DE102015006646A1 - Exhaust gas aftertreatment system and method of execution therewith - Google Patents

Abstract

Exhaust gas aftertreatment system (5) having an exhaust gas-throughflowable SCR catalyst stage (13) and a reducing agent metering device (15) for the provision of NH3 in the SCR catalyst stage (13); wherein the exhaust aftertreatment system (5) is configured to adjust a reductant dosing amount based on an NH 3 concentration determined on the output side of the SCR catalyst stage (13). Also proposed is a method of execution with the exhaust aftertreatment system.

Description

The present invention relates to an exhaust aftertreatment system according to the preamble of claim 1. Furthermore, the invention relates to a method according to the preamble of claim 11.

In the prior art, it is well known to meet emission requirements, such as. B. ANIMAL 4 To use exhaust aftertreatment devices. In particular, with diesel engines, z. B. Genset area, exhaust aftertreatment devices with SCR system (SCR: Selective catalytic reduction) are provided, which have an SCR catalyst and a reducing agent metering device. With the reducing agent NH ₃ (ammonia) is provided in the exhaust gas in the SCR catalyst.

The addition of the reducing agent is usually controlled by means of two NOx sensors, so that a constant comparison of actual and setpoint is possible. As a result, the NOx control could provide the best possible nitrogen oxide conversion at each operating point. The problem with such NOx sensors, however, is their tolerance, which at high NOx conversion rates at the SCR catalytic converter - to comply with low emission limits - are usually too imprecise for actual value detection. As a result of the inadequately determined actual value, excessively large amounts of reducing agent can subsequently be metered into the exhaust gas, so that the SCR catalyst can reach the state of complete NH ₃ charge (in which case excess NH ₃ (ammonia) in the SCR catalytic converter Amount is stored). Then, as the SCR catalyst heats up, for example, due to a load increase such as an acceleration of an internal combustion engine used with the exhaust aftertreatment system, a large amount of NH ₃ stored in the SCR ceramic becomes due to a concomitant decrease in the NH ₃ storage capacity of the SCR catalyst - Shock wave - expelled.

However, as part of a proposed according to the prior art, downstream barrier catalyst this expelled NH ₃ is converted back to NO x, so that in particular low NOx emission limits, which are associated with high conversion rates and so far high NH ₃ metering, can not be met regularly.

Proceeding from this, the object of the present invention is to specify an exhaust aftertreatment system and a method with which emission limits can be maintained in an improved manner.

This object is achieved with an exhaust aftertreatment system with the features of claim 1 and with a method having the features of claim 11.

Advantageous developments and embodiments of the invention are specified in the further claims.

According to the invention, an exhaust aftertreatment system with an SCR catalyst stage which can flow through the exhaust gas and a reducing agent metering device for the provision of NH ₃ (NH ₃ : ammonia) in the SCR catalyst stage or in the exhaust gas through the SCR catalyst stage is proposed. The exhaust aftertreatment system is intended for use with an internal combustion engine, preferably with a diesel engine (diesel engine) or with a diesel engine, in particular a large engine. The internal combustion engine or the exhaust aftertreatment system can be provided for example for a motor vehicle such as a ship, a locomotive or a commercial vehicle, or for a stationary device, for. As for a combined heat and power plant, an (emergency) generator set or for industrial applications. In the context of the present invention, such an internal combustion engine, in addition to or as an alternative to use with diesel fuel z. B. also be provided for use with heavy oil, or bio-oil.

The SCR catalyst stage is preferably disposed in an exhaust line of the exhaust aftertreatment system and further preferably formed by means of an SCR catalyst disk. When viewed in the exhaust gas flow direction, the SCR catalytic converter stage here forms, in particular, a first or input catalytic converter stage of an SCR catalytic converter of the exhaust aftertreatment system (which can also have further exhaust gas treatment components, such as a (diesel) oxidation catalytic converter or a (diesel) particle filter).

Now with respect to the reductant dosing device, it is provided for the supply of NH ₃ in the exhaust gas - in a known manner - for the addition of reducing agent to the exhaust gas supplied to the SCR catalyst stage, in particular of reducing agent in the form of ^AdBlue® or HWL (HWL: urea-aqueous solution). Here, the reducing agent metering device may also be part of an SCR system, which further comprises the SCR catalyst. For injection of the reducing agent into the exhaust gas, the reducing agent metering device z. B. have an injector.

