Method for operating an exhaust system with an SCR catalyst for an internal combustion engine, internal combustion engine with an

Abstract

The invention relates to a method for operating an exhaust system (20) for an internal combustion engine (10) of a motor vehicle (K), which exhaust system is loaded with exhaust gas (14) emitted by the internal combustion engine (10). In the method, the exhaust gas (14) is supplied to at least a first SCR catalyst (30) of the exhaust system (20). The method involves a cold start of the internal combustion engine (10), in which at least one pre-operating state of the at least first SCR catalyst (30), which is temporally prior to the cold start and which is ascertained by means of a computing device (12), is utilized. In addition to the amount of water adsorbed on the first SCR catalyst (30) prior to the cold start, an amount of reducing agent, which is assigned to the pre-operating state and is also referred to as the pre-amount of reducing agent, adsorbed on the first SCR catalyst (30) prior to the cold start is used in order to determine the actual amount of reducing agent on the first SCR catalyst (30). As a result, a demanded metering of a reducing agent compensation amount which characterizes the difference between the desired reducing agent amount and the actual reducing agent amount and which is mixed into the exhaust gas (14) upstream of the first SCR catalyst (30) can be achieved. Further aspects of the invention relate to an internal combustion engine (10) having an exhaust system (20) and a motor vehicle (K) having an internal combustion engine (10).

Description

Technical Field

[0001] The invention relates to a method for operating an exhaust system for an internal combustion engine of a motor vehicle, the exhaust system being loaded with exhaust gas emitted by the internal combustion engine and in which the exhaust gas is supplied to at least a first SCR catalytic converter of the exhaust system. Other aspects of the invention relate to an internal combustion engine having an exhaust system and a motor vehicle having an internal combustion engine. Background Art

[0002] For exhaust gas aftertreatment of exhaust gases emitted by modern internal combustion engines, systems for selective catalytic reduction are usually used, which are designed to reduce nitrogen oxides in the exhaust gases. It is expedient to use ammonia, which is released in the context of thermal decomposition and subsequent hydrolysis of a urea solution used as a reducing agent, which is added to the exhaust gas upstream of the SCR catalyst.

[0003] Various systems and methods are known from the prior art which contribute, for example, to improving exhaust gas aftertreatment and, additionally or alternatively, to reducing the consumption of urea.

[0004] For example, DE102008059078A1 describes an exhaust gas aftertreatment system for a self-igniting internal combustion engine, which comprises: a denitration catalytic converter installed in an exhaust gas line, a particle filter and an introduction device for a reducing agent. The burner, the particle filter, the introduction device and the denitration catalytic converter are arranged in this order in the exhaust gas line.

[0005] In contrast, EP 2 061 957 A0 discloses a method for operating an SCR catalytic converter system for an internal combustion engine having an SCR catalytic converter which is suitable for storing a reducing agent and reducing nitrogen oxides (NOx) of the exhaust gas with the participation of the SCR catalytic converter. The reducing agent or a chemical precursor of the reducing agent is stored in a storage container and supplied from there to the SCR catalytic converter via a metering and delivery system. At least one partial flow of the air flowing into the storage container or the air present in the storage container is conditioned so that the relative humidity and / or specific humidity in the storage container is reduced. Summary of the invention

[0006] The object of the present invention is to provide a method for improved exhaust gas aftertreatment of exhaust gases emitted by an internal combustion engine, in particular after a cold start of the internal combustion engine, in which method the exhaust gas flows through an exhaust system of the internal combustion engine which is provided for exhaust gas aftertreatment. Furthermore, the object of the present invention is to provide an internal combustion engine having such an exhaust system and a motor vehicle having an internal combustion engine.

[0007] This object is achieved by a method having the features of claim 1, by an internal combustion engine having the features of claim 6 and by a motor vehicle having the features of claim 7. Advantageous embodiments and expedient refinements of the invention are specified in the dependent claims.

