Method for operating SCR system

Abstract

The invention relates to a method for operating an SCR system having a suction line and a return line of an internal combustion engine, in which at least one dosing valve doses a reagent upstream of at least one SCR catalyst, the reagent being fed from a tank to the dosing valve at a constant pressure by means of an actuatable pump, the method is characterized by the following steps: determining the rigidity for the SCR system; -determining a performance factor for a pump of the SCR-system; determining a first consumption deviation; -determining a correction function as a function of the temperature of the urea liquid, the rigidity and the performance factor; determining a corrected consumption deviation from the first consumption deviation and the correction function; the corrected consumption deviation is compared with a second consumption deviation which can be specified, and whether the urea liquid is dispensed excessively or insufficiently for the SCR system is determined from the comparison.

Description

Technical Field

[0001] The invention relates to a method for detecting a blockage of one or more injection openings of a dosing valve of an SCR system 25. The invention also relates to a computer program which, when run on a computer, performs all the steps of the method according to the invention. The invention also relates to a computer program product having a program code which is stored on a machine-readable carrier for executing the method when the program is executed on a computer or a controller. Background Art

[0002] In order to comply with increasingly stringent emission regulations, it is necessary to reduce nitrogen oxides (NO x ). It is known that an SCR catalytic converter (Selective Catalytic Reduction) is arranged in the exhaust gas region of an internal combustion engine. In the SCR catalytic converter, the nitrogen oxides contained in the exhaust gas of the internal combustion engine are reduced to nitrogen in the presence of a reducing agent. This can significantly reduce the proportion of nitrogen oxides in the exhaust gas. Ammonia (NH3) is required in the reduction process, which is mixed into the exhaust gas. Therefore, NH3 or an NH3-releasing agent is used as a reducing agent. Usually, an aqueous urea solution (urea water solution; HWL) is used for this purpose, which is injected into the exhaust system downstream of the SCR catalytic converter. Ammonia is formed from this solution, which acts as a reducing agent. 32% aqueous urea solution is commercially available under the trade name get.

[0003] A dosing system for an SCR catalytic converter according to the prior art comprises a reducing agent tank, a delivery module and a dosing valve. The reducing agent solution is delivered from the reducing agent tank via the delivery module to the dosing valve. The delivery module contains a pump which is connected to the dosing valve and, if necessary, to a reducing agent return by means of a pipeline. Such a pipeline system is referred to as a hydraulic system.

[0004] The California Air Resources Board (CARB) requires the identification of deviations in the dosing quantity in the catalytic converter system that lead to exceeding the defined nitrogen oxide emission limit values ​​of the exhaust system of the diesel engine. This can occur in particular due to a blockage of one or more injection openings of the dosing valve of the SCR system 25. Therefore, a reduction medium must be carried out. The identification between the required and actually dosed mass of the SCR system 25 is particularly due to the presence of a reductant return in the delivery module of the SCR system 25 and due to the fact that a certain mass of reductant is dosed. The influence of the air volume contained in the system on the pressure curve at the same time makes it impossible to detect metering quantity deviations by analyzing the gradient of the pressure curve in the hydraulic system or by deviations in the pump speed.

[0005] Therefore, in order to detect deviations in the dosing quantity of the urea-water solution due to blockage of one or more injection openings of the dosing valve, an additional mass flow sensor must be used in the hydraulic system of the SCR system 25 . Summary of the invention

[0006] According to the invention, an SCR system having an intake and return line of an internal combustion engine is provided with the features of the independent patent claims, as well as a computing unit and a computer program for its execution. Advantageous embodiments are the subject of the dependent claims and the following description.

[0007] In a first aspect, the invention relates to a method according to the invention for operating an SCR system having an intake and return line of an internal combustion engine, wherein at least one metering valve meters a reagent upstream of at least one SCR catalytic converter, wherein the reagent is conveyed from a tank to the metering valve at a constant pressure by means of a controllable pump, characterized by the following steps:

[0008] - Determine the rigidity for the SCR system;

[0009] - Determine the performance factor of the pump for the SCR system;

[0010] - determining a first consumption deviation;

[0011] - determining a correction function according to the temperature, rigidity and performance factor of the urea liquid;

[0012] - determining a corrected consumption deviation based on the first consumption deviation and the correction function;

[0013] wherein the corrected consumption deviation is compared with a second predefinable consumption deviation,

[0014] - and from the comparison it is determined whether the urea liquid for the SCR system is overdosed or underdosed.

