecting reduction of NOx storage capability during deterioration of the catalyst is investigated. Furthermore, effective method of detecting deterioration was verified by driving test using various deteriorated catalysts. As the result, "NOx storage ratio in catalyst during NOx storage interval" which can be calculated based on the outputs from the two NOx sensors installed at the inlet and outlet side of the catalyst and the exhaust gas flow amount, was suggested as an index for NOx storage capability of the NOx storage-reduction catalyst. It has been shown that this index is effective for detecting decrease of NOx storage capability of the catalyst. Also, during driving actual road, driving interval is determined for average speed in every driving conditions, "NOx storage ratio in catalyst during NOx storage interval" is continuously calculated for each condition. As an average of these values, "average of NOx storage ratio during NOx storage interval" is determined. It was indicated that by monitoring this average periodically, deterioration detection of the NOx storage-reduction catalyst is possible.