Patent Publication Number: US-2007113540-A1

Title: Catalyst body and an exhaust system for a small-size engine and a method for producing the catalyst body

Description:
A catalyst body and an exhaust system for a small-size engine and a method for producing the catalyst body  
      The invention relates to an annular catalyst body that can be flowed through in a radial manner for installation in a muffler housing, an exhaust system for a small-size engine and a method for producing the catalyst body.  
      Such small-size engines are used for example in lawnmowers, motor saws, pistol-grip cross-cut saws, power units and in bikes. As a result of increasing awareness for environmental issues, a demand has developed to continually improve exhaust gas cleaning and sound muffling of small-size engines. The increasingly stricter emission regulations demand that components are adjusted continually to changing requirements.  
      It is known in the exhaust gas treatment in such devices to use catalyst bodies of radial or axial configuration for reducing the pollutants contained in the exhaust gas. The terms of radial and axial shall be understood as the direction of main flow of the exhaust gas in which it flows through the catalyst body. It is further known to provide such catalyst bodies in the form of hollow cylinders. This shape is obtained as a result of production, as a result of flow configuration or as a result of the type of fastening. Especially annular catalyst bodies are used in practice. Such annular bodies currently mostly consist of pressed woven metal wire cloth or bent metal wire cloth. It has been noticed in practical application however that such wire structures have a low mechanical stability, and upsetting and bulging can occur especially when used in a muffler housing as a result of the stresses caused by oscillations and the flow and the elasticity of the wire structure. Additional loads on the catalyst body occur in the case of a catalyst body fixed under pre-tension in the muffler housing. As a result of the elastic and plastic deformations of individual wires and the entire ring body, the catalytic active coating and the washcoat will detach, thus reducing the performance of the catalyst and reducing the efficiency of pollutant reduction. Individual wires may also rub against each other, thus leading to the coating of the wires crumbling away or cracking off. When wire cloth is used, bending and bulging will occur even in the case that the wires are soldered together. Moreover, closed ring structures consisting of wire require a high amount of effort in production. The transport of such prefabricated annular bodies proves to be expensive because a large amount of space is required as a result of the inner cavity.  
      A muffler unit is built into the muffler housing in addition to the catalyst body as a further component in known exhaust systems of small-size appliances. The muffler unit usually consists of a so-called reflection body. The exhaust gas is introduced into the pipe and the openings situated in the pipe cause a refraction of sound and thus a reduction in noise. Concerning the arrangement of catalyst body and muffler unit, it is common practice to switch them in series, which means that the exhaust gas, after entering the muffler housing, is supplied first to the reflection body and thereafter flows through the catalyst body in an axial or radial manner. When a radial catalyst is used, the same is often arranged about the reflection body coaxially to and spaced from the same. This requires an adjustment of the geometric dimensions of the components to be arranged, a plurality of means for fastening the components and a time-consuming assembly. Moreover, the catalyst bodies and muffler units cannot be used in all types of exhaust systems of a small-size engine as a replacement for used components.  
      The invention is thus based on the object of providing an annular catalyst body which can be flowed through in a radial manner, is easy to produce, requires little space in transport and simultaneously fulfils the requirements concerning the catalytic effect and sufficient stability, so that it remains dimensionally stable during the use in an exhaust system. It is a further object of the invention to provide an exhaust system which offers a lower complexity in assembly and also offers an improved exchangeability of the components. It is also an object of the present invention to provide a method for producing the catalyst body with which a timely and cost-effective production is possible.  
      The object is achieved for an annular catalyst body which can be flowed through in a radial manner in such a way that it consists of a perforated metal foil. A catalyst body with high mechanical stability is thus created which is also easy to produce in a cost-effective manner. Moreover, the metal foil has a low volume prior to mounting and can thus be transported in a compact manner. The stability of the metal foil also contributes to the fact that consumed catalytic coatings will hold permanently.  
      The perforations of the perforated metal foil are adjusted in a preferred embodiment for sound reduction in such a way that the metal foil simultaneously fulfils the function of a conventional reflection body. The metal foil which is adjusted for sound reduction as a replacement for a conventional muffling unit also comprises favorable stability properties in addition to the catalytic effect. Because the perforations are adjusted for reduction of pollutants and sound, the reflection body can be omitted, as a result of which one component of the exhaust system can be omitted.  
      The term “metal foil” shall be understood as any metallic layer which is configured in such a way that the intended perforations can be introduced and the same can be shaped into a ring. The term “metal foil” shall also include sheet metal. All these materials can also be provided with a catalytic coating. A strip of a certain dimension is obtained from metallic base material and is brought into the form of a closed circle.  
      The term “perforation” relating to the invention shall designate any type of opening in the metal foil. It can have a round, angular, circular, elliptic, slotted and further linear or non-linear shapes and can be introduced in the known manner into the metal foil.  
