METHOD FOR DETERMINING AN EXHAUST GAS SENSOR FAULT AND EXHAUST GAS SENSOR

Description

[0001] The present invention relates to a method for determining a fault of an exhaust gas sensor, for example a nitrogen oxide sensor with extended ammonia measurement, and an exhaust gas sensor, in particular a method for self-diagnosis of the exhaust gas sensor.

[0002] Exhaust gas sensors, such as nitrogen oxide sensors, lambda sensors, and oxygen sensors, can be based on the amperometric measurement principle, i.e., an electrochemical method for the quantitative determination of chemical substances. Specifically, an electric current is applied to an electrode of the exhaust gas sensor in such a way that a constant electrochemical potential is established over time. For example, nitrogen oxide sensors allow the nitrogen oxide concentration to be measured in the exhaust gas of internal combustion engines, such as gasoline or diesel engines. This enables, for example, optimal control and regulation as well as diagnostics of nitrogen oxide catalysts by the engine control system.

[0003] Such exhaust gas sensors comprise a main body made of a solid electrolyte, in which cavities with associated electrodes are provided. Furthermore, a heating device is arranged in the main body, which is designed to heat the main body to a predetermined operating temperature and maintain it at that temperature, for example, approximately 850°C.

[0004] JP 2019 / 203837 A relates to a nitrogen oxide sensor with an additional mixed-potential electrode for ammonia measurement. The resulting nitrogen oxide sensor also includes a self-diagnosis function to monitor the ammonia measurement. For this purpose, the performance value is determined by calculating the difference between the ammonia value measured by the ammonia sensor and the ammonia value estimated by the nitrogen oxide sensor.

[0005] US 2020 / 0088665 A1 discloses a diagnostic device for gas sensors. This device analyzes ammonia drift during a coasting phase. This takes advantage of the fact that the heating temperature of the sensor element increases with the greater the electrode aging.

[0006] US 10 527 569 B2 discloses a method for determining the aging of a mixed potential electrode.

[0007] DE 199 82 982 B4 discloses a gas sensor with self-diagnosis and a method for self-diagnosis. Figure 1The gas sensor comprises a main body with an oxygen pump cell for controlling the oxygen concentration in a first gas chamber. An oxygen measuring electrode is provided to measure the oxygen concentration present there. A second oxygen measuring electrode is provided in a second gas chamber for the gas atmosphere to be detected. An oxygen concentration signal is read out in each of the first gas chamber and the gas atmosphere to be detected (second gas chamber). These two signals are compared and used for diagnosis. The document does not disclose a mixed potential electrode on the outside of the main body with which an ammonia value is determined and thus ammonia slip is detected.

[0008] DE 10 2019 004 190 A1 describes a gas sensor and a gas concentration measurement method.

[0009] The present invention is based on the object of providing a method and an exhaust gas sensor with which a fault in the exhaust gas sensor can be determined.

[0010] This object is achieved by a method according to independent claim 1 and an exhaust gas sensor according to independent claim 9. Advantageous embodiments are specified in the subclaims.

[0011] The present invention is essentially based on the idea of providing a method for self-diagnosis of an exhaust gas sensor, in particular a nitrogen oxide sensor with an extended ammonia measurement function. In particular, the invention takes advantage of the fact that a nitrogen oxide sensor with an additional mixed-potential electrode for ammonia measurement has the ability to determine two independently measured oxygen values, which can then be compared for self-diagnosis. If the two measured oxygen values deviate from each other by more than a predetermined oxygen threshold, a fault in the exhaust gas sensor can be detected.More precisely, the oxygen value determined by the nitrogen oxide sensor during a predetermined operating state of the internal combustion engine can be compared with the oxygen value determined by means of the mixed potential electrode and, if the two determined oxygen values deviate from each other, a fault in the exhaust gas sensor, in particular in the mixed potential electrode, can be detected.

