Abstract
The invention concerns a method for monitoring an SCR catalytic converter in an exhaust line of an internal combustion engine, into which a reducing agent solution for the reduction of nitrogen oxides is dosed, wherein the SCR catalytic converter is diagnosed as defective if a measured variable is below a corresponding threshold and wherein the diagnosis of the SCR catalytic converter occurs when enabling criteria are met, wherein the enabling criteria are selected depending on a BPU model and a WPA model such that when the SCR catalytic converter (3) conforms to the BPU model, ammonia slip occurs through this SCR catalytic converter (3), and when the SCR catalytic converter corresponds to the WPA model, no ammonia slip occurs through this SCR catalytic converter.
Claims
1. A method for monitoring an SCR catalytic converter (3) in an exhaust system (2) of an internal combustion engine (1), into which a reducing agent solution for reducing nitrogen oxides is dosed, the method comprising: selecting (20), based on a BPU model and a WPA model, enabling criteria; and when one or more of the enabling criteria is met, diagnosing the SCR catalytic converter (3) as defective (80) when a measured value (η.sub.mess) is below a corresponding threshold (η.sub.OBD), wherein the enabling criteria is selected such that ammonia slip occurs through the SCR catalytic converter (3) when the SCR catalytic converter (3) conforms to the BPU model, and no ammonia slip occurs through the SCR catalytic converter (3) when the SCR catalytic converter (3) corresponds to the WPA model.
2. The method according to claim 1, characterized by an enabling criterion that is met when the SCR catalytic converter (3) has a maximum ammonia storage capacity (F.sub.maxBPU) according to the BPU model which is insufficient to store all of the ammonia converted by the reducing agent and if the SCR catalytic converter according to the WPA model has a maximum ammonia storage capacity (F.sub.maxWPA) sufficient to store all of the ammonia converted by the reducing agent.
3. The method according to claim 1, characterized by an enabling criterion that is met when the absolute temperature of the SCR catalytic converter (1) is above a temperature threshold.
4. The method according to claim 1, characterized by an enabling criterion that is met when the gradient of the temperature of the SCR catalytic converter (3) is above a first temperature gradient threshold.
5. The method according to claim 4, characterized by an enabling criterion that is met if the gradient of the temperature of the SCR catalytic converter (3) is below a second temperature gradient threshold that is above the first temperature gradient threshold.
6. The method according to claim 1, characterized by an enabling criterion that is met when a modeled ammonia fill level value determined from a model for the ammonia potential of the SCR catalytic converter (3) lies above a first ammonia fill level threshold that represents the maximum ammonia storage capacity (F.sub.maxBPU) of the SCR catalytic converter (3) according to the BPU model, and below a second ammonia storage threshold value that represents the maximum ammonia storage capacity (F.sub.maxWPA) of the SCR catalytic converter (3) according to the WPA model.
7. The method according to claim 1, characterized by an enabling criterion that is met when a dosed reducing agent mass is above an upper dosing threshold at which a maximum ammonia storage capacity (F.sub.maxBPU) of the SCR catalytic converter (3) in accordance with the BPU model is insufficient to store all of the ammonia converted by the reducing agent and the maximum ammonia storage capacity (F.sub.maxWPA) of the SCR catalytic converter (3) according to the WPA model is sufficient to store all the ammonia converted by the reducing agent.
8. The method according to claim 1, characterized by an enabling criterion that is met when a model for the ammonia slip according to the BPU model predicts ammonia slip of the SCR catalytic converter (3).
9. The method according to claim 1, characterized by an enabling criterion that is met when a difference between an ammonia slip model for the BPU model and an ammonia slip model for the WPA model exceeds a model threshold.
10. The method according to claim 1, characterized in that the measured variable is one or more of the following variables or is determined directly therefrom: an ammonia concentration downstream of the SCR catalytic converter (3); an ammonia mass flow downstream of the SCR catalytic converter (3); a sensor signal of a nitrogen oxide sensor downstream of the SCR catalytic converter (3); and/or a sensor signal of a nitrogen oxide sensor upstream of the SCR catalytic converter (3).
