DE102015206838A1 - Method for operating an exhaust aftertreatment device of a motor vehicle - Google Patents

Abstract

The invention relates to a method for operating an exhaust gas aftertreatment device (6) of a motor vehicle having an internal combustion engine (6) operated in normal lean burn mode with a first NOx storage catalytic converter (8) and a second NOx storage catalytic converter (10) connected downstream in the exhaust gas flow direction (A). and at least one broadband lambda probe (12) arranged between the first NOx storage catalytic converter (8) and the second NOx storage catalytic converter (10), comprising the steps of: performing a first regeneration step with a rich mixture in order to charge the first NOx storage catalytic converter (8 ), and performing a second regeneration step with a rich mixture to regenerate the second NOx storage catalyst (10). Furthermore, the invention relates to an exhaust aftertreatment device (6) and a motor vehicle, in particular passenger car, with such an exhaust aftertreatment device (6).

Description

The invention relates to a method for operating an exhaust gas aftertreatment device of a motor vehicle with an internal combustion engine operated in normal operation in lean burn mode. Furthermore, the invention relates to an exhaust aftertreatment device and a motor vehicle, in particular passenger car, with such an exhaust aftertreatment device.

With exhaust aftertreatment devices, combustion gases, after they have left the combustion chamber or the combustion chamber of an internal combustion engine driving the motor vehicle, cleaned by mechanical, catalytic or chemical means, so as to be able to comply with legal pollutant limits.

Modern lean burn gasoline engines operate in a lean burn mode with an excess of oxygen (λ> 1) to increase engine efficiency. Conventional catalysts can therefore not be used. Although the oxidation of CO (carbon monoxide) and C _m H _n (incompletely combusted hydrocarbons) in the presence of excess oxygen is still possible analogous to the conventional three-way catalyst, but NOx (nitrogen oxides) must be cached. Their catalytic reduction succeeds only in a stoichiometric to rich exhaust gas mixture. Therefore, catalysts with additional chemical elements are required, which allow a storage of NOx, so-called NOx storage catalysts. In order to comply with future emission standards, cars with diesel engines will also be equipped with NOx storage catalytic converters in the future.

In order to achieve this intermediate storage of the nitrogen oxides in the NOx storage catalysts, a noble metal catalyst such as platinum and a NOx storage component, which is usually an alkaline earth metal such as barium, are applied to suitable supports. In the lean, that is oxygen-rich, atmosphere, the nitrogen oxides are oxidized under the action of the noble metal catalyst, absorbed to form nitrates such as barium nitrate in the catalyst and thus removed from the exhaust gas stream. By regular, brief "enrichment" of these reactions take place in the opposite direction, whereby the NOx molecules are released back into the exhaust stream and further reduced by the existing in the rich atmosphere reducing components such as C _m H _n and / or CO.

If the absorption capacity of the NOx storage catalytic converter is exhausted, the engine electronics briefly set a rich, reducing exhaust gas mixture. In this short, fat regeneration cycle, the NOx temporarily stored in the catalytic converter is reduced to nitrogen, thereby preparing the NOx storage catalytic converter for the next storage cycle. This procedure makes it possible to minimize the pollutant emissions of lean-burn engines and to comply with the applicable limit values of the Euro standards.

For monitoring the absorption capacity of the NOx storage catalytic converter, it is known to evaluate sensor signals from two broadband lambda probes, one of which is arranged on the input side and the other on the output side of the NOx storage catalytic converter. A comparison of the signal characteristics of the two broadband lambda probes makes it possible to determine a reductant slip, from which in turn a time characteristic of a slip ratio can be determined in order to determine the remaining absorption capacity of the NOx storage catalytic converter. However, this requires stable measured values. This procedure is from the DE 10 2012 218 726 known.

Further, for monitoring the intake capacity of the NOx trap catalyst, it is known to perform an oxygen storage determination immediately after a rich mixture operation, in which oxygen is rapidly stored in the NOx trap catalyst immediately after the transition to the lean burn mode. In order to determine and compare the stored oxygen it is necessary that the regeneration has been completed, i. H. the oxygen regeneration is complete, so that the oxygen storage begins with an empty NOx storage catalyst.

Furthermore, z. B. from the US 2001/0037643 A1 , of the US 02010/0229535 A1 , and the US 6,938,412 B2 an exhaust gas treatment device with two downstream in the flow direction NOx storage catalytic converter known. Due to the resulting different distances between the two NOx storage catalytic converters, different operating temperatures result for both NOx storage catalytic converters, so that both NOx storage catalytic converters can complement each other at a given operating temperature.

