DE102006043102A1 - Method for activating exhaust gas converter, comprises redirecting of gas flow through reducing agent generating system - Google Patents

Abstract

An SCR catalyst converter (33) is positioned in the direction of the exhaust flow. The reducing agent generating system (60) comprises a multitron unit (62) assembled of a NOx-generator, a mixing chamber, and an oxidation reformation unit (63) as well as a combined NOx-storage and ammonia generating unit (64). The gas flow in the gas duct (65) of the reducing agent generating system (60) is at least intermittently redirected and at least some of the exhaust gas of the internal combustion engine (10) is guided through the reducing agent generating system (60). Independent claims are given for an arrangement using the method and the use of the arrangement.

Description

State of technology

The invention relates to a method for starting an exhaust gas aftertreatment system of an internal combustion engine with an exhaust system in which an SCR catalyst is provided in the flow direction of the exhaust gas, wherein a reducing agent generation system, a Multitron unit consisting of a NO _x production unit, a mixture formation chamber and a Oxidation reforming consists, as well as a combined NO _x storage / ammonia generating unit in a gas path of the reducing agent generation system, wherein for the reduction of nitrogen oxides in the exhaust gas upstream of the SCR catalyst from the reducing agent generation system generated ammonia is fed as a reducing agent of the exhaust system and wherein the Multitron unit via a compressor via a connection Vedichter / Multitron air and via a fuel supply fuel as starting materials for the production of ammonia are at least temporarily supplied. The invention further relates to a corresponding device.

In connection with future legal requirements regarding the nitrogen oxide emission of motor vehicles, a corresponding exhaust aftertreatment is required. The selective catalytic reduction (SCR) can be used to reduce the NO _x emission (denitrification) of internal combustion engines, in particular diesel engines, with temporally predominantly lean, ie oxygen-rich exhaust gas. In this case, a defined amount of a selectively acting reducing agent is added to the exhaust gas. This can be, for example, in the form of ammonia, which is metered in directly as a gas or is also obtained from a precursor substance in the form of urea or from a urea-water solution (HWL).

In the DE 10139142 A1 is described an exhaust gas purification system of an internal combustion engine, in which an SCR catalyst is used to reduce the NO _x emission, which reduces the nitrogen oxides contained in the exhaust gas with the reagent ammonia to nitrogen. The ammonia is recovered in a urea-water solution (HWL) hydrolysis catalyst upstream of the SCR catalyst. The hydrolysis catalyst converts the urea contained in the HWL to ammonia and carbon dioxide. In a second step, the ammonia reduces the nitrogen oxides to nitrogen, with water being produced as a by-product. The exact procedure has been adequately described in the specialist literature (see WEISSWELLER in CIT (72), pages 441-449, 2000). The HWL is provided in a reagent tank.

adversely in this method is that HWL in the operation of the internal combustion engine is consumed. The consumption is about 4% of fuel consumption. The supply of urea-water solution should correspondingly large area, for Example at gas stations, be assured. Another disadvantage of the method is in the necessary operating temperature range. The thermolysis reaction of the urea-water solution takes place only from temperatures around 130 ° C instead of and the hydrolysis reaction for the conversion of hydrogen and Nitrogen oxide on the hydrolysis catalyst to ammonia only in the range of 200 ° C to 220 ° C. These Temperatures in the exhaust gas, for example, in diesel engines only after a long time Operating time reached. Because of deposits, it can be at temperatures below 200 ° C come to blockages on the dosing unit, which the supply the urea-water solution at least hinder in the exhaust tract. Furthermore, a metered addition the urea-water solution at temperatures below 200 ° C due to a polymerization to inhibit the necessary catalytic Properties on hydrolysis or SCR catalyst lead.

