DE102019107544A1 - Method for operating an exhaust aftertreatment system and an exhaust aftertreatment system - Google Patents

Abstract

The invention relates to a method for operating an exhaust gas aftertreatment system for an internal combustion engine (10), the internal combustion engine (10) having its outlet (16) connected to an exhaust system (20) in which, in the flow direction of an exhaust gas from the internal combustion engine (10), an exhaust gas system (10) is close to the engine first SCR catalyst (28, 30) and downstream of the first SCR catalyst (28, 30) a second SCR catalyst (32) are arranged. A first metering element (36) is arranged upstream of the first SCR catalytic converter (28, 30), with which a reducing agent (48) can be metered into the exhaust gas duct (22) upstream of the first SCR catalytic converter (28, 30). The method comprises the following steps: - determining a component temperature of the second SCR catalytic converter (32) and / or an exhaust gas temperature (TEG) on the second SCR catalytic converter (32), - comparison of the determined component temperature or the determined exhaust gas temperature with a respective threshold temperature (TS1 , TS2), - determining an ammonia level on the second SCR catalytic converter (32); - lowering the ammonia level of the first SCR catalytic converter (28, 30) when the ammonia level on the second SCR catalytic converter (32 ) exceeds a critical load condition and the component temperature or the exhaust gas temperature (TEG) is above the respective threshold value (TS1, TS2), - conversion of the nitrogen oxides in the exhaust gas flow of the internal combustion engine (10) by the second SCR catalytic converter (32), whereby the level of the second SCR catalytic converter (32) is reduced. The invention also relates to an exhaust gas aftertreatment system for carrying out such a method.

Description

The invention relates to a method for operating an exhaust gas aftertreatment system with at least two SCR catalytic converters connected in series, as well as an exhaust gas aftertreatment system for carrying out such a method according to the preamble of the independent claim.

The current exhaust gas legislation, and one that will become increasingly strict in the future, place high demands on the engine-related raw emissions and the exhaust gas aftertreatment of internal combustion engines. The demands for a further decrease in consumption and the further tightening of the exhaust gas standards with regard to the permissible nitrogen oxide emissions represent a challenge for the engine developers. In gasoline engines, exhaust gas cleaning is carried out in the known manner using a three-way catalytic converter and a three-way catalytic converter - and further downstream catalysts. Exhaust aftertreatment systems are currently used in diesel engines which have an oxidation catalytic converter or a NOx storage catalytic converter, a catalytic converter for the selective catalytic reduction of nitrogen oxides (SCR catalytic converter) and a particle filter for separating out soot particles and, if necessary, further catalytic converters. Ammonia is preferably used as the reducing agent. Because dealing with pure ammonia is complex, a synthetic, aqueous urea solution is usually used in vehicles, which is mixed with the hot exhaust gas flow in a mixing device upstream of the SCR catalytic converter. As a result of this mixing, the aqueous urea solution is heated, with the aqueous urea solution releasing ammonia in the exhaust gas duct. A commercially available, aqueous urea solution generally consists of 32.5% urea and 67.5% water.

Exhaust gas aftertreatment systems are known from the prior art which have a first SCR catalytic converter close to the engine, in particular a particulate filter with an SCR coating, and a second SCR catalytic converter which is arranged in an underbody position of the motor vehicle remote from the engine. The close-coupled particle filter with the SCR coating can be heated up to its operating temperature more quickly after a cold start of the internal combustion engine and can thus be used for an efficient conversion of the nitrogen oxide emissions shortly after a cold start of the internal combustion engine. The second SCR catalytic converter remote from the engine is used at high engine loads and / or during regeneration of the particulate filter, in which the close-coupled particulate filter with the SCR coating is operated above the temperature range required for selective, catalytic reduction and thus only for insufficient conversion of nitrogen oxides or to provide an additional catalyst volume for the selective catalytic reduction of nitrogen oxides at high engine loads. The arrangement of the particulate filter with the SCR coating close to the engine also means that there is only a short mixing section for mixing the reducing agent and exhaust gas before entering the particulate filter, thus limiting the uniform distribution over the cross section of the particulate filter. This effect is intensified in the case of high exhaust gas mass flows and the associated high flow velocities in the exhaust gas duct. This can mean that, regardless of the temperature, the conversion capacity of the particulate filter is insufficient and the conversion capacity of the second SCR catalytic converter is also necessary.