An NOx storage catalyst stores not only NOx but also oxygen. During regeneration of the NOx trap catalyst, HC and CO are used in addition to NOx to convert oxygen.

The conversion of NOx has a slower reaction rate than the conversion of oxygen. This may be with a downstream of the NOx storage catalyst arranged lambda probe can be detected during regeneration.

Oxygen is stored quickly, within one to two seconds, in the NOx storage catalyst immediately after regeneration. Simultaneously with the oxygen storage takes place a NOx storage, which, however, extends over a longer period of time, usually a few minutes before the NOx storage catalyst is saturated.

After a long NOx storage time, when a relatively large amount of NOx is stored in the NOx storage catalyst, typically, the sensor signal of the output side lambda probe slowly decreases below unity during regeneration due to the slow NOx conversion.

In an exhaust gas treatment device with two NOx catalysts connected in series in the flow direction, the sensor signal of the output-side lambda probe of the first NOx storage catalytic converter is the input-side sensor signal of the second NOx storage catalytic converter. However, the sensor signal is not suitable to determine the reducing agent slip, since the determination at a lambda of one is not robust to sensor errors and the lambda value during the regeneration difficult to control, in particular, the aging state of the NOx storage catalyst can not be robust determine.

On the other hand, in the case of oxygen storage determination, the amount of the reductants supplied to the second NOx storage catalyst is small, so that the regeneration would take a long time to achieve complete regeneration. However, during a driving operation, such a long period of time is practically not given.

There is therefore a need to show a way, such as an exhaust gas aftertreatment device of a motor vehicle with a first NOx storage catalyst and downstream in the exhaust gas flow direction second NOx storage catalyst and at least one arranged between the first NOx storage and the second NOx storage catalyst broadband lambda probe reliably can be operated.

The object of the invention is achieved by a method for operating a Abgasbachbehandlungsvorrichtung a motor vehicle with a normal operation in lean burn operation internal combustion engine, with a first NOx storage catalyst and downstream in exhaust gas flow direction of the second NOx storage catalyst and at least one between the first NOx storage catalyst and the second NOx catalytic converter arranged broadband lambda probe in which a first regeneration step is performed with a rich mixture to regenerate the first NOx storage catalyst, and a second regeneration step is performed with a rich mixture to regenerate the second NOx storage catalyst. Here, under a lean burn operation, an operation with an excess of oxygen (λ> 1) is understood to increase the engine efficiency, while a rich mixture is understood to mean a mixture with an oxygen deficiency (λ <1). After a short NOx storage period, when only a minor amount of NOx is stored in the NOx storage catalyst but oxygen storage is still taking place, the broadband lambda sensor signal falls rapidly below a value of one during regeneration due to the fast oxygen conversion. Now, the sensor signal of the broadband lambda probe can be used to monitor the absorption capacity of the second NOx storage catalyst, since it is deep enough for this purpose. Thus, information is provided, which can be used after its evaluation for monitoring a degeneration of the second NOx storage catalyst.

According to one embodiment, in an intermediate step between the first regeneration step and the second regeneration step, a lambda value measured with the broadband lambda probe is compared with a threshold value less than one, and the method is continued with the second regeneration step only if the measured lambda value is less than the threshold is. This ensures that the method is continued with the second regeneration step only if the lambda value has fallen to a value less than one due to the oxygen conversion. This increases operational safety.

According to a further embodiment, the signal of the broadband lambda probe is evaluated in a further step in order to determine an aging state of the second NOx storage catalytic converter. Thus, targeted regeneration can be initiated in order to maintain the operability of the second NOx storage catalytic converter.

In another embodiment, a reductant slip is determined and evaluated to determine an aging condition of the second NOx storage catalyst. This is now possible because the lambda value is deep enough. This makes it possible to reliably determine a degeneration of the second NOx storage catalyst.

According to another embodiment, an oxygen storage determination is performed and evaluated to determine an aging condition of the second NOx storage catalyst. This is now possible because oxygen in the second NOx storage catalyst is now rapidly degraded. This also makes it possible to reliably determine a degeneration of the second NOx storage catalyst.

Further, the invention includes an exhaust aftertreatment device and a motor vehicle, in particular a car, with such an exhaust aftertreatment device.

