Patent Publication Number: US-2010124496-A1

Title: Turbomachine

Description:
The present invention relates to turbomachinery, such as for instance a power turbine or turbocharger for an internal combustion engine. In particular, the present invention relates to the limitation of damage which may occur to the turbocharger as a result of low cycle fatigue failure of the turbine wheel. 
     Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine chamber defined by a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the intake manifold of the engine, thereby increasing engine power. 
     The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor housing. It is well known that providing an effective sealing system to prevent oil leakage from the central bearing housing into the turbine housing is problematic. It is however important to prevent oil leaking into the turbine housing where it will mix with the exhaust gas and increase exhaust emissions and may cause damage to downstream components such as a catalytic converter. 
     The turbine wheel comprises a central body or hub, having a back face which faces towards the bearing housing. Turbine blades extend generally radially from the hub and generally axially relative to the back face. The turbine wheel may be friction welded to a seal boss at the end of the turbocharger shaft, the seal boss having a larger diameter than the shaft for rotation within an passage through a housing wall separating the bearing housing from the turbine housing. Known oil seal arrangements comprise a seal ring located around the seal boss within the passage providing a seal in the manner of a piston ring. 
     It is also common that a heat shield, frequently made of sheet metal, is provided intermediate the turbine wheel and the housing wall so as to reduce the amount of heat transfer (due to hot exhaust gases) to the bearing housing 
     It is known that thermal and/or mechanical loading conditions on parts of the turbocharger, particularly the turbine, may cause premature failure of materials concerned. This is known as fatigue. One type of fatigue is low cycle fatigue, which is generally caused by repeated loading cycles as the turbocharger changes between low and high operating speeds. In some cases low cycle fatigue may cause the turbine wheel to fail and fracture, which may result in catastrophic damage to the turbocharger. In particular, damage to the bearing housing may result in oil leakage from the bearing cavity into the turbine presenting a fire risk. 
     It is an object of the present invention to obviate or mitigate the problems of oil leakage from the turbocharger bearing housing into the turbocharger turbine housing due to a turbine wheel failure. 
     According to the present invention there is provided a turbomachine comprising:
         a housing defining a bearing cavity;   a turbine wheel mounted to a shaft for rotation about an axis;   a housing wall disposed between the bearing cavity and the turbine wheel; the shaft extending into the bearing cavity through a shaft passage provided in said housing wall; the housing wall including:   a first portion defining an air gap between the housing wall and the turbine wheel;   a second portion which is annular and protrudes from the first portion into said air gap to define the opening to said passage; and   at least one third portion which is radially distanced from second portion and which protrudes from said first portion into said air gap;       

     wherein the third portion is configured to support the turbine wheel in a failure condition in which the wheel rotates off-axis to thereby at least reduce the radial load which would otherwise be applied to the annular second portion by the shaft. 
     Accordingly, the third portion of the housing wall is configured, e.g. sized and located, to bear load (directly or indirectly) from the turbine wheel in a failure condition which in turn reduces off-axis load on the second annular portion of the housing wall via the shaft. In some embodiments the turbine wheel may directly contact the third portion of the housing wall. In other embodiments a heat shield may be located between the third portion and the turbine wheel which is sandwiched between the third portion and the wheel when the wheel fails. 
     Turbine wheels may suffer low cycle fatigue in an asymmetric manner. This is particularly the case when the failure originates from the back face of the wheel (usually that which is adjacent the bearing housing) or when a large portion of the wheel, for example a blade, becomes detached. It is believed that when such a failure occurs, the remainder of the turbine wheel (which is still attached to the shaft) reacts by moving radially with respect to the axis of rotation of the turbine wheel, and thereafter will become highly unbalanced. The high torque on the shaft may even cause it to snap. The turbine wheel and attached shaft may act as a lever, particularly if the shaft has snapped, which applies a force to the housing wall and/or any oil seal arrangement within the shaft passage through the housing wall, causing possible fracture of the housing wall around the passage and/or failure of the seal arrangement. Either fracture of the bearing housing or failure of the seal arrangement would result in high-pressure oil within the bearing cavity leaking into the turbine and mixing with the exhaust gas flow. As previously mentioned, this may severely contaminate downstream emissions equipment, and may cause a fire. 
