Abstract:
A silencer for internal combustion engines provides a form factor of a rectangular prism, and includes a box-shaped outer shell lined with a thermal and acoustic isolation material, and containing a set of acoustic elements in a serpentine path inside that layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is based on, and claims the benefit of U.S. provisional application Serial No. 60/267,465 filed Feb. 8, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a device for acoustically attenuating the exhaust noise emitted from internal combustion engines.  
         DESCRIPTION OF THE RELATED ART  
         [0003]    The basic structure of silencers, such as mufflers used in automobiles is well known in the art. Typically, a “silencer” is the traditional name given to higher quality exhaust noise attenuation systems. They tend to be larger, heavier, and much more complex in construction than a “muffler”, and have been more commonly used in stationary applications. Mufflers have generally been lower performance, mass produced, and smaller in size and weight.  
           [0004]    Generally, exhaust silencers can be classified into the categories of “absorptive”, “reactive”, or a “combination” of absorptive and reactive elements. The absorption type of silencer is generally of the “straight through” design, sometimes known as a “glass pack”, which employs a perforated tube passing through a chamber packed with an absorptive material. It is designed to absorb the acoustic energy, in effect converting it into heat. This type of silencer is fairly effective in suppressing higher frequency noise, although there is a wide variance in performance across the range of octave band center frequencies. Reactive silencers generally depend upon area change, and reversals of direction to reflect the sound energy of the exhaust gas prior to discharge. This is achieved through the utilization of the attenuation properties of a combination of expansion chambers, baffles and perforated tubes in order to “contain” the noise within the silencer. The most effective silencer systems will combine absorption and reactive effects to provide superior “broad-band” attenuation characteristics. This can be achieved by simply coupling the silencers in series, with the primary silencer being typically a reactive design. For the purposes of reducing installation costs and minimizing the package size, it is generally more effective to use a silencer system incorporating both reactive and absorptive elements.  
           [0005]    The combination silencers available for industrial applications today, are of a cylindrical exterior aspect, and are generally very large, with the outer diameter of the cylinder being significantly larger than the exhaust pipe diameter. They are available in a wide range of configurations and performance specifications, but in order to attain the higher levels of acoustic attenuation required for today&#39;s applications, a very large length and outer diameter is required. In addition to the fact that these large cylindrical units take up a lot of space when fitted, they are also exceedingly difficult to handle and time consuming to install. With the ever-increasing demands for industrial equipment to blend in with the natural surroundings, and the drive towards urban and rural beautification, the large cylinders are often considered too unsightly to leave exposed. Architects will often specify “decorative” enclosures for the silencers in order to “hide” the silencer, which in turn adds cost to the installation and significantly increases the volumetric size of the installation. There are also “line of sight” rules followed by certain planning authorities, which limit the overall height of some installations, forcing contractors to look for alternatives to the cylindrical silencer.  
           [0006]    What is needed, is a method of enclosing all the elements of a high performance combination silencer in a package, which is smaller in dimension, easier to handle and install than the existing cylindrical arrangements, and which has a low profile when installed.  
           [0007]    The present invention solves these and other problems currently in the art.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    The silencer incorporates a number of noise-reducing or attenuating elements arranged, positioned, and enclosed in such a manner as to provide a flat, rectangular aspect from all sides on the exterior. This “flat sided” construction means that it can be installed and supported much easier than with a cylindrical shape, and allow it to be mounted in closer proximity to adjacent objects. It can be supported easily from a wall or positioned within a rectangular opening, to allow it to be mounted in an enclosure or plant room roof, with the lower profile allowing a lower assembled elevation.  
           [0009]    The attenuating elements described, may be of a “reactive” type, “absorptive” type, or “combination” of reactive and absorptive types, dependant on attenuation requirements and particular application.  
           [0010]    The attenuating elements are inter-connected by means of chambers, and incorporate deflectors, which duct the exhausted gas from one element to the intake of the following element.  
           [0011]    The inlet and outlet to the silencer are provided by means of connection flanges, which are attached to the exterior walls of the unit, and which enter the primary and final chambers respectively. The design is such that the inlet and outlet flanges can be optionally made on the top, bottom, or sides of the unit, thus allowing for flexibility in application.  