[0012] Accordingly, according to a first aspect of the present invention, a method for detecting a fault in an exhaust gas sensor having a main body is disclosed, which is designed to be arranged in an exhaust system of an internal combustion engine. The exhaust gas sensor has a pumping cavity arranged in the main body and connected to the exhaust gas, in which a pumping electrode is arranged, a mixed potential electrode arranged on the outside of the main body and coming into contact with the exhaust gas, and a reference cavity arranged in the main body and connected to the ambient air, in which a reference electrode is arranged.The method according to the invention comprises determining a predetermined operating state of the internal combustion engine in which the exhaust gas is substantially ammonia-free, determining a first oxygen value based on a pumping current applied to the pumping electrode such that an electrode voltage developing between the pumping electrode and the reference electrode is kept constant at a predetermined voltage value, determining a second oxygen value based on a mixed potential voltage developing between the mixed potential electrode and the reference electrode, and determining an error of the exhaust gas sensor if the first oxygen value deviates from the second oxygen value by more than a predetermined oxygen threshold value.The method according to the invention also comprises determining an ammonia value based on a mixed potential voltage developing between the mixed potential electrode and the reference electrode and determining an ammonia slip in the exhaust system of the internal combustion engine if the determined ammonia value is greater than a predetermined ammonia threshold value.

[0013] Thus, a diagnosis of the exhaust gas sensor, in particular of the mixed-potential electrode, can be performed by comparing the first oxygen value determined by the pumping current applied to the pumping electrode with the second oxygen value determined based on the mixed-potential voltage. If the second oxygen value deviates from the first oxygen value by more than the predetermined oxygen threshold value, a fault in the mixed-potential electrode can be identified. Above all, it can be assumed that, due to the routine self-diagnosis of the nitrogen oxide sensor and thus of the pumping electrode, the first oxygen value should be error-free and can therefore be used as a comparison measure for the second oxygen value.

[0014] Preferably, a mixed-potential electrode fault is detected if the first oxygen value deviates from the second oxygen value by more than the predetermined oxygen threshold. A mixed-potential electrode fault occurs, for example, if the mixed-potential electrode is excessively aged. Furthermore, a mixed-potential electrode fault may occur if the mixed-potential electrode is contaminated and thus less or no longer sensitive.

[0015] Preferably, the predetermined ammonia threshold is 1 ppm.

[0016] Therefore, if the exhaust gas sensor indicates an ammonia value that is greater than the predetermined ammonia threshold, it can be assumed that there is increased ammonia slip through the SCR catalyst, which can be detected by the exhaust gas sensor.

[0017] In a further preferred embodiment, the method according to the invention additionally comprises determining that a fault in the exhaust gas sensor cannot be detected if the determined first oxygen value deviates from an oxygen reference value by more than a predetermined oxygen threshold. The oxygen reference value preferably lies in a range between approximately 20% and approximately 21%, which corresponds to the approximate normal proportion of oxygen in the air.

[0018] If the first oxygen value deviates from the oxygen reference value by more than an oxygen threshold value, such as 1%, a self-diagnosis method according to the present invention can no longer be carried out properly, since the exhaust gas is not ammonia-free and thus a diagnosis is not possible due to the cross-sensitivity of the mixed potential electrode.

[0019] The predetermined operating state of the internal combustion engine is advantageously overrun or engine coasting. Furthermore, an overheated SCR catalyst can indicate an ammonia-free SCR catalyst and thus a predetermined operating state of the internal combustion engine, for example, if no reducing agent has been injected for an extended period of time. In particular, during these operating states of the internal combustion engine, the exhaust gas is essentially ammonia-free, which is why the mixed potential electrode, which is cross-sensitive to oxygen, is only sensitive to the oxygen in the exhaust gas. Thus, the signal from the mixed potential electrode can be used to determine the oxygen content in the exhaust gas and compare it with the first oxygen value determined by the pump electrode.

[0020] Advantageously, the predetermined oxygen threshold is approximately 2%, preferably approximately 1.5%, and more preferably approximately 1%. In further preferred embodiments, the predetermined oxygen threshold may be approximately 0.5%.