According to the invention, the exhaust aftertreatment system is set up in a characteristic manner, to adjust a reducing agent metering amount based on a NH ₃ concentration, which is determined on the output side of the SCR catalyst stage, in particular sensed is. In this case, the exhaust aftertreatment system is preferably set up to adjust the reducing agent metering quantity within the scope of a reducing agent metering control or NH ₃ control based on the NH ₃ concentration.

The proposed exhaust gas aftertreatment system makes it possible in an advantageously simple manner to achieve improved compliance with low NOx emission limit values insofar as it is no longer necessary to resort to tolerance-related NOx sensors of the type described above. With the proposed exhaust aftertreatment system, which now continues to regulate an NH ₃ breakthrough at the SCR catalyst stage (output side) for the adjustment of the reducing agent metering quantity, a reducing agent metering amount in a development of the invention can furthermore be metered precisely and according to need and furthermore Storage of NH _{3 can} be effectively avoided at a further, downstream SCR catalyst stage.

For the adjustment of the reducing agent metering amount as a function of the determined NH ₃ concentration is provided in the invention that the exhaust aftertreatment system has at least one NH ₃ sensor, by means of which the NH ₃ concentration (continuously) is determined. The NH ₃ sensor is preferably arranged in the exhaust gas stream immediately after the SCR catalyst stage, more preferably between the SCR catalyst stage and another SCR catalyst stage (which will be discussed in more detail below). In this case, the at least one NH ₃ sensor is preferably arranged and / or configured in such a way that a maximum NH ₃ concentration value in the flow cross section at the outlet of the SCR catalyst stage by means of the NH ₃ sensor in the exhaust gas can be determined, in particular operating point-dependent. Such NH ₃ concentration value is expected to z. B. in the middle of the exhaust stream at the output of the SCR catalyst stage.

Preferred embodiments of the invention further provide that the exhaust aftertreatment system is adapted to automatically calibrate the NH ₃ sensor, in particular before the start of an NH _{3 control} according to the invention. The calibration is provided in this case, for example, such that sensor ₃ is moved to a lower measurement range limit of NH, for the first time to a value measured for NH ₃ detected by the NH ₃ sensor. For example, the SCR catalyst stage for this purpose first without NH ₃ dosing and then with increasing reducing agent dosage or NH ₃ concentration (due to increasing reducing agent dosing) exhaust gas flow. If, for the first time, an NH ₃ rise or NH ₃ breakthrough is detected by the sensor, this value can be set as the zero point for the adjustment of the reducing agent metering or NH ₃ control (which zero point can also be reached with the achievement of a maximum or maximum charge) 100% NOx conversion rate at the SCR catalyst stage corresponds (when the SCR catalyst stage is run to its maximum conversion point)).

In general, the exhaust aftertreatment system with the invention is preferably set up for a zero point control in the context of the reducing agent metering setting or NH _{3 control} , ie in particular corresponding to a determined NH ₃ breakthrough and / or an NH ₃ concentration increase at the outlet the SCR catalyst stage to reduce a reductant dosage as part of the reductant dosage and further preferably also to increase a reductant dosage amount until an NH ₃ breakthrough and / or NH ₃ concentration increase at the exit of the SCR catalyst stage is determined , In that regard, with the reducing agent metering setting, achieving a maximum or 100% NOx conversion rate at the SCR catalyst stage is always provided, the achievement of which with the invention - spatially - advantageously coinciding with the end of the SCR catalyst stage. A storage of excess NH ₃ at the other SCR catalyst stage (in the end) is thus effectively avoided with the aforementioned features.

In a further embodiment of the invention it is further proposed that the exhaust aftertreatment system (as noted above) has a further, the SCR catalyst stage downstream or second SCR catalyst stage. Like the first SCR catalyst stage, the second SCR catalyst stage is also preferably provided as an SCR catalyst disk, preferably together with the first SCR catalyst stage in an SCR catalyst, for example. B. an SCR system arranged.

The further SCR catalyst stage is provided in particular for NH ₃ (off) storage, ie for NH ₃ storage in the event of any NH ₃ excess at the outlet or after the first SCR catalyst stage. For example, such an excess may occur if, due to a temperature increase, eg. B. with an acceleration phase of an internal combustion engine used with the exhaust aftertreatment system, comes to a reduction of the NH ₃ storage capacity of the first SCR catalyst stage and recorded NH _{3 is} increasingly expelled.

