Abstract:
An exhaust system purifies a gas flow of harmful substances. The exhaust system contains at least device for supplying a reducing agent, a first catalytic converter, and a particle trap, in the direction of flow of the gas flow through the exhaust system. At least one other exhaust purification component is provided and/or there is a distance of at least 0.5 meters between the first catalytic converter and the particle trap. In addition, a mixer and a second catalytic converter are positioned directly upstream of the particle trap. A regeneration of the particle trap disposed in the exhaust system is carried out. A reducing agent is introduced into the exhaust gas system, only upstream of the turbocharger, for carrying out the regeneration process of the particle trap.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This is a continuing application, under 35 U.S.C. §120, of copending international application No. PCT/EP2004/004543, filed Apr. 29, 2004, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German patent application No. 103 21 105.5, filed May 9, 2003; 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 an exhaust system for the purification of a gas stream of pollutants, which contains a particle trap which is regenerated discontinuously, using a reducing agent. Furthermore, a method for the regeneration of a particle trap is described.  
         [0003]     Owing to statutory provisions which place increasingly more stringent requirements on the exhaust systems in motor vehicle construction, exhaust systems have in the past been the subject of constant development. In this context, a multiplicity of components are employed, which in each case perform different functions within the exhaust system. Thus, for example, starting catalytic converters or preturbo catalytic converters are known, which have a particularly small volume and, after a cold start of the internal combustion engine, therefore quickly reach their starting temperature required for catalytic conversion. Furthermore, electrically heatable catalytic converters are known, which likewise allow an improved cold starting behavior of the exhaust system. Adsorbers, as they are known, have the task, in the exhaust system of an internal combustion engine, of adsorbing for a certain period of time specific pollutants contained in the exhaust gas, these preferably being stored until a following catalytic converter has reached its operating temperature. Moreover, particularly in the exhaust system of diesel engines, particle traps or particle filters are used, which intercept soot particles and/or other solid impurities contained in the exhaust gas. The intercepted particle accumulations may, in principle, be converted continuously or discontinuously, for example by the supply of high thermal energy.  
         [0004]     To reduce the particle emissions in the exhaust gas, particularly where diesel engines are concerned, particle traps are known, which are constructed from a ceramic substrate. These have ducts, so that the exhaust gas to be purified can flow into the particle trap. The adjacent ducts are closed alternately, so that the exhaust gas flows into the duct on the inlet side, passes through at least one ceramic wall and escapes again through the adjacent duct on the outlet side. Particle traps of this type are known as “closed” particle filters. They attain an effectiveness of approximately 95% over the entire range of particle sizes which occur.  
         [0005]     Another type of particle trap which has a high thermal load-bearing capacity and a markedly lower pressure loss may be gathered from published, non-prosecuted German patent application DE 101 53 283, corresponding to U.S. patent publication No. 2004/019440 A1. This publication describes a particle trap which is designated as an “open” filter system. In such an open system, a structural reciprocal closing of the filter ducts is dispensed with. The duct walls are formed at least partially of porous or highly porous material. The flow ducts of the open filter have deflecting or guiding structures which steer the exhaust gas having the particles contained in it to the regions formed of porous or highly porous material. A particle filter is designated as being open when particles can basically run completely through it, specifically even particles which are considerably larger than the actual particles to be filtered out. As a result, such a filter cannot become blocked during operation, even if there is an agglomeration of particles. A suitable method for measuring the openness of a particle filter is, for example, the test to ascertain the diameter up to which spherical particles can still trickle through such a filter. In the present applications, a filter is open, in particular, when spheres with a diameter larger than or equal to 0.1 mm can still trickle through, preferably spheres with a diameter above 0.2 mm.  
         [0006]     Irrespective of the type of particle trap used, a reliable and, if possible, complete regeneration of the particle filters in the exhaust system of a motor vehicle must be ensured. Such a regeneration of the particle trap is required, since the increasing accumulation of particles in the duct wall have a through flow effect resulting in a constantly rising pressure loss which entails adverse effects on the engine power. Regeneration concerns the brief heating of a particle trap or of the particles accumulated in it, so that the soot particles are converted into gaseous constituents.  
         [0007]     Previously, particle traps of this type were heated directly, for example by ohmic resistance heating. It was also known to convert the accreted soot particles by a separate burner. Subsequent devices for regenerating the particle filters are distinguished in that, upstream of such a particle trap, a reducing agent is supplied, which ultimately brings about a chemical conversion of the soot particles accreted in the particle trap. In this context, two different systems have come to the fore: discontinuous and continuous regeneration.  