[0015] The unique advantage of the present invention is that it is possible to draw an over- or under-dosing of the reagent from a comparison of the corrected consumption deviation with a second predeterminable consumption deviation. The influence of the temperature of the urea liquid, the rigidity of the SCR system and the performance factor of the pump are taken into account by means of a correction function for the first consumption deviation. As a result, a more accurate consumption deviation of the SCR system is obtained. This can further improve the dosing accuracy of the urea liquid of the SCR system and improve the exhaust gas aftertreatment. As a result, it is also possible to prevent over-dosing of urea liquid in the exhaust system, so that there is no ammonia leakage or ammonia leakage is reduced. Therefore, in an advantageous improvement scheme, it can also be concluded that the dosing valve and / or the SCR system is blocked or leaking.

[0016] In a further refinement, an overdosage of urea liquid is detected if the corrected consumption deviation exceeds a predefinable second consumption deviation.

[0017] The detection of an overdose of a reagent is particularly advantageous, since an adaptation of the dosed amount can thereby be performed. Thus, for example, the dosing strategy can be corrected so that, for example, an overdose of Adblue does not occur.

[0018] In a special embodiment, underdosing of urea liquid is detected if the first consumption deviation lies below a predefinable consumption deviation.

[0019] Identifying an under-dosing of an agent is particularly advantageous, since an adaptation of the dosing amount can thereby be performed. Thus, for example, the dosing strategy can be corrected so that, for example, under-dosing of Adblue does not occur. This allows compliance with emission regulations and prevents undesired pollutant emissions.

[0020] In an advantageous embodiment, the method is initiated when a constant system pressure is present for the SCR system, in particular within a predeterminable period of time.

[0021] In another refinement, the performance factor is determined as a function of the current pump frequency.

[0022] In an advantageous embodiment, the correction function is determined as a function of the rigidity of the SCR system, the temperature of the urea fluid and the current performance factor.

[0023] By taking into account the rigidity of the SCR system, the temperature of the urea liquid and the performance factor, a more precise consumption deviation can be determined. By further addition, a correction function can be determined, with which a known consumption deviation of the SCR system can be corrected.

[0024] In a special embodiment, the first consumption deviation for an SCR system having a suction and return line is determined with the aid of the following formula:

[0025]

[0026] wherein c represents the rigidity of the SCR system (25), p(t3) represents a third pressure value at a third time point (t3), p(t4) represents a fourth pressure value at a fourth time point (t4), and represents the average value between the third and fourth pressure values ​​(p(t3); p(t4)).

[0027] In an advantageous embodiment, the rigidity for an SCR system having a suction and return line is determined with the aid of the following formula:

[0028]

[0029] Among them, A b，eff represents the cross-sectional area of ​​the throttle valve 320 and ρ represents the density of the urea liquid (105), p(t1) represents a first pressure value at a first time point (t1), p(t2) represents a second pressure value at a second time point (t2), and p sys represents a predeterminable system pressure of the SCR system (25).

[0030] In an alternative embodiment, the rigidity for an SCR system having a suction and return line is determined with the aid of the following formula:

[0031]

[0032] Among them, the parameters a, b, x and y are:

[0033]

[0034] Among them, A b，eff represents the effective cross section of the return line (103), p sys denotes a predeterminable system pressure, p0 denotes a fourth pressure of the SCR system (25) at a fourth time (t0), p tf Indicates that at the fifth time point (t f ) of the SCR system (25), p p denotes the pump factor of the pump (120) and p denotes the density of the dosing medium (105).

[0035] When the computer program according to the invention is run on a computer, it can implement all the steps of the method according to the invention. This makes it possible to implement the method according to the invention in existing SCR-catalyst systems without having to make structural changes to them. For this purpose, the computer program product according to the invention has a program code stored on a machine-readable carrier and for executing the method according to the invention when the program is implemented on a control unit or a computer. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description.

[0037] Figure 1 shows the SCR-system,

[0038] Figure 2 Schematic representation of an SCR system with intake and return lines for an internal combustion engine,

[0039] Figure 3 shows a measurement example of the pressure profile for an SCR system with suction and return lines,

[0040] Figure 4 A first exemplary embodiment of the method according to the invention for an SCR system having a suction and return line is shown using a combined flow chart / block diagram. DETAILED DESCRIPTION

[0041] Figure 1 An SCR system 25 is shown having a return line 103 of an internal combustion engine (not shown), wherein only the elements of such a reduction system that are relevant to the present invention are shown here.

