WO2011042686A2

WO2011042686A2 - Turbomachine

Info

Publication number: WO2011042686A2
Application number: PCT/GB2010/001852
Authority: WO
Inventors: Tom J. Roberts; James Alexander Mcewen; Robert L. Holroyd; Tim Denholm; John F. Parker
Original assignee: Cummins Ltd
Priority date: 2009-10-06
Filing date: 2010-10-04
Publication date: 2011-04-14
Also published as: CN102713159A; EP2486242A2; IN2012DN02857A; WO2011042686A3; BR112012007832A2

Abstract

A variable geometry turbine comprising: a turbine wheel mounted for rotation about a turbine axis within a housing. The housing defines an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls. A cylindrical sleeve is axially movable across the annular inlet to vary the size of a gas flow path through the inlet. The annular inlet is divided into axially adjacent annular portions by at least one annular baffle which is axially spaced from the first and second inlet sidewalls. Inlet vanes extend axially across at least two of said annular portions defined by the or each baffle so as to divide said annular inlet into at least two axially offset inlet passages. The configuration of the inlet vanes extending into one of the inlet portions differs from the configuration of the inlet vanes extending into another of the inlet portions and the inner diameter of the sleeve is greater than the outer diameter of the inlet passages.

Description

TURBOMACHINE

The present invention relates to a turbine suitable for, but not limited to, use in turbochargers and variable geometry turbochargers.

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 a housing in which is provided an exhaust gas driven turbine wheel mounted on a rotatable shaft connected downstream of an engine outlet manifold. A compressor impeller wheel is mounted on the opposite end of the shaft such that rotation of the turbine wheel drives rotation of the impeller wheel. In this application of a compressor, the impeller wheel delivers compressed air to the engine intake manifold. A power turbine also comprises an exhaust gas driven turbine wheel mounted on a shaft, but in this case the other end of the shaft is not connected to a compressor. For instance, in a turbocompound engine, two turbines are provided in series, both driven by the exhaust gases of the engine. One turbine drives a compressor to deliver pressurised air to the engine and the other, the "power turbine", generates additional power which is then transmitted to other components via a mechanical connection, such as a gear wheel to transmit power to the engine crankshaft, or via other types of connection, for instance a hydraulic or electrical connection.

It is an object of the present invention to obviate or mitigate one or more of the problems associated with existing turbines.

According to a first aspect of the present invention there is provided a variable geometry turbine comprising:

a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; and

a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet;

the annular inlet divided into axially adjacent annular portions by at least one annular baffle which is axially spaced from the first and second inlet sidewalls;

inlet vanes extending axially across at least two of said annular portions defined by the or each baffle so as to divide said annular inlet into at least two axially offset inlet passages; wherein the configuration of the inlet vanes extending into one of the inlet portions differs from the configuration of the inlet vanes extending into another of the inlet portions and wherein the inner diameter of the sleeve is greater than the outer diameter of the inlet passages.

According to a second aspect of the present invention there is provided a variable geometry turbine comprising:

the annular inlet divided into axially adjacent annular portions by at least two annular baffles which are axially spaced from the first and second inlet sidewalls;

inlet vanes extending axially across at least two of said annular portions defined by the or each baffle so as to divide said annular inlet into at least two axially offset inlet passages;

wherein the configuration of the inlet vanes extending into one of the inlet portions differs from the configuration of the inlet vanes extending into another of the inlet portions.

In the first and/or second aspects of the present invention it is preferred that the sleeve is movable towards the second inlet sidewall so as to narrow the gas flow path through the inlet, and the gas flow path through the inlet passage that is closer to the second inlet sidewall has a cross-sectional area perpendicular to the direction of gas flow along said path that is smaller than the corresponding cross-sectional area of the gas flow path through the inlet passage that is further away from the second inlet sidewall.

The vanes may be provided in annular arrays within each annular portion. Preferably an array of vanes in a first annular portion defines a plurality of first inlet passages having a first total cross-sectional area perpendicular to the direction of gas flow and another array of vanes in a second annular portion, which is axially offset from the first annular portion, defines a plurality of second inlet passages having a second larger total cross-sectional area perpendicular to the direction of gas flow.

In a preferred embodiment the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes defining a plurality of inlet passages having a total cross-sectional area perpendicular to the direction of gas flow which decreases progressively between adjacent pairs of annular arrays.

In a further preferred embodiment the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within an intermediate array defining a plurality of inlet passages having a total cross-sectional area perpendicular to the direction of gas flow that is larger or smaller than the total cross-sectional area perpendicular to the direction of gas flow of inlet passages defined by the arrays of vanes on either side of the intermediate array.

Tthe inlet passages having the smallest total cross-sectional area perpendicular to the direction of gas flow may be provided in the annular portion nearest to the second inlet sidewall where the gas flow path through the inlet is narrowest or substantially closed.

Preferably the sum of the minimum circumferential separations between adjacent vanes within the inlet passage closer to the second inlet sidewall is lower than the sum of the minimum circumferential separations between adjacent vanes within the inlet passage further away from the second inlet sidewall.

In the first and/or second aspects of the present invention it is preferred that at least one vane in one of said annular portions has a greater maximum circumferential thickness than at least one vane in another of the annular portions.

The vanes may be provided in annular arrays within each annular portion. Preferably an array of vanes in a first annular portion incorporates a plurality of vanes of a maximum circumferential thickness and another array of vanes in a second annular portion, which is axially offset from the first annular portion, incorporates a plurality of vanes of a larger maximum circumferential thickness.

It is preferred that the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes having a maximum circumferential thickness which decreases progressively between adjacent pairs of annular arrays. Alternatively, it is preferred that the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within an intermediate array having a maximum circumferential thickness that is larger or smaller than the maximum circumferential thickness of vanes in the arrays of vanes on either side of the intermediate array.

Preferably the vanes having larger maximum circumferential thickness are provided in the annular portion(s) nearer to a closed position of the sleeve where the gas flow path through the inlet is narrowest. It is particularly preferred that vanes having the largest maximum circumferential thickness are provided in the annular portion nearest to the closed position of the sleeve where the gas flow path through the inlet is narrowest or substantially closed.

It is preferred in the first and/or second aspects of the present invention that at least one vane in one of said annular portions has a greater leading edge thickness than at least one vane in another of the annular portions.

Said vanes may be provided in annular arrays within each annular portion. An array of vanes in a first annular portion preferably incorporates a plurality of vanes of a first leading edge thickness and another array of vanes in a second annular portion, which is axially offset from the first annular portion, incorporates a plurality of vanes of a second larger leading edge thickness.

In a preferred embodiment the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes having a leading edge thickness which decreases progressively between adjacent pairs of annular arrays.

In a further embodiment the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within an intermediate array having a leading edge thickness that is larger or smaller than the leading edge thickness of vanes in the arrays of vanes on either side of the intermediate array.

It is preferred that vanes having a larger leading thickness are provided in the annular portion(s) nearer to a closed position of the sleeve where the gas flow path through the inlet is narrowest. Vanes having the largest leading edge thickness are preferably provided in the annular portion nearest to the closed position of the sleeve where the gas flow path through the inlet is narrowest or substantially closed.

It is preferred in the first and/or second aspects of the present invention that at least one vane in one of said annular portions has a greater maximum outer diameter than at least one vane in another of the annular portions.

The vanes may be provided in annular arrays within each annular portion. Preferably an array of vanes in a first annular portion incorporates a plurality of vanes of a first maximum outer diameter and another array of vanes in a second annular portion, which is axially offset from the first annular portion, incorporates a plurality of vanes of a second larger maximum outer diameter.

One preferred embodiment provides that the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes having a maximum outer diameter which decreases progressively between adjacent pairs of annular arrays.

Another preferred embodiment provides that the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within an intermediate array having a maximum outer diameter that is larger or smaller than the maximum outer diameter of vanes in the arrays of vanes on either side of the intermediate array.

The vanes having a larger maximum outer diameter may be provided in the annular portion(s) nearer to a closed position of the sleeve where the gas flow path through the inlet is narrowest. Preferably vanes having the largest maximum outer diameter are provided in the annular portion nearest to the closed position of the sleeve where the gas flow path through the inlet is narrowest or substantially closed.

In the first and/or second aspects of the present invention it is preferred that at least one vane in one of said annular portions has a greater maximum inner diameter and defines a greater radial clearance between said vane and the turbine wheel than at least one vane in another of the annular portions. Said vanes may be provided in annular arrays within each annular portion. Preferably an array of vanes in a first annular portion incorporates a plurality of vanes of a first maximum inner diameter which define a first radial clearance between said vanes and the turbine wheel and another array of vanes in a second annular portion, which is axially offset from the first annular portion, incorporates a plurality of vanes of a second larger maximum inner diameter which define a second larger radial clearance between said vanes and the turbine wheel.

In a preferred embodiment the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes having a maximum inner diameter which define a radial clearance between said vanes and the turbine wheel which both decrease progressively between adjacent pairs of annular arrays.

In an alternative preferred embodiment the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within an intermediate array having a maximum inner diameter which defines a radial clearance between said vanes and the turbine wheel both of which are larger or smaller than the maximum inner diameter of vanes and a radial clearance between said vanes and the turbine wheel in the arrays of vanes on either side of the intermediate array.

Vanes having a larger maximum inner diameter and which define a larger radial clearance between said vanes and the turbine wheel are preferably provided in the annular portion(s) nearer to a closed position of the sleeve where the gas flow path through the inlet is narrowest. It is preferred that said vanes with a larger maximum inner diameter and which define a larger radial clearance between said vanes and the turbine also define a larger swirl angle. Vanes having the largest maximum inner diameter and which define the largest radial clearance between said vanes and the turbine wheel are preferably provided in the annular portion nearest to the closed position of the sleeve where the gas flow path through the inlet is narrowest or substantially closed. It is particularly preferred that vanes with the largest maximum inner diameter and which define the largest larger radial clearance between said vanes and the turbine also define the largest swirl angle. In the first and/or second aspects of the present invention it is preferred that the number of inlet vanes extending into one of the inlet portions differs from the number of inlet vanes extending into another of the inlet portions. Said inlet portions may be adjacent one another.