      The thickness of the metal foil can be adjusted according to the requirements concerning pollutant treatment, noise reduction and the stresses resulting from the gas pressure and the fastening in the muffler housing.  
      The metal foil in accordance with the invention provides a catalyst body which is easy to produce and can also be used as a replacement for a conventional reflection body and also allows easy handling during mounting. The metal foil, which is plane at first, enables easy transport with little need for space. Sufficient stability for use in a muffler housing is achieved after the final shaping into a closed circle.  
      A further preferred embodiment is that webs are arranged between the perforations for stiffening the metal foil. They are adapted in such a way that they prevent any bulging of the metal foil in the built-in state. These connecting webs or material bridges are thus obtained in combination with the arrangement of the perforations and are directly dependent upon the same. The dimensions and thicknesses of the webs are dimensioned in such a way that they can permanently withstand all stresses occurring after the installation in the muffler housing, i.e. both in the idle state as well as in operation of the device. Such webs ensure the dimensional stability of the catalyst body. In a further preferred embodiment this is achieved in such a way that the webs are aligned according to the forces acting upon the catalyst body. Pressure forces usually occur in the direction of the longitudinal axis of the annular catalyst body, especially when the same is fastened in the muffler housing by being subjected to a pre-tension, whereas tensile forces can occur perpendicular to the same. The webs can also extend in an oblique manner over the circumference of the annular catalyst body. The thus formed grating-like bracing stabilizes the catalyst body and prevents squashing or barrel-like bulging. In a further appropriate embodiment, the webs are aligned coaxially to the longitudinal axis and/or in the circumferential direction of the catalyst body. Optimal bracing is achieved especially in the case of directions of the webs standing perpendicular with respect to each other.  
      Certain requirements placed on the thickness of the metal foil will arise from the aforementioned requirements concerning the occurring loads and with respect to exhaust gas treatment and assembly. In a further preferred embodiment, the metal foil has a thickness of 0.05 mm to 2 mm and preferably of 0.3 mm to 1 mm. The thickness also depends on the desired inside hollow cross section of the ring or according to the desired inside diameter in the case of circular configuration. It should be chosen on the one hand in such a way that the metal foil is still sufficiently pliant so as to form the circular shape on the one hand and also still has a sufficient dimensional stability on the other hand. The metal foil should further be configured in a heat-resistant manner and can comprise shares of chromium or aluminum for example for this purpose.  
      The metal foil is appropriately provided with a helical, star-like, trapezoid or wave-like shape for optimizing the catalytic and/or muffling effect. Such shapes can be punched into the originally planar metal foil by means of the respective tools. The eddies formed on the leading edges increase the contact of the exhaust gas with the catalytic layer and contribute to the refraction of sound.  
      For simplifying the assembly and reducing the apparatuses necessary for this purpose, it is provided for in a further embodiment of the invention that the ends of the metal foil are joined with a detachable interlocking connection. It is advantageous in this respect when the ends of the metal foil are provided with a hook-like configuration and can thus be brought into engagement with a few manipulations. The ends of the metal foil can be bent over in opposite directions for forming the hooks.  
      It is also appropriate as a detachable interlocking connection that the one end of the metal foil comprises a recess and the other end of the metal foil comprises a counterpart for the engagement in the recess. The counterpart can have a larger extension than the recess once it has been introduced into the recess and is in engagement with the same, so that the connection is no longer able to detach or open by itself. Especially in the case of a configuration of the ends of the metal foil with a recess or a counterpart, they can be easily integrated in the same during the punching or cutting of the metal strip. The recess and the counterpart can have a shape of any linear or non-linear type which are adjusted to each other, e.g. they can have rectangular, oval or bent shapes. The said interlocking connections can also be secured by joining measures if necessary, such that one or several spot welds are made in the corner regions of the one end for permanent fixing with the other end of the metal foil.  
      Concerning the configuration of the perforations, they are formed in a further preferred embodiment by bending away the metal foil. At least a part of the contour outline of the later perforation is predetermined at first by punching or cutting on the metal foil which is plane at first. Thereafter the material of the perforation to be produced at the respective location is bent away in such a way that the bent portion of the metal foil still has a connection with the metal foil or the adjacent web. The bending away can occur in different directions. For the purpose of forming the spacers simultaneously, the bending away occurs in one direction, e.g. to the outside. As an alternative it is possible to produce the perforations also in such a way that the material still present at the location is fully removed by punching for example. A symmetrical pattern in the form of rows with or without offset extending parallel in the circumferential direction of the ring is chosen with respect to the arrangement pattern of the perforations. An asymmetric or irregular distribution of the perforations is also possible.  
      The object in accordance with the invention is achieved for an exhaust system in such a way that the catalyst body consists of a perforated metal foil. The catalyst body can thus be produced more easily and cost-effectively and can be installed better in the exhaust system.  