[0008] A first aspect of the present invention relates to a method for operating an exhaust system for an internal combustion engine of a motor vehicle, the exhaust system being loaded with exhaust gas discharged by the internal combustion engine, and in the method exhaust gas is supplied to at least a first SCR catalytic converter of the exhaust system. In the method, at least one pre-operating state of at least the first SCR catalytic converter is utilized. The at least one pre-operating state ascertained by means of a computing device is temporally prior to a cold start of the internal combustion engine and therefore also prior to a cold start of the exhaust system. The computing device can be configured as an engine controller of the internal combustion engine, referred to as ECU for short. It is also conceivable that the computing device can be assigned to a motor vehicle, which can have an internal combustion engine. The expression "pre-operating state" can be understood according to the present invention as a state of at least the first SCR catalytic converter, which has already had this state before a cold start, for example before a combustion operation of the internal combustion engine starting a cold start. The pre-operating state can be generated, for example, by a cooling phase, for example, by a cold start (also referred to as pre-cold start) that is temporally prior to a cold start, which is merely an example. The cold start may begin when the internal combustion engine starts combustion operation, ie, when it starts operation by combustion of fuel, and end, for example, when the starting temperature (also called light-off temperature) of the first SCR catalytic converter is reached.

[0009] The method comprises at least the following steps performed after a cold start of the internal combustion engine:

[0010] In step a), the first SCR catalytic converter is charged with exhaust gas, so that nitrogen oxides, which can also be referred to as nitrogen oxide components, are adsorbed by means of the first SCR catalytic converter, and water is condensed from the exhaust gas at least in an exhaust gas duct arranged upstream of the first SCR catalytic converter of the exhaust system and supplying the first SCR catalytic converter with exhaust gas. In this case, exhaust gas can be generated due to the combustion operation of the internal combustion engine and can be supplied to the first SCR catalytic converter. Due to a cold start, at least a portion of the water contained in the exhaust gas (water content) can condense out on the cold exhaust gas duct.

[0011] In step b), the exhaust gas duct is heated, so that the water condensed from the exhaust gas evaporates upstream of the first SCR catalytic converter, is at least partially supplied to the first SCR catalytic converter together with the exhaust gas, and is adsorbed on the first catalytic converter surface of the first SCR catalytic converter. As the duration after the start of the cold start increases, due to the operation-related heating of the exhaust system and therefore also due to the heating of the exhaust gas duct, at least part of the water condensed in step a) can evaporate, so that the water can be transferred from the liquid phase to the gas phase and can be supplied to the first SCR catalytic converter together with the water content actually contained in the exhaust gas. The evaporated water and the water content contained in the exhaust gas can then be adsorbed on the first catalytic converter surface of the first SCR catalytic converter.

[0012] In step c), the nitrogen oxide components stored on the first catalyst surface are at least partially reduced due to the heating of the catalyst surface, which is caused by an exothermic reaction of water on the catalyst surface and by an increase in the exhaust gas temperature due to the combustion operation of the internal combustion engine. The exothermic reaction can be achieved on the first catalyst surface by water contained in the exhaust gas and by water evaporating from the exhaust gas duct. The heating of the first SCR catalyst can be achieved not only by the exothermic reaction but also by the increased operating time of the internal combustion engine after the start of the cold start.

[0013] In step d), the actual amount of reducing agent adsorbed on the first catalyst surface and assigned to the first SCR catalyst is determined by means of a computing device based on the amount of water on the first catalyst surface, the mass flow of the exhaust gas, the catalyst temperature of the first SCR catalyst, the amount of nitrogen oxide components adsorbed in the first SCR catalyst, and a preliminary amount of reducing agent adsorbed on the first catalyst surface assigned to a preliminary operating state. For example, the actual amount of reducing agent can be modeled by means of the computing device, i.e. the actual amount of reducing agent is determined by means of a model. Preferably, during such modeling, a NOx storage characteristic, i.e. a nitrogen oxide storage characteristic of at least the first SCR catalyst, can be determined.

