DE102017204301A1 - Method for correcting an offset of an ammonia sensor - Google Patents

Abstract

The invention relates to a method for correcting an offset of an ammonia sensor, which is arranged in an SCR system downstream of at least one SCR catalytic converter. The method comprises the following steps: First, ammonia slip is detected by means of the ammonia sensor, then a modeled ammonia level (FNH3mod) is set to a maximum ammonia level (FNH3max), followed by underdosing of a reducing agent for the SCR system. When the modeled ammonia level (FNH3mod) has returned to a nominal ammonia level (FNH3max), the underdosing is terminated. This is followed by determining a current offset and correcting the offset by means of a difference between the current offset and an anticipated offset.

Description

The present invention relates to a method for correcting an offset of an ammonia sensor in an SCR system. Furthermore, the invention relates to a computer program that performs each step of the method when it runs on a computing device, and a machine-readable storage medium that stores the computer program. Finally, the invention relates to an electronic control device which is set up to carry out the method according to the invention.

State of the art

Today, SCR catalysts (Selective Catalytic Reduction) are used to reduce nitrogen oxides (NOx) in the exhaust of motor vehicles. Here are nitrogen oxide molecules, which are located on an SCR catalyst surface, in the presence of ammonia (NH ₃ ) as a reducing agent, reduced to elemental nitrogen. The reducing agent is in the form of a urea-water solution, is cleaved from the ammonia, known commercially as AdBlue ^®, provided and injected by means of a metering module upstream of the SCR catalyst in an exhaust line. The determination of a desired metering rate takes place in an electronic control unit, in which strategies for operation and monitoring of the SCR system are stored.

The SCR catalysts known today store ammonia at their catalyst surface. The storage capacity is significantly dependent on a temperature of the catalyst surface and decreases with increasing temperature. The more ammonia is bound to the catalyst surface and available for reduction, the higher the nitrogen oxide conversion rate. As long as the storage capacity of the SCR catalyst is not exhausted, excessively metered reductant is stored. On the other hand, if the dosing unit makes available less reducing agent than would be necessary for the complete reduction of the nitrogen oxides present in the exhaust gas, the ammonia level is reduced by the further reduction of the nitrogen oxides taking place at the catalyst surface. Today's usual dosing strategies for SCR systems usually work with a slight overdose in order to achieve the maximum nitrogen oxide conversion rate. It must be ensured that excessively metered ammonia does not pass unused the catalyst surface. The passing ammonia mass is also referred to as ammonia slip.

From the relationship between ammonia level and nitrogen oxide conversion rate results in an increase in the nitrogen oxide conversion rate, when the overdose of the reducing agent causes an increase in the ammonia mass stored in the SCR catalyst. On the other hand, the nitrogen oxide conversion rate remains the same if the SCR catalytic converter is already optimally operated. If the measured nitrogen oxide conversion rate decreases in this case, that is, the sensor signal described above increases, this can be attributed to ammonia downstream of the SCR catalyst. In this case, it can be assumed that the maximum storage capacity of the SCR catalyst has been exhausted and thus excessively metered ammonia unused passes through the catalyst surface, ie an ammonia slip takes place. The metered reducing agent mass is always corrected by regulations.

The DE 10 2010 002 620 A1 describes a regulation of the metering mass by means of an adaptation factor, which indicates the ratio between nominal and actual metering mass. This directly changes a precursor mass of the reducing agent and serves to regulate a nitrogen oxide concentration measured by the sensor downstream of the SCR catalytic converter to a modeled nitrogen oxide value. The control adapts to the respective system and to longer-lasting environmental influences with the aid of an I-controller and can thus reduce the number of necessary adaptation interventions in the event of systematic errors. In addition, the regulation can also take into account very large and spontaneous changes, for example when refueling with the wrong reducing agent. The I-controller acts precisely, but also slowly.

In the DE 10 2012 221 574 A1 a level observer is shown. This fast P-controller permanently compensates the modeled nitrogen oxide concentration downstream of the SCR catalytic converter with the corresponding signal from the corresponding nitrogen oxide sensor. In the event of a deviation, the P-controller can perform the level control within a few seconds until the nominal efficiency is reached again.

