Patent Publication Number: US-8979485-B2

Title: Variable geometry turbine

Description:
RELATED APPLICATION 
     The present application is related to, and claims priority to United Kingdom Patent Application No. 1015679.2 filed on Sep. 20, 2010, which is incorporated herein by reference. 
     The present invention relates to a variable geometry turbine. Particularly, but not exclusively, the present invention relates to a variable geometry turbine for a turbocharger or other turbomachine. 
     A turbomachine comprises a turbine. A conventional turbine comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. Rotation of the turbine wheel drives either a compressor wheel mounted on the other end of the shaft within a compressor housing to deliver compressed air to an engine intake manifold, or a gear which transmits mechanical power to an engine flywheel or crankshaft. The turbine shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a bearing housing. 
     Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric pressure (boost pressures). Turbochargers comprise a turbine having a turbine housing which defines a turbine chamber within which the turbine wheel is mounted; an annular inlet passageway defined between opposite radial walls arranged around the turbine chamber; an inlet arranged around the inlet passageway; and an outlet passageway extending from the turbine chamber. The passageways and chambers communicate such that pressurised exhaust gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine and rotates the turbine wheel. Turbine performance can be improved by providing vanes, referred to as nozzle vanes, in the inlet passageway so as to deflect gas flowing through the inlet passageway towards the direction of rotation of the turbine wheel. 
     Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the inlet passageway can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suite varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level which ensures efficient turbine operation by reducing the size of the annular inlet passageway. Turbochargers provided with a variable geometry turbine are referred to as variable geometry turbochargers. 
     In one known type of variable geometry turbine, an array of vanes, generally referred to as a “nozzle ring”, is disposed in the inlet passageway and serves to direct gas flow towards the turbine. The axial position of the nozzle ring relative to a facing wall of the inlet passageway is adjustable to control the axial width of the inlet passageway. The nozzle ring vanes extend into the inlet and through vane slots provided in a “shroud” defining the facing wall of the inlet passageway to accommodate movement of the nozzle ring. Thus, for example, as gas flow through the turbine decreases, the inlet passageway width may be decreased to maintain gas velocity and optimise turbine output. This arrangement differs from another type of variable geometry turbine in which a variable guide vane array comprises adjustable swing guide vanes arranged to pivot so as to open and close the inlet passageway. 
     The known shroud comprises an annular plate which seats in the mouth of an annular shroud cavity. The shroud plate is held in position by a retaining ring located in a circumferential groove provided in the outer periphery of the shroud plate and extending into a circumferential groove provided in the turbine housing around the mouth of the shroud cavity. The retaining ring is a split ring of a form commonly referred to as a “piston ring”. 
     The nozzle ring may typically comprise a radially extending wall (defining one wall of the inlet passageway) and radially inner and outer axially extending walls or flanges which extend into an annular cavity behind the radial face of the nozzle ring. The cavity is formed in a part of the turbocharger housing (usually either the turbine housing or the turbocharger bearing housing) and accommodates axial movement of the nozzle ring. The flanges may be sealed with respect to the cavity walls to reduce or prevent leakage flow around the back of the nozzle ring. 
     In one arrangement of a variable geometry turbine the nozzle ring is supported on rods extending parallel to the axis of rotation of the turbine wheel and is moved by an actuator which axially displaces the rods. Nozzle ring actuators can take a variety of forms, including pneumatic, hydraulic and electric and can be linked to the nozzle ring in a variety of ways. The actuator will generally adjust the position of the nozzle ring under the control of an engine control unit (ECU) in order to modify the airflow through the turbine to meet performance requirements. 
     During the lifetime of a turbine the shroud retaining ring and/or the shroud itself may be subject to wear and fatigue. It is an object of the present invention to reduce such wear/fatigue. 
