FR2878900A1 - Nitrogen oxide accumulator catalyst regeneration triggering method for internal combustion engine, involves combining partial decision values to obtain decision value, and triggering catalyst regeneration by comparing value with threshold - Google Patents

Abstract

The method involves determining a load of a nitrogen oxide accumulator catalyst (30). Two partial decision values are formed based on the load in nitrogen oxide of the catalyst and operation parameters of an internal combustion engine (10), respectively. The two values are combined to obtain a decision value. Regeneration of the catalyst is triggered based on the result of comparison of the decision value with a threshold. An independent claim is also included for a control apparatus of triggering the regeneration of a nitrogen oxide accumulator catalyst in an exhaust gas of an internal combustion engine.

Description

Field of the invention

  The present invention relates to a method for initiating the regeneration of an NOx nitrogen oxide storage catalyst in the exhaust gas of an internal combustion engine comprising the following steps: the catalyst charge is determined, a decision value dependent on the load is determined, the decision value is compared with a threshold, and the regeneration is triggered according to the result of the comparison.

  The invention also relates to a control apparatus executing such a method.

  State of the art Such a method and such a control apparatus are already known. The combustion of fuel in internal combustion engines with excess air forms more nitrogen oxides than the combustion of a stoichiometric mixture of fuel and air. To convert these higher nitrogen oxide emissions, current internal combustion engines include an accumulator catalyst that takes the nitrogen oxides emitted when operating with excess air in the exhaust and stores them. . The nitrogen oxide fraction taken from the exhaust gases of the total quantity of nitrogen oxides contained in the exhaust gas depends on the temperature of the accumulator catalyst, the volume of the exhaust gas, the composition exhaust gas and in particular the amount of nitrogen oxides already accumulated.

  To re-establish the decreasing accumulation capacity as the nitrogen oxide charge of the accumulator catalyst increases, the accumulator catalyst must be regenerated from time to time in an exhaust gas atmosphere. In this reducing atmosphere of exhaust gas and in a certain temperature window, the accumulated nitrogen is released in the form of molecular nitrogen. The reducing atmosphere of the exhaust gas is for example obtained by increasing the dosage of the fuel. An exhaust gas reducing atmosphere is generated by the reducing agents contained in the exhaust gas and which are added to the exhaust gas inside the engine, that is to say, for example by increasing the dosage. fuel combustion chambers or by an exhaust gas mixture. As a reducing agent, there is for example CO and / or H2 and / or hydrocarbons.

  While the regeneration has a duration of the order of a few seconds, the storage corresponds to durations of the order of minutes. On average over time, the internal combustion engine can thus operate mainly with an excess of air.

  If the accumulator catalyst is regenerated too infrequently and in a manner which is not sufficiently intensive, there is an unnecessarily high emission of nitrogen oxides downstream of the accumulator catalyst. If, on the other hand, the regeneration is too frequent and / or too intensive, this unnecessarily increases the consumption of reducing agent and there is a risk of an increase in hydrocarbon emission downstream of the accumulator catalyst.

  It is thus generally desirable to carry out the regeneration always precisely when the storage capacity of the NOx nitrogen oxide storage catalyst has decreased to a defined minimum value or, which is synonymous, if the charge of the accumulator catalyst exceeds a defined threshold.

  A regeneration controlled in time and which depends only on an operating point accepts in a manner incompatible the disadvantages mentioned above. In order to correctly enter the charge state of the accumulator catalyst, either continuous calculations (load model in the control unit) or one or two suitable sensors for NOx nitrogen oxides are required. This allows triggering the regeneration for a limit value of the load that has been fixed beforehand. The state of charge of the accumulator catalyst has been modeled up to now from the operating parameters of the internal combustion engine and the accumulator catalyst by calculation and / or using the signals of the nitrogen oxide sensors. NOx installed in the exhaust system. If a predefined load threshold is exceeded, the regeneration of the accumulator catalyst is triggered.

  In the known processes, too high a temperature rise of the NOx nitrogen oxide storage catalysts and deterioration of the operation of the internal combustion engine have been observed.

  OBJECT OF THE INVENTION In view of this state of the art, the object of the present invention is to develop a method for triggering the regeneration of a NOx nitrogen oxide storage catalyst that overcomes these drawbacks or less reducing them. The invention also aims to develop a control apparatus for the execution of such a method.

