DE102018218200B4 - Method for operating an exhaust system of an internal combustion engine, exhaust system and motor vehicle - Google Patents

Abstract

Method for operating an exhaust system (1) of an internal combustion engine (2) with an LNT catalytic converter (3) arranged in a first control circuit RK1 and an energy storage device (4) arranged in a second control circuit RK2, which is used to supply an electrical heating device (5) with electric current is formed, with a nitrogen oxide storage state SSZ of the LNT catalytic converter (3) being regulated in the first control circuit RK1 and a state of charge LZ of the energy storage device (4) being regulated in the second control circuit RK2, and with the first and second control circuits RK1, RK2 being linked to one another, having: - determining the nitrogen oxide storage state SSZ in the first control circuit RK1, - using the determined nitrogen oxide storage state SSZ as an input variable for controlling a lower target value for the state of charge USWL of the energy storage device (4) in the second control circuit RK2, the determined nitrogen oxide storage state SSZ having a minimum value for the nitrogen oxide storage state S SZmin is compared, when the minimum value for the nitrogen oxide storage state SSZminder lower setpoint for the state of charge USWL is exceeded or left at an increased value and when the minimum value for the nitrogen oxide storage state SSZminder lower setpoint for the state of charge USWL is reached or exceeded, it is reduced or left at a reduced value .

Description

The present invention relates to methods for operating an exhaust system of an internal combustion engine, exhaust systems and a motor vehicle with such an exhaust system.

Ideally, systems for the aftertreatment of exhaust gases from a combustion engine should be fully functional in all possible operating situations in order to keep the emission of air pollutants as low as possible and to be able to comply with legal requirements for pollutant emissions.

A cold start phase of the internal combustion engine is particularly problematic. In which, due to the low exhaust gas temperatures, the light-off temperature of the catalysts in the exhaust gas aftertreatment system has not yet been reached and catalytic conversion can therefore not take place or can only take place to a limited extent.

In order to minimize the emission of air pollutants, especially nitrogen oxides, in the cold start phase, two different strategies are currently used. On the one hand, the catalytic converter or the exhaust gas reaching the catalytic converter can be heated electrically ( DE 10 2008 023 394 A1 ) and on the other hand, a nitrogen oxide storage catalytic converter, also known as an LNT catalytic converter (lean oxide trap), can be used to temporarily store nitrogen oxides and thus remove them from the exhaust gas for a certain period of time.

However, the disadvantage of electrical heating is the additional energy requirement, while an LNT catalytic converter can only store a limited amount of nitrogen oxides before a so-called regeneration of the LNT catalytic converter is required, in which the stored nitrogen oxides are reduced to non-toxic substances and converted back into be discharged into the exhaust stream.

Despite these two strategies, which can also be used in combination, it is currently not yet possible to remove nitrogen oxides from the exhaust gas in all operating situations to such an extent that legally prescribed limit values can be observed and the environmental pollution is low.

Such an operating situation can occur, for example, during a cold start phase of the combustion engine, in which on the one hand the storage capacity of the LNT catalytic converter is exhausted and consequently no more nitrogen oxides can be stored and on the other hand the charge level of the battery is so low that electricity is required to heat the Exhaust gas or catalyst can no longer be made available sufficiently.

An exhaust gas control system for a hybrid vehicle with an LNT catalytic converter and an electronic control unit is known from US 2018/0 281 774 A1. If the battery charge level is sufficient, the control unit controls the combustion engine during the regeneration of the LNT catalytic converter in such a way that the engine speed is reduced. The electric motor is controlled in such a way that the required torque is compensated. On the other hand, if the charge level of the battery is low, the regeneration of the LNT catalytic converter is carried out during normal operation of the combustion engine.

the DE 10 2016 219 038 A1 proposes a method for controlling an exhaust gas cleaning system in a motor vehicle with an internal combustion engine, an exhaust system, at least one LNT catalytic converter, at least one battery and at least one heating device in the exhaust system. The following method steps are provided: operating the internal combustion engine, determining the temperature of the LNT catalytic converter, determining the state of charge of the battery, switching on the electrical heating device to heat the LNT catalytic converter as long as the state of charge of the battery is above a first predetermined level and the temperature of the LNT catalyst are below a threshold, increasing engine torque as long as the battery state of charge is below a second predetermined level, starting a regeneration of the LNT catalyst and turning off the electric heater when the temperature of the LNT catalyst has reached a predetermined value . US 2007/0 205 028 A1 discloses a hybrid vehicle propulsion system having an internal combustion engine, an LNT catalyst and an energy storage device. The internal combustion engine is operated in a spark ignition mode when the energy storage device is at a low state of charge and in an auto-ignition mode at a higher state of charge. The nitrogen oxide storage state of the LNT catalytic converter can be taken into account here.

