GB2548582A

GB2548582A - Turbine diffuser and method of manufacture of a turbine diffuser

Info

Publication number: GB2548582A
Application number: GB1604777.1A
Authority: GB
Inventors: Donaldson James
Original assignee: Cummins Ltd
Current assignee: Cummins Ltd
Priority date: 2016-03-22
Filing date: 2016-03-22
Publication date: 2017-09-27
Anticipated expiration: 2036-03-22
Also published as: GB201604777D0; GB2548582B

Abstract

A turbine diffuser 45 comprises an annular outer wall 30, an inner wall 46 disposed radially inwardly thereof and defining a flow path therebetween, a support assembly attaching the radially outer surface of the inner wall to the radially inner surface of the outer wall, wherein the inner wall defines at least one aperture that is spaced from the support assembly. The support assembly may comprise a plurality of radial struts 47 that are preferably solid struts. The cross-sectional area of the outer wall may increase linearly from an upstream end 25 to a downstream end 26. The diffuser may form part of a turbine assembly of a turbocharger (figure 8). A method of manufacture of the turbine diffuser of the invention comprises defining a mould cavity having the shape of the inner wall, arranging at least one core (52, figure 7) in a core cavity, solidifying molten metal to form at least the inner wall and the support assembly and removing the at least one core through an aperture defined by the inner wall.

Description

Turbine Diffuser and Method of Manufacture of a Turbine Diffuser

Field of the invention

The present invention relates to a method of manufacture of a turbine diffuser, in particular of a diffuser of a turbine of a turbocharger. The present invention also relates to a turbine diffuser, in particular to a diffuser of a turbine of a turbocharger.

Background of the invention

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 turbo charger 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. The turbo charger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems.

In known turbo chargers, the turbine stage comprises a turbine chamber within which a turbine wheel is mounted, an annular inlet passageway defined between facing radial walls arranged around the turbine chamber, an inlet arranged around the inlet passageway and an outlet passageway extending from the turbine chamber.

The passageways and chamber communicate such that pressurised exhaust gas admitted to the turbine inlet flows through the inlet passageway to the outlet passageway via the turbine chamber and rotates the turbine wheel. It is known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passageway so as to direct gas flowing through the inlet passageway towards the direction of rotation of the turbine wheel.

Turbines may be of the 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 by the turbine can be varied to suit varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level which ensures efficient turbine operation by reducing the size of the annular inlet passageway. Turbochargers provided with a variable geometry turbine are referred to as variable geometry turbocharges.

In one known type of the variable geometry turbine an axially movable wall member, generally referred to as a “nozzle ring” defines one wall of the inlet passageway. The position of the nozzle ring relative to a facing wall of the inlet passageway is adjustable to control the axial width of the inlet passageway. Thus, for example, as gas flow through the turbine decreases, the inlet passageway width may be decreased to maintain gas velocity and optimise turbine output. The nozzle ring is provided with vanes which extend into the inlet and through slots provided in a “shroud” defining the facing wall of the inlet passageway to accommodate movement of the nozzle ring. The vanes are at a fixed angle relative to the nozzle ring. A variable geometry turbocharger including such a variable geometry turbine is for instance disclosed in US 5,868,552.

In different arrangements the nozzle ring is fixed and the shroud is axially movable so as to control the axial width of the inlet passageway.

It is known to provide the turbine with a diffuser that extends from an inlet, provided at the outlet of the turbine, to an outlet. A known turbine diffuser comprises an annular outer wall, that encircles a longitudinal axis, and an annular inner wall disposed within the annular outer wall such that an annular flowpath is defined between a radially inner surface of the outer wall and a radially outer surface of the inner wall. The flowpath extends from the diffuser inlet to the diffuser outlet. The inner wall is generally solid and has a longitudinal axis that is coaxial with the longitudinal axis of the outer body.

The respective diameters of the radially inner surface of the outer wall and the radially outer surface of the inner wall vary with axial position, from the diffuser inlet to the diffuser outlet, such that the flowpath generally increases in cross-sectional area from the inlet to the outlet. This increasing cross-sectional area increases the static pressure of the flow, which thereby decreases the ratio of the total pressure to the static pressure in the flow (the total pressure remains constant). In this respect, the dynamic pressure of the flow decreases as the flow is slowed down as it passes through the diffuser.

In this way, the diffuser acts to expand the flow and thereby recover energy from the flow.

The inner wall also forces the flow radially outwardly, which allows the angle of the radially inner surface of the outer wall, relative to the longitudinal axis, to be relatively large, thereby providing a more efficient diffusion process. A turbine diffuser is conventionally manufactured using a sand casting process. In sand casting a die is located around a sand core. A suitable bonding agent (usually clay) is typically mixed with the sand and the mixture is moistened, typically with water, but sometimes with other substances, to provide the strength and plasticity of the core suitable for moulding. The sand is compacted within, or around, a mould to provide the required shape of the core.

