DE102015219114B4 - Process for exhaust aftertreatment of an internal combustion engine - Google Patents

Abstract

Method for exhaust gas aftertreatment of an internal combustion engine (10) with an exhaust gas duct (12), with a three-way catalytic converter (14) arranged in the exhaust gas duct (12), with a first NOx storage catalytic converter (16) and with an exhaust gas through the Exhaust duct (12) downstream of the first NOx storage catalytic converter (16) and downstream of the three-way catalytic converter (14) arranged second NOx storage catalytic converter (18), downstream of the first NOx storage catalytic converter (16) and upstream of the second NOx storage catalytic converter (18) an injection valve (20) for introducing hydrocarbons into the exhaust gas duct (12) is arranged, comprising the following steps: - operating the internal combustion engine (10) with an over-stoichiometric, lean combustion air ratio λE> 1, the first NOx storage catalytic converter (16 ) and the second NOx storage catalytic converter (18) are loaded with nitrogen oxides, - switching the internal combustion engine (10) to a sub-stoichiometric, rich combustion air ratio λE<1 for regeneration of the first NOx storage catalytic converter (16) with simultaneous metering of fuel into the exhaust gas duct ( 12) downstream of the first NOx storage catalytic converter (16) and upstream of the second NOx storage catalytic converter (18), - operation of the internal combustion engine (10) with a stoichiometric combustion air ratio (λE= 1), with fuel being transported downstream of the first NOx storage catalytic converter (16 ) and upstream of the second NOx storage catalytic converter (18),- regenerating the second NOx storage catalytic converter (18),- switching the internal combustion engine (10) to an over-stoichiometric, lean combustion air ratio and reloading the first NOx storage catalytic converter (16) and of the second NOx storage catalytic converter (18).

Description

The invention relates to a method for treating exhaust gas from an internal combustion engine.

The current exhaust gas legislation and one that will become increasingly strict in the future places high demands on engine raw emissions and the exhaust gas aftertreatment of combustion engines. For example, with the introduction of the EU6 emissions legislation, a particle limit value for gasoline engines will be prescribed. This can lead to the use of an Otto Particulate Filter (OPF) being necessary for some motor vehicle models. Vehicle and engine manufacturers are also required to reduce the consumption of combustion engines and the associated CO2 emissions. Among other things, this means that consumption-optimized combustion processes are being developed for internal combustion engines. One way of reducing the consumption of a gasoline engine is a lean burn process, i.e. a combustion process in which the internal combustion engine is operated with a super-stoichiometric combustion air ratio as far as possible. Since the NOx emissions can no longer be sufficiently reduced from the exhaust gas with a conventional three-way catalytic converter in a lean-burn process in over-stoichiometric operation, additional catalytic converters such as NOx storage catalytic converters are required. The NOx emissions are stored as nitrates in the NOx storage catalytic converter. These NOx storage catalytic converters must be periodically regenerated with the help of an engine rich phase. The design of this fat phase significantly influences the tailpipe emissions. In order to keep NOx emissions low, exhaust gas aftertreatment systems with NOx storage catalytic converters arranged one after the other in the flow direction of an exhaust gas of the internal combustion engine have been developed. The regeneration of these exhaust aftertreatment systems can lead to emission slippage and the formation of nitrous oxide (N ₂ O formation).

From the DE 10 2013 221 439 A1 An exhaust aftertreatment system for an internal combustion engine is known in which the exhaust system is designed with two flows in a first section after an outlet of the internal combustion engine and in a subsequent section the two flows are combined to form a common exhaust gas duct. A NOx storage catalytic converter is arranged in each flow of the exhaust gas duct and in the common exhaust gas duct, the NOx storage catalytic converters in the individual flows being regenerated separately and at different times from one another. The disadvantage of such a method, however, is that lean exhaust gas flows against the NOx storage catalytic converter in the common exhaust gas duct when one of the NOx storage catalytic converters in the two flows of the exhaust gas duct is regenerated, so that there is no regeneration of the NOx storage catalytic converter in the common exhaust gas duct is possible or the NOx storage catalytic converter in the common exhaust gas duct is flown with a stoichiometric exhaust gas and this can lead to NOx desorption at the NOx storage catalytic converter in the common exhaust gas duct and thus to increased tailpipe emissions.

