Patent Publication Number: US-9404509-B2

Title: Turbocharger and method of producing thereof

Description:
CROSS-REFERENCE 
     This application claims priority to Japanese patent application no. 2012-207890 filed on Sep. 21, 2012, the contents of which are entirely incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a turbocharger in which a back plate is disposed between a compressor housing and a bearing housing, and the invention also relates to a method of producing the turbocharger. 
     2. Description of the Related Art 
     A turbocharger installed in a car or the like is configured such that air sucked in a compressor is compressed and discharged toward an internal combustion engine. That is, a discharge scroll chamber into which compressed air discharged from an impeller flows is formed in an air flow passage formed inside of a compressor housing, the discharge scroll chamber guides compressed air into a discharge port, and the compressed air from the discharge port is discharged toward the internal combustion engine. Especially, a shape of the discharge scroll chamber largely influences performance of the compressor, and it is required to finish the discharge scroll chamber into an appropriate shape in accordance with required performance. 
     Here, there is a method of producing the compressor housing by gravity die casting for example. In this case, since casting can be carried out using a so-called core, flexibility in shape is high and a complicated shape can be formed. However, since a casting cycle is long, productivity is poor and costs are high. Further, if a sand mold is used, a surface roughness becomes large and thus, there is a problem that efficiency of the compressor is deteriorated. 
     There is also a method of forming the compressor housing by die casting. In this case, since the casting cycle is short as compared with the gravity die casting, the productivity is superior and costs are low. However, since the compressor housing can be formed only when a molded shape can be extracted from a mold and thus, flexibility in shape is poor and a complicated shape can not be formed. Hence, there is a compressor housing formed by assembling three pieces, i.e., a scroll piece, a shroud piece and an outer peripheral annular piece as disclosed in Patent Document 1. According to this, the pieces are formed into shapes which can easily be formed by die casting, and flexibility in shape of a discharge scroll chamber of the compressor housing is secured. 
     In a turbocharger described in Patent Document 2, a compressor housing is composed of two pieces, i.e., a scroll piece and a shroud piece. An outer periphery of a back plate disposed on a side opposite to a suction side in the compressor housing is provided with a curved surface, and this curved surface is formed as a part of an inner wall surface of a discharge scroll chamber. 
     PATENT DOCUMENTS 
     
         
         Patent Document 1: JP 4778097 B1 
         Patent Document 2: JP 2002-180841 A 
       
    
     SUMMARY OF THE INVENTION 
     According to the compressor housing described in Patent Document 1, however, the number of parts is increased, producing man-hours are increased and producing costs become high. Further, if positional accuracy of the outer peripheral annular piece is not extremely high, there is a possibility that smooth flow of compressed air discharged from a diffuser portion to the discharge scroll chamber is hindered in a seam in its inner end. 
     In the turbocharger described in Patent Document 2, a shape of the curved surface portion formed on the back plate is constant over its circumferential direction. Hence, flexibility in shape of the discharge scroll chamber is limited, and there is a limit on an ideal shape. 
     The present invention has been accomplished in view of the background, and the invention provides a turbocharger capable of reducing its costs and enhancing its performance, and provides a method of producing the turbocharger. 
     One aspect of the invention resides in a turbocharger, including 
     a compressor housing provided therein with an air flow passage in which an impeller is disposed; 
     a bearing housing rotatably supporting a rotor shaft which is connected to the impeller; and 
     a back plate which is disposed between the bearing housing and the compressor housing and which faces a part of the air flow passage; 
     the air flow passage includes a suction port from which air is sucked toward the impeller, and a discharge scroll chamber which is formed on an outer peripheral side of the impeller in a circumferential direction, and which guides compressed air discharged from the impeller to a discharge port, the discharge scroll chamber has such a shape that a cross-sectional area thereof gradually increases toward the discharge port in the circumferential direction, 
     the compressor housing includes a scroll piece and a shroud piece which are mutually assembled, 
     the scroll piece includes a cylindrical suction port-forming portion forming the suction port, a suction-side concave surface configuring a suction-side wall surface of the discharge scroll chamber, and a scroll outer periphery disposed on an outer peripheral side of the discharge scroll chamber, 
     the shroud piece includes a cylindrical shroud fit-in portion which is fitted into the scroll piece, an inner peripheral concave surface configuring an inner peripheral wall surface of the discharge scroll chamber, a shroud surface facing to the impeller, and a diffuser surface extending from the shroud surface toward the discharge scroll chamber, 
     the back plate includes a facing surface which is facing to the diffuser surface, an outer peripheral annular fit-in portion which is fitted into the scroll outer periphery of the scroll piece, and an outer peripheral concave surface configuring an outer peripheral wall surface of the discharge scroll chamber, and 
     the outer peripheral concave surface has such a shape that a cross sectional shape thereof formed by a plane including a rotation axis of the impeller gradually changes along the circumferential direction. 
