DE102016200158A1 - Method for monitoring an exhaust aftertreatment system of an internal combustion engine and control device for an exhaust aftertreatment system - Google Patents

Abstract

In a method according to the invention for monitoring an exhaust gas aftertreatment system (3) of an internal combustion engine (1) designed for lean operation, which comprises a NOx storage catalytic converter (5), nitrogen oxides stored in a regeneration phase (12) in the NOx storage catalytic converter (5) are supplied by supplying a Reduces reducing means, after the end of the regeneration phase (12) in the lean operation of the internal combustion engine (1) an oxygen content of the exhaust stream downstream of the NOx storage catalyst (5) is detected by means of at least a first oxygen sensor, which is arranged downstream of the NOx storage catalyst (5) and is formed as a narrow-band oxygen sensor, an oxygen content of the exhaust stream upstream of the NOx storage catalytic converter (5) is detected, as determined by the NOx storage catalyst amount of oxygen or mass oxygen uptake of the NOx storage catalyst (5) is up to a detected by the Sauerstoffgeh old determined downstream of the NOx storage catalyst (5) certain time (t2), and from the determined oxygen uptake is concluded on the functioning of the NOx storage catalytic converter (5). The invention also relates to a corresponding control device for an exhaust aftertreatment system (3) of an internal combustion engine (1) designed for a lean operation (11, 13).

Description

The invention relates to a method for monitoring an exhaust aftertreatment system of an internal combustion engine according to the preamble of claim 1 and to a corresponding control device for an exhaust aftertreatment system.

Internal combustion engines often generate considerable amounts of nitrogen oxides (NO _x ) during operation. In particular, in diesel and gasoline engines used in motor vehicles, the amounts of nitrogen oxides in the exhaust gas are generally above the permissible limits, so that an exhaust aftertreatment to reduce NO _x emissions is necessary. In many engines, the reduction of nitrogen oxides by the non-oxidized constituents contained in the exhaust gas, namely by carbon monoxide (CO) and unburned hydrocarbons (HC), using a three-way catalyst. However, this method is not available in particular in the case of diesel and Otto lean-burn engines, since the reduction of NO _x does not or hardly occurs due to the high proportion of oxygen in the exhaust gas. For lean-burn engines, therefore, according to a widespread method, an NO _x storage catalytic converter (Lean NO _x trap, LNT) is used, which receives and stores the nitrogen oxides contained in the exhaust gas of the internal combustion engine. From time to time, there is a regeneration of the NO _x storage catalytic converter, for which a fuel excess is generated in the exhaust gas conducted through the NO _x storage catalytic converter.

However, the functionality of the NO _x storage catalyst decreases with increasing operating time, which is due, inter alia, to contamination of the storage catalyst with the sulfur contained in the exhaust gas, as well as thermal aging as a result of high temperatures, such as occur in a regular to be carried out desulfurization. It is therefore necessary to monitor the functioning of a provided in the exhaust system NO _x storage.

From the European patent application EP 1 936 140 A1 a method for monitoring an exhaust aftertreatment system of an internal combustion engine is known, upstream and downstream of the exhaust aftertreatment system is arranged in each case a lambda probe and to check the functioning of the exhaust aftertreatment system, the internal combustion engine is converted into an operation in which the exhaust gases have a high concentration of unburned hydrocarbons. It is assumed that the exhaust aftertreatment system is inoperable if the air conditions detected by the two lambda probes and due to the high HC concentration are substantially the same.

According to DE 10 2012 218 728 A1 For checking the functionality of a NO _x storage catalytic converter of an internal combustion engine, the internal combustion engine is converted into a substoichiometric operation (λ <1) and the air ratio is detected by a respective lambda probe arranged upstream or upstream of the storage catalytic converter. In this case, the enrichment, ie the enrichment of the exhaust gas with unburned hydrocarbons, limited so that the probes operate error-free. In a fully functional storage catalyst unburned hydrocarbons located in the exhaust due to the enrichment upstream of the catalyst are completely oxidized as it flows through the catalyst, so that there are no unburned hydrocarbons in the exhaust gas downstream of the catalyst. If the functionality of the NO _x storage catalyst is limited, no or less in the exhaust gas located unburned hydrocarbons are oxidized by release of stored nitrogen oxides. From the time course of the detected by the downstream of the storage cylinder lambda probe air ratio during the phase of enrichment and in particular from the time course of the determined from the signals of the probes, integrated over a short time interval mass flows of unburned hydrocarbons in the exhaust gas can be statements regarding the functionality of the storage catalytic converter.

