Patent Description:
Turbochargers are well-known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric pressure (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 inlet 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 wheel housing.

In known turbochargers, the turbine stage comprises a turbine chamber within which the turbine wheel is mounted; an annular inlet passage defined between facing radial walls arranged around the turbine chamber; an inlet arranged around the inlet passage; and an outlet passage extending axially from the turbine chamber. The passages and chambers communicate such that pressurised exhaust gas admitted to the inlet chamber flows through the inlet passage to the outlet passage via the turbine and rotates the turbine wheel.

It is known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passage so as to deflect gas flowing through the inlet passage towards the direction of rotation of the turbine wheel. Each vane is generally laminar, and is positioned with one radially outer surface arranged to oppose the motion of the exhaust gas within the inlet passage, i.e. the radially inward component of the motion of the exhaust gas in the inlet passage is such as to direct the exhaust gas against the outer surface of the vane, and it is then redirected into a circumferential motion.

Turbines may be of a fixed or variable geometry type. Variable geometry type turbines differ from fixed geometry turbines in that the geometry of the inlet passage 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 form of a variable geometry turbocharger, a nozzle ring carries a plurality of axially extending vanes, which extend into the air inlet, and through respective apertures ("slots") in a shroud which forms a radially-extending wall of the air inlet. The nozzle ring is axially movable by an actuator to control the width of the air passage. Movement of the nozzle ring also controls the degree to which the vanes project through the respective slots. The shroud is ring-shaped and encircles the rotational axis.

An example of such a variable geometry turbocharger is shown in <FIG>, taken from <CIT>. The illustrated variable geometry turbine comprises a turbine housing <NUM> defining an inlet chamber <NUM> to which gas from an internal combustion engine (not shown) is delivered. The exhaust gas flows from the inlet chamber <NUM> to an outlet passage <NUM> via an annular inlet passage <NUM>. The inlet passage <NUM> is defined on one side by the face of a movable annular wall member <NUM> which constitutes the nozzle ring, and on the opposite side by an annular shroud <NUM>, which covers the opening of an annular recess <NUM> in the facing wall. The shroud <NUM> is a ring-shaped member (a one-piece unit) defining a central aperture and encircling the rotational axis. The facing wall is defined by a portion <NUM> of the turbine housing <NUM>. The shroud <NUM> is connected to the portion <NUM> of the turbine housing <NUM> by a bracket <NUM> at the radially-outer side of the shroud <NUM>. In some arrangements a retention ring (not shown) is provided partially inserted into a radially-outwardly facing recess in the bracket <NUM>, and a radially outer portion of the retention ring is retained by the portion <NUM> of the turbine housing <NUM>.

Gas flowing from the inlet chamber <NUM> to the outlet passage <NUM> passes over a turbine wheel <NUM> and as a result torque is applied to a turbocharger shaft <NUM> supported by a bearing assembly <NUM> that drives a compressor wheel <NUM>. Rotation of the compressor wheel <NUM> about rotational axis <NUM> pressurizes ambient air present in an air inlet <NUM> and delivers the pressurized air to an air outlet <NUM> from which it is fed to an internal combustion engine (not shown). The speed of the turbine wheel <NUM> is dependent upon the velocity of the gas passing through the annular inlet passage <NUM>. For a fixed rate of mass of gas flowing into the inlet passage, the gas velocity is a function of the width of the inlet passage <NUM>, the width being adjustable by controlling the axial position of the nozzle ring <NUM>. As the width of the inlet passage <NUM> is reduced, the velocity of the gas passing through it increases. <FIG> shows the annular inlet passage <NUM> closed down to a minimum width, whereas in <FIG> the inlet passage <NUM> is shown fully open.

The nozzle ring <NUM> supports an array of circumferentially and equally spaced vanes <NUM>, each of which extends across the inlet passage <NUM>. The vanes <NUM> are orientated to deflect gas flowing through the inlet passage <NUM> towards the direction of rotation of the turbine wheel <NUM>. When the nozzle ring <NUM> is proximate to the annular shroud <NUM> and to the facing wall, the vanes <NUM> project through suitably configured slots in the shroud <NUM> and into the recess <NUM>. Each vane has an "inner" major surface which is closer to the rotational axis <NUM>, and an "outer" major surface which is further away. Both the nozzle ring <NUM> and the shroud <NUM> are at a fixed angular position about the axis <NUM>. The vanes <NUM> are illustrated in <FIG> as having a chamfered end portion (towards the right of the figures), but in most modern arrangements the vanes are either longitudinally symmetric along their whole length, or else composed of two sections which are each longitudinally symmetric but which have a different profile from each other as viewed in the axial direction.

A pneumatically or hydraulically operated actuator <NUM> is operable to control the axial position of the nozzle ring <NUM> within an annular cavity <NUM> defined by a portion <NUM> of the turbine housing via an actuator output shaft (not shown), which is linked to a stirrup member (not shown). The stirrup member in turn engages axially extending guide rods (not shown) that support the nozzle ring <NUM>. Accordingly, by appropriate control of the actuator <NUM> the axial position of the guide rods and thus of the nozzle ring <NUM> can be controlled. It will be appreciated that electrically operated actuators could be used in place of a pneumatically or hydraulically operated actuator <NUM>.

The nozzle ring <NUM> has axially extending inner and outer annular flanges <NUM> and <NUM> respectively that extend into the annular cavity <NUM>, which is separated by a wall <NUM> from a chamber <NUM>. Inner and outer sealing rings <NUM> and <NUM>, respectively, are provided to seal the nozzle ring <NUM> with respect to inner and outer annular surfaces of the annular cavity <NUM>, while allowing the nozzle ring <NUM> to slide within the annular cavity <NUM>. The inner sealing ring <NUM> is supported within an annular groove <NUM> formed in the inner surface of the cavity <NUM> and bears against the inner annular flange <NUM> of the nozzle ring <NUM>, whereas the outer sealing ring <NUM> is supported within an annular groove <NUM> provided within the annular flange <NUM> of the nozzle ring <NUM> and bears against the radially outermost internal surface of the cavity <NUM>. It will be appreciated that the inner sealing ring <NUM> could be mounted in an annular groove in the flange <NUM> rather than as shown, and/or that the outer sealing ring <NUM> could be mounted within an annular groove provided within the outer surface of the cavity rather than as shown. A first set of pressure balance apertures <NUM> is provided in the nozzle ring <NUM> within the vane passage defined between adjacent apertures, while a second set of pressure balance apertures <NUM> are provided in the nozzle ring <NUM> outside the radius of the nozzle vane passage.

Note that in other known turbomachines, the nozzle ring is axially fixed and an actuator is instead provided for translating the shroud in a direction parallel to the rotational axis. This is known as a "moving shroud" arrangement.

In known variable geometry turbo-machines which employ vanes projecting through slots in a shroud, a clearance is provided between the vanes and the edges of the slots to permit thermal expansion of the vanes as the turbocharger becomes hotter. As viewed in the axial direction, the vanes and the slots have the same shape, but the vanes are smaller than the slots. In a typical arrangement, the vanes are positioned with an axial centre line of each vane in a centre of the corresponding slot, such that in all directions away from the centre line transverse to the axis of the turbine, the distance from the centre line to the surface of the vane is the same proportion of the distance from the centre line to the edge of the corresponding slot. The clearance between the vanes and the slots is generally arranged to be at least about <NUM>% of the distance of a centre of the vanes from the rotational axis (the "nozzle radius") at room temperature (which is here defined as <NUM> degrees Celsius) around the entire periphery of the vane (for example, for a nozzle radius of <NUM> the clearance may be <NUM>, or <NUM>% of the nozzle radius). This means that, if each of the vanes gradually thermally expands perpendicular to the axial direction, all points around the periphery of the vane would touch a corresponding point on the slot at the same moment. At all lower temperatures, there is a clearance between the entire periphery of the vane and the edge of the corresponding slot.

<CIT> discloses a variable geometry turbine including a shroud <NUM> with a flange <NUM> extending to the rear of the shroud cavity for receiving forces generated by gas pressure. <CIT> discloses a variable geometry turbine assembly in which a shroud and a nozzle ring are mounted on the same axial side of an inlet passageway.

The present invention aims to provide new and useful vane assemblies for use in a turbo-machine, as well as new and useful turbo-machines (especially turbo-chargers) incorporating the vane assemblies.