From the DE 10 2010 014 468 A1 discloses a method for regenerating two NOx storage catalytic converters arranged one behind the other in the exhaust gas duct, with a bypass being provided for the NOx storage catalytic converter located closer to the internal combustion engine in the direction of flow in order to direct sub-stoichiometric exhaust gas past the closer NOx storage catalytic converter for the purpose of regenerating the further NOx storage catalytic converter and to regenerate the further NOx storage catalytic converter before the closer NOx storage catalytic converter. The disadvantage of such a solution, however, is that it requires a relatively expensive and complex exhaust gas duct, and that stoichiometric exhaust gas nevertheless flows through the further NOx storage catalytic converter during the regeneration of the closer NOx storage catalytic converter.

The DE 101 62 383 A1 discloses an exhaust system and a method for exhaust gas aftertreatment of a diesel engine, in which upstream of a device for removing particles and nitrogen oxides, a means for reducing nitrogen oxides connected via an exhaust line to the device near the diesel engine is used. This means ensures a reduction of nitrogen oxides in operating situations of the internal combustion engine in which the device for this is not active, in particular at low load or low speed of the diesel engine.

US 2013/0 202 507 A1 describes a method for the aftertreatment of exhaust gases from an internal combustion engine. In the flow direction of an exhaust gas stream of the internal combustion engine, a NOx storage catalytic converter and downstream of the NOx storage catalytic converter an SCR catalytic converter and downstream of the SCR catalytic converter a further catalytic converter with an oxygen storage capacity are arranged. The exhaust aftertreatment system is alternately charged with a sub-stoichiometric and a super-stoichiometric exhaust gas from the internal combustion engine.

From the DE 199 18 756 A1 an exhaust gas aftertreatment system for cleaning an exhaust gas of an internal combustion engine is known, which has a catalyst system arranged in an exhaust gas duct for reducing a nitrogen oxide content of the exhaust gas. In this case, it is provided that the catalytic converter system comprises a first NOx storage catalytic converter in the flow path of the exhaust gas and a second NOx storage catalytic converter at a distance from it.

The invention is now based on the object of proposing a simple and inexpensive exhaust gas aftertreatment device with which an exhaust gas aftertreatment method can be carried out in which the NOx emissions can be converted from the exhaust gas into harmless exhaust gas components without increased slip during the regeneration of the NOx storage catalytic converters.

• - Betreiben der Brennkraftmaschine mit einem überstöchiometrischen, mageren Verbrennungsluftverhältnis λ_E > 1, wobei der erste NOx-Speicherkatalysator sowie der zweite NOx-Speicherkatalysator mit Stickstoffoxiden beladen werden,
• - Umschalten der Brennkraftmaschine auf ein unterstöchiometrisches, fettes Verbrennungsluftverhältnis λ_E < 1 zur Regeneration des ersten NOx-Speicherkatalysators bei gleichzeitigem Eindosieren von Kraftstoff in den Abgaskanal stromab des ersten NOx-Speicherkatalysators und stromauf des zweiten NOx-Speicherkatalysators,
• - Regenerieren des zweiten NOx-Speicherkatalysators,
• - Umschalten der Brennkraftmaschine auf ein überstöchiometrisches, mageres Verbrennungsluftverhältnis und erneutes Beladen des ersten NOx-Speicherkatalysators sowie des zweiten NOx-Speicherkatalysators.