In order to cast a particular component, a die is positioned to enclose the sand core to define a mould cavity between an inner surface of the die and an outer surface of the sand core, the mould cavity having the shape of the component. Molten metal is then injected into the mould cavity. Once the molten metal cools and solidifies, the die is removed and the sand core is removed from the inside of the component, for example by tipping the sand particles out through a suitable outlet path through the component.

In order to use sand casting to make a one-piece diffuser having the arrangement of an outer wall and an inner wall as described above, it is necessary to use multiple cores. Typically, a number of sand cores are arranged to define a mould cavity that has the shape of the outer wall and inner wall, joined together by a plurality of radial struts that extend from a radially outer surface of the inner wall to a radially inner surface of the outer wall.

One of the cores is provided radially inwardly of the other cores such that the section of the mould cavity defined between a radially outer surface of the inner core and a radially inner surface of the outer cores forms the shape of a wall. This wall becomes the inner wall of the turbine diffuser.

The formed inner wall is substantially hollow and, once it has been formed, it is necessary to remove the sand core from within the moulded inner wall. This is achieved by making the radially extending struts, which extend between the inner wall and the outer wall, hollow in order to allow the sand to be tipped out of the inner wall, through the struts and out of the diffuser. In order to do this, it is necessary to make the struts relatively large, in order that the opening provided in the struts is sufficiently large to allow the sand to be tipped out through the struts easily.

However, the use of radial struts having a relatively large diameter is disadvantageous in that it inhibits the flow through the turbine diffuser, due to the obstruction presented to the flow.

In addition, casting the turbine diffuser is relatively expensive due to the requirement for multiple sand cores, each of which could fail, or become misaligned during the casting process.

Summary of the invention

It is an object of the present invention to obviate or mitigate at least some of the problems discussed above. It is also an object of the present invention to provide an improved, or alternative, method of manufacture of a turbine diffuser. It is also an object of the present invention to provide an improved or alternative turbine diffuser.

According to a first aspect of the present invention there is provided a method of manufacture of a turbine diffuser comprising an annular outer wall encircling a longitudinal axis of the turbine diffuser, an inner wall disposed radially inwardly of the outer wall, and a support assembly including at least one support member that attaches a radially outer surface of the inner wall to a radially inner surface of the outer wall, the inner wall encircling a core cavity, and the longitudinal axis of the turbine diffuser extending through the core cavity from an upstream end to a downstream end, with a flowpath being defined between a radially inward surface of the outer wall and an opposed radially outward surface of the inner wall, said method comprising: (a) defining a mould cavity by arranging at least one or more cores, the mould cavity comprising a portion having the shape of the inner wall and a portion having the shape of the support assembly, the core cavity being filled by at least a portion of a first one or more of the cores; (b) providing a molten metal in the mould cavity and solidifying the molten metal to form at least the inner wall and the support assembly; and (c) removing said first one or more cores from the core cavity along at least one core path which extends from the core cavity to the exterior of the inner wall through at least one aperture defined by the inner wall; wherein, for at least at a portion of the radial length of the support assembly from a radially inner end of the support assembly to a radially outer end of support assembly, the core path does not pass through the support assembly. Preferably, the core path does not pass through any of the support assembly, and it may even be spaced from all points of the support assembly.

The above method of manufacture means that the first core(s) do not have to be supported during the casting process by portions of the first core(s) which are internal to the support assembly in the resulting turbine diffuser. This means that the first core(s) can be both simpler and supported more securely, which leads to a reduced risk of the turbine diffuser being defective. In other words, the average cost of producing each non-defective turbine diffuser is reduced.

Furthermore, the above method of manufacture is advantageous in that, once the inner wall has been formed, the first core(s) may be removed from the core cavity, through the at least one aperture. Since the at least one aperture is at least partially external to the support assembly, this removes the need for a bore to be provided in the support assembly, that passes along the radial length of the support assembly and out through the outer wall, in order to allow removal of the core from the core cavity. This increases the structural integrity of the support assembly, which allows it to be reduced in size (e.g. diameter). This reduces the negative effect (“drag”) of the support assembly on the flow through the diffuser.

Furthermore, embodiments of the invention can be formed in which there are fewer cores than in known systems, such as just a single first core and two other cores. The reduced number of cores leads to reduced cost, since there is a cost for producing each core.

Furthermore, as there are fewer cores, which typically also means that they are larger for a given size of the turbine diffuser, they are less likely to fall out of alignment with each other, creating a defective turbine diffuser.

Optionally at least a part of the cross-sectional area of the at least one aperture opens into a region external to the support assembly.

Optionally, the core path may pass through the inner wall. That is, the core path may comprise a bore that passes radially through the inner wall. In this case, the core cavity may be substantially closed at the upstream and downstream ends.

However, more preferably, the at least one aperture is formed at an end of the longitudinal axis, such as at a downstream end of the inner wall and/or at an upstream end of the inner wall.