In an earlier patent application (<CIT>, which was unpublished at the priority date of the present application), the present applicant proposed that in the turbine of a turbomachine of the kind in which, at a gas inlet between a nozzle ring and a shroud, vanes project from the nozzle through slots in the shroud, one "conformal" portion of a lateral surface of each vane (i.e. a surface including a direction parallel to the rotational axis) substantially conforms to the shape of a corresponding "conformal" portion of a lateral surface of the corresponding slot at room temperature, so as to enable the respective conformal portions of the surfaces to be placed relative to each other with only a small clearance between them. An advantage of this is that gas flow between the respective conformal portions of the surfaces of the vane and the slot can be substantially reduced. This reduces leakage of gas into or out of a recess on the other side of the shroud from the nozzle ring. Such leakage reduces the circumferential redirection of the gas caused by the vanes, and has been found to cause significant losses in efficiency.

In such an arrangement, the conformal portions of the vane surface and slot surface can be positioned close to each other, or even in contact, at low temperature (such as room temperature). At higher temperatures, if the shroud and nozzle ring expand uniformly, this contact is maintained. However, uneven thermal expansion of the components of the turbine in use may cause the vanes and the slots to press against one another, making it harder to move the vanes axially relative to the slots. To some extent this effect may be reduced by any free play in the mounting of the shroud and nozzle ring, which permits the vane to retract away from the inner surface of the shroud, to prevent the respective surfaces being pressed together with high force. Any such free play is not due to design but rather the result of tolerances in the formation of components. It varies from one turbomachine to another, and the present inventors have found experimentally that such free play permits relative rotation of the nozzle ring with respect to the shroud by significantly less than <NUM> degrees, e.g. up to <NUM> degrees.

In general terms, the present invention proposes that a turbine (for example of a turbo-charger) permits the nozzle ring to move relative to the shroud in the circumferential direction by a larger angular amount (at least. <NUM> degrees), to relieve pressure between the vanes and the edges of the respective slots.

A specific expression of the invention is a turbine according to claim <NUM>. Another expression of the invention is a turbocharger according to claim <NUM>.

The shroud and nozzle are each supported within the turbine housing, but the nozzle is rotatable relative to the turbine housing about the axis by at least <NUM> degree. Typically, the shroud is mounted on the turbine housing such that it is angularly rotatable about the axis with respect to the housing by an amount less than <NUM> degree.

The concept of arranging for the nozzle ring to be rotatable relative to the shroud is referred to here as "clocking".

Typically, the nozzle ring and shroud are relatively rotatable about the axis of the turbine by at least <NUM> degrees, at least <NUM> degrees, at least <NUM> degree, at least <NUM> degrees, or at least <NUM> degrees.

We refer to a connection between the turbine housing and either the shroud or nozzle ring which permits relative rotation respectively of the shroud or nozzle ring with respect to the turbine housing by at least <NUM> degree, as a coupling mechanism.

In one possibility, the coupling mechanism may substantially fix the axial position of the shroud ring, and/or maintain a centre of the shroud/nozzle substantially on the axis of the turbine wheel, but permits the nozzle ring to rotate about the axis of the turbine wheel relative to the turbine housing. The coupling mechanism permits rotation of the nozzle ring relative to the turbine housing through a fixed range of angles which is at least <NUM> degree, or freely (i.e. by an unlimited angular amount). In the latter case the rotation of the nozzle ring relative to the turbine housing may be limited only by interaction between the vanes of the nozzle ring and the slots of the shroud.

The turbine preferably further includes an actuator for displacing one of the nozzle ring or shroud axially with respect to the other. The actuator may be typically mounted on the turbine housing. In one possibility, the coupling mechanism couples the nozzle ring or the shroud to the turbine housing via the actuator.

In a first possibility, the coupling mechanism connects the actuator to the nozzle ring, while permitting the nozzle ring to move rotationally relative to the actuator. The shroud may be substantially fast with (that is, in mounted in fixed positional relationship with) a housing of the turbo-machine. The turbine housing may comprise a limit element which bears against a circumferentially-facing surface of the shroud and limits rotation of the shroud about the axis. The limit element may for example be provided as a pin element which projects from the turbine housing, the shroud having a wall defining a gap containing the pin element. A circumferentially-facing surface of the wall may bear against the pin element in use to limit rotational motion of the shroud.

The coupling mechanism may include at least one guide coupling. Each guide coupling may include: (i) a first element fast with one of the nozzle ring and actuator, and (ii) a second element fast with the other of the nozzle ring and actuator, and being arranged to move within a limited region defined by the first element. The region may be sized to permit the second element to rotate circumferentially relative to the first element about the axis by at least <NUM> degrees. For example, the first element may define a control surface extending in a circumferential direction about the axis (e.g. an edge of an elongate circumferential slot), and the second element being arranged to move along a path defined by the control surface. The path may be at least <NUM> degrees in length. In a variation, the region may be defined by an aperture which is large enough to permit the rotational motion, but which does not include a control surface to guide the rotation to be along a path.

In a second possibility, the coupling mechanism connects the actuator to the shroud, while permitting the shroud to move rotationally relative to the actuator.

A rotation mechanism is provided for urging the shroud and nozzle ring to rotate relatively around the axis in a predefined sense. In principle, the rotation mechanism may comprise an externally-controllable actuator. In other possibilities the rotation mechanism could be provided comprising at least one resilient spring element, and/or at least one magnetic element. The rotation mechanism may urge lateral surfaces of the vanes and respective lateral surfaces of respective slots against each other, thereby reducing gas flow between those surfaces. This is particularly, but not exclusively, useful if the lateral surfaces of the vanes and the respective slots conform to each other closely in shape.

In a preferred case, the rotation mechanism comprises gas interaction elements on one of the shroud and the nozzle, arranged to develop a rotational force in use due to flow of the gas against the gas interaction elements. The vanes themselves may serve as gas interaction elements for urging the nozzle ring to rotate relative to the turbine housing, so that no additional rotation mechanism is required.

In the case of gas interaction element(s) provided on the shroud, one or more of the gas interaction element(s) may be on a face of the shroud opposite to the nozzle ring.

If a face of the shroud includes a land surface (e.g. a surface which is transverse to the rotational axis), the gas interaction element(s) may, for example, include a respective ridge element of the face of the shroud which is upstanding from (e.g. further away from the nozzle ring than) the land surface. The ridge element(s) may be elongate. The ridge element(s) may comprise a top surface which is substantially transverse to the axial direction, and/or two opposed wall surfaces which include the axial direction. Typically, rotational force is developed due to flow of the gas against one of the wall surfaces. Additionally, rotational force is developed by flow of the gas against other surfaces of the shroud, such as the inwardly facing surfaces of the slot which extend between the faces of the shroud and which define the edge of the slot. The net rotational force on the shroud is the sum of the rotational forces imparted by the gas onto all the surfaces of the shroud.

At least one respective ridge element may be provided for one or more of the slots of the shroud, such as each of the slots. A respective ridge element for a slot may have a shape matching a shape of an edge of the slot. A respective ridge element for a slot may be provided proximate an edge of the slot, for example within a distance from the slot about the rotational axis of less than <NUM> microns, or less than <NUM> microns. Indeed, an axially extending surface of the raised portion may be substantially flush with an inwardly facing surface of the slot which defines the edge of the slot. For example, it may be a continuous axial extension of a portion of the inwardly-facing surface of the slot (i.e. a projected slot surface).

Some or all of the ridge elements may extend radially inward of a radially inward end of the slot, for example to join an inner rim portion of the shroud face which is upstanding from the land surface and encircles the rotational axis radially inwardly of the slots. Alternatively or additionally, some or all of the ridge elements may extend radially outward of a radially outward end of the slot, for example so as to join (e.g. be formed integrally with) an outer rim portion of the shroud face which is upstanding from the land surface and encircles the rotational axis radially outwardly of the slots. In this case, the ridge elements partition the land surface of the shroud into respective portions of each of the slots.

The inner and/or outer rim(s) may be considered as rib elements (i.e. upstanding elements which extend circumferentially to join a plurality of the ridge elements). The ridge elements may be connected together by other rib element(s) upstanding from the face of the shroud. The rib element(s) may make the ridge elements easier to form with high precision, since, if corresponding rib elements connect to one or both ends of the ridge elements, it may be unnecessary to form corners for the ridge elements at their ends.

