DE102017200542A1 - Method for determining a nitrogen oxide mass flow - Google Patents

Abstract

The invention relates to a method for determining a nitrogen oxide mass flow in an exhaust line of an internal combustion engine upstream of an SCR catalytic converter. The nitrogen oxide mass flow is determined from an ammonia level change of the SCR catalyst in at least one period (72, 73, 74) in which an ammonia mass is metered into the SCR catalytic converter, which falls below an ammonia threshold, and from a nitrogen oxide mass flow downstream of the SCR catalytic converter. The ammonia level change is measured by means of microwaves.

Description

The present invention relates to a method for determining a nitrogen oxide mass flow in an exhaust line of an internal combustion engine upstream of an SCR catalytic converter. Furthermore, the present invention relates to a computer program executing each step of the method and to a machine-readable storage medium storing the computer program. Finally, the invention relates to an electronic control device which is set up to carry out the method.

State of the art

For the reduction of nitrogen oxides in the exhaust of motor vehicles SCR catalysts are used (Selective Catalytic Reduction). Nitrogen oxide molecules are reduced to elemental nitrogen on the catalyst surface in the presence of ammonia as a reducing agent. The reducing agent is in the form of a ammoniakabspaltenden urea solution (HWL) provided, which is commercially available under the name AdBlue ^®. This is injected into the exhaust line through a metering valve upstream of the SCR catalyst.

The use of SCR catalysts also requires the use of at least one nitrogen oxide sensor. Frequently, even a respective nitrogen oxide sensor is used upstream or downstream of the SCR catalyst. The legislation of many states requires close monitoring of these nitric oxide sensors for drift.

Drifting in the characteristic line of a nitrogen oxide sensor upstream of the SCR catalytic converter can be carried out by comparing the sensor value with a nitrogen oxide model value. Both values are integrated over a given period of time. By evaluating the integrals, a decision can be made as to whether the sensor is still intact. The accuracy of the nitrogen oxide modeling depends on the tolerances of the air and injection system as well as environmental conditions such as humidity in particular. In order to achieve a sufficient accuracy of modeling, therefore, often additional humidity sensors must be provided.

Disclosure of the invention

The method for determining a nitrogen oxide mass flow in an exhaust line of an internal combustion engine upstream of an SCR catalyst provides that the nitrogen oxide mass flow from an ammonia level change of the SCR catalyst in at least one period in which an ammonia mass is metered into the SCR catalyst, which falls below an ammonia threshold , and is determined from a nitrogen oxide mass flow downstream of the SCR catalyst. In particular, no ammonia is metered into the SCR catalyst at all during the period. The ammonia level change is measured by means of microwaves. A method for determining the ammonia level of an SCR catalyst by means of microwaves are, for example, from DE 103 58 495 B4 and from the DE 10 2010 034 983 A1 known. These documents are incorporated by reference into this disclosure. The precise determination of the ammonia level by means of microwaves allows a very good correlation with the nitrogen mass flow.

For determining the nitrogen oxide mass flow, the contribution of the reducing agent metering into the exhaust gas line is also relevant. It should be taken into account, which proportion of the metered amount of ammonia is stored in the SCR catalyst. This would allow for possible losses such as ammonia slip or ammonia oxidation at high temperatures as well as poor hydrolysis at lower temperatures. In order to reduce the influence of the dosing amount, the determination of the stock oxide mass flow in different embodiments of the method is carried out when no dosing takes place anyway or the dosing is reduced or switched off for the purpose of detection for a certain time.

One embodiment of the method in which the dosage is actively engaged provides for a reduction or suspension of the dosage amount under suitable conditions and for a given period of time. The ammonia level change of the SCR catalyst is measured in a period in which a metering of ammonia in the SCR catalyst is actively suspended or reduced compared to a normal operation of the SCR catalyst until an integral of the reacted in the SCR catalyst nitrogen oxide mass exceeds a nitrogen oxide threshold. In this case, a complete suspension of metering is preferred. In this case, the amount of nitrogen oxide stored in the SCR catalyst depends exclusively on the nitrogen oxide mass flow upstream of the SCR catalyst and the conversion of the SCR catalyst. At suitable temperatures and under favorable operating conditions, a complete conversion of almost 100% can be assumed. The decrease in the level thus correlates directly with the nitrogen oxide mass upstream of the SCR catalyst. The nitrogen oxide conversion and thus the expected decrease in the ammonia level in the SCR catalyst can be from the nitrogen oxide mass flow downstream of the SCR catalyst be calculated. This can be measured in particular by means of a nitrogen oxide sensor which is arranged downstream of the SCR catalytic converter in the exhaust gas line. The decrease of the nitrogen oxide level measured via the microwave signal is compared with the expected decrease. If the measured difference lies below or above the expected value by a predefinable threshold value, an error is detected.