In the DE 199 22 961 C2 is an exhaust gas purification system for purifying the exhaust gas of a combustion source, in particular an automotive internal combustion engine, at least nitrogen oxides contained therein with an ammonia generation catalyst for generating ammonia using constituents of at least a portion of the exhaust gas emitted from the combustion source during ammonia generation operating phases and downstream of the ammonia generation catalyst Nitrogen oxide reduction catalyst for the reduction of nitrogen oxides contained in the emitted exhaust gas of the combustion source using the generated ammonia as Re duktionsmittel described. In this case, a combustion-source-external nitrogen oxide generation unit is provided for enriching the exhaust gas supplied to the ammonia generation catalyst with nitrogen oxide produced by it during the ammonia generation operating phases. As a nitrogen oxide generating unit, for example, a plasma generator for plasma-technical oxidation of nitrogen contained in a gas stream supplied in nitrogen oxide is proposed. The hydrogen needed for ammonia production is generated during the ammonia generation operating phases by the operation of the combustion source with a rich, ie fuel-rich, air ratio.

One Plasmachemisches process for producing a hydrogen-rich Gas mixture is described in WO 01/14702 A1. It will be in an arc a rich fuel-air mixture, preferably under POx conditions.

Around to carry with you to avoid another fuel has now been in an unpublished one Document of the applicant a plasma method for on-board generation proposed by reducing agents. This will be the reduction The nitrogen oxides required ammonia from non-toxic substances as needed produced in the vehicle and then fed to the SCR process. A in terms of the fuel consumption acceptable solution offers a discontinuous operated method for ammonia production, as well proposed in this document. This method or This device is referred to below as the RGS method (Reductant Generating System) or reductant generation system.

A disadvantage of this method is that especially in the start phase, the reducing agent generation system is reached a sufficiently high operating temperature only very slowly, in which an optimal operation is guaranteed. The previous strategy provides a burner functionality, which makes it possible to make the system, in particular the catalytic components at about 650 ° C and the ammonia generating unit at about 250 ° C ready for operation. For this, diesel fuel combustion in a flame, possibly assisted by catalytic combustion within the catalytic components to raise the temperature in the area of the plasma reactor within the NO _x production unit is provided. A disadvantage is the additional energy consumption until the operational readiness of the reducing agent generation system (RGS), which results in a rather high fuel consumption.

It It is therefore an object of the invention to provide a method on the one hand the fast achievement of an optimal operating temperature of the reducing agent generation system and on the other hand the necessary energy requirement is minimized. It is still a task the invention to provide a suitable device for this purpose.

Advantages of invention

These Task is in the method with the features of the claim 1 and solved in the device with the features of claim 14.

The The object of the invention relating to the method is solved by at least temporarily the direction of flow the gas flow in the gas path of the reductant generation system turned over and at least part of the exhaust gas of the internal combustion engine from the exhaust system through the reductant generation system is passed.

The The object of the invention relating to the device is achieved in that the flow direction the gas flow in the gas path of the reductant generation system through a valve system or by reversing the pumping direction of the compressor at least temporarily reversible.

With the method and the device according to the invention it can be achieved that the heat energy contained in the exhaust gas flow from the internal combustion engine can be used in certain operating phases and in particular for the system start of the reducing agent generation system for raising the temperature of RGS components or of the entire reducing agent generation system. As a result, no or only a small amount of additional energy for heating the reducing agent generation system to the required operating temperature has to be provided. In particular, the NO _x storage / ammonia generating unit, which connects directly to the exhaust system of the internal combustion engine, is convectively heated by the passed exhaust gas flow and brought to operating temperature without additional fuel consumption and additional energy. The residual heat can be used to preheat the Multitron unit with the catalytic oxidation reforming unit.

The NO _x mass flow contained in the exhaust gas guided by the reducing agent generation system may be proportionately to completely stored in the NO _x storage / ammonia generating unit and used for conversion to ammonia, resulting in a reduction of the emitted amount of nitrogen oxide. By thus obtained additional ammonia nitrogen oxide is converted from the exhaust again in the subsequent selective catalytic reaction on the SCR catalyst, which further improves the positive balance of the process and the device in terms of energy consumption and conversion of nitrogen oxides.

He follows the reversal of the flow direction of Gas flow in the reducing agent generation system by switching of gas paths by means of a valve system in front of the compressor, the Pumping direction of the compressor to be maintained. The invention can be so implement with all known and used compressors.

In a preferred embodiment according to the method of the invention can be provided that ver in an ammonia production phase through the valve system, a fresh air supply to an input and the connection compressor / Multitron with an output of the compressor ver is connected and that in a heating phase through the valve system, the fresh air supply line to the output and the connection compressor / Multitron is connected to the input of the compressor.

