DE102017205322A1 - Method for detecting damage to an SCR catalyst - Google Patents

Abstract

The invention relates to a method for detecting damage to an SCR catalyst. In periods (t ₁ , t ₂ , t ₃ ) in which a model value for an ammonia loading (NH3 _mod ) of the SCR catalyst is above a predefinable release threshold (S), an ammonia charge (NH3 _mik ) of the SCR measured by means of a microwave _{sensor is determined} -Catalyst or derived therefrom monitoring factor with a predetermined threshold (D) compared. From the result of the comparison, it is concluded that the SCR catalytic converter has been damaged.

Description

The present invention relates to a method for detecting damage to an SCR catalyst. 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 (NOx) 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.

OBD legislation requires very close monitoring of the SCR catalyst in many markets. The requirements are often quoted as multiples of the emission limit. In Europe, fixed OBD limits are to be monitored. If the SCR catalytic converter ages, a warning to the driver must be given to the driver before the OBD limit value is exceeded.

Currently, aged or damaged SCR catalysts are monitored by evaluating the nitrogen oxide mass flows upstream and downstream of the catalyst. The necessary nitrogen oxide concentrations are measured with the help of nitrogen oxide sensors. However, these are subject to tolerances. In addition, ammonia slip, which often occurs at the high conversion rates required to meet the stringent emissions standards, is measured by nitric oxide sensors as nitric oxide.

In cases where the passive process is too inaccurate, a time consuming process is used that determines the ammonia storage capacity of the SCR catalyst by actively intervening in the dosing amount. This correlates very well with thermal damage or chemical damage poisoning of the SCR catalyst. Due to the modulation of the dosing this often leads to phases with reduced nitrogen oxide conversion. This has a negative influence on the nitrogen oxide emissions.

Disclosure of the invention

The method for detecting damage to an SCR catalytic converter expires in periods in which a model value for ammonia loading of the SCR catalytic converter is above a predefinable release threshold. During these periods, an ammonia charge of the SCR catalytic converter measured by means of a microwave sensor or a monitoring factor derived therefrom is compared with a predefinable threshold value. From the result of the comparison, it is concluded that the SCR catalytic converter has been damaged. The method can be used as passive monitoring of the SCR catalyst during operation without having to intervene in the dosing of the SCR catalyst. It makes it possible to detect damage, which in particular can also be due to age, so that it makes it possible to robustly and quickly determine the aging state of an SCR catalytic converter.

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 model value for the ammonia loading can in particular be calculated on the basis of a nitrogen oxide signal upstream of the SCR catalytic converter. This signal can either come from a nitrogen oxide emissions model of an internal combustion engine, the exhaust gases are passed into the SCR catalyst or from a nitrogen oxide sensor, which is arranged upstream of the SCR catalyst in an exhaust line. If another nitrogen oxide sensor is arranged in the exhaust gas line downstream of the SCR catalytic converter, its signal can likewise be taken into account in the model. Such a model value of the ammonia loading calculated in this way is calculated anyway in conventional operating strategies of SCR catalysts and can be compared with the release threshold in one embodiment of the method. In another embodiment of the method, however, it may alternatively be provided that a further model value of the ammonia loading is carried out according to a calculation method which is optimized for the method and takes into account, for example, OBD-specific parameters.

By comparing the model value of the ammonia loading with the release threshold, it is ensured that the SCR catalytic converter is diagnosed only at such a high ammonia load that a reliable statement can be made about the occurrence of damage. This is because in these operating points the expected Ammonia loading is much greater for an undamaged SCR catalyst than for a damaged SCR catalyst. For this purpose, the release threshold is specified in particular as a function of an ammonia minestore storage capability of the SCR catalyst which represents the expected maximum NH3 level of a damaged SCR catalytic converter. Since this is temperature-dependent, it is preferred that the release threshold is predetermined as a function of a temperature of the SCR catalyst.

In addition to exceeding the release threshold, further release conditions can be specified, which must be fulfilled before the comparison between the measured ammonia load or the monitoring factor takes place with a predefinable threshold value. Such a release condition may in particular be that no error relevant to the method, such as in particular a fault of a sensor evaluated in the method, is present. At high temperatures of the SCR catalyst this has a very low ammonia storage capacity, so that the model value of the ammonia loading can not exceed the release threshold. It comes at such temperatures to a kind of reset the ammonia level. A further preferred release condition therefore consists in that, after the temperature of the SCR catalytic converter has first exceeded a predefinable temperature threshold and then fallen below the threshold value again, the comparison between the measured ammonia load or the monitoring factor is suspended with a predefinable threshold value for a predeterminable period of time so that a sufficiently reliable model of ammonia loading can be considered again. Only with sufficient reliability of the model value can also be trusted in the reliability of the test, whether this is above the release threshold.

In the method, a defect threshold may be used that represents an ammonia load that should always be exceeded by an intact SCR catalyst in the periods when the model value is above the release threshold. The defect threshold is preferably set depending on a temperature and hydrocarbon loading of the SCR catalyst since both high temperature and high hydrocarbon loading degrade the ammonia storage capability of the SCR catalyst.