Out KR 10 2011 006 3140 A It is known that from the signals of two lambda probes, one of which is arranged upstream and the other downstream of a NO _x storage catalyst, the amount of the reducing agent, which is supplied to the storage catalyst, and the amount of the reducing agent after passing is present in the exhaust gas through the storage catalyst. From the thus determined through or slip ratio (slip ratio), the aging of the NO _x storage catalytic converter is determined.

According to US 2004/0182068 A1 For example, the oxygen storage capability of an NO _x storage catalyst (LNT) is estimated based on the signal from a sensor located downstream of the storage catalyst. As a result, the end of a regeneration phase can be predicted more accurately in order to reduce pollutant emissions.

Out DE 103 05 452 A1 a method for the diagnosis of a catalyst in the exhaust gas stream of an internal combustion engine is known, wherein the oxygen storage capacity of a NO _x storage catalyst is determined by the storage catalytic converter is acted upon after previous lean operation with rich exhaust gas until downstream of the storage catalytic converter by Lambdamessung a reducing agent breakthrough is detected. With renewed transition in the lean operation is concluded from a time delay between the exceeding of a predetermined lean lambda value before and after the storage catalyst on the oxygen storage capacity of the catalyst.

In US 2007/0084195 A1 is a method for determining an _NOx storage capacity of a catalytic device, such as a NO _x storage catalytic converter disclosed exhaust gas treating an internal combustion engine, wherein the oxygen storage capacity of NO _x storage catalytic converter is determined by the signals arranged one upstream and one downstream of the catalyst UEGO sensors during a transition from rich to lean mixture. For this purpose, after the end of a regeneration cycle, a diagnostic air-fuel ratio is maintained which is closer to a stoichiometric ratio than in normal lean operation.

Since the behavior of an NO _x storage catalyst depends to a large extent on the temperature of the storage catalytic converter, the known monitoring methods provide reliable information about the functionality of the NO _x storage catalytic converter only in a relatively narrow temperature range. This is particularly problematic because to monitor the functionality of the NO _x storage catalytic converter and to comply with the prescribed monitoring procedures (In-Use Performance Requirements, IUPR) the appropriate monitoring measures during operation of a equipped with the NO _x storage vehicle motor vehicle at least with a predetermined Frequency must be performed so that compliance with the narrow temperature range can not always be guaranteed. Furthermore, the known methods are associated with a considerable effort, which is particularly due to the necessary sensors.

It is an object of the present invention to provide an alternative or improved method for monitoring an exhaust aftertreatment system of an internal combustion engine, in particular a designed for lean operation internal combustion engine, and a corresponding control device for such an exhaust aftertreatment system, in particular such a method or such a control device, wherein the above-mentioned disadvantages can be avoided as possible.

This object is achieved by a method and by a control device as indicated in the independent claims.

An inventive method for monitoring an exhaust aftertreatment system of an internal combustion engine relates in particular to an engine designed for lean operation, for example, a diesel engine or a gasoline lean-burn engine, in particular a diesel or gasoline engine with direct injection. Preferably, it is the internal combustion engine and the exhaust aftertreatment system of a motor vehicle. The term "lean operation" means that the internal combustion engine is operated with excess air, ie that the lambda value (air ratio) assumes a value λ> 1. The exhaust aftertreatment system comprises a NO _x storage catalytic converter for reducing the nitrogen oxides (NO _x ) contained in the exhaust gas of the internal combustion engine. The inventive method is used in particular for monitoring the functionality of the NO _x storage catalytic converter.

