EP1247963B1 - Method for controlling of an operating mode of a lean burn internal combustion engine - Google Patents

Description

The invention relates to a method for controlling an operating mode of a lean-running internal combustion engine having the features of the preamble of claim 1.

To reduce fuel consumption today's internal combustion engines are operated over the widest possible operating ranges in a lean operating mode, that is, with an air-fuel mixture with excess air (λ> 1). In the case of internal combustion engines having a direct fuel injection, by implementing a so-called stratified charge mode in which the injected fuel concentrates substantially in the region of a spark plug of a cylinder, particularly lean air-fuel mixtures and thus particularly high fuel consumption advantages can be represented. A certain problem of the lean operating mode is known to be nitrogen oxides NO _{x of} the exhaust gas, which can not be completely converted to environmentally neutral nitrogen N ₂ due to the oxygen excess in the lean exhaust gas on conventional oxidation or 3-way catalysts. To overcome this problem, it is known to use NO _x storage catalysts which, in addition to a 3-way catalytic component, have an NO _x storage component for absorbing NO _x in the lean exhaust gas. For the purpose of a NO _x regeneration NO _x storage catalysts are alternately applied in discontinuous operation with lean and rich exhaust gas, wherein in the rich intervals the stored in lean mode NO _{x is} released and reduced. However, these required due to a finite NO _x storage capacity of a storage catalyst rich operating intervals lead to a partial compensation of the consumption advantage achieved in lean operation.

A NO _x storage and conversion capability of the storage catalyst depends on various operating parameters and also changes with aging of the catalyst, as disclosed in WO 01 06105 A. In particular, sufficient storage capacity is available only in a certain temperature range of the NO _x storage catalytic converter. Therefore, for example, from DE 199 32 290 A1 and DE 199 29 292 A1 discloses an approval of a lean operation of the internal combustion engine of the catalyst temperature of the NO _x storage catalytic converter to set a lower and upper temperature threshold for the allowable lean operating range to prevent NO _x breakthroughs. DE 198 50 786 A1 further discloses a method according to which this upper and lower temperature threshold is adapted to the NO _x storage capacity of the catalyst, which is monitored on the basis of a duration of a previous lean interval or NO _x regeneration interval.

However, it has been found that the catalyst temperature alone is not a sufficient criterion for the NO _x storage and conversion capability of the catalyst and thus for the approval of the lean operation mode. Rather, the NO _x storage behavior of the catalyst depends on other factors. For example, an aged storage catalyst at an average catalyst temperature of 350 ° C at operating points with low NO _x -Rohemissionen still a sufficient NO _x storage and having conversion capacity, whereas at high NO _x -Rohemissionen the storage capacity so far decrease that This causes high regeneration frequency completely consumes the consumption advantage gained by the lean operation. In such a case, the adaptation of the temperature thresholds does not show the desired success.

The object of the present invention is to provide a method for controlling an operating mode of a lean-running internal combustion engine, which overcomes the described deficiencies of the prior art and allows more accurate tuning of a lean operating mode of the internal combustion engine to a current operating point and to a state of the NO _x storage catalytic converter.

• ein Magerbetriebsmodus der Verbrennungskraftmaschine mit λ > 1 in Abhängigkeit von vorgebbaren Schwellenwerten für eine Temperatur des NO_x-Speicherkatalysators und für mindestens einen weiteren Betriebsparameter der Verbrennungskraftmaschine und/oder des Abgastraktes zugelassen wird und
• mindestens ein Schwellenwert in Abhängigkeit einer aktuellen NO_x-Speicher-und/oder NO_x-Konvertierungsfähigkeit des NO_x-Speicherkatalysators variiert wird, erfolgt eine Zulassung des Magerbetriebsmodus unter sehr genauer Berücksichtigung des aktuellen Betriebspunktes sowie eine ständige Aktualisierung der Zulassungsbedingungen an eine tatsächliche NO_x-Speicher- und/oder -Konvertierungsfähigkeit des NO_x-Speicherkatalysators. Im Ergebnis kann somit eine NO_x-Endemission deutlich reduziert werden und gleichzeitig der Magerbetriebsmodus über weite Betriebsbereiche eines durch die Verbrennungskraftmaschine angetriebenen Fahrzeuges eingesetzt werden, wodurch optimale Kraftstoffverbrauchswerte erzielt werden können.

