WO2001006106A1

WO2001006106A1 - Method and device for desulfating a nox accumulator catalyst

Info

Publication number: WO2001006106A1
Application number: PCT/EP2000/006517
Authority: WO
Inventors: Christos Coutsochristos; Ngoc-Hoa Huynh; Alf Degen; Lorenz Salzer; Peter Gerl
Original assignee: Dr. Ing. H.C.F. Porsche Aktiengesellschaft; Audi Aktiengesellschaft; Bayerische Motoren Werke Aktiengesellschaft; Volkswagen Aktiengesellschaft; Daimlerchrysler Ag
Priority date: 1999-07-15
Filing date: 2000-07-10
Publication date: 2001-01-25
Also published as: DE19933029A1; EP1200723A1

Abstract

The invention relates to a method for desulfating an NOx accumulator catalyst (4) in the exhaust gas system of an internal combustion engine (1) with direct injection. Said method comprises the following stages: Executing a retarded injection into the exhaust gas during the exhaust cycle of the internal combustion engine (1) in a desulfation mode and supplying an excess of oxygen to the exhaust gas in the desulfation mode, in such a way that post-combustion occurs in the NOx accumulator catalyst (4) to aid desulfation.

Description

Method and device for desulfating a NO storage catalyst

The present invention relates to a method and a device for desulfating a NO _x storage catalyst.

The emission of pollutants from gasoline engines can be effectively prevented by a catalytic aftertreatment. It is essentially a matter of completely burning the fuel that has not yet been completely burned. A catalyst requires the post-combustion of reactive CO and HC to harmless carbon dioxide (C0 ₂ ) and water (H ₂ 0) and at the same time reduces nitrogen oxides (NO _x ) in the exhaust gas to neutral nitrogen (N ₂ ).

The three-way catalytic converter, for example, is common, which breaks down all three pollutants CO, HC and N0 _X at the same time. It has a tube structure made of a ceramic, which is coated with precious metals, preferably with platinum and rhodium, the latter accelerating the chemical breakdown of the pollutants.

The three-way catalytic process assumes that the mixture has a stochiometric composition. A stochiometric mixture composition is characterized by an air ratio = 1.00. With this mixture composition, the catalyst works with a very high efficiency. Even a deviation of just one percent significantly affects the effectiveness of the pollutant conversion.

The known λ probe supplies a signal about the current mixture composition to the control unit for use in the mixture control. The λ probe is installed in the exhaust pipe of the engine at a point where the exhaust gas homogeneity required for the system to function is available over the entire operating range of the engine. In order to fully utilize the consumption potential, especially of a direct-injection gasoline engine, it is necessary to operate the engine with excess air, ie lean (λ> 1). In this lean operating mode, however, the known three-way catalytic converters are useless since they can no longer convert the nitrogen oxides (NO _x ).

As an alternative way of removing NO _x from the lean exhaust gas, the known NO _x storage catalytic converter is a promising alternative. In the new state of such a NO _x storage catalytic converter, known or applicable exhaust gas limit values can be maintained today.

However, the NO _x storage catalyst, which stores N0 _X in the form of nitrates, is sensitive to sulfur, since the much rarer but more chemically stable sulfates occupy the storage locations for the nitrates. This has the consequence that the storage catalytic converter poisons during operation depending on the sulfur content of the fuel and completely loses its NO _x storage function over time. If the fuel contains 30 ppm sulfur (clean fuel according to US California Phase 2 standard), the storage tank must be cleaned of sulfates after approx. 70 hours or approx. 2000 km.

The effective operating range for detoxification is proven to be at monolith temperatures> 600 ° C in rich exhaust gas (λ <0.98), whereby the prevailing temperature determines the necessary detoxification duration. However, damage

Temperatures of over 750 ° C the previously known N0 _x storage material thermal. Such favorable desulfation conditions occur in vehicles

Customer operation only when driving at full throttle for several minutes.

Such occur particularly in vehicles with powerful engines

Operating conditions in the field almost never arise. It is then an object of the present invention to provide a method and a device for desulfating a NO _x storage catalytic converter, as a result of which the desulfating can be carried out independently of the operating states specified by the driver.