It is therefore the object of the invention to provide options with which the aforementioned disadvantages can be eliminated. In particular, the emission of nitrogen oxides should be reliably reduced in a cold start phase of an internal combustion engine.

This problem is solved by the subject matter of the independent claims. Advantageous developments of the invention are specified in the dependent claims.

The basic idea of the invention is the parameter "state of charge" of the energy storage device, the electrical power for electrical heating, z. B. the catalyst and / or the exhaust, provides to link with the parameter "nitrous oxide storage state" of the LNT catalyst.

In this way, it can be prevented that, during a cold start of the internal combustion engine, neither sufficient nitrogen oxides can be stored nor electrical current can be made available for heating the catalytic converter and/or exhaust gas.

According to a method according to the invention for operating an exhaust system of an internal combustion engine with an LNT catalytic converter arranged in a first control loop and an energy storage device arranged in a second control loop, which is designed to supply an electrical heating device with electrical current, a nitrogen oxide storage state of the LNT Catalyst and regulated in the second control circuit, a state of charge of the energy storage device, wherein the first and the second control circuit are linked.

An internal combustion engine, sometimes also referred to as an internal combustion engine, is to be understood as meaning an internal combustion engine for converting chemical energy contained in the fuel into mechanical work. Exhaust gas is formed during the combustion process required for this. The internal combustion engine can be designed, for example, as a self-igniting or spark-ignited internal combustion engine. Motor petrol or diesel can be used as fuel, for example.

In addition to the internal combustion engine, there can be an electric machine that can be operated either as an electric motor or as a generator. Combustion engine and electric machine together can form a hybrid drive.

The exhaust system is formed by an exhaust line through which exhaust gas flows and in which catalysts, filters, sensors, etc. are arranged, so that the exhaust gas can also flow through or around the catalysts, filters, sensors, etc. and the exhaust gas is post-treated can be. Stated flow directions refer to the flow direction of the exhaust gas from the combustion engine in the direction of the exhaust.

As initially described, the LNT catalytic converter is a nitrogen oxide storage catalytic converter which can store nitrogen oxides in accordance with its nitrogen oxide storage capacity. If the nitrogen oxide storage capacity is exhausted, the LNT catalytic converter must be regenerated, e.g. B. by the LNT catalyst unburned or partially burned fuel is supplied as a reducing agent.

The nitrogen oxide storage status SSZ of the LNT catalytic converter indicates how much of the nitrogen oxide storage capacity can still be used, i. H. how many nitrogen oxides can still be stored. The nitrogen oxide storage state SSZ can, for example, express the still usable nitrogen oxide storage capacity as a percentage. A low nitrogen oxide storage state SSZ of z. B. 20% means that only a few nitrogen oxides can be stored and z. B. only 20% of the original nitrogen oxide storage capacity are available. Conversely, a high nitrogen oxide storage state SSZ of z. B. 90% accordingly that sufficient nitrogen oxides can be stored and z. B. 90% of the original nitrogen oxide storage capacity are still available.

The nitrogen oxide storage state SSZ is regulated by means of the first control loop. For this purpose, the current nitrogen oxide storage state SSZ is determined as an actual value and compared to a lower target value for the nitrogen oxide storage state USWs. If the current nitrogen oxide storage state SSZ falls below the lower target value for the nitrogen oxide storage state USWs, sufficient nitrogen oxides can no longer be stored and regeneration of the LNT catalytic converter is initiated. The control of the first control loop can be implemented accordingly as a two-point control, with the LNT catalytic converter being regenerated if the lower setpoint for the nitrogen oxide storage state USWs is not reached and nitrogen oxides can continue to be stored if the lower setpoint for the nitrogen oxide storage state USWs is reached or exceeded.