According to another aspect of the present invention there is provided a variable geometry turbine comprising:

a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls;

the sleeve being movable towards the second inlet sidewall so as to narrow the gas flow path through the inlet;

wherein the gas flow path through the inlet passage that is closer to the second inlet sidewall has a cross-sectional area perpendicular to the direction of gas flow along said path that is smaller than the corresponding cross-sectional area of the gas flow path through the inlet passage that is further away from the second inlet sidewall.

The vanes are orientated to deflect gas flowing through the annular inlet towards the direction of rotation of the turbine wheel. Gas is deflected along inlet passages defined between neighbouring vanes and adjacent baffles or sidewalls. The "throat area" of the annular inlet, which may be thought of as the maximum gas "swallowing capacity" of the turbine, is the total cross-sectional area perpendicular to the direction of gas flow of all of the inlet passages defined across the annular inlet. One of the parameters which contributes the definition of the throat area is the minimum circumferential separation between circumferentially adjacent vanes within each annular portion. It is thus preferred that the sum of the minimum circumferential distances between adjacent vanes within the inlet passage closer to the second inlet sidewall is lower than the sum of the minimum circumferential distances between adjacent vanes within the inlet passage further away from the second inlet sidewall. By using baffles to divide the annular inlet into two or more annular portions the throat area of each annular portion can be independently defined by the arrangement of the vanes within each annular portion and the axial width of each annular portion. In this way, the throat area of the annular inlet can be varied between the first and second inlet sidewalls. Preferably the gas flow path through the annular inlet is more constricted nearer to the second inlet sidewall, where the gas flow path through the inlet is narrowest or substantially closed, than closer to the first inlet sidewall. The variation in the degree of constriction may be progressive across the axial width of the annular inlet or may vary discontinuously with intermediate annular portions being less constricted than neighbouring annular portions provided that the gas flow path through an inlet passage closer to the second inlet sidewall is more constricted than the gas flow path through an inlet passage that is further away from the second inlet sidewall.

Control of the degree of constriction to the gas flow path through the annular inlet by the arrangement of the vanes can be achieved in a number of ways. For example, one or more, or all, of the vanes within one annular portion may have a thickened leading edge, a larger circumferential thickness, or both, as compared to vanes in other annular portions. In a preferred embodiment, vanes with a thicker leading edge are provided in the annular portion(s) nearer to the second inlet sidewall, i.e. the closed position of the sleeve where the gas flow path through the inlet is at its narrowest, since this is where a greater variation in gas incidence angle is to be expected. By way of a further example, a greater number of vanes may be provided in one annular portion than another. For instance, an annular array of fifteen vanes may be included in the same nozzle assembly as an annular array of only eight vanes. Other arrays may have a different number of vanes, greater than fifteen or fewer than eight, or somewhere in between, e.g. twelve. In another example, the swirl angle of vanes in one annular portion may be greater than that in another annular portion. Moreover, the radial extent, outer and/or inner maximum diameter of vanes in one annular portion may be different to that in another annular portion to provide a different degree of constriction in the two annular portions. It will be appreciated that any one or more of the above modifications in vane structure, arrangement or orientation may be employed to achieve the desired variation in throat area across the axial width of the annular inlet.

The vanes are preferably provided in annular arrays within each annular portion. An array of vanes in a first annular portion may define a plurality of first inlet passages having a first total cross-sectional area perpendicular to the direction of gas flow and another array of vanes in a second annular portion, which is axially offset from the first annular portion, may define a plurality of second inlet passages having a second larger total cross- sectional area perpendicular to the direction of gas flow. The first inlet passages are preferably provided closer to the second inlet sidewall than the second inlet passages.

In one embodiment the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes define a plurality of inlet passages having a total cross- sectional area perpendicular to the direction of gas flow which decreases progressively between adjacent pairs of annular arrays. In an alternative embodiment in which the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, the vanes within an intermediate array define a plurality of inlet passages having a total cross-sectional area perpendicular to the direction of gas flow that is larger or smaller than the total cross-sectional area perpendicular to the direction of gas flow of inlet passages defined by the arrays of vanes on either side of the intermediate array.

In a preferred embodiment the inlet passages within the turbine having the smallest total cross-sectional area perpendicular to the direction of gas flow are provided in the annular portion nearest to the second inlet sidewall where the gas flow path through the inlet is narrowest or substantially closed.

In one preferred embodiment there may be provided at least one annular array of vanes consisting of a relatively small number of vanes configured to define a relatively high swirl angle but which are relatively "thick" and extend to a relatively small internal radius as compared to other arrays of vanes within the same annular inlet so as to provide a relatively small radial clearance around that region of the turbine wheel. With such an arrangement it is easier for an actuator to achieve high resolution control of the cross- sectional flow area because it varies less for a given sleeve movement. The increased swirl may be useful for a vane array positioned to correspond to relatively small inlet widths, which could provide an improvement in efficiency.

There is further provided a variable geometry turbine comprising a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalis; and a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; wherein the annular inlet is divided into a first annular inlet portion and a second annular inlet portion axially offset from the first inlet portion, inlet vanes extending axially into each of the first and second inlet portions, the inlet vanes defining axially adjacent inlet passages; wherein the configuration of the inlet vanes extending into the first inlet portion differs from the configuration of the inlet vanes extending into the second inlet portion.

The inlet vanes may have any suitable configuration, and may for example have a similar general aerofoil configuration to that of known inlet vanes, or they may have any alternative configuration selected to define a particular arrangement and configuration of inlet passages. Since the vanes and inlet baffles together define the configuration and orientation of the inlet passages, a wide variety of different inlet passage configurations can be achieved by appropriate design of the individual nozzle vanes in combination with the inlet baffles. Moreover, the designs can be such that there may be differently configured inlet passages in one annular portion as compared to another annular portion within an annular inlet, or there may be differently configured inlet passages within a single annular portion, or both. For instance, the vanes extending across a first annular portion of the inlet may define a different swirl angle to the vanes extending across a second annular portion of the inlet.

For certain engine applications (such as for exhaust gas recirculation, "EGR") it may be desirable to reduce the turbine efficiency in one or more of the arrays of inlet passageways. For example, it may be desirable to reduce efficiency at relatively open inlet widths in some applications. Such reduced efficiency could for instance be achieved by reducing the radial extent of the vanes (as discussed above), increasing the circumferential width of the vanes, or otherwise configure the vanes to reduce the effective inlet area, i.e. the throat area of the annular inlet.

In some embodiments relatively small "splitter vanes" may be located between adjacent pairs of "main" vanes. This arrangement may have the effect of increasing the total number of vanes compared with other embodiments, but the vanes may be provided with a reduced radial extent so that there is a greater radial clearance between the vanes and the turbine wheel. The splitter vanes may be advantageous in some embodiments to reduce vibration excited in the turbine blades. In some embodiments, the vanes may have a "cut-off' configuration in the region of the trailing edge rather than a full airfoil configuration which can be expected to provide reduced efficiency but which may be useful in some applications. In addition, obstructions may be located between adjacent vanes which could further reduce efficiency.

The trailing edges of at least some of the vanes extending across a first annular portion of the inlet may lie on a different radius to the trailing edges of at least some of the vanes extending across a second annular portion of the inlet. In some embodiments the trailing edges of all of the vanes extending across a first annular portion of the inlet lie on a radius different to that of the trailing edges of all of the vanes extending across a second annular portion of the inlet. In some embodiments the trailing edges of vanes of one annular portion of the inlet lie on a minimum radius which is different to that of vanes extending across any other annular portion of the inlet.

The trailing edges of at least a majority of vanes extending across an annular portion of the inlet may lie on a radius greater than the internal radius of a baffle defining the annular portion. In some embodiments all of the vanes extending across an annular portion may have a trailing edge lying at a radius greater than the internal radius of a baffle defining the annular portion. In some embodiments each annular baffle may have an internal radius smaller than the radius of the leading edge of any vane in the annular inlet.

According to a further aspect of the present invention there is provided a variable geometry turbine comprising:

wherein at least one vane in one of said annular portions has a greater maximum circumferential thickness than at least one vane in another of the annular portions. The vanes are orientated to deflect gas flowing through the annular inlet towards the direction of rotation of the turbine wheel. A thickened vane can be useful in accommodating greater variation in gas incidence angle without causing flow separation and turbulent flow (efficiency loss). It will be appreciated that thicker vanes reduce the "throat area" of the annular inlet, i.e. the maximum swallowing capacity of the turbine. As a result, the maximum thickness of a vane or vanes in each annular portion of the inlet or "nozzle section" may be optimized to suit its axial location within the annular inlet and a particular application. By way of example, in a preferred embodiment thicker vanes are provided in the annular portion(s) nearer to the closed position of the sleeve, i.e. where the gas flow path through the inlet is at its narrowest, since this is where a greater variation in gas incidence angle is to be expected.

There is further provided a variable geometry turbine comprising a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; and a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; wherein the annular inlet is divided into a first annular inlet portion and a second annular inlet portion axially offset from the first inlet portion, inlet vanes extending axially into each of the first and second inlet portions, the inlet vanes defining axially adjacent inlet passages; wherein the configuration of the inlet vanes extending into the first inlet portion differs from the configuration of the inlet vanes extending into the second inlet portion.

The inlet vanes may be provided in annular arrays within each annular portion. The vanes in two or more annular arrays may have different maximum circumferential thicknesses. That is, an array of vanes in a first annular portion may incorporate a plurality of vanes of a first maximum circumferential thickness and another array of vanes in a second annular portion, which is axially offset from the first annular portion, may incorporate a plurality of vanes of a different second circumferential maximum thickness, the first maximum circumferential thickness being larger than the second or vice versa as appropriate. In embodiments incorporating three or more annular portions and therefore three or more annular arrays of vanes, the variation in maximum circumferential thickness of the vanes may decrease progressively between adjacent pairs of annular arrays, or an intermediate array may possess vanes having a maximum circumferential thickness that is larger or smaller than the arrays of vanes on either side.

