AT522752A1 - Method for functional testing of an exhaust gas aftertreatment system of an internal combustion engine and an exhaust gas aftertreatment system - Google Patents

Abstract

The invention relates to a method for checking the functionality of an exhaust gas aftertreatment system (1) of an internal combustion engine with an internal combustion engine (2), the exhaust gas aftertreatment system (1) comprising an SCR system (17) with at least two SCR units (3, 4), wherein in an aftertreatment fluid containing urea, in particular, is metered into the exhaust gas flow and reacts with the exhaust gas flow in the SCR units (3, 4), an NH3 concentration between a first SCR unit (3) and the second SCR unit (4) being achieved by a NH3 sensor (6) is determined. The invention also relates to an exhaust gas aftertreatment system (1) for an exhaust gas flow from an internal combustion engine with an internal combustion engine (2).

Description

Method for functional testing of an exhaust aftertreatment system of a

Internal combustion engine and exhaust aftertreatment system

The invention relates to a method for checking the function of an exhaust gas aftertreatment system of an internal combustion engine with an internal combustion engine, the exhaust gas aftertreatment system comprising an SCR system with at least two SCR units, with an in particular urea-containing aftertreatment fluid being metered into the exhaust gas flow, which is in the SCR units

reacts with the exhaust gas flow.

The invention also relates to an exhaust gas aftertreatment system of an exhaust gas flow of an internal combustion engine with an internal combustion engine, the exhaust gas aftertreatment system comprising at least two SCR units, with at least one injection device for metering a particularly urea-containing aftertreatment fluid into the exhaust gas flow and a control unit for regulating an amount of the injected exhaust gas aftertreatment fluid, wherein the

Control unit is connected to the injection device.

Different methods for functional testing of exhaust gas aftertreatment systems are known from the prior art. For example, methods are known in which an SCR catalytic converter is first completely loaded with ammonia and then the SCR storage capacity of the SCR catalytic converter is determined by completely emptying it. A mass balance is used to calculate how much

The SCR catalytic converter could still store ammonia.

In addition, methods are known in which urea is metered into a completely emptied SCR catalytic converter until an ammonia slip is detected after the SCR catalytic converter - that is, the SCR catalytic converter is fully loaded. This means that a mass balance can be used to determine how much ammonia is still in the

SCR catalyst is storable.

In addition, methods known from the state of the art are often insufficient to meet all legal requirements, especially when SCR-

Units have different coatings.

The object of the invention is to overcome the disadvantages of the prior art. In particular, the object of the invention is to create a method by means of which the functionality of an exhaust gas aftertreatment system can be checked quickly and reliably

can be checked.

It is also an object of the invention to provide an exhaust gas aftertreatment system

in which the function of SCR units can be reliably checked.

The object of the invention is achieved in particular in that, in a method of the type mentioned at the outset, between a first SCR unit and the

second SCR unit, an NH3 concentration is determined by an NH3: sensor.

In the method according to the invention, it is particularly advantageous that an amount of ammonia is also determined between the two SCR units, as a result of which an NH3 slip can be measured downstream of the first SCR unit and thus a storage capacity of the first SCR unit can be determined. Subsequently, based on the storage capacity of the SCR unit, aging and / or poisoning of the same can be recognized and then responded accordingly. For this purpose, for example, a load control or a

Desulfurization can be carried out.

The method according to the invention makes it possible to use the NH3: sensor to identify at an early stage an aged and / or damaged SCR unit, which has a limited storage capacity, so that reversible damage can be reversed and irreversible damage can be detected

and / or an SCR unit can be defined as defective.

The NHs measurement between the two SCR units can also be used to infer the loading of a second SCR unit downstream of the first SCR unit. As a result, the functionality of the second SCR unit can also be determined. This is preferably done via the NHs sensor in combination with at least one NOx

Sensor.

The method according to the invention can therefore be used to diagnose a state of an SCR unit, the amount of NHs that are converted (i.e. reacts with NOx),

stored and oxidized, as well as the amount which is measured as slip,

This means that the metering of the operating fluid can also be set,

controlled and / or regulated.

If several SCR units are provided, it can be advantageous if an NH3 sensor is used to determine an NH3 concentration or an NH3 amount between each SCR unit. However, it can also be provided that an NH3 amount is determined only once, this being in particular upstream before a last one

(arranged downstream in the exhaust tract) SCR unit takes place.