For the appropriate control, the exact determination of the nitrogen oxide concentration by a nitrogen oxide sensor downstream of the SCR catalyst is crucial. Common nitric oxide sensors show a cross sensitivity to ammonia, ie their sensor signal contains not only the nitrogen oxide concentration, but shows a composite signal from nitrogen oxide and ammonia. Therefore, it can not be differentiated in the control based solely on the nitrogen oxide sensor, if the dosing mass was chosen too small, therefore nitrogen oxide was not converted, or if it was chosen too large, therefore free ammonia passes the SCR catalyst, ie ammonia Slippage occurs. The latter Often occurs at the significant overdose for the maximum rate of nitric oxide conversion.

In order to separate the sum signal from nitrogen oxide and ammonia, an additional ammonia sensor is used at the position of the nitrogen oxide sensor. The ammonia sensor measures the ammonia concentration, allowing it to be subtracted from the summed signal, so the SCR system is robustly tuned to its maximum nitric oxide conversion rate. In addition, the ammonia sensor can be used to determine whether ammonia slip occurs and to reduce the metering mass of the reducing agent accordingly.

In order to use the ammonia sensor, it is essential to monitor its offset and shift the offset over the life of the ammonia sensor. For the nitric oxide sensor such monitoring is already known. However, in the case of nitrogen oxide sensors, operating points are utilized at which the nitrogen oxide emission is suspended. When transferred to the ammonia sensor, this means that the monitoring of the offset or its displacement is only carried out if there is no ammonia slip. As described above, the ammonia sensor is usually used in SCR systems with permanently set weak overdosage, in which ammonia slip can hardly be ruled out.

Disclosure of the invention

The method relates to a correction of an ammonia sensor in an SCR system, which is arranged downstream of at least one SCR catalytic converter. It includes the following steps: Initially, ammonia slip is detected by the SCR catalyst by means of a signal from the ammonia sensor. Accordingly, at this time, an ammonia level of the SCR catalyst must have reached a maximum value (hereinafter referred to as maximum ammonia level). To take this into account, a modeled ammonia level, which is used inter alia for controlling a metering mass of a reducing agent for the SCR catalyst, set to the maximum ammonia level. Subsequently, an underdosing of the reducing agent is carried out, i. the metering mass of the reducing agent is reduced until it lies below a metering mass necessary for a complete conversion of nitrogen oxides currently flowing into the SCR catalyst through the SCR. Accordingly, the ammonia level drops, which is also reflected in a decrease in the modeled ammonia level.

If the modeled ammonia level reaches a nominal ammonia level for the present operating conditions, the underdosing is ended and the dosing mass is adjusted again. The ammonia level has been reduced to such an extent by the underdosing that, at least for a correction time, it can be assumed that there is no ammonia slip. As a result, the signal of the ammonia sensor only reflects a current offset during the correction time. This current offset is determined and used in the following for the correction of the offset of the ammonia sensor. The offset is corrected by means of a difference, which is formed between the current offset and an anticipated offset. Advantageously, the difference can be added to the previous offset to obtain a corrected offset. When repeating the method, this corrected offset may preferably be used as an expected offset. This method has the advantage that offsets of the offset are detected quickly and corrected accordingly.

Such an ammonia sensor is often used in high nitrogen oxide emission SCR systems. If no underdosage is carried out, these SCR systems are at least temporarily operated with a slight overdosage of the reducing agent in order to achieve a maximum nitrogen oxide conversion rate. In the case of such SCR systems, the method described above is particularly advantageous since, in the case of overdosage, ammonia slip must be assumed.

Advantageously, after the modeled ammonia level has reached the nominal ammonia level and thus the underdosing is terminated, a debounce time is awaited before the current offset is determined. The ammonia level does not decrease evenly distributed over the entire SCR catalyst, but begins to sink primarily to an exhaust gas inlet of the SCR catalyst. Due to inertia and run time of the gas through the SCR catalyst, a delay occurs until the ammonia level has decreased throughout the entire SCR catalyst. The debounce time is designed to compensate for this delay.

In order to reduce the risk of ammonia slip, the following conditions for releasing at least the correction of the offset, preferably also the determination of the offset and more preferably also the debounce time, can be checked. In the first condition, a temperature of the SCR catalyst is measured and a temperature gradient formed therefrom is compared with a first threshold value. When the temperature gradient of the SCR Catalyst is above the first threshold, at least the correction of the offset, preferably also the determination of the offset and more preferably also the debounce time, can be disabled. The comparison of the temperature gradient with the first threshold value can be carried out at any time, preferably permanently, until the offset is corrected. The storage capacity of the SCR catalyst depends on the temperature, so if the temperature rises too high, there is a risk that ammonia slip will occur during the determination of the current offset.

