Patent Publication Number: US-10309248-B2

Title: Variable geometry system turbocharger

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of International Application No. PCT/JP2014/083079, filed on Dec. 15, 2014, which claims priority to Japanese Patent Application No. 2014-014481, filed on Jan. 29, 2014, the entire contents of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a variable geometry system turbocharger equipped with a variable nozzle unit which adjusts a passage area (a throat area) for exhaust gas to be supplied to a turbine wheel side. 
     2. Description of the Related Art 
     A variable geometry system turbocharger is equipped with a variable nozzle unit which adjusts a passage area (a throat area) for exhaust gas to be supplied to a turbine wheel side (see Japanese Patent Application Laid-open Publication Nos. 2013-130116 and 2013-194546). The variable nozzle unit is disposed between a turbine scroll passage and the turbine wheel in a turbine housing adjacent to a bearing housing. Configurations of the conventional variable nozzle unit and its surrounding components are as follows. 
     A first nozzle ring is disposed in the turbine housing. A second nozzle ring is provided integrally with the first nozzle ring at a position away from the first nozzle ring in an axial direction (an axial direction of the turbine wheel). The first nozzle ring has a surface (an opposed surface) which is opposed to the second nozzle ring. Likewise, the second nozzle ring has a surface (an opposed surface) which is opposed to the first nozzle ring. Moreover, variable nozzles are disposed between the opposed surface of the first nozzle ring and the opposed surface of the second nozzle ring. The variable nozzles are disposed at intervals in a circumferential direction (a predetermined circumferential direction). Each variable nozzle is rotatable in forward and reverse directions (opening and closing directions) around a shaft center which is parallel to a shaft center of the turbine wheel. A link mechanism is disposed on an opposite surface side from the opposed surface of the first nozzle ring. The link mechanism rotates the variable nozzles synchronously in the forward and reverse directions. The passage area (the throat area) of the exhaust gas to be supplied to the turbine wheel side is increased when the link mechanism rotates the variable nozzles synchronously in the forward direction (the opening direction). On the other hand, the passage area is reduced when the link mechanism rotates the variable nozzles synchronously in the reverse direction (the closing direction). 
     The bearing housing has a side surface which is opposed to a back surface of the turbine wheel. An annular protrusion is formed at a central part of the side surface. The protrusion protrudes toward the back surface of the turbine wheel. Meanwhile, an annular heat shield plate is fitted to an outer peripheral surface of the protrusion of the bearing housing. The heat shield plate shields heat from the back surface side of the turbine wheel. A biasing member is provided at a position on the outer peripheral surface of the protrusion of the bearing housing, the position being adjacent to the heat shield plate. The biasing member is formed from a wave washer or the like, which biases the heat shield plate in a direction to bring the heat shield plate into pressure contact with an inner peripheral edge portion of the first nozzle ring. Here, a press-contacting portion between the first nozzle ring and the heat shield plate is a sealing portion for suppressing a leakage of the exhaust gas from the opposite surface side from the opposed surface of the first nozzle ring to an inlet side of the turbine wheel. 
     SUMMARY 
     The biasing member such as the wave washer is necessary for suppressing the above-mentioned leakage of the exhaust gas. However, this means that biasing force of the biasing member acts on the variable nozzle unit as force in an axial direction. Accordingly, the biasing force tends to complicate a thermal deformation of the variable nozzle unit when the variable geometry system turbocharger is in operation. Then, when the variable geometry system turbocharger is in operation, the parallelism between the opposed surfaces of the first nozzle ring and the second nozzle ring is likely to be deteriorated depending on an operation condition of an engine. For this reason, a nozzle side clearance is usually set somewhat large so as to sufficiently ensure operational reliability of the variable nozzles, or in other words, operational reliability of the variable geometry system turbocharger even if the parallelism is deteriorated. 