In normal operation of the internal combustion engine, which is the lean operation, the NO _x storage catalytic converter stores the nitrogen oxides contained in the exhaust gas stream. For regeneration, ie for renewal of the storage capacity of the NO _x storage catalytic converter, occasional regeneration phases are necessary in which the nitrogen oxides stored in the NO _x storage catalytic converter are reduced by means of a reducing agent supplied to the exhaust gas flow and released in the form of harmless gases. In particular, fuel can serve as a reducing agent, for which purpose the exhaust gas stream conducted through the NO _x storage catalyst is enriched with unburnt fuel in the regeneration phase, for example by fuel injection into the exhaust aftertreatment system upstream of the NO _x storage catalytic converter or by appropriate control of the internal combustion engine, in particular an injection system of the internal combustion engine , This means that there is a substoichiometric oxygen concentration in the regeneration phase, ie that the lambda value is less than 1, λ <1. Such regeneration is also referred to as "rich purge".

According to the invention, after the end of the regeneration phase, in particular immediately after the end of the regeneration phase, in the lean operation of the internal combustion engine, an oxygen content of the exhaust gas flow downstream of the NO _x storage catalytic converter is determined. For this purpose, the exhaust aftertreatment system comprises at least one first oxygen sensor, which is arranged downstream of the NO _x storage catalytic converter, ie in particular in the exhaust gas stream downstream of the NO _x storage catalytic converter, wherein the oxygen content of the exhaust gas downstream of the NO _x storage catalytic converter is determined from the signal of the at least one first oxygen sensor becomes. The at least one first oxygen sensor is designed as a narrow-band oxygen sensor, in particular, the at least one first oxygen sensor is a narrow-band lambda probe. Further, an oxygen content of the exhaust gas flow upstream of the NO _x storage catalyst is determined and, using this oxygen content, an oxygen uptake of the NO _x storage catalyst is determined until a time determined by the detected oxygen content downstream of the NO _x storage catalytic converter. In particular, this point in time is determined by the fact that the lambda value determined from the signal of the first oxygen sensor exceeds the value λ = 1, ie said time marks the transition from an oxygen deficiency to an oxygen excess in the exhaust gas stream downstream of the NO _x storage catalytic converter; This time is also known as oxygen breakthrough. The determined oxygen uptake is in particular the amount of oxygen or mass taken up by the NO _x storage catalytic converter.

Further carried out after the end of the regeneration phase, oxygen uptake of the NO _x storage catalyst according to the invention from the thus determined, closed on the functional capability of the NO _x storage catalytic converter. It is exploited that during the regeneration of the stored oxygen in the NO _x storage catalyst is consumed. After the end of the regeneration phase, ie when the exhaust gas stream which is fed to the NO _x storage catalytic converter again has an excess of oxygen (λ> 1), oxygen is available for storage in the NO _x storage catalytic converter. The oxygen uptake of the NO _x storage catalytic converter after the transition to operation with excess oxygen therefore permits a conclusion as to its functionality. In particular, the aging state of the NO _x storage catalytic converter can be concluded by comparison with reference values of the aging-dependent oxygen uptake of NO _x storage catalysts and, for example, an aging parameter can be determined which represents a measure of the aging or of the functionality of the NO _x storage catalytic converter.

