Abstract:
The present specification discloses a muffler for attenuating exhaust noise from an internal combustion engine. The muffler includes a main chamber, an intermediate pipe, a diffuser chamber, a spark arrester, and an exhaust pipe. Exhaust gases from the engine enter the main chamber after which they flow through the intermediate pipe. The intermediate pipe extends back through the main chamber to the diffuser chamber. The spark arrester is a screen held within the exhaust muffler and removably coupled to the exhaust pipe. Gases flow through the screen to separate hot particles. The exhaust pipe extends within the main chamber, completing a 180° bend within the chamber before exiting the main chamber and the muffler. An outer enclosure may surround the muffler. A method of attenuating exhaust gas noise is also disclosed.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to exhaust mufflers for internal combustion engines and, more specifically, to a compact exhaust muffler for small vehicles. 
     BACKGROUND OF THE INVENTION 
     Mufflers with spark arresters attenuate exhaust gas noise and sparks from internal combustion engines. However, mufflers add size, weight, and power-robbing backpressure to an engine. Small vehicles, such as all terrain vehicles (ATVs), motorcycles, and snowmobiles, need high power-to-weight ratios for optimum performance. Small size and low weight help the driver maneuver the vehicle through off-road obstacles. A high power-to-weight ratio helps the vehicle climb and accelerate, and increases available load-carrying capacity. While an effective muffler adds weight and reduces horsepower, quiet operation without exhaust spark emissions not only enables the vehicle to meet mandatory government regulations, but increases the vehicle acceptance by both the user and others wishing to enjoy the outdoor environment. 
     Typical commercially available mufflers reduce noise levels to regulation standards. However, the noise level may still not be acceptable to users and others in the vicinity. Manufacturers may be reluctant use mufflers that decrease the noise level much below government mandates since such reductions usually result in an increase in muffler weight, size, and/or backpressure. Muffler noise reductions may be increased with additional baffles and chambers or by increasing size—the very changes that also increase weight and backpressure. Such changes also increase the complexity and cost of manufacture. 
     Backpressure occurs when exhaust gases do not have a direct, easy-flow exit route. The engine must push the gases out through a tortuous (although noise-canceling) path of chambers, baffles, tubes, and turns. Thus backpressure restricts power that would have been available to propel the vehicle. 
     Therefore, a need exists for a muffler that decreases backpressure without increasing noise, or conversely, that decreases noise without increasing backpressure. Further advantages would result with simplified manufacturing and lighter weights. 
     SUMMARY OF THE INVENTION 
     The present invention provides a muffler for a small vehicle of a size that fits within the envelope of the current systems. The muffler reduces noise, weight, and complexity. It also decreases exhaust gas backpressure. 
     The present invention provides a muffler for attenuating exhaust gas noise from an internal combustion engine. The muffler includes a main chamber and an exhaust pipe. Exhaust gases are channeled into the main chamber from the engine. The main chamber, in the preferred embodiment, encloses at least a third of the volume of the muffler. The exhaust pipe is in fluid communication with the main chamber. The exhaust pipe extends through the main chamber and receives exhaust gases from the main chamber (preferably via a diffusion chamber). 
     In one aspect of the invention, an intermediate pipe is provided. The intermediate pipe is in fluid communication with the main chamber (preferably via a transition chamber) at its first end and with the exhaust pipe at its second end. The intermediate pipe extends through the main chamber. 
     In a further aspect of the invention, the exhaust pipe includes a bend in a mid portion thereof as it extends within the main chamber. Preferably, the bend substantially completes a 180° turn within the main chamber. With such bend, the exhaust pipe is longer than the main chamber. 
     In still a further aspect of the invention, a diffusion chamber is situated adjacent the main chamber. Gases enter the diffusion chamber after leaving the main chamber and before entering the exhaust pipe. The diffusion chamber may be smaller than the main chamber. 
     The preferred embodiment of the invention also includes a spark arrester disposed within the diffusion chamber and connected to the exhaust pipe. Gases pass through the spark arrester before entering the exhaust pipe. In one aspect of the invention, the spark arrester is removable from the outside of the muffler. 
