Patent Publication Number: US-2006011409-A1

Title: Exhaust system of a turbo-charged engine

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an exhaust system of a turbo-charged engine, and more particularly, relates to a structure of a muffler thereof.  
      2. Background of the Invention  
      The following two points are required for the exhaust system of a turbo-charged engine from a viewpoint of performance:  
      1) Low exhaust back pressure  
      With the turbo-charged engine, due to its large amount of exhaust gas, a large capacity of a catalyst needs to be provided upstream of a muffler, so that an engine exhaust back pressure is likely to be high. In order to increase an engine output, the engine exhaust back pressure is required to be lowered.  
      2) Lowering a flow noise having a high-frequency  
      With the turbo-charged engine, due to its large amount of exhaust gas and its high velocity of exhaust gas, a relatively large gas flow noise (a high-frequency noise) is generated, so that a subsidiary muffler  1  (a muffler having a high-frequency resonance chamber, a front muffler  1  illustrated in  FIGS. 8 and 9 ) is required to be provided. For example, though a muffler disclosed in Japanese Utility Model Publication HEI 6-25506 has a high-frequency resonance chamber, since an inner pipe  5  is throttled at an inlet of the muffler relative to an exhaust pipe, a pressure loss is generated at the throttled inlet and, as a result, the low exhaust back pressure required to the exhaust system of the turbo-charged engine cannot be obtained.  
      Further, in order to decrease a columnar resonance noise (a low-frequency noise) generated due to a length of the exhaust pipe at a deceleration time and to cut a noise transmission through a pipe, a main muffler  2  (a rear muffler  2  illustrated in  FIGS. 8 and 10 ) having an enlarged chamber  3  (and a low-frequency resonance chamber  7 ) and a discontinuous pipe  6  is provided. However, with the main muffler  2  having an enlarged chamber, a gas flow noise (a high-frequency noise) is generated in the main muffler due to turbulence of the gas flow at the inlet of the pipe  6   a,  so that in the main muffler or downstream of the main muffler, another muffler  4  for decreasing the high-frequency noise is needed.  
      For the conventional main muffler (rear muffler), a muffler having an enlarged chamber and having the same structure and size as those of a usual engine (non turbo-charged engine) is in order to unify parts. As a result, the following problems are likely to happen:  
      a) It is difficult to obtain a low exhaust back pressure  
      In the main muffler  2  having the enlarged chamber  3 , since the exhaust gas flow passage has a U-turn portion and changes in cross-sectional area, a pressure loss is large so that it is difficult to obtain a low exhaust back pressure.  
      b) A gas flow noise (a high-frequency noise) is likely to be generated.  
      A gas flow noise (a high-frequency noise) is generated in the rear main muffler. Particularly, at the inlet of the pipe  6   a  the gas flow is turbulent and generates a gas flow noise. In order to decrease the noise, a high-frequency noise decreasing muffler  4  is installed in the main muffler, accompanied by an increase in the size of the muffler  2  and a complicated structure of the muffler  2 .  
      c) A volume of the muffler is large and a noise decreasing efficiency of the muffler is low.  
      Despite that an exhaust energy is absorbed at the turbine of the turbo-charged engine and a noise of an engine combustion order (low-frequency noise) is unlikely to be transmitted to a downstream portion, the conventional main muffler of the turbo-charged engine has substantially the same structure as that of a usual engine (non turbo-charged engine) which is designed so as to decrease the noise of the engine combustion order. This means that the volume of the conventional main muffler of the turbo-charged engine is unnecessarily large and the noise decreasing efficiency per a unit volume is low.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide an exhaust system of a turbo-charged engine thereby obtaining a low exhaust pressure, decreasing a gas flow noise, and decreasing a muffler volume.  
      The above object can be attained by an exhaust system of a turbo-charged engine according to the present invention as follows:  
      (1) An exhaust system of a turbo-charged engine according to the present invention includes an exhaust pipe connected to a turbo-charged engine, and a front muffler and a rear muffler disposed in the exhaust pipe in an order of the front muffler and the rear muffler in a direction of flow of exhaust gas.  
      Each muffler of the front muffler and the rear muffler includes an inner pipe. The inner pipe extends straight and continuously from an inlet to an outlet of the each muffler and is not throttled in diameter between the inlet and the outlet of the each muffler. The each muffler does not include an enlarged chamber in the each muffler.  
      (2) The front muffler includes a high-frequency resonance chamber outside the inner pipe of the front muffler, and the rear muffler includes a high-frequency resonance chamber and a low-frequency resonance chamber outside the inner pipe of the rear muffler.  