     Another aspect of the invention resides in a method of producing the turbocharger, in which the scroll piece, the shroud piece and the back plate are individually casted, the scroll piece and the shroud piece are assembled to each other to obtain the compressor housing, the back plate is fastened to the bearing housing, and the compressor housing is assembled to the back plate to produce the turbocharger, wherein 
     a mold for casting the back plate includes a body mold for forming the facing surface, the outer peripheral annular fit-in portion and the outer peripheral concave surface, and a central mold for forming the plate central portion, the central mold is detachably attached to the body mold, and the central mold is attached to the body mold such that a circumferential phase of the central mold is changed with respect to the body mold. 
     In the turbocharger, since the compressor housing can be configured using the two pieces, i.e., the scroll piece and the shroud piece, the number of parts of the compressor housing can be relatively small. As a result, the producing man-hours of the turbocharger can be reduced and the producing costs can be reduced. 
     Further, a part of the wall surface of the discharge scroll chamber can be formed by the outer peripheral concave surface of the back plate. That is, in a turbocharger in which a back plate is disposed between a bearing housing and a compressor housing, a part of the wall surface of a discharge scroll chamber is formed into a concave shape utilizing a part of the back plate which is an existing part. Hence, it is possible to enhance flexibility in shape of the discharge scroll chamber without increasing the number of parts. 
     The outer peripheral concave surface has such a shape that a cross sectional shape formed by a plane including a rotation axis of the impeller is gradually changed along a circumferential direction. Hence, a shape of a wall surface of the discharge scroll chamber on its outer peripheral side can gradually be changed along the circumferential direction. According to this, an ideal shape of the discharge scroll chamber can be obtained, and an ideal air flow in the compressor can be realized. As a result, it is possible to enhance the performance of the turbocharger. 
     The outer peripheral concave surface is formed in the back plate together with a facing surface thereof which is facing to the diffuser surface. Hence, a seam is not formed between the facing surface and the outer peripheral concave surface. Therefore, it is possible to easily and reliably realize a smooth flow of compressed air which is discharged from a space (diffuser portion) between the diffuser surface and the facing surface into the discharge scroll chamber. From this point of view also, it is possible to enhance the performance of the turbocharger. 
     In the producing method, the back plate is casted using a mold in which the central mold is detachably attached to the body mold. The central mold can be attached to the body mold such that a phase of the central mold in the circumferential direction can be changed with respect to the body mold. Hence, it is possible to produce the back plate by appropriately changing the phase of the outer peripheral concave surface with respect to the plate central portion. A phase of the plate central portion when it is assembled into the turbocharger is determined by necessity by a position of an oil discharging port. On the other hand, the phase of the outer peripheral concave surface is set in accordance with a position of the discharge port, but a position of the outer peripheral concave surface differs depending upon types of a vehicle in which the turbocharger is mounted. Hence, a phase difference between the plate central portion and the outer peripheral concave surface in the circumferential direction can differ depending upon the corresponding vehicle type. A mold for casting the back plate is configured such that the phase of the central mold in the circumferential direction can be changed and attached with respect to the body mold as described above. According to this configuration, a plurality of kinds of back plates suitable for a corresponding vehicle type can be produced using one kind of mold. Hence, according to this producing method, it is possible to produce a plurality of kinds of back plates with excellent production efficiency, and the production efficiency of the turbocharger can be enhanced. 
     As described above, according to the present invention, it is possible to provide a turbocharger capable of reducing its costs and enhancing its performance and to provide a method of producing the turbocharger. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a compressor portion in a first embodiment taken along a line A-A in  FIG. 3 . 