With the second or further SCR catalyst stage, it is possible, on the one hand, to absorb the excessively present NH ₃ and, on the other hand, to lower the reducing agent metering amount, ie with the aim of further storing NO x then the stored NH ₃ in the second SCR -Catalyst stage spend (preferably until fully implemented). In the context of the invention, this can be done for example with an exhaust aftertreatment system, which has a higher-level control.

With such a superimposed control, implemented by means of z. B. a control device such as a control unit, in particular a NOx sensor, preferably at the output or after the second SCR catalyst stage, provided, the superimposed control may follow the following principle of procedure: the NOx sensor detects no increase in NOx emissions - above a predetermined threshold - although the NH ₃ sensor z. B. has indicated a threshold NH ₃ threshold and reducing agent has been reduced accordingly - Stored NH ₃ in the second SCR catalyst stage is still available to a sufficient extent and the reducing agent Zudosiermenge remains reduced. On the other hand, if the NOx value at the NOx sensor rises above a threshold value, the stored NH ₃ in the second catalyst stage is converted by means of NO x and the reducing agent metering quantity is increased again (in particular until an NH ₃ breakthrough at the outlet of the first one again) SCR catalyst stage is determined in the context of NH ₃ adjustment or regulation).

According to a further aspect of the proposed exhaust aftertreatment system, the exhaust aftertreatment system may also be configured for (mere) NO x control in the context of the reducing agent metering. For example, a conventional NOx control may be provided which z. B. acts over a first operating range in which the (first) SCR catalyst stage is not driven with high or maximum NOx conversion rates. In a further, second operating range in which the SCR catalyst stage is then operated in particular with high or maximum NOx conversion rates (preferably 100%), the NH _{3 control} or NH ₃ -based reducing agent metering can then be used. Setting to be changed.

The invention also proposes a method for the embodiment with an exhaust aftertreatment system as described above, in the context of which reference is made to the statements made above on the exhaust aftertreatment system, which essentially all of them also apply to the method.

The method provides a first step in which an NH ₃ concentration at the outlet of the SCR catalyst stage is determined, that is to say in particular with at least one NH ₃ sensor as described above. In a second step, the reducing agent dosage is then adjusted based on the determined NH ₃ concentration. The method is in particular carried out when a maximum possible NOx conversion is to be achieved at the SCR catalyst stage, more particularly in the context of a reducing agent metering control or NH _{3 control} . As described above for the exhaust aftertreatment system, the method may be advantageously carried out in combination with NOx control, for example in the context of a superimposed control, or (depending on operating range) alternatively thereto. The first step can be preceded by a calibration step for adjusting the NH ₃ sensor, see above.

Code for executing the method can be stored in a computer program product, for example in a control unit, a volatile or a fixed memory, or generally on a data carrier.

Further features and advantages of the invention will become apparent from the following description of embodiments of the invention, with reference to the figures of the drawings, which show details essential to the invention, and from the claims. The individual features can be realized individually for themselves or for several in any combination in a variant of the invention.

Preferred embodiments of the invention are explained below with reference to the accompanying drawings. Show it:

1 exemplarily and schematically an SCR catalyst according to the prior art, illustrated in a situation with maximum NOx conversion rate.

2 exemplary and schematically an exhaust aftertreatment system according to a possible embodiment of the invention.

3 by way of example and schematically shows an SCR catalyst according to a possible embodiment of the invention, illustrated in an exemplary situation with maximum NOx conversion rate.

4 a diagram illustrating the NH ₃ sensor tolerance bands of a suitable NH ₃ sensor.

5 exemplary and schematically an exhaust aftertreatment system according to another possible embodiment of the invention, in particular with superimposed NOx control.

In the following description and the drawings, the same reference numerals correspond to elements of the same or comparable function. Elements of the prior art are provided with primed reference numerals.

1 shows an example and schematically an SCR catalyst 1' according to the prior art in a situation with approximately 100% NOx conversion rate (NOx: nitrogen oxides, solid arrows in 1 ). By means of the dashed arrows, an NH ₃ distribution (NH ₃ : ammonia;) is furthermore shown, as is typically the case in the catalyst 1' can adjust. The catalyst 1' is partially in breakthrough.

