Patent Publication Number: US-2017370378-A1

Title: Compressor and supercharging system of internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Japan application serial no. 2016-127492, filed on Jun. 28, 2016. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a compressor and a supercharging system of an internal combustion engine equipped with the compressor. 
     Description of Related Art 
     The compressor of a supercharger includes a compressor housing, which constitutes a part of an intake flow passage of an internal combustion engine, and a compressor impeller disposed rotatably in the compressor housing. The compressor impeller is connected to a turbine impeller, which is disposed rotatably in a turbine housing that constitutes a part of an exhaust flow passage of the internal combustion engine, through a rotating shaft. When the turbine impeller rotates due to the energy of an exhaust air, the compressor impeller rotates as well, and the intake air is discharged toward an annular scroll passage formed around the compressor impeller, so as to be boosted. 
     Patent Literature 1 has disclosed a technique of creating a swirling flow in the same direction as the rotation direction of the compressor impeller with respect to the intake air that flows into the compressor impeller. According to the technique of Patent Literature 1, the swirling flow with respect to the intake air that flows into the inlet of the compressor impeller via a main intake flow passage is created through formation of a swirling intake flow passage that goes over the entire circumference around the main intake flow passage leading to the inlet of the compressor impeller. The range of the intake flow rate (also referred to as “flow rate range” hereinafter) that the compressor can supercharge has a lower limit. If it drops below the lower limit, a stall may occur, but it is considered possible to lower the lower limit by creating such a swirling flow. 
     According to the technique of Patent Literature 1, a branch intake flow passage that branches from the main intake flow passage to the swirling intake flow passage is disposed to introduce a part of the main flow flowing through the main intake flow passage to the swirling intake flow passage, so as to create the swirling flow described above. In addition, according to the technique of Patent Literature 1, an intake flow passage regulating valve is disposed in the portion where the main intake flow passage and the branch intake flow passage branch off, and an inclination angle of the intake flow passage regulating valve with respect to the main flow is varied in a range of 0° to 90°, so as to adjust the amount of the intake air introduced from the main flow into the branch intake flow passage, i.e., the speed of the swirling flow. 
     PRIOR ART LITERATURE 
     Patent Literature 
     Patent Literature 1: Japanese Patent Publication No. 2011-111988 
     SUMMARY OF THE INVENTION 
     Problem to be Solved 
     In the technique of Patent Literature 1 as described above, the swirling flow is generated by introducing a part of the main flow of the intake air that flows into the turbine impeller into the branch intake flow passage. Therefore, the generated swirling flow may not reach a sufficient speed. Moreover, since the technique of Patent Literature 1 disposes the intake flow passage regulating valve in the main intake flow passage to introduce a part of the main flow into the branch intake flow passage, it enhances the pressure drop in the intake flow passage and raises the concern that a sufficient amount of air may not be supplied to the internal combustion engine. 
     The invention provides a compressor and a supercharging system of an internal combustion engine including the compressor, wherein the compressor is capable of generating a swirling flow at a sufficient speed with respect to a main flow of a fluid that flows into a compressor impeller without hindering the main flow. 
     Solution to the Problem 
     (1) A compressor (for example, the compressors  6  and  6 ′ which will be described later) is for compressing a fluid that flows through a fluid flow passage. The compressor includes: an impeller (for example, the compressor impeller  8  which will be described later) being rotatable around a rotating shaft (for example, the rotating shaft  21  which will be described later); a shroud (for example, the shroud  721  which will be described later) covering a side portion (for example, the tip end edge  843  which will be described later) of the impeller and constituting a part of the fluid flow passage; a fluid duct (for example, the intake duct  73  which will be described later) being tubular and extending along an axial direction of the impeller and introducing the fluid to a front edge (for example, the front edge portion  841  which will be described later) of the impeller; a scroll flow passage (for example, the scroll flow passage  773  which will be described later) being annular around the rotating shaft, wherein a flow passage cross-sectional area of the scroll flow passage gradually decreases along a circumferential direction of the impeller from a base end side (for example, the side of the base end portion  771  which will be described later) where a fluid introduction portion (for example, the swirling gas introduction portion  774  which will be described later) is disposed toward a distal end side (for example, the side of the distal end portion  772  which will be described later); and a fluid ejection passage (the swirling gas ejection passage  78  which will be described later) extending along a radial direction of the impeller and connecting an inside of the scroll flow passage and an inside of the fluid duct. The fluid introduction portion is connected to a portion on a downstream side of the front edge of the impeller in the fluid flow passage. 
     (2) In this case, preferably an angle formed by an extending direction of the fluid ejection passage and an inner peripheral surface in the fluid duct is an acute angle. 
     (3) In this case, preferably the flow passage cross-sectional area of the scroll flow passage gradually decreases along a direction the same as a rotation direction of the impeller from the base end side toward the distal end side. 
     (4) In this case, preferably the flow passage cross-sectional area of the scroll flow passage gradually decreases along a direction opposite to a rotation direction of the impeller from the base end side toward the distal end side. 