As noted above, it is preferable if a portion of the surface of each vane is conformal with an opposed portion of the surface of the respective slot, where the two conformal portions of the respective surfaces are urged together by the rotation mechanism. In one specific expression of this concept, each of the vanes has an axially-extending vane surface which includes (i) a vane outer surface facing an outer surface of the corresponding slot, (ii) an opposed vane inner surface facing an inner surface of the corresponding slot. The vane further includes a median line between the vane inner surface and the vane outer surface extending from a first end of the vane to a second end of the vane. The vane surface includes a conformal portion, extending along at least <NUM>% of the length of the median line, and facing a corresponding conformal portion of the slot surface, wherein, at room temperature, the respective profiles of the conformal portion of the vane surface and the corresponding conformal portion of the slot surface diverge from each other by no more than <NUM>% of the nozzle radius, and preferably no more than <NUM>%, <NUM>% or even <NUM>% of the nozzle radius.

The conformal portion of the vane surface may extend along at least <NUM>%, at least <NUM>%, at least <NUM>%, at least <NUM>%, at least <NUM>%, or at least <NUM>% of the length of the median line.

In this document the statement that two lines diverge from each other by no more than a certain distance x may be understood to mean that the lines can be placed such that the lines do not cross and such that no point along either one of the lines is further than a distance x from the other of the lines. The statement that the conformal portion of the vane surface and the corresponding conformal portion of the slot surface diverge from each other by no more than a certain distance x refers to the parts of the conformal portion of the vane surface and the portion of the conformal portion of the slot surface which are in axial register with each other, and which appear as respective lines when viewed in the axial direction. In such a view, these lines diverge from each other by no more than the distance x.

Preferably, at room temperature, the conformal portion of the vane surface of the vane and the corresponding conformal portion of the slot surface can be positioned with a gap of no more than <NUM>%, no more than <NUM>%, no more than <NUM>% or even no more than <NUM> % of the nozzle radius (e.g. for a <NUM> nozzle radius, a gap of no more than <NUM>, no more than <NUM>, or even no more than <NUM>) between them along the whole of their respective lengths. Thus, leakage of gas between the vane inner surface and the slot inner surface can be reduced. If the conformal portion of the vane surface is shorter (e.g. at least <NUM>% or <NUM>% of the length of the median line, but not more than <NUM>% or even no more than <NUM>%) the divergence is preferably no more than <NUM>% or even <NUM>% of the nozzle radius (i.e. for a <NUM> nozzle radius, no more than <NUM> or no more than <NUM>). The divergence may, for example, be in the range <NUM> micron to <NUM>, or even <NUM> micron to <NUM>.

Note that this is in contrast to the known vane and slot arrangement discussed above, in which the vane and slot have the same general shape as viewed in the axial direction, but have different sizes at room temperature, so that each portion of the vane surface of has a different radius of curvature from the nearest portion of the slot surface.

In some embodiments, the conformal portion of the vane is positionable in contact with the corresponding portion of the edge of the slot along substantially the whole of the length of the conformal portion. For example, there may be more than two points of contact between them, and the maximum distance of any point of the conformal portion of the vane surface from the slot surface is no greater than <NUM>%, <NUM>% or even <NUM>% of the nozzle radius. For example, in the case of a nozzle radius of <NUM>, the vane may be positionable such that the maximum distance of any point of the conformal portion of the vane surface from the slot surface is no greater than <NUM>, <NUM> or even <NUM>.

The conformal portion of the vane surface may include a portion of one of the convex end portions of the vane surface. If the conformal portion of the vane surface is on the inner face of the vane, this is typically a conformal portion at a leading edge of the vane. If the conformal surface is on the outer face of the vane, this is typically at a trailing edge of the vane. Preferably, the conformal portion of the vane surface includes at least the portion of the convex end portion of the vane surface between a first major vane surface and the median line.

Embodiments of the invention will now be described for the sake of example only, with reference to the following drawings in which:.

Referring to <FIG>, a nozzle ring is shown which could be used in the known turbocharger of <FIG>. The nozzle ring is viewed in an axial direction from the right as viewed in <FIG> (this direction is also referred to here as "from the turbine end" of the turbocharger), from a position between the nozzle ring <NUM> and the shroud <NUM>.

The axis of the shaft about which the turbine wheel <NUM> (not shown in <FIG>, but visible in <FIG>) and compressor wheel <NUM> (also not shown in <FIG>, but visible in <FIG>) rotate is denoted as <NUM>.

Viewed in this axial direction, the substantially-planar annular nozzle ring <NUM> encircles the axis <NUM>. From the nozzle ring <NUM>, vanes <NUM> project in the axial direction. Defining a circle <NUM> centred on the axis <NUM> and passing through the centroids of the profiles of the vanes <NUM>, we can define the nozzle radius <NUM> as the radius of the circle <NUM>.

Gas moves radially inwardly between the nozzle ring <NUM> and the shroud <NUM>. In some turbines, the radially outer surface of the vanes <NUM> is a "high pressure" surface, while the radially inward surface of the vanes <NUM> is a "low pressure" surface. In other turbines, these roles are reversed.

The nozzle ring <NUM> is moved axially by an actuator <NUM> (not shown in <FIG>, but visible in <FIG>) within an annular cavity (also not shown in <FIG>, but visible in <FIG>) defined by a portion <NUM> of the turbine housing. Each vane <NUM> is optionally longitudinally-symmetric (that is, its profile as viewed in the axial direction, may be same in all axial positions), although in some embodiments only a portion of the vane <NUM> is longitudinally-symmetric.

The actuator exerts a force on the nozzle ring <NUM> via two axially-extending guide rods. In <FIG>, a portion <NUM> of the nozzle ring <NUM> is omitted, making it possible to view the connection between the nozzle ring <NUM> and a first of the guide rods. The guide rod is not shown, but its centre is in a position labelled <NUM>. The guide rod is integrally formed with a bracket <NUM> (commonly called a "foot") which extends circumferentially from the guide rod to either side. The bracket <NUM> contains two circular apertures <NUM>, <NUM>. The surface of the nozzle ring <NUM> which faces away from the shroud <NUM> is formed with two bosses <NUM>, <NUM> which project from the nozzle ring <NUM>. Each of the bosses <NUM>, <NUM> has a circular profile (viewed in the axial direction). The bosses <NUM>, <NUM> are inserted respectively in the apertures <NUM>, <NUM>, and the bosses <NUM>, <NUM> are sized such that the boss <NUM> substantially fills the aperture <NUM>, while the boss <NUM> is narrower than the aperture <NUM>. The connection between the boss <NUM> and the aperture <NUM> fixes the circumferential position of the nozzle ring <NUM> with respect to the bracket <NUM> (in typical realizations, the relative circumferential motion of the nozzle ring <NUM> and the shroud <NUM> about the axis <NUM> is no more than <NUM> degrees). However, the clearance between the boss <NUM> and the aperture <NUM> permits the bracket <NUM> to rotate slightly about the boss <NUM> if the guide rods move apart radially due to thermal expansion. For that reason, the boss <NUM> is referred to as a "pivot".

The location, as viewed in the axial direction, at which a second of the guide rods is connected to the nozzle ring <NUM> is shown as <NUM>. The connection between the nozzle ring <NUM> and the second guide rod is due to a second bracket (not visible in <FIG>) integrally attached to the second guide rod. The second bracket is attached to the rear surface of the nozzle ring <NUM> in the same way as the bracket <NUM>. The pivot for the second bracket is at the location <NUM>.

Holes <NUM>, <NUM> are balance holes provided in the nozzle rings for pressure equalisation. They are provided to achieve a desirable axial load (or force) on the nozzle rings.

Facing the nozzle ring <NUM>, is the shroud <NUM> illustrated in <FIG> is a view looking towards the shroud <NUM> from the nozzle ring <NUM> (i.e. towards the right side of <FIG>). The shroud defines slots <NUM> (that is, through-holes) for receiving respective ones of the vanes <NUM>. The edge of each slot is an inwardly-facing lateral (i.e. transverse to the axis <NUM>) slot surface. Note that in <FIG> the slots <NUM> are not illustrated as having the same profile as the vanes <NUM> of <FIG>, but typically the respective profiles do have substantially the same shape although the slots are of greater size than the vanes.