[0014] In step e), a comparison of the reducing agent setpoint quantity assigned to the first SCR catalytic converter with the actual reducing agent quantity is carried out by means of the computing device. The reducing agent setpoint quantity can be a setpoint value which depends on an operating state of the internal combustion engine and / or the first SCR catalytic converter, in particular a momentary operating state which is present after a cold start and during warm-up of the internal combustion engine, and which can be ascertained by means of the computing device (engine control unit, ECU).

[0015] The reductant nominal amount can be achieved, for example, according to modeling, as is known from the following public document: “Global Kinetic modeling of NH3-SCR with two sites of NH3 storage on Cu-SSZ-13” by Zhiming et al., Chemical Engineering Journal 406(2021) 127120.

[0016] In step f), a reducing agent compensation amount is metered by means of a first metering unit of the exhaust system, the reducing agent compensation amount representing the difference between the reducing agent setpoint amount and the reducing agent actual amount, the first metering unit being controlled by a computing device, and the first metering unit mixing the reducing agent compensation amount into the exhaust gas upstream of the first SCR catalyst due to being controlled by the computing device. The computing device can thus control the first metering unit as a function of the difference, so that the reducing agent compensation amount that can be represented by the difference can be mixed into the exhaust gas by means of the first metering unit.

[0017] The invention is based on the recognition that the less nitrogen oxide components can be stored and converted in the first SCR catalytic converter, the greater the amount of water on the catalytic converter surface. By mixing in the reducing agent compensation amount determined by means of a calculation device, it is possible to prevent too little or too much reducing agent from being added upstream of the first SCR catalytic converter, so that a particularly advantageous exhaust gas aftertreatment can be achieved overall.

[0018] In a further advantageous embodiment of the invention, the actual amount of reducing agent is determined by a calculation device based on at least one water quantity adsorbed on the first SCR catalytic converter and assigned to a preliminary operating state and / or based on at least one nitrogen oxide component adsorbed on the first SCR catalytic converter and assigned to a preliminary operating state. The actual amount of reducing agent can be determined particularly accurately using the water quantity and, in addition or alternatively, the nitrogen oxide component quantity. In particular, it is therefore taken into account which state the first SCR catalytic converter had before the cold start, i.e., which state the first SCR catalytic converter had at the beginning of the cold start.

[0019] In a further advantageous development of the invention, a humidity reservoir of the exhaust system is used which is arranged upstream of the first SCR catalytic converter and which adsorbs at least a portion of the water condensed from the exhaust gas on its humidity reservoir surface. This is advantageous because a prolonged storage period (adsorption period) of the water is achieved, so that the time of the exothermic reaction at the first SCR catalytic converter can be shifted later in time. As a result, the first SCR catalytic converter can absorb more nitrogen oxides after a cold start and during the warming up of the internal combustion engine, while at the same time a portion of the condensed water can be stored in the humidity reservoir, and overall drier exhaust gas reaches the first SCR catalytic converter during a cold start and during the warming up of the internal combustion engine.

[0020] In a further advantageous embodiment of the invention, an oxidation catalytic converter and / or a nitrogen oxide storage catalytic converter is used as the humidity storage device. This is advantageous because, on the one hand, water storage and, therefore, also exhaust gas aftertreatment is achieved, if an oxidation catalytic converter is used as the humidity storage device and, in addition or as an alternative, a nitrogen oxide storage catalytic converter is used.

[0021] In a further advantageous development of the invention, a second SCR catalytic converter arranged downstream of the first SCR catalytic converter is used for exhaust gas aftertreatment. The second SCR catalytic converter enables particularly advantageous exhaust gas aftertreatment over a wider range of applications.

[0022] A second aspect of the invention relates to an internal combustion engine comprising an exhaust gas system which is designed to carry out the method according to the first aspect of the invention. The exhaust gas system allows particularly low-pollutant operation of the internal combustion engine.