      In a preferred embodiment the catalyst body consists of a perforated metal foil, with the perforations of the perforated metal foil being adjusted for sound reduction in such a way that the metal foil simultaneously assumes the function of a conventional muffling unit. The number of components for exhaust treatment thus required in an exhaust system of a small-size engine is reduced. This leads to a reduction of the production costs and the required assembly work and to a better exchangeability of the components. Reference is hereby also made to the information provided with respect to the first mentioned solution for configuring the catalyst body in accordance with the invention.  
      Preferably, the catalyst body is arranged coaxially to the inflow direction of the exhaust gas into the muffler housing about an inlet of the muffler housing. Suitable flow conditions are created within the muffler housing and especially with respect to the radial flow against the metal foil from the inside, thus leading to favorable results with respect to silencing and pollutant reduction.  
      In a further preferred embodiment of the invention, the catalyst body is tensioned with a pretension against the muffler housing. For fixing the position of the catalyst body before and after bracing, embossed portions shaped according to the ends of the catalyst body are provided in the respective parts of the muffler housing and/or additional parts such as holding rings or holding plates. The fastening means used for bracing such as screws for example can be used at the same time to fasten the muffler housing on or close to the small-size engine or on the respective device. By bracing the catalyst body between the housing parts or additional parts of the muffler housing, complex fastening measures within the muffler housing can be avoided.  
      The object in accordance with the invention is achieved for the production of the catalyst body in accordance with the invention in such a way that the catalyst body is bent in a ring-like manner from a perforated metal foil strip and the ends of the strip are joined by means of an interlocking connection.  
      In a preferred production method, the perforated metal foil strip is cut prior to this from a continuous product, punched or broken from previously applied perforation points. The production from a continuous product also allows a continuous coating process and especially considerable cost savings. The foils or strips of sheet metal can also be produced individual in sections.  
      The metallic base material which is plane in its initial state can also be provided first with a catalytically active coating. The coating can also occur after obtaining the metal foil strips. The ends of the metal foil are hooked together with an interlocking connection for example. They can also be pressed together or spot-welded for permanently fixing the ends of the metal foil.  
      The object is achieved for a catalyst body which can be flowed through perpendicular to its upper side and is provided for installation in a muffler housing in such a way that the catalyst body comprises a metal foil provided with cuts. Cuts can be made in a metal foil very easily. Such a metal foil represents a product that can be stacked easily for transport and has a low transport volume. A metal foil provided with cuts further ensures that it can be formed very easily in the regions between the cuts. Ducts can thus be formed by which gas to be treated catalytically is deflected to a desired direction of flow.  
      The metal foil is provided in its longitudinal direction with a helical, star-like, trapezoid or wave-like configuration in a preferred embodiment of the invention. Such shapes can be embossed easily into the originally planar foil. The mechanical stability of the metal foil thus machined can be increased. Moreover, additional leading edges are thus formed which lead to further swirling and increase the probability that the exhaust gas comes into contact with the coated metal foil.  
      The metal foil is configured with a single layer in the simplest embodiment. The size of the surface of the metal foil is determined by the desired cleaning effect. If in the case of a single-layer configuration the requirements placed on exhaust gas cleaning are not fulfilled and there is still sufficient space in the muffler housing, the metal foil can be provided with a fanfold configuration in accordance with a further embodiment of the invention. A fanfold metal foil can be produced very easily by folding. The individual foil layers are always joined to each other and the width of each layer can be configured variably depending on the available space. If the layer widths differ considerably from each other and a fanfold is difficult to produce, the catalyst body can comprise metal foils in accordance with a further configuration of the invention which are stacked in layers. It is not necessary that the individual metal foils are fixedly joined to each other in a mechanical respect. Conventional fixing methods such as soldering or welding increase the stability of the catalyst body especially in numerously stacked layers.  
      If the metal foil is provided with a plane configuration in its longitudinal direction at its two ends, the ends can be used in a preferred manner for fixing the metal foil. A simple and cost-effective fixing is achieved when the metal foil is mounted at its end by a frictional connection with the muffler housing. This can occur by clamping. One possibility is clamping the ends of the metal foil between the joining surfaces of the housing. No further mounting elements are necessary in this case. If other components such as spacers are provided in then interior space of the muffler housing, the ends of the foil can be clamped between the components and the housing. A friction-locked clamping connection should be configured in such a way that additional mounting elements such as screws are not necessary. This simplifies the configuration of the muffler housing and the catalyst body and contributes towards the cost-effective structure.  
      The metal foil is configured in accordance with a preferred embodiment in such a way that in a section formed between two cuts the metal foil is orientated in its shape in a direction which is opposite to the shape of the metal foil adjacent to the section. In the case of a wave-like metal foil, the foil can be bent over in a wave trough in a section between two cuts in such a way that a crest of a wave is obtained. With an unchanged foil height and foil width it is thus possible to obtain a doubling of the leading edges and the swirling, so that the cleaning effect of the catalyst body is increased.  