     In the meantime, if the nozzle side clearance is set somewhat large, there is a concern of reduction in turbine efficiency of the variable geometry system turbocharger due to an increase in leakage flow (clearance flow) from the nozzle side clearance. When there is such a concern, a further improvement in turbine efficiency of the variable geometry system turbocharger is desired. Here, the nozzle side clearance means a gap between the first nozzle ring and the variable nozzles or a gap between the second nozzle ring and the variable nozzles. To be more precise, the nozzle side clearance means a gap between the opposed surface of the first nozzle ring and side surfaces of the variable nozzles on one side in the axial direction or a gap between the opposed surface of the second nozzle ring and side surfaces of the variable nozzles on the other side in the axial direction. The former gap will be referred to as a nozzle side clearance on the first nozzle ring side and the latter gap will be referred to as a nozzle side clearance on the second nozzle ring side. 
     An object of the present disclosure is to provide a variable geometry system turbocharger which can solve the aforementioned problem. 
     An aspect of the present disclosure is a variable geometry system turbocharger comprising a variable nozzle unit disposed between a turbine scroll passage and a turbine wheel in a turbine housing adjacent to a bearing housing and configured to adjust a passage area for exhaust gas to be supplied to the turbine wheel side. The variable nozzle unit includes: a first nozzle ring disposed in the turbine housing; a second nozzle ring provided integrally with the first nozzle ring at a position away from and opposed to the first nozzle ring in an axial direction; variable nozzles disposed at intervals in a circumferential direction between the first nozzle ring and the second nozzle ring, and configured to be rotatable in forward and reverse directions around shaft centers parallel to a shaft center of the turbine wheel; a link mechanism configured to rotate the variable nozzles synchronously in opening and closing directions; an annular heat shield plate provided at a side surface of the bearing housing opposed to a back surface of the turbine wheel, and configured to shield heat from the turbine wheel side; and a seal member provided between an inner peripheral surface of the first nozzle ring and an outer peripheral surface of the heat shield plate. 
     In the specification and claims of the present application, the expression “disposed” is intended to include a state of being disposed indirectly with a different member interposed in between as well as a state of being disposed directly on, and the expression “provided” is intended to include a state of being provided indirectly with a different member interposed in between as well as a state of being provided directly on. Meanwhile, the expression “axial direction” means an axial direction of the turbine wheel (in other words, an axial direction of the first nozzle ring or the second nozzle ring). Moreover, the expression “attachment bolt” is intended to include a countersunk bolt, a button bolt, a hexagon socket bolt, and the like. Further, the expression “radial direction” means a radial direction of the turbine wheel (in other words, a radial direction of the first nozzle ring or the second nozzle ring). 
     According to the present disclosure, a nozzle side clearance can be set small while suppressing a leakage of exhaust gas from the opposite surface side from the opposed surface of the first nozzle ring toward the inlet side of the turbine wheel and sufficiently ensuring the parallelism between the opposed surface of the first nozzle ring and the opposed surface of the second nozzle ring when the variable geometry system turbocharger is in operation. Thus, it is possible to further improve turbine efficiency of the variable geometry system turbocharger while sufficiently ensuring operational reliability of the variable geometry system turbocharger and reducing a leakage flow from the nozzle side clearance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing characteristic portions of a variable geometry system turbocharger according to an embodiment of the present disclosure. 
         FIG. 2  is an enlarged view of a portion viewed along an arrow II in  FIG. 3 . 
         FIG. 3  is a front sectional view of the variable geometry system turbocharger according to the embodiment of the present disclosure. 
         FIG. 4  is a cross-sectional view showing characteristic portions of a variable geometry system turbocharger according to a first modified example of the embodiment of the present disclosure. 
         FIG. 5  is a cross-sectional view showing characteristic portions of a variable geometry system turbocharger according to a second modified example of the embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An embodiment of the present disclosure will be described below with reference to  FIG. 1  to  FIG. 3 . Here, as shown in the drawings, “L” indicates a left direction, “R” indicates a right direction, “D 1 ” indicates an axial direction, and “D 2 ” indicates a radial direction. 