To comply with very low NOx emission limits, very high NOx conversion rates are required. However, the usual feedback NOx sensors are too imprecise for actual value detection. As 1 Because of sensor tolerances of the NOx sensors and as a result of overdosed NH ₃ addition amounts resulting therefrom, NOx can already be produced significantly before the actual end of the catalytic converter 1' be reacted to 100%, leaving NH ₃ towards the end of the catalyst 1' , partially prevails in large quantities, z. B. centered with radially outwardly decreasing concentration. This excess NH ₃ tends to be in the SCR catalyst 1' to be stored, if necessary, until full NH ₃ loading is reached, ie no further NH ₃ can be stored. Excess NH ₃ is thus at the output 3 ' released, s. dashed arrows in 1 ,

Due to the tolerances of the NOx sensor system is the SCR catalyst 1' Consequently, during load phases with 100% NOx conversion, it is regularly loaded with a considerable amount of stored NH ₃ . Increases the temperature of the SCR catalyst 1' For example, in the context of transient acceleration processes of an internal combustion engine, the output side dissolves an NH ₃ wave, which disadvantageously in turn leads to increased NOx emissions during after-treatment in a barrier catalyst according to the prior art, thus causing problems to reliably comply with low emission limits can.

2 now illustrates an inventive exhaust aftertreatment system 5 , in particular in connection with an internal combustion engine used therewith 7 , preferably in the form of a diesel engine.

The exhaust aftertreatment system 5 includes an exhaust system 9 in which, for example, a first exhaust treatment stage 11 in z. B. form of a (diesel) oxidation catalyst can be arranged. Furthermore, the exhaust aftertreatment system includes 5 an SCR catalyst 1 (SCR: selective catalytic reduction) with an SCR catalytic converter stage through which exhaust gas can flow 13 (first SCR catalyst stage 13 ), which is formed in particular by means of an SCR catalyst disk. The SCR catalyst 1 is still assigned a reducing agent metering device 15 the exhaust aftertreatment system for the provision of NH ₃ in the SCR catalyst stage 13 ie, to provide NH ₃ by injecting reducing agent into the exhaust, e.g. B. in the form of Adblue ^® or HWL (urea-aqueous solution).

The SCR catalyst 1 furthermore preferably comprises a further or second SCR catalyst stage 17 , for example, again provided in the form of an SCR catalyst disk, which in the exhaust line 9 or Abgaströmungsweg the first SCR catalyst stage 13 is subordinate. Downstream of the SCR catalyst 1 can the exhaust system 9 to an exhaust outlet 19 be guided, for example, to the environment.

As 2 further illustrates the proposed exhaust aftertreatment system 5 a sensor 21 on, which as NH ₃ sensor 21 is provided. The NH ₃ sensor is between the first 13 and the second 17 SCR catalytic converter stage in the exhaust system 9 and to that extent also in the SCR catalyst 1 arranged, in particular after the first SCR catalyst stage 13 at the downstream side 3 , The NH ₃ sensor is provided in order to determine (in particular continuously) an NH ₃ concentration in the exhaust gas, ie on the output side at the (first) SCR catalytic converter stage 13 , Here, the NH ₃ sensor is preferably positioned and set up in such a way always to determine the highest NH ₃ concentration over the flow cross-section, in the present case, for example. B. centrally in the middle of the exhaust flow path 7 ,

Connected is the NH ₃ sensor in the context of a reducing agent metering adjustment according to the invention, in particular regulation, with a control device 23 the exhaust aftertreatment system 5 , Preferably, a control device, which continues to serve, based on sensor signals of the NH ₃ sensor 21 and insofar as a determined NH ₃ concentration, the reducing agent metered amount in the exhaust gas via the reducing agent metering device 15 adjust. This is the control unit 23 furthermore signaling with the reducing agent metering device 15 connected. The control unit 23 can z. B. also the internal combustion engine 7 be assigned.

3 now illustrates an SCR catalyst designed according to the present invention 1 closer, in particular in an operating situation, as may result in a target NOx conversion rate of almost 100%, especially when used with an exhaust aftertreatment system 5 according to 2 or 5 (NH ₃ is exemplified by the dashed arrows in 3 represented NOx by means of the solid arrows in 3 ).

3 first illustrates the first SCR catalyst stage 13 and the second downstream SCR catalyst stage 17 in the SCR catalyst 1 between which the NH ₃ sensor 21 in the exhaust flow path 9 is arranged. The second SCR catalyst stage 13 is particularly provided to absorb any excess NH ₃ or store, as this z. B. may spontaneously increase in transient processes with particular increase in temperature such as an increase in load or acceleration.

Now it will be even closer to the working principle of 3 shown SCR catalyst 1 in the context of the inventively proposed exhaust aftertreatment system 5 and a proposed method.