This object is achieved by a method having the features of claim 1. As a result of that

• a lean operating mode of the internal combustion engine with λ> 1 is permitted as a function of predefinable threshold values for a temperature of the NO _x storage catalytic converter and for at least one further operating parameter of the internal combustion engine and / or the exhaust gas tract, and
• at least one threshold value is varied as a function of a current NO _x storage and / or NO _x conversion capability of the NO _x storage catalytic converter, an approval of the lean operation mode takes place under very close consideration of the current operating point as well as a constant updating of the admission conditions to an actual NO _x storage and / or conversion capability of the NO _x storage catalytic converter. As a result, thus, a NO _x emissions can be significantly reduced and at the same time the lean operation mode can be used over wide operating ranges of a vehicle driven by the internal combustion engine, whereby optimum fuel consumption values can be achieved.

It is particularly preferably provided that the lean _operating mode is made dependent on a raw NO _x emission of the internal combustion engine, since this decisively influences a NO _x storage rate of the storage catalytic converter. Furthermore, raw emissions of further exhaust gas components, in particular of unburned hydrocarbons HC and / or carbon dioxide CO ₂ and / or carbon monoxide CO, can be taken into account. These hinder the storage of NO _x in the catalyst in high concentrations. Accordingly, for the raw emission of each of these components or a sum of the components, an upper threshold value can be set, above which the lean operating mode is blocked. In this context, raw emission means a concentration and / or a mass flow of the respective exhaust gas component upstream of a first catalytic converter connected downstream of the internal combustion engine. In individual cases, a specification of individual threshold values for the concentration and the mass flow may be expedient. The raw emission of NO _x or the further exhaust gas components can either be measured by means of gas sensors arranged correspondingly in the exhaust gas tract or preferably in dependence on a current operating point of the internal combustion engine, in particular a current engine speed and / or engine load. In this case, if the internal combustion engine is currently in a stoichiometric or rich operating mode with λ ≤ 1, a lean operating mode is assumed for the modeling. Such a modeling of raw emissions, for example on the basis of stored maps, is well known and will not be explained in detail here.

With regard to the temperature of the NO _x storage catalyst, a local temperature of a coating of the catalyst and / or of a catalyst support and / or a temperature distribution over the entire catalyst length can be considered. In this case, the catalyst temperature can be determined from an exhaust gas temperature measured upstream and / or downstream of the storage catalytic converter and / or measured by means of a temperature measuring point arranged in the catalytic converter itself. However, it is also known and appropriate, the catalyst temperature in dependence of an operating point of the Calculate internal combustion engine using appropriate maps with good accuracy.

To make an adaptation of the threshold values for the permissible lean operating range to actual requirements, the variation is at least a threshold value when the NO _x storage and / or conversion capacity of the catalyst drops below a predetermined limit value, that is behind a, for a fresh NO _x Storage catalyst derived or tuned to an already irreversibly partially damaged catalyst performance expectancy remains. To assess the NO _x storage and / or conversion capacity, various strategies are available. Specifically, the lean operation can vary over a downstream of the NO _x storage catalytic converter, for example by means of a _NOx sensor measured NO _x content in the exhaust gas controlled and terminated when a temporary or accumulated on the lean operating phase NO _x content exhaustion of the storage capacity of the catalyst displays. In this case, a duration of a thus controlled lean operating phase can be used as a measure of an incorporated NO _x mass and thus for the storage and conversion capability of the catalyst. Likewise, the measured, in particular the cumulated NO _x content or its course can be used to assess the NO _x storage and / or conversion ability. Alternatively, it is also possible to use a duration of at least one preceding NO _x regeneration phase as a criterion for the storage and / or conversion capability of the catalytic converter if the regeneration is controlled as a function of an oxygen content measured downstream of the catalytic converter, for example by means of a lambda probe. Again, instead of the duration of the regeneration phase and the measured oxygen content or its course serve as a criterion. Furthermore, the NO _x storage and / or conversion capability of the storage catalytic converter can be determined on the basis of fuel consumption and / or based on a frequency of the NO _x regeneration. Regardless of the choice of rating, the behavior of the NO _x storage catalyst is always compared to that of a fresh, undamaged catalyst or, if irreversible damage has already been identified, to best expected behavior.

Preferably, that threshold value of an operating parameter of the internal combustion engine and / or of the exhaust gas tract is always varied, in the region of which the operating state was during the preceding lean phase or at least predominantly, with corresponding limits for the area being specified. If the possibly averaged operating state was within the limit of several threshold values in the observation period, then preferably all affected threshold values are varied.