The inventive method for desulfating a NO _x storage catalyst with the features of claim 1 and the corresponding device according to claim 11 have the advantage that the detoxification by means of specific functions of direct injection can be carried out independently of the operating states of the internal combustion engine desired by the driver and, moreover, not is noticeable by the driver as disturbing.

The present invention makes use of the possibility restricted to direct injection engines to add fuel to the burned exhaust gas again during the push-out phase or exhaust phase, that is to say with the exhaust valve open, which is also referred to as late injection. Another advantage of such a direct injection is that the composition of the air-fuel mixture, i.e. the λ value, for generating the engine power can be freely selected within system-dependent limits. A distinction is made between the λ value in the engine (λ _M ), which is the mixture composition in the combustion that takes place to generate engine power, and the λ value of the exhaust gas (λ _A ), which shows the mixture composition at different times at different locations describes the exhaust system, or the λ value of the catalyst (λ _κ ), which means the mixture composition effective in the catalyst.

In a first variant it is proposed to select the value λ _M at each operating point in the detoxification mode such that the λ value λ _κ at least to the upper limit value of the λ value for detoxification is set λ _κ = 0.98 and the late injection quantity used is suitable for generating afterburning in the NO _x storage catalyst, by means of which the normal operating temperature of the catalyst is increased at least up to that temperature (600 ° C.), which is required for desulfation. At operating points with a low load and a correspondingly low operating temperature, a λ value λ _M with a relatively large excess of oxygen must be selected, so that a correspondingly large amount of fuel can be added through the late injection and a correspondingly high calorific value can be achieved in order to bring about the large temperature increase required. In principle, this method can be applied to any operating point, so that desulphation can be implemented regardless of customer habits. By deliberately releasing the required calorific value directly in the storage catalytic converter, where the temperature increase is to take place, the process is optimal in terms of its efficiency.

However, in this first variant it should be noted that the NO _x storage catalytic converter must be the first catalytically active conversion device or conversion device downstream of the exhaust valve, but because of its thermal sensitivity it is not suitable for installation close to the engine. A starting catalytic converter close to the engine in conventional three-way technology cannot be used with this procedure, since it would immediately release the calorific value carried along in the exhaust gas stream in the process described above. On the other hand, it is difficult to comply with the increasingly strict exhaust gas regulations without a catalytic converter close to the engine.

According to a second variant, the present invention offers the possibility, with the aid of a secondary air pump, of using the method according to the invention also with a pre-catalyst. The additional fuel required to heat the NO storage catalytic converter becomes the same as in the first variant added to the exhaust gas. Through a suitable control or regulation of the main combustion with excess fuel, ie rich mixture (λ <1) required for power generation, however, the exhaust gas, which originates from the main combustion, contains only a little oxygen (0 ₂ ), so that it is added later with late injection added fuel can pass the respective pre-catalyst without any significant chemical reaction. The oxygen required to release the calorific value in the NO _x storage catalytic converter is only added to the exhaust gas after the pre-catalytic converter by means of the secondary air pump, so that the desired heating effect only occurs in the NO _x storage catalytic converter.

While in the second variant, due to the use of the pre-catalytic converter and the secondary air pump, a slightly increased construction effort is necessary, a third variant of the invention, which consists of a sensible combination of the first and second variant, describes a method and a device in which the Desulfation of the required temperature increase on the NO _x storage catalytic converter can be achieved even with systems with a pre-catalytic converter without additional construction costs by means of the engine control which is present anyway.

Since it is customary for direct injection engines to split the exhaust gas routing close to the engine, taking into account the ignition sequence, between two pre-catalysts for reasons of gas exchange and combustion, the presence of two pre-catalysts can be assumed to be state of the art. The third variant of the invention therefore proposes to operate the cylinder groups, which each consist of one or more cylinders, each of which is connected to a precatalyst, separately in terms of control technology. This opens up the possibility of operating the one group of cylinders in a stoichiometric manner without substantial oxygen components in the exhaust gas. Any quantity of unburned gasoline can be added to the corresponding oxygen-poor exhaust gas by means of spar injection, without an uncontrolled conversion in the pre-catalyst takes place. The other group of cylinders is operated overstoichiometrically, so that the exhaust gas generated there has a high concentration of oxygen. The two

Processes are to be coordinated in such a way that the mixture after passing through the precatalysts in the main catalytic converter to be desulfated produces the ideal exhaust gas composition of λ <0.98 for desulfating and the exhaust gas exactly the amount required to reach the minimum temperature of 600 ° C

Carries fuel with it.