The fact that after the end of the regeneration phase, an oxygen uptake of the NO _x storage catalytic converter is detected and closed from the detected oxygen uptake on the functioning of the NO _x storage catalytic converter, a possibility for monitoring the functionality of the NO _x storage catalyst is provided, which is independent of during the regeneration phase is performed measurements. It has been found that, as a result, a reliable determination of the aging state of the NO _x storage catalytic converter is possible even at relatively high temperatures, the known methods no longer providing a reliable result. Since the NO _x storage catalyst after the end of the regeneration phase and the transition to an operation in which the exhaust gas flow has an excess of oxygen, with the exhaust gas flow oxygen is supplied, which is available for uptake by the NO _x storage, can be detected by detecting the Oxygen content in the exhaust stream upstream of the NO _x storage catalyst, an oxygen uptake of the NO _x storage catalytic converter are detected. Characterized in that the oxygen uptake is detected up to a determined from the signal of the first oxygen sensor time, the oxygen uptake can be detected until a time in which it is substantially complete, in particular until the oxygen breakthrough. This makes it possible to draw conclusions about the functionality of the NO _x storage catalytic converter. To determine the said time, a narrow-band oxygen sensor is sufficient, which is arranged to detect the oxygen content of the exhaust gas downstream of the NO _x storage catalytic converter. As a result, a monitoring of the functionality of the NO _x storage catalytic converter is made possible in a particularly simple and cost-effective manner. Furthermore, the fact that the oxygen uptake is detected in lean operation, ie in particular during normal operation of the internal combustion engine, makes it possible to operate the internal combustion engine again in normal operation immediately after the end of the regeneration phase, so that a limitation of the operation of the internal combustion engine caused by the monitoring is avoided or at least can be minimized and, for example, a diagnostic phase can be avoided, during which the internal combustion engine is operated even after the end of the regeneration phase in a manner deviating from normal operation.

Advantageously, the exhaust aftertreatment system comprises at least one second oxygen sensor which is arranged upstream of the NO _x storage catalytic converter, ie in the exhaust gas flow upstream of the NO _x storage catalytic converter, and from whose signal the oxygen content of the exhaust gas upstream of the NO _x storage catalytic converter is determined. As a result, the accuracy of monitoring the operation of the NO _x storage catalytic converter can be improved. The second oxygen sensor may also be designed as a narrow-band oxygen sensor or narrow-band lambda probe, whereby a further simplified and more cost-effective design of the exhaust aftertreatment system is made possible. However, the second oxygen sensor can also be, for example, a broadband oxygen sensor or a UEGO (Universal Exhaust Gas Oxygen Sensor) sensor, which enables particularly accurate detection of the oxygen uptake of the NO _x storage catalytic converter.

Alternatively, it may be provided that the oxygen content of the exhaust gas present upstream of the NO _x storage catalytic converter is determined from an air mass flow and data from the engine control, in particular from the injection parameters becomes. As a result, a particularly cost-effective solution can be achieved.

Preferably, the oxygen storage, ie the amount of oxygen stored or mass and thus the oxygen absorption capacity of the NO _x storage catalytic converter is determined and from this infer the functionality of the NO _x storage catalytic converter. The oxygen storage or the oxygen uptake capacity can be determined, for example, by determining an oxygen uptake or a reduction of the oxygen content of the exhaust gas as it flows through the NO _x storage catalytic converter from the end of the regeneration phase to a point in time which substantially eliminates any further oxygen uptake takes place, in particular until the oxygen breakthrough. It can be assumed that the amount of oxygen that can be taken up by the storage catalytic converter has been absorbed by the storage catalytic converter so that the stored oxygen mass and thus the uptake capacity of the NO _x storage catalytic converter can be determined from this, which makes it possible to infer their reliability ,

In a particularly advantageous manner, the oxygen absorption capacity of the NO _x storage catalytic converter by integrating an upstream of the NO _x storage catalytic converter detected oxygen mass flow is determined from the end of the regeneration phase up to a time point at which a value determined from the signal of the first oxygen sensor λ-value of the value λ = 1 exceeds. The relevant mass flow can in particular be determined from the oxygen content in the exhaust gas flow upstream of the NO _x storage catalytic converter and an air mass flow, for example from the signal of the at least one second oxygen sensor and / or data of the engine control and the air mass flow, whose value can also be provided by the engine control , The beginning of the integration period can be determined with the aid of the second oxygen sensor, ie when a λ value of the second oxygen sensor exceeds the value λ = 1, or also approximately from data of the engine control. The end of the integration period is defined by the fact that the λ value of the first oxygen sensor changes from a value λ ≦ 1 to a value λ> 1. This transition, which can be reliably detected by the arranged downstream of the NO _x storage catalytic narrow-band oxygen sensor, in particular a narrow-band lambda probe, characterizes the breakthrough of oxygen through the NO _x storage catalyst and thus the time at which the intake of oxygen is essentially completed after the end of the regeneration phase. As a result, a determination of the oxygen absorption capacity of the NO _x storage catalytic converter and thus monitoring of its functionality is made possible in a particularly simple manner.

