Abstract:
A muffler for an internal combustion engine has a casing forming an expansion chamber therein. An inlet duct is connected to the casing and projects into the expansion chamber and discharges exhaust fluid into the expansion chamber. The casing tapers in an upstream direction to form a pocket for receiving reverse flow of the exhaust gases and to minimize reverse flow from flowing back into the inlet duct in an upstream direction. In the discharge end of the inlet duct are a set of primary vanes and a set of secondary vanes. The vanes are secured to the walls of the duct and extend radially toward the central area of the duct. The vanes are angled in order to deflect the exhaust fluid flow into a swirling movement as it is discharged into the expansion chamber and maintains that swirling movement while passing out of the casing through the outlet duct.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to mufflers for internal combustion engines of the type commonly used in motor vehicles. More particularly, the present invention relates to such mufflers which provide improved engine combustion efficiency resulting in improved performance and reduced toxic exhaust emissions levels. 
         [0002]    Motor vehicles utilizing internal combustion engines continue to be the favored form of transportation to most people in the developed countries of the world. This is in spite of their many disadvantages the more important of which include toxic exhaust emissions and exhaust noise. Although mufflers can substantially reduce or perhaps even eliminate the exhaust noise, it is commonly believed that they do so at the expense of reduced power output and reduced fuel economy. 
         [0003]    Designers of exhaust systems have recognized that improving the effectiveness of exhaust gas flow out of the engine can provide improved combustion efficiency and thereby reduced toxic exhaust emissions. There have consequently been many exhaust system designs that have sought to increase the velocity of exhaust gas flow through the exhaust system and thereby scavenge exhaust gases from the combustion chamber and exhaust ports. Some exhaust header system designs position exhaust pipes around the inner circumference of a collector pipe to produce swirling of the exhaust gases from the collector pipe in a vortex flow and thereby enhance exhaust gas flow therefrom. Such systems have been very effective in improving exhaust as well as intake fluid flow and thereby improving combustion. However, such systems require retuning of the engine and replacement of major engine system components and are thus impractical for many motor vehicle owners. 
         [0004]    The particular muffler design is also important in that it can substantially affect the combustion efficiency of the engine. Since muffler replacement is easier than exhaust manifold and pipe replacement, automotive engineers have sought improve the muffler design in order to provide improved exhaust flow and thereby improved combustion efficiency. A muffler is also a very important component of motor vehicles because it reduces their exhaust noise making their use in crowded cities more tolerable. Consequently, many mufflers are designed with the dual purpose of both increasing exhaust flow and attenuating exhaust noise. 
         [0005]    There are two basic designs of muffler in contemporary use in modern motor vehicles. These designs are the dissipative type and the reactive type. A dissipative muffler absorbs the sound energy as the exhaust fluid passes through the muffler via fibers or other sound deadening material packed therein. Two primary disadvantages of dissipative mufflers are that they lose their effectiveness over time and are expensive to manufacture. Moreover, dissipative mufflers do not produce good low frequency sound attenuation. The reactive type of muffler attenuates the sound energy by reflecting the sound back toward the source. A reactive muffler is inexpensive to manufacture and provides good low frequency sound attenuation but has the disadvantage of producing high backpressure. Variations on these two basic designs have sought to produce desired sound attenuation without substantially or unacceptably increasing the back pressure on the engine since, as is commonly known, excessive muffler induced back pressure will substantially compromise engine combustion efficiency and reduce engine performance. 