The object is achieved by a method according to the invention for exhaust gas aftertreatment of an internal combustion engine with an exhaust gas duct, with a three-way catalytic converter arranged in the exhaust gas duct, with a first NOx storage catalytic converter and with an exhaust gas downstream of the first NOx storage catalytic converter and downstream in the flow direction of an exhaust gas through the exhaust gas duct of the three-way catalytic converter arranged second NOx storage catalytic converter, wherein downstream of the first NOx storage catalytic converter and upstream of the second NOx storage catalytic converter an injection valve for introducing hydrocarbons into the exhaust gas duct is arranged, the method according to the invention comprising the following steps:

• - Operating the internal combustion engine with an over-stoichiometric, lean combustion air ratio λ _E > 1, with the first NOx storage catalytic converter and the second NOx storage catalytic converter being loaded with nitrogen oxides,
• - Switching the internal combustion engine to a sub-stoichiometric, rich combustion air ratio λ _E <1 to regenerate the first NOx storage catalytic converter while at the same time metering fuel into the exhaust gas duct downstream of the first NOx storage catalytic converter and upstream of the second NOx storage catalytic converter,
• - regeneration of the second NOx storage catalyst,
• - Switching of the internal combustion engine to an over-stoichiometric, lean combustion air ratio and reloading of the first NOx storage catalytic converter and the second NOx storage catalytic converter.

The method according to the invention enables simultaneous regeneration of the first NOx storage catalytic converter and the second NOx storage catalytic converter without emission-increasing secondary effects, as in the regeneration methods known from the prior art. This prevents a stoichiometric exhaust gas from flowing against the second NOx storage catalytic converter during regeneration of the first NOx storage catalytic converter and desorption of the nitrogen oxides on the second NOx storage catalytic converter during regeneration of the first NOx storage catalytic converter. A higher overall conversion of harmful exhaust gas components can thus be achieved. By regenerating both NOx storage catalytic converters at the same time, the emission slip can be reduced and the regeneration time can be shortened.

Advantageous improvements of the method specified in the independent claim are possible as a result of the measures specified in the dependent claims.

According to a variant of the method that is not according to the invention, it is provided that after the first NOx storage catalytic converter has been completely regenerated, the metering of fuel into the exhaust gas duct is switched off. Although in principle additional metering of fuel into the exhaust gas duct between the first NOx storage catalytic converter and the second NOx storage catalytic converter is also possible during the regeneration of the second NOx storage catalytic converter, after complete regeneration of the first NOx storage catalytic converter downstream of the first NOx Storage catalytic converter already has a sub-stoichiometric, rich exhaust gas, so that there is already sufficient reducing agent to regenerate the second NOx storage catalytic converter. An additional dosing of fuel into the exhaust duct is therefore not necessary and reduces the conversion due to an unfavorable composition of the reducing agent.

According to the invention, it is advantageously provided that after complete regeneration of the first NOx storage catalytic converter, the internal combustion engine is operated with a stoichiometric combustion air ratio λ _E =1 and at the same time fuel is metered into the exhaust gas duct downstream of the first NOx storage catalytic converter and upstream of the second NOx storage catalytic converter. This enables efficient exhaust gas cleaning by the three-way catalytic converter, but fuel has to be metered into the exhaust gas duct over a longer period of time.

A further improvement of the method is that the regeneration of the NOx storage catalytic converters is initiated when an increase in NOx emissions is measured in the exhaust gas duct downstream of the second NOx storage catalytic converter. This can be done, for example, by an NOx sensor downstream of the second NOx storage catalytic converter gates take place. If the storage capacity of the NOx storage catalytic converters is reached, the NOx concentration increases downstream of the second NOx storage catalytic converter. No further NOx emissions can be stored as nitrates in the NOx storage catalytic converters. An increase in the NOx concentration thus signals the need for regeneration of the storage catalytic converters and can be easily processed as a signal by a control unit of the internal combustion engine in order to switch to a sub-stoichiometric combustion air ratio and thus initiate regeneration. Alternatively, the regeneration of the NOx storage catalytic converters can also be initiated by a calculation model stored in a control unit of the internal combustion engine.

According to a further improvement of the method, it is provided that the regeneration of the NOx storage catalytic converters is completed and the internal combustion engine is operated again with a lean combustion air ratio that is more than stoichiometric if downstream of the second NOx storage catalytic converter a substoichiometric exhaust gas (a rich breakthrough through the second NOx storage catalytic converter ) is measured. Sub-stoichiometric exhaust gas downstream of the second NOx storage catalytic converter occurs when the nitrates in the NOx storage catalytic converters have been completely broken down and the NOx storage catalytic converters are thus completely regenerated. A rich breakthrough through the second NOx storage catalytic converter can thus be easily detected by a lambda probe arranged in the exhaust gas duct as a signal for the completion of the regeneration.

