GB2542229A - Method for determining a state of aging of an NOx storage catalyst of an exhaust gas aftertreatment system of an internal combustion engine designed for - Google Patents

Abstract

In a method for determining a state of ageing of a NOx storage catalyst, eg LNT, (5, fig.1) of an exhaust gas aftertreatment system (3) of an internal com­bustion engine (1) designed for lean-mixture operation, eg a diesel engine or a direct injection spark-ignition engine, an ageing parameter of the NO storage catalyst is determined on the basis of a function measurement of the NO storage catalyst, wherein the state of ageing is determined repeatedly and a state of ageing of the NO storage catalyst is determined from a plurality of values of the ageing parameter determined within an averaging interval. The inven­tion also relates to a corresponding control device for an exhaust gas aftertreatment system (3) of an internal combustion engine (1) designed for lean-mixture operation. The ageing parameter may be determined on the basis of oxygen absorption of the NOx storage catalyst or air ratio downstream of the catalyst. The function measurement may be carried out in time relationship with a regeneration phase.

Description

Method of Determining a State of Ageing of a NOx Storage Catalyst of an Exhaust Gas Aftertreatment System of an Internal Combustion Engine Designed for Lean-Mixture Operation, and Control Device

The invention relates to a method for determining a state of ageing of a N0X storage catalyst of an exhaust gas aftertreatment system of an internal combustion engine designed for lean-mixture operation in accordance with the preamble of claim 1, and to a corresponding control device for an exhaust gas aftertreatment system of this kind.

During operation, internal combustion engines often produce considerable quantities of nitrogen oxides (NOx). Particularly in the case of diesel and spark ignition engines used in motor vehicles, the quantities of nitrogen oxide in the exhaust gas are generally above the permitted limits, and therefore there is a need for exhaust gas aftertreatment to reduce NOx emissions. In the case of many engines, the nitrogen oxides are reduced by the unoxidized components of the exhaust gas, namely by carbon monoxide (CO) and unburnt hydrocarbons (HC), with the aid of a three-way catalytic converter. However, in the case of lean-mixture diesel and spark ignition engines, in particular, this method is not available owing to the small quantities of unoxidized exhaust gas components. In the case of lean-mixture engines, therefore, use is made of a NOx storage catalyst (lea NOx trap, LNT) in an expanded method, said catalyst adsorbing and storing the nitrogen oxides contained in the exhaust gas from the internal combustion engine. From time to time, the NOx storage catalyst is regenerated, for which purpose an excess of fuel is produced in the exhaust gas passed through the NOx storage catalyst, for example.

However, the functioning capacity of the NOx storage catalyst decreases with increasing time in operation, this being attributable inter alia to contamination of the storage catalyst by sulfur contained in the exhaust gas and to thermal ageing due to high temperatures such as those encountered particularly during a desulfurization process, which has to be carried out at regular intervals. It is therefore necessary to monitor the functioning capacity of a NOx storage catalyst provided in the exhaust system.

One known practice is to detect the temperature of the NOx storage catalyst and to infer a state of ageing therefrom, in particular by integration of the temperature exposure during the life of the NOx storage catalyst up to that point. However, such temperature-based determination of the state of ageing is relatively inaccurate since the actual functioning or performance capacity of the NOx storage catalyst is not taken into account in the process.

European Patent Application EP 1 936 140 A1 discloses a method for monitoring an exhaust gas aftertreatment system of a combustion engine, wherein respective lambda probes are arranged upstream and downstream of the exhaust gas aftertreatment system and, to check the ability to function of the exhaust gas aftertreatment system, the combustion engine is switched to a mode in which the exhaust gases have a high concentration of unburnt hydrocarbons. Here, the exhaust gas aftertreatment system is assumed to be nonfunctional if the air ratios detected by the two lambda probes, said ratios being incorrect owing to the high HC concentration, are substantially equal.

According to US 6,922,985 B2, in the case of an engine of a motor vehicle, measured values are recorded periodically over a test block period by respective oxygen sensors upstream and downstream of an exhaust gas catalyst, and absolute differences between successive pairs of measured values detected upstream and downstream are calculated. The ratio of the sums of the absolute differences is used to infer whether ageing of the exhaust gas catalyst has occurred. According to US 6,116,021, an oxygen storage capacity of a catalyst is calculated using signals from an exhaust gas sensor arranged upstream and an exhaust gas sensor arranged downstream, and this capacity is compared with threshold values in order to determine the capacity of the catalyst. US 7,325,393 B2 discloses the practice of calculating a value that expresses the difference between the oxygen concentration on the upstream side and the downstream side of a catalyst during lean-mixture operation of an engine. If this value is lower than a deterioration identification threshold, deterioration of the catalyst is inferred.