The invention will now be explained with reference to a drawing. Show it:

1 an engine assembly having an internal combustion engine and an exhaust aftertreatment device for carrying out an embodiment of the method according to the invention.

2 Various signal waveforms from the input side and output side arranged on a NOx storage catalytic converter broadband lambda probes.

3a Signal curves of an input side of a NOx storage catalytic converter arranged NOx storage catalytic converter.

3b Signal curves of an output side of a NOx storage catalytic converter arranged NOx storage catalytic converter.

3c shows a slip ratio, formed by evaluation of 3a and 3b ,

3d shows the influence of degeneration on the hatching ratio.

3e shows the influence of degeneration on reducing agent slip.

4a shows a waveform of a lambda value in an oxygen storage determination.

4b shows the decrease in oxygen storage capacity as a function of degeneration of a NOx storage catalyst.

The 1 shows an engine assembly 2 as a drive of a motor vehicle, such as a car, in the present embodiment, an internal combustion engine 4 , an exhaust gas turbine turbine 14 an exhaust gas turbocharger (not shown) and an exhaust aftertreatment device 6 ,

The internal combustion engine 4 is designed in the present embodiment as a lean burn diesel engine, ie the lean burn diesel engine is operated in normal operation with an excess of oxygen (λ> 1) to increase the engine efficiency. Deviating from this, the internal combustion engine 4 also be designed as a lean burn gasoline engine with direct injection. Furthermore, the internal combustion engine needs 4 not necessarily be charged by means of an exhaust gas turbocharger.

The in the exhaust gas flow direction A of the exhaust gas turbocharger turbine 14 downstream exhaust aftertreatment device 6 has in the present embodiment, a first NOx storage catalyst 8th and a second NOx storage catalyst 10 on.

The first NOx storage catalyst 8th and the second NOx storage catalyst 10 are designed to store NOx (nitrogen oxides). They have the same structure with a suitable support with a noble metal catalyst such as platinum and a NOx storage component such. As an alkaline earth metal such as barium, on. Here, in the present embodiment, the first NOx storage catalyst 8th near the internal combustion engine 4 disposed while the second NOx storage catalyst 10 further apart from the internal combustion engine 4 is arranged, for. B. at a position in the underfloor area.

Furthermore, in the present embodiment, the exhaust aftertreatment device 6 a particle filter 16 and an SCR catalyst 18 on.

However, the particle filter 16 and the SCR catalyst 18 not mandatory.

The particle filter 16 is designed to retain soot particles in the exhaust stream.

The SCR catalyst 18 (Selective Catalytic Reduction) is designed to convert the exhaust gas component NOx selectively without formation of undesirable by-products to nitrogen and water. The conversion is carried out using a synthetically prepared, aqueous urea solution, which is carried in an additional tank.

Furthermore, the exhaust aftertreatment device 6 in the present embodiment, a first broadband lambda probe 12 on, between the first NOx storage catalyst 8th and the second NOx storage catalyst 10 is arranged. The first broadband lambda probe 12 is therefore the input side of the second NOx storage catalyst 10 arranged. The first broadband lambda probe 12 is a sensor that is in the Combustion exhaust gas of the internal combustion engine 4 the residual oxygen content is recorded. While a simple lambda probe is only capable of capturing lambda values by one, the broadband lambda probe is 12 suitable for detecting lambda values of 0.8 or higher, as in an operation of the internal combustion engine 4 occur with a rich mixture.

Further, in the present embodiment, a second broadband lambda probe 22 provided, the output side of the second NOx storage catalyst 10 is arranged. The second broadband lambda probe 22 has the same structure as the first broadband lambda probe 12 on.

The motor assembly 2 is formed in the present embodiment for exhaust gas recirculation (EGR, English: exhaust gas recirculation, EGR). In this case, controlled combustion gases are returned to the combustion chamber of the internal combustion engine 4 directed. By returning a portion of the exhaust gases into the combustion chamber, the combustion temperature in the cylinders of the internal combustion engine 4 lowered, thus reducing the formation of NOx. In this case, the exhaust gas recirculation may be formed as low-pressure or high-pressure exhaust gas recirculation. In the case of low-pressure exhaust gas recirculation, the removal of combustion gases takes place after the exhaust aftertreatment device 6 and the introduction before the turbocompressor of the exhaust gas turbocharger. In the case of the high-pressure exhaust gas recirculation, on the other hand, the removal takes place in front of the exhaust gas turbine 14 the exhaust gas turbocharger and the exhaust aftertreatment device 6 and the introduction to the charge air cooler and a throttle valve, if present.

