GB2504482A

GB2504482A - Variable geometry turbine for a turbocharger

Info

Publication number: GB2504482A
Application number: GB1213456.5A
Authority: GB
Inventors: John Michael Bywater
Original assignee: Cummins Ltd
Current assignee: Cummins Ltd
Priority date: 2012-07-27
Filing date: 2012-07-27
Publication date: 2014-02-05
Also published as: GB201213456D0

Abstract

A variable geometry turbine of the kind used in a turbocharger has a variable geometry element, such as a nozzle ring mounted on guide rods 126, which is operated by an actuator. The actuator comprises a yoke defining a pair of arms 132 which extend into engagement with the guide rods. The actuator further comprises a motor to drive a toothed gear wheel 137 which is operatively connected to a sector gear 136 fixedly mounted on the yoke. The sector gear is defined by a rim that extends radially from the yoke pivot axis to one side of a plane which extends substantially parallel to the turbine axis and which contains the yoke pivot axis, and the arms of the yoke extend to the other side of said plane. Operation of the motor drives rotation of the yoke via the gears, which in turn drives the guide rods in translation.

Description

A VARIABLE GEOMETRY TURBINE

The present invention relates a variable geometry turbine and to a turbomachine, such as a turbocharger, incorporating such a turbine.

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 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 engine intake manifold. 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 wheel housing.

In known turbochargers, the turbine stage comprises a turbine chamber defined by the turbine housing and within which the turbine wheel is mounted, an annular inlet passageway 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 chamber and rotates 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 suit varying engine demands.

In one common type of variable geometry turbine, one wall of the inlet passageway is defined by a movable wall member (generally referred to as a nozzle ring"). The position of the nozzle ring relative to a facing wall (sometimes referred to as the shroud") of the inlet passageway is adjustable to control the width of the inlet passageway. For instance, as gas flowing through the turbine decreases the inlet passageway width may also be decreased to maintain gas velocity and optimise turbine output. Typically the nozzle ring is provided with vanes, which extend into the inlet passageway and through slots

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provided on the facing wall of the inlet passageway to accommodate movement of the movable nozzle ring. Alternatively, vanes may extend from a fixed wall through slots provided in the nozzle ring. The nozzle ring is generally supported on rods extending parallel to the axis of rotation of the turbine wheel and is moved by an actuator, which is operable to displace the rods in an axial direction.

Various types of actuators may be used to move the nozzle ring including, for example, a pneumatic actuator or a motor and gear transmission which are generally mounted on the outside of the housing. The actuator is coupled to the nozzle ring by a yoke pivotally supported on a shaft that is journalled in the housing, the yoke defining two spaced apart elongate arms which extend on opposite sides of the turbine axis to engage portions of the support rods extending outside the housing. The end of each arm has a pin that extends into a sliding block that is in turn received in a slot defined in a respective support rod. The actuator is driven by a motor via a crank coupled to one end of the shaft.

Operation of the motor causes the yoke to pivot about the shaft such that the pins on the arms describe an arc of a circle and that in turn causes the blocks to move axially and slide vertically within the slots defined in the support rods. Axial movement of the nozzle ring can thus be achieved by rotation of the yoke about the shaft.

In the variable geometry turbines of the kind described above, the yoke shaft is often located in the hostile environment outside the housing. Moreover, when an electric motor is used to actuate the yoke, typically via a gear transmission, the motor, gears and a controller are usually mounted on the outside of the bearing housing, commonly in a water-cooled module. The various components of the actuator therefore occupy a significant volume around the space already occupied by the housing. It would be desirable to reduce the space-claim of the housing and associated nozzle ring actuator.

