CN112983611A

CN112983611A - Diagnostic method for an SCR catalytic converter

Info

Publication number: CN112983611A
Application number: CN202011437863.7A
Authority: CN
Inventors: C·纳格尔; A·弗兰兹; B·葛尔默
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2019-12-14
Filing date: 2020-12-11
Publication date: 2021-06-18
Also published as: DE102019219645A1

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) as a function of the quotient of two integrals. The first integral is an integral of the modeled nitrogen oxide amount downstream of the SCR catalytic converter and a difference of the measured nitrogen oxide amount downstream of the SCR catalytic converter and the ammonia amount. The second integral is an integral of the amount of ammonia dosed into the SCR catalytic converter or the amount of ammonia overdosed into the SCR catalytic converter.

Description

Diagnostic method for an SCR catalytic converter

Technical Field

The invention relates to a diagnostic method for an SCR catalytic converter. The invention also relates to: a computer program implementing each step of the method; and 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.

Background

For reducing nitrogen oxides in the exhaust gas of internal combustion engines, in particular diesel engines, SCR catalytic converters (selective catalytic reduction) can be used. In the SCR catalytic converter, nitrogen oxide molecules on the surface of the catalytic converter are reduced to elemental nitrogen in the presence of ammonia as a reducing agent. The reducing agent is injected into the exhaust gas 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).

In many countries, OBD (On Board diagnostics) regulations require monitoring of SCR catalytic converters. If the SCR catalytic converter ages or if it breaks down, so that its ability to reduce nitrogen oxides is reduced, this must be communicated to the driver of the motor vehicle in which it is built, so that the driver can go to a repair shop. The aged or damaged SCR catalytic converter is monitored mostly 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 by means of a nitrogen oxide sensor, which however has a cross-sensitivity to ammonia and therefore displays the sum signal of nitrogen oxide and ammonia. An increase in the sensor signal of the nitrogen oxide sensor downstream of the SCR catalytic converter can therefore indicate an increase in the nitrogen oxide concentration due to a decrease in the nitrogen oxide conversion but also an increase in the ammonia concentration due to the permeation of pure ammonia (Durchbruch). Because it is not possible to directly distinguish between nitrogen oxides and ammonia, the amount of nitrogen oxide conversion may appear worse than it actually is, so that a false diagnosis may occur.

Methods for monitoring SCR catalytic converters can be divided into passive and active methods. In the passive method, the dosing strategy and thus in particular the ammonia level of the SCR catalytic converter are not influenced. In this case, the diagnosis is carried out in an operating phase in which a sufficiently good distinction can be made between intact SCR catalytic converters and defective SCR catalytic converters. If the accuracy of the passive method is not sufficient to distinguish between an intact SCR catalytic converter and a defective SCR catalytic converter in a sufficiently robust manner, it is possible to switch to an active method in which the advantages for a robust diagnosis are established by means of active intervention in the dosing quantity of the reducing agent.

An active monitoring strategy is described in DE 102007040439 a 1. The method utilizes the following characteristics of the SCR catalytic purifier: the NH3 storage capacity of the SCR catalytic converter decreases with age. The SCR catalytic converter is first filled with reducing agent by means of a stoichiometric excess of the reducing agent charge, also referred to as overdosing, up to the maximum ammonia storage capacity that can be achieved. The achievement of the maximum storage capacity is detected by the permeation of pure ammonia behind the SCR catalytic converter. This is also known as ammonia slip and can be measured based on the cross-sensitivity of the nitrogen oxide sensor to ammonia. Subsequently, the reducing agent portion is reduced compared to the normal portion, so that an under-filling occurs or the reducing agent portion is completely cut off. In this evacuation test, the amount of stored ammonia is in turn gradually reduced due to the reduction of nitrogen oxides. By determining the SCR efficiency during the purge test, the available ammonia storage capacity may be indirectly determined.