      If the catalyst body is arranged in the muffler housing with a front housing part and a rear housing part in a zone between two pins joining the housing parts, the catalyst body can be guided laterally by the pins. Especially in the case of a fanfold-like or coated catalyst body it is thus ensured that any bulging or bending under pressure load in the longitudinal axis of the folded or stacked catalyst body is prevented This guidance by the pins does not require any additional fastening means for the catalyst body, but does not exclude them when an oscillation node is to be produced in the catalyst body.  
      The pins are preferably installed in the interior space of the muffler housing. This allows building an entirely enclosed housing which comprises mounting elements and filters exclusively in its interior space. If pins are provided at the edge of the housing in order to screw the housing together from the outside, the metal foil would need to be guided up to the edge of the housing and be clamped there by means of a frictional connection The metal foil would thus assume the entire inner width of the housing. This can be prevented by the embodiment in accordance with the invention, so that the metal foil is capable of assuming only a limited and defined portion of the interior space.  
      If the catalyst body is built into the muffler housing in such a way that it fills a distance which is formed in the interior space of the muffler housing from the front housing part to the rear housing part, a maximum available flow path is made available with maximum cleaning effect. The air drawn in can thus pass through the catalyst body arranged in a stacked manner for example starting from the inlet of the housing and can leave the catalyst body only directly prior to the outlet from the muffler housing.  
      According to a further embodiment of the invention, a region which is formed outside of the zone between two pins joining the housing parts and within the muffler housing is not filled up by the catalyst body. A flow resistance of the muffler and thus the silencing can be influenced. The muffler housing is thus not only provided for receiving a metal foil for cleaning the exhaust gas, which foil needs to fill a large part of the interior space for a sufficient cleaning effect. In addition, a space which can be configured in a different manner in comparison with the catalyst body can be provided in addition by the space not filled by the catalyst body, which space can be adjusted in an optimal manner for an even more effective refraction of sound.  
      The invention relates further to an exhaust system for small-size engines with a muffler housing, with the exhaust system comprising a catalyst body as described above. By using such a catalyst body, an overall simple and cost-effective production of an exhaust system is enabled.  
      The invention further relates to a method for producing a catalyst body for installation in a muffler housing of a small-size engine, in which the catalyst body is folded in a fanfold-like manner from a metal foil provided with cuts and is joined to the muffler housing at the ends of the metal foil by means of a frictional connection. An effective mounting of a catalyst body is achieved with respect to time and cost.  
    
    
      The invention is explained below in closer detail on the basis of several preferred embodiments by reference to the drawings which show schematically:  
       FIG. 1  shows an exploded view of an exhaust system with a first embodiment of the metal foil as an annular catalyst body that can be flowed through in a radial manner;  
       FIG. 2  shows a top view of a second embodiment of the metal foil;  
       FIG. 3   a  shows a top view of a third embodiment of the metal foil;  
       FIG. 3   b  shows a perspective view of a projection of the metal foil according to  FIG. 3   a;    
       FIG. 3   c  shows a cross-sectional view of a further projection of the metal foil according to the invention;  
       FIG. 3   d  shows a cross-sectional view of a further projection of the metal foil in accordance with the invention;  
       FIG. 3   e  shows a cross-sectional view of a further projection of the metal foil in accordance with the invention;  
       FIG. 3   f  shows a top view of several patterns of cutting lines in a metal foil in accordance with the invention for forming the projections or perforations;  
       FIG. 4   a  shows a side view of the metal foil according to  FIG. 3   a;    
       FIG. 4   b  shows a side view of a fourth embodiment of the metal foil;  
       FIG. 4   c  shows a side view of a fifth embodiment of the metal foil;  
       FIG. 4   d  shows a side view of a sixth embodiment of the metal foil;  
       FIG. 5   a  shows a cross-sectional view of a first interlocking connection of a metal foil;  
       FIG. 5   b  shows a cross-sectional view of a second interlocking connection of a metal foil;  
       FIG. 6  shows an exploded view of a further exhaust system with a metal foil in accordance with the invention as an annular catalyst body that can be flowed through in a radial manner;  
       FIG. 7  shows a perspective view of a metal foil provided with cuts prior to forming and after forming;  
       FIG. 8  shows a metal foil with a profile that is wave-like in the longitudinal direction;  
       FIG. 9  shows a side view of a metal foil folded in a fanfold-like manner with sections bent over opposite to the wave form of the metal foil;  
       FIG. 10  shows a catalyst body with an intermediate layer;  
       FIG. 11  shows a catalyst body with ends configured in a planar way;  
       FIG. 12  shows a sectional view of an endless loop of three superimposed metal foils;  
       FIG. 13  shows a cross-sectional view through a muffler housing with a single-layer metal foil in the interior space of the muffler housing, and  
       FIG. 14  shows a cross-sectional view through a muffler housing with a multi-layer metal foil in the interior space of the muffler housing. 
