CN108625951B

CN108625951B - Method for correcting offset of ammonia sensor

Info

Publication number: CN108625951B
Application number: CN201810213903.6A
Authority: CN
Inventors: C.海曼; F.施魏策尔
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2017-03-15
Filing date: 2018-03-15
Publication date: 2021-11-30
Anticipated expiration: 2038-03-15
Also published as: DE102017204301A1; CN108625951A; KR20180105585A

Abstract

The invention relates to a method for correcting an offset of an ammonia sensor, which is arranged downstream of at least one SCR catalyst in an SCR system. The method comprises the following steps: firstly, ammonia slip is detected by means of an ammonia sensor; then, the modeled ammonia-fill level (FNH 3 mod) is set to the maximum ammonia-fill level (FNH 3 max); and subsequently, a low dosing of the reducing agent is performed for the SCR-system. The low metering is ended when the modeled ammonia-fill level (FNH 3 mod) returns to the nominal ammonia-fill level (FNH 3 max). Next, a current offset is determined and the offset is corrected by means of a difference between the current offset and the expected offset.

Description

Method for correcting offset of ammonia sensor

Technical Field

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

Background

To reduce the nitrogen oxides (NOx) in the exhaust gas of motor vehicles, SCR catalysts (are used today), among other thingsSelective Catalytic RAcquisition). Here, ammonia (NH) is present as a reducing agent₃) Nitrogen oxide molecules are reduced to elemental nitrogen, which are located on the SCR-catalyst surface. The reducing agent is urea-water solution (also commercially available as AdBlue)^®Known) is provided and injected into the exhaust pipe by a dosing module upstream of the SCR-catalyst, separating ammonia from said urea-water-solution. Performing the pairing in the electronic control deviceThe determination of the desired metering rate, in which a strategy for operating and monitoring the SCR system is stored.

Currently known SCR-catalysts store ammonia on their catalyst surface. The storage capacity is largely dependent on the temperature of the catalyst surface and decreases as the temperature increases. The more ammonia that is bound at the catalyst surface and available for reduction, the higher the nitrogen oxide-conversion. The excess metered reducing agent is stored as long as the storage capacity of the SCR catalyst is not exhausted. If, on the other hand, the dosing unit supplies less reducing agent than is necessary for complete reduction of the nitrogen oxides present in the exhaust gas, the ammonia filling level is reduced as a result of the further reduction of the nitrogen oxides that takes place at the catalyst surface. The metering strategies currently available for SCR systems generally operate with a low excess metering in order to achieve maximum nox conversion. In this case, it must be ensured that the ammonia metered in excess passes the catalyst surface without being used. The mass of ammonia passing through is also referred to as ammonia slip.

From the relationship between the ammonia filling level and the nitrogen oxide conversion, on the one hand, an increase in the nitrogen oxide conversion is obtained when the excess of reducing agent causes an increase in the mass of ammonia stored in the SCR catalyst. On the other hand, the nitrogen oxide conversion remains unchanged if the SCR catalyst has already been operated optimally. If the measured nitrogen oxide conversion in this case decreases, that is to say the aforementioned sensor signal increases, this can be attributed to ammonia downstream of the SCR catalytic converter. In this case, it can be assumed that the maximum storage capacity of the SCR catalyst is exhausted and therefore ammonia metered in excess passes through the catalyst surface without being used, i.e. ammonia slip occurs. The metered reducing agent mass is continuously corrected by the regulation.

DE 102010002620 a1 describes the adjustment of a metering mass by means of an adaptation factor which describes the ratio between a nominal metering mass and an actual metering mass. This adaptation factor directly changes the pre-control quality of the reducing agent and is used to adjust the nitrogen oxide concentration, which is measured by a sensor downstream of the SCR catalytic converter, to the modeled nitrogen oxide value. By means of the I-regulator, the regulation is adapted to the respective system and to the long-lasting environmental influences, and the number of adaptation interventions (adaptistance) necessary in the event of a system error is therefore reduced. Furthermore, the regulation can also take into account very large and spontaneous changes, for example when the reducing agent is being filled incorrectly. In this case, the I-regulator functions precisely, but also correspondingly slowly.

A fill level viewer is shown in DE 102012221574 a 1. This rapid P-regulator permanently compensates the modeled nox concentration downstream of the SCR catalytic converter using the signal of the associated nox sensor. In the event of deviations, the P-regulator can perform filling level regulation within a few seconds until the nominal efficiency is reached again.