         FIG. 2  is another sectional view of the compressor portion in the first embodiment taken along a line B-B in  FIG. 3 . 
         FIG. 3  is a plan view of a compressor housing in the first embodiment as viewed from a suction side. 
         FIG. 4  is a sectional view of the compressor housing in the first embodiment. 
         FIG. 5  is a sectional view of a back plate in the first embodiment. 
         FIG. 6  is an explanatory diagram showing an entire shape of a scroll chamber in the first embodiment. 
         FIG. 7  is a perspective view of the compressor housing in the first embodiment. 
         FIG. 8  is a perspective view of the back plate in the first embodiment. 
         FIG. 9  is a rear view of the back plate in the first embodiment as viewed from a bearing housing. 
         FIG. 10  is a front view of the back plate in the first embodiment as viewed from a compressor. 
         FIG. 11A  is a sectional view taken along a line C-C in  FIG. 10 ,  FIG. 11B  is a sectional view taken along line D-D in  FIG. 10 ,  FIG. 11C  is a sectional view taken along line E-E in  FIG. 10 , and  FIG. 11D  is a sectional view taken along line F-F in  FIG. 10 . 
         FIG. 12  is a sectional view of a mold in the first embodiment. 
         FIG. 13A  is a plan view of a mold (lower mold) in which a central mold is attached to a body mold, and  FIG. 13B  is a plan view of the mold (lower mold) in which the central mold is attached to the body mold with a phase which is different from that of  FIG. 13A  in the first embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the turbocharger, a “circumferential direction” means a rotation direction of the impeller, and an “axial direction” means a direction of a rotation axis of the impeller. 
     The cross sectional shape of the outer peripheral concave surface may preferably be formed such that a radius of curvature thereof gradually increases toward the discharge port in the circumferential direction. In this case, it is possible to more easily form the discharge scroll chamber into an ideal shape. That is, since the outer peripheral concave surface configuring a part of a wall surface of the discharge scroll chamber has the above-described shape, in the shape of the discharge scroll chamber in which a cross-sectional area thereof gradually becomes larger toward the discharge port in the circumferential direction, it becomes easy to form cross sections of various portions into ideal shapes. When a cross sectional shape of the outer peripheral concave surface is an arc, the radius of curvature is a radius of curvature of the arc, but the cross sectional shape of the outer peripheral concave surface is not the arc, the radius of curvature means an average radius of curvature of the entire cross sectional shape. 
     The outer peripheral concave surface may preferably be formed such that a size thereof in an axial direction of the turbocharger gradually increases from its one end toward the other end in the circumferential direction. In this case, like the above-described case, it is possible to more easily form the discharge scroll chamber into the ideal shape. 
     The outer peripheral concave surface may preferably be formed such that a size thereof in a radial direction gradually increases toward the discharge port in the circumferential direction. In this case also, like the above-described case, it is possible to more easily form the discharge scroll chamber into the ideal shape. 
     The back plate is fastened to the bearing housing through a bolt, a shaft hole through which the rotor shaft is inserted, a bolt hole through which the bolt is inserted and an oil discharge port through which lubricant is discharged may be formed in a plate central portion of the back plate on an inner peripheral side thereof as compared with the facing surface. In this case, when the back plate is produced, if its accuracy is enhanced, it is possible to easily assemble the turbocharger while keeping positional accuracy of the outer peripheral concave surface in the circumferential direction. That is, if the back plate is produced such that a positional relation (phase difference) in the circumferential direction between a position of the bolt hole and a shape of the outer peripheral concave surface becomes a predetermined positional relation (phase difference), a position of the outer peripheral concave surface is determined by fastening the back plate in the bolt hole through a bolt and fixing the back plate to the bearing housing. If the compressor housing is fitted to the back plate while aligning a circumferential direction thereof with the outer peripheral concave surface, the discharge port can be disposed at a position as designed, i.e., the discharge port can be disposed at a position suitable for a position of a pipe to be connected. Therefore, when the turbocharger is assembled, the discharge scroll chamber and the discharge port can easily be disposed at appropriate circumferential positions. 
