Abstract:
An object of the present invention is to provide an exhaust muffling device having a simple structure, which is capable of suppressing air column resonance favorably, irrespective of a length of an outlet pipe of an exhaust pipe which communicates with a muffler, and with which it is possible to achieve appropriate reductions in weight and cost. The exhaust muffling device of the present invention comprises a muffler having an expansion chamber whose length is L in an exhaust direction, and an outlet pipe being in communication with the expansion chamber and whose pipe length is approximately 2nL in which n is a natural number.

Description:
BACKGROUND  
       [0001]     The present invention relates to an exhaust muffling device for reducing exhaust noise by suppressing air column resonance.  
         [0002]     Conventionally, in this type of exhaust muffling device installed in an internal combustion engine, a plurality of mufflers are arranged-over an exhaust pipe which communicates with an exhaust manifold in order to suppress air column resonance generated in the exhaust pipe. For example, a constitution is known whereby a length from a convergence point of an exhaust manifold to a terminal outlet of an exhaust pipe is set as a reference length (X), a first muffler is provided in a position of 3X/5 from the terminal outlet, and a second muffler is provided in a position of 2X/5 from the terminal outlet. This type of exhaust muffling device is disclosed in Japanese Laid-Open Patent Publication No.S 59-226222.    
         [0003]     By providing a plurality of mufflers in appropriate locations, this type of conventional exhaust muffling device is capable of muffling exhaust noise favorably. However, the overall weight is increased by the provision of the plurality of mufflers, which is likely to lead to an increase in cost. To consider this problem simply, if the second muffler of the exhaust muffling device described in Japanese Laid-Open Patent Publication No.S 59-226222  were omitted, for example, then a pipe length from the first muffler to the terminal outlet would increase, and especially if this length were to reach or exceed 1.5 m, for example, air column resonance would occur in the normal rotation region of the internal combustion engine, causing the sound pressure level of the exhaust gas to increase.  
       SUMMARY  
       [0004]     An object of the present invention is to provide an exhaust muffling device having a simple structure, which is capable of suppressing air column resonance favorably, irrespective of the length of an outlet pipe of an exhaust pipe connected to a muffler, and with which it is possible to achieve appropriate reductions in weight and cost.  
         [0005]     To attain the above object, an exhaust muffling device according to the present invention comprises a muffler having an expansion chamber whose length is L in an exhaust direction, and an outlet pipe which is communication with the expansion chamber and whose pipe length is approximately 2nL in which n is a natural number.  
         [0006]     According to this constitution, when air column resonance is generated inside the outlet pipe at a specific frequency corresponding to the pipe length of the outlet pipe, the length ratio is set as described above, and hence the air column resonance is suppressed effectively by means of an interference effect inside the expansion chamber.  
         [0007]     More specifically, a fundamental wavelength λ 1  of the air column resonance generated inside the outlet pipe is approximately double the pipe length of the outlet pipe, and hence to effectively suppress this air column resonance by means of an interference effect between the incident waves and reflected waves inside the expansion chamber, the length L of the expansion chamber should be set to ¼ of the fundamental wavelength λ 1 . In other words, a relationship of expansion chamber length:pipe length of outlet pipe being approximately 1:2 should be established. Similarly, a wavelength λ 2  of air column resonance at a second harmonic is approximately equal to the pipe length, and hence by setting the length L of the expansion chamber to ¼ of λ 2  in order to obtain an interference effect, a relationship of expansion chamber length: pipe length of outlet pipe being approximately 1:4 is established. Similarly with a third harmonic and fourth harmonic, relationships of 1:6 and 1:8 are established respectively.  
         [0008]     Thus in the present invention, the relationship of expansion chamber length:pipe length of outlet pipe being approximately 1:2n is established, and hence even when the pipe length of the outlet pipe is great, air column resonance of the n th  harmonic can be suppressed favorably. Note that the “approximately” of “approximately 2nL” is a concept incorporating open end correction. Namely, the pipe length of the outlet pipe defines 2nL plus δL in which δL is the open end correction (compensation).  
         [0009]     Preferably, the muffler further comprises a resonance chamber adjacent to the expansion chamber. More preferably the exhaust muffling device further comprises an inlet pipe which communicates with the resonance chamber and expansion chamber.  
         [0010]     Another exhaust muffling device according to the present invention comprises a muffler having an expansion chamber whose length is L in an exhaust direction, and an outlet pipe having a first open end face, a second open end face and a through hole formed in a peripheral wall of the outlet pipe near the first open end face. The first open end face is in communication with the expansion chamber, the through hole is in communication with an inside of the muffler, and a length of the outlet pipe from the through hole to the second open end face is approximately 2nL in which n is a natural number.  
