Abstract:
A method for regenerating at least one particle agglomerator of an exhaust gas after-treatment system of an internal combustion engine of a motor vehicle, includes operating the internal combustion engine in at least one operating phase in such a way that a sufficient portion of nitrogen dioxides is directly produced in the exhaust gas in order to ensure a conversion of particles containing carbon in the at least one particle agglomerator. A motor vehicle suitable for carrying out the method is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This is a continuation, under 35 U.S.C. §120, of copending International Application No. PCT/EP2008/057038, filed Jun. 5, 2008, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German Patent Application DE 10 2007 032 734.1, filed Jul. 13, 2007; the prior applications are herewith incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The present invention relates to a method for regenerating at least one particle agglomerator of an exhaust-gas aftertreatment system of an internal combustion engine of a motor vehicle. The invention also relates to a motor vehicle having an internal combustion engine and an exhaust-gas aftertreatment system which is formed with at least one continuously regenerable particle agglomerator. In this respect, the invention relates in particular to the elimination of soot particles from mobile internal combustion engines, such as for example diesel engines. 
         [0003]    It is known that the particles which are entrained in the exhaust gas flow and substantially contain carbon can be thermally burned or converted through the use of nitrogen dioxide (NO 2 ) which is also formed in the exhaust-gas aftertreatment system. For that purpose, it is known to provide particle agglomerators, for example filters, particle separators and the like, in which the entrained particles are at least temporarily trapped and accumulated. During a thermal regeneration, the particle agglomerator is heated up to such an extent (for example to above 800° C.) that a conversion of the carbon with oxygen entrained in the exhaust gas is initiated. For that purpose it is, for example, possible for burners, heating elements, electrically heatable filters or an exothermic conversion of hydrocarbons to be considered as a source for the heat energy. In contrast, the so-called continuously regenerative conversion of particles (the so-called CRT process) is based on a conversion of the carbon-containing particles at low temperatures, for example below 400° C., using nitrogen dioxide. For that purpose, it is known to conduct the exhaust gas generated by the engine through an oxidation catalytic converter, and to thereby oxidize nitrogen oxides which are already contained in the exhaust gas in order to be able to provide sufficient nitrogen dioxide for the conversion of the soot particles. The nitrogen dioxide has a high affinity to carbon, such that carbon dioxide and nitrogen are regularly formed in the event of the nitrogen dioxide coming into contact with the soot particles. 
         [0004]    In the known method and devices, with regard to the passively regenerable particle agglomerator (CRT process), an oxidation coating is provided upstream of the particle agglomerator or directly in the particle agglomerator. However, that coating, which often contains platinum, is expensive and requires, if appropriate, additional exhaust-gas treatment devices which result in more complex exhaust-gas aftertreatment systems. 
       SUMMARY OF THE INVENTION 
       [0005]    It is accordingly an object of the invention to provide a method for regenerating at least one particle agglomerator and a motor vehicle including an exhaust gas after-treatment system, which overcome the hereinafore-mentioned disadvantages and at least partially solve the highlighted problems of the heretofore-known methods and devices of this general type. In particular, the invention is intended to specify a practicable and cost-effective method for regenerating at least one particle agglomerator which particularly permits a tailored passive regeneration. In addition, the invention is also intended to specify a device which is suitable for such a method, in which the device is distinguished by a low pressure drop and a particularly high level of effectiveness in the case of small particles (for example with a mean diameter of at most 500 nanometers). 
         [0006]    With the foregoing and other objects in view there is provided, in accordance with the invention, a method for regenerating at least one particle agglomerator of an exhaust-gas aftertreatment system of an internal combustion engine of a motor vehicle. The method comprises operating the internal combustion engine at least in one operating phase to cause a proportion of nitrogen dioxides being sufficient to ensure a conversion of carbon-containing particles in the at least one particle agglomerator to be directly generated in the exhaust gas. 