           [0012]    The entire outer wall of the silencer is lined, on the inner surface with a thermo-acoustic layered assembly. This method of construction reduces radiant noise from the walls of the silencer, and aids in the absorption of the exhaust acoustic energy. It also provides lower temperatures on the outer surfaces of the unit, reducing radiant heat to the plant room or enclosure negating need for additional thermal lagging on the exterior of the silencer.  
         DETAILED DESCRIPTION OF THE INVENTION  
         [0013]    The description, component list, and associated sketches which follow (Refer to FIGS.  1  to  4 ), relate to the apparatus of this invention.  
           [0014]    It should be noted that the arrangement of the invention is such that by selecting certain elements, a range of performance specifications are available. This would be achieved as follows: -  
           [0015]    Lower Acoustic Performance Specification.  
           [0016]    This would utilize primary chamber C 1  as a gas entry chamber, primary reactive unit R 1  as the attenuating element, and chamber C 2  to exit the exhaust gas from the apparatus.  
           [0017]    Mid-Range Acoustic Performance Specification.  
           [0018]    This would utilize primary chamber C 1  as a gas entry chamber, primary reactive unit R 1  as the first attenuating element, chambers C 2  and C 3  to direct the gas from R 1  into the secondary reactive unit R 2 , and chamber C 4  to exit the exhaust gas from the apparatus.  
           [0019]    Highest Acoustic Performance Specification.  
           [0020]    This would utilize primary chamber C 1  as a gas entry chamber, primary reactive unit R 1  as the first attenuating element, chambers C 2  and C 3  to direct the gas from R 1  into the secondary reactive unit R 2 , chambers C 4  and C 5  to direct the gas from R 2  into the absorptive attenuating element A 1 , and chamber C 6  to exit the exhaust gas from the apparatus.  
           [0021]    For the purposes of this description, the construction of the highest performance, and hence most complex apparatus is detailed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a top view of the apparatus, viewed with the top cover and associated components removed for clarity;  
         [0023]    [0023]FIG. 2 is a section view X-X, as specified on FIG. 1;  
         [0024]    [0024]FIG. 3 is a section view Y-Y, as specified on FIG. 1;  
         [0025]    [0025]FIG. 4 is a section view Z-Z, as specified on FIG. 1. 
     
    
     DETAILED CONSTRUCTION AND OPERATION OF THE APPARATUS  
       [0026]    The arrangement of the apparatus is generally of a sheet metal construction with mild steel material being used for the development work. Stainless steel would be a more suitable material to be used throughout the assembly if a longer than normal service life is specified. The arrangement is in the shape of a six-sided box, which forms the main external structure. This exterior “shell” is constructed of a sheet steel material with a selected thickness suitable for the design pressure specified. Internally, the structure is reinforced and strengthened by using longitudinal baffles and cross members, listed as items  7 ,  10  &amp;  18 . These members, in addition to providing strength and support for other components, act as the dividing walls of the assembly, separating the enclosure into a specific number of six-sided chambers and acoustic elements. It should be noted that although the acoustic elements provide the largest portion of the attenuation of the exhaust noise, some attenuation is provided by the absorptive nature of the walls of the chambers, the function of which is mainly to direct the flow of the gas through the apparatus. It should also be noted that within the assembly, the relative scale of the components to each other remains constant, thus allowing the design of the apparatus to be scaled up or down to suit a range of applications.  
         [0027]    The apparatus is constructed of up to six chambers and three noise attenuating elements. Each chamber and element is considered in turn.  
         [0028]    Chamber, C 1 : -  
         [0029]    The engine exhaust gas will enter chamber C 1  of the apparatus via the inlet flange and inlet tube (items  20  &amp;  12 ). The flange pattern is based on ANSI, or other industry standards, so mating flanges should be readily available. The inlet flange-pipe arrangement may be positioned on any of the perimeter walls (top, bottom, end, or side), dependant on specific requirements, thus providing flexibility in application. Chamber C 1  is a six-sided box shape, which is lined internally on four of it&#39;s sides as detailed in Appendix A. The two non-lined sides are made up of cross member (item  10 ), and longitudinal baffle (item  18 ). An opening of suitable size for the required gas flow is provided in item  10 , thus allowing the exhaust gas to pass into the primary reactive element, R 1 .  