For an advantageous regulation, it is crucial to determine the nox concentration precisely by means of a nox sensor downstream of the SCR catalytic converter. Conventional nox sensors show a lateral sensitivity for ammonia, i.e. their sensor signal includes not only the nox concentration but also a sum signal made up of nox and ammonia. Thus, with regard to the regulation based solely on the nox sensor, it is not possible to distinguish between: whether the metering mass is selected too small, so that nitrogen oxides are not converted; alternatively, the metering mass is selected too large, so that free ammonia passes through the SCR catalyst, i.e. ammonia slip occurs. The latter usually occurs at a relatively weak excess, which is essential for maximum nitrogen oxide conversion.

In order to separate the sum signal, which is composed of nitrogen oxide and ammonia, an additional ammonia sensor is used at the location of the nitrogen oxide sensor. The ammonia sensor measures the ammonia concentration, which can be subtracted from the sum signal, so that the SCR system is robustly adjusted to its maximum nox conversion. Furthermore, ammonia sensors can also be used for: it is determined whether ammonia slip has occurred and the dosing quality of the reducing agent is correspondingly reduced.

In order to be able to use the ammonia sensor, it is necessary to monitor its offset and the passage of the offset over the service life of the ammonia sensor. Such monitoring is already known for nox sensors. In the case of nox sensors, however, operating points are used at which nox emissions are interrupted. By means of the ammonia sensor, this means that the monitoring of the offset or its displacement is carried out only if there is no ammonia slip. As already mentioned, ammonia sensors are often used in relatively weak overdetermined SCR systems with permanent regulation, wherein ammonia slip can hardly be ruled out.

Disclosure of Invention

The method involves correcting an ammonia sensor in the SCR system, which is arranged downstream of at least one SCR catalyst. It comprises the following steps: at the beginning, the ammonia slip is detected by the signal of the ammonia sensor through the SCR catalyst. At this point in time, therefore, the ammonia fill level of the SCR catalyst must reach a maximum value (hereinafter, referred to as maximum ammonia fill level). In this regard, the modeled ammonia-fill level is set to a maximum ammonia-fill level that is used, among other things, to adjust the dosing quality of the reductant for the SCR-catalyst. Subsequently, a low dosing of the reducing agent is carried out, i.e. the dosing mass of the reducing agent is reduced to such an extent that it is lower than the dosing mass necessary for complete conversion of the nitrogen oxides currently flowing into the SCR catalyst by the SCR. Correspondingly, the ammonia fill level decreases, which is also reflected in the modeled decrease in ammonia fill level.

If the modeled ammonia fill level reaches the nominal ammonia fill level for the current operating conditions, the low metering is ended and the metering quality is adjusted again. The ammonia fill level is reduced by the low metering to such an extent that, at least during the correction time, the absence of ammonia slip can be assumed. Thus, during the calibration time, the signal of the ammonia sensor reflects only the current offset. This current offset is determined and used hereinafter to correct the offset of the ammonia-sensor. The offset is corrected by means of a difference, which is formed between the current offset and the predicted offset. Advantageously, the difference can be added to the previous offset in order to obtain a corrected offset. This corrected offset can preferably be used as the expected offset when repeating the method. This method provides the advantage of: the passage of the offset is quickly identified and correspondingly corrected.

Such ammonia sensors are often used in SCR systems with high nox emissions. When no under-dosing is performed, the SCR systems are operated at least temporarily with a weaker overdosing of the reducing agent in order to achieve a maximum nox conversion. The aforementioned method is particularly advantageous with such SCR systems, since here, in the case of an overdosing, an ammonia slip must be assumed.

Advantageously, the modeled ammonia-fill level then reaches the nominal ammonia-fill level and the low metering is then ended, waiting for the debounce time (entprelzeit), before determining the current offset. The ammonia filling level does not decrease evenly distributed over the entire SCR catalyst, but first begins to drop at the exhaust gas inlet of the SCR catalyst. A delay occurs due to the inertia and the operating time of the gases through the SCR-catalyst until the ammonia-filling level across the entire SCR-catalyst decreases. The debounce time is constructed to compensate for this delay.