         [0007]    This means, in particular, that the first particle agglomerator which is disposed following the internal combustion engine is regenerated in the way proposed herein. In this case, thermal regeneration is dispensed with, in such a way that the conversion from carbon-containing particles takes place at temperatures below 400° C. or even below 300° C. The particle agglomerator can fundamentally be formed in the manner of a filter, a particle separator or similar simple devices for temporarily trapping the particles. The internal combustion engine is preferably a lean-burn engine in which combustion takes place predominantly with an excess of air, such as for example in a diesel engine or a so-called lean-burn engine. In other words, it is thus proposed in this case that the internal combustion engine be operated, at least in a certain operating phase (regeneration phase) such as for example in a low-load situation, in such a way that a sufficiently high proportion of nitrogen dioxides is directly generated by the internal combustion engine. A “regeneration phase” is understood to be a time interval in which the amount of particles in the particle agglomerator is reduced, in particular by at least 20% by weight, if appropriate by at least 40% by weight or even by at least 80% by weight. The individual mechanisms of how the internal combustion engine can be correspondingly regulated are discussed in detail below. In this connection, it is thus proposed firstly that the internal combustion engine itself be used as a nitrogen oxide source for the regeneration of the particle agglomerator, in such a way that additional nitrogen oxide sources such as for example upstream oxidation catalytic converters, can be dispensed with. 
         [0008]    In accordance with another mode of the method of the invention, the internal combustion engine places a proportion of the nitrogen dioxides (NO 2 ) in a range of from 25% by volume to 60% by volume of all of the nitrogen oxides (NO x ) present. The conditions in the combustion chamber of the internal combustion engine are thus in particular set in such a way that the proportion of the nitrogen dioxides in relation to all of the nitrogen oxides generated reaches a significant range, in particular of more than 30% by volume or even 45% by volume (these ratios can, if appropriate, likewise be considered in mol.-% for regulation). This relates specifically to the nitrogen oxide proportion during the operating phase in which the regeneration of the particle accumulator takes place. The 25% by volume can be considered in this case as a lower limit and/or as a mean value during the operating phase. It is preferably also proposed that the nitrogen dioxide proportion substantially does not exceed 60% by volume in order to still be able to generate sufficient power through the use of the internal combustion engine. 
         [0009]    In accordance with a further mode of the method of the invention, up to the at least one particle agglomerator, solely the internal combustion engine actively generates nitrogen dioxide (NO 2 ). In other words, this means in particular that, between the internal combustion engine and the particle agglomerator in question, the exhaust-gas aftertreatment system does not have any device or measures for the targeted enrichment of the exhaust gas with nitrogen dioxide. The method and the device can therefore be of particularly simple construction, and a targeted regeneration of the particle agglomerator can be regulated through the use of corresponding operation of the internal combustion engine. Redox processes of course cannot be prevented in the exhaust gas itself, although they are often not suitable for bringing about a corresponding active, significant generation of nitrogen dioxide. 
         [0010]    In accordance with an added mode of the invention, the method can be refined in such a way that an increase in the proportion of an exhaust-gas flow recirculated into the internal combustion engine is carried out in the operating phase. For this purpose, the exhaust-gas aftertreatment system is formed, for example, with a so-called exhaust-gas recirculation (EGR) in such a way that the exhaust gas generated by the internal combustion engine is (partially) supplied to the internal combustion engine again, in particular before the exhaust gas reaches the at least one particle agglomerator. A targeted increase in the exhaust-gas recirculation rate can lead to a significant increase in the nitrogen dioxide proportion in the exhaust gas and can thereby promote the regeneration proposed in this case. The rate of the recirculated flow is preferably in the range of up to 60% by volume, in particular in a range of from 20% by volume to 50% by volume. 
         [0011]    In accordance with an additional mode of the method of the invention, a reduction of the combustion chamber temperature in the internal combustion engine is carried out in the operating phase. It has been found that a high nitrogen dioxide proportion is conventionally produced in the exhaust gas in combustion processes carried out at a relatively low temperature. In particular, the combustion chamber temperature is regulated for this purpose, in terms of a peak temperature of the combustion, in a range below 450° C. 
         [0012]    In accordance with yet another mode of the method of the invention, it is also considered to be advantageous that, alternatively or in addition to the possibilities specified above, an increase in the charge pressure in the internal combustion engine is carried out in the operating phase. In this case, the exhaust-gas aftertreatment system is, for example, formed with an exhaust-gas turbocharger which results in a compression of the intake air flow. The charge pressure, that is to say the pressure in the combustion chamber of the internal combustion engine, of the fuel-air mixture, is conventionally in a range of from 30 to 50 bar. For the regeneration phase, it is now proposed, in particular, that an increase in the charge pressure by, for example, at least 15%, if appropriate even 25%, of the previously regulated charge pressure is carried out. With the increase in charge pressure, the peak temperature of the combustion in the combustion chamber and therefore the nitrogen oxide formation are also influenced. 