According to DE 10 2012 218 728 A1, the ability to function of a NOx storage catalyst of a combustion engine is checked by switching the combustion engine to a substoichiometric mode (λ < 1) and detecting the air ratio by means of respective lambda probes arranged upstream and downstream of the storage catalyst. Here, the enrichment, i.e. the enrichment of the exhaust gas with unburnt hydrocarbons, is limited in such a way that the probes operate correctly. In the case of a fully functional storage catalyst, the unburnt hydrocarbons which are present in the exhaust gas owing to the enrichment upstream of the catalyst are fully oxidized as they flow through the catalyst, with the result that there are no unburnt hydrocarbons in the exhaust gas downstream of the catalyst. If the ability to function of the NOx storage capacity is limited, fewer unburnt hydrocarbons in the exhaust gas or no hydrocarbons are oxidized by the release of stored nitrogen oxides. From the time characteristic of the air ratio during the phase of enrichment, which is detected by the lambda probe arranged downstream of the storage catalyst, and, in particular, from the time characteristic of the mass flows of unburnt hydrocarbons in the exhaust gas, which are determined from the signals of the probes and are integrated over a short time interval, it is possible to draw conclusions about the ability to function of the storage catalyst.

German Patent Applications DE 10 2015 200 761.8, DE 10 2015 200 751.0, DE 10 2015 200 762.6 and DE 10 2015 200 752.9, which are not prior publications, disclose methods for monitoring an exhaust gas aftertreatment system of an internal combustion engine, wherein an air ratio or oxygen adsorption of the NOx storage catalyst is detected by means of at least one oxygen sensor during or after the end of a regeneration phase, and the functioning capacity of the NOx storage catalyst is inferred therefrom.

It is the object of the present invention to indicate an improved method for determining a state of ageing of a NOx storage catalyst of an exhaust gas aftertreatment system of an internal combustion engine designed for lean-mixture operation, and to indicate a corresponding control device for an exhaust gas aftertreatment system of this kind.

This object is achieved by a method and by a control device as indicated in the dependent claims. A method according to the invention for determining a state of ageing of a NOx storage catalyst of an exhaust gas aftertreatment system of an internal combustion engine designed for lean-mixture operation relates, in particular, to a diesel engine or to a lean-mixture spark-ignition engine, in particular to a diesel or spark-ignition engine with direct injection. This is preferably the internal combustion engine and the exhaust gas aftertreatment system of a motor vehicle. The expression "lean-mixture operation" means that the internal combustion engine is operated with excess air, i.e. that the lambda value (air ratio) assumes a value λ > 1. The exhaust gas aftertreatment system comprises a NOx storage catalyst for reducing the nitrogen oxides (NOx) contained in the exhaust gas of the internal combustion engine. The exhaust gas aftertreatment system can comprise a plurality of NOx storage catalysts.

According to the method of the invention, an ageing parameter of the NOx storage catalyst is determined on the basis of a function measurement of the NOx storage catalyst. At a point in time determined, for example, by operating parameters of the internal combustion engine, a measurement of one or more measured variables affected by the functioning of the NOx storage catalyst is performed here. A measured variable of this kind can be correlated with the NOx storage, NOx liberation, oxygen storage and/or oxygen adsorption of the NOx storage catalyst during operation or in a regeneration phase, for example. From the at least one measured, function-dependent measured variable, a current value of the ageing parameter is determined. If the function measurement or determination of the ageing parameter depends on further conditions, e.g. on the temperature of the NOx storage catalyst, determination of the ageing parameter can be restricted to a permissible range, i.e. to the conditions in which sufficiently accurate measurement of the measured variable and, on that basis, reliable determination of the ageing parameter is possible.

According to the invention, the ageing parameter is determined repeatedly and a state of ageing of the NOx storage catalyst is determined from a plurality of values of the ageing parameter determined within an averaging interval, in particular by averaging. In this case, an average of the values determined for the ageing parameter within the averaging interval can be calculated and the state of ageing determined from the average, for example, or a state of ageing can be determined from each value for the ageing parameter determined in the averaging interval and the state of ageing of the NOx storage catalyst calculated by averaging from a plurality of states of ageing determined in this way and based on individual measurements. During averaging, it is possible, in particular, for an arithmetic mean to be formed, or outliers or values determined outside permissible conditions, e.g. outside a permissible temperature range, can first be eliminated and the remaining values used as a basis for averaging, for example.