Corresponding a preferred embodiment according to the device According to the invention, it may be provided that in an ammonia production phase through the valve system, a fresh air supply with an input and the connection compressor / Multitron with an output of the compressor is connected and that in a heating phase through the valve system the fresh air supply with the output and the connection compressor / Multitron connected to the input of the compressor. During the ammonia production phase, in which the reducing agent generation system to operating temperature is, we supplied in a known gas flow direction fresh air of the Multitron unit. During the Heating phase is by the compressor at the same direction of rotation of the blower Exhaust over the gas path of the reducing agent generation system through the along the gas path provided components from the exhaust system of the Internal combustion engine sucked and pumped into the fresh air supply line.

According to one Method variant, the switching of the flow direction by means of one or multiple 2/2 valves and / or one or more 3/2 valves and / or a 4/2 valve performed become. This can be done according to a variant the valve system of one or more 2/2-valves, preferably of four 2/2 valves and / or of one or more 3/2 valves, preferably from two 3/2 valves, and / or from a 4/2 valve be constructed. The different variants allow a variable adaptation of the invention to the circumstances of existing Reducing agent generation systems. 2/2 valves are inexpensive standard components, the wall opposite Activation and integration in the pipeline network is something for this higher. When implementing with 4/2 valves, only one valve is to be integrated and head for.

is it provided that the reversal of the flow direction of the gas flow in the reductant generation system By reversing the pumping direction of the compressor, there are no additional Provide valves and gas lines. For this purpose it can be provided that the pumping direction of the compressor by reversing the operating voltage is reversible.

According to one preferred embodiment variant of the invention it is provided that the reversal of the flow direction according to an RGS heating flow direction and the introduction at least a portion of the exhaust gas into the reductant generation system in cold and / or re-start phases of the reductant generation system he follows. By reversing the gas flow is a desired modulierbarbarer operation of the reducing agent generation system possible in the ammonia production discontinuously and in dependence from the operating conditions of the internal combustion engine and the resulting resulting amount needed Ammonia takes place. For small ammonia requirements for the reducing agent generation system and thus long pauses during which no ammonia is produced, is a reboot and the necessary heating on the introduced exhaust gas realized. The reductant generation system can be enough in the short term Ammonia available put what an increased Pollutant emissions during this Prevents phases.

Becomes after reaching an operating temperature of one or more components the reductant generation system or the entire Reducing agent generation system, the flow direction back to a Ammonia-generation flow direction switched, so the ammonia generation immediately after the take place known methods.

If it is provided that the switching takes place as a function of the temperature of the NO _x storage / ammonia generating unit, then it can be ensured that the NO _x storage / ammonia generating unit has a storage unit for storing the product from the Multitron storage unit. Unit generated nitrogen oxide has necessary operating temperature. This makes it possible to start the arc in the Multitron unit and thus initiate NO _x production without NO _x slipping out of the system.

In a variant of the method, it is provided that the fresh air extraction in switched ammonia generation flow direction of one and / or the exhaust gas inlet with switched RGS heating flow direction in an intake the internal combustion engine between an air filter and the internal combustion engine he follows.

To can according to a preferred embodiment the fresh air supply to the intake of the engine be connected. The switched RGS heating flow direction In the intake tract of the internal combustion engine pumped exhaust gas is from sucked this and can be used as a cooled low-pressure exhaust gas recirculation become. As a result, a high-pressure exhaust gas recirculation can be reduced and thus the exhaust gas flow over to enlarge a turbocharger, whereby the possible Boost pressure increased becomes.

is it provided that the fresh air supply in the flow direction after an air filter is connected to the intake, so is filtered ammonia generation flow direction filtered air pumped into the reductant generation system, whereby the Pollution of the system is significantly reduced. As of anyway existing air filter of the internal combustion engine is used No additional Components for air purification in the fresh air supply of the reducing agent generation system required.

According to one Another preferred variant of the method may be provided that the exhaust gas removal with switched RGS heating flow direction from the exhaust system takes place in the exhaust direction after a particle filter.

To can be provided according to a preferred embodiment of the device be that the gas path in the exhaust direction after a particle filter with the exhaust system connected is. This ensures that the exhaust gas, which is the Flow through the reducing agent generation system, is particle-free.