     The present invention may also be defined as a muffler for an internal combustion engine including a chamber, a first pipe portion, a second pipe portion, and a muffler exit. Exhaust gases are channeled into the chamber from the engine. The chamber includes a chamber exit. The first pipe portion is in fluid communication with the chamber exit. The first pipe portion extends through at least half of the length of the chamber. The second pipe portion is also in fluid communication with the first pipe portion. The second pipe portion also extends through at least half of the length of the chamber. The muffler exit is in fluid communication in the second pipe portion. Exhaust gases are channeled from the chamber through the first pipe portion to an opposite end of the chamber and then through the second pipe portion and out of the muffler. 
     The present invention further includes a method of attenuating exhaust noise of an internal combustion engine. The method includes channeling exhaust gases from the engine into the main chamber. The gases are then channeled from the main chamber into an exhaust pipe that extends through the main chamber. The exhaust pipe is longer than the length of the main chamber, as the pipe includes a bend within the main chamber. Finally, the gases are channeled from the pipe to an exhaust exit. 
     In the preferred method of the invention, the gases are channeled through an intermediate pipe between the main chamber and the exhaust pipe. The gases are also channeled through a transition aperture to a diffusion chamber between the intermediate pipe and the exhaust pipe. Thus, the gases flow from the engine to the main chamber out of the main chamber exit (e.g., through the transition aperture and transition chamber), through the intermediate pipe into the diffuser, through a spark arrester disposed between the main chamber and the exhaust pipe in the diffuser, and through the exhaust pipe including surrounding the bend within the exhaust pipe to exit the external portion of the muffler. 
     In still a further aspect of the invention, an apparatus for exhaust noise attenuation is provided. The apparatus includes a muffler and an outer enclosure. The muffler has an exhaust entrance port, a first chamber, and an exhaust exit port. The outer enclosure is spaced from and at least substantially encloses a majority of the muffler. The muffler exhaust port dumps exhaust into the outer enclosure. The outer enclosure further includes an enclosure exit port. 
     The further aspect of this embodiment of the invention, the muffler includes at least two walls. The outer enclosure is separated from the two walls. The outer enclosure forms a chamber between the two walls and the outer enclosure. Preferably, the enclosure exit port is disposed through a wall of the enclosure removed from the exhaust exit pipe port. 
     The muffler with the spark arrester arrangement may also be defined as including an exhaust entrance port, a diffuser chamber, a diffuser chamber exit port, and a spark arrester. The diffuser chamber is in fluid communication with the entrance port. The spark arrester is at least partially disposed within the diffuser chamber. It is coupled to the diffuser chamber exit port. The spark arrester includes a screen through which exhaust gases pass to flow through the diffuser chamber exit port. The screen is removably coupled to the diffuser chamber such that it can be removed from the muffler. 
     Further aspects of the spark arrester included with the muffler include accessibility from outside of the muffler. The spark arrester forms a tube shape having two ends. One end of the spark arrester engages the diffuser chamber exit port. The other end has a cap thereon. The end with the cap is adjacent the diffuser chamber sidewall. The cap engages the end of the diffuser chamber and end of the muffler. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
         FIG. 1  is a cross-sectional view of two halves of a prior-art muffler illustrating the various chambers; 
         FIG. 2  is a side elevational view of the prior-art muffler shown in  FIG. 1 ; 
         FIG. 3  is a perspective view of a muffler of the present invention showing the outer shell in phantom; 
         FIG. 4  is a cross-sectional view of the muffler; 
         FIG. 5  is an isometric view of the spark arrester element; 
         FIG. 6  is a side elevational view of the cage portion of the spark arrester; 
         FIG. 7  is a schematic view of the muffler including the outer enclosure; 
         FIG. 8  is a schematic view of a muffler with a complete enclosure; 
         FIG. 9  illustrates the enclosure of  FIG. 8  with the addition of a removable spark arrester; 
         FIG. 10  illustrates the details of the spark arrester illustrated in  FIG. 9 ; 
         FIG. 11  is an isometric view of the spark arrester; 
         FIG. 12  is a side elevational view of an alternate spark arrester with a tapered screen; 
         FIG. 13  is a schematic view of an enclosed muffler and pipe; 
         FIG. 14A  is a schematic view of an enclosed muffler having a catalytic converter; 
         FIG. 14B  is a schematic view a variation of the muffler of  FIG. 14A ; and 
         FIG. 14C  is a schematic view a further variation of the muffler of  FIG. 14A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An understanding of a typical prior-art muffler helps develop an appreciation for the present invention. A standard muffler  10  such as that used with ATVs is illustrated in  FIGS. 1 and 2 . Muffler  10  has a generally cylindrical shape with an inlet pipe  12  and an outlet pipe  26  at opposing ends thereof. Various chambers are situated between inlet pipe  12  and outlet pipe  26 . In the illustrated example, six chambers are included. Inlet pipe  12  extends through an outer shell  40  to direct exhaust gases from the engine into first chamber  14 . First chamber  14  is generally cylindrical in shape and is bounded by an inner shell  42 , which is secured to outer shell  40 , as well as the first end of outer shell  40  and a wall between first chamber  14  and second chamber  16 . The diameter of first chamber  14  is about twice its length. First chamber  14  creates a small expansion chamber for hot gases from inlet pipe  12 . These gases are then directed through second chamber inlet pipe  28  into second chamber  16 . 