It is also known that the conversion performance of the two SCR catalytic converters can be increased if a defined amount of ammonia is stored on the catalytically active surface of the respective SCR catalytic converter. Depending on the current operating state of the internal combustion engine, reducing agent is metered into the exhaust gas duct upstream of the coated particle filter, upstream of the SCR catalytic converter in the underbody position or upstream of both SCR catalytic converters.

The disadvantage of the method known from the prior art, however, is that the loading of the second SCR catalytic converter can lead to ammonia slip due to changing operating conditions of the internal combustion engine. Therefore, when the second SCR catalytic converter is loaded, only a comparatively low load is set, which, however, restricts the conversion performance of the second SCR catalytic converter.

From the DE 10 2015 221 982 A1 An exhaust aftertreatment system for a diesel engine is known in which an oxidation catalyst is arranged in the exhaust system, a particle filter with an SCR coating is arranged downstream of the oxidation catalyst and a second SCR catalyst is arranged downstream of the particle filter with the SCR coating. To inject liquid reducing agent for the SCR catalytic converter and / or for the SCR-coated particle filter, a first injection position is upstream of the SCR-coated particle filter in the form of a first metering device and a second injection position is upstream of the SCR catalytic converter and downstream of the SCR-coated particle filter provided in the form of a second metering device. The injection positions for the injection of liquid reducing agent are selected depending on the operating states of the SCR catalytic converter system.

The invention is based on the object of being able to monitor the fill level of the SCR catalytic converters in an exhaust gas aftertreatment system with several SCR catalytic converters and thus to enable the most efficient possible conversion of the nitrogen oxides in the exhaust gas.

• - Ermitteln einer Bauteiltemperatur des zweiten SCR-Katalysators und/oder einer Abgastemperatur am zweiten SCR-Katalysator;
• - Vergleichen der ermittelten Bauteiltemperatur oder der ermittelten Abgastemperatur mit einer jeweiligen Schwellentemperatur;
• - Ermitteln des Ammoniak-Füllstands auf dem zweiten SCR-Katalysator;
• - Absenken des Ammoniak-Füllstands des ersten SCR-Katalysators, wenn der Ammoniak-Füllstand des zweiten SCR-Katalysators eine kritische Beladung übersteigt und die Bauteiltemperatur oder die Abgastemperatur oberhalb des jeweiligen Schwellenwertes liegt,
• - Konvertieren der Stickoxide im Abgasstrom des Verbrennungsmotors durch den zweiten SCR-Katalysator, wodurch der Füllstand des zweiten SCR-Katalysators reduziert wird.

According to the invention, this object is achieved by a method for operating an exhaust gas aftertreatment system for an internal combustion engine, the outlet of which is connected to an exhaust system in which, in the flow direction of an exhaust gas of the internal combustion engine, a first SCR catalyst close to the engine and a second SCR downstream of the first SCR catalyst close to the engine -Catalyst is arranged, dissolved. A first metering element is arranged upstream of the first SCR catalytic converter, with which a reducing agent, in particular aqueous urea solution, can be metered into the exhaust gas duct upstream of the first SCR catalytic converter. A second metering element can be arranged downstream of the first SCR catalytic converter and upstream of the second SCR catalytic converter, with which the reducing agent can be metered into the exhaust gas duct downstream of the first SCR catalytic converter and upstream of the second SCR catalytic converter. The procedure consists of the following steps:

• Determining a component temperature of the second SCR catalytic converter and / or an exhaust gas temperature on the second SCR catalytic converter;
• - Comparing the determined component temperature or the determined exhaust gas temperature with a respective threshold temperature;
• - Determining the ammonia level on the second SCR catalytic converter;
• - Lowering the ammonia fill level of the first SCR catalytic converter if the ammonia fill level of the second SCR catalytic converter exceeds a critical load and the component temperature or the exhaust gas temperature is above the respective threshold value,
• - Converting the nitrogen oxides in the exhaust gas flow of the internal combustion engine through the second SCR catalytic converter, whereby the fill level of the second SCR catalytic converter is reduced.