     According to a first aspect of the present invention there is provided a variable geometry turbine comprising: a housing; a turbine wheel supported in the housing for rotation about a turbine axis; an annular inlet passage upstream of said turbine wheel defined between respective inlet surfaces defined by an annular nozzle ring and a facing annular shroud; the nozzle ring being axial movable to vary the size of the inlet passage; a circumferential array of inlet vanes supported by the nozzle ring and extending across the inlet passage; the shroud covering the opening of a shroud cavity defined by the housing inlet passage and inboard of the shroud, and defining a circumferential array of vane slots, the vane slots and shroud cavity being configured to receive said inlet vanes to accommodate axial movement of the nozzle ring; wherein the annular shroud comprises an outer flange around its radially outer periphery, the outer flange defining a circumferential flange groove for receiving a retaining ring for securing the shroud in the opening of the shroud cavity, the flange groove being defined on an inboard side by a radially extending flange wall; wherein an annular flange rim extends axially inboard from said radial flange wall. 
     Preferably the annular shroud rim is a continuation of an axially extending annular flange wall which defines an annular base of the flange groove and extending axially beyond said radial flange wall. 
     An annular gap is preferably defined between the shroud flange rim and inner surface of the housing defining a portion of the shroud cavity, wherein said annular gap increases in radial width along the length of the flange rim towards the inboard end of the flange rim. 
     The annular flange rim may have a radially outer surface and a radially inner surface, and wherein the radius of the radial outer surfaces reduces towards the inboard end of the rim. 
     The radius of the inner surface of the flange rim may be substantially constant, so that the flange rim tapers along its length towards its inboard end. 
     According to a second aspect of the present invention there is provided a variable geometry turbine comprising: a housing; a turbine wheel supported in the housing for rotation about a turbine axis; an annular inlet passage upstream of said turbine wheel defined between respective inlet surfaces defined by an annular nozzle ring and a facing annular shroud; the nozzle ring being axial movable to vary the size of the inlet passage; a circumferential array of inlet vanes supported by the nozzle ring and extending across the inlet passage; the shroud covering the opening of a shroud cavity defined by the housing inlet passage and inboard of the shroud, and defining a circumferential array of vane slots, the vane slots and shroud cavity being configured to receive said inlet vanes to accommodate axial movement of the nozzle ring; wherein the annular shroud comprises an outer flange around its radially outer periphery, the outer flange defining a circumferential flange groove for receiving a retaining ring for securing the shroud in the opening of the shroud cavity, the flange groove being defined on an inboard side by a radially extending flange wall; wherein the retaining ring is a substantially annular split ring having a radially inner portion received within the flange groove and the radially outer portion received within an annular groove defined by the housing to thereby key the shroud in position in the mouth of the shroud cavity; the housing groove having an outboard sidewall, a base and an inboard side wall; wherein the outboard face of the radially outer portion of the retaining ring and the outboard sidewall of the housing groove define corresponding frusto-conical surfaces which cooperate to bias the retaining ring in an inboard direction under a radial spring force of the retaining ring, thereby urging a portion of the shroud into contact with an abutment surface defined by the housing to secure the shroud in position in the mouth of the shroud cavity; and wherein the axial width of the housing groove is such that the inboard wall of the housing groove is spaced from the inboard surface of the radially outer portion of the retaining ring so that there is no contact between the two. 
     Preferably the axial spacing between the inboard wall of the radially outer portion of the retaining ring and the inboard wall of the housing groove is at least equal to the maximum width of the retaining ring. 
     It is preferred that the inboard wall of the housing groove extends to a smaller radius than the outer radius of the shroud, and wherein an axial gap is defined between said inboard wall of the housing groove and the outer flange of the shroud. 
     The portion of the shroud which is urged against an abutment surface of the housing may be at the radially inner periphery of the shroud. Said portion of the shroud which is urged into contact with an abutment surface of the housing, may be an axially extending inboard flange at the radially inner periphery of the shroud. 
     The portion of the shroud urged into contact with a abutment surface of the housing is preferably a portion of the radially outer flange. 
