Abstract:
To provide an exhaust system ( 100 ) for a combustion engine having a housing ( 10 ) in which at least one aperture for an exhaust gas inlet ( 11 ) and an additional aperture for an exhaust gas outlet ( 12 ) are provided, and having a catalytic element that is arranged in the housing ( 10 ) of the exhaust system ( 100 ) for cleaning exhaust gases from the combustion engine, and which can be manufactured simply and inexpensively, yet still achieves a high conversion rate to satisfy increasingly stringent environmental regulations, it is suggested to arrange at least one cleaning unit ( 13 ) including at least the described catalytic element ( 14 ) and a reflection means ( 15 ) in housing ( 10 ) of the exhaust system ( 100 ), in which some or all of the exhaust gases are directed through the catalytic element ( 14 ) of the cleaning unit ( 13 ) before reaching the reflection means ( 15 ) disposed behind it, which directs the exhaust gases ( 22 ) through the catalytic element ( 14 ) again, this time from the other side.

Description:
RELATED APPLICATIONS 
   This Application claims priority to German Utility Model No. 20 2004 009 506.8 filed on Jun. 17, 2004, and German Utility Model No. 20 2004 019 896.7 filed on Dec. 20, 2004. 
   BACKGROUND OF THE INVENTION 
   The invention relates to an exhaust system for a combustion engine, in which the engine may be either a four-stroke or a two-stroke petrol engine. The use of an exhaust system in two-stroke engines is especially advantageous, however. Because of its compact construction, this exhaust system may also be used in hand-operated tools, such as petrol-driven chainsaws, hedge trimmers or similar. The exhaust systems includes a housing in which at least one aperture is provided for an exhaust gas inlet, and a further aperture is provided for an exhaust gas outlet. To prevent the exhaust gases from escaping into the atmosphere without being cleaned, at least one catalytic element is also arranged in the housing, so that some or all of the exhaust gases flow through the catalytic element before they escape into the atmosphere through the gas outlet aperture. 
   It is known from the prior art to used catalytic elements in exhaust systems to reduce emissions of pollutants from combustion engines. These catalytic elements enable subsequent treatment of the exhaust gas with the components contained in the gas. The hydrocarbons are converted to carbon dioxide or carbon monoxide using the residual oxygen content. For lower conversion rates, i.e. if not all hydrocarbons are converted into carbon dioxide or carbon monoxide, coated metal meshes may be useful instead of the usual honeycomb catalytic converters. In a broader comparison with honeycomb catalytic converters, the production of 2-dimensional catalytic element is particularly simple and correspondingly cheaper. 
   In this context, the object of the invention was to provide an exhaust system having a catalytic element that may be manufactured easily and inexpensively but with which a high conversion rate may be achieved, to satisfy increasingly stringent environmental regulations. 
   This object is solve by the features listed in claim  1 , which are particularly significant for the following reasons. 
   SUMMARY OF THE INVENTION 
   The exhaust system according to the invention for a petrol combustion engine includes a housing, in which at least one aperture for an exhaust gas inlet and a further aperture for an exhaust gas outlet are provided. A cleaning unit is provided inside the housing of the exhaust system and includes at least one catalytic element and one reflection means. The cleaning unit may be simply attached to the inside of the housing by a welded joint, a positive and/or non-positive locking fixture. The arrangement of the catalytic element with regard to the reflection means is equivalent to the principle of a container, the opening of which is partly or completely covered by a sieve. The cleaning unit itself is arranged in the housing in such manner that at least the inflowing exhaust gases are directed partly or entirely through the catalytic element in the cleaning unit. In order to ensure that these partially converted exhaust gases are passed through the same catalytic element again, the reflection means is arranged behind the catalytic element. When they have passed through the catalytic element, the exhaust gases are forces against the reflection means, by which they are reflected back towards the catalytic element from which they have just emerged, and must pass through it again, this time from the other side. Then, at least a portion of the repeatedly converted exhaust gases is able to escape into the atmosphere or the environment through the aperture for exhaust gas outlet. Another fraction of the converted exhaust gases is mixed with the inflowing exhaust gas and is directed through the catalytic element in the cleaning unit again. This mixing of cold, inflowing exhaust gas with the hot, partially converted exhaust gas, the thermal load on the catalytic element is lowered. This in turn increases the operating life of the catalytic element. Since the partially converted exhaust gas is also forced to pass through the catalytic element the cleaning unit multiple times, conversion is also improved. In addition, the residence time of the exhaust gases in the exhaust system is increased artificially. In this way, optimum cleaning effect may be achieved with just cone catalytic element. The frequent passes through the catalytic element mean that the exhaust system may be quite small, thereby also reducing expenses for materials. As a result, the exhaust system according to the invention may be installed or retrofitted without difficulty in existing exhaust designs. In summary—with the measure described previously—the conversion rate of the exhaust system may be significantly improved with a simple construction and small dimensions. Furthermore, use of the cleaning provides an enhance noise damping effect, which is based on deliberate reflection of the exhaust gases. 
