EP1247963A2 - Method for controlling of an operating mode of a lean burn internal combustion engine - Google Patents

Abstract

Process for controlling an operating modulus of a lean burn IC engine (10) having an exhaust gas tract (14) with a NOx storage catalyst (20) and a gas sensor (24) comprises allowing a lean operating modulus of the engine with lambda more than 1 depending on threshold values for a temperature of the catalyst and for a further operating parameter of the engine and/or exhaust gas tract; and varying at least one threshold value depending on an actual NOx storability and/or NOx convertibility of the catalyst. Preferred Features: The further parameter comprises a NOx crude emission of the engine and/or a crude emission of a further exhaust gas component. The further exhaust gas component comprises non-combustible hydrocarbons, carbon dioxide and/or carbon monoxide.

Description

The invention relates to a method for controlling an operating mode lean-burn internal combustion engine with the features of the preamble of Claim 1.

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

A NO _x storage and convertibility of the storage catalytic converter depends on various operating parameters and also changes with aging of the catalytic converter. In particular, there is sufficient storage capacity only in a certain temperature range of the NO _x storage catalytic converter. It is therefore known, for example from DE 199 32 290 A1 and DE 199 29 292 A1, to make approval for lean operation of the internal combustion engine dependent on the catalytic converter temperature of the NO _x storage catalytic converter and to specify a lower and upper temperature threshold for the permissible lean operating range in order to specify NO Avoid _X breakthroughs. DE 198 50 786 A1 also discloses a method according to which these upper and lower temperature thresholds are adapted to the NO _x storage capacity of the catalytic converter, which is monitored on the basis of a duration of a previous lean interval or NO _x regeneration interval.

However, it has been shown that the catalytic converter temperature alone is not a sufficient criterion for the NO _x storage and convertibility of the catalytic converter and thus for the approval of the lean operating mode. Rather, the NO _X storage behavior of the catalyst depends on other influencing variables. For example, an aged storage catalytic converter with an average catalytic converter temperature of 350 ° C in operating points with low raw NO _x emissions can still have sufficient NO _x storage and conversion capacity, whereas with high NO _x raw emissions the storage capacity can decrease to such an extent that it is caused by it high regeneration frequency completely consumes the consumption advantage gained through lean operation. In such a case, the adjustment 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 deficiencies of the prior art and allows a lean operating mode of the internal combustion engine to be adjusted more precisely 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-Speicherund/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 with the features of claim 1. As a result of that

• a lean operating mode of the internal combustion engine with λ> 1 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 is permitted 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,

the lean operating mode is approved taking into account the current operating point very precisely and the approval conditions are constantly updated to reflect the actual NO _x storage and / or conversion capacity of the NO _x storage catalytic converter. As a result, NO _X -Endemission can thus be reduced considerably and at the same time the lean operation mode be used over wide operating ranges of a driven by the internal combustion engine vehicle, whereby optimum fuel consumption 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. 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 also be taken into account. In high concentrations, these hinder the incorporation of NO _x into the catalyst. Accordingly, an upper threshold value can be specified for the raw emission of each of these components or a sum of the components, the lean operating mode of which is blocked when exceeded. Raw emission is understood to mean 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, it may be useful to specify individual threshold values for the concentration and the mass flow. The raw emission of NO _x or the further exhaust gas components can either be measured by means of gas sensors arranged in the exhaust tract or preferably modeled as a function of a current operating point of the internal combustion engine, in particular a current engine speed and / or engine load. If the internal combustion engine is currently in a stoichiometric or rich operating mode with λ 1 1, a lean operating mode is assumed for the modeling. Such modeling of raw emissions, for example on the basis of stored characteristic maps, is sufficiently known and is not explained in more detail here.