     According to a third aspect of the present invention there is provided a variable geometry turbine comprising: a housing; a turbine wheel supported in the housing for rotation about a turbine axis; an annular inlet passage upstream of said turbine wheel defined between respective inlet surfaces defined by an annular nozzle ring and a facing annular shroud; the nozzle ring being axial movable to vary the size of the inlet passage; a circumferential array of inlet vanes supported by the nozzle ring and extending across the inlet passage; the shroud covering the opening of a shroud cavity defined by the housing inlet passage and inboard of the shroud, and defining a circumferential array of vane slots, the vane slots and shroud cavity being configured to receive said inlet vanes to accommodate axial movement of the nozzle ring; wherein the annular shroud comprises a radially extending outer flange wall around its radially outer periphery; wherein the housing defines an internally screw threaded annular surface around the opening of the shroud cavity; and wherein the shroud is retained in position by a retaining ring provided with a screw threaded outer surface which engages said screw threaded surface of the housing and wherein a portion of the retaining ring bears against the outer flange of the shroud. 
     Preferably the retaining ring has a radially extending outboard portion and an axially extending inboard portion, wherein said inboard portion defines said screw threaded surface for engagement with the screw threaded surface of the housing, and wherein the radially extending outboard portion bears against the outer flange of the shroud. 
     The outer flange of the shroud may be trapped between the radially extending portion of the retaining ring and an annular support ring located within the opening of the shroud cavity. 
     It is preferred that the shroud has an inner annular flange extending radially inboard at its inner periphery, and wherein the inboard end of the inner flange is urged against an abutment surface of the housing by axial force applied to the shroud by the retaining ring. 
     The radially extending outer flange of the shroud preferably extends radially from the inboard end of an axially extending shroud flange wall. A radial outboard surface of the retaining ring may be substantially aligned with the radial outboard surface of the shroud. 
     According to a fourth aspect of the present invention there is provided a variable geometry turbine comprising: a housing; a turbine wheel supported in the housing for rotation about a turbine axis; an annular inlet passage upstream of said turbine wheel defined between respective inlet surfaces defined by an annular nozzle ring and a facing annular shroud; the nozzle ring being axial movable to vary the size of the inlet passage; a circumferential array of inlet vanes supported by the nozzle ring and extending across the inlet passage; the shroud covering the opening of a shroud cavity defined by the housing inlet passage and inboard of the shroud, and defining a circumferential array of vane slots, the vane slots and shroud cavity being configured to receive said inlet vanes to accommodate axial movement of the nozzle ring; wherein the shroud comprises an annular wall defining said vane slots and having radial outboard and inboard surfaces; the outboard surface of the annular shroud wall having a radial width A; the annular shroud wall having an axial thickness C between its outboard and inboard surfaces; wherein an axial flange extends inboard of the shroud wall around its radial inner periphery, said inner flange extending a distance B from the inboard surface of the radial shroud wall; wherein the ratio A:B is equal to or less than about 5 and/or the ratio B:C is equal to or greater than about 1.5. 
     The ratio A:B may be at least 3. The ratio B:C may be less than 5. 
    
    
     
       Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is an axial cross-section through a known variable geometry turbocharger; 
         FIG. 2A  is a front view of a prior art shroud for use in a variable geometry turbine; 
         FIG. 2B  is a cross-sectional view taken along line G-G of the shroud of  FIG. 2A ; 
         FIG. 3  is a schematic illustration of the prior art shroud of  FIGS. 2   a  and  2   b  installed in a turbine housing; 
         FIGS. 4   a  and  4   b  are sectional views of a first embodiment of a shroud according to the present invention; 
         FIG. 5  is a sectional view of part of a turbocharger turbine including the shroud of  FIGS. 4   a  and  4   b  in accordance with the present invention; 
         FIG. 6  is a schematic sectional view of a second embodiment of the present invention; 
         FIG. 7  is a schematic view of a third embodiment of the present invention; 
         FIG. 8  is a sectional view of a fourth embodiment of the present invention; and 
         FIG. 9  is a sectional view illustrating a fifth embodiment of the present invention. 
     
    
    
     Referring to  FIG. 1 , this illustrates a known variable geometry turbocharger comprising a variable geometry turbine housing  1  and a compressor housing  2  interconnected by a central bearing housing  3 . A turbocharger shaft  4  extends from the turbine housing  1  to the compressor housing  2  through the bearing housing  3 . A turbine wheel  5  is mounted on one end of the shaft  4  for rotation within the turbine housing  1 , and a compressor wheel  6  is mounted on the other end of the shaft  4  for rotation within the compressor housing  2 . The shaft  4  rotates about turbocharger axis  4   a  on bearing assemblies located in the bearing housing  3 . 