Optionally the method comprises passing the core as a single piece through the at least one aperture to the exterior of the inner wall.

Alternatively, the method comprises breaking the core up into a plurality of portions and passing the portions of the core through the at least one aperture to the exterior of the inner wall.

The core may be passed out through the at least one aperture under the action of gravity. Alternatively, or additionally, the core may be passed through the at least one core path by a pressure differential applied between the inlet and outlet apertures of the at least one core path. In this respect, the core may be sucked or blown along the at least one core path.

Optionally the one or more core(s) are of a particulate material. The particulate material may be sand or powder for example. It will be appreciated that any suitable particulate material may be used. Furthermore, a non-particulate material could be used. For example, the core(s), and in particular the first core(s), may be made of metal, so they can be used multiple times.

The support member may be substantially solid along at least a portion of its radial length between the outer surface of the inner wall and the inner surface of the outer wall. The support member may be substantially solid along at least 20%, preferably at least 40%, even more preferably at least 60%, still even more preferably at least 80% of its radial length between the outer surface of the inner wall and the inner surface of the outer wall. Indeed, the support member may be substantially solid along substantially its entire radial length between the outer surface of the inner wall and the inner surface of the outer wall.

The at least one support member may be elongate, extending along a longitudinal axis. The longitudinal axis of the at least one support member may extend in the radial direction. In this respect, the longitudinal axis of the at least one support member may be substantially parallel to the radial direction (the radial direction at the circumferential location of the support member) or may be inclined relative to said radial direction.

The support assembly may comprise a plurality of said support members disposed spaced apart circumferentially (and/or spaced apart axially) about the inner wall. The plurality of support members may be substantially axially aligned. The plurality of support members may be radially opposed to each other about the longitudinal axis of the inner wall.

The mould cavity may be at least party defined by at least one die that is arranged with the one or more cores.

Optionally the outer wall is formed by arranging for the mould cavity to further include a portion having the shape of the outer wall, so that the outer wall is formed when the mould cavity is filled with molten material and the molten material is solidified. At least a part of the surface of the portion of the mould cavity having the shape of the outer wall may be provided by the die.

The first core(s) may be supported, following said step of defining the mould cavity, using the at least one aperture. This is advantageous in that the core is supported during the casting process, which prevents the core from becoming mis-aligned during the casting process. For example, the one or more first cores may extend through the at least one aperture, and be supported externally of the core cavity (by other cores and/or by the die).

Optionally as the molten metal is provided in the mould cavity, a surface of the first core(s) is supported along at least a circumferential portion of the first core(s). Optionally, the first cores are supported substantially along the entire circumference of the first cores.

Optionally the cross-sectional area of the inner wall, about its longitudinal axis, increases from its upstream end to its downstream end. The cross-sectional area may increase linearly or non-linearly.

The inner wall may have a substantially circular cross-sectional shape about the longitudinal axis. Alternatively, the inner wall may have an elliptical, oval or any other suitable cross-sectional shape.

The inner wall may be substantially hollow. The core cavity defined by the inner wall may extend from the upstream end to the downstream end of the inner wall. The inner wall may comprise a substantially annular wall, with the core cavity disposed radially inwardly of the annular wall.

The outer wall may be substantially centred on a longitudinal axis that is substantially co-axial with the longitudinal axis of the turbine diffuser. Similarly, the inner wall may be substantially centred on a longitudinal axis that is substantially co-axial with the longitudinal axis of the turbine diffuser

The outer wall may be substantially circular about its longitudinal axis. Alternatively, the outer wall may be oval, elliptical or have any other curved cross-sectional shape about its longitudinal axis.

The outer wall may increase in cross-sectional area from an upstream end to a downstream end of the outer wall. The increase in cross-sectional area may be linear or non-linear.

The radially outer surface of the inner wall and the radially inner surface of the outer body may be arranged such that the flow path defined between these surfaces increases in cross-sectional area from the upstream end to the downstream end of the inner wall.

According to a second aspect of the present invention there is provided a turbine diffuser manufactured according to the method of the first aspect of the invention.

According to a third aspect of the present invention there is provided a turbine diffuser comprising an annular outer wall and an inner wall disposed radially inwardly of the outer wall, the inner wall encircling a longitudinal axis which extends from an upstream end to a downstream end, with a flowpath defined between a radially outer surface of the inner wall and a radially inner surface of the outer wall, the diffuser further comprising a support assembly that attaches the radially outer surface of the inner wall to the radially inner surface of the outer wall, the inner wall defining at least one aperture which is spaced from the support assembly, the inner wall and support assembly being respective portions of a single moulded one-piece unit. The aperture(s) are preferably spaced axially from the support assembly.

The aperture(s) may be at longitudinal end(s) of the inner wall. Indeed the inner wall may define a body in which the leading and/or trailing edges are substantially open.