With regard to the temperature of the NO _x storage catalytic converter, a local temperature of a coating of the catalytic converter and / or a catalytic converter support and / or a temperature distribution over the entire catalytic converter length can be considered. The catalytic converter 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 expedient to calculate the catalytic converter temperature with good accuracy as a function of an operating point of the internal combustion engine using corresponding characteristic maps.

In order to adapt the threshold values for the permissible lean operating range to actual requirements, at least one threshold value is varied when the NO _x storage and / or conversion capacity of the catalytic converter falls below a predetermined limit value, that is to say behind a threshold value for a fresh NO _x - Storage catalytic converter derived performance or matched to an already irreversibly partially damaged catalytic converter performance performance remains. To evaluate the _NOx storage-and / or conversion capacity, various strategies are available. In particular, 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 controlled in the exhaust gas 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, the duration of a lean operating phase controlled in this way can be used as a measure of an embedded NO _x mass and thus of the storage and conversion capacity of the catalytic converter. Likewise, the measured, in particular the accumulated, NO _x content or its course can be used to assess the ability to store and / or convert NO _x . Alternatively, a duration of at least one previous NO _X regeneration phase can also be used as a criterion for the storage and / or convertibility of the catalyst if the regeneration is controlled depending on an oxygen content measured downstream of the catalyst, for example by means of a lambda probe. Here too, instead of the duration of the regeneration phase, the measured oxygen content or its course can 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 on the basis of a frequency of NO _x regeneration. Regardless of the choice of the assessment size, the behavior of the NO _x storage catalytic converter is always compared with that of a fresh, undamaged catalytic converter or - if irreversible damage has already been found - with a behavior that can be expected at best.

That threshold value of an operating parameter is preferably always the Internal combustion engine and / or the exhaust tract varies, in the area of which Operating state was during the previous lean phase or at least prevailing, with appropriate limits to be specified for the area. lag the possibly averaged operating status in the observation period in the limit range several threshold values, all the threshold values concerned are preferably varied.

If, on the other hand, the operating state was not in a range of a threshold value, at least one threshold value can advantageously be varied. In particular, it is provided to lower an upper threshold value for a raw emission permissible in the lean operating mode if, during the previous lean operating phase, the mean raw NO _x emission of the internal combustion engine was in the range of this upper threshold value and the NO _x storage and / or conversion capacity was within the specified range Falls below the limit. Conversely, this threshold can be raised if there is sufficient storage and conversion capacity. An analogous procedure is followed for the other threshold values under consideration. 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 expedient to take account of 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 direct fuel injection and is capable of stratified loading in lean operating mode. There is an in in stratified charge mode fuel injected into a cylinder at the ignition point essentially in the region of a Spark plug concentrated before, while in the rest of the cylinder's combustion chamber practically clean Air prevails. In this way, particularly lean air-fuel ratios can be achieved in stratified charge mode and thus realize a particularly low fuel consumption. The Training of the stratified charge cloud and its transport to the spark plug can be done in a known manner Way through wall-guiding measures, for example by a trough-shaped Design of a piston crown are supported. Furthermore, are air leading Measures known and useful, such as in the form of a suction pipe of the Cylinder-arranged charge movement flap can be realized and special Cause air currents in the combustion chamber.

Further advantageous refinements of the invention result from the others, in the Characteristics mentioned subclaims.

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 is explained in more detail below in exemplary embodiments with reference to the associated drawings. Show it:

Figure 1

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

Figure 2

a permissible range for a lean operating mode as a function of a catalyst temperature and a NO _x raw emission and

Figure 3

a flow diagram of a preferred embodiment of the method.