If it is provided that the heating behavior of the combined NO _x storage / ammonia generating unit is regulated by adjusting the blower output of the compressor, the necessary operating temperature of the NO _x storage / ammonia generating unit can be determined by regulating the amount of the reducing agent Generation system conducted exhaust gas can be accurately adjusted.

Around To achieve this, it can be provided that the blower power of the compressor is adjustable.

In addition to the previously described variants of the method, it can be provided for further temperature increase in the reducing agent generation system that, when the ammonia generating flow direction is switched, fuel combustion in a flame and / or catalytic combustion at the oxidation reforming unit are carried out. As a result, in particular the oxidation reforming unit is further heated until it has the operating temperature necessary for pulsed operation. In addition, the downstream within the reducing agent generation system NO _x storage / ammonia Erzeungungseinheit is further heated by this measure.

Overheating of the compressor when the RGS heating flow direction is switched can be avoided by providing a gas cooler in the gas path between the multitron unit and the NO _x storage / ammonia production unit. In this case, the gas cooler can be used with switched ammonia generation flow direction for regulating the temperature of the NO _x storage / ammonia production unit.

In preferred embodiment the exhaust aftertreatment system has a control unit, with the valves of the valve system are switchable, wherein the control unit on the input side with at least one temperature sensor within the reducing agent generation system or a component of the reducing agent generation system connected is. This can on the one hand a monitoring of the temperature in the Reducing agent generation system and on the other hand a scheme take place, in which the introduction of a part of the exhaust gas stopped when the components of the reductant generation system have reached their required operating temperatures. temperature exceeded can to be avoided.

is the control unit integrated in the reducing agent generation system or part of a parent Motor control, can also complex control tasks within the exhaust aftertreatment system be realized, including signals from additional sensors in the control unit can be processed.

The described method and the device described can be preferred to use in diesel engines or lean-burn engines, the one Have reducing agent generation system.

drawings

The Invention will be described below with reference to the figures shown in the figures Embodiments explained in more detail. It demonstrate:

1 a schematic representation of an exhaust aftertreatment system of an internal combustion engine with a reducing agent generation system with a 4/2-valve for switching the gas paths,

2 a schematic representation of the exhaust aftertreatment system with a reducing agent generation system with four 2/2-valves for switching the gas paths and

3 a schematic representation of the exhaust aftertreatment system with a reducing agent generation system with two 3/2 valves for switching the gas paths.

description the embodiments

1 schematically shows the technical environment using the example of a diesel engine, in which the inventive method can be applied.

Shown is an exhaust aftertreatment system 1 for an internal combustion engine 10 whose exhaust gases via an exhaust system 30 be guided, wherein in the flow direction of the exhaust gas, a diesel oxidation catalyst 31 (DOC), a particle filter 32 and a downstream SCR catalyst 33 are provided. For the reduction of nitrogen oxides is before the SCR catalyst 33 from a reductant generation system 60 Ammonia can be fed as a reducing agent. SCR catalysts 33 work on the principle of selective catalytic reduction, in which by means of the reducing agent ammonia in oxygen-containing exhaust gases nitrogen oxides are reduced to nitrogen and water.

The reductant generation system 60 has a multitron unit 62 consisting of an unrepresented NO _x production unit, a mixture forming chamber, not shown, and an oxidation reforming unit 63 (POx) exists. In the embodiment shown, the oxidation reforming unit is 63 executed separately. The reductant generation system 60 continue to know a combined NO _x storage / ammonia production unit 64 on, over a gas path 65 with the Multitron unit 62 and the associated oxidation reforming unit 63 connected is.

The multitron unit 62 Can via a fuel feeder 40 Fuel to be supplied. It is still connected via a compressor / Multitron connection 68 with a compressor 61 connected. The compressor 61 is via a fresh air supply 22 with the intake tract 20 the internal combustion engine 10 connected, the fresh air supply 22 after an air filter 21 at the intake tract 20 is arranged. In the fresh air supply 22 and the compound Compressor / Multitron 68 is a valve system 70 arranged, which in the illustrated embodiment as a 4/2 valve 71 is executed. The 4/2 valve 71 is via a control line with a control unit 50 connected. In the compressor 61 an unillustrated blower is arranged which a gas flow from an input 61.1 to an exit 61.2 of the compressor causes.