     As shown in  FIG. 3 , a variable geometry system turbocharger  1  according to the embodiment of the present disclosure supercharges (compresses) air to be supplied to an engine (not shown) by using energy of exhaust gas from the engine. 
     The variable geometry system turbocharger  1  includes a bearing housing (a center housing)  3 . A radial bearing  5  and a pair of thrust bearings  7  are provided in the bearing housing  3 . Moreover, a rotor shaft (a turbine shaft)  9  extending in a right-left direction is rotatably provided to the bearings  5  and  7 . In other words, the rotor shaft  9  is rotatably provided to the bearing housing  3  through the bearings  5  and  7 . 
     A compressor housing  11  is provided adjacent to a right side of the bearing housing  3 . The compressor housing  11  includes a shroud  11   s  in the inside thereof. A compressor wheel  13  is provided in the compressor housing  11 . The compressor wheel  13  is provided rotatably around its shaft center (in other words, a shaft center of the rotor shaft  9 ) C, and compresses the air by using centrifugal force. Moreover, the compressor wheel  13  includes a compressor disk  15  which is integrally connected to a right end portion of the rotor shaft  9 . A hub surface  15   h  of the compressor disk  15  extends outward in the radial direction (outward in the radial direction of the compressor wheel  13 ) and toward the left side. Furthermore, compressor blades  17  are integrally provided to the hub surface  15   h  of the compressor disk  15  at intervals in a circumferential direction. A tip end edge (an outer edge)  17   t  of each compressor blade  17  extends along the shroud  11   s  of the compressor housing  11 . Here, different compressor blades (not shown) having a shorter axial length than that of the compressor blades  17  may be used in addition to the compressor blades  17 . In this case, the different compressor blades (not shown) are integrally provided to the hub surface  15   h  of the compressor disk  15  and are arranged alternately with the compressor blades  17 . 
     An air intake port  19  for taking in the air is formed on an inlet side (an upstream side as viewed in a flowing direction of a mainstream of the air) of the compressor wheel  13  in the compressor housing  11 . The air intake port  19  is connected to an air cleaner (not shown) which cleans the air. On the other hand, an annular diffuser passage  21  to increase pressure of the compressed air is formed on an outlet side (a downstream side in the flowing direction of the air) of the compressor wheel  13  between the bearing housing  3  and the compressor housing  11 . In addition, a spiral-shaped compressor scroll passage  23  is formed inside the compressor housing  11 . The compressor scroll passage  23  communicates with the diffuser passage  21 . Moreover, an air exhaust port  25  for discharging the air that is compressed (the compressed air) is formed at an appropriate position of the compressor housing  11 . The air exhaust port  25  communicates with the compressor scroll passage  23  and is connected to an intake manifold (not shown) of the engine. 
     As shown in  FIG. 2  and  FIG. 3 , a turbine housing  27  is provided adjacent to a left side of the bearing housing  3 . The turbine housing  27  includes a shroud  27   s  in the inside thereof. Moreover, a turbine wheel  29  is provided in the turbine housing  27 . The turbine wheel  29  is provided rotatably around its shaft center (in other words, the shaft center of the rotor shaft  9 ) C, and the turbine wheel  29  generates rotational force (rotation torque) by using pressure energy of the exhaust gas. The turbine wheel  29  includes a turbine disk  31  which is integrally provided to a left end portion of the rotor shaft  9 . A hub surface  31   h  of the turbine disk  31  extends outward in the radial direction (outward in the radial direction of the turbine wheel  29 ) and toward the right side (one side in the axial direction of the turbine wheel  29 ). Furthermore, turbine blades  33  are integrally provided to the hub surface  31   h  of the turbine disk  31  at intervals in a circumferential direction. A tip end edge (an outer edge)  33   t  of each turbine blade  33  extends along the shroud  27   s  of the turbine housing  27 . 