     The present invention reduces the likelihood of such catastrophic failure of the turbine by provision of the third portion (or portions) of the housing to provide some support to a failed turbine wheel that may be rotating in an asymmetric manner. Such support will reduce any off-axis, or lever, forces exerted on the second portion of the housing wall around the shaft passage (and on any seal arrangement present in the passage) thereby reducing the risk of fracture to the housing wall (or damage of the seal arrangement), thus reducing the likelihood of oil leakage into the turbine. 
     In some embodiments the present invention further reduces the likelihood of catastrophic failure of the turbine by configuration of the third portion of the housing such that it reinforces the second portion of the housing defining the aperture shaft. That is, any axially forces exerted by the shaft as a result of off-axis rotation of the turbine wheel will be transmitted to the or each reinforcing portion. The or each third portion may for instance be positioned to transmit any such force to a more robust portion of the housing, such as a housing wall having at least a substantial component extending in the axially direction. 
     In some embodiments, the or each third portion may be configured to reinforce the second portion without necessarily being configured to directly support the turbine wheel in a failure condition. One aspect of the present invention provides a turbo machine comprising:
         a housing defining a bearing cavity;   a turbine wheel mounted to a shaft for rotation about an axis;   a housing wall disposed between the bearing cavity and the turbine wheel; the shaft extending into the bearing cavity through a shaft passage provided in said housing wall; the housing wall including:   a first portion defining an air gap between the housing wall and the turbine wheel;   a second portion which is annular and protrudes from the first portion into said air gap to define the opening to said passage; and   at least one third portion which protrudes from said first portion into said air gap;       

     wherein the third portion is configured (e.g. sized and located) to reinforce the second portion of the housing defining the aperture shaft. 
     For instance, the or each third portion of the housing may comprise a rib or web portion connecting the second portion of the housing to the first portion of the housing or to another portion of the housing. 
     The second portion may be defined by a generally domed portion of the housing wall. 
     The or each third portion may be positioned at a radius less than or equal to the radius of the turbine wheel. 
     The second portion may be spaced from the turbine wheel by a minimum distance D and the or each third portion is spaced from the turbine wheel by minimum distance d, wherein d is less than or equal to D. 
     In some embodiments the turbomachine may comprise a bearing housing which defines at least a part of the bearing cavity and a turbine which defines a turbine chamber within which the turbine wheel rotates. The housing wall may be a wall of the bearing housing which separates the bearing cavity from the turbine chamber in which the turbine wheel rotates, the air gap between the two reducing heat transfer to the bearing housing. In such embodiments a heat shield may be located in the air gap between the housing wall and the turbine wheel. Any such heat shield is preferably spaced from the second portion of the housing wall to limit heat transfer to the housing. In other embodiments the housing wall may itself comprise a heat shield, for instance integrally cast with the bearing housing. 
     The or each third portion of the housing wall may be annular. 
     The or each third portion of the housing wall may be defined by a rib or other protrusion which has a height extending from the housing wall towards the turbine wheel. For instance, the or each third portion may be a rib with a length extending in a linear or curved direction away from the first portion of the housing wall. Such a rib may extend to the second portion of the housing or may terminate at a location spaced from the second portion of the housing. 
     The height of height of the or each third portion may be non-uniform to define a region or regions of the respective third portion that will deform preferentially under impact. 
     The or each third portion may extend in a generally radial direction away from the second portion of the housing wall. In some embodiments at least one third portion may extend in a generally tangential direction to the first portion. 
     The housing wall may comprise a plurality of said third portions which may, for instance, be arranged circumferentially around said second portion. 
     Whereas in some embodiments the or each third portion may be defined by a discreet protrusion, such as for example a rib or the like, provided on the surface of the housing wall, in other embodiments the or each third portion may be defined by a contour of the housing wall. 
     A typical turbine wheel may comprise a central body or hub supporting turbine wheel blades, the hub having a back face which faces towards the housing wall. Other forms of turbine wheel may comprise a back plate supporting the wheel blades and which defines the wheel back face. The minimum distance d may be defined between the or each third portion of the housing wall and the turbine wheel back face. 
     The or each third portion of the housing wall may define a contact surface (which is contacted in the event of turbine wheel failure) which lies in a plane substantially normal to the axis or which lies on a conical surface defined about the axis—for instance corresponding to the likely orientation of an asymmetrically rotating turbine wheel following wheel failure as described above. 
     In some embodiments the housing wall defines a portion of the bearing cavity and the shaft may be sealed with respect to said passage to prevent or obstruct leakage of oil through said passage to the turbine wheel. 