An energy storage device, for example a rechargeable battery, is arranged in a second control loop, the state of charge LZ of which is controlled by means of the second control loop. The energy storage device can be designed, for example, as a lead-acid battery or as a lithium-ion battery. The energy storage device can, for example, be assigned to the aforementioned electric machine, so that the energy storage device can be charged by means of the electric machine acting as a generator and/or discharged by means of the electric machine acting as an electric motor.

The energy storage device powering an electric heater. The electric heater is used to heat the catalytic converters in the exhaust system. For this example, a catalyst such. B. an SCR catalytic converter (SCR, engl. Selective Catalytic Reduction, German Selective Catalytic Reduction), directly and / or indirectly heated by the heating device directly and / or the catalyst supplied exhaust gas heated. The heating device can, for example, be part of a so-called e-catalyst.

The temperature of the catalytic converters is increased by means of the electric heating device so that their light-off temperature is reached quickly and the air pollutants contained in the exhaust gas can be effectively post-treated.

The state of charge LZ of the energy storage device indicates how much electrical charge is still stored or can be removed. The state of charge LZ can be expressed, for example, as a percentage of a maximum possible amount of charge to be stored. A low state of charge LZ of z. B. 20% means that only little electrical charge is stored and z. B. only 20% of the originally stored electrical charge are available. Conversely, a high state of charge LZ of z. B. 90% accordingly that sufficient electrical charge is stored and z. B. still 90% of the originally stored amount of charge are available.

The state of charge LZ is regulated by means of the second control circuit in that the current state of charge LZ is determined as the actual value and compared to a lower target value for the state of charge USW _L. If the current state of charge falls below the lower target value for the state of charge USW _L , the energy storage device has to be recharged.

According to the invention, the first and the second control circuit are linked to one another. For example, the actual value of the nitrogen oxide storage state SSZ can be used to determine the lower target value for the state of charge USW _L and/or the actual value of the state of charge LZ to determine the lower target value for the state of nitrogen oxide storage USWs.

This can ensure that at least either the charge state LZ is sufficient for electrical heating or the nitrogen oxide storage state SSZ allows sufficient storage of nitrogen oxides. Consequently, nitrogen oxides contained in the exhaust gas can be sufficiently removed from the exhaust gas either by being stored in the LNT catalyst or by being after-treated in the heated catalyst. The emission of air pollutants, in particular nitrogen oxides, into the environment can be largely avoided and legal requirements regarding the emission of pollutants can be complied with.

Furthermore, the method according to the invention enables optimal use of the energy storage device, since its lower setpoint value for the state of charge USW _L is not changed constantly, but only when necessary. If this case of need does not occur, the energy storage device can be operated in a larger range of the state of charge LZ, so that, for example, in the case of an electric motor assigned to the internal combustion engine, a drive can take place over a longer period of time by means of the electric motor and consequently fuel for operating the internal combustion engine can be saved.

In addition, use of the internal combustion engine for heating the catalytic converters, in that the internal combustion engine is either operated under increased load to increase the exhaust gas temperature or is used to drive the electric machine acting as a generator to power the electric heating device, can be avoided. Both variants would in fact require increased fuel consumption and consequently increase the amount of nitrogen oxides to be treated.

According to the invention, the method includes determining the nitrogen oxide storage state SSZ in the first control loop, ie the actual value of the controlled variable nitrogen oxide storage state SSZ is determined. The determined nitrogen oxide storage state SSZ is then used as an input variable for controlling a lower target value for the state of charge USW _L in the second control loop.

In other words, the lower target value for the state of charge USW _L can be defined as a function of the nitrogen oxide storage state SSZ.

The determined nitrogen oxide storage state SSZ is compared with a minimum value for the nitrogen oxide storage state SSZ _min . If the minimum value for the nitrogen oxide storage state SSZ _min is not reached, the lower setpoint for the state of charge USW _L of the energy storage device is increased in the second control circuit or left at an increased value and if the minimum value for the nitrogen oxide storage state SSZ _min is reached or exceeded, the lower setpoint for the State of charge USW _L of the energy storage device in the second control loop is reduced or left at a reduced value.