For reasons of smooth running, it may be necessary to use the same λ _M in both cylinder groups. In this case, by providing an overstoichiometric λ _A by adding fuel by means of late injection, the same effect can be achieved with a somewhat reduced efficiency only on one cylinder group, i.e. the overstoichiometric exhaust gas flows as it comes from the engine in the first cylinder group and in the second cylinder group is λ _A «1 by late injection.

In order to avoid thermal overloading of a pre-catalytic converter, the operating mode can be changed between the cylinder groups at short intervals

Advantageous developments and improvements of the invention can be found in the subclaims.

Exemplary embodiments of the invention are shown in the drawings and are explained in more detail in the description below.

Show it-

Fig. 1 is a schematic representation of an exhaust system of an internal combustion engine with direct injection as the first exemplary embodiment of the present

Invention, Fig. La shows the temperature increase with a variation of the λ value in the first exemplary embodiment to illustrate the inventive

2, a schematic representation of an exhaust system of an internal combustion engine with direct injection as the second exemplary embodiment of the present

Invention, Fig. 2a shows the temperature increase with a variation of the λ value in the second exemplary embodiment to illustrate the inventive

3, a schematic representation of an exhaust system of an internal combustion engine with direct injection as the third exemplary embodiment of the present

Invention, Fig. 4 is a schematic representation of an exhaust system of an internal combustion engine with direct injection as the fourth embodiment of the present

Invention, Fig. 5 is a schematic representation of an exhaust system of an internal combustion engine with direct injection as the fifth exemplary embodiment of the present

Invention, Fig. 6 is a schematic representation of an exhaust system of an internal combustion engine with direct injection as the sixth exemplary embodiment of the present invention, Fig. 7 is a schematic representation of an exhaust system of an internal combustion engine with direct injection as the seventh exemplary embodiment of the present invention, Fig. 8 is a schematic representation of an exhaust system an internal combustion engine with direct injection as the eighth exemplary embodiment of the present

Invention and 9 shows a schematic illustration of an exhaust system of an internal combustion engine with direct injection as the ninth exemplary embodiment of the present invention.

In the figures, identical reference symbols designate identical or functionally identical elements.

1 shows a schematic illustration of an exhaust system of an internal combustion engine with direct injection as the first exemplary embodiment of the present invention.

In Fig. 1, 1 denotes an internal combustion engine with four cylinders A to D, 2 an air filter, 20 intake lines and 40 swirl flaps for turbulence generation for the respective cylinders A to D, 10 an exhaust line, 4 an NO _x storage catalytic converter and 4a a downstream three-way Catalyst and 6 a rear silencer.

In this first embodiment, the desulfation mode is carried out approximately every 70 hours or every 2000 km. This can be achieved by using a suitable timer circuit. To assess the need for desulfation, the NO _x storage behavior can also be detected using a NO _x sensor downstream of the NO _x storage catalytic converter to be monitored. Internal model calculations can be used as a rating.

In the disinfection mode, a suitable control of the direct injection causes a late injection into the exhaust gas during the push-out cycle.

When controlling the mixture composition for the main combustion, such an excess of oxygen is provided in the exhaust gas that afterburning for desulfation takes place in the NO _x storage catalyst 4, in which NO _x storage catalyst 4 a temperature of at least 600 ° C and a λ value <0.98 prevails.

FIG. 1 a shows a representation of the temperature increase when the λ value is varied in the first embodiment to illustrate the method according to the invention.

The λ _M value is plotted on the x-axis and the associated temperature increase on the y-axis. Line G shows an example of an operating point with a speed of 2000 rpm and a specific work w _p of 0.2 kJ / dm ³ .

In normal operation, λ _M = λ _A = λ _κ = 3.2, as a result of which an exhaust gas temperature T _A in the exhaust manifold of 320 ° C. and a catalyst temperature T _κ of 250 ° C. is achieved.

The Zιel-λ _κ or Zιel-λ _A for desulfation is 0.98 and the target catalyst temperature T _κ at 700 ° C.