11. The method according to claim 10, characterized in that the measured variable is a measured conversion rate (η.sub.mess) of the SCR catalytic converter (3) that is determined from a sensor signal of a nitrogen oxide sensor (5) or an ammonia sensor downstream of the SCR catalytic converter (3) and a sensor signal of a nitrogen oxide sensor (4) or an ammonia sensor upstream of the SCR catalytic converter (3).
12. The method according to claim 1, characterized in that the monitoring is terminated when an integrated exhaust gas mass exceeds an associated exhaust gas mass threshold value.
13. A non-transitory machine-readable storage medium having a computer program that is configured to perform each step of the method according to claim 1.
14. An electronic control unit that is configured to carry out monitoring of an SCR catalytic converter (3) by the method according to claim 1.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description.
(2) FIG. 1 shows a schematic representation of an SCR catalytic converter in an exhaust system of a combustion engine, which is monitored by means of an embodiment of the method according to the invention.
(3) FIG. 2 shows a flow chart of an exemplary embodiment of the method according to the invention with the use of the additional enabling condition designed for NH.sub.3.
(4) FIG. 3 shows a graph of the maximum ammonia storage capacity of an SCR catalytic converter according to a WPA model and according to a BPU model as a function of the temperature of the SCR catalytic converter and an exemplary ammonia slip situation due to a temperature increase.
(5) FIG. 4 shows a diagram of the maximum ammonia storage capacity of an SCR catalytic converter according to a WPA model and according to a BPU model as a function of the temperature of the SCR catalytic converter and with an exemplary ammonia slip situation due to an increase in the ammonia fill level predetermined by a dosing strategy.
(6) FIG. 5 shows a diagram of the maximum ammonia storage capacity of an SCR catalytic converter according to a WPA model and according to a BPU model as a function of the temperature of the SCR catalytic converter and with an exemplary ammonia slip situation due to a temporary overdose.
DETAILED DESCRIPTION
(7) FIG. 1 shows a schematic representation of an internal combustion engine 1, an exhaust gas line 2 and an exhaust aftertreatment system with an SCR catalytic converter 3 that is monitored by means of an embodiment of the method according to the invention. An exhaust mass flow Q.sub.A emitted by the internal combustion engine 1 is transported from the internal combustion engine 1 via the exhaust gas line 2 to the exhaust gas aftertreatment system. The internal combustion engine 1 is controlled by an electronic control unit 6. The urea water solution required for the reduction of nitrogen oxides in the SCR catalytic converter 3 is injected into the exhaust line 2 upstream of the SCR catalytic converter 3 via a dosing valve 8 by means of a known transport and dosing system 7. The transport and dosing system 7 and the dosing valve 8 are actuated by the electronic control unit 6. In addition, a first nitrogen oxide sensor 4 upstream of the SCR catalytic converter 3 is disposed upstream of the dosing valve 8. The first nitrogen oxide sensor 4 measures the nitrogen oxide concentration in the exhaust gas upstream of the SCR catalytic converter 3 and forwards the measurement result to the electronic control unit 6. Furthermore, a second nitrogen oxide sensor 5 is disposed downstream of the SCR catalytic converter 3 and measures the nitrogen oxide concentration in the exhaust gas downstream of the SCR catalytic converter 3, wherein the nitrogen sensor 5 has a cross-sensitivity to ammonia, and also passes the measurement result to the electronic control unit 6. In addition, a first temperature sensor 9 upstream of the SCR catalytic converter 3 is disposed upstream of the dosing valve 8, and a second temperature sensor 10 is disposed downstream of the SCR catalytic converter 3. By means of the control unit 6, the transport and dosing system 7 and the dosing valve 8 are controlled and the required mass of urea-water solution is dosed into the exhaust line 2 as a function of the determined nitrogen concentrations in the exhaust gas.
(8) FIG. 2 shows a flow chart of an exemplary embodiment of the method according to the invention for the monitoring the SCR catalytic converter 3 depending on a selected dosing strategy. First, a model 10 for maximum ammonia storage capacity F.sub.maxWPA according to a WPA model and a model 11 for a maximum ammonia storage capacity F.sub.maxBPU according to a BPU model are provided. Said models 10, 11 can be calculated as part of a diagnostic function. Optionally, the model 10 may also be calculated as part of a dosing strategy according to the WPA model. Alternatively, the modeled ammonia fill level can also be determined independently of the dosing strategy.