• - Der mittels Mikrowellen im SCR-Katalysator gemessene Ammoniakfüllstand überschreitet einen vorgegebenen Füllstandsschwellenwert. Auf diese Weise wird gewährleistet, dass ein ausreichend hoher Umsatz von Stickoxiden während der Durchführung des Verfahrens erfolgt.
• - Eine Temperatur und ein Temperaturgradient des SCR-Katalysators liegen jeweils unter einem ersten Temperaturschwellenwert und einem ersten Temperaturgradientenschwellenwert. Bei hohen Temperaturen nimmt die Ammoniakspeicherfähigkeit des SCR-Katalysators ab, was die Durchführung des Verfahrens unmöglich machen kann. Steigende Temperaturen deuten darauf hin, dass die Temperatur in unmittelbarer Zukunft zu hoch für die Durchführung des Verfahrens sein kann. Es ist daher bevorzugt, dass der Temperaturgradientenschwellenwert in Abhängigkeit von der aktuellen Temperatur des SCR-Katalysators vorgegeben wird.
• - Die Temperatur und der Temperaturgradient des SCR-Katalysators liegen jeweils über einem zweiten Temperaturschwellenwert und einem zweiten Temperaturgradientenschwellenwert. So kann sichergestellt werden, dass der SCR-Katalysator eine ausreichend hohe Konvertierungsfähigkeit besitzt. Negative Temperaturgradienten deuten darauf hin, dass die Temperatur in unmittelbarer Zukunft für die Durchführung des Verfahrens zu niedrig sein kann. Daher ist es bevorzugt, dass der zweite Temperaturgradientenschwellenwert in Abhängigkeit von der aktuellen Temperatur des SCR-Katalysators vorgegeben wird.
• - Der Abgasmassenstrom des Verbrennungsmotors liegt in einem vorgegebenen Bereich. Insbesondere ein zu hoher Abgasmassenstrom kann die Konvertierungsfähigkeit des SCR-Katalysators und somit das Ergebnis der Ermittlung negativ beeinflussen.
• - Ein Stickoxidsensor ist messbereit, damit sein Signal in dem Verfahren ausgewertet werden kann.
• - Es liegt kein Systemfehler vor. Die für dieses Überwachungskriterium zu berücksichtigenden Systemfehler können dabei aus Systemfehlern ausgewählt werden, welche für den Betrieb des SCR-Katalysators relevant sind, wie beispielsweise ein Fehler eines Hitzdrahtanemometers (HFM).

A release of the active suspension or reduction of the ammonia dosing is preferably carried out only if one or more monitoring criteria are met. Suitable monitoring criteria are selected in particular from the following list:

• - The measured by microwaves in the SCR catalyst ammonia level exceeds a predetermined level threshold. In this way it is ensured that a sufficiently high conversion of nitrogen oxides takes place during the implementation of the method.
• A temperature and a temperature gradient of the SCR catalytic converter are each below a first temperature threshold value and a first temperature gradient threshold value. At high temperatures, the ammonia storage capacity of the SCR catalyst decreases, which may make the performance of the process impossible. Rising temperatures indicate that the temperature may be too high for the process in the immediate future. It is therefore preferred that the temperature gradient threshold value is predefined as a function of the current temperature of the SCR catalytic converter.
• The temperature and the temperature gradient of the SCR catalyst are each above a second temperature threshold value and a second temperature gradient threshold value. This ensures that the SCR catalyst has a sufficiently high conversion capability. Negative temperature gradients indicate that the temperature may be too low in the immediate future to carry out the process. Therefore, it is preferable that the second temperature gradient threshold is given as a function of the current temperature of the SCR catalyst.
• - The exhaust gas mass flow of the internal combustion engine is within a predetermined range. In particular, an excessive exhaust gas mass flow can adversely affect the conversion capability of the SCR catalytic converter and thus the result of the determination.
• - A nitric oxide sensor is ready to measure, so that its signal can be evaluated in the process.
• - There is no system error. The system errors to be considered for this monitoring criterion can be selected from system errors that are relevant to the operation of the SCR catalytic converter, such as a hot-wire anemometer (HFM) fault.

If one of the release conditions is no longer met before the integral of the reacted nitrogen oxide mass exceeds the nitrogen oxide threshold, the monitoring may be suspended from the ammonia level change until there is again clearance for the process and the process can then be completed in another phase.