     A gas intake port  35  for taking in the exhaust gas is formed at an appropriate position of the turbine housing  27 . The gas intake port  35  is connected to an exhaust manifold (not shown) of the engine. A spiral-shaped turbine scroll passage  37  is formed on an inlet side (an upstream side as viewed in a flowing direction of a mainstream of the exhaust gas) of the turbine wheel  29  inside the turbine housing  27 . The turbine scroll passage  37  communicates with the gas intake port  35 . Moreover, a gas exhaust port  39  for discharging the exhaust gas is formed on an outlet side (a downstream side as viewed in the flowing direction of the mainstream of the exhaust gas) of the turbine wheel  29  in the turbine housing  27 . The gas exhaust port  39  is connected to an exhaust emission control system (not shown) which cleans the exhaust gas. 
     The variable geometry system turbocharger  1  is equipped with a variable nozzle unit  41  which adjusts a passage area (a throat area) of the exhaust gas to be supplied to the turbine wheel  29  side. The variable nozzle unit  41  is disposed between the turbine scroll passage  37  and the turbine wheel  29  in the turbine housing  27 . 
     Next, a specific configuration of the variable nozzle unit  41  will be described. 
     As shown in  FIG. 1  and  FIG. 2 , a first nozzle ring  43  is disposed between the turbine scroll passage  37  and the turbine wheel  29  in the turbine housing  27 . The first nozzle ring  43  is disposed concentrically with the turbine wheel  29  through a cup-shaped support ring  45 . Meanwhile, bottomless (penetrating) first support holes  47  (only one of which is illustrated) are formed at equal intervals in a circumferential direction (a predetermined circumferential direction) in the first nozzle ring  43 . An annular first stepped portion  49  is provided in an inner peripheral surface of the first nozzle ring  43 , and is formed to recede outward in the radial direction. Here, an outer peripheral edge portion of the support ring  45  is sandwiched between a left side portion of the bearing housing  3  and a right side portion of the turbine housing  27 . The first support holes  47  are arranged at equal intervals in a circumferential direction. However, the intervals do not have to be equal. 
     A second nozzle ring  51  is provided at a position away from and opposed to the first nozzle ring  43  in the axial direction (the axial direction of the turbine wheel  29 , or in other words, the right-left direction). The second nozzle ring  51  is provided integrally and concentrically with the first nozzle ring  43  with connecting pins  53  (only one of which is illustrated) interposed in between. The second nozzle ring  51  surrounds an annular protrusion  55 . Here, the protrusion  55  is formed to protrude in the right direction (the one side in the axial direction) between the turbine scroll passage  37  and the turbine wheel  29  in the turbine housing  27 . In other words, the second nozzle ring  51  is located on the outside in the radial direction of the annular protrusion  55  of the turbine housing  27 . Bottomed second support holes  57  (only one of which is illustrated) are formed in the second nozzle ring  51  in such a way as to align with the first support holes  47  in the first nozzle ring  43 . An annular second stepped portion  59  is provided in an inner peripheral surface of the second nozzle ring  51 , and is formed to recede outward in the radial direction. The first nozzle ring  43  has a surface (an opposed surface) which is opposed to the second nozzle ring  51 . Likewise, the second nozzle ring  51  has a surface (an opposed surface) which is opposed to the first nozzle ring  43 . The connecting pins  53  have a function to set an interval between the opposed surface of the first nozzle ring  43  and the opposed surface of the second nozzle ring  51 . 
     Here, an opposite surface (a right side surface) side from the opposed surface of the first nozzle ring  43  communicates with the turbine scroll passage  37  via a through-hole (not shown) formed in the support ring  45 . Meanwhile, an inner diameter of the first nozzle ring  43  and an inner diameter of the second nozzle ring  51  are set to an equal dimension, while an outer diameter of the first nozzle ring  43  and an outer diameter of the second nozzle ring  51  are set to an equal dimension. Note that the term “equal” means to be substantially equal and includes a margin of error to the extent not causing structural or operational problems. Such a margin of error is set in a range of ±5 mm, for example. 