     (5) In this case, preferably the compressor further includes a compressor housing (for example, the compressor housing  7  which will be described later) formed with the fluid duct, the shroud, the scroll flow passage, and a high pressure flow passage (for example, the diffuser chamber  74  and the main scroll flow passage  75  which will be described later), which is a part of the fluid flow passage and through which a fluid discharged from a rear edge (for example, the rear edge portion  842  which will be described later) of the impeller flows. The fluid introduction portion is connected to the shroud or the high pressure flow passage in the compressor housing. 
     (6) In this case, preferably the fluid introduction portion is connected to the shroud. 
     (7) In this case, preferably a diffuser chamber (for example, the diffuser chamber  74  which will be described later) is disposed in the compressor housing and the diffuser chamber is a part of the high pressure flow passage and decelerates the fluid discharged from the rear edge of the impeller in the radial direction, and the fluid introduction portion is connected to the diffuser chamber. 
     (8) In this case, preferably a main scroll flow passage (for example, the main scroll flow passage  75  which will be described later) that is annular around the rotating shaft is disposed in the compressor housing and the main scroll flow passage is a part of the high pressure flow passage and the fluid discharged from the rear edge of the impeller in the radial direction flows through the main scroll flow passage, and the fluid introduction portion is connected to the main scroll flow passage. 
     (9) A supercharging system (for example, the supercharging system S which will be described later) of an internal combustion engine includes: a compressor (for example, the compressor  6 ′ which will be described later) disposed in an intake flow passage (for example, the intake flow passage  92  which will be described later) of the internal combustion engine (for example, the internal combustion engine  91  which will be described later); a turbine (for example, the turbine  3  which will be described later) disposed in an exhaust flow passage (for example, the exhaust flow passage  93  which will be described later) of the internal combustion engine; and a rotating shaft (for example, the rotating shaft  21  which will be described later) connecting an impeller (for example, the compressor impeller  8  which will be described later) of the compressor and an impeller (for example, the turbine impeller  5  which will be described later) of the turbine. The compressor described in (1) or (2) is used as the compressor, and the fluid introduction portion is connected to an upstream side of the impeller of the turbine in the exhaust flow passage. 
     Effects of the Invention 
     (1) According to the invention, the tubular fluid duct that extends along the axial direction of the impeller and introduces the main flow of the fluid to the front edge of the impeller, the scroll flow passage which is annular around the rotating shaft and has the flow passage cross-sectional area that gradually decreases along the circumferential direction of the impeller from the base end side where the fluid introduction portion is disposed toward the distal end side, and the fluid ejection passage which extends along the radial direction of the impeller and connects the inside of the scroll flow passage and the fluid duct through which the main flow flows are disposed. Thus, the fluid introduced from the fluid introduction portion to the scroll flow passage is accelerated along the circumferential direction while flowing through the scroll flow passage, and is ejected into the fluid duct via the fluid ejection passage, so that a swirling flow is created along the circumferential direction with respect to the main flow that flows through the fluid duct. Thus, the fluid easily flows to the front edge of the impeller, so that the lower limit of the flow rate range of the compressor can be lowered. Moreover, according to the invention, the fluid introduction portion, which is the inlet of the scroll flow passage, is connected to the portion on the downstream side of the front edge of the impeller in the fluid flow passage. Here, the portion on the downstream side of the front edge of the impeller in the fluid flow passage has a total pressure, which is a combination of a static pressure and a dynamic pressure, higher than that inside the fluid duct. Accordingly, the invention generates the swirling flow by using the fluid recirculated due to such differential pressure. Thus, according to the invention, the swirling flow can be generated without using a part of the main flow that flows through the fluid duct. Therefore, the generated swirling flow has a sufficient speed as compared with the conventional art. Furthermore, according to the invention, since the swirling flow is generated by recirculating the fluid, there is no need to dispose a device hindering the main flow in the fluid duct. Thus, the pressure drop in the fluid duct is not worsened. 
     (2) According to the invention, the angle formed by the extending direction of the fluid ejection passage, which connects the inside of the scroll flow passage and the inside of the fluid duct, and the inner peripheral surface of the fluid duct is set to an acute angle, by which a swirling flow having an axial velocity component can be ejected from the fluid ejection passage. Thus, the lower limit of the flow rate range can be further lowered. 
     (3) According to the invention, the flow passage cross-sectional area of the scroll flow passage is gradually decreased along the same direction as the rotation direction of the impeller. Thus, the fluid introduced from the fluid introduction portion to the scroll flow passage is accelerated in the same direction as the rotation direction of the impeller in the process of flowing from the base end side toward the distal end side, and is ejected into the fluid duct via the fluid ejection passage, so as to create a swirling flow in the same direction as the rotation direction of the impeller with respect to the main flow that flows through the fluid duct. When such a swirling flow is created in the fluid flowing to the front edge of the impeller, as will be described later with reference to  FIG. 6  and  FIG. 7 , the relative inflow angle of the fluid on the radial outer side of the impeller decreases and the fluid flows easily to the front edge of the impeller. Thus, the lower limit of the flow rate range of the compressor can be further lowered. 