[0021] According to a further aspect of the present invention, an exhaust gas sensor for arrangement in an exhaust system of an internal combustion engine is disclosed. The exhaust gas sensor according to the invention comprises a main body, a pumping cavity arranged in the main body and connected to the exhaust gas, in which a pumping electrode is arranged, a mixed potential electrode arranged on the outside of the main body and coming into contact with the exhaust gas, a reference cavity arranged in the main body and connected to the ambient air, in which a reference electrode is arranged, and a control unit configured to carry out a method according to the invention for determining a fault in the exhaust gas sensor.

[0022] Further features and objects of the invention will become apparent to those skilled in the art by practicing the present teachings and viewing the accompanying drawings in which: Fig. 1 shows a schematic sectional view through an exhaust gas sensor for an internal combustion engine of a vehicle, and Fig. 2 shows an exemplary flow diagram of a method for determining a fault of an exhaust gas sensor for an internal combustion engine.

[0023] Within the scope of the present disclosure, amperometric sensors, such as nitrogen oxide sensors, lambda sensors, and oxygen sensors, are characterized in that their measuring principle is based on amperometry, i.e., an electrochemical method for the quantitative determination of chemical substances. In particular, an electric current is adjusted at a working electrode such that a temporally constant and predetermined electrochemical potential is established.

[0024] In the context of the present disclosure, mixed-potential sensors relate to sensors in which an electrical potential develops between two electrodes due to electrochemical processes, which can be used as a measure of the composition of the gas to which the electrodes are exposed. As with amperometric sensors, this results in the so-called Nernst potential.

[0025] Furthermore, in the context of the present disclosure, the term "control" encompasses the control-related terms "control" and "regulation." Those skilled in the art will recognize when control-related control and when control-related regulation are to be applied.

[0026] The Fig. 1shows an exemplary exhaust gas sensor 10, such as a nitrogen oxide sensor with an additional mixed potential electrode 23 for ammonia measurement. Furthermore, the present invention is applicable to exhaust gas sensors 10 that have a ceramic base plate with attached electrodes for oxygen measurement.

[0027] With reference to the Fig. 1 a schematic sectional view of an exemplary nitrogen oxide sensor 10 is shown, which is designed to be arranged in an exhaust system of an internal combustion engine (not shown) and to quantitatively detect the nitrogen oxide content or the oxygen content in the exhaust gas of the internal combustion engine.

[0028] The nitrogen oxide sensor 10 has a main body 12 made of a solid electrolyte, which is preferably formed from a solid solution of zirconium oxide and yttrium oxide and / or a solid solution of zirconium oxide and calcium oxide. Additionally, a solid solution of hafnium oxide, a solid solution of perovskite-based oxides, or a solid solution of trivalent metal oxide, such as aluminum oxide (Al 2 O 3 ), can be used. The main body 12 forms a sensor element of the exhaust gas sensor 10.

[0029] Within the main body 12 of the nitrogen oxide sensor illustrated by way of example, a first pump cavity 20, a second pump cavity 30, and a measuring cavity 40 are provided. The first pump cavity 20 is connected to the exterior of the main body 12 via a connecting path 15. In particular, exhaust gas can flow into the first pump cavity 20 through the connecting path 15. The second pump cavity 30 is connected to the first pump cavity 20 via a first diffusion path 25.

[0030] The measuring cavity 40 is connected to the second pump cavity 30 via a second diffusion path 35.

[0031] Also formed in the main body 12 is a reference cavity 50 that communicates directly with the exterior of the main body 12. A reference electrode 52 is arranged within the reference cavity 50. In particular, the reference cavity 50 communicates with the ambient air, i.e., not with the exhaust gas, and is designed to form an oxygen reference for the various electrodes arranged in the nitrogen oxide sensor.

[0032] An exhaust gas electrode 22 is arranged on an outer side of the main body 12. In particular, during a measuring operation of the nitrogen oxide sensor, by applying a reference current to the exhaust gas electrode 22, the oxygen present in the exhaust gas can be ionized and diffuse through the main body 12 as oxygen ions to the reference electrode 52, where it can be converted back into oxygen molecules to form an oxygen reference.