The outer wall may have a gradually increasing (or at least not substantially decreasing), cross-sectional area (measured perpendicular to the longitudinal axis) along the longitudinal axis from the upstream to the downstream end.

Similarly, the inner wall may have a gradually increasing (or at least not substantially non-decreasing) cross-sectional area (measured perpendicular to the longitudinal axis) along the longitudinal axis from the upstream to the downstream end. For example, preferably the cross-sectional area of the inner wall decreases in the downstream direction by less than 20%, or more preferably less than 10%, from the position of maximum cross-sectional area.

This shape for the inner wall may reduce the drag on gas flowing through the turbine extractor, as compared to an inner wall (as provided in a known turbine diffuser) in which the cross-sectional area decreases significantly towards the upstream end. This is because, by omitting a downstream portion of the inner wall with a reducing cross-section, the turbine diffuser can be made shorter in the longitudinal direction than the known turbine diffuser.

The turbine diffuser of the third aspect may have any of the features of the turbine diffuser described above in relation to the method of the first aspect of the invention.

According to a fourth aspect of the invention there is provided a turbine assembly comprising: a turbine comprising an inlet and an outlet, a turbine chamber disposed between the inlet and the outlet, with a turbine wheel rotatably mounted in the turbine chamber, wherein the turbine outlet is in gas communication with the inlet of a turbine diffuser according to the third aspect of the invention.

The turbine may be of any suitable type, including a twin entry turbine, such as a double flow turbine and a twin flow turbine. Double flow turbines and twin flow turbines have an inlet which includes two separate flow passages separated by a dividing wall. The two separate flow passages which define at least a part of a volute of the turbine meet at the generally annular inlet passageway. In the case of a twin flow turbine, the two separate flow passages meet at the generally annular inlet passageway such that each flow passage supplies a respective portion of the inlet passageway, the two respective portions being axially spaced from one another. In the case of a double flow turbine, the two separate flow passages meet at the generally annular inlet passageway such that each flow passage supplies a respective portion of the inlet passageway, the two respective portions being substantially in the same plane perpendicular to the axis, but being circumferentially separate (which may also be referred to as circumferentially segmented).

According to a fifth aspect of the invention there is provided a turbocharger comprising a turbine according to the third aspect of the invention and a compressor, the compressor comprising a housing defining an inlet and an outlet and an impeller wheel chamber provided between the inlet and the outlet, with an impeller wheel rotatably mounted within the impeller wheel chamber, wherein the turbine wheel is coupled to the impeller wheel such that rotation of the turbine wheel rotates the impeller wheel.

Any features of any of the above aspects of the invention may be combined with any features of any other aspect of the invention in any combination.

Brief description of the figures

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-sectional view of a first known variable geometry turbocharger;

Figure 2 is a perspective view of a known turbine diffuser;

Figure 3 is a cross-sectional view of the turbine diffuser of Figure 2;

Figure 4 illustrates a process for forming the turbine diffuser of Figure 2;

Figure 5 is a perspective view of a turbine diffuser which can be produced by a method according to the invention;

Figure 6 is a cross-sectional view of the turbine diffuser of Figure 5;

Figure 7 illustrates schematically a step of the method according to the invention for forming the turbine diffuser of Figure 5; and

Figure 8 is a cross-sectional view of a turbocharger incorporating a second turbine diffuser which can be produced by a method according to the invention.

Detailed description of the embodiments

Referring to figure 1, this illustrates a variable geometry turbocharger comprising a variable geometry turbine housing 1 and a compressor housing 2 interconnected by a central bearing housing 3. A turbocharger shaft 4 extends from the turbine housing 1 to the compressor housing 2 through the bearing housing 3. A turbine wheel 5 is mounted on one end of the shaft 4 for rotation within the turbine housing 1, and a compressor wheel 6 is mounted on the other end of the shaft 4 for rotation within the compressor housing 2. The shaft 4 rotates about turbocharger axis 4a on bearing assemblies located in the bearing housing 3.

The turbine housing 1 defines an inlet volute 7 to which gas from an internal combustion engine is delivered via an exhaust path 501 (the engine 500 and exhaust path 501 are shown schematically in Figure 1). The exhaust gas flows from the inlet volute 7 to an axial outlet passageway 8 via an annular inlet passageway 9 and the turbine wheel 5. The inlet passageway 9 is defined on one side by a face of a radial wall of a movable annular wall member 11, comprising an annular shroud 12, and on the opposite side by a second wall member, also referred to as a nozzle ring 10, which forms the wall of the inlet passageway 9 facing the annular shroud 12. The shroud 12 defines an annular recess 13 in the annular wall member 11. An end surface of the axial outlet passageway 8 is labelled 24.

The nozzle ring 10 supports an array of circumferentially and equally spaced inlet vanes 14 each of which extends across the inlet passageway 9. The vanes 14 are orientated to deflect gas flowing through the primary inlet passageway 9 towards the direction of rotation of the turbine wheel 5. When the annular shroud 12 is proximate to the nozzle ring 10 the vanes 14 project through suitably configured slots in the shroud 12, into the recess 13.

