METHOD FOR OPERATING AN EXHAUST GAS AFTERTREATMENT SYSTEM OF A MOTOR VEHICLE

Abstract

In the case of a method for operating an exhaust gas aftertreatment system of a motor vehicle, the exhaust gas aftertreatment system comprises at least one NOx storage catalyst (10) and at least one SCR catalyst (30). According to the invention, when an inadequate function of the NOx storage catalyst (10) or of the SCR catalyst (30) is identified, at least one auxiliary measure is initiated which leads to a reduction of the NOx emissions of the motor vehicle.

Claims

1. A method for operating an exhaust gas aftertreatment system of a motor vehicle, wherein the exhaust gas aftertreatment system comprises at least one NOx storage catalyst (10) and at least one SCR catalyst (30), wherein, when an inadequate function of the NOx storage catalyst (10) or of the SCR catalyst (30) is identified, at least one auxiliary measure for reducing the NOx emissions of the motor vehicle is introduced.

2. The method according to claim 1, wherein, in the event of an inadequate function of the SCR catalyst (30), a more frequent regeneration of the NOx storage catalyst (10) in comparison to the normal mode is carried out.

3. The method according to claim 2, wherein the more frequent regeneration of the NOx storage catalyst (10) is initiated by changing a threshold for an operating condition at which a regeneration is intended to be carried out.

4. The method according to claim 1, wherein, in the event of an inadequate function of the NOx storage catalyst (10), at least one auxiliary measure is initiated which improves the operating conditions for the SCR catalyst (30).

5. The method according to claim 4, wherein a measure for heating up the exhaust gas aftertreatment system is initiated as the auxiliary measure.

6. The method according to claim 4, wherein the auxiliary measure is an adaptation of at least one operating variable of the SCR catalyst (30).

7. The method according to claim 6, wherein, as auxiliary measure, a switch is made from a pilot-controlled mode to a closed-loop-control mode of the SCR catalyst (30), a switching from a closed-loop-control mode to a pilot-controlled mode of the SCR catalyst (30) is blocked, or both.

8. The method according to claim 4, wherein, in the event of a mode of the SCR catalyst (30) under NH.sub.3 level regulation, a higher NH.sub.3 desired level of the SCR catalyst (30) is set as the auxiliary measure.

9. A computer program which is designed to carry out the steps of a method according to claim 1.

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

11. An electronic control device which is designed to carry out the steps of a method according to claim 18.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Further features and advantages of the invention emerge from the description below of exemplary embodiments in conjunction with the drawings. The individual features can be realized here in each case by themselves or in combination with one another.

[0015] In the drawings:

[0016] FIG. 1 shows a schematic illustration of an exemplary refinement of an exhaust gas aftertreatment system from the prior art, in which the operating method according to the invention can be used, and

[0017] FIG. 2 shows a schematic flow diagram of an exemplary embodiment of the method according to the invention.

DETAILED DESCRIPTION

[0018] FIG. 1 shows, in a schematic manner, an exemplary construction of an exhaust gas aftertreatment system from the prior art, in which the method according to the invention can be used. The exhaust tract of an internal combustion engine (not shown specifically) of a motor vehicle is illustrated. The exhaust gas of the internal combustion engine flows through the exhaust tract in the arrow direction. The exhaust gas aftertreatment system comprises an NOx storage catalyst (NSC) 10, a diesel particle filter (cDPF) 20 and an SCR catalyst (SCR) 30. A metering point 40 for the required liquid reducing agent solution (for example AdBlue®) is located upstream of the SCR 30. A lambda probe 11, 12 is in each case located upstream and downstream of the NSC 10. An NOx sensor 31, 32 is in each case located upstream and downstream of the SCR 30. The NOx sensor 31 which is located upstream of the SCR 30 can optionally be replaced by a calculated model value. In such a combined exhaust gas aftertreatment system with an NSC 10 and an SCR 30, the two catalysts contribute their part in maintaining the NOx limit values. If one of the two catalysts 10, 30 is no longer capable of functioning correctly, the NOx emissions which occur can no longer be adequately compensated for. In general, the defective component is indeed displayed. However, in the conventional situation, an increase of the NOx emissions of the motor vehicle occurs until the defective component is repaired. These increased NOx emissions are avoided or at least reduced with the operating method according to the invention since, according to the method according to the invention, in the event of an error of one of the two catalysts 10 or 30, suitable replacement measures (auxiliary measures) are taken in order to use the other, still functioning exhaust gas aftertreatment components in each case to compensate for insufficient exhaust gas aftertreatment. The operating conditions for such a combined system are customarily designed in such a manner that both the NSC 10 and the SCR 30 are operated in one driving cycle. According to the invention, in the event of an error of the NSC 10 or of the SCR 30 (escalation situation), the operating conditions for the other catalyst in each case are selected and influenced in such a manner that the functioning component is increasingly used for reducing the NOx emissions.