[0033] A first pumping electrode 24 is arranged within the first pumping cavity 20. In particular, during the measuring operation of the nitrogen oxide sensor, by applying a first pumping current IP0 to the first pumping electrode 24, the oxygen present in the exhaust gas can be ionized within the first pumping cavity 20 and migrate or pass through the main body 12 as oxygen ions. Due to the oxygen ions discharged from the first pumping cavity 20, a first electrode voltage or first Nernst voltage V0 is indirectly formed between the first pumping electrode 24 and the reference electrode 52. More precisely, the first electrode voltage or first Nernst voltage V0 is formed directly from the residual oxygen still present in the first pumping cavity 20.

[0034] A second pump electrode 34 is arranged within the second pump cavity 30. During the measuring operation of the nitrogen oxide sensor, by applying a second pump current IP1 to the second pump electrode 34, the oxygen present in the gas mixture within the second pump cavity 30 can be ionized and migrate or pass through the main body 12 as oxygen ions. Due to the oxygen ions discharged from the second pump cavity 30, a second electrode voltage or second Nernst voltage V1 is indirectly formed between the second pump electrode 34 and the reference electrode 52. More precisely, the second electrode voltage or second Nernst voltage V1 is formed directly from the residual oxygen still present in the second pump cavity 30.

[0035] A measuring electrode 44 is arranged within the measuring cavity 40. This measuring electrode is designed to ionize the oxygen and / or nitrogen oxides present within the measuring cavity 40 upon application of a measuring current IP2 during the measuring operation of the nitrogen oxide sensor, so that the oxygen ions can migrate or pass through the main body 12. Due to the oxygen ions discharged or pumped out of the measuring cavity 40, a third electrode voltage or third Nernst voltage V2 forms between the measuring electrode 44 and the reference electrode 52, which is kept at a constant value by applying the measuring current IP2 to the measuring electrode 44. More precisely, the third electrode voltage or third Nernst voltage V2 is formed directly from the residual oxygen still present in the measuring cavity 40. The applied measuring current IP2 is then an indication of the nitrogen oxide content within the exhaust gas.

[0036] The pump currents IP0, IP1 applied to the first and second pump electrodes 24, 34 are set such that preferably only the oxygen is ionized, but not the nitrogen oxides. In particular, the first pump electrode 24 is designed to pump almost all of the oxygen out of the exhaust gas during normal operation of the nitrogen oxide sensor or to allow a predetermined oxygen slip from the first pump cavity 20 into the second pump cavity 30. The second pump electrode 34 is designed to ionize and discharge the oxygen not yet pumped out of the first pump cavity 20, so that almost only nitrogen oxides are present in the measuring cavity 40. The measuring electrode 44 is designed to ionize the nitrogen oxides, wherein the measuring current IP2 applied to the measuring electrode 44 is a measure of the nitrogen oxide content in the exhaust gas.

[0037] Furthermore, a heating device 60 is arranged within the main body 12, which is designed to heat the main body 12 to a predetermined operating temperature and to maintain it at this temperature, for example at approximately 850°C.

[0038] The operating method for determining the nitrogen oxide content in the exhaust gas of the internal combustion engine using the disclosed nitrogen oxide sensor is already known from the prior art, to which reference is made here. The control principle for the nitrogen oxide sensor of the Fig. 1 is characterized in that the respective electrode voltages or Nernst voltages V0, V1, V2 are kept at a constant level by applying and adjusting the pump currents IP0, IP1 and the measuring current IP2.

[0039] The exhaust gas sensor 10 of the Fig. 1 furthermore has the functionality of determining the ammonia content (NH3 content) in the exhaust gas of the internal combustion engine. For this purpose, the exhaust gas sensor 10 of the Fig. 1 Additionally, a mixed potential electrode (or measuring electrode) 23 is arranged on an outer side of the main body 12 and can come into direct contact with the exhaust gas of the internal combustion engine. The mixed potential electrode 23 is particularly sensitive to ammonia, but also exhibits cross-sensitivity to other rich gases, such as hydrocarbons, carbon monoxide, and hydrogen. This means that with the mixed potential sensor integrated in the exhaust gas sensor 10, which includes the mixed potential electrode 23, the ammonia content and oxygen content in the exhaust gas can be detected from the output signal (= Nernst voltage V3, see below) using a function.

