Abstract:
A sound absorbing muffler used to attenuate noise carried by the exhaust gases of an internal combustion engine includes a straight-through flow tube of constant diameter and cross-section. The muffler utilizes both reactive and dissipative components and includes two annular sound absorbing end chambers, an outer annular resonating chamber, and an inner sound absorbing means. The muffler&#39;s configuration produces effective noise attenuation, yet low backpressure.

Description:
This is a continuation of United States patent application Ser. No. 07/825,536, filed Jan. 24, 1992, entitled LOW BACKPRESSURE STRAIGHT-THROUGH REACTIVE AND DISSIPATIVE MUFFLER, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to an acoustic muffler, and more particularly to a muffler for use in conjunction with a motor vehicle internal combustion engine to reduce exhaust noise, although it also may be used in other applications of silencing a fluid flow. 
     A muffler is generally connected to the outlet of an internal combustion engine exhaust gas system to reduce exhaust noise from the engine. There are two general classifications of mufflers, reactive and dissipative. Reactive mufflers are usually composed of several chambers of different volumes and shapes connected together with pipes. Reactive mufflers tend to reflect the sound energy back to the source. Dissipative mufflers are usually composed of ducts or chambers which are filled with acoustic absorbing materials. These materials absorb the acoustic energy and transform it into thermal energy. 
     Reactive mufflers are most useful when the source noise is composed of pure tones at specific, fixed frequencies, and when the fluid to be muffled is a hot, dirty, high-speed gas flow. Reactive mufflers are particularly useful for low frequency applications and for those installations where high temperatures or flammable gasses restrict the use of dissipative materials. Reactive mufflers are often constructed of baffles, reverse flow passages, or multiple tubes. These configurations produce a relatively high pressure drop, causing a back pressure at the exhaust of an engine, thus restricting engine performance. Back pressure produced by passenger car mufflers can range as high as three to seven pounds per square inch at maximum engine power. As a result it is desirable to employ a &#34;straight-through&#34; configuration to effectively eliminate back pressure. 
     Dissipative mufflers are useful when the source produces noise in a broader frequency band. They are particularly effective at high frequencies, but precautions must be taken if the fluid flow has a high velocity or temperature or if it contains particles or is corrosive. The most simple dissipative muffler is constructed by lining the interior of a duct with sound absorbing material. At high velocity, facing materials such as wire screens or perforated metal sheets are necessary to prevent erosion of the sound absorbing material. Dissipative mufflers produce a relatively low pressure drop with high attenuation at predominately middle and high frequencies. A typical dissipative muffler may be effective above approximately 500 Hertz. The approximate attenuation per linear foot is a function of the acoustical absorption coefficient of the absorbing material, the volume of the duct, as well as the frequency of the sound. Mufflers are generally tuned to attenuate specified frequencies. This tuning is volume dependent. A longer muffler will attenuate lower effective frequencies. The maximum efficiency for a simple dissipative muffler occurs at a frequency at which the width of the duct is between one half and twice the wavelength of the sound. 
     Mufflers which consist of a combination of the reactive and dissipative types are known in the art in a variety of configurations. 
     SUMMARY OF THE INVENTION 
     The present invention provides a muffler having both reactive and dissipative elements. The muffler has multiple dissipative sound absorbing chambers and a reactive annular outer gap. The muffler includes a straight-through flow tube having a constant cross-section and diameter, and creating low backpressure. The flow tube allows a fluid flow, preferably comprising the exhaust gases of an internal combustion engine, to pass entirely through the muffler. The muffler requires no baffles, multiple tubes, or reverse flow sections. 
     The present invention comprises an annular housing, an inlet to the housing, and outlet from the housing, an elongated flow tube which communicates between the inlet and the outlet, two annular end chambers containing sound absorbing material, an annular resonating chamber between the two end chambers, and a relatively thin annular sleeve containing sound absorbing material, disposed around the flow tube and contained entirely within the resonating chamber. Perforations allow communication between the flow tube and the annular sleeve, between the annular sleeve and the resonating chamber, as well as between the resonating chamber and the end chambers. The total volume of the muffler, length of the muffler, and length of each end chamber may be varied in order to tune the muffler to eliminate specified frequencies of noise. 
