A NOx Sensor for Feedback Control and Emissions Reduction

Current emission control systems utilize a catalytic converter employing a three-way catalyst (TWC), composed of a mixture of noble metals to minimize the three main pollutant classes of NO_x, CO, and HC. The TWC is most efficient when the air-to-fuel ratio (A/F) is at stoichiometry (i.e. A/F ≈ 14.7). The stoichiometric set-point region is maintained by the use of oxygen sensors composed of the solid-electrolyte yttria stabilized zirconia (YSZ) in an electronic feedback loop. As combustion gets leaner a different exhaust sensor can be utilized to give a measure of the level of pollutants. A NO_x sensor is an alternative for an oxygen sensor that can be used for feedback control of engine combustion or exhaust NO_x traps. A solid electrolyte disk composed of YSZ having two Pt electrodes with one being covered by a microporous zeolite material was tested as a sensor for combustion produced gases such as NO and NO₂ in the presence of O₂. At 500°C it was found that the sensor responded to the NO_x gases and was hardly affected by changing O₂ levels in the range of 1-10%. A modified sensor design was used in a prototype assembly constructed to allow in-situ testing in an I-4 automobile engine when the engine cycle was perturbed. The sensor showed a response with changing gas concentrations and was correlated with an on-line FTIR gas analyzer. The sensor assembly design may show promise for catalytic converter monitoring or engine control diagnostics.