    
    
       FIG. 1  shows an exploded view of an annular catalyst body  1  which can be flowed through in a radial manner and which is a part of an exhaust system  11  in accordance with the invention. The catalyst body  1  consists of a metal foil  2  which comprises a plurality of perforations  3  and webs  5 . The metal foil  2  and especially its perforations  3  and webs  5  are adapted for sound reduction. The configuration of the components of metal foil  2  and its material properties, surface structure and dimensions are adjusted in such a way that the metal foil  2  simultaneously assumes the function of a conventional reflection body.  
       FIGS. 2 through 5  will be used below for explaining the configuration of the metal foil  2  perse.  FIG. 2  shows the unwinding of a metal foil  30  in a top view. The thickness of the metal foil is 230 μm. The rectangular perforations  33  are arranged in three rows and in a symmetrical manner. Material bridges or webs  35  extending in the longitudinal and transversal directions are situated between the perforations  33 . The webs  35  between the uppermost and lowest row of perforations  33  and the respective longitudinal edges  38  and  39  are provided with a wider configuration than the webs  35  between the perforations  33  in order to increase stiffening in the transversal direction. This is also used for providing a sufficient stability concerning the later fastening of the metal foil  30 . Based on the illustrated planar form, the metal foil  30  is bent into an oval or elliptic ring. The ends  36  and  37  are then joined with each other, so that the catalyst body  1  is obtained. For installation in a muffler housing  4 , the annular metal foil  30  is placed with its edges  38  and  39  on the rear walls  24  of the housing parts  18  and  19  or on additional parts and is then clamped in the muffler housing (also see  FIG. 1 ).  
       FIG. 3   a  shows a metal foil  40  in a further embodiment. The same applies as in  FIG. 2  concerning the thickness of the metal foil  40 , the arrangement of the perforations  43  and webs  45 . The openings or perforations  43  are provided with an arched configuration and are formed by upwardly bending away the metal foil  40 . The projections  44  thus formed are used for increasing the surface of the metal foil  40  in order to produce a swirling of the exhaust gas for pollutant reduction and for a refraction of sound. They can also be used simultaneously as spacers. The metal foil  40  is again bent into a ring for forming the catalyst body  1 . The ends  46  and  47  are then joined to each other.  FIG. 3   b  shows a perspective view of a section of the metal foil  40  with two arched perforations  43  and two interposed brackets  48 . The brackets  48  or projections  44  are bent in the same direction from the metal foil  40 . In order to enable their pressing away, the metal foil is cut or punched in advance at the predetermined places. The brackets  48  or projections  44  can assume any linear or non-linear shape.  
       FIGS. 3   c  to  3   f  show further embodiments of the perforations and projections of the metal foil in accordance with the invention. The embodiments in the  FIGS. 3   c  through  3   e  are shown in a cross-sectional view.  FIG. 3   f  shows a top view of six embodiments of the applied cutting lines or perforation points. They allow pressing away metal foil regions situated between the cutting lines, thus producing the perforations and projections according to  FIGS. 3   c  through  3   e.    
       FIG. 3   c  shows in detail a roof-like or V-shaped configuration of a projection  91 . The exhaust gas flowing from below passes through in a vertical or inclined manner the horizontal opening cross section situated beneath the horizontal opening cross section and hits the lower side of the two legs  94 . The exhaust gas is deflected in a virtually horizontal way from there and flows away upwardly via triangular opening cross sections aligned vertically or perpendicularly above the metal foil plane.  FIG. 3   d  shows a projection  92 , with the leg  95  which is bent away in an inclined manner from the metal foil plane aligned horizontally at its free end. The exhaust gas flowing from below against the same is thus deflected even more strongly in a horizontal way. In the case of a spiral winding of the metal foil, said projection  92  can simultaneously act as a spacer.  FIG. 3   e  shows a further projection  93  which is obtained by bending away the respective metal foil region in opposite directions. The inclined legs  96  and  97  lead to an impingement and deflection of the exhaust gas. The transition of all legs  94 ,  95 ,  96 ,  97  among each other and into the metal foil plane can be provided with an edged or rounded configuration. The exhaust gas can also flow in from above.  
       FIG. 3   f  shows a top view of several patterns  100  to  105  for forming the projections or perforations in a metal foil in accordance with the invention. These patterns of cutting lines are formed by the notching, incision and severing lines (referred to below as cutting lines) shown with the unbroken lines. The broken lines mark the bending points obtained by pressing away the metal foil regions. The patterns  100 ,  101 ,  102  of the cutting lines must be chosen for forming the projections  91 ,  92 ,  93  of  FIGS. 3   c ,  3   d ,  3   e . The cutting lines therefore allow a pressing away of the metal foil regions delimited by the cutting and bending lines, thus producing the projections which project in a leg-like manner from the remainder of the metal foil. The cutting line pattern  104  is suitable for forming the projection  44  according to  FIG. 3   b . The patterns  103  and  105  represent additional alternatives. The possibilities as shown in  FIGS. 3   b  to  3   f  for forming the projections can be used in any of the metal foils described within the scope of the invention in a uniform or mixed manner.  