[0023] A third aspect of the invention relates to a motor vehicle comprising an internal combustion engine according to the second aspect of the invention. The motor vehicle can be driven in a particularly low-pollutant manner by means of the internal combustion engine.

[0024] Preferred embodiments and advantages described with respect to one of the aspects apply correspondingly to the corresponding other aspects of the invention, and vice versa.

[0025] The features and feature combinations mentioned above in the description and the features and feature combinations mentioned below in the description of the drawings and / or the features and feature combinations shown individually in the drawings may be used not only in the respectively described combination but also in other combinations or individually, without departing from the scope of the present invention.

[0026] Further advantages, features and details of the invention emerge from the claims, from the subsequent description of preferred embodiments and with the aid of the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will be explained again below with the aid of specific embodiments. The accompanying drawings for this purpose are as follows:

[0028] Figure 1 A schematic diagram shows a motor vehicle, which comprises an internal combustion engine having an exhaust system, through which exhaust gas discharged by means of the internal combustion engine flows, which exhaust gas passes through an exhaust gas duct of the exhaust system and successively through a humidity accumulator of the exhaust system designed as an oxidation catalytic converter, through a first SCR catalytic converter, through a second SCR catalytic converter and through a silencer;

[0029] Figure 2 showing a diagram in which, during a cold start of the internal combustion engine, the vehicle speed of the motor vehicle, the first catalytic converter temperature of the first SCR catalytic converter, the second catalytic converter temperature of the second catalytic converter, and the cumulative nitrogen oxide emissions of the exhaust gas before exhaust gas aftertreatment by the oxidation catalytic converter, the cumulative nitrogen oxide emissions of the exhaust gas after the first SCR catalytic converter, and the cumulative nitrogen oxide emissions of the exhaust gas after the second SCR catalytic converter are depicted over time by way of example; and

[0030] Figure 3 A flow chart is shown which illustrates various method steps of a method for operating an exhaust system of an internal combustion engine for driving a motor vehicle. DETAILED DESCRIPTION

[0031] Figure 1 By way of example, a motor vehicle K is shown which has an internal combustion engine 10. The internal combustion engine 10 has an exhaust system 20. With the aid of the internal combustion engine 10 and the exhaust system 20, a method for operating the exhaust system 20 can be carried out, the essential method steps of which are described in Figure 3 Schematically described in .

[0032] In a method for operating an exhaust system 20 of an internal combustion engine 10, the exhaust system 20 is charged with exhaust gas 14, which is discharged by the internal combustion engine 10. The exhaust gas 14 guided by the exhaust gas conduit 22 of the exhaust system 20 first flows through a humidity storage 24 of the exhaust system 20. The humidity storage 24 can be configured as an oxidation catalytic converter and additionally or alternatively as a nitrogen oxide storage catalytic converter. Subsequently, the exhaust gas 14 flows successively through a first SCR catalytic converter 30 close to the engine, a second SCR catalytic converter 40 (which is currently configured as a bottom SCR catalytic converter) also used for exhaust gas aftertreatment, and a silencer 50. The exhaust system 20 may also include other catalytic converters or filters, but they are not further shown at present. In order to mix a reducing agent, especially an aqueous urea solution, a first metering unit 32 is arranged upstream of the first SCR catalytic converter 30 and a second metering unit 42 is arranged upstream of the second SCR catalytic converter 40. Internal combustion engine 10 has a computing device 12 , also referred to as ECU, by means of which, for example, two metering units 32 , 42 can be controlled.

[0033] After a cold start of the internal combustion engine 10, Figure 3 In the method, use is made of a preliminary operating state of at least the first SCR catalytic converter 30 , which is present in time before the cold start and is ascertained by means of the computing device 12 of the internal combustion engine 10 .