According to a further advantageous improvement of the method, it is provided that the injection quantity of the fuel metered in downstream of the first NOx storage catalytic converter and upstream of the second NOx storage catalytic converter is regulated via lambda probes arranged in the exhaust gas duct upstream and downstream of the second NOx storage catalytic converter. As a result, the injection quantity can be selected in such a way that sufficient reducing agent is available to break down the nitrates, but no unburned fuel flows through the second NOx storage catalytic converter, thereby increasing tailpipe emissions.

A device suitable for carrying out the method for exhaust gas aftertreatment of an internal combustion engine has an exhaust gas duct, a three-way catalytic converter arranged in the exhaust gas duct, a first NOx storage catalytic converter and, in the flow direction of an exhaust gas through the exhaust gas duct, downstream of the first NOx storage catalytic converter and downstream of the three-way Way catalyst arranged second NOx storage catalyst, wherein downstream of the first NOx storage catalyst and upstream of the second NOx storage catalyst, an injection valve for introducing hydrocarbons into the exhaust duct is arranged. As a result, the device is suitable for carrying out a method according to the invention in a simple manner. In the context of this patent application, a first NOx storage catalytic converter is to be understood as meaning both a NOx storage catalytic converter and a three-way catalytic converter with an integrated NOx storage catalytic converter.

According to a preferred embodiment of the device, it is provided that the exhaust gas duct has a single-flow design at least in the region between the three-way catalytic converter and the second NOx storage catalytic converter. A single-flow design of the exhaust gas duct is simple and inexpensive. In addition, there are no bypass ducts through which an exhaust gas can flow past an exhaust gas cleaning element and thus could lead to increased tailpipe emissions in the event of a faulty actuating element. In addition, a single-flow design of the exhaust gas duct reduces the costs for exhaust gas cleaning elements that would otherwise have to be implemented twice, while at the same time reducing the installation effort.

According to an advantageous development of the device, it is provided that the injection valve and the internal combustion engine are connected to a common fuel tank via fuel lines. A simple supply of fuel to the injection valve is thus possible. In addition, there is no need for an additional reservoir, such as is necessary for storing a liquid for the selective catalytic reduction of nitrogen oxides.

According to a further advantageous development, the first NOx storage catalytic converter or a three-way catalytic converter with integrated NOx storage catalytic converter is arranged close to the engine and the second NOx storage catalytic converter is arranged remote from the engine, in particular in an underbody position of a motor vehicle. A position close to the engine is understood to mean a mean exhaust gas path of at most 50 cm, in particular at most 30 cm, after the outlet of the internal combustion engine. A position remote from the engine is understood to mean a mean exhaust gas path of at least 100 cm, in particular at least 120 cm, after the outlet of the internal combustion engine. By positioning the first NOx storage catalytic converter close to the engine, it is possible to reach an operating temperature of the NOx storage catalytic converter more quickly, so that the first NOx storage catalytic converter reaches the temperature window more quickly after a cold start of the internal combustion engine, in which efficient storage of NOx emissions is possible. This allows the operating range of the NOx storage catalytic converters are expanded, since the exhaust gas has a higher temperature when flowing through the first NOx storage catalytic converter than when flowing through the second NOx storage catalytic converter and thus at least one of the NOx storage catalytic converters works in the temperature window in which efficient storage of NOx -Emissions in the storage catalyst is possible. The position of the second NOx storage catalytic converter remote from the engine also makes it easier to install the second NOx storage catalytic converter. In addition, there is usually more installation space at this position, so that the second NOx storage catalytic converter can be made larger. This lengthens the period of time until the second NOx storage catalytic converter is fully loaded and thus lengthens the period of time between two necessary regeneration cycles.