       FIG. 4   a  shows a side view of the metal foil of  FIG. 3   a  in a helical winding, with the projections  44  also being used as spacers. The side views in the  FIGS. 4   b  to  4   d  show a wave-like metal foil  50 , a star-like metal foil  60  and a trapezoid metal foil  70 . Their perforations  3  can be formed by punching, cutting, bending or pressing away. The exhaust gas need not necessarily flow through the respective perforation  3  in a radial manner. It can also be slightly deflected in its vicinity. If the perforation  3  is formed by bending away the metal foil  2  in such a way that the pass-through area of the opening or perforation  3  extends in an inclined or vertical manner, the exhaust gas will be briefly deflected in this region in order to allow a flow through the perforation  3 . The perforations  3  can then extend in an inclined or vertical manner for example when the material that is pressed away assumes the shape of a roof. Such V- or U-shaped roofs pressed out of the plane of the metal foil can be arranged in the peaks or valleys of the metal foils  40 ,  50 ,  60  or  70 .  
       FIGS. 5   a  and  5   b  show two possibilities for forming a detachable interlocking connection  8  for fastening the two ends  6  and  7  of the metal foil  2 . This also applies accordingly for all other aforementioned metal foils. In  FIG. 5   a  the interlocking connection  8  is enabled by a hook-like configuration of the ends  6  and  7  of the metal foil  2 . The metal foil  2  can be bent into a ring with a few manipulations prior to the assembly of the exhaust system and can be hooked together afterwards. In  FIG. 5   b , the one end  6  of the metal foil  2  is provided with a recess  9  and the other end  7  is provided with a counterpart  10  for engagement in the recess  9 . The rectangular recess  9  is created by bending away a part of the metal foil  2  by forming a bracket  75 . The other end  7  of the metal foil  2  is bent by forming a bracket  76  as the counterpart  10 , so that said end  7  is in engagement with the other end  6  after threading the counterpart  10  into the recess  9 . The interlocking connection  8  can be mounted and dismounted with a few manipulations. There is principally also the possibility to fix the interlocking connection  8  by one or several spot welds for permanent fixing.  
       FIG. 1  further shows the assembly of the exhaust gas system  11  in accordance with the invention. The catalyst body  1  is formed by the metal foil  2 . Instead of the metal foil  2  it is possible to use in the exhaust system  11  any other embodiment of the metal foils  30 ,  40 ,  50 ,  60  or  70  as described within the scope of the invention. For reasons of simplicity merely the embodiment with the metal foil  2  will be explained.  
      In addition to the metal foil  2  which simultaneously forms the muffler unit of a conventional exhaust system, one can further see the muffler housing  4  consisting of the housing parts  18  and  19  and two metal pins  16 . The metal pins  16  are penetrated by screws or other fastening means for clamping the housing parts  18  and  19  with interposed metal foil  2 . The screws are used at the same time for fastening the muffler housing  4  on or close to a small-size motor (not shown). The housing part  18  is provided on its rear wall  24  with a central region  22  which is embossed in a plate-like manner. It contains the inlet  14  and two openings  23  for passing through the screws. The other housing part  19  also comprises on its rear wall  24  a plate-like central region  22  and two openings  23 . For discharging the exhaust gas from the muffler housing  4 , an outlet  15  is arranged on a side wall of the housing part  19 . For simplified handling during the assembly, the outside diameter of the annularly bent metal foil  2  shall not be larger than the inside diameter of the circular central region  22 , so that the metal foil  2  can be used at least temporarily for fixing the position during the assembly of the exhaust system  11  in the central plate-like region  22  which is sunk in comparison with the ambient rear wall. After introduction of a screw each or a similar fastening means along the axis  25  into the openings  23  of the one housing part  18 , the metal pins  16  can be pushed over the screws in order to insert thereafter the screw ends into the openings  23  of the other housing part  19  and to push the same up to the limit stop on the circumferential edge  20  of the housing part  18 . The circumferential edges  20  and  21  of the housing parts  18  and  19  are each formed as web-like beads, so that a gastight seal of the housing parts  18  and  19  is achieved. Further sealing measures such as rubber seals for example can be attached for this purpose. The openings  23  and the connection of the inlet  14  to the motor are also provided with a gas-tight configuration. The beads  20  and  21  can also be shaped in such a way that one housing part overlaps the other housing part at least in part.  
