Diagnostic Methods for SCR Catalytic Converters

Abstract

The invention relates to a diagnostic method for an SCR catalytic converter, in which the storage efficiency of the SCR catalytic converter is determined (26) based on the quotient of two integrals. The first integral is the integral of the difference between the modeled nitrogen oxide amount downstream of the SCR catalytic converter and the measured sum of the nitrogen oxide amount and the ammonia amount downstream of the SCR catalytic converter. The second integral is the integral of the ammonia amount dosed into the SCR catalytic converter or the ammonia amount overdosed into the SCR catalytic converter.

Description

Technical Field

[0001] The present invention relates to a diagnostic method for an SCR catalytic converter. The present invention also relates to a computer program that implements each step of the method, and a machine-readable storage medium storing the computer program. Finally, the present invention relates to an electronic control device configured to implement the method. Background Art

[0002] To reduce nitrogen oxides in the exhaust gases of internal combustion engines, particularly diesel engines, an SCR (selective catalytic reduction) catalytic converter can be used. In this SCR catalytic converter, nitrogen oxide molecules on the catalytic converter surface are reduced to elemental nitrogen in the presence of ammonia as a reducing agent. The reducing agent is injected into the exhaust tract of the internal combustion engine upstream of the SCR catalytic converter in the form of an ammonia-releasing reducing agent solution (urea-water solution; HWL).

[0003] In many countries, OBD (On Board Diagnostics) regulations require monitoring of SCR catalytic converters. If an SCR catalytic converter ages or becomes damaged, resulting in a decrease in its ability to reduce nitrogen oxides, this must be communicated to the driver of the vehicle in which the SCR catalytic converter is installed so that the driver can visit a repair shop. Monitoring an aged or damaged SCR catalytic converter is usually performed by analyzing the nitrogen oxide mass flow upstream and downstream of the SCR catalytic converter. The nitrogen oxide concentration required for this purpose is measured using a nitrogen oxide sensor, which, however, has cross-sensitivity to ammonia and therefore displays a combined nitrogen oxide and ammonia signal. Therefore, an increase in the sensor signal of the nitrogen oxide sensor downstream of the SCR catalytic converter can indicate not only an increase in nitrogen oxide concentration due to a decrease in nitrogen oxide conversion, but also an increase in ammonia concentration due to pure ammonia crossover. Since it is impossible to directly distinguish between nitrogen oxide and ammonia, the nitrogen oxide conversion may appear lower than it actually is, potentially leading to misdiagnosis.

[0004] Methods for monitoring SCR catalytic converters can be divided into passive and active methods. In the passive method, the dosing strategy, and therefore in particular the ammonia level of the SCR catalytic converter, is not intervened. Diagnosis is performed during an operating phase in which a sufficiently good distinction can be made between intact and defective SCR catalytic converters. If the passive method is not accurate enough to make a sufficiently robust distinction between intact and defective SCR catalytic converters, a switch can be made to an active method in which favorable conditions for robust diagnosis are established by actively intervening in the reducing agent dosing.

[0005] An active monitoring strategy is described in DE 10 2007 040 439 A1. This method utilizes the following property of the SCR catalytic converter: the NH3 storage capacity of the SCR catalytic converter decreases as it ages. The SCR catalytic converter is first filled with reducing agent by a stoichiometric excess reducing agent dosage, up to the maximum achievable ammonia storage capacity, which is also called overdosing. Whether the maximum storage capacity has been reached is detected by the penetration of pure ammonia after the SCR catalytic converter. This is also called ammonia slip and can be measured based on the cross-sensitivity of the nitrogen oxide sensor to ammonia. Subsequently, the reducing agent dosage is reduced compared to the normal dosage, so that underdosing occurs or the reducing agent dosage is completely cut off. During this emptying test, the amount of stored ammonia is gradually reduced again due to nitrogen oxide reduction. By determining the SCR efficiency during the emptying test, the available ammonia storage capacity can be indirectly determined. Summary of the Invention

[0006] The diagnostic method for an SCR catalytic converter is based on the understanding that a value known as the storage efficiency of an SCR catalytic converter is a better indicator for assessing whether an SCR catalytic converter is intact or defective than characteristic variables used in previous diagnostic methods. The storage efficiency of an SCR catalytic converter is a value determined by the quotient of two integrals. The first integral is the integral of the amount of ammonia downstream of the SCR catalytic converter, which is typically located in the numerator of the quotient. The second integral is the integral of the amount of ammonia dosed or overdosed into the SCR catalytic converter, which is typically located in the denominator of the quotient. The overdosed ammonia amount is defined as the difference between the amount of ammonia required for nitrogen oxide reduction in the SCR catalytic converter according to a model and the amount of ammonia actually dosed. When determining the dosed or overdosed ammonia amount, the hydrolysis capacity (BPU) is preferably taken into account. This takes into account the fact that an aged SCR catalytic converter can only partially hydrolyze HWL into ammonia. The dosed HWL quantity can then be converted via a temperature-dependent characteristic curve into the amount of ammonia dosed or overdosed in the SCR catalytic converter.

