Abstract:
A silencer ( 10 ) includes a polymeric casing ( 12 ) spaced from at least one inlet pipe and at least one outlet pipe by polymeric spacers ( 52 ). The polymeric spacers are made from a different material to that of the polymeric casing. The polymeric spacers are heat-resistant and have a higher melting point than the polymeric casing. The polymeric spacers are made of a softer material than the polymeric casing, and the polymeric spacers are capable of being deformed to accommodate irregularities in the shape of the inlet and outlet pipes.

Description:
BACKGROUND TO THE INVENTION 
     THIS invention relates to a silencer for a motor vehicle. 
     Free flow exhaust systems are widely used in the motor industry as they tend to improve the performance of a motor vehicle when compared to conventional exhaust systems. A drawback associated with free flow exhaust systems, however, is that the exhaust noise levels are considerably higher than for conventional exhaust systems, and are often higher than legal limits. 
     Attempts have been made to provide silencers or mufflers to reduce the noise of free flow exhaust systems to acceptable levels. Traditionally, this has entailed providing an elongate silencer which is positioned around a perforated section of the exhaust pipe to dampen the noise levels. As with conventional silencers, these free flow exhaust silencers are made from stainless steel or similar metallic materials. As a result, they generally have to be fitted by specialist fitters. They are also susceptible to corrosion and generally have to be replaced fairly often. 
     SUMMARY OF THE INVENTION 
     According to the invention a silencer includes a polymeric casing connected to at least one inlet pipe and to at least one outlet pipe by polymeric spacers. 
     The polymeric spacers are preferably heat-resistant, preferably have a higher melting point than the polymeric casing, are preferably made of a different material to that of the polymeric casing, are preferably flexible relative to the polymeric casing and are preferably softer than the polymeric casing. 
     The polymeric spacers are capable of deformation to accommodate irregularities in the shape of the inlet and/or outlet pipes. 
     The inlet and outlet pipes are preferably made of metal which is preferably steel. 
     The casing preferably has collars, with the polymeric spacers being located between the collars and the inlet and outlet pipes. The housing preferably narrows towards the collars. 
     A sealant may be interposed between the polymeric spacers and their respective inlet and outlet pipes. 
     Preferably the polymeric spacers anchor the casing to the inlet and outlet pipes. 
     The polymeric spacers may be made from polytetrafluoroethylene. 
     The housing may include two sections secured to one another. 
     The two sections are preferably secured to one another by a connector. The two sections and the connector may have clip formations so that the two sections can be clipped to the connector. The connector may be a spacer to increase the length of the housing. Preferably the two sections are identical. Each section may include an integrally formed collar. 
     The polymeric casing is preferably made of a glass filled nylon. 
     The inlet and outlet pipes preferably form part of a continuous pipe which extends through the housing, with at least part of the continuous pipe located within the housing being perforated. Insulating material preferably surrounds the continuous pipe. The insulating material may be a continuous length of glass wool wrapped around the continuous pipe. 
     The silencer may however have a plurality of inlet and outlet pipes. The inlet and outlet pipes may be formed by a plurality of continuous pipes extending through the housing, with at least part of each continuous pipe located within the housing being perforated. 
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
     The invention will now be described by way of non-limiting examples with reference to the accompanying drawings in which: 
     FIG. 1 is an exploded, perspective view of a silencer according to the invention; 
     FIG. 2 is a cross-sectional view on line  2 — 2  of the silencer of FIG. 1, fitted to a section of exhaust pipe; and 
     FIG. 3 is a cross-sectional plan view of a silencer with twin exhaust pipes. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A composite silencer  10  includes a nylon casing  12  of a three piece construction. The nylon casing  12  consists of two identical sections  14  and  16  connected together by a connector  18 . The connector is positioned between openings  20  and  22  of the sections  14  and  16 . 
     The two sections  14  and  16  and the connector  18  are clipped together via complementary clip formations  24  and  26 . The clip formations  24  on the sections  14  and  16  are in the form of ring-like projections or lips surrounding openings  20  and  22 . The clip formations  26  on the connector  18  are in the form of complementary receiving channels. Although this clip arrangement is preferred, it is envisaged that other appropriate arrangements such as male and female formations or bayonet-type formations could be used so that the two sections can be connected directly to one another. 
     Each of the sections  14  and  16  have integrally formed end walls  28  and  30 , respectively, which narrow towards annular collars  32  and  34 , respectively. The collars  32  and  34  have apertures  36  and  38  which are axially aligned and which allow an elongate exhaust pipe  40  to extend through the nylon casing  12 . The exhaust pipe  40  has perforations  42  to allow exhaust gas passing through the exhaust pipe  40  to enter a chamber  44  defined by the casing  12 . 