The 4/2 valve 71 has two operating positions. Thus, according to the invention in a first operating position, the fresh air supply 22 with the entrance 61.1 of the compressor 61 and the connection compressor / Multitron 68 with the exit 61.2 be connected to the compressor. In a second operating position, the fresh air supply 22 with the exit 61.2 of the compressor 61 and the connection compressor / Multitron 68 with the input of the compressor 61 get connected. The gas paths within the 4/2 valve 71 are therefore crossed in the second operating position relative to the first operating position. With constant pumping direction of the compressor 61 is achieved in that two opposite flow directions, an ammonia production flow direction 66 in the first operating position and an RGS heating flow direction 67 in the second operating position of the 4/2 valve 71 in the gas path 65 of the reductant generation system 60 be generated. The control of the switching of the gas paths is carried out by the control unit 50 ,

During the first operating position of the 4/2 valve 70 at switched ammonia generation flow direction 66 is the multitron unit 62 from the air filter 21 filtered air from the intake system 21 the internal combustion engine 10 via the fresh air supply 22 , the compressor 61 and the connection compressor / Multitron 68 and about the fuel supply 80 Fuel as starting materials for the production of ammonia, at least temporarily fed.

The Ammonia is made from air and in the example shown from diesel fuel generated. These are a hydrogen generating unit and a nitrogen oxide generating unit intended.

In the example shown, in the hydrogen generating unit, the hydrocarbon of the fuel becomes hydrogen and carbon monoxide by means of an oxidation reforming reaction in the oxidation reforming unit 63 generated. The required mixture formation chamber is in the Multitron unit 62 integrated.

The production of ammonia occurs within the reductant generation system 60 in which nitrogen monoxide NO is generated in a lean phase (λ> 1) in an arc plasma within the multitron of air. These nitrogen oxides flow through the separate oxidation reforming unit 63 (POx) and then the combined NO _x storage / ammonia production unit 64 supplied and stored. In a second phase of operation following the lean phase, the fat phase (0.33 <λ <1), liquid fuel is metered into the evaporation and mixture-forming zone in the area of the multitron and in the oxidation reforming unit 63 (POx) to a hydrogen and carbon monoxide-containing gas mixture, which then in the range of NO _x storage / ammonia generating unit 63 converts the previously stored nitrogen oxides to ammonia. This generated gaseous ammonia is then in the exhaust gas flow of the exhaust system 30 before the SCR catalyst 33 added.

Because the SCR catalyst 33 has an ammonia storage capacity, it is possible to continuously achieve via a batch process for ammonia production, the reduction of nitrogen oxides by means of the SCR process in the exhaust stream. In the temperature range between 150 ° C and 450 ° C, catalysts of titanium dioxide (TiO ₂ ) and vanadium pentoxide (V ₂ O ₅ ) convert the nitrogen oxides with the ammonia produced at a high rate.

During the second operating position of the 4/2 valve 70 with switched RGS heating flow direction 67 in the gas path 65 of the reductant generation system 60 is from the compressor 61 at least a portion of the exhaust gas from the exhaust system 30 the internal combustion engine 10 along the Gasweg 65 through the components NO _x storage / ammonia production unit 62 and Multitron unit 62 with oxidation reforming unit 63 sucked and then on the fresh air supply 22 in the intake tract 20 the internal combustion engine 10 pumped. The warm exhaust gas is primarily the NO _x storage / ammonia production unit 64 heated convectively. With the residual heat contained in the derived exhaust then the Multitron unit 62 and in particular the associated oxidation reforming unit 63 warmed up.

During the switched RGS heating flow direction 67 Accordingly, the components of the reducing agent generation system 60 , in particular the NO _x storage / ammonia production unit 64 , from the waste heat of the internal combustion engine 10 be brought to operating temperature, without requiring an additional energy source and thus additional fuel consumption would be necessary. The warming up of the components is especially in the cold and restart phases of the reducing agent generation system 60 necessary. The control of the 4/2 valve by the control unit 50 takes place as a function of temperature values that are transmitted to it by corresponding, not shown, sensors on the respective components. If the amount of energy that can be taken from the exhaust gas is insufficient to achieve the necessary operating temperature for all components, then during the subsequent ammonia generation flow direction 66 known methods such as additional fuel combustion in a flame, possibly assisted by catalytic combustion at the oxidation reforming unit 63 , used to increase the temperature.