         [0013]    In accordance with yet a further mode of the method of the invention, it is also possible for an increase in the oxygen content in the internal combustion engine to be carried out in the operating phase. Accordingly, the combustion is, for example, carried out with an even greater excess of air. In this way, the oxygen content in the fuel-air mixture can, for example, be increased by a value of at least 1%, and in particular in a lambda range of from 1.05 to 1.1 (approximately 1% oxygen and 2% oxygen, respectively). The so-called combustion air ratio (lambda) places the air mass m (AIR,actual)  which is actually available for a combustion in relation to the minimum necessary stoichiometric air mass m (AIR,stoichiometric)  which is required for a complete combustion. This effect can also, in particular temporarily, lead to the desired generation of nitrogen dioxides. 
         [0014]    In accordance with yet an added mode of the method of the invention, for an equally effective conversion of the carbon-containing particles with a simultaneously small volume of the provided particle agglomerator, it is also proposed that the internal combustion engine be operated in such a way that carbon-containing particles, the majority of which have a mean diameter of at most 200 nanometers [nm], are generated in the exhaust gas. The internal combustion engine is very particularly preferably operated in such a way that the mean diameter is at most 100 nanometers. This fundamentally also applies in an operating state of the internal combustion engine which does not correspond to the operating phase for regenerating the particle agglomerator (regeneration phase). The very small particles can particularly favorably be converted with the provided nitrogen dioxide to form carbon monoxide and elementary nitrogen. In order to provide the particles of this size, it is necessary in particular for the outlet of the combustion chamber and the exhaust line to be adapted so as to prevent an excessive agglomeration of particles up to a size above the limit value specified herein. 
         [0015]    In accordance with yet an additional mode of the method of the invention, it is also proposed that an active temperature increase of the exhaust gas be carried out at least in the operating phase. This means, in particular, that the exhaust gas in the exhaust-gas aftertreatment system is placed in contact with additional temperature-increasing measures, in such a way that the exhaust gas, at the latest when it comes into contact with the particles to be converted, is at a nominal temperature for significantly carrying out the CRT process. The temperature-increasing measures include in particular (uncoated) (electrically operated) heating bodies, heat exchangers and the like. The concept of the targeted or regulated (non-catalytic and/or catalytic) temperature increase of the exhaust gas in order to improve the oxidation of nitrogen monoxides in the exhaust-gas aftertreatment system can generally bring significant advantages in carrying out the CRT process, and is accordingly desirable, if appropriate, even independently of the method according to the invention described herein. 
         [0016]    With the objects of the invention in view, there is also provided a motor vehicle, comprising an exhaust-gas aftertreatment system having at least one continuously regenerable particle agglomerator being a bypass flow filter (also referred to as a semi-filter), and an internal combustion engine being a sole active nitrogen dioxide source up to the at least one particle agglomerator. 
         [0017]    The motor vehicle proposed herein can be operated, in particular, according to the method of the invention described herein, in such a way that a non-thermal regeneration of the at least one particle agglomerator is possible in desired operating phases. The motor vehicle proposed herein is distinguished by its particularly simply-constructed exhaust-gas aftertreatment system, with corresponding control of the internal combustion engine resulting in reliable regeneration of the particle agglomerator, in such a way that a blockage of the particle agglomerator and therefore a pressure rise across the particle agglomerator is prevented. 
         [0018]    With regard to the configuration of the internal combustion engine as a sole (exclusive) active nitrogen oxide source, reference is made substantially to the explanations given above. With regard to the particle agglomerator proposed herein, it is specified that the latter include a bypass flow filter. A bypass flow filter of this type is distinguished in that it provides a plurality of flow paths for the exhaust gas, with the exhaust gas (theoretically) having the possibility of flowing through the particle agglomerator without coming into contact with a filter material, or flowing through the latter. For this purpose, the bypass flow filter can be constructed in the manner of a honeycomb body which is formed, for example, with channel walls which are formed at least partially from a gas-impermeable material and can optionally also include a filter medium. The gas-impermeable material (preferably a sheet metal foil) is now formed with elevations and guide blades which at least partially close off (or deflect) the channel and thereby bring about a deflection of at least a part of the exhaust gas flow towards the channel wall (or towards the filter medium). In this case, the elevations are formed in such a way that they do not completely close off the channel at any point, and thereby permit a secondary flow flowing past the elevation. One possible construction of a bypass flow filter of that type can be gathered, for example, from International Publication No. WO 01/80978 A1, corresponding to U.S. Patent Application Publication No. US 2003/0072694 A1, or from International Publication No. WO 02/00326 A1, corresponding to U.S. Pat. No. 6,712,884, in such a way that reference can be made, in particular, to those documents for explanation. 