According to an advantageous embodiment, it is provided that the metering of fuel into the exhaust gas duct is stopped at the latest when the second NOx storage catalytic converter has been completely regenerated. This avoids a sub-stoichiometric exhaust gas downstream of the second NOx storage catalytic converter and associated increased HC emissions.

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

• 1 ein erstes Ausführungsbeispiel einer Vorrichtung zur Abgasnachbehandlung einer Brennkraftmaschine,
• 2 ein zweites Ausführungsbeispiel einer Vorrichtung zur Abgasnachbehandlung,
• 3 ein weiteres Ausführungsbeispiel einer Vorrichtung zur Abgasnachbehandlung,
• 4 ein weiteres Ausführungsbeispiel einer Vorrichtung zur Abgasnachbehandlung mit einem zusätzlichen Partikelfilter,
• 5 ein weiteres Ausführungsbeispiel einer Vorrichtung zur Abgasnachbehandlung mit einem zusätzlichen Partikelfilter,
• 6 ein weiteres Ausführungsbeispiel einer Vorrichtung zur Abgasnachbehandlung mit einem zusätzlichen Partikelfilter,
• 7 ein Ablaufdiagramm eines erfindungsgemäßen Verfahrens zur Regeneration der beiden NOx-Speicherkatalysatoren sowie die dabei auftretenden Verbrennungsluftverhältnisse bzw. Abgasluftverhältnisse,
• 8 ein Diagramm zur NOx-Konzentration im Abgas, wenn entgegen der Erfindung der zweite NOx-Speicherkatalysator während der Regeneration des ersten NOx-Speicherkatalysators mit einem stöchiometrischen Abgas durchströmt wird.

The invention is explained below in exemplary embodiments with reference to the associated drawings. Show it:

• 1 a first embodiment of a device for exhaust gas aftertreatment of an internal combustion engine,
• 2 a second embodiment of a device for exhaust gas aftertreatment,
• 3 a further exemplary embodiment of a device for exhaust gas aftertreatment,
• 4 a further exemplary embodiment of a device for exhaust gas aftertreatment with an additional particle filter,
• 5 a further exemplary embodiment of a device for exhaust gas aftertreatment with an additional particle filter,
• 6 a further exemplary embodiment of a device for exhaust gas aftertreatment with an additional particle filter,
• 7 a flowchart of a method according to the invention for regenerating the two NOx storage catalytic converters and the combustion air ratios or exhaust gas air ratios that occur,
• 8th a diagram of the NOx concentration in the exhaust gas when, contrary to the invention, a stoichiometric exhaust gas flows through the second NOx storage catalytic converter during the regeneration of the first NOx storage catalytic converter.

1 shows an internal combustion engine 10 with a device for exhaust gas aftertreatment of the internal combustion engine 10, with an exhaust gas duct 12, a three-way catalytic converter with an integrated NOx storage catalytic converter 22 being arranged in the flow direction of the exhaust gas of the internal combustion engine 10 through the exhaust gas duct 12. The internal combustion engine 10 is preferably designed as a spark-ignited internal combustion engine, which is preferably operated with a lean-burn process. This internal combustion engine can be designed either as a supercharged internal combustion engine or as a naturally aspirated engine. The three-way catalytic converter with an integrated NOx storage catalytic converter 22 includes a three-way catalytic converter 14 and a first NOx storage catalytic converter 16, which are designed as a common component. A second NOx storage catalytic converter 18 is arranged in the exhaust gas duct 12 of the internal combustion engine 10 downstream of the three-way catalytic converter with an integrated NOx storage catalytic converter 22 . Between the three-way catalytic converter with an integrated NOx storage catalytic converter 22 and the second NOx storage catalytic converter 18 is an injection valve 20 for metering hydrocarbons into the exhaust gas duct 12 downstream of the first NOx storage catalytic converter 16 or downstream of the three-way catalytic converter with an integrated NOx storage catalyst 22 and upstream of the second NOx storage catalyst 18 is provided. Furthermore, three lambda probes 28, 30, 32 are arranged in the exhaust gas duct 12, the first lambda probe 28 being located downstream of the three-way catalytic converter with integrated NOx storage catalytic converter 22 and upstream of the injection valve 20, the second lambda probe 30 downstream of the Injection valve 20 and upstream of the second NOx storage catalyst 18 and the third lambda probe 32 downstream of the second NOx storage catalyst 18 is arranged. A NOx sensor 34 for measuring the NOx concentration in the exhaust gas duct 12 is also arranged in the exhaust gas duct 12 downstream of the second NOx storage catalytic converter 18 . The three-way catalytic converter with integrated NOx storage catalytic converter 22 is preferably arranged close to the engine, while the second NOx storage catalytic converter 18 is preferably arranged remote from the engine in an underbody layer of a motor vehicle.