     (First Embodiment) 
     A first embodiment of the turbocharger and the method of producing thereof will be described with reference to  FIGS. 1 to 13 . 
     As shown in  FIG. 1 , a turbocharger  1  of the first embodiment includes a compressor housing  2  provided therein with an air flow passage  10  in which an impeller  13  is disposed, a bearing housing  6  which rotatably supports a rotor shaft  14  which is connected to the impeller  13 , and a back plate  3  which is disposed between the bearing housing  6  and the compressor housing  2  and which faces a part of the air flow passage  10 . 
     The air flow passage  10  includes a suction port  11  from which air is sucked toward the impeller  13 , and a discharge scroll chamber  12  which is formed on an outer peripheral side of the impeller  13  in its circumferential direction, and which guides compressed air discharged from the impeller  13  to a discharge port  15  (see  FIG. 3 ). As shown in  FIG. 6 , the discharge scroll chamber  12  has such a shape that a cross-sectional area thereof gradually increases toward the discharge port  15  in the circumferential direction. 
     As shown in  FIGS. 1, 2, 4 and 7 , the compressor housing  2  is composed of a scroll piece  4  and a shroud piece  5  which are mutually assembled. 
     The scroll piece  4  includes a cylindrical suction port-forming portion  41  forming the suction port  11 , a suction-side concave surface  42  configuring a suction-side wall surface of the discharge scroll chamber  12 , and a scroll outer periphery  43  disposed on the outer peripheral side of the discharge scroll chamber  12 . 
     The shroud piece  5  includes a cylindrical shroud fit-in portion  51  which is fitted into the scroll piece  4 , an inner peripheral concave surface  52  configuring an inner peripheral wall surface of the discharge scroll chamber  12 , a shroud surface  53  facing to the impeller  13 , and a diffuser surface  54  extending from the shroud surface  53  toward the discharge scroll chamber  12 . 
     As shown in  FIGS. 1, 2 and 5 , the back plate  3  includes a facing surface  31  which is facing to the diffuser surface  54 , an outer peripheral annular fit-in portion  32  which is fitted into the scroll outer periphery  43  of the scroll piece  4 , and an outer peripheral concave surface  33  configuring an outer peripheral wall surface of the discharge scroll chamber  12 . 
     As shown in  FIGS. 1, 2, 10 and 11 , the outer peripheral concave surface  33  has such a shape that a cross sectional shape thereof formed by a plane including a rotation axis of the impeller  13  (when it is merely mentioned “cross sectional shape” in the following description, this means a cross sectional shape formed by this plane unless otherwise specified) gradually changes along the circumferential direction. 
     More specifically, the cross sectional shape of the outer peripheral concave surface  33  is formed such that a radius of curvature of the outer peripheral concave surface  33  gradually increases as approaching the discharge port  15  in the circumferential direction. 
     An axial size h of the outer peripheral concave surface  33  gradually becomes larger toward the discharge port  15  in the circumferential direction. A radial size w of the outer peripheral concave surface  33  also becomes larger toward the discharge port  15  in the circumferential direction. Symbols  3   a ,  3   b ,  3   c  and  3   d  in  FIGS. 11A, 11B, 11C and 11D  show cross sections of portions of the back plate  3  at positions further, in this order, from the discharge port  15  as a passage of compressed air. That is, the symbols  3   a ,  3   b ,  3   c  and  3   d  in  FIG. 11  show cross sections of the portions of the back plate  3  forming parts of cross sections  12   a ,  12   b ,  12   c  and  12   d  of the discharge scroll chamber  12  shown in  FIGS. 1 and 2 . 
     As shown in  FIGS. 1 and 2 , a cross sectional shape of the suction-side concave surface  42  of the scroll piece  4  gradually changes along the circumferential direction. A cross sectional shape of the inner peripheral concave surface  52  of the shroud piece  5  also gradually changes along the circumferential direction. The cross sectional shapes of the suction-side concave surface  42  and the inner peripheral concave surface  52  are formed such that radii of curvature thereof gradually become larger toward the discharge port  15  in the circumferential direction. 
