Patent Publication Number: US-2006010860-A1

Title: Catalyst-attached muffler for internal combustion engine

Description:
FIELD  
      The present invention relates to a muffler with catalytic converter which is provided with an expansion chamber and with an exhaust gas-purifying catalyst. In particular, the present invention relates to a muffler with catalytic converter which is suited for use in a small air-cooled two-stroke gasoline engine for use in a portable power working machine such as a brush cutter or a chain saw.  
     BACKGROUND INFORMATION  
      In recent years, even in an internal combustion engine such as a small air-cooled two-stroke gasoline engine to be employed in a portable power working machine such as a brush cutter or a chain saw, it is strongly demanded, as in the case of on-vehicle engine, to minimize the quantity of hydrocarbon (HC), carbon monoxide (CO), etc. in the exhaust gas. Accordingly, it has been proposed to mount an exhaust gas-purifying catalyst inside the muffler installed on the exhaust gas system of the engine so as to enable the exhaust gas to pass through the catalyst, thereby purifying (minimizing HC, CO, etc.) the exhaust gas (see JP Laid-open Patent Publication (Kokai) No.9-184418 (1997) and International Patent Publication No.9-507898 (1997)).  
       FIG. 4  illustrates, together with part of internal combustion engine  1 ′, one example of the conventional catalyst-installed muffler which is constructed as described above. The internal combustion engine  1 ′ shown in  FIG. 4  is a small air-cooled two-stroke gasoline engine of Schnürle type scavenging system which can be mounted as a power source on a portable power working machine such as a brush cutter or a chain saw. The displacement of the engine  1 ′ may be about 23 to 45 mL for example. This engine  1 ′ is provided with a cylinder  2 ′ having a domed combustion chamber  5 ′. The cylinder  2 ′ is provided with a large number of cooling fins  6 ′ on the outer circumference thereof Moreover, as shown in  FIG. 5 , the cylinder  2 ′ is provided with an exhaust port  10 ′ which is rectangular in cross-section having nearly rounded corners and which is tapered in configuration, i.e. the cross-sectional area of the exhaust port  10 ′ is gradually increased from the inner end (cylinder bore  2   a ′ side) to the outer end thereof. Although not shown in the drawing, the cylinder  2 ′ is further provided with an air-intake port and a scavenging port which are disposed in the known manner along the inner circumferential wall of the cylinder bore  2   a′.    
      On the outer side of the exhaust port  10 ′ of the cylinder  2 ′, there is attached a muffler  20 ′. This muffler  20 ′ is provided with a couple of outer wall panels  21 ′ and  22 ′, i.e. a left side outer wall panel  21 ′ and a right side outer wall panel  22 ′, which are hermetically jointed together with each other by means of flat lock seam, thereby defining an expansion chamber  25 ′. The left side outer wall panel  21 ′ located on the exhaust port  10 ′ side is provided with an opening  21   a ′ having approximately the same configuration as the outer end portion  10   a ′ of the exhaust port  10 ′. A reinforcing plate  28 ′ is fixedly mounted along the opening  21   a ′ by means of welding, etc. Further, the right side outer wall panel  22 ′ is provided with a recessed portion  22   a ′ which is located just in front of the exhaust port  10 ′. A closed-end cylindrical catalyst holder  40 ′ is interposed between the recessed portion  22   a ′ and the reinforcing plate  28 ′ so as to interconnect the recessed portion  22   a ′ and the reinforcing plate  28 ′. A heat-insulating/shock-absorbing plate  27 ′ is interposed between the catalyst holder  40 ′ and the recessed portion  22   a′.    
      At an inner end portion  41 ′ (on the exhaust port  10 ′ side) of the catalyst holder  40 ′, there is provided a mounting hole  40   a ′ for fitting and fixedly holding (by means of welding, etc.) a short cylindrical exhaust gas-purifying catalyst  30 ′. At an outer end portion  42 ′ (on the recessed portion  22   a ′ side) of the catalyst holder  40 ′, there are provided a suitable number of exhaust gas-passing holes  43 ′. The catalyst  30 ′ is formed of an oxidation catalyst comprising a gas-permeable metallic carrier and enclosed by a cylindrical shell  32 ′. This catalyst  30 ′ is held by the catalyst holder  40 ′ in such a manner that the central axis “O” of catalyst  30 ′ extending along the passing direction of exhaust gas is approximately aligned with the central line of the exhaust port  10 ′ (i.e. the outer end portion  10   a ′ thereof) and also with the central line of the catalyst holder  40 ′.  