Disclosure of Invention

The diagnostic method for an SCR catalytic converter is based on the recognition that: the value of the SCR catalytic converter, which is referred to as the storage efficiency, is a better characteristic for assessing whether the SCR catalytic converter is intact or defective than the characteristic variables used in the diagnostic methods up to now. The storage efficiency of the SCR catalytic converter is a value determined by the quotient of two integrals. The first integral, which is the integral of the ammonia amount downstream of the SCR catalytic converter, is located in particular in the numerator of the quotient. The second integral is the integral of the amount of ammonia dosed into the SCR catalytic converter or the amount of ammonia overdosed into the SCR catalytic converter, which second integral is located in particular in the denominator of the quotient. The ammonia amount dosed in excess is understood here as the difference between the ammonia amount required for the reduction of nitrogen oxides in the SCR catalytic converter according to the model and the actual ammonia amount dosed. In the case of determining the amount of ammonia dosed or the amount of ammonia dosed in excess, the hydrolysis capacity of BPU (best partially unacceptable) is preferably taken into account. This takes into account the fact that an aged SCR catalytic converter can only partially hydrolyze HWL to ammonia. The dosed quantity of HWL can then be converted by means of a temperature-dependent characteristic curve into a dosed or overdosed quantity of ammonia in the SCR catalytic converter.

In the case of conventional measurement of the nitrogen oxide concentration upstream and downstream of the SCR catalytic converter to be monitored by means of a nitrogen oxide sensor, the value used up to now for the SCR diagnosis corresponds in the conventional sense to the nitrogen oxide conversion ratio of the SCR catalytic converter if no ammonia is present in the exhaust gas downstream of the SCR catalytic converter to be monitored. However, if ammonia is present in the exhaust gas, this proportion is incorrectly interpreted by the sensor as nitrogen oxide and therefore the actual nitrogen oxide conversion is not calculated, but rather a lower value. For example, model-based correction of the nox sensor signal is error-prone and leads to a greater spread of the monitoring results. This effect is reduced by analyzing the dosing or overdosing phase, since here the detection is usually mainly carried out on ammonia.

The diagnostic method using the storage efficiency is particularly well suited for monitoring an SCR catalytic converter close to the engine, for example an SCR catalytic converter (SCRF) arranged on a particle filter, in an exhaust system with two SCR catalytic converters built one behind the other, since the ammonia fraction in the exhaust gas after the first SCR catalytic converter is higher than the ammonia fraction after the second SCR catalytic converter or after a single SCR catalytic converter, and therefore the inaccuracy of the efficiency calculation tends to be higher. In addition, the ammonia content of the exhaust gas upstream of the first SCR catalytic converter in the exhaust gas is known under normal conditions, since this ammonia content is directly obtained from the dosing into the exhaust gas line. However, the diagnostic method can in principle also be used for the second SCR catalytic converter in a system with two SCR catalytic converters. This is particularly relevant when a second dosing valve is used between the two SCR catalytic converters.

The method can be implemented in particular in four different embodiments:

in a first embodiment as a passive diagnostic method, the passive diagnostic method is carried out if an ammonia overdosing phase of the SCR catalytic converter is identified. In this case, the second integral is the integral of the amount of ammonia that is overdosed into the SCR catalytic converter. This embodiment of the method is particularly suitable for identifying an aged SCR catalytic converter which is built as a first SCR catalytic converter in an SCR catalytic converter system having a plurality of SCR catalytic converters.

In a further passive embodiment of the diagnostic method, the diagnostic method is carried out if an ammonia dosing phase of the SCR catalytic converter is identified. However, the second integral is the integral of the amount of ammonia dosed into the SCR catalytic converter as a whole. This embodiment of the method is particularly suitable for detecting a complete failure of an SCR catalytic converter which is built as a second SCR catalytic converter in a system having a plurality of SCR catalytic converters.

In these two passive embodiments of the diagnostic method, it is preferred that: an ammonia overdosing phase or an ammonia dosing phase is identified within a first time period. The first and second integrals are then each determined over a second time period, which is longer than the first time period, in order to achieve an evaluation time that enables a 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.

In a third embodiment of the diagnostic method, ammonia is actively over-dosed into the SCR catalytic converter in order to carry out the diagnostic method. The second integral is an integral of the amount of ammonia that is overdosed into the SCR catalytic converter. This embodiment of the diagnostic method is particularly suitable for identifying an aged SCR catalytic converter which is built as a first SCR catalytic converter in a system having a plurality of SCR catalytic converters.

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 amount of ammonia dosed into the SCR catalytic converter as a whole. This embodiment of the diagnostic method is particularly suitable for detecting a complete failure of an SCR catalytic converter which is built as a second SCR catalytic converter in an SCR catalytic converter system having a plurality of SCR catalytic converters.

In an active embodiment of the diagnostic method, the ammonia overdosing or dosing is preferably carried out over a first time period, wherein the first integral and the second integral are determined over a second time period, which is longer than the first time period. In this way, a sufficiently long analysis time for robust execution of the diagnostic method is achieved.