     Variable nozzles  61  are disposed between the opposed surface (a left side surface) of the first nozzle ring  43  and the opposed surface (a right side surface) of the second nozzle ring  51 . The variable nozzles  61  are disposed at equal intervals in a circumferential direction (a predetermined circumferential direction) in such a way as to surround the inlet side of the turbine wheel  29 . Each variable nozzle  61  is rotatable in opening and closing directions (forward and reverse directions) around a shaft center which is parallel to the shaft center C of the turbine wheel  29 . A first nozzle shaft  63  is integrally formed at a right side surface (a side surface on one side in the axial direction) of each variable nozzle  61 . Each first nozzle shaft  63  is rotatably supported by the corresponding first support hole  47  in the first nozzle ring  43 . Moreover, a second nozzle shaft  65  is integrally formed at a left side surface (a side surface on another side in the axial direction) of each variable nozzle  61  and concentrically with the corresponding first nozzle shaft  63 . Each second nozzle shaft  65  is rotatably supported by the corresponding second support hole  57  in the second nozzle ring  51 . Here, the variable nozzles  61  are arranged at equal intervals in the circumferential direction. However, the intervals do not have to be equal. Each variable nozzle  61  may be provided with the first nozzle shaft  63  and the second nozzle shaft  65  to be supported from both sides thereof, or deprived of the second nozzle shaft  65  to be supported from one side thereof. 
     A link mechanism  69  is disposed in an annular link chamber  67  formed on the opposite surface side from the opposed surface of the first nozzle ring  43 . The link mechanism  69  is connected to the first nozzle shafts  63  of the variable nozzles  61  and rotates the variable nozzles  61  synchronously in the opening direction or the closing direction. Meanwhile, the link mechanism  69  is formed from a publicly known configuration as disclosed in Japanese Patent Application Publications No. 2009-243300 and No. 2009-243432, for example. The link mechanism  69  is connected to a rotating actuator (not shown) such as a rotary motor or a rotary cylinder, which rotates the variable nozzles  61  in the opening direction or the closing direction, through a power transmission mechanism  71 . Here, the link mechanism  69  may be disposed on the opposite surface side from the opposed surface of the second nozzle ring  51  instead of being disposed on the opposite surface side from the opposed surface of the first nozzle ring  43  (in the link chamber  67 ). 
     Next, configurations of components surrounding the variable nozzle unit  41  will be described. 
     As shown in  FIG. 1 , the bearing housing  3  includes a side surface which is opposed to a back surface  31   b  of the turbine disk  31 . An annular heat shield plate  73  is integrally provided at a central part of the side surface by being fastened with countersunk bolts  75  (only one of which is illustrated). The heat shield plate  73  shields heat from the turbine wheel  29  side. The heat shield plate  73  is located concentrically with the turbine wheel  29 . An outer peripheral surface of the heat shield plate  73  is fitted to the inner peripheral surface of the first nozzle ring  43 . An annular fitting flange  77  is provided at an inner peripheral edge portion of the heat shield plate  73 , and is formed to protrude in the right direction. The fitting flange  77  is fitted to a circular fitting groove  79  which is formed in a central part of the side surface of the bearing housing  3  mentioned above. Here, the circular fitting groove  79  is formed concentrically with the turbine wheel  29 . Moreover, the heat shield plate  73  includes a side surface which is opposed to the back surface  31   b  of the turbine disk  31 . In this side surface, an annular housing recess (a housing stepped portion)  80  is formed. The housing recess  80  is formed to recede in the right direction (the one side in the axial direction) and houses an outer edge portion of the turbine disk  31  (part of the turbine wheel  29 ). A circular fitting groove  81  is formed in an outer peripheral surface of the heat shield plate  73 . Here, a fastened portion F between the bearing housing  3  and the heat shield plate  73  is an annular sealing portion which suppresses a leakage of the exhaust gas from a back surface  73   b  side of the heat shield plate  73  to the back surface  31   b  side of the turbine disk  31 . In other words, the fastened portion F is formed from portions of the bearing housing  3  and the heat shield plate  73  which come into contact with each other by means of fastening. This contact suppresses the leakage of the exhaust gas. 