In the second condition, a gradient of an ammonia sensor signal of the ammonia sensor may be formed during or, optionally, after the underdosing is carried out, and the gradient of the ammonia sensor signal may be compared with a second threshold value. If the gradient of the ammonia sensor signal is above the second threshold value, at least the correction of the offset, preferably also the determination of the offset and particularly preferably the debounce time, can be disabled. If there is too great a gradient of the ammonia sensor signal, it can be concluded during or after the underdosing of the present ammonia slip, although it should be assumed that there is no ammonia slip until the current offset has been determined.

In the third condition, after the underdosing has been completed, integration of an exhaust mass flow may be performed and the integrated exhaust mass flow may be compared to a third threshold. If the integrated exhaust gas mass flow is above the third threshold value, at least the correction of the offset, preferably also the determination of the offset and particularly preferably also the debounce time, can be blocked. If the integrated exhaust gas mass flow is below the first threshold value, it is ensured over this period that no ammonia slip is to be expected due to the low exhaust gas mass and the associated dosage. In addition, this period specifies a maximum debounce time and if necessary correction time.

To determine the current offset, in particular the ammonia sensor signal can be filtered via the correction time. This has the advantage of removing unwanted artifacts for the current offset. The current offset is then determined within the correction time.

Advantageously, a weighting function can be used in the correction of the offset of the ammonia sensor. The weighting function is calculated with the difference between the current offset and the expected offset, for example in the form of a curve or its rating. In this way, individual erroneous corrections of the offset do not weigh disproportionately, so that the offset is not negatively influenced by them.

The computer program is set up to perform each step of the method, in particular when it is performed on a computing device or controller. It allows the implementation of the method in a conventional electronic control unit without having to make any structural changes. For this purpose it is stored on the machine-readable storage medium.

By loading the computer program on a conventional electronic control unit, the electronic control unit is obtained, which is adapted to perform the correction of the offset of the ammonia sensor.

list of figures

• 1a zeigt ein Diagramm eines Ammoniak-Füllstands abhängig von der Temperatur für einen herkömmlichen SCR-Katalysator.
• 1b zeigt ein Diagramm einer Stickoxid-Konvertierungsrate abhängig vom Ammoniak-Füllstand aus 1a des herkömmlichen SCR-Katalysators sowie einen Ammoniak-Schlupf.
• 2 zeigt Diagramme eines modellierten Ammoniak-Füllstands, eines nominalen Ammoniak-Füllstands, der Temperatur und eines Signals eines Ammoniak-Sensors während fünf World Harmonized Transient Cycles (WHTC) über der Zeit, gemäß einer Ausführungsform der Erfindung.
• 3 stellt in einem Ausschnitt III des Diagramms aus 2 den vierten WHTC detaillierter dar.
• 4 zeigt ein Ablaufdiagramm einer Ausführungsform des erfindungsgemäßen Verfahrens.
• 5 stellt in einem weiteren Ausschnitt V des Diagramms aus 2 einen Übergangsbereich zwischen dem vierten und dem fünften WHTC dar, bei dem eine Ausführungsform des erfindungsgemäßen Verfahrens angewendet wurde.

Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description.

• 1a shows a graph of ammonia level depending on the temperature for a conventional SCR catalyst.
• 1b shows a diagram of a nitrogen oxide conversion rate depending on the ammonia level 1a the conventional SCR catalyst and ammonia slip.
• 2 Figure 11 shows plots of modeled ammonia level, nominal ammonia level, temperature, and ammonia sensor signal during five World Harmonized Transient Cycles (WHTC) versus time, according to an embodiment of the invention.
• 3 shows in a section III of the diagram 2 the fourth WHTC in more detail.
• 4 shows a flowchart of an embodiment of the method according to the invention.
• 5 shows in another section V of the diagram 2 a transition region between the fourth and the fifth WHTC, in which an embodiment of the method according to the invention has been applied.

Embodiment of the invention

1a shows in a diagram the relationship between an ammonia level FNH3 and the temperature T for a conventional SCR catalyst. In the diagram are respectively depending on the temperature T, a minimum ammonia level FNH3min, a nominal ammonia level FNH3nom and a maximum ammonia level FNH3max are shown. If the ammonia level FNH3 falls below the minimum ammonia level FNH3min, nitrogen oxides flowing through the SCR catalytic converter are not completely converted by the SCR. If, however, the ammonia level FNH3 exceeds the maximum ammonia level FNH3max, ammonia slip occurs, which in 1b is clarified again. Therefore, when controlling an SCR system, the ammonia level FNH3 should be kept between the maximum ammonia level FNH3max and the minimum ammonia level FNH3min. It is highlighted a marked temperature T ₁ , which in the embodiment in 1b is used.