These target values are achieved by λ _M = 1, 8 and one by

Late injection caused λ _κ change Δλ, which causes a catalyst temperature increase ΔT of 380 K.

In this case, in the desulfation mode for the main combustion in the combustion chamber, an exactly lean mixture composition is provided such that the calorific value of the post-injection for setting the λ _κ to 0.98 together with the oxygen contained in the exhaust gas expelled in the post-combustion in the NO _x storage catalyst 4 is accurate the desulfation temperature is set to> 600 ° C. 2 shows a schematic representation of an exhaust system of an internal combustion engine with direct injection as a second exemplary embodiment of the present invention.

In this exemplary embodiment, in contrast to the first exemplary embodiment, a respective three-way pre-catalyst 3a, 3b is provided in the exhaust pipe branch of the cylinders B, C and A, D, respectively. Furthermore, a secondary air pump 7 is connected downstream of the pre-catalytic converters 3a, 3b after the point of union of the two exhaust gas line branches via a secondary air line 30

The additional fuel required to heat the NO _x storage catalytic converter 4 is added to the exhaust gas, as in the first exemplary embodiment. Through a suitable control or regulation of the main combustion with excess fuel, i.e. rich mixture, required for power generation, however, the exhaust gas contains only a little oxygen (0 ₂ ) here, so that the fuel subsequently added with late injection without the respective pre-catalyst or starting catalyst 3a, 3b significant chemical reaction can happen

The oxygen required to release the calorific value in the NO _x storage catalytic converter 4 is only added to the exhaust gas after the starting catalytic converters 3a, 3b by means of the secondary air pump 7, so that the desired efficiency-optimized heating effect of the NO _x storage catalytic converter 4 is exactly the same as in the previously described case with only one NO _x -Speed catalyst enters.

2a shows a representation of the temperature increase in the case of a variation of the λ value in the second embodiment to illustrate the method according to the invention The λ _M value is in turn plotted on the x-axis and the associated temperature increase is plotted on the y-axis. Straight line G also shows the operating point with a speed of 2000 rpm and a specific work w _e of 0.2 kJ / dm ³ .

In normal operation, as in the previous example, λ _M = λ _A = λ _κ = 3.2, as a result of which an exhaust gas temperature T _A of 320 ° C. and a catalyst temperature T _κ of 250 ° C. are achieved.

These target values are achieved by λ _M = 1, 0, λ _A = 0.82 and λ _κ = 0.98, a λ _κ change Aλ _v caused by a late injection and a λ _A change Δλ ₂ caused by the secondary air addition a catalyst temperature increase ΔT of 380 K is effected.

3 shows a schematic representation of an exhaust system of an internal combustion engine with direct injection as a third exemplary embodiment of the present invention.

In this third exemplary embodiment, two monoliths 4, 4 'arranged one behind the other form the NO _x storage catalyst. Here, a secondary air supply between the two monoliths 4, 4 ', which can be controlled by means of the clock valves 8a or 8b, creates the possibility of releasing the calorific value by adding the secondary air from the secondary air pump 7 through the secondary air line branches 30a or 30b from upstream of the first monolith 4 to downstream of the first monolith 4, ie immediately before the second monolith 4 '. 4 shows a schematic illustration of an exhaust system of an internal combustion engine with direct injection as the fourth exemplary embodiment of the present invention.

In contrast to the third exemplary embodiment, in this fourth exemplary embodiment the two clock valves 8a and 8b are replaced by a single changeover valve 9.

5 shows a schematic illustration of an exhaust system of an internal combustion engine with direct injection as a fifth exemplary embodiment of the present invention.

In this fifth exemplary embodiment, in addition to the reference numerals 5 already introduced, designate an exhaust gas heat exchanger enveloping the NO _x storage catalytic converters, 50 a partition wall and 49 an inlet opening above the second monolith 4 '. The exhaust gas heat exchanger 5 is connected by a further secondary air line 30c to an inlet in the exhaust line above the first monolith 4.

Here, in order to reduce the fuel consumption to the lowest possible value, the secondary air is preheated by transferring part of the calorific value released to the secondary air downstream of the catalysts 4, 4 ′ to be regenerated from the exhaust gas stream through the exhaust gas heat exchanger 5.

