阅读说明：本技术 用于运行scr催化器的方法 (Method for operating an SCR catalyst ) 是由 A.弗里奇 R.魏因曼 S.拉德曼 T.赫夫肯 于 2019-06-20 设计创作，主要内容包括：本发明涉及一种用于运行SCR催化器的方法。其中,获知(34)还原剂溶液的浓度与额定值的偏差(?c)并且根据所述偏差进行(35)注入的还原剂溶液的计量量相对于要求的计量量的改变(?m)。(The invention relates to a method for operating an SCR catalyst. Wherein the deviation (Δ c) of the concentration of the reducing agent solution from the setpoint value is determined (34) and the change (Δ m) of the metering quantity of the reducing agent solution injected is carried out (35) as a function of the deviation relative to the requested metering quantity.)

1. Method for operating an SCR catalyst (12), wherein a deviation (Δ c) of the concentration of the reducing agent solution (21) from a target value is determined (34) and a change (Δ m) of the metered quantity of the reducing agent solution injected is determined (35) as a function of the deviation relative to a requested metered quantity.

2. Method according to claim 1, characterized in that the change (Δ m) is made with a linear dependency on the offset (Δ c).

3. Method according to claim 1, characterized in that the change is made only when the value of the offset (Δ c) exceeds a threshold value.

4. Method according to claim 3, characterized in that the change (Δ m) is made with a linear dependency on the difference between the offset and the threshold.

5. Method according to any of claims 1 to 4, characterized in that the change (Δ m) is made only when the offset (Δ c) is negative.

6. Method according to any of claims 1 to 5, characterized in that the sensor values of the quality sensor (20) are used as for signal processingInput values, the signal processing having two filters (32, 33) with different time constants (k)₁、k₂) And the offset (Δ c) is known from the difference between the two filters (32, 33).

7. Method according to claim 6, characterized in that the time constant (k) of the slower filter (33) is₂) The selection (36) is based on a nitrogen oxide-adapted control rate of the SCR catalyst (12).

8. Computer program arranged to perform each step of the method according to any one of claims 1 to 7.

9. A machine-readable storage medium on which the computer program according to claim 8 is stored.

10. Electronic control device (22) which is provided for operating an SCR catalyst (12) by means of a method according to one of claims 1 to 7.

Technical Field

The invention relates to a method for operating an SCR catalyst. Furthermore, the invention relates to a computer program which carries out each step of the method, and to a machine-readable storage medium which stores the computer program. Finally, the invention relates to an electronic control device which is provided for carrying out the method.

Background

In order to meet the increasingly stringent exhaust gas legislation, it is necessary to reduce the nitrogen oxides in the exhaust gases of internal combustion engines, in particular diesel motors. For this purpose, it is known to arrange an SCR catalyst (selective catalytic reduction) in the exhaust gas line, which reduces the nitrogen oxides contained in the exhaust gas of the internal combustion engine to nitrogen in the presence of a reducing agent. The proportion of nitrogen oxides in the exhaust gas can thereby be greatly reduced. Ammonia, which is mixed into the exhaust gas, is required for the reaction process. An aqueous urea solution (urea-water solution; HWL) is generally used, which is injected before the SCR catalyst in the exhaust line and which serves as a reagent for separating off ammonia. An aqueous 32.5% urea solution is commercially available under the trade name AdBlue.

Legislation on emissions protection requires monitoring the quality of the HWL used by means of a urea quality sensor in order to identify, in particular, possible false refills (fehlbankung) by the vehicle driver. Mis-priming may be performed, for example, with water or diluted HWL. If such a wrong filling is recognized, the driver is warned and finally the operation of the vehicle is reduced in the so-called "Inducement" (inductive). It is described in DE 102015215388 a1 how the dilution of the content water of a HWL tank can be recognized with a higher accuracy than the absolute measurement accuracy of a quality sensor. For this purpose, the difference between the sensor signal filtered with a small time constant and the sensor signal filtered with a large time constant is taken into account. A large difference can be created by mixing water to rapidly change the urea concentration. If the threshold is exceeded in this case, an error is set and a "lead" is introduced.