Will during the switched RGS heating flow direction 67 the operating temperature of the NO _x storage / ammonia production unit 64 of about 250 ° C, so is the start of the arc in the Multitron unit 62 in the subsequent ammonia production phase without NO _x slipping out of the system possible as this from the NO _x storage / ammonia generating unit 64 stored and then converted to ammonia.

During the switched RGS heating flow direction 67 Nitrogen oxide is released from the by the reducing agent generation system 60 guided exhaust gas at least partially in the NO _x storage / ammonia generating unit 64 stored and converted in the subsequent ammonia production phase to ammonia, which in turn to the SCR catalyst 33 Nitrogen oxide is reduced from the exhaust gas. This allows the NO _x emission of the internal combustion engine 10 and the exhaust aftertreatment system 1 in particular during the cold or re-start phases of the reductant generation system 60 , be noticeably reduced.

With switched ammonia production flow direction 66 Fresh air is after an air filter 21 from the intake tract 20 the internal combustion engine 10 Accordingly, the air is sufficiently filtered to prevent premature contamination of the reductant generation system 60 to avoid.

With switched RGS heating flow direction 67 becomes exhaust after a particulate filter 32 , which is designed here as a diesel particulate filter, the exhaust system 30 taken and the intake tract 20 the internal combustion engine 10 fed. The removed exhaust gas is therefore sufficiently free of particles. That in the intake tract 20 the internal combustion engine 10 pumped exhaust gas is sucked by this and used as cooled low-pressure exhaust gas recirculation. This has the consequence that a high-pressure exhaust gas recirculation, not shown, reduced and thus the exhaust gas flow can be increased via a turbocharger, not shown. This increases the possible boost pressure.

2 shows a variant of the in 1 shown arrangement. Deviating from 1 takes place in 2 the switching between the ammonia generation flow direction 66 and the RGS heating flow direction 67 through four 2/2 valves ( 72 . 73 . 74 . 75 ). At this time, the ammonia generation flow direction becomes 66 achieved when the 2/2 valves 72 . 75 opened and the 2/2 valves 73 . 74 are closed. The RGS heating flow direction 67 This is achieved by the 2/2 valves 72 . 75 closed and the 2/2 valves 73 . 74 are open. This arrangement offers opposite to in 1 shown embodiment the advantage that simply constructed valves can be used. A disadvantage is the increased circuit complexity for connection to the control unit 50 as well as two additionally required gas lines.

3 shows a further embodiment of the in 1 shown arrangement. Deviating from 1 the switchover takes place between the ammonia generation flow direction 66 and the RGS heating flow direction 67 through two 3/2 valves 76 . 77 , During the ammonia generation flow direction 66 is in the 3/2 valve 76 a gas path from a port a 76.1 to a connection b 76.2 opened while connecting to port c 76.3 closed is. At the 3/2 valve 77 is a gas path from port a 77.1 to the port b 77.2 opened while connecting to port c 77.3 closed is. Accordingly, the entrance 61.1 of the compressor 61 with the fresh air supply 22 and the exit 61.2 of the compressor 61 with the connection compressor / Multitron 68 connected, the compressor 61 Fresh air is pumped from the intake tract 20 the internal combustion engine 10 to the multitron unit 62 ,

To switch the RGS heating flow direction 67 will be in the 3/2 valve 76 a gas path from port a 76.1 to the port c 76.3 opened while the connection to the port b 76.2 is closed. At the 3/2 valve 77 is a gas path from port a 77.1 to the port c 77.3 opened while the connection to the port b 77.2 closed is. In this circuit is the input 61.1 of the compressor 61 with the connection compressor / Multitron 68 and thus the multitron unit 62 connected. The exit 61.2 of the compressor 61 is with fresh air supply 22 connected. Thus, exhaust gas from the exhaust system 30 the internal combustion engine 10 by the reductant generation system 60 in the intake tract 20 the internal combustion engine 10 pumped.

