Oxygen sensor monitoring a deterioration of a three-way catalyst in natural gas fueled engines

Two oxygen sensors are used to provide high-precision air/fuel ratio control for a three-way catalyst system of natural gas fueled engines. An optimized design for the sensor casing prevented sensor degradation, and an introduction of a subsidiary sensor in the exit side of the catalyst with a window tracking effect stabilized the emission control performance. The window tracking effect was explained as follows: the three-way catalyst deteriorates when used for long time in natural gas fueled engines. In most cases, the methane oxidation activity gradually decreased by the sintering of platinum. The window tracking-type oxygen sensor, which has electrode with low methane oxidation activity, detects the shifted window of the deteriorated catalyst because the window shift was brought about by the residual methane in the exhaust gas.