In operation, the aerodynamic flow of exhaust gas through the turbine inlet exerts a significant load on the nozzle ring and this is transmitted via the rods to the ends of yoke arms. If this back-dnve" is not resisted the nozzle ring is forced to the position where the annular inlet passageway is fully open. Since the arms of the yoke are relatively long this in turn imparts a significant torque on the yoke shaft that has to be reacted by the application of continuous torque from the motor in order for the nozzle ring to be maintained in position against the force applied by the gas. The magnitude of the yoke shaft torque for a given force applied to the nozzle ring is proportional to the distance of the ends of the yoke arms from the yoke shaft rotational axis. Although the ends of the yoke arms are typically equidistant from the turbine axis, as mentioned above, the motor and crank which drives rotation of the yoke shaft is usually operatively connected to one end of the yoke shaft resulting in a imbalance in the torque applied by the motor to the yoke shaft to counter the back-drive experienced by the arms of the yoke. It would be desirable to improve the balance of the torque applied to the yoke shaft.

Actuators have been developed for use in variable geometry turbines which seek to address one or more of the problems outlined above. For example, PCT/0B2008/001791 describes an actuator that is connected to a yoke shaft at a position which is equidistant between the two arms of the yoke. The actuator comprises a lead screw which engages a lead screw nut. Either the screw or the nut is held captive relative to the housing such that rotation of the free component drives translational movement of the captive component in the direction of the screw axis, which in turn pivots the yoke and drives translational movement of the rods. The lead screw / lead screw nut arrangement necessitates significant modification to the basic structure of the yoke to incorporate a second pair of radially extending arms between which the lead screw nut is supported.

Additionally, the lead screw nut defines a pair of slots for receipt of sliding blocks which are pivotally connected to the second pair of arms connected to the yoke. While this is a convenient means for translating linear movement of the lead screw or nut into rotational movement of the yoke it introduces additional components and wear surfaces in between the motor and the yoke as compared to prior art arrangements which drive the yoke via a gear coupled to the end of the yoke shaft. A further example of a modified actuator system is described in PCT1GB20091000645. This system replaces the conventional yoke arrangement with a motor driven shaft that defines a pair of worm gears, one at each end of the shaft, to engage with complementary racks defined by ends of the rods supporting the nozzle ring. While this system may reniove the wear surfaces defined by the sliding blocks employed in many conventional actuator systems and the system described in PCT/GB2008/001791, and it may increase the torque capacity of the actuator motor without adding too much to the overall space claim of the turbine and the actuator, it entirely replaces the conventional design of yoke with a more complicated design requiring a greater degree of precision both during manufacture and assembly. It also necessitates a significant modification to the conventional design of the rods that support the nozzle ring, further increasing the cost and complexity of the manufacture and assembly of a turbine incorporating this actuator mechanism. It is desirable to provide an actuator that can achieve at least some of the benefits over conventional actuators provided by the actuators described in PCT/GB2008/001791 and PCT/GB2009/000645, whilst minimising the space-claim of the actuator and the extent to which other components of the turbine to need to be modified to accommodate the improved actuator.

It would also be desirable to achieve these benefits with an actuator that is more simple and robust in construction, and which would be cheaper to manufacture and assemble.

It is one object of the present invention to obviate or mitigate the aforementioned disadvantages with prior art actuators for variable geometry turbines.

According to a first aspect of the present invention there is provided a variable geometry turbine comprising: a turbine wheel mounted within a housing for rotation about a turbine axis; a gas flow inlet passage upstream of said turbine wheel; a gas flow control mechanism located upstream of said turbine wheel and operable to control gas flow through said gas flow inlet passage; and an actuator assembly for operating the control mechanism; the control mechanism comprising a movable member for varying the size of the gas flow inlet passage, the movable member being mounted on at least one guide member that is translatable in a direction substantially parallel to the turbine axis; the actuator assembly comprising a yoke defining at least one arm which extends into engagement with a respective guide member, a rotary drive member configured to drive a toothed gear wheel in rotatation about a drive axis, the toothed gear wheel arranged to drive rotation of a sector gear fixed to the yoke, rotation of the sector gear causing the yoke to pivot about a yoke pivot axis to drive the or each respective guide member in translation; wherein the sector gear is defined by a rim that extends radially from the yoke pivot axis to one side of a plane which extends substantially parallel to the turbine axis and which contains the yoke pivot axis, and said at least one arm of the yoke extends to the other side of said plane.