Overall, with the process variants and the device embodiments described a rapid heating of the reduction Mitel generation system 60 be achieved with its components, resulting in a faster system startup of the exhaust aftertreatment system 1 is possible. By using the hot exhaust gas as a heat source, an otherwise additional fuel consumption is avoided or at least significantly minimized.

In principle, the apparatus and method may be applied to all diesel or lean-burn engines powered by other fuels which use a reductant generation system 60 is used.

Claims (25)

Method for starting an exhaust aftertreatment system ( 1 ) an internal combustion engine ( 10 ) with an exhaust system ( 30 ), in which in the flow direction of the exhaust gas, an SCR catalyst ( 33 ), wherein a reducing agent generation system ( 60 ) a multitron unit ( 62 ), which consists of a NO _x production unit, a mixture formation chamber and an oxidation reforming unit ( 63 ) and a combined NO _x storage / ammonia production unit ( 64 ) in a gas path ( 65 ) of the reducing agent generation system ( 60 ), wherein for the reduction of nitrogen oxides in the exhaust gas direction upstream of the SCR catalyst ( 33 ) of the reductant generation system ( 60 ) generated ammonia as a reducing agent of the exhaust system ( 30 ) and wherein the multitron unit ( 62 ) via a compressor ( 61 ) via a connection Vedichter / Multitron ( 68 ) Air and via a fuel supply ( 40 ) Fuel are fed as starting materials for generating the ammonia at least temporarily, characterized in that at least at times the flow direction of the gas flow in the gas path ( 65 ) of the reducing agent generation system ( 60 ) and at least a part of the exhaust gas of the internal combustion engine ( 10 ) from the exhaust system through the reducing agent generation system ( 60 ). A method according to claim 1, characterized in that the reversal of the flow direction of the gas stream in the reducing agent generation system ( 60 ) by switching gas paths by means of a valve system ( 70 ) in front of the compressor ( 61 ) he follows. A method according to claim 1 or 2, characterized in that in an ammonia production phase by the valve system ( 70 ) a fresh air supply ( 22 ) with an input ( 61.1 ) and the connection compressor / Multitron ( 68 ) with an output ( 61.2 ) of the compressor ( 61 ) and that in a heating phase through the valve system ( 70 ) the fresh air supply ( 22 ) with the output ( 61.2 ) and the connection compressor / Multitron ( 68 ) with the entrance ( 61.1 ) of the compressor ( 61 ) is connected. Method according to one of the preceding claims, characterized in that the switching of the flow direction by means of one or more 2/2-valves ( 72 . 73 . 74 . 75 ) and / or one or more 3/2 valves ( 76 . 77 ) and / or a 4/2 valve ( 71 ) is carried out. A method according to claim 1, characterized in that the reversal of the flow direction of the gas stream in the Reduktionsmitel generation system ( 60 ) by reversing the pumping direction of the compressor ( 61 ) he follows. Method according to one of the preceding claims, characterized in that the reversal of the flow direction in accordance with an RGS heating flow direction ( 67 ) and the introduction of at least a portion of the exhaust gas into the reductant generation system ( 60 ) in cold and / or restart phases of the reductant generation system ( 60 ) he follows. Method according to one of the preceding claims, characterized in that after reaching an operating temperature of one or more components of the reducing agent generation system ( 60 ) or the entire reducing agent generation system ( 60 ) the flow direction back to an ammonia production flow direction ( 66 ) is switched. A method according to claim 7, characterized in that the switching as a function of the temperature of the NO _x storage / ammonia generating unit ( 64 ) he follows. Method according to one of the preceding claims, characterized in that the fresh air removal at switched ammonia-generating flow direction ( 66 ) from one and / or the exhaust gas inlet with switched RGS heating flow direction ( 67 ) into an intake tract ( 20 ) of the internal combustion engine ( 10 ) between an air filter ( 21 ) and the internal combustion engine ( 10 ) he follows. Method according to one of the preceding claims, characterized in that the exhaust gas removal with switched RGS heating flow direction ( 66 ) from the exhaust system ( 30 ) in the exhaust gas direction after a particle filter ( 32 ) he follows. Method according to