     In other embodiments the housing wall comprises a heat shield with a second housing wall which defines a portion of the bearing cavity being disposed between the bearing cavity and the heat shield. The shaft may be sealed with respect to a second passage through said second housing wall to prevent or obstruct leakage of oil to the turbine wheel. 
     The third portion of the housing wall may be radially spaced from the dome or other feature defining the second portion, albeit that the second portion may be defined by a feature of the housing wall which contacts the dome. 
     The housing wall may be defined by, or connected to, a housing comprising a further wall portion which extends in a direction having a significant component parallel to said axis. The further wall portion may meet said housing wall at a radial location corresponding to the radial location of the third portion of the housing wall such that any substantially axial force applied to the second portion of the housing wall is transmitted to the further housing wall portion. The further housing wall portion will thus support the or each third portion of the housing wall in the event of impact by the turbine wheel. 
     The or each third portion of the housing wall may be an integral feature of the housing wall (eg integrally cast with the housing wall or machined into the housing wall) or may be fabricated as a separate component which is subsequently attached to the housing wall. 
     The turbomachine may be a turbocharger or may for instance be a power turbine. 
     Other preferred and particularly advantageous features of the invention will be apparent from the following description. 
    
    
     
       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 a cross-section through a known turbocharger; 
         FIG. 2  is an expanded view of the turbine end bearing and oil seal assemblies of the turbocharger of  FIG. 1 ; 
         FIG. 3  shows details of part of another known turbocharger bearing housing structure; 
         FIG. 4  shows a fractured dome portion of a known turbocharger; 
         FIGS. 5   a  and  5   b  show details of a first embodiment of present invention; 
         FIG. 6  is a cross-section through a turbocharger in accordance with an embodiment of the present invention; 
         FIG. 7  is a simplified enlarged cross-sectional view of a turbine wheel and bearing housing in accordance with the present invention; 
         FIG. 8  is an end on view of a bearing housing in accordance with a second embodiment of the invention; and 
         FIG. 9  is a cross-section through a turbine wheel and bearing housing in accordance with the same embodiment of the invention as shown in  FIG. 6 . 
     
    
    
     Referring to  FIGS. 1 and 2 , the illustrated turbocharger comprises a turbine  1  joined to a compressor  2  via a central bearing housing  3 . The turbine  1  comprises a turbine wheel  4  rotating within a turbine housing  5 . Similarly, the compressor  2  comprises a compressor wheel  6  which rotates within a compressor housing  7 . The turbine wheel  4  and compressor wheel  6  are mounted on opposite ends of a common turbocharger shaft  8  which extends through the central bearing housing  3 . 
     The turbine housing  5  has an exhaust gas inlet volute  9  located annually around the turbine wheel  4  and an axial exhaust gas outlet  10 . The compressor housing  7  has an axial air intake passage  11  and a compressed air outlet volute  12  arranged annually around the compressor wheel  6 . 
     Intermediate the turbine wheel  4  and the bearing housing  3  there is a heat shield  13   a , made of sheet metal, which is installed in the turbine housing  5  behind the turbine wheel  4 . The purpose of the heat shield  13   a  is to prevent overheating of the bearing housing  3 , due to the hot exhaust gases in the turbine housing  5 , which can for instance result in oil coking in the bearing housing  3 . The heat shield  13   a  is spaced from the turbine wheel  4 , not only such that the heat shield  13   a  does not impede the movement of the wheel  4 , but also so that no heat can be conducted directly from the wheel  4  to the heat shield  13   a . A portion of the heat shield  13   a  is also spaced from the bearing housing  3  to minimise heat conduction to the bearing housing. 
     The turbine wheel  4  typically comprises a generally cylindrical main body or hub  4   a , having a back face  4   c  which faces towards the bearing housing/heat shield. A plurality of turbine blades extend generally radially from the hub and generally axially relative to the back face. 
     In use, the turbine wheel  4  is rotated by the passage of exhaust gas from the annular exhaust gas inlet  9  to the exhaust gas outlet  10 , which in turn rotates the compressor wheel  6 , which thereby draws intake air through the compressor inlet  11  and delivers boost air to the intake of an internal combustion engine via the compressor outlet volute  12 . 