     The turbine housing  1  defines an inlet volute  7  to which gas from an internal combustion engine (not shown) is delivered. The exhaust gas flows from the inlet volute  7  to an axial outlet passageway  8  via an annular inlet passageway  9  and the turbine wheel  5 . The inlet passageway  9  is defined on one side by a face  10  of a radial wall of a movable annular wall member  11 , referred to as a “nozzle ring”, and on the opposite side by a second wall member comprising an annular shroud  12  which forms the wall of the inlet passageway  9  facing the nozzle ring  11 . The shroud  12  covers the opening of an annular recess, or shroud cavity,  13  in the turbine housing  1 . 
     The nozzle ring  11  supports an array of circumferentially and equally spaced inlet vanes  14  each of which extends across the inlet passageway  9 . The vanes  14  are orientated to deflect gas flowing through the inlet passageway  9  towards the direction of rotation of the turbine wheel  5 . The vanes  14  project through suitably configured slots in the shroud  12 , and into the shroud cavity  13 , to accommodate movement of the nozzle ring  11 . 
     The position of the nozzle ring  11  is controlled by an actuator assembly of the type disclosed in U.S. Pat. No. 5,868,552. An actuator (not shown) is operable to adjust the position of the nozzle ring  11  via an actuator output shaft (not shown), which is linked to a yoke  15 . The yoke  15  in turn engages axially extending actuating rods  16  that support the nozzle ring  11 . Accordingly, by appropriate control of the actuator (which may for instance be pneumatic or electric), the axial position of the rods  16  and thus of the nozzle ring  11  can be controlled. The speed of the turbine wheel  5  is dependent upon the velocity of the gas passing through the annular inlet passageway  9 . For a fixed rate of mass of gas flowing into the inlet passageway  9 , the gas velocity is a function of the width of the inlet passageway  9 , the width being adjustable by controlling the axial position of the nozzle ring  11 .  FIG. 1  shows the annular inlet passageway  9  fully open. The inlet passageway  9  may be closed to a minimum by moving the face  10  of the nozzle ring  11  towards the shroud  12 . 
     The nozzle ring  11  has axially extending radially inner and outer annular flanges  17  and  18  that extend into an annular cavity  19  provided in the turbine housing  1 . Inner and outer sealing rings  20  and  21  are provided to seal the nozzle ring  11  with respect to inner and outer annular surfaces of the annular cavity  19  respectively, whilst allowing the nozzle ring  11  to slide within the annular cavity  19 . The inner sealing ring  20  is supported within an annular groove formed in the radially inner annular surface of the cavity  19  and bears against the inner annular flange  17  of the nozzle ring  11 . The outer sealing ring  20  is supported within an annular groove formed in the radially outer annular surface of the cavity  19  and bears against the outer annular flange  18  of the nozzle ring  11 . 
     Gas flowing from the inlet volute  7  to the outlet passageway  8  passes over the turbine wheel  5  and as a result torque is applied to the shaft  4  to drive the compressor wheel  6 . Rotation of the compressor wheel  6  within the compressor housing  2  pressurises ambient air present in an air inlet  22  and delivers the pressurised air to an air outlet volute  23  from which it is fed to an internal combustion engine (not shown). 
     The shroud  12  of the turbocharger of  FIG. 1  is shown in greater detail in  FIGS. 2A and 2B . The shroud is an annular plate comprising a radially extending shroud wall  24  provided with vane slots  25  for the receipt of the vanes  14  of the nozzle ring  11 . The vane slots  25  are best seen in  FIG. 2A , each slot having a leading end  25   a  and a trailing end  25   b . The trailing end  25   b  of two of the slots  25  is visible in the cross-section of  FIG. 2   b . The radially inner periphery of the annular shroud wall  24  is formed with an axially extending flange  26 , which extends in an inboard direction away from the turbine inlet  9  when the shroud  12  is in position in the turbine housing, and provides means for seating the inner periphery of the shroud  12  in the mouth of the shroud cavity  13 . 
