GB2458191A

GB2458191A - Variable geometry turbine for a turbocharger

Info

Publication number: GB2458191A
Application number: GB0812016A
Authority: GB
Inventors: Timothy James William Proctor
Original assignee: Cummins Turbo Technologies Ltd
Current assignee: Cummins Turbo Technologies Ltd
Priority date: 2008-03-14
Filing date: 2008-07-01
Publication date: 2009-09-16
Anticipated expiration: 2028-07-01
Also published as: GB0812016D0; GB2458191B; GB0804780D0

Abstract

A variable geometry turbine 1 of the kind used in a turbocharger comprises a turbine wheel 4 supported in a housing 5 having a gas flow inlet passageway 13 defined between an annular wall of a movable wall member, such as a nozzle ring 14, and a facing wall such as a shroud plate 15. The nozzle ring 14 is moveable axially relative to the housing 5 to vary a width of the inlet passageway 13. An annular array of vanes 17 extends across the inlet passageway 13, the vanes 17 having an axial height extending from a root 26 to a tip 27. Each of the vanes 17 is receivable in a corresponding slot 18 provided in one of the walls. Some, most or all of the vanes 17 are tapered along their axial extent, eg by increasing the machining tolerance towards the vane tip, such that they are thinner at the tip 27 than the root 26 to avoid sticking of the vanes when the nozzle ring 14 is moved to more open positions. The amount of taper may vary eg non-linearly.

Description

A VARIABLE GEOMETRY TURBINE

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

Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shafi within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housing.

In known turbochargers, the turbine stage comprises a turbine chamber defined by the turbine housing and within which the turbine wheel is mounted, an annular inlet passageway arranged around the turbine chamber, an inlet arranged around the inlet passageway, and an outlet passageway extending from the turbine chamber. The passageways and chambers communicate such that pressurised exhaust gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine chamber and rotates the turbine wheel.

Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the inlet passageway can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suit varying engine demands.

In one common type of variable geometry turbine, one wall of the inlet passageway is defined by a moveable wall member (generally referred to as a "nozzle ring"). The position of the nozzle ring relative to a facing wall (sometimes referred to as the shroud) of the inlet passageway is adjustable to control the width of the inlet passageway. For instance, as gas flowing through the turbine decreases the inlet passageway width may also be decreased to maintain gas velocity and optimise turbine output. Typically the nozzle ring is provided with vanes, which extend into the inlet passageway and through s'ots provided on the facing wall of the inlet passageway to accommodate movement of the moveable nozzle ring. Alternatively, vanes may extend from a fixed wall through slots provided in the nozzle ring. In each case the slots are in the form of slotted apertures i.e. the slots have a closed peripheral edge. When the vanes are received by the slots there is a small clearance between the surface of the vanes and the corresponding edge of the slots. The nozzle ring is generally supported on rods extending parallel to the axis of rotation of the turbine wheel and is moved by an actuator, which is operable to displace the rods in an axial direction.

When the width of the inlet passageway is reduced to a minimum by virtue of the nozzle ring having moved to a position where it is immediately adjacent to the other wall, the gap between the shroud and the nozzle ring affords some measure of leakage flow of exhaust gas to the turbine wheel. For a fixed gas flow rate the velocity of the gas is dependent on the width of the passageway and therefore approaches a maximum value when the nozzle ring is in this position.

The vanes of a nozzle ring have an aerofoil shape with a pair of major vane surfaces meeting at leading edge and a trailing edge, the leading edge being disposed radially outboard of the latter with respect to the turbine axis. The distance between leading and trailing edges in a vane is known as the chord or chordal length whereas the length from the root of the vane to a tip is known as the span. The distance between the major vane surfaces at any point is known as the thickness.

A nozzle vane generally has the same aerodynamic sectional profile along its axial length from root to tip (often referred to as the vane "height" instead of span), that is, in a direction extending substantially parallel to the axis of the turbocharger.

Such a vane would ordinarily provide a consistent small clearance between its surfaces and the edges of the slot regardless of its axial position. In practice it has been found that vanes are generally slightly thicker at the tip compared to the root as a result of deflection of the tip end of the vane away from the cutting tool during the machining process. Given that the clearance between the slot and the surface of the vane is typically less than 0.25mm there is a risk of vanes "sticking" in the slots at high temperatures owing to small differences in behaviour of the vanes and slots under thermal expansion. Simply making the clearance larger to avoid this problem is not an option as it would lead to unacceptable reductions in efficiency when the inlet passageway is at its minimum width.