     The turbocharger shaft  8  rotates on fully floating journal bearings  13  and  14  housed towards the turbine end and compressor end respectively of the bearing housing  3 . The compressor end bearing assembly  14  further includes a thrust bearing  15  which interacts with an oil seal assembly including an oil slinger  16 . Details of the compressor end bearing and oil seal are not important to an understanding of the present invention and will not be described further. Oil is supplied to the bearing housing from the oil system of the internal combustion engine via oil inlet  17  and is fed to the bearing assemblies by oil passageways  18 . 
     The turbine wheel  4  is joined to the end of the turbocharger shaft  8  at a seal boss  19 . Generally, the seal boss  19  is formed integrally with the shaft  8  and is joined (for instance by friction welding) to a boss portion on the turbine wheel  4 . The seal boss  19  extends through a passage  20  in a bearing housing wall  3   a  and into the turbine housing. The bearing housing wall  3   a  is such that it is generally convex in nature, the radially innermost portion (with respect to the axis of shaft  8 ), which defines the opening of the aperture  20 , being closer to the back face than any other portion of the bearing housing wall. The radially innermost portion is sometimes referred to as the “dome”  3   b . The seal boss  19  is sealed with respect to the passage  20  by a seal ring  21  (piston ring), which sits in an annular groove defined by the seal boss. 
     In more detail (referring in particular to  FIG. 2 ) the passage  20  through the bearing housing wall  3   a  is radially stepped having a relatively narrow diameter inboard portion  20   a  and a relatively large diameter outboard portion  20   b . This provides an annular abutment shoulder  22  for the ring seal  21  which sits within an annular groove  23  provided in the outer surface of the seal boss  19 . The seal ring  21  is stationary with respect to the bearing housing  3  and is provided to prevent the leakage of air/oil through the passage  20 . The abutment shoulder  22  prevents the seal ring  21  creeping inboard towards the bearing housing  3 . In order to provide an abrupt, non-radiused change of diameter of the passage  20 , a slight annular recess  24  is cut back in to the surface of the passage  20  to define the shoulder  22 . 
     The turbine end journal bearing  13  is located between circlips  25  and  26 . Oil is fed to the bearing  13  via oil passageway  18  and the bearing  13  is provided with circumferentially spaced radial holes  27  for oil to pass to the turbocharger shaft  8 . An annular oil return groove  28  is radially recessed into the bearing housing wall adjacent the passage  20  through the housing wall  3   a . The oil return groove  28  surrounds the shaft  8  and has an entrance  29 . 
     The seal boss  19  extends slightly into the bearing housing  18  beyond the inner surface of the bearing housing wall  3   a  and axially overlaps the entrance  29  to the oil groove  28 . The inboard end of the seal boss  19  forms a radial shoulder around the shaft  8  having an annular face  30 . As the turbocharger shaft  8  rotates, oil reaching the annular face  30  is radially dispelled and propelled from the face  30  of the boss  19  is into the oil groove  28  from which it drains back to the engine crank case via an oil drain hole  31  (shown in  FIG. 1 ). The provision of the oil groove  28  thus prevents oil from accumulating in the region of the passage  20 , and similarly ensuring that the boss  19  protrudes into the bearing housing  3  ensures that oil is projected into the oil groove  28  and not towards the annular gap defined where the boss  19  passes through the passage  20 . 
       FIG. 3  shows part of another known turbocharger. The same reference numerals are used to correspond to those provided above. With the turbocharger of  FIG. 3  it can be seen that the bearing housing dome  3   b  is very much more pronounced that with the housing design of  FIGS. 1 and 2 . 
     It has been found that under certain fatigue conditions, such as low-cycle fatigue, the turbine wheel  4  may fail whilst it is in use. Such a failure may involve the turbine wheel  4  cracking, or in extreme cases, separating into a plurality of pieces. This type of failure tends to be asymmetric in nature and is particularly the case when the failure occurs from the back face of the turbine wheel  4  or when a large portion of the wheel, for example a blade  4   b , becomes detached. 
     Either fracture of the bearing housing or failure of the seal arrangement would result in high-pressure oil within the bearing housing passing into the turbine housing, and hence the engine exhaust system. As previously mentioned, this may severely contaminate downstream emissions equipment, or may cause a fire. 