     It is an object of the present invention to effectively attenuate noise over a broad band of frequencies. 
     It is a further object of the present invention to provide a straight-through flow tube, low backpressure muffler. 
     It is a further object of the present invention to provide a muffler having both reactive and dissipative elements. 
     It is a further object of the present invention to provide a muffler configured and tuned to effectively muffle the exhaust noise of an automobile internal combustion engine. 
    
    
     These and other advantages and features will become apparent from the following description and claims in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a muffler arranged according to the principles of the present invention; 
     FIG. 2 is a sectional view along line 2--2 in FIG. 1; 
     FIG. 3 is an enlarged partial sectional view of one aspect of the present invention; 
     FIG. 4 is a sectional view of a first alternative embodiment of the present invention; 
     FIG. 5 is a sectional view of a second alternative embodiment of the present invention; 
     FIG. 6 is a sectional view along line 6--6 in FIG. 5; and 
     FIG. 7 is a sectional view of a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the embodiment of FIGS. 1-3, muffler 10 is connected to an exhaust pipe by a coupling means (not shown). The fluid, normally air and other exhaust gases, flowing through the exhaust pipe carries sound waves generated during the operation of the internal combustion engine. The majority of these sound waves are considered undesirable noise which is to be muffled. 
     With reference to the drawings, FIG. 1 shows a muffler 10 having a straight-through flow tube 12. Two annular outer end plates 14,16 are mounted to the flow tube 12, and comprise doughnut-shaped disks with no other perforations than the one allowing assembly on the flow tube 12. An outer shell 18 Is mounted about the flow tube 12 and spans the space between the outer end plates 14,16. The outer shell 18 is affixed to the perimeter of the outer end plates 14,16 so as to define a muffling chamber 20 which is imperforate, allowing no gas or sound waves to escape. Two annular inner end plates 22,24 define, in conjunction with outer end plates 14,16, two end chambers 26,28, as well as a central annular resonating chamber 30. Muffling chamber 20 therefore consists of resonating chamber 30 and both end chambers 26,28. 
     Preferably, an annular sound absorbing sleeve 32 surrounds a portion of the flow tube 12 contained within the resonating chamber 30. The end chambers 26,28 both contain sound absorbing material 34, as does the central annular sleeve 32. This material is preferably fiberglass, and may also be wire mesh, steel wool, basalt wool or similar material. The annular sleeve 32 and the end chambers 26,28 may each be filled with a different type of sound absorbing material 34. The construction material for the flow tube 12, outer end plates 14,16, inner end plates 22,24, outer shell 18 and annular sleeve 32 is preferably a metal, such as stainless steel or aluminized coated or low carbon steel. 
     The flow tube 12 is preferably a straight round cylinder passing entirely through the muffler 10 and having a constant diameter and cross-section. The flow tube 12 has a smooth and continuous interior surface, with no baffles or flow barriers. The flow tube 12 is formed with perforations 36 around its perimeter to allow the sound waves to communicate with the muffling chamber 20. The flow tube 12 in the preferred embodiment is formed with apertures 36 only along the length which is surrounded by the annular sleeve 32. In addition, the inner end plates 22,24, and inner annular sleeve 32 are formed with apertures 38,40,42, respectively, comprising holes allowing fluid communication through each respective member. In the preferred embodiment, these apertures 36,38,40,42 are formed as louvers 44, rather than through holes, as shown in FIG. 3. Louvers 44 may be formed in various configurations, and the louvers 44 shown in FIG. 3 serve only as an example. 
     The annular sleeve 32 which surrounds a portion of the flow tube 12 is contained entirely within the resonating chamber 30. The length of the annular sleeve 32 is less than or equal to the length of the resonating chamber 30. The ends of the sleeve 32 do not extend to reach the inner end plates 22,24. This central sound absorbing means 32,34 is preferably formed as a &#34;pinch can&#34; which is constructed of a tube, the ends of which are compressed into contact with the outer surface of the flow tube. The annular sleeve 32 may also be formed of a finely perforated screen. 