     Second chamber  16  is also cylindrical in shape, but has a length approximately equal to its diameter. Furthermore, second chamber  16  includes second chamber inlet pipe  28  and second chamber outlet pipes  30  therein. These pipes extend nearly the entire length of the chamber. Exhaust gases enter second chamber inlet pipe  28 , pass through the pipe and into second chamber  16  at the far end thereof. The gases then circle back to the front end of the chamber to enter into the three second chamber outlet pipes  30 . These pipes are smaller in diameter than inlet pipe  28 . However, three pipes are included to channel the gases out of second chamber  16  and into third chamber  18 . 
     Third chamber  18  is much smaller than second chamber  16 . Third chamber  18  is bounded on its downstream end by a diffuser wall  32 . Diffuser wall  32  includes diffusion openings to circulate the air into fourth chamber  20 . 
     Fourth chamber  20  is approximately the same size as third chamber  18 , but includes a separation wall  34  to separate any sparks from the remaining exhaust gases. The debris (e.g., carbon deposits) is collected beneath the separator wall  34  and may be removed from muffler  10  with soot plug  46  as illustrated in  FIG. 2 . Exhaust gases leave fourth chamber  20  through fifth chamber inlet pipe  36 . 
     Fifth chamber inlet pipe  36  directs gases into fifth chamber  22  in a manner similar to second chamber inlet pipe  28  with regard to second chamber  16 . Likewise, fifth chamber  22  also includes fifth chamber outlet pipes  38  that channel the exhaust gases through another wall and into sixth chamber  24 . One inlet pipe  36  is utilized and three fifth-chamber outlet pipes  38  are provided. 
     Sixth chamber  24  is used to collect the exhaust gases for channeling them to outlet pipe  26 . 
     Note that an inner shell  42  is secured to outer shell  40  with spacing between the two shells. An absorption layer  44  is placed within the spacing for noise attenuation. Absorption layer  44  may include standard glass packing materials. 
     The prior-art muffler accomplishes the goals of reducing noise levels to within legal limits and provides a spark arrester feature with a soot cleaning plug  46 . The muffler is also of a size that allows it to fit within the space required on the back of a small vehicle such as an all-terrain vehicle (ATV). However, further reductions in noise levels may be preferable for users and others in the vicinity of the user of the vehicle. Furthermore, the muffler introduces excessive back pressure such that the horsepower of the engine is adversely affected. Opening up the muffler to decrease the back pressure may be accomplished, but not without noise increases. 
     Furthermore, in situations where certain amount of back pressure is desired for optimum engine performance, the prior-art mufflers require that different designs be created and produced to tune the muffler for each engine application. This is the case, for example, with a family of ATVs of differing engines and engine sizes. Back pressure requirements would differ in each case. Thus, a different muffler must be designed and manufactured for each ATV. This will increase the cost of manufacture of the mufflers since each will necessarily be produced in lower quantities. This may not be an issue in many instances since the back pressure introduced by the muffler in order to create sufficient noise reduction is such that it exceeds the back pressure needed for optimum engine performance. In such instance, engine performance (horsepower) suffers, as explained above. 
     The present invention provides a muffler that has low back pressure with a device for changing the back pressure dependent upon the engine requirements for optimum horsepower. The system is also lighter weight and cheaper to manufacture than prior-art mufflers. This muffler and its various components are illustrated in  FIGS. 3-6 . 