[0040] During operation of the exhaust gas sensor 10, a third Nernst voltage or electrode voltage (hereinafter also referred to as mixed potential voltage) V3 develops between the mixed potential electrode 23 and the reference electrode 52 due to the electrochemical processes in the main body 12. This third Nernst voltage or electrode voltage (hereinafter also referred to as mixed potential voltage) V3 can be used to determine the oxygen concentration in the exhaust gas and the ammonia concentration in the exhaust gas. To evaluate the mixed potential voltage V3, analytical methods can be used to determine the oxygen value and the ammonia value separately, with a first analytical method outputting the ammonia value in the exhaust gas and a second analytical method outputting the oxygen value in the exhaust gas. In particular, the determination of the second oxygen value using the mixed potential voltage V3 is an oxygen value determined independently of the first oxygen value and can be carried out separately.In particular, the corresponding procedure is implemented in software at the factory and can then contribute to increasing the accuracy of the exhaust gas sensor 10 during operation.

[0041] The Fig. 2 shows an exemplary flow chart of a method according to the invention for determining a fault of an exhaust gas sensor, such as the exhaust gas sensor 10 of the Fig. 1 .

[0042] The procedure of Fig. 2starts at step 200 and then goes to step 210, where it is determined whether the internal combustion engine is in a predetermined operating state. The determination of whether the internal combustion engine is in a predetermined operating state preferably takes place in the control system of the internal combustion engine, wherein the control system provides a signal indicating the determined operating state to the exhaust gas sensor 10. The predetermined operating state is characterized in that the exhaust gas is essentially ammonia-free during this operating state. For example, the overrun state or the engine run-on state can be regarded as a predetermined operating state of the internal combustion engine, since the exhaust gas is essentially ammonia-free during these operating states. The method remains at step 210 until a predetermined operating state has been determined.

[0043] If it is determined in step 210 that the internal combustion engine is in a predetermined operating state, the method proceeds to step 220, at which a first oxygen value is determined based on the pumping current IP0 applied to the pumping electrode 42.

[0044] In a subsequent step 230, a second oxygen value is determined based on the mixed potential voltage V3 developing between the mixed potential electrode and the reference electrode. Preferably, steps 220 and 230 run concurrently.

[0045] In a subsequent step 240, a check is carried out to determine whether the determined first oxygen value is suitable for serving as a comparison value for the determined second oxygen value. In particular, in step 240, a check is carried out to determine whether the determined first oxygen value deviates from an oxygen reference value by no more than a predetermined oxygen threshold value. In particular, in step 240, a check can therefore be carried out to determine whether the first oxygen value corresponds to the expected approximately 21% oxygen content in the air. If it is determined in step 240 that the determined first oxygen value deviates from the oxygen reference value by more than an oxygen threshold value, the method proceeds to step 250, where it is determined that it is not possible to determine a fault in the exhaust gas sensor 10 according to the present invention before the method ends in step 290.If the exhaust gas is not ammonia-free, it cannot be stated with sufficiently high certainty whether the mixed potential electrode 23 has reacted to the oxygen in the exhaust gas or to the ammonia in the exhaust gas.

[0046] However, if it is determined in step 240 that the determined first oxygen value does not deviate from the oxygen reference value by more than a predetermined threshold, the method proceeds to step 260, where the determined first oxygen value is compared with the determined second oxygen value. In particular, in step 260, it is checked whether the first oxygen value deviates from the second oxygen value by more than a predetermined oxygen threshold.

[0047] If it is determined in step 260 that the first oxygen value does not deviate from the second oxygen value by more than a predetermined oxygen threshold value, the method proceeds to step 270, at which a fault-free exhaust gas sensor 10, in particular a fault-free mixed potential electrode 23, is determined before the method ends again in step 290.

[0048] However, if it is determined in step 260 that the first oxygen value deviates from the second oxygen value by more than the predetermined oxygen threshold value, the method proceeds to step 280, at which a fault in the exhaust gas sensor 10, in particular the mixed potential electrode 23, is determined before the method again ends in step 290.