         [0006]    Exhaust noise is appreciably reduced by friction effects produced by muffler internal structures and noise-wave effects produced by resonance chambers. However, utilization of such structures increases the complexity, cost and size of the exhaust system. But, a large exhaust system is often very undesirable as motor vehicle space may be limited and ground clearance may need to be high especially for sport utility vehicles. Thus, compactness is a very desirable feature in a muffler used in modern motor vehicles. Compactness and concomitantly reduced weight are especially important for high performance vehicles wherein reduced weight can desirably improve acceleration. In attempts to provide both exhaust efficiency and compactness, many mufflers incorporate various internal structures designed to either improve sound attenuation or improve exhaust flow efficiency. An example of a compact, sound attenuating muffler specifically designed for compactness is disclosed in U.S. Pat. No. 4,574,914 to Flugger. The Flugger muffler is especially useful for high performance motor vehicles because it achieves sound attenuation without significant decrease in engine performance. The Flugger muffler is a reactive type which includes partitions as well as convergently and divergently shaped structures which change the direction of exhaust flow. The Flugger muffler is effective in both preserving exhaust flow efficiency and providing sound attenuation. Nevertheless, its size and shape render it unsuitable for some types of motor vehicles. 
         [0007]    Some mufflers are specifically designed to reduce back pressure and thereby improve exhaust gas flow as well as intake induction and combustion efficiency. The goal is improved performance and perhaps fuel economy. An example of such a muffler is disclosed in U.S. Pat. No. 6,213,251 to Kesselring. The Kesselring muffler includes restrictor disk holes and a helical passageway therein to enhance the exhaust gas flow therethrough. The specific goal is moderate backpressure at low rpm and little or negative backpressure at high rpm. Such types of mufflers, however, have inordinate complexity. 
         [0008]    Many muffler designs incorporate apertures in the exhaust tubes therein in order to gradually expand the gas stream flowing through the muffler. However, such designs are not very effective at this because since the tubes are straight tubes the major portion of the gas stream flows through and out of the tube and only a small portion flows out through the muffling apertures. Nevertheless, such apertured tubes are in common use in mufflers and some mufflers have used such apertures to provide a swirling exhaust gas stream in order to enhance exhaust gas flow through the muffler. An example of such a muffler pipe design is disclosed in U.S. Pat. No. 6,385,9678 to Chen. The Chen pipe has a conical structure to accelerate the gas stream and spiral portions spaced around the conical structure. However, the Chen pipe is only a part of a muffler and many types of muffler casings would not be suitable for such a pipe. 
         [0009]    Despite the prevalence of many types of mufflers, what is needed is a muffler that can curtail reverse flow of exhaust gas toward the engine. What is also needed is a muffler that can provide adequate sound attenuation as well as fuel economy. It is also desirable that these features be provided without sacrificing power output. 
       SUMMARY OF THE INVENTION 
       [0010]    It is a principal object of the present invention to provide a muffler having structural components that impart a swirling motion to the exhaust fluid flowing therethrough. 
         [0011]    It is another object of the present invention to provide a muffler which prevents reverse flow of exhaust fluid therein. 
         [0012]    It is also an object of the present invention to provide a muffler that reduces back pressure while providing exhaust sound attenuation. 
         [0013]    It is also an object of the present invention to provide a muffler having exhaust fluid swirling components that are shaped to provide minimal restriction of fluid flow therethrough. 
         [0014]    Exhaust systems generally are compromised by an inherent exhaust flow inefficiency caused by valve overlap of the internal combustion engine. At the end of the exhaust stroke of the engine&#39;s piston, the piston starts to move down while the intake valve is opening to allow the air/fuel charge into the combustion chamber. However, the valve overlap design of modern engines has the exhaust valve also open at this crucial time thereby allowing the combustion chamber to draw exhaust gases directly from the exhaust system. This is especially problematic if the exhaust gas velocity is low and exhaust system pressure is high whereupon the exhaust gases will readily flow backward into the combustion chamber rather than out from the exhaust system. Exhaust gases entering the combustion chamber will dilute the intake fluid with unburnable gases and occupy needed combustion chamber space. This can result in reduced power since there is a lower quantity of fresh air/fuel mixture in the combustion chamber than there otherwise would be. Additionally, the presence of the hot exhaust gases in the combustion chamber may raise the temperature of the mixture above the fuel&#39;s knock resistance accelerating engine wear and possibly damaging internal engine components. Consequently, it is imperative that an exhaust system have high fluid flow velocity and therefore low pressure in order to prevent or minimize these effects. 