The present invention provides a variable geometry turbine with an actuator of relatively compact design which can be employed in an essentially conventional variable geometry turbine without needing to significantly modify the other components of the turbine. Such an arrangement is eminently suitable for use in variable geometry turbines incorporating moving walls and which require an actuator that can be driven from above of below the nozzle ring. The actuator of the present invention may also be more robust than many prior art actuators due to its relatively simple construction and the reduced number of interconnected components compared to more complex actuator systems, such as those described in PCT/GB2008/001791 and PCT!3B2009/000645. Consequently, a variable geometry turbine incorporating an actuator of this kind according to the present invention is likely to be cheaper to manufacture and assemble than many prior art turbines.

The sector gear may be fixedly mounted on the yoke or it may be formed integrally with the yoke. One end of the rim may define the sector gear and another end of the rim may be connected to or formed integrally with the yoke. For example, the radially outer end of the rim may define the sector gear while the radially inner end of the rim may be connected to or integrally formed with the yoke. The radially extending rim and the at least one arm of the yoke preferably extend in generally opposite radial directions from the yoke pivot axis.

The drive axis about which the toothed gear wheel rotates during operation of the rotary drive member may be transverse to the yoke pivot axis. Preferably the drive axis is substantially perpendicular to the yoke pivot axis.

The toothed gear wheel and the sector gear may be any appropriate type of gear. It is preferred that at least one of the toothed gear wheel and the sector gear is a bevel gear, more preferably both of the gears are bevel gears. One or both of the gears may be spur gears, spiral bevel gears or hypoid gears. To provide a compact actuator arrangement it is preferred that the toothed gear wheel and the sector gear are mounted within said housing.

In a preferred embodiment there is provided a pair of guide members and the yoke defines a pair of arms which extend into engagement with a respective guide member of the pair of guide members. Each arm may define or be connected to an end formation that engages a formation defined by its respective guide member. By way of example, each arm may be connected to a block via a pin, the block then being slidingly received in a slot defined by the end of the respective guide member.

The sector gear is preferably fixed to the yoke at a position which is in between the end formations defined by the pair of arms. In this way, the possibility of applying an uneven torque to the guide members is significantly reduced as compared to prior art actuators that drive rotation of the yoke from an end of the yoke shaft. The drive axis of the rotary drive member may lie in a plane that bisects the yoke pivot axis at a point that is in between the end formations defined by the pair of arms. Said point is preferably approximately equidistant from said end formations, thereby minimising the torque required to effect translation of the guide members and avoiding the possibility of applying an uneven torque to the guide members.

The yoke may be pivotally mounted on a yoke shaft supported by the housing. The yoke may floats on said yoke shaft which is non-rotatably supported by the housing.

The rotary drive member may be a motor having an output shaft which drives the toothed gear wheel. The output shaft may extend into the housing, in which case, it is preferred that the toothed gear wheel is mounted on a portion of the output shaft located within the housing. The output shaft of the motor may be disposed between the yoke and the movable member to provide a compact arrangement.

The at least one guide member may be a guide rod arranged for axial translation along its length. In one embodiment there are a pair of such guide rods that may be arranged on each side of the turbine axis. The motor output shaft is preferably disposed between the guide rods. The guide rods each have a translational axis and the axis of the motor shaft is disposed in close proximity thereto to allow for a compact arrangement in which the torque required from the motor to resist the force applied to the movable member is

relatively low compared to prior art arrangements.

The movable member may be a substantially annular wall member, which may have a central axis arranged to be substantially coaxial with the axis of the turbine.

The movable member may be disposed opposite a facing wall of the housing, the distance between the movable member and the facing wall determining the size of the gas flow inlet passage.

The substantially annular wall member may support an array of vanes that extend in a direction away from the at least one guide member in a direction substantially parallel to the axis of the turbine.