[0007] In conventional measurements of NOx concentrations upstream and downstream of the monitored SCR catalytic converter using a NOx sensor, if no ammonia is present in the exhaust gas downstream of the monitored SCR catalytic converter, the value previously used for SCR diagnosis conventionally corresponds to the NOx conversion rate of the SCR catalytic converter. However, if ammonia is present in the exhaust gas, the sensor incorrectly interprets this contribution as NOx, and therefore calculates a lower value instead of the actual NOx conversion rate. Model-based corrections to the NOx sensor signal, for example, are prone to errors and can lead to a greater spread in the monitoring results. This effect is mitigated by analyzing the dosing or overdosing phase, as ammonia is typically primarily detected in these phases.

[0008] The diagnostic method using storage efficiency is particularly well-suited for monitoring the engine-proximal SCR catalytic converter in an exhaust system with two SCR catalytic converters installed one behind the other, such as an SCR catalytic converter located on a particle filter (SCRF). This is because the ammonia content in the exhaust gas downstream of the first SCR catalytic converter is higher than that downstream of the second SCR catalytic converter or downstream of a single SCR catalytic converter, and therefore the efficiency calculation tends to be more imprecise. Furthermore, under normal circumstances, the ammonia content of the exhaust gas upstream of the first SCR catalytic converter is known because it is directly derived from the metering into the exhaust line. However, the diagnostic method can also be used for the second SCR catalytic converter in a system with two SCR catalytic converters. This is particularly useful when a second metering valve is used between the two SCR catalytic converters.

[0009] The method can be implemented in four different embodiments:

[0010] In a first embodiment, the passive diagnostic method is executed if an ammonia overdosing phase of the SCR catalytic converter is detected. In this case, the second integral is the integral of the amount of ammonia overdosed into the SCR catalytic converter. This embodiment of the method is particularly suitable for identifying an aged SCR catalytic converter that is installed as the first SCR catalytic converter in an SCR catalytic converter system having multiple SCR catalytic converters.

[0011] In another passive embodiment of the diagnostic method, the method is executed if an ammonia dosing phase of the SCR catalytic converter is detected. However, the second integral is the integral of the total amount of ammonia dosed to the SCR catalytic converter. This embodiment of the method is particularly suitable for detecting a complete failure of an SCR catalytic converter that is installed as the second SCR catalytic converter in a system with multiple SCR catalytic converters.

[0012] In both passive embodiments of the diagnostic method, it is preferred that an ammonia overdosing phase or an ammonia dosing phase be identified within a first time period. Subsequently, the first integral and the second integral are each determined within a second time period that is longer than the first time period in order to achieve an analysis time that enables robust diagnosis. The length of the first time period can be determined, in particular, by the amount of ammonia dosed into the SCR catalytic converter, the exhaust gas mass flow, the nitrogen oxide mass flow, or the nitrogen oxide concentration in the exhaust gas upstream of the SCR catalytic converter.

[0013] In a third embodiment of the diagnostic method, ammonia is actively overdosed into the SCR catalytic converter to perform the diagnostic method. The second integral is the integral of the amount of ammonia overdosed into the SCR catalytic converter. This embodiment of the diagnostic method is particularly suitable for detecting an aged SCR catalytic converter that is installed as the first SCR catalytic converter in a system having multiple SCR catalytic converters.

[0014] In a fourth embodiment of the diagnostic method, ammonia is actively dosed into the SCR catalytic converter. The second integral is the integral of the total amount of ammonia dosed into the SCR catalytic converter. This embodiment of the diagnostic method is particularly suitable for detecting complete failure of an SCR catalytic converter that is installed as the second SCR catalytic converter in an SCR catalytic converter system having multiple SCR catalytic converters.

[0015] In an active embodiment of the diagnostic method, ammonia overdosing or ammonia dosing is preferably performed within a first time period, wherein the first integral and the second integral are determined within a second time period that is longer than the first time period. This results in a sufficiently long analysis time for robust execution of the diagnostic method.