In order to increase the nitrogen oxide mass flow upstream of the SCR catalytic converter, it is preferred in this embodiment of the method that an exhaust gas recirculation rate in the at least one period is reduced.

In another embodiment of the method, this is done passively. This means that the ammonia level change is measured during periods in which the ammonia dosage falls below a dosing amount threshold in an operating strategy of the SCR catalytic converter until an integral of the nitrogen oxide mass reacted in the SCR catalytic converter exceeds a threshold value. In particular, a signal of the nitrogen oxide sensor arranged downstream of the SCR catalytic converter in the exhaust gas line is used for the calculation of the integral. Also in the passive method, the conditions of the active method for the respective periods can be queried. The reliability of the detection result increases the lower the dosing amount of ammonia in the periods is. In order to keep the influence of the uncertainty of the dosing quantity low, the evaluation therefore takes place in periods in which the ammonia dosing falls below the dosing quantity threshold value. This can be selected depending on the current nitrogen oxide mass flow upstream of the SCR catalytic converter. A suitable for passive process control low ammonia dosage occurs, for example, with increasing temperatures of the SCR catalyst. As a result of the resulting ammonia storage capacity of the SCR catalyst, an excess of ammonia is present in the latter in comparison with a desired value. To reduce the level, the dosage is reduced or even turned off completely.

When the process is performed in a temperature range in which the Ammonia storage capacity of the SCR catalyst is low, which can be determined by the fact that the temperature falls below a third temperature threshold, may be required as a further release condition, in particular, that a stoichiometric Reduktionsmitteldosierung done, ie just as much urea water solution is metered into the exhaust line, as necessary is to reduce the current nitrogen oxide mass flow. In this case, it is expected that the ammonia level will not change. However, if it increases, when the integral of the reacted nitrogen oxide mass exceeds its threshold, it can be assumed that the nitrogen oxide mass flow upstream of the SCR catalyst has been measured or modeled too high. If it falls, however, then it can be concluded that the nitrogen oxide mass flow measured or modeled is too low, based on the actual nitrogen oxide mass flow.

In one embodiment of the method, the nitrogen oxide mass flow is compared with a nitrogen oxide mass flow, which was measured by a nitrogen oxide sensor arranged in the exhaust gas line upstream of the SCR catalytic converter. In this way, this nitrogen oxide sensor can be monitored for larger characteristic or offset errors.

In another embodiment, the nitrogen oxide mass flow is compared with a model of nitrogen oxide emissions of the internal combustion engine. In this way, the model can be made plausible or corrected. In this embodiment, the speed and / or fuel injection quantity of the internal combustion engine can be used as further release conditions. If these have too high dynamics, so that their gradient exceeds a predefinable threshold, it is preferred that the ammonia level change is not measured in this period.

The method can in particular also be carried out in two stages. By default, passive monitoring of the nitrogen oxide sensor or of the model takes place. If the result is correct, no further diagnosis is carried out. On the other hand, if the result is not correct, in a second step the active diagnosis is carried out, the exact result of which is used for the final evaluation of a possible error.

When a nitrogen oxide storage catalyst (NSC) is disposed upstream of the SCR catalyst, it is preferable to inhibit the NSC regeneration for the period of measuring the ammonia level change.

If two SCR catalysts are arranged in the exhaust gas line, the method can be applied to only one of the two SCR catalysts. For this purpose, a model of the ammonia level change has to be designed specifically for this SCR catalytic converter. Alternatively, the method can be applied to both SCR catalysts together. For this purpose, the fill levels of the SCR catalysts determined by means of microwaves are balanced and deviations of the nitrogen oxide mass flows determined therefrom are added by expected values from a nitrogen oxide sensor signal or from a model.

The computer program is set up to perform each step of the process, especially when running on an electronic control unit. It allows the implementation of different embodiments of the method in an electronic control unit, without having to make structural changes. This 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 determine by the method, a nitrogen oxide mass flow in an exhaust line of an internal combustion engine upstream of an SCR catalyst.

list of figures

• 1 zeigt schematisch einen Abgasstrang, in dem ein Stickoxidmassenstrom mittels Ausführungsbeispielen des erfindungsgemäßen Verfahrens ermittelt werden kann.
• 2 zeigt ein Ablaufdiagramm eines Ausführungsbeispiels des erfindungsgemäßen Verfahrens.
• 3 zeigt in einem Diagramm die zeitliche Änderung von Ammoniakfüllständen in einem Ausführungsbeispiel des erfindungsgemäßen Verfahrens mit aktiver Ammoniakunterdosierung.
• 4 zeigt in einem Diagramm den zeitlichen Verlauf von Ammoniakfüllständen in einer passiven Ausführungsform des erfindungsgemäßen Verfahrens.
• 5 zeigt schematisch einen weiteren Abgasstrang, in dem mittels Ausführungsbeispielen des erfindungsgemäßen Verfahrens ein Stickoxidmassenstrom ermittelt werden kann.