     (4) According to the invention, the flow passage cross-sectional area of the scroll flow passage is gradually decreased along the direction opposite to the rotation direction of the impeller. Thus, the fluid introduced from the fluid introduction portion to the scroll flow passage is accelerated in the direction opposite to the rotation direction of the impeller in the process of flowing from the base end side toward the distal end side, and is ejected into the fluid duct via the fluid ejection passage, so as to create a swirling flow in the direction opposite to the rotation direction of the impeller with respect to the main flow that flows through the fluid duct. Here, in a state close to a stall, that is, when the flow rate of the fluid is close to the lower limit, the main flow in the fluid duct in the vicinity of the shroud on the radial outer side of the impeller tends to follow the rotation direction of the impeller. On the other hand, the invention creates the swirling flow in the direction opposite to the rotation direction of the impeller to reduce the following. Therefore, the lower limit of the flow rate range of the compressor can be further lowered. However, in the case where the direction of the scroll flow passage is set opposite to the rotation direction of the impeller as in the invention, in order to reduce only the following and not to cause great influence on the entire main flow flowing through the fluid duct, the flow rate of the fluid ejected from the fluid ejection passage into the fluid duct is preferably set to about 10% or less of the flow rate of the entire fluid that flows into the impeller. 
     (5) According to the invention, the compressor housing is formed with the fluid duct, the shroud, the scroll flow passage, and the high pressure flow passage, and the fluid introduction portion of the scroll flow passage is connected to the shroud or the high pressure flow passage. Since the fluid can be recirculated in the compressor housing, the size of the entire compressor can be reduced. That is, in a case where a fluid supply source connected to the fluid introduction portion is disposed outside the compressor housing, it is necessary to dispose piping independent of the compressor housing. According to the invention, however, such piping is not required. Moreover, in the fluid flow passage, the shroud and the high pressure flow passage are in the vicinity of the impeller, and the total pressure is higher than the other portions. Accordingly, by recirculating the fluid from such portions, a fast swirling flow can be generated. 
     (6) According to the invention, a fast swirling flow can be generated by connecting the fluid introduction portion and the shroud. In addition, because the shroud, the fluid duct, and the scroll flow passage are disposed at positions close to one another, according to the invention, the flow passage connecting the fluid introduction portion and the shroud can be shortened. Thus, the pressure drop in the flow passage can be suppressed. 
     (7) According to the invention, a fast swirling flow can be generated by connecting the fluid introduction portion and the diffuser chamber. In addition, because the diffuser chamber, the fluid duct, and the scroll flow passage are disposed at positions close to one another, according to the invention, the flow passage connecting the fluid introduction portion and the diffuser chamber can be shortened. Thus, the pressure drop in the flow passage can be suppressed. 
     (8) According to the invention, a fast swirling flow can be generated by connecting the fluid introduction portion and the main scroll flow passage. 
     (9) In the supercharging system of the internal combustion engine of the invention, the fluid introduction portion, which is the inlet of the scroll flow passage, is on the downstream side of the front edge of the impeller in the fluid flow passage, and is further connected to the upstream side of the turbine impeller in the exhaust flow passage. That is, according to the invention, the so-called high pressure external EGR gas is supplied to the fluid introduction portion to generate a swirling flow. Thus, in addition to the effect of generating a high-speed swirling flow, it is also possible to achieve effects, such as reduction of NOx in the exhaust air and improvement of fuel efficiency, that are expected by recirculating a part of the exhaust air to the intake air. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing the configuration of a supercharger that uses a compressor according to the first embodiment of the invention. 
         FIG. 2  is a perspective view of a compressor impeller. 
         FIG. 3  is a cross-sectional view of a compressor housing taken along the line III-III. 
         FIG. 4  is a perspective cross-sectional view of the compressor housing taken along a plane including the axis. 
         FIG. 5  is a cross-sectional view of the compressor housing, which schematically shows a position for disposing a gas acquisition port. 
         FIG. 6  is a diagram schematically showing a variation of a velocity triangle at a front edge portion of a compressor impeller when a swirling flow is generated in an axial flow passage by an axial flow swirler. 
         FIG. 7  is a diagram showing a radial distribution of a relative inflow angle at the front edge portion of the compressor impeller. 
         FIG. 8  is a diagram showing the configuration of a supercharging system of an internal combustion engine according to the second embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     The first embodiment of the invention is described hereinafter with reference to the figures.  FIG. 1  is a cross-sectional view showing the configuration of a supercharger  1  that uses a compressor according to the present embodiment. 
     The supercharger  1  includes a bearing housing  2 , a turbine  3  assembled to one end side of the bearing housing  2 , and a compressor  6  assembled to the other end side of the bearing housing  2 . The bearing housing  2  includes a rod-shaped rotating shaft  21  and a bearing  22 . The rotating shaft  21  extends between the turbine  3  and the compressor  6 . The bearing  22  rotatably supports the rotating shaft  21 . 