      Further, the right side outer wall panel  22 ′ is provided, at a lower portion thereof, with an outlet port  29 ′ for discharging exhaust gas out of the muffler  20 ′. Although not shown in the drawing, a spark arrester screen, an exhaust gas-guiding member for regulating the discharge direction of exhaust gas, etc. may be mounted at this outlet port  29 ′.  
      In the muffler constructed as described above, the exhaust gas to be blown out of the exhaust port  10 ′ at nearly the sound velocity is guided, through the inner end portion  41 ′ of the catalyst holder  40 ′ positioned in the muffler  20 ′, to the catalyst  30 ′ and forced to pass through the interior of catalyst  30 ′ as shown by a solid arrow in  FIG. 4 . As the exhaust gas passes through the catalyst  30 ′, the quantities of HC, CO, etc. in the exhaust gas are minimized by the catalyst  30 ′. The exhaust gas that has passed through the catalyst  30 ′ is blown into the expansion chamber  25 ′ through the exhaust gas-passing holes  43 ′ of the catalyst holder  40 ′ and forced to expand and diffuse. As a result, the exhaust sound can be attenuated. The exhaust gas thus expanded and diffused in the expansion chamber  25 ′ is then permitted to move downward and discharged out of the muffler  20 ′ through the outlet port  29 ′.  
      In the case of the conventional catalyst-installed muffler  20 ′ however, since the cross-sectional configuration of the catalyst  30 ′ differs from the shape of opening of the exhaust port  10 ′ and from the cross-sectional configuration of the catalyst holder  40 ′ (i.e. the inner end portion  41 ′ thereof) as shown in  FIG. 5 , most of the exhaust gas is caused to pass through a central portion of the catalyst  30 ′, so that the catalyst  30 ′ existing at the peripheral portion thereof is substantially kept away from the flow of exhaust gas. Additionally, since the catalyst  30 ′ is located far away from the outer end portion  10   a ′ of the exhaust port  10 ′, it is impossible to take in the exhaust gas of higher temperature. Thus, the conventional catalyst-installed muffler  20 ′ is accompanied with the problem that the purifying capacity of the catalyst  30 ′ is not sufficiently utilized.  
      The present invention has been made in view of overcoming the aforementioned problem, and therefore an object of the present invention is to provide a catalyst-attached muffler for internal combustion engine, which is capable of utilizing the purifying capacity of catalyst as much as possible.  
     BRIEF SUMMARY OF THE INVENTION  
      With a view to realize the aforementioned object, the catalyst-installed muffler according to the present invention is basically constructed such that it comprises an expansion chamber, and a gas-permeable exhaust gas-purifying catalyst, that exhaust gas blown out of the exhaust port of an internal combustion engine is introduced into the expansion chamber through the catalyst, and that one end portion of the catalyst is fitted in an outer end portion of the exhaust port.  
      Preferably, the catalyst is cylindrical or prismatic in configuration, and an outer circumferential cross-sectional configuration of the catalyst is identical with an inner circumferential cross-sectional configuration of the outer end portion of the exhaust port.  
      Preferably, the catalyst is an oxidation catalyst formed of metallic carrier.  
      Preferably, the exhaust port is tapered in configuration, wherein the cross-sectional area thereof is expanded gradually from the inner end to the outer end thereof.  
      Preferably, the engine is a small air-cooled two-stroke gasoline engine.  