In this exemplary embodiment, too, the clock valves 8a and 8b, which offer an alternative access to the two chambers of the exhaust gas heat exchanger 5 separated by the partition 50, provide a choice for the detoxification of the monoliths 4 and 4 '. 6 shows a schematic illustration of an exhaust system of an internal combustion engine with direct injection as the sixth exemplary embodiment of the present invention.

In contrast to the fifth exemplary embodiment, the two clock valves 8a and 8b are replaced by a single changeover valve 9 in this sixth exemplary embodiment.

7 shows a schematic illustration of an exhaust system of an internal combustion engine with direct injection as the seventh exemplary embodiment of the present invention.

In addition to the reference symbols 5 'already introduced, FIG. 7 denotes a further exhaust gas heat exchanger which is arranged downstream of the two monoliths 4, 4'. A secondary air line 30d leads from it into the first exhaust gas heat exchanger 5. A second partition 51 is provided therein, the inlet 49 now being in the chamber defined between the first and second partition 50 and 51, respectively. The changeover valve 9 and the further secondary air lines 30e, 30f, 30g also provide a choice for the detoxification of the monoliths 4 and 4 '.

8 shows a schematic illustration of an exhaust system of an internal combustion engine with direct injection as the eighth exemplary embodiment of the present invention.

In contrast to the previous seventh exemplary embodiment, it is possible in this eighth exemplary embodiment by means of additional clock valves 8c and 8d to have the addition of secondary air either preheated or unheated. The unheated case is an excellent means, the NO storage catalyst after to cool down again after a desulfation in order to return to consumption-optimized lean operation as quickly as possible, which is only possible for emission reasons after the catalyst temperature has been reduced to approximately 400 ° C. or below. Until then, operation with λ _M = 1 is required.

FIG. 9 shows a schematic illustration of an exhaust system of an internal combustion engine with direct injection as the ninth exemplary embodiment of the present invention.

This ninth exemplary embodiment takes advantage of the fact that the internal combustion engine 1 has a first (A, D) and a second (B, C) cylinder group, the exhaust gases of which are brought together at a point upstream of the NO _x storage catalytic converter, with each cylinder group upstream in the exhaust branch Point a corresponding pre-catalyst 3a, 3b is provided, in which no post-combustion has to take place.

In one cylinder group (A, D), a first mixture composition with λ _M <1 is provided for the main combustion in the combustion chamber and a late injection into the exhaust gas of these cylinders is carried out during the push-out cycle of the internal combustion engine 1. Therefore, the oxygen content of the exhaust gas pushed out is not sufficient for a significant post-combustion in the pre-catalyst 3b. In this group of cylinders, the air-fuel ratio of the exhaust gas is therefore substochiometric, ie oxygen is missing for post-combustion in the pre-catalyst.

In the other cylinder group (B, C), a first mixture composition with λ _M > 1 is provided for the main combustion in the combustion chamber and no late injection into the exhaust gas of these cylinders is carried out during the push-out cycle of the internal combustion engine 1 Therefore, the exhaust gas is oxygen-rich, ie λ _A > 1, and the oxygen content of the exhaust gas pushed out is also not sufficient for a significant post-combustion in the pre-catalyst 3a. The air / fuel ratio in this cylinder group is therefore superstoichiometric, ie fuel is missing for post-combustion in the pre-catalytic converter.

The first and the second mixture composition are dimensioned such that the

Oxygen content of the combined exhaust gas of the first and second

Cylinder group for the afterburning in the NO _x storage catalyst 4 the desulphation λ <0.98 is reached and the temperature of the catalyst is set to> 600 ° C.

Although the present invention has been described above on the basis of preferred exemplary embodiments, it is not limited to this but can be modified in a variety of ways.

The methods described above can be further optimized by dividing the process into two phases. A first phase consists in heating the NO _x storage catalyst, the actual catalyst temperature being below the ideal desulfation temperature. In this phase it is advantageous if as much heating energy as possible is added to the catalyst. After reaching the ideal desulfation temperature, the second phase is initiated, in which the ideal desulfation temperature is only maintained. The implementation of the first phase is associated with the risk of overheating for the NO _x storage catalyst. It is therefore proposed to change the phase with the aid of a temperature measurement

To control the catalyst temperature, which in many cases will be required for onboard diagnostic purposes in the future anyway. The same temperature measurement can be used in the second phase to regulate the fuel addition. This allows the greatest possible efficiency to be combined with optimal operational safety. Of course, the invention is not limited to a four-cylinder internal combustion engine, but can be applied to any number of cylinders.