By virtue of the fact that the ageing parameter is determined on the basis of a function measurement of the NOx storage catalyst, the actual functioning or performance capacity of the NOx storage catalyst can be taken into account. By virtue of the fact that the function measurement is carried out repeatedly within the averaging interval, that an ageing parameter is in each case determined from the respective measured value of the at least one function-dependent measured variable, and that the state of ageing is calculated from a plurality of parameter values determined, a statistically broader base for determining the state of ageing is created. By using a plurality of measurements carried out within the averaging interval, in particular a multiplicity of such measurements, it is thus possible to determine the state of ageing with increased accuracy. Here, it is possible to exploit the fact that the state of ageing of the NOx storage catalyst generally changes only very slowly in the course of operation of the internal combustion engine.

According to the invention, it is, on the one hand, possible to judge with particularly high reliability whether the performance capacity of the NOx storage catalyst still meets the requirements set. The more accurate determination, according to the invention, of the state of ageing of the NOx storage catalyst furthermore makes it possible, on the other hand, to control the exhaust gas aftertreatment system in a manner that is optimum for the respective state of ageing of the NOx storage catalyst, for example, and to minimize the additional consumption of fuel during a regeneration of the NOx storage catalyst or to minimize ammonia consumption in an additional active SCR (Selective Catalytic Reduction) system, for example. Moreover, the state of ageing determined can be used as a basis for a more accurate prediction of a residual life of the NOx storage catalyst, for example.

The function measurement, i.e. the detection of the at least one measured variable correlated with the functioning of the NOx storage catalyst, is preferably carried out in each case in time relationship with a regeneration phase, in particular during and/or following a regeneration phase, e.g. immediately following a regeneration phase. During normal operation of the internal combustion engine, which is lean-mixture operation, the NOx storage catalyst stores the nitrogen oxides contained in the exhaust gas flow. For regeneration, i.e. to renew the storage capacity of the NOx storage catalyst, occasional regeneration phases can be carried out, in which the nitrogen oxides stored in the NOx storage catalyst are reduced with the aid of a reducing agent fed into the exhaust gas flow and are released in the form of harmless gases. Fuel can be used, in particular, as a reducing agent, for which purpose the exhaust gas flow passed through the NOx storage catalyst is enriched with unburnt fuel in the regeneration phase, e.g. by fuel injection into the exhaust gas aftertreatment system upstream of the NOx storage catalyst or by appropriate control of the internal combustion engine, in particular of an injection system of the internal combustion engine. This means that there is a substoichiometric oxygen concentration in the regeneration phase, i.e. that the lambda value is less than 1, λ < 1. Regeneration of this kind is also referred to as "rich purge". By virtue of the fact that the measurement of the at least one function-dependent measurement variable on the basis of which a value of the ageing parameter is determined is carried out in time relationship with a regeneration phase, determination of a current value of the ageing parameter during the operation of the internal combustion engine is made possible in a simple manner.

In particular, the ageing parameter can be determined on the basis of detection of the oxygen adsorption of the NOx storage catalyst, i.e. the function measurement is, in particular, detection of the oxygen adsorption of the NOx storage catalyst or serves for detection of the oxygen adsorption. In this case, it is possible, after the end of a regeneration phase for example, to detect an oxygen content of the exhaust gas flow downstream of the NOx storage catalyst by means of a first oxygen sensor, which is arranged downstream of the NOx storage catalyst, to detect an oxygen content of the exhaust gas flow upstream of the NOx storage catalyst by means of a second oxygen sensor, which is arranged upstream of the NOx storage catalyst, and to determine the oxygen adsorption of the NOx storage catalyst by comparing the oxygen content of the exhaust gas flow upstream and downstream of the NOx storage catalyst. In this process, use can be made of the fact that the oxygen adsorption by the NOx storage catalyst which takes place after the transition to operation with an excess of oxygen allows a conclusion to be drawn about the functioning capacity of said catalyst and enables an ageing parameter to be determined, e.g. by comparison with reference values for the aging-dependent oxygen adsorption of NOx storage catalysts, said ageing parameter being a measure of the ageing or functioning capacity of the NOx storage catalyst. At least one of the oxygen sensors can be designed as a narrow-band oxygen sensor. Such methods for determining an ageing parameter are described in German Patent Applications DE 10 2015 200 761.8 and DE 10 2015 200 752.9, which in this respect are incorporated by reference into the present application. In this way, simple and informative determination of a current value of the ageing parameter is made possible, it being possible, in particular, to use the sensors available in the exhaust gas aftertreatment system for monitoring the functioning capacity of the NOx storage catalyst.