     In the turbocharger  1 , a turbine is rotated by exhaust gas discharged from an internal combustion engine of a car or the like, suction air is compressed in a compressor utilizing this rotation force, and the compressed air is sent from the discharge port  15  into the internal combustion engine. Therefore, the turbocharger  1  is provided with a turbine housing (not shown) on a side opposite, in the axial direction, to the compressor housing  2  which configures an outer shell of the compressor. An exhaust gas flow passage is formed inside of the turbine housing, and a turbine impeller is disposed in the exhaust gas flow passage. The turbine impeller is fixed to the rotor shaft  14 . That is, the impeller  13  of the compressor and the turbine impeller are connected to each other through the rotor shaft  14 . According to this, as the turbine impeller rotates, the impeller  13  of the compressor rotates. 
     The bearing housing  6  rotatably supports the rotor shaft  14 . The bearing housing  6  is disposed between the compressor housing  2  and the turbine housing. As shown in  FIGS. 1, 2, 8 and 9 , the substantially disk-shaped back plate  3  is fixed to one end of the bearing housing  6  in the axial direction. 
     The back plate  3  is fastened to the bearing housing  6  through a bolt (not shown). As shown in  FIGS. 5 and 8 to 10 , a shaft hole  341  through which the rotor shaft  14  is inserted, bolt holes  342  through which bolts are inserted, and an oil discharge port  343  through which lubricant is discharged are formed in a plate central portion  34  of the back plate  3 . The plate central portion  34  is located on an inner peripheral side thereof as compared with the facing surface  31 . 
     In this embodiment, four bolt holes  342  are formed, and the one oil discharge port  343  is formed. As shown in  FIG. 9 , the oil discharge port  343  is disposed in a surface of the back plate  3  on the side of the bearing housing  6  between two of the bolt holes  342  which are adjacent to each other in the circumferential direction. The oil discharge port  343  is dented in a thickness direction of the back plate  3 , and the oil discharge port  343  guides lubricant which drips from the shaft hole  341  into an oil discharge passage (not shown) in the bearing housing  6 . 
     As shown in  FIGS. 8 and 10 , the facing surface  31  and the outer peripheral concave surface  33  are annularly formed on a surface of the back plate  3  on the side of the compressor. As shown in  FIG. 5 , the facing surface  31  and the outer peripheral concave surface  33  are mutually continuously formed. That is, the facing surface  31  is formed as a flat surface intersecting with the axial direction at right angles, but the outer peripheral concave surface  33  is formed such that it curves, toward the compressor, from an outer peripheral end of the facing surface  31  continuously without through a step or the like. As described above, the cross sectional shape of the outer peripheral concave surface  33  gradually changes in the circumferential direction. 
     The discharge scroll chamber  12  is formed by the outer peripheral concave surface  33  of the back plate  3  formed as described above, the scroll outer periphery  43  of the scroll piece  4  formed such that its cross sectional shape gradually changes in the circumferential direction, and the inner peripheral concave surface  52  of the shroud piece  5  ( FIGS. 1 and 2 ). 
     As shown in  FIG. 6 , the discharge scroll chamber  12  is substantially annularly formed, and the discharge port  15  projects from the discharge scroll chamber  12  in a tangential direction of the circumferential direction. Across-sectional area of the discharge scroll chamber  12  gradually increases toward the discharge port  15  along the circumferential direction. Symbols  12   a ,  12   b ,  12   c  and  12   d  in  FIGS. 1 and 2  show cross sections of the discharge scroll chamber  12  at positions further, in this order, from the discharge port  15  as a passage of compressed air. 
     Hence, cross sectional shapes of the suction-side concave surface  42  of the scroll piece  4 , the inner peripheral concave surface  52  of the shroud piece  5  and the outer peripheral concave surface  33  of the back plate  3  which form the discharge scroll chamber  12  gradually change not rotation symmetrically but along the circumferential direction. As shown in  FIGS. 1, 2, 10 and 11 , the back plate  3  has such a shape that a size (size h in axial direction and size w in radial direction) of the cross sectional shape of the outer peripheral concave surface  33  and an average radius of curvature gradually increase in the circumferential direction. That is, the sizes h and w and the average radius of curvature increase in the circumferential direction and toward the discharge port  15 . 