         [0008]     The system for the continuous regeneration of filters is called continuous regeneration trap (CRT) and is described, for example, in U.S. Pat. No. 4,902,487. In such a system, the particles are converted at temperatures of above 200° C. by oxidation by being brought into contact with nitrogen dioxide (NO 2 ). The nitrogen dioxide required for this purpose is often generated by an oxidation catalytic converter which is disposed upstream of the filter. In this case, however, as regards application in motor vehicles with diesel fuel, the problem arises that only an insufficient fraction of nitrogen monoxide (NO) which can be converted into the desired nitrogen dioxide is present in the exhaust gas. Consequently, it has not been possible hitherto to ensure that a continuous regeneration of the particle trap takes place in the exhaust system. It is therefore often also customary to supply the exhaust system with urea or similar reducing agents which allow a continuous regeneration of the filters. Such systems have the disadvantage of a high outlay in technical terms and also the fact that separate consumption equipment or operating equipment has to be carried in the motor vehicle.  
         [0009]     In the discontinuous regeneration of particle traps, it is known for the particle trap to be preceded by an oxidation catalytic converter to which unsaturated or unburnt hydrocarbons (HC) are supplied. When the unsaturated hydrocarbons are in contact with the oxidation catalytic converter, this gives rise to a particularly exothermal reaction which results in a significant increase in the temperature of the exhaust gas. In this case, temperatures are reached which lie in a range where it is possible to convert the particle agglomerations stored in the particle traps. Temperatures of above 600° C. often have to be reached in this case. The supply of reducing agent may in this case take place separately, but it is also known to introduce unburnt fuel fractions from the internal combustion engine directly into the exhaust gas line, so that these impinge onto the oxidation catalytic converter.  
         [0010]     The wish, initially outlined, to bring about a catalytic conversion of exhaust gas even directly after the cold starting of the internal combustion engine can be implemented, using starting catalytic converters which are distinguished by a small volume (for example, smaller than 20% of the piston-swept volume of the internal combustion engine) and by their proximity to the engine. In this case, the technical problem arises that a supply of unsaturated hydrocarbons, which is to bring about regeneration in a downstream particle trap arranged at a marked distance from the starting catalytic converter, is no longer possible. The fuel acting as a reducing agent would impinge onto the starting catalytic converter and lead to an exothermal reaction. Since the particle trap is arranged at a very long distance from the starting catalytic converter or additional components for exhaust gas purification are arranged between the starting catalytic converters and the particle trap, the required temperature increase is not brought about in the particle trap.  
       SUMMARY OF THE INVENTION  
       [0011]     It is accordingly an object of the invention to provide a method for regenerating a particle trap and an exhaust system which overcomes the above-mentioned disadvantages of the prior art devices and methods of this general type, so that a discontinuous regeneration of the particle trap can be ensured even when long distances between the starting catalytic converter and the particle trap have to be covered by the exhaust gas or temperature-sensitive components with the conversion of specific exhaust gas constituents are disposed between the starting catalytic converter and the particle trap. Moreover, the exhaust system is to have a simple construction and regeneration is to be capable of being carried out in a simple way.  
         [0012]     The exhaust system for the purification of a gas stream having pollutants contains, in a direction of flow of the gas stream through the exhaust system, at least a device for supplying a reducing agent, a first catalytic converter, a particle trap, at least one further exhaust-gas purification component and/or a distance of at least 0.5 m between the first catalytic converter and the particle trap being provided. According to the invention, a mixer and a second catalytic converter directly precede the particle trap.  
         [0013]     To explain the terms used here, the meaning is explained in more detail below individually. The term “direction of flow of the gas stream” is to be understood as meaning the direction of the gas stream which the flow assumes from an internal combustion engine toward the exhaust or outlet into the atmosphere. In this context, a main direction of flow is meant, that is to say, in particular, local flow turbulences or the like remain ignored. The configuration of the individual devices in the direction of flow through the exhaust system results in that the gas stream comes into contact first with the device for supplying a reducing agent, subsequently with the first catalytic converter and finally with the particle trap. What remains unaffected by this is that the gas stream comes into contact, between these individual components, with further components of the exhaust system, such as, for example, further adsorbers, exhaust gas lines, etc. Furthermore, the reference that “at least” the devices listed are provided also coontains the fact that the devices may be directly or indirectly arranged multiply one behind the other.  