As an alternative or in addition, the ageing parameter can be determined on the basis of an air ratio detected downstream of the NOx storage catalyst, i.e. the function measurement is or includes, in particular, detection of the air ratio downstream of the NOx storage catalyst. For example, a first air ratio can be detected downstream of the NOx storage catalyst by means of an oxygen sensor in a regeneration phase, a breakthrough time, at which the first air ratio undershoots a predeterminable threshold value, can be determined, and the functioning capacity of the NOx storage catalyst can be inferred from at least one characteristic variable dependent on the breakthrough time, and a value of the ageing parameter can be determined. The at least one characteristic variable can be the time interval between a starting time of the regeneration phase and the breakthrough time, for example, or it can be the quantity of reducing agent supplied from the starting time of the regeneration phase up to the breakthrough time. The oxygen sensor can be designed as a narrow-band oxygen sensor. Methods of this kind for determining an ageing parameter are described in Patent Applications DE 10 2015 200 751.0 and 10 2015 200 762.6, which in this respect are incorporated by reference into the present application. It is furthermore possible to determine an ageing parameter of the NOx storage catalyst in accordance with the method described in Laid-Open Application DE 10 2012 218 728 A1, which in this respect is likewise incorporated by reference into the present application. In this way too, simple and informative determination of a current value of the ageing parameter is made possible, likewise, in particular, using the sensors available for monitoring the functioning capacity of the NOx storage catalyst.

In a particularly advantageous manner, it is possible to envisage that the ageing parameter can be determined both on the basis of detection of the oxygen adsorption of the NOx storage catalyst and on the basis of an air ratio detected downstream of the NOx storage catalyst. Whether the ageing parameter determined from the relevant measurement is determined on the basis of detection of the oxygen adsorption or on the basis of the air ratio detected downstream can depend here on the temperature of the NOx storage catalyst prevailing when a respective measurement is made, for example. Particularly accurate and reliable determination of the state of ageing of the NOx storage catalyst is thereby made possible.

According to a preferred embodiment of the invention, the ageing parameter is a normalized ageing parameter. In particular, the ageing parameter is normalized with respect to an expected life of the NOx storage catalyst and, for example, indicates the current life reached in relation to the expected life or the residual life in relation to the expected life. The expected life of the NOx storage catalyst can be a nominal value specified for a specific type, indicating the life up to incapacity to function under typical operating conditions, i.e. up to the point where the required exhaust gas limits are no longer achieved. By virtue of the fact that the ageing parameter is a normalized ageing parameter, calculation of the state of ageing is made possible in a particularly simple manner by averaging the values of the ageing parameter determined in the averaging interval.

The averaging interval preferably extends in each case up to a current determination of the ageing parameter, i.e. it is a matter of sliding average formation with an averaging interval which is always at least approximately equal and is based on the last measurements carried out in each case. Simple and reliable calculation of the state of ageing is thereby made possible.

In particular, the internal combustion engine is an internal combustion engine of a motor vehicle, and the averaging interval is determined by a number of driving cycles and/or a driving distance traveled. Since regeneration generally takes place after a fixed number of driving cycles or after a predeterminable driving distance of the motor vehicle, particularly simple and informative determination of the ageing parameter and hence simple and reliable determination of the state of ageing can thereby be enabled.

It is advantageous if the averaging interval is short relative to the expected life of the NOx storage catalyst. In particular, the averaging interval can be about 10%, preferably about 6%, particularly preferably about 4%, or preferably even just 1% of the number of driving cycles or of the driving distance which corresponds to the expected life of the NOx storage catalyst. Since, therefore, on the one hand, only a correspondingly small change in the ageing parameter is to be expected and, on the other hand, a sufficient number of function measurements and determinations, based thereon, of the respectively current value of the ageing parameter are carried out within the averaging interval, particularly accurate calculation of the state of ageing is made possible in this way.