The vanes within each annular array may have the same radial extent such that the arrays of vanes are essentially continuous across the full width of the annular inlet. Alternatively, the vanes in two or more annular arrays may have different radial extents. For example, the leading edges of all of the vanes across the different arrays may lie on the same outer radius, while the radius of the trailing edges of the different arrays of vanes may differ. In an embodiment including three or more axially spaced annular arrays of vanes the radial position of the trailing edge of each annular array of vanes may decrease from a first annular array to an adjacent second annular array and then further decrease from the second annular array to an adjacent third annular array of vanes.

The number of vanes in each annular array may differ. For instance, an annular array of fifteen vanes may be included in the same nozzle assembly as an annular array of only eight vanes. Other arrays may have a different number of vanes, greater than fifteen or fewer than eight, or somewhere in between, e.g. twelve. In addition, vanes having different radial extents, and different swirl angles may be used, e.g. some vanes swept forwards to a greater extent than others, and as such defining a greater swirl angle.

For certain engine applications (such as for exhaust gas recirculation, "EGR") it may be desirable to reduce the turbine efficiency in one or more of the arrays of inlet passageways. For example, it may be desirable to reduce efficiency at relatively open inlet widths in some applications. Such reduced efficiency could for instance be achieved by reducing the radial extent of the vanes (as discussed above), increasing the circumferential width of the vanes, or otherwise configure the vanes to reduce the effective inlet area.

In some embodiments relatively small "splitter vanes" may be located between adjacent pairs of "main" vanes. This arrangement may have the effect of increasing the total number of vanes compared with other embodiments, but the vanes may be provided with a reduced radial extent so that there is a greater radial clearance between the vanes and the turbine wheel. The splitter vanes may be advantageous in some embodiments to reduce vibration excited in the turbine blades.

In some embodiments, the vanes may have a "cut-off' configuration in the region of the trailing edge rather than a full airfoil configuration which can be expected to provide reduced efficiency but which may be useful in some applications. In addition, obstructions may be located between adjacent vanes which could further reduce efficiency.

Another aspect of the present invention provides a variable geometry turbine comprising: a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls;

wherein at least one vane in one of said annular portions has a greater leading edge thickness than at least one vane in another of the annular portions.

The vanes are orientated to deflect gas flowing through the annular inlet towards the direction of rotation of the turbine wheel. As is well known to the skilled person, a vane has a leading edge and a trailing edge. The leading edge is the portion of the vane which is orientated to face the incident gas flowing through the inlet and therefore is the portion of the vane which the oncoming gas strikes first. The trailing edge is the portion of the vane which the gas flowing through the inlet contacts last before flowing on to the turbine wheel. A vane with a thicker leading edge can be useful in accommodating greater variation in gas incidence angle without causing flow separation and turbulent flow (efficiency loss).

It will be appreciated that vanes having a thicker leading edge can reduce the "throat area" of the annular inlet, i.e. the maximum swallowing capacity of the turbine. As a result, the thickness of the leading edge of a vane or vanes in each annular portion of the inlet or "nozzle section" may be optimized to suit its axial location within the annular inlet and a particular application. By way of example, in a preferred embodiment vanes with thicker leading edges are provided in the annular portion(s) nearer to the closed position of the sleeve, i.e. where the gas flow path through the inlet is at its narrowest, since this is where a greater variation in gas incidence angle is to be expected.

The use of vanes with varying leading edge thickness may be combined with vanes of varying maximum circumferential thickness. For example, vanes with thicker leading edges and may also have larger maximum circumferential thicknesses as compared to other vanes present in the same annular inlet. Alternatively, by appropriate design vanes with thicker leading edges may have smaller maximum circumferential thicknesses as compared to other vanes present in the same annular inlet. The leading edge thickness of the vanes provided within an annular inlet may vary in a generally similar manner to that of the vanes' maximum circumferential thickness, i.e. both may increase progressively from the first inlet sidewall to the second inlet sidewall, or the leading edge thickness of the vanes may vary independently of the variation in maximum circumferential thickness of the vanes across the annular inlet, or the maximum circumferential thickness of all of the vanes provided within the annular inlet may be the same in spite of the vanes having differing leading edge thicknesses.

In one preferred embodiment there may be provided at least one annular array of vanes consisting of a relatively small number of vanes configured to define a relatively high swirl angle but which have relatively "thick" leading edges and extend to a relatively small internal radius as compared to other arrays of vanes within the same annular inlet so as to provide a relatively small radial clearance around that region of the turbine wheel. With such an arrangement it is easier for an actuator to achieve high resolution control of the cross-sectional flow area because it varies less for a given sleeve movement. The increased swirl may be useful for a vane array positioned to correspond to relatively small inlet widths, which could provide an improvement in efficiency.

The inlet vanes may be provided in annular arrays within each annular portion. The vanes in two or more annular arrays may have different leading edge thicknesses. That is, an array of vanes in a first annular portion may incorporate a plurality of vanes of a first leading edge thickness and another array of vanes in a second annular portion, which is axially offset from the first annular portion, may incorporate a plurality of vanes of a different second leading edge thickness, the first leading edge thickness being larger than the second or vice versa as appropriate. In embodiments incorporating three or more annular portions and therefore three or more annular arrays of vanes, the variation in leading edge thickness of the vanes may decrease progressively between adjacent pairs of annular arrays, or an intermediate array may possess vanes having a leading edge thickness that is larger or smaller than the arrays of vanes on either side.

For certain engine applications (such as for exhaust gas recirculation, "EGR") it may be desirable to reduce the turbine efficiency in one or more of the arrays of inlet passageways. For example, it may be desirable to reduce efficiency at relatively open inlet widths in some applications. Such reduced efficiency could for instance be achieved by reducing the radial extent of the vanes (as discussed above), increasing the circumferential width and/or leading edge thickness of the vanes, or otherwise configure the vanes to reduce the effective inlet area.

According to a still further aspect of the present invention there is provided a variable geometry turbine comprising:

wherein at least one vane in one of said annular portions has a greater maximum outer diameter than at least one vane in another of the annular portions.

The vanes are orientated to deflect gas flowing through the annular inlet towards the direction of rotation of the turbine wheel.

The inlet vanes may be provided in annular arrays within each annular portion. The leading edges of at least some of the vanes extending across a first annular portion of the inlet may lie on a different radius to the leading edges of at least some of the vanes extending across a second annular portion of the inlet. In some embodiments the leading edges of all of the vanes extending across a first annular portion of the inlet lie on a radius different to that of the leading edges of all of the vanes extending across a second annular portion of the inlet. In some embodiments the leading edges of vanes of one annular portion of the inlet lie on a maximum radius which is different to that of vanes extending across any other annular portion of the inlet.

The vanes in two or more annular arrays may have different maximum outer diameters. That is, an array of vanes in a first annular portion may incorporate a plurality of vanes of a first maximum outer diameter and another array of vanes in a second annular portion, which is axially offset from the first annular portion, may incorporate a plurality of vanes of a different second maximum outer diameter, the first maximum outer diameter being larger than the second or vice versa as appropriate. In embodiments incorporating three or more annular portions and therefore three or more annular arrays of vanes, the variation in maximum outer diameter of the vanes may decrease progressively between adjacent pairs of annular arrays, or an intermediate array may possess vanes having a maximum outer diameter that is larger or smaller than the arrays of vanes on either side.

The vanes within one or more annular arrays may have different radial extents as compared to that of one or more annular arrays within the same inlet. For example, while at least one vane in one of said annular portions has a greater maximum outer diameter than at least one vane in another of the annular portions, said vanes may have substantially the same maximum inner diameter such that the trailing edges of the vanes are essentially continuous across the full width of the annular inlet. In an embodiment including three or more axially spaced annular arrays of vanes the radial position of the trailing edge of each annular array of vanes may decrease from a first annular array to an adjacent second annular array and then further decrease from the second annular array to an adjacent third annular array of vanes.

There is further provided a variable geometry turbine comprising a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; and a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; wherein the annular inlet is divided into a first annular inlet portion and a second annular inlet portion axially offset from the first inlet portion, inlet vanes extending axially into each of the first and second inlet portions, the inlet vanes defining axially adjacent inlet passages; wherein the configuration of the inlet vanes extending into the first inlet portion differs from the configuration of the inlet vanes extending into the second inlet portion. The inlet vanes may have any suitable configuration, and may for example have a similar general aerofoil configuration to that of known inlet vanes, or they may have any alternative configuration selected to define a particular arrangement and configuration of inlet passages. Since the vanes and inlet baffles together define the configuration and orientation of the inlet passages, a wide variety of different inlet passage configurations can be achieved by appropriate design of the individual nozzle vanes in combination with the inlet baffles. Moreover, the designs can be such that there may be differently configured inlet passages in one annular portion as compared to another annular portion within an annular inlet, or there may be differently configured inlet passages within a single annular portion, or both. For instance, the vanes extending across a first annular portion of the inlet may define a different swirl angle to the vanes extending across a second annular portion of the inlet.

In some embodiments relatively small "splitter vanes" may be located between adjacent pairs of "main" vanes. This arrangement may have the effect of increasing the total number of vanes compared with other embodiments, but the vanes may be provided with a reduced radial extent so that there is a greater radial clearance between the vanes and the turbine wheel. The splitter vanes may be advantageous in some embodiments to reduce vibration excited in the turbine blades. In some embodiments, the vanes may have a "cut-off configuration in the region of the trailing edge rather than a full airfoil configuration which can be expected to provide reduced efficiency but which may be useful in some applications. In addition, obstructions may be located between adjacent vanes which could further reduce efficiency.

A yet further aspect of the present invention provides a variable geometry turbine comprising:

inlet vanes extending axially across at least two of said annular portions defined by the or each baffle so as to divide said annular inlet into at least two axially offset inlet passages; wherein at least one vane in one of said annular portions has a greater maximum inner diameter and defines a greater radial clearance between said vane and the turbine wheel than at least one vane in another of the annular portions.

The vanes are orientated to deflect gas flowing through the annular inlet towards the direction of rotation of the turbine wheel. A radial clearance is defined between each vane and the turbine wheel.