         [0015]    As exhaust gas temperature equalizes in the exhaust system, pressure tends to move in a reverse direction i.e., toward the combustion chamber. This normally happens during deceleration and can cause spent exhaust gases to enter the combustion chamber as a result of the valve overlap (also known as positive overlap). As a result of the reduction in the quantity of power producing fresh air/fuel mixture in the combustion chamber, there will be a slight flat spot during re-acceleration and a reduction in fuel economy. 
         [0016]    The muffler of the present invention is specifically designed to prevent reverse flow of exhaust gases that typically occurs during deceleration by providing a pocket within the muffler expansion chamber. The pocket in effect traps the exhaust gases thereby precluding their reentry into the inlet duct of the muffler. During engine operation, the hot exhaust gases discharged into the muffler expansion chamber expand to the walls thereof and upon commencement of the reverse flow move into the pocket where their movement is stopped. As a result, the exhaust gases are trapped in the pocket. The exhaust gases thus collect in the expansion chamber instead of moving back into the inlet duct. When reacceleration takes place, the pocket is emptied as the exhaust flow velocity increases producing a pressure drop in the chamber which draws the gases out of the pocket. 
         [0017]    Backpressure which is basically resistance to fluid flow is also necessary to avoid or minimize because high backpressure causes the exhaust gases to remain in the exhaust system too long. When the exhaust gases back up in the system there is an increased tendency for the gases to reverse flow. Thus, it is advantageous for an exhaust system to produce very low backpressure or, more preferably, a vacuum within the system to induce scavenging of the exhaust gases and to thereby aid exhaust flow. 
         [0018]    The muffler of the present invention is also specifically designed to aid fluid flow through the exhaust system by causing the fluid to swirl as it moves through the system. The swirl reduces the decrease in exhaust gas velocity that would otherwise occur yielding reduced backpressure. Consequently, this improved flow reduces the tendency of the fluid to reverse flow during deceleration. Overall performance and power output are improved as a result. 
         [0019]    The muffler of the present invention achieves its goal of swirling the fluid flow by incorporating vanes which are positioned in the exhaust flow stream. More specifically, the vanes are secured to the inside of the inlet duct. The vanes are angled so that they deflect the fluid laterally into a rotational movement. The vanes thus impart a swirling movement to the exhaust fluid discharged into the expansion chamber thereby enhancing fluid flow in the space utilized to prevent flow reversion. 
         [0020]    The vanes are specially curved (at their edges) and shaped for maximal efficiency in producing the swirl effect with minimal fluid flow restriction. The vanes are longitudinally longer at the inner surfaces of the walls of the inlet duct than at the central area of the duct. Thus, the peripheral portions of the vanes are larger and therefore provide more deflection than the smaller, more centrally located portions of the vanes. This is desirable because it more efficiently yields the desired swirl. This is because the swirl produced is essentially exhaust gas rotation about a central axis with the more peripheral gas at peripheral areas of the duct (or chamber) rotating more than the gas at more centrally located areas. Consequently, flow deflection at the peripheral portions of the duct is more effective in producing the desired fluid rotation about the central axis of the duct (and chamber). Similarly, near the central area of the housing the vane portions are smaller producing less deflection and concomitantly less fluid flow restriction at the duct area where swirl can less effectively be produced. 
         [0021]    The lower or trailing edges of the vanes are also curved to streamline the vanes for reduced fluid flow resistance. The curvature is in a direction of from the periphery to the center of the duct (or chamber). Since the peripheral ends of the primary vanes are longer than the central (or inner) ends, the lower or trailing edge is angled in the direction of fluid flow and the curvature thereof is also curved in this direction. 
         [0022]    Additionally, lower end portions and lower medial end portions of the vanes are bent in the direction of the deflection of the fluid flow. The lower end portions and lower medial end portions are thus angled laterally to enhance deflection of the fluid flow. This deflection provided by these lower portions is also very effective in producing swirl because the fluid flow has been previously deflected by upper portions of the primary vanes and has been moving downwardly alongside the vanes until it reaches these lower portions where it is further deflected to add more lateral movement and thereby more rotational movement to the fluid flow. 