In accordance with another aspect of the present invention there is provided a turbomachine, such as a turbocharger, comprising a variable geometry turbine as defined above and a compressor driven by said turbine.

The turbine wheel may be rotatable on a turbine shaft and the housing may comprise a turbine housing portion in which the turbine wheel is housed and a bearing housing portion in which bearings for the turbine shaft of the turbine are housed. The yoke of the actuator assembly may be received in the bearing housing. The output shaft of the rotary drive member motor may extend into the bearing housing.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a longitudinal sectioned view through a turbocharger fitted with a known variable geometry turbine; Figure 2 is an enlarged perspective view of components of the nozzle ring actuator assembly of the turbocharger of Figure 1; and Figure 3 is an enlarged perspective view of components of a nozzle ring actuator according to a preferred embodiment of the present invention.

Referring to figure 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 4a 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 chamber 7 to an axial outlet passageway 8 via an annular inlet passageway 9 and turbine wheel 5. The inlet passageway 9 is defined on one side by the face 10 of a radial wall of a movable annular wall member 11, commonly referred to as a "nozzle ring", and on the opposite side by 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 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. When the nozzle ring 11 is proximate to the annular shroud 12 the vanes 14 project through suitably configured slots in the shroud 12 into the recess 13.

The position of the nozzle ring 11 is controlled by an actuator assembly of the type disclosed in US 5,868,552 referred to above. 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 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 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 chamber 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 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, 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. Figure 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.

Figure 2 illustrates components of a nozzle ring and nozzle ring actuator assembly of the general type shown in Fig. 1. These components are shown removed from the turbocharger for clarity. Specifically, Fig. 2 shows the back side of the nozzle ring 11 (facing away from the turbine inlet) supported on rods 16 mounted within bushes 24 for movement parallel to the axis of the turbocharger. Each arm of yoke 15 is connected to a respective rod 16 via a pivot pin 25 (only one of which is visible in Figure 2) and sliding block 26. Each pivot pin 25 pivotally connects an end of an arm of the yoke 15 to a respective sliding block 26 which is received within a slot defined in the respective support rod 16. The yoke 15 is clamped to a yoke shaft 27 by bolt 28. The yoke shaft 27 is rotatably supported within bearings 29 which are mounted in the bearing housing wall (the bearing housing is not shown in Fig. 2). One end of the yoke shaft 27 is formed with a crank 30 appropriate for connection to an actuator. In the example illustrated in Fig. 2 the crank 30 is a sector gear defining teeth 31 suitable for connection to a gear wheel assembly driven by a rotary electric actuator (not shown).

In operation, rotary motion of the electric actuator is transferred to the crank 30 which rotates the yoke shaft 27 about its axis within the bushes 29. This in turn rotates the yoke causing the pins 25 to describe an arc of a circle. This causes the blocks 26 to move axially with the rods 16, whilst sliding within the slots to accommodate off axis movement of the pins 25. The nozzle ring 11 is thereby moved along the axis of the turbocharger by rotation of the yoke 15.

Figure 3 shows a nozzle ring actuator forming part of a variable geometry turbine according to a preferred embodiment of the present invention. In Figure 3 parts corresponding to those shown in Figures 1 and 2 take the same reference numbers but have been increased by 100. The actuator shown in Figure 3 employs a yoke 115 of generally similar construction to the yoke 15 employed in the actuator shown in Figures 1 and 2. The yoke 115 of Figure 3 incorporates a yoke shaft (not visible) whose ends are rotatably received within bearings 129 mounted within the bearing housing 103. The yoke defines a pair of arms 132 (only one arm being visible in Figure 3) which extend radially from the yoke shaft so as to engage the ends of guide rods 126 which support the nozzle ring (not shown). The fundamental difference between the actuator shown in Figure 2 and the actuator of the present invention shown in Figure 3 is the means by which the yoke 115 is driven to pivot about the axis defined by the yoke shaft. In the arrangement shown in Figure 3, the yoke 115 defines a rim 133 which extends radially to the opposite side of the yoke shaft to the yoke arms 132. A radially outer section 134 of a radial face 135 of the rim 133 defines a series of teeth which combine to define a sector gear 136. The sector gear 136 is mounted within the bearing housing 103 opposite a bevel gear 137 that is rotationally mounted on a drive shaft 138 which defines a drive axis. The drive axis lies in a plane that bisects the yoke pivot axis at the midpoint between the arms 132 of the yoke 115 while the rim 133 extends from a point on the yoke that is in between the yoke arms 132 but offset from the midpoint between the arms 132 along the yoke pivot axis to accommodate the central positioning of the bevel gear 137. In the specific embodiment shown in Figure 3 the drive axis is perpendicular to the yoke pivot axis, although it will be appreicated that this does not have to be the case. By suitable arrangement of the sector gear 136 and the bevel gear 137 different orientations of the drive shaft 138 relative to the yoke pivot axis may be accommodated.