If no faulty filling is detected, the dosing strategy of the HWL into the SCR catalyst is carried out independently of the urea content of the HWL. In this case, a pre-controlled metering or a regulated metering can be carried out. In the case of pilot metering, the currently required quantity of HWL is determined from a suitable characteristic field and metered in. The nitrogen oxide conversion or ammonia level of the SCR catalyst is determined with a nitrogen oxide sensor positioned downstream of the SCR catalyst in the case of a regulated dosing and the adaptation of the dosing quantity is known from a suitable model (adaptation). The adaptation is performed relatively slowly, i.e. on a time scale of hours.

Disclosure of Invention

In a method for operating an SCR catalyst, a deviation of the concentration of the reducing agent solution from a target value is first determined. The reducing agent solution is in particular HWL, which is stored in a reducing agent tank of the SCR catalyst. The setpoint value corresponds in particular to the expected concentration of HWL available on the market, i.e. 32.5%. If a metered quantity of reducing agent solution is required, which is to be injected into the exhaust line upstream of the SCR catalyst, a change of the metered quantity actually injected relative to the required metered quantity is carried out in the method on the basis of a previously known concentration deviation.

Conventional methods for operating an SCR catalyst can only react with "induced" measures to a wrong filling with water or diluted HWL and thus with a negative deviation of the concentration of the reducing agent solution from the setpoint value. However, it is not possible to completely avoid that a vehicle with an SCR catalyst arranged in its exhaust line emits higher nitrogen oxide emissions than permitted by legislation when using a dilute HWL. In contrast, the present invention makes it possible to increase the metered amount of diluted HWL to such an extent that the actual metered amount of urea is in turn in accordance with the requirements and thus an optimized reduction of nitrogen oxides by the SCR catalyst can be achieved.

In a particularly simple embodiment of the method, this is achieved in that the change in the injected metered quantity relative to the required metered quantity is effected in a linear dependence on the concentration deviation. If the deviation is negative, which indicates a diluted HWL, a positive change is made, i.e. the metered amount is increased. At a positive deviation, i.e. an increased HWL concentration, the dosing amount is reduced relative to the request.

In a further embodiment of the method, the change is only made if the value of the deviation exceeds a threshold value. The threshold value is in particular selected such that it corresponds to the natural fluctuation width of the sensor signal of the urea quality sensor used. In this way, it is prevented that the method is involved in the regulation of the metering variable due to natural fluctuations in the sensor signal and thus additional metering deviations occur. When the value of the deviation exceeds the threshold value, then in this embodiment it is preferred that the change in the injected metered quantity is made with a linear dependence on the difference between the deviation and the threshold value. This results in: as the threshold is exceeded, the metering change proceeds slowly from the zero value at the time the threshold is reached.

Thus avoiding a sudden jump in the change when the threshold is exceeded.

A misfueling generally results only in a decrease in the urea concentration of the HWL, but not in an increase. Therefore, concentrations of HWL that are too high are not believed. For this purpose, it is preferred that the change of the injected metered quantity is carried out only when the deviation is negative and that a positive deviation does not lead to a change of the metered quantity.

The deviation can preferably be determined in such a way that the sensor value of the quality sensor is used as an input value for the signal processing. The signal processing has two filters with different time constants. And thus a faster filter and a slower filter. The offset can be known from the difference between the two filters. The difference between the filters is herein understood to be the difference between a sensor value obtained by filtering with a faster filter and the same sensor value obtained by filtering with a slower filter. This procedure makes it possible to identify deviations with a higher accuracy than the absolute measurement accuracy of the sensor.

The two filters are designed in particular as PT1 filters. This is advantageous because the PT1 filter has a simple and well-known transmission behavior.

It is also preferred that the upper and lower frequencies of the raw signal of the sensor are cut off (abgeschnitten) by two filters and only the intermediate frequency range required for the evaluation is taken into account. This procedure has the advantage that the amount of data to be processed is reduced and thus shorter calculation times are achieved.

If a regulated metering of the reducing agent solution is carried out, the long-term average concentration of the reducing agent solution approaches the current value and the dynamic deviation correspondingly subsides in the method. But at the same time becomes effective by a slow adaptation of the nox sensor to the metering quantity. It is therefore preferred that the time constant of the slow filter is selected as a function of the nitrogen oxide adaptation regulation speed of the SCR catalytic converter, so that undesirable metering deviations are not produced by this method.