A first SCR catalytic converter close to the engine is to be understood in this context as an SCR catalytic converter whose inlet is spaced from the outlet of the internal combustion engine with an exhaust gas run length of a maximum of 80 cm, preferably a maximum of 50 cm. A method according to the invention makes it possible to control the fill level of the SCR catalytic converters and to prevent ammonia breakthrough through the second SCR catalytic converter in a reliable manner. Furthermore, the proposed method enables a higher loading of the SCR catalytic converters with ammonia, as a result of which the conversion performance with regard to nitrogen oxide emissions is improved. In addition, the reducing agent consumption can be reduced, since no reducing agent is lost as ammonia slip through the second SCR catalytic converter.

The features listed in the dependent claims enable advantageous improvements and non-trivial further developments of the method listed in the independent claim for operating an exhaust gas aftertreatment system for an internal combustion engine.

In a preferred embodiment of the invention it is provided that alternating thermal loading of the first SCR catalyst leads to a release of ammonia, the released ammonia being stored in the second SCR catalyst and increasing the fill level of the second SCR catalyst. As a result, the second SCR catalytic converter can also be loaded without an additional metering in of reducing agent by the second metering element. An overcharging of the second SCR catalytic converter with ammonia is reliably prevented by a method according to the invention, since the ammonia reservoir is emptied in good time due to the consumption of reducing agent during the selective catalytic reduction.

Another improvement of the method provides that a model for the ammonia storage capacity of the first SCR catalytic converter and the second SCR catalytic converter is stored in an engine control unit of the internal combustion engine, the loading of the ammonia store of the second SCR catalytic converter from the determined ammonia slip is calculated via the first SCR catalytic converter and the amount of the reducing agent metered in by the second metering element. By adapting the model, the amount of reducing agent metered in can be adapted in order to avoid an ammonia breakthrough through the second SCR catalytic converter and thus an increase in tailpipe emissions. Alternatively, the loading and unloading of the ammonia storage of the SCR catalytic converters can also be monitored by NOx sensors, which are cross-sensitive to ammonia. In particular, an ammonia breakthrough through the first SCR catalytic converter can be detected, which leads to a loading of the ammonia reservoir of the second SCR catalytic converter.

In an advantageous improvement of the method, it is provided that the first SCR catalytic converter close to the engine in normal operation from an ammonia level of 60% - 100% of the maximum ammonia storage capacity and the second SCR catalytic converter to an ammonia level of 50% - 90 %, preferably from 60% to 80% are conditioned. This makes it possible to ensure that the nitrogen oxides are efficiently converted, with an ammonia breakthrough through the second SCR catalytic converter being reliably prevented.

In a preferred embodiment of the method it is provided that the lowering of the ammonia fill level of the first SCR catalytic converter takes place by a regeneration of a particle filter. The first SCR catalytic converter itself is preferably designed as a particle filter with a coating for the selective, catalytic reduction of nitrogen oxides. Alternatively, a particle filter can also be connected upstream or downstream of the first SCR catalytic converter. When the particle filter is regenerated, a temperature level is reached at the first SCR catalytic converter which leads to thermal desorption of ammonia stored in the first SCR catalytic converter. This ammonia is stored in the second SCR catalytic converter, which increases its fill level. In order to lower the fill level of the second SCR catalytic converter after a regeneration, it makes sense to forego metering in reducing agent and to first use the ammonia stored in the second SCR catalytic converter to convert the nitrogen oxides in order to increase the fill level of the second SCR catalytic converter to reduce.

In an advantageous embodiment of the method, it is provided that the threshold value for the component temperature of the second SCR catalytic converter is 250.degree. From this temperature an efficient conversion of the nitrogen oxide emissions by the second SCR catalytic converter can be expected. This ensures that the light-off temperature required for the selective catalytic reduction of nitrogen oxides is reached on the second SCR catalytic converter.