     The result of such failure is to cause the rotating turbine  4  and shaft  8  to become unstable. The turbine  4  and shaft  8  experience a force which causes them to become highly unbalanced and move radially outwards relative to their normal axis of rotation. The torque produced by this movement, in combination with any forces which may result from the turbine wheel  4  and shaft  8  abutting the turbine housing  5  and/or bearing housing  3 , may cause the shaft  8  to break or deform and thereafter will become highly unbalanced. Deformation or breaking of the shaft usually occurs in the region of the journal bearings  13 ,  14  (particularly the turbine end bearing  13 ) or within the dome  3   b . Once the shaft  8  has deformed or broken, the wheel  4  and any portion of the shaft  8  attached thereto, continues to move radially outward. This exerts radial force on the bearing housing and or seal  21  in the region of the passage  20 . The shaft  8  may effectively form a lever, which as the turbine  4  moves radially outwards, causes a large force to be applied to the dome  3   b , which can break it open and/or cause failure of the seal arrangement  21 . 
     A dome  3   b  which has been broken in the above manner is shown in  FIG. 4 . If the dome  3   b  or seal  21  are damaged, pressurised oil, which is fed to the bearing  13  via inlet  17  and passageway  18 , will flow into the turbine and mix with the hot exhaust gas flow. Exposure of the engine exhaust system, which is typically at a high temperature, to oil is potentially hazardous as the oil may ignite. In addition, the ingress of oil into the engine exhaust system may lead to external oil leaks and/or contamination of downstream emissions equipment, for instance a catalytic converter. 
       FIGS. 5   a  and  5   b  illustrate part of a turbocharger bearing housing in accordance with a first embodiment of the present invention. The illustrated housing structure in accordance with the present invention is a modification of that shown in  FIG. 3  and like reference numerals are used where appropriate. In accordance with the present invention the bearing housing is modified by the provision of radially extending ribs or buttresses  3   c  which are raised above the housing wall. In the illustrated example the height of the ribs  3   c  above the housing wall is less than that of the dome  3   b  but in other embodiments the height of the ribs  3   c  may be equal to or greater than the height of dome  3   b . Should a turbine failure of the type discussed above occur, any radial and/or levering movement of the turbine  4  and attached portion of shaft  8  will be limited by the abutment of the turbine  4  with the ribs  3   c . The ribs  3   c  protrude a sufficient distance towards the turbine  4  such that any nutating motion of a failed turbine  4  results in the turbine  4  abutting the ribs  3   c , inhibiting significant contact between the dome  3   b  and the back face of the turbine  4 . Limiting the radial movement of the turbine wheel  4  reduces, and may even prevent, off axis forces on the dome  35  and/or seal  21 . This may prevent the dome  3   b  from breaking and/or the seal from being destroyed and may, for instance, prevent the shaft  8  from breaking. This will thereby prevent or minimise the leakage of oil into the exhaust system in the event of turbine wheel failure 
     Furthermore, in this embodiment the ribs  3   c  in the illustrated embodiment are arranged to reinforce the dome  3   b  and to transmit the force towards a radially outer portion of the housing to be borne by a housing wall which extends generally axially and is thereby better orientated to bear the load. Tests have shown that this embodiment of the invention greatly reduces any tendancy of the dome to fracture under turbine wheel failure conditions. 
       FIG. 6  shows a turbocharger in accordance with a second embodiment of the present invention, wherein the bearing housing  3  has a raised annular portion  32  provided radially outward of the dome  3   b . Again, where appropriate, the same reference numbers are used to identify corresponding features to those in previous figures. Due to the convex dome shape of the bearing housing  3  the spacing between the back face of the turbine wheel  4  and the bearing housing  3  increases as radial distance from the axis of the shaft  8  increases. The annular portion  32  protrudes from the bearing housing  3  towards, depending on the radial location of the annular portion, either the back face of the turbine wheel  4  or the blades  4   b  of the turbine wheel  4 . In the embodiment shown, the portion  32  is annular and is concentric with the turbine wheel  4 . The heat shield  13   a  is intermediate the containment portion  32  and the turbine wheel  4 . It is desirable that, as shown, the containment portion  32  is such that it does not contact the heat shield  13   a , so that there is no direct conduction of heat therebetween. 