In the context of the invention, the second SCR unit is arranged in particular immediately downstream of the first SCR unit. These preferably have a

different coatings, for example iron zeolite and copper zeolite.

A physical sensor is provided between the SCR units to determine an NH3s quantity or an NH3 concentration between the SCR units. I.e. an NHs concentration is actually measured at this point; the NH: sensor delivers a real measured value. If more than two SCR units are planned, NHs-

Sensors arranged as described above.

According to the invention, it is advantageous if, in normal operation, in particular in normal operation, an operating material suitable for selective catalytic reduction, such as in particular a urea-containing aftertreatment fluid, a urea solution or AdBlue®, is metered in upstream of the SCR units. The fuel can contain a reducing agent, such as in particular ammonia (NHs), or it can be converted into a reducing agent, such as in particular NHs. A urea-containing mixture, in particular a urea-water solution such as AdBlue®, is preferably used as the fuel, the fuel optionally being converted into the reducing agent by the reactions shown below,

especially NH3, is converted: Thermolysis: (NH2) 2CO 3 NH3 + HNCO hydrolysis: HNCO + H2O0> NHs + CO2

In a first step, the urea (NH2) 2CO in

Ammonia NHz3 and isocyanic acid HNCO are converted. In a second

During the hydrolysis reaction, the isocyanic acid HNCO can be mixed with water H20O in step

Ammonia NHz3 and carbon dioxide CO2 are converted.

The reducing agent, in particular NHs, can optionally be at least temporarily stored and / or stored in at least one SCR unit. If necessary, the ammonia accumulates in the active centers of the SCR system. The at least temporarily stored reducing agent, in particular the ammonia NHes, can then reduce nitrogen oxides NOx, such as in particular nitrogen monoxide NO and nitrogen dioxide NO2.

The metering of the fuel can be done via a metering device, such as in particular

via an injector or an injection nozzle.

In the context of the invention, an aftertreatment fluid is to be understood as an operating material which contains a reducing agent or can be converted into a reducing agent

is. The reducing agent is at least temporarily stored in the SCR units.

In the context of the invention, an aftertreatment fluid is understood to mean a liquid, gaseous or solid fluid. Amminex®, for example, is used as the gaseous fluid. The aftertreatment fluid can also be partially liquid and / or partially gaseous and / or partially solid. In particular, it contains urea. We particularly prefer a given amount of ammonia

dosed.

In the context of the present disclosure, an SCR unit can in particular be understood to mean an sSDPF catalytic converter, an SCR catalytic converter and / or an ASC catalytic converter

will. In particular, it can be provided that the second SCR unit comprises an SCR catalytic converter and an ASC catalytic converter, and / or that the second SCR unit

is formed from an SCR catalytic converter and an ASC catalytic converter.

Furthermore, the SCR unit and / or the SCR units can be the device for metering in the operating material and, if appropriate, also the operating material and / or

include the operating fluid container as such.

In the context of the invention, the internal combustion engine is in particular a diesel internal combustion engine, albeit also a gasoline engine

Internal combustion engine can be provided.

Aging and / or damage to the first SCR unit can occur due to the comparison of modeled NH3 values and real NH3 values measured in the

normal operation can be determined passively.

It is beneficial if the SCR system is defined as functional if no reducing agent slip is detected by the NHs sensor after the first SCR unit during the entire course of metering in a predetermined diagnostic amount of aftertreatment fluid, which means that essentially all of the aftertreatment fluid is at least temporarily is stored in the first SCR unit, and that the SCR system is defined as limited or non-functional if a reducing agent slip is detected by the NHs sensor after the first SCR unit during the course of metering the predetermined diagnostic amount of aftertreatment fluid before the entire predefined diagnostic amount of aftertreatment fluid is metered in, with which a reduced aftertreatment fluid storage capacity of the first SCR unit is detected. The predetermined diagnostic amount of fuel that is metered in defines the amount of reducing agent introduced since the fuel can be converted into the reducing agent and / or the fuel contains the reducing agent. If the SCR system is defined as having limited or non-functional functionality, this means that the introduced amount of reducing agent, in particular NHs, cannot be stored by the first SCR unit. If necessary, the reduced reducing agent storage capacity, in particular the NHs storage capacity, of the SCR unit can be reduced due to aging of the SCR unit and / or due to a defect in the SCR unit.