<FIG> is another view looking in the axial direction from the nozzle ring <NUM> towards the shroud <NUM> (i.e. towards the right side of <FIG>), showing a representative vane <NUM> inserted into a respective representative slot <NUM>. The vane <NUM> has a generally arcuate (crescent-shaped) profile, although in other forms the vanes are substantially planar. Specifically, the vane <NUM> has a vane inner surface <NUM> which is closer to the wheel. The vane inner surface <NUM> is typically generally concave as viewed in the axial direction, but may alternatively be planar. The vane <NUM> also has a vane outer surface <NUM> which is closer to the exhaust gas inlet of the turbine. Each of the vane inner and outer surfaces <NUM>, <NUM> is a major surface of the vane. The vane outer surface <NUM> is typically convex as viewed in the axial direction, but may also be planar. The major surfaces <NUM>, <NUM> of the vane <NUM> face in generally opposite directions, and are connected by two axially-extending end surfaces <NUM>, <NUM> which, as viewed in the axial direction, each have smaller radii of curvature than either of the surfaces <NUM>, <NUM>. The end surfaces <NUM>, <NUM> are referred to respectively as the leading edge surface <NUM> and the trailing edge surface <NUM>.

In most arrangements, the vane outer surface <NUM> is arranged to oppose the motion of the exhaust gas the inlet passage, i.e. the motion of the exhaust gas in the inlet passage is such as to direct the exhaust gas against the vane outer surface. Thus, the vane outer surface <NUM> is typically at a higher pressure than the vane inner surface <NUM>, and is referred to as the "high pressure" (or simply "pressure") surface, while the vane inner surface <NUM> is referred to as the "low pressure" (or "suction") surface. These oppose corresponding portions of the inwardly-facing surface which define the edge of the slot <NUM>, and which are given the same respective name.

In some possible arrangements, it is the vane inner surface <NUM> which redirects the flow of the gas. In this case, the vane inner surface <NUM> is typically at a higher pressure than the vane outer surface <NUM>, and is referred to as the "high pressure" (or simply "pressure") surface, while the vane outer surface <NUM> is referred to as the "low pressure" (or "suction") surface. Again, they oppose corresponding portions of the inwardly-facing surface which define the edge of the slot <NUM>, and which are given the same respective name.

As viewed in the axial direction, each vane <NUM> has a median line <NUM> which extends from one end of the vane to the other (half way between the vane inner and outer surfaces <NUM>, <NUM> when viewed in the axial direction), and this median line has both a radial and a circumferential component. We refer to the surface of the slot which the vane inner surface <NUM> faces as the slot inner surface <NUM>, and the surface of the slot which the vane outer surface <NUM> faces as the slot outer surface <NUM>. As shown in <FIG>, there is a gap of substantially constant width between the periphery of the vane <NUM> and the surface of the slot <NUM>. This gap includes four portions: between the vane inner surface <NUM> and the slot inner surface <NUM>; between the vane outer surface <NUM> and the slot outer surface <NUM>; and between the vane's leading and trailing edge surfaces <NUM>, <NUM>, and respective leading and trailing portions <NUM>, <NUM> of the edge of the slot. The surfaces <NUM>, <NUM>, <NUM> and <NUM> together constitute the inwardly-facing slot surface which defines the slot.

Turning to <FIG>, a first possible positional arrangement is shown between a vane and shroud slot in a turbine which is an embodiment of the invention. The turbine has the form illustrated in <FIG> and <FIG>, with the difference that the vanes and/or slots in the shroud are differently shaped and sized. In <FIG>, elements corresponding to elements of <FIG> are given reference numerals <NUM> higher. Thus, a representative vane <NUM> is depicted within a representative slot <NUM>. The vane outer surface <NUM> faces a slot outer surface <NUM>, and a vane inner surface <NUM> faces a slot inner surface <NUM>. Optionally, the vane <NUM> may be longitudinally-symmetric along the whole of its length (i.e. with the same profile, as viewed in the axial direction, in all axial positions). In another possibility, only a part of the vane <NUM> may be axially symmetric, e.g. including the portion which can be inserted into the slot <NUM> when the vane <NUM> is in its most advanced position. In this case, the portion of the vane shown in <FIG> is part of this axially symmetric portion of the vane. The vane <NUM> is integrally formed with the nozzle ring <NUM>, as a one-piece unit, for example by casting and/or machining.

In contrast to the known vanes of <FIG>, the vane <NUM> of <FIG> has a narrower clearance between the vane inner surface <NUM> and the opposed slot inner surface <NUM>. By contrast, a much wider gap exists between the vane outer surface <NUM> and the corresponding portion <NUM> of the slot outer surface <NUM>. This means that exhaust gas entering the shroud recess <NUM> between the outer vane surface <NUM> and the slot outer surface <NUM> is largely prevented from exiting the shroud recess between the vane inner surface <NUM> and the slot inner surface <NUM>.

To encourage this effect, the vane surface and slot surface are formed with a conformal portion <NUM> which extends along at least about <NUM>%, at least about <NUM>%, at least about <NUM>%, at least about <NUM>%, at least about <NUM>%, at least about <NUM>%, or at least about <NUM>% of the length of the median line <NUM>, or even at least <NUM>% or <NUM>% of the length of the median line <NUM>. As illustrated in <FIG>, the conformal portion <NUM> of the vane surface in <FIG> includes substantially all of the vane inner surface <NUM>. The profile (that is the shape, as viewed in the axial direction) of the vane inner surface <NUM> and a corresponding portion of the slot inner surface <NUM> are very similar to each other, so that they can be placed against each other with a very small (e.g. negligible) gap between them along the whole length of the conformal portion <NUM>. Specifically, the profile of the vane inner surface <NUM> and the corresponding portion of the slot inner surface <NUM> at room temperature are such that they may be positioned against each other with a gap between them which, e.g. transverse to the median line <NUM>, is no more than <NUM>% of the nozzle radius <NUM>, and preferably no more than <NUM>% or <NUM>% of the nozzle radius <NUM>. On average over the conformal portion <NUM> of the vane surface, the gap between the vane inner surface <NUM> and the slot inner surface <NUM> is no more than <NUM>%, or no more than <NUM>% of the gap between the vane outer surface <NUM> and the slot outer surface <NUM>. The vane's leading edge surface <NUM> is spaced from the corresponding portion of the inner surface of the slot <NUM>.

Turning to <FIG>, a second possible positional arrangement is shown between a vane <NUM> and shroud slot <NUM> in a turbine which is an embodiment of the invention. Elements having the same meaning as in <FIG> are given reference numerals <NUM> higher. The vane surface and slot surface are formed with a conformal portion <NUM> which extends along at least about <NUM>% of the length of the median line <NUM>. The conformal portion <NUM> of the vane surface in <FIG> includes substantially all of the vane inner surface <NUM> and also the majority of the vane leading end surface <NUM> which faces a leading edge surface <NUM> of the slot. At room temperature, the profile of the vane inner surface <NUM> and a corresponding portion of the slot inner surface <NUM> are substantially identical to within machining tolerances, so that they can be placed against each other with substantially no gap between them along the whole length of the conformal portion <NUM>. There is a gap between the outer surface <NUM> of the vane <NUM> and the facing portion <NUM> of the slot <NUM>.

Turning to <FIG>, a third possible positional arrangement is shown between a vane <NUM> and shroud slot <NUM> in a turbine which is an embodiment of the invention. In this arrangement, the conformal portion <NUM> of the vane <NUM> is at the vane outer surface <NUM>, and similarly the conformal portion <NUM> of the slot <NUM> is at the slot outer surface <NUM>. The conformal portion <NUM> of the vane <NUM> includes most of the outer surface <NUM> of the vane <NUM>, which lies against the slot outer surface <NUM> along at least <NUM>% of the length of the median line <NUM>. It further includes the trailing surface <NUM> which lies against the corresponding portion <NUM> of the slot edge up to a position which is radially inward of the intersection of the median line <NUM> with the trailing surface <NUM>. This positional arrangement impedes gas flow from the outer surface <NUM> of the vane <NUM> to the inner surface <NUM> by substantially preventing gas leaking between the vane outer surface <NUM> and the slot outer surface <NUM>.