     The radially outer periphery of the shroud plate  24  is formed with a grooved flange  27 . The flange  27  extends axially inboard from the shroud plate wall  24  to a greater extent than the inner shroud  26 , and defines an annular groove  28  around the radially outer periphery of the shroud. In more detail, the grooved flange  27  comprises an axially extending flange wall  27   a  and a radially extending flange wall  27   b , the groove  28  being defined between the outer periphery of the shroud wall  24  and the radially extending flange wall  27   b , the base of the groove  28  being defined by the axially extending flange wall  27   a . The overall configuration is generally “h” shaped. 
       FIG. 3  schematically illustrates mounting of the known shroud plate  12  of  FIGS. 2   a  and  2   b  to a turbine housing  1 . Specifically,  FIG. 3  schematically illustrates the manner in which the outer periphery of the shroud  12  is secured in the opening, or mouth, of the shroud cavity  13 . A retaining ring  29  (which may have the from of a conventional “piston ring”) is located within the groove  28  of the shroud  12 . The retaining ring is a split ring which can be radially compressed to allow the shroud  12  to be slid into the mouth of the shroud cavity  13 . As the shroud  12  is fitted in position, the groove  28  aligns with an annular groove  30  defined around the mouth of the shroud cavity  13 . The housing  1  is also formed with a radial extending annular shoulder  1   a . With the grooves  28  and  30  aligned, the retaining ring  29  springs radially outwards to engage the groove  30  and secure the shroud  12  in position. The radially outer periphery of the retaining ring  29  tapers defining a conical outboard surface  32  which engages with a complimentary conical surface defined by an outboard side wall  33  of the groove  30 . Interaction of the surfaces  32  and  33  as the retaining ring  29  radially expands into the groove  30  biases the shroud  12  axially inwards into the mouth of the shroud cavity  13  to ensure the shroud  12  is firmly located in position. 
       FIG. 4   a  is a cross-section of a shroud  40  in accordance with an embodiment of the present invention.  FIG. 4   b  is an enlarged view of detail of the shroud  40 . It can be see that the shroud  40  has many features in common with the shroud  12 . That is, shroud  40  is an annular plate comprising a radially extending shroud wall  41  provided with an axial extending flange  42  at its inner periphery, and a grooved shroud flange  43  at its outer periphery. Moreover, flange  43  comprises an axially extending flange wall  43   a  and a radially extending flange wall  43   b , with a flange groove  44  defined between the shroud wall  41  and the radially extending flange wall  43   b.    
     In accordance with a first aspect of the present invention the flange wall  43   a  extends axially inboard beyond the radially extending flange wall  43   b , to form an axially extending annular flange rim  43   c . The radially inner surface of the rim  43   c  is a continuation of the radial inner surface of flange wall  43   a . The radially outer surface of the rim  43   c  is tapered, reducing in diameter towards the axial end of the rim  43   c.    
     In accordance with a fourth aspect of the present invention the radially inner flange  42  is axially extended relative to the inner flange  26  of the prior art shroud  12 . 
       FIG. 5  illustrates the shroud of  FIGS. 4   a  and  4   b  fitted to a turbocharger turbine, showing part of a turbocharger turbine of the general type illustrated in  FIG. 1 , and thus reference numerals used in  FIG. 1  will be used in  FIG. 5  where appropriate. The shroud  40  according to the present invention is shown fitted within the mouth of the shroud cavity  13  defined by a turbine housing  1 . The radial shroud plate wall  41  defines one side wall of the turbine inlet  9 , the opposing side wall being defined by nozzle ring  11 . Nozzle vanes  14  are supported by the nozzle ring  11  and extend across the inlet  9  through the shroud vane slots  25 , and into the shroud cavity  13 . Operation of this variable geometry turbine is the same operation of the variable geometry turbine of  FIG. 1 . 