If an ammonia sensor is arranged in the exhaust gas line downstream of the SCR catalytic converter or if a multi-gas sensor is arranged there, which can determine the ammonia amount in particular, this value can be used directly in the first integration. If only one nitrogen oxide sensor with cross-sensitivity to ammonia is present downstream of the SCR catalytic converter, the ammonia quantity can be calculated as the difference between the nitrogen oxide quantity in the exhaust line and the sum of the nitrogen oxide quantity downstream of the SCR catalytic converter and the ammonia quantity measured by means of the nitrogen oxide sensor. In the case of the execution of the diagnostic method in the overdose phase, the nitrogen oxide amount in the exhaust line which is used for calculating 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 the oxynitride model of WPA (worst part acceptable). In a further preferred embodiment of the method, the model is a nitroxide model of BPU (best partially unacceptable). If, on the other hand, the diagnostic method is carried out during the dosing phase, the nitrogen oxide quantity in the exhaust line which is used for calculating the first integral is the measured nitrogen oxide quantity upstream of the SCR catalytic converter.

The computer program is set up as: in particular, each step of the method is performed when the computer program runs on a computing device or on an electronic control device. The computer program enables implementation of the different embodiments of the method on an electronic control device without structural changes in this respect. To this end, the computer program is stored on a machine-readable storage medium. By loading the computer program onto a conventional electronic control unit, an electronic control unit is obtained which is set up to diagnose the SCR catalytic converter by means of the diagnostic method.

Drawings

Embodiments of the invention are illustrated in the drawings and are further described in the following description.

Fig. 1 shows a schematic representation of an SCR catalytic converter, which can be diagnosed 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 shows a flow chart of a further embodiment of the method according to the invention.

Fig. 4 shows a flow chart of a further embodiment of the method according to the invention.

Detailed Description

An internal combustion engine 10 of a motor vehicle, which is shown in fig. 1, has an SCR catalytic converter 12 in its exhaust gas line 11. In the reducing agent tank 13 there is stored a reducing agent solution 14 in the form of an aqueous urea solution. The feed module 15 at the bottom of the reducing agent tank 13 is designed to transport the reducing agent solution 14 to the dosing module 16. In the exhaust line 11, the dosing module is arranged upstream of the SCR catalytic converter 12. In the exhaust line 11, a first nox sensor 17 is arranged upstream of the dosing module 16. In the exhaust line 11, a second nitrogen oxide sensor 18 is arranged downstream of the SCR catalytic converter 12. Both

nox sensors

17, 18 have a cross-sensitivity to ammonia. The internal combustion engine 10 and the dosing module 16 are controlled by an electronic control device 19. The electronic control unit also receives data from the

nox sensors

17, 18. The following example of the diagnostic method according to the invention is described in terms of this simple SCR catalytic converter system. However, all the following embodiments of the diagnostic method can also be applied at a more complex SCR catalytic converter system in which a further SCR catalytic converter is arranged in the exhaust line 11 downstream of the SCR catalytic converter 12, and in which a further dosing module can optionally be arranged between the two SCR catalytic converters.

The flow of the first embodiment of the diagnostic method is shown in fig. 2. After the method has been started 20, a value α for a dosing phase of, for example, 100mg of ammonia is calculated 21. This value is derived from equation 1:

(formula 1)

Here, mNH3_sollRepresenting the amount of ammonia required to reduce the nitrogen oxides emitted by the internal combustion engine 10 in accordance with the measurement of the first nitrogen oxide sensor 17, mNH3_istWhich represents the amount of ammonia actually dosed into the SCR catalytic converter 12 by means of the dosing module 12.

Next, it is checked 22 whether the general admissibility conditions for the diagnosis are fulfilled. These conditions of permission include: the appropriate operating mode of the internal combustion engine 10; opening of the

nitrogen oxide sensors

17, 18; a predetermined temperature range of the SCR catalytic converter 12; maximum filtration temperature gradient of the SCR catalytic converter 12; a predetermined range of the exhaust gas mass flow or the nitrogen oxide mass flow and the nitrogen oxide concentration at the first nitrogen oxide sensor 17; and a predetermined range of nitrogen levels in the SCR catalytic converter 12.