      (3) The low-frequency resonance chamber and the high-frequency resonance chamber outside the inner pipe of the rear muffler are disposed in an order of the low-frequency resonance chamber and the high-frequency resonance chamber in the direction of flow of the exhaust gas.  
      (4) Each of the inner pipe of the front muffler and the inner pipe of the rear muffler includes, at a portion of the each inner pipe outside which the high-frequency resonance chamber is located, a louver hole formed therein for letting an interior of the each inner pipe and the high-frequency resonance chamber outside the portion of the each inner pipe communicate with each other.  
      (5) An interior of each of the high-frequency resonance chamber outside the inner pipe of the front muffler and the high-frequency resonance chamber outside the inner pipe of the rear muffler is a vacant space which is not filled with any sound absorbing material.  
      With the exhaust system of a turbo-charged engine according to item (1) above, since the inner pipe extends straight and is not throttled in diameter from the inlet to the outlet of the muffler, a pressure loss is small and the exhaust back pressure is very low. As a result, the output of the turbo-charged engine is improved to a great extent.  
      Further, since there is no enlarged chamber in the muffler, a flow turbulence which is generated at the inlet of the inner pipe from the enlarged chamber in the conventional muffler is unlikely to be generated, so that a gas flow noise is decreased.  
      Furthermore, since there is no enlarged chamber in the muffler, the volume of the muffler can be minimized by selecting a volume of the low-frequency resonance chamber as small as possible.  
      With the exhaust system of a turbo-charged engine according to item (2) above, since the front muffler includes a high-frequency resonance chamber outside the inner pipe of the front muffler, and the rear muffler includes a high-frequency resonance chamber and a low-frequency resonance chamber outside the inner pipe of the rear muffler, both a gas flow noise (i.e., a high-frequency noise) and a noise due to a columnar resonance at a deceleration time (i.e., a low-frequency noise) can be decreased.  
      With the exhaust system of a turbo-charged engine according to item (3) above, since the low-frequency resonance chamber and the high-frequency resonance chamber outside the inner pipe of the rear muffler are disposed in an order of the low-frequency resonance chamber and the high-frequency resonance chamber in the direction of flow of the exhaust gas, a gas flow noise issued from a rear end of the exhaust pipe can be suppressed.  
      With the exhaust system of a turbo-charged engine according to item (4) above, since the inner pipe includes a louver hole formed therein for letting an interior of the inner pipe and the high-frequency resonance chamber outside the inner pipe communicate with each other, in spite of that there is no sound absorption material such as glass wool, substantially the same noise muffling effect as that of a high-frequency chamber filled with sound absorption material can be obtained by use of the louver hole.  
      With the exhaust system of a turbo-charged engine according to item (5) above, since no sound absorption material is provided, there is no fear of scattering of sound absorption material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features, and advantages of the present invention will become apparent and will be more readily appreciated from the following detailed description of the preferred embodiments of the present invention in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a system diagram of an exhaust system of a turbo-charged engine according to the present invention;  
       FIG. 2  is a schematic cross-sectional view of a front muffler of the exhaust system of the turbo-charged engine according to the present invention;  
       FIG. 3  is a schematic cross-sectional view of a rear muffler of the exhaust system of the turbo-charged engine according to the present invention;  
       FIG. 4  is a plan view of the exhaust system of a turbo-charged engine according to the present invention;  
       FIG. 5  is system diagrams of three cases A, B and C, specifications of which are different from each other;  
       FIG. 6  is a graph of sound pressure level versus engine speed of cases A, B and C of  FIG. 5 ;  
       FIG. 7  is a graph of sound pressure level versus frequency, based on data taken out at 4000 rpm of engine speed of  FIG. 6 ;  
       FIG. 8  is a system diagram of a conventional exhaust system of a turbo-charged engine;  
       FIG. 9  is a schematic cross-sectional view of a front muffler of the conventional exhaust system of the turbo-charged engine; and  
       FIG. 10  is a schematic cross-sectional view of a rear muffler of the conventional exhaust system of the turbo-charged engine. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      An exhaust system of a turbo-charged engine according to the present invention will be explained with reference to  FIGS. 1-7 .  
      An exhaust system  10  of a turbo-charged engine according to the present invention includes an exhaust pipe  11  connected to a turbo-charged engine, and a muffler  12  and/or  13  disposed in the exhaust pipe  11 . The muffler  12  and/or  13  includes at least one muffler of a front muffler (a main muffler)  12  and a rear muffler (a subsidiary muffler)  13 . The muffler  12  and/or  13  may be one rear muffler  13 . In a case where both the front muffler  12  and the rear muffler  13  are provided, the front muffler  12  and the rear muffler  13  may be provided separately from each other or may be provided integrally with each other.  