         [0014]     The term “catalytic converter” is to be understood as meaning a multiplicity of known carrier bodies for catalytically active material. In this context, the carrier bodies may be constructed predominantly from metal and/or ceramic. Where metallic catalytic converter carrier bodies are concerned, as is known, at least partially structured sheet metal foils are wound together with one another in such a way that ducts capable of having a fluid flowing through them are formed. It is also known to produce metallic carrier bodies by extrusion. Furthermore, ceramic carrier bodies are known which acquire their honeycomb form likewise by an extruding and sintering operation. Such a honeycomb form has proved particularly advantageous because a particularly large surface is thereby provided which results in intimate contact with the gas stream.  
         [0015]     The term “particle trap” includes both classic filter systems with alternately closed ducts and the “open” filter systems described above.  
         [0016]     The term “exhaust-gas purification component” is a general term for a multiplicity of different components for the treatment of exhaust gas, in particular honeycomb bodies, water traps, heating elements, silencers, adsorbers, storage devices, etc.  
         [0017]     The term “distance” between the first catalytic converter and the particle trap is to be understood as meaning, in particular, their spacing along the flow path of the gas stream. Therefore, for this purpose, the distance along the exhaust gas line which connects the first catalytic converter and the particle trap along the shortest path is to be determined.  
         [0018]     A “mixer” within the meaning of this disclosure describes a device which brings about an eddying or a significant flow deflection of part gas streams. In particular, the fraction of deflected part gas streams lies above 50%, in particular 80%, preferably above 95%. In this case, it is particularly advantageous that the part exhaust gas streams are not deflected substantially parallel to one another, but move at least partially toward one another, so that intermixing takes place. A mixing element of the type described in German patent DE 199 38 840 may be mentioned as an example here. All other known mixers may, of course, also be used, as long as they fulfill the above criteria.  
         [0019]     As regards the second catalytic converter, it may be pointed out that this again is a type of exhaust-gas treatment component, such as was described in respect of the first catalytic converter. However, the second catalytic converter is not configured as a starting catalytic converter, that is to say it is not located in proximity to the engine.  
         [0020]     By the exhaust system according to the invention, it is possible, as also explained in more detail below with regard to the method, to use fuel as a reducing agent for regenerating the particle trap, the fuel passing through the first catalytic converter substantially without a complete exothermal reaction. The fuel/gas mixture is then treated by the mixture in such a way that the desired exothermal reaction takes place in the second catalytic converter and brings about the temperature increase required for the regeneration of the particle trap. An aspect of the invention is that the fuel required for regeneration is led, concentrated in a portion or part volume flow of the exhaust gas stream, through the first catalytic converter, so that oxygen, which is required for catalytic conversion, is not available sufficiently for a considerable fraction of the entrained fuel. Thus, catalytically motivated reactions occur only in the edge regions of the part gas stream having a high fuel content, but a large part of the additionally injected fuel quantity passes through the first catalytic converter without being converted.  
         [0021]     The mixer has the effect, then, that the fuel-enriched part gas stream is mixed with the remaining exhaust gas which, precisely in the case of diesel engines, is particularly lean, that is to say oxygen-rich. As a result of the mixing action, a dissolving of the part gas stream having a high fuel content takes place, so that the fuel flows, finely dispersed with the exhaust gas stream, toward the downstream particle trap. In this connection, it is not appreciably important whether the mixed exhaust gas stream flows through further exhaust-gas purification components (having a nonoxidizing action) or not on its way toward the second catalytic converter. The mixed exhaust gas stream ultimately impinges onto the second catalytic converter which, in turn, has a catalytically active surface and then brings about a conversion of the exhaust-gas/fuel dispersion.  
         [0022]     Since the second catalytic converter precedes the particle trap directly (or immediately, that is to say without further exhaust-gas purification components being arranged between), the temperature increase is transferred directly to the particle trap on account of the exothermal reaction. This, then, ensures a complete regeneration of the particle trap. It is particularly advantageous, in this case, that the second catalytic converter and the particle trap are disposed in relation to one another in such a way that the exhaust gas can discharge as large an energy quantity as possible to the particle trap. This may be ensured, for example, in that the catalytic converter and the particle trap have only a small spacing with respect to one another, in particular this spacing amounts to less than 10 cm, in particular less than 5 cm and preferably less than 2 cm. The spacing in this case describes the distance which the exhaust gas covers after it emerges from the second catalytic converter and until it enters the particle trap. In particular, it is advantageous if the exhaust gas line between the second catalytic converter and the particle trap is thermally insulated or has no additional structural parts, such as flaps, guide plates, probes or the like, or else curved portions.  