   Further advantageous configurations of the exhaust system are described in subordinate claims  2  to  18 . 
   To improve the conversion rate of the exhaust system according to the invention yet further, the exhaust gases may be directed multiple times from both sides through the same catalytic element. To this end, the cleaning unit must be arranged in such manner in the housing that a large portion of the reflected exhaust gases is directed back to the cleaning unit by the housing or the housing wall, In this way, the exhaust gases are able to be almost completely converted with just one, particularly flat catalytic element, before they escape into the atmosphere through the outlet aperture. 
   In a further embodiment of the exhaust system, the reflection means is configured in a bowl or parabolic shape. The catalytic element is then arranged at least partly in front of an aperture in the reflection means. The bowl-shaped or parabolic design of the reflection means causes the exhaust gases to be reflected in controlled manner, so that a flow path through the exhaust system may be controlled. By this means, a portion of the exhaust gases may be reflected in a predictable and defined manner, so that it flows through the catalytic element from both sides several times. 
   A further embodiment of the exhaust system provides that the entire aperture of the reflection means is covered by the catalytic element, so that all reflected exhaust gases are directed through the catalytic element at least twice. The catalytic may be flat, arcuate, corrugated or some other similar shape. Increasing the surface area of the catalytic element also serves to improve the chemical conversion process taking place in the exhaust gases. 
   To prevent all reflected exhaust gases from flowing through the same point in the catalytic element, which would result in achieving only a moderate conversion rate, a further variant of the exhaust system is suggested in which a floor of the reflection means has a corrugated or relief profile. The individual rises or irregularities in the floor may prevent the accumulation of reflected exhaust gases, so that the exhaust gases are able to flow in even distribution over the entire surface of the catalytic element. 
   A simple an inexpensive variant of the exhaust system according to the invention provides that the reflection means constitutes a part of the housing. In this case, the entire reflection means does not need to be formed by the housing, but for example the floor of the reflection means may be replaced by a housing wall. This measure serves to cut material requirements and weight. Also, the already partly converted and therefore hot exhaust gases may be cooled better in the cleaning unit, so that the operating life of the catalytic element is extended. To this end, the corresponding housing wall might also be conformed with corrugations or in relief, to dissipate heat over a larger surface area. 
   In a particularly resilient design of the exhaust system according to the invention, the reflection means is constructed partly or entirely from stainless steel. The use of stainless steel serves to prevent corrosion of the reflection means, which is particularly exposed to the chemically aggressive exhaust gases. In this way, the overall operating life of the exhaust system may be prolonged. If the reflection means does not form an integral part (like the variant described previously) with the housing, the entire housing may be fabricated from a simple steel panel. Of course the housing and other components of the exhaust system may also be galvanized to extend their operating life. 
   In a special embodiment, the cleaning unit is disposed opposite the aperture for the exhaust gas inlet. In this way, it is possible to ensure that as far as possible all inflowing exhaust gases pass into the cleaning unit and consequently are directed through the catalytic element of the cleaning unit at least twice. This also ensures that a large portion of the reflected exhaust gases is mixed with the inflowing, cold exhaust gases. Under the effects of the prevailing flow and pressure conditions, the mixed exhaust gases are passed back to the cleaning unit that is facing the aperture for the exhaust gas inlet. Consequently, this exhaust system demonstrates a high conversion rate. 
   To improve this conversion rate yet further, a cleaning unit may contain multiple catalytic elements. For example, with two or three catalytic elements arranged side by side, the efficiency of the cleaning unit is enhanced considerably. In the same way, it is conceivable that a cleaning unit may also contain multiple reflection means, which are covered by one or more catalytic elements. In an arrangement with multiple reflection means, the inflowing exhaust gases may be reflected more precisely and more variably, i.e. in many different directions. 
   In a further embodiment of the exhaust system, at least two cleaning units are arranged inside the housing. In this way, the exhaust gases may be reflected from one cleaning unit to the other and vice versa. A particularly simple arrangement of the two cleaning units is achieved if the second cleaning unit is disposed opposite the first cleaning unit. In this way, a constantly repeating cycle or a “ping-pong” effect may be achieved, in which the exhaust gases pass through the same cleaning units multiple times before they finally escape from the exhaust system. 