  DESCRIPTION AND ADVANTAGES OF THE INVENTION For this purpose, the invention relates to a method of the type defined above, characterized in that the step of determining the decision value comprises the following other steps: a first partial value is formed based on the NOx nitrogen oxide charge of the accumulator catalyst; - a second decision partial value dependent on the operating parameters of the internal combustion engine; and - the two partial decision values for get the decision value.

  The problem is also solved by means of a control apparatus of the type defined above, characterized in that the control apparatus forms a first decision partial value for the determination of the decision value. first partial value of the NOx oxide charge of the accumulator catalyst, then forms a second decision partial value dependent on the operating parameter of the internal combustion engine, and combines the two partial decision values to obtain the decision value.

  Advantages of the invention The invention is based on the consideration that regeneration triggered solely as a function of the load takes place during actual operation frequently under adverse conditions. In the case of an internal combustion engine the operating parameters are for example unfavorable when the temperature of the exhaust gas is insufficient or when there are not enough reducing agents resulting from an addition to the inside the engine in the combustion chambers of it.

  By combining the partial decision values stated above, regeneration is no longer triggered when a reduced load limit is reached. Instead, regeneration is triggered based on a decision value that flexibly processes the different partial decision values. For a first small partial decision value but a large second partial decision value, for example, it is already possible to trigger a regeneration.

  By combining these two partial decision values to form the decision value, it will be possible to trigger a regeneration of the accumulator catalyst for favorable operating parameters of the internal combustion engine already when the charging of the accumulator catalyst does not in itself require any regeneration. As a result, the accumulator catalyst is re-generated more often and more frequently at advantageous operating points of the internal combustion engine. Conversely, regenerations made under unfavorable conditions will be rarer.

  This improves the average of the conversion of nitrogen oxides established over many accumulation cycles and decreases the increase in consumption and the sliding of the reducing means. In addition, regenerations will be shortened and avoided which, because of the subsequent heavy heating of the accumulator catalyst, would unnecessarily reduce the accumulation efficiency unnecessarily during the next accumulation phase.

  For the implementation of the method, it is advantageous for the combination to represent the first decision partial value in the decision value so that an increasing load promotes the initiation of a regeneration and at least three values of the first value. partial decision are associated with different states of charge.

  In contrast to known solutions using a fixed load limit as a trip criterion, this embodiment provides a soft influence quantity for controlling the influences of the load with a fineness defined by the number of pairs of a value of the value. partial decision and a charge. In the ideal case, a continuous function of the first decision value with respect to the load is used.

  It is furthermore advantageous that the second partial decision value is a measure of the quantity or suitability of the composition (ratio of CO and HC hydrocarbons and the residual oxygen content) of the reducing agent which can be assayed. in the exhaust gas via at least one of the combustion chambers of the internal combustion engine, and that the combination represents the second decision partial value in the decision value for an increasing amount of reducing agents promotes the initiation of regeneration.

  In addition, switching to operating mode at certain operating points for noise modification or torque stability will be less troublesome than at other operating points. Without the combination with the second decision partial value, operating points that are frequently unused but better suited for regeneration will be neglected while the regeneration will be triggered more frequently at unused operating points.

  Thanks to the addition of reducing agent downstream of the engine, the distribution, evaporation and the activity of the reducing agent in different sizes in the different operating pumps.

  The quantity and the composition of the reducing agent supplied to the internal combustion engine by internal engine-specific measurements depend strongly on the operating point of the internal combustion engine which is defined, for example, by the charge of the combustion chamber. , engine speed and other parameters such as temperature. At certain operating points, it is relatively difficult to generate large amounts of CO oxides and H2 hydrogen while maintaining the residual oxygen content and the HC hydrocarbon fraction hardly oxidizable at a low level. By promoting the initiation of regeneration for potentially increasing amounts of reducing agent available, the unfavorable conditions discussed above are largely avoided. This is why in the sum of many regenerations, there will be undisturbed operation of the internal combustion engine and better execution of the regeneration operations.

  It is also advantageous that the step of determining the decision value comprises the following steps: - a first partial decision value dependent on the NOx nitrogen oxide charge of the accumulator catalyst is formed, - a second partial value is formed A third decision value dependent on the temperature of the catalyst is formed according to the operating parameters of the internal combustion engine, and the three partial decision values are combined to form the decision value.

  An excessively high catalyst temperature is unfavorable for the accumulator catalyst because the exothermic regeneration results in a further troublesome temperature rise of the accumulator catalyst. The temperature may for example be involved in the third decision partial value for an excessively high temperature of the accumulator catalyst to act to delay the initiation of a regeneration.