According to a particularly preferred embodiment of the invention, the state of ageing of the NOx storage catalyst, determined as described above, is used to correct a determination of the state of ageing based on detection of the temperature of the NOx storage catalyst, e.g. on the basis of time integration of the temperature exposure of the NOx storage catalyst, or on the basis of a prediction, based thereon, of the residual life. In this way, the detection, known per se, of the temperature of the NOx storage catalyst and the determination of the state of ageing based thereon can be supplemented by a determination of the state of ageing which takes account of the actual functioning or performance capacity of the NOx storage catalyst. Increased accuracy of the determination of the state of ageing and of the prediction of the residual service life can thereby be achieved. A control device according to the invention for an exhaust gas aftertreatment system of an internal combustion engine designed for lean-mixture operation is designed to carry out the above-described method for monitoring the exhaust gas aftertreatment system. The control device can have storage means for storing reference values of the measured variables and of the measured values detected and the values of the ageing parameter determined therefrom. The control device can furthermore comprise processor means for determining the ageing parameter, for eliminating unusual values, for carrying out averaging and for calculating the state of aging. The control device can furthermore be designed for controlling one or more sensors for detecting the function-dependent measured variable and/or the temperature of the NOx storage catalyst. A state of ageing of the NOx storage catalyst determined in this way can be provided for a display for a driver of a motor vehicle fitted with the internal combustion engine, for example, and/or can be stored in a fault memory. The control device can furthermore be designed to control the exhaust gas aftertreatment system in accordance with the state of ageing determined in order, for example, to bring about injection of reducing agent, in particular fuel, into the exhaust line upstream of the NOx storage catalyst in a manner that is optimum for the state of ageing determined, or can be designed for appropriate communication with an engine control device of the internal combustion engine in order to bring about optimum enrichment of the exhaust gas flow with fuel for regeneration of the NOx storage catalyst, e.g. by control of the injection system of the internal combustion engine. The control device can be part of an electronic engine controller of the internal combustion engine.

The invention is explained in greater detail by way of example below by means of the drawings, in which:

Figure 1 shows, in symbolic form, an internal combustion engine having an exhaust gas aftertreatment system which comprises a NOx storage catalyst;

Figure 2 shows, by way of example, the reducing agent slip ratio during the regeneration phase in accordance with the prevailing temperature of the NOx storage catalyst for various states of ageing of the storage catalyst;

Figure 3 shows, by way of example, the oxygen storage by the NOx storage catalyst immediately after the end of the regeneration phase, likewise in ac- cordance with the prevailing temperature for various states of ageing of the storage catalyst; and

Figure 4 shows a simplified sequence diagram for one illustrative embodiment of a method according to the invention.

As shown symbolically by way of example in figure 1, the exhaust gases of an internal combustion engine 1 of a motor vehicle are passed via an exhaust manifold 2 to an exhaust gas aftertreatment system 3. In the exhaust line 4, which comprises a plurality of pipe sections, the exhaust gas aftertreatment system 3 has a NOx storage catalyst 5, through which the exhaust gas flow is passed. A first lambda probe 7 is arranged downstream of the NOx storage catalyst 5, and a second lambda probe 6 is arranged upstream of the NOx storage catalyst 5. The lambda probes 6 and 7 detect an oxygen content of the exhaust gas flow before entry to and after exit from the NOx storage catalyst 5, respectively. From the signals of the lambda probes 6, 7, a respective air ratio λ can be determined with the aid of a control device (not shown). The exhaust gas aftertreatment system 3 can comprise further components (likewise not shown). In particular, further filters or catalysts can be present, as can further sensors, e.g. two first lambda probes 7 can be provided downstream of the NOx storage catalyst 5 instead of the first lambda probe 7. In general, however, one first lambda probe 7 is sufficient.

During normal operation of the exhaust gas aftertreatment system 3 and of the internal combustion engine 1, which corresponds to lean-mixture operation of the internal combustion engine 1, an excess of oxygen is present in the exhaust gas flow, i.e. λ > 1. To regenerate the NOx storage catalyst 5, the exhaust gas is enriched by controlling an injection system of the internal combustion engine 1 in such a way, for example, that the exhaust gas flow is enriched with unburnt fuel or by injecting fuel into the exhaust manifold 2 or into the exhaust line 4 upstream of the NOx storage catalyst 5. The λ value determined on the basis of the signal of the second lambda probe 6 arranged upstream of the NOx storage catalyst 5 therefore falls to values λ < 1. Such a regeneration phase typically lasts for a few seconds, e.g. about 2 to 6 seconds. After the end of the regeneration phase, the λ value measured by the second lambda probe 6 rises again to values above 1, i.e. the internal combustion engine 1 and the exhaust gas aftertreatment system 3 are once again in the lean-mixture mode.