In 1b is a diagram of a nitrogen oxide conversion rate NOxKonv depending on the ammonia level FNH3 for the conventional SCR catalyst at the in 1a marked temperature T ₁ shown. The higher the ammonia level FNH3, the higher the nitrogen oxide conversion rate NOxKonv, since more reactants are available for the SCR. At the nitrogen oxide conversion rate NOxKonv NOxKonv is an operating point with increasing nitrogen oxide conversion rate 10 for the nominal ammonia level FNH3nom, an operating point 11 for the minimum ammonia level FNH3min and an operating point 12 for the maximum ammonia level FNH3max for operating the SCR catalyst. If the ammonia level FNH3 exceeds the maximum ammonia level FNH3max, the ammonia is no longer stored on the SCR catalytic converter.

An ammonia mass passing through the SCR catalyst 20 , which corresponds to the ammonia slip, is also shown in the diagram. However, it can be seen that the nitrogen oxide conversion rate NOxKonv continues to increase in this range, so that more nitrogen oxide is converted by the SCR. In order to comply with legal regulations, the SCR catalytic converter becomes particularly at high nitrogen oxide emissions in one operating point 13 above the maximum ammonia level FNH3max operated, ie there is an overdose instead. The passing ammonia mass 20 , ie the ammonia slip, must, however, remain below legally prescribed limits. As a result, the SCR system must be controlled so that the highest possible nitrogen oxide conversion rate NOxKonv at the same time low ammonia slip or in other words expressed as high as possible ammonia FNH3 with the lowest possible passing ammonia mass 20 is realized. Unless otherwise stated, the SCR system used in this exemplary embodiment is operated in the case of overdosing of the reducing agent, in particular if not provided otherwise in a method step of the method according to the invention. In addition, it is an operating point 15 at an underdose 32 , which will be used subsequently. At the underdose 32 is metered less reducing agent than required for the minimum ammonia level FNH3min, so that the ammonia level FNH3 decreases.

In 2 five consecutively executed World Harmonized Transient Cycles (WHTC) WHTC1 to WHTC5 for the SCR catalyst are shown in a diagram over time t. The storage capacity for ammonia of the SCR catalyst is significantly reduced in this example by aging effects. The diagram shows in the upper part a temperature T of the SCR catalyst, which was averaged for better usability, in the middle part a modeled ammonia level FNH3mod and the nominal ammonia level FNH3nom and in the lower part a signal YNH3 of a downstream ammonia sensor of the SCR catalyst is arranged. The nominal ammonia level FNH3nom essentially follows the course of the temperature T. over the five WHTC1 to WHTC5. The signal YNH3 of the ammonia sensor is in a low range during the first WHTC1. Due to the reduced storage capacity of the SCR catalyst, the metered amount is increased by a control to convert sufficient nitrogen oxide. Accordingly, the signal YNH3 of the ammonia sensor increases over the following two WHTC2 and WHTC3 until it rises so high in the fourth WHTC4 that it becomes a threshold 21 for ammonia slip.

A section of III designated with the 2 sets the fourth WHTC4 in 3 in more detail, wherein the representation of the diagram of the 3 those in 2 equivalent. As can be seen clearly here, the signal YNH3 of the ammonia sensor exceeds the threshold 21 for the ammonia slip at about 6800 seconds. Various methods for detecting ammonia slip are known in which, in addition to exceeding the threshold 21 In addition, an exhaust gas mass, which must flow at least through the SCR catalyst, is taken into account. Therefore, the ammonia slip becomes the threshold a short time after the signal YNH3 of the ammonia sensor becomes the threshold 21 has exceeded detected at about 6880 subsidence. At this point, the modeled ammonia level FNH3mod is set to the maximum ammonia level FNH3max and an underdosage is carried out so that the modeled ammonia level FNH3mod drops again. At the end of the fourth WHTC4 and in the fifth WHTC5, the signal YNH3 has returned to approximately zero.