If the SCR system is defined as having limited or non-functional functionality, it is useful if the exhaust gas aftertreatment system is desulfurized. As a result, after a reduced storage capacity of the SCR

Unit (s) has been recognized - reversible damage is healed.

In addition, the NH3 measurement downstream of the first SCR unit and upstream of the second SCR unit makes it possible to differentiate between normal, generic aging of the SCR units and poisoning of the SCR units and between a defective SCR unit . No methods are known from the prior art by means of which the occurrence of an NHs slip can be detected at such an early stage. A particularly important area of application of the method according to the invention is in the functional testing of an exhaust gas aftertreatment system with two or more SCR units, with at least

two of them have a different coating.

The predetermined amount of aftertreatment fluid generally corresponds to an amount of reducing agent which must still be storable in the SCR unit so that a limit value, in particular a legal limit value for pollutant emissions, is reached with the SCR unit, the SCR system and / or the exhaust gas aftertreatment system , in particular a NOx emission, can be met. This can mean that if the predetermined amount of reducing agent can no longer be stored in the SCR unit, the SCR catalytic converter is not able to provide the legally required

specified limit values to meet the required nitrogen oxide conversion rate.

A reducing agent slip is to be understood as meaning, in particular, an NHs slip. The reducing agent slip is detected at least by the NHs sensor. It can also be provided that the NHs slip is additionally detected by an increased measured value of the NOx sensor, since the concentration of NH: Influence on a NOx

Sensor can have.

When defining the SCR system as functional or not functional or only functional to a limited extent, a diagnostic mode can optionally be activated in advance. In the so-called emptying phase, the supply of aftertreatment fluid is reduced or stopped so that no or only very little new reducing agent is supplied to the SCR system. The emptying of the first and / or second SCR unit can be based on the fact that the reducing agent still contained in the SCR units is consumed by the reduction of nitrogen oxides (NOx), that is to say that less reducing agent is introduced than is consumed. It can be provided that the amount of reducing agent is always introduced

which is required for sufficient cooling of the dosing device. It can

it can be provided that the reducing agent still contained in the SCR units takes place through an emptying mode, such as, in particular, an increase in the temperature of the SCR catalytic converter. After the first and / or second SCR units have been emptied, they can be tempered, in particular heated, in order to bring the SCR system to a predetermined temperature. Provision can be made for the SCR system to be brought to a previously defined temperature during emptying, which means that emissions can be kept low. This tempering step can be omitted if the temperature of the SCR system is already within a predetermined temperature window. Aftertreatment fluid is then metered in again in the so-called loading phase. The reducing agent contained in the aftertreatment fluid or formed from the operating material is possibly at least temporarily absorbed and / or at least temporarily stored by the SCRE units. The determined functionality of the exhaust gas aftertreatment system can then be output as status information and / or saved. Subsequently

the diagnostic mode can be ended if necessary.

It is advantageous if the metering in of the aftertreatment fluid is stopped before the predetermined diagnostic amount of aftertreatment fluid has been metered in when a reducing agent slip is detected after the first SCR unit, whereby the reducing agent emissions are minimized. Since the metering in of the aftertreatment fluid and thus also the introduction of reducing agent is stopped as soon as a reducing agent slip is detected after the first SCR unit, emissions, in particular reducing agent emissions, can essentially be prevented or reduced. This means that possibly not the entire predetermined diagnostic amount of operating material during the method

is or must be dosed.

The SCR system can be judged to be non-functional if an NHs slip is detected after the first SCR unit before the predetermined diagnostic amount of fuel is dosed. This makes it possible to establish that the reducing agent storage capacity of the SCR unit is reduced and, with such an SCR unit, a limit value, in particular a legal limit value, may be reduced

pollutant emission, can no longer be complied with.