     Referring first to  FIG. 3 , the internal construction of muffler  100  is illustrated. Muffler  100  includes an outer shell  102  (shown in phantom in  FIG. 3 ). Outer shell  102  includes a first end wall  104  at the upstream end of muffler  100  and a second end wall  106  at the downstream end. First and second inner walls  108  and  110  lying in plains parallel to end walls  104  and  106  are also provided within outer shell  102 . 
     End walls  104  and  106  along with inner walls  108  and  110  divide muffler  100  into three chambers: a main chamber  112 , a transition chamber  114 , and a diffusion chamber  116 . Main chamber  112  is the largest of the chambers and preferably comprises one-third to one-half or more of the total volume of muffler  100 . Transition chamber  114  is at one end of the muffler being bounded by first end wall  104  and first inner wall  108 . Diffusion chamber  116  is at the opposite end of muffler  100  being bounded by second inner wall  110  and second end wall  106 . Alternatively, any type of diffuser may be employed, whether a diffusion chamber, a diffuser wall, or other device. In the preferred embodiment a diffusion chamber is used as illustrated and described herein. 
     As can be seen in  FIGS. 3 and 4 , numerous pipes are situated within muffler  100 . An inlet pipe  118  is extends through first end wall  104  and first inner wall  108  to channel exhaust gases from the pipe leading from the engine of the vehicle into main chamber  112 . At the opposite end of muffler  100  exhaust pipe  120  allows the gases to escape muffler  100 . Two intermediate pipes  122  extend within muffler  100  and channel the gases between the transition chamber and the diffusion chamber. Intermediate pipes  122  are in fluid communication within transition chamber  114  and diffusion chamber  116 . These pipes extend through main chamber  112 , but are not open thereto. 
     An intermediate wall  124  is provided between intermediate pipes  122  at the portion thereof that is not adjacent exhaust pipe  120 . Intermediate wall  124  prevents early back flow through transitional aperture  138  as explained below. Exhaust pipe  120  extends from diffusion chamber  116  through main chamber  112 . A bend  126  allows exhaust pipe  120  to extend back through second inner wall  110 , through diffusion chamber  116  to exit through second end wall  106  at an external portion  128 . Exhaust pipe  120  is preferably stamped in halves and fitted together. Stamping the halves allows the straightforward manufacture of a pipe with a bend having a larger cross section so as to not restrict air flow. Alternate embodiments include a simple bent pipe to form pipe  120 . 
     A spark arrester  130  is secured to the end of exhaust pipe  120  that is in fluid communication with diffusion chamber  116 . An arrester aperture  132  is formed in second end wall  106  to allow spark arrester  130  to be removed and replaced within muffler  100 . 
     An inner shell  134  is provided within main chamber  112 , spaced from outer shell  102 . An absorption layer  136  is positioned between inner and outer shells  134  and  102 . Inner shell  134  includes perforations therein for effective noise attenuation with absorption layer  136 . 
     Note also that transitional aperture  138  is formed within first inner wall  108  to allow the flow of gases from main chamber  112  to transition chamber  114 . Transition aperture  138  is preferably at least as large as the diameter of inlet pipe  118 . 
     The basic flow of gases through muffler  100  will now be described. Exhaust gases from the engine enter inlet pipe  118 . Inlet pipe  118  directs the gases past transition chamber  114  into main chamber  112 . The volume of main chamber  112  is large enough to effectively create an expansion chamber for the gases. This is where the main noise attenuation occurs. Thus, this is the place where inner shell  134  and absorption layer  136  are positioned for noise attenuation. The gases travel throughout main chamber  112 , aided by intermediate wall  124 . The gases then must pass around the outer walls of exhaust pipe  120  and intermediate pipes  122  before they pass through transition aperture  138  to enter transition chamber  114 . Transition chamber  114  is relatively open and provides for little restriction to the flow of exhaust gases to then enter into intermediate pipes  122 . Intermediate pipes  122  flow through the body of main chamber  112  without being open thereto. Intermediate pipes  122  channel the exhaust gases from transition chamber  114  at one end of muffler  100  to diffusion chamber  116  at the other end. The exhaust gases then flow through spark arrester  130  and into exhaust pipe  120 . Exhaust pipe  120  then extends through the middle of main chamber  112  bending therein to exit back through diffusion chamber  116  and out second end wall  106 . The exhaust gases then exit external portion  128 . 