[0019] An exemplary flow chart for the approach according to the method according to the invention in the event of an error and the auxiliary measures which are going to be initiated is illustrated in FIG. 2. During the driving cycle, the OBD monitoring functions are active here both for the NSC and for the SCR catalyst. Accordingly, after the start 200 of the method, the functions of the NSC and of the SCR catalyst are checked in step 201. In this context, it is initially enquired in step 202 whether the function of the NSC is in order. If this is the case, it is enquired in step 203 whether the function of the SCR catalyst is in order. If this is the case, it is output in step 204 that the exhaust gas aftertreatment system is in order in respect of the degradation of nitrogen oxides. However, if the enquiry in step 203 reveals that the function of the SCR catalyst is not in order, one or more auxiliary measures are requested in step 205, said auxiliary measures leading to the deficient NOx conversion in the SCR catalyst being compensated for via the NSC. In particular, a more frequent or additional regeneration of the NSC is requested here. By this means, the NOx charging of the NSC can be degraded, and therefore the NSC is ready for increased storage of NOx. The request in step 205 thus leads to the auxiliary measure 206, wherein the NSC is emptied more frequently because of the increased regeneration, and therefore the NOx charging of the NSC is reduced and the NSC can make a greater contribution to degrading the nitrogen oxides.

[0020] If the enquiry in step 202 reveals that the function of the NSC is not in order, it is enquired in step 207 whether the function of the SCR catalyst is in order. If this is the case, a different type of operation for the SCR catalyst is requested in step 208, as a result of which the operating conditions for the SCR catalyst are improved, and therefore the SCR catalyst can compensate for the deficient reduction of the nitrogen oxides by the defective NSC. In particular, a heating mode for the exhaust gas aftertreatment system can be requested here, as a result of which the temperature in the SCR catalyst is increased as an auxiliary measure in step 209. By this means, a suitable and optimum temperature level for the SCR catalyst is set, and therefore the SCR catalyst can make a greater contribution to degrading the nitrogen oxides. If the request in step 207 reveals that the function of the SCR catalyst is also not in order, it is output in step 210 that both the NSC and the SCR catalyst are defective. Further auxiliary measures are not expedient in this case.

[0021] Depending on the result of the monitoring functions, in principle four scenarios can therefore occur:

[0022] Scenario 1 (step 206): The SCR catalyst is no longer completely capable of functioning and the NSC is intact and can contribute, according to the invention, to the further reduction of the NOx emissions. The NSC is used here to an increased extent for NOx reduction throughout the entire driving cycle. For this purpose, an intervention is made in the normal operation and corresponding auxiliary measures taken for the inadequate SCR operation. So that the NSC can make a greater contribution to reducing the nitrogen oxide emissions, the NOx stored in the NSC is reduced by increased and more frequent regeneration of the NSC, and therefore further NOx absorption in the NSC is possible. In comparison to the normal operation, increased NSC regenerations are carried out here. This intervention can take place, for example, by reducing the NOx mass threshold during which NSC regeneration is initiated. Furthermore, it is possible to carry out the regeneration by changed operating conditions both of the engine and of the NSC. For example, the thresholds for the NSC temperature, for the exhaust gas mass flow, for the engine operating point or for the predicted additional fuel consumption can be changed in order to adapt the frequency of the NSC regeneration to the changed conditions.

[0023] Scenario 2 (step 204): In this case, both the NSC and the SCR catalyst are in order. That is to say that the NOx emissions can be reduced as intended. No replacement measures are necessary.

[0024] Scenario 3 (step 209): The NSC is no longer completely capable of functioning. However, the SCR catalyst is intact and can contribute to the further reduction of the NOx emissions. For this purpose, the SCR catalyst is increasingly used for the NOx conversion throughout the entire driving cycle. For this purpose, an intervention is made in the normal operation and corresponding auxiliary measures are taken for the inadequate NSC mode. In particular, the exhaust gas system can be correspondingly heated up and an increased temperature level for better SCR operation can be maintained for a period sufficient so that the operation of the SCR catalyst can make a greater contribution to reducing the NOx emissions. In addition to an increase of the SCR temperature, the operating variables of the SCR can also be adapted to an increased NOx conversion. For example, a change can be made from a pilot-controlled mode of the SCR catalyst to a closed-loop-control mode, or an optionally provided standard change from a closed-loop-control mode to an open-loop-control mode can be suppressed. Furthermore, it is possible, as a replacement measure, to change to a higher NH.sub.3 desired level of the SCR catalyst.

[0025] Scenario 4 (step 210): In this case, both the NSC and the SCR catalyst are defective. That is to say that the NOx emissions cannot be reduced as intended. In this case, the replacement measures according to the method according to the invention are not expedient.

[0026] In conventional diagnosis functions for the NSC and the SCR catalyst, two individual error tests are often carried out, the results of which make it possible to draw a conclusion about the degree of possibly present damage of the NSC or of the SCR catalyst. For example, there is a first error code for total failure or for removal of the component and there is a second error code which provides information about a certain aging and reduction of the functioning capability. In which of these two or these four error cases which auxiliary measure is initiated according to the method according to the invention can be selected in adaptation to the respective applications. For example, the described auxiliary measures can be initiated if only a certain aging and reduction of the functioning capability of one of the two exhaust gas aftertreatment components is determined. In other applications, the initiation of auxiliary measures can be restricted to the situations in which one of the two exhaust gas aftertreatment components completely fails.