According to the invention, an exhaust gas aftertreatment system for an internal combustion engine, which is connected with its outlet to an exhaust system, is proposed in which a first SCR catalyst close to the engine and downstream of the first SCR catalyst, preferably in an underbody position of a motor vehicle, in the flow direction of an exhaust gas of the internal combustion engine second SCR catalyst are arranged. A first metering element is arranged upstream of the first SCR catalytic converter, with which a reducing agent can be metered into the exhaust gas duct upstream of the first SCR catalytic converter. A second metering element can be arranged downstream of the first SCR catalytic converter and upstream of the second SCR catalytic converter, with which the reducing agent can be metered into the exhaust gas duct downstream of the first SCR catalytic converter and upstream of the second SCR catalytic converter. Alternatively, the second SCR catalytic converter can also be loaded with ammonia by overdosing the reducing agent through the first metering element. The exhaust gas aftertreatment system can be operated via an engine control unit which is set up to carry out a method according to the invention when a machine-readable program code is executed by the engine control unit. An exhaust gas aftertreatment system according to the invention makes it possible to minimize nitrogen oxide emissions from the internal combustion engine and to optimize the use of reducing agents. In particular, the exhaust gas aftertreatment system according to the invention can optimize the loading state of the SCR catalytic converters, in particular of the second SCR catalytic converter, depending on the operating situation of the internal combustion engine in order to avoid an overdose of ammonia and to ensure an efficient conversion of nitrogen oxides.

In a preferred embodiment of the exhaust gas aftertreatment system it is provided that the second SCR catalytic converter has an ammonia barrier catalytic converter or an ammonia barrier catalytic converter is connected downstream of the second SCR catalytic converter. An ammonia barrier catalytic converter can prevent an overdosing of reducing agent from increasing the ammonia tailpipe emissions. In this way, it can be achieved in an operationally reliable manner that even in the event of ammonia slip through the first SCR catalytic converter, in particular a particle filter with an SCR coating, the ammonia present in the exhaust gas duct downstream of the first SCR catalytic converter is converted.

In a preferred embodiment of the exhaust gas aftertreatment system it is provided that a turbine of an exhaust gas turbocharger is arranged in the exhaust system, an oxidation catalytic converter or a NOx storage catalytic converter being arranged downstream of the turbine of the exhaust gas turbocharger and upstream of the first metering element. The ratio of nitrogen monoxide and nitrogen dioxide can be changed by means of an oxidation catalytic converter or a NOx storage catalytic converter which has an oxidation component, whereby the efficiency of the selective catalytic reduction of nitrogen oxides can be increased.

In a further improvement of the exhaust gas aftertreatment system, it is provided that the first SCR catalytic converter is designed as a particle filter with an SCR coating, with a low-pressure exhaust gas recirculation branching off from the exhaust gas duct downstream of the particle filter and upstream of the second SCR catalytic converter. The particle filter allows soot particles and other solid particles to be filtered out of the exhaust gas flow so that they do not pass through the Low-pressure exhaust gas recirculation are fed back into the air supply system and can lead to damage there, in particular to damage to the compressor of the exhaust gas turbocharger. In addition, exhaust gas recirculation via the low-pressure exhaust gas recirculation can minimize the raw nitrogen oxide emissions of the internal combustion engine in a known manner, whereby the use of reducing agents in exhaust gas aftertreatment can also be reduced.

The various embodiments of the invention mentioned in this application can be advantageously combined with one another, unless stated otherwise in the individual case.

• 1 einen Verbrennungsmotor mit einem erfindungsgemäßen Abgasnachbehandlungssystem; und
• 2 ein Ablaufdiagramm zur Durchführung eines erfindungsgemäßen Verfahrens zum Betreiben eines Abgasnachbehandlungssystems mit mindestens zwei SC R - Katalysatoren.

The invention is explained below in exemplary embodiments with reference to the accompanying drawings. The same components or components with the same function are identified with the same reference numbers in the different figures. Show it:

• 1 an internal combustion engine with an exhaust gas aftertreatment system according to the invention; and
• 2 a flowchart for performing a method according to the invention for operating an exhaust gas aftertreatment system with at least two SC R catalytic converters.