      In  FIG. 1 , the turbo-charged engine of  FIG. 1  is an eight- or six-cylinder diesel or gasoline engine of a V-type having right and left banks. On an exhaust pipe from each bank, a turbine of a turbocharger is disposed, and a diesel particulate removal apparatus and a catalytic converter are disposed downstream of the turbine in an exhaust gas flow direction. Only the catalytic converter may be disposed. Downstream of the catalytic converter, the front muffler  12  is disposed. Downstream of the front muffler  12 , the exhaust pipes from the right and left banks are collected into a single pipe, and in the single pipe the rear muffler  13  is disposed. The engine is not limited to a V-type engine and may be an in-line engine. The number of cylinders is not limited to eight or six and may be, for example, twelve and four. The turbine and the front muffler may be disposed in the single pipe downstream of a collecting point of the right and left pipes of V-type engine or may be disposed on the single pipe of the in-line engine.  
      As illustrated in  FIGS. 2 and 3 , each muffler of the front muffler  12  and the rear muffler  13  includes an inner pipe  14  and an outer casing  15  provided outside the inner pipe  14 .  
      The inner pipe  14  extends straight and continuously from an inlet  16  to an outlet  17  of each muffler  12 ,  13 . The inner pipe  14  is not throttled in diameter between the inlet  16  and the outlet  17  of each muffler  12 ,  13 . Enlarging the cross-sectional area of the inner pipe is permitted but throttling the cross-sectional area of the inner pipe is not permitted from the viewpoint of suppressing a pressure loss. The inner pipe  14  does not have a plate extending perpendicular to a flow direction.  
      Each of the front muffler  12  and the rear muffler  13  does not have an enlarged chamber in the muffler.  
      A space between the inner pipe  14  and the outer casing  15  provided outside the inner pipe  14  defines a resonance chamber.  
      The front muffler  12  includes a high-frequency resonance chamber  18  outside the inner pipe  14  and does not include a low-frequency resonance chamber. The rear muffler  13  includes a low-frequency resonance chamber  19  and a high-frequency resonance chamber  18  outside the inner pipe  14 .  
      The low-frequency resonance chamber  19  and the high-frequency resonance chamber  18  outside the inner pipe  14  of the rear muffler  13  are disposed in an order of the low-frequency resonance chamber  19  and the high-frequency resonance chamber  18  in the direction of flow of the exhaust gas.  
      The high-frequency resonance chamber  18  muffles a gas flow noise (a high-frequency noise equal to or higher than 1 kHz sounding “sha-”). The low-frequency resonance chamber  19  muffles a noise at an engine deceleration time and a noise of an engine combustion order (a low-frequency noise lower than 1 kHz, for example, 150-200 Hz, sounding “bo-”).  
      Each of the inner pipe  14  of the front muffler  12  and the inner pipe  14  of the rear muffler  13  includes, at a portion of the each inner pipe outside which the high-frequency resonance chamber  18  is located, a plurality of louver holes  20  formed therein for letting an interior of the inner pipe  14  and the high-frequency resonance chamber  18  outside the portion of the inner pipe  14  communicate with each other. The louver holes  20  are directed outwardly in a radial direction of the inner pipe  14 . The louver holes  20  are manufactured by forming slits in a flat plate or a pipe and then imposing a tension to the flat plate or the pipe in a direction perpendicular to the slits. The louver holes  20  have a hole shape other than a circle.  
      An interior of each of the high-frequency resonance chamber  18  outside the inner pipe  14  of the front muffler  12  and the high-frequency resonance chamber  18  outside the inner pipe  14  of the rear muffler  13  is a vacant space which is not filled with any sound absorbing material such as glass wool.  
      The low-frequency resonance chamber  19  and the high-frequency resonance chamber  18  outside the inner pipe  14  of the rear muffler  13  are separated from each other by a partition  21 .  
      The low-frequency resonance chamber  19  outside the inner pipe  14  of the rear muffler  13  includes a column defining space defined between the inner pipe  14  and a skirt  22  coupled to the inner pipe  14 . One end of the column defining space is open to a space defined between the inner pipe  14  and the outer casing  15 , and the other end of the column defining space is closed and communicates with an exhaust gas passage inside the inner pipe  14  through a communicating opening  23  formed in the inner pipe  14 . The communicating opening  23  may be a circular hole or a slit extending a circumferential direction of the inner pipe  14 .  