         [0011]     According to this constitution, when air column resonance is generated inside the outlet pipe in accordance with a partial length (to be referred to hereinafter as the “substantial air column length”) of the outlet pipe from the through hole formed in the peripheral wall thereof to the second open end face, a relationship of expansion chamber length: substantial air column length being approximately 1:2n is established, and hence air column resonance of the n th  harmonic can be suppressed effectively by means of an interference effect inside the expansion chamber in a similar manner to that described above.  
         [0012]     Note that the through hole formed in the peripheral wall may communicate with the expansion chamber inside the muffler in order to function as a so-called bypass hole. Alternatively, the through hole may communicate with a resonance chamber provided separately inside the muffler in order to reduce exhaust noise of a different frequency to the frequency of the air column resonance to be suppressed. When a plurality of the through holes are formed in a longitudinal direction of the outlet pipe, the through hole which is closest to the second open end face serves as a reference for the substantial air column length.  
         [0013]     In these cases, the natural number n is preferably 1 or 2.  
         [0014]     According to this constitution, air column resonance having a long wavelength, which is typically difficult to muffle, can be suppressed appropriately.  
         [0015]     According to the exhaust muffling device of the present invention, air column resonance can be suppressed favorably using a simple structure, irrespective of the length of the outlet pipe, thereby enabling an improvement in the muffling performance. As a result, a separate sub-muffler does not have to be provided in the outlet pipe, enabling appropriate reductions in weight and cost. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0016]      FIG. 1A  is a schematic diagram showing an outline of an exhaust muffling device according to a first embodiment of the present invention;  
         [0017]      FIG. 1B  is a view showing a relational expression between an expansion chamber and an outlet pipe in  FIG. 1A ;  
         [0018]      FIG. 2A  is a view illustrating a first acoustic mode of air column resonance generated inside the outlet pipe;  
         [0019]      FIG. 2B  is a view illustrating a second harmonic of the air column resonance generated within the outlet pipe;  
         [0020]      FIG. 2C  is a view illustrating a third harmonic of the air column resonance generated within the outlet pipe;  
         [0021]      FIG. 2D  is a view illustrating a fourth harmonic of the air column resonance generated within the outlet pipe;  
         [0022]      FIG. 3  is a schematic diagram showing an outline of an exhaust muffling device according to a second embodiment of the present invention;  
         [0023]      FIG. 4  is a schematic diagram showing an outline of an exhaust muffling device according to a third embodiment of the present invention; and  
         [0024]      FIG. 5  is a schematic diagram showing an outline of an exhaust muffling device according to a fourth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0025]     An exhaust muffling device according to preferred embodiments of the present invention will be described below with reference to attached drawings. The exhaust muffling device is used to reduce exhaust noise by setting the length of an expansion chamber in a muffler and a pipe length of an outlet pipe to a predetermined ratio such that air column resonance is suppressed effectively. In the following description, an example is provided in which the exhaust muffling device is applied to an exhaust system of an internal combustion engine in an automobile.  
         [0026]     (First Embodiment)  
         [0027]     As shown in  FIG. 1 , an exhaust muffling device  1  comprises a muffler  2  for muffling the exhaust noise of exhaust gas discharged from an internal combustion engine, an inlet pipe  3  of an exhaust pipe for introducing the exhaust gas into the muffler  2  from the internal combustion engine, and an outlet pipe  4  of the exhaust pipe for discharging the exhaust gas outside from the muffler  2 .  
         [0028]     A single expansion chamber  11  having a length of L 1  in an exhaust direction is disposed in an interior of the muffler  2 . The expansion chamber  11  is formed with a larger exhaust gas passage sectional area than the inlet pipe  3 , and hence the pressure of the exhaust gas that flows into the expansion chamber  11  from the inlet pipe  3  decreases. The length L 1  of the expansion chamber  11  in this case corresponds to a distance between inner walls  12 ,  13  facing each other in the exhaust direction of the muffler  2 .  
         [0029]     One open end portion of the inlet pipe  3  communicates with an exhaust manifold, not shown in the drawing, in the internal combustion engine, and the other open end portion  21  communicates with the expansion chamber  11  via the inner wall  12  of the muffler  2 . A pipe length of the outlet pipe  4  is set to L 2 . One open end portion  31  of the outlet pipe  4  communicates with the expansion chamber  11  via the inner wall  13  of the muffler  2 , and the other open end portion  32  communicates with the outside, and is therefore open to the atmosphere.  
         [0030]     When the internal combustion engine is operative, an exhaust pulse generates air column resonance in the outlet pipe  4  in accordance with the pipe length L 2  of the outlet pipe  4 . In other words, as shown in  FIG. 2 , the first acoustic mode of the air column resonance is set with the pipe length L 2  as a half wavelength, and air column resonance is generated in wavelengths of natural number multiples of this half wavelength.  