In other words, the lower target value for the state of charge USW _L can be increased and thus an earlier charging of the energy storage device can be initiated if the nitrogen oxide storage state SSZ reaches the minimum value for the nitrogen oxide storage stands SSZ _min below, ie only a few nitrogen oxides can be stored. By charging the energy storage device earlier, it can be ensured that sufficient electricity is available for electrical heating and, for example, the light-off temperature of a catalytic converter, e.g. B. an SCR catalyst, can be achieved quickly.

The minimum value for the nitrogen oxide storage state SSZ _min is preferably higher than the lower target value for the nitrogen oxide storage state USWs. As a result, the lower target value for the state of charge USW _L can be changed when the nitrogen oxide storage state SSZ falls, before regeneration of the LNT catalytic converter is triggered when the nitrogen oxide storage state USWs falls below the lower target value. On the one hand, this advantageously avoids too frequent regeneration of the LNT catalyst and, on the other hand, the advantages associated with the method according to the invention can be realized.

Another method according to the invention includes determining the state of charge LZ of the energy storage device instead of determining the nitrogen oxide storage state SSZ in the first control circuit and the subsequent method steps. The state of charge LZ can be determined, for example, using an electrical voltage of the energy storage device at a defined load. The determined state of charge LZ is then used as an input variable for controlling a lower target value for the nitrogen oxide storage state USWs in the first control loop.

In other words, the lower target value for the nitrogen oxide storage state USWs can be defined as a function of the state of charge LZ.

For example, the determined state of charge LZ can be compared with a minimum value for the state of charge LZ _min . If the minimum value for the state of charge LZ _min is not reached, the lower target value for the nitrogen oxide storage state USWs of the energy storage device is increased in the first control circuit or left at an increased value and if the minimum value for the state of charge LZ _min is reached or exceeded, the lower target value for the nitrogen oxide storage state USWs of the energy storage device reduced in the first control loop or left at a reduced value.

In other words, the lower target value for the nitrogen oxide storage state USWs can be increased and thus an earlier regeneration of the LNT catalytic converter can be initiated if the state of charge LZ falls below the minimum value for the state of charge LZ _min , i.e. only little electrical current can be made available for electrical heating and sufficient heating can no longer be ensured. The earlier regeneration of the LNT catalytic converter can ensure that sufficient nitrogen oxides can be stored, especially after a cold start of the internal combustion engine.

The minimum value for the state of charge LZ _min is preferably higher than the lower target value for the state of charge USW _L . As a result, the lower setpoint for the nitrogen oxide storage state USWs can be changed when the state of charge LZ falls, before recharging of the LNT catalytic converter is triggered when the state of charge USW _L falls below the lower setpoint. On the one hand, this advantageously avoids too frequent regeneration of the LNT catalyst and, on the other hand, the advantages associated with the method according to the invention can be realized.

According to various embodiment variants, the method can include determining the state of charge LZ of the energy storage device and using the determined state of charge LZ as an input variable for controlling a lower target value for the nitrogen oxide storage state USWs in the first control loop.

The determined state of charge LZ can be compared with a minimum value for the state of charge LZ _min . If the minimum value for the state of charge LZ _min is not reached, the lower setpoint for the nitrogen oxide storage state USWs can be increased or left at an increased value, and if the minimum value for the state of charge LZ _min is reached or exceeded, the lower setpoint for the nitrogen oxide storage state USWs can be reduced or at be left at a reduced value.

According to further embodiment variants, the methods can include determining an end of operation of the internal combustion engine. The lower setpoint value for the state of charge USW _L and/or the state of nitrogen oxide storage USWs is increased or left at an increased value only when the end of operation of the internal combustion engine is imminent.

The end of operation of the internal combustion engine can take place in a vehicle, for example, based on a route calculation to a destination, it being possible to assume that the end of operation is imminent shortly before the destination is reached. Alternatively, the end of operation can also be based on a time if the end of operation is scheduled for a specific time.

In other words, the controls of the lower target value for the state of charge USW _L and/or the nitrogen oxide storage state USWs described above can be carried out under the condition that an end of operation of the internal combustion engine is imminent.