      With the conventional exhaust system, the rear muffler having an enlarged chamber is larger in volume than the front muffler, and the rear muffler is a main muffler and the front muffler is a subsidiary muffler.  
      In contrast, with the exhaust system according to the present invention, as illustrated in  FIG. 4 , since the rear muffler does not have an enlarged chamber and is made compact, the rear muffler  13  is smaller in volume than each of the two front mufflers  12 . As a result, the front muffler  12  is a main muffler and the rear muffler  13  is a subsidiary muffler. For example, in the present invention, a volume of each of the two front mufflers  12  is 9.3 L (where L is Liter which is equal to 1,000 cm 3 ), and a volume of the single rear muffler  13  is 7.5 L (where L is Liter).  
      Tests for investigating exhaust back pressure were conducted using a V-type eight-cylinder diesel engine. In the case of the exhaust system of the conventional turbo-charged engine, the engine back pressure exceeds 50 kPa, while in the case of the exhaust system of the turbo-charged engine  10  according to the present invention where a diesel particulate purification apparatus (DPNR), an oxidation catalyst apparatus ( 20 R), the main muffler  12  (M/M), the subsidiary muffler  13  (S/M) were disposed in that order, the engine back pressure was decreased by about 8 kPa compared with that of the conventional engine whereby a very low engine back pressure was obtained.  
      Further, compared with the pressure loss at the main muffler (the rear muffler having an enlarged chamber) of the conventional exhaust system, the pressure loss at the main muffler (the front muffler having no enlarged chamber) of the exhaust system  10  according to the present invention was lowered to a pressure as low as about 5 kPa. The pressure loss at the DPNR and the pressure loss at the oxidation catalyst ( 20 NR) were equal to each other with the exhaust system according to present invention and the conventional one.  
       FIGS. 5-7  illustrate the test results about exhaust noise using a plurality of cases A, B and C having different specifications. Engines used in the test were a V-type eight cylinder diesel engine having an intake air volume of 296 g/s, an output of 210 kW and a torque of 660 Nm.  
      As illustrated in  FIG. 5 , case A was a case having the subsidiary muffler (the rear muffler  13 ), and the diameter of the inner pipe in the subsidiary muffler was 61 mm.  
      Case B was a case having no subsidiary muffler (no rear muffler  13 ) and the diameter of the exhaust pipe was 70 mm.  
      Case C was a case having no subsidiary muffler (no rear muffler  13 ) and the diameter of the exhaust pipe was 60.5 mm.  
      As shown in  FIG. 6 , in acceleration, a higher exhaust noise decreasing effect was obtained in case A (where the subsidiary muffler was provided and the pipe diameter was 61 mm) than in case C (where no subsidiary muffler was provided and the pipe diameter was 60.5 mm), and a higher exhaust noise decreasing effect was obtained in case B (where no subsidiary muffler was provided and the pipe diameter was 70 mm) than in case A (where the subsidiary muffler was provided and the pipe diameter was 61 mm). It can be understood from  FIG. 6  that, in acceleration, the gas flow noise was predominant, and the noise was smaller in case A having the subsidiary muffler than in case C having no subsidiary muffler. Further, it can be understood that when the pipe diameter was large (that is, when the pipe was not throttled), the exhaust noise decreasing effect became high.  
      As shown in  FIG. 6 , in deceleration, a higher noise (noise of an engine combustion order) decreasing effect was obtained in case A (where the subsidiary muffler was provided and the pipe diameter was 61 mm) than in case B (where no subsidiary muffler was provided and the exhaust pipe diameter was 61 mm). It can be understood from  FIG. 6  that, in deceleration, providing the subsidiary muffler (the rear muffler  13 ) is effective for decreasing the exhaust noise.  
       FIG. 7  illustrates the results of frequency analysis at the time of a full load at 4000 rpm and at the time of no load, in  FIG. 6 .  
      From  FIG. 7 , it can be seen that, in the high-frequency region (gas flow noise region) higher in frequency than 1 kHz, a higher exhaust noise decreasing effect was obtained in case A (where the subsidiary muffler was provided and the pipe diameter was 61 mm) than in case C (where no subsidiary muffler was provided and the pipe diameter was 60.5 mm), and a higher exhaust noise decreasing effect was obtained in case B (where no subsidiary muffler was provided and the pipe diameter was 70 mm) than in case A (where the subsidiary muffler was provided and the pipe diameter was 61 mm). It can be understood from  FIG. 7  that, in acceleration, the gas flow noise was predominant, and that when the pipe diameter was large (that is, when the pipe was not throttled), the exhaust noise decreasing effect became high.  