[0034] In step a), the first SCR catalytic converter 30 is charged with the exhaust gas 14, so that nitrogen oxides are adsorbed by means of the first SCR catalytic converter 30, and water is condensed from the exhaust gas 14 in an exhaust gas duct 22 of the exhaust system, which is arranged upstream of the first SCR catalytic converter 30 and supplies the exhaust gas 14 to the first SCR catalytic converter 30. The humidity reservoir 24 of the exhaust system 20, which is arranged upstream of the first SCR catalytic converter 30, can adsorb at least a portion of the water condensed from the exhaust gas 14 on a humidity reservoir surface of the humidity reservoir 24.

[0035] In step b), the exhaust gas duct 22 is heated, which is caused by the combustion operation of the internal combustion engine 10, so that water condensed from the exhaust gas 14 evaporates upstream of the first SCR catalyst 30, is at least partially supplied to the first SCR catalyst 30 together with the exhaust gas 14, and is adsorbed on the first catalyst surface of the first SCR catalyst 30.

[0036] In step c), the nitrogen oxide components stored on the first catalytic converter surface are at least partially reduced due to the heating of the first catalytic converter surface (which in the present case is designed as a zeolite surface), which is caused by an exothermic reaction of water on the first catalytic converter surface and by an increase in the exhaust gas temperature due to the combustion operation of the internal combustion engine.

[0037] In step d), the actual amount of reducing agent adsorbed on the first catalytic converter surface and assigned to the first SCR catalytic converter 30 is ascertained by means of the computing device 12 as a function of the amount of water on the first catalytic converter surface, as a function of the mass flow of the exhaust gas 14, as a function of the (first) catalytic converter temperature T1 of the first SCR catalytic converter 30, as a function of the amount of nitrogen oxide components adsorbed in the first SCR catalytic converter 30 and as a function of a preliminary amount of reducing agent adsorbed on the first catalytic converter surface assigned to a preliminary operating state.

[0038] Furthermore, the actual amount of reducing agent can be determined by the calculation device 12 based on at least one default amount of water adsorbed on the first SCR catalytic converter 30 and assigned to a preliminary operating state and additionally or alternatively based on at least one default amount of a nitrogen oxide component adsorbed on the first SCR catalytic converter 30 and assigned to a preliminary operating state.

[0039] The stored nitrogen oxide component may include, on the one hand, nitrogen oxides caused by previous operating conditions, in particular a preliminary amount of nitrogen oxide components, which may have been stored in the first SCR catalytic converter 30 before the cold start, and, on the other hand, nitrogen oxides additionally stored in the first SCR catalytic converter 30 after the cold start and during the warm-up of the internal combustion engine 10. The same applies to water adsorbed on the first SCR catalytic converter 30, which may consist of a preliminary amount of water (caused by previous operating conditions) and a water amount adsorbed on the first SCR catalytic converter 30 starting from the cold start (on the first catalytic converter surface).

[0040] In step e), the desired reducing agent quantity assigned to the first SCR catalytic converter 30 is compared with the actual reducing agent quantity by means of the computing device 12 .

[0041] In step f), finally, a reducing agent compensation amount representing the difference between the set amount of reducing agent and the actual amount of reducing agent is metered into the exhaust system 20 by means of a first metering unit 22, which is controlled by the computing device 12 and which, as a result of being controlled by the computing device 12, mixes the reducing agent compensation amount into the exhaust gas 14 upstream of the first SCR catalytic converter 30.

[0042] It is clear that method steps a) to f) performed with respect to the first SCR catalytic converter 30 can also be performed with respect to the second SCR catalytic converter 40. The exhaust gas flow from the first SCR catalytic converter 30 and into the second SCR catalytic converter 40 can be understood here as exhaust gas entering the second SCR catalytic converter 40. It is taken into account that not only the humidity storage device 24 but also the first SCR catalytic converter 30 absorbs water from the exhaust gas during the cold start. The exothermic reaction caused by water on the second SCR catalytic converter 40 is accordingly time-shifted and therefore takes place later than the exothermic reaction started on the first SCR catalytic converter 30.