     The flow system described creates very little back pressure while creating superior noise attenuation. Noise attenuation is accomplished by routing the pipes through the main chamber where the gas first enters muffler  100  through inlet pipe  118 . The long pipes extending therethrough tend to attenuate the noise while not appreciably increasing back pressure. The system is also easy to manufacture from pipes and stamped metal channels. Furthermore, the muffler system created is lightweight. The system is also tunable for different engines by simply changing the spark arrester as described below in connection with  FIGS. 5 and 6 . 
       FIGS. 5 and 6  illustrate the details of spark arrester  130 . Spark arrester  130  includes a screen  140  wrapped around a cage  142 . Cage  142  is fixed to an endplate  144  that engages with second end wall  106  with fasteners  153  through holes  152 . A band  146  helps secure screen  140  to cage  142 . 
     Cage  142  is constructed with struts  148  and end rings  150 . The size of struts  148  are widened or narrowed in order to tune the back pressure of muffler  100  for optimum engine performance in a specific engine application. This is the only piece that need be changed in order to tune the muffler for a specific engine. Thus, muffler  100  can be used for an entire family of ATVs by simple changes to spark arrester  130 . This helps decrease the manufacturing costs as more units are produced that are the same. Thus, a specific spark arrester cage  142  may be different from one engine to the next, but the balance of muffler  100  is the same from one to another. 
     End ring  150  at the inner end of cage  142  is sized to fit over the outer diameter of the entrance to exhaust pipe  120 . The openings in screen  140  are such as to provide proper spark arrester function. The ability to remove spark arrester  130  also allows debris and soot material to be expelled from diffusing chamber  116  as may be required. Suitable fasteners  153  through holes  152  may be used to secure endplate  144  to second end wall  106 , which may include fastener receptacles therein. 
     Turning now to  FIG. 7 , an additional aspect of the present invention will be described.  FIG. 7  illustrates a muffler  154  that includes an outer enclosure  156  surrounding a majority thereof. Muffler  154  may be a standard muffler such as that illustrated in  FIGS. 1 and 2  or may be a muffler as described in connection with  FIGS. 3 and 4 . 
     Muffler  154  includes an inlet pipe  158 , a main body, and an outlet pipe  160 . Mounting brackets  162  are preferably secured to the top and side thereof for securement to a vehicle such as an ATV. 
     Enclosure  156  includes enclosure walls  164  that create an outer chamber  166 . Outer chamber  166  encloses a majority of muffler  154  and creates a place where exhaust gases are dumped from outlet pipe  160  before exiting into the environment. Outer chamber  166  is of a size sufficiently large such that expansion of gases is easily accomplished and additional back pressure is minimal. An enclosure exit  168  is provided. Enclosure exit  168  may be situated anywhere on enclosure walls  164 . Preferably, it is not in the direct line of exhaust exit from outlet pipe  160 . Thus, exhaust gases enter muffler  154  then exit through outlet pipe  160  to enter outer chamber  166  where the gases are further cooled and quieted before exiting enclosure exit  168 . This arrangement provides additional noise attenuation as well as a cooler interface between the user and the hot exhaust gases. Enclosure  156  also reduces heat at the user and machine parts interface to the extent that it preferably completely replaces any remote heat shields that are customarily attached to the outer wall of muffler  156  or to other parts of the machine to deflect heat from the muffler. External heat shields are often used to protect parts of the machine or vehicle that may overheat or even melt, such as the carburetor and plastic parts. The heat shielding effect of enclosure  156  reduces or eliminates the need for such additional shields. Noise reduction is accomplished through a reduction in exiting exhaust gas noise as well as attenuation of muffler shell noise. 
     Enclosure walls  164  may completely encompass muffler  154 , as illustrated in  FIG. 8 , or may partially encompass the muffler as illustrated in  FIG. 7 . In the case of complete enclosure, struts  170  between muffler  154  and enclosure walls  164  preferably hold the spaced relationship between the two members with mounting brackets  162  secured on enclosure walls  164 . Other arrangements are also possible that accomplish the purposes of having an outer chamber into which the exhaust gases are dumped before exiting the muffler system. 