[0016] If an ammonia sensor is arranged in the exhaust gas line downstream of the SCR catalytic converter, or if a multi-gas sensor capable of determining the ammonia amount is arranged there, this value can be directly used in the first integral. If only one nitrogen oxide sensor with cross-sensitivity to ammonia is located downstream of the SCR catalytic converter, the ammonia amount can be calculated as the difference between the nitrogen oxide amount in the exhaust gas line and the sum of the nitrogen oxide and ammonia amounts downstream of the SCR catalytic converter measured by the nitrogen oxide sensor. When the diagnostic method is performed during an overdosing phase, the nitrogen oxide amount in the exhaust gas line used to calculate the first integral is, in this case, the modeled nitrogen oxide amount downstream of the SCR catalytic converter. In a preferred embodiment of the method, the model used for this purpose is a WPA (worst part acceptable) nitrogen oxide model. In another preferred embodiment of the method, the model is a BPU (best part unacceptable) nitrogen oxide model. If, however, the diagnostic method is carried out during the dosing phase, the nitrogen oxide quantity in the exhaust gas line used for calculating the first integral is the nitrogen oxide quantity measured upstream of the SCR catalytic converter.

[0017] The computer program is configured to execute each step of the method, particularly when it is run on a computing device or an electronic control unit. The computer program enables implementation of various embodiments of the method on the electronic control unit without requiring structural changes. For this purpose, the computer program is stored on a machine-readable storage medium. Loading the computer program onto a conventional electronic control unit results in an electronic control unit configured to diagnose an SCR catalytic converter using the diagnostic method. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description.

[0019] Figure 1 An SCR catalytic converter is schematically shown, which can be diagnosed using an exemplary embodiment of the method according to the present invention.

[0020] Figure 2 A flow chart of an embodiment of the method according to the present invention is shown.

[0021] Figure 3 A flow chart showing another exemplary embodiment of the method according to the present invention is shown.

[0022] Figure 4 A flow chart showing a further embodiment of the method according to the present invention is shown. DETAILED DESCRIPTION

[0023] Motor vehicles in Figure 1The internal combustion engine 10 shown in FIG. 1 has an SCR catalytic converter 12 in its exhaust gas line 11. A reducing agent solution 14 in the form of an aqueous urea solution is stored in a reducing agent tank 13. A feed module 15 at the bottom of the reducing agent tank 13 is configured to transport the reducing agent solution 14 to a dosing module 16. The dosing module is arranged upstream of the SCR catalytic converter 12 in the exhaust gas line 11. A first nitrogen oxide sensor 17 is arranged upstream of the dosing module 16 in the exhaust gas line 11. A second nitrogen oxide sensor 18 is arranged downstream of the SCR catalytic converter 12 in the exhaust gas line 11. Both nitrogen oxide sensors 17, 18 have cross-sensitivity to ammonia. The internal combustion engine 10 and the dosing module 16 are controlled by an electronic control unit 19. The electronic control unit also receives data from the nitrogen oxide sensors 17, 18. The following embodiment of the diagnostic method according to the present invention is described based on this simple SCR catalytic converter system. However, all of the following embodiments of the diagnostic method can also be applied to more complex SCR catalytic purification systems in which a further SCR catalytic purifier is arranged downstream of the SCR catalytic purifier 12 in the exhaust line 11 and in which a further dosing module can optionally be arranged between the two SCR catalytic purifiers.

[0024] exist Figure 2 The flow of a first embodiment of the diagnostic method is shown in FIG. After starting 20 the method, a value α for a dosing phase of, for example, 100 mg of ammonia is calculated 21. This value is obtained according to formula 1:

[0025] (Formula 1)

[0026] Here, mNH3 soll represents the amount of ammonia required for reducing nitrogen oxides emitted by the internal combustion engine 10 according to the measurement value of the first nitrogen oxide sensor 17, and mNH3 ist φ represents the amount of ammonia actually dosed into the SCR catalytic converter 12 by means of the dosing module 12 .

[0027] Next, a check 22 is performed to determine whether general diagnostic authorization conditions are met. These authorization conditions include: a suitable operating mode of the internal combustion engine 10; activation of the nitrogen oxide sensors 17 and 18; a predetermined temperature range of the SCR catalytic converter 12; a maximum filter temperature gradient of the SCR catalytic converter 12; predetermined ranges for the exhaust gas mass flow or nitrogen oxide mass flow and nitrogen oxide concentration at the first nitrogen oxide sensor 17; and a predetermined range for the nitrogen filling level of the SCR catalytic converter 12.