         [0031]     More specifically, a wavelength λ 1  of air column resonance at the first acoustic mode (first harmonic) is approximately double the pipe length L 2 , taking into account open end correction of the outlet pipe  4  (see  FIG. 2A ). To suppress this air column resonance effectively by means of an interference effect between the incident waves and reflected waves inside the expansion chamber  11 , the length L 1  of the expansion chamber  11  should be set to ¼ of the wavelength λ 1 . In other words, by establishing a relationship of L 1 :L 2  being approximately 1:2, exhaust noise generated by air column resonance at the first acoustic mode is reduced.  
         [0032]     As shown in  FIG. 2B , a wavelength λ 2  of second harmonic air column resonance is approximately equal to the pipe length L 2 , and hence to obtain a similar interference effect, the length L 1  of the expansion chamber  11  should be set to ¼ of λ 2 . In other words, by establishing a relationship of L 1 :L 2  being approximately 1:4, exhaust noise generated by second harmonic air column resonance is reduced. Likewise, as shown in  FIGS. 2C and 2D , at the third harmonic, L 1  should be set to ¼ of λ 3 , and at the fourth harmonic, L 1  should be set to ¼ of λ 4 . Hence, by establishing relationships of L 1 :L 2  being approximately 1:6 and L 1 :L 2  being approximately 1:8 respectively, exhaust noise generated by air column resonance corresponding to the third and fourth harmonic is reduced.  
         [0033]     Accordingly, in this embodiment a relationship of L 2  being approximately 2nL 1  (where n is a natural number) is established, as shown in  FIG. 1B . Thus air column resonance of the nth harmonic can be suppressed favorably, enabling exhaust noise to be muffled in totality. A particularly useful muffling effect is obtained when the natural number “n” is set to 1 or 2, which corresponds to air column resonance having a long wavelength (low frequency) that is usually difficult to muffle. More specifically, when the pipe length L 2  of the outlet pipe  4  is great, for example 1.5 m or more, low frequency air column resonance can be generated in a normal rotation region of the internal combustion engine, but this air column resonance can be suppressed effectively.  
         [0034]     According to the exhaust muffling device 1  of this embodiment, the effective length of the expansion chamber  11  is set in relation to the outlet pipe  4 , and hence the muffling performance can be increased sufficiently without providing a separate muffler (sub-muffler) in the outlet pipe  4 . As a result, sub-mufflers can be omitted from the constitution of the exhaust muffling device  1 , enabling reductions in weight and cost. From the point of view of an automobile, these effects enable a reduction in the space occupied by the exhaust muffling device  1 , and hence on the automobile side, convenience is enhanced.  
         [0035]     Note that in the embodiment described above, a single expansion chamber  11  is provided in the muffler  2 , but a plurality of expansion chambers may be provided in the exhaust direction by dividing the interior of the muffler  2  using partition walls to form a so-called multistage expansion constitution. In this case, two adjacent expansion chambers may be considered as a single expansion chamber by providing an opening with a large opening ratio in the partition wall, thereby enabling communication between the adjacent expansion chambers on either side of the opening. By setting the pipe length L 2  of the outlet pipe  4  at a approximately even-numbered multiples of the total length L 2  of the plurality of expansion chambers in the exhaust direction (L 2  being approximately 2nL,), identical effects to those described above can be obtained.  
         [0036]     Next, other embodiments of the exhaust muffling device  1  according to the present invention will be described. Note that the descriptions of each of the following embodiments focus on differences with the first embodiment, and hence identical members and target lengths have been allocated identical reference symbols.  
         [0037]     (Second Embodiment)  
         [0038]     As shown in  FIG. 3 , the interior of the muffler  2  in the exhaust muffling device  1  according to a second embodiment is separated into a resonance chamber  42  disposed on the inlet pipe  3  side, and the expansion chamber  11  disposed on the outlet pipe  4  side, using a separator  41 . The length L 1  of the expansion chamber  11  in the exhaust direction corresponds to a distance from the separator  41  to the inner wall  13  of the muffler  2  near the outlet pipe  4 . The resonance chamber  42  serves to resonate with oscillation of a specific frequency in order to reduce the level of the exhaust noise caused by this oscillation. A length L 3  of the resonance chamber  42  in the exhaust direction is set to a length obtained by subtracting the length L 1  of the expansion chamber  11  from an entire length L 0  of the muffler  2  in the exhaust direction. An open end portion  21  of the inlet pipe  3  on the muffler  2  side communicates with the expansion chamber  11  via the resonance chamber  42 , and a plurality of punch holes  44  formed in the peripheral wall of the open end portion  21  face the resonance chamber  42 , and thus communicate with the resonance chamber  42 .  