In the positional relationships of <FIG> and <FIG>, if there is differential thermal expansion between the vanes <NUM>, <NUM>, <NUM> and the shroud (for example, because they are formed from different materials and/or experience different temperatures), the conformal portion of the vane <NUM>, <NUM>, <NUM> may be forced against the slot inner surface <NUM>, <NUM> or slot outer surface <NUM>. Frictional force between them may then prevent axial motion of the vane relative to the shroud. However, even if, as in the system of <FIG>, the nozzle ring and shroud were mounted in a "fixed" angular position, then there would be a certain free play in the system (for example, due to the coupling of the nozzle ring <NUM> to the rods illustrated in <FIG>, the nozzle ring may have a certain inherent freedom to rotate about the axis <NUM>), and experimentally we have found that this may be up to <NUM>°. This would allow the vanes <NUM>, <NUM>, <NUM> to retract to a certain extent from the conformal portion of the surface of the slot. However, the extent of this retraction would be limited, and since it depends on the tolerances of the components it may be inconsistent from one turbine unit to another. Accordingly, in embodiments of the present invention (described below) the nozzle ring and the shroud are arranged to be relatively rotatable with respect to each other by a greater degree. The turbine is however arranged to generate a rotational force which urges the respective conformal portions of the surfaces of the nozzle ring and slot together.

Specifically, <FIG> illustrates a nozzle ring in a first embodiment of the invention. Elements corresponding to elements of <FIG> are given reference numerals <NUM> higher. The nozzle ring of <FIG> can again be used in a system such as the known one of <FIG>, with the vane arrangement positioned within a chamber defined by a portion <NUM> of the turbine housing.

As in the nozzle ring of <FIG>, the nozzle ring <NUM> of the embodiment of <FIG> includes a plurality of equally circumferentially-spaced, axially-extending vanes <NUM> for insertion into slots of a shroud <NUM> having the same appearance as the known shroud <NUM> of <FIG>. The vanes <NUM> and slots may have the profiles and positional arrangement illustrated in any of <FIG>, such that a conformal portion of the surface of one of the vanes <NUM> may be placed against a corresponding conformal portion of the edge of the corresponding slot, or with a small clearance between them. The centroids of the vanes <NUM> lie on a circle <NUM> which has a radius <NUM>, which is the nozzle radius.

Like Fig. <NUM>(a), <FIG> shows how the vane arrangement would appear as viewed in the axial direction from a position between the nozzle ring <NUM> and shroud <NUM>. As for the known arrangement of <FIG>, the nozzle ring <NUM> is movable in either axial direction within an annular cavity (not shown, but of the same construction as shown in <FIG>) defined by a portion <NUM> of the turbocharger housing by an actuator (not shown, but of the same construction as shown in <FIG>), by means of two axially-extending guide rods which the actuator can move in either axial direction. Holes <NUM>, <NUM> in the nozzle ring <NUM> are balance holes provided in the nozzle ring <NUM> for pressure equalisation. They are provided to achieve a desirable axial load (or force) on the nozzle. In use, within an arrangement such as that of <FIG>, exhaust gas moves radially inwardly towards the turbine wheel in the direction A. Thus, the radially outer surfaces of the vanes <NUM> are high pressure surface, and their radially inner surfaces are low pressure surfaces. Thus, the exhaust gas exert a force on the outer surface of the vanes <NUM> which urges the vanes to move in the clockwise direction of <FIG>.

The connection between the nozzle ring <NUM> and a first of the guide rods is illustrated in <FIG> by neglecting a portion <NUM> of the front of the nozzle ring <NUM>, to reveal a bracket <NUM> ("foot") which is fixedly mounted to the first of the guide rods. The surface of the nozzle ring <NUM> which faces away from the shroud <NUM> is formed with two bosses <NUM>, <NUM> which project from the nozzle ring <NUM> in the axial direction away from the turbine wheel. Each of the bosses <NUM>, <NUM> has a circular profile (viewed in the axial direction). The bracket <NUM> includes a circular aperture <NUM> into which the boss <NUM> is inserted. The aperture <NUM> has a larger radius than the boss <NUM>, thus permitting the bracket <NUM> to rotate slightly about the boss <NUM> if the guide rods move apart radially due to thermal expansion. For that reason, the boss <NUM> is referred to as a "pivot".

<FIG> is an enlarged portion of <FIG>, showing that the bracket <NUM> includes an arcuate slot <NUM>, instead of the circular aperture <NUM> of the known system of <FIG>. The arcuate slot <NUM> has a curved central axis extending in the circumferential direction about the axis <NUM>. The boss <NUM> is inserted into the arcuate slot <NUM>. Transverse to the central axis, the width of the arcuate slot <NUM> is only slightly larger than the diameter of the boss <NUM>, so the edge of the slot provides a control surface to guide the boss along a path. The connection between the boss <NUM> and the aperture <NUM> fixes the radial position of the boss <NUM>, but permits relative circumferential motion of the nozzle ring <NUM> with respect to the bracket <NUM>. The amount of this circumferential movement is limited by the length of the arcuate slot. In typical realizations, the relative circumferential motion of the nozzle ring <NUM> and the shroud <NUM> about the axis <NUM> is by at least <NUM> degrees, and may be at least <NUM> degree, at least <NUM> degrees, and up to about two degrees. Note that in a variation, instead of an arcuate slot <NUM>, the bracket <NUM> may include an (e.g. circular) aperture within which the boss <NUM> moves, so that the combination of the boss and aperture permits relative circumferential motion of the nozzle ring <NUM> and the shroud <NUM> by at least <NUM> degrees. The boss <NUM> remains within a region defined by the aperture, but the edge of aperture does not limit the position of the boss <NUM> to be a location on a path defined by the aperture.

The connection between the nozzle ring <NUM> and the second guide rod is due to a second bracket (not visible in <FIG>) integrally attached to the second guide rod, and having the same shape as the bracket <NUM>. The location, as viewed in the axial direction, of the second guide rod is shown as <NUM>. The second bracket is attached to the rear surface of the nozzle ring <NUM> in the same way as the bracket <NUM>. The position of the boss for the second bracket which corresponds to the boss <NUM> of the bracket <NUM>, is indicated as <NUM>; this boss lies within a circumferentially-extending arcuate slot of the second bracket, so that the boss and slot cannot move relatively in the radial direction, but can move relatively in the circumferential direction. The length of the arcuate slot may the same as that of the arcuate slot <NUM>.

Thus, the brackets <NUM> and bosses <NUM> together form a coupling mechanism which permits the shroud <NUM> and nozzle ring <NUM> to move relatively in the circumferential direction. However, the centres of the nozzle ring <NUM> and shroud <NUM> remain on the axis <NUM>, and the overall plane of each of the nozzle ring <NUM> and shroud <NUM> remains substantially transverse to the axis <NUM>.

Due to the force applied by the exhaust gas in the circumferential direction to the vanes <NUM>, the vanes <NUM> are urged in this direction. This motion is permitted by the connection between the brackets <NUM> and the respective bosses <NUM>, so that the inner surface of each vane <NUM> is pressed against the corresponding slot inner surface. Relative circumferential motion of the nozzle ring <NUM> and the shroud is referred to as "clocking". This motion is possible because the bosses <NUM> slide within the slots <NUM> of the brackets <NUM>, so that the nozzle ring <NUM> can move circumferentially even though the guide rods do not. The shroud in this case is mounted so as not to be moveable relative to the turbine housing.

Since, as explained above with reference to <FIG>, a conformal portion of the inner surface of vane <NUM> has substantially the same profile (i.e. the same shape and same dimensions) as a corresponding conformal portion of the inner edge of the respective slot, the vane <NUM> and the slot edge lie very close together, or even substantially in contact, along the whole of the conformal portion of the vane <NUM>. In particular, the conformal portion of the vane <NUM> may include the entire vane inner surface which exactly coincides with a corresponding portion of the slot inner surface.

Thus, the embodiment benefits from the force of the exhaust gas to ensure that the conformal portion of the vane surface is pressed against the corresponding conformal portion of the edge of the slot, with little or no clearance between them. This reduces, or even eliminates leakage of gas between the conformal portion of the vane surface and the corresponding conformal portion of the edge of the slot out of the recess <NUM>.

