Catalytic exhaust muffler for motorcycles
BACKGROUND OF THE INVENTION 
     This invention relates to mufflers and particularly to mufflers for motorcycles which include structure for fluid treatment in addition to structure for silencing. Catalytic converters for treating automotive exhaust gases are available in a variety of configurations. Although such devices are commonly provided as a separate unit in addition to the usual muffler, it is known to provide a combined muffler and catalytic converter unit as taught by U.S. Pat. No. 3,445,196, for example. Providing a catalytic converter for use on motorcycles has been very difficult for several reasons. There is very limited space available in the region of the engine exhaust manifold such that an attempt to locate a catalytic converter near the manifold, where it operates most effectively, could result in too much heat too close to the driver&#39;s body and exhaust pulses of such magnitude and frequency that it would be most difficult to protect the catalyst element from being damaged thereby. Because of space limitations, a desire to shield the driver and any rider from excessive heat, and esthetic considerations, it would seem desirable to mount a catalyst element within a muffler housing. However, many problems are presented. These include reduction of the space available for sound treatment, increased backpressure, damage to relatively fragile catalyst elements by the exhaust pulses which are especially severe, and a difference in the coefficients of thermal expansion between the muffler housing and the catalyst element. 
     SUMMARY 
     It is among the objects of the present invention to provide an improved muffler which overcomes the problems of the prior art. The improved muffler incorporates a catalyst element and includes structure which dampens the exhaust pulses going to the catalyst element. The invention uses a radial flow catalyst element, preferably of the type having a wound ceramic yarn substrate. Such an element contributes a minimum of backpressure while aiding in noise suppression by changing the direction of flow of the exhaust gases. In one embodiment, where the muffler housing has an oval cross-section, inlet and outlet tubes are welded to a metal housing. Substantial portions of the tubes which extend toward each other within the housing are perforated so that exhaust gases may flow radially back and forth between the tubes and the chambers formed between the outside surface of the tubes and the inside surface of the housing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal sectional side view of a muffler device containing a monolithic catalyst element and having a circular cross-sectional configuration; 
     FIG. 2 is a longitudinal sectional side view of a muffler device containing a monolithic catalyst element and having an oval cross-sectional configuration; 
     FIG. 3 is a sectional view taken on line 3--3 of FIG. 2; and 
     FIG. 4 is a longitudinal sectional side view of a muffler device containing a pellet type catalyst element and having a circular cross-sectional configuration. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, the muffler device indicated generally at 10 includes a tubular housing or wrapper 12 which may be formed of type 304 stainless steel or other material suitable for the high temperatures and corrosive environment to which the device is subjected. A tubular heat shield 14 is preferably welded to the ends of wrapper 12 and radially spaced therefrom to provide insulation. The inlet end of the muffler 10 comprises an inlet tube 16 for receiving exhaust gases which is joined to the wrapper 12 by an inlet transition and end cap member 18. The member 18 is welded to the inlet tube at 20 and to the wrapper at 22. The inner end 16&#39; of the tube 16 is welded at 24 to a first bulkhead member 26 which closes off its inner end and centers the tube 16 relative to the wrapper 12. Perforations 28 in the tube 16 and perforations 30 in bulkhead 26 permit exhaust gases inside tube 16 to move radially outwardly into a first chamber 32 and thence axially forward into second chamber 34. An axially extending flange 36 on bulkhead member 26 is free to slide along the inner wall of the wrapper 12 in response to dimensional changes produced by thermal differences. The bulkhead 26 is welded at 38 to an annular ring member 40 which is welded at 42 to a second bulkhead member 44. The second bulkhead member 44 is unperforated and has an outer flange portion 46 which can slide along the inner wall of wrapper 12 and a recess portion 48 which receives the inlet end of a monolithic catalyst element 50. The area between the first and second bulkheads 26, 44 defines the second chamber 34 while the areas inside and outside the catalyst element 50 define third and fourth chambers 54, 56. The downstream end of the catalyst element 50 is supported in a recess 58 in an end cap member 60 which blocks the axial flow of exhaust gases and forces them to travel radially outwardly through the catalyst 50 from chamber 54 to chamber 56. The end cap 60 is carried at one end of a perforated tube member 62 to which it is welded at 64. A third bulkhead 66, which includes perforations 67, is welded to tube 62 near its center and includes a flange portion 68 in sliding contact with the inner wall of wrapper 12. A fourth bulkhead 70, which includes openings 72 is welded to tube 62 at 74 and includes a flange portion in sliding contact with the inner wall of wrapper 12. An outlet pipe 76 telescopes inside tube 62 and is welded thereto at 74. The inlet end of outlet pipe 76 is closed by a plate 78. The pipe 76 includes side openings 80 and is welded to the outlet transition and end cap member 82 at 84. The holes 67 in bulkhead 66 permit the passage of a portion of the gases while the remainder of the gases leaving chamber 56 move radially inwardly through a first set of sound abatement perforations 86 to chamber 88 formed in the inside of tube 62. The gases then move radially outwardly through a second set of perforations 90 to chamber 92 from whence they pass through axial openings 72 in bulkhead 70 to chamber 94. The gases then again move radially inwardly through elongated openings 80 in outlet tube 76 and are exhausted from the outer end 76&#39; of tube 76. 