   A particularly interesting embodiment of the exhaust system is achieved if at least one additional catalytic element is arranged in the housing besides the existing cleaning unit, and through which at least some of the exhaust gases are directed. This is preferably to be arranged in the housing in such manner that all inflowing and outflowing exhaust gases must flow through this additional catalytic element. To this end, the additional catalytic element may enclose the aperture of the exhaust gas inlet in the interior of the housing, so that the inflowing exhaust gases must pass through the additional catalytic element first. Therefore, reference will also be made in the following text to an additional, first catalytic element. The location of the additional, first catalytic element behind the inlet aperture has the advantage that the exhaust gas counterpressure necessary for the operation of a two-stroke engine is created. If the first catalytic element has an curved shape, at the same time the exhaust gas flow may be directed optimally to the existing cleaning unit, since the speed and inertia of the exhaust gas flow created at this point renders this particularly simple. It is also conceivable to arrange the additional catalytic element in front of the exhaust gas outlet aperture, though this arrangement has the disadvantage that the outflowing exhaust gases are heated again before the escape to the outside. 
   To construct an exhaust system as inexpensively as possible, mesh-type or perforated panel catalytic elements such as are commercially available as inexpensive semi-finished parts may be used, since the exhaust gases are forced through at least the catalytic element of the cleaning unit multiple times anyway. The use of 2-dimensional catalytic elements is a practical solution for the catalytic element in the cleaning unit and for the additional catalytic elements. Because of the special design of the exhaust system according to the invention, this enables high conversion rates to be achieve, such as would only be achievable otherwise with expensive honeycomb catalysts. Accordingly, conversion rates of more than 50% were measured in an exhaust system of the described construction (such as was described in the previous section, see also  FIGS. 1 and 2 ) with mesh-type or perforated panel catalytic elements. Moreover, the 2-dimensional, mesh-type or perforated panel catalytic elements have the advantage that they do not heat up as much as the comparable honeycomb catalytic converters and they have a lower flow resistance. As a result, the 2-dimensional catalytic elements have a longer operating life. In addition, the performance of the combustion engine is scarcely affected. 
   In an further embodiment of the exhaust system, the catalytic element may be constructed to be self-supporting. In this way, additional support or retaining constructions for positioning the catalytic elements inside the housing or for affixing them on or above the reflection means may be dispensed with. In addition, the possible uses of the catalytic elements are expanded thereby, since they allow almost unlimited shape variations. 
   In a further embodiment, the catalytic elements may be welded or soldered or otherwise permanently joined to each other at the contact points of the mesh-forming elements, to improve mechanical strength. 
   Also, partially coated catalytic elements may be used in a particularly inexpensive embodiment of the exhaust system. For example, it is not necessary to coat the points where the catalytic elements contact the housing or the reflection means, because no exhaust gases are able to pass over these points anyway. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, the invention is explained in greater detail with reference to the attached drawings and in various embodiments. In the drawings: 
       FIG. 1  shows a three-dimensional view of an exhaust system according to the invention, with a cleaning unit and a first, arcuate catalytic element, 
       FIG. 2  shows a longitudinal section through the exhaust system according to the invention of  FIG. 1 , and 
       FIG. 3  shows a similar longitudinal section to that of  FIG. 2 , but through a different embodiment of the exhaust system according to the invention with multiple cleaning units, and 
       FIG. 4   a  shows a schematic representation of a catalytic mesh for a catalytic element, with simple, alternating weaving of strands, and 
       FIG. 4   b  shows a schematic representation of a catalytic mesh for a catalytic element with paired weaving of strands, and 
       FIG. 4   c  shows a three-dimensional view of the catalytic mesh of  FIG. 4   a , with simple weaving of strands. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a three-dimensional view of a first embodiment of exhaust system  100  according to the invention. A housing  10  of this exhaust system  100  has an essentially cuboid shape. In order to show the internal construction of exhaust system  100  as well, particularly the arrangement of catalytic elements  14 ,  18  and reflection means  15 , exhaust system  100  is shown in partial cutaway. In this particularly advantageous embodiment, a first, arcuate catalytic element  18  is arranged in enclosing or containing manner in front of the aperture for exhaust gas inlet  11 . As a result, all inflowing exhaust gases  21  are forced to pass through the first catalytic element before they flow farther into the interior  19  of housing  10 . Cleaning unit  13  is positioned directly opposite first catalytic element  18  and inlet aperture  11 . This cleaning unit  13  contains a 2-dimensional catalytic element  14  in the form of a perforated panel  14  behind which a bowl-shaped reflection means is arranged. Cleaning unit  13  and catalytic element  18  may be secured in fixed manner inside housing  10  by a welded connection, a clamp, or a positive locking fixture (not shown). In the present case, aperture  16  of reflection means  15  is not partly, but entirely covered by catalytic element  14 . To this end, catalytic element  14  may also be affixed to reflection means  15  via a welded connection, a clamp or a positive locking fixture. As was indicated previously, at least a large fraction of the inflowing exhaust gases  21  is directed into cleaning unit  13  via first catalytic element  18 . There, the gases then flow through catalytic element  14  in the cleaning unit for the first time. The partially converted exhaust gases are then reflected back to catalytic element  14  by reflection means  15 . Consequently, the reflected exhaust gases  22  must pass through catalytic element  14  again to reach cleaning unit  13 . Now, a portion of exhaust gases  23  may escape to the outside through the aperture for exhaust gas outlet  12 , which is provided in housing  10  in the upper area of exhaust system  100  and opposite inlet aperture  11 . The remaining portion of the exhaust gases is mixed with the cool, inflowing exhaust gases  21  and sent through cleaning unit  13  again. This embodiment is particularly suitable for two-stroke engines. 