If the vane <NUM> thermally expands, the vane can expand into the clearance at the outer surface of the vane <NUM>. This causes the nozzle ring <NUM> to move circumferentially (in the anti-clockwise direction in <FIG>) relative to the shroud <NUM>, and relative to the actuator <NUM> and the guide rods. This motion is opposed by the pressure of gas on the outer surfaces of the vanes <NUM>, which urges the respective conformal portions of the surfaces of the vane and slot together. Thus, despite the differential thermal expansion of the nozzle ring and shroud, a close connection between the conformal portion of the vane <NUM> and the edge of the respective slot is maintained, without an excessive force being developed between them.

As discussed above, the first embodiment shown in <FIG> can be employed in a known turbocharger as illustrated in <FIG>. However, <FIG> and <FIG> illustrate portions of two respective novel turbines (such as turbines of turbochargers or other turbo-machines) in which the nozzle mechanism of <FIG> can also be advantageously employed.

Specifically, <FIG> shows a turbine housing <NUM> having a portion <NUM> for defining a recess <NUM> and for retaining a ring-shaped shroud <NUM> covering the recess <NUM>. The portion <NUM> of the turbine housing <NUM> defines, on its surface facing towards the bearing housing, an aperture <NUM>. The aperture <NUM> is the opening of a circular-cylindrical cavity having a rotational axis extending approximately in the axial direction (i.e. parallel to the rotational axis). <FIG> shows a circular-cylindrical pin element <NUM> which can be inserted into the aperture <NUM>, e.g. so as to substantially fill it, with a rotational axis of the pin element <NUM> extending in the axial direction. The pin element <NUM> may be longer than the depth of the aperture <NUM>, and extends out of the aperture <NUM>.

<FIG> is a cross-sectional view of the turbine housing <NUM> when it is supporting the shroud <NUM>, whereas <FIG> is a cut-away perspective view of the turbine housing <NUM> and the shroud <NUM>. In both views the bearing housing and the nozzle ring are omitted. The radially-inner portion of the shroud <NUM> defines a bracket <NUM>, having inner and outer annular walls <NUM>, <NUM>. Between the annular walls <NUM>, <NUM> is positioned a retaining ring <NUM>. The retaining ring <NUM> extends radially-inwardly out of the gap between the annular walls <NUM>, <NUM>, and its inner portion is retained by an annular lip <NUM> of the portion <NUM> of the turbine housing. Providing the retaining ring <NUM> at the radially inner portion of the shroud <NUM>, has been found to provide excellent resistance to gas leakage at the radially-inner edge of the shroud <NUM> from the recess <NUM> into the inlet passage <NUM>.

In a radially-outer portion of the shroud <NUM> is provided a wall <NUM> extending in the axial direction away from the inlet passage <NUM>.

<FIG> is a plan view of the shroud <NUM> looking axially from the direction of the bearing housing. The wall <NUM> is on the reverse of the shroud <NUM> so it is not visible in <FIG> but its outline is indicated by the line <NUM>. Similarly, <FIG> marks the position of the pin element <NUM>, although it too is on the rear of the shroud <NUM>. The wall <NUM> extends around the majority of angular positions about the axis of the turbine, but the wall <NUM> includes a gap <NUM> between circumferentially-facing surfaces <NUM>, <NUM> of the wall <NUM>. When the shroud <NUM> is supported by the portion <NUM> of the turbine housing <NUM>, the pin element <NUM> is within the gap <NUM> in the wall <NUM>. Thus, the pin element <NUM> firmly prevents the shroud <NUM> from rotating in the anti-clockwise direction as viewed in <FIG>. Note that this achieved without requiring high tolerance in the shape of the shroud <NUM>. This is becuase the exact extent of the gap <NUM> is not relevant. Provided it is significantly larger than the diameter of the pin element <NUM> (e.g. at least <NUM>% larger), the pin element <NUM> can be inserted into it when the shroud <NUM> is attached to the portion <NUM> of the turbine housing <NUM>. Only the surface <NUM> of the shroud <NUM> impacts on the pin element <NUM>.

<FIG> shows three variants within the scope of the claims of the embodiment of <FIG>, respectively in <FIG>, <FIG> and <FIG>. Turning firstly to <FIG>, elements having the same meaning as those in <FIG> are given a reference numeral which is the same but followed by the letter "a". As shown in <FIG>, in this form of the turbine housing 401a, the portion 428a of the turbine housing 401a is formed with a shoulder <NUM> (instead of with an aperture). The shoulder <NUM> may be radially outward from the recess 408a.

As shown in the perspective views of <FIG>, the shroud 406a is formed with a recess <NUM> at its radially-outer edge for receiving the shoulder <NUM>. Thus, the shroud 406a is prevented from rotation about the rotational axis of the turbine. <FIG>, which is a perspective view of the shroud 406a mounted on the portion 428a of the turbine housing <NUM>, shows the shoulder <NUM> inserted into the recess <NUM>. Thus, in use, the shoulder <NUM> prevents rotation of the shroud <NUM> around the axis of the turbine. Thus, the shoulder <NUM>, like the pin element <NUM> of the arrangement of <FIG>, acts as a limit element of the turbine which bears against a circumferentially-facing surface of the shroud (the surface defining the recess <NUM>) and limits rotation of the shroud 406a about the axis.

As in the arrangement of <FIG>, the shroud 406a is provided with an annular retaining ring 485a at its radially inner side. The retaining ring 485a may be inserted between two walls 483a, 484a of a bracket 429a defined by the radially-inner portion of the shroud 406a. The radially-inner retaining ring 485a is effective at preventing gas leakage from the recess 408a into the inner passage 404a.

Turning to <FIG>, a further variant is shown. The cyclindrical pin element <NUM> of <FIG> is replaced by a pin element 482b shown in <FIG>, which is composed of two portions <NUM>, <NUM>. These are each illustrated as substantially cuboidal. The portion <NUM>, which is illustrated as larger than the portion <NUM>, defines an aperture <NUM> which may be a substantially circular-cylindrical through hole. The portion <NUM> has a rounded <NUM>, e.g. a non-circular cylindrical surface, discussed below.

<FIG> shows the pin element 482b in use with a shroud <NUM> which is substantially the same as in <FIG>, and so is designated by the same reference numeral. Whereas the pin element <NUM> of <FIG> in use extends axially out of the aperture <NUM>, in the arrangement of <FIG> the longest dimension of the pin element 482b extends radially. That is, the portion <NUM> is a radially-inner portion, and the portion <NUM> is a radially-outer portion. The radially-outer portion <NUM> has a greater circumferential width than the inner portion <NUM>. The radially-inner portion <NUM> is circumferentially recessed relative to the surface <NUM> of the radially-outer portion <NUM>. A second pin <NUM> (shown looking along its length axis in <FIG>) passes through the aperture <NUM>, and extends in the axial direction of the turbine into an aperture in the turbine housing which may be the aperture <NUM> of <FIG>. This secures the pin element 482b to the turbine housing.

In <FIG> the shroud <NUM> is viewed from the rear (i.e. looking towards the nozzle ring). The radially-outer portion <NUM> is located within the gap <NUM>, with the face <NUM> facing the surface <NUM> of the wall <NUM>, which extends axially from the shroud <NUM>. Thus, both surfaces <NUM> and <NUM> face circumferentially. Rotational motion of the shroud <NUM>, in the clockwise direction as seen in <FIG>, is limited by the surface <NUM> of the pin element 482b.

The surface <NUM> is substantially flat, thus reducing the contact pressure compared to the round pin element <NUM>. However, it is preferably not exactly flat, but instead may be convex and slightly curved, e.g. with a radius of curvature much greater (e.g. <NUM> times greater) than the circumferential extent of the pin element 482b. Thus, the contact between the surface <NUM> and the surface <NUM> is not at a corner of either element, but between the rounded surface <NUM> and the flat surface <NUM>. In a variation, the surface <NUM> also might be rounded, or be the only rounded surface. Note that the radially-inner portion <NUM> of the pin element 482b, which is radially inward of the wall <NUM>, may lie against the rear surface of the shroud <NUM> or be axially separated from it. Its circumferentially-facing surfaces do not limit the motion of the shroud. However, the inner portion <NUM> can increase the strength of the pin element 482b.

<FIG> illustrates the installation of the pin element 482b in the assembly process of the turbine. The pin element 482b is held in correspondingly-shaped gap in an assembly tool <NUM>, and moved by moving the assembly tool <NUM> to an appropriate position relative to the portion <NUM> of the turbine housing. Then the pin <NUM> can be threaded through the through-hole <NUM>, to secure the pin element 482b to the tubine housing.