  It is also advantageous if the third temperature dependent decision value of the accumulator catalyst is a measure for a predictable effect of a regeneration for that temperature and the combination of the third decision partial value is involved in the decision value for that an increase in the predictable effect favors the initiation of a regeneration.

  This development takes into account the necessary duration of a regeneration operation which strongly depends on the temperature of the accumulator catalyst. For comparable charges of an accumulator catalyst, the time required for regeneration is shorter as the Washcoat temperature of the monolith is high. In the case of a driving mode with dynamic alternation, it frequently happens that for the regeneration there are very advantageous conditions in the accumulator catalyst when it is not yet loaded to the maximum. A brief regeneration then makes it possible to completely empty the accumulator and thus significantly increase the efficiency with an implementation of reduced means.

  According to another preferred development, the first decision partial value is furthermore formed as a function of the temperature of the accumulator catalyst.

  The first decision partial value depends first of all on the charge of the accumulator catalyst. This dependence must make it possible to trigger the regeneration of the accumulator catalyst if its capacity to accumulate nitrogen oxides decreases because of its excessive load. However, it has been found that the accumulation capacity of the nitrogen oxides does not depend only on the charge but also on the temperature prevailing at the time of charging. The development evoked takes into account this relationship of dependence.

  It is also advantageous that the first partial decision value which depends on the NOx nitrogen oxide charge of the accumulator catalyst is formed from the signal of a NOx nitrogen oxide sensor installed downstream of the accumulator catalyst in the the direction of flow of the exhaust gas and that the second partial decision value dependent on the operating parameters of the internal combustion engine is formed as a measure of NOx emissions of nitrogen oxides of the internal combustion engine and that the decision value is the efficiency of the accumulator catalyst and that either the decision value or the threshold serving as a reference efficiency is formed as a function of the temperature of the accumulator catalyst and as a function of the space velocity of the exhaust gases.

  Spatial velocity means in known manner a volume flow rate of exhaust gas normalized with respect to the volume of the accumulator catalyst. Low temperatures, high temperatures and a high space velocity decrease the charge efficiency of the NOx nitrogen oxide storage catalyst, the other conditions remaining the same. In connection with the other characteristics which give the real efficiency of the load, it is possible to avoid an unnecessary triggering of a regeneration if the load is still too low but for example if the space velocity is high (because for example the target efficiency decreases at high space velocity). This improves the effectiveness of the reducing agent used. Conversely, under favorable loading conditions, that is to say for example for an average temperature and a low space velocity, it is ensured that a regeneration is triggered in time.

  It is furthermore advantageous to form the actual yield as the difference between the first partial value and the second decision partial value, the normalized difference with respect to the first decision partial value and to predefine the dependence of the threshold or the decision value for that a first temperature at which the first predictable efficiency of a regeneration is greater than that at a second temperature, promotes the initiation of regeneration and that a lower space velocity favors the triggering of a regeneration more strongly than a higher space velocity.

  This very concrete development develops the advantages described in the paragraph above.

  It is also advantageous that the threshold provided for comparison with the decision value depends on the state of aging of the accumulator catalyst and its instantaneous sulfur load.

  Both influences reduce the buildup efficiency with respect to the new condition. Therefore, it is recommended that the decision to trigger a re-generation of these two properties be made dependent. The dependency is for example designed so that if the efficiency of the accumulator catalyst decreases, the regeneration is triggered earlier. Alternatively or additionally, a thermally damaged storage catalyst no longer regenerates at low temperatures, because at that time too much reducing agent would pass unnecessarily through the accumulator catalyst.

  From the point of view of the design of the control unit, it is advantageous for it to perform at least one of the developments given above.

  Drawings The present invention will be described below in more detail with the aid of exemplary embodiments shown in the drawings and described hereinafter in more detail. Thus: FIG. 1 shows the technical environment of the invention; FIG. 2 shows a first exemplary embodiment of a method according to the invention; FIG. 3 shows a variant of the exemplary embodiment of FIG. FIG. 4 shows the evolution depending qualitatively on the temperature of a first decision partial value TEW_ 1 as a function of the charge B of an accumulator catalyst, - FIG. 5 qualitatively shows the evolution of a second partial decision value TEW_2 according to the operating parameter L of an internal combustion engine, - Figure 6 qualitatively shows the evolution of a third decision partial value TEW_3 as a function of the temperature T of an accumulator catalyst, and FIG. 7 shows another embodiment of a method according to the invention.