         [0023]     According to further refinements of the invention, the mixer is a turbocharger. Precisely where the most recent diesel engines are concerned, which operate on the principle of direct injection, the use of an exhaust gas turbocharger for the compression of the intake air has proved appropriate. Such a compressor for the intake air is operated by the exhaust gas flowing through the turbocharger. When it flows through the turbocharger, the exhaust gas experiences a pronounced eddying, so that the turbocharger fulfills completely the criteria which were illustrated above with reference to the mixer. That is to say, for example, the first catalytic converter can then be followed by only one turbocharger which is followed, in turn, by a second catalytic converter and the particle trap. Precisely in such a configuration of the exhaust-gas purification components or of the turbocharger, the method described below is advantageous, since it prevents the first catalytic converter, which is preferably configured as a starting catalytic converter, from generating such high temperatures in the exhaust gas that the directly following turbocharger is damaged. Thus, it is possible for the exhaust gas to be led through at temperatures tolerable to the turbocharger, and for the exhaust gas subsequently to be heated by the second catalytic converter to a temperature such that a regeneration of the particle trap is ensured.  
         [0024]     According to a further refinement of the exhaust system, the device for supplying the reducing agent contains at least one injection nozzle for the provision of fuel in the combustion space of a mobile internal combustion engine. Therefore, in particular, that only one or a plurality of injection nozzles, which are intended for supplying the internal combustion engine with fuel, are used for the provision of reducing agent for the regeneration of the particle trap. That is to say, in other words, the at least one injection nozzle injects into the cylinders of the internal combustion engine fuel which emerges, essentially unburnt, from the cylinders and passes through the first catalytic converter (and, if appropriate, also the turbocharger), and finally an exothermal reaction for the conversion of the fuel takes place only as a result of contact with the second catalytic converter. It is thereby possible to have a particularly simple construction of the exhaust system, and, finally, additional lines or nozzles and the like for introducing the reducing agent may be dispensed with.  
         [0025]     Further, it is proposed that the injection nozzle be disposed in such a way that the fuel can be introduced into an outlet duct of the internal combustion engine. In this case, it is clear to a person skilled in the art that, if appropriate, further measures are required for this purpose. Thus, it must be remembered that the injection nozzle is usually oriented in such a way that a particularly good compression or combustion behavior of the fuel/air mixture in the cylinder of the internal combustion engine is ensured. In order to ensure that the fuel reaches the outlet duct, it is necessary, if appropriate, to have predetermined positions of the piston or of the valve.  
         [0026]     In order, for example, to obtain an easily retrofittable embodiment of the exhaust system which does not require a modification of the cylinder or of the combustion space, it is proposed that at least one separate supply line be provided in or on an outlet duct of the internal combustion engine and/or of the exhaust system. What is meant by this is that, for example, an additional line is provided from the fuel supply toward the engine, and the fuel is supplied to the exhaust gas stream between the combustion space or the engine cylinder and the first catalytic converter. In this context, it must be borne in mind that this takes place in such a way as to generate a relatively narrowly limited part gas stream which has a particularly high concentration of fuel. This ensures that the oxygen required for carrying out an exothermal reaction is displaced, and the part gas stream having a high fuel content flows both through the first catalytic converter and, if appropriate, following components, without experiencing significant chemical conversion.  
         [0027]     According to a further refinement, it is proposed that the device for supplying a reducing agent be connected to a reducing agent reservoir and to a control unit, so that an intermittent supply of reducing agent can be carried out. As regards the reducing agent reservoir, separate containers or storage spaces may be provided, but it is also possible for this to be directly the fuel tank. The control unit assumes the task of regulating or controlling, as required, the opening times or prevailing pressures in the injection nozzle or other nozzles. This must take place, in particular, as a function of the piston position or outlet valve position of the cylinder within the internal combustion engine.  
         [0028]     It is advantageous, further, that the first catalytic converter has a first contact face promoting the oxidation of at least one pollutant contained in the gas stream. That is to say, in particular, unsaturated hydrocarbons are converted into less harmful constituents by the first catalytic converter. Precisely because there is always particular public interest in this respect, it is proposed that the second catalytic converter, too, has a second contact face promoting the oxidation of at least one pollutant contained in the gas stream. In this case, under certain circumstances, it is possible that both the first catalytic converter and the second catalytic converter have the same catalytically active material on or in the contact face. This is surprising in as much as the device proposed here or the method explained below ensures that the reducing agent, on the one hand, passes through the same coating, but, on the other hand, is converted by an exothermal reaction as a result of a considerable increase in temperature of the exhaust gas.  