However, it has also been shown that ammonia is already present on the ammonia sensor, namely at high and low temperature T of the SCR catalyst. As a result, correction of the offset of the ammonia sensor based on only the temperature T is not effective.

In 4 a flowchart of an embodiment of the inventive method for correcting the offset of the ammonia sensor is shown. In a first step, the signal YNH3 of the ammonia sensor is recorded. The signal YNH3 becomes, for example, as described above, a recognition 31 of ammonia slip 31. In order to reduce the ammonia level FNH3 again, the modeled ammonia level FNH3mod is set to the maximum ammonia level FNH3max 32. Accordingly, the underdosage subsequently takes place 33 instead - as in the description too 1b already executed - and the modeled ammonia FNH3mod level drops. The resulting course of the modeled ammonia level FNH3mod is already in 2 and more detailed in 3 shown.

If the modeled ammonia level FNH3mod has fallen to the nominal ammonia level FNH3nom, this will be in a query 34 tested and then the underdosing 33 terminated 35. The low ammonia level minimizes the likelihood of ammonia slip. In addition to this condition, in this embodiment, three more conditions 40 . 50 and 60 checked before proceeding through a release 36. The conditions 40 . 50 and 60 refer to risk factors for ammonia slip, which should be minimized.

The first condition 40 refers to the temperature T of the SCR catalyst. During the duration of the previous procedure, a measurement 41 the temperature T and a filtering 42 through a PT1 filter, eg a low-pass filter. From the measured temperature T, a temperature gradient T is calculated 43. If the temperature gradient T is in a first comparison 44 above a first threshold value S _T for the temperature T, the further method is disabled 37. Otherwise, the first condition applies 40 for the release 36 as fulfilled.

At the second condition 50 will after the underdose 33 takes place, the received signal YNH3 of the ammonia sensor filtered by a combination of DT1 filter (s) and PT1 filter (s) 51 and calculates therefrom a gradient Y of the signal YNH3 52. In a second comparison 53 the gradient Y of the signal YNH3 is compared with a second threshold value S _Y for the signal YNH3. If the gradient Y of the signal YNH3 is above the second threshold value S _Y , the further method is disabled 37. Otherwise, the second condition applies 50 for the release 36 as fulfilled.

Will in the query 33 confirms that the modeled ammonia level FNH3mod has decreased to the nominal ammonia level FNH3nom, is in the third condition 60 investigated exhaust gas mass flowed through the SCR catalyst. For this purpose, an exhaust gas mass flow q _{m is} first recorded 61 and then an integration 62 carried out. The integrated exhaust gas mass flow ∫q _m , which in principle reflects the exhaust gas mass flow, then becomes a third comparison 63 with a third threshold value S _m for the integrated exhaust gas mass flow ∫q _m or for the exhaust mass compared. If the integrated exhaust gas mass flow ∫q _m above the third threshold value S _m , the further procedure is blocked 37. Otherwise, the third condition applies 60 for the release 36 as fulfilled.

Are in the release 36 all conditions 40 . 50 and 60 is satisfied, it is assumed that there is no ammonia slip. Subsequently, a debounce time t _E , in which the ammonia level FNH3 compensates over the entire SCR catalyst, waited 70 before the signal YNH3 of the ammonia sensor by a PT1 filter, for example by a low-pass filter, over a correction time t _{K is} filtered 71. In a further step, a current offset O _a for the ammonia sensor is then determined 72. Between the current offset O _a and a preliminary offset, a difference D is formed 73. The provisional offset is at a first pass of the inventive method for the sensor used specific offset, which is either specified by the manufacturer or determined by a method or taught. In a further embodiment, the current offset O _{a is selected} as the provisional offset O _v during the first pass of the method, so that the difference D with which the offset is corrected yields zero in the first pass. In a repetition of the method according to the invention, an offset O _k corrected in the previous run of the method is selected as a provisional offset O _v . About a weighting function 74 , For example, in the form of a curve or their Bedatung, the difference D is weighted so that too large deviations are mitigated due to an erroneously determined current offset O _a , so as not to negatively affect the over a long time corrected offset O _k . Finally, the correction 75 of the offset by adding the difference D to the preliminary offset O _v so as to obtain the corrected offset O _k .