The position of the annular wall member 11 is controlled by an actuator assembly of the type disclosed in US 5,868,552. An actuator (not shown) is operable to adjust the position of the annular wall member 11 via an actuator output shaft (not shown), which is linked to a yoke 15. The yoke 15 in turn engages axially extending actuating rods 16 that support the annular wall member 11. Accordingly, by appropriate control of the actuator (which may for instance be pneumatic, hydraulic or electric), the axial position of the rods 16 and thus of the annular wall member 11 can be controlled. The speed of the turbine wheel 5 is dependent upon the velocity of the gas passing through the inlet passageway 9. For a fixed rate of mass of gas flowing into the inlet passageway 9, the gas velocity is a function of the width of the inlet passageway 9, the width being adjustable by controlling the axial position of the annular wall member 11. For a fixed rate of mass of gas flowing into the inlet passageway 9, up until the point at which the vanes 14 choke the inlet passageway 9 the narrower the width of the inlet passageway 9, the greater the velocity of the gas passing through the inlet passageway 9.

Figure 1 shows the inlet passageway 9 fully open. The inlet passageway 9 may be closed to a minimum by moving the annular shroud 12 of the annular wall member 11 towards the nozzle ring 10. When the separation between the annular shroud 12 of the annular wall member 11 and the nozzle ring 10 is a minimum (such that the width of the inlet passageway is a minimum), the annular wall member 11 may be said to be in a closed position.

The annular wall member 11 has axially extending radially inner and outer annular flanges 17 and 18 that extend into an annular cavity 19 provided in the turbine housing 1. Inner and outer sealing rings 20 and 21 are provided to seal the annular wall member 11 with respect to inner and outer annular surfaces of the annular cavity 19 respectively, whilst allowing the annular wall member 11 to slide within the annular cavity 19. The inner sealing ring 20 is supported within an annular groove formed in the radially inner annular surface of the cavity 19 and bears against the inner annular flange 17 of the annular wall member 11. The outer sealing ring 20 is supported within an annular groove formed in the radially outer annular surface of the cavity 19 and bears against the outer annular flange 18 of the annular wall member 11.

Gas flowing from the inlet volute 7 to the outlet passageway 8 passes over the turbine wheel 5 and as a result torque is applied to the shaft 4 to drive the compressor wheel 6. Rotation of the compressor wheel 6 within the compressor housing 2 pressurises ambient air present in an air inlet 22 and delivers the pressurised air to an air outlet volute 23 from which it is fed to an internal combustion engine (not shown).

The known turbo-charger of Fig. 1 is to be used with a known turbine diffuser shown in perspective view in Fig. 2, and in cross-section in Fig. 3. The turbine diffuser has a central axis A, an inlet 25 and an outlet 26. The turbine diffuser includes an outer wall 30 and an inner wall 35, both encircling the central axis A. The outer wall 30 of the turbine diffuser of Fig. 2 comprises a first outer frusto-conical portion 31 for insertion into the axial outlet passageway 8, a flange 32 for bearing against the surface 24 of the turbine housing, a second outer frusto-conical portion 33, and a cylindrical outer shroud 34. The inner wall 35 comprises a first inner frusto-conical portion 36, a second inner frusto-conical portion 37, and two end walls 38, 39. The conical portions 36, 37, and end walls 38, 39 together enclose a central space 40. The inner wall 35 is supported from the outer wall 30 by four radially-extending struts 41 (though in variants of the diffuser, the number can be varied), which are equally axially spaced about the central axis A and each contain a respective central void 42. The central void 42 is opens at its radially inner end into the space 40, and at its radially outer end to the outer surface of the outer wall 30. Two of these struts 41 are shown in the cross-sectional view of Fig. 3, which also shows the opening of a central void 42 in a third strut into the central space 40. Figure 2 shows the opening of two of the central voids 42 at the outer surface of the first frusto-conical portion 33 of the outer wall.

Referring to Fig. 4, there is shown the known mould assembly for forming the known turbine diffuser of Figs. 2 and 3. The view is similar to that of Fig. 3: a cross-section in a plane containing the line A which will become the central longitudinal axis of the turbine diffuser. Five sand cores labelled “core 1”, “core 2”, “core 3”, “core 4” and “core 5” are positioned within a die 43 which is shown in cross-section by dashed lines. A suitable bonding agent (usually clay) is typically mixed with the sand and the mixture is moistened, typically with water, but sometimes with other substances, to provide the strength and plasticity of the core suitable for moulding. The sand is compacted within, or around, a mould to provide the required shape of the core.