  Description of the embodiments

  FIG. 1 shows an internal combustion engine 10, the combustion chambers 12 of which are supplied with air by an air addition system 14. From the mass of sucked air, measured by a mass air flow meter 16 and taking into account the signals of other sensors, a control or management apparatus 18 forms the injection pulse widths controlling the injectors 20. As other sensors, there is in particular a rotational speed sensor 22, a driver of desire of the driver 24, a temperature sensor 26 and a NOx nitrogen oxide sensor 28. It should be noted that this enumeration has only an example character and as a variant or in addition to the sensors indicated ci above, other sensors can also be used to form the injection pulse widths and other controls for the internal combustion engine 10.

  The temperature sensor 26 captures the temperature of a NOx nitrogen oxide storage catalyst 30 installed in the exhaust system 32 and thus passed through the exhaust gas emitted by the internal combustion engine 10. nitrogen oxide sensor NOx 28 is installed downstream of the nitrogen oxide storage catalyst NOx 30 in the direction of passage of the exhaust gas in the exhaust system 32. By actions on the control of the internal combustion engine 10 in particular by interventions on the fuel metering, the control apparatus 18 also manages the charging and regeneration of NOx nitrogen oxide storage catalyst 30.

  Figure 2 shows the progress of a load control and regeneration program. Step 34 corresponds to the main control program of the internal combustion engine 10 by which, for example, the injection pulse widths are calculated and transmitted. In the intervals in which the internal combustion engine 10 operates with excess air and therefore the NOx nitrogen oxide storage catalyst 30 is charged with nitrogen oxides, the program leaves the main program in the first stage. step 34 in a predefined manner, for example periodically for a step 36 during which a first partial decision value TEW_ 1 is formed. The formation of the decision partial value TEW_ 1 is made as a function of the charge of NOx nitrogen oxide storage catalyst 30 with nitrogen oxides. The charge can be determined, for example, by the control apparatus 18 by using a charge model to determine the raw emissions of nitrogen oxides by the internal combustion engine 10 as a function of the parameters of the internal combustion engine 10. From such load models are known per se. Step 36 then continues with step 38 in which a second decision partial value TEW _2 is formed as a function of the operating parameters of the internal combustion engine. Thereafter, the formation is made so that the operating conditions of the internal combustion engine 10 for regenerating the NOx nitrogen oxide storage catalyst 30 are represented in the decision partial value TEW_2.

  Next, the decision partial values TEW_i are added in step 40 to obtain the decision value EW. In these relations i, in FIG. 2, represents 1 or 2. As a variant, the decision value EW can also be the weighted product of the partial decision values. In step 42, the decision value EW is compared with a threshold E_S. The threshold E_S is preferably a function of the instantaneous charge of the accumulator catalyst with sulfur and / or as a function of the prior thermal stresses, that is to say as a function of page or previous damage. These influences can be modeled in the example of the control device. The comparison is made for example so that an exceeding of the threshold E_S by the decision value EW triggers the start of a regeneration in step 44. Such a regeneration can also be made for example by a modification of the fuel dosage thanks to the reducing agent contained in the exhaust gas of the internal combustion engine 10. If on the other hand we do not pass the threshold E_S, we return to step 42 in the main program in step 34. The loop formed by steps 34, 36, 38, 40, 42 is thus traveled until a threshold exceeding in step 42 triggers the start of a regeneration in step 44. The regeneration is done for example for the duration of a period calculated by the control unit and at the end of this period, the internal combustion engine 10 again operates with excess air. Alternatively, it is also possible to terminate the regeneration with a defined signal provided by a sensor, for example an HC hydrocarbon sensor.

  FIG. 3 shows a development of the method of FIG. 2 with an additional step 46 traversed between the steps 38 and 40 of FIG. 2. In step 46, a partial decision value TEW_3 is formed, depending on the temperature of the accumulated catalyst. NOx of nitrogen oxides 30. Then, in step 40 according to FIG. 3, and following the partial decision values TEW_i (i = 1, 2, 3), these partial values are added to obtain the value of EW decision.

  The succession of the steps of FIGS. 2 and 3 has in common that, in particular, a partial decision value TEW_ 1 of small amplitude can lead to the triggering of a regeneration if at least one of the other two partial decision values TEW_2 or TEW_3 is important enough.