In order to reduce the risk of ammonia slip, conditions can be checked for releasing at least the correction of the offset, preferably also the determination of the offset, and more preferably for releasing the debounce time. Under a first condition, the temperature of the SCR catalyst is measured and the temperature gradient formed thereby is compared with a first threshold value. At least the correction of the offset, preferably also the determination of the offset and particularly preferably also the debounce time, is blocked when the temperature gradient of the SCR catalyst is above a first threshold value. The comparison of the temperature gradient with the first threshold value can be carried out at any time, preferably permanently, until a correction for the offset is carried out. The storage capacity of the SCR catalytic converter is temperature-dependent, so that there is a risk of a sharp rise in temperature: during the determination of the current offset, an ammonia slip occurs.

In the second condition, during or if necessary after the low metering is performed, a gradient of the ammonia sensor signal of the ammonia sensor can be formed and compared with a second threshold value. When 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 also the debounce time, can be blocked. If there is an excessive gradient of the ammonia sensor signal, the current ammonia slip can be inferred during or after the under-metering, although it should be assumed that: until no ammonia slip is present after the current offset is determined.

Under a third condition, after the low metering is ended, integration of the exhaust gas mass flow is carried out and the integrated exhaust gas mass flow is compared with a third threshold value. When the integrated exhaust gas mass flow is above the third threshold value, at least the correction of the offset amount, preferably also the determination of the offset amount 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 during this time period that: due to the lower exhaust gas mass and the metering associated therewith, no ammonia slip should then be expected. Furthermore, this time period predetermines a maximum debounce time and, if necessary, a correction time.

In order to determine the current offset, in particular the ammonia sensor signal can be filtered during the correction time. This provides the advantage of: the unwanted artifacts (artifacts) for the current offset are removed. Then, the current offset is determined within the correction time.

Advantageously, a weighting function is used in correcting the offset of the ammonia sensor. The weighting function is calculated using the difference between the current offset and the expected offset, for example in the form of a curve or its parameters (Bedatung). In this way, individual, erroneous corrections to the offset do not disproportionately affect them, so that the offset is not negatively affected by them.

A computer program is provided for carrying out each step of the method, in particular when it is executed on a computing device or control device. It enables the method to be implemented in conventional electronic control devices without structural changes being necessary here. For this purpose, it is stored on a storage medium that can be read by a machine.

By running the computer program on a conventional, electronic control device, the following electronic control device is obtained: the electronic control device is configured to perform a correction for the offset of the ammonia-sensor.

Drawings

Embodiments of the invention are illustrated in the accompanying drawings and set forth in more detail in the following description.

Fig. 1a shows a temperature-dependent ammonia fill level diagram for a conventional SCR catalyst.

Fig. 1b shows a graph of the nitrogen oxide conversion of a conventional SCR catalyst as a function of the ammonia filling level from fig. 1a, and the ammonia slip.

Fig. 2 shows a graph of modeled ammonia-fill level, nominal ammonia-fill level, temperature, and ammonia-sensor signal over time according to an embodiment of the present invention during five World Harmonious Transient Cycles (WHTC).

Figure 3 shows the fourth WHTC in more detail in section III of the graph from figure 2.

Fig. 4 shows a flow chart of an embodiment of the method according to the invention.

Fig. 5 shows in a further section V from the diagram of fig. 2 the transition region between the fourth and fifth WHTCs, in which an embodiment of the method according to the invention has been applied.

Detailed Description

FIG. 1a shows in a diagram the relationship between the ammonia fill level FNH3 and the temperature T for a conventional SCR catalyst. In the graph, the minimum ammonia-filling level FNH3min, the nominal ammonia-filling level FNH3nom, and the maximum ammonia-filling level FNH3max are shown respectively according to the temperature T. If the ammonia fill level FNH3 is below the minimum ammonia fill level FNH3min, the nitrogen oxides flowing through the SCR catalyst are not completely converted by the SCR. However, if the ammonia-fill level FNH3 exceeds the maximum ammonia-fill level FNH3max, ammonia slip results, which is again illustrated in fig. 1 b. Therefore, when adjusting the SCR system, the ammonia-fill level FNH3 should be maintained between the maximum ammonia-fill level FNH3max and the minimum ammonia-fill level FNH3 min. Highlights the marked temperature T₁Said temperature is used in the embodiment of fig. 1 b.