Alternatively, likewise in a particularly advantageous manner by 1 reduced λ value, the oxygen absorption capacity of the NO _x storage catalytic converter determined by integration of an upstream of the NO _x storage catalytic converter are determined from the end of the regeneration phase up to the time at which the out of the signal of the first Oxygen sensor determined λ value exceeds the value λ = 1. By the integral of the λ value upstream of the NO _x storage catalyst, based on a value λ = 1, the excess oxygen in the exhaust gas is detected, which enters the NO _x storage catalyst and is available for inclusion in these, and in particular Inclusion of the air mass flow the entire available and essentially absorbed by the NO _x storage oxygen mass. As with the method described above, the beginning of the integration period can be determined, for example, by means of the second oxygen sensor or data from the engine control, and the end of the integration period is also given by the λ value of the first oxygen sensor increasing from a value λ ≦ 1 a value λ> 1, that is, that oxygen not taken up by the NO _x storage catalyst passes through it and thus can be assumed that the absorption capacity of the NO _x storage catalytic converter is substantially reached. This also makes it possible in a particularly simple manner to determine the oxygen absorption capacity of the NO _x storage catalytic converter and thus to monitor its functionality.

According to a preferred embodiment of the invention, the internal combustion engine has a low-pressure exhaust gas recirculation, and the at least one first oxygen sensor arranged downstream of the NO _x storage catalytic converter, which is a narrow-band oxygen sensor, is arranged in an intake tract of the internal combustion engine. As a result, the possibility is created to use such existing in the intake system sensor to monitor the operation of the NO _x storage catalytic converter. This is particularly possible in an advantageous manner when a regeneration takes place in an operating state of the internal combustion engine, in which a high proportion of the exhaust gas or even the entire exhaust gas flow is recycled, such as in a braking or deceleration phase of the motor vehicle. As a result, monitoring of the exhaust aftertreatment system or of the NO _x storage catalytic converter is made possible in a particularly simple and cost-effective manner.

A control device according to the invention for an exhaust aftertreatment system of an internal combustion engine, which is designed for a lean operation, is set up for carrying out the above-described method for monitoring the exhaust aftertreatment system. The control device may comprise storage means for storing reference values of the oxygen uptake of a NO _x storage catalytic converter of the exhaust gas aftertreatment system as a function of an aging state of the NO _x storage catalytic converter and processor means for determining the oxygen uptake of the NO _x storage catalytic converter and, for example, an aging parameter of the storage catalytic converter. In particular, the control device is configured to control the at least one first and / or second oxygen sensor or the lambda probes serving as such sensors in order to process the respective sensor signals and to determine, for example, values for the oxygen content of the exhaust gas flow and / or the proportion of unburned fuel in the exhaust gas flow and further evaluate them in the manner described above. Such a determined result of the monitoring of the functionality of the NO _x storage catalytic converter or a determined aging parameter can be provided, for example, for a display for a driver of a motor vehicle equipped with the internal combustion engine and / or stored in an error memory. The controller may be further configured to control an injection of a reducing agent, in particular fuel, into the exhaust line upstream of the NO _x storage catalytic converter in order to cause the regeneration of the NO _x storage catalytic converter. Alternatively or additionally, the control device may be designed for communication with an engine control device of the internal combustion engine in order to effect an enrichment of the exhaust gas flow with fuel for the regeneration of the NO _x storage catalytic converter by driving the internal combustion engine or the injection system of the internal combustion engine. The control device may be part of an electronic engine control of the internal combustion engine.

The invention will be explained in more detail by way of example with reference to the drawings. Show it:

1 symbolically an internal combustion engine having an exhaust aftertreatment system comprising a NO _x storage catalyst;

2 exemplifies the time profile of the air ratio λ upstream or downstream of the NO _x storage catalyst before, during and after a regeneration phase, and

3 For example, the oxygen storage by the NO _x storage catalyst immediately after the end of the regeneration phase as a function of the prevailing temperature for different aging states of the storage catalyst.