In this example, the lean-running internal combustion engine 10 shown in FIG. 1 has four cylinders 12, each of which has a direct fuel injection system (not shown). The internal combustion engine 10 can also be operated in a stratified charge mode by means of a wall and air-guided mixture preparation method. An exhaust gas generated by the internal combustion engine 10 is aftertreated in an exhaust tract designated overall by 14. The exhaust section 14 consists essentially of a installed in an exhaust passage 16 catalyst system with a small volume and close-coupled pre-catalyst 18, for example a 3-way or oxidizing catalyst, and a typically arranged on an underbody position NO _X storing catalyst 20, the NO _X - In addition to a 3-way catalyst component, storage catalytic converter 20 includes a NO _x absorber for storing nitrogen oxides NO _{x which are} not fully 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 regulation of an air-fuel ratio to be supplied to the cylinders 12. A further gas sensor 24, which is preferably a NO _x sensor, is installed downstream of the storage catalytic converter 20. The NO _x sensor 24 detects, for example, a NO _x breakthrough during a lean operation and thus regulates a discontinuous lean / fat loading of the storage catalytic converter 20 for the purpose of its NO _x regeneration. A temperature measuring point 26 measures an exhaust gas temperature upstream of the NO _x storage catalytic converter 20 and permits conclusions to be drawn about the temperature of the storage catalytic converter 20. Alternatively, the catalytic converter temperature can also be modeled in a manner known per se using selected operating parameters of the internal combustion engine 10. All sensor signals and operating parameters of the internal combustion engine 10 and the exhaust tract 14 are transmitted to an engine control 28. Here, the signals and data are evaluated and the internal combustion engine 10 is controlled using stored algorithms and maps.

The ability of the NO _x storage catalytic converter 20 to store nitrogen oxides is not sufficient at every operating point to ensure NO _x emission in accordance with permissible limit values. In particular, the NO _x storage catalytic converter 20 has sufficient NO _x storage and conversion capability only in a certain temperature window, which is why an upper and a lower threshold value are usually specified for a catalytic converter temperature that is permissible in the lean operating mode. If the catalyst temperature lies outside the range limited by the threshold values, the lean operation is blocked and the internal combustion engine 10 is operated in a stoichiometric or rich operation. According to the invention, in addition to the catalytic converter temperature, additional threshold values for at least one further operating parameter of the internal combustion engine 10 or the exhaust tract 14 are specified, in particular for a maximum permissible NO _x raw emission of the internal combustion engine 10. Furthermore, all specified threshold values can preferably be adapted if the NO _x storage and / or conversion capacity of the storage catalytic converter 20 is below a requested value.

FIG. 2 shows the dependency of an admissibility range for the lean operating mode on a catalytic converter temperature TSK of the NO _x storage catalytic converter 20 and on a raw NO _x emission NORE of the internal combustion engine 10. The area permitted for the lean mode with λ> 1 is shown in white and the area hatched in which the internal combustion engine 10 must be operated with a rich or stoichiometric mixture with λ dargestellt 1. 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 blocked when the NO _X raw emission NORE is above a maximum threshold value NOREMX. A specification of further threshold values is conceivable. For example, an upper threshold value for a raw emission of exhaust gas components such as HC, CO ₂ and CO could be provided in a third dimension, not shown, which hinder the incorporation of NO _x into the catalytic converter.

All predefined threshold values are designed to be variable and can be varied within predefinable limits depending on a current NO _x storage and / or conversion capacity of the storage catalytic converter 20. In this case, that threshold value is preferably changed, in the border area of which an operating point was in the period under consideration of a previous lean operating phase. For example, as shown in point 100, if the possibly averaged raw NO _x emission of the previous lean operating phase was close to the upper threshold value NOREMX, this threshold value NOREMX is reduced if there is insufficient NO _x storage capacity or conversion capacity, for example based on a length of the lean operating phase or a NO _x emission measured downstream of the storage catalytic converter 20 is detected. On the other hand, the point NOREMX can be raised under point 100 under certain conditions to be explained, if there is sufficient NO _X storage capacity. On the other hand, if the operating point was within the limit range of the upper temperature threshold TSKMX (point 102) in the interval under consideration, this temperature threshold is lowered if the storage capacity is insufficient and if necessary increased if the storage capacity is sufficient. If the operating point is in the vicinity of several threshold values, all of the threshold values concerned can also be adapted. If, as shown in point 104, the operating point is not in the limit range of a threshold value, several, but at least one threshold value can be varied if the NO _x storage and / or NO _x conversion capability is insufficient. Since the catalyst temperature TSK and the NO _X raw emission NORE have the greatest influence on the storage capacity, these threshold values are preferably adjusted. In the case of an operating point near the lower temperature limit TSKMN (point 106), the threshold value TSKMN is increased if the NO _x storage and NO _x conversion capability is insufficient. In this case, however, provision is preferably made for the position of the threshold value TSKMN to also be dependent on an aging condition (irreversible damage) determined using a diagnostic method, since the threshold value TSKMN corresponding to a light-off temperature of the catalytic converter mainly depends on the aging condition.