The inlet vanes may be provided in annular arrays within each annular portion. The trailing edges of at least some of the vanes extending across a first annular portion of the inlet may lie on a different radius to the trailing edges of at least some of the vanes extending across a second annular portion of the inlet. In some embodiments the trailing edges of all of the vanes extending across a first annular portion of the inlet lie on a radius different to that of the trailing edges of all of the vanes extending across a second annular portion of the inlet. In some embodiments the trailing edges of vanes of one annular portion of the inlet lie on a maximum radius which is different to that of vanes extending across any other annular portion of the inlet.

The vanes in two or more annular arrays may have different maximum inner diameters and define correspondingly different radial clearances between those vanes and the turbine wheel. That is, an array of vanes in a first annular portion may incorporate a plurality of vanes of a first maximum inner diameter which define a first radial clearance between the vanes and the turbine wheel and another array of vanes in a second annular portion, which is axially offset from the first annular portion, may incorporate a plurality of vanes of a different second maximum inner diameter which define a correspondingly different second radial clearance between the vanes and the turbine wheel, the first maximum inner diameter and radial clearance being larger than the second or vice versa as appropriate. In embodiments incorporating three or more annular portions and therefore three or more annular arrays of vanes, the variation in maximum inner diameter of the vanes and the corresponding radial clearance between the vanes and the turbine wheel may decrease progressively between adjacent pairs of annular arrays, or an intermediate array may possess vanes having a maximum inner diameter and which define a corresponding radial clearance that is larger or smaller than the arrays of vanes on either side. The vanes within one or more annular arrays may have different radial extents as compared to that of one or more annular arrays within the same inlet. For example, while at least one vane in one of said annular portions has a greater maximum inner diameter than at least one vane in another of the annular portions and defines a greater radial clearance between itself and the turbine wheel than the vane in the other annular portion, said vanes may have substantially the same maximum outer diameter such that the leading edges of the vanes are essentially continuous across the full width of the annular inlet. In an embodiment including three or more axially spaced annular arrays of vanes the radial position of the trailing edge of each annular array of vanes may decrease from a first annular array to an adjacent second annular array and then further decrease from the second annular array to an adjacent third annular array of vanes.

The baffle(s), inlet formations(s) and/or sliding sleeve may be formed from a material that is a ceramic, a metal or a cermet (a ceramic/metal composite). The metal could be any steel, or a nickel based alloy, such as inconel. Any or all of these components may be provided with a coating, for example on the sliding interface of the nozzle and the sleeve there could be a coating of diamond-like-carbon, anodisation, or tribaloy or a substitute wear resistant coating. The aerodynamic surfaces may be provided with a coating to promote smoothness or resist corrosion. Such coatings could include non-deposited coatings such as a plasma-electrolytic-oxide coating or substitute coatings. A catalyst coating to hinder or prevent the build-up of unwanted sooty deposits could be provided on any surface within the turbine housing, for example any surface of the baffle(s), inlet formation(s) and/or sleeve, which comes into contact with exhaust gases during operation.

In certain embodiments it is preferred that the axially movable sleeve can be moved across substantially the fully axial width of the annular inlet so as to substantially close or entirely close gas flow path through the annular inlet.

In the first aspect of the present invention in addition to the sleeve with an inner diameter that is greater than the outer diameter of the inlet passages a second sleeve may be provided on or adjacent to the inner diameter of one or more of the annular baffle(s), on or adjacent to one or more of the outer diameter of the annular baffle(s), or at any intermediate diameter. In the second aspect of the present invention the sleeve may be provided on or adjacent to the inner diameter of one or more of the annular baffle(s), on or adjacent to one or more of the outer diameter of the annular baffle(s), or at any intermediate diameter, however, it is preferred that the sleeve has an inner diameter that is greater than the outer diameter of the inlet passages.

Preferably the sleeve is moveable with respect to the baffle(s). Thus it is preferred that the baffle(s) is/are substantially fixed in position during operation of the turbine such that variation in the axial width of the annular inlet of the turbine is achieved by axial displacement of the sleeve rather than any movement in the baffle(s).

It is preferred that the sleeve is moveable with respect to the inlet formations, i.e. the vane(s) and/or any other kind of flow-guiding structure provided in the annular inlet, such as a honeycomb-type flow-guide. Thus, the inlet formations are preferably substantially fixed in position during operation of the turbine such that variation in the axial width of the annular inlet of the turbine is achieved by axial displacement of the sleeve rather than any movement in the inlet formations.

In the first aspect of the present invention there may be a single baffle so as to divide the annular inlet into two axially offset inlet portions. Alternatively, there may be two axially offset baffles disposed within the annular inlet so as to define three axially offset inlet portions. As a further alternative there may be two or more axially offset baffles disposed within the annular inlet, as in the second aspect of the present invention, so as to define three or more axially offset inlet portions.

The number of vanes in each annular array may differ. For instance, an annular array of fifteen vanes may be included in the same nozzle assembly as an annular array of only eight vanes. Other arrays may have a different number of vanes, greater than fifteen or fewer than eight, or somewhere in between, e.g. twelve. In addition, vanes having different radial extents, and different swirl angles may be used, e.g. some vanes swept forwards to a greater extent than others, and as such defining a greater swirl angle. In a preferred embodiment the swirl angle of a vane or annular array of vanes is larger than the swirl angle of a vane or annular array of vanes that is axially offset.

It is preferred that the relationship between the swirl angle of one array of vanes compared to an axially offset array of vanes is generally similar to the variation in vane maximum inner diameter and clearance between the vanes and the turbine wheel in so far as an increase in one parameter is accompanied by an increase the other two parameters. By way of example, where a first array of vanes defines a first maximum inner diameter, a first radial clearance between the vanes and the turbine wheel and a first swirl angle, a second axially offset array of vanes may define a second maximum inner diameter, a second radial clearance and a second swirl angle in which all of said first parameters are larger than all of the corresponding second parameters. In a preferred embodiment the three parameters progressively increase from one side of the inlet to the opposite side, most preferably from the "open side" of the inlet, i.e. the side nearest to the annular portion furthest from a closed position of the sleeve where the gas flow path through the inlet is narrowest, towards the "closed side" of the inlet, i.e. the annular portion closest to the closed position of the sleeve. For certain engine applications (such as for exhaust gas recirculation, "EGR") it may be desirable to reduce the turbine efficiency in one or more of the arrays of inlet passageways. For example, it may be desirable to reduce efficiency at relatively open inlet widths in some applications. Such reduced efficiency could for instance be achieved by reducing the radial extent of the vanes (as discussed above), increasing the circumferential width of the vanes, or otherwise configure the vanes to reduce the effective inlet area.

The leading edges of at least some of the vanes extending across a first annular portion of the inlet may lie on a different radius to the leading edges of at least some of the vanes extending across a second annular portion of the inlet. In some embodiments the leading edges of all of the vanes extending across a first annular portion of the inlet lie on a radius different to that of the leading edges of all of the vanes extending across a second annular portion of the inlet. In some embodiments the leading edges of vanes of one annular portion of the inlet lie on a minimum radius which is different to that of vanes extending across any other annular portion of the inlet.

According to another aspect of the present invention there is provided a variable geometry turbine comprising a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; and a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; wherein the annular inlet is divided into a first annular inlet portion and a second annular inlet portion axially offset from the first inlet portion, inlet vanes extending axially into each of the first and second inlet portions, the inlet vanes defining axially adjacent inlet passages; wherein the configuration of the inlet vanes extending into the first inlet portion differs from the configuration of the inlet vanes extending into the second inlet portion.

It will be appreciated that axially offset inlet passages include inlet passages with different axial positions and/or inlet passages with different axial extents. Axially offset inlet passages may be spaced apart, adjacent or axially overlapping.

The first and second inlet portions may be adjacent one another.

The configuration of the inlet vanes extending into the first inlet portion may differ from the configuration of the inlet vanes extending into the second inlet portion in that the number of inlet vanes extending into the first inlet portion differs from the number of inlet vanes extending into the second inlet portion.

It will be appreciated that features of any one or more of the above defined aspects of the present invention, and optional features thereof, may be combined together in any desirable arrangement in a variable geometry turbine, subject of course to technical constraints that would be evident to the skilled person.

The baffle(s), vane(s) and/or sliding sleeve may be formed from a material that is a ceramic, a metal or a cermet (a ceramic/metal composite). The metal could be any steel, or a nickel based alloy, such as inconel. Any or all of these components may be provided with a coating, for example on the sliding interface of the nozzle and the sleeve there could be a coating of diamond-like-carbon, anodisation, or tribaloy or a substitute wear resistant coating. The aerodynamic surfaces may be provided with a coating to promote smoothness or resist corrosion. Such coatings could include non-deposited coatings such as a plasma-electrolytic-oxide coating or substitute coatings.

It should be appreciated that exhaust gas typically flows to the annular inlet from a surrounding volute or chamber. The annular inlet is therefore defined downstream of the volute, with the downstream end of the volute terminating at the upstream end of the annular inlet. As such, the volute transmits the gas to the annular inlet, while the gas inlet passages of the present invention receive gas from the volute. In some embodiments, the first and second inlet sidewalls which define the annular inlet are continuations of walls which define the volute. The annular inlet may be divided into at least two axially offset inlet passages by one or more baffles located in the annular inlet, and which are therefore positioned downstream of the volute.

The turbine of the present invention has been illustrated in the figures using a single flow volute, however it is applicable to housings that are split axially, whereby gas from one or more of the cylinders of an engine is directed to one of the divided volutes, and gas from one or more of the other cylinders is directed to a different volute. It is also possible to split a turbine housing circumferentially to provide multiple circumferentially divided volutes, or even to split the turbine housing both circumferentially and axially. It should be appreciated, however, that an axially or circumferentially divided volute is distinguished from the multiple gas inlet passages present in the turbine of the present invention. For example, the gas inlet passages relate to a nozzle structure arranged to accelerate exhaust gas received from the volute towards the turbine, and optionally to adjust or control the swirl angle of the gas as it accelerates. The multiple gas inlet passages forming part of the present invention may be further distinguished from a divided volute arrangement in that, while the gas inlet passages receive gas from the volute (or divided volute), and split the gas into an array of paths directed on to the turbine, a divided volute receives gas from the exhaust manifold so as to retain the gas velocity in gas pulses resulting from individual engine cylinder opening events.