         [0023]    Also included are secondary vanes for maximal efficiency in producing the swirl effect with minimal fluid flow restriction. The secondary vanes are mounted in the duct and attached to the walls thereof. The secondary vanes are also angled the same as the primary vanes for producing the desired deflection of the fluid flow. But, the secondary vanes are shorter in width and thus extend only a short distance toward the center and into the inner area of the duct so that they are essentially located only in the inner peripheral area of the duct where there is maximal effectiveness in producing the fluid flow rotational movement. 
         [0024]    The present invention obviates the need for a central support structure by interconnecting lateral inner ends of the vanes at the central area of the duct. The central area of the duct is thus open, and there is therefore nothing to impede fluid flow through the center of the duct. Thus, the present invention provides improved exhaust fluid flow over prior art comparable structures. Moreover, elimination of a central member does not result in reduction in the efficiency of the structures in producing fluid swirl because the swirl produced is essentially fluid rotation about a central axis i.e., the center of the duct, with the more peripheral fluid at peripheral areas of the passageway rotating more than the fluid at more centrally located areas. The overall fluid movement is thus in the shape of a spiral as it moves through the passageway. Consequently, the swirl cannot typically be effectively accomplished by means of structures located at the center of the duct but can instead be effectively accomplished by means of structures located at more peripheral portions of the duct. Indeed, maximal twisting or turning of the fluid flow is accomplished by means of structures such as the secondary vanes and structure portions such as the larger peripheral portions of the primary vanes both of which are located at the area of the inner perimeter of the duct. 
         [0025]    The muffler of the present invention thus provides an exhaust system component that prevents or reduces reverse flow of the exhaust fluid as well as enhancing fluid flow through the exhaust system. The present invention thus eliminates or minimizes hesitation during acceleration thereby improving performance and improves fuel economy by ensuring a fresh air/fuel mixture in the combustion chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0026]      FIG. 1  is a perspective view of the muffler of the present invention shown with a portion cut away to illustrate the components thereof. 
           [0027]      FIG. 2  is a longitudinal-sectional view of the muffler of the present invention. 
           [0028]      FIG. 3  is cross-sectional view of the muffler of the present invention illustrating inner vane components thereof and taken along lines  3 - 3  of  FIG. 2 . 
           [0029]      FIG. 4  is a schematic drawing of the muffler of the present invention showing the movement of exhaust gas therethrough. 
           [0030]      FIG. 5  is a side plan view of a representative primary vane of the muffler of the present invention. 
           [0031]      FIG. 6  is a top view of a representative primary vane of the muffler of the present invention. 
           [0032]      FIG. 7  is a rear end view of a representative primary vane of the muffler of the present invention showing the angled lower end portion thereof and also showing the fluid flow passing thereagainst and proximal thereto. 
           [0033]      FIG. 8  is a side plan view of a representative secondary vane of the muffler of the present invention. 
           [0034]      FIG. 9  is a top view of a representative secondary vane of the muffler of the present invention. 
           [0035]      FIG. 10  is a rear end view of a representative secondary vane of the muffler of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0036]    Referring to the drawings, the muffler of the present invention is generally designated by the numeral  10 . The muffler  10  includes a casing  12 . The casing  12  is hollow and includes an inlet port  14  and an outlet port  16  at opposite ends thereof. The casing  12  also includes a main body  18  defining an expansion chamber  20  in the casing  12  and located between the inlet port  14  and outlet port  16 . An inlet duct  22  is connected to the inlet port  16  and has an inlet end  24  for connection to an exhaust pipe (not shown) to allow exhaust fluid  26  from an internal combustion engine (not shown) to enter the casing  12 . The inlet duct  22  extends through the inlet port  14  and has an outlet (or discharge) end  28  for discharge of the exhaust fluid  26  into the expansion chamber  20  which has a larger cross-sectional area than the inlet duct  22  (and inlet port  14 ), as is typical for conventional mufflers. The casing  12  also includes an outlet duct  30  connected to the outlet port  16  for allowing emission of exhaust fluid  26  therefrom and out of the chamber  20 . The outlet duct  30  and outlet port  16  are preferably larger in cross-sectional area than the inlet port  14  and inlet duct  22  to reduce resistance to fluid flow. 