         [0029]     According to yet a further refinement of the exhaust system, the second catalytic converter and the particle trap form a structural unit. Therefore, in particular, that the second catalytic converter and the particle trap are connected not only via the exhaust gas line surrounding them. Thus, for example, it is possible to arrange the second catalytic converter and the particle trap in a common casing tube which is in contact with the exhaust gas line. It is also possible, however, for the second catalytic converter and the particle trap not only to be connected to one another over the circumference, but that, if appropriate, contacting takes place via the end faces, for example via pins, sheet metal foils or the like. Further, it is also possible, for example, with regard to the structural unit, to provide a thermal insulation acting in a circumferential direction, so that the exothermal energy generated in the second catalytic converter is discharged almost completely to the particle trap.  
         [0030]     According to a further advantageous refinement of the exhaust system, the second catalytic converter and the particle trap together form a body through which a fluid is capable of flowing and which has, in the direction of flow, first a catalytically active coating and subsequently a device for the accretion of particles. That is to say, for example, the second catalytic converter and the particle trap are produced with the same carrier body. That is to say, in other words, for example, the duct walls which are formed, as a rule, by ceramic material or by metal sheets extend over an entire length jointly in the direction of flow of the second catalytic converter and of the particle trap. A subdivision of the carrier body itself then does not have to occur. However, clearances, deformations, material accumulations or the like may be carried out in part portions of the body or the duct walls, so that the portion of the body is adapted to the respective function of catalytic converter, on the one hand, and of particle trap, on the other hand. In principle, however, it is possible for these portions of carrier body or body to be distinguished from one another (only or additionally) by different coatings. In this case, there may also be an overlap region of the portion which constitutes the second catalytic converter and the portion which forms the particle trap.  
         [0031]     According to a further aspect of the invention, a method for regeneration of a particle trap disposed in an exhaust system is proposed, the exhaust system having (as seen in the direction of flow of a gas stream) at least one first catalytic converter, one turbocharger, one second catalytic converter and the particle trap. In this case, a reducing agent is introduced into the exhaust system, downstream of the turbocharger, in order to carry out a process of regenerating the particle trap. For this purpose, the reducing agent is supplied to a part gas stream of the exhaust system in a concentration such that no or only a very slight exothermal reaction takes place when the part gas stream flows through the first catalytic converter. This part gas stream, still having a high fuel content, is then led through the turbocharger, a particularly intensive mixing with the part exhaust gas streams from other cylinders of the internal combustion engine taking place. Since these part exhaust gas streams from the other cylinders constitute a particularly lean (oxygen-rich) mixture, the part gas stream which still has a high fuel fraction is then enriched with oxygen. The result of this is that, when the part gas stream subsequently impinges onto an oxidation catalytic converter, the desired exothermal reaction occurs. The thermal energy released in this case is used for burning the soot particles which have accreted in the following particle trap. This prevents flow paths (which the exhaust gas assumes through the particle trap) from becoming clogged, thus leading to an increase in the flow resistance of the particle trap. The resulting pressure drop of the exhaust gas stream across the particle trap has adverse effects on the engine power, which are reliably avoided in the method described here.  
         [0032]     In this case, it is particularly advantageous that the supply of reducing agent takes place intermittently. This applies particularly when the supply of reducing agent takes place by at least one injection nozzle, fuel being introduced into a combustion space of a mobile internal combustion engine. In this context, diesel engines, in particular, are to the forefront.  
         [0033]     According to a further advantageous refinement of the method, a subsequent injection of the fuel into the combustion space takes place, so that unburnt part volume flows of the fuel pass into an outlet duct of the internal combustion engine. The term “subsequent” refers to, in this sense, that the injection nozzle injects fuel at two different time points during a working cycle of the piston in the cylinder. At the first time point, the quantity of fuel required for auto ignition or combustion is injected into the combustion space of the cylinder, compressed and burnt. During the upward movement of the piston, the exhaust gas which has occurred during combustion is expelled through an open outlet valve into the outlet duct and further on to the exhaust gas line. At this time point, that is to say, in particular after the conclusion of combustion in the combustion space, a predeterminable or calculatable quantity of fuel (or of another reducing agent) is introduced into the combustion space via the injection valve and flows through the outlet duct or the exhaust gas line together with or after the expelled exhaust gas part stream.  