During operation, rotation of the drive shaft 138 by a suitable drive means, such as a motor (not shown), rotates the bevel gear which in turn drives the sector gear 136 in rotation about the yoke shaft axis. Since the rim 133 defining the sector gear 136 is fixedly mounted to the yoke 115, in this case by welding, although other means of connection may be used, rotation of the rim 133 about the yoke axis directly drives pivoting of the yoke 115 about the yoke axis. Consequently, the yoke arms 132 formed integrally with the yoke 115 pivot about the yoke axis in the same rotational direction as the sector gear 133. Rotation of the yoke arms 132 drives translational movement of the guide rods 126 mounted to the nozzle ring in the manner described above in relation to Figure 1 and 2.

Many engine arrangements require the use of a moving wall actuator positioned above the variable geometry turbine. While this could be acheived using an actuator arrangement of the kind shown in Figures 1 and 2, the nozzle ring would have to turned through 90 ° about the turbine axis. As a result the guide rods are no-longer side-by-side but are instead now one above the other. As a result the upper guide rod bears a significantly different load to the lower guide rod. The nozzle ring may also drop' from its centred position relative to the guide rods. It will be appreciated that the actuator of the present invention overcomes these problems and represents an eminently suitable arrangement for use when a top mounted actuator is required. Further advantages of the actuator of the present invention arise from providing the sector gear on the yoke in between the arms of the yoke. This enables the yoke shaft to be bolted into the bearing housing, which reduces oil leaks paths and allows more accurate alignment of the shaft than if the shaft had to be mounted within cross-drilled bores within the bearing housing.

Numerous modifications and variations to the embodiment described above may be made without departing from the scope of the invention as defined in the appended claims. For example, the actuator may be provided with any appropriate gearing by appropriate modification to the teeth of the sector and/or bevel gears to drive translation of the nozzle ring via the guide rods. Moveover, the connection between the arms of the yoke and the guide rods may take any appropriate form. For example, the conventional rectangular sliding block arrangement employed in Figure 3 may be substituted with blocks having a V side profile which are received in complementary formations defined by the ends of the guide rods. Furthermore, the variable geometry mechanism, including the nozzle ring, may vary from that shown provided that a movable wall portion is driven directly or indirectly by the drive mechanisms described above. In one example, the positions of nozzle ring (with vanes fixed thereto) and shroud plate may be interchanged with the nozzle ring being fixed and the shroud plate being movable by the drive mechanism.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the scope of the inventions as defined in the claims are desired to be protected. It should be understood that while the use of words such as preferable, preferably, preferred or more preferred utilized in the description above indicate that the feature so described may be more desirable, it nonetheless may not be necessary and embodiments lacking the same may be contemplated as within the scope of the invention, the scope being defined by the claims that follow. In reading the claims, it is intended that when words such as "a," "an," "at least one," or "at least one portion" are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. When the language "at least a portion" and/or "a portion" is used the item can include a portion and/or the entire item unless specifically stated to the contrary.