Advantageously, the lower setpoint for the state of charge USW _L and/or the lower setpoint for the state of nitrogen oxide storage USWs are only increased or left at an increased value if a cold start must subsequently be expected due to an imminent end of operation of the internal combustion engine, in which due to If the temperature of the catalytic converters is too low, adequate exhaust aftertreatment would otherwise not be guaranteed. If the end of operation is not imminent, the lower target value for the state of charge USW _L and/or the lower target value for the state of nitrogen oxide storage USWs, on the other hand, can be reduced or left at a reduced value, so that the number of regenerations of the LNT catalytic converter can be kept as low as possible and the storage capacity of the energy storage device is as complete as possible, e.g. B. for driving the electric motor can be exploited.

Overall, therefore, a reduction in the emission of air pollutants is achieved without significantly increasing fuel consumption.

An exhaust gas system according to the invention for an internal combustion engine has an LNT catalytic converter arranged in a first control circuit, with a nitrogen oxide storage state SSZ of the LNT catalytic converter being controlled in the first control circuit. Furthermore, the exhaust system has an energy storage device which is arranged in a second control circuit and is designed to supply an electrical heating device with electricity, with a state of charge LZ of the energy storage device being controlled in the second control circuit.

In addition, the exhaust system has a device for determining the nitrogen oxide storage state SSZ of the LNT catalytic converter and a control unit for controlling a lower target value for the state of charge USW _L in the second control circuit depending on the determined nitrogen oxide storage state SSZ, the determined nitrogen oxide storage state SSZ having a minimum value for the nitrogen oxide storage state SSZ _min is compared, if the minimum value for the nitrogen oxide storage state SSZ _min is not reached, the lower target value for the state of charge USW _L is increased or left at an increased value and if the minimum value for the nitrogen oxide storage state SSZ _min is reached or exceeded, the lower target value for the state of charge USW _L is reduced or left at a reduced value, is set up and configured.

An alternative exhaust gas system for an internal combustion engine has an LNT catalytic converter arranged in a first control circuit, with a nitrogen oxide storage state SSZ of the LNT catalytic converter being controlled in the first control circuit. Furthermore, the exhaust system has an energy storage device which is arranged in a second control circuit and is designed to supply an electrical heating device with electricity, with a state of charge LZ of the energy storage device being controlled in the second control circuit.

In addition, the alternative exhaust system has a device for determining the state of charge LZ of the energy storage device and a control unit that is set up and designed to control a lower target value for the nitrogen oxide storage state USWs in the first control circuit depending on the determined state of charge LZ.

The exhaust gas systems according to the invention can, for example, be suitable for carrying out the methods according to the invention explained above. The above explanations for explaining the methods according to the invention therefore also serve to describe the exhaust gas systems according to the invention. The advantages of the exhaust systems according to the invention correspond to those of the methods according to the invention and their design variants.

The control unit receives input signals from the devices for determining the nitrogen oxide storage state SSZ of the LNT catalyst and/or the state of charge LZ of the energy storage device, processes them and triggers actuators in response to the processed input signals based on instructions or a code programmed in the control system according to one or more routines .

The control unit can be realized in terms of hardware and/or software and can be designed physically in one or more parts.

Optionally, the control unit can be designed and set up to determine an end of operation of the internal combustion engine. The control of the lower setpoint for the state of charge USW _L in the second control circuit depending on the determined nitrogen oxide storage state SSZ and / or the control of the lower setpoint for the state of nitrogen oxide storage USWs in the first control circuit depending on the determined state of charge LZ can then take place under the condition that an end of operation of the internal combustion engine since a subsequent cold start can then be expected.

A motor vehicle according to the invention has an exhaust system as described above.

The advantages of the motor vehicle according to the invention therefore correspond to those of the exhaust system according to the invention. In addition, the invention has a particularly advantageous effect in a motor vehicle, since it enables compliance with strict legal requirements with regard to the permissible emission of air pollutants.

A motor vehicle is a motor-driven vehicle, e.g. B. to understand a land, air or water vehicle.