In FIG. 1b, the temperature T marked in FIG. 1a is shown₁Graph of the nitrogen oxide conversion NOxKonv for a conventional SCR catalyst, as a function of the ammonia fill level FNH 3. The following is applicable: the higher the ammonia-fill level FNH3, the greater the nitrogen oxide-conversion NOxKonv, since more reactant is available for SCR. The operating point 10 for the nominal ammonia fill level FNH3nom, the operating point 11 for the minimum ammonia fill level FNH3min and the operating point 12 for the maximum ammonia fill level FNH3max are characterized by an increased nitrogen oxide conversion NOxKonv for operating the SCR catalyst. If the ammonia-fill level FNH3 exceeds the maximum ammonia-fill level FNH3max, ammonia is no longer stored at the SCR-catalyst.

Also shown in the diagram is the ammonia mass 20 passing through the SCR catalyst, which corresponds to the ammonia slip. However, it can be seen that the nox conversion NOxKonv continues to increase in this region, so that more nox is converted by means of SCR. In order to comply with legal regulations, in particular in the case of high nox emissions, the SCR catalytic converter is operated at an operating point 13 above the maximum ammonia fill level FNH3max, i.e., an overdosing takes place. However, the passing ammonia mass 20 (i.e. the ammonia slip) must remain below the legally prescribed limit values. The SCR system must therefore be set such that the highest possible nitrogen oxide conversion NOxKonv is achieved with a low ammonia slip, or in other words the highest possible ammonia fill level FNH3 is achieved with the lowest possible ammonia mass 20 passing through. The SCR system used in this exemplary embodiment is operated in the event of an overdosing of the reducing agent, if not otherwise stated, i.e. in particular if no further provisions are made with regard to the method steps of the method according to the invention. Furthermore, the working point 15 is indicated at a low dose 32, which is used hereinafter. At low dose 32, less reductant is dosed than is required for the minimum ammonia-fill level FNH3min, so that the ammonia-fill level FNH3 is reduced.

In fig. 2, five consecutive World Harmonious Transient Cycles (WHTC) WHTC1 to WHTC5 for the SCR catalyst are graphically represented over time t. In this example, the storage capacity of ammonia for the SCR-catalyst is significantly reduced over the aging effect. The diagram shows in the upper part the temperature T of the SCR catalyst, which is determined for better usability, in the middle part the modeled ammonia filling level FNH3mod and the nominal ammonia filling level FNH3nom, and in the lower part the signal YNH3 of the ammonia sensor, which is arranged downstream of the SCR catalyst. Across from WHTC1 to WHTC5, the nominal ammonia-filling level FNH3nom is essentially inversely proportional to the course of the temperature T. During the first WHTC1, the signal YNH3 of the ammonia-sensor is in the low range. Due to the reduced storage capacity of the SCR catalytic converter, the metering quantity is increased by regulation in order to sufficiently convert nitrogen oxides. Correspondingly, the ammonia sensor signal YNH3 rises in the two subsequent WHTC2 and WHTC3 until it rises so sharply in the fourth WHTC4 that it exceeds the threshold value 21 for ammonia slip.

The part of figure 2 denoted in III shows the fourth WHTC4 in figure 3 in more detail, wherein the view of the diagram of figure 3 corresponds to that in figure 2. As can be clearly seen here, the signal YNH3 of the ammonia sensor exceeds the threshold value 21 for ammonia slip at approximately 6800 seconds. Different methods for detecting ammonia slip are known, in which, in addition to exceeding threshold value 21, the exhaust gas mass, which must flow at least through the SCR catalytic converter, is additionally taken into account. Therefore, shortly after the ammonia sensor signal YNH3 has exceeded the threshold value 21, an ammonia slip is recognized at about 6880 seconds. At this point in time, the modeled ammonia-fill level FNH3mod is set to the maximum ammonia-fill level FNH3max, and the under-dosing is performed such that the modeled ammonia-fill level FNH3mod drops again. At the end of the fourth WHTC4 and in the fifth WHTC5, the signal YNH3 falls back to approximately zero.

However, it has also been shown that ammonia was already present at the ammonia sensor, specifically at a high and a low temperature T of the SCR catalyst. As a result, the correction of the offset amount of the ammonia-sensor based on the temperature T alone does not achieve the target.