It is one object of the present invention, amongst others, to obviate or mitigate the aforementioned disadvantage. Another object of the present invention is to provide for an alternative, or an improved, variable geometry turbine.

According to a first aspect of the present invention there is provided a variable geometry turbine comprising a turbine wheel supported in a housing for rotation about a turbine axis, a gas flow inlet passageway upstream of said turbine wheel and extending towards the turbine wheel, the inlet passageway being defined between a first wall and a second wall facing the first wall, at least one of the first and second walls being moveable relative to the housing to vary a width of the inlet passageway, an array of vanes extending around the axis and across the inlet passageway, the vanes being fixed relative to the first wall at a root and receivable in corresponding slotted apertures defined in the second wall, the vanes having an axial length extending substantially parallel to the axis of rotation of the turbine wheel from the root to a tip, the vanes having first and second major surfaces between which is defined a vane thickness, wherein the vanes are tapered along at least a portion of their axial length between the root and the tip such that they decrease in thickness from a first axial position between the root and the tip, the thickness of the vanes at the tip being less than that at the first axial position.

Thus the taper is inwardly directed along at least part of the vane height or span so that the thickness (i.e. the distance between major surfaces of the vanes) decreases along an imaginary axial line. Put another way the vanes are tapered such that they decrease in thickness from the first axial position to a second axial position that is between the first axial position and the tip.

The vanes each have a chordal length extending between a radially outer leading edge and a radially inner trailing edge (with respect to the turbine axis). The chordal length may vary along the axial length of the vane.

The slotted apertures of the second wall are each defined by a closed edge that surrounds the periphery of the corresponding vane with a clearance between the edge and the vane periphery.

It is to be understood that there may be additional vanes besides the array of vanes, those additional vanes not being tapered or at least not being tapered in the same manner as those of the array.

The tapered part of each vane extends along a portion of the axial length between the root and the tip, that is it does not include the root which is generally of a tapered configuration.

The vanes may be continuously tapered along the portion of their axial length or indeed along their entire axial length. Alternatively, they may have a discontinuous taper.

Any tapered portion of the vane may be in a linear progression or alternatively in a non-linear progression. For example, the vane may be tapered along a first portion nearest to the root and then a second portion spaced from the second portion, there being no taper between the first and second portions.

The vanes may be fixed to the first wall, which is moveable, with the second wall being fixed. Alternatively, the vanes may be fixed to the second wall and the first wall may be movable over them. In the latter version the vanes may be fixed relative to the housing.

According to another aspect of the present invention there is provided a turbocharger comprising a compressor and a variable geometry turbine as defined above, the compressor being driven by the variable geometry turbine.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a longitudinal sectioned view through a turbocharger fitted with a variable geometry turbine in accordance with the present invention; Figure IA is an enlarged view of a nozzle ring vane and a shroud of the variable geometry turbine of figure 1; Figure 2 is an end view of a nozzle ring that forms part of the variable geometry turbocharger of figure 1; Figure 3 is a schematic representation of the nozzle ring shown in section from above, an axial taper of the vanes being exaggerated; Figure 4 is an enlarged view of part of the nozzle ring of figure 2, illustrating a tapered vane; Figure 5 is a perspective view of the vane of figure 4, looking towards a leading edge of the vane; and Figure 6 is a graph illustrating different tapered vane profiles.

Referring to Figures 1 and 1A, the illustrated turbocharger comprises a turbine I joined to a compressor 2 via a central bearing housing 3. The turbine 1 comprises a turbine wheel 4 rotating within a turbine housing 5. Similarly, the compressor 2 comprises a compressor wheel 6 that rotates within a compressor housing 7. The turbine wheel 4 and compressor wheel 6 are mounted on opposite ends of a common turbocharger shaft 8 that extends through the central bearing housing 3 and rotates about axis 9.

As is conventional, the bearing housing 3 has a central portion which houses journal bearing assemblies located towards the compressor and turbine ends of the bearing housing respectively.