In 2 an alternative exemplary embodiment of the device for exhaust gas aftertreatment is shown. With the structure being largely the same, only the differences from the embodiment according to FIG 1 received. The execution form of insurance 2 additionally has a further three-way catalytic converter 24 which is arranged in the exhaust gas duct upstream of the three-way catalytic converter with an integrated NOx storage catalytic converter 22 .

In 3 a further exemplary embodiment of a device for exhaust gas aftertreatment is shown. With largely the same structure as in 1 in the following only the differences to the embodiment according to FIG 1 received. Instead of a three-way catalytic converter with an integrated NOx storage catalytic converter 22, a separate three-way catalytic converter 14 and, downstream of the three-way catalytic converter 14, a separate first NOx storage catalytic converter 16 are provided in this embodiment.

In the 4 until 6 further exemplary embodiments of a device for exhaust gas aftertreatment are shown. With largely the same structure as in the 1 until 3 a particle filter 26 is arranged in each case in the exhaust gas duct 12 downstream of the second NOx storage catalytic converter 18 . The particle filter 26 can be designed as a particle filter with a coating for the selective catalytic reduction (SCR) of nitrogen oxides, with such an SCR coating enabling even better exhaust gas cleaning, but at the same time increasing the exhaust gas back pressure and thus consumption.

The method according to the invention for the exhaust aftertreatment of the internal combustion engine 10 is shown in the flowchart in FIG 7 clear and can be divided into three phases. In a first phase, internal combustion engine 10 is operated with an over-stoichiometric combustion air ratio λ _E >1, for example as in FIG 7 shown with λ _E = 1.5. The NOx emissions occurring in this mode of operation cannot be removed from the exhaust gas by the three-way catalytic converter 14, 24 or the three-way catalytic converter with integrated NOx storage catalytic converter 22, since no reducing agent for the NOx emissions is required in this mode of operation present in the exhaust gas. Therefore, the NOx emissions that occur in this mode of operation are initially stored in the three-way catalytic converter with an integrated NOx storage catalytic converter 22 or in the first NOx storage catalytic converter 16 . After increasing loading of the three-way catalytic converter with integrated NOx storage catalytic converter 22 or the first NOx storage catalytic converter 16, the NOx emissions are also stored in the second NOx storage catalytic converter 18 in the underbody of the motor vehicle. Overall, however, only a limited storage capacity is available for storing NOx emissions as nitrates, so that the NOx storage catalytic converters 16, 18 or the three-way catalytic converter with an integrated NOx storage catalytic converter 22 must be regenerated periodically.

If the loading capacity of the second NOx storage catalytic converter 18 in the underbody position of the motor vehicle is reached, which can be detected, for example, by an increase in the NOx concentration at the NOx sensor 34 downstream of the second NOx storage catalytic converter 18, the regeneration of the NOx storage catalytic converters 16, 18 or the three-way catalytic converter with integrated NOx storage catalytic converter 22. Alternatively, the regeneration can also be initiated by a calculation model stored in a control unit of internal combustion engine 10 . For this purpose, internal combustion engine 10 is operated in a second phase with a sub-stoichiometric combustion air ratio λ _E <1, preferably 0.9<λ _E <0.98, for example λ _E =0.92. By introducing a sub-stoichiometric exhaust gas into a NOx storage catalytic converter 16, 18 or a three-way catalytic converter with an integrated NOx storage catalytic converter 22, the nitrates stored there are decomposed and the NOx is released again, with a reducing agent present in the substoichiometric exhaust gas , In particular carbon monoxide (CO) or hydrogen (H ₂ ) can be reduced to nitrogen (N ₂ ).