      According to the catalyst-installed muffler of the present invention, since the exhaust gas-purifying catalyst is cylindrical or prismatic in configuration, and an outer circumferential cross-sectional configuration of the catalyst is made identical with an inner circumferential cross-sectional configuration of the outer end portion of the exhaust port, it is possible to enable one end portion of the catalyst (i.e. the exhaust gas-introducing side thereof) to fit substantially hermetically in an outer end portion of the exhaust port, thereby making it possible to make the entire region of the catalyst available for the purification of the exhaust gas. As a result, it is now possible to utilize the purifying capacity of the catalyst to a maximum extent and hence to greatly enhance the exhaust gas purifying performance of the catalyst.  
      Further, since part of the catalyst is introduced into the exhaust port, the temperature of exhaust gas to be introduced into the catalyst can be kept higher as compared with the conventional muffler. As a result, the reactive capacity (activation) of catalyst can be enhanced, thus making it possible to further enhance the exhaust gas purifying capacity of the catalyst.  
      Furthermore, since the catalyst is positioned close to the exhaust port (combustion chamber), the distance between the distal end (exhaust gas discharging side) of the catalyst and the exhaust outlet port of muffler can be elongated. As a result, the temperature of exhaust gas at the exhaust outlet port of muffler can be much lowered. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional view schematically illustrating one embodiment of the catalyst-installed muffler according to the present invention together with part of the internal combustion engine to which this muffler is applied;  
       FIG. 2  is a cross-sectional view taken along the line II-II in  FIG. 1 ;  
       FIG. 3  is an enlarged view showing the exhaust port and peripheries thereof in the internal combustion engine shown in  FIG. 1 ;  
       FIG. 4  is a cross-sectional view schematically illustrating one example of the conventional catalyst-installed muffler together with part of the internal combustion engine to which this muffler is applied; and  
       FIG. 5  is a cross-sectional view taken along the line V-V in  FIG. 4 . 
    
    
     DETAILED DESCRIPTION  
      Next, one embodiment of the muffler according to the present invention will be explained with reference to the drawings.  
      Just like the conventional internal combustion engine shown in  FIG. 4 , an internal combustion engine  1  shown in  FIG. 1  is a small air-cooled two-stroke gasoline engine of Schnürle type scavenging system which can be mounted as a power source on a portable power working machine such as a brush cutter or a chain saw. The displacement of the engine  1  may be about 23 to 45 mL for example. This engine  1  is provided with a cylinder  2  having a domed combustion chamber  5 . The cylinder  2  is provided with a large number of cooling fins  6  on the outer circumference thereof. Moreover, the cylinder  2  is provided with an exhaust port  10  which is circular in cross-section as shown in  FIG. 2  and generally tapered in configuration (bell-like), i.e. the cross-sectional area of the exhaust port  10  is gradually increased from the inner end (cylinder bore  2   a  side) to the outer end thereof, excepting the outer end portion  10   a  of the exhaust port  10  whose inner circumferential cross-section is made cylindrical (having an inner diameter of Φx as shown in  FIG. 3 ). Although not shown in the drawing, the cylinder  2  is further provided with an air-intake port and a scavenging port, both of which are disposed in the known manner along the inner circumferential wall of the cylinder bore  2   a.    
      On the outer side of the exhaust port  10  of the cylinder  2 , there is attached a muffler  20 . This muffler  20  is provided with a couple of outer wall panels  21  and  22 , i.e. a left side outer wall panel  21  and a right side outer wall panel  22 , which are hermetically jointed together with each other by means of flat lock seam, thereby defining an expansion chamber  25 . The left side outer wall panel  21  located on the exhaust port  10  side is provided with an opening  21   a  having approximately the same configuration as the outer end portion  10   a  of the exhaust port  10 . Further, the right side outer wall panel  22  is provided with a recessed portion  22   a  which is located just in front of the exhaust port  10 . A closed-end cylindrical catalyst holder  40  is interposed between the recessed portion  22   a  and a terminal fringe portion of the opening  21   a  of the left side outer wall panel  21  so as to interconnect the recessed portion  22   a  and the terminal fringe portion of the opening  21   a . A heat-insulating/shock-absorbing plate  27  is interposed between the catalyst holder  40  and the recessed portion  22   a.    