It is advantageous if the predetermined diagnostic amount of aftertreatment fluid for the diagnostic mode is dimensioned in such a way that, with a functional SCR unit, the reducing agent introduced by the diagnostic amount of aftertreatment fluid can be stored at least temporarily in the SCR unit, so that, if the SCR unit is functional, no reducing agent emissions are planned occur. As soon as the predetermined amount of aftertreatment fluid has been metered in, the aftertreatment fluid supply can optionally be stopped or greatly reduced, so that no or only very little reducing agent is introduced. If no NHs slip is detected downstream of the first and upstream of the second SCR unit while the predetermined diagnostic amount of aftertreatment fluid is being metered in, the SCR system can be assessed as functional. This means that as a result the essentially entire amount of reducing agent, which is defined by the amount of fuel introduced, has been stored at least temporarily in the SCR units. It may therefore not be necessary to introduce so much fuel or reducing agent in front of (upstream) the SCR units that a reducing agent slip necessarily occurs after the first SCR unit during the functional check. In this way, on the one hand, emissions can be reduced or prevented, since no reducing agent emissions occur as planned with a functioning SCR system. On the other hand, the functionality of the SCR system can be quickly determined because, in contrast to conventional methods, less fuel has to be metered in and thus also the metering time and duration

the functional check is shorter than with conventional methods.

It is advantageous if the SCR system is emptied if the SCR system is emptied by stopping or reducing the supply of aftertreatment fluid until a parameter is detected that provides information that there is no longer any aftertreatment fluid in the SCR system is stored, this parameter being detected by the NHs sensor. As soon as a parameter is detected which indicates that no more reducing agent is stored or present in the SCR system, the emptying of the SCR unit (s) can be regarded as complete and the emptying phase can be ended. In particular, the measured values of the NHs sensor and

possibly also the NOx sensors, which if necessary before and

are arranged according to the SCR system. For example, it can be assumed that essentially no more reducing agent is stored in the SCR unit if the concentration of nitrogen oxides upstream of the SCR system essentially corresponds to the concentration of nitrogen oxides downstream of the SCR system. In such a case, the lack of reducing agent means that nitrogen oxides can no longer be reduced by the SCR units. From this it can be concluded

that the SCR units have been drained.

It is advantageous if the reactions of the first SCR unit that are decisive for the method are calculated in addition to the real operation in a first kinetic model, the first kinetic model in particular corresponding to a mathematical mapping of the physical model of the first SCR unit, the Responses of the second SCR unit that are decisive for the method are calculated in addition to the real operation in a second kinetic model, the second kinetic model corresponding in particular to a mathematical mapping of the physical model of the second SCR unit, and where a desired total load of the first SCR unit and of the second SCR unit and a desired loading amount of the second SCR unit

can be specified.

If necessary, it is provided that the predetermined diagnostic amount of aftertreatment fluid is determined or calculated by the kinetic model and in particular corresponds to an amount of reducing agent which, according to the model calculation, can be stored in its entirety at least temporarily in the SCR units if it is functional. If the SCR system is defined as having only limited functionality, the kinetic model is advantageously adapted

and / or adapted.

The reactions that are decisive for the process can be calculated in a mathematical and / or physical model. For example, such a kinetic model is in "Hollauf, Bernd: Model-Based Closed-Loop Control of SCR Based DeNOx Systems. Master’s thesis, University of Applied Science Technikum Kärnten, 2009. "revealed. It is preferably provided that the relevant reactions are mapped mathematically and physically by the kinetic models. The

Reactions can therefore be based on physical conditions, whereby

Estimates and / or uncertainties can be reduced and thereby the accuracy of the modeled values can be increased. For example, the oxidation of the reducing agent, in particular the oxidation of NHs, can also be mapped with the kinetic models. With conventional methods without kinetic models, the oxidation of the reducing agent, if this is taken into account at all, can usually only be estimated, which is associated with great uncertainties or

is imprecise.

The kinetic model can be used to determine an amount of aftertreatment fluid which should be storable in a functional SCR unit. With the kinetic model, it may also be possible to use the predicted and / or calculated reducing agent storage capacity to determine a limit reducing agent storage capacity, the so-called limit loading, of the SCR unit, which must at least be achieved in order for the SCR units can be defined as functional. If necessary, it is provided that the amount of fuel is reduced by the limiting reducing agent storage capacity of the SCR

Unit is determined.

It is advantageous if, as input variables for calculating the relevant reactions in the kinetic models, an exhaust gas mass flow, an exhaust gas temperature, a NOx concentration downstream of the internal combustion engine and / or a NOx concentration downstream of the exhaust gas treatment system, in particular downstream of an ammonia slip catalytic converter, a NO2 concentration, an NH3 concentration and an outside temperature can be used. As a result, the exhaust gas mass flow, the exhaust gas temperature, the NOx concentration downstream of the internal combustion engine and / or the NOx concentration downstream of the exhaust gas aftertreatment system and an NH: concentration can be taken into account in the kinetic models. In particular, the time course of the measured values serves as an input variable for

the kinetic models.