At this stage, the portions of Fig. 4 which are cross-hatched areas are a mould cavity. Molten metal is then injected into the mould cavity, filling the mould cavity. Note that the inner wall 35 and the struts 41 are formed by metal which is moulded around the outside of Core 1. The outer wall 30 is formed by metal which fills the portion of the mould cavity between cores 4 and 5, and between cores 2 and 3. Once the molten metal cools and solidifies, the die is removed and the cores are removed, thereby revealing the turbine diffuser of Figs. 2 and 3. Core 1 is removed from the inside of the inner wall by tipping the sand particles out through the spaces 42.

Turning to Figs. 5 and 6, there is shown a turbine diffuser 45 which can be produced by a method according to the invention, and which may be used in combination with the turbocharger of Fig. 1. Elements which have the same significance as in the known turbine diffuser of Fig. 2 are labelled by the same reference numerals. The turbine diffuser 45 comprises an annular outer wall 30 having a first outer frusto-conical portion 31 for insertion into the axial outlet passageway 8 of a turbine housing (for example, the one of Fig. 1), a flange 32 for bearing against the surface 24 of the turbine housing, and a second frusto-conical portion 33. The annular inner wall 46 in this case is frusto-conical. The annular inner wall 46 and outer wall are both substantially rotationally symmetric about a central axis A, which is the longitudinal axis of the turbine diffuser. The turbine diffuser is intended for gas flow from the left of Figs 5 and 6 (the upstream end of the longitudinal axis) to the right of Figs. 5 and 6 (the downstream end of the longitudinal axis), between the inner wall 46 and the outer wall 30.

Note that at each point along the longitudinal axis, the cross-section of the inner wall 46 is a circle. Considering successive points along the longitudinal axis in the upstream to downstream direction, the area of the circle centred on each point (i.e. the cross-section of the inner wall) gradually increases. This leads to less drag than the inner wall 35 in the turbine diffuser of Fig. 2, in which the cross-sectional area of the inner wall 30 gradually reduces in the region 37.

Solid struts 47 extend between the second frusto-conical portion 33 and the inner wall 46, and function as a support assembly, which supports the inner wall 46 from the second frusto-conical portion 33 of the outer wall 30. Although two of the struts 47 are visible in Fig. 6, the number of struts may be higher or lower than this. The turbine diffuser defines a flowpath between its inlet 25 (“upstream”) and its outlet 26 (“downstream”). In the direction along the longitudinal axis A from left to right in the image, the cross-section of the flowpath gradually increases.

At the downstream end of the inner wall 46, the inner wall 46 defines a “downstream aperture” facing towards the outlet 26 of the turbine diffuser 45. The downstream aperture is the portion of the volume encircled by the inner wall 46 (the “core cavity”) which has the greatest radial extent. At the upstream end of the inner wall 46, the inner wall 46 defines an “upsteam aperture” facing towards the inlet 25 of the turbine diffuser 45. The upstream aperture is the portion of the core cavity which has the smallest radial extent. Thus, the core cavity is open at both ends, with respective apertures at each end facing in opposite directions along the longitudinal axis.

Referring to Fig. 7 there is shown an axial cross-sectional view of a mould assembly 50 for forming the turbine diffuser 45 of Figs. 5 and 6.

The mould assembly 50 comprises a mould die 51, a first core 52 and second and third cores 53, 54. Note that only three cores are used, in contrast to the five cores of Fig. 4, so the cost of providing the cores is reduced, and the chance that one will be misaligned is reduced. This chance is reduced also by the fact that the cores 52, 53, 54 are simpler and larger (for a given size of the resulting turbine diffuser) than the cores of Fig. 4. Thus, the mould assembly 50 has less chance of producing a defective product than that of Fig. 4.

The die 51 is a substantially annular member that extends in a circumferential direction about a longitudinal axis 60 of the mould assembly 50, which will become the longitudinal axis A of the turbine diffuser 45. The die 51 extends in the direction of the longitudinal axis 60 from a first end 61 to a second end 62. The die 51 forms a continuous ring about the longitudinal axis 60. A radially inner surface 63 of the die 51 has a shape corresponding to that of a radially outer surface of the turbine diffuser 45 to be formed by the mould assembly 50. The inner surface 63 is substantially annular, extending about the longitudinal axis 60. The inner surface 63 has first and second sections 63’, 63”, provided axially either side of an intermediate section 63’”. The first and second sections 63’, 63” tend end away from the longitudinal axis 60 as they extend in the direction from the first end 61 to the second end 62 of the die 51, such that the cross-sectional area of the first and second sections 63’, 63” increase in the axial direction from the first end 61 to the second end 62 of the die 51.

The intermediate section 63’” of the inner surface 63 extends radially in the radially outwards direction from a position between the first and second sections 63’, 63”. The intermediate section 63’” has a shape corresponding to the outer surface of the flange 32 of the turbine diffuser 45 formed by the mould assembly 50.