FIG. 3 shows a flowchart of the method according to the invention according to operating point 100 of FIG. 2. The process flow can be divided into two main sections, namely checking the NO _x storage and NO _x conversion capability of the storage catalytic converter 20 and determining the threshold values of the lean operating range on the one hand (steps S1 to S5) and approval or blocking of lean operation at a current operating point on the other hand (S6 to S11). Initially, initialization takes place in S1, in which, among other things, 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 catalytic converter 20 are specified. These specifications are based on empirical values of an undamaged and fully regenerated storage catalytic converter. In a first query in S2, the storage and conversion capability of the storage catalytic converter 20 is checked. 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 catalytic converter and NO _x emissions aufintegrierten 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 by the increment ΔNORE in S3. It is assumed here that the threshold value NOREMX was set too high and that the NO _x storage catalytic converter 20 did not have sufficient storage capacity due to the high raw NO _x emission (see point 100 in FIG. 2). If, on the other hand, the query in S2 is denied and thus sufficient NO _x storage and NO _x conversion capability is determined, the method goes to 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 was NOREMX. If this query is answered in the affirmative, the NO _X emission threshold NOREMX is increased in S5 by the increment ΔNORE. The loop in steps S4 and S5 ensures that the permissible lean range is not unnecessarily restricted. If the query in S4 is negated, there is no variation of the NOREMX emission threshold, but rather it is retained.

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 operating mode, a lean operating mode is assumed for this calculation. The calculation can be carried out, for example, on the basis of stored characteristic diagrams 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, the NO _X raw emission NOREF determined in this way can be averaged over a minimum time. A query S7 checks whether the raw emission NOREF determined in S6 is below the emission threshold value NOREMX defined in S2 to S5. If this is the case, the current catalytic converter temperature TSK of the NO _x storage catalytic converter 20 is determined in S8 by measurement or modeling. S9 then queries whether this current catalyst temperature TSK is in the permissible temperature range, that is to say is greater than the lower temperature threshold TSKMN and less than the upper temperature threshold TSKMX. If this query is also answered in the 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. As a result, the lean operating mode is permitted in S10. If one of the queries S7 and S9 is answered in the negative, at least one operating parameter lies outside the permissible range, so that the lean operating range is blocked in S11 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 catalytic converter

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 catalytic converter

NOHKGW

Limit value for NO _x emissions

NORE

NO _X raw emissions

NOREF

current (measured or modeled) raw NO _x emission

NOREMX

Upper threshold for raw NO _x emissions

ΔNORE

Increment of the threshold value for the raw NO _x emission

TSK

Catalytic converter temperature of the NO _x storage catalytic converter

TSKMN

lower catalyst temperature threshold

TSKMX

upper threshold for the catalyst temperature

Claims (18)