  FIG. 4 shows a curve beam 48, 50, 52 qualitatively showing the behavior of the charge dependent fractional value TEW_ 1 B of the accumulator catalyst 30, the temperature T of the accumulator catalyst being the parameter. The arrow bearing the reference T is directed towards the curves corresponding to the high temperatures of the parameters. From the evolution of each of the curves 48, 50, 52, it can be seen that the amplitude of the decision partial value TEW_1 increases as the load B of the accumulator catalyst 30 increases, which favors the triggering of a regeneration. FIG. 4 thus shows the continuous relation between the first partial decision value TEW_1 and the load W. Alternatively, the dependency relation can also be conceived so that at least one of the three values al, a2, a3 is associated with different loads b1, b2, b3, and then al represents for example all the charges B less than b l.

  FIG. 5 qualitatively shows the evolution of the decision partial value TEW_2 according to an operating parameter of the internal combustion engine 10; in FIG. 5 the operating parameter is the load L. The evolution of the curve 54 is predefined so that the decision partial value TEW_2 is a measure of the quantity of reducing agents assayed by at least one combustion chamber 12 of the internal combustion engine 10 or by downstream addition of the engine of the reducing agent to the exhaust gas. From the qualitative evolution of the curve 54 of FIG. 5, it can be seen, for example, that for the average loads L of the internal combustion engine, more reducing agents can be provided or that the switching to the rich mode is less troublesome than for low loads and high loads.

  FIG. 6 qualitatively shows the evolution of a partial decision value TEW_3 depending on the temperature T of the accumulator catalyst 30. The evolution of this partial decision value TEW_3 represents the predictable effect of the regeneration for the various temperatures As shown by the evolution of the curve 56, this predictable effect at the average temperatures T is greater than the low temperatures T and the high temperatures T.

  As the triggering of a regeneration depends on the sum of the three partial decision values TEW_1, TEW_2 and TEW_3, small values of one of the partial values can be compensated by high values of the other two partial values, so that for example for medium loads L and average temperatures T, a regeneration will be carried out even for low charges B. Thus, the accumulator catalyst 30 will be regenerated under the advantageous conditions before reaching its maximum possible load B. FIG. 7 shows another exemplary embodiment of a method according to the invention. In this method, steps 40 and 42 of FIG. 2 are replaced by steps 58, 60, 62 described hereinafter. In the description above of Figures 2 to 6, the calculation of the load B made in the control apparatus 18 and based on a load model was used. The charge thus calculated has a great influence on the accumulation capacity of the accumulator catalyst and this capacity decreases as the load increases. However, since the instantaneous capacity of the accumulation also depends on the space velocity, that is to say on the volume flow rate of the exhaust gas, which is normalized with respect to the volume of catalyst as well as the temperature of the accumulator catalyst 30, it is not necessary to can conclude automatically from a low accumulation capacity, that the accumulator catalyst 30 is heavily-loaded and that it must be regenerated. In other words: for a high space velocity and / or a very low or very high temperature of the accumulator catalyst 30, the instantaneous storage capacity can be very low even though the catalyst is completely empty. In this case, it would be undesirable to decide a regeneration based on the low instantaneous accumulation capacity. The other exemplary embodiment according to FIG. 7 presents a possibility to take into account the influence of the space velocity VR and the temperature T of the accumulator catalyst 30.

  For this purpose and after forming the decision partial value TEW_1 and TEW_2, in step 58, a decision value EW is defined as the difference between the decision partial value TEW_1 and TEW_2 normed with respect to the partial decision value TEW_1. This first partial decision value TEW_ 1, which depends on the charge of the NOx nitrogen oxide storage catalyst 30, is formed from the NOx 28 nitrogen oxide sensor signal installed downstream of the oxidation accumulator catalyst. NOx nitrogen 30 in the direction of passage of the exhaust gas.

  The second partial decision value TEW_2 which depends on the operating parameters of the internal combustion engine 10 is formed by calculation from the operating parameters of the internal combustion engine as a measure of the raw NOx emissions by the engine. Internal combustion 10. The decision value EW formed in step 58 thus represents the efficiency of the accumulator catalyst 30 for its storage capacity. In step 60, the threshold E_S is formed as a function of the temperature T of the accumulator catalyst 30 and of the space velocity VR. In forming the threshold E_S, account is taken of the dependence relationships qualitatively represented by the characteristic fields 64, 66. In other words: the influence of the temperature acts outside a mean temperature range in the direction of a reduction on the threshold E_S which further decreases with the increase of the space velocity VR.