[0042] The tank 100 stores a reducing agent solution, for example, a urea-water solution 105, which is also sold under the product name "AdBlue". The urea-water solution 105 is conveyed to the metering medium unit 300 by means of a pump 120 via a suction line 101 (pump mass flow ), through which the urea-water solution is injected into the exhaust gas channel 400 before the catalytic converter K (dosing mass flow ).

[0043] The pressure in the SCR system 25 is detected by a pressure sensor 130 arranged in the suction line 101. This signal is converted in a pressure-voltage converter 132 and fed to a control device 200, for example a motor controller. The pump 120 can also be controlled by the controller 200, more precisely, so that the pressure in the SCR system 25 is kept constant. For this purpose, a system pressure p is predetermined for the SCR system 25. sys , the system pressure is in particular between 4 and 12 bar. Thus, if, for example, the reducing agent flows out via the metering medium unit 300 and from there into the exhaust gas, the pump 120 is actuated so that the reducing agent is conveyed from the tank 100 and the pressure in the SCR system 25 remains virtually constant. In particular, for the method according to the invention, the pump frequency f of the pump 120 is pThe dosing medium unit 300 has its own dosing valve 310 and a throttle valve 320, which can be controlled by the control device 200 via an electromagnet 312 arranged together with the dosing valve 310 in the housing 309, and the throttle valve is arranged in the return line 103, which leads to the tank 100 and realizes the return of the return medium to the tank 100 (return mass flow rate The return line 103 serves to cool a metering valve 310 which is arranged directly upstream of the catalytic converter K in the hot exhaust gas.

[0044] Figure 2 The course of the pressure p in the SCR system 25 for one embodiment of the present invention, the actuation PM of the pump 120 in the delivery module of the SCR system 25 and the actuation DV of the metering valve 310 of the SCR system 25 are shown, respectively, based on the time point t. In the time period from t0 to t1, the pump 120 is constantly actuated. Depending on the requirement of the SCR catalytic converter, the metering valve is actuated more or less strongly in order to meter the urea-water solution 105 into the SCR catalytic converter. The pressure in the SCR system 25 of the SCR system 25 fluctuates similarly to the actuation of the metering valve 310. At the time point t1, the internal combustion engine connected to the SCR catalytic converter system is turned off and no longer requires the urea-water solution 105. Therefore, neither the pump 120 nor the metering valve 310 is actuated. The pressure p in the SCR system 25 gradually decreases via the throttle valve 320. At the time point t2, the method according to the present invention is now started for detecting a blockage of one or more injection openings of the metering valve 310. For this purpose, the actuation causes a sudden speed change, which leads to a sudden increase in pressure p in the SCR system 25 to its system pressure p sys In this case, the metering valve 310 remains closed. Based on the first pressure profile, the rigidity c of the SCR system 25 is modeled according to Formula 1 based on the first pressure p(t1) at the first time t1 and the second pressure p(t2) at the second time t2:

[0045]

[0046] Here, A b，eff denotes the cross-sectional area of ​​the throttle valve 320 , and ρ denotes the density of the urea aqueous solution.

[0047] At the third time t3, the pump 120 is closed and the metering valve 310 is opened. As a result, a drop in the pressure p in the SCR system 25 occurs via the metering valve 310 and via the throttle valve 320. The metering valve 310 is then closed again at the fourth time t4. Based on this second pressure profile and the already calculated stiffness c, it is determined based on the third pressure p(t3) at the third time t3 and the fourth pressure p(t4) at the fourth time t4 on the basis of modeling whether one or more injection openings of the metering valve 310 are blocked according to formula 2:

[0048]

[0049] Here, A d，eff represents the cross-sectional area of ​​the dispensing valve 310. By dividing the cross-sectional area A thus determined d，eff By comparison with the cross-sectional area of ​​the permeable dispensing valve 310 , it can be concluded whether there is a blockage of one or more injection openings of the dispensing valve 310 .

[0050] The pump 120 of the pump 120 is then constantly controlled. As soon as the pressure p is set to a constant value again, the SCR catalytic converter system is ready for new dosing of the urea-water solution 105 .

[0051] Figure 3 The flow chart for an SCR system 25 with a suction and return line 101 ; 103 is shown based on measurements. sys As a starting point, the pressure profile p of the SCR system 25 is shown, wherein at a third time t inj Urea liquid is metered into the exhaust gas channel 400 by means of the metering valve 310 , preferably a test injection. During the entire period, the pump 120 of the SCR system 25 continues to pump at a constant pump frequency f p is controlled and the urea liquid is delivered via the suction and return lines 101; 103. inj At the beginning, the pressure p decreases until the sixth time point t0. That is, at the fifth time point t inj , the metering valve 310 opens and closes again at the sixth time point t0. From the sixth time point p0 onwards, the pressure p increases again.