         [0037]    The main body  18  of the casing  12  includes a front portion  32 , a front medial portion  34 , a medial portion  36 , a rear medial portion  38  and a rear portion  40 . The front portion  32  and the rear portion  40  are located at opposite longitudinal ends of the casing  12 . The front medial portion  34 , the medial portion  36  and the rear medial portion  38  are preferably unitary and preferably cylindrical. However, other suitable shapes may also be utilized instead. 
         [0038]    The inlet duct  22  extends into the expansion chamber  20  such that the outlet end  28  is located at the medial portion  36  of the expansion chamber  20 . The expansion chamber  20  includes a pocket  42  which is located at the inlet port  14  area. The inlet duct  22  includes an outlet end portion  44 , and the outlet end portion  36 , the front portion  38  and the front medial portion  34  together define the pocket  42 . The front portion  32  is convergently tapered toward the inlet port  14  and in an upstream direction relative to the direction of flow of the exhaust fluid  26 . Similarly, the rear portion  40  is convergently tapered toward the outlet port  16  and in a downstream direction relative to the direction of flow of the exhaust fluid  26 . The front portion  32  and the rear portion  40  are preferably frusto-conical. 
         [0039]    During the crucial deceleration phase of engine operation, the exhaust fluid  26  tends to reverse direction and move rearward. But, the fluid  26  tends to move into the pocket  42  rather than into the inlet duct  22  because there is less pressure in the pocket  42  than in the outlet end  28  and because the expansion of the fluid outwardly from the outlet end  28  as it is discharged therefrom tends to promote flow laterally outwardly and thereby rearwardly into the pocket  42 . Once the fluid is moving into the pocket  42 , the inlet duct  22  and the front medial portion  36  (and to a certain extent the front portion  32 ) block lateral movement of the fluid such that it becomes trapped in the pocket  42 . As a result of the taper of the front portion  38 , the pocket  42  has a smaller cross-sectional area at the inlet port  14  than at the front medial portion  34 . The smaller cross-sectional area of the inlet port  14  area of the pocket  42  tends to compress fluid entering therein so that the total quantity of fluid in the pocket is thereby maximized. Consequently, there is a maximal quantity of fluid  26  in the pocket  42  concomitantly minimizing the quantity of fluid available to reverse flow into the outlet end  28  of the inlet duct  22 . After deceleration is terminated and reacceleration is commenced, the velocity of the stream of fluid  26  flowing out of the outlet end  28  causes a pressure drop in the expansion chamber  20  so that this pressure drop in conjunction with the higher pressure of the fluid in the pocket  42  due to its compression facilitates fluid flow out of the pocket and subsequently out of the muffler  10 . 
         [0040]    The muffler  10  also incorporates a set of vanes  46  which impart a swirling motion to the exhaust fluid in order to improve the flow of exhaust fluid  26  through the exhaust system. The set of vanes include a plurality of primary vanes  48  which are preferably mounted in the inlet duct  22 . The primary vanes  48  are located at the outlet end portion  44  of the inlet duct  22 . The primary vanes  48  are preferably securely attached to the inner surfaces  50  of the walls  52  of the inlet duct  22  via welding or other suitable attachment means. 
         [0041]    The set of vanes  46  also include a plurality of secondary vanes  54  which are also preferably mounted in the inlet duct  22 . The secondary vanes  54  are similarly located at the outlet end portion  44  of the inlet duct and preferably securely attached to the inner surfaces  50  of the walls  52  of the inlet duct  22  via welding or other suitable attachment means. Each of the secondary vanes  54  are situated between the primary vanes  48  such that the vanes  48  and  54  alternate about the circumference of the inner surfaces  50  of the walls  52  of the inlet duct  22 . Both the primary vanes  48  and the secondary vanes  54  are in the path of the exhaust fluid  26  flow. 