     Two first seal rings  83  serving as an example of a seal member are brought into pressure contact, by their own elastic force, with a bottom surface  49   u  of the first stepped portion  49  of the first nozzle ring  43  (part of the inner peripheral surface of the first nozzle ring  43 ). The first seal rings  83  suppress the leakage of the exhaust gas from the opposite surface side from the opposed surface of the first nozzle ring  43  to the inlet side of the turbine wheel  29 . Meanwhile, an inner peripheral edge portion of each first seal ring  83  is fitted in the circular fitting groove  81  in the heat shield plate  73 . In other words, the first seal rings  83  are held by their own elastic force between the inner peripheral surface of the first nozzle ring  43  and the outer peripheral surface of the heat shield plate  73 . 
     A circular fitting groove  85  is formed in an outer peripheral surface of the protrusion  55  of the turbine housing  27 . Two second seal rings  87  (an example of a different seal member) serving as an example of a different seal member are brought into pressure contact, by their own elastic force, with a bottom surface  59   u  of the second stepped portion  59  of the second nozzle ring  51  (part of the inner peripheral surface of the second nozzle ring  51 ). The second seal rings  87  suppress the leakage of the exhaust gas from the opposite surface side from the opposed surface of the second nozzle ring  51  to the inlet side of the turbine wheel  29 . Meanwhile, an inner peripheral edge portion of each second seal ring  87  is fitted in the circular fitting groove  85  in the protrusion  55  of the turbine housing  27 . In other words, the second seal rings  87  are held by their own elastic force between the inner peripheral surface of the second nozzle ring  51  and the outer peripheral surface of the protrusion  55  of the turbine housing  27 . 
     Next, operation and effect of the embodiment of the present disclosure will be described. 
     The exhaust gas is taken in from the gas intake port  35  and flows from the inlet side to the outlet side of the turbine wheel  29  via the turbine scroll passage  37 . In the course of the flow of the exhaust gas, rotational force (rotation torque) is generated by using pressure energy of the exhaust gas, so that the rotor shaft  9  and the compressor wheel  13  can be rotated integrally with the turbine wheel  29 . Thus, the air taken in from the air intake port  19  can be compressed and discharged from the air exhaust port  25  via the diffuser passage  21  and the compressor scroll passage  23 , and the air to be supplied to the engine can be supercharged (compressed) accordingly. 
     If the number of revolutions of the engine is in a high-revolution range and a flow rate of the exhaust gas is high when the variable geometry system turbocharger  1  is in operation, the variable nozzles  61  are rotated synchronously in the forward direction (the opening direction) while operating the link mechanism  69  by the drive of the rotary actuator. Thus, it is possible to supply a large amount of the exhaust gas to the turbine wheel  29  side by increasing the passage area (the throat area) of the exhaust gas to be supplied to the turbine wheel  29  side. 
     If the number of revolutions of the engine is in a low-revolution range and the flow rate of the exhaust gas is low, the variable nozzles  61  are rotated synchronously in the reverse direction (the closing direction) while operating the link mechanism  69  by the drive of the rotary actuator. Thus, it is possible to increase a flow velocity of the exhaust gas by reducing the passage area for the exhaust gas to be supplied to the turbine wheel  29  side, and to yield a sufficient work output from the turbine wheel  29 . 
     The heat shield plate  73  is fastened with the countersunk bolts  75  and thus integrally provided to the side surface of the bearing housing  3  opposed to the back surface  31   b  of the turbine disk  31 . Moreover, the two first seal rings  83  are provided between the inner peripheral surface of the first nozzle ring  43  and the outer peripheral surface of the heat shield plate  73 . Accordingly, the leakage of the exhaust gas from the opposite surface side from the opposed surface of the first nozzle ring  43  to the inlet side of the turbine wheel  29  can be suppressed without using the biasing member such as the wave washer, or in other words, without constantly applying the force in the axial direction to the variable nozzle unit  41 . Thus, it is possible to set the nozzle side clearance small by suppressing complication of a thermal deformation of the variable nozzle unit  41  and sufficiently ensuring the parallelism between the opposed surface of the first nozzle ring  43  and the opposed surface of the second nozzle ring  51  when the variable geometry system turbocharger  1  is in operation. 