[0052] Pump factor p p According to the pressure difference p sys- p from the characteristic field P p，map Here, the characteristic field P p，map The pump factor p is determined in the application phase of the SCR system 25 used depending on the different system pressures (especially between 0 and 12 bar). pThe performance of the pump 120 at different system pressures is described and thus forms a measure for the performance of the pump 120. In particular, the system pressure p sys and determine the pressure p avg =(p0+p tf ) / 2 is used to determine the pump factor p p As for the characteristic field P p，map The input parameter of .

[0053] Effective cross section A of the return line 103 b，eff and the density ρ of the urea liquid 105 are present as predefined values ​​in the controller 200. They are already stored in the controller 200 in particular during the application phase.

[0054] In another embodiment, the rigidity c of the SCR system 25 with the intake and return lines 101 ; 103 can alternatively be determined with the aid of the following formula:

[0055]

[0056] From the fifth time point t0 to the sixth time point t f In particular, a fluid is dispensed, in particular a test injection, before a time period of t0. That is, the dispensing valve 310 is opened and the closing time of the dispensing valve 310 corresponds to the fifth time point t0. The predeterminable time t0 is stored in the controller 200. app , in particular between 0.3 ms and 2 s. For an SCR system 25 with a return line 103, the predeterminable time t app Characterizes the evaluation period after the preceding dispensing.

[0057] Therefore, the sixth time point t f Defined as t f =t0+t app .

[0058] The parameters a, b, x, and y are defined as follows:

[0059]

[0060] Among them, p sys represents the predetermined system pressure, p0 represents the pressure at the fifth time point t0, and p tf Indicates that at the sixth time point t f The pressure, and p p Indicates the pump factor at the current pressure p. In particular, the system pressure p sys and determine the pressure p avg =(p0+p tf ) / 2 is used to determine the pump factor pp as the characteristic field Pp，map The input parameter of .

[0061] The first consumption deviation CDM for the SCR system 25 is calculated according to the following formula cal :

[0062]

[0063] Among them, A d，eff A represents the effective cross section of the metering valve 310 of the SCR system 25 b,eff denotes the effective cross section of the return line 130 or the throttle valve 320 of the SCR system 25 , c denotes the rigidity, p(t3) denotes a third pressure value at a third time t3 , p(t4) denotes a fourth pressure value at a fourth time t3 , and Represents the average value of the third and fourth pressure values ​​p(t3); p(t4).

[0064] Furthermore, the temperature of the aqueous urea solution 105 is continuously measured, in particular by a temperature sensor (not further shown) placed in the urea liquid 105 , and is received and stored by the controller 200 .

[0065] Correction function f(c, T, d p ) is also stored in the control unit 200, wherein during the operating time a correction factor f of the control deviation CDM is determined by means of a correction function. This calculation is determined as a function of the determined rigidity c, the current temperature T of the urea liquid 105 and a performance factor dp which is dependent on the current pressure p.

[0066] By means of the polynomial method, the correction function f(c, T, d p ) can be described as follows:

[0067] f(c, T, d p )=D0(T)+D1(T)·c+D2(T)·c 2 +D3(T)·c 3 +d p (T) (9)

[0068] Wherein, D0, D1, D2 and D3 represent temperature-related coefficients, T represents the current temperature of the urea liquid 105, c represents the rigidity, and d p (T) represents the performance factor of the pump 120 as a function related to the current temperature T of the urea liquid 105 .

[0069] The temperature-dependent coefficients D0, D1, D2 and D3 are calculated during the application phase as a function of the temperature T of the urea liquid 105, the rigidity c determined for the SCR system 25 and the temperature-dependent performance factor d pThe polynomial method of the function f has the advantage that the rigidity c, the current temperature T of the urea liquid 105 and the performance factor d can be taken into account for calculating the current consumption deviation. p in order to determine the consumption deviation more precisely.

[0070] Correct the first consumption deviation CDM according to the following formula cal , and obtain the corrected consumption deviation CDM of the SCR system 25 Cor :

[0071]

[0072] Figure 4 An exemplary sequence of the method according to the invention for an SCR system 25 having a suction line 101 is shown in FIG.