     The turbine  3  includes a turbine housing  4  and a turbine impeller  5 . The turbine housing  4  constitutes a part of an exhaust flow passage through which an exhaust air of an internal combustion engine (not shown) flows, and the turbine impeller  5  is disposed in the turbine housing  4 . The turbine  3  converts energy of the exhaust air that flows through the exhaust flow passage into mechanical power. 
     An exhaust introduction duct (not shown) connected to the exhaust flow passage of the internal combustion engine, an annular turbine scroll flow passage  42  through which the exhaust air introduced from the exhaust introduction duct flows, a tubular turbine impeller chamber  43  formed to be surrounded by the turbine scroll flow passage  42 , and an annular exhaust flow passage  45  communicating the turbine scroll flow passage  42  with a base end side of the turbine impeller chamber  43  are disposed in the turbine housing  4 . 
     The turbine impeller  5  is disposed to be rotatable in the turbine impeller chamber  43  in a state of being connected to the one end side of the rotating shaft  21 . In the exhaust flow passage  45 , a plurality of blade-shaped nozzle vanes  46  are arranged at equal intervals along a circumferential direction of the rotating shaft  21  at a predetermined angle with respect to the circumferential direction, so as to surround the base end side of the turbine impeller chamber  43 . 
     The exhaust air of the internal combustion engine introduced into the turbine scroll flow passage  42  via the exhaust introduction duct is accelerated in the circumferential direction as it flows through the turbine scroll flow passage  42 , and flows inward in a radial direction of the rotating shaft  21  to the base end side of the turbine impeller  5  via the exhaust flow passage  45 . The turbine impeller  5  is rotated by the energy of the exhaust air introduced as described above. 
     The compressor  6  includes a compressor housing  7  and a disk-shaped compressor impeller  8 . The compressor housing  7  constitutes a part of an intake flow passage of the internal combustion engine. The compressor impeller  8  is disposed to be rotatable around the rotating shaft  21  in a compressor impeller chamber  72  formed in the compressor housing  7  in a state of being connected to the other end side of the rotating shaft  21 . With these, the compressor  6  compresses intake air flowing through the intake flow passage. 
       FIG. 2  is a perspective view of the compressor impeller  8 . The compressor impeller  8  includes a conical wheel  81 , and a plurality of plate-shaped main blades  84  and splitters  86  disposed on an outer peripheral surface of the wheel  81 . 
     The wheel  81  has a hub surface  82  and a shaft mounting hole  83 . The hub surface  82  extends smoothly outward in the radial direction from a distal end side  81   a , which is in an axial direction parallel to an axis C, to a base end side  81   b.  The shaft mounting hole  83  penetrates the center of the wheel  81  from the base end side  81   b  to the distal end side  81   a.  The rotating shaft connected to the turbine impeller is connected to the wheel  81  by screwing a cap (not shown) while the rotating shaft is inserted into the shaft mounting hole  83 . Thereby, the compressor impeller  8  and the turbine impeller are connected together via the rotating shaft. 
     The main blades  84  are disposed on the hub surface  82  of the wheel  81  at equal intervals along the circumferential direction. Each of the main blades  84  has a plate shape that extends in a predetermined angular distribution from a front edge portion  841  of the distal end side  81   a,  which is an inlet of the intake air, toward a rear edge portion  842  of the base end side  81   b  which is an outlet of the intake air, on the hub surface  82 . A tip end edge  843  of the main blade  84  is formed along the surface shape of an opposing shroud  721 , which will be described later (see  FIG. 1 ), when the compressor impeller  8  is housed in the compressor impeller chamber  72 . 
     The splitter  86  is disposed between two pieces of the main blades  84  that are adjacent to each other on the hub surface  82 . Each of the splitters  86  has a plate shape that extends in a predetermined angular distribution from a front edge portion  861  of the distal end side  81   a  toward a rear edge portion  862  of the base end side  81   b  on the hub surface  82 . A tip end edge  863  of the splitter  86  is formed along the surface shape of the shroud  721  (see  FIG. 1 ) in the same manner as the tip end edge  843  of the main blade  84 . 
     The compressor impeller  8  configured as described above rotates clockwise in  FIG. 2  as the turbine impeller connected to it by the rotating shaft is blown by the exhaust air to rotate. When the compressor impeller  8  rotates in the state of being disposed in the compressor impeller chamber, the intake air flowing in from the distal end side  81   a  flows along the axial direction from the front edge portions  841  of the main blades  84  and the front edge portions  861  of the splitters  86  and flows between the main blades  84  and the splitters  86 , and is discharged outward in the radial direction from the respective rear edge portions  842  and  862 . 
     Reverting to  FIG. 1 , the compressor impeller chamber  72 , an intake duct  73 , a diffuser chamber  74 , a main scroll flow passage  75 , and an axial flow swirler  76  are formed in the compressor housing  7 . The compressor impeller chamber  72  houses the compressor impeller  8 . The intake duct  73  is connected to the intake flow passage (not shown) of the internal combustion engine and introduces the intake air flowing through the intake flow passage into the compressor impeller chamber  72 . The diffuser chamber  74  decelerates the intake air discharged from the compressor impeller chamber  72 . The main scroll flow passage  75  is for the intake air discharged from the diffuser chamber  74  to flow through. The axial flow swirler  76  generates a swirling flow with respect to the main flow of the intake air that flows into the compressor impeller chamber  72  via the intake duct  73 . 