     FIG. 2  provides a clearer illustration of the operating principle of the exhaust system  100  according to the invention, since here the inflowing, reflected and outflowing exhaust gases  21 ,  22  and  23  are represented as arrows. This figure shows a longitudinal section through exhaust system  100  in the area of inlet and outlet apertures  11 ,  12  of  FIG. 1 . As may be clearly seen, inflowing exhaust gases  21  must pass through catalytic element  18  to reach the interior  19  of housing  10 . The arcuate design of first catalytic element  18  causes the major portion of the exhaust gases to be directed to the additional catalytic element  14  in cleaning unit  13 . Moreover, catalytic element  14  extends above and below arcuate catalytic element. In this way, it may be ensured that most of the inflowing exhaust gases  21  indeed does also pass into cleaning unit  13 . In the present case, cleaning unit  13  is not designed as an integral component of housing  10 . Therefore, bowl-shaped reflection means  15  has its own floor  17 . This is essentially conformed to the shape of housing  10  behind it  10 . As a result, it does not have a corrugated or relief shape, because it is flat. The partially converted exhaust gases, which have already passed through catalytic element  14 ,  18  twice, are directed through catalytic element  14  again, but this time from the inner side. A fraction of these reflected exhaust gases  22  is mixed with fresh, inflowing exhaust gases  21 , another part is directed to the outside through the aperture for exhaust gas outlet  12 , and a third fraction passes through arcuate first catalytic element  18  again and is then reflected back to cleaning unit  13  by the housing wall. This constant reflection of the exhaust gases in housing  10  also provides good noise damping. If further noise damping is desired, additional reflection or absorption means may be provided in the free, upper area of interior space  19 . Since the exhaust gases generally flow through cleaning unit  13  several times before escaping through exhaust system  100 . A high conversion rate is achieved even with 2-dimensional catalytic elements  14 ,  18 . 
   The embodiment of  FIG. 3  illustrates an exhaust system  100  according to the invention that includes a total of three cleaning units  13 ,  13 ′. Of these, as in the configuration of  FIGS. 1 and 2  described previously, cleaning unit  13  is arranged opposite the aperture for exhaust gas inlet  11 . But cleaning unit  13  is constructed as an integral part of housing  10 . In this case, housing  10  serves as floor  17  of reflection means  15 . The bottom curvature of bowl-shaped reflection means  15  is also replaced by the existing curvature of the housing (see bottom left corner of housing  10 ). Only the top curvature of reflection means  14  must be provided as an extra part in housing  10 . This extra part may be permanently secured to the housing wall as a curved panel member. This provision not only enables the amount of materials used to be reduced, it also provides a means for improved heat dissipation over the cool surface of the housing. To this end, floor  17  may also be corrugated, so that the surface and therewith the heat dissipation is increased. Additionally, a corrugated or relief-shaped floor  17  prevents reflected exhaust gases  22  from being forced through catalytic element  14  in varying volumes. 