If, on the other hand, the operating state was not in any range of a threshold value, advantageously at least one threshold value can be varied. In particular, it is envisaged to lower an upper threshold value for a raw emission allowable in the lean operating mode if, during the preceding lean operating phase, the average internal combustion NO _x emissions of the internal combustion engine were in the range of this upper threshold value and the NO _x storage and / or conversion capability was predetermined Limit value falls below. Conversely, with sufficient storage and conversion capability, this threshold can be increased. The procedure is analogous for the other thresholds considered. In addition, in particular in the case of a lower threshold value for a catalyst temperature permissible in the lean operating mode, it may be useful to take into account a measured or calculated irreversible damage to the NO _x storage catalyst instead of or in addition to the currently measured catalyst behavior.

The internal combustion engine preferably has a direct fuel injection and is capable of charging in the lean operating mode. In the stratified charge mode, a fuel injected into a cylinder at the time of ignition is substantially concentrated in the region of a spark plug, while practically pure air prevails in the remaining combustion chamber of the cylinder. In this way, particularly lean air-fuel ratios and thus a particularly low fuel consumption can be realized in the stratified charge mode. The formation of the stratified charge cloud and its transport to the spark plug can be supported in a known manner by wall-guiding measures, for example by a trough-shaped design of a piston crown. Furthermore, air-conducting measures are known and expedient, which can be realized approximately in the form of a arranged in a suction pipe of the cylinder charge motion flap and cause special air currents in the combustion chamber.

Further advantageous embodiments of the invention will become apparent from the remaining features mentioned in the dependent claims.

Figur 1

eine Blockdarstellung einer Verbrennungskraftmaschine mit zugeordneter Abgasanlage;

Figur 2

einen in Abhängigkeit von einer Katalysatortemperatur und einer NO_x-Rohemission zulässigen Bereich für einen Magerbetriebsmodus und

Figur 3

ein Fließschema einer bevorzugten Ausführung des Verfahrens.

The invention will be explained in more detail in embodiments with reference to the accompanying drawings. Show it:

FIG. 1

a block diagram of an internal combustion engine with associated exhaust system;

FIG. 2

a range permissible for a lean operation mode depending on a catalyst temperature and a NO _x raw emission, and

FIG. 3

a flow chart of a preferred embodiment of the method.

The lean burn internal combustion engine 10 shown in Figure 1 has in this example four cylinders 12, each having an unillustrated direct fuel injection system. The internal combustion engine 10 is further operable by a wall and air-guided mixture treatment process in a stratified charge mode. An exhaust gas produced by the internal combustion engine 10 is aftertreated in an exhaust tract, designated overall by 14. The exhaust gas tract 14 consists essentially of a catalyst system installed in an exhaust gas duct 16, with a small volume and close to the engine arranged precatalyst 18, for example, a 3-way or oxidation catalyst, and a typically arranged at an underbody position NO _x storage catalyst 20. The NO _x - Storage catalytic converter 20 comprises, in addition to a 3-way catalytic converter component, a NO _x absorber for storing nitrogen oxides NO _{x which are} not completely convertible in the lean operating mode. A lambda probe 22 arranged downstream of the internal combustion engine 10 measures an oxygen content of the exhaust gas and thus enables a regulation of an air-fuel ratio to be supplied to the cylinders 12. Downstream of the storage catalyst 20, a further gas sensor 24 is installed, which is preferably a NO _x sensor. The NO _x sensor 24 detects, for example, a NO _x breakthrough during a lean operation and thus controls a discontinuous lean / rich loading of the storage catalyst 20 for the purpose of its NO _x regeneration. A temperature measuring point 26 measures upstream of the NO _x storage catalytic converter 20 an exhaust gas temperature and allows conclusions about the temperature of the storage catalyst 20. Alternatively, the catalyst temperature can also be modeled in a conventional manner based on selected operating parameters of the internal combustion engine 10. All sensor signals and operating parameters of the internal combustion engine 10 and the exhaust gas tract 14 are transmitted to a motor controller 28. Here an evaluation of the signals and data and a control of the internal combustion engine 10 based on stored algorithms and maps.