     The shroud  20  is secured in position by retaining ring  19  which operates in the same manner as the retaining ring  19  of prior art shroud  12 . The axially extended inner shroud flange  42  abuts against a radially extending annular shoulder  1   b  defined by the housing  1 . It will be noted that the radially extending flange wall  43   b  does not abut against the housing shoulder  1   a , but the axially extending inner flange  42  does abut against the housing shoulder  1   b . The spring action of the retaining ring  19 , and the interaction of the outboard conical surfaces of the retainer ring  19  and the housing groove  18 , bias the shroud inwardly effectively maintaining the shroud in position against the reactive force exerted by housing shoulder  1   b  on the inner shroud flange  42 . 
     The flange rim  43   c  extends into the shroud cavity  13  beyond the housing shoulder  1   a , a radial spacing between the flange rim  43   c  and the cavity wall increasing along the axial length of the rim  43   c  by virtue of its tapered configuration. 
     The inventors have found that certain wear exhibited in the known shroud  12  in the region of the retaining ring  19  can surprisingly be attributed to flexing of the shroud plate wall  24  in an axial direction illustrated by arrows A-A of  FIG. 3 , causing a rocking motion at the periphery of the shroud plate as illustrated by arrows B-B in  FIG. 3 . Moreover, the inventors have demonstrated that provision of the axially extended flange rim  43   c  sufficiently stiffens the flange  41  against such movement to at least significantly reduce wear in the shroud according to the first aspect of the present invention. 
     The inventors have also surprisingly found that the above mentioned flexing of the shroud plate can be the cause of crack formation in the region of the trailing edge of the shroud vane slots  25   b  in the prior art shroud  12 . Moreover, the inventors have found that this can be substantially prevented by axially extending the inner shroud flange  42  in accordance with the fourth aspect of the present invention as illustrated. 
     Whereas the embodiment of the invention illustrated in  FIGS. 4 and 5  incorporates both the first and fourth aspects of the invention, a shroud plate according to the present invention could incorporate only one of these two aspects of the invention. For instance a shroud plate could include the flange rim  43   c  but with a conventionally sized inner flange  42 , or could include the radially extended inner flange  42  with a conventional slotted flange at its outer periphery as illustrated schematically in  FIG. 6 . 
     Referring to  FIG. 6 , three dimensions of a shroud plate according to a second embodiment of the fourth aspect the invention are illustrated, namely the radial extent of the shroud plate A, the axial thickness of the shroud plate wall C, and the axial extent of the inner flange  42  inboard the shroud plate wall B. In the prior art shroud  12 , the ratio A:B is typically about 21 and the ratio B:C is typically about 0.75. The present inventors have found that extending the inner flange  42  to a length such that the ratio A:B is about 5 or less and/or the ratio B:C is about 1.5 or greater, substantially prevents crack formation at the vane slot trailing edge  25   b  in accordance with the present invention. 
     Both the first and fourth aspects of the invention provide advantages over the prior art shroud without requiring the radial shroud wall to be generally thickened which would be undesirable as it would increase the thermal mass of the shroud and could also be more expensive to manufacture as the vane slots have to be cut through the shroud wall. With embodiments which combine both the first and fourth aspects of the invention as for instance illustrated in  FIGS. 4 and 5 , the thermal mass at both the radially inner and outer peripheries of the shroud  40  can be balanced to improve thermal fatigue and durability. 
     A second aspect of the present invention is schematically illustrated in  FIG. 7 . This aspect of the invention may be applied to a conventional shroud plate  12  as illustrated, and the same reference numerals as used in  FIGS. 3 to 5  will be used where appropriate. In  FIG. 7  the shroud  12  is schematically illustrated in the manner of  FIG. 3  and is shown fitted to a turbine housing  1  to define one wall of a turbine inlet  9 , the opposing wall of which is defined by nozzle ring  11  which supports nozzle vanes  14 . Nozzle vanes  14  extend through the shroud  12  into shroud cavity  13 . 