     Should a turbine failure of the type discussed above occur, any radial and/or levering movement of the turbine  4  and attached portion of shaft  8  will be limited by the abutment of the turbine  4  with the annular portion  32  (albeit via the heat shield  13   a ). As with ribs  3   c  of the earlier embodiment, the annular portion  32  protrudes a sufficient distance towards the turbine  4  such that any nutating motion of a failed turbine  4  results in the turbine  4  abutting the annular portion  32 , inhibiting significant contact between the dome  3   b  and the back face of the turbine  4 . As described above, limiting the radial movement of the turbine wheel  4  in this way reduces, and may even prevent, off axis forces on the dome  35  and/or seal  21  and thereby prevent the dome  3   b  from breaking and/or the seal from being destroyed (and may, also prevent the shaft  8  from breaking) 
     As previously discussed, the turbine  4  comprises a main body  4   a  with a back face  4   c  and blades  4   b . As seen best in  FIG. 6 , the annular portion  32  is sized and positioned such that should the turbine wheel  4  abut the annular portion  32  if the turbine  4  fails, then it is the back face of the hub  4   a  of the turbine wheel  4  which abuts the containment portion  32 . This is advantageous in that the main body  4   a  is capable of withstanding a greater force than the blades  4   b . As such, a greater reaction force can be imparted to the turbine wheel  4  via the annular portion  32 , so as to oppose the force which urges the failed turbine  4  radially outwards. The annular portion  32  may for instance lie at the same radius as the outer periphery of the turbine wheel back face  4   c . Alternatively, in other embodiments the annular portion  32  may be radially inward or outward of the periphery of the turbine wheel back face. 
     In some embodiments, such as those illustrated in  FIGS. 6 and 7 , the annular portion  32  is also sized and shaped such that should the turbine  4  fail and impart a force to the portion  32 , the transmission of such a force will be through a side wall portion  3   c  of the bearing housing  3  which extends generally axially away from the turbine. Any transmitted force will thus have a significant component parallel to the direction of the extension of the side wall  3   c  away from the turbine, as shown by arrow A. The transmission of any reaction force in this manner is advantageous as it minimises the risk of fracturing the housing  3  whist it is under the load of a failed turbine  4 . 
     In a further embodiment of the invention shown in  FIGS. 8 and 9  the raised portion comprises a plurality of parallel ribs  33  rather than the annular rib  32 . The ribs  33  are adjacent the dome  3   b  and are connected thereto with ribs  33   a  extending radially from the dome and ribs  33   b  extending tangentially to the dome  3   b . The dome  3   b  comprises a central nose portion  34 , the surface of which is substantially parallel to the back face of the turbine wheel  4 . The axial height of the ribs  33  is the same as that of the nose portion  34 . The individual ribs  33  are sized and positioned (in relation to the turbine  4  and each other) such that a nutation of a failed turbine  4  in any direction will result in the turbine  4  abutting the containment portion  32 . As with the previous embodiment of the invention, the abutment of the turbine  4  with the ribs  33  inhibits significant off-axis forces on the dome  3   b  and seal  21  in the event of turbine wheel failure. This structure allows the bearing housing to be cast economically using a conventional two-part cast process 
     In the above-described embodiments, the housing wall  3   a , which separates the turbine wheel  4  chamber from the bearing cavity, is provided by the bearing housing  3 . In other embodiments the housing wall may be provided by the turbine housing  5 . In yet further embodiments the heat shield  13   a , may be sufficiently robust as to bear the load of a failed turbine wheel. For example in some embodiments the heat shield may be an integrally cast wall of the bearing housing, and as such may be the housing wall which is provided with the portion  32  or ribs  33  etc. 
     Although embodiments of the invention described above have a support portion provided as a protrusion from the housing wall, is also envisaged that in other embodiments of the invention such a support portion could be provided by a less prominent feature of the shape or configuration of the bearing housing wall  3   a . For instance, the wall  3   a  may have a generally concave curving towards the turbine wheel at a radius beyond the passage  20  or dome  3   b  if present. Discreet protrusions are however preferred as they typically involve only a small increase in weight of the turbocharger housing 
     As mentioned above, the configuration and location of the reinforcing or support portion of the housing wall may vary from those shown in the illustrated embodiments of the invention. For instance the annular protrusion  32  may be replaced by an annular array of circumferentially spaced discreet protrusions. For instance such discreet protrusions may be arced ribs lying on a circle circumscribing the axis, or may be ribs curving generally towards or away from the axis in the direction of rotation of the turbine wheel. The radial width of the annular protrusion, or any such ribs, may vary as may the orientation of the face of any such protrusion. For instance an annular protrusion may define a surface facing towards the turbine wheel which lies substantially perpendicular to the axis or which lies on the surface of a cone corresponding to the orientation the turbine wheel might assume if it fails and contacts the protrusion. 