[0043] This is also achieved, for example, by Figure 2 visible, Figure 2 A diagram is shown in which the following variables are plotted over a time s in seconds: temperature T in units of ° C.; vehicle speed v in units of km / h; cumulative nitrogen oxide emissions cNOx (unitless).

[0044] Figure 2It is shown that at the beginning of a cold start at t=0, the first catalytic converter temperature T1 of the first SCR catalytic converter 30 rises significantly before the second catalytic converter temperature T2 of the second SCR catalytic converter 40. The time-shifted rise of the second catalytic converter temperature T2 is caused by the fact that the exothermic reaction caused by water starts later on the second SCR catalytic converter 40 than on the first SCR catalytic converter 30. The respective catalytic converter temperatures T1, T2 can be present on the respective catalytic converter surfaces of the respective SCR catalytic converters 30, 40. The catalytic converter temperatures T1, T2 can preferably be modeled or can be measured, for example, by temperature sensors of the exhaust system 20 that are respectively connected to the computing device 12 in a signal-transmitting manner.

[0045] The longer the exhaust gas conduit 22 is and the greater the water storage capacity of the humidity reservoir 24 is, the longer the condensed water can be retained before it reaches the first SCR catalytic converter 30 and subsequently the second SCR catalytic converter 40. In other words, the respective time of the start of the respective water-induced exothermic reaction in the respective SCR catalytic converter 30, 40 can be delayed. This start of the respective exothermic reaction is achieved by means of the Figure 2 This can be seen in the corresponding steep rise in the corresponding catalytic converter temperatures T1 , T2 in FIG.

[0046] In this regard, refer to Figure 2 , see curves CNOx0, CNOx1 and CNOx2. Here, CNOx0 indicates the nitrogen oxide concentration of the original emission in the exhaust gas 14 of the internal combustion engine 10, which is present upstream of the humidity storage device 24. In addition, CNOx1 represents the nitrogen oxide concentration of the exhaust gas 14 after the first SCR catalyst 30 and before the second SCR catalyst 40, while CNOx2 indicates the nitrogen oxide concentration of the exhaust gas 14 after the second SCR catalyst 40.

[0047] Due to the time offset, i.e. due to the delay of the exothermic reaction, drier exhaust gas reaches the corresponding SCR catalytic converter 30, 40 over a longer period of time after the start of the cold start. Therefore, the first SCR catalytic converter 30 or the second SCR catalytic converter 40 can absorb more nitrogen oxides after the cold start and during the hot operation of the internal combustion engine 10, and thus improved exhaust gas purification can be achieved.

[0048] This method makes it possible to achieve a particularly suitable metering of reducing agent in these cases by the metering units 32, 42. With the aid of this method, the nitrogen oxide storage characteristics of the first SCR catalytic converter 30 and the second SCR catalytic converter 40 can be determined particularly accurately as a function of the already mentioned variables, such as the variables assigned to the preliminary operating state and the variables present during the cold start (such as the catalyst temperatures T1, T2). Reference numerals list