During the regeneration of the three-way catalytic converter with integrated NOx storage catalytic converter 22 or the first NOx storage catalytic converter 16, a stoichiometric exhaust gas results without additional measures downstream of the three-way catalytic converter with integrated NOx storage catalytic converter 22 or the first NOx Storage catalyst 16. Therefore, the loaded second NOx storage catalyst 18 would flow without additional measures with a stoichiometric exhaust gas. This would already lead to increased desorption of the NOx emissions stored in the second NOx storage catalytic converter 18 and increased tailpipe emissions, since no reducing agent is available for the released NOx emissions with a stoichiometric exhaust gas. Such an increase in NOx emissions, which is avoided by the method according to the invention, is in 8th shown.

In order to prevent NOx desorption at the second NOx storage catalytic converter 18 during the regeneration of the first NOx storage catalytic converter 16 or the regeneration of the three-way catalytic converter with an integrated NOx storage catalytic converter 22, during the regeneration of the first NOx storage catalytic converter 16 or of the three-way catalytic converter with integrated NOx storage catalytic converter 22 fuel downstream of the first NOx storage catalytic converter 16 or of the three-way catalytic converter with integrated NOx storage catalytic converter 22 and upstream of the second NOx storage catalytic converter 18 in the Exhaust channel 12 metered. Thus, a sub-stoichiometric exhaust gas is present at the same time on the first NOx storage catalytic converter 16 or on the three-way catalytic converter with integrated NOx storage catalytic converter 22 and on the second NOx storage catalytic converter 18, so that with NOx storage catalytic converters 16, 18 or the three- Path catalyst with integrated NOx storage catalyst 22 and the second NOx storage catalyst 18 are regenerated at the same time.

If the regeneration of the first NOx storage catalytic converter 16 or the three-way catalytic converter with integrated NOx storage catalytic converter 22 is completely completed, in a third phase the internal combustion engine 10 continues to be operated with a sub-stoichiometric combustion air ratio, with the metering of the fuel upstream of the second NOx -Storage catalyst 18 can be turned off. At this point in time, the sub-stoichiometric exhaust gas from the internal combustion engine 10 reaches the second NOx storage catalytic converter 18 and the regeneration of the second NOx storage catalytic converter 18 is continued. For this it makes sense that the storage capacity for NOx emissions on the second NOx storage catalyst 18 is greater than on the first NOx storage catalyst 16 or the three-way catalyst with integrated NOx storage catalyst 22, so that the regeneration of the second NOx storage catalytic converter takes longer than the regeneration of the first NOx storage catalytic converter 16 or the three-way catalytic converter with integrated NOx storage catalytic converter 22. Alternatively, the fuel metering via the injection valve 20 is stopped at the latest when the second NOx storage catalytic converter 18 is completely regenerated in order to avoid HC emissions downstream of the second NOx storage catalytic converter 18, which can no longer be oxidized in the exhaust gas duct 12.

Alternatively, in this third phase, the metering of fuel through the injection valve 20 upstream of the second NOx storage catalytic converter 18 can also be continued and the internal combustion engine 10 can be operated with a stoichiometric combustion air ratio. As a further alternative, in this third phase, internal combustion engine 10 can also continue to be operated with a sub-stoichiometric combustion air ratio and additional fuel can be metered into exhaust gas duct 12 upstream of second NOx storage catalytic converter 18 .