In use, the turbine wheel 4 is rotated by the passage of exhaust gas passing over it from the internal combustion engine. This in turn rotates the compressor wheel 6 that draws intake air through a compressor inlet 10 and delivers boost air to the inlet manifold of an internal combustion engine via an outlet volute I Oa.

The turbine housing 5 defines an inlet chamber 11 (typically a volute) to which the exhaust gas from an internal combustion engine is delivered. The exhaust gas flows from the inlet chamber 11 to an axially extending outlet passageway 12 via an annular inlet passageway 13 and turbine wheel 4. The inlet passageway 13 is defined on one side by the face of a radial wall of a movable annular wall member 14, commonly referred to as a "nozzle ring", and on the opposite side by an annular shroud plate 15 that forms the wall of the inlet passageway 13 facing the nozzle ring 14. The shroud plate 15 covers the opening of an annular recess 16 in the turbine housing 5.

The speed of rotation the turbine wheel 4 is dependent upon the velocity of the gas passing through the annular inlet passageway 13. For a fixed rate of mass of gas flowing into the inlet passageway, the gas velocity is a function of the gap between the nozzle ring 14 and the shroud 15 that defines the passageway 13 and is adjustable by controlling the axial position of the nozzle ring 14 (as the inlet passageway 13 gap is reduced, the velocity of the gas passing through it increases).

The nozzle ring 14, shown in more detail in figures 2 to 5, supports an array of circumferentially and equi-angularly spaced inlet vanes 17 each of which extends substantially axially across the inlet passageway 13. The vanes 17 are orientated to have a pre-selected angle of attack so as to deflect gas flowing through the inlet passageway towards the direction of rotation of the turbine wheel 4. When the nozzle ring 14 is proximate to the annular shroud plate 15, the vanes 17 project through suitably configured slots 18 (see figure 1 A) in the shroud plate, into the recess 16. The slots are in the form of apertures with a closed edge having a profile almost identical to the profile of the thickest part of the vane but slightly larger so as to provide a clearance The nozzle ring 14 is movable in an axial direction on a pair of diametrically opposed rods 20 that extend in the bearing housing 3 in a direction that is substantially parallel to that of the turbocharger shaft 8. The nozzle ring 14 is mounted on the ends of the rods 20 and, when it is axially disposed such that the gap is at its maximum, it is received in a cavity 21 defined in an end of the bearing housing 3. The rods are movable axially by means of a suitable linkage mechanism driven by an actuator such as a pneumatic actuator or a motor.

Each vane 17 has an aerofoil type profile with opposed first and second major surfaces 22, 23 extending between a leading edge 24 disposed proximate the inlet passageway 13 arid a trailing edge 25 proximate the outer periphery of the turbine wheel 4. Each vane 17 is joined integrally to the radial wall of the nozzle ring 14 at a root 26 and extends in an axial direction (i.e. substantially parallel to the axis of the turbocharger shaft) to a tip 27. The distance from the root 26 to the tip 27 is often referred to as the vane height. The distance between the first and second major surfaces 22, 23 is defined as the vane thickness.

In figure 3 the representation of the vanes is simplified to convey the nature of a tapered feature of the vanes. The section is shown through two vanes in a plane that is substantially perpendicular to the major surfaces 23, 23 of the vanes so as to show the variation in thickness between the surfaces at a given plane between the leading and trailing edges. Each vane 17 is continuously tapered along its axial length from a position adjacent to the root 26 to the tip 27 such that the thickness of the vane at any imaginary axial line along its height reduces in the axial direction. In the embodiments shown the taper is provided by inclination of both the first and second major surfaces 22, 23 of the vane in the axial direction at substantially the same angle (represented by a in figure 3). However, it is to be understood that the angles may be different and, indeed, one of the surfaces may extend substantially parallel to the axis of the shaft 9. Besides the schematic representation in figure 3, the taper can be seen from the reduction in thickness apparent at the leading edge 24 in the direction from root 26 to tip 27 in the view of figure 5.

The clearance between a vane 17 and the edges of the shroud that define its respective slot 18 will be at a minimum when the nozzle ring 14 is moved to the position where the inlet passageway 13 is at the minimum width and increases as the nozzle ring 14 moves away from the shroud 15 to open up the inlet. As the leakage flow through the clearance between the vane and the shroud becomes less dominant at more open nozzle ring positions, the increased size of the clearance has a less significant impact on turbine performance and efficiency. The tapering of the vane can be provided, at least in part, by increasing the machining tolerance towards the vane tip.