         [0030]    Reactive Attenuating Element, R 1 : -  
         [0031]    R 1  is lined on three sides as detailed in Appendix A. The three non-lined sides are made up of a cross member (item  10 ), longitudinal baffle (item  18 ), and cross member (item  7 ). R 1  is divided into primary and secondary sections by primary reactive baffle (item  22 ). The exhaust gas will enter the primary portion of R 1  through the opening in item  10  into the area between the reactive tubes (item  8 ), where it is forced to enter the tubes through the holes provided. The tubes are constructed of 16-gage sheet steel, which has been perforated with small holes to an approximate 50% open area. Some larger holes are also provided towards the inlet ends of the reactive tubes. These are primarily for the purpose of providing some pressure relief, should the silencer be subjected to higher than normal pressure “spikes”, but also provide an additional back-pressure safety factor, should some contamination and blockage of the tubes occur over an extended period of time. The outlet from the primary section of R 1  is via the ends of the tubes, which protrude through item  22  and exit to the secondary section of R 1 . In the secondary section of R 1 , the gas is allowed to expand in the larger area, before passing through a suitably sized opening in item  7 , which allows the gas to escape into chamber C 2 .  
         [0032]    Chamber, C 2 : -  
         [0033]    C 2  is lined on four sides as detailed in Appendix A. The two non-lined sides are made up of cross member (item  7 ), and longitudinal baffle (item  18 ). The gas enters C 2  via the opening in item  7 , and is forced to turn into chamber C 3  through a large opening in item  18 . A deflector (item  26 ) is positioned at an angle in order to assist the gas in making this turn, and to prevent any possible erosion of the insulation material on the interior surface of the silencer, directly opposite the opening in item  7 .  
         [0034]    Chamber, C 3 : -  
         [0035]    C 3  is lined on three sides as detailed in Appendix A, with the three non-lined sides being made up of both longitudinal baffles (item  18 ), and cross member (item  7 ). The gas enters C 3  by means of the opening in the longitudinal baffle from C 2 , and as the only other opening to C 3  is via the holes in item  7 , the gas is forced to turn and exit through these openings into the secondary reactive element, R 2 . Chambers C 2  and C 3  therefore act to turn the exhaust gas flow through 180 degrees.  
         [0036]    Reactive Attenuating Element, R 2 : -  
         [0037]    R 2  is not lined on any side. The gas enters from C 3  via the openings in item  7 , which allow the gas to fill the cavity around the secondary reactive tube (item  16 ). This tube is of a similar design to the primary reactive tubes (item  8 ), and is also manufactured from 16 gage sheet metal which is perforated with small openings to around 50% open area. Again, and for similar purposes to those in item  8 , larger pressure relief openings are provided at the inlet end of the tube. The gas is forced through the holes in the wall of the tube, and into its inner area. As the tube protrudes through item  10 , the gas is allowed to freely exit from the end of the tube and expand into chamber C 4 .  
         [0038]    Chamber, C 4 : -  
         [0039]    C 4  is lined on three sides as detailed in Appendix A, with the three non-lined sides being made up of longitudinal baffles (items  18 ), and cross member (item  10 ). The gas enters C 4  from the end of item  16 , which passes through item  10 , and is forced to turn and exit C 4  into chamber C 5  through a large opening in item  18 . A deflector (item  26 ) is positioned at an angle in order to assist the gas in making this turn, and to prevent the high velocity gas from causing any possible erosion of the insulation material directly opposite the end of item  16 .  
         [0040]    Chamber, C 5 : -  
         [0041]    C 5  is lined on four sides as detailed in Appendix A. The two non-lined sides are made up of cross member (item  10 ), and longitudinal baffle (item  18 ). The gas enters C 5  via the opening in item  18 , and is forced to turn into the end of the absorption tube (item  17 ), and hence into the absorptive attenuating element, A 1 . A deflector (item  25 ) is positioned at an angle in order to assist the gas in making this turn, and to prevent the high velocity gas from eroding the insulation material directly opposite the opening in item  18 . Chambers C 4  and C 5  therefore act to turn the exhaust gas flow through  180  degrees.  