     An annular protrusion/rib or annular array of protrusions/ribs may lie at a radius substantially corresponding to the circumference of the turbine wheel back face, or may lie between the circumference of the back face and the axis or between the circumference of the back face and the outer extremity of the turbine wheel blades. In some embodiments a protrusion or protrusions positioned at a radius corresponding to that of the main body  4   a  of the turbine wheel  4  my be most effective in enabling the maximum reaction force to be exerted on a failed turbine wheel  4 , without significantly increasing the risk of further blade fracture or deformation. In some embodiments the possible location of the protrusion or other reinforcing or support portion, might be to limited by other features of the configuration of the bearing housing, turbine housing and/or heat shield. 
     In embodiments in which the support portion comprises ribs, such ribs may extend radially away from the axis (similar to ribs  33   a  a shown in  FIG. 6 ) or tangentially to a circle centred on the axis (similar to ribs  33   b  shown in  FIG. 6 ), or may extend in a direction between these two directions. Such ribs may define a surface facing the turbine wheel which lies in plane normal to the axis or lies on the surface of a cone corresponding to the possible orientation assumed by a failed turbine wheel contacting the protrusions. In some embodiments such ribs may be combined with an annular or similar protrusion or protrusions as described above. Where the bearing housing wall has a dome, as illustrated, ribs may contact the dome or be radially spaced therefrom. 
     Where the support portion comprises a protrusion or plurality of protrusions, such protrusions may be integrally formed (e.g. cast or machined) with the housing wall. Alternatively the protrusions may comprise separately fabricated structures which are subsequently attached to the housing wall (e.g. by welding or securing with bolts, rivets or other fasteners). This would for instance enable the protrusions to be formed of a different material to that of the housing wall and may allow more flexibility in the configuration of the protrusions. 
     It is also envisaged that the cross-sectional profile (perpendicular to its longitudinal axis) of any protrusion which forms part of the reinforcing or support portion may take different forms. The protrusions of the above described embodiments have substantially parallel side walls extending axially (and which are radiused where they meet the bearing housing wall) and an end surface facing towards the turbine wheel which is arcuate. The end surface could alternatively be flat to increase the surface area which will contact and support a failed turbine wheel. This not only spreads any reaction force exerted by the reinforcing or support portion on the turbine  4  over a greater area, but also further limits the radial movement of a failed turbine  4 . In addition, if it is desired that the abutment of the reinforcing or support portion with a failed turbine wheel  4  should cause the rotation of the turbine  4  to stop, the stopping distance of the failed turbine  4  will be reduced. The flat profile of the tip portion of the protrusion may, for instance, be perpendicular to the axis of rotation of the shaft  8 , or may be at a fixed angle relative to the axis of rotation of the shaft  8  which corresponds to that of the back face of a notional failed turbine  4  as mentioned above 
     It is additionally envisaged that any protrusion which forms part of the support portion may be such that its radial distance from the shaft  8  axis may decrease as it extends towards the turbine  4 . This may lead to improved transfer of impact (or reaction) force to a load-bearing portion of the housing  3 , for example the housing sidewall  3   c.    
     In some of the above-described embodiments, the minimum axial separation between the support portion and the turbine wheel  4  is less than that the minimum axial separation between the nose of the dome  3   b  (i.e. the portion of the dome defining the opening of the passage  20 ) and the turbine wheel  4 . The difference in separation may vary between different embodiments of the invention. In some embodiments the difference may be relatively small, and in others it may be larger. The appropriate difference may to some extent be dictated by the configuration of the bearing housing and/or heat shield and/or distance between the turbine wheel and housing wall and/or diameter of the turbine wheel relative to the radial position of the protrusion or other housing feature defining the reinforcing portion. 
     In some embodiments the protrusion or other housing feature which forms the support portion may include a ‘crumple zone’, i.e. a portion which will deform preferentially when a failed turbine  4  impacts on the support portion. Such a crumple zone may, for instance, take the form of a portion of the protrusion or other feature which has a fractionally greater axial height (i.e. extends closer to the turbine wheel) than adjacent portions of the protrusion or other support feature. Such a crumple zone may for instance flatten on impact to absorb some of the force of the impact 
     Whereas the present invention has been described in relation to a turbocharger turbine it will be appreciated that it may be applied to turbines of other turbomachinery such as for instance power turbines. As will be known to the skilled person, a power turbine is a turbine which drives a gear or other coupling for deriving work from the turbine rather than a compressor wheel. 
     Other possible modifications and applications of the invention will be readily apparent to the appropriately skilled person.