The motor vehicle can be used as a hybrid electric vehicle, e.g. B. be designed as a mild hybrid electric vehicle or full hybrid electric vehicle. For example, the energy storage device of the exhaust system of the motor vehicle according to the invention can be a traction battery of a hybrid electric vehicle.

• 1 ein Abgassystem in einer beispielhaften Ausgestaltung; und
• 2 ein beispielhaftes Ablaufschema eines Verfahrens zum Betreiben des Abgassystems.

The invention is explained in more detail below with reference to the figures and the associated description. Show it:

• 1 an exhaust system in an example configuration; and
• 2 an exemplary flow chart of a method for operating the exhaust system.

In the 1 an exhaust system 1 according to the invention is shown schematically in an exemplary embodiment. The exhaust system 1 is connected to an internal combustion engine 2 which, during its operation, generates an exhaust gas 13 which is fed to the exhaust system 1 . In the exemplary embodiment, the internal combustion engine 2 is shown with four cylinders, but it can also have a different number of cylinders, e.g. B. have 2, 3, 6 or 8 cylinders.

The internal combustion engine 2 can be a diesel engine, for example.

The internal combustion engine 9 is assigned an electric machine 9, which can be operated either as a generator or as an electric motor. The internal combustion engine 2 and the electric machine 9 together form a hybrid drive, z. B. the hybrid drive of a hybrid electric vehicle.

The electric machine 9 is electrically connected to an energy storage device 4, which is a lithium-ion battery in the exemplary embodiment. Alternatively, however, other rechargeable accumulators can also be used as the energy storage device 4 .

If the electric machine 9 is operated as a generator, then a current flow 11 is formed from the electric machine 9 to the energy storage device 4 and the energy storage device is charged. If, on the other hand, the electric machine 9 is operated as an electric motor, then a current flow 11 takes place from the energy storage device 4 to the electric machine 9.

The exhaust system 1 has an LNT catalytic converter 3 and an SCR catalytic converter 10 downstream of the LNT catalytic converter 3 . There is also an electrical heating device 5 with which the exhaust gas 13 can be heated upstream of the LNT catalytic converter 3 and consequently the LNT catalytic converter 3 through which the exhaust gas 13 flows and the SCR catalytic converter 10 can also be heated. The heating device 5 is electrically connected to the energy storage device 4 so that a current flow 11 is formed between the energy storage device 4 and the heating device 5 and the heating device 5 can be supplied with electrical current.

The LNT catalytic converter 3 is arranged in a first control circuit RK1 (not shown), which is used to regulate the nitrogen oxide storage state SSZ of the LNT catalytic converter 3 . A device 6 for determining the nitrogen oxide storage state SSZ is also part of the first control loop RK1. If the nitrogen oxide storage status SSZ is too low, i. H. If the nitrogen oxide storage state SSZ falls below a lower target value for the nitrogen oxide storage state USWs, regeneration of the LNT catalytic converter 3 is initiated in order to increase the nitrogen oxide storage state SSZ again and to enable nitrogen oxides to be stored.

The energy storage device 4 is arranged in a second control circuit RK2 (not shown), which is used to regulate the state of charge LZ of the energy storage device 4 . A device 7 for determining the state of charge LZ is also part of the second control loop RK2. If the state of charge LZ is too low, i.e. if the state of charge LZ falls below a lower target value for the state of charge USW _L , charging of the energy storage device 4 is initiated in order to increase the state of charge LZ again and, among other things, to provide sufficient electrical energy for operating the heating device 5 be able.

Both the device 6 for determining the nitrogen oxide storage state SSZ and the device 7 for determining the state of charge LZ are connected to the control unit 8 in a signal-conducting manner, so that the control unit 8 can receive input signals 12 of the two devices 6, 7 and a connection of the first control circuit to the second control circuit RK2 is made possible.

Control unit 8 controls the lower setpoint for the state of charge USW _L in the second control circuit RK2 as a function of the determined nitrogen oxide storage state SSZ, with the determined nitrogen oxide storage state SSZ being compared to a minimum value for the nitrogen oxide storage state SSZ _min when the minimum value for the nitrogen oxide storage state SSZ _min is undershot lower target value for the state of charge USW _L is increased or left at an increased value and when the minimum value for the nitrogen oxide storage state SSZ _min is reached or exceeded, the lower target value for the state of charge USW _L is reduced or left at a reduced value.