By employing a taper rather than increasing the clearance between the vane and the edges of the slot in the shroud, the turbine performance is maintained at small inlet gaps whilst sticking of the vanes is avoided when the nozzle ring is moved to more open positions.

It is to be appreciated that in order to the invention to work most, if not all, of the vanes in the array need to be tapered so as to prevent sticking of the nozzle ring.

However, it is not necessarily the case that the vanes all have to be tapered in the same manner. Moreover, it is intended that the invention is to cover an arrangement in which there may be other vanes provided in the array which are configured so as not to stick but which do not have the taper. For example, a few of the vanes may be configured so as to have sufficient clearance whilst not impacting significantly on the overall leakage flow provided by the nozzle ring when in the closed (or almost closed) position.

It is to be understood that whilst a continuous and linear taper is depicted in the figures other configurations could be used to the same effect. For example, the amount of taper could vary along the axial length. Different examples are illustrated in the graph of figure 7 in which the axial distance d along the vane from the root (x-axis) is plotted against the clearance between the vane and the edge of the slot in the shroud. Plot A is where the vanes have no taper, that is, an imaginary axially extending line on each major surface extends in parallel to the axis of the turbocharger shaft direction so that the clearance remains constant irrespective of the nozzle ring position. Plot B shows the position for the tapered vanes shown in figures 1 to 5, the clearance increasing linearly with nozzle ring retraction. Plot C represents a steeper taper than that represented by plot B. Plot D represents an arrangement in which the taper is a non-linear progression. Plot E shows a vane in which it initially has no taper (adjacent to the root) and is then followed by a constant taper extending towards the tip. An alternative is shown in E' where the constant taper is then followed by a parallel portion extending to the tip.

The amount of taper can be defined by a taper ratio equal to the thickness of the vane at specific position along the tip chord divided by the thickness of the vane at a corresponding axially aligned location on the root chord.

Numerous modifications and variations to the embodiment described above may be made without departing from the scope of the invention as defined in the appended claims. For example, the vanes may be fixed to the housing and the nozzle ring may be moveable over the vanes. Furthermore, the height of the vanes may vary across the width of the vanes by having, for example, a cut-out portion that reduces the vane chord at the tip end in the manner shown in our US patent No. 6,652,224.

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

Claims

CLAIMSA variable geometry turbine comprising a turbine wheel supported in a housing for rotation about a turbine axis, a gas flow inlet passageway upstream of said turbine wheel and extending towards the turbine wheel, the inlet passageway being defined between a first wall and a second wall facing the first wall, at least one of the first and second walls being moveable relative to the housing to vary a width of the inlet passageway, an array of vanes extending around the axis and across the inlet passageway, the vanes being fixed relative to the first wall at a root and receivable in corresponding slotted apertures defined in the second wall, the vanes having an axial length extending in a direction substantially parallel to the axis of rotation of the turbine wheel from the root to a tip, the vanes having first and second major vane surfaces between which is defined a vane thickness, wherein the vanes are tapered along at least a portion of their axial length between the root and the tip such that they decrease in thickness from a first axial position between the root and the tip, the thickness of the vanes at the tip being less than that at the first axial position.
2. A variable geometry turbine according to claim 1, wherein the vanes are continuously tapered along the portion of their axial length.
3. A variable geometry turbine according to claim 2, wherein the vanes are continuously tapered along their entire axial length.
4. A variable geometry turbine according to claim 1, wherein the vanes are discontinuously tapered along the portion of their axial length.
5. A variable geometry turbine according to claim 1, 2 or 3, wherein the vanes are tapered in a linear progression.
6. A variable geometry turbine according to any one of claims 1 to 4, wherein the vanes are tapered in a non-linear progression.
7. A variable geometry turbine according to any preceding claim, wherein the vanes are fixed to the first wall which is moveable and the second wall is fixed.
8. A variable geometry turbine according to any preceding claim, wherein the vanes each have a chordal length extending between a radially outer leading edge and a radially inner trailing edge, the chordal length varying along the axial length.
9. A turbocharger comprising a compressor and a variable geometry turbine according to any one of the preceding claims, the compressor being driven by the variable geometry turbine.