Method for the dynamic monitoring of a NOx sensor

Abstract

The invention relates to a method for monitoring a NOx sensor (10) having an oxygen-ion-conducting solid electrolyte and having at least one cavity (12), wherein at least one cavity (12) of the NOx sensor is flooded with a defined oxygen concentration during a self-diagnosis of the NOx sensor. The gradient of a pump current resulting therefrom is evaluated and, in the case of a deviation in comparison with reference values, possibly impaired dynamics of the NOx sensor are inferred.

Claims

1. A method for monitoring a NOx sensor (10) having an oxygen-ion-conducting solid electrolyte, at least one cavity (12), a Nernst cell (21), and a NOx pump electrode (22) that is arranged within at least one cavity (12), the method comprising flooding, during an internal diagnosis of the NOx sensor, the at least one cavity (12) of the NOx sensor with a defined oxygen concentration by reducing a voltage of the Nernst cell (21); analyzing a time curve of a resulting NOx pump current (300, 400), wherein at least one variable representative of the time curve of the resulting NOx pump current (300, 400), is analyzed; and inferring an impaired dynamic response of the NOx sensor (10) in response to determining a deviation of the at least one variable in comparison to one or more reference values.

2. The method as claimed in claim 1, characterized in that the time curve is analyzed based on a variable representing a gradient in a case of a rising pump current (300, 400) during the internal diagnosis.

3. The method as claimed in claim 1, characterized in that the time curve is analyzed based on a variable representing a gradient in a case of a falling pump current (300, 400) during the internal diagnosis.

4. The method as claimed in claim 1, wherein inferring an impaired dynamic response of the NOx sensor (10) further includes inferring either or both a poisoned NOx electrode (22) in the NOx sensor and a clogged NOx diffusion barrier (15) in the NOx sensor.

5. The method as claimed in claim 1, wherein NOx sensor (10) is configured to be arranged in an exhaust system of an internal combustion engine of a motor vehicle.

6. The method of claim 5, wherein the NOx sensor is arranged downstream of a turbocharger associated with the internal combustion engine.

7. The method of claim 5, wherein the NOx sensor is arranged downstream of a nitrogen oxide storage catalytic converter.

8. The method of claim 5, wherein the NOx sensor is arranged downstream of a diesel particle filter.

9. The method of claim 5, wherein the NOx sensor is arranged upstream of an SCR catalytic converter.

10. The method of claim 5, wherein the NOx sensor is arranged downstream of the SCR catalytic converter.

11. A non-transitory computer readable medium comprising program code to perform each step of the method as claimed in claim 1.

12. An electronic control unit which is configured to carry out the steps of a method as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the invention result from the following description of exemplary embodiments in conjunction with the drawings. In this case, the individual features can each be implemented alone or in combination with one another.

(2) In the drawings:

(3) FIG. 1 shows a schematic illustration of the construction of a NOx sensor, based on the Nernst principle, from the prior art;

(4) FIG. 2 shows an exemplary signal curve (NOx and Nernst voltage) in the case of the internal diagnosis of a NOx sensor according to the prior art; and

(5) FIG. 3 shows an exemplary signal curve in the case of the dynamic response monitoring according to the invention of a NOx sensor.

DETAILED DESCRIPTION

(6) FIG. 1 illustrates an exemplary design of a NOx sensor 10 known per se, which is constructed according to the Nernst principle. The sensor 10 is implemented in a layered structure on the basis of an oxygen-ion-conducting solid electrolyte (zirconium dioxide) and insulating layers made of aluminum oxide, wherein a first cavity 11, a second cavity (measuring chamber) 12, and a reference gas space 13 are provided. The exhaust gas flows in the direction of the arrow via a first diffusion barrier 14 into the first cavity 11. The second cavity 12 is separated from the first cavity 11 via a second diffusion barrier 15. An outer pump electrode (APE) 16 subjected to the exhaust gas and an inner pump electrode (IPE) 17 in the first cavity 11 form the oxygen pump cell 18. Furthermore, a Nernst electrode (NE) 19 is located in the first cavity 11. The corresponding reference electrode (RE) 20 is located in the reference gas space 13. This pair forms the Nernst cell 21. A NOx pump electrode (NOE) 22 is arranged in the second cavity (measuring chamber) 12. Its counter-electrode (NOCE) 23 is located in the reference gas space 13. These two electrodes form the NOx pump cell 24. All electrodes in the first and in the second cavity 11, 12 have a common return conductor (COM) 25. Furthermore, a heater 26 for the sensor 10 is provided.