     The sound absorbing means, consisting of the annular sleeve 32 and sound absorbing material 34, operates to reduce pressure pulsations flowing from inside the flow tube 12 into the muffling chamber 20. This annular sleeve sound absorbing means 32,34 acts as a mechanical filter to dampen high pressure spikes. In another embodiment of the present invention, the sound absorbing material 34 and annular sleeve 32 may be eliminated. 
     The cross-section of the muffler 10 is preferably an oval shape, but may also be round, or even square or rectangular. An oval muffler 10 produces better noise attenuation and causes little shell ringing. A square or rectangular muffler transmits high frequency sound and may resonate, producing a kettle drum or bell-like ringing sound. 
     A first alternative embodiment of the present invention is shown in FIG. 4, wherein the flow tube 12&#39; is not axially aligned with the centroid of the oval muffling chamber 20&#39; defined by outer shell 18&#39;. This off-center configuration enables the present invention to fit within the volume available in the particular application, usually the undercarriage of an automobile. 
     A second alternative embodiment is shown in FIGS. 5 and 6, wherein the central annular sound absorbing means indicated generally at 50 is constructed of an inner shell 52 which surrounds the flow tube 12&#34; intermediate the outer shell 18&#34;. The inner shell 52 is welded to and spans the distance between the inner end plates 22&#34;,24&#34;. The inner shell 52 defines an inner dissipative chamber 54 and an outer annular resonating gap 56. The inner shell 52 is preferably composed of a metal and is formed with perforations 58. The dissipative chamber 54 is filled with sound absorbing material 34&#34;. A wire screen 60 may be wrapped around the flow tube 12&#34; between the inner end plates 22,24 to operate as a mechanical filter to reduce air pressure pulsations. 
     A third alternative embodiment is shown in FIG. 7, which depicts a muffler having the sound absorbing means 50 described as in the second alternative embodiment above, and wherein the flow tube 12&#39;&#39;&#39; is not axially aligned with the centroid of the oval muffling chamber 20&#39;&#39;&#39; defined by outer shell 18&#39;&#39;&#39;. 
     All embodiments of the present invention may be tuned to eliminate specific ranges of noise frequencies. The tuning process is necessarily empirical, and is based on trial and error. The length of the end chambers 26,28 may be altered to tune the muffler, and further the length of each end chamber 26,28 may be different. In addition, the ratio of the volume of the annular sleeve 32,52 or dissipating chamber 54 to the volume of the resonating chamber 30 or annular gap 56 may be set to tune the muffler. Depending on the desired noise frequencies for attenuation, the volume ratio may range from approximately 10% to 90%. An annular sleeve 32 or &#34;pinch can&#34; as shown in FIGS. 1 and 2 is preferable for a volume ratio on the order of 10% to 50%. An inner shell 52 as shown in FIGS. 5 and 6 is preferable for a volume ratio on the order of 50% to 90%. 
     All embodiments of the present invention operate in substantially the same manner. In operation, exhaust gas enters the inlet 46 to the flow tube 12 of the muffler 10, and may flow straight though the flow tube 12 and exit from the outlet 48. High pressure pulses of exhaust gas may flow from the flow tube 12 though the sound-absorbing material 34 contained in annular sleeve 32,52 through the perforations 42,58 in the annular sleeve 32,52 and into the resonating chamber 30. Exhaust gas may further flow from resonating chamber 30 into the end chambers 26,28. High pressure pulses are damped by the sound-absorbing chambers 26,28,32,54 as well as by the finite volume enclosed by the outer shell 18 and outer plates 14,16 of the muffling chamber 20. Exhaust gas tends to flow straight through the flow tube 12, and not escape through the perforations 36 on the flow tube 12, because the gas cannot escape the muffler 10 by any other means than the outlet 48. 
     Acoustic noise carried by exhaust gas is attenuated by absorption and reflection. The sound-absorbing material 34 contained in annular sleeve 32 and the end chambers 26,28 operates to absorb the sound waves, transforming mechanical acoustic energy into thermal energy. The resonating chamber 30 operates to reflect specific frequencies of sound through the flow tube 12, back out the inlet 46 of the muffler 10. 
     It should be understood that various modifications of the preferred embodiments of the present invention will become apparent to those skilled in the art after a study of the specification, drawings, and the following claims.