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

• 1 schematically shows an exhaust line, in which a nitrogen oxide mass flow can be determined by means of embodiments of the method according to the invention.
• 2 shows a flowchart of an embodiment of the method according to the invention.
• 3 shows a diagram of the temporal change of ammonia levels in an embodiment of the method according to the invention with active Ammoniaunterdosierung.
• 4 shows a diagram of the time course of Ammoniakfüllständen in a passive embodiment of the method according to the invention.
• 5 schematically shows a further exhaust gas line, in which by means of embodiments of the method according to the invention, a nitrogen oxide mass flow can be determined.

Embodiments of the invention

An internal combustion engine 10 of a motor vehicle used in 1 is shown, emits exhaust gases in an exhaust line 20 , In this one after another a nitrogen oxide storage catalyst 31 , a coated diesel particulate filter 32 and an SCR catalyst 33 arranged. A metering valve 21 between the diesel particulate filter 32 and the SCR catalyst 33 is arranged to urea water solution in the exhaust line 20 inject. A first nitrogen oxide sensor 41 is between the diesel particulate filter 32 and the SCR catalyst 33. A second nitrogen oxide sensor 42 is located downstream of the SCR catalyst 33. A microwave sensor 51 is on the SCR catalyst 33 arranged. It is set up for the ammonia level of the SCR catalyst 33 to eat. An electronic control unit 11 of the internal combustion engine 10 also controls the dosing valve 21 , It receives data from the nitrogen oxide sensors 41 . 42 and from the microwave sensor 51 ,

The first nitrogen oxide sensor 41 provides a nitrogen oxide concentration in exhaust gas, which can be converted by means of the known exhaust gas mass flow into a measured nitrogen oxide mass flow. For its plausibility can be a start 50 an embodiment of the method according to the invention carried out by an active diagnosis is performed. For this purpose, first an examination 51 whether all release conditions of the diagnosis have been fulfilled. Only if this is the case, a continuation takes place 52 of the procedure. As in 3 is now an active underdose 61 of HWL by means of the metering valve 21 switched on until the signal from the second nitrogen oxide sensor 42 It can be recognized that in the SCR catalyst 33 reacted nitrogen oxide mass exceeds a predetermined nitrogen oxide threshold. In a time span 71 from the beginning of the underdosing at time t ₁ to the end of the underdosing at time t ₂ , the decrease of the means of the microwave sensor 51 measured ammonia level NH ₃ _Mik in the SCR catalyst 33 and the decrease of a modeled ammonia level NH ₃ _Mod determined. If another test 53 shows that the required nitrogen oxide mass in the period 71 was reduced, then carried out an evaluation 54 the decreases of the two ammonia levels NH ₃ _Mik and NH ₃ _Mod. Gives another test 55 in that the measured ammonia level NH ₃ _Mik lies within a predetermined diagnostic window around the model value NH ₃ _Mod, which is shown in the in 3 illustrated embodiment is the case, then the result 56 get that first nitric oxide sensor 41 okay. Otherwise it will be an error 57 of the first nitrogen oxide sensor 41 recognized.

In another embodiment of the method according to the invention this is carried out passively. During the exam 51 whether the conditions of the diagnosis are met, there is also a check whether the metering of HWL in the exhaust system 20 in the current operating strategy of the SCR catalyst 33 falls below a predetermined Dosiermengenschwellenwert. In 4 is shown that this condition 62 in three periods 72 . 73 . 74 is satisfied. Only in the third period 74 reaches the integral of the reduced nitrogen oxide mass NOx_red a predetermined nitrogen oxide threshold NOx_S. In all three periods 72 . 73 . 74 In each case, the change in the measured ammonia level NH ₃ _Mik and the modeled ammonia level NH ₃ _mod is determined. These are in 4 represented as the integral of their negative changes. Only after the end of the third period 74 If an examination 53 determines whether the reduced nitrogen oxide mass NOx_Red exceeds its nitrogen oxide threshold value NOx_S, the method proceeds with an evaluation 54 the determined values continued. In this case, only the ammonia level changes are considered up to a time t ₃ at which the nitrogen oxide threshold NOx_S was reached. If the following exam 55 reveals that the measured ammonia level change _Mik NH ₃ NH ₃ is _Mod within the diagnostic window around the model value, which in the in 4 illustrated embodiment, this leads to the result that the first nitrogen oxide sensor 41 okay. Otherwise it will be an error 57 of the first nitrogen oxide sensor 41 recognized.