In fig. 4, a flow chart of an embodiment of the method according to the invention for correcting the offset of the ammonia sensor is shown. In a first step, the signal YNH3 of the 30 ammonia sensor is recorded. For example, as described above, the ammonia slip 31 is detected 31 by the signal YNH 3. To again reduce the ammonia-fill level FNH3, the modeled ammonia-fill level FNH3mod is set to 32 to the maximum ammonia-fill level FNH3 max. As already explained in the description with respect to fig. 1b, correspondingly, the low metering 33 then takes place and the modeled ammonia fill level FNH3mod drops. The resulting course of the modeled ammonia filling level FNH3mod is shown in more detail in fig. 2 and in fig. 3.

If the modeled ammonia-fill level FNH3mod returns to the nominal ammonia-fill level FNH3nom, this is checked in query 34 and the low gauge 33 is then ended 35. Due to the low ammonia-fill level, the possibility of ammonia-slip is minimized. In addition to this condition, three

conditions

40, 50 and 60 are also checked in this example before continuing the method by releasing 36.

Conditions

40, 50 and 60 relate to risk factors for ammonia-slip, which should be minimized.

The first condition 40 relates to the temperature T of the SCR catalyst. During the duration of the method so far, the measurement of the temperature T is carried out and passes PT1-A filter (e.g., a low pass filter) to perform filtering 42. Calculating 43 a temperature gradient from the measured temperature T

. If the temperature gradient in the first comparison 44

Above a first threshold value S for a temperature T_TFurther methods are blocked 37. Otherwise, the first condition 40 for the release 36 is deemed to be satisfied.

After the occurrence of the low metering 33, the received signal YNH3 of the ammonia sensor is filtered 51 by a combination of a DT1 filter and a PT1 filter for the second condition 50, and the gradient of the signal YNH3 is calculated 52 therefrom

. In a second comparison 53, the gradient of the signal YNH3

And a second threshold S for signal YNH3_YA comparison is made. If the gradient of signal YNH3

Above a second threshold S_YFurther methods are blocked 37. Otherwise, the second condition 50 for release 36 is deemed satisfied.

If it is ascertained in query 33 that the modeled ammonia fill level FNH3mod returns to the nominal ammonia fill level FNH3nom, the exhaust gas mass flowing through the SCR catalyst is investigated in a third condition 60. For this purpose, first of all an exhaust gas mass flow q is received 61_mAnd then performs integration 62. The integrated exhaust gas mass flow is then compared in a third comparison 63

And a third threshold value S_mA comparison is made, the integrated exhaust gas mass flow in principle reflecting the exhaust gasMass flow, the third threshold being used for the integrated exhaust gas mass flow

Or for exhaust gas quality. If the integrated exhaust gas mass flow

Above a third threshold value S_mFurther methods are blocked 37. Otherwise, the third condition 60 for release 36 is deemed satisfied.

If all of the

conditions

40, 50 and 60 are fulfilled in the release 36, the absence of ammonia slip is assumed. Then, at the correction time t_kWait 70 for debounce time t before filtering 71 ammonia-sensor signal YNH3 with PT 1-filter (e.g., with low pass filter)_EThe ammonia fill level FNH3 over the entire SCR catalytic converter is compensated for in the debounce time. Then, in a further step, a current offset O for the ammonia sensor is determined 72 therefrom_a. At the current offset O_aAnd the temporary (vorl ä ufig) offset, form 73 a difference D. In the case of the first round of the method according to the invention, the temporary offset is a specific offset for the sensor used, which is predefined by the manufacturer or determined or learned by the method. In a further embodiment, for the first round of the method according to the invention, a temporary offset O is selected_vAs the current offset O_aSo that the difference D becomes zero in the first run, the offset is corrected using the difference. When repeating the method according to the invention, the offset O to be corrected during the previous execution of the method_kOffset O selected as temporary_v. The difference D is weighted by a weighting function 74 (for example in the form of a curve or a set of parameters thereof) in such a way that the current offset O determined in an error manner is reduced_aResulting in too large a deviation so as not to adversely affect the corrected offset O over a longer time_k. Finally, a correction 75 of the offset is performed by adding the difference D to the temporary offset O_vIn order to thus obtain a corrected offset O_k。