(9) In a further process step 20, relevant enabling criteria, which are explained in the description of FIGS. 3 to 5, are determined for an SCR catalytic converter according to the WPA model and according to the BPU model. Relevant enabling criteria may be, for example, the temperature T of the SCR catalytic converter 3, the gradient of the temperature of the SCR catalytic converter 3, the modeled ammonia and the overdose mass. As part of a comparison 40 the modeled maximum ammonia storage capacity F.sub.maxWPA according to the WPA model and the maximum ammonia storage capacity F.sub.maxBPU according to the BPU model are compared with the enabling criteria. If an enabling criterion is met, the diagnosis is carried out. In further exemplary embodiments, it may be provided that several and, as a special case, all enabling criteria are met before the diagnosis takes place. If, on the other hand, no enabling criteria or insufficient enabling criteria are met, no diagnosis is carried out. In this case, the comparison 40 is performed again. In further exemplary embodiments, the enabling criteria can be changed or replaced. During the diagnosis, in a next process step 50, a current conversion rate η.sub.mess is measured by means of the nitrogen oxide sensors 4, 5 in the exhaust line and compared in a further comparison 60 with an associated threshold η.sub.OBD. If the measured conversion rate η.sub.mess is above the threshold η.sub.OBD or if it corresponds to it, the SCR catalytic converter 3 is diagnosed as functioning 70. If the conversion rate η.sub.mess is below the threshold η.sub.OBD, the SCR catalytic converter 3 is diagnosed as defective 80.
(10) FIG. 3 shows a graph of the ammonia fill level F of an SCR catalytic converter 3 as a function of the temperature T of the SCR catalytic converter 3. The temperature-dependent curve of the maximum ammonia storage capacity F.sub.maxWPA of the SCR catalytic converter 3 is shown according to the WPA model and the temperature-dependent curve of the maximum ammonia storage capacity F.sub.maxBPU of the SCR catalytic converter 3 is shown according to the BPU model. The curve of the maximum ammonia storage capacity F.sub.maxWPA according to the WPA model lies above the curve of the maximum ammonia storage capacity F.sub.maxBPU according to the BPU model, since it is generally assumed that an SCR catalytic converter according to the WPA model can store more ammonia than the SCR catalytic converter according to the BPU model. The graph shows two operating points 100, 102. The first operating point is at a first temperature T.sub.1 and at an associated first ammonia fill level F.sub.1 and lies below the curve of the maximum ammonia storage capacity F.sub.maxBPU of an SCR catalytic converter 3 according to a BPU model, so that at this first operating point 100 no ammonia slip is expected. If the temperature of the SCR catalytic converter 3 increases to a second temperature T.sub.2, the ammonia fill level of the SCR catalytic converter 3 decreases insignificantly along the arrow 101 from the first ammonia fuel level F.sub.1 to the second ammonia fill level F.sub.2. This is now the second operating point 102 at the second temperature T.sub.2 and the second ammonia fill level F.sub.2. The second operating point 102 thus lies in a corridor above the curve of the maximum ammonia storage capacity F.sub.maxBPU of an SCR catalytic converter 3 according to a BPU model and below the curve of the maximum storage capacity F.sub.maxWPA of an SCR catalytic converter 3 according to a WPA model. As a result, a high ammonia slip is to be expected in the case of the SCR catalytic converter 3 in accordance with a BPU model and at most a slight slip in the case of an SCR catalytic converter 3 according to a WPA model. At the second operating point 102, an enabling criterion for the enabling of the diagnosis is consequently met.