In other embodiments of the method according to the invention, both the active diagnosis and the passive diagnosis can be carried out on an exhaust gas system, which in 5 is shown. In this is between the nitrogen oxide storage catalyst 31 and the SCR catalyst 33 instead of the diesel particulate filter 32 a SCRF catalyst (SCR on filter) 34 is arranged. This has a second microwave sensor 52 on. Between the NOx storage catalyst 31 and the SCRF catalyst 34 are a second metering valve 22 and a third nitrogen oxide sensor 43 arranged. Embodiments of the inventive method may be used to diagnose the first nitrogen oxide sensor 41 using the results of the second nitrogen oxide sensor 42 and the first microwave sensor 51 perform. This corresponds to the diagnosis made in the 2 to 4 is shown. Using the measurement results of the first nitrogen oxide sensor 41 and the second microwave sensor 52 may also in a similar manner, a diagnosis of the third nitrogen oxide sensor 43 respectively. Alternatively, it is also possible to record on the measurement results of both microwave sensors 51, 52 and further use of the measurement result of the second nitrogen oxide sensor 42 the third nitrogen oxide sensor 43 be diagnosed.

Instead of a diagnosis of a nitrogen oxide sensor, in further exemplary embodiments of the method according to the invention in the absence of a nitrogen oxide sensor upstream of the respective SCR sensor, Catalyst also a model of nitrogen oxide emissions of the internal combustion engine 10 be checked.

In all embodiments of the method according to the invention is a regeneration of the nitrogen oxide storage catalyst 31 blocked for the duration of the diagnosis.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 10358495 B4 [0005]
• DE 102010034983 A1 [0005]

Claims (10)

Method for determining a nitrogen oxide mass flow in an exhaust line (20) of an internal combustion engine (10) upstream of an SCR catalytic converter (33, 34), characterized in that the nitrogen oxide mass flow from an ammonia level change of the SCR catalytic converter (33, 34) in at least one period ( 71, 72, 73, 74) in which an ammonia mass is metered into the SCR catalyst (33, 34), which falls below an ammonia threshold, and from a nitrogen oxide mass flow downstream of the SCR catalyst (33, 34) is determined, wherein the ammonia level change is measured by means of microwaves. Method according to Claim 1 characterized in that the ammonia level change of the SCR catalyst (33, 34) is measured over a period of time (71) in which metered addition of ammonia into the SCR catalyst (33, 34) is actively suspended or reduced until an integral of the nitrogen oxide mass reacted in the SCR catalytic converter (33, 34) exceeds a nitrogen oxide threshold value. Method according to Claim 2 , characterized in that an exhaust gas recirculation rate in the period (71, 72, 73, 74) is reduced. Method according to Claim 1 , characterized in that the measurement of the ammonia level change so long in periods (72, 73, 74) takes place, in which in an operating strategy of the SCR catalyst (33, 34) a metering of ammonia into the SCR catalyst (33, 34) falls below a metering amount threshold until an integral (NOx_red) of the nitrogen oxide mass reacted in the SCR catalytic converter (33, 34) exceeds a nitrogen oxide threshold value (NOx_S). Method according to one of Claims 2 to 4 characterized in that a signal of a nitrogen oxide sensor (42) arranged downstream of the SCR catalytic converter (33, 34) in the exhaust gas line (10) is used for a calculation of the integral (NOx_red). Method according to one of Claims 1 to 5 , characterized in that the nitrogen oxide mass flow is compared with a nitrogen oxide mass flow which has been measured by a nitrogen oxide sensor (41, 43) arranged upstream of the SCR catalytic converter (33, 34) in the exhaust gas line. Method according to one of Claims 1 to 6 , characterized in that the nitrogen oxide mass flow is compared with a model of nitrogen oxide raw emissions of the internal combustion engine (10). Computer program, which is set up, each step of the procedure according to one of Claims 1 to 7 perform. Machine-readable storage medium on which a computer program is based Claim 8 is stored. Electronic control unit (11), which is set up to operate by means of a method according to one of Claims 1 to 7 to determine a nitrogen oxide mass flow in an exhaust line (20) of an internal combustion engine (10) upstream of an SCR catalytic converter (33, 34).

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