(7) The operation of the sensor 10 is carried out in a way known per se by means of a sensor control unit (SCU) 100. The inner pump electrode 17 only has weak catalytic activity, for example, due to an alloy of platinum with gold. The pump voltage applied during the conventional measuring operation is therefore only sufficient to split (dissociate) oxygen molecules. NO is only dissociated slightly at the adjusted pump voltage and passes the first cavity 11 with only minor losses. As a strong oxidizing agent, NO.sub.2 is converted directly into NO at the inner pump electrode 17. Ammonia reacts at the inner pump electrode 17 in the presence of oxygen and at temperatures of, for example, 650 C. to form NO and water. Because of the higher voltage which is applied at the NOx pump electrode 22 and due to the admixture of, for example, rhodium, by which the catalytic activity of the NOx pump electrode 22 is enhanced, NO is completely dissociated at the NOx pump electrode 22. The oxygen formed in this case is pumped out through the solid electrolyte. The resulting pump current is a measure of the nitrogen oxides in the exhaust gas.

(8) In order to fulfill the OBD-II legislation in particular, a NOx sensor has to enable various diagnoses for the engine control unit. For this purpose, a differentiation is made between electrical diagnoses, which detect short-circuits between the various contacts to ground or the battery, and plausibility checks, to monitor the offset of the sensor (error at 0 ppm), and the so-called internal diagnosis (NOx cell status function), which can detect a possible sensor fault at high NOx concentrations. Carrying out the internal diagnosis known per se is illustrated on the basis of FIG. 2. To simulate a defined NOx pump current >0, the Nernst voltage (VS) is reduced, for example from 425 mV to 225 mV. A small share of the O.sub.2 molecules from the exhaust gas is thus no longer pumped out at the oxygen pump cell 18, but rather passes through the second diffusion barrier 15 up to the NOx pump electrode 22. After several seconds, a stable NOx pump current has resulted, which corresponds to approximately 300 ppm in this example. The NOx signal is integrated over several seconds, so that the result is an integral value. Subsequently, the Nernst voltage is increased to 425 mV again, and the sensor returns into the normal operating mode, wherein the NOx signal is enabled again after a short waiting time. During an initial calibration of the system (new part), the measured integral value is typically stored as a reference value, so that the integral value measurable in later operation can be compared to this reference value.

(9) FIG. 3 illustrates the signal curve during the internal diagnosis according to the method according to the invention, wherein the integral of the NOx signal after the reduction of the Nernst voltage is not observed, but rather the time curve of the resulting pump current is observed, to be able to infer a possibly impaired dynamic response of the NOx sensor. The illustrated NO/O.sub.2 signal results from the internal diagnosis which is carried out, comparable to the method illustrated on the basis of FIG. 2. A signal curve 300, which is to be expected in the case of a NOx sensor which is operating correctly, is shown in FIG. 3. Furthermore, a signal curve 400 is shown, from which a sensor which is not operating correctly may be inferred. The rise and fall of the signal 300 are substantially steeper than in the case of the signal 400. According to the invention, the dynamic curve of the respective signal is analyzed, for example, on the basis of a gradient of the signal curve. For example, a value may be ascertained which represents the gradient in the case of the rising pump current or the gradient in the case of the pump current falling again after the increase of the Nernst voltage. The respective value can be compared to a suitable reference value or expected value, so that a possibly existing impaired dynamic response can be inferred, as in the signal curve 400. In this way, in particular damage at the NOx pump electrode (poisoned electrode) and/or at a diffusion barrier (clogged NOx diffusion barrier) within the NOx sensor may be established. For example, if the NOx pump electrode is greatly damaged because of poisoning during operation, the reaction is delayed at the NOx pump cell, so that this can be recognized by analyzing the gradient during the internal diagnosis. An evaluation of the signal gradient can take place in both directions in this case. Firstly, the gradient can be analyzed during a rise of the pump current, which occurs during the reduction of the Nernst voltage (at the beginning of the internal diagnosis). Secondly, the gradient can be analyzed during the fall of the pump current as a result of the increase of the Nernst voltage (end of the internal diagnosis).

(10) The method according to the invention is not restricted to NOx sensors such as those schematically illustrated in FIG. 1. Rather, the method according to the invention can also be used in the case of other NOx sensors in which an internal diagnosis is carried out by flooding with oxygen.