As in 1 exemplified symbolically, the exhaust gases of an internal combustion engine 1 via an exhaust manifold 2 to an exhaust aftertreatment system 3 directed. The exhaust aftertreatment system 3 points in the exhaust system 4 comprising a plurality of pipe sections, a NO _x storage catalyst 5 through which the exhaust stream is passed. Downstream of the NO _x storage catalytic converter 5 is a first lambda probe 7 , which is a narrow-band lambda probe, and upstream of the NO _x storage catalytic converter 5 a second lambda probe 6 , which is a broadband lambda probe in the example described here. The lambda probes 6 . 7 each detect an oxygen content of the exhaust gas stream before entering and leaving the NO _x storage catalytic converter 5 , From the signals of the lambda probes 6 . 7 can be determined by means of a control device, not shown, in each case an air ratio λ. The exhaust aftertreatment system 3 may include other components, also not shown. In particular, further filters or catalysts may be present as well as further sensors, for example, instead of the first lambda probe 7 two first lambda probes 7 downstream of the NO _x storage catalytic converter 5 be provided. In general, however, is a first lambda probe 7 sufficient.

In 2 are from the signal upstream of the NO _x storage catalytic converter 5 the exhaust aftertreatment system 3 arranged second lambda probe 6 determined λ value (curve 8th ) and the signals from two downstream of the NO _x storage catalytic converter 5 arranged first lambda probes 7 determined λ values (curves 9 . 10 ) over time t, with time t given in units of 100 ms. In normal operation of the exhaust aftertreatment system 3 or the internal combustion engine 1 is in the exhaust stream, an excess of oxygen present, ie λ> 1. This area, the lean operation of the internal combustion engine 1 corresponds and already well before the in 2 Period shown is in 2 with the reference number 11 characterized. For the regeneration of the NO _x storage catalytic converter 5 the exhaust gas is enriched by, for example, an injection system of the internal combustion engine 1 is controlled so that the exhaust gas stream is enriched with unburned fuel, or by in the exhaust manifold 2 or in the exhaust system 4 upstream of the NO _x storage catalytic converter 5 Fuel is injected. This is shown by the fact that due to the signal upstream of the NO _x storage catalytic converter 5 arranged second lambda probe 6 determined λ value falls to values below 1. The regeneration phase, by Values λ <1 of the upstream second lambda probe 6 (Curve 8th ) is in 2 with the reference number 12 marked. Typically, the regeneration phase lasts 12 a few seconds, for example about 2 to 6 seconds. After the end of the regeneration phase 12 the λ value of the second lambda probe increases 6 (Curve 8th ) back to values above 1, ie the internal combustion engine 1 and the exhaust aftertreatment system 3 are again in lean operation. This area, which is well into the end of the in 2 lasts in the period shown is in 2 with the reference number 13 indicated.

The signals from the first lambda probes 7 determined λ values (curves 9 . 10 ) are in the initial lean operation 11 also above λ = 1, ie the exhaust gas flow is also after passing through the NO _x storage catalytic converter 5 an excess of oxygen. At the beginning of the regeneration phase 12 and during the majority of the duration of the regeneration phase 12 is the λ value of the first lambda probes 7 (curves 9 . 10 ) above the λ value of the second lambda probe 6 (Curve 8th ), ideally at about λ = 1, ie during the regeneration phase 12 into the NO _x storage catalytic converter 5 Existing exhaust gas flow existing unburned fuel is used to reduce the NO _x storage catalytic converter 5 stored nitrogen oxides and consumed by reaction with the oxygen also stored therein. However, they are near the end of the regeneration phase 12 in the NO _x storage catalytic converter 5 stored nitrogen oxides completely or substantially reduced, so fuel passes through it, and the signals downstream of the NO _x storage catalytic converter 5 arranged first lambda probes 7 also result in a fuel surplus (λ <1). The λ values of the first lambda probes 7 (curves 9 . 10 ) then approach that of the second lambda probe 6 (Curve 8th ) or even under the second lambda probe 6 lie.