         [0034]     In internal combustion engines with a plurality of cylinders, each with a combustion space, it is particularly advantageous that the injection of the reducing agent into the cylinders takes place alternately. This contains, on the one hand, the fact that the individual cylinders each assume an injection of the reducing agent in succession, but it is also possible that individual cylinders are bypassed, individual injection nozzles assume the injection of the reducing agent multiply one after the other and/or no rigidly predetermined alternation with regard to the cylinders is carried out. The latter is the case particularly when the injection into the respective cylinders takes place as a function of detected measurement values which reflect the operating state of the internal combustion engine or of the exhaust system. This, on the one hand, ensures that the remaining quantities of fuel which may possibly remain in an individual cylinder after the injection of the reducing agent are repeatedly burnt. There is therefore uniform combustion in all the cylinders.  
         [0035]     According to a further refinement of the method, the trigger time point for an injection of the reducing agent is determined as a function of a detected and/or calculated parameter which characterizes the functionality of the particle trap. Therefore devices (sensors, probes, etc.) are provided which monitor the functionality of the particle trap. Suitable measurement values are in this case the pressure drop across the particle trap, the temperature in the particle trap, the concentration of at least one pollutant in the exhaust gas after emergence from the particle trap, etc. When the pressure drop reaches, for example, a predetermined limit value, this may be used as an indication of the triggering of a regeneration cycle. In this case, under certain circumstances, it is also necessary to take into account the duration which the fuel quantity injected near the engine requires in order to reach the particle trap. This must take place in such a way that a temperature increase in the particle trap is brought about before the latter has a detectable adverse effect, for example, on the engine power.  
         [0036]     According to yet a further refinement of the method, the location of the injection of the reducing agent is selected as a function of a detected and/or calculated parameter which characterizes the temperature of the gas stream in a part region of the exhaust system. Therefore, for example, that the injection nozzle of the plurality of cylinders is selected as a function of specific temperatures of the gas stream or of the exhaust system and/or of the internal combustion engine. If, for example, a remaining quantity of fuel in the cylinder subsequently leads to increased thermal load during the next combustion, it may be advantageous, when a predetermined limit temperature is reached, to carry out an injection of the reducing agent via the other injection nozzles only. Under certain circumstances, it is also possible that, by virtue of the configuration of the flow paths in the exhaust gas line, the flow in each case arrives at different regions of the exhaust-gas purification components to an increased extent as a function of the injection location, so that, in particular, the temperature of the exhaust gas stream is represented as a thermal load at these regions. Here, too, adaption in order to ensure the functionality of the exhaust-gas treatment components may be provided.  
         [0037]     Other features which are considered as characteristic for the invention are set forth in the appended claims.  
         [0038]     Although the invention is illustrated and described herein as embodied in a method for regenerating a particle trap and exhaust 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.  
         [0039]     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 DRAWINGS  
       [0040]      FIG. 1  is a diagrammatic, illustration of an exhaust system;  
         [0041]      FIG. 2  is a diagrammatic, sectional view of part of a direct-injection diesel internal combustion engine;  
         [0042]      FIG. 3  is a diagrammatic, sectional view of a subsequent injection of a reducing agent;  
         [0043]      FIG. 3A  is a diagrammatic, sectional view of a detail of the subsequent injection of the reducing agent shown in  FIG. 3 ;  
         [0044]      FIG. 4  is a diagrammatic, perspective view of an exemplary embodiment of a first catalytic converter;  
         [0045]      FIG. 5  is a diagrammatic, perspective view of an exemplary embodiment relating to a structural unit containing a second catalytic converter and of a particle trap;  
         [0046]      FIG. 5A  is a diagrammatic, perspective view of a detail shown in  FIG. 5 ; and  
         [0047]      FIG. 6  is a diagrammatic, perspective view of a detail of a particle trap as illustrated in  FIG. 5 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0048]     Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a diagrammatic, illustration of an exhaust system  1  for purifying a gas stream  2  of pollutants  3 . The exhaust system  1  contains, in a direction of flow  4  of the gas stream  2  through the exhaust system  1 , at least one first catalytic converter  5 , one mixer  6 , one second catalytic converter  7  and a particle trap  8 . Further, a device for supplying a reducing agent is provided, which is disposed only upstream of the mixer  6 . In this case, in an internal combustion engine  12 , which is preferably a diesel engine for a passenger car, fuel  10  is injected into combustion spaces  11  of the various cylinders  24  ( FIG. 2 ). The fuel  10  is burnt with highly compressed intake air and is subsequently expelled into the surroundings via an exhaust gas line  26 .  