It is advantageous if the input variables for calculating the relevant reactions in the kinetic models are real and / or simulated measured values, the values being recorded by at least one real NHs sensor of the exhaust gas aftertreatment system. As a real sensor, the

A physical sensor understood within the scope of the invention. This allows values

in particular real measured values, preferably values recorded over time, are included or taken into account in the calculation of the kinetic models. However, it can also be provided that further values are additionally or alternatively also modeled, for example the NOx value downstream and / or upstream of the engine as well as several temperature values. This means that values can either be exclusively real measured values or exclusively modeled values or a combination of real and modeled values. A single one can do this

Value can be composed of measured value and modeled value.

The further goal is achieved if, in an exhaust gas aftertreatment system of the type mentioned at the beginning, an NHs sensor for determining an NHs concentration is arranged between the SCR units between the SCR units

Determine reducing agent slip downstream of a first SCR unit.

An exhaust gas aftertreatment system according to the invention thus brings the same advantages as have been described in detail with regard to the method according to the invention. The advantages of the invention can be in particular

can be achieved in that an NHs sensor is arranged between the SCR units.

It is useful if the aftertreatment fluid, which in particular contains urea, is metered into the exhaust gas flow via at least one injection device, the injection device being arranged upstream of the SCR units. It can also be beneficial if there is another in the exhaust gas aftertreatment system

Injection device, upstream of the first injection device, is provided.

It is advantageous if the injection device is controlled via a control unit, the control unit prescribing an amount of aftertreatment fluid to be dosed in such a way that a desired total charge amount and the charge amount of the second SCR unit are achieved. In principle, it can also be provided that a

desired load of the first SCR unit is reached.

It is favorable if the first SCR unit and the second SCR unit have different coatings, these in particular in a common housing

are arranged. It is particularly preferred that these form a common facility,

both units are coated differently. For coating, for example, a first side of the SCR device (first SCR unit) can be immersed in a first solution and a second side of the SCR device (second SCR unit) in a second solution. Particularly preferably, an ASC is also arranged in the common housing of the two SCR units downstream of the second SCR unit. It can also be provided that the SCR device in addition to the

SCR units also includes an ASC.

The NHs sensor is a real, physical sensor that measures an NHs concentration downstream of the first SCR unit and upstream of the second SCR unit

determined or this specifies the kinetic model.

It is particularly advantageous if, based on the functional check, a factor for the deterioration of the SCR system is calculated, the metering in of aftertreatment fluid being adapted by the factor. For this purpose, the metering can either be influenced directly or actively by the factor. The regulation and / or the kinetic model can also be adapted by the calculated factor, with the adaptation being able to take place continuously. The factor for the deterioration of the SCR system can be calculated actively or passively. It is advantageous that, following the function check, it is also possible to react to a function of the SCR units. Either a factor for the deterioration of the entire SCR system and / or a factor for each SCR unit can be calculated. It is advantageous if a factor is calculated in each case so that each SCR unit can be checked and for a corresponding state

can be responded to individually.

Further advantages, features and effects are the following exemplary embodiment

described. It shows

Fig. 1 is a schematic representation of an inventive

Exhaust aftertreatment system.

1 shows a schematic illustration of an exhaust gas aftertreatment system 1 according to the invention. The exhaust gas aftertreatment system 1, soft

an internal combustion engine is designed with an internal combustion engine 2,

adjoins, comprises a first optional diesel oxidation catalyst 8, a first SCR unit 3, a second SCR unit 4, an NH3: sensor 6, an injection device 5, a further injection device 9, an SCR-ASC device 10, a second diesel oxidation catalyst 12, several temperature sensors 13, several NOx sensors, a pressure sensor 16, an ASC 15 and a housing 7 for the SCR units 3, 4 and the ASC 15. The SCR units 3, 4 and the ASC 15 form an SCR -System 17.

During normal operation, an operating substance, such as AdBlue® in particular, is metered in upstream of the first SCR unit 3 via the injection device 9. The fuel contains a reducing agent or can be converted into a reducing agent. According to this embodiment, the reducing agent is ammonia (NHa}). The reducing agent is at least temporarily in at least one SCR unit 3, 4

saved.