1 shows the schematic representation of an internal combustion engine 10 . The internal combustion engine 10 is designed as a direct injection diesel engine and has several combustion chambers 12 on. At the combustion chambers 12 each is a fuel injector 14th for injecting a fuel into the respective combustion chamber 12 arranged. The internal combustion engine 10 is with its outlet 16 with an exhaust system 20th connected. The internal combustion engine 10 can also have a high-pressure exhaust gas recirculation with a high-pressure exhaust gas recirculation valve, via which an exhaust gas of the internal combustion engine 10 from the outlet 16 can be returned to the inlet. At the combustion chambers 12 inlet valves and outlet valves are arranged with which a fluidic connection to the combustion chambers 12 or from the combustion chambers 12 to the exhaust system 20th can be opened or closed.

The exhaust system 20th includes an exhaust duct 22nd , in which in the flow direction of an exhaust gas of the internal combustion engine 10 through the exhaust duct 22nd a turbine 24 of an exhaust gas turbocharger 18th is arranged, which drives a compressor, not shown, in the intake tract via a shaft. The exhaust gas turbocharger 18th is preferably used as an exhaust gas turbocharger 18th designed with variable turbine geometry. To do this are a turbine wheel of the turbine 24 adjustable guide vanes are connected upstream, via which the flow of the exhaust gas onto the blades of the turbine 24 can be varied. Downstream of the turbine 24 are several exhaust aftertreatment components 26th , 28 , 30th , 32 , 58 intended. This is immediately downstream of the turbine 24 the first component of exhaust gas aftertreatment is an oxidation catalytic converter 26th or a NOx storage catalytic converter 58 is arranged. Downstream of the oxidation catalyst 26th or of the NOx storage catalytic converter 58 is a first SCR catalytic converter close to the engine 28 , 30th , especially a particulate filter 30th with a coating for the selective catalytic reduction of nitrogen oxides (SCR coating). Downstream of the first SCR catalytic converter close to the engine 28 , 30th is preferably a second SCR catalytic converter in the underbody position of a motor vehicle 32 in the exhaust duct 22nd arranged. The second SCR catalytic converter 32 can use an oxidation catalytic converter, in particular an ammonia barrier catalytic converter 34 exhibit. Alternatively, the second SCR catalytic converter 32 an oxidation catalyst or an ammonia barrier catalyst 34 be downstream. Downstream of the oxidation catalyst 26th or of the NOx storage catalytic converter 58 and upstream of the first SCR catalytic converter 28 , 30th is a first metering element 36 , in particular a water-cooled metering element, for metering in a reducing agent 48 in the exhaust duct 22nd intended. Downstream of the first SCR catalyst 28 , 30th , especially downstream of the particulate filter 30th With the SCR coating, an exhaust gas recirculation line branches off a low-pressure exhaust gas recirculation system from the exhaust gas duct 22nd from. Downstream of the junction and upstream of the second SCR catalyst 32 is a second metering element 38 , in particular an air-cooled or water-cooled metering element, arranged around the reducing agent 48 in the exhaust duct 22nd upstream of the second SCR catalyst 32 bring in. The first metering element 36 and the second metering element 38 are each via a reducing agent line 44 with a common reducing agent tank 46 connected in which the reducing agent 48 is in stock. It also includes the exhaust system 20th an exhaust flap with which the exhaust gas recirculation can be controlled via the low pressure exhaust gas recirculation.

In the exhaust system 20th can be downstream of the turbine 24 and upstream of the oxidation catalyst 26th or a NOx sensor 54 can be arranged in the NOx storage catalytic converter 58. Downstream of the first SCR catalyst 28 , 30th and a further NOx sensor 54 may be arranged upstream of the junction. Furthermore, downstream of the second SCR catalytic converter 32 another NOx sensor 54 can be arranged. The particle filter 30th a differential pressure sensor is assigned with which a pressure difference Δp across the particle filter 30th can be determined. In this way, the loading state of the particle filter 30th determined and regeneration of the particle filter when a defined load level is exceeded 30th be initiated. Also in the exhaust system 20th a temperature sensor 50 and if necessary. further exhaust gas sensors 52 provided to determine the exhaust gas temperature T _EG .