     The inner diameter of the first nozzle ring  43  and the inner diameter of the second nozzle ring  51  are set to an equal dimension, while the outer diameter of the first nozzle ring  43  and the outer diameter of the second nozzle ring  51  are set to an equal dimension. As a consequence, it is possible to reduce a difference in thermal expansion in the radial direction between the first nozzle ring  43  and the second nozzle ring  51  when the variable geometry system turbocharger  1  is in operation, and to suppress a misalignment between each first support hole  47  in the first nozzle ring  43  and the corresponding second support hole  57  in the second nozzle ring  51 . Moreover, since the two seal rings  87  are provided between the inner peripheral surface of the second nozzle ring  51  and the outer peripheral surface of the protrusion  55  of the turbine housing  27 , it is possible to suppress the leakage of the exhaust gas from the opposite surface side from the opposed surface of the second nozzle ring  51  to the inlet side of the turbine wheel  29 . 
     The outer edge portion of the turbine disk  31  is housed in the annular housing recess  80 . Accordingly, it is possible to suppress the leakage of the exhaust gas from the variable nozzle  61  side to the back surface  31   b  side of the turbine disk  31 . Moreover, the fastened portion F between the bearing housing  3  and the heat shield plate  73  is the annular sealing portion. Accordingly, it is possible to suppress the leakage of the exhaust gas from the back surface  73   b  side of the heat shield plate  73  to the back surface  31   b  side of the turbine disk  31 . 
     Each first support hole  47  in the first nozzle ring  43  is bottomless (is a penetrating hole), so that the opposite surface side of the opposed surface of the first nozzle ring  43  communicates with the turbine scroll passage  37 . Meanwhile, each second support hole  57  in the second nozzle ring  51  is bottomed. Accordingly, in each variable nozzle  61 , pressure that acts on an end surface of the first nozzle shaft  63  can be made sufficiently larger than pressure that acts on an end surface of the second nozzle shaft  65  when the variable geometry system turbocharger  1  is in operation. In this way, it is possible to bring each variable nozzle  61  closer to the opposed surface side of the second nozzle ring  51  by using the difference in pressure and thus to reduce a leakage flow from the nozzle side clearance on the opposed surface side of the second nozzle ring  51  when the variable geometry system turbocharger  1  is in operation. 
     According to this embodiment, the leakage of the exhaust gas from the opposite surface side from the opposed surface of the first nozzle ring  43 , and the like, to the inlet side of the turbine wheel  29  can be suppressed. In addition, it is possible to set the nozzle side clearance small by sufficiently ensuring the parallelism between the opposed surface of the first nozzle ring  43  and the opposed surface of the second nozzle ring  51  when the variable geometry system turbocharger  1  is in operation. As a consequence, the turbine efficiency of the variable geometry system turbocharger  1  can be further improved by reducing the leakage flow from the nozzle side clearance while sufficiently ensuring operational reliability of the variable geometry system turbocharger  1 . 
     A misalignment between each first support hole  47  in the first nozzle ring  43  and the corresponding second support hole  57  in the second nozzle ring  51  can be suppressed when the variable geometry system turbocharger  1  is in operation. Accordingly, it is possible to ensure the operational reliability of the variable geometry system turbocharger  1  even more sufficiently. In addition, the leakage flow from the nozzle side clearance on the opposed surface side of the second nozzle ring  51  can be reduced while suppressing the leakage of the exhaust gas from the variable nozzle  61  side and the like to the back surface  31   b  side of the turbine disk  31 . Thus, it is possible to further improve the turbine efficiency of the variable geometry system turbocharger  1  by stabilizing a flow of the exhaust gas on the shroud  27   s  side of the turbine housing  27  in the turbine wheel  29 . 