Fig. 5 shows the section of the diagram denoted by V in fig. 2, which shows the transition region between the fourth WHTC4 and the fifth WHTC5 that is important for the correction of the offset. In the lower part of this diagram, the signal YNOx of a nitrogen oxide sensor is additionally plotted, which is arranged downstream of the SCR catalytic converter and has a lateral sensitivity for ammonia. Similar to fig. 2 and 3, the identification 31 of ammonia-slip is performed at about 6880 seconds, and the modeled ammonia-fill level FNH3mod is set 32 to a maximum ammonia-fill level FNH3 max. With the low metering 33 performed, the modeled ammonia-fill level FNH3mod drops until it returns again to the nominal ammonia-fill level FNH3nom at about 7070 seconds. The signal YNH3 of the ammonia sensor is reduced with a delay due to the inertia and the operating time of the exhaust gas through the SCR catalytic converter. The signal YNOx of the nox sensor rises again sharply in the 7150 s range until it finally falls off with delay. De-jitter time t indicated here_EFrom a point in time at about 7070 seconds, at which the modeled ammonia-fill level FNH3mod again returns to the nominal ammonia-fill level FNH3nom, to a point in time at about 7240 seconds, at which the ammonia-sensor signal YNH3 drops to zero. Furthermore, the nitrogen oxide sensor receives a low concentration of nitrogen oxides, which are not converted by the SCR catalyst, so that the signal YNOx of the nitrogen oxide sensor does not drop to zero. From a later point in time, at least at a correction time t_kThe starting point is the absence of ammonia slip, where the correction time extends to about 7320 seconds. Accordingly, at the correction time t_kMeanwhile, the current offset O is added_aDetermination of (c) and correction for offset 75.

Claims

1. Method for correcting (75) an offset of an ammonia sensor arranged downstream of at least one SCR catalyst in an SCR system, comprising the steps of:

I. -identifying (31) an ammonia slip by means of the ammonia sensor;

setting (32) the modeled ammonia-fill level (FNH 3 mod) to a maximum ammonia-fill level (FNH 3 max);

performing a low dosing (33) of reductant for the SCR-system;

ending (35) the low metering (33) when the modeled ammonia-fill level (FNH 3 mod) returns to a nominal ammonia-fill level (FNH 3 nom);

v. determining (72) a current offset (O)_a) (ii) a And is

Correcting (75) the offset by means of a difference (D) at the current offset (O)_a) And an expected offset (O)_v) The difference between them.

2. Method according to claim 1, characterized in that the SCR-system is operated at least temporarily with a weak overdosing of the reducing agent when no underdosing (33) is performed.

3. Method according to claim 1 or 2, characterized in that after method step IV and before method step V, a debounce time (t) is waited (70)_E）。

4. Method according to claim 1, characterized in that the following steps are additionally performed:

-measuring (41) a temperature (T) of the SCR-catalyst;

-forming (43) a temperature gradient of the SCR-catalyst (c)

）；

-subjecting the temperature gradient of the SCR-catalyst(s) ((

) And a first threshold value (S)_T) Performing a comparison (44); and is

-when the temperature gradient of the SCR-catalyst: (

) Above the first threshold value (S)_T) At least blocking (37) said method step VI.

5. Method according to claim 1, characterized in that, after method step III, the following steps are additionally performed:

-forming (52) a gradient of the signal (YNH 3) of the ammonia-sensor (YNH 3) ("YNH-sensor")

）；

-said gradient of said signal (YNH 3) (y;)

) And a second threshold value (S)_Y) Performing a comparison (53); and is

-when said gradient of said signal (YNH 3)

) Blocking (37) at least the method step VI above the second threshold.

6. Method according to claim 1, characterized in that after method step IV the following steps are additionally performed:

for exhaust gas mass flows (q)_m) Performing an integration (62);

-integrating the exhaust gas mass flow (

) And a third threshold value (S)_m) Performing a comparison (63); and is

When integrated exhaust gas mass flow (

) Above the third threshold value (S)_m) At least blocking (37) said method step VI.

7. The method according to any one of claims 4, 5 or 6, characterized in that when blocking (37) the method step VI, the method step V is also blocked (37).

8. Method according to claim 1, characterized in that for determining (72) the current offset, at the determined correction time (t)_k) The ammonia sensor signal (YNH 3) is filtered (71).

9. Method according to claim 1, characterized in that the correction (75) of the offset of the ammonia-sensor is performed by means of a weighting function (74).

10. Method according to claim 1, characterized in that the corrected offset (O) is used when repeating the method_k) As an expected offset (O)_v）。

11. A machine-readable storage medium, on which a computer program is stored, the computer program being arranged to perform each step of the method according to any one of claims 1 to 10.

12. Electronic control device, which is provided for carrying out the correction (75) of the offset of the ammonia-sensor by means of a method according to any one of claims 1 to 10.