[0073] In a first step 500, the activation conditions of the method are monitored in the controller 200. The method is activated when the controller 200 for the SCR system 25 detects a pressure-stable system state. A pressure-stable system state is present when the pressure buildup is successful with a stable system pressure psys, preferably between 6 and 12 bar. In particular, a stable system state can be present when the controller 200 detects that the system pressure psys is reached within a predeterminable time. For this purpose, the controller 200 can monitor the pressure p and perform the activation when a predeterminable system pressure psys is applied within a predeterminable time.

[0074] If the rigidity c for the SCR system 25 has already been calculated and stored in the control unit 200 , the method can also be continued in step 540 .

[0075] If no rigidity c is present in the control unit 200 for the SCR system 25 , the method is continued in step 510 .

[0076] In step 510, the stiffness c for the SCR system 25 is determined. This can be performed in the decompression phase using formula 1. For this purpose, the pump 120 is deactivated at a first time t1. Figure 2 As described in , the pressure p decreases slowly via the throttle valve 320. At a second time t2, the pump 120 is activated again, so that the pressure p in the SCR system 25 suddenly increases to its system pressure p sys The metering valve 310 remains closed during this period. The stiffness c of the SCR system 25 is determined based on the first pressure profile according to formula 1 based on the first pressure p(t1) at the first time t1 and the second pressure p(t2) at the second time t2 on the basis of modeling.

[0077] Alternatively, the rigidity c can also be determined according to Formula 3. inj , the metering valve 310 opens, and at a later fourth time t0, the metering valve 310 closes again. During this metering, the pressure p in the SCR system 25 drops. Here, this pressure is continuously received and stored by the controller 200. During this metering, the pump frequency f p Keep it constant.

[0078] When the metering valve 310 is closed at the fourth time t0, a fourth pressure p0 of the SCR system 25 is determined. When the metering valve 310 is closed, the pressure p of the SCR system 25 increases again. Then, the controller 200 waits for a predeterminable time Δt until a fifth time t f , and determine a fifth pressure p of the SCR system 25 tf In addition, according to the pump frequency f p From the characteristic field d p , map determines the performance factor d p The stiffness c for the SCR system 25 is then determined by the controller 200 according to equation 3.

[0079] The method then continues in step 520 .

[0080] In step 520, a third pressure value pi at a third time t3 and a fourth pressure value pf at a fourth time t4 are determined within the pump stroke determined in step 510. Here, the fourth pressure value pf at the fourth time t4 is f The third and fourth time points t3, t4 are preferably separated by a predeterminable time t of at least 10 ms. app .

[0081] The method then continues in step 530 .

[0082] In step 530 , the method is continued when the pressure of the SCR system 25 has again reached the system pressure psys. p The dosing of the urea liquid 105 is kept constant and performed. For this purpose, the dosing valve 320 is opened at the third time t3 and a third pressure value p(t3) is determined. At the fourth time t4, the dosing valve 320 is closed again and a fourth pressure value p(t4) is determined. In addition, an average value is determined from the third and fourth pressure values ​​p(t3); p(t4)

[0083] According to the following formula 8,

[0084]

[0085] A first consumption deviation CDM for the SCR system 25 can be determined cal .

[0086] The method then continues at step 540 .

[0087] In step 540, the current temperature T of the urea liquid 105 is determined by the controller 200. Based on the determined rigidity c, the current temperature T of the urea liquid 105, and the determined performance factor d p Determine the correction function according to formula 9:

[0088] f(c, T, d p )=D0(T)+D1(T)·c+D2(T)·c 2 +D3(T)·c 3 +d p (T)

[0089] Wherein, D0, D1, D2 and D3 represent temperature-related coefficients, T represents the current temperature of the urea liquid 105, c represents the rigidity, and d p (T) represents the performance factor of the pump 120 as a function related to the current temperature T of the urea liquid 105 .

[0090] The method then continues in step 550 .

[0091] In step 560, the corrected consumption deviation CDM is determined according to formula 10 cor :

[0092]

[0093] Among them, the correction function is f(c, T, d p ) and the first consumption deviation is CDM cal .

[0094] The method then continues in step 550 .

[0095] In step 550, the corrected consumption deviation CDM cor from a predeterminable consumption deviation CDM for the SCR system 25 app Comparison. The predeterminable consumption deviation, i.e. the parameter CDM app This corresponds to the effective cross section A for the metering valve 310 stored in the control unit 200 during the application phase. d，eff and the effective cross section A for the throttle valve 320 b,eff The parameter of the quotient.