The ability of the NO _x storage catalyst 20 to store nitrogen oxides is not sufficient at each operating point to ensure NO _x emissions consistent with allowable limits. In particular, the NO _x storage catalyst 20 has sufficient NO _x storage and conversion ability only in a certain temperature window, and therefore usually upper and lower thresholds for one in the Lean operating mode allowable catalyst temperature can be specified. If the catalyst temperature is outside the range limited by the threshold values, the lean operation is inhibited and the internal combustion engine 10 is operated in a stoichiometric or rich operation. According to the invention, in addition to the catalyst temperature, additional threshold values are specified for at least one further operating parameter of the internal combustion engine 10 or the exhaust gas tract 14, in particular for a maximum permissible NO _x raw emission of the internal combustion engine 10. Further, all predefined threshold values can preferably be adapted if the NO _x storage and / or conversion capability of the storage catalyst 20 is below a requested value.

FIG. 2 shows the dependence of a permissible range for the lean operating mode on a catalyst temperature TSK of the NO _x storage catalytic converter 20 and on a NO _x raw emission NORE of the internal combustion engine 10. The range permitted for lean operation with λ> 1 is shown in white and the range, in which the internal combustion engine 10 must be operated with a rich or stoichiometric mixture with λ ≤ 1, shown hatched. The lean operating range is limited by a lower temperature threshold TSKMN and an upper temperature threshold TSKMX for the catalyst temperature TSK. On the other hand, the lean operation is disabled when the NO _x raw emission NORE is above a maximum threshold value NOREMX. A specification of further threshold values is conceivable. For example, in a third dimension, not shown, an upper threshold for raw emissions of exhaust gas constituents such as HC, CO ₂ and CO could be provided which hinder the incorporation of NO _x into the catalyst.

All predetermined threshold values are designed to be variable and can be varied within predefinable limits as a function of a current NO _x storage and / or conversion capability of the storage catalytic converter 20. In this case, preferably, that threshold value is changed, in the border region of which an operating point was in the considered period of a preceding lean operating phase. For example, as shown in point 100, if the possibly averaged NO _x raw emission of the previous lean operation phase was close to the upper threshold NOREMX, this threshold NOREMX is lowered if insufficient NO _x storage ability or conversion capability, for example, based on a lean operation phase length or a measured downstream of the storage catalyst 20 NO _x emission is detected. On the other hand, at point 100, under certain conditions, which are still to be explained, an increase in the threshold value NOREMX can take place if a sufficient NO _x storage capacity is present. By contrast, if the operating point in the considered interval was in the upper limit of the upper temperature threshold TSKMX (point 102), this temperature threshold is lowered if there is insufficient storage capacity and, if necessary, an increase with sufficient storage capacity. If the operating point is close to several threshold values, all affected threshold values can also be adapted. If the operating point, as shown in point 104, is not in any limit range of a threshold value, then, if the NO _x storage and / or NO _x conversion capability is inadequate, several, but at least one threshold value can be varied. Since the catalyst temperature TSK and the NO _x -Rohemission NORE have the greatest impact on the storage capacity, preferably takes an adjustment of these thresholds. In the case of an operating point close to the lower temperature limit TSKMN (point 106), an increase of the threshold value TSKMN takes place when the NO _x storage capacity and NO _x conversion capability are inadequate. In this case, however, it is preferably provided to predetermine the position of the threshold value TSKMN also as a function of an aging state (irreversible damage) determined by means of a diagnostic method, since the threshold value TSKMN corresponding to a light-off temperature of the catalyst mainly depends on the state of aging.

3 shows a flowchart of the method according to the invention in accordance with the operating point 100 of FIG. 2. The method sequence can be divided into two main sections, namely checking of the NO _x storage and NO _x conversion capability of the storage catalytic converter 20 and determination of the threshold values of the lean operating range on the one hand (steps S1 to S5) and permission or inhibition of lean operation at a current operating point on the other hand (S6 to S11). Initially, an initialization takes place in S1, in which, inter alia, the threshold values for the lean operating range and a limit value for NO _x storage and NO _x conversion capability of the NO _x storage catalyst 20 are specified. These specifications are based on empirical values of an undamaged and completely regenerated storage catalytic converter. In a first query in S2, a check of the storage and conversion capability of the storage catalytic converter 20 is performed. In this example, this check is made 20 measured with reference to a downstream in the preceding lean phase by means of the _NOx sensor 24 of the NO _x storage catalyst and aufintegrierten NO _x emission NOHK. If this NO _x emission NOHK is above a limit value NOHKGW specified in S1, the upper threshold value NOREMX for the NO _x raw emission of the internal combustion engine 10 is lowered in S3 by the increment ΔNORE. It is assumed here that the threshold value NOREMX was set too high and the NO _x storage catalytic converter 20 due to the high NO _x raw emission (see point 100 in FIG 2) did not have sufficient storage capacity. If, on the other hand, the query in S2 is answered in the negative and a sufficient NO _x storage and NO _x conversion capability determined, the method proceeds to the query S4, where it is checked whether the NO _x raw emission NORE of the previous lean phase is very close to the upper emission threshold NOREMX. If this inquiry is affirmed, the NO _x -Emissionsschwelle NOREMX is increased in S5 by the increment ΔNORE. The loop of steps S4 and S5 ensures that the allowable lean area is not unnecessarily restricted. If the query is denied in S4, there is no variation of the emission threshold NOREMX, this is rather maintained.