         [0042]    The vanes  48  have top edges  56  that are in misalignment with bottom edges  58  thereof, and vanes  54  have upper edges  60  that are in misalignment with lower edges  62  thereof. This misalignment is with reference to the direction of fluid flow  64  passing through the muffler  10  during the acceleration phase of engine operation (or longitudinally with reference to the casing  12 ). 
         [0043]    The primary vanes  48  are situated so that the bottom edges  58  are flush with the discharge end edge  29  of the inlet duct  22 . However, the secondary vanes  54  are medially situated on the walls  52 . Thus, the lower edges  62  are not flush with the discharge end edge  29  of the inlet duct  22 . 
         [0044]    The primary vanes  48  thus are preferably oriented at an angle such that the flat planar outer surfaces  66  thereof face the fluid flow  64 . The secondary vanes  54  are similarly oriented at an angle such that the flat planar outer surfaces  68  thereof face the fluid flow  64 . The fluid flow  64  impinging on the surfaces  66  and the surfaces  68  thus is deflected laterally. The vanes  48  and  54  are preferably oriented at an angle of twenty-five degrees with reference to the axis  70  of the casing  12 . More specifically, the angular orientation of the vanes  48  is with reference to a plane which includes the axis  70  and the top edge  56  of the particular vane  48 . This orientation is with reference to a line or plane which connects the top edges  56  and bottom edges  58  of each particular vane  48 . Similarly, the angular orientation of the vanes  54  is with reference to a plane which includes the axis  70  and the upper edge  60  of the particular vane  54 . Since the axis  70  coincides with the direction of the fluid flow  64 , the angular orientation is also relative to the direction of fluid flow  64  entering the casing  12 . Furthermore, the vanes  48  and  54  are also oriented at an angle which is laterally clockwise from a vantage point of fluid flow  64  entering the inlet port  14 . Thus, this particular orientation of the vanes  48  and  54  deflects the fluid flow  64  laterally thereby essentially turning it and rotating it in a clockwise direction. This clockwise rotational movement of the fluid flow results in a spiral shaped movement of the fluid flow  64  that exits from the outlet end  28 . 
         [0045]    The primary vanes  48  have main portions  72 , inner lower medial end vane portions  74 , outer lower medial end vane portions  76 , inner lower end vane portions  78  and outer lower end vane portions  80  which are all flat planar. Each of the main portions  72  are angled twelve degrees with reference to the plane of their respective top edges  56  and axis  70 . The lower medial end vane portions  74  and  76  are bent along bend lines  82  so that portions  74  and  76  are angled horizontally in a clockwise direction from the vantage point of the fluid flow entering the inlet port  14  with reference to the plane that includes the top edge  56  and the axis  70  (or direction of fluid flow  64  into the inlet port  14 ). Thus, the lower medial end vane portions  74  and  76  are oriented in the same direction as main portions  72  of vanes  48 . However, in addition to being angled twelve degrees with reference to their respective main portions  72 , these lower medial end vane portions  74  and  76  are angled in the same direction as the main portions  72 , as described in detail hereinabove. Similarly, the lower end vane portions  78  and  80  are bent along bend lines  84  and  86  respectively so that portions  78  and  80  are angled horizontally in a clockwise direction from the vantage point of the fluid flow entering the inlet port  14  with reference to the plane that includes the upper edge  60  and the axis  70  (or direction of fluid flow  64  into the inlet port  14 ). Thus, as with lower medial end vane portions  74  and  76 , the lower end vane portions  78  and  80  are oriented in the same direction as main portions  72  of vanes  48 . The lower end vane portions  78  and  80  are angled twelve degrees with reference to their respective lower medial end vane portions  74  as well as angled in the same direction as the main portions  72 . Thus, the fluid flow that has been deflected horizontally by the main portions  72  is further deflected horizontally by the lower medial end vane portions  74  and  76  and subsequently by the lower end vane portions  78  and  80 . The fluid flow  64  which passes alongside the main portions  72  and thereby diverted from its previously solely longitudinal direction of movement into a horizontal direction acquires a certain degree of directional stability by the support provided by the angled main portions  72 . This directional stability of the fluid flow stream can be relatively easily changed by deflection via the lower medial end vane portion  74  and  76  and the lower end vane portions  78  and  80  in the same horizontal direction thereby increasing the degree of rotational movement imparted to the fluid flow  64 . The fluid flow  64  exiting the inlet duct  22  thus swirls to a greater degree due to the angled portions  74 ,  76 ,  78  and  80  than otherwise. Deflection of the fluid flow  64  successively in three steps is also more effective than simply angling the entire vane  48  at the same angular orientation as the lower end vane portions  78  and  80 . The bend line  86  is preferably perpendicular to the directional line of fluid flow  64 . The line  84  is preferably angled at a forty-five degree angle in the direction of fluid flow while the bend line  82  is preferably angled at a sixty degree angle in the direction of fluid flow  64 . 