     In the compressor impeller chamber  72 , the shroud  721  is formed to cover a side portion of the compressor impeller  8 . The shroud  721  has a shroud surface in a shape that is along the tip end edge  843  from the front edge portion  841  to the rear edge portion  842  of the compressor impeller  8 . More specifically, when the compressor impeller  8  rotates around the rotating shaft  21 , the shape of the shroud surface is substantially equal to an envelope surface formed by the tip end edge  843  of the compressor impeller  8 . The shroud  721  covers the tip end edge  843 , which is the side portion of the compressor impeller  8 , with this shroud surface. A side of the shroud  721  near the front edge portion  841  of the compressor impeller  8  becomes an intake inlet that has an inner diameter substantially equal to an outer diameter of the front edge portion  841 . Moreover, a side of the shroud  721  near the rear edge portion  842  of the compressor impeller  8  becomes an annular intake outlet that has a width substantially equal to a height of the rear edge portion  842 . 
     The intake duct  73  is formed with an axial flow passage  71  that extends to the intake inlet of the compressor impeller chamber  72  along the axial direction parallel to the axis C of the rotating shaft  21 . The axial flow passage  71  is divided into a reduced diameter portion  711  and a straight portion  712 . An inner diameter of the reduced diameter portion  711  gradually decreases from an upstream side toward the side of the intake inlet, which is a downstream side. The straight portion  712  has an inner diameter substantially equal to the intake inlet of the shroud  721 . The axial flow passage  71  is connected to the intake flow passage of the internal combustion engine (not shown). The intake air of the internal combustion engine is accelerated in the process of flowing through the reduced diameter portion  711  and then introduced to the front edge portion  841  of the compressor impeller  8  disposed at the intake inlet. 
     The diffuser chamber  74  is annular and is formed to surround the intake outlet of the compressor impeller chamber  72 . In the diffuser chamber  74 , line blade rows are formed and erected at predetermined intervals along the circumferential direction of the compressor impeller  8 . Accordingly, the intake air that has been discharged outward in the radial direction via the intake outlet from the rear edge portion  842  due to rotation of the compressor impeller  8  is decelerated in the process of flowing and spreading along the blade rows formed in the diffuser chamber  74  and is thereby compressed. 
     The main scroll flow passage  75  is annular and is formed to surround the diffuser chamber  74 . A flow passage cross-sectional area of the main scroll flow passage  75  gradually increases along the same direction as the rotation direction of the compressor impeller  8  (see  FIG. 3  which will be described later, for example). Thus, the intake air that has been discharged outward in the radial direction from the diffuser chamber  74  is further decelerated in the process of flowing through the main scroll flow passage  75  and then introduced to a combustion chamber of the internal combustion engine (not shown). 
     In the compressor housing  7  configured as described above, except for the axial flow swirler  76  which will be described later, the axial flow passage  71  of the intake duct  73 , the shroud  721  of the compressor impeller chamber  72 , the diffuser chamber  74 , and the main scroll flow passage  75  constitute a part of the intake flow passage of the internal combustion engine. 
     Next, the configuration of the axial flow swirler  76  is described with reference to  FIG. 1  and  FIG. 3  to  FIG. 5 .  FIG. 3  is a cross-sectional view of the compressor housing  7  taken along the line III-III (see  FIG. 1 ).  FIG. 4  is a perspective cross-sectional view of the compressor housing  7  taken along a plane including the axis C. 
     The axial flow swirler  76  includes an annular swirling flow passage  77  accelerating a swirling gas along the circumferential direction of the compressor impeller  8 , a swirling gas ejection passage  78  ejecting the swirling gas accelerated in the circumferential direction by the swirling flow passage  77  into the axial flow passage  71 , and a swirling gas supply device  79  supplying the swirling gas to the swirling flow passage  77 . 
     The swirling flow passage  77  includes a scroll flow passage  773  and a swirling gas introduction portion  774 . The scroll flow passage  773  extends along the circumferential direction of the compressor impeller  8  from a base end portion  771  toward a distal end portion  772 , and the swirling gas introduction portion  774  extends outward along a tangential direction from the base end portion  771 . The scroll flow passage  773  communicates the base end portion  771  and the distal end portion  772 , and is annular in a plan view as shown in  FIG. 3 . In addition, a flow passage cross-sectional area of the scroll flow passage  773  gradually decreases along the circumferential direction from the base end portion  771  toward the distal end portion  772 , and more specifically, along the same direction as the rotation direction of the compressor impeller  8  (that is, clockwise in  FIG. 3 ). Accordingly, when the swirling gas is supplied from the swirling gas supply device  79  to the swirling gas introduction portion  774  in the tangential direction of the scroll flow passage  773 , the swirling gas is accelerated in the same direction as the rotation direction of the compressor impeller  8  in the process of flowing through the scroll flow passage  773  from the base end portion  771  to the distal end portion  772 . 