     From the preceding description, it is readily evident that the hot exhaust gases continually change their direction. Each direction change absorbs some of the energy in the gas and thus reduces the sound of the gases in addition to evening out the exhaust pulses and reducing the impact of the gases on the catalyst element 50. Furthermore, since the internal elements are only welded to the outer housing ends 18, 82 at 20 and 84, the forces which might be applied to rigidly affixed internal bulkheads as a result of the extreme temperature differences produced by the catalytic converter 50, are obviated. The metal members which are internal to the housing 12 could be expected to become hotter during use and thus expand axially toward the ceramic element 50 at a greater rate than the radially adjacent portions of the housing. However, the ceramic element 50 would have a lower temperature coefficient of expansion than the portions of the metal housing which are radially adjacent to it and would thus tend to offset the higher expansion of the inner metal elements which support it. Since it is desirable to maintain the monolith retaining elements 48, 58 in firm contact with the monolith 50, the axial lengths of the various metal and ceramic elements are selected so that the axial expansion of the inner metal and ceramic members at the operating temperature of the unit will be equal to or slightly greater than the expansion of the outer housing 12. Since motorcycle engines can tolerate very little back pressure in the exhaust system, the various openings in the unit are sized to minimize back pressure. For example, in the disclosed embodiment, where the inlet and outlet tubes 16, 76 each have an open area of 2.76 in 2 , the inlet tube perforations 28 have an open area of 4.2 in 2 , the spaced perforations 30 have an open area of 3.7 in 2  and the ceramic element 50 has an I.D. open area of 6.3 in 2  and an O.D. open area of 8.8 in 2 . The annular area of chamber 56 is 4.2 in 2 . The open area of holes 67 and 72 in each of the bulkheads 66, 70 is 3.7 in 2  while the area of the sound abatement holes 86 and 90 totals 6.0 in 2  and the area of openings 80 totals 10.1 in 2 . 
     The embodiment 100 of FIG. 2 is quite similar to the embodiment 10 of FIG. 1 but is oval, as seen in the FIG. 3 section view, rather than round so that it may be used in locations where a smaller dimension in one direction is required. The gas flow varies somewhat from FIG. 1 in that the incoming gases move radially outwardly through perforations 128 in tube 116, pass through varying size apertures 130, 130&#39; in bulkhead 131 and return inwardly through perforations 133 in tube 135 before passing through catalyst element 150 and openings 137 in bulkhead 139. The gases exiting the openings 137 diffuse partially through openings 167 in bulkhead 166 and holes 190 and partially through sound abatement holes 186 before exiting from outlet tube 176. The even number reference characters from 110-190 shown in FIG. 2 correspond to the like number reference characters 10-90 in FIG. 1. As in FIG. 1, the catalyst element 156 is mounted in between a pair of metallic retaining members 144, 160 which are free to expand relative to the housing 112 with increases in temperature. Suitable open areas for the various portions of the FIGS. 2 and 3 design include 2.4 in 2  for each of the tubes 116, 176; 3.0 in 2  for each of the sets of holes 128, 133; 3.5 in 2  for openings 130, 137 and 167; and 5.2 in 2  for the inner surface of ceramic monolith 150; 8.6 in 2  for the outer monolith surface; and 4.2 in 2  for each of the sets of holes 186, 190. 
     The embodiment 210 shown in FIG. 4 is similar to the embodiments of FIGS. 1 and 2 in that the gas flow is caused to move alternately in a radially inward and radial outward fashion from the time it enters inlet tube 216 until the time it leaves outlet tube 276. However, the catalytic converter member 250 is not a ceramic monolith, but rather, a pellet type including an outer perforated screen 251, an inner perforated screen 253, and a plurality of catalyst coated pellets 255 packed between the two screens. The inlet bulkheads 257,259 which support the inlet tube 216 for free axial expansion relative to housing 212 have openings 261,263 through which gases passing outwardly through apertures 265 may be directed through openings 267 in converter support bulkhead 269 to outer chamber 271 and thence through the converter to inner chamber 273. Although the chamber 273 communicates directly with the outlet tube 276, a plurality of apertures 275 permit a portion of the gases to flow into and out of closed chamber 277 to provide some sound attenuation. As in the embodiments of FIGS. 1 and 2, provisions for accommodating thermal expansion are also important in the FIG. 3 embodiment. The right end of the outer converter screen 251 and the left end of outlet tube 276 are welded to bulkhead member 279. The left end of the screen 251 is free to slide axially along the walls 269&#39; of a cupped recess portion of bulkhead 269. The inner screen 253 is welded to outer screen 251 at its left end but is free to slide axially inside bulkhead 279 at its right end. The bulkhead member 279 is generally corrugated, the corrugations providing a degree of flexibility serving to accommodate axial movement of tube 276, and inner screen 251. Suitable open areas for the various portions of the FIG. 3 design include 1.48 in 2  for each of the tubes 216,276; 1.55 in 2  for the openings 261,263 in each of the bulkheads 257,259; 3.2 in 2  for the openings 267 in bulkhead 269; 17.3 in 2  for the open area of screen 251; 8.2 in 2  for the open area of screen 253; and 3.6 in 2  for the apertures 275 in outlet tube 276. Although the housing 212 is not shown as being mounted within an outer heat shield, as shown in FIGS. 1 and 2 at 14,114, such a shield could certainly be used, depending on the operating environment.