The downstream catalytic converter downstream of the NO _x storage catalytic converter can also be a conventional three-way catalytic converter.

Claims

claims

1. A method for desulfating a NO _x storage catalyst in the exhaust system of an internal combustion engine with direct injection, comprising the steps:

^■ a transit through a Spateinspritzung in the exhaust gas during the Aussschiebetaktes of the internal combustion engine in a Desulfatisierungsmodus and

^■ Providing such an excess of oxygen in the exhaust gas in the desulfation mode that afterburning takes place in the NO "storage catalyst for desulfation.

2. The method according to claim 1, characterized in that in the desulfation mode for the main combustion in the combustion chamber such a lean mixture composition is provided that the oxygen content of the exhaust gas exhausted is sufficient for the afterburning in the NO _x storage catalyst.

3. The method according to claim 1, characterized in that in the desulfation mode, the oxygen content necessary for the afterburning is achieved by a subsequent addition of oxygen into the exhaust gas pushed out.

4. The method according to claim 3, wherein the internal combustion engine in the exhaust system upstream of the NO _x storage catalyst has at least one further catalyst in which no post-combustion has to take place, characterized in that such a rich mixture composition is provided in the desulfation mode for the main combustion in the combustion chamber, that the oxygen content of the exhaust gas pushed out is not sufficient for the afterburning in the further catalyst and the subsequent addition of oxygen to the exhaust gas takes place between the further catalyst and the NO _x storage catalyst.

5. The method of claim 3, wherein the internal combustion engine in the exhaust system at least two hmtereinandergeschaltete NO _x -Speιcherkatalysatoren, characterized in that is provided in the Desulfatisierungsmodus for main combustion in the combustion chamber such a rich mixture composition that the oxygen content of the pushed-out exhaust gas for post-combustion in a Pre-catalytic converter is not sufficient and the subsequent addition of oxygen into the exhaust gas takes place either upstream of the near-engine NO _x storage catalytic converter or between the two NO _x storage catalytic converters.

6. The method according to any one of claims 3 to 5, characterized in that the subsequent addition of oxygen is carried out by adding preheated air, which is preferably taken from the part of the exhaust system located downstream of the NO _x storage catalyst.

7. The method according to any one of the preceding claims, characterized in that the afterburning for desulfating is carried out in a first and second phase, the NO _x storage catalyst in the first phase being brought to a temperature required for desulfating, preferably above 600 ° C and maintained at temperature in the second phase for a predetermined period of time.

8. The method according to claim 1 or 2, wherein the internal combustion engine has a first and a second cylinder group, the exhaust gases of which are brought together at a point upstream of the NO _x storage catalytic converter, wherein in the exhaust branch of each cylinder group upstream of the point in each case at least one further catalytic converter is provided, in which no post-combustion has to take place, characterized in that in the desulfation mode in the case of one cylinder group for the main combustion in the combustion chamber, such a first mixture composition it is provided that the oxygen content of the exhaust gas pushed out is not sufficient for the afterburning in the further catalytic converter and a late injection into the exhaust gas is carried out during the push-out cycle of the internal combustion engine and in the other cylinder group for the main combustion in the combustion chamber, such a second one, preferably different from the first one Mixture composition is provided that the fuel content of the exhaust gas pushed out is not sufficient for the afterburning in the further catalyst and no late injection into the exhaust gas is carried out during the push-out cycle of the internal combustion engine, the first and the second mixture composition being dimensioned such that the oxygen content of the combined exhaust gas first and second cylinder group is sufficient for the afterburning.

9. The method according to any one of the preceding claims, characterized in that the desulfating mode is carried out approximately every 70 hours or every 2000 km, in particular after assessing the need for desulfating using a NO _x sensor downstream of the NO _x storage catalyst to be monitored

10 The method according to any one of the preceding claims, characterized in that in the desulfation mode in the afterburning in the NO _x storage catalyst, a temperature greater than 600 ° C and a λ value <0.98 prevail.