         [0042]    Absorptive Attenuating Element, A 1 : -  
         [0043]    The exhaust gas enters the end of the absorption tube, which protrudes through item  10  into chamber C 5 . The absorption element is composed of the perforated sheet metal tube (item  17 ) of approximately 50% open area. It is wrapped entirely on the outer surface with high temperature lagging fabric as detailed in Appendix A, which is in turn wrapped with a layer of absorptive material of appropriate thickness for the diameter of the tube. The lagging fabric acts as a shield for the absorptive material, preventing the erosion of the fibrous insulation by the high velocity exhaust gas. The insulation is packed, and mechanically retained, within the confines of A 1 . The insulation used is a mechanically bonded glass fiber “blanket” capable of withstanding very high temperatures. The material was obtained from BGF Industries, Inc., Greensboro, N.C., although similar materials are available from other sources. The exit end of item  17  protrudes through item  7 , thus allowing the gas to pass directly into chamber C 6 .  
         [0044]    Chamber, C 6 : -  
         [0045]    Similar to chamber C 1 , C 6  is lined on four sides as detailed in Appendix A. The two non-lined sides are made up of cross member (item  7 ), and longitudinal baffle (item  18 ). The exhaust gas enters C 6  via item  17 , which protrudes through item  7 . The gas can then be exited from C 6 , and from the apparatus, on any of the perimeter walls (top, bottom, side or end), via the outlet tube (item  15 ), and outlet flange (item  20 ), and in a similar means to the inlet described earlier.  
       Appendix A  
       [0046]    All perimeter walls are lined on the inside with a high density, thermo-acoustic insulation material, which is faced with a high temperature lagging fabric and perforated sheet metal.  
         [0047]    The insulation material used is a basaltic rock fiber, pre-formed into semi-rigid “slabs” with an approximate density of 8 lb/cu.ft. A range of densities is available however, which may be selected depending on required thermal and acoustic insulation properties. The thickness of the slabs used may also be selected to derive the required thermal and acoustic properties, although ½″ increments are the most readily available. The specific material used has been obtained from Fibrex Insulations, Inc., Ontario, Canada, although this type of material is available from a number of sources worldwide.  
         [0048]    In view of the fibrous, semi-rigid nature of the basaltic and fiberglass insulation, and of the high velocity of the gas passing through the apparatus, it is likely that if the insulation were to be directly exposed to the gas flow, that erosion of the insulation material would result. This would in turn lead to premature failure of the silencing characteristics, and of the thermal insulation qualities of the apparatus. For this reason, the basaltic insulation material is faced on the exhaust gas side with a high temperature lagging fabric. The material chosen is a close-woven cloth of fiberglass yams which is capable of withstanding continuous exposure to extremely high temperatures, and which acts to “shield” the fibrous basalt material from the high velocity gas flow. The lagging fabric was obtained from BGF Industries, Inc., Greensboro, N.C., although similar materials are available from other sources.  
         [0049]    The insulation is mechanically retained to the walls of the apparatus using a perforated sheet metal material. This material is a mild steel of 16 gage thickness, which has been perforated to provide an approximate 50% open area. It is expected that a stainless steel material could be used for this application if a longer service life was required.  
                                       ITEM   QTY.   COMPONENT DESCRIPTION                    1   1   Top Cover (Not Shown)        2   1   Silencer Body        3   1   End Plate       4, 5, 6, 9,   12    Insulation Retaining Material       11, 13, 14,   Total       21       7, 10   1 Each   Cross Members        8   4   Primary Reactive Tubes       12   1   Inlet Tube       15   1   Outlet Tube       16   1   Secondary Reactive Tube       17   1   Absorption Tube       18   2   Longitudinal Baffles       19   1   Inlet End Plate       20   2   Inlet and Outlet Flanges (Outlet Flange Not Shown)       22   1   Primary Reactive Baffle       23   1   Absorptive Wrap       24   5   Absorptive Wrap Retaining Ties (Not Shown)       25, 26, 27   4 Total   Deflectors       28   4   Lifting Lugs