[0028] If the admission condition is met, the value α is compared 23 with a threshold value, which in the present case is, for example, 1.2. If this threshold value is exceeded, it is recognized that an overdosing phase of the SCR catalytic converter 12 has begun, and the method is continued within a first time period 31. In a first calculation step 24, the overdosing amount is calculated. Subsequently, the overdosing amount is compared 25 with an analysis threshold, which in the present case is, for example, 300 mg of ammonia. If this analysis threshold is not reached or exceeded, the method is interrupted. Otherwise, the method is continued within an additional time period 32, so that the first time period 31 and the additional time period 32 together form a second time period, which is longer than the first time period 31. If the exhaust gas mass accumulated since the start of the second time period 32 exceeds a threshold value, which in the present case is, for example, 0.5 kg, the additional time period 32 ends. In the additional time period 32, an additional analysis phase occurs, at the end of which the amount of NH3 is calculated according to formula 2. Eff Storage efficiency expressed as:

[0029] (Formula 2)

[0030] Here, mNH3 OvrDos Indicates the excess amount of ammonia. mNH3 Ds represents the amount of ammonia downstream of the SCR catalytic converter 12. This amount of ammonia can be calculated using Formula 3:

[0031] (Formula 3)

[0032] Here, m(NOx+NH3) Mess mNOx represents the sum of the amount of nitrogen oxides and ammonia downstream of the SCR catalytic converter 12 measured by means of the second nitrogen oxide sensor 18. Mod represents the amount of nitrogen oxides at the second nitrogen oxide sensor 18 that can be determined with the aid of the model. In this case, the two integrals are determined over the first time period 31 and the additional time period 32. Subsequently, the storage efficiency NH3 Eff A comparison 27 is performed with a threshold value, which serves to differentiate between an intact and a defective SCR catalytic converter 12. Depending on the result of this comparison, the SCR catalytic converter 12 is diagnosed 28 as functional or 29 as defective.

[0033] Alternatively, in comparison 25, a threshold value can also be predefined for the modeled ammonia fill level of the SCR catalytic converter as a function of the temperature and the exhaust gas mass flow for the transition to the additional time period 32. In another alternative, a threshold value for the overdosing quantity is queried, but the transition to the additional time period 32 is not performed until after the current overdosing phase has ended.

[0034] In a second embodiment of the method, the authorization condition checked in step 22 additionally includes the following: the difference between the actual ammonia level in the SCR catalytic converter and the maximum ammonia level at which no ammonia slip can be expected under WPA conditions, which has a value of at least 200 mg in the current case, for example. The current actual level is derived from a model of the SCR catalytic converter 12 calculated for the current dosing strategy. Alternatively, the actual level can be determined in a separately calculated model for diagnosis. The maximum ammonia level at which no ammonia slip can be expected under WPA conditions is known from a performance map based on the temperature of the SCR catalytic converter and the exhaust gas mass flow. Alternatively, the actual level is set in the separately calculated diagnostic model during initialization of the control unit 19 to the maximum possible value for ammonia loading, for example, 6 g in the current case, thereby achieving a maximum estimate of the level. As the temperature of the SCR catalytic converter 12 increases, the ammonia level based on the maximum storage capacity decreases. When the temperature subsequently decreases, storage space becomes available, and diagnosis can be authorized. The possible conversion of nitrogen oxides in the SCR catalytic converter 12 can also reduce the fill level, whereby ammonia storage space also becomes free. For comparison 23, instead of using a threshold value, the aforementioned ammonia amount is integrated as soon as the dosing amount at the dosing valve 12 is greater than a first threshold value, for example 5 mg / s in the present case, and remains so until the dosing amount again falls below a second threshold value, for example 3 mg / s in the present case. As a result, no overdosing is detected, but only the normal dosing of ammonia into the SCR catalytic converter 12 is detected. If the threshold value of the dosed ammonia is exceeded, a transition is made from the first time period 31 to the additional time period 32, which threshold value is selected as an average over the diagnostic time period or the maximum temperature of the SCR catalytic converter 12 depending on the temperature of the SCR catalytic converter 12 and the exhaust gas mass flow. The other method steps are carried out as in the first embodiment of the method, wherein, however, the storage efficiency NH3 is calculated according to formula 4 in step 26. Eff :

[0035] (Formula 4)

[0036] Among them mNH3 Dos Represents the amount of ammonia dosage. Replaces the modeled nitrogen oxide amount mNOx in Equation 3 Mod , in order to calculate mNH3 using formula 4 Ds , using the nitrogen oxide amount mNOx measured by means of the first nitrogen oxide sensor 17 upstream of the SCR catalytic converter 12 according to Formula 5 Mess :

[0037] (Formula 5)

[0038] exist Figure 3 A third embodiment of the method according to the present invention is shown in FIG. During a first time period 31, each time the conditions of steps 22 and 23 are met, the excess dosage amount is calculated 24, and the method is then restarted. Only when one of the release conditions according to step 22 is no longer met, or when the value α no longer exceeds the threshold value in step 23, is the excess dosage amount calculated during the last occurrence of step 24 compared with an analysis threshold 25. If the analysis threshold is exceeded, the method continues in the same manner as in the first two embodiments, and the integral over the first time period 31 and the additional time period 32 is taken into account in equation 2. However, if the analysis threshold is not exceeded in step 25, all integrators are reset 40, and the diagnostic method is restarted.