To ensure effective compliance with low NOx emission limits, the NH ₃ sensor is commissioned with the exhaust aftertreatment system 5 initially (automatically) adjusted or calibrated. For this purpose, the first SCR catalyst stage 13 first approached without reductant metered addition or exhaust gas flowing through. The NH ₃ sensor 21 In this case, no NH ₃ concentration will be determined (the determined sensor output value corresponds to 0 ppm NH ₃ ). As a result, reducing agent is metered in until the NH ₃ sensor 21 after reaching a maximum NOx conversion rate (as in 3 illustrates) an NH ₃ breakthrough at the exit of the first SCR catalyst stage 13 detected or a NH ₃ -concentration increase (which results from a now present excess of dosed reducing agent). This breakthrough value is set in the sequence for the NH _{3 control} or reducing agent metering setting according to the invention as setpoint or desired metering for the maximum achievable turnover rate (to which point it is regulated).

4 exemplifies tolerance bands of an NH ₃ sensor suitable for NH ₃ concentration measurement 21 , which advantageously detects a change in measured value even with a negative measuring tolerance.

With the appropriately calibrated sensor 21 Now, the reducing agent metering control can advantageously be such that an excess of NH ₃ towards the end of the first SCR catalyst stage is effectively avoided, thus also a NH ₃ storage after the NH ₃ sensor. The exhaust aftertreatment system is designed 5 This therefore, in the context of the NH ₃ -based reducing agent metering setting always maximum NOx conversion in the first SCR catalyst stage 13 to achieve, which is further achieved immediately at the end of the first catalyst stage, s. 3 , Once the NH ₃ sensor 21 detects an increase in NH ₃ concentration (breakthrough), the reducing agent dosage is reduced in the context of reducing agent dosage corresponding and in particular immediately, ie based on the determined NH ₃ concentration, the NH ₃ concentration drops (compared to the set zero point) is correspondingly increased metered reducing agent.

Nevertheless, excess NH ₃ precipitates at the outlet 3 on, for. B. in transient processes, this is in the context of the invention in the second SCR catalyst stage 17 stored and spent for subsequent conversion by means of NOx, z. B. by suitable reduction of the reducing agent dosage over a period required for the reaction.

Within the scope of the proposed method, in a first step, an NH ₃ concentration is produced at the outlet 3 the SCR catalyst stage 13 determined and adjusted in a second step, the reducing agent dosage based on the determined NH ₃ concentration.

5 Numeral shows a preferred embodiment of an exhaust aftertreatment system 5 , which in particular in the context of a higher-level control further comprises a NOx sensor.

The exhaust aftertreatment system 5 according to 5 is set up for a superposed control in which a NOx actual value downstream of the second SCR catalyst stage 17 or the SCR catalyst 1 is determined. To determine the NOx actual value, the exhaust aftertreatment system 5 a NOx sensor 25 on, which signals with the control device 23 connected is. The NOx actual values are here by the control device 23 used to adjust the reducing agent dosage suitable for the needs, in particular next to the NH ₃ sensor 21 determined actual values for the NH ₃ concentration.

This will be discussed in more detail below.

With the exhaust aftertreatment system 5 according to 5 becomes like in the embodiment after 2 the second SCR catalyst stage 17 intended to accumulate any accumulating, excess NH ₃ (so that a blocking catalyst as may be unnecessary in the prior art). In the case of excess NH _3, the NH ₃ sensor detects 21 in this case an increase in concentration, which to the control device 23 is communicated. As a result, the control device reduces 23 the reducing agent metering or interrupts the same, the NOx sensor 25 the determined NOx concentration continuously to the control device 23 communicated.

As long as the determined NOx concentration in this case falls below a threshold, ie now that in the second SCR catalyst stage 17 stored NH ₃ reacted with NOx reacting and thus degraded, the reduced reducing agent dosage can be maintained. By contrast, if the determined NOx concentration increases beyond the threshold value, the stored NH _{3 is} the second SCR catalytic converter stage 17 implemented in the intended manner, whereupon the control device 23 , the reducing agent dosage increases again, especially until a NH ₃ increase on the NH ₃ sensor 21 is detected.

It should be noted finally that in the context of the present invention can also be provided to provide an NOx control scheme, alternatively, in addition to NH _3, the NH ₃ scheme, for example, for operating ranges of an internal combustion engine 7 can be provided, in which maximum or 100% NOx conversion rates at the first SCR catalyst stage 13 should be achieved. In areas of lower utilization, only the NOx control can take effect.