[0096] If the corrected consumption deviation CDM cor falls below a predeterminable consumption deviation CDMapp , it is detected that the urea liquid 105 is insufficiently dosed to the SCR system 25 .

[0097] If the corrected consumption deviation CDM cor Excess consumption deviation CDM app , an overdose of urea liquid 105 for the SCR system 25 is detected.

[0098] In special designs, it is possible to adjust the consumption deviation CDM around a predeterminable app The value of defines a threshold value band, for example around a predeterminable consumption deviation CDM app If the corrected consumption deviation CDM cor If the urea content of the SCR system 25 is within the threshold value band, then the dosing of the urea liquid 105 of the SCR system 25 is normal. The result of the comparison can preferably be used to correct or adapt the dosing amount m determined by the dosing strategy. dos Or the distribution mass flow m D To this end, the controller 200 determines the corrected dosing quantity m dos,corr Or the corrected mass flow rate And then metering takes place via the metering valve 310 .

[0099] The method can then be ended or started from the beginning.

Claims

1. A method for operating an SCR system (25) having an intake and return line (101; 103) of an internal combustion engine, in which at least one metering valve (310) meters a reagent upstream of at least one SCR catalytic converter, wherein: The reagent (105) is delivered from the tank (100) to the dispensing valve (310) at a constant pressure by means of a controllable pump (120), characterized by the following steps: - determining the stiffness (c) for the SCR system (25); - Determining the performance factor (d) of the pump (120) for the SCR system (25) p ); - Determine the first consumption deviation (CDM cal ); - According to the temperature (T) of the urea liquid (105), the rigidity (c) and the performance factor (d p )Determine the correction function (f(c,T,d p )); -According to the first consumption deviation (CDN cal ) and the correction function (f(c, T, d p )) Determine the corrected consumption deviation (CDN cor ); - Among them, the corrected consumption deviation (CDM cor ) and a predeterminable second consumption deviation (CDM app ) for comparison, - and from the comparison it is determined whether the urea liquid (105) for the SCR system (25) is overdosed or underdosed.

2. The method according to claim 1, characterized in that When the corrected consumption deviation (CDM cor ) exceeds the predeterminable second consumption deviation (CDM app ), it is recognized that the urea liquid (105) is excessively dosed.

3. The method according to claim 1, characterized in that When the first consumption deviation (CDM cal ) is lower than the predeterminable consumption deviation (CDM app ), insufficient dosing of the urea liquid (105) is detected.

4. The method according to claim 1, characterized in that: When there is a constant system pressure (p sys ), in particular within a predeterminable time period, the method is activated.

5. The method according to claim 1, characterized in that According to the current pump frequency (f p )Determine the performance factor (d p ).

6. The method according to claim 1, characterized in that According to the rigidity (c) of the SCR system (25), the temperature (T) of the urea liquid (105) and the current performance factor (d p ) determines the correction function (f(c, T, d p )).

7. The method according to claim 1, characterized in that A first consumption deviation for an SCR system (25) having a suction and return line (101; 103) is determined using the following formula: wherein c represents the rigidity of the SCR system (25), p(t3) represents a third pressure value at a third time point (t3), p(t4) represents a fourth pressure value at a fourth time point (t4), and represents the average value between the third and fourth pressure values ​​(p(t3); p(t4)).

8. The method according to claim 1, characterized in that The rigidity (c) for an SCR system (25) having a suction and return line (101; 103) is determined with the aid of the following formula: Among them, A b,eff represents the cross-sectional area of ​​the throttle valve 320 and ρ represents the density of the urea liquid (105), p(t1) represents a first pressure value at a first time point (t1), p(t2) represents a second pressure value at a second time point (t2), and p sys represents a predeterminable system pressure of the SCR system (25).

9. The method according to claim 1, characterized in that: The rigidity (c) for an SCR system (25) having a suction and return line (101; 103) is determined with the aid of the following formula: Among them, the parameters a, b, x and y are: Among them, A b，eff represents the effective cross section of the return line (103), p sys denotes a predeterminable system pressure, p0 denotes a fourth pressure of the SCR system (25) at a fourth time (t0), p tf Indicates that at the fifth time point (t f ) of the SCR system (25), p p denotes the pump factor of the pump (120) and p denotes the density of the dosing medium (105). 10 . Computer program configured to carry out the method according to claim 1 .

11. Electronic storage medium having a computer program according to claim 10.

12. A device, in particular a control unit (200), which is configured to carry out the method according to any one of claims 1 to 9.

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