In S6, a current NO _x raw emission NOREF is determined by measuring the NO _x concentration upstream of the catalyst system 18, 20 or by calculation based on current operating parameters of the internal combustion engine 10. If the internal combustion engine 10 is currently in a stoichiometric or lean mode of operation, a lean mode of operation is assumed for this calculation. The calculation can be carried out, for example, on the basis of stored characteristic maps which contain information about the expected NO _x raw emission as a function of a current engine speed and / or engine load and / or other operationally relevant variables. To avoid dynamic effects, an averaging of the NO _x raw emissions NOREF determined in this way can take place over a minimum time. In a query S7, it is checked whether the raw emission NOREF determined in S6 lies below the emission threshold value NOREMX defined in S2 to S5. If this is the case, the current catalyst temperature TSK of the NO _x storage catalytic converter 20 is determined by measurement or modeling in S8. It is then queried in S9 whether this current catalyst temperature TSK is within the permissible temperature range, ie greater than the lower temperature threshold TSKMN and smaller than the upper temperature threshold TSKMX. If this query is also affirmative, the current operating point of the internal combustion engine 10 and of the exhaust system 14 is in the range permissible for the lean operating mode. Consequently, the lean operation mode is permitted in S10. If one of the queries S7 and S9 is denied, at least one operating parameter is outside the permissible range, so that in S11 the lean operating range is blocked and the internal combustion engine 10 is charged with a stoichiometric or rich air-fuel mixture.

LIST OF REFERENCE NUMBERS

10

Internal combustion engine

12

cylinder

14

exhaust tract

16

exhaust duct

18

precatalyzer

20

NO _x storage catalyst

22

lambda probe

24

Gas sensor / NO _x sensor

26

Temperature measuring point

28

motor control

λ

Air-fuel ratio lambda

NOHK

NO _x emission behind NO _x storage catalyst

NOHKGW

Limit value for NO _x emissions

NORE

NO _x crude emission

NOREF

current (measured or modeled) NO _x raw emission

NOREMX

upper threshold for NO _x raw emissions

ΔNORE

Increment of the NO _x raw emission threshold

TSK

Catalyst temperature of the NO _x storage catalyst

TSKMN

lower threshold for the catalyst temperature

TSKMX

upper threshold for the catalyst temperature

Claims (18)

1. Method for controlling an operating mode of an internal combustion engine (10) which is capable of running in a lean-burn mode and has an exhaust section (14), which comprises at least one NO_x storage catalytic converter (20) and a gas sensor (24) connected downstream of it, in which method

   - a lean-burn operating mode of the internal combustion engine (10) with λ > 1 is permitted above a lower threshold value (TSKMN) and below an upper threshold value (TSKMX) for a temperature (TSK) of the NO_x storage catalytic converter (20) and as a function of at least one further threshold value for at least one further operating parameter of the internal combustion engine (10) and/or of the exhaust section (14), and