     In accordance with the second aspect of the invention, flexing of the shroud  12  which may otherwise cause wear to the shroud plate is accommodated by enlarging the retaining ring receiving groove  50  defined by the housing  1 . In particular, the groove  50  has a conical outboard sidewall  51  in common with the groove  18  of the known turbocharger, which interacts with the tapered retaining ring  19  to urge the shroud  12  in an inboard direction (relative to the shroud cavity  13 ), but the opposing inboard sidewall  52  of the groove  50  is sufficiently spaced from the retaining ring  19  that the two will not contact as a result of flexing in the shroud  12 . 
     A radially extending annular shoulder  1   b  is defined around the mouth of the cavity  13  at the region of the inner peripheral edge of the shroud  12  and provides an abutment surface for the shroud inner flange  42 . The shroud  12  is thus held firmly in position in the manner of the first embodiment of the invention described above. That is, there is no need for the retaining ring  1   a  to bear against the inboard sidewall of the groove  50  in order to retain the shroud in the correct position. 
     It will be appreciated that the second aspect of the invention could be combined with either, or both, of the first and fourth aspects of the invention by providing the shroud with an extended outer flange rim and/or axially extended inner flange. 
     As a modification to the third embodiment of the invention, the shroud could be maintained in position by abutment of the radially extending flange wall  27   b  with a modified annular shoulder  1   a  of the housing, rather than abutment of the inner shroud flange  42  with the radial shoulder  1   b  of the housing. 
     In accordance with a third aspect of the invention, the shroud retaining ring is replaced by use of a threaded locking ring in conjunction with a modified shroud as illustrated for instance in  FIGS. 8 and 9 . Both  FIGS. 8 and 9  are cross-sections through a turbine housing  1  in accordance with two different embodiments of the third aspect of the invention. 
     Referring first to  FIG. 8 , a modified shroud  60  comprises a radially extending shroud wall  61  and axially extending inner and outer flanges  62  and  63  respectively. In addition, the outer periphery of the shroud  60  is provided with a radial flange wall  64  extending outwardly from the outer flange wall  63 . In the illustrated embodiment of the inner flange  62  is also axially extended in accordance with the fourth aspect of the invention. 
     The shroud  60  is secured in position in the mouth of a shroud cavity  13  by a screw threaded retaining ring  65  which screws into the mouth of the shroud cavity  13  to clamp the outer periphery of the shroud  60  against an annular supporting ring  66 . In more detail, the radially inner surface of the mouth of the shroud cavity  13  provides a seat for the shroud flange  32 , and the radially outer surface of the mouth of the shroud cavity  13  is provided with an internal screw thread  67 . The retaining ring  65  is generally L-shaped in cross-section having an axially extending screw threaded portion  65   a  and a radially extending portion  65   b . The axially extending portion  65   a  screws into engagement with the threaded portion  67  of the housing  1 , and the radially extending portion  65   b  clamps the radially extending flange wall  64  against the support ring  66  which is trapped between the flange wall  64  and an annular abutment shoulder  1   a  of the housing  1 . At the inner periphery of the shroud  60 , the shroud flange  62  abuts against an annular shoulder  1   b  of the housing. 
     The embodiment of  FIG. 9  differs from the embodiment of  FIG. 8  in that it omits the support ring  66 , the shroud  60  being held in position by the inward (inboard) force exerted on radial shroud flange  64  by the retaining ring  65 , and the outward (outboard) force exerted on the inner shroud flange  62  by the housing shoulder  1   b.    
     In some embodiments of the invention the retaining ring  65  may hold the outer periphery of the shroud  60  in position without exerting a clamping force sufficient to prevent rotation of the shroud  60 . That is, the shroud  60  may be allowed to rotate except to the extent that such rotation would be prevented by inlet vanes which extend through the shroud plate. 
     It will be appreciated that whereas the embodiments of the third aspect of the invention illustrated in  FIGS. 8 and 9  also include an inner shroud flange in accordance with the fourth aspect of the invention, this need not necessarily be the case. 
     Whereas the present invention has been illustrated in relation to the turbine of a turbocharger, it will be appreciated that the invention may be applied to other turbines and turbomachines, such as for instance a variable geometry power turbine. 
     Other modifications which may be made to the illustrated embodiments of the invention will be readily apparent to the appropriately skilled person.