The computer program is provided for carrying out each step of the method, in particular when the method is run on a computing device or an electronic control device. To this end, the computer program is stored on a machine-readable storage medium. By uploading the computer program onto a conventional electronic control unit, an electronic control unit is obtained which is provided for operating the SCR catalytic converter by means of the method.

Drawings

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

Fig. 1 schematically shows an SCR catalyst, which can be operated by means of an embodiment of the method according to the invention.

Fig. 2 shows a flow chart of an embodiment of the method according to the invention.

Fig. 3 graphically shows the correlation between the change in the concentration deviation of the reducing agent solution and the metered amount of reducing agent solution injected in an exemplary embodiment of the method.

Fig. 4 graphically shows the correlation between the deviation of the concentration of the reducing agent solution and the change in the metered amount of reducing agent solution injected in a further exemplary embodiment of the method according to the invention.

Fig. 5 graphically shows the correlation between the deviation of the concentration of the reducing agent solution and the change in the metered amount of reducing agent solution injected in a further exemplary embodiment of the method according to the invention.

Detailed Description

Fig. 1 shows a metering device of an SCR catalyst system in an exhaust gas line 10 of an internal combustion engine 11 of a motor vehicle for metering in an aqueous urea solution (HWL) as a reducing agent solution 21. The SCR catalyst system is used in a known manner for reducing nitrogen oxides in the exhaust gas of the internal combustion engine 11 by means of Selective Catalytic Reduction (SCR). For reduction, a reducing agent solution 21 is injected into the exhaust line 10 upstream of the SCR catalyst 12 via a metering valve 13. The reducing agent solution 21 is stored in the reducing agent tank 14. For extracting the reducing agent solution, a suction line 15 is provided, which is connected to a displacement pump 16. Which directs the reducing agent solution 21 through the pressure line 17 to the metering valve 13. A first nox sensor 18 is arranged in the exhaust line 10 between the internal combustion engine 11 and the metering valve 13. A second nitrogen oxide sensor 19 is arranged in the exhaust line 10 downstream of the SCR catalyst 12. A quality sensor 20 is disposed in the reducing agent tank 14 to measure the concentration of the reducing agent solution 21 stored therein. The quality sensor transmits its sensor data to the electronic control unit 22, to which the data of the two nox sensors 18, 19 are also supplied.

As shown in fig. 2, the first embodiment of the method according to the invention starts with a start 30 of the combustion engine 11. The quality sensor 20 provides a sensor raw signal relating to the concentration of the reducing agent solution 21 and transmits it to the electronic control unit 22. There, the sensor raw signal has a time constant k₁A faster filter 32 of e.g. 30 seconds and with a time constant k₂Filtering is performed for a slower filter 33 of, for example, 10 hours. The two filters 32, 33 are PT1 filters that cut off the upper and lower frequencies of the raw sensor signal. The difference between the filtered values is formed and the deviation of the concentration of the reducing agent solution 21 from the setpoint value of 32.5% is determined from the difference. When the metered amount of the reducing agent solution 21 is requested in the electronic control device 22, the requested metered amount is changed Δ m to calculate the metered amount to be actually injected. The metering valve 13 is operated based on the actual metered amount. The change Δ m is here based on a linear dependency on the deviation c, as it is shown in fig. 3, and it is saved as a characteristic line in the electronic control device 22. The time constant k of the slower filter 33 is followed₂The slower filter then serves as a basis for further filtering of the raw sensor signal 36. The method is then ended 37.

In a second embodiment of the method, the characteristic line for changing the relevance of Δ m to the offset c corresponds to the illustration in FIG. 4. A threshold value is predefined here, which excludes the intervention of the method into the metering quantity in the case of a small deviation Δ c. As long as the value of the deviation Δ c does not exceed the threshold, that is, as long as the negative deviation Δ c is not lower than the threshold having a negative number and the positive deviation Δ c does not exceed the threshold having a positive number, the change of the injected metering amount Δ m is not performed. The change m of the injected metering amount is set only if the deviation Δ c is still larger in value, wherein the value of the change m linearly increases with the further decreased deviation Δ c for the negative deviation Δ c and linearly falls with the further increased deviation Δ c for the positive deviation Δ c.

In a third embodiment of the method, the characteristic line for the negative offset Δ c corresponds to the characteristic line in the second embodiment. On the contrary, the error Δ c for the correct setting is always set, and the metering amount for the injection is not changed. This is shown in fig. 5.