[0039] In the fourth embodiment of the diagnostic method, if normal dosing is performed instead of overdosing, the diagnostic method can also be used according to Figure 3 Here, the check 22 and the comparison 23 are performed as in the second embodiment. Then, the storage efficiency is calculated in step 26 by means of formula 4 as in the second embodiment.

[0040] exist Figure 4 shows the sequence of a fifth embodiment of the method according to the present invention. In this case, in step 22, not only the authorization conditions of the second embodiment are checked, but also whether the conditions for the overdosing phase are met. If this is the case, ammonia is initially actively overdosed 50 within a first time period 31, in the present case, for example, at a value of α=1.5. In step 24, the overdosed amount is integrated upwards, and in step 25, this overdosed amount is compared with an analysis threshold. If this analysis threshold is reached or exceeded, the method is continued as in the first embodiment using steps 26 to 29, with the calculation in step 26 being performed according to Formula 2. Otherwise, the method is restarted.

[0041] In the fifth exemplary embodiment, overdosing can alternatively also be performed by adding an overshoot of, in the present case, for example, 20 mg / s to the ammonia quantity mNH3 required for reducing the nitrogen oxides emitted by the internal combustion engine 10. soll superior.

[0042] In a sixth embodiment of the diagnostic method, no active overdosing is performed for diagnosis, but only active dosing is performed for diagnosis. Figure 4In the flowchart of FIG. 2 , in addition to the general permission conditions, a check is then performed in step 22 to determine whether the dosing conditions are met. In step 50 , instead of overdosing, only normal dosing is performed. In step 24 , the dosing amount is integrated upwards, and in step 25 , this dosing amount is compared with the analysis threshold. Method steps 26 to 29 are performed as in the second embodiment of the diagnostic method, wherein in step 26 , formula 4 is used to calculate the storage efficiency NH 3 Eff .

Claims

1. A diagnostic method for an SCR catalytic converter (12), wherein the storage efficiency of the SCR catalytic converter (12) is determined (26) based on the quotient of two integrals, wherein - the first integral is the integral of the amount of ammonia downstream of the SCR catalytic converter (12); and - a second integral is the integral of the amount of ammonia dosed into the SCR catalytic converter (12) or the amount of ammonia overdosed into the SCR catalytic converter (12), the amount of ammonia overdosed being understood as the difference between the amount of ammonia required for nitrogen oxide reduction in the SCR catalytic converter according to the model and the amount of ammonia actually dosed; wherein if an ammonia overdosing phase of the SCR catalytic converter (12) is identified, the diagnostic method is performed, wherein the second integral is the integral of the amount of ammonia overdosed into the SCR catalytic converter (12); or wherein if an ammonia dosing phase of the SCR catalytic converter (12) is identified, the diagnostic method is performed, wherein the second integral is the integral of the amount of ammonia dosed into the SCR catalytic converter (12); If an ammonia overdosing phase or an ammonia dosing phase is detected within a first time period (31), the diagnostic method is carried out, wherein the first integral and the second integral are each determined within a second time period, which is longer than the first time period.

2. The diagnostic method according to claim 1, wherein During the execution of the diagnostic method, ammonia is actively overdosed into the SCR catalytic converter (12), wherein the second integral is the integral of the amount of ammonia overdosed into the SCR catalytic converter (12).

3. The diagnostic method according to claim 1, wherein During the execution of the diagnostic method, ammonia is actively dosed into the SCR catalytic converter (12), wherein the second integral is the integral of the amount of ammonia dosed into the SCR catalytic converter (12).

4. The diagnostic method according to claim 2 or 3, characterized in that If active ammonia overdosing or ammonia dosing occurs within a first time period (31), the diagnostic method is performed, wherein the first integral and the second integral are each determined within a second time period, which is longer than the first time period.

5. A computer program product comprising a computer program configured to execute each step of the method according to any one of claims 1 to 4. 6 . A machine-readable storage medium having a computer program stored thereon, the computer program being configured to execute each step of the method according to claim 1 .

7. An electronic control unit (19) configured to diagnose an SCR catalytic converter (12) using the diagnostic method according to claim 1.

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