At the beginning of the regeneration phase and during the predominant part of the duration of the regeneration phase, the λ value of the first lambda probe 7 is above the λ value of the second lambda probe 6, ideally being about λ = 1, i.e. the unburnt fuel present in the exhaust gas flow entering the NOx storage catalyst 5 during the regeneration phase is consumed in reducing the nitrogen oxides stored in the NOx storage catalyst 5 and by reacting with the oxygen likewise stored therein. If, however, the nitrogen oxides stored in the NOx storage catalyst 5 have been completely or largely reduced toward the end of the regeneration phase, fuel passes through the catalyst, and the signals of the first lambda probe 7 arranged downstream of the NOx storage catalyst 5 likewise show an excess of fuel (λ < 1). From the ratio of the proportion of reducing agent in the exhaust gas flow after exit from the NOx storage catalyst 5 to the proportion before entry to the NOx storage catalyst 5, which is referred to as the reducing agent slip ratio (SR), it is possible to infer the state of ageing of the NOx storage catalyst 5. From the time within a regeneration phase at which the reducing agent passes through the NOx storage catalyst 5, which is also referred to as the "breakthrough time", and/or from the quantity of reducing agent supplied up to the breakthrough time, it is likewise possible to infer the functioning capacity of the NOx storage catalyst 5 and to determine a corresponding ageing parameter.

After the end of the regeneration phase, the λ value determined from the signal of the second lambda probe 6 arranged upstream of the NOx storage catalyst 5 rises rapidly again to values above 1 and thus indicates an excess of oxygen. In contrast, the λ value of the first lambda probe 7 initially remains at λ < 1. Only at a later time, typically a few seconds after the end of the regeneration phase, does an excess of oxygen also prevail again in the exhaust gas flow passing through the NOx storage catalyst 5, this being indicated by a rise in the λ values of the first lamda probe 7 to values λ > 1. The delayed rise in the λ values determined on the basis of the signals of the first lambda probe 7 is due to the fact that the oxygen stored in the NOx storage catalyst 5 has been consumed during the regeneration phase, and that oxygen is adsorbed again at the beginning of lean-mixture operation, when there is oxygen available again in the exhaust gas flow. From the delay in the rise of the λ values of the first lambda probe 7 and/or from the λ value of the first lambda probe 7, which is initially still low at the beginning of lean-mixture operation, as compared with the λ value of the second lambda probe 6, it is possible to determine the oxygen adsorption and, from the latter, the oxygen storage or oxygen adsorption capacity of the NOx storage catalyst 5. From the oxygen adsorption M02 determined in this way, it is likewise possible to draw a conclusion about the functioning capacity of the NOx storage catalyst 5 and to determine a corresponding ageing parameter.

The abovementioned methods of determining the functioning capacity and/or determining an ageing parameter of a NOx storage catalyst are described in Patent Applications DE 10 2015 200 761.8 and 10 2015 200 752.9, which are incorporated by reference in this respect.

Figure 2 shows the reducing agent slip ratio (SR) during the regeneration phase in accordance with the prevailing temperature for NOx storage catalysts with different states of aging. Here, as presented at the top right in figure 3, the different states of ageing have been produced by prior heating of the NOx storage catalyst to 650°C, 750°C, 850°C or 950°C for a period of ten hours in each case. The respective measured values, represented by points, for the reducing agent slip ratio SR are grouped around curves 8, 9, 10, 11, wherein the bottom curve 8 corresponds to a NOx storage catalyst with the least ageing (at 650°C), the central curves 9, 10 correspond to moderate states of ageing (aging at 750°C and 850°C) and the top curve 11 relates to measured values for the greatest ageing (at 950°C). As can be seen from figure 2, it is possible, in principle, to distinguish between the different states of ageing by means of the reducing agent slip ratio SR determined during the regeneration phase, especially in the temperature range below about 300°C. Above 300-350°C, in particular above 350°C, curves 8, 9, 10, 11 converge, i.e. the reducing agent slip ratios SR approach one another irrespective of the state of aging. The reducing agent slip ratio SR therefore allows determination of an ageing parameter of the NOx storage catalyst, especially in a temperature range below about 300-350°C. Here, the indicated temperature of the NOx storage catalyst is the temperature in an entry section of the NOx storage catalyst, which can be detected by a temperature sensor arranged there or upstream thereof.