  In the following step 62, the decision value EW is compared with the threshold E_S. The threshold E_S corresponds to a minimum capacitance capacity dependent on the space velocity VR and the temperature T of the accumulator catalyst 30. As long as the effective accumulation capacity represented by the decision value EW is greater than the capacity the minimum accumulation represented by the threshold E_S, we return to step 34 from step 62 and in this step 34, the main management program of the internal combustion engine is processed. On the other hand, a regeneration is triggered by passing to step 44 if the decision value EW representing the effective storage capacity falls below the minimum value represented by the threshold E S.

Claims (2)

  1) Process for triggering (44) the regeneration of an NOx nitrogen oxide storage catalyst (30) in the exhaust gas of an internal combustion engine (10) comprising the following steps: catalyst load (30), - a load-dependent decision value (40) is determined, - the decision value is compared (42) with a threshold, and - the regeneration is triggered (44) depending on the result of the decision. the comparison, characterized in that the step (40) of determining the decision value comprises the following other steps: forming (36) a first decision partial value which depends on the NOx nitrogen oxide load of the catalyst accumulator, -on form (38) a second decision partial value dependent on the operating parameters of the internal combustion engine (10), and - the two partial decision values are combined (40) to obtain the decision value.   2) Method according to claim 1, characterized in that the combination (40) represents the first decision partial value in the decision value so that increasing load promotes the triggering (44) of a regeneration, and at least three values of the first decision partial value are associated with different loads.   3) Method according to claim 1, characterized in that the second decision partial value is a measure of a quantity of reducing agent that can be determined in the exhaust gas at least through the chamber of combustion (12) of the internal combustion engine (10), and the combination represents the second decision partial value in the decision value for an increase in the amount of reducing agent to promote the initiation of a regeneration.   4) Process according to claim 1, characterized in that the step (40) for determining the decision value comprises the following other steps: - (36) a first partial decision value dependent on the charge of oxides of NOx nitrogen of the accumulator catalyst (30), - forming (38) a second decision partial value dependent on the operating parameters of the internal combustion engine (10), - forming (46) a third partial decision value depending on the temperature of the catalyst (30), and - the three partial decision values are combined (40) to form the decision value. 5) The method according to claim 4, characterized in that the third decision partial value is a measurement of an effect of a predictable regeneration at the temperature, and the combination represents the third decision partial value in the decision value for an increase in the predictable effect to favor the triggering a regeneration.   6) Process according to claim 1, characterized in that the first decision partial value is further formed depending on the temperature of the catalyst (30).   7) Process according to claim 1, characterized in that the first partial decision value dependent on the NOx oxide feedstock of the catalyst (30) is formed from the signal of a NOx nitrogen oxide sensor (28). ) installed downstream of the catalyst (30) in the direction of passage of the exhaust gas, forming the second decision partial value depending on the operating parameters of the internal combustion engine (10) as a measurement of oxide emissions nitrogen NOx of the internal combustion engine (10), the decision value is formed as the actual yield of the catalyst (30), and either the decision value or the threshold serving as a reference efficiency is formed as a function of the temperature of the catalyst (30) and as a function of the space velocity of the exhaust gas.   8) Process according to claim 7, characterized in that the actual yield is formed as the difference between the first partial value and the second partial decision value, the normalized difference with respect to the first partial decision value, and the dependence is predefined. the threshold or the decision value so that a first temperature at which a first predictable effect of a regeneration is greater than that of a second temperature, promotes the initiation of the regeneration, and a lower space velocity promotes the initiation of regeneration more strongly than a higher space velocity.   9) Process according to claim 1, characterized in that the comparison provided for step (42) with the threshold provided for the decision value depends on the aging state of the accumulator catalyst and its momentary sulfur load .   10) A control apparatus (18) initiating the regeneration of a NOx nitrogen oxide storage catalyst (30) installed in the exhaust gas of an internal combustion engine (10) and defining the charge of the catalyst ( 30) and a load dependent decision value, comparing the decision value with a threshold and triggering the regeneration according to the result of the comparison, characterized in that the control device (18) forms a first partial decision value for determining the decision value, this first partial value depending on the NOx oxide charge of the accumulator catalyst (30), then forming a second decision partial value depending on the operating parameter of the internal combustion engine ( 10), and combines the two partial decision values to obtain the decision value.   11) Control device (18) according to claim 10, characterized in that it carries out a method according to any one of claims 2 to 9.