     The swirling gas ejection passage  78  extends along the radial direction of the compressor impeller  8  and connects the inside of the scroll flow passage  773  with the straight portion  712  of the axial flow passage  71  formed inside the intake duct  73 . The swirling gas ejection passage  78  is annular around the rotating shaft  21  and connects a radially inner portion inside the scroll flow passage  773  and the straight portion  712  over the entire circumference. Moreover, as shown in  FIG. 1 , the swirling gas ejection passage  78  is inclined with respect to the axial flow flowing through the straight portion  712 . That is, an angle α (referred to as “axial inclination angle” hereinafter) (see FIG.  4 ) formed by an extending direction of the swirling gas ejection passage  78  and an inner peripheral surface of the straight portion  712  is an acute angle. 
     With the configuration as described above, the swirling gas supplied from the swirling gas supply device  79  to the swirling gas introduction portion  774  is ejected from the swirling gas ejection passage  78  toward the inside of the straight portion  712  while flowing through the scroll flow passage  773  from the base end portion  771  toward the distal end portion  772 . At this time, because the swirling gas is accelerated in the same direction as the rotation direction of the compressor impeller  8  in the process of flowing through the scroll flow passage  773 , the ejection flow of the swirling gas in the swirling gas ejection passage  78  has a velocity component in the same direction as the rotation direction. In addition, because the swirling gas ejection passage  78  is inclined with respect to the axial flow, the ejection flow of the swirling gas also has a velocity component in the same direction as the axial flow. By ejecting the swirling gas having such velocity components to the inside of the straight portion  712 , a swirling flow is created in the axial flow that flows through the straight portion  712 . 
     A reduction rate of the flow passage cross-sectional area of the scroll flow passage  773  is correlated to a magnitude of a rotation direction component of the ejection flow of the swirling gas in the swirling gas ejection passage  78 . More specifically, as the reduction rate of the flow passage cross-sectional area of the scroll flow passage  773  is increased, a radial velocity of the swirling gas in the scroll flow passage  773  also increases. Therefore, the reduction rate of the flow passage cross-sectional area is adjusted so that the angle of the ejection flow in the swirling gas ejection passage  78  with respect to the axis C is, for example, 30 degrees or more. In addition, the above-described axial inclination angle α is set in a range of 15 degrees to 60 degrees, for example. 
     The swirling gas supply device  79  includes a gas acquisition port  791  and a gas supply passage  792 . The gas acquisition port  791  is formed in a portion defined as a swirling gas supply source in the intake flow passage or the exhaust flow passage of the internal combustion engine. The gas supply passage  792  connects the gas acquisition port  791  and the swirling gas introduction portion  774 . The swirling gas supply device  79  acquires the intake air or exhaust air in the supply source from the gas acquisition port  791  to serve as the swirling gas and supplies it to the swirling gas introduction portion  774  via the gas supply passage  792 . Here, a flow rate regulating valve may be disposed in the gas supply passage  792  for adjusting the flow rate of the swirling gas that flows from the gas acquisition port  791  to the swirling gas introduction portion  774 . 
     Here, in order to generate a sufficiently strong swirling flow in the axial flow passage  71 , the gas acquisition port  791  needs to be disposed at least in a portion where a total pressure, which is a combination of a static pressure and a dynamic pressure, is higher than that inside the straight portion  712  where the ejection flow of the swirling gas is formed, so that the swirling gas flows in the scroll flow passage  773  from the base end portion  771  toward the distal end portion  772 . Thus, the gas acquisition port  791  is disposed in a portion on the downstream side of the front edge portion  841  of the compressor impeller  8  in the entire flow passage that includes the intake flow passage and the exhaust flow passage of the internal combustion engine, that is, the portion where the total pressure is higher than that of the straight portion  712  in the axial flow passage  71 , and as a result, the swirling gas introduction portion  774  and the portion with the high total pressure are connected via the gas supply passage  792 . 
       FIG. 5  is a cross-sectional view of the compressor housing  7 . In the compressor housing  7 , as described above, the shroud  721 , the diffuser chamber  74 , and the main scroll flow passage  75  respectively constitute a part of the intake flow passage on the downstream side of the front edge portion  841  of the compressor impeller  8 , and the total pressure in each portion is higher than that inside the straight portion  712  during rotation of the compressor impeller  8 . Therefore, the shroud  721  (that is, the section indicated by the arrow  5   a  in  FIG. 5 ), the diffuser chamber  74  (that is, the section indicated by the arrow  5   b  in  FIG. 5 ), and the main scroll flow passage  75  are all eligible as the position for disposing the gas acquisition port. Particularly, among the three portions, the sections indicated by the arrows  5   a  and  5   b  are close to the swirling gas introduction portion of the swirling flow passage  77  and therefore have the advantage of shortening the gas supply passage connecting these portions and consequently suppressing pressure drop in the flow passage. 
       FIG. 6  is a diagram schematically showing a variation of a velocity triangle at the front edge portion of the compressor impeller when the swirling flow is generated in the axial flow passage by the axial flow swirler described above. 