After the end of the regeneration phase 12 increases the λ value, which results from the signal of the upstream of the NO _x storage catalytic converter 5 arranged second lambda probe 6 is determined (curve 8th ) quickly returns to levels above 1, indicating an excess of oxygen. At time t ₁ , which indicates the end of the regeneration phase, the upstream of the NO _x storage catalyst 5 determined λ value the value λ = 1. In contrast, remain the λ values of the first lambda probes 7 (curves 9 . 10 ) initially at λ ≤ 1. Only at a later time, typically a few seconds after the end of the regeneration phase 12 , also prevails in that by the NO _x storage catalytic converter 5 passing through exhaust gas flow again an excess of oxygen, as by the steep increase in the λ values of the two first lambda probes 7 (curves 9 . 10 ) is displayed at values λ> 1 in the further course. The point in time at which a λ value of the first lambda probes 7 exceeds the value λ = 1 is in 2 denoted by t ₂ . This time t ₂ can be reliably detected even with a narrow-band lambda probe. The delayed increase due to the signals of the first lambda probes 7 determined λ values is due to the fact that during the regeneration phase 12 in the NO _x storage catalytic converter 5 stored oxygen has been consumed and at the beginning of lean operation 13 if oxygen is available again in the exhaust gas flow, it is resumed. From the course of the λ value of the second lambda probe 6 In the period between the times t ₁ and t ₂ , therefore, the oxygen uptake and from it the oxygen storage or the oxygen uptake capacity of the NO _x storage catalytic converter 5 be determined.

For this purpose, the time integral L of the amount 1 decreased λ value, which is determined from the signal of the second lambda probe, from the time t ₁ , when the λ value of the second lambda probe 6 (Curve 8th ) exceeds the value λ = 1 until the time t ₂ when the mean value of the λ values of the first lambda probes 7 (curves 9 . 10 ) exceeds the value λ = 1, calculated:

From the Integral L a conclusion can be drawn on the functionality of the NO _x storage catalytic converter 5 or on its aging state. By calculating the integral over (λ-1), it becomes the NO _x storage catalyst 5 available excess oxygen over the period mentioned. The integral L usually enables a sufficiently accurate estimate of the oxygen uptake and, taking into account the known from the data of the engine control air mass flow, the absorbed oxygen mass M _O2 and thus a sufficiently accurate determination of the oxygen uptake capacity of the NO _x storage catalytic converter 5 that depends on its state of aging. In this case, the signal of the first lambda probes 7 only needed to determine the end of the integration period, namely the time t ₂ . This allows the use of a low-cost narrow-band lambda sensor as the first lambda probe 7 ,

In the in 2 Example shown are two sensors, namely two lambda probes 7 , downstream of the NO _x storage catalytic converter 5 arranged to a redundant or a more accurate determination of the end time t ₂ of Integration period; For this purpose, for example, an average of the signals of the two first lambda probes 7 or the λ values determined therefrom (curves 9 . 10 ) be used. In a correspondingly simpler form, the method can also be used with only a first lambda probe 7 be performed.

In 3 is the determined in the manner described oxygen storage, immediately after the end of the regeneration phase 12 and the transition to lean operation 13 takes place, ie the absorbed oxygen mass M _O2 , for NO _x storage catalysts shown with different aging states as a function of the temperature of the NO _x storage catalytic converter. The indicated temperature is the temperature in an inlet section of the NO _x storage catalytic converter, which can be detected by a temperature sensor arranged there or upstream thereof. The different aging states are as shown in the upper left corner of 3 have been produced by a prior heating of the NO _x storage catalyst for each of a period of ten hours to different high temperatures.

As in 3 1 and 2, the oxygen concentration M _O2 measured values represented by dots, which cover a wide range of operating conditions, are grouped in different ranges, with the measured values for a least aging NO _x storage catalyst (at 650 ° C) being in an upper range For medium aging conditions (aging at 750 ºC and 850 ºC respectively) in the middle ranges and the measurements at the highest aging (at 950 ºC) in a lower range. In particular, at temperatures above 250 ° C, a distinction is thus made between the different aging states based on the oxygen storage M _O2 after the end of the regeneration phase 12 readily possible. This allows monitoring of the functionality of the NO _x storage catalytic converter 5 take place, in particular in a temperature range above 250 ° C.