If the permission condition is fulfilled, the value α is compared 23 with a threshold value, which in the present case is, for example, 1.2. If the threshold value is exceeded, it is recognized that the over-dosing phase of the SCR catalytic converter 12 has already started and the method is continued for a first period of time 31. In a first calculation step 24, the overdosing amount is calculated. Subsequently, the overdose is compared 25 with the analysis threshold, which in the present case is, for example, 300mg of ammonia. If the analysis threshold is not met or exceeded, then the method is runAnd (7) breaking. Otherwise, the method is continued for an additional time period 32, such 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. The additional time period 32 ends if the cumulative exhaust gas mass since the beginning of the second time period 32 exceeds a threshold value, which in the present case is, for example, 0.5 kg. During the additional period of time 32, an additional analysis phase takes place, at the end of which NH3 is calculated according to equation 2_EffStorage efficiency expressed:

(formula 2)

Here, mNH3_OvrDosIndicating an excess charge of ammonia. mNH3_DsRepresenting the amount of ammonia downstream of the SCR catalyst 12. The ammonia amount can be calculated by means of equation 3:

(formula 3)

Here, m (NOx + NH3)_MessWhich represents the sum of the amount of nitrogen oxides and the amount of ammonia downstream of the SCR catalytic converter 12 measured by means of the second nitrogen oxide sensor 18. mNOx_ModWhich represents the nitrogen oxide amount at the second nitrogen oxide sensor 18, which can be determined by means of a model. In this case, the two integrals are determined over a first time period 31 and an additional time period 32. Then, the storage efficiency NH3 is determined_EffA comparison 27 is made with a threshold value, which serves to distinguish between intact and defective SCR catalytic converters 12. Depending on the result of this comparison, the SCR catalytic converter 12 is diagnosed 28 as functional or the SCR catalytic converter 12 is diagnosed 29 as damaged.

In comparison 25, a threshold can alternatively also be predefined for the ammonia level of the SCR catalytic converter, which is modeled for the transition to additional time period 32, as a function of the temperature and the exhaust gas mass flow. In another alternative, the threshold for the overdosing amount is queried but not shifted to the additional time period 32 until after the end of the current overdosing phase.

In a second embodiment of the method, the admissions conditions checked in step 22 additionally comprise: the difference between the actual ammonia level of the SCR catalytic converter and the maximum ammonia level at which ammonia slip cannot yet be expected in the case of WPA has at least in the present case a value of, for example, 200 mg. The current actual filling level is determined from a model of the SCR catalytic converter 12, which is calculated for the current dosing strategy. Alternatively, the actual filling level can be determined in a model calculated separately for the diagnosis. The maximum ammonia level at which ammonia slip is not yet expected in the case of WPA is known from the overall characteristic curve as a function of the temperature of the SCR catalytic converter and the exhaust gas mass flow. Alternatively, the actual fill level is set in a separately calculated diagnostic model at the initialization of the control device 19 to the maximum possible value for the ammonia charge, which is, for example, 6g in the present case, as a result of which a maximum estimate of the fill level is achieved. As the temperature of SCR catalytic converter 12 increases, the ammonia level based on the maximum storage capacity decreases. In the event of an immediately decreasing temperature, the storage location becomes free and a diagnosis can be approved. The possible nitrogen oxide conversion in the SCR catalytic converter 12 can also be reduced in level, whereby the ammonia storage space likewise becomes free. For comparison 23, instead of using a threshold value, integration of the above-mentioned ammonia amount is started as soon as the dosage amount at the dosage valve 12 is greater than the first threshold, which in the present case is, for example, 5mg/s, and is maintained until the dosage amount again falls below the second threshold, which in the present case is, for example, 3 mg/s. Thus, no overdosing is identified, but only the normal dosing of ammonia into the SCR catalytic converter 12. If a threshold value for dosed ammonia is exceeded, which is selected on average over a diagnostic period or a maximum temperature of the SCR catalytic converter as a function of the temperature of the SCR catalytic converter 12 and the exhaust gas mass flow, a transition is made from the first period 31 to an additional period 32. The further method steps are carried out as in the first exemplary embodiment of the method, wherein however in step 26 the storage efficiency NH3 is calculated according to equation 4_Eff：

(formula 4)

Wherein mNH3_DosThe amount of ammonia charge is shown. Substituting the modeled nitrogen oxide quantity mNOx in equation 3_ModIn the case of using equation 4, to calculate mNH3_DsThe quantity of nitrogen oxides mcnox measured by means of the first nitrogen oxide sensor 17 upstream of the SCR catalytic converter 12 is used according to equation 5_Mess：

(formula 5)

A third embodiment of the method according to the invention is shown in fig. 3. During a first time period 31, the overdose amount 24 is calculated and the method is then restarted each time the conditions of

steps

22 and 23 are met. The overdose amount calculated at the last run through step 24 is compared 25 with the analysis threshold only if one of the permission conditions according to step 22 is no longer fulfilled or if the value α no longer exceeds the threshold value in step 23. If this analysis threshold is exceeded, the method continues in the same way as in the first two embodiments and the integrals over the first 31 and additional 32 time periods are 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.