The first, second and third cores, 52, 53, 54 are arranged within the die 51, relative to each other, as well as relative to the inner surface 63 of the die 51 such that a mould cavity 65 is defined between opposed surfaces of the die 51 and the cores 52, 53, 54. This cavity is the cross-hatched area of Fig. 7, and is initially empty. The first core 52 has a substantially circular cross-sectional shape that is substantially centred on the longitudinal axis 60, with the cross-sectional area of the circular cross-sectional shape varying with axial position.

The first core 52 comprises substantially cylindrical first and second end portions 52’, 52”, spaced apart along the axis 60, with a frusto-conical intermediate portion 52”' extending between the first and second portions 52’, 52”. The intermediate portion 52’” extends from a first end touching the first portion 52’, to a second end touching the portion 52”, and as it does so, the diameter of the circular void surrounded by the inner wall 46 (referred to, once the inner wall 46 is formed, as the core cavity) increases linearly. A radially outer surface of the first core 52 has a shape corresponding to a radially inner surface of the inner wall 46 of the turbine diffuser 45 formed by the mould assembly 50.

Each core piece 53, 54’ extends in the circumferential direction about the axis 60 to form a semi-circular shape when viewed along the axis 60. The cores 53, 54 together form a complete circle in the circumferential direction. When viewed in an axial plane, such as the plane of Fig. 7, the cores 53, 54 extend in a line which diverges from the axis 60, typically at an acute angle.

Each core 53, 54 has a respective semi-circular cylindrical axially-extending portion 55, 56. The portions 55, 56 touch on the plane transverse to the plane of Fig. 7 and including the axis 60. Similarly, the cores 53, 54 have respective semi-circular cylindrical axially-extending portions 55’, 56’, which touch on the plane transverse to the plane of Fig. 7 and including the axis 60.

Between the portions 55, 55’, and between the portions 56, 56’, the respective cores 53, 54 each have respective inner surfaces and respective outer surfaces. The two outer surfaces together define a frusto-conical surface which corresponds to the inner surface of the outer wall 30 of the turbine diffuser 45, and the two inner surfaces together define a frusto-conical surface which corresponds to the outer surface of the inner wall 46 of the turbine diffuser 45.

In order to manufacture the turbine diffuser 45, the mould die 62 is arranged as shown in Fig. 7 with the inner core 52, the first core 53 and the second core 54 together forming a mould cavity 65 (the dark region shown in Fig. 7) for receiving molten metal to form the turbine diffuser. Assuming that the gravitational direction is the down direction in Fig. 7, the first cover (inner core) 52 is supported at its longitudinal ends by the third core 54. Similarly, the second core 53 is supported at its ends by the third core 54. However, between the portions 55, 56 and the portions 55’, 56', there is a clearance between the first core and the frusto-conical combination of the second core 53 and third core 54. This clearance 101, which is where the inner wall 46 of the turbine diffuser will be formed, extends circumferentially substantially about the entire circumferential extent of the longitudinal axis 60.

Similarly, there is a radial clearance 102 between the outer surface of the second and third cores 53, 54 and the inner surface of the die 62. This clearance 102 is where the outer wall of the turbine diffuser 45 will be formed, and it extends circumferentially substantially about the entire circumferential extent of the longitudinal axis 60. A radially-extending linearly-symmetric space 103 is formed (e.g. punched, machined or pressed through) in the core 53. Space 103 extends radially outwards as far as the outer surface of the core 53, and extends radially inwards as far as the inner surfaces of the core 53. Similarly, radially-extending linearly-symmetric space 104 is formed (e.g. punched, machined or pressed through) in the core 54. The space 104 extends radially outwards as far as the outer surface of the core 54, and extends radially inwards as far as the inner surface of the core 54. In other words, each space 103,104 is a through-hole between a portion 102 of the mould cavity 65 where the outer wall of the turbine diffuser will be formed, and a portion 101 of the mould cavity 65 where the inner wall of the turbine diffuser will be formed. Each such space 103, 104 will be where a corresponding one of the struts 47 will be formed. The spaces 103, 104 may be substantially radial in their length direction, and may be substantially cylindrical (e.g. circularly cylindrical).

Thus, the mould cavity 65 is composed of the clearances 101, 102, and the spaces 103,104 (and possibly other such radial spaces which are not visible in Fig. 7 because they are out of the plane of the diagram; each such space produces a respective strut in the turbine diffuser).

In order to form the turbine diffuser, a molten metal is injected into the clearances 101, 102 and spaces 103,104, which are fluidly connected to each other. The molten metal is then re-cooled and solidified to form the turbine diffuser 45.

Once the molten metal has solidified to form the turbine diffuser, second and third cores 53, 54 are removed. Then, the inner mould 52 is removed through the downstream aperture in the inner wall 46 (i.e. by moving it to the right as seen in Fig. 7). The downstream aperture is large enough that the inner core 52 can be slidably removed through the downstream aperture without having to break up the inner core 52.