     Turning now to  FIG. 9 , an embodiment of a completely enclosed muffler is illustrated. However, in this embodiment, a spark arrester is held within the enclosure body. A separate spark arrester may or may not be included within the main muffler  154 . Enclosure  156   a  includes an opening into which fasteners  153  secure spark arrester assembly  172 . Spark arrester assembly  172  is similar to spark arrester  130  except that assembly  172  also includes an exit pipe  174  secured directly to the outer end thereof. Spark arrester assembly  172  includes an endplate  144   a  with an aperture there through to which exit pipe  174  is secured, preferably by a weld. 
     The details of construction of spark arrester assembly  172  are illustrated in  FIGS. 10 and 11 . Spark arrester assembly  172  is constructed with a cage  142   a  having an inner cap  176 . Cap  176  may alternatively be at least partially replaced with screen material similar to screen  140   a  that surrounds cage  142   a . Clamps  146   a  secure screen  140   a  to cage  142   a . Fasteners are secured through holes  152   a  and mounting plate  144   a.    
     Exhaust gases within enclosure  156   a  go through screen  140   a , through endplate  144   a  and exit pipe  174 . 
     An alternate embodiment of a spark arrester assembly is also illustrated in  FIG. 12 . In this embodiment, the cage is eliminated. A screen tube is provided having an inner, closed end compressed together by rolling the screen material. Other fixation means to close the end may alternatively be employed. The outer end includes a clamp  146   b  that secures screen  140   b  to exit pipe  174   b . Exit pipe  174   b  is welded to mounting plate  144   b , but extends through such that the outer end of screen  140   b  can be clamped thereto. 
       FIG. 13  illustrates an alternate embodiment similar to  FIG. 9  with an elongated tail pipe  160   c  within outer enclosure  156   c . Tail pipe  160   c  extends from one end of muffler  154   c  some distance within outer enclosure  156   c , preferably to the other end of outer enclosure  156   c . A bend is preferably formed in pipe  160   c  such that a long length of pipe fits within outer enclosure  156   c . Spark arrester assembly  172   c  is situated out of direct alignment with the exhaust gas exit from pipe  160   c . Thus in this embodiment it is on the opposite side of outer enclosure  156   c . This long-pipe arrangement within enclosure  156   c  further quiets the exhaust noise of the entire muffler assembly with little additional backpressure. The additional silencing achieved with enclosure  156   c  may allow more design flexibility for muffler  154   c . Thus, a simpler, less expensive design may be employed. Muffler  154   c  in one embodiment includes an internal elongated pipe, while another embodiment omits such pipe. 
       FIGS. 14A-C  illustrate a muffler design variation including a catalytic converter  178 . Thus, in  FIG. 14A  catalyst  178   d  is positioned within muffler  154   d  between inlet pipe  158   d  and exhaust pipe  120   d . As exhaust gases run through catalyst  178   d , not only are pollutants (such as carbon monoxide and nitrous oxides) reduced, but noise is also diminished. In this embodiment, exhaust pipe  120   d  forms a 180 degree bend within muffler main  154   d . Exhaust pipe  120   d  joins with outlet pipe  160   d , which runs within enclosure  156   d  outside of muffler  154   d . Outlet pipe  160   d  preferably forms a 180 degree bend within enclosure  156   d  before dumping exhaust gases into enclosure  156   d  for exit through spark arrester assembly  172   d.    
     In a variation of the arrangement described above ( FIG. 14B ), a catalyst  178   e  is secured directly to an inlet pipe  158   e . Exhaust gases run through catalyst  178   e  as they enter muffler  154   e . The gases run through muffler  154   e , after which they exit through outlet pipe  160   e  and spark arrester assembly  172   e . In the preferred embodiment, outlet pipe  160   e  includes a 180 degree bend and an elongated pipe similar to that discussed above in connection with  FIG. 13 . 
     In a further variation ( FIG. 14C ), a catalyst  178   f  is secured to outlet pipe  160   f . In this embodiment, exhaust gases pass through catalyst  178   f  as they exit muffler  154   f . Again in this preferred embodiment, outlet pipe  160   f  includes a 180 degree bend and an elongated pipe. 
     The placement of catalyst  178  in any of the embodiments discussed herein may be made depending on the particular engine and its tuning requirements. In each embodiment herein catalyst  178  acts as an additional silencer in addition to its pollution control capabilities. 
     While the preferred embodiments of the invention have been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. For example, alternate shapes and pipes may be used. Different numbers and arrangements of pipes may also be employed. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined by reference to the claims that follow.