Downstream of the first metering element 36 and upstream of the first SCR catalyst 28 , 30th , especially the particulate filter 30th with the SCR coating, a first exhaust mixer can 40 be provided in order to mix the exhaust gas flow of the internal combustion engine 10 and reducing agents 48 before entering the first SCR catalytic converter 28 , 30th to improve and shorten the length of the mixing section. Downstream of the second metering element 38 and upstream of the second SCR catalyst 32 can use a second exhaust mixer 42 be arranged to the mixing of exhaust gas flow and reducing agent 48 to improve and the evaporation of the reducing agent 48 in the exhaust duct 22nd to support.

The internal combustion engine 10 is with an engine control unit 56 connected, which via signal lines, not shown, to the exhaust gas sensors 52 , NOx sensors 54, the differential pressure sensor, the temperature sensor 50 as well as with the fuel injectors 14th of the internal combustion engine 10 and the dosing elements 36 , 38 connected is.

In 2 a flow chart for performing a method according to the invention for operating such an exhaust gas aftertreatment system is shown. In a first method step <100>, a component temperature of the second SCR catalytic converter is 32 and ammonia loading of the second SCR catalyst 32 determined. Alternatively or additionally, an exhaust gas temperature T _EG can also be used at the second SCR catalytic converter 32 be determined. In a method step <110>, the determined component temperature and / or the exhaust gas temperature T _{EG is} compared with a respective threshold temperature T _S1 , T _S2 . Is the determined ammonia loading of the second SCR catalytic converter 32 Above a critical limit loading and component temperature or the exhaust gas temperature T _EG above the respective threshold temperature T _S1 , T _S2 , then in a method step <120> the loading of the ammonia store of the first SCR catalytic converter 28 , 30th lowered. As a result, nitrogen oxides get into the exhaust gas duct downstream of the first SCR catalytic converter 28 , 30th , since a complete conversion of the nitrogen oxides is no longer possible with the lowered ammonia level. In a method step <130>, the nitrogen oxides are mixed with that in the second SCR catalytic converter 32 stored ammonia implemented, whereby the level of the second SCR catalytic converter 32 reduced. In a method step <140>, a load model of the SCR catalytic converters 28 , 30th , 32 or the level of the second SCR catalytic converter by measuring the nitrogen oxides and / or ammonia in the exhaust system accordingly 32 determined. Is the ammonia storage of the second SCR catalytic converter 32 sufficiently emptied, so there is a risk of ammonia breakthrough through the second SCR catalytic converter 32 can be excluded, then in a method step <150> the loading of the first SCR catalytic converter 28 increased again with ammonia, including the reducing agent 48 through the first metering element 36 back into the exhaust duct 22nd is metered in until the ammonia reservoir of the first SCR catalytic converter is essentially completely charged 28 is reached. The ammonia storage of the first SCR catalytic converter is thereby used 28 , 30th conditioned to a level of 50% - 100%, preferably 80% - 100%, in order to ensure an efficient conversion of the nitrogen oxides in the exhaust gas flow.

List of reference symbols

10

Internal combustion engine

12th

Combustion chamber

14th

Fuel injector

16

Outlet

18th

Exhaust gas turbocharger

20th

Exhaust system

22nd

Exhaust duct

24

turbine

26th

Oxidation catalyst

28

first SCR catalytic converter

30th

Particle filter with SCR coating

32

second SCR catalytic converter

34

Ammonia barrier catalyst

36

first metering element

38

second metering element

40

first exhaust mixer

42

second exhaust mixer

44

Reducing agent line

46

Reducing agent tank

48

Reducing agent

50

Temperature sensor

52

Exhaust gas sensor

54

NOx sensor

56

Engine control unit

58

NOx storage catalytic converter

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 102015221982 A1 [0006]

Claims (10)