5 puts the in 2 V is a section of the diagram, which shows a transition range between the fourth WHTC4 and the fifth WHTC5 that is relevant for the correction of the offset. In the lower part of this diagram, in addition, a signal YNOx of a nitrogen oxide sensor arranged downstream of the SCR catalytic converter is shown, which has a cross-sensitivity to ammonia. Analogous to 2 and 3 the recognition takes place 31 of the ammonia slip at approx. 6880 seconds and the modeled ammonia level FNH3mod is set to the maximum ammonia level FNH3max 32. By the executed underdosage 33 The modeled ammonia level FNH3mod drops until it has returned to the nominal ammonia level FNH3nom at approx. 7070 seconds. Due to inertia and a running time of the exhaust gas through the SCR catalyst, the signal YNH3 of the ammonia sensor decreases in a delayed manner. The signal YNOx of the nitrogen oxide sensor rises in the range 7150 Seconds again strongly, before it finally decays also delayed. The debounce time t _E indicated here extends from the time at which the modeled ammonia fill level FNH3mod has returned to the nominal ammonia fill level FNH3nom at approximately 7070 seconds until the time when the signal YNH3 of the ammonia sensor reaches zero fell, at about 7240 seconds. The nitrogen oxide sensor continues to receive low concentrations of nitrogen oxide that has not been converted by the SCR catalyst so that its YNOx signal does not drop to zero. From the latter point in time, it is assumed that no ammonia slip is present at least over the correction time t _K , which extends here up to approximately 7320 seconds. Accordingly, the determination takes place during the correction time t _K 72 the current offset O _a and the offset offset 75.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• DE 102010002620 A1 [0005]
• DE 102012221574 A1 [0006]

Claims (13)

A method of correcting (75) an offset of an ammonia sensor disposed in an SCR system downstream of at least one SCR catalyst, comprising the steps of: I. detecting (31) ammonia slip by means of the ammonia sensor; II. Setting (32) a modeled ammonia level (FNH3mod) to a maximum ammonia level (FNH3max); III. Performing an underdosing (33) of a reductant for the SCR system; IV. Ending (35) the underdosing (33) when the modeled ammonia level (FNH3mod) has returned to a nominal ammonia level (FNH3nom); V. determining (72) a current offset (O _a ); and VI. Correcting (75) the offset by means of a difference (D) between the current offset (O _a ) and an expected offset (O _v ). Method according to Claim 1 , characterized in that the SCR system, if no underdosing (33) is carried out, at least temporarily operated with a weak overdose of the reducing agent. Method according to Claim 1 or 2 , characterized in that after step IV and before step V a debounce time (t _E ) is awaited (70). Method according to one of the preceding claims, characterized in that in addition the following steps are carried out: - measuring (41) a temperature (T) of the SCR catalyst; - forming (43) a temperature gradient (T) of the SCR catalyst; Comparing (44) the temperature gradient (T) of the SCR catalyst with a first threshold value (S _T ); and - blocking (37) at least the method step VI when the temperature gradient (T) of the SCR catalyst is above the first threshold value (S _T ). Method according to one of the preceding claims, characterized in that after method step III additionally the following steps are carried out: - forming (52) a gradient (Ẏ) of a signal YNH3 of the ammonia sensor; Comparing (53) the gradient (Ẏ) of the signal YNH3 with a second threshold value (S _Y ); and - blocking (37) at least the method step VI when the gradient (Y) of the signal YNH3 is above the second threshold value. Method according to one of the preceding claims, characterized in that the following steps are carried out after method step IV: - integration (62) of an exhaust gas mass flow (q _m ); - comparing (63) the integrated exhaust mass flow (∫q _m ) with a third threshold (S _m ); and - blocking (37) at least the method step VI when the integrated exhaust gas mass flow (∫q _m ) is above the third threshold value (S _m ). Method according to one of Claims 4 . 5 or 6 , characterized in that when the method step VI is disabled (37), also the method step V is disabled (37). Method according to one of the preceding claims, characterized in that for determining (72) of the current offset a signal (YNH3) of the ammonia sensor over a fixed correction time (t _K ) is filtered (71). Method according to one of the preceding claims, characterized in that the correction (75) of the offset of the ammonia sensor by means of a weighting function (74). Method according to one of the preceding claims, characterized in that in repetitions of the method, a corrected offset (O _k ) is used as the prospective offset (O _v ). Computer program, which is set up, each step of the procedure according to one of Claims 1 to 10 perform. Machine-readable storage medium on which a computer program is based Claim 11 is stored. Electronic control unit, which is adapted to operate by means of a method according to one of Claims 1 to 10 to perform the correction (75) of the offset of the ammonia sensor.

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