         [0019]    In accordance with a concomitant feature of the motor vehicle of the invention, the at least one particle agglomerator includes, in the flow direction of the exhaust gas, at least one first zone and a second zone, with the second zone extending to a downstream end side and with the second zone including an oxidation catalytic converter. This means, in particular, that the particle agglomerator can be divided into at least two zones which extend in the axial direction and over the entire cross section of the particle agglomerator, with the downstream zone, which extends to the downstream end of the particle agglomerator, being provided with an oxidation catalytic converter. In this case, the first zone is preferably catalytically inactive, that is to say, for example, free from a coating. The oxidation catalytic converter can be formed, for example, in the manner of a conventional washcoat coating with high-grade metal doping. 
         [0020]    Other features which are considered as characteristic for the invention are set forth in the appended claims, noting that the features listed individually in the claims can be combined in any desired technologically meaningful way and highlight further embodiments of the invention. 
         [0021]    Although the invention is illustrated and described herein as embodied in a method for regenerating at least one particle agglomerator and a motor vehicle including an exhaust gas after-treatment system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
         [0022]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0023]      FIG. 1  is a diagrammatic, plan view of a first embodiment variant of an exhaust-gas aftertreatment system of a motor vehicle; 
           [0024]      FIG. 2  is a graph showing a possible curve or profile of nitrogen dioxide concentration during operation of the internal combustion engine; 
           [0025]      FIG. 3  is a fragmentary, perspective view showing details of the construction of an advantageous particle agglomerator; and 
           [0026]      FIG. 4  is a cross-sectional view of a further embodiment of a particle agglomerator. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Referring now to the figures of the drawings in detail and first, particularly, to  FIG. 1  thereof, there is seen a diagrammatic illustration of one possible construction of an exhaust-gas aftertreatment system  2  of an internal combustion engine  3  of a motor vehicle  4 , which construction is fundamentally suitable for carrying out the method described herein. The motor vehicle  4  therefore firstly has the internal combustion engine  3 , in particular a diesel engine, which has a plurality of combustion chambers  21  in which a supplied fuel-air mixture is burned and from which exhaust gas is discharged into the atmosphere through an exhaust line  19 . 
         [0028]    The exhaust-gas aftertreatment system  2  shown herein has a branch for an exhaust-gas recirculation  12 , downstream of the internal combustion engine  3  in a flow direction  7 , in such a way that a part of the exhaust-gas flow can be supplied again in a regulated fashion to the combustion chambers  21  of the internal combustion engine  3 . A particle agglomerator  1  is illustrated further downstream in the flow direction  7 . The particle agglomerator  1  is followed further downstream by a turbocharger  13 , so that as exhaust gas flows through the turbocharger  13 , a turbine is simultaneously driven and the turbine compresses an air quantity which is supplied through an intake tract or part  20  to the internal combustion engine  3 . 
         [0029]    After the exhaust gas has flowed further through the exhaust line  19  in the flow direction  7 , for example to an underbody region of the motor vehicle  4 , the exhaust gas undergoes further removal of pollutants through the use of further exhaust-gas aftertreatment units  24 . In the case illustrated herein, the exhaust gas flows in the flow direction  7  through an oxidation catalytic converter  11 , a filter  22  and an SCR catalytic converter  23  (for the selective catalytic reaction of nitrogen oxide), with the exhaust gas being mixed upstream of the SCR catalytic converter  23  with a reducing agent which is introduced through the use of a corresponding addition of reducing agent  25 . The exhaust gas which is purified and converted in this way then finally flows through the exhaust line  19  into the environment. 
         [0030]    The construction of the exhaust-gas aftertreatment system  2  shown herein permits, in particular, a discontinuous, targeted regeneration of the particle agglomerator  1  with nitrogen dioxides, which are provided in a targeted fashion through the use of the internal combustion engine  3 . 
         [0031]      FIG. 2  shows graphically and by way of example different curves or profiles of a nitrogen dioxide concentration of the exhaust gas generated by the internal combustion engine for a regeneration of the particle agglomerator. In this case, the abscissa  30  denotes time, while the ordinate  31  substantially illustrates the nitrogen oxide concentration. 