Method for operating an exhaust gas aftertreatment system of an internal combustion engine (10), the internal combustion engine (10) having its outlet (16) connected to an exhaust system (20) in which a first SCR catalytic converter (10) close to the engine is located in the flow direction of an exhaust gas of the internal combustion engine (10). 28, 30) and downstream of the first SCR catalyst (28, 30) a second SCR catalyst (32) are arranged, and upstream of the first SCR catalyst (28, 30) a first metering element (36) with which a Reducing agent (48) can be metered into the exhaust gas duct (22) upstream of the first SCR catalytic converter (28, 30), which comprises the following steps: determining a component temperature of the second SCR catalytic converter (32) and / or an exhaust gas temperature (T _EG ) on the second SCR catalyst (32); - Comparison of the determined component temperature or the determined exhaust gas temperature with a respective threshold temperature (T _S1 , T _S2 ); - Determining a fill level of the second SCR catalytic converter (32); - Lowering the ammonia fill level of the first SCR catalytic converter (28, 30) when the second SCR catalytic converter (32) exceeds a critical fill level and the component temperature or the exhaust gas temperature (T _EG ) is above the respective threshold value (T _S1 , T _S2 ) lies; - Conversion of the nitrogen oxides in the exhaust gas flow of the internal combustion engine (10) by the second SCR catalytic converter (32), whereby the fill level of the second SCR catalytic converter (32) is reduced. Method for operating an exhaust aftertreatment system according to Claim 1 , characterized in that alternating thermal loading of the first SCR catalyst (28, 30) leads to a release of ammonia, the released ammonia being stored in the second SCR catalyst (32) and the fill level of the second SCR catalyst (32 ) elevated. Process for exhaust aftertreatment according to Claim 1 or 2 , characterized in that a model for the ammonia storage capacity of the first SCR catalyst (28, 30) and the second SCR catalyst (32) is stored in an engine control unit (56) of the internal combustion engine (10), the loading of the ammonia storage of the second SCR catalytic converter (32) is calculated from the determined ammonia slip via the first SCR catalytic converter (28, 30) and the amount of the reducing agent (48) metered in by the second metering element (38). Process for exhaust gas aftertreatment according to one of the Claims 1 to 3 , characterized in that the first SCR catalyst (28, 30) in normal operation to an ammonia level of 60% - 100% of the maximum ammonia storage capacity and the second SCR catalyst (32) to an ammonia level of 50% to 90 % are conditioned. Process for exhaust gas aftertreatment according to one of the Claims 1 to 4th , characterized in that the lowering of the ammonia fill level of the first SCR catalytic converter (28, 30) takes place by a regeneration of a particle filter. Process for exhaust gas aftertreatment according to one of the Claims 1 to 5 , characterized in that the threshold value for the component temperature of the second SCR catalytic converter is 250 ° C. Exhaust aftertreatment system for an internal combustion engine (10), which is connected with its outlet (16) to an exhaust system (20), in which in the flow direction of an exhaust gas of the internal combustion engine (10) a first SCR catalyst (28, 30) close to the engine and downstream of the first SCR catalyst (28, 30) a second SCR catalyst (32) are arranged, and upstream of the first SCR catalyst (28, 30) a first metering element (36) with which a reducing agent (48) upstream of the first SCR -Catalyst (28, 30) can be metered into the exhaust gas duct (22), and with an engine control unit (56) which is set up to implement a method according to one of the Claims 1 to 6th perform when a machine-readable program code is executed by the engine control unit (56). Exhaust aftertreatment system Claim 7 , characterized in that the second SCR catalytic converter (52) has an ammonia barrier catalytic converter (54) or an ammonia barrier catalytic converter (54) is connected downstream of the second SCR catalytic converter (52). Exhaust aftertreatment system Claim 7 or 8th , characterized in that a turbine (24) of an exhaust gas turbocharger (18) is arranged in the exhaust system (20), with an oxidation catalyst (26) or an oxidation catalyst (26) or upstream of the turbine (24) of the exhaust gas turbocharger (18) and upstream of the first metering element (36) a NOx storage catalytic converter (58) is arranged. Exhaust aftertreatment system according to one of the Claims 7 to 9 , characterized in that the first SCR catalytic converter (28, 30) is designed as a particle filter (30) with a coating for the selective, catalytic reduction of nitrogen oxides.

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