A further variation is shown in <FIG>. This variation includes a pin element 482c which is is equivalent to the pin element 482b of <FIG>, but omits the inner portion <NUM>. One circumferentially-facing surface 498a of the pin element 482c is for implacting, and limiting the motion of, the surface <NUM> of the shroud <NUM> of <FIG>. The pin element 482c has the same cross-section (shape and size) in all planes parallel to the page. Thus, the surface 498a includes straight lines extending into the page, but the intersection of these lines with the page is a curved line 498b. In other words, the surface 498a is substantially flat, but more exactly is a convex (non-circular) cylindrical surface with a radius of curvature much greater (e.g. <NUM> times greater) than the circumferential extent of the pin element 482c. In <FIG> the pin element 482c is shown during the assembly process of the turbine, being supported in an appropriately sized gap within an assembly tool 494a.

Turning to <FIG>, a shroud <NUM> is shown of a comparative example. This example is again a turbocharger with the general form of <FIG>, and elements of the embodiment other than the shroud <NUM>, and its coupling to the turbine housing, are identical to the known turbocharger of <FIG>, and therefore will be referred here by the same respective reference numerals. In particular, the nozzle ring <NUM> of the turbocharger may be as shown in <FIG>, and is arranged for axial motion under the control of an actuator <NUM> as illustrated in <FIG>. Like the shroud <NUM> of the known turbocharger of <FIG>, the shroud <NUM> of the comparative example is mounted in the turbine housing <NUM> in such a way that it is maintained at a fixed axial position (the same position illustrated in <FIG>), and with its overall plane held perpendicular to the rotational axis <NUM>. However, in contrast to the known arrangement of <FIG>, the coupling between the shroud <NUM> and the turbine housing <NUM> permits the shroud <NUM> to rotate freely about the rotation axis <NUM> of the turbine wheel. Its rotation is limited only by interaction with the vanes of the nozzle ring.

The shroud <NUM> is viewed in <FIG> in a perspective view, looking at its face which, in use, is away from the nozzle ring <NUM>. The shroud <NUM> is formed with a land surface <NUM> which is planar and transverse to the axis <NUM>. The land surface <NUM> is formed with plurality of slots <NUM> which are through-holes. The land surface <NUM> extends between an outer rim <NUM> and an inner rim <NUM>. Each of the slots <NUM> is defined by (i.e. has an edge which is) an inwardly-facing surface which at all points contains the axial direction. In other words, the slot <NUM> has longitudinal symmetry in the axial direction.

The outer rim <NUM> is typically where the shroud <NUM> is coupled to the turbine housing <NUM>. The outer rim <NUM> may, for example, be trapped in a toroidal space defined between a circular surface of the turbine housing <NUM> and a toroidal plate (not shown) mounted to the turbine housing <NUM>, such that the outer rim <NUM> is able to rotate in the toroidal space about the rotational axis <NUM>.

<FIG> is an enlarged view of a portion of <FIG>, and shows that each slot <NUM> is provided with a respective ridge element <NUM> which is upstanding from the land surface <NUM> in the axial direction away from the nozzle ring <NUM>. The ridge element <NUM> extends along a portion of the edge of the slot <NUM>. The ridge element <NUM> is elongate and curved. It extends between a trailing (radially inner) end <NUM> and a leading (radially outer) end <NUM>. Looking along an extension direction of the ridge element <NUM> (i.e. in the direction from inner end <NUM> towards outer end <NUM>), the ridge element <NUM> has a rectangular form. It is defined between two wall surfaces <NUM>, <NUM>, which each extend in the axial direction <NUM>, and a top surface <NUM> which is transverse to the axial direction <NUM>. The wall surface <NUM> is on the side of the ridge element <NUM> facing towards the slot <NUM>. Each part of the wall surface <NUM> which is towards the slot <NUM> is flush with the closest portion of the radially inner surface of the slot <NUM>, i.e. each portion of the wall surface <NUM>, and the respective closest portion of the radially inner surface of the slot <NUM>, form a continuous surface in which lines in the axial direction extend continuously on both the portion of the wall surface <NUM> and the respective closest portion of the inner surface of the slot <NUM>.

Each slot <NUM> is for receiving a respective vane <NUM>. The vanes <NUM> and the corresponding slot surfaces, are formed with conformal portions as illustrated in <FIG>.

The turbo-charger of the comparative example is of a type in which the radially outer surfaces of the slot and vane are the high pressure side, and the radially inner surfaces are the suction side. In use, when a vane <NUM> is received in the slot <NUM>, the ridge element <NUM> is on the side of the vane <NUM>. The wall surface <NUM> faces towards the vane inner surface, and the portion of the slot surface closest to the wall surface <NUM> is the slot inner surface (the suction surface). The flow of the gas generates forces on various surfaces of the shroud <NUM>. In particular, compared to the conventional shroud <NUM> of <FIG>, a rotational force is developed on the wall surface <NUM> which urges the shroud <NUM> to rotate in the anti-clockwise direction as viewed in <FIG>, as indicated by the large arrow. A simulation we performed showed that a rotational force on the shroud <NUM> in the anti-clockwise direction existed even in the absence of the ridge elements <NUM>, but the rotational force was about <NUM>% greater as a result of the ridge elements <NUM> when the vane <NUM> is in a central position within the slot <NUM>. When the ridge elements are in this position, the efficiency of the turbine only slightly increased (by less than <NUM>%) relative to the known shroud. However, the force urges the slots <NUM> and vanes <NUM> to adopt an arrangement as illustrated in <FIG>. That is, due to the ridge elements <NUM>, the respective conformal portions of the vane <NUM> and slot <NUM>, are urged together, so as to inhibit, or even prevent, flow of the gas between them. In one of these two positions, the efficiency of the comparative example would be significantly higher than if the ridge element <NUM> is not present.

Turning to <FIG>, a shroud <NUM> is shown of a second comparative example. This example is again a turbocharger with the general form of <FIG>, and elements of the second comparative example other than the shroud <NUM>, and its coupling to the turbine housing, are identical to the known turbocharger of <FIG>, and therefore will be referred here by the same respective reference numerals. In particular, the nozzle ring <NUM> of the turbocharger may be as shown in <FIG>, and is arranged for axial motion under the control of an actuator <NUM> as illustrated in <FIG>. Like the shroud <NUM> of the known turbocharger of <FIG>, the shroud <NUM> of the second comparative example is mounted in the turbine housing <NUM> in such a way that it is maintained at a fixed axial position (the same position illustrated in <FIG>), and with its overall plane held perpendicular to the rotational axis <NUM>. However, as in the first comparative example, the coupling between the shroud <NUM> and the turbine housing <NUM> permits the shroud <NUM> to rotate freely about the rotation axis <NUM> of the turbine wheel. Its rotation is limited only by interaction with the vanes of the nozzle ring.

The shroud <NUM> is viewed in <FIG> in a perspective view, looking at its face which, in use, is away from the nozzle ring <NUM>. It is formed with a land surface <NUM> which is planar and transverse to the axis <NUM>. The land surface <NUM> is formed with plurality of slots <NUM> which are through-holes. The land surface <NUM> extends between an outer rim <NUM> and an inner rim <NUM>. Each of the slots <NUM> is defined by (i.e. has an edge which is) an inwardly-facing surface which at all points contains the axial direction <NUM>. In other words, the slot <NUM> has longitudinal symmetry in the axial direction.

<FIG> is a view of a portion of the shroud <NUM> in the axial direction, looking towards the nozzle ring, and <FIG> are perspective views of respective portions of the same face of the shroud <NUM> from different respective directions. They show that each slot <NUM> is provided with a respective ridge element <NUM> which is upstanding from the land surface <NUM> in the axial direction away from the nozzle ring <NUM>. The ridge element <NUM> extends along a portion of the edge of the slot <NUM>. The ridge element <NUM> is elongate and curved. At an outer end it joins the outer rim <NUM>, and at an inner end it joins the inner rim <NUM>. Thus, the ridge elements <NUM> partition the land surface <NUM> into respective portions, one for each slot <NUM>.