LIST OF REFERENCE NUMBERS

1

 internal combustion engine

2

 exhaust

3

 aftertreatment system

4

 exhaust gas line

5

NO _x storage catalyst

6

 lambda probe

7

 lambda probe

8th

 Curve

9

 Curve

10

 Curve

11

 lean operation

12

 regeneration phase

13

 lean operation

L

 integral

t ₂

 time

t ₂

 time

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

• EP 1936140 A1 [0004]
• DE 102012218728 A1 [0005]
• KR 1020110063140 A [0006]
• US 2004/0182068 A1 [0007]
• DE 10305452 A1 [0008]
• US 2007/0084195 A1 [0009]

Claims (9)

Method for monitoring an exhaust aftertreatment system ( 3 ) of a combustion engine designed for lean operation ( 1 ) containing a NO _x storage catalyst ( 5 ), wherein in a regeneration phase ( 12 ) in the NO _x storage catalytic converter ( 5 ) are reduced by supplying a reducing agent, characterized in that after the end of the regeneration phase ( 12 ) in lean operation of the internal combustion engine ( 1 ) an oxygen content of the exhaust gas stream downstream of the NO _x storage catalytic converter ( 5 ) is detected by means of at least one first oxygen sensor downstream of the NO _x storage catalytic converter ( 5 ) is arranged and which is designed as a narrow-band oxygen sensor, an oxygen content of the exhaust gas stream upstream of the NO _x storage catalytic converter ( 5 ), an oxygen quantity or mass determined as the amount of oxygen or mass taken up by the NO _x storage catalytic converter, is determined by the oxygen absorption of the NO _x storage catalytic converter ( 5 ) to one by the detected oxygen content downstream of the NO _x storage catalytic converter ( 5 ) determined time (t ₂ ) and from the determined oxygen uptake on the functionality of the NO _x storage catalytic converter ( 5 ) is closed. A method according to claim 1, characterized in that the oxygen content of the exhaust gas stream upstream of the NO _x storage catalytic converter ( 5 ) by means of at least one second oxygen sensor upstream of the NO _x storage catalytic converter ( 5 ) is detected is detected. A method according to claim 2, characterized in that the second oxygen sensor is designed as a narrow-band oxygen sensor. A method according to claim 1, characterized in that the oxygen content of the exhaust gas stream upstream of the NO _x storage catalytic converter ( 5 ) is determined from an air mass flow and data of the engine control. Method according to one of the preceding claims, characterized in that an oxygen absorption capacity of the NO _x storage catalytic converter ( 5 ) is determined. A method according to claim 5, characterized in that the oxygen uptake capacity of the NO _x storage catalytic converter ( 5 ) by integration of an upstream of the NO _x storage catalytic converter ( 5 ) determined oxygen mass flow from the end of the regeneration phase ( 12 ) is determined until a time (t ₂ ) at which a λ value determined from the signal of the first oxygen sensor exceeds the value λ = 1. Method according to one of the preceding claims, characterized in that the oxygen absorption capacity of the NO _x storage catalytic converter ( 5 ) by integration of an upstream of the NO _x storage catalytic converter ( 5 ) by 1 decreased λ-value from the end of the regeneration phase ( 12 ) is determined until a time (t ₂ ) at which a λ value determined from the signal of the first oxygen sensor exceeds the value λ = 1. Method according to one of the preceding claims, characterized in that the internal combustion engine ( 1 ) has a low-pressure exhaust gas recirculation and that the at least one first oxygen sensor in an intake tract of the internal combustion engine ( 1 ) is arranged. Control device for an exhaust aftertreatment system ( 3 ) one for a lean operation ( 11 . 13 ) designed internal combustion engine ( 1 ), the exhaust aftertreatment system ( 3 ) an NO _x storage catalyst ( 5 ), characterized in that the control device for carrying out the method for monitoring the exhaust aftertreatment system ( 3 ) is set up according to one of the preceding claims.

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