[0049] According to the present invention, an exhaust gas sensor can be tested for its functionality. In particular, a nitrogen oxide sensor that is expanded to include the functionality of an ammonia measurement by means of an additional mixed-potential electrode 23 can be tested to determine whether the mixed-potential electrode 23 of the nitrogen oxide sensor 10 is functioning properly. Preferably, the electrical signals of the exhaust gas sensor 10 are first assigned the respective concentration values, which can then be compared with one another. This means that, for example, based on the mixed-potential voltage V3, the oxygen value and / or the ammonia value are first determined according to their standard assignment rule before a respective comparison is made with reference values or other values.The same applies to the pump current IP0, which is first converted to the corresponding first oxygen value before a comparison with other determined oxygen values can be made.

[0050] The Fig. 2 The sequence of method steps shown is not limited to the example shown; in particular, step 240 can follow directly on from step 220, so that the second oxygen value is only determined when the first oxygen value is suitable for fault diagnosis of the mixed potential sensor.

Claims

1. Method for ascertaining a fault of an exhaust gas sensor (10), which comprises a main body (12) and is configured to be arranged in an exhaust gas tract of an internal combustion engine, wherein the exhaust gas sensor (10) comprises a pump cavity (20)being arranged in the main body (12) and being connected to the exhaust gas, a pump electrode (24) being arranged in said pump cavity, a mixed potential electrode (23) being arranged on the outer side of the main body (12) and coming into contact with the exhaust gas, and a reference cavity (50) being arranged in the main body (12) and being connected to ambient air, wherein a reference electrode (52) being arranged in said reference cavity (50), the method comprising: - ascertaining a predetermined internal combustion engine operating state in which the exhaust gas is substantially free of ammonia, - ascertaining a first oxygen value on the basis of a pump current (IP0) applied to the pump electrode (24) in such a way that an electrode pump voltage (V0) formed between the pump electrode (24) and the reference electrode (52) is kept constant at a predetermined voltage value, - ascertaining a second oxygen value on the basis of a mixed potential voltage (V3) formed between the mixed potential electrode (23) and the reference electrode (52), - ascertaining a fault of the exhaust gas sensor (10) if the first oxygen value differs from the second oxygen value by more than a predetermined oxygen threshold value, - ascertaining an ammonia value on the basis of a mixed potential voltage (V3) formed between the mixed potential electrode (23) and the reference electrode (52), and - ascertaining ammonia slip in the exhaust gas tract of the internal combustion engine if the ammonia value ascertained is greater than a predetermined ammonia threshold value.

2. Method according to Claim 1, wherein a fault of the mixed potential electrode (23) is ascertained if the first oxygen value differs from the second oxygen value by more than the predetermined oxygen threshold value.

3. Method according to any one of the preceding claims, wherein the predetermined ammonia threshold value is approximately 1 ppm.

4. Method according to any one of the preceding claims, further comprising: - determining that it is not possible to ascertain a fault of the exhaust gas sensor (10) if the first oxygen value ascertained differs from an oxygen reference value by more than a predetermined oxygen threshold value.

5. Method according to Claim 4, wherein the oxygen reference value is in a range between approximately 20 % and approximately 21 %.

6. Method according to any one of the preceding claims, wherein the predetermined internal combustion engine operating state includes fuel cut-off or run-on.

7. Method according to any one of the preceding claims, wherein the predetermined oxygen threshold value is approximately 2 %, preferably approximately 1.5 % and most preferably approximately 1 %.

8. Exhaust gas sensor (10) for arrangement in an exhaust gas tract of an internal combustion engine, the exhaust gas sensor (10) comprising: - a main body, - a pump cavity (20) being arranged in the main body (12) and being connected to the exhaust gas, a pump electrode (24) being arranged in said pump cavity, - a mixed potential electrode (23) being arranged on an outer side of the main body (12) and coming into contact with the exhaust gas, - a reference cavity (50) being arranged in the main body (12) and being connected to ambient air, a reference electrode (52) being arranged in said reference cavity, and - a control unit being configured to execute a method according to any one of the preceding claims.