      At an inner end portion  41  (on the exhaust port  10  side) of the catalyst holder  40 , there is mounted a short cylindrical exhaust gas-purifying catalyst  30  in such a manner that one end portion  30   a  thereof is substantially hermetically introduced into the interior of the outer end portion  10   a  of the exhaust port  10  and the other end portion  30   b  thereof (exhaust gas discharging side) is inserted into the inner end portion  41  of the catalyst holder  40  and fixedly secured thereto by means of welding for example. At an outer end portion  42  (on the recessed portion  22   a  side) of the catalyst holder  40 , there are provided a suitable number of exhaust gas-passing holes  43 . The catalyst  30  is formed of an oxidation catalyst comprising a gas-permeable metallic carrier and enclosed by a cylindrical shell  32 . This catalyst  30  is held by the catalyst holder  40  in such a manner that the central axis “O” of catalyst  30  extending along the passing direction of exhaust gas is approximately aligned with the central line of the exhaust port  10  (i.e. the outer end portion  10   a  thereof) and also with the central line of the catalyst holder  40 .  
      Further, the right side outer wall panel  22  is provided, at a lower portion thereof, with an outlet port  29  for discharging exhaust gas out of the muffler  20 . Although not shown in the drawing, a spark arrester screen, an exhaust gas-guiding member for regulating the discharge direction of exhaust gas, etc. may be mounted at this outlet port  29 .  
      In the muffler constructed as described above, the exhaust gas to be blown out of the exhaust port  10  at nearly the sound velocity is guided to the catalyst  30  which is partially inserted into the exhaust port  10  and forced to pass therethrough along the central axis “O” as shown by a solid arrow in  FIG. 1 . As the exhaust gas passes through the catalyst  30 , the quantities of HC, CO, etc. in the exhaust gas are minimized by the catalyst  30 . The exhaust gas that has passed through the catalyst  30  is blown into the expansion chamber  25  through the exhaust gas-passing holes  43  of the catalyst holder  40  and forced to expand and diffuse. As a result, the exhaust sound can be attenuated. The exhaust gas thus expanded and diffused in the expansion chamber  25  is then permitted to move downward and discharged out of the muffler  20  through the outlet port  29 .  
      According to the catalyst-installed muffler  20  of this embodiment, since the exhaust gas-purifying catalyst  30  is cylindrical in configuration and an outer circumferential cross-sectional configuration (circular) of the catalyst  30  (the shell  32 ) is made identical with an inner circumferential cross-sectional configuration (circular) of the outer end portion  10   a  of the exhaust port, it is possible to enable one end portion of the catalyst (i.e. the exhaust gas-introducing side thereof) to fit substantially hermetically in an outer end portion  10   a  of the exhaust port  10 , thereby making it possible to make the entire region of the catalyst available for the purification of the exhaust gas. As a result, it is now possible to utilize the purifying capacity of the catalyst  30  to a maximum extent and hence to greatly enhance the exhaust gas purifying performance of the catalyst  30 .  
      Further, since part of the catalyst  30  is introduced into the exhaust port  10 , the temperature of exhaust gas to be introduced into the catalyst  30  can be kept higher as compared with the conventional muffler. As a result, the reactive capacity (activation) of catalyst can be enhanced, thus making it possible to further enhance the exhaust gas purifying capacity of the catalyst.  
      Furthermore, since the catalyst  30  is positioned close to the exhaust port  10  (combustion chamber), the distance between the distal end (exhaust gas discharging side) of the catalyst  30  and the exhaust outlet port  29  of muffler  20  can be elongated. As a result, the temperature of exhaust gas at the exhaust outlet port  29  of muffler  20  can be much lowered.  
      While, in the foregoing description, one embodiment of the present invention has been explained in details for the purpose of illustration, it will be understood that the construction of the device can be variously modified without departing from the spirit and scope of the invention claimed in claims attached herewith.  
      For example, the muffler of the present invention can be also applied likewise to a four-stroke engine other than the air-cooled two-stroke gasoline engine which is employed in the aforementioned embodiment.  
      Further, with respect to the exhaust gas-purifying catalyst, it is also possible to employ a catalyst in the form of a foamed molded body or a ceramic carrier, other than the aforementioned example (oxidation catalyst formed of metallic carrier).