         [0032]    With regard to a first curve or profile  26 , it can be seen that the nitrogen dioxide concentration is disposed mostly below a predefined regeneration field  28  during operation of the internal combustion engine  3 . If a regeneration of the particle agglomerator is now to take place, then the nitrogen dioxide concentration in the exhaust gas is adjusted through the use of a regeneration phase  29  or an operating phase of the internal combustion engine in such a way that the concentration lies in the regeneration field  28 . If the demands on the internal combustion engine change (for example power demand, load range, . . . ) or the regeneration of the particle agglomerator is to be ended, the internal combustion engine  3  can be operated with a relatively low nitrogen dioxide proportion in the exhaust gas again. It is thereby possible for a discontinuous, and at predefined and/or calculated times non-thermal, regeneration of the particle agglomerator to be carried out. 
         [0033]    Furthermore, it is also possible for the nitrogen dioxide proportion in the exhaust gas to fundamentally be regulated in such a way that the proportion lies in the region of the regeneration field  28  at regular intervals and/or permanently, as shown in particular by a second curve or profile  27  illustrated through the use of dashed lines. 
         [0034]      FIG. 3  shows a portion of an embodiment variant of a particle agglomerator  1 . The latter is formed with substantially smooth extra-fine wire layers  15  in the manner of a metallic nonwoven, between which are provided structured metal foils  14 , in such a way that channels  16  are formed which extend in the flow direction  7  or along a corresponding axis of the particle agglomerator  1 . Channel narrowing points  17  are formed in the interior of the channels  16 , through the use of guiding faces  32  in the metal foil  14 . The channel narrowing points  17  bring about a (partial) deflection of the exhaust gas flow towards the extra-fine wire layer  15 . In this case, the channel narrowing points  17  or the guide faces  32  are formed in such a way that the channel  16  is not completely closed off but rather a secondary flow  33  is still permitted. As a result of the turning-up of the guide face  32  out of the metal foil  14 , a passage opening  18  is formed which permits the passage of exhaust gas to adjacent channels  16 . 
         [0035]    Furthermore, it is shown in  FIG. 3  that the exhaust gas, which contains nitrogen dioxide (NO 2 ), carbon (C) and oxygen (O 2 ), enters into the particle agglomerator  1  and there initiates a conversion of carbon-containing particles  5  contained therein with the nitrogen dioxide, in such a way that nitrogen monoxides (NO), nitrogen (N 2 ), carbon dioxide (CO 2 ) and oxygen (O 2 ) finally leave the particle agglomerator  1  again. The probability of the reaction of nitrogen oxide and soot particles is considerably increased through the use of the particle agglomerator, in such a way that the relatively high conversion rates can be realized with a low pressure loss of the exhaust gas and a blockage of the particle agglomerator is reliably prevented. 
         [0036]      FIG. 4  illustrates a particle agglomerator  1  which firstly has a first zone  8  and thereafter a second zone  9  which extends to a rear end side  10 , in the flow direction  7 . The particle agglomerator  1  is formed over its entire length with smooth extra-fine wire layers  15  and structured metal foils  14 . The metal foils  14  have alternating (oppositely disposed) tapering channel narrowing points  17 , in adjacent channels  16 , which simultaneously permit a secondary flow  33  and bring about a flow of part of the exhaust gas towards the extra-fine wire layer  15 . In this way, the particles  5 , preferably with a diameter  6  of less than 200 nm, are accumulated in or on the walls (or the extra-fine wire layer) of the particle agglomerator  1  and are converted through the use of the nitrogen dioxide which is provided. In this case, the first zone  8  has no oxidatively active coating, while the second zone  9  has a correspondingly provided oxidation catalytic converter  11 , through the use of which new nitrogen oxide is generated again in situ for the regeneration of the particle agglomerator in the rear part. 
         [0037]    It is, of course, possible for various modifications to the system proposed herein to be carried out directly, without departing from the concept of the invention described herein. It is, for example, possible for other particle agglomerators to be used, although it is also possible to position the particle agglomerator  1 , for example, downstream of a turbocharger  13 . The downstream exhaust-gas aftertreatment units  24  can also be combined and supplemented in any desired manner. Furthermore, the invention can also be used with some other internal combustion engine, such as for example a direct-injection spark-ignition engine.