at least the preceding claims, characterized in that the heating behavior of the combined NO _x storage / ammonia production unit ( 64 ) by adjusting the blower capacity of the compressor ( 61 ) is regulated. Method according to one of the preceding claims, characterized in that when switched ammonia generating flow direction ( 66 ) fuel combustion in a flame and / or catalytic combustion at the oxidation reforming unit ( 63 ) is carried out. Application of the method according to one of the preceding claims, characterized in that the method is applied to diesel engines or to lean-burn engines which use a reducing agent generation system ( 60 ) exhibit. Device for starting an exhaust aftertreatment system ( 1 ) an internal combustion engine ( 10 ) with an exhaust system ( 30 ), in which in the flow direction of the exhaust gas, an SCR catalyst ( 33 ), wherein a reducing agent generation system ( 60 ) a Mutlitron unit ( 62 ), which consists of a NO _x production unit, a mixture formation chamber and an oxidation reforming unit ( 63 ) and a combined NO _x storage / ammonia production unit ( 64 ) in a gas path ( 65 ) of the reducing agent generation system ( 60 ), wherein for the reduction of nitrogen oxides in the exhaust gas direction upstream of the SCR catalyst ( 33 ) of the reductant generation system ( 60 ) produced ammonia as a reducing agent in the exhaust system ( 30 ) and wherein the multitron unit ( 62 ) via a compressor ( 61 ) via a connection Vedichter / Multitron ( 68 ) Air and via a fuel supply ( 40 ) Fuel is supplied as starting materials for generating the ammonia at least temporarily, characterized in that the flow direction of the gas flow in the gas path ( 65 ) of the reducing agent generation system ( 60 ) by a valve system ( 70 ) or by reversing the pumping direction of the compressor ( 61 ) is at least temporarily reversible. Apparatus according to claim 14, characterized in that in an ammonia production phase by the valve system ( 70 ) a fresh air supply ( 22 ) with an input ( 61.1 ) and the connection compressor / Multitron ( 68 ) with an output ( 61.2 ) of the compressor ( 61 ) and that in a heating phase through the valve system ( 70 ) the fresh air supply ( 22 ) with the output ( 61.2 ) and the connection compressor / Multitron ( 68 ) with the entrance ( 61.1 ) of the compressor ( 61 ) connected is. Device according to claim 14 or 15, characterized in that the valve system consists of one or more 2/2 valves ( 72 . 73 . 74 . 75 ), preferably of four 2/2 valves ( 72 . 73 . 74 . 75 ) and / or one or more 3/2 valves ( 76 . 77 ), preferably two 3/2 valves ( 76 . 77 ), and / or from a 4/2 valve ( 71 ) is constructed. Device according to one of claims 14 to 16, characterized in that the pumping direction of the compressor ( 61 ) is reversible by reversing the operating voltage. Device according to one of claims 14 to 17, characterized in that the fresh air supply line ( 22 ) with the intake tract ( 20 ) of the internal combustion engine ( 10 ) connected is. Apparatus according to claim 18, characterized in that the fresh air supply line ( 22 ) in Flow direction after an air filter ( 21 ) with the intake tract ( 20 ) connected is. Device according to one of claims 14 to 19, characterized in that the gas path ( 65 ) in the exhaust gas direction after a particle filter ( 32 ) with the exhaust system ( 30 ) connected is. Device according to one of claims 14 to 20, characterized in that in the gas path ( 65 ) between the Mutitron unit ( 62 ) and the NO _x storage / ammonia generating unit ( 64 ) A gas cooler is provided. Device according to one of claims 14 to 21, characterized in that the blower output of the compressor ( 61 ) is controllable. Device according to one of claims 14 to 22, characterized in that the exhaust aftertreatment system ( 1 ) a control unit ( 50 ), with which the valves ( 71 . 72 . 73 . 74 . 75 . 76 . 77 ) of the valve system ( 70 ) are switchable, wherein the control unit ( 90 ) on the input side with at least one temperature sensor within the reducing agent generation system ( 60 ) or a component of the reducing agent generation system ( 60 ) connected is. Device according to claim 23, characterized in that the control unit ( 50 ) in the reductant generation system ( 60 ) is integrated or is part of a higher-level engine control. Application of the device according to one of claims 14 to 24, characterized in that the device is used in diesel engines or lean-burn engines which use a reducing agent generation system ( 60 ) exhibit.