To the engine assembly 2 also includes a control unit 20 in that the respective lambda values of the first broadband lambda sensor 12 and the second broadband lambda sensor 22 reads and evaluates and causes a change from a lean burn operation to a rich mix operation and vice versa, as explained in detail below. For this purpose, the control unit on hardware and / or software components.

It will now be described with additional reference to 2 to 4 An embodiment of a method according to the invention for operating such an exhaust aftertreatment device 6 described.

In normal operation, the internal combustion engine 4 in the partial load range with a lambda value greater than one, ie operated in the lean mix. To a regeneration of the first NOx storage catalyst 8th to effect a first regeneration step is performed. This first regeneration step is performed by the controller 20 for controlling the motor assembly 2 initiated. The control unit controls this 20 the internal combustion engine 4 such that it is operated with a mixture having a lambda value greater than one to regenerate the first NOx storage catalyst.

During regeneration, due to the rapid conversion of oxygen, the lambda value drops rapidly to a value less than one, as it does 2 shows.

In an intermediate step is by the control unit 20 one with the broadband lambda probe 12 measured lambda value compared with a threshold value less than one. The procedure will only be done by the controller 20 continued when the measured lambda value is less than the threshold value. This ensures that the process is continued only when the first NOx storage catalyst 8th was completely regenerated.

It then forwards the controller 20 a return to normal operation of the internal combustion engine 4 in, in which the internal combustion engine again in lean-burn mode, ie with a lambda value greater than one operation.

If the threshold comparison has shown that the lambda value is less than one, the controller will conduct 20 a second regeneration step with a rich mixture to the second NOx storage catalyst 10 to regenerate. Here, NOx is returned to the exhaust stream and further reduced by the existing in the rich atmosphere reducing components such as C _m H _n (incompletely burned hydrocarbons) and / or CO (carbon monoxide).

In a further step, the respective signals of the first broadband lambda probe 12 and the second broadband lambda probe 22 evaluated to an operating state of the second NOx storage catalyst 10 to determine, ie to what extent a degeneration of the second NOx storage catalyst 10 has progressed.

This can be done by determining a reductant slip, in which a time characteristic of a slip ratio is determined in order to determine the remaining absorption capacity of the NOx storage catalytic converter. A method for determining the reducing agent slip is known from DE 10 2012 218 726 known, the disclosure of which is fully incorporated herein.

Alternatively or additionally, an oxygen storage determination may be made immediately after operation with a rich mixture so as to maintain the remaining capacity of the NOx storage catalyst to determine or increase the accuracy of the determination.

The 3a to 3e refer to methods for determining reductant slip.

The 3a shows the time course of the sensor signal on the input side of the second NOX storage catalytic converter 10 arranged first broadband probe 12 ,

The 3b shows the time course of a sensor signal on the output side of the second NOX storage catalytic converter 10 arranged second broadband lambda probe 22 ,

A comparison of 3a and 3b shows that the second NOX storage catalyst 10 shows the transmission behavior of a deadtime element, wherein, depending on the degeneration, the storage capacity of the second NOX storage catalytic converter 10 decreases and thus the dead time is smaller.

The 3c shows the evaluation by determining a slip ratio of the 3a and 3b during operation with a rich mixture. It can be seen that the decreasing dead time leads to an increasing slip ratio.

The 3d and 3e illustrate the change in the slip ratio and the reducing agent slip in the direction of arrow P due to the degeneration of the second NOx storage catalyst 10 again.

Thus, the reducing agent slip is determined and evaluated to the operating state of the second NOx absorber 10 to determine.

The 4a and 4b relate to the method for oxygen storage determination, wherein the timing of the exhaust aftertreatment device 6 is evaluated after operation with a rich mixture to the operating state of the second NOx absorber 10 or to increase the accuracy of the determination in combination with the method of determining reductant slip.

The 4a shows the waveform of a lambda value in such an oxygen storage determination. The oxygen storage determination is performed immediately after a rich mixture operation in which oxygen is rapidly stored in the NOx trap catalyst immediately after the transition to lean mode.

It can be seen that due to the transmission behavior analogous to a deadtime element, a kind of phase shift is established, which can be evaluated, the operating state of the second NOx absorber 10 to determine.

