NOx SENSOR AND NOx SENSOR OPERATION METHOD

A NOx sensor operation method of operating a NOx sensor that includes a sensor element formed of an oxygen-ion conductive solid electrolyte, and is capable of sensing NOx in a measurement gas, wherein

• the sensor element includes:
• a plurality of internal spaces communicating sequentially under predetermined diffusion resistance from an inlet through which the measurement gas is introduced;
• a plurality of inner electrodes arranged to face the respective internal spaces;
• an out-of-space pump electrode disposed at a location other than the plurality of internal spaces;
• a reference electrode disposed to be contactable with a reference gas;
• a plurality of electrochemical pump cells configured to pump in or out oxygen between the plurality of internal spaces and an outside of the sensor element by applying pump voltages across the respective inner electrodes and the out-of-space pump electrode from predetermined pump power supplies;
• and
• a plurality of electrochemical sensor cells configured to cause potential differences between the respective inner electrodes and the reference electrode in accordance with oxygen concentrations in the respective internal spaces,
• one of the inner electrodes arranged in the respective internal spaces is a measurement electrode capable of reducing NOx,
• the plurality of electrochemical pump cells include:
• a measurement pump cell including the measurement electrode;
• and
• at least one oxygen concentration control pump cell including an inner electrode other than the measurement electrode,
• the plurality of electrochemical sensor cells include:
• a measurement sensor cell including the measurement electrode;
• and
• at least one oxygen concentration sensing sensor cell including the inner electrode other than the measurement electrode, and
• the method comprises
• a) determining whether a predetermined determination target value exceeds a predetermined threshold during a predetermined determination time, the determination target value being an indicator of operation of pumping in oxygen of a determination target pump cell included in the at least one oxygen concentration control pump cell,
• unless the determination target value exceeds the predetermined threshold in the step a), the NOx sensor is controlled in a basic mode in which a concentration of NOx is identified based on a measurement pump current flowing between the measurement electrode and the out-of-space pump electrode of the measurement pump cell in accordance with the concentration of NOx due to reduction of NOx by the measurement electrode while an oxygen concentration in each of the plurality of internal spaces is maintained constant, and
• when the determination target value exceeds the predetermined threshold in the step a), the basic mode is stopped, and the NOx sensor is operated in a protected execution mode in which the measurement electrode is protected against over-reduction.