         [0049]     In direct proximity to the internal combustion engine  12 , in particular at a spacing of less than 70 cm, a plurality of first catalytic converters  5  are provided, in each case a first catalytic converter  5  being integrated in a tube of the exhaust manifold. In the embodiment illustrated, a reducing agent  23  is supplied to the exhaust gas stream via a separate supply line  14 , upstream of the mixer  6  which is configured here as a turbocharger. The reducing agent  23  flows through the mixer  6  or the turbocharger and subsequently impinges onto the second catalytic converter  7 . The second catalytic converter  7  has a conical configuration and is disposed in a widening of the exhaust gas line  26 . Directly after the second catalytic converter, the particle trap  8  is positioned with a spacing  44  which is preferably less than 5 cm. The particle trap is followed by a three-way catalytic converter  27  of the known type of construction. Between the first catalytic converter  5  and the particle trap  8 , there is a distance  43  which amounts to at least 0.5 m, preferably even to more than 1 m. In this case, the arrow identified by  43  is to be understood merely diagrammatically, the actual distance  43  being determined by the flow path of the gas stream  2  from the outlet of the first catalytic converter  5  until it enters the particle trap  8 .  
         [0050]      FIG. 2  shows diagrammatically, obviously not true to scale, a combustion space  11 , such as is to be encountered, for example, in a direct-injection diesel internal combustion engine. The cylinder  24  contains a piston  32 , the cylinder  24  and the piston  32  at least partially delimiting a combustion space  11 , also called a piston-swept volume. Furthermore, the engine block of the internal combustion engine  12  has disposed in it an injection nozzle  9  which is connected both to a fuel tank  15  and to a control unit  16 . The task of the injection nozzle  9  is to inject, as required, into the combustion space  11  a predefined or predetermined quantity of fuel  10  which is subsequently ignited by highly compressed intake air. The ignition of the fuel/air mixture results in an expansion of the gas mixture, as a result of which the piston  32  is pressed downward. After combustion, a valve  33  is moved upward, and the exhaust gas located in the combustion space  11  is expelled through an outlet duct  13  in the direction of flow  4 . In the form illustrated, the outlet valve  33  is closed, and therefore the injection nozzle  9  injects, finely dispersed, the required quantity of fuel  10  which is required for actual combustion or power generation.  
         [0051]      FIG. 3  shows diagrammatically, and by a detail shown in  FIG. 3A , the subsequent injection of fuel as reducing agent. Once again, the cylinder  24  and the piston  32  which delimit the combustion space  11  are indicated diagrammatically. In the snapshot shown here, the valve  33  is in a position in which the exhaust gas stream can flow from the combustion space  11  into the outlet duct  13 . This is brought about in that the piston  32  moves upward. The desired quantity of fuel required for reducing the particle trap is then injected into the combustion space by the injection nozzle  9 . The fuel  10  is, if possible, introduced into the exhaust gas duct  13  in such a way that a kind of “fatty disk” occurs. This preferably is a part volume flow  25  which has a particularly high concentration of hydrocarbons. Oxygen depletion prevails in this part volume flow, this being a state which does not normally occur in diesel exhaust gases on account of the lean combustion. The enlarged detail in  FIG. 3A  indicates diagrammatically that the gas stream  2  or the exhaust gas stream contains pollutants  3  and particles  22  which are propagated through the outlet duct  13  in the direction of flow  4 . Whereas, in the part region indicated, in which pollutants  3  and particles  22  have accumulated, a relatively high concentration of oxygen is provided for a catalytic reaction, virtually no oxygen molecules or a fraction markedly below 50%, preferably less than 30%, are to be encountered in the part volume flow  25 . This ensures that the part volume flow  25  flows through the first catalytic converter  5 , without giving rise there to already highly exothermic reactions which would possibly result in damage to the turbocharger arranged downstream.  
         [0052]      FIG. 4  shows diagrammatically a perspective view of an embodiment of the first catalytic converter  5 , such as is to be encountered, for example, for use in a tube of an exhaust manifold. The first catalytic converter  5  contains a housing  31  in which a plurality of sheet metal foils  28  are disposed in such a way that ducts  29  through which the gas stream  2  is capable of flowing are formed. Thus, in spite of the small volume, a relatively large first contact face  17  is formed. The sheet metal foils  28  are partially structured and are disposed in such a way that ducts running substantially parallel to one another are formed. Inside the housing  31 , a kind of honeycomb body is formed in that smooth and corrugated sheet metal foils  28  are first stacked and subsequently wound in an S-shaped manner (or in involute form) and are introduced into the housing  31 . To fix the sheet metal foils  28  to the housing  31  or to fasten the sheet metal foils  28  to one another, a brazing technique is predominantly employed.  