11 Device for desulfating a NO _x storage catalyst in the exhaust system of an internal combustion engine with direct injection with:

a direct injection control device for initiating a late injection into the exhaust gas during the push-out cycle of the internal combustion engine in a desulfation mode, and ^■ Mixture composition control device for providing such an excess of oxygen in the exhaust gas in the desulfation mode that afterburning takes place in the NO _x storage catalyst for desulfation.

12. The apparatus according to claim 11, characterized in that the mixture composition control device is designed such that it provides such a lean mixture composition for the main combustion in the combustion chamber that the oxygen content of the exhaust gas exhausted is sufficient for the afterburning.

13. The apparatus according to claim 11, characterized in that the mixture composition Steuereinπchtung comprises a secondary air pump, by means of which the oxygen content necessary for the afterburning can be achieved by a subsequent addition of oxygen into the exhaust gas pushed out.

14. The apparatus of claim 13, wherein the internal combustion engine in the exhaust system upstream of the NO _x storage catalyst has at least one further catalyst in which no post-combustion has to take place, characterized in that the mixture composition control device is designed such that it is used for the main combustion in the combustion chamber such a rich mixture composition provides that the oxygen content of the exhaust gas pushed out is not sufficient for the afterburning in the further catalytic converter and the subsequent addition of oxygen by the secondary air pump into the exhaust gas takes place between the further catalytic converter and the NO storage catalytic converter.

15. The apparatus according to claim 13, wherein the internal combustion engine in the exhaust system has at least two hermteremandergierter NO _x storage catalysts, characterized in that the mixture composition Steueremπchtung is designed such that it is such a fat for the main combustion in the combustion chamber Mixture composition provides that the oxygen content of the exhaust gas pushed out is not sufficient for the afterburning in a pre-catalytic converter and the subsequent addition of oxygen into the exhaust gas by means of the secondary air pump can be controlled via a valve device optionally upstream of the engine

NO _x storage catalyst or between the two NO _x storage catalysts.

16. The device according to one of claims 13 to 15, characterized in that the subsequent addition of oxygen is carried out by adding air preheated by a heat exchanger, which is preferably removed from the part of the exhaust system located downstream of the NO _x storage catalyst.

17. Device according to one of the preceding claims 11 to 16, characterized in that the direct injection control device and the mixture composition Steuereinπchtung are designed such that the post-combustion for desulfurization can be carried out in a first and second phase, the NO _x storage catalyst in the first phase can be brought to a temperature required for desulfation of preferably above 600 ° C. and in the second phase can be kept at the temperature for a predetermined period of time.

18. The apparatus of claim 11 or 12, wherein the internal combustion engine has a first and a second cylinder group, the exhaust gases are brought together at a point upstream of the NO _x storage catalytic converter, wherein at least one further catalyst is provided in the exhaust branch of each cylinder group upstream of the point, in which no post-combustion has to take place, characterized in that the mixture composition control device is designed such that in one cylinder group for the main combustion in the combustion chamber such a first mixture composition can be provided that the oxygen content of the exhaust gas pushed out is not sufficient for the afterburning in the further catalytic converter and a late injection into the exhaust gas can be carried out during the push-out cycle of the internal combustion engine and, in the other cylinder group for the main combustion in the combustion chamber, such a second, preferably from the first mixture composition that can be provided, the fuel content of the exhaust gas pushed out is not sufficient for the afterburning in the further catalyst and no late injection into the exhaust gas can be carried out during the push-out cycle of the internal combustion engine, the first and the second mixture composition being dimensioned such that the oxygen content of the combined exhaust gas of the first and second cylinder groups is sufficient for the afterburning.

19. Device according to one of the preceding claims 11 to 18, characterized in that a timer device is provided which has the effect that the desulfating mode can be carried out approximately every 70 hours or every 2000 km

20. Device according to one of the preceding claims 11 to 19, characterized in that a NO _x sensor for assessing the need for desulphation is provided downstream of the NO _x storage catalyst to be monitored.

21. Device according to one of the preceding claims, characterized in that the direct inspection control device and the

Mixture composition Steuereinπchtung are designed such that in the

Desulfation mode in the afterburning in the NO _x storage catalytic converter a temperature greater than 600 ° C and a λ value <0.98 prevail