Method for controlling an operating mode of a lean-burn internal combustion engine (10) with an exhaust tract (14), which comprises at least one NO _x storage catalytic converter (20) and a gas sensor (24) connected downstream thereof, wherein a lean operating mode of the internal combustion engine (10) with λ> 1 as a function of predeterminable threshold values for a temperature (TSK) of the NO _x storage catalytic converter (20) and for at least one further operating parameter of the internal combustion engine (10) and / or the exhaust gas tract (14) will 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 (20). Method according to claim 1, characterized in that the at least one further operating parameter comprises a raw NO _x emission (NORE) of the internal combustion engine (10) and / or a raw emission of a further exhaust gas component. A method according to claim 2, characterized in that the further exhaust gas component comprises unburned hydrocarbons (HC) and / or carbon dioxide (CO ₂ ) and / or carbon monoxide (CO). Method according to claim 2 or 3, characterized in that the NO _x raw emission (NORE) and / or the raw emission of the further exhaust gas component is measured or modeled depending on an operating point of the internal combustion engine (10) assuming the lean operating mode. Method according to one of claims 2 to 4, characterized in that the raw emission comprises a concentration and / or a mass flow of NO _x or the other exhaust gas component upstream of a first catalyst (18) connected downstream of the internal combustion engine (10). 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 a catalyst carrier and / or a temperature distribution. Method according to Claim 6, characterized in that the temperature (TSK) is measured by means of a temperature measuring point (26) arranged upstream and / or downstream and / or in the NO _x storage catalytic converter (20) and / or as a function of an operating point of the internal combustion engine (10) is calculated. Method according to one of the preceding claims, characterized in that the at least one threshold value is varied when the NO _x storage and / or NO _x conversion capacity of the NO _x storage catalytic converter (20) falls below a predetermined limit value. Method according to claim 8, characterized in that the lean operation is controlled as a function of a 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 ability of the NO _x Storage catalyst (20) is determined on the basis of a duration of at least one previous lean operating phase and / or on the basis of the measured NO _x content or its course. A method according to claim 8 or 9, characterized in that a NO _x regeneration is controlled depending on 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 ability of the NO _X storage catalytic converter (20) is determined on the basis of a duration of at least one previous NO _X regeneration phase and / or on the basis of the measured oxygen content or its course. Method according to one of claims 8 to 10, characterized in that the NO _x storage and / or NO _x conversion ability of the NO _x storage catalytic converter (20) is determined on the basis of fuel consumption and / or on the basis of a frequency of NO _x regeneration , Method according to one of the preceding claims, characterized in that the threshold value of an operating parameter of the internal combustion engine (10) and / or the exhaust tract (14) is varied, in the area of which an operating state was during the previous lean operating phase. Method according to Claim 12, characterized in that an upper threshold value (NOREMX) for a raw NO _x emission (NORE) permissible for the lean operating mode is reduced if the raw NO _x emission (NORE) of the internal combustion engine (10) increases during the previous lean operating phase was in the range 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) falls below the predetermined limit value. 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) permissible for the lean operating mode is reduced if the catalytic converter temperature (TSK) changes during the previous lean operating phase Range of the upper threshold value (TSKMX) was found and the NO _x storage and / or NO _x conversion capacity of the NO _x storage catalytic converter (20) falls below the predetermined limit value. 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) permissible for the lean operating mode is raised if the catalytic converter temperature (TSK) changes during the previous lean operating phase Range of the lower threshold value (TSKMN) was found and the NO _x storage and / or NO _x conversion ability of the NO _x storage catalytic converter (20) falls below the predetermined limit value or a measured or calculated irreversible damage to the NO _x storage catalytic converter (20) is determined. Method according to one of the preceding claims, characterized in that the lean 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 less than the upper temperature threshold value (TSKMX). Method according to one of the preceding claims, characterized in that the lean operating mode is permitted if the current, measured or calculated NO _X raw emission (NORE) of the internal combustion engine (10) is the upper threshold value (NOREMX) for the NO _X -Rough emission below. Method according to one of the preceding claims, characterized in that the internal combustion engine (10) has a direct fuel injection and is capable of stratified charging in the lean operating mode.

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