     Although this embodiment relates to a single, straight exhaust pipe  40 , other exhaust pipe configurations can be used. As shown in FIG. 3, two exhaust pipes pass through the casing. In FIG. 3, the suffix  1  is used for parts corresponding to those FIGS. 1 and 2. However the exhaust pipe configuration could consist of a single pipe entering the casing from one end, branching into two pipes via a Y-branch, the two branched pipes exiting the casing through the opposite end. Alternatively, this configuration can be reversed so that two exhaust pipes enter the casing with a single exhaust pipe exiting the casing. In each of these configurations, the respective casing sections  14  and  16  are adapted to accommodate the different configurations. The sections  14  and  16  and the connector  18  are injection moulded from a glass filled nylon material known as nylon 66. This material consists of approximately 60 to 65% nylon and approximately 30 to 35% glass. Although it is envisaged that any other suitable heat resistant polymeric material may be used, nylon 66 is preferred because of its heat resistant properties. It can withstand working temperatures ranging from about −70° C. up to about 200° C., and has a melting point of 261° C. It is also highly shock absorbent and is therefore able to withstand vibrations and other body fatigue during use. The polymeric material absorbs sound to a greater extent than stainless steel, because it is less dense. 
     Located between an outer surface  46  of the exhaust pipe  40  and inner walls  48  and  50  of the collars  32  and  34 , respectively, are a pair of heat resistant spacers in the form of bushes  52 . The bushes  52  are formed of a heat resistant polymeric material, in this case polytetrafluoroethylene (P.T.F.E.). The bushes  52  are pressed into position and form an interference fit between the inner walls  48  and  50  of the collars, and between the bores of the bushes  52  and the exhaust pipe  40 . The bushes  52  substantially seal the ends of the chamber  44  and provide resistance to heat transfer from the exhaust pipe  40  to the casing  12  from hot exhaust gases passing through the exhaust pipe  40 . Silicon sealant  54  is provided between the bushes and the exhaust pipe  40 . This is necessary in certain situations as the bushes  52  and the casing  12  expand under heat. Since they have different heat expansion co-efficients, expansion of these components may result in gases leaking from the chamber  44 , which is undesirable. The seals between the bushes and the collars and between the bushes and the exhaust pipe have withstood pressures of up to two bar above atmospheric pressure without leaking. 
     Although it is envisaged that other materials can be used for the bushes  52 , P.T.F.E bushes are preferred because of their heat resistant properties. The P.T.F.E bushes  52  have a working temperature ranging from about −269° C. to about 270° C., and have a melting point of about 380° C. The P.T.F.E. bushes can withstand heat spikes of up to 330° C., and is believed that they can withstand heat spikes of up to 370° C. This material therefore has the ability to withstand the high temperatures of the stainless steel exhaust and resist heat transfer to the casing  12 . In addition, the bushes  52  are flexible and soft relative to the casing  12 . The bushes  52  are therefore capable of deformation to conform to irregularities in the shape of the exhaust pipe  40  which may not be perfectly round. 
     A continuous length of glass wool  56  is wrapped around the exhaust pipe  40  within the chamber  44  to provide additional damping or muffling of the exhaust noise. The glass wool can withstand temperatures of up to 700° C. 
     Connectors  18  of variable widths can be used. As a result, the overall length of the casing  12  can be adjusted by using connectors  18  of different widths. This allows a user to change the length of the casing to suit a particular motor vehicle by simply changing a single part. The connectors  18  of different widths are colour coded to simplify the choice of connector  18 . 
     The applicant envisages that the silencer  10  may be offered for sale in the form of a kit comprising the identical sections  14  and  16 , at least one connector  18 , the bushes  52 , glass wool, and the stainless steel perforated exhaust pipe section  40 . The kit can be readily assembled by winding the glass wool around the exhaust pipe  40 , positioning the bushes  52  within the respective collars  32  and  34 , sliding the two sections  14  and  16  over the pipe  40  with the connector  18  positioned between them, and clipping the pieces together. Typically, where additional sealing is required, the silicon sealant  54  will be put in place prior to clipping the pieces together. A section of an existing exhaust pipe is then replaced by the exhaust pipe  40  with the assembled silencer  10  fitted thereto. Alternatively, the silencer may be assembled in situ on an existing exhaust pipe that has been perforated. 
     The silencer according to the invention is believed to have a number of advantages over conventional stainless steel mufflers. Because the silencer of the invention is made from nylon, which is less dense than stainless steel, it tends to dampen or muffle sound to a greater extent than stainless steel. Further, the simple, yet effective, manner of clipping the various pieces together and frictionally fitting it to the free flow exhaust pipe will enable a user to fit the silencer a do-it-yourself (DIY) manner. No welding or special adhesives are required to assemble the silencer or fit it to the exhaust pipe. In addition, the user can vary the length of the silencer, to suit a particular make of vehicle, by interchanging connectors of different widths. Further, the silencer is smaller than conventional silencers because of the excellent noise dampening qualities of the nylon, and the silencer is between 35% and 55% lighter than equivalent steel exhausts. 
     It will be appreciated that many modifications or variations of the invention are possible without departing from the spirit or scope of the invention.