     As shown in  FIG. 10 , a discharge communication concave portion  35  is formed in a circumferential part of the back plate  3 . The discharge communication concave portion  35  connects the discharge scroll chamber  12  and the discharge port  15  to each other. The discharge communication concave portion  35  is formed so as to be parallel to the discharge port  15  between a portion of the outer peripheral concave surface  33  where the axial size h is the largest and a portion of the outer peripheral concave surface  33  where the axial size h is the smallest. That is, the sizes h and w and the radius of curvature of the outer peripheral concave surface  33  gradually increase in the circumferential direction from a first adjacent portion  351  to a second adjacent portion  352  with respect to the discharge communication concave portion  35  shown in  FIG. 10  over a substantially entire circumference of the back plate  3  except a portion thereof where the discharge communication concave portion  35  is formed. 
     Further, as shown in  FIGS. 8 and 10 , a positioning step  36  for positioning the back plate  3  and the compressor housing  2  in the circumferential direction is formed on an outer peripheral side of the outer peripheral concave surface  33  and at a position adjacent to the discharge communication concave portion  35  in the circumferential direction. That is, as show in  FIG. 7 , a positioning step  46  is formed also on the compressor housing  2  at a position corresponding to the positioning step  36  of the back plate  3 . 
     The scroll piece  4  and the shroud piece  5  configuring the compressor housing  2  are formed of aluminum die casted articles. As shown in  FIG. 4 , the suction port-forming portion  41  of the scroll piece  4  is formed into a cylindrical shape centering around a rotation axis of the impeller  13 . A scroll wall surface-forming portion  420  having the suction-side concave surface  42  is formed from an end of the suction port-forming portion  41  on a side opposite to the suction side (suction side is called “tip end side” and its opposite side is called “base end side” hereinafter) such as to spread toward the outer peripheral side. The scroll outer periphery  43  is provided on an outer peripheral portion of the scroll wall surface-forming portion  420  such as to extend to the base end side. 
     The shroud fit-in portion  51  of the shroud piece  5  is formed into a cylindrical shape centering around the rotation axis of the impeller  13 , and a suction passage which is in communication with the suction port  11  is formed in the shroud fit-in portion  51 . The shroud fit-in portion  51  is fitted into the suction port-forming portion  41  of the scroll piece  4 . 
     An inner surface of the shroud fit-in portion  51  is formed such that it spreads outward from the base end side, thereby forming the shroud surface  53 . The shroud surface  53  is connected to the diffuser surface  54  which spreads in a direction intersecting with the axial direction at right angles on its outer peripheral side. 
     As shown in  FIGS. 1 and 2 , the impeller  13  is disposed on the inner peripheral side of the shroud piece  5 . The impeller  13  is formed by projecting a plurality of blades which are arranged in the circumferential direction from an outer peripheral surface of a hub. The blades are facing to the shroud surface  53  of the shroud piece  5 . 
     When a compressor portion of the turbocharger  1  is assembled, the shroud fit-in portion  51  is press-fitted into the suction port-forming portion  41 , thereby assembling the shroud piece  5  into the scroll piece  4 , and the compressor housing  2  shown in  FIGS. 4 and 7  is formed. The back plate  3  is fastened to the bearing housing  6  and fixed thereto. 
     Thereafter, the compressor housing  2  is assembled to the back plate  3 . That is, as shown in  FIGS. 1 and 2 , the compressor housing  2  is assembled to the back plate  3  which is fixed to the bearing housing  6  such that the impeller  13  is disposed inside of the compressor housing  2 . At this time, the outer peripheral annular fit-in portion  32  in the back plate  3  is press-fitted into the scroll outer periphery  43  in the scroll piece  4 . 
     At this time, the positioning step  46  of the scroll piece  4  is abutted against the positioning step  36  of the back plate  3  in the circumferential direction, thereby positioning, in the circumferential direction, the back plate  3  and the compressor housing  2 . 
     According to this, the diffuser surface  54  of the shroud piece  5  and the facing surface  31  of the back plate  3  are facing to each other at a predetermined distance from each other, and a diffuser portion  16  is formed therebetween. On the outer peripheral side of the diffuser portion  16 , the discharge scroll chamber  12  is formed by the suction-side concave surface  42  of the scroll piece  4 , the inner peripheral concave surface  52  of the shroud piece  5  and the outer peripheral concave surface  33  of the back plate  3 . 