     (First Modified Example) 
     A first modified example of the embodiment of the present disclosure will be described with reference to  FIG. 4 . Here, as shown in the drawing, “L” indicates the left direction, “R” indicates the right direction, “D 1 ” indicates the axial direction, and “D 2 ” indicates the radial direction. 
     As shown in  FIG. 4 , instead of the countersunk bolts  75  (see  FIG. 1 ), button bolts  89  may be used as fastening members for providing the heat shield plate  73  integrally at the central part of the side surface of the bearing housing  3 . In this case, the heat shield plate  73  is allowed to be displaced in the radial direction with respect to the button bolts  89  and the like. As mentioned above, the shape of each bolt head is not limited to a particular shape. For example, though the illustration is omitted, hexagon bolts, hexagon socket bolts, lobular socket bolts, and the like are also applicable. 
     The first modified example of the embodiment of the present disclosure has operation and effect similar to the above-described embodiment of the present disclosure. 
     (Second Modified Example) 
     A second modified example of the embodiment of the present disclosure will be described with reference to  FIG. 5 . As shown in the drawing, “L” indicates the left direction, “R” indicates the right direction, “D 1 ” indicates the axial direction, and “D 2 ” indicates the radial direction. 
     Here, the fastening members for providing the heat shield plate  73  integrally at the central part of the side surface of the bearing housing  3  may be omitted. In this case, as shown in FIG.  5 , an annular heat shield plate  91  is provided integrally at the central part of the side surface of the bearing housing  3  by laser welding. The heat shield plate  91  is located concentrically with the turbine wheel  29 , and an outer peripheral surface of the heat shield plate  91  is fitted to the inner peripheral surface of the first nozzle ring  43 . Here, a welded portion W between the bearing housing  3  and the heat shield plate  91  is an annular sealing portion which suppresses the leakage of the exhaust gas from a back surface  91   b  side of the heat shield plate  91  to the back surface  31   b  side of the turbine disk  31 . Note that other welding methods including TIG welding, MIG welding, electron beam welding, friction welding, and the like may be used instead of laser welding. 
     An annular fitting flange  93  is provided at an inner peripheral edge portion of the heat shield plate  91 , and is formed to protrude inward in the radial direction. The fitting flange  93  of the heat shield plate  91  is fitted to an outer peripheral surface of a fitting protrusion  95 . The fitting protrusion  95  is provided at the central part of the side surface of the bearing housing  3  opposed to the back surface  31   b  of the turbine disk  31 , and is formed to protrude toward the turbine disk  31  side. Meanwhile, a circular fitting groove  97  is formed in the outer peripheral surface of the heat shield plate  91 . An inner peripheral edge portion of each first seal ring  83  is fitted in the circular fitting groove  97  in the heat shield plate  91 . 
     The second modified example of the embodiment of the present disclosure has operation and effect similar to the above-described embodiment of the present disclosure. 
     It is to be understood that the present disclosure is not limited only to the description of the embodiment mentioned above, but can be embodied in various other aspects as follows, for example. 
     Specifically, instead of providing the heat shield plate  73  integrally at the central part of the side surface of the bearing housing  3  opposed to the back surface  31   b  of the turbine disk  31  either by means of fastening with the attachment bolts or by means of welding, the heat shield plate  73  may be integrally provided by means of a press-fitting action using a press-fitting recess formed in one of the bearing housing  3  and the heat shield plate  73  and a press-fitting protrusion formed at the remaining one of the bearing housing  3  and the heat shield plate  73 . Meanwhile, the number of the first seal rings  83  or the second seal rings  87  may be changed into one or more than two. A shape of a joint of the first seal ring  83  or the second seal ring  87  may adopt any of straight cut, step cut, angle cut, and the like. Furthermore, instead of the first seal rings  83  or the second seal rings  87 , metal O-rings or metal gaskets (metal gaskets having a U-shaped cross section, metal gaskets having a V-shaped cross section, metal gaskets having a C-shaped cross section, and the like) may be used as the seal members or the different seal members. 
     It is to be noted that the scope of rights encompassed by the present disclosure is not limited to these embodiments.