The 4b shows that it is due to the degeneration of the second NOx storage catalyst 10 there is a decrease in oxygen storage capacity (OSC).

In both cases, the sensor signal of the broadband lambda probe 12 for monitoring the absorption capacity of the second NOx storage catalytic converter 10 be used because it is deep enough for this. This allows, with the determination of the reducing agent slip or the oxygen storage determination, the degeneration of the second NOx storage catalytic converter 10 reliable to determine.

LIST OF REFERENCE NUMBERS

2

 motor assembly

4

 Internal combustion engine

6

 exhaust aftertreatment device

8th

 first NOx storage catalyst

10

 second NOx storage catalyst

12

 input-side broadband lambda probe

14

 The exhaust gas turbocharger turbine

16

 particulate Filter

18

 SCR catalyst

20

 control unit

22

 output broadband lambda probe

A

 Exhaust gas flow direction

P

 arrow

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102012218726 [0006, 0051]
• US 2001/0037643 A1 [0008]
• US 02010/0229535 A1 [0008]
• US 6938412 B2 [0008]

Claims (11)

Method for operating an exhaust aftertreatment device ( 6 ) of a motor vehicle with an internal combustion engine in normal operation in the lean burn mode ( 6 ), with a first NOx storage catalyst ( 8th ) and a downstream in the exhaust gas flow direction (A) second NOx storage catalyst ( 10 ) and at least one between the first NOx storage catalyst ( 8th ) and the second NOx storage catalyst ( 10 ) arranged broadband lambda probe ( 12 ), comprising the steps of: performing a first regeneration step with a rich mixture to produce the first NOx storage catalyst ( 8th ) and performing a second regeneration step with a rich mixture to treat the second NOx storage catalyst ( 10 ) to regenerate. Method according to claim 1, wherein, in an intermediate step between the first regeneration step and the second regeneration step, one with the broadband lambda probe ( 12 ) is compared with a threshold value less than one, and the method is continued with the second regeneration step only if the measured lambda value is less than the threshold value. Method according to claim 1 or 2, wherein in a further step the signal of the broadband lambda probe ( 12 ) is evaluated to an aging state of the second NOx storage catalyst ( 10 ). The method of claim 3, wherein a reductant slip is determined and evaluated to determine an aging condition of the second NOx trap catalyst ( 10 ). The method of claim 3, wherein an oxygen storage determination is performed to determine an operating condition of the second NOx storage catalyst ( 10 ). Exhaust aftertreatment device ( 6 ) for a motor vehicle with an internal combustion engine operated in normal operation in the lean-burn mode ( 6 ), with a first NOx storage catalyst ( 8th ) and a downstream in the exhaust gas flow direction (A) second NOx storage catalyst ( 10 ) and at least one between the first NOx storage catalyst ( 8th ) and the second NOx storage catalyst ( 10 ) arranged broadband lambda probe ( 12 ), wherein the exhaust aftertreatment device ( 6 ) is adapted to perform a first regeneration step with a rich mixture to the first NOx storage catalyst ( 8th ) and perform a second regeneration step with a rich mixture to recover the second NOx storage catalyst ( 10 ) to regenerate. Exhaust aftertreatment device ( 6 ) according to claim 6, wherein the exhaust aftertreatment device ( 6 ) is formed in an intermediate step between the first regeneration step and the second regeneration step one with the broadband lambda probe ( 12 ), and to continue the method with the second regeneration step only if the measured lambda value is smaller than the threshold value. Exhaust aftertreatment device ( 6 ) according to claim 6 or 7, wherein the exhaust aftertreatment device ( 6 ) is configured, in a further step, the signal of the broadband lambda probe ( 12 ) to an operating state of the second NOx storage catalyst ( 10 ). Exhaust aftertreatment device ( 6 ) according to claim 8, wherein the exhaust aftertreatment device ( 6 ) is configured to determine and evaluate a reducing agent slip in order to determine an operating state of the second NOx storage catalytic converter ( 10 ). Exhaust aftertreatment device ( 6 ) according to claim 8, wherein the exhaust aftertreatment device ( 6 ) is configured to perform an oxygen storage determination to determine an operating state of the second NOx storage catalytic converter ( 10 ). Motor vehicle with an exhaust aftertreatment device ( 6 ) according to one of claims 6 to 10.

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