     The back plate  3  is also formed of aluminum die casted articles. 
     As shown in  FIG. 12 , a mold  7  used when the back plate  3  is casted includes body molds  711  and  712  for forming the facing surface  31 , the outer peripheral annular fit-in portion  32  and the outer peripheral concave surface  33 , and central molds  721  and  722  for forming the plate central portion  34 , and the central molds  721  and  722  are detachably attached to the body molds  711  and  712 . As shown in  FIGS. 13A and 13B , the central molds  721  and  722  can be attached such that their phases in the circumferential direction can be changed with respect to the body molds  711  and  712 . 
     The mold  7  includes a cavity  70  between a lower mold formed by attaching the central mold  721  to the body mold  711  and an upper mold formed by attaching the central mold  722  to the body mold  712 . 
     A mold surface for forming a surface of the back plate  3  on the side of the bearing housing  6  is formed on the lower mold (body mold  711  and central mold  721 ). A mold surface for forming a surface of the back plate  3  on the side of the compressor housing  2  is formed on the upper mold (body mold  712  and central mold  722 ). The mold surface of the central mold  721  has a projection  723  for forming the oil discharge port  343 . Various convex and concave shapes are formed on the central molds  721  and  722  in addition to the projection  723  but these convex and concave shapes are omitted in  FIG. 12 . Illustration of the mold surfaces of the body molds  711  and  712  are also simplified. 
     As shown in  FIG. 13 , the central mold  721  as viewed from a normal direction of the mold surface is circular in shape. According to this, the central mold  721  can be attached to the body mold  711  from any orientation in the circumferential direction. The central mold  722  as viewed from a normal direction of the mold surface is also circular in shape, and the central mold  722  can be attached to the body mold  712  from any orientation in the circumferential direction. 
     Hence, if an attaching orientation of the central mold  721  ( 722 ) with respect to the body mold  711  ( 712 ) is changed between a state shown in  FIG. 13A  and a state shown in  FIG. 13B  and back plates  3  are die casted, different kinds of back plates  3  can be obtained. Here, the different kinds of back plates  3  mean back plates  3  having phase difference (positional relation in circumferential direction) differ between the plate central portion  34  (oil discharge port  343 ) and the outer peripheral concave surface  33  (positioning step  36 ). 
     This embodiment has the following advantageous effects. 
     In the turbocharger  1 , since the compressor housing  2  can be configured using the two pieces, i.e., the scroll piece  4  and the shroud piece  5 , the number of parts of the compressor housing  2  can be relatively small. As a result, the producing man-hours of the turbocharger  1  can be reduced and the producing costs can be reduced. 
     Further, apart of the wall surface of the discharge scroll chamber  12  can be formed by the outer peripheral concave surface  33  of the back plate  3 . That is, in a turbocharger in which a back plate is disposed between a bearing housing and a compressor housing, a part of the wall surface of a discharge scroll chamber  12  is formed into a concave shape utilizing a part of the back plate  3  which is an existing part. Hence, it is possible to enhance flexibility in shape of the discharge scroll chamber  12  without increasing the number of parts. 
     The outer peripheral concave surface  33  has such a shape that its cross sectional shape gradually changes along the circumferential direction. Hence, the shape of the wall surface of the discharge scroll chamber  12  on the outer peripheral side can gradually be changed along the circumferential direction. According to this, the discharge scroll chamber  12  can be formed into an ideal shape, and it is possible to realize ideal air flow in the compressor. As a result, it is possible to enhance performance of the turbocharger  1 . 
     The outer peripheral concave surface  33  is formed in the back plate  3  together with a facing surface  31 . Hence, a seam is not formed between the facing surface  31  and the outer peripheral concave surface  33 . Therefore, it is possible to easily and reliably realize a smooth flow of compressed air which is discharged from a diffuser portion  16  into the discharge scroll chamber  12 . From this point of view also, it is possible to enhance the performance of the turbocharger  1 . 