     First, in a case of not using the axial flow swirler, the axial flow is generated along the axial direction of the compressor impeller in the axial flow passage. That is, in this case, an absolute velocity vector U 1  at the front edge portion is parallel to the axial direction. Moreover, in a case where the compressor impeller is rotating with the arrow ω as the rotation direction, a tangential velocity vector V 1  at the front edge portion is in a direction opposite to the arrow ω and has a length proportional to the rotation velocity. Accordingly, a relative velocity vector W 1  obtained by combining the two vectors U 1  and V 1  is inclined by an angle θ 1  with respect to the axial direction. 
     Next, in a case of using the axial flow swirler, the swirling flow generated in the axial flow passage has a velocity component in the same direction as the rotation direction and a velocity component the same as the axial direction. Accordingly, in this case, an absolute velocity vector U 2  at the front edge portion is inclined by the velocity component in the rotation direction (swirler rotation direction component) with respect to the absolute velocity vector U 1  when the axial flow swirler is not used, and the absolute velocity vector U 2  is longer than the absolute velocity vector U 1  by the velocity component in the axial direction (swirler axial direction component). Accordingly, a relative velocity vector W 2  obtained by combining the absolute velocity vector U 2  and the tangential velocity vector V 1  is inclined by an angle θ 2 , which is smaller than the aforementioned angle θ 1 , with respect to the axial direction. That is, the swirling flow generated by the axial flow swirler has an effect of reducing a relative inflow angle of the intake air at the front edge portion of the compressor impeller. 
       FIG. 7  is a diagram showing a radial distribution of the relative inflow angle at the front edge portion of the compressor impeller. In  FIG. 7 , the broken line indicates a distribution when the axial flow swirler is not used while the solid line indicates a distribution when the axial flow swirler is used. 
     As shown in  FIG. 7 , in the case of not using the axial flow swirler, that is, when the axial flow is generated along the axis in the axial flow passage, the relative inflow angle increases as it goes toward the radial outer side. The reason is that the rotation velocity increases toward the radial outer side. Further, in the case of using the axial flow swirler, the swirling flow is generated in the vicinity of the wall surface in the axial flow passage. In addition, as described above, this swirling flow has the effect of reducing the relative inflow angle and facilitating inflow of the intake air to the compressor impeller. Accordingly, as shown in  FIG. 7 , in the case of using the axial flow swirler, the relative inflow angle is reduced partially on the radial outer side, more specifically, in the vicinity of the wall surface of the axial flow passage. Therefore, by using the axial flow swirler, the inflow of the intake air to the compressor impeller in the vicinity of the wall surface of the axial flow passage is facilitated, so that the lower limit of the flow rate range of the compressor can be lowered. 
     According to the compressor  6  of the present embodiment, the following effects are achieved. 
     (1) According to the compressor  6  of the present embodiment, the intake air introduced to the scroll flow passage  773  from the swirling gas introduction portion  774  flows through the scroll flow passage  773  while being accelerated along the same direction as the rotation direction of the compressor impeller  8 , and is ejected into the axial flow passage  71  of the intake duct  73  via the swirling gas ejection passage  78  and creates the swirling flow along the same direction as the rotation direction with respect to the axial flow that flows through the axial flow passage  71 . By creating such a swirling flow in the intake air flowing to the front edge portion  841  of the compressor impeller  8 , the relative inflow angle at the front edge portion  841  decreases and the intake air flows easily to the front edge portion  841 . Thus, the lower limit of the flow rate range of the compressor  6  can be lowered. Moreover, in the compressor  6 , the swirling gas introduction portion  774 , which is the inlet of the scroll flow passage  773 , is connected to the portion on the downstream side of the front edge portion  841  of the compressor impeller  8  in the entire flow passage that includes the intake flow passage and the exhaust flow passage of the internal combustion engine, that is, the portion where the total pressure is higher than that inside the straight portion  712 , and the swirling flow is generated by using the swirling gas recirculated by the differential pressure. As a result, in the compressor  6 , the swirling flow can be generated without using a part of the main flow that flows through the axial flow passage  71 . Therefore, the generated swirling flow has a sufficient speed as compared with the conventional art. Furthermore, in the compressor  6 , because the swirling flow is generated by recirculating the swirling gas, there is no need to dispose a device hindering the main flow in the axial flow passage  71 . Thus, the pressure drop in the axial flow passage  71  is not worsened. 
     (2) In the compressor  6 , the angle formed by the extending direction of the swirling gas ejection passage  78 , which connects the inside of the scroll flow passage  773  and the inside of the axial flow passage  71 , and the inner peripheral surface of the axial flow passage  71  is set to an acute angle, by which a swirling flow having an axial velocity component can be ejected from the swirling gas ejection passage  78 . Thus, the lower limit of the flow rate range can be further lowered. 