      Concerning the geometrical dimensions of the components to be assembled along the axis  25 , the shell-like housing parts  18  and  19  have a certain depth, so that after the insertion of the metal foil  2  into the two plate-like central regions  22  the ends of the metal foil  2  which are directed along the axis  25  each rest on the rear wall  24  and the beads  20  and  21  also rest on each other. The screws extending along the axis  25  penetrate the metal pins  16  and the two housing parts  18  and  19  and project beyond the muffler housing  4 . In order to clamp the metal foil  2  and thus the catalyst body  1  in the muffler housing  4  or against the housing parts  18  and  19 , the metal pins  16  therefore have a shorter length than the metal foil  2 . The thus assembled and pretensioned exhaust gas system  11  can finally be attached by means of the projecting ends of the screws to a small-size engine or close to the same.  
      For reducing the pollutants and for reducing the sound, the exhaust gas flows via the inlet  14  into the interior of the muffler housing  4  and reaches an inner cylindrical cavity  26  which is delimited by the metal foil  2  and the rear walls  24  of the housing parts  18  and  19 . The exhaust gas flows from there radially to the outside and meets the metal foil  2 . Depending on the configuration of the perforations  3  and the webs  5 , a contact is established with the catalytically active coating of metal foil  2  and a reduction of the pollutants in the exhaust gas occurs. The impingement of the exhaust gas stream on the metal foil  2  also leads to a refraction of the sound waves and to a noise reduction. Once the exhaust gas has flowed through the metal foil  2 , it reaches the ambient environment via the outlet  15 .  
       FIG. 6  shows an exploded view of a further embodiment of an exhaust system with the catalyst body  1  according to  FIG. 1 . The same parts as in  FIG. 1  are designated with the same reference numerals. The same components as in  FIG. 1  are used especially for the metal foil  2  and the housing parts  18  and  19 . It is also possible to use all other aforementioned metal foils  30 ,  40 ,  50 ,.  60  or  70 . The configuration of the components of metal foil  2  and its material properties, surface structure and dimensions are adjusted in such a way that the metal foil  2  assumes the function of a catalyst. Further components of this embodiment are shown as additional parts in the form of two holding plates  82  and  83  and a reflection pipe  80 . The reflection pipe  80  consists of sheet metal and comprises several perforations  81  for sound reduction. The holding plates  82  and  83  each comprise an opening  84  in the middle for attachment to the reflection pipe  80 , which opening corresponds to the outside shape of the cross-sectional shape of the reflection pipe  80 . The outer edges of the holding plates  82 ,  83  each comprise a circumferential recess or groove (not shown) for fixing the metal foil  2 .  
      During the assembly of the exhaust system according to  FIG. 6 , the individual components are pushed into each other along the axis  25 . The holding plate  82  is pushed onto the reflection pipe  80  in such a way that it comes to lie at a certain distance from the end of reflection pipe  80  which is opposite of the housing part  18 . Thereafter the annular metal foil  2  is guided over the reflection pipe  80  in a manner coaxial to and spaced from the latter, and it is pressed into the recess of holding plate  82 . Then the second holding plate  83  is pushed over the reflection pipe  80  until the still free edge of the metal foil  2  reaches the recess of the holding plate  83 . The longitudinal dimensions of the metal foil  2  and the reflection pipe  80  which are directed in the direction of axis  25  can be chosen in such a way that at least one of the holding plates  82 ,  83  comes to lie on a face side of the reflection pipe. The opening  84  can be omitted when the holding plate  82  or  83  is fastened to a face-side end. The holding plates  82 ,  83  can also both be arranged on the face sides of the reflection pipe  80  or at a distance on the same with space from the respective end of the reflection pipe  80  and can optionally also be welded onto the same. For the purpose of the subsequent clamping of the exterior housing parts  18  and  19 , the same measures are taken as already explained in  FIG. 1 . The configuration of the circumferential edges  20  and  21  of the housing parts  18  and  19  occurs again in the form of web-like beads, so that a gas-tight seal of the housing parts  18  and  19  is achieved. As in  FIG. 1 , the rear walls  24  of the housing parts  18 ,  19  can also be provided with sunk central regions according to the ends of the reflection pipe. These sunk parts or recesses can face towards the inside or outside.  
      For the purpose of sound and pollutant reduction, the exhaust gas flows via inlet  14  into the interior of the muffler housing  4  and reaches the reflection pipe  80 . From there the exhaust gas flows radially to the outside and meets the reflection pipe  80 . Depending on the configuration of the bores  81 , a certain reflection or refraction of the sound waves occurs. Thereafter the exhaust gas flows through the perforations  81 , continues to flow radially to the outside and meets the metal foil  2 . A contact occurs there with the catalytically active coating of the metal foil  2  and a reduction of the pollutants in the exhaust gas occurs. The impingement of the exhaust gas stream onto the metal foil  2  also leads to a refraction of the sound waves and thus also to a sound reduction. Once the exhaust gas has flowed through the perforations  3  of the metal foil  2  it reaches the ambient environment via the outlet  15 .  