[0049] 10 Internal combustion engine

[0050] 12Computing Device

[0051] 14 Exhaust

[0052] 20 Exhaust equipment

[0053] 22 Exhaust gas guide pipe

[0054] 24 humidity memory

[0055] 30 First SCR Catalyst

[0056] 32 First metering unit

[0057] 40 Second SCR catalyst

[0058] 42 Second Measuring Unit

[0059] 50 Silencer

[0060] KMotor Vehicle

[0061] t time

[0062] T Temperature

[0063] T1 first catalyst temperature

[0064] T2 Second catalyst temperature

[0065] vVehicle speed

[0066] cNOx Cumulative nitrogen oxide emissions

[0067] cNOx0 Nitrogen oxide concentration in raw emissions

[0068] cNOx1 nitrogen oxide concentration

[0069] cNOx2 nitrogen oxide concentration

Claims

1. A method for operating an exhaust system (20) of an internal combustion engine (10) for a motor vehicle (K), the exhaust system being loaded with exhaust gas (14) emitted by the internal combustion engine (10), and in which the exhaust gas (14) is supplied to at least a first SCR catalytic converter (30) of the exhaust system (20), the method comprising at least the following steps which are carried out after a cold start of the internal combustion engine (10), in which at least one preliminary operating state of the at least first SCR catalytic converter (30) which is located before the cold start and is ascertained by means of a computing device (12): a. loading the first SCR catalyst (30) with exhaust gas (14), so that nitrogen oxides are adsorbed by means of the first SCR catalyst (30) and water is condensed from the exhaust gas (14) in at least an exhaust gas guide pipe (22) arranged upstream of the first SCR catalyst (30) of the exhaust system and supplying the exhaust gas (14) to the first SCR catalyst (30); b. heating the exhaust gas guide pipe (22), so that water condensed from the exhaust gas (14) evaporates upstream of the first SCR catalyst (30), is at least partially supplied to the first SCR catalyst (30) together with the exhaust gas (14), and is adsorbed on the first catalyst surface of the first SCR catalyst (30); c. the nitrogen oxide components stored on the first catalyst surface are at least partially reduced due to heating of the catalyst surface, the heating being caused by an exothermic reaction of water on the catalyst surface and by an increase in the exhaust gas temperature due to combustion operation of the internal combustion engine (10); d. an actual amount of reducing agent adsorbed on the first catalytic converter surface and assigned to the first SCR catalytic converter (30) is determined by means of a computing device (12) based on an amount of water on the first catalytic converter surface, based on a mass flow of the exhaust gas (14), based on a catalytic converter temperature (T1) of the first SCR catalytic converter (30), based on an amount of nitrogen oxide components adsorbed in the first SCR catalytic converter (30) and based on a preliminary amount of reducing agent adsorbed on the first catalytic converter surface assigned to a preliminary operating state; e. the rated amount of reducing agent assigned to the first SCR catalytic converter (30) is compared with the actual amount of reducing agent by means of a computing device (12); f. Metering a reducing agent compensation amount by means of a first metering unit (32) of the exhaust system (20), the reducing agent compensation amount representing the difference between a set reducing agent amount and an actual reducing agent amount, the first metering unit being controlled by a computing device (12) and the first metering unit mixing the reducing agent compensation amount into the exhaust gas (14) upstream of the first SCR catalytic converter (30) due to being controlled by the computing device (12).

2. The method according to claim 1, characterized in thatThe actual amount of reducing agent is determined by a calculation device (12) based on at least one preset amount of water adsorbed on the first SCR catalytic converter (30) and assigned to a preliminary operating state and / or based on at least one preset amount of a nitrogen oxide component adsorbed on the first SCR catalytic converter (30) and assigned to a preliminary operating state.

3. The method according to claim 1 or 2, characterized in that: A humidity storage device (24) of the exhaust system (20) is used, which is arranged upstream of a first SCR catalytic converter (30) and which absorbs at least a portion of the water condensed from the exhaust gas (14) on its humidity storage device surface.

4. The method according to claim 3, characterized in that: An oxidation catalytic converter and / or a nitrogen oxide storage catalytic converter is used as humidity storage device ( 24 ).

5. The method according to any one of the preceding claims, characterized in that The second SCR catalytic converter (40) of the exhaust gas device (20) is arranged downstream of the first SCR catalytic converter (30) and is used for exhaust gas aftertreatment.

6. An internal combustion engine (10) comprising an exhaust system (20), the internal combustion engine being designed to carry out the method according to any one of claims 1 to 5.

7. A motor vehicle (K) comprising an internal combustion engine (10) according to claim 6.

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