      Effects and technical advantages of the present invention will now be explained.  
      In the exhaust system  10  of a turbo-charged engine according to the present invention, since the inner pipe  14  extends straight and is not throttled in diameter from the inlet to the outlet of the muffler  12 ,  13  compared with the exhaust pipe, a pressure loss at the muffler  12 ,  13  is small, whereby the exhaust back pressure is very low and can be lower than an objective back pressure of 50 kPa. As a result, the output of the turbo-charged engine is improved to a great extent.  
      In the conventional turbo-charged engine, since exhaust energy is retrieved by a turbine and a compressor is driven by the retrieved energy thereby charging a large amount of air to the engine, an output of the engine is large and an exhaust gas volume is large. Therefore, if a main muffler having an enlarged chamber is installed in the exhaust pipe as in the conventional exhaust system, a pressure loss at the main muffler becomes too large, so that it is difficult to effectively obtain a large engine output. In contrast, in the exhaust system  10  according to the present invention, since a pressure loss at the muffler  12 ,  13  is small, the exhaust back pressure of the engine is very low, whereby the engine output is extremely improved.  
      Further, since there is no enlarged chamber in the muffler  12 ,  13 , a flow turbulence which is generated at the inlet of the inner pipe from the enlarged chamber in the conventional muffler is unlikely to be generated, so that a gas flow noise which is generated in the conventional muffler having the enlarged chamber is decreased. In the conventional muffler of the type of the enlarged chamber, in order to decrease a gas flow noise, a high-frequency noise decreasing muffler of a large size having sound absorbing material therein is provided downstream of the enlarged chamber. In contrast, according to the present invention, the rear muffler  13  can be made compact and the high-frequency resonance chamber  18  of the rear muffler  13  can be made compact.  
      Further, since there is no enlarged chamber in the muffler  12 ,  13 , the volume of the muffler can be minimized by selecting a volume of the low-frequency resonance chamber  19  as small as possible.  
      Since a noise of an engine combustion order (low-frequency noise) is unlikely to pass through the turbine and to be transmitted through an exhaust pipe downstream of the turbine, the low-frequency resonance chamber of a large volume does not need to be provided. Despite that, in the conventional engine, the enlarged chamber-type muffler having an unnecessarily large low-frequency resonance chamber is used for unifying parts between the turbo-charged engine and a non-turbo-charged engine. In contrast, in the present invention, by selecting the volume of the low-frequency resonance chamber  19  in the rear muffler  13  as small as possible, the volume of the muffler can be minimized. As a result, mounting the muffler to a vehicle is easy.  
      Since the front muffler  12  includes the high-frequency resonance chamber  18  outside the inner pipe  14  of the front muffler  12 , and the rear muffler  13  includes the high-frequency resonance chamber  18  and the low-frequency resonance chamber  19  outside the inner pipe  14  of the rear muffler  13 , both a gas flow noise (i.e., a high-frequency noise) at an acceleration time and a noise due to a columnar resonance at a deceleration time (i.e., a low-frequency noise) can be decreased.  
      Since the low-frequency resonance chamber  19  and the high-frequency resonance chamber  18  outside the inner pipe  14  of the rear muffler  13  are disposed in the order of the low-frequency resonance chamber  19  and the high-frequency resonance chamber  18  in the direction of flow of the exhaust gas, a gas flow noise issued from a rear end of the exhaust pipe can be effectively suppressed.  
      Since the inner pipe  14  includes a louver hole or holes  20  formed therein for letting an interior of the inner pipe  14  and the high-frequency resonance chamber  18  outside the inner pipe  14  communicate with each other, in spite of that there is no sound absorption material such as glass wool, substantially the same noise muffling effect as that of a high-frequency chamber  18  filled with sound absorption material can be obtained by the louver hole  20 .  
      Further, since no sound absorption material is provided, there is no fear of scattering of the sound absorption material and of deterioration of a noise absorbing characteristic due to scattering of the sound absorption material.  
      Although the present invention has been described above with reference to specific exemplary embodiments, it will be appreciated by those skilled in the art that various modification and alterations can be made to the particular embodiments shown without materially departing from the novel teachings and advantages of the present invention. Accordingly, it is to be understood that all such modifications and alterations are included within the sprit and scope of the present invention as defined by the following claims.