         [0039]     Exhaust gas from the internal combustion engine is introduced into the expansion chamber  11  and expands therein, whereupon the exhaust gas is discharged into the atmosphere from the outlet pipe  4 . Hence in this embodiment also, by setting the relationship of L 2  being approximately 2nL, similarly to the first embodiment, air column resonance of the n th  harmonic generated inside the outlet pipe  4  can be favorably suppressed. Since the resonance chamber  42  is provided separately inside the muffler  2 , exhaust noise of a predetermined frequency, such as high frequency sound waves, can also be reduced inside the muffler  2 .  
         [0040]     (Third Embodiment)  
         [0041]     As shown in  FIG. 4 , in the exhaust muffling device  1  according to a third embodiment, the inlet pipe  3  and outlet pipe  4  communicating with the expansion chamber  11  inside the muffler  2  are not disposed coaxially. Further, these pipes  3 ,  4  are inserted more deeply into the muffler  2  than in the first embodiment such that they overlap. An entire length of the outlet pipe  4  is set to L 4 , and an open end portion  31  thereof near one open end face  33 . communicates with the expansion chamber  11 . The outlet pipe  4  comprises a plurality of bypass holes  51  (through holes) formed in a peripheral wall thereof in positions on the inner wall  13  side near the open end portion  31 . The plurality of bypass holes  51  communicate with the expansion chamber  11 .  
         [0042]     In this embodiment, air column resonance is generated inside the outlet pipe  4  in accordance with a pipe length from an open end face  34  near the other open end portion  32  of the outlet pipe  4 , which communicates with the outside, to the closest bypass hole  51  to the open end face  34 , and hence this pipe length (“the substantial air column length”) is set as L 2  (being approximately 2nL,) as described above. The exhaust gas which passes through the inlet pipe  3  and expands in the expansion chamber  11  flows into the outlet pipe  4  from the open end face  33  of the outlet pipe  4 , whereupon a part of the exhaust gas flows into the outlet pipe  4  through the plurality of bypass holes  51 . In other words, the exhaust gas flowing into the outlet pipe  4  converges at the positions of the bypass holes  51 , and is then discharged into the atmosphere.  
         [0043]     According to this embodiment, the substantial air column length of the outlet pipe  4  and the length of the expansion chamber  11  are set similarly to the first embodiment such that L 2  being approximately 2nL 1 , and hence air column resonance of the n th  harmonic can be suppressed effectively by means of an interference effect inside the expansion chamber  11 . Furthermore, by inserting the inlet pipe  3  and outlet pipe  4  into the expansion chamber  11  and employing the plurality of bypass holes  51  in the manner described above, exhaust noise can be muffled in a wider frequency range than that of the first embodiment.  
         [0044]     (Fourth Embodiment)  
         [0045]     As shown in  FIG. 5 , the interior of the muffler  2  in the exhaust muffling device  1  according to a fourth embodiment is separated into the expansion chamber  11 , which is disposed on the inlet pipe  3  side, and a resonance chamber  42 , which is disposed on the outlet pipe  4  side, using a separator  41 . The length L 1  of the expansion chamber  11  in the exhaust direction corresponds to a distance from the inner wall  12  of the muffler  2  on the inlet pipe  3  side to the separator  41 . The length L 3  of the resonance chamber  42  in the exhaust direction corresponds to a length obtained by subtracting the length L 1  of the expansion chamber  11  from the entire length L 0  of the muffler  2  in the exhaust direction.  
         [0046]     The open end face  33  of the outlet pipe  4  near the muffler  2  communicates with the expansion chamber  11 , and the plurality of punch holes  44  (through holes) formed in the peripheral wall of the open end portion  31  at the open end face  33  face the resonance chamber  42 , and thus communicate with the resonance chamber  42 . Similarly to the third embodiment, air column resonance is generated inside the outlet pipe  4  in accordance with a pipe length from the other open end face  34  of the outlet pipe  4 , which communicates with the outside, to the closest punch hole  44  to the open end face  34 , and hence this pipe length (“the substantial air column length”) is set as L 2 , which is approximately equal to 2nL 1 .  
         [0047]     The exhaust gas which expands in the expansion chamber  11  flows into the outlet pipe  4  through the open end face  33 , and is then discharged into the atmosphere. A part of the exhaust gas flowing into the outlet pipe  4  flows into the resonance chamber  42  through the punch holes  44 , and it is here that the exhaust gas resonance effect occurs.  
         [0048]     Accordingly in this embodiment, similarly to the third embodiment, the relationship of L 2  being approximately 2nL, is established, and therefore air column resonance of the n th  harmonic can be favorably suppressed. Furthermore, similarly to the second embodiment, the resonance chamber  42  is provided separately inside the muffler  2 , enabling exhaust noise of a predetermined frequency, such as high frequency sound waves, to be reduced inside the muffler  2 .  
         [0049]     The entire disclosure of Japanese Patent Application No.2003-415415 filed on Dec. 12, 2003 including specification, claims, drawings and summary are incorporated herein by reference in its entirety.