Claims

CLAIMSA variable geometry turbine comprising: a turbine wheel mounted within a housing for rotation about a turbine axis; a gas flow inlet passage upstream of said turbine wheel; a gas flow control mechanism located upstream of said turbine wheel and operable to control gas flow through said gas flow inlet passage; and an actuator assembly for operating the control mechanism; the control mechanism comprising a movable member for varying the size of the gas flow inlet passage, the movable member being mounted on at least one guide member that is translatable in a direction substantially parallel to the turbine axis; the actuator assembly comprising a yoke defining at least one arm which extends into engagement with a respective guide member, a rotary drive member configured to drive a toothed gear wheel in rotatation about a drive axis, the toothed gear wheel arranged to drive rotation of a sector gear fixed to the yoke, rotation of the sector gear causing the yoke to pivot about a yoke pivot axis to drive the or each respective guide member in translation; wherein the sector gear is defined by a rim that extends radially from the yoke pivot axis to one side of a plane which extends substantially parallel to the turbine axis and which contains the yoke pivot axis, and said at least one arm of the yoke extends to the other side of said plane.
2. A variable geometry turbine according to claim 1, wherein the sector gear is fixedly mounted on the yoke.
3. A variable geometry turbine according to claim 1, wherein the sector gear is formed integrally with the yoke.
4. A variable geometry turbine according to any preceding claim, wherein the drive axis is transverse to the yoke pivot axis.
5. A variable geometry turbine according to any preceding claim, wherein the drive axis is perpendicular to the yoke pivot axis.
6. A variable geometry turbine according to any preceding claim, wherein at least one of the toothed gear wheel and the sector gear is a bevel gear.
7. A variable geometry turbine according to any preceding claim, wherein the toothed gear wheel and the sector gear are mounted within said housing.
8. A variable geometry turbine according to any preceding claim, wherein there is provided a pair of guide members and the yoke defines a pair of arms which extend into engagement with a respective guide member of the pair of guide members.
9. A variable geometry turbine according to claim 8, wherein each arm defines or is connected to an end formation that engages a formation defined by its respective guide member.
10. A variable geometry turbine according to claim 9, wherein the sector gear is fixed to the yoke at a position which is in between the end formations defined by the pair of arms.
11. A variable geometry turbine according to claim 8 or 9, wherein the drive axis lies in a plane that bisects the yoke pivot axis at a point that is in between the end formations defined by the pair of arms.
12. A variable geometry turbine according to claim 11, wherein said point is approximately equidistant from said end formations.
13. A variable geometry turbine according to any preceding claim, wherein the yoke is pivotally mounted on a yoke shaft supported by the housing.
14. A variable geometry turbine according to claim 13, wherein the yoke floats on said yoke shaft which is non-rotatably supported by the housing.
15. A variable geometry turbine according to any preceding claim, wherein the rotary drive member is a motor having an output shaft which drives the toothed gear wheel.
16. A variable geometry turbine according to any preceding claim, wherein the rotary drive member is a motor having an output shaft that extends into the housing.
17. A variable geometry turbine according to any preceding claim, wherein the rotary drive member is a motor having an output shaft that is disposed between the yoke and the movable member.
18. A variable geometry turbine according to any preceding claim, wherein the at least one guide member is a guide rod.
19. A variable geometry turbine according to any preceding claim, wherein the movable member is a substantially annular wall member.
20. A variable geometry turbine according to any preceding claim, wherein the movable member is disposed opposite a facing wall of the housing, the distance between the movable member and the facing wall determining the size of the gas flow inlet passage.
21. A variable geometry turbine according to claim 20, wherein the substantially annular wall member supports an array of vanes that extend in a direction away from the at least one guide member.
22. A variable geometry turbine according to any preceding claim in which the housing comprises a turbine housing in which the turbine wheel is housed for rotation on a turbine shaft and a bearing housing in which bearings for supporting rotation of the turbine shaft are housed, the yoke being received in the bearing housing.
23. A turbocharger comprising a variable geometry turbine according to any preceding claim and a compressor driven by said turbine.