As can likewise be seen in figure 2, there is relatively wide scatter in the individual measured values for the reducing agent slip ratio SR around the respective curves 8, 9, 10, 11, often making it impossible to assign an individual measured value reliably to a particular curve 8, 9, 10 or 11. Determining the state of ageing of the NOx storage catalyst from a single measured value for the reducing agent slip ratio is therefore subject to considerable uncertainty or even impossible with one of the methods described above. On the other hand, it is possible to achieve a considerable increase in accuracy of determination of the state of ageing by averaging over a multiplicity of measured values. Here, a further increase in accuracy can be achieved if the measured values used for averaging are restricted to a narrow temperature range and/or if the respective temperature during the determination of an ageing parameter is taken into account and the state of ageing of the NOx storage catalyst is calculated from the relevant ageing parameters by averaging.

Figure 3 shows the oxygen storage M02, i.e. the quantity of oxygen adsorbed, which takes place immediately after the end of the regeneration phase and the transition to lean-mixture operation, likewise for NOx storage catalysts with different states of ageing in accordance with the prevailing temperature of the NOx storage catalyst. In this case, as presented at the top right in figure 3, the different states of ageing have been produced by previous heating of the NOx storage catalyst to 650°C, 850°C or 950°C in each case for a time period of ten hours. The respective measured values for the oxygen storage M02, which are represented by points, are grouped in accordance with the respective state of aging, wherein the measured values for a NOx storage catalyst with the least ageing (at 650°C) are in the upper area and the measured values in the case of the greatest ageing (at 950°C) are in a lower area. In principle, therefore, the different states of ageing can be distinguished by the amount of oxygen adsorbed after the end of the regeneration phase, particularly in the temperature range above about 300°C. However, there is scatter in the individual measured values for the oxygen storage M02, even for a given state of aging, and therefore an increase in accuracy when determining the state of ageing can be achieved by averaging over a multiplicity of measured values. A further increase in accuracy can be achieved here if the measured values used for averaging are restricted to a temperature range above about 300°C and/or the respective temperature is taken into account when determining an ageing parameter and the state of ageing of the NOx storage catalyst is calculated from the relevant ageing parameters by averaging. Particularly accurate determination of the state of ageing is possible if the ageing parameter is determined on the basis of the reducing agent slip ratio SR at a temperature below about 300-350°C (see figure 2) and on the basis of the quantity of oxygen adsorbed at a temperature above about 300°C (see figure 3).

In figure 4, the sequence of a method for determining a state of ageing of the NOx storage catalyst 5 according to one illustrative embodiment of the invention is shown in simplified form. As described above, one or more measured variables dependent on the functioning of the NOx storage catalyst 5 are detected in time relationship with a regeneration phase, in particular during the regeneration phase or immediately after the end of the regeneration phase. Thus, for example, the reducing agent slip ratio SR, a breakthrough time, a reducing agent quantity supplied in the regeneration phase up to the breakthrough time and/or oxygen storage M02 are detected from the measured λ values downstream and upstream of the NOx storage catalyst 5. At the same time, the temperature of the NOx storage catalyst 5 is measured by means of a temperature sensor. Taking into account the measured temperature, an ageing parameter, which is a function of a state of ageing defined as above by a heating temperature, for example, or is identical with the temperature of a ten-hour heating period, which produces corresponding aging, is determined from the at least one measured value of said function-dependent measured variable. The value determined can be a value normalized with respect to the expected life of the NOx storage catalyst 5, for example. The value determined for the ageing parameter is stored in a memory of the control device of the exhaust gas aftertreatment system 3. In detecting the measured value or determining the ageing parameter, a selection can be made according to the temperature, detected during the respective measurement, of the NOx storage catalyst 5, by which the stored values of the normalized ageing parameter are restricted to those at which a particularly high accuracy can be achieved on the basis of the prevailing temperature.