Optionally, the lower setpoint for the nitrogen oxide storage state USWs can also be controlled in the first control circuit RK1 as a function of the determined state of charge LZ.

Optionally, the control unit 8 can be designed and set up to determine an end of operation of the internal combustion engine 2 . This makes it possible to take into account an impending end of operation when controlling the lower setpoint values for the state of charge USW _L and possibly for the state of nitrogen oxide storage USWs.

With the exhaust system 1 according to the invention, for example, below with reference to 2 described procedures are carried out.

In a first step S1, the impending end of operation of the internal combustion engine 2 is determined, e.g. B. by checking the approach to a destination.

In a next step S2, it is checked whether the end of operation of the internal combustion engine 2 is imminent. If this is not the case, the method goes back to step S1.

If the end of operation of the internal combustion engine is imminent, the nitrogen oxide storage state SSZ is determined in step S3 and a subsequent step S4 checks whether the nitrogen oxide storage state SSZ falls below a minimum value for the nitrogen oxide storage state SSZ _min . If this is not the case, ie sufficient nitrogen oxides can still be stored in the LNT catalytic converter 3, the method proceeds to step S6 and the lower target value for the state of charge USW _L is reduced or left at a reduced value. Decreased value means that the value is lower than the increased value mentioned below. The reduced value can be, for example, the previously customary lower setpoint value for the state of charge USW _L in the second control loop RK2.

If, on the other hand, it is determined in step S4 that the nitrogen oxide storage state SSZ falls below the minimum value for the nitrogen oxide storage state SSZ _min , i.e. SSZ <SSZ _min , the lower setpoint for the state of charge USW _L is increased in step S5 or left at an increased value. The increased value prevents the energy storage device 4 from being discharged too much and no longer having sufficient electrical energy for the heating device 5 . If, during a subsequent cold start of the internal combustion engine 2, the nitrogen oxide storage state SSZ of the LNT catalytic converter 3 is not sufficient for storing nitrogen oxides, the heating device can be used in any case to quickly heat up the catalytic converters, so that in this case too nitrogen oxides are emitted can be largely avoided.

Optionally, method steps S1 and S2 can be omitted.

In a corresponding manner, when the charge state LZ _min falls below a minimum value, the lower setpoint value for the nitrogen oxide storage state USWs can be increased.

Reference List

SSZ

nitric oxide storage status

SSZmin

Minimum value for the nitrogen oxide storage state

ETC

lower setpoint for the nitrogen oxide storage state

LZ

state of charge

LZmin

Minimum value for the state of charge

USWL

lower target value for the state of charge

RK1

first loop

RK2

second control loop

1

exhaust system

2

combustion engine

3

LNT catalyst

4

energy storage device

5

heating device

6

Device for determining the nitrogen oxide storage status

7

Device for determining the state of charge

8th

control unit

9

electric machine

10

SCR catalytic converter

11

current flow

12

input signal

13

exhaust

S1 to S6

process steps

Claims (10)