(11) FIG. 4 shows a graph of the ammonia fill level F of an SCR catalytic converter 3 as a function of the temperature T of the SCR catalytic converter 3. The temperature-dependent curve of the maximum ammonia storage capacity F.sub.maxWPA of the SCR catalytic converter 3 according to the WPA model is shown and the temperature-dependent curve of the maximum ammonia storage capacity F.sub.maxBPU of the SCR catalytic converter 3 according to the BPU model is shown. The curve of the maximum ammonia storage capacity F.sub.maxWPA of an SCR catalytic converter 3 according to the WPA model lies, as already described in FIG. 3, above the curve of the maximum storage capacity F.sub.maxBPU of an SCR catalytic converter 3 according to the BPU model. The graph shows two further operating points 200, 202. The first operating point is located at a third temperature T.sub.3 and an associated third ammonia fill level F.sub.3 and is below the curve of the maximum ammonia storage capacity F.sub.maxBPU of the SCR catalytic converter 3 according to a BPU model, so again no ammonia slip is expected. A defined mass of the urea-water solution is dosed into the exhaust gas line and the temperature T of the SCR catalytic converter 3 decreases from the third temperature T.sub.3 to the fourth temperature T.sub.4. As a result, the ammonia fill level of the SCR catalytic converter 3 rises along the arrow 201 from the third ammonia fill level F.sub.3 to the fourth ammonia fill level F.sub.4. This is now the fourth operating point 202 at the fourth temperature T.sub.4 and the fourth ammonia fill level F.sub.4. The fourth operating point 202 thus lies in a corridor above the curve of the maximum ammonia storage capacity F.sub.maxBPU of an SCR catalytic converter 3 according to a BPU model and below the curve of the maximum storage capacity F.sub.maxWPA of an SCR catalytic converter 3 according to a WPA model. As a result, a high slip is to be expected in the case of the SCR catalytic converter 3 according to a BPU model and at most a slight slip in the case of an SCR catalytic converter 3 according to a WPA model. At the fourth operating point 202, an enabling criterion for the enabling of the diagnosis is thus met.
(12) FIG. 5 shows a graph of the ammonia fill level F of an SCR catalytic converter 3 as a function of the temperature T of the SCR catalytic converter 3. The temperature-dependent curve of the maximum ammonia storage capacity F.sub.maxWPA of the SCR catalytic converter 3 according to the WPA model is shown and the temperature-dependent curve of the maximum ammonia storage capacity F.sub.maxBPU of the SCR catalytic converter 3 according to the BPU model is shown. The curve of the maximum ammonia storage capacity F.sub.maxWPA of the SCR catalytic converter 3 according to the WPA model lies above a curve of the maximum storage capacity F.sub.maxBPU of the SCR catalytic converter 3 in accordance with the BPU model, since it is assumed that the SCR catalytic converter 3 according to the WPA model can store more ammonia than the SCR catalytic converter according to the BPU model. The graph shows two further operating points 300, 302. The first operating point is at a fifth temperature T.sub.5 and an additional fifth ammonia fill level F.sub.5 and lies below the curve of the maximum ammonia storage capacity F.sub.maxBPU of the SCR catalytic converter 3 according to the BPU model, so that again no ammonia slip is to be expected. A defined mass of urea-water solution is dosed into the exhaust line that is greater than the mass of urea-water solution required for a conversion of nitrogen oxides in the exhaust gas. As a result, the ammonia fill level of the SCR catalytic converter 3 increases sharply from the fifth ammonia fill level F.sub.5 to the sixth ammonia fill level F.sub.6 along the arrow 301. At the same time, the temperature T of the SCR catalytic converter 3 decreases from the fifth temperature T.sub.5 to the sixth temperature T.sub.6. This is now the sixth operating point 302 at the sixth temperature T.sub.6 and the sixth ammonia fill level F.sub.6. The sixth operating point 302 thus lies in a corridor above the curve of the maximum ammonia storage capacity F.sub.maxBPU of an SCR catalytic converter 3 according to the BPU model and below the curve of the maximum ammonia storage capacity F.sub.maxWPA of the SCR catalytic converter 3 according to the WPA model. As a result, a high slip is expected in the SCR catalytic converter 3 according to the BPU model, whereas it can be assumed that the SCR catalytic converter according to the WPA model can still store the excess ammonia. At the sixth operating point F.sub.6, an enabling criterion for the enabling of the diagnosis is thus met.