     The cross sectional shape of the outer peripheral concave surface  33  is formed such that the radius of curvature thereof gradually increases and the size h in the axial direction and the size w in the radial direction gradually increase toward the discharge port  15  in the circumferential direction. According to this, the discharge scroll chamber  12  can more easily be formed into the ideal shape. That is, the outer peripheral concave surface  33  configuring a part of the wall surface of the discharge scroll chamber  12  has the above-described shape. According to this, the discharge scroll chamber  12  has such a shape that its cross-sectional area gradually increases toward the discharge port  15  in the circumferential direction, and it becomes easy to form the cross sectional shapes of various portions of the discharge scroll chamber  12  into ideal shapes. 
     The shaft hole  341 , the bolt holes  342  and the oil discharge port  343  are formed in the plate central portion  34  of the back plate  3 . According to this, when the back plate  3  is produced, if its accuracy is enhanced, it is possible to easily assemble the turbocharger  1  while keeping the positional accuracy of the outer peripheral concave surface  33  in the circumferential direction. That is, if the back plate  3  is produced such that a positional relation (phase difference) in the circumferential direction between a position of the bolt holes  342  and a shape of the outer peripheral concave surface becomes a predetermined positional relation (phase difference), a position of the outer peripheral concave surface  33  is determined by fastening a bolt to the back plate  3  through the bolt holes  342  and fixing the back plate  3  to the bearing housing  6 . If a circumferential position is aligned to the outer peripheral concave surface  33  and the compressor housing  2  is fitted to the back plate  3 , the discharge port  15  can be disposed at a position as designed, i.e., at a position corresponding to a position of a pipe to be connected. That is, the positioning step  46  of the scroll piece  4  is abutted against the positioning step  36  of the back plate  3  in the circumferential direction, and the compressor housing  2  is assembled to the back plate  3 . According to this, an accurate discharge scroll chamber  12  can be formed by the outer peripheral concave surface  33  and the suction-side concave surface  42 , and the discharge port  15  can be directed to an appropriate direction. As described above, when the turbocharger  1  is assembled, it is possible to easily dispose the discharge scroll chamber  12  and the discharge port  15  at appropriate circumferential positions. 
     In the above producing method, the back plate  3  is casted using the mold  7  in which the central molds  721  and  722  are detachably attached to the body molds  711  and  712 . The central molds  721  and  722  can be attached to the body molds  711  and  712  such that the phases of the central molds  721  and  722  can be changed in the circumferential direction with respect to the body molds  711  and  712 . Hence, the back plate  3  can be produced while appropriately changing the phase of the outer peripheral concave surface  33  with respect to the plate central portion  34 . The phase of the plate central portion  34  when it is assembled into the turbocharger  1  is determined by necessity by the position of the oil discharge port  343 . Although the phase of the outer peripheral concave surface  33  is set in accordance with the position of the discharge port  15 , the position differs depending upon a type of a vehicle or the like in which the turbocharger  1  is to be mounted. Thus, a phase difference between the plate central portion  34  and the outer peripheral concave surface  33  in the circumferential direction differs depending upon a corresponding vehicle type. Therefore, since the mold  7  for casting the back plate  3  is configured such that the central molds  721  and  722  can be attached to the body molds  711  and  712  with the phases of the central molds  721  and  722  changed in the circumferential direction with respect to the body molds  711  and  712 , a plurality of kinds of back plates  3  suitable for corresponding vehicle types can be produced using one kind of mold  7 . Hence, according to the producing method, it is possible to produce a plurality of kinds of back plates  3  with excellent production efficiency, and the production efficiency of the turbocharger  1  can be enhanced. 
     As described above, according to this embodiment, it is possible to provide a turbocharger capable of reducing its costs and enhancing its performance, and to provide a method of producing the turbocharger. 
     Although the scroll piece and the shroud piece are made of aluminum by die casting in the embodiment, a material and a producing method of these pieces are not limited to these, and these pieces can be made of other material and by other producing method. For example, the shroud piece may be a resin molded article. 
     Although the back plate is also made of aluminum by die casting in the embodiment, a material and a producing method of the back plate are not limited to these. 
     In the mold shown in the embodiment, it is not necessary that the central molds are attached to both the upper and lower molds. That is, the central mold may be detachably attached to only the lower mold or the upper mold.