In one embodiment of the method, damage to the SCR catalyst is concluded when the comparison reveals that the measured ammonia loading is not above the defect threshold. The defect threshold thus acts as a threshold value. In this embodiment, the absolute values of the measured ammonia load and the threshold value are compared with each other. This is a particularly simple embodiment of the method to be implemented.

So that small disturbances of the measured ammonia loading do not directly lead to the detection of damage, it is preferred in this embodiment that a temporal debouncing occurs between the measured ammonia loading and the defect threshold.

In another embodiment of the method, a monitoring factor is compared with a monitoring threshold. The monitoring factor F is calculated according to formula 1: $$F=\frac{F_1}{F_2}$$

$$F_2={\displaystyle \int \left(NH{3}_{mod}-D\right)}$$

In this case, F ₁ denotes a factor counter, which is calculated according to formula 2 and F ₂ a factor denominator, which is calculated according to formula 3: $$F_1={\displaystyle \int \left(NH{3}_{mik}-D\right)}$$

$$F_2={\displaystyle \int \left(NH{3}_{mOd}-D\right)}$$

Here, NH3 _mik denotes the ammonia load measured by means of microwaves, NH3 _mod the model value for the ammonia load and D the defect threshold. The integration for calculating the factor counter and the factor denominator takes place in the periods in which the model value is above the release threshold. If the monitoring factor exceeds a monitoring threshold, then damage to the SCR catalyst is inferred. The method according to this embodiment enables particularly precise detection of damage. However, it requires more computing steps than the first embodiment. In order to ensure that the two integrals are based on a sufficiently large database in order to enable a very reliable detection of the damage, it is preferred that damage to the SCR catalyst is only concluded if the factor denominator exceeds a denominator threshold.

The computer program is set up to perform each step of the process, especially when running on a computing device or electronic controller. It allows the implementation of the method on a conventional electronic control unit without this to make structural changes. For this purpose it is stored on the machine-readable storage medium. By applying the computer program to a conventional electronic control unit, the electronic control unit is obtained, which is set up to detect damage to an SCR catalytic converter by means of the method.

list of figures

• 1 zeigt schematisch einen SCR-Katalysator, welcher mittels Ausführungsbeispielen des erfindungsgemäßen Verfahrens auf eine Schädigung untersucht werden kann.
• 2 zeigt in zwei Diagrammen den zeitlichen Verlauf mehrerer Werte in Ausführungsbeispielen des erfindungsgemäßen Verfahrens für einen intakten SCR-Katalysator.
• 3 zeigt in zwei Diagrammen den zeitlichen Verlauf mehrerer Werte in Ausführungsbeispielen des erfindungsgemäßen Verfahrens für einen geschädigten SCR-Katalysator.
• 4 zeigt ein Ablaufdiagramm eines Ausführungsbeispiels des erfindungsgemäßen Verfahrens.
• 5 zeigt ein Ablaufdiagramm eines anderen Ausführungsbeispiels des erfindungsgemäßen Verfahrens.

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

• 1 schematically shows an SCR catalyst, which can be examined by means of embodiments of the method according to the invention for damage.
• 2 shows in two diagrams the time course of several values in embodiments of the inventive method for an intact SCR catalyst.
• 3 shows in two diagrams the time course of several values in embodiments of the inventive method for a damaged SCR catalyst.
• 4 shows a flowchart of an embodiment of the method according to the invention.
• 5 shows a flowchart of another embodiment of the method according to the invention.

Embodiments of the invention

In 1 is an SCR catalyst 10 shown, which in an exhaust line 20 is arranged. The SCR catalyst 10 has a microwave sensor 11 to measure its ammonia load and a temperature sensor 12 to measure its temperature. Upstream of the SCR catalyst 10 is in the exhaust line 20, a first nitrogen oxide sensor 21 disposed and downstream of the SCR catalyst 10 is a second nitrogen oxide sensor 22 arranged. Between the first nitrogen oxide sensor 21 and the SCR catalyst 10 is a metering valve 30 arranged to inject a urea water solution into the exhaust line 20. It is powered by an electronic control unit 40 which also controls the measuring signals of the sensors 11 . 12 . 21 . 22 receives.

The microwave sensor 11 continuously provides a signal for a measured ammonia load NH3 _{mic of} the SCR catalyst 10 , From the measurement results of the nitrogen oxide sensors 21 . 22 and the control of the metering valve 30 creates the electronic control unit 40 continuously modeled NH3 _mod for the ammonia loading of the SCR catalyst 10 , The time course of these values for an intact SCR catalyst 10 is in 2 shown. There, the time course of a release threshold S is further shown, by the electronic control unit 40 continuously depending on a temperature of the SCR catalyst 10 is calculated, which is measured by means of the temperature sensor 12. Furthermore, the electronic control unit 40 calculates from the temperature of the SCR catalyst and a modeled hydrocarbon loading of the SCR catalyst 10 a defect threshold D, whose time course is also in 2 is shown. In the exemplary embodiments of the method according to the invention described below, the detection of damage to the SCR catalytic converter is released only in the periods t ₁ , t ₂ , t ₃ in which the model value NH3 _mod exceeds the release threshold S. In these periods t ₁ , t ₂ , t ₃ an evaluation takes place A.