To check a function of the exhaust gas aftertreatment system 1, in particular the SCR units 3, 4, the amount of NHs that is converted (reacts with NOx) or the amount of NHs that "disappears" due to oxidation or slip

determined.

Claims (10)

Claims 1. A method for checking the function of an exhaust gas aftertreatment system (1) of an internal combustion engine with an internal combustion engine (2), the exhaust gas aftertreatment system (1) comprising an SCR system (17) with at least two SCR units (3, 4), with an in particular urea-containing one in the exhaust gas flow Aftertreatment fluid is metered in, which reacts in the SCR units (3, 4) with the exhaust gas flow, characterized in that between a first SCR unit (3) and the second SCR unit (4) an NHs concentration is achieved by an NHs- Sensor (6) is determined. 2. The method according to claim 1, characterized in that the SCR System (17) is defined as functional if, during the entire course of metering in a predetermined diagnostic amount of aftertreatment fluid, no reducing agent slip is detected by the NHs sensor (6) after the first SCR unit (3), so that essentially all of the aftertreatment fluid is at least is temporarily stored in the first SCR unit (3), and that the SCR system (17) is defined as partially functional or non-functional if a reducing agent slip through the NHs: sensor during the course of metering the predetermined diagnostic amount of aftertreatment fluid (6) is detected after the first SCR unit (3) before the entire predefined diagnostic amount of aftertreatment fluid is metered in, whereby a reduced aftertreatment fluid storage capacity of the first SCR unit (3) is detected. 3. The method according to claim 1 or 2, characterized in that the metering in of the aftertreatment fluid is stopped before the predetermined diagnostic amount of aftertreatment fluid is metered in when a reducing agent slip is detected after the first SCR unit (3), whereby no reducing agent emissions occur as planned. 4. The method according to any one of claims 1 to 3, characterized in that the metering of the aftertreatment fluid is stopped when the predetermined diagnostic amount of aftertreatment fluid is metered and no reducing agent slip is detected after the first SCR unit (3), whereby no reducing agent emissions occur as planned. 5. The method according to any one of claims 1 to 4, characterized in that the predetermined diagnostic amount of aftertreatment fluid for the diagnostic mode is dimensioned in such a way that in a functional SCR unit (3, 4) the reducing agent introduced by the diagnostic amount of aftertreatment fluid is at least temporarily in the SCR unit (3, 4) can be stored, so that when the SCR unit (3, 4) is functional, no reducing agent emissions are planned occur. 6. The method according to any one of claims 1 to 5, characterized in that the emptying of the SCR system (17) by stopping or reducing the aftertreatment fluid supply takes place until a parameter is detected that provides information that there is no more aftertreatment fluid in the SCR system (17) is stored, this parameter being detected by the NHs: sensor (6). 7. The method according to any one of claims 1 to 6, characterized in that the reactions of the first SCR unit (3) relevant for the method are calculated in addition to the real operation in a first kinetic model, the first kinetic model in particular a mathematical mapping of the physical model of the first SCR unit (3), the reactions of the second SCR unit (4) relevant to the method being calculated in addition to the real operation in a second kinetic model, the second kinetic model in particular being a mathematical mapping of the physical model of the second SCR unit (4) corresponds, and wherein a desired total load of the first SCR unit (3) and the second SCR unit (4) and a desired load of the second SCR unit (4) is specified will. 8. The method according to claim 7, characterized in that the input variables for calculating the relevant reactions in the kinetic models are real and / or simulated measured values, the values of at least one real NH3 sensor (6) of the exhaust gas aftertreatment system (1) can be added. 9. The method according to any one of claims 1 to 8, characterized in that based on the functional check a factor for the deterioration of the SCR System (17) is calculated, whereby the metering of Post-treatment fluid is adapted. 10. Exhaust aftertreatment system (1) of an exhaust gas stream of an internal combustion engine with an internal combustion engine (2), the exhaust aftertreatment system (1) comprising at least two SCR units (3, 4), with at least one injection device (5) for metering an in particular urea-containing aftertreatment fluid into the Exhaust gas flow and a control unit for controlling an amount of the injected exhaust gas aftertreatment fluid, the control unit being connected to the injection device, characterized in that between the SCR units (3, 4) an NHs sensor (6) for determining an NH3 concentration between the SCR units (3, 4) is arranged to a Determine reducing agent slip downstream of a first SCR unit (3).