It will be appreciated that axially offset inlet passages include inlet passages with different axial positions and/or inlet passages with different axial extents. Axially offset inlet passages may be spaced apart, adjacent or axially overlapping. Advantageous and preferred features of the invention will be apparent from the following description. Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is an axial cross-section through a conventional turbocharger.

Figure 2a is an axial cross-section through a turbine volute and annular inlet of a turbine according to an embodiment of the present invention;

Figure 2b is an axial cross-section through a turbine volute and annular inlet of a turbine according to a further embodiment of the present invention;

Figure 2c is an axial cross-section through a turbine volute and annular inlet of a turbine according to another embodiment of the present invention;

Figure 2d is an axial cross-section through a turbine volute and annular inlet of a turbine according to a still further embodiment of the present invention;

Figure 2e is an axial cross-section through a turbine volute and annular inlet of a turbine according to a yet further embodiment of the present invention;

Figure 3 is a perspective illustration of components of a section of a nozzle structure forming part of a turbine according to the embodiment of figure 2a composed of an inlet sidewall, baffles, vanes and an axially slidable sleeve;

Figure 4 is an illustration of components of a section of a nozzle structure forming part of a turbine according to the embodiment of figure 2b composed of an inlet sidewall, baffles, vanes and an axially slidable sleeve - (A) is a perspective view of said section of the nozzle structure, (B) shows radial cross-sectional views of the three arrays of vanes and their respective sidewall or baffle, and (C) shows detail views of a vane in each of said three arrays of vanes;

Figure 5 is an illustration of components of a section of a nozzle structure forming part of a turbine according to the embodiment of figure 2c composed of an inlet sidewall, baffles, vanes and an axially slidable sleeve - (A) is a perspective view of said section of the nozzle structure, (B) shows radial cross-sectional views of the three arrays of vanes and their respective sidewall or baffle, and (C) shows detail views of a vane in each of said three arrays of vanes;

Figure 6 is an illustration of components of a section of a nozzle structure forming part of a turbine according to the embodiment of figure 2d composed of an inlet sidewall, baffles, vanes and an axially slidable sleeve - (A) is a perspective view of said section of the nozzle structure, and (B) shows radial cross-sectional views of the three arrays of vanes and their respective sidewall or baffle;

Figure 7 is an illustration of components of a section of a nozzle structure forming part of a turbine according to the embodiment of figure 2e composed of an inlet sidewall, baffles, vanes and an axially slidable sleeve - (A) is a perspective view of said section of the nozzle structure, and (B) shows radial cross-sectional views of the three arrays of vanes and their respective sidewall or baffle;

Figures 8a to 8c are schematic illustrations of further embodiments of the present invention;

Figures 9a to 9c are schematic illustrations of further embodiments of the present invention;

Figures 10 to 10e schematically illustrate components of a further embodiment of the present invention;

Figures 11a to 1 1e schematically illustrate components of a further embodiment of the present invention;

Figures 12a to 12e schematically illustrate components of a further embodiment of the present invention; and

Figures 13a to 13f, are each schematic illustrations of a radial view around a portion of the circumference of a respective inlet structure in accordance with various embodiments of the present invention. Referring to Figure 1 , the turbocharger comprises a turbine 1 joined to a compressor 2 via a central bearing housing 3. The turbine 1 comprises a turbine wheel 4 for rotation within a turbine housing 5. Similarly, the compressor 2 comprises a compressor wheel 6 which can rotate within a compressor housing 7. The turbine wheel 4 and compressor wheel 6 are mounted on opposite ends of a common turbocharger shaft 8 which extends through the central bearing housing 3.

The turbine housing 5 has an exhaust gas inlet volute 9 located annularly around the turbine wheel 4 and an axial exhaust gas outlet 10. The compressor housing 7 has an axial air intake passage 11 and a compressed air outlet volute 12 arranged annularly around the compressor wheel 6. The turbocharger shaft 8 rotates on journal bearings 13 and 14 housed towards the turbine end and compressor end respectively of the bearing housing 3. The compressor end bearing 14 further includes a thrust bearing 15 which interacts with an oil seal assembly including an oil slinger 16. Oil is supplied to the bearing housing from the oil system of the internal combustion engine via oil inlet 17 and is fed to the bearing assemblies by oil passageways 18.

In use, the turbine wheel 4 is rotated by the passage of exhaust gas from the annular exhaust gas inlet 9 to the exhaust gas outlet 10, which in turn rotates the compressor wheel 6 which thereby draws intake air through the compressor inlet 11 and delivers boost air to the intake of an internal combustion engine (not shown) via the compressor outlet volute 12.

In figure 2a there is shown a turbine volute 20 and annular inlet 21 of a turbine 22 according to an embodiment of the present invention. Equiaxially spaced across the inlet 21 are two annular baffles 23a, 23b which, together with inner and outer sidewalls 24, 25 of the inlet, define three axially offset annular inlet portions 26a, 26b, 26c of equal axial width. Extending axially across each of the three inlet portions 26a-c are respective annular arrays of vanes 27a, 27b, 27c having differing arrangements so as to constrict the area accessible to gas flowing through the annular arrays 27a-c to differing extents.

Figure 3 is an illustration of components of a section of a nozzle structure forming part of a turbine according to the embodiment of figure 2a. A perspective view of the nozzle structure is shown which comprises of an inlet sidewall 30, first and second axially spaced baffles 31 a, 31 b, three annular arrays of axially extending vanes 32a, 32b, 32c and an axially slidable sleeve 33. Each array of vanes 32a-c is comprised of a plurality of vanes 34a, 34b, 34c. Of the three arrays 32a-c, the array 32c furthest from the "closed position" of the sleeve 33, i.e. when the sleeve 33 covers the entire turbine inlet and overlies the sidewall 30, includes the smallest number of vanes 34c. The middle array 32b contains more vanes 32b, while the array 32a closest to the "closed position" of the sleeve 33, i.e. the array 32a which lies in the annular inlet portion which is bordered on one side by the inlet sidewall 30, contains the largest number of vanes 34a. In this way, the array 32a closest to the "closed position" of the sleeve 33 presents the greatest constriction to gas flowing through the annular inlet, while the array 32c lying furthest away from the "closed position" of the sleeve 33 presents the least constriction to gas flow through the annular inlet.

In figure 2b there is shown a turbine volute 120 and annular inlet 121 of a turbine 122 according to an embodiment of the present invention. Equiaxially spaced across the inlet 121 are two annular baffles 123a, 123b which, together with inner and outer sidewalls 124, 125 of the inlet, define three axially offset annular inlet portions 126a, 126b, 126c of equal axial width. Extending axially across each of the three inlet portions 126a-c are respective annular arrays of vanes 127a, 127b, 127c of differing maximum circumferential thickness, i.e. width in radial cross-section, for instance as viewed in figure 4B or 4C.

Figure 4 is an illustration of components of a section of a nozzle structure forming part of a turbine according to the embodiment of figure 2b. A perspective view of the nozzle structure is shown in Figure 4(A) and comprises of an inlet sidewall 130, first and second axially spaced baffles 131a, 131 b, three annular arrays of axially extending vanes 132a, 132b, 132c and an axially slidable sleeve 133. Figure 4(B) shows radial cross-sectional views of the three annular arrays of vanes 132a-c comprised in the nozzle structure shown in Figure 4(A). Figure 4(C) shows a detailed radial cross-sectional view of a respective vane 134a, 134b, 134c in each of the three arrays of vanes 132a-c. The circumferential thickness of each vane 134a-c in each array 132a-c is indicated by a double headed arrow within each vane 134a-c in figure 4(C).

As can be observed from figures 4(B) and 4(C) the vanes 134c in the array 132c furthest from the "closed position" of the sleeve 133, i.e. when the sleeve 133 covers the entire turbine inlet and overlies the sidewall 130, are circumferentially thinner and thereby define a smaller radial cross-sectional area than the vanes 134b in the middle array 132b, which are in turn, circumferentially thinner than the vanes 134a in the array 132a closest to the "closed position" of the sleeve 33, i.e. the vanes 34a which lie in the annular inlet portion which is bordered on one side by the inlet sidewall 130. In the embodiment illustrated in figure 4 the three arrays of vanes 132a-c each contain the same total number of vanes 134a-c and each define a similar swirl angle. It will be appreciated however that in alternative embodiments the number of vanes in an array may vary from one array to another, and/or the swirl angle defined by vanes in an array may vary from that defined by vanes in other arrays in the same nozzle structure.

In figure 2c there is shown a turbine volute 220 and annular inlet 221 of a turbine 222 according to an embodiment of the present invention. Equiaxially spaced across the inlet 221 are two annular baffles 223a, 223b which, together with inner and outer sidewalls 224, 225 of the inlet, define three axially offset annular inlet portions 226a, 226b, 226c of equal axial width. Extending axially across each of the three inlet portions 226a-c are respective annular arrays of vanes 227a, 227b, 227c of differing maximum circumferential thickness, i.e. width in radial cross-section, for instance as viewed in figure 5B or 5C.

Figure 5 is an illustration of components of a section of a nozzle structure forming part of a turbine according to the embodiment of figure 2c. A perspective view of the nozzle structure is shown in Figure 5(A) and comprises of an inlet sidewall 230, first and second axially spaced baffles 231 a, 231 b, three annular arrays of axially extending vanes 232a, 232b, 232c and an axially slidable sleeve 233. Figure 5(B) shows radial cross-sectional views of the three annular arrays of vanes 232a-c comprised in the nozzle structure shown in Figure 5(A). Figure 5(C) shows a detailed radial cross-sectional view of a respective vane 234a, 234b, 234c in each of the three arrays of vanes 232a-c. The thickness of each respective leading edge 235a, 235b, 235c of each vane 234a-c in each array 232a-c is directly related to a respective angle 236a, 236c, 236c defined as shown in figure 5(C). The maximum circumferential thickness of each vane 234a-c in each array 232a-c is indicated by a double headed arrow within each vane 234a-c in figure 5(C).