Method for operating an exhaust system (1) of an internal combustion engine (2) with an LNT catalytic converter (3) arranged in a first control circuit RK1 and an energy storage device (4) arranged in a second control circuit RK2, which is used to supply an electrical heating device (5) with electric current is formed, with a nitrogen oxide storage state SSZ of the LNT catalytic converter (3) being regulated in the first control circuit RK1 and a state of charge LZ of the energy storage device (4) being regulated in the second control circuit RK2, and with the first and second control circuits RK1, RK2 being linked to one another, having: - determining the nitrogen oxide storage state SSZ in the first control circuit RK1, - using the determined nitrogen oxide storage state SSZ as an input variable for controlling a lower target value for the state of charge USW _L of the energy storage device (4) in the second control circuit RK2, the determined nitrogen oxide storage state SSZ having a minimum value for the nitrogen oxide storage status nd SSZ _min is compared, if the minimum value for the nitrogen oxide storage state SSZ _min is not reached, the lower target value for the state of charge USW _L is increased or left at an increased value and if the minimum value for the nitrogen oxide storage state SSZ _min is reached or exceeded, the lower target value for the State of charge USW _L is reduced or left at a reduced value. Method for operating an exhaust system (1) of an internal combustion engine (2) with an LNT catalytic converter (3) arranged in a first control circuit RK1 and an energy storage device (4) arranged in a second control circuit RK2, which is used to supply an electrical heating device (5) with electric current is formed, with a nitrogen oxide storage state SSZ of the LNT catalytic converter (3) being regulated in the first control circuit RK1 and a state of charge LZ of the energy storage device (4) being regulated in the second control circuit RK2, and with the first and second control circuits RK1, RK2 being linked to one another, having: - Determining the state of charge LZ of the energy storage device (4), - Use of the determined state of charge LZ as an input variable for controlling a lower target value for the nitrogen oxide storage state USWs in the first control circuit RK1. procedure after claim 2 , whereby the determined state of charge LZ is compared with a minimum value for the state of charge LZ _min , if the minimum value for the state of charge LZ _min is not reached, the lower target value for the state of nitrogen oxide storage USW _S is increased or left at an increased value and if the Minimum value for the state of charge LZ _min , the lower setpoint for the nitrogen oxide storage state USWs is reduced or left at a reduced value. procedure after claim 2 or 3 comprising: - determining the nitrogen oxide storage state SSZ in the first control circuit RK1, - using the determined nitrogen oxide storage state SSZ as an input variable for controlling a lower target value for the state of charge USW _L of the energy storage device (4) in the second control circuit RK2. procedure after claim 4 , whereby the determined nitrogen oxide storage state SSZ is compared with a minimum value for the nitrogen oxide storage state SSZ _min , if the minimum value for the nitrogen oxide storage state SSZ _min is not reached, the lower target value for the state of charge USW _L is increased or left at an increased value and if the Minimum value for the nitrogen oxide storage state SSZ _min , the lower target value for the state of charge USW _L is reduced or left at a reduced value. Method according to one of the preceding claims, comprising: - determining an end of operation of the internal combustion engine (2), the lower setpoint for the state of charge USW _L and/or the lower setpoint for the state of nitrogen oxide storage USWs being increased only when the end of operation of the internal combustion engine (2) is imminent or at be left at the increased value. Exhaust system (1) for an internal combustion engine (2), comprising: - an LNT catalytic converter (3) arranged in a first control circuit RK1, a nitrogen oxide storage state SSZ of the LNT catalytic converter (3) being controlled in the first control circuit RK1, - one in one energy storage device (4) arranged in the second control circuit RK2, which is used to supply an electrical heating device (5). electric current is formed, a state of charge LZ of the energy storage device (4) being regulated in the second control circuit RK2, - a device (6) for determining the nitrogen oxide storage state SSZ of the LNT catalytic converter (3) and - a control unit (8), furnished and formed for controlling a lower target value for the state of charge USW _L in the second control circuit RK2 as a function of the determined nitrogen oxide storage state SSZ, the determined nitrogen oxide storage state SSZ being compared to a minimum value for the nitrogen oxide storage state SSZ _min , if the minimum value for the nitrogen oxide storage state SSZ _min is undershot, the lower target value for the state of charge USW _L is increased or left at an increased value and when the minimum value for the nitrogen oxide storage state SSZ _min is reached or exceeded, the lower target value for the state of charge USW _L is reduced or left at a reduced value. Exhaust system (1) for an internal combustion engine (2), comprising: - an LNT catalytic converter (3) arranged in a first control circuit RK1, a nitrogen oxide storage state SSZ of the LNT catalytic converter (3) being controlled in the first control circuit RK1, - an energy storage device (4) arranged in a second control circuit RK2, which is designed to supply an electrical heating device (5) with electric current, a state of charge LZ of the energy storage device (4) being controlled in the second control circuit RK2, - A device (7) for determining the state of charge LZ of the energy storage device (4) and - A control unit (8), set up and designed to control a lower target value for the nitrogen oxide storage state USWs in the first control circuit RK1 depending on the determined state of charge LZ. Motor vehicle with an exhaust system (1). claim 7 or 8th . motor vehicle after claim 9 , designed as a hybrid electric vehicle.

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