         [0046]    The vanes  48  are preferably interconnected at front or inner end portions  88  via interconnection members  90 . Vanes  48  are thus formed into pairs of vanes  48 . Interconnection members  90  are preferably laterally curved while longitudinally straight such that they are semi-cylindrical in shape. The interconnection members  90  are preferably located proximal to or more preferably adjacent to the central area  92 . The members  90  are preferably oriented at an angle of twenty-five degrees relative to the plane including the top edge  56  and the axis  70 , as with the vanes  48  and  54 . Since the interconnection members  90  interconnect the vanes  48  providing structural rigidity thereto, there is no need for a support structure at the center of the inlet duct  22  to attach the vanes  48  to and thereby provide structural support thereto. Consequently, the central area  92  of the inlet duct  22  is open allowing exhaust fluid  26  to pass freely therethrough. Since the center of the inlet duct  22  cannot pragmatically incorporate structures that can effectively provide swirl to the fluid flow, the lack of a central support structure does not reduce the swirl effect provided but instead minimizes fluid flow restriction of the inlet duct  22 . 
         [0047]    The vanes  48  are preferably longitudinally longer at peripheral areas  94  of the inlet duct  22  than at the central area  92 . Thus, the rear end vane portions  96  are longer than the front end vane portions  98 . More specifically, the front end vane portions  98  are twenty-five percent of the length of the rear end vane portions  96 . Basically, this difference in length reduces the longitudinal length of the vanes  48  at the more central area where the vane  48  is less effective in producing swirl. In addition, front upper edges  99  of the vanes  48  are curved in the direction of fluid flow  64  and bottom edges  58  are also curved in the direction of fluid flow  64 . Edges  99  and  58  are curved toward each other into a converging direction so that the vanes  48  are substantially smaller at the central area  92  than at the peripheral area  94 . The front upper edges  99  and the top (or leading) edges  56  first meet the fluid flow  64  so the leading edge  56  is straight to provide larger vane  48  area at the peripheral area  94  where the vanes  48  can more effectively provide swirl while the front upper edge  99  is curved downwardly to provide smaller vane  48  surface area at the central area  94  where the vanes cannot relatively provide swirl. 
         [0048]    The vanes  54  are preferably rectangular in shape. Vanes  54  are also preferably longitudinally shorter and laterally (or axially with reference to the casing  12 ) shorter than the vanes  48 . 
         [0049]    The vanes  48  and  54  are preferably composed of stainless steel. However, other suitable materials may also be used. Similarly, the casing  12  is preferably composed of stainless steel. However, it may also be composed of galvanized steel or other suitable material. 
         [0050]    Accordingly, there has been provided, in accordance with the invention, a muffler for preventing reverse flow of exhaust fluid therethrough and for swirling the fluid flow passing therethrough that fully satisfies the objectives set forth above. It is to be understood that all terms used herein are descriptive rather than limiting. Although the invention has been described in conjunction with the specific embodiment set forth above, many alternative embodiments, modification and variations will be apparent to those skilled in the art in light of the disclosure set forth herein. Accordingly, it is intended to include all such alternatives, embodiments, modifications and variations that fall within the spirit and scope of the invention set forth in the claims hereinbelow.