   - at least one of the threshold values is varied as a function of a current NO_x storage and/or NO_x conversion capacity of the NO_x storage catalytic converter (20).
2. Method according to Claim 1, characterized in that the at least one further operating parameter comprises an untreated NO_x emission (NORE) from the internal combustion engine (10) and/or an untreated emission of a further exhaust-gas component.
3. Method according to Claim 2, characterized in that the further exhaust-gas component comprises unburnt hydrocarbons (HC) and/or carbon dioxide (CO₂) and/or carbon monoxide (CO).
4. Method according to Claim 2 or 3, characterized in that the untreated NO_x emission (NORE) and/or the untreated emission of the further exhaust-gas component is measured or is modelled as a function of an operating point of the internal combustion engine (10) assuming the lean-burn operating mode.
5. Method according to one of Claims 2 to 4, characterized in that the untreated emission comprises a concentration and/or a mass flow of NO_x or of the other exhaust-gas component upstream of a first catalytic converter (18) connected downstream of the internal combustion engine (10).
6. Method according to one of the preceding claims, characterized in that the temperature (TSK) of the NO_x storage catalytic converter (20) comprises a local temperature of a catalyst coating and/or of a catalyst support and/or a temperature distribution.
7. Method according to Claim 6, characterized in that the temperature (TSK) is measured by means of a temperature-measuring position (26) arranged upstream of and/or downstream of and/or in the NO_x storage catalytic converter (20) and/or is calculated as a function of an operating point of the internal combustion engine (10).
8. Method according to one of the preceding claims, characterized in that the at least one threshold value is varied if the NO_x storage and/or NO_x conversion capacity of the NO_x storage catalytic converter (20) drops below a predetermined limit value.
9. Method according to Claim 8, characterized in that the lean-burn operation is controlled as a function of an NO_x content in the exhaust gas measured downstream of the NO_x storage catalytic converter (20), and the NO_x storage and/or NO_x conversion capacity of the NO_x storage catalytic converter (20) is determined on the basis of a duration of at least one preceding lean-burn operating phase and/or on the basis of the measured NO_x content or its profile.
10. Method according to Claim 8 or 9, characterized in that an NO_x regeneration is controlled as a function of an oxygen content in the exhaust gas measured downstream of the NO_x storage catalytic converter (20), and the NO_x storage and/or NO_x conversion capacity of the NO_x storage catalytic converter (20) is determined on the basis of a duration of at least one preceding NO_x regeneration phase and/or on the basis of the measured oxygen content or its profile.
11. Method according to one of Claims 8 to 10, characterized in that the NO_x storage and/or NO_x conversion capacity of the NO_x storage catalytic converter (20) is determined on the basis of a fuel consumption and/or on the basis of a frequency of the NO_x regeneration.
12. Method according to one of preceding claims, characterized in that the threshold value of an operating parameter of the internal combustion engine (10) and/or of the exhaust section (14) which is varied is the one in whose vicinity an operating state was during the preceding lean-burn operating phase.
13. Method according to Claim 12, characterized in that an upper threshold value (NOREMX) for an untreated NO_x emission (NORE) which is permissible for the lean-burn operating mode is lowered if during the preceding lean-burn operating phase the untreated NO_x emission (NORE) from the internal combustion engine (10) was in the vicinity of the upper threshold value (NOREMX) and the NO_x storage and/or NO_x conversion capacity of the NO_x storage catalytic converter (20) drops below the predetermined limit value.
14. Method according to Claim 12 or 13, characterized in that an upper threshold value (TSKMX) for a catalytic converter temperature (TSK) of the NO_x storage catalytic converter (20) which is permissible for the lean-burn operating mode is lowered if during the preceding lean-burn operating phase the catalytic converter temperature (TSK) was in the vicinity of the upper threshold value (TSKMX) and the NO_x storage and/or NO_x conversion capacity of the NO_x storage catalytic converter (20) drops below the predetermined limit value.
15. Method according to Claim 12 or 13, characterized in that a lower threshold value (TSKMN) for a catalytic converter temperature (TSK) of the NO_x storage catalytic converter (20) which is permissible for the lean-burn operating mode is raised if during the preceding lean-burn operating phase the catalytic converter temperature (TSK) was in the vicinity of the lower threshold value (TSKMN) and the NO_x storage and/or NO_x conversion capacity of the NO_x storage catalytic converter (20) drops below the predetermined limit value or a measured or calculated irreversible damage to the NO_x storage catalytic converter (20) is established.
16. Method according to one of the preceding claims, characterized in that the lean-burn operating mode is permitted if the current catalytic converter temperature (TSK) of the NO_x storage catalytic converter (20) is greater than the lower temperature threshold value (TSKMN) and lower than the upper temperature threshold value (TSKMX).
17. Method according to one of the preceding claims, characterized in that the lean-burn operating mode is permitted if the current, measured or calculated (assuming lean-burn operation) untreated NO_x emission (NORE) from the internal combustion engine (10) drops below the upper threshold value (NOREMX) for the untreated NO_x emission.
18. Method according to one of the preceding claims, characterized in that the internal combustion engine (10) has direct fuel injection and is capable of stratified-charge operation in the lean-burn operating mode.

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