As can be observed from figures 5(B) and 5(C) the vanes 234c in the array 232c furthest from the "closed position" of the sleeve 233, i.e. when the sleeve 233 covers the entire turbine inlet and overlies the sidewall 230, have thinner leading edges 235c, which in turn have thinner leading edges 235b than the vanes 234a in the array 232a closest to the "closed position" of the sleeve 233, i.e. the vanes 234a which lie in the annular inlet portion which is bordered on one side by the inlet sidewall 230. In spite of the difference in leading edge thickness, the vanes 234a-c in the three arrays of vanes 232a-c all possess substantially the same circumferential thickness (indicated by a double headed arrow within each vane in figure 5(C)). In an alternative embodiment, the vanes 234a-c in the three arrays 232a-c may have different maximum circumferential thicknesses, for instance, the array of vanes 232a with the thickest leading edges 235a may also possess the largest maximum circumferential thickness as compared to the other two arrays 232b- c. In the embodiment illustrated in figure 5 the three arrays of vanes 232a-c each contain the same total number of vanes 234a-c and each define a similar swirl angle. It will be appreciated however that in alternative embodiments the number of vanes in an array may vary from one array to another, and/or the swirl angle defined by vanes in an array may vary from that defined by vanes in other arrays in the same nozzle structure.

In figure 2d there is shown a turbine volute 320 and annular inlet 321 of a turbine 322 according to an embodiment of the present invention. Equiaxially spaced across the inlet 321 are two annular baffles 323a, 323b which, together with inner and outer sidewalls 324, 325 of the inlet, define three axialiy offset annular inlet portions 326a, 326b, 326c of equal axial width. Extending axialiy across each of the three inlet portions 326a-c are respective annular arrays of vanes 327a, 327b, 327c of differing maximum outer diameter, i.e. width in radial cross-section. As can be seen in figure 2d, the vane 327a has a smaller radial extent and thus defines a smaller maximum outer diameter than the two other vanes 327b-c. This is further described below in relation to figure 6.

Figure 6 is an illustration of components of a section of a nozzle structure forming part of a turbine according to the embodiment of figure 2d. A perspective view of the nozzle structure is shown in Figure 6(A) and comprises of an inlet sidewall 330, first and second axialiy spaced baffles 331 a, 331 b, three annular arrays of axialiy extending vanes 332a, 332b, 332c and an axialiy slidable sleeve 333. Figure 6(B) shows radial cross-sectional views of the three annular arrays of vanes 332a-c comprised in the nozzle structure shown in figure 6(A). Each array of vanes 332a-c is comprised of a plurality of equiangularly spaced vanes 334a, 334b, 334c of similar radial cross-sectional profile in that the leading edge of each vane 334a-c is the same thickness, the maximum circumferential thickness of each vane 334a-c is the same, and the radial cross-sectional area of each vane 334a-c is the same.

As can be observed from figure 6(B) the vanes 334b-c in the arrays 332b-c furthest from the "closed position" of the sleeve 333, i.e. when the sleeve 333 covers the entire turbine inlet and overlies the sidewall 330, extend radially outwards to a greater extent and thereby define a greater maximum outer diameter than the vanes 334a in the array 332a closest to the "closed position" of the sleeve 333, i.e. the vanes 334a which lie in the annular inlet portion which is bordered on one side by the inlet sidewall 330. In the embodiment shown in figure 6 the vanes 334a-c in the three arrays 332a-c all possess trailing edges lying on the same inner radius, i.e. defining the same maximum inner diameter. This does not have to be the case, however. One or more arrays 332a-c may define a greater maximum inner diameter than one or more other arrays 332a-c. Moreover, in a further alternative embodiment the arrays of vanes 332a-c may each define a different maximum outer diameter.

In the embodiment illustrated in figure 6 the three arrays of vanes 332a-c each contain the same total number of vanes 334a-c and each define a similar swirl angle. It will be appreciated however that in alternative embodiments the number of vanes in an array may vary from one array to another, and/or the swirl angle defined by vanes in an array may vary from that defined by vanes in other arrays in the same nozzle structure.

In figure 2e there is shown a turbine volute 420 and annular inlet 421 of a turbine 422 according to an embodiment of the present invention. Equiaxially spaced across the inlet 421 are two annular baffles 423a, 423b which, together with inner and outer sidewalls 424, 425 of the inlet, define three axially offset annular inlet portions 426a, 426b, 426c of equal axial width. Extending axially across each of the three inlet portions 426a-c are respective annular arrays of vanes 427a, 427b, 427c of differing maximum inner diameter, i.e. width in radial cross-section. As can be seen in figure 2e, the array of vanes 427a has a smaller radial extent and defines a greater maximum inner diameter and a greater radial clearance between the vanes 427a and the turbine wheel 428 than the middle vanes 427b. In a similar way, the middle array of vanes 427b has a smaller radial extent and defines a greater maximum inner diameter and a greater radial clearance between the vanes 427b and the turbine wheel 428 than the vanes 427c. This is further described below in relation to figure 7.

Figure 7 is an illustration of components of a section of a nozzle structure forming part of a turbine according to the embodiment of figure 2e. A perspective view of the nozzle structure is shown in Figure 7(A) and comprises of an inlet sidewall 430, first and second axially spaced baffles 431 a, 431 b, three annular arrays of axially extending vanes 432a, 432b, 432c and an axially slidable sleeve 433. Figure 7(B) shows radial cross-sectional views of the three annular arrays of vanes 432a-c comprised in the nozzle structure shown in Figure 7(A). Each array of vanes 432a-c is comprised of a plurality of equiangularly spaced vanes 434a, 434b, 434c of similar radial cross-sectional profile in that the leading edge of each vane 434a-c is the same thickness, the maximum circumferential thickness of each vane 434a-c is the same, and the radial cross-sectional area of each vane 434a-c is the same.

As can be observed from figure 7(B) the vanes 434c in the array 432c furthest from the "closed position" of the sleeve 433, i.e. when the sleeve 433 covers the entire turbine inlet and overlies the sidewall 430, extend radially inwards to a greater extent and thereby define a smaller maximum inner diameter than the vanes 434b in the middle array 432b, which, in turn, define a smaller maximum inner diameter than the vanes 434a in the array 432a closest to the "closed position" of the sleeve 433, i.e. the vanes 434a which lie in the annular inlet portion which is bordered on one side by the inlet sidewall 430. Moreover, the radial clearance defined between the trailing edges of the vanes 434a-c and the turbine wheel (not shown in figure 7) increases progressively from the array 434c furthest from the closed position of the sleeve to the array 434a closest to the closed position of the sleeve. By virtue of the different orientation of the vanes 432a-c within each array 434a-c the swirl angle generated by the arrays of vanes 434a-c also increases progressively from the array 434c furthest from the closed position to the array 434a closest to the closed position.

In the embodiment shown in figure 7 the vanes 434a-c in the three arrays 432a-c all possess leading edges lying on the same outer radius, i.e. defining the same maximum outer diameter. This does not have to be the case, however. One or more arrays 432a-c may define a greater maximum outer diameter than one or more other arrays 432a-c. Moreover, in a further alternative embodiment two of the arrays of vanes 432a-c may define a first maximum inner diameter which is different to that of the other of the arrays 432a-c.

In the embodiment illustrated in figure 7 the three arrays of vanes 432a-c each contain the same total number of vanes 434a-c. It will be appreciated however that in alternative embodiments the number of vanes in an array may vary from one array to another in the same nozzle structure. Referring now to figure 8a, it can be seen that vanes 537 are not continuous across the full width of the turbine annular inlet, but rather vanes defining each of the annular arrays of inlet passages 539a - 539d have different radial extents. Whilst the leading edges of all of the vanes 537 lie on the same outer radius, the radius of the trailing edges of the vanes differ, in that the radial position of the trailing edge of each annular array of vanes decreases progressively from the first annular array 539a to the fourth annular array 539d. In addition, it can be seen that the inlet baffles 538a - 538c have a greater radial extent than at least some of the vanes 537 (in the illustrated embodiment it is greater than that of any of the vanes). That is, whilst they have substantially the same outer radius as the vanes 537, the inner radius of the baffles 538a - 538c is significantly less than that of the vanes 537, so that the baffles 538a - 538c extend further towards the turbine wheel 505 than the vanes 537. In this particular embodiment each of the baffles 538a - 538c has the same radial dimension but this may not be the case in other embodiments. In addition, embodiments in which the baffles extend closer to the turbine wheel than the vanes may include embodiments in which the vanes all have the same radial extent. To offer a significant turbine efficiency improvement, the baffles preferably have a radial extent greater than 1 10% of that of at least those vanes that do not extend as close to the wheel as the baffle, more preferably greater than 120%. Where at least some of the gas passages have a relatively radial swirl direction (e.g. at an average angle of greater than 40 degrees to the circumferential direction) the baffles preferably have a radial extent greater than 120% of that of at least those vanes that do not extend as close to the wheel as the baffle, more preferably greater than 140%. Where at least some of the gas passages have a very radial swirl direction (e.g. at an average angle of greater than 60 degrees to the circumferential direction) the baffles preferably have a radial extent greater than 140% of that of at least those vanes that do not extend as close to the wheel as the baffle, more preferably greater than 160%.

Also apparent from figure 8a, the axial spacing of the inlet baffles 538a - 538c is irregular so that whilst the width of the annular arrays of inlet passages 539b and 539c is the same, the axial width of the annular array 539a is greater than that of 538b and 538c, and the axial width of annular array 539d is less than that of axial arrays 538b and 538c.

Although not apparent from figure 8a, but illustrated in figures 8b and 8c, the number of vanes in each of the annular arrays 539a to 539d may differ. For instance figure 8b shows an annular array of fifteen vanes and figure 8c shows an annular array of only eight vanes which may be included in the same nozzle assembly. Other arrays may have a different number of vanes, greater than fifteen or fewer than eight, or somewhere in between (e.g. twelve). In addition, figures 8b and 8c show the vanes having different radial extents, and different swirl angles (that is the vanes visible in 8c are swept forwards to a greater extent than the vanes shown in figure 8b, and as such have a greater swirl angle).