     (3) In the compressor  6 , the intake duct  73 , the shroud  721 , the swirling flow passage  77 , the diffuser chamber  74 , and the main scroll flow passage  75  are formed in the compressor housing  7 , and the swirling gas introduction portion  774  of the swirling flow passage  77  is connected to any one of the shroud  721 , the diffuser chamber  74 , and the main scroll flow passage  75 . Since the swirling gas can be recirculated in the compressor housing  7 , the size of the entire compressor  6  can be reduced. Moreover, in the entire flow passage including the intake flow passage and the exhaust flow passage, the shroud  721 , the diffuser chamber  74 , and the main scroll flow passage  75  are in the vicinity of the compressor impeller  8 , and the total pressure is higher than the other portions. Accordingly, by recirculating the swirling gas from such portions, a fast swirling flow can be generated. 
     Second Embodiment 
     Next, the second embodiment of the invention is described with reference to the figures.  FIG. 8  is a diagram showing the configuration of a supercharging system S of the internal combustion engine according to the present embodiment. 
     The supercharging system S includes an intake flow passage  92  that introduces intake air to a combustion chamber of an internal combustion engine  91 , an exhaust flow passage  93  that introduces an exhaust air discharged from the combustion chamber of the internal combustion engine  91 , a supercharger  1 ′ formed by combining a compressor  6 ′ disposed in the intake flow passage  92  and a turbine  3  disposed in the exhaust flow passage  93 , an intercooler  96  that cools the intake air compressed by the compressor  6 ′ by using cooling water or outside air, an EGR flow passage  94  that recirculates a part of the exhaust air flowing through the exhaust flow passage  93  to the intake flow passage  92 , and an EGR cooler  97  that cools the exhaust air flowing through the EGR flow passage  94  by using cooling water or outside air. The supercharger  1 ′ disposed in the supercharging system S differs from the supercharger  1  described in the first embodiment in the configuration of the compressor  6 , more specifically the configuration of the swirling gas supply device, and the configurations of the other portions are the same. 
     The EGR flow passage  94  connects the portion of the exhaust flow passage  93 , which is on the upstream side of the turbine impeller  5  of the turbine  3 , with the swirling gas introduction portion  774  formed in the compressor  6 ′, so as to supply a part of the exhaust air flowing through the exhaust flow passage  93  as a swirling gas to the swirling gas introduction portion  774 . 
     According to the supercharging system S of the present embodiment, the following effect (3) is achieved in addition to the aforementioned effects (1) to (2). (3) In the supercharging system S, the swirling gas introduction portion  774 , which is the inlet of the swirling flow passage, is connected to the portion, which is on the downstream side of the front edge portion of the compressor impeller  8  in the entire flow passage including the intake flow passage  92  and the exhaust flow passage  93  and further on the upstream side of the turbine impeller  5  in the exhaust flow passage  93 , by the EGR flow passage  94 . That is, in the supercharging system S, the so-called high pressure external EGR gas is supplied to the swirling gas introduction portion  774  to generate a swirling flow. Thus, in addition to the effect of generating a high-speed swirling flow, it is also possible to achieve effects, such as reduction of NOx in the exhaust air and improvement of fuel efficiency, that are expected by recirculating a part of the exhaust air to the intake air. 
     Although the embodiments of the invention have been described above, the invention is not limited thereto. The configuration of the details may be modified where appropriate without departing from the scope of the spirit of the invention. 
     For example, in the embodiments described above, the flow passage cross-sectional area of the scroll flow passage  773  gradually decreases along the same direction as the rotation direction of the compressor impeller  8  from the base end portion  771  toward the distal end portion  772 , so as to accelerate the swirling gas in the same direction as the rotation direction of the compressor impeller  8 . Nevertheless, the invention is not limited thereto. The flow passage cross-sectional area of the scroll flow passage may gradually decrease along the direction opposite to the rotation direction of the compressor impeller from the base end portion toward the distal end portion to accelerate the swirling gas in the direction opposite to the rotation direction of the compressor impeller  8 . In a state close to a stall, the main flow in the intake duct  73  in the vicinity of the shroud  721  tends to follow the rotation direction of the compressor impeller  8 . Whereas, by configuring the scroll flow passage as described above and accelerating the swirling gas in the direction opposite to the rotation direction of the compressor impeller  8 , a swirling flow in the direction opposite to the rotation direction of the compressor impeller  8  can be created to reduce the following, so that the lower limit of the flow rate range can be further lowered. However, in the case where the direction of the scroll flow passage is set opposite to the rotation direction of the compressor impeller  8  as described above, in order to reduce only the following and not to cause great influence on the entire main flow flowing through the intake duct  73 , the flow rate of the intake air ejected from the swirling gas ejection passage  78  into the intake duct  73  is preferably set to about 10% or less of the flow rate of the entire intake air that flows into the compressor impeller  8 . 
     For example, the embodiments described above illustrate that the compressor of the invention is applied to a supercharger that compresses intake air sucked in by the internal combustion engine, but the invention is not limited thereto. The compressor of the invention is applicable not only to the supercharger of the internal combustion engine but also to the so-called turbo machine, such as jet engine and pump, that performs conversion between fluid energy and mechanical energy by using an impeller.