Looking along an extension direction of the ridge element <NUM>, the ridge element <NUM> has a rectangular form. It is defined between two wall surfaces <NUM>, <NUM> which each include at all points the axial direction <NUM>, and a top surface which is transverse to the axial direction <NUM>. The wall surface <NUM> is on the side of the ridge element <NUM> facing towards the slot <NUM>. Each part of the wall surface <NUM> which is towards the slot <NUM> is flush with the closest portion of the inner surface of the slot <NUM>, i.e. each portion of the wall surface <NUM>, and the respective closest portion of the inner surface of the slot <NUM>, form a continuous surface in which lines in the axial direction extend continuously on both the portion of the wall surface <NUM> and the respective closest portion of the inner surface of the slot <NUM>.

The turbo-charger of the second comparative example is of a type in which the radially inner surfaces of the slot and vane are the suction (low pressure) side, and the radially outer surfaces are on the high pressure side. In use, when a vane <NUM> is received in the slot <NUM>, the ridge element <NUM> is on the low pressure side of the vane <NUM>. The wall surface <NUM> faces towards the vane inner surface, and the portion of the slot surface closest to the wall surface <NUM> is the slot inner surface. The slot outer surface <NUM> is the pressure surface.

The flow of the gas generates forces on various surfaces of the shroud <NUM>. In particular, compared to the conventional shroud <NUM> of <FIG>, a greater net rotational force (torque) is developed which urges the shroud <NUM> to rotate in the anti-clockwise direction as viewed in <FIG>, as indicated by the large arrow. Positive (anti-clockwise) torques are developed on the slot pressure surface <NUM>, the outer rim <NUM>, and the wall surface <NUM>. These are greater than negative torques on the wall surface <NUM>, the slot suction surface and the shroud plate extended fine <NUM>. The net torque urges the slots <NUM> and vanes <NUM> to adopt an arrangement as illustrated in <FIG>. That is, due to the ridge elements <NUM>, the respective conformal portions of the vane <NUM> and slot <NUM>, are urged together, so as to inhibit, or even prevent, flow of the gas between them. Simulations we performed showed that the net torque on the shroud is about <NUM>% higher than in a known shroud as shown in <FIG>. This comparison is performed when the vanes are in a central position within the slot. Accordingly, the rotational force on the shroud <NUM> is significantly greater than for the shroud <NUM> of the first embodiment. Even in this position, the efficiency of the second comparative example is about <NUM>% higher than with the conventional shroud.

When the vanes are at an angular position as shown in <FIG>, the simulation showed the torque being <NUM>% higher. and the efficiency of the turbine was <NUM>% higher.

Turning to <FIG>, shrouds of six further comparative examples are shown in Figs. <NUM>(a)-(f) respectively. All these comparative examples are turbochargers of the "moving shroud" type, in which an actuator (not shown) is mounted on the turbine housing (not shown) to translate the shroud axially. This actuator replaces the actuator <NUM> of the turbocharger of <FIG>. It is known for the actuator of a turbocharger of the "moving shroud" type to be connected to the shroud by an arrangement resembling <FIG>. That is, the shroud is mounted on guide rods using a bracket (foot) similar to the bracket <NUM>. The axial position of the guide rods is controlled by the actuator.

<FIG> illustrates the shroud of a third comparative example. The shroud has the same appearance as a shroud of a conventional "moving shroud" turbine, including a number of slots <NUM> for receiving vanes (not shown). The radially inner side of each slot <NUM> is the low pressure surface. However, in the example of <FIG>, in contrast to a known "moving shroud" turbine, a coupling mechanism (not shown) is provided between the actuator and the shroud to permit the shroud to rotate about the circumferential axis of the turbine, i.e. perpendicular to face <NUM>, which faces towards the nozzle ring. Although this coupling mechanism is not shown, it may resemble the coupling of <FIG>, in which the bracket which conventionally connects a moving shroud to the guide rods is replaced by a bracket resembling the bracket <NUM> of <FIG>.

Furthermore, in the example of <FIG>, the lateral surfaces of the vanes (not shown) and the slots <NUM> are formed with opposed conformal portions as illustrated in any of <FIG>.

<FIG> shows the shroud of a fourth comparative example. The shroud is viewed looking towards a face of the shroud which faces away from the nozzle ring. The face includes a land surface <NUM>. The example of <FIG> is identical to the example of <FIG> (and accordingly corresponding elements are given the same reference numerals), except that as illustrated in <FIG> a ridge element <NUM> is provided along an edge of the slot <NUM>, upstanding from the land surface <NUM>.

<FIG> shows the shroud of a fifth comparative example. The example of <FIG> is identical to the example of <FIG> (and accordingly corresponding elements are given the same reference numerals), except that as illustrated in <FIG> a loop-like ridge element <NUM> is provided around the entire edge of the slot <NUM>, upstanding from the face <NUM> (which can be considered as a land surface).

<FIG> shows the shroud of a sixth comparative example. The example of <FIG> is identical to the example of <FIG> (and accordingly corresponding elements are given the same reference numerals), except that as illustrated in <FIG> a loop-like ridge element <NUM> is provided around the entire edge of the slot <NUM>, upstanding from the land surface <NUM> which faces away from the nozzle ring.

<FIG> shows the shroud of a seventh comparative example. The example of <FIG> is identical to the example of <FIG> (and accordingly corresponding elements are given the same reference numerals), except that as illustrated in <FIG> the shroud includes a number of blades <NUM>, which are arranged to provide a "waterwheel" arrangement. The blades <NUM> are gas interaction elements, which develop a rotational force on the shroud due to gas flow in the recess on the side of the shroud opposite the nozzle ring.

<FIG> shows the shroud of an eighth comparative example. The example of <FIG> is identical to the example of <FIG> (and accordingly corresponding elements are given the same reference numerals), except that as illustrated in <FIG> a ridge element <NUM> is provided along a radially inward edge of the slot <NUM>, upstanding from the land surface <NUM> which faces away from the nozzle ring. The radially outer end of the slot curls around a radially outer end of the slot <NUM>.

In simulations, we have demonstrated that gas flow in all these comparative examples develops a positive torque, where the positive direction is the anti-clockwise direction as viewed on <FIG> (i.e. the clockwise direction viewed from the turbine end). This positive torque would tend to produce a vane-slot arrangement as illustrated in <FIG>, with the radially inner side of the slot <NUM> pressed against the radially inner side of the vane.

However, the example of <FIG> produces a less positive torque than the example of <FIG>, and the examples of <FIG> are only slightly more positive than the example of <FIG>. In the case of the example of <FIG> this is because the blades <NUM> are in a position in which the gas flow tends to be slow. By contrast, the example of <FIG> produces a positive torque which is about <NUM>% higher than the example of <FIG>, due to a high pressure difference, on the radially inward side of the loop-like ridge element <NUM>, between the inwardly facing surface of the ridge element <NUM> (a low pressure position) and the opposite outwardly-facing wall surface of the ridge element <NUM>.

The example of <FIG> produces a positive torque approximately twice that of the example of <FIG>. This is because the ridge element <NUM> has the same surfaces as the ridge element <NUM> of example of <FIG> but without the radially-outer portion of the loop-shaped ridge element (i.e. without a portion which corresponds to the ridge element <NUM> of <FIG> which, as noted above, tends to reduce the positive torque). Thus, it can be concluded that ridge elements at the suction side of these examples were most effective in generating torque in the desired clocking direction.

Claim 1:
A turbine comprising:
(i) a turbine wheel (<NUM>) having an axis (<NUM>),
(ii) a turbine housing (<NUM>) for defining a chamber for receiving the turbine wheel for rotation of the turbine wheel about an axis, the turbine housing further defining a gas inlet (<NUM>), and an annular inlet passage (<NUM>) from the gas inlet to the chamber,
(iii) a ring-shaped shroud (<NUM>; <NUM>; 406a; <NUM>; <NUM>) defining a plurality of slots (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>) and encircling the axis; and
(iv) a nozzle ring (<NUM>; <NUM>) supporting a plurality of vanes (<NUM>; <NUM>; <NUM>; <NUM>; <NUM>) which extend from the nozzle ring parallel to the axis, and project through respective ones of the slots;
the shroud and nozzle ring being positioned on opposite sides of the inlet passage and rotatable relative to each other about the axis by an angular amount of at least <NUM> degrees,
the turbine being characterized in that:
the nozzle ring is rotatable relative to the turbine housing about the axis by at least <NUM> degree; and
a rotation mechanism is provided for, in use, urging the nozzle ring to rotate around the axis in a predefined sense.