         [0053]      FIG. 5  shows diagrammatically a perspective view of an exemplary embodiment of a second catalytic converter  7  and a particle trap  8  which together form a structural unit  19 . The structural unit  19  is also distinguished in that the second catalytic converter  7  and the particle trap  8  are disposed in a common casing tube  34 . In the variant illustrated, the second catalytic converter  7  and the particle trap  8  are formed by a body  20  which contains a plurality of sheet metal foils  28  which are at least partially structured in such a way that ducts  29  through which a fluid is capable of flowing are formed. This also results in, for example, that, in principle, specially configured metallic honeycomb bodies, the general form of construction of which is already known, may be used as such a structural unit  19 .  
         [0054]     A distinction is made, above all, between two typical forms of construction of metallic honeycomb bodies. An earlier form of construction, of which published, non-prosecuted German patent application DE 29 02 776 A1 shows typical examples, is the spiral form of construction, in which a smooth and a corrugated sheet metal ply are laid one onto the other and are wound spirally, as also illustrated in  FIG. 5 . In another form of construction, the honeycomb body is constructed from a multiplicity of alternately disposed smooth and corrugated or differently corrugated sheet metal plies, the sheet metal plies first forming one or more stacks which are coiled together with one another. In this case, the ends of all the sheet metal plies come to lie on the outside and can be connected to a housing or casing tube, thus giving rise to numerous connections which increase the durability of the honeycomb body. Typical examples of these forms of construction are described in European patent EP 0 245 737 B1 or International patent disclosure WO 90/03220. It has also been known for a long time to equip the sheet metal plies with additional structures in order to influence the flow and/or to achieve cross mixing between the individual flow ducts. Typical examples of such embodiments are known from International patent disclosures WO 91/01178, WO 91/01807 and WO 90/08249. Finally, there are also honeycomb bodies in a conical form of construction, if appropriate also with further additional structures for influencing the flow. Such a honeycomb body is described, for example, in WO 97/49905. Furthermore, it is also known, in a honeycomb body, to leave a clearance free for a sensor, in particular to accommodate a lambda probe. An example of this is described in German Utility Model DE 88 16 154 U1.  
         [0055]     On the gas inlet side, which is illustrated on the left side in  FIG. 5A , the body  20  has a catalytically active coating  21 . The catalytically active coating  21 , in conjunction with the second contact face  18  which is formed partially by the catalytic coating  21 , ensure an effective conversion of the quantities of reducing agent, thermal energy being generated which increases the entire body  20  or the exhaust gas located in it markedly, for example to temperatures of above 600° C. The sheet metal foils  28  shown here are provided with a thickness  35  which lies in the range of 0.02 to 0.11 mm.  
         [0056]      FIG. 6  shows an embodiment of the particle trap  8 , such as may be present, for example, in the structural unit  19  shown in  FIG. 5 . The sheet metal foil is called a corrugated ply  36  here, since this corrugated ply  36  has additional structures for the interception of solid constituents in the exhaust gas stream. In principle, however, the sheet metal foil  29  may at the same time also be the corrugated ply  36 . The arrows in  FIG. 6  represent the direction of flow  4  and illustrate the flow paths which the exhaust gas containing particles  22  can follow. At least in the part region of the body  20  which constitutes the particle trap  8 , a fiber ply  37  is disposed in the immediate vicinity of the corrugated ply  36 , the fiber ply having pores  38  for absorbing the particles  22 . The corrugated ply  36  forms a multiplicity of ducts  29  making it possible for the exhaust gas to flow freely through the particle trap  20  (the “open filter” principle). To influence the flow, the corrugated ply  36  has guide faces  40  which are delimited at least partially by orifices  39 . Adjacent ducts  29  are connected to one another by the orifices  39 , so that an exchange of part gas streams in adjacent ducts  29  becomes possible. The guide faces  40  form steadying points  41  and eddying points  42  which ensure that the particles  22 , on the one hand, are deflected toward the fiber ply  37  and, on the other hand, can collect in part regions until regeneration takes place.  
         [0057]     The device described here or the method explained here makes it possible by simple measures to have a reliable regeneration of a particle filter by use of fuel, even when further structural elements or exhaust-gas purification components are positioned in the flow path of the fuel toward the particle trap or the oxidation catalytic converter positioned directly in front of this. The proposed method is particularly effective precisely in connection with exhaust systems which have an exhaust-gas turbocharger.