The present invention therefore provides a great degree of flexibility in optimising various features of the nozzle to particular requirements and efficiency profiles. For instance, in one embodiment of the invention as illustrated in figures 8a to 8c, there may be eight vanes in the array 539d, twelve vanes in each of the arrays 539b and 539c, and fifteen vanes in the array 539a. The swirl angle may be greatest in the array 539d and decrease progressively to the array 539a. This is just one example and it will be appreciated that many other variations are possible. Various factors may influence the particular nozzle design, which may include minimising turbine high-cycle fatigue (i.e. minimising the forcing function on the blades), and optimising or otherwise tailoring the efficiency and swallowing capacity of the turbine (e.g. providing low efficiency at wide inlet openings which is useful in some applications such as e.g. EGR engines as described below).

For instance, in an embodiment in which the sleeve 530 is actuated from the turbine housing side of the inlet, so that its free end moves towards the bearing housing side of the inlet as the inlet is closed (this possibility is discussed in more detail further below) the arrays of inlet channels 539c and 539d are less able to stimulate vibration and fatigue in the turbine blades because the hub end of the turbine leading edge is more rigidly connected to the turbine hub (by virtue of it being closer to the turbine wheel back face). In some applications of the invention it may be desirable to maximise turbine efficiency at smaller inlet openings and thus the vane arrays 539c and 539d may have a reduced clearance with respect of the turbine wheel (as illustrated) to boost efficiency given that this may not result in any significant vibration/fatigue problem as the turbine blades are more rigidly supported in this region. In addition, increasing the swirl angle of the vanes in the array 539d can offer a slight efficiency increase when the sleeve is at nearly closed positions (in which the leading edge of the sleeve 530 extends beyond the location of the inlet baffle 538c). This would have the additional effect of reducing the rate that the cross-sectional flow area changes as a function of sleeve motion, when the sleeve is nearly closed, which allows the actuator to control the cross-sectional flow area more precisely. For certain engine applications (such as for EGR) it may be desirable to reduce the turbine efficiency in one or more of the arrays of inlet channels 539a - 539d. For instance, it may be desirable to reduce efficiency at relatively open inlet widths in some applications. Such reduced efficiency could for instance be achieved by reducing the radial extent of the vanes (as illustrated) and/or by increasing the circumferential width or otherwise configured of the vanes to reduce the effective inlet area. The inlet area could be reduced further by providing other obstacles to flow, for instance posts extending axially into the channel. The axial width of the array can be reduced to increase effective friction losses, and the swirl angle of the vanes could be configured to provide mixed swirl. Other examples (not illustrated) could include a ring of similar and evenly spaced posts, two or more concentric rings of posts, a ring of unevenly and randomly distributed posts, or even a ring of vanes arranged to reverse the swirl angle of the gas (i.e. to rotate gas in the opposite direction to the turbine).

Other possible examples of vane arrays that might define any given annular array of inlet passages are illustrated in figures 9a - 9c which are axial sections showing an inlet baffle 538 supporting vanes 537. In figure 9a there is a relatively small number of vanes 537 with a relatively high swirl angle. In addition, the vanes are relatively "thick" and extend to a relatively small internal radius to provide a relatively small radial clearance around the turbine wheel. With such an arrangement it is easier for an actuator to achieve high resolution control of the cross-sectional flow area because it varies less for a given sleeve movement. The increased swirl may be useful for a vane array positioned to correspond to relatively small inlet widths, which could provide a small efficiency improvement.

In the embodiment of figure 9b, relatively small "splitter vanes" 537a are located between adjacent pairs of main vanes 537. In this case there are an increased number of vanes compared with the embodiment of figure 9a, but the vanes have a reduced radial extent so that there is a greater radial clearance between the vanes and the turbine wheel. The splitter vanes may be advantageous in some embodiments to reduce vibration excited in the turbine blades.

In the embodiment of figure 9c, the vanes have a "cut-off configuration rather than a full airfoil configuration which can be expected to provide reduced efficiency which may be useful in some applications. In addition, obstructions 537b are located between adjacent vanes 537 which can further reduce efficiency. Further possible embodiments of a nozzle assembly according to the invention illustrated in figures 10a - 10e, 1 1a - 1 1 e, and 12a - 12e. In each case, each of the figures a - d is an axial section showing the vanes of a particular annular array of inlet passages 539 which together constitute five adjacent annular arrays of inlet passages in the nozzle assembly as a whole. Each figure e is an illustration of the combined locations of all the vanes from figures a - d.

Referring first to figures 10a - 10e, it can be seen that each of the annular arrays 539a - 539d comprise different numbers of vanes, which for some embodiments may have different configurations such as curvature and/or swirl angle and/or radial extent and/or thickness etc. However, in each of the arrays there is a vane with a leading edge at 0° (the top of the vane array is seen in the figures) and also at 120° and 240°. This provides support edges across the width of the assembly as a whole (and thus across the width of the inlet as a whole) which can be useful for guiding the sleeve used to vary the inlet width. With a conventional nozzle array, in which vanes extend across the full width of the inlet and are equi-spaced around the circumference of the inlet, the turbine blade produces an even pattern of vane wakes as it sweeps past the trailing edges of the vanes and is thus subjected to one or more main frequencies of vibration. Depending upon the turbine speed these frequencies of vibration may match a resident vibration mode of the blade leading to resonant excitation which contributes to metal fatigue. However, with the illustrated embodiment of the present invention, there are several different patterns of vane wakes, each of which could excite blade vibration at certain speeds, but less strongly than if the blades were aligned circumferentially.

Referring now to the embodiments of figures 12a to 12e, it can be seen that this is very similar to the embodiment of figures 10a to 10e except that the vane at 120° has been moved to 1 12.5° and the vane at 240° has been moved to 225° (it will be appreciated that these are non-limiting example positions, and other position could be chosen including a reverse arrangement with the angles shifted slightly above 120° / 240°).

Accordingly the positions of some of the vanes (between 0° and 240°) are shifted together slightly, while other vanes are shifted apart (from 240° up to 360°/0°). This can alleviate vibration induced by the turbine blade passing each vane and corresponding wake (i.e. 9^th order excitation for the array in fig 12a, 12^th order for that in fig 12b, 15^th order in figure 12d). This is because if the first (squeezed) set of vanes are passed at a rate that begin to induce vibration, these will be followed by a second (stretched) set of vanes that are passed at a different frequency which does not excite the vibration. This is then followed by the first (squeezed) set of vanes again that induces vibration at the resonant frequency but at the wrong phase angle and so forth.

The amount of flow obstruction presented by the vanes is now lower in the top left of each of figures 12a, 12b and 12d. This would ordinarily induce considerable 1 ^st order vibration (1 ^st order vibration is caused by variation in the gas flow between one side of the turbine and the other, so vibration would be induced if the turbine is rotating at one of the resonant frequencies of it's blades). If this is problematic, one option is to provide at least one of the vane arrays (in this case the third array shown in figure 12c) with an extra vane in the "stretched" region so that in this region the vanes are instead "compressed" together. This will for instance be effective when the sliding sleeve is at one or a small number of positions.

The embodiment of figures 1 1 a to 1 1e shows a modification which may be provided in addition to or as an alternative to that illustrated in figure 12a to 12e. Here the vanes in the stretched region (240° to 360°) are thickened to compensate for the reduction in the angular density of vanes. Alternatively or in addition the vanes in the compressed region (120° to 240°) may be thinner. Rather than changing the blade thickness, it would be possible to vary other characteristics of the blades, such as for instance the blade length.

In the embodiments of the invention described above, each inlet baffle is annular and as such extends around the full circumference of the inlet. Each inlet baffle may however be considered to comprise an annular array of adjacent baffle portions defined between adjacent inlet vanes (or vane portions). In some embodiments (not shown), the baffle "portions" of each baffle 538 may be aligned to define the respective annular baffle. However, in alternative embodiments it may for instance be desirable to effectively omit some baffle portions, and in some embodiments it may no longer be possible to identify the equivalent of a single inlet baffle extending annularly around the full circumference of in the inlet.

Non limiting examples of various alternative embodiments are illustrated in Figs 13a to 13f. These figures are schematic radial views of un-rolled portions of the circumference of the respective embodiments. Figure 13a illustrates an embodiment in which inlet vane portions 537a-537d extend between adjacent inlet baffles 538 and between in the baffles 538 and side walls 532 and 533. No single inlet vane 537 is continuous across a baffle 538, with the effect that individual inlet passages 539 are arranged in circumferentially staggered annular arrays 539a-539b (there is circumferential overlap between axially adjacent passages 539).

Figure 13b is a modification of the embodiment shown in figure 8a, in which some vanes 537 do extend across the full width of the inlet, whereas other vane portions extend only between neighbouring baffles 538 or between a baffle 538 and enabling inlet wall 532/533. There are again four annular arrays of circumferentially adjacent inlet passages 539a-539d, but in this case each annular array includes inlet passages 539 of different sizes, in this case some have a rectangular cross-section whereas others have a square cross-section.

Fig 13c illustrates an embodiment of the invention in which inlet vanes 537 extend from the side walls 532 and 533 respectively, but in which no single inlet vane 537 extends the full width of the inlet. The effect in this case is to create four annular arrays of circumferentially adjacent in the passages 539a-539b, wherein the passages adjacent each side wall 532 and 533 have a rectangular cross-section and the passages 539b and 539c define between the baffles 538 have a generally square cross-section.

Fig 3d illustrates an embodiment in which inlet vanes 537 extend only half way across the full width of the inlet, in this case extending from side wall 532 to a central inlet baffle 538b. In this case there only two annular arrays of inlet passages 539a and 539b whereas the "arrays" of 539c and 539d are each replaced by a single annular passage way 539c and 539d respectively.

Although a single 'vaneless' space 539d may be provided without any vanes or other structures crossing it, if two vaneless spaces are provided (as shown in figure 13d) then the baffle separating them will require support. This could for instance be in the form of at least three small axially extending struts spaced around the turbine inlet between that central baffle and a neighbouring baffle or a side wall.