   In addition, the two further cleaning units  13 ′ are arranged on the housing wall above and below the aperture for exhaust gas inlet  11  (opposite cleaning unit  13 ). These further cleaning units  13 ′ further increase the number of desirable reflections in housing  10 . In this way, the conversion rate may also be improved. The two further cleaning units  13 ′ each include a catalytic element  14 ′ and a reflection means  15 ′. However, it is also conceivable that the two cleaning units  13 ′ include only one catalytic element  14 ′ that extends fully over both apertures (of reflection means  15 ′). The advantage of this would be that, exactly in the embodiment of  FIGS. 1 and 2 , inflowing exhaust gases  21  would be forced to pass through a catalytic element at least once at all events before leaving exhaust system  100  again. The two further cleaning units  13 ′ are each designed as integral components of housing  10 . 
   Also in  FIG. 3 , a protrusion  20  is shown, via which cleaning units  13 ,  13 ′ are clamped or hooked inside housing  10 . However, these protrusions  20  might also be replaced by a welded seam, via which catalytic elements  14 ,  14 ′ and/or cleaning units  13 ,  13 ′ themselves are permanently affixed to the housing. 
   Various mesh-like catalytic elements  14 ,  14 ′,  18  are represented in detail in  FIG. 4   a  to  c .  FIGS. 4   a  and  4   b  show cutaway sections of the respective meshes in views from above and from the left. These show respectively a horizontal and a vertical section through the corresponding meshes in  FIG. 4   a  and  4   b . Meshes  30  are themselves made up of horizontal retaining strands  31  and vertical retaining strands  32  that are interlaced or interwoven with each other. 
   In mesh  30  in the embodiment in  FIG. 4   a , two adjacent, horizontal retaining strands  31  are simply woven together by vertical retaining strands  32  extending transversely thereto. In this example, a horizontal retaining strand  31  passes alternatingly over and under vertical retaining strands  32 , which are arranged directly beside each other. The same also applies for vertical retaining strands  32 , which pass over and under two adjacent horizontal retaining strands  31 . As a result, this mesh  30  has quite normal woven structures, wherein for example horizontal retaining strands  31  resemble a monofilament weft. 
   To increase the mechanical strength and resilience of mesh-like catalytic element  14 ,  14 ′,  18 , horizontal retaining strands  31  are permanently connection to vertical retaining strands  32  at their common contact points  33 . This may be effected for example by welding, soldering or similar. These measures may serve to prevent damaging and noisy resonances in mesh  30 . 
   In  FIG. 4   c , mesh  30  of  FIG. 4   a  is shown in three dimensions. In this way, a single representation serves to illustrate clearly the paths of the individual horizontal and vertical retaining strands  31 ,  32  with respect to each other. 
   In the variant of mesh  30  shown in  FIG. 4   b , each horizontal retaining strand  31  passes over two adjacent, vertical retaining strands  32  and then passes under the next two adjacent vertical retaining strands  32 . Two adjacent horizontal retaining strands  31  pass round opposite sides of one vertical retaining strand  32 , thus substantially improving the stability of mesh  30 . Even these vertical and horizontal retaining strands  31 ,  32  may also be attached permanently to each other at their contact points  33 . This lends woven structures to mesh  30  also. 
   Of course, other arrangements for weaving and interlacing the horizontal and vertical retaining strands  31 ,  32  with each other are conceivable and feasible for creating a mesh  30 . A combination of perforated panel and mesh  30  may also be used for catalytic elements  14 ,  14 ′,  18 . 
   It should also be noted that the exhaust system  100  according to the invention may be used as a complete exhaust system, or also as a starting, middle, or ending installation for an existing partial exhaust system. In the same way, two or more flat or planar catalytic elements  22  may be used at the same time. Many combinations of the various embodiments are possible, unless they are incompatible for technical reasons. 
   In conclusion, it should be noted that the exhaust gas system  100  according to the invention may also be equipped with technical features other than those described here, which however fulfil the same function. 
   KEY TO LEGEND 
   
       
         100  Exhaust system 
         10  Housing 
         11  Aperture for exhaust gas inlet 
         12  Aperture for exhaust gas outlet 
         13  Cleaning unit 
         13 ′ Additional cleaning unit 
         14  Catalytic element of  13   
         14 ′ Catalytic element of  13 ′ 
         15  Reflection means of  13   
         15 ′ Reflection means of  13 ′ 
         16  Aperture of  15   
         17  Floor of  15   
         18  Additional catalytic element 
         19  Interior or interior space of housing  10   
         20  Protrusion 
         21  Arrow indicating inflowing exhaust gases 
         22  Arrow indicating reflected exhaust gases 
         23  Arrow indicating outflowing exhaust gases 
         30  Mesh 
         31  Vertical retaining strand 
         32  Horizontal retaining strand 
         33  Contact points of  31  and  32