LIST OF REFERENCE NUMBERS

1, 1 '

SCR catalyst

3, 3 '

Exit (downstream side)

5

aftertreatment system

7

Internal combustion engine

9

exhaust gas line

11

Exhaust gas treatment stage

13

SCR catalyst stage

15

Reducing agent dosing

17

another SCR catalyst stage

19

exhaust outlet

21

NH ₃ sensor

23

control device

25

NOx sensor

Claims (14)

Exhaust aftertreatment system ( 5 ) with an SCR catalytic converter stage through which exhaust gas can flow ( 13 ) and a reducing agent metering device ( 15 ) for the provision of NH ₃ in the SCR catalyst stage ( 13 ); characterized in that - the exhaust aftertreatment system ( 5 ) is adapted to adjust a reducing agent metering amount based on a NH ₃ concentration, which at the output side at the SCR catalyst stage ( 13 ) is determined. Exhaust aftertreatment system ( 5 ) according to claim 1, characterized in that - the exhaust aftertreatment system ( 5 ) is set to adjust the reducing agent dosage amount in the context of a reducing agent metering control based on the determined NH ₃ concentration. Exhaust aftertreatment system ( 5 ) according to one of the preceding claims, characterized in that - the exhaust aftertreatment system ( 5 ) another, the SCR catalyst stage ( 13 ) downstream SCR catalyst stage ( 17 ), which at an NH ₃ excess at the output ( 3 ) of the SCR catalyst stage ( 13 ) is provided for NH ₃ storage. Exhaust aftertreatment system ( 5 ) according to one of the preceding claims, characterized in that - the SCR catalyst stage ( 13 ) and the further SCR catalyst stage ( 17 ) per stage of a single SCR catalyst ( 1 ), the stages ( 13 . 17 ) are each provided in each case in the form of catalyst disks. Exhaust aftertreatment system ( 1 ) according to one of the preceding claims, characterized in that - the exhaust aftertreatment system ( 5 ) an NH ₃ sensor ( 21 ), by means of which the NH ₃ concentration is determined. Exhaust aftertreatment system ( 5 ) according to one of the preceding claims, characterized in that - the NH ₃ sensor ( 21 ) in the flow path ( 9 ) between the SCR catalyst stage ( 13 ) and the further SCR catalyst stage ( 17 ) is arranged. Exhaust aftertreatment system ( 5 ) according to one of claims 5 and 6, characterized in that - the NH ₃ sensor ( 21 ) is arranged and / or configured such that a respective highest NH ₃ concentration value in the flow cross-section at the outlet ( 3 ) of the SCR catalyst stage ( 13 ) by means of the NH ₃ sensor ( 21 ) can be determined. Exhaust aftertreatment system ( 5 ) according to one of claims 5 to 7, characterized in that - the exhaust aftertreatment system ( 5 ) is set up to automatically calibrate the NH ₃ sensor. Exhaust aftertreatment system ( 5 ) according to one of the preceding claims, characterized in that - the exhaust aftertreatment system ( 5 ) is arranged, corresponding to a determined NH ₃ breakthrough and / or NH _{3 increase in} concentration at the outlet ( 3 ) of the SCR catalyst stage ( 13 ) to reduce a reducing agent dosage amount in the context of the NH ₃ -based reducing agent dosage; and / or - the exhaust aftertreatment system is adapted to increase a reducing agent metering amount within the scope of the NH ₃ -based reducing agent metering until an NH ₃ breakthrough and / or NH _{3 increase in} concentration at the outlet ( 3 ) of the SCR catalyst stage ( 13 ) is determined. Exhaust aftertreatment system ( 5 ) according to one of the preceding claims, characterized in that - the exhaust aftertreatment system ( 5 ) is further set up for superimposed control in the context of reducing agent dosing; and / or the exhaust aftertreatment system ( 5 ) is arranged to a NOx control in the context of the reducing agent metering, wherein the NOx control is alternatively to the NH ₃ -based setting or in conjunction with it executable. Method of execution with an exhaust aftertreatment system ( 5 ) according to any one of the preceding claims, characterized in that - in the method in a first step, an NH ₃ concentration at the exit ( 3 ) of the SCR catalyst stage ( 13 ) is determined; and - in a second step, the reducing agent dosage is adjusted based on the determined NH ₃ concentration. A method according to claim 10, characterized in that - the process with maximum possible NOx conversion at the SCR catalyst stage ( 13 ) is stirred. Method according to one of claims 10 to 12, characterized in that - the method is carried out as part of a reducing agent metering control. Computer program product, characterized in that - the computer program product has code for carrying out the method according to one of claims 11 to 13.

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