A single vaneless space 519c between one of the side walls 532 or 533 and the annular arrays of passages (i.e. at one axial end of the turbine inlet) may be very beneficial. By including a vaneless space to be exposed when the sleeve is fully open, the flow range of the variable geometry turbine can be considerably increased. Optionally the radially outboard inlet of the vaneless space may be axially wider than the radially inboard outlet (not illustrated).

The embodiments of figures 13e and 13f also comprise at least one annular inlet passage absent any vanes. In the embodiment of Fig 13e, there is a single inlet baffle 538 and vanes 537 extend from side wall 532 to the inlet baffle 538, but do not extend from the inlet baffle 538 to the side wall 533. This creates a first annular array of adjacent inlet passages 539a and a single annular inlet passage 539b. Figure 13f is an extreme example of the embodiments shown in Fig 13e, in which there is only a single vane 537 shown which extends from side wall 532 to the single inlet baffle 538. Where the figure shows only a single vane 537 it is to be understood that there is a diametrically opposed vane 537 so that there are two adjacent semi-circular inlet portions 539a in a first annular array, and a axially adjacent single annular inlet passageway 539b. In practice, there are unlikely to be any applications to the present invention which will require only a single pair of diametrically opposed vanes 537.

In some embodiments there may be at least 6 vanes to help ensure the ends of the vanes are close enough together without being impractically long and inducing excessive gas friction. This may also help the gas to swirl in relatively homogenously (e.g. constant swirl angle around the circumference) which may be difficult to achieve with fewer than 6 vanes. In some embodiments there may be at least 9 vanes, preferably at least 12 and normally at least 14. For instance, such a turbine inlet could have 9-18 vanes, with very small turbocharger turbines suiting perhaps 13-16 vanes and very large automotive ones suiting perhaps 15-18 vanes.

In some embodiments of the invention the skin friction induced by the baffles may be reduced by reducing the radial extent of the baffles and vanes, and hence reducing the vane length. If necessary or desired the number of vanes can be increased to increase the "vane solidity".

It will be appreciated that these are just some of the many different arrangements made possible by the present invention.

Claims

1. A variable geometry turbine comprising:

wherein the configuration of the inlet vanes extending into one of the inlet portions differs from the configuration of the inlet vanes extending into another of the inlet portions and wherein the inner diameter of the sleeve is greater than the outer diameter of the inlet passages.

2. A variable geometry turbine comprising:

3. A turbine according to claim 1 or 2, wherein the sleeve is movable towards the second inlet sidewall so as to narrow the gas flow path through the inlet, and the gas flow path through the inlet passage that is closer to the second inlet sidewall has a cross- sectional area perpendicular to the direction of gas flow along said path that is smaller than the corresponding cross-sectional area of the gas flow path through the inlet passage that is further away from the second inlet sidewall.

4. A turbine according to claim 3, wherein said vanes are provided in annular arrays within each annular portion.

5. A turbine according to claim 4, wherein an array of vanes in a first annular portion defines a plurality of first inlet passages having a first total cross-sectional area perpendicular to the direction of gas flow and another array of vanes in a second annular portion, which is axially offset from the first annular portion, defines a plurality of second inlet passages having a second larger total cross-sectional area perpendicular to the direction of gas flow.

6. A turbine according to claim 4 or 5, wherein the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes defining a plurality of inlet passages having a total cross-sectional area perpendicular to the direction of gas flow which decreases progressively between adjacent pairs of annular arrays.

7. A turbine according to claim 4 or 5, wherein the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within an intermediate array defining a plurality of inlet passages having a total cross-sectional area perpendicular to the direction of gas flow that is larger or smaller than the total cross-sectional area perpendicular to the direction of gas flow of inlet passages defined by the arrays of vanes on either side of the intermediate array.

8. A turbine according to any one of claims 3 to 7 wherein the inlet passages having the smallest total cross-sectional area perpendicular to the direction of gas flow are provided in the annular portion nearest to the second inlet sidewall where the gas flow path through the inlet is narrowest or substantially closed.

9. A turbine according to claim 3, wherein the sum of the minimum circumferential separations between adjacent vanes within the inlet passage closer to the second inlet sidewall is lower than the sum of the minimum circumferential separations between adjacent vanes within the inlet passage further away from the second inlet sidewall.

10. A turbine according to claim 1 or 2, wherein at least one vane in one of said annular portions has a greater maximum circumferential thickness than at least one vane in another of the annular portions.

11. A turbine according to claim 10, wherein said vanes are provided in annular arrays within each annular portion.

12. A turbine according to claim 11 , wherein an array of vanes in a first annular portion incorporates a plurality of vanes of a maximum circumferential thickness and another array of vanes in a second annular portion, which is axially offset from the first annular portion, incorporates a plurality of vanes of a larger maximum circumferential thickness.

13. A turbine according to claim 11 or 12, wherein the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes having a maximum circumferential thickness which decreases progressively between adjacent pairs of annular arrays.

14. A turbine according to claim 11 or 12, wherein the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within an intermediate array having a maximum circumferential thickness that is larger or smaller than the maximum circumferential thickness of vanes in the arrays of vanes on either side of the intermediate array.

15. A turbine according to any one of claims 10 to 14, wherein vanes having larger maximum circumferential thickness are provided in the annular portion(s) nearer to a closed position of the sleeve where the gas flow path through the inlet is narrowest.

16. A turbine according to claim 15, wherein vanes having the largest maximum circumferential thickness are provided in the annular portion nearest to the closed position of the sleeve where the gas flow path through the inlet is narrowest or substantially closed.

17. A turbine according to claim 1 or 2, wherein at least one vane in one of said annular portions has a greater leading edge thickness than at least one vane in another of the annular portions.

18. A turbine according to claim 17, wherein said vanes are provided in annular arrays within each annular portion.

19. A turbine according to claim 18, wherein an array of vanes in a first annular portion incorporates a plurality of vanes of a first leading edge thickness and another array of vanes in a second annular portion, which is axially offset from the first annular portion, incorporates a plurality of vanes of a second larger leading edge thickness.

20. A turbine according to claim 18 or 19, wherein the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes having a leading edge thickness which decreases progressively between adjacent pairs of annular arrays.

21. A turbine according to claim 18 or 19, wherein the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within an intermediate array having a leading edge thickness that is larger or smaller than the leading edge thickness of vanes in the arrays of vanes on either side of the intermediate array.

22. A turbine according to any one of claims 17 to 21 , wherein vanes having a larger leading thickness are provided in the annular portion(s) nearer to a closed position of the sleeve where the gas flow path through the inlet is narrowest.

23. A turbine according to claim 22, wherein vanes having the largest leading edge thickness are provided in the annular portion nearest to the closed position of the sleeve where the gas flow path through the inlet is narrowest or substantially closed.

24. A turbine according to claim 1 or 2, wherein at least one vane in one of said annular portions has a greater maximum outer diameter than at least one vane in another of the annular portions.

25. A turbine according to claim 24, wherein said vanes are provided in annular arrays within each annular portion.

26. A turbine according to claim 25, wherein an array of vanes in a first annular portion incorporates a plurality of vanes of a first maximum outer diameter and another array of vanes in a second annular portion, which is axially offset from the first annular portion, incorporates a plurality of vanes of a second larger maximum outer diameter.

27. A turbine according to claim 25 or 26, wherein the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes having a maximum outer diameter which decreases progressively between adjacent pairs of annular arrays.

28. A turbine according to claim 25 or 26, wherein the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within an intermediate array having a maximum outer diameter that is larger or smaller than the maximum outer diameter of vanes in the arrays of vanes on either side of the intermediate array.

29. A turbine according to any one of claims 24 to 28, wherein vanes having a larger maximum outer diameter are provided in the annular portion(s) nearer to a closed position of the sleeve where the gas flow path through the inlet is narrowest.

30. A turbine according to claim 29, wherein vanes having the largest maximum outer diameter are provided in the annular portion nearest to the closed position of the sleeve where the gas flow path through the inlet is narrowest or substantially closed.

31. A turbine according to claim 1 or 2, wherein at least one vane in one of said annular portions has a greater maximum inner diameter and defines a greater radial clearance between said vane and the turbine wheel than at least one vane in another of the annular portions.

32. A turbine according to claim 31 , wherein said vanes are provided in annular arrays within each annular portion.

33. A turbine according to claim 32, wherein an array of vanes in a first annular portion incorporates a plurality of vanes of a first maximum inner diameter which define a first radial clearance between said vanes and the turbine wheel and another array of vanes in a second annular portion, which is axially offset from the first annular portion, incorporates a plurality of vanes of a second larger maximum inner diameter which define a second larger radial clearance between said vanes and the turbine wheel.

34. A turbine according to claim 32 or 33, wherein the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes having a maximum inner diameter which define a radial clearance between said vanes and the turbine wheel which both decrease progressively between adjacent pairs of annular arrays.

35. A turbine according to claim 32 or 33, wherein the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within an intermediate array having a maximum inner diameter which defines a radial clearance between said vanes and the turbine wheel both of which are larger or smaller than the maximum inner diameter of vanes and a radial clearance between said vanes and the turbine wheel in the arrays of vanes on either side of the intermediate array.

36. A turbine according to any one of claims 31 to 35, wherein vanes having a larger maximum inner diameter and which define a larger radial clearance between said vanes and the turbine wheel are provided in the annular portion(s) nearer to a closed position of the sleeve where the gas flow path through the inlet is narrowest.

37. A turbine according to claim 36, wherein said vanes with a larger maximum inner diameter and which define a larger radial clearance between said vanes and the turbine also define a larger swirl angle.

38. A turbine according to claim 36 or 37, wherein vanes having the largest maximum inner diameter and which define the largest radial clearance between said vanes and the turbine wheel are provided in the annular portion nearest to the closed position of the sleeve where the gas flow path through the inlet is narrowest or substantially closed.

39. A turbine according to claim 38, wherein said vanes with the largest maximum inner diameter and which define the largest larger radial clearance between said vanes and the turbine also define the largest swirl angle.

40. A turbine according to claim 1 or 2, wherein the number of inlet vanes extending into one of the inlet portions differs from the number of inlet vanes extending into another of the inlet portions.

41. A turbine according to claim 40, wherein said inlet portions are adjacent one another.