In 3 is the time course of in 2 shown values for an SCR catalyst 10, which is damaged by aging.

The sequence of a first embodiment of the method according to the invention is shown in FIG 4 shown. After the start 50 the procedure is as long as a comparison 51 of the model value NH3 _mod with the release threshold S until the model value NH3 _mod exceeds the release threshold S. At the same time it checks if there is no error of the sensors 11 . 12 . 21 . 22 is present and whether by means of the temperature sensor 12 measured temperature of the SCR catalyst 10 after exceeding a temperature threshold, was below this temperature threshold for a given period of time. All these release conditions are in the in the 2 and 3 shown periods t ₁ , t ₂ and t ₃ met. Subsequently, a comparison is made 52 the measured ammonia load NH3 _mic with the defect threshold D. For an intact SCR catalyst 10, as in the. 2 is shown in the periods t ₁ , t ₂ and t ₃ respectively found that the measured ammonia load NH3 _{mik is} above the defect threshold D. Thereupon it is detected 53 that the SCR catalyst 10 is intact. In the case in 3 is shown, in the periods t ₁ , t ₂ and t _3, the defect threshold is temporarily below each of the measured ammonia load NH3 _mic and it is a damage 54 of the SCR catalyst 10 recognized.

In another embodiment of the method according to the invention, which in 5 is shown, takes place after the start 60 of the procedure, first a test 61 on the fulfillment of the same release conditions, which in the test 51 of the first embodiment are checked. Once these are met, a calculation is made 62 one Factor counter F _{1 of} a factor denominator F ₂ and a monitoring factor F using the formulas 1 to 3 , In a further test 63 it is checked whether the factor denominator F _{2 is} above a denominator threshold N. If this release condition is not fulfilled, a return to the check is made 61 the other release conditions. Otherwise, the monitoring factor F is compared with a monitoring threshold Ü 64. For the course of the measured ammonia load NH3 _mik and the model value NH3 _mod according to 2 results from the integrals of the formulas 1 to 3 a monitoring factor F, which is at least as large as the monitoring threshold Ü. From this it is recognized 65 that the SCR catalyst is intact. If, on the other hand, the measured ammonia load is NH3 _mik and the model value NH3 _mod in 3 shown monitoring, the monitoring factor F is below the monitoring threshold Ü, resulting in damage 66 of the SCR catalyst 10 is closed.

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 [0007]
• DE 102010034983 A1 [0007]

Claims (10)

Method for detecting damage (54, 66) of an SCR catalytic converter (10), wherein in periods (t ₁ , t ₂ , t ₃ ) in which a model value for an ammonia load (NH _{3 mod} ) of the SCR catalytic converter (10) Above a predefinable release threshold (S) is located (51, 61), a by means of a microwave sensor (11) measured ammonia load (NH3 _mik ) of the SCR catalyst (10) or a derived from this monitoring factor (F) with a predetermined threshold (D, Ü) is compared (52, 64) and from the result of the comparison (52, 64) on damage (54, 66) of the SCR catalyst (10) is closed. Method according to Claim 1 , characterized in that the release threshold (S) in dependence on a temperature of the SCR catalyst (10) is predetermined. Method according to Claim 1 or 2 , characterized in that a damage (54) of the SCR catalyst (10) is concluded (54), if the comparison (52) shows that the measured ammonia load (NH3 _mik ) is not above a defect threshold (D). Method according to Claim 3 , characterized in that the comparison between the measured ammonia load (NH3 _mik ) and the defect threshold (D) is time debounced. Method according to Claim 1 or 2 , characterized in that according to the following formulas a monitoring factor F is calculated for a factor counter F ₁ and a factor denominator F ₂ (62) $$\begin{array}{ccc}F=\frac{F_1}{F_2}.& F_1={\displaystyle \int \left(NH{3}_{mik}-D\right).}& F_2={\displaystyle \int \left(NH{3}_{mOd}-D\right)}\end{array}$$

wherein NH3 _mik measured by means of microwaves ammonia loading, NH3 _mod the model value for ammonia loading and D denotes a defect threshold, and that a damage (66) of the SCR catalyst (10) is closed when the comparison (64) indicates that the Monitoring factor F exceeds a monitoring threshold (Ü). Method according to Claim 5 , characterized in that only when damage (66) of the SCR catalyst (10) is closed when the factor denominator (F ₂ ) exceeds a denominator threshold (N) (63). Method according to one of Claims 3 to 6 , characterized in that the defect threshold (D) in dependence on a temperature and of a hydrocarbon loading of the SCR catalyst (10) is predetermined. 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. An electronic control unit (40) adapted to operate by a method according to any one of Claims 1 to 7 to detect damage (54, 66) of an SCR catalyst (10).

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