Detection of the at least one function-dependent measured variable and the determination of the, optionally normalized, ageing parameter based thereon and on the measured temperature is repeated several times, and the values determined for the ageing parameter are stored in the memory of the control device. Thus, the function-dependent measured variable can be measured in each regeneration phase and the, optionally normalized, ageing parameter can be determined therefrom, for example. Based on a given number of stored ageing parameters, an average is then formed, e.g. from the normalized ageing parameters of the 100 last measurements in each case. The memory can be designed in such a way, for example, that the 100 last measurements in each case or the ageing parameters determined therefrom are stored and the values based on measurements further in the past are no longer retained in memory. If, for example, the ageing parameter is determined in each third driving cycle and one driving cycle corresponds to a driving distance of on average 20 km, the averaging interval of 100 measurements corresponds to a driving distance of about 6000 km. This is so short relative to the expected life of the NOx storage catalyst 5, which can be at least 160,000 km, for example, that the state of ageing does not change significantly within the averaging interval. A restriction to a predetermined temperature range of the NOx storage catalyst 5 can also be imposed in the averaging.

From the average calculated in this way, a state of ageing of the NOx storage catalyst 5 is then determined, e.g. as a ratio of the expired life to a predetermined life of the NOx storage catalyst 5, wherein the life can in each case be indicated as a driving distance or as a number of driving cycles. If the ageing parameter is already normalized with respect to the expected life of the NOx storage catalyst (5), the state of ageing determined by averaging is likewise already normalized with respect to the expected life. The state of ageing determined is then compared with a state of ageing determined from temperature detection of the NOx storage catalyst 5, and a corrected state of ageing is determined, taking into account both values. This has a higher accuracy than a state of ageing determined by just one of the methods mentioned and, for example, can be used to predict the anticipated residual life of the NOx storage catalyst 5 for a corresponding indication to an operator or driver of the motor vehicle or, alternatively, for control of the exhaust gas aftertreatment system 3 in a manner corresponding to the expired life and a corresponding performance capacity of the NOx storage catalyst 5.

List of reference signs 1 internal combustion engine 2 exhaust manifold 3 exhaust gas aftertreatment system 4 exhaust line 5 NOx storage catalyst 6 lambda probe 7 lambda probe 8 curve 9 curve 10 curve 11 curve

Claims (10)

Claims

1. A method of determining a state of ageing of a NOx storage catalyst (5) of an exhaust gas aftertreatment system (3) of an internal combustion engine (1) designed for lean-mixture operation, wherein an ageing parameter of the NOx storage catalyst (5) is determined on the basis of a function measurement of the NOx storage catalyst (5), wherein the state of ageing is determined repeatedly and a state of ageing of the NOx storage catalyst (5) is determined from a plurality of values of the ageing parameter determined within an averaging interval.

2. The method as claimed in claim 1, wherein nitrogen oxides stored in the NOx storage catalyst (5) in a regeneration phase are reduced by supplying a reducing agent, and the function measurement is carried out in time relationship with the regeneration phase.

3. The method as claimed in claim 2, wherein the ageing parameter is determined on the basis of detection of the oxygen adsorption of the NOx storage catalyst (5).

4. The method as claimed in claims 2 or 3, wherein the ageing parameter is determined on the basis of an air ratio detected downstream of the NOx storage catalyst (5).

5. The method as claimed in any of the preceding claims, wherein the ageing parameter is a normalized ageing parameter.

6. The method as claimed in any of the preceding claims, wherein the averaging interval extends in a sliding manner as far as a respectively current determination of the ageing parameter.

7. The method as claimed in claim 6, wherein the internal combustion engine (1) is an internal combustion engine of a motor vehicle, and the averaging interval is determined by a number of driving cycles and/or a driving distance traveled.

8. The method as claimed in claim 7, wherein the averaging interval is short in comparison with the expected life of the NOx storage catalyst (5), in particular is about 10%, preferably about 6%, particularly preferably about 4%, or preferably about 1% of the number of driving cycles or of the driving distance which corresponds to the expected life of the NOx storage catalyst (5).

9. The method as claimed in any of the preceding claims, wherein the state of ageing determined is used to correct a state of ageing determined on the basis of detection of the temperature of the NOx storage catalyst (5).

10. A control device for an exhaust gas aftertreatment system (3) of an internal combustion engine (1) designed for lean-mixture operation, wherein the exhaust gas aftertreatment system (3) comprises a NOx storage catalyst (5), wherein the control device is designed to carry out the method for determining the state of ageing of the NOx storage catalyst (5) as claimed in one of the preceding claims.