The duration of the regeneration of the NOx storage catalysts 16, 18 or the three-way catalyst with integrated NOx storage catalyst 22 and the second NOx storage catalyst is chosen so long that the nitrates in the three-way catalyst with integrated NOx storage catalyst 22 or in the first NOx storage catalytic converter 16 and in the second NOx storage catalytic converter 18 and the NOx storage catalytic converters 16, 18 or the three-way catalytic converter with integrated NOx storage catalytic converter 22 can be regarded as completely regenerated. This state can be determined, for example, by means of the lambda probe 32 downstream of the second NOx storage catalytic converter 18 when the oxygen concentration in the exhaust gas falls (rich breakthrough through the second NOx storage catalytic converter 18). As soon as there are no longer any nitrates to be reduced, this lambda probe 32 indicates a sub-stoichiometric exhaust gas, so that the internal combustion engine 10 can again be operated with a super-stoichiometric combustion air ratio and the NOx storage catalytic converters 16, 18 or the three-way catalytic converter with it integrated NOx storage catalyst 22 can be loaded again with NOx emissions. Alternatively, the end of the regeneration can also be ended by the balancing model stored in the control unit of internal combustion engine 10 .

Reference List

10

internal combustion engine

12

exhaust duct

14

Three-way catalytic converter

16

first NOx storage catalyst

18

second NOx storage catalyst

20

injector

22

Three-way catalytic converter with integrated NOx storage catalytic converter

24

Three-way catalytic converter

26

particle filter

28

Lambda probe

30

Lambda probe

32

Lambda probe

34

NOx sensor

λE

combustion air ratio

λafterTWNSC

Exhaust air ratio downstream of the three-way catalytic converter with integrated NOx storage catalytic converter or downstream of the first NOx storage catalytic converter

λbefore NPC

Exhaust air ratio upstream of the second NOx storage catalyst

λafterNSC

Exhaust air ratio downstream of the second NOx storage catalyst

Claims (4)

Method for exhaust gas aftertreatment of an internal combustion engine (10) with an exhaust gas duct (12), with a three-way catalytic converter (14) arranged in the exhaust gas duct (12), with a first NOx storage catalytic converter (16) and with an exhaust gas through the Exhaust duct (12) downstream of the first NOx storage catalytic converter (16) and downstream of the three-way catalytic converter (14) arranged second NOx storage catalytic converter (18), downstream of the first NOx storage catalytic converter (16) and upstream of the second NOx storage catalytic converter (18) an injection valve (20) for introducing hydrocarbons into the exhaust gas duct (12) is arranged, comprising the following steps: - operating the internal combustion engine (10) with an over-stoichiometric, lean combustion air ratio λ _E > 1, the first NOx storage catalytic converter ( 16) and the second NOx storage catalytic converter (18) are loaded with nitrogen oxides, - Switching the internal combustion engine (10) to a sub-stoichiometric, rich combustion air ratio λ _E < 1 to regenerate the first NOx storage catalytic converter (16) with simultaneous metering of fuel into the Exhaust gas duct (12) downstream of the first NOx storage catalyst (16) and upstream of the second NOx storage catalyst (18), - Operating the internal combustion engine (10) with a stoichiometric combustion air ratio (λ _E = 1), with fuel being used at the same time downstream of the first NOx storage catalytic converter (16) and upstream of the second NOx storage catalytic converter (18), - regeneration of the second NOx storage catalytic converter (18), - switching of the internal combustion engine (10) to an over-stoichiometric, lean combustion air ratio and reloading of the first NOx storage catalytic converter ( 16) and the second NOx storage catalytic converter (18). Process for exhaust aftertreatment claim 1 , characterized in that the regeneration of the NOx storage catalytic converters (16, 18) is initiated when an increase in NOx emissions is measured in the exhaust gas duct downstream of the second NOx storage catalytic converter (18). Process for exhaust aftertreatment claim 1 or 2 , characterized in that the regeneration of the NOx storage catalytic converters (16, 18) is completed and the internal combustion engine (10) is again adjusted to an over-stoichiometric combustion air ratio λ _E > 1 when downstream of the second NOx storage catalytic converter (18) a sub-stoichiometric exhaust gas ( Fat breakthrough through the second NOx storage catalyst) is measured. Process for exhaust gas aftertreatment according to one of Claims 1 until 3 , characterized in that the injection quantity of the fuel metered into the exhaust gas duct (12) downstream of the first NOx storage catalytic converter (16) and upstream of the second NOx storage catalytic converter (18) is controlled via at least one lambda probe (28, 30, 32) or is regulated.

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