In a fourth embodiment of the diagnostic method, the diagnosis can also be performed according to the flow chart of fig. 3 if not overdosing but only normal dosing is performed. Here, the check 22 and the comparison 23 are performed as in the second embodiment. Next, the storage efficiency is calculated in step 26 by means of equation 4 as in the second embodiment.

Fig. 4 shows a flow chart of a fifth exemplary embodiment of the method according to the present invention. In this case, in step 22, it is checked not only the admissible conditions of the second exemplary embodiment, but also whether the conditions for the overdosing phase are fulfilled. If this is the case, in the first time period 31, the ammonia is first dosed positively with an overdosing 50 in the present case, for example, with a value of α = 1.5. The overdose is integrated upwards in step 24 and compared with an analysis threshold in step 25. If the analysis threshold is reached or exceeded, the method continues as in the first exemplary embodiment with steps 26 to 29, wherein the calculation is carried out in step 26 according to equation 2. Otherwise, the method is restarted.

In the fifth exemplary embodiment, an overdosing can alternatively also be carried out by adding an overshoot, which in the present case is, for example, 20mg/s, to the ammonia quantity mNH3 required for reducing the nitrogen oxides emitted by internal combustion engine 10_sollThe above.

In a sixth embodiment of the diagnostic method, no active overdose is performed for diagnosis, but only active dosing. In the flow chart according to fig. 4, it is then checked in step 22 whether the conditions for the ingredients are fulfilled in addition to the general approval conditions. In step 50, instead of overdosing, only normal dosing is performed. The dose is integrated upwards in step 24 and compared with an analysis threshold in step 25. Method steps 26 to 29 are carried out as in the second exemplary embodiment of the diagnostic method, equation 4 being used in step 26 to calculate the storage efficiency NH3_Eff。

Claims

1. A diagnostic method for an SCR catalytic converter (12), in which the storage efficiency of the SCR catalytic converter (12) is determined (26) as a function of the quotient of two integrals, wherein

-the first integral is an integral of the amount of ammonia downstream of the SCR catalytic converter (12); and is

-the second integral is an integral of the amount of ammonia dosed into the SCR catalyst (12) or the amount of ammonia dosed in excess into the SCR catalyst (12).

2. Diagnostic method according to claim 1, wherein the diagnostic method is performed if an ammonia over-dosing phase of the SCR catalyst (12) is identified, wherein the second integral is an integral of the amount of ammonia that is over-dosed into the SCR catalyst (12).

3. Diagnostic method according to claim 1, characterized in that the diagnostic method is performed if an ammonia dosing phase of the SCR catalyst (12) is identified, wherein the second integral is an integral of the amount of ammonia dosed into the SCR catalyst (12).

4. Diagnostic method according to claim 2 or 3, wherein the diagnostic method is performed if an ammonia overdose phase or an ammonia dose phase is identified within a first time period (31), wherein the first integral and the second integral are each determined within a second time period, the second time period being longer than the first time period.

5. The diagnostic method of claim 1, wherein ammonia is actively over-dosed into the SCR catalyst (12) during execution of the diagnostic method, wherein the second integral is an integral of the amount of ammonia over-dosed into the SCR catalyst (12).

6. The diagnostic method according to claim 1, characterized in that ammonia is dosed actively into the SCR catalyst (12) during the execution of the diagnostic method, wherein the second integral is an integral of the amount of ammonia dosed into the SCR catalyst (12).

7. The diagnostic method according to claim 5 or 6, wherein the diagnostic method is performed if an active ammonia overdose or ammonia dosing has taken place within a first time period (31), wherein the first integral and the second integral are each determined within a second time period, the second time period being longer than the first time period.

8. A computer program set up to perform each step of the method according to any one of claims 1 to 7.

9. A machine readable storage medium having stored thereon a computer program according to claim 8.

10. An electronic control device (19) which is set up for diagnosing an SCR catalytic converter (12) by means of a diagnostic method as claimed in one of claims 1 to 7.