       FIG. 7  shows a metal foil  110  with cuts  111  which are introduced into the metal foil parallel with respect to each other. A cut does not represent a perforation here where material is removed. Ideally, the material is only severed at the intended positions. In a subsequent production step the metal foil can be deformed in a section  114  which is arranged between two steps. If the metal foil  110  is bent outwardly in this section  114 , an elevation  115  is obtained by which a gas flowing on the upper side  119  of the metal foil can be deflected in the direction towards the lower side  120  of the metal foil (see arrows in  FIG. 7 ). If the metal foil is provided with a wave-like configuration (see  FIG. 8 ), the metal foil  110  can be oriented in its shape in a direction in the section  114  which is opposite to the shape of the metal foil adjacent to the section  114 . In the case of an overall size of the metal foil  110  as predetermined by the wave form, further leading edges and thus possibilities for swirling the gas can be provided by providing elevations  115  in the section  114 .  
      If the exhaust gas flows perpendicularly against the upper side  119  of the metal foil (see  FIG. 9 ), the gas can pass from the upper side  119  in the direction towards the bottom side  120  as a result of the elevations  115  (see arrows in  FIG. 9 ). In the case of a metal foil which is configured in a wave-like manner and is folded as a fanfold and whose wave crests meet each other, the gas flowing through a first fanfold layer flows laterally in the direction towards a wave trough of the fanfold layer situated below. It can meet a further elevation  115  there, beneath which it passes, so that a gas flowing perpendicular to the upper side of the first fanfold layer is guided successively from one layer to the next layer and also laterally. The gas can also flow perpendicular to the drawing plane in  FIG. 9  in the intermediate space between wave troughs and wave crests, see in  FIG. 9  marking with point as a symbol for a flow from the drawing plane upwardly away or a marking with a cross as a symbol for a flow in the drawing plane downwardly into the same. As a result of this plurality of directions of flow, sufficient swirls are produced in order to bring the gas into contact with the catalytically coated surface of the metal foil.  
      Oscillations in the housing and in the catalyst body can lead to the consequence that a wave crest of one layer penetrates a wave trough of an adjacent foil layer and thus considerably reduces the flow cross section. A planar intermediate layer  121  can contribute obstructing this process. In the case of several foil layers it is therefore advantageous to provide planar intermediate layers  121  which will prevent the wave-like metal foils from sliding into each other (see  FIG. 10 ).  
       FIG. 11  shows a catalyst body which comprises two wave-like metal foils  110  and an interposed gas-permeable planar intermediate layer  121 . The metal foils and the intermediate layer are each configured in such a way that they have a planar configuration in their longitudinal direction at their planar ends  112  and  113 . These non-wave-like or non-uneven points are especially suitable for a clamping connection with the muffler housing for example. The planar ends  112 ,  113  can also be provided in an endless strip with individual foil layers (see  FIG. 12 ). It is advantageous to press the stacked sheet-metal layers in predetermined intervals flat on each other and to join these points by means of spot welding. These flat pressed areas are especially suitable in order to sever individual catalyst bodies from the endless strip. Respective severing planes  122  are schematically displayed in  FIG. 11 .  
      Such endless strips made of individual foil layers or with several stacked foil layers are coated with catalyst material preferably after providing the openings and optionally after joining the individual foils before the severing into individual catalyst bodies is performed.  
      The simplest embodiment of a catalyst body is a single-layer metal foil.  FIG. 13  shows a metal foil  110  in a muffler housing with the front housing part  18  and rear housing part  19  directly on the gas inlet  14  in a zone  116  between two pins  16  joining the two housing parts. The pins are provided in the interior space of the housing, with the metal foil  110  being joined at its planar ends  112 ,  113  in a non-positive manner with the housing part  18  by means of the pins  16 . No further mounting elements are thus required for the metal foil. The gas flows perpendicular to the upper side  119  onto the metal foil  110 . To ensure that the gas can pass through the metal foil, the metal foil needs to comprise openings such as perforations or elevations  115 . After passing through the metal foil the gas continues to flow through the muffler housing and exits through outlet  22 .  
      When the metal foil is provided with a fanfold-like configuration or when several metal foil are stacked above one another they can form a catalyst body which fills the interior space of the muffler housing from the front housing part  18  to the rear housing part  19  along a distance  117  (see  FIG. 14 ). A maximum filtering effect is thus achieved. In this embodiment too, the catalyst body is joined in a non-positive way to the housing by means of pins  16 . The region  118  which is outside of the zone  116  but still within the interior space of the muffler housing as predetermined by housing parts  18 ,  19  is not filled by the catalyst body. The region  118  can be completely free or be filled with sound-absorbing elements, so that the muffler also achieves a high muffling property in addition to the filtering effect by the catalyst body.  
      A possibility is created with the metal foil in accordance with the invention to control the reduction of pollutants and sound of a small-size engine especially by forming the perforations and elevations contained in the metal foil and their free cross section. The free cross section of the perforations can also be brought into a relationship with the opening cross section of the engine outlet. A suitable ratio of the cross section of the engine outlet to the free section of the perforations is one to two for example.