This method of manufacture is advantageous in that, once the turbine diffuser 45 has been formed, the inner core 52 can be removed through the downstream aperture formed by the inner wall 46 of the turbine diffuser 45. Since this aperture is provided in a surface of the turbine diffuser 45 that is external to the support struts 47, this allows the support struts 47 to be substantially solid. This increases the structural integrity of the struts 47, compared to the struts 41 in the turbine diffuser of Fig. 2. This allows the support struts 47 to be reduced in size (e.g. diameter), which reduces the negative effect of the support struts 47 on the flow through the diffuser.

In the described embodiment, the inner core 52 is removed from inside the inner wall 46 by being slidably removed along a “core path” through the downstream aperture in the downstream end of the inner wall 46. Alternatively, or additionally, the inner core 52 may be removed through the upstream aperture provided at the upstream end of the inner wall 46. In this regard, the inner core 52 may be broken up and passed as separate pieces through the upstream aperture which has a smaller diameter than that of the maximum diameter of the inner core 52. In other words, the “core path” would in this case be through the upstream aperture.

Alternatively, or additionally, one or more apertures may be provided at other locations, as at least one through-hole in a surface of the inner wall 46 that is external to the support struts 47, and preferably spaced from the struts 47.

Referring to Fig. 8, a turbine diffuser 70 which may be produced by the method described above, is shown connected to the turbine housing 71 of a known turbocharger 72, which is functionally equivalent to the turbocharger of Fig. 1. The inlet of the tubine diffuser 70 is inserted into the outlet 73 of the known turbocharger 72. The turbine diffuser 70 differs from the turbine diffuser 45 of Figs. 5-7 only in that the flange 74 has a smaller radial extent than the flange 32 of the turbocharger 45. Brackets 75 connect the flange 74 to a ridge 76 on the turbine housing in an gastight fashion, so that the gas expelled by the turbine 77 travels through the turbine diffuser 70 between the inner wall 78 and the outer wall 79, to the outlet 80 of the turbine diffuser 70.

Although the explanation above is in terms of a turbocharger, it will be appreciated that the turbine diffuser may be used with a turbine of any suitable turbo machine and is not limited to use with turbo chargers.

Claims

CLAIMS:

1. A method of manufacture of a turbine diffuser comprising an annular outer wall encircling a longitudinal axis of the turbine diffuser, an inner wall disposed radially inwardly of the outer wall, and a support assembly including at least one support member that attaches a radially outer surface of the inner wall to a radially inner surface of the outer wall, the inner wall encircling a core cavity, and the longitudinal axis of the turbine diffuser extending through the core cavity from an upstream end to a downstream end, with a flowpath being defined between a radially inward surface of the outer wall and an opposed radially outward surface of the inner wall, said method comprising: (a) defining a mould cavity by arranging at least one or more cores, the mould cavity comprising a portion having the shape of the inner wall and a portion having the shape of the support assembly, the core cavity being filled by at least a portion of a first one or more of the cores; (b) providing a molten metal in the mould cavity and solidifying the molten metal to form at least the inner wall and the support assembly; and (c) removing said first one or more cores from the core cavity along at least one core path which extends from the core cavity to the exterior of the inner wall through at least one aperture defined by the inner wall; wherein, for at least at a portion of the radial length of the support assembly from a radially inner end of the support assembly to a radially outer end of support assembly, the core path does not pass through the support assembly.

2. A method of manufacture according to claim 1 wherein the at least one core path does not pass through the support assembly along any of the radial length of the support assembly.

3. A method of manufacture according to claim 2 wherein the at least one core path is spaced from the support assembly at all points along the radial length of the support assembly.

4. A method of manufacture according to any preceding claim wherein the method comprises passing the core as a single piece along the at least one core flow path to the exterior of the inner wall.

5. A method of manufacture according to any of preceding claim wherein the method comprises breaking the core up into a plurality of portions and passing the portions of the core along the core flow path along the at least one core path to the exterior of the inner wall.

6. A method of manufacture according to any preceding claim wherein the core is of a particulate material.

7. A method of manufacture according to any preceding claim wherein the at least one support member assembly is substantially solid along at least 20%, preferably at least 40%, even more preferably at least 60%, still even more preferably at least 80% of its radial length between the outer surface of the inner wall and the inner surface of the outer wall.

8. A method of manufacture according to any preceding claim wherein the at least one support member is substantially solid along substantially its entire radial length between the outer surface of the inner wall and the inner surface of the outer wall.

9. A method of manufacture according to any preceding claim wherein the at least one support member is elongate, having a longitudinal axis that extends in the radial direction.

10. A method of manufacture according to any preceding claim wherein the support assembly comprises a plurality of said support members disposed circumferentially about the inner wall.

11. A method of manufacture according to any preceding claim wherein the at least one aperture is provided at the downstream end of the inner wall.

12. A method of manufacture according to any preceding claim wherein the at least one aperture is provided at the upstream end of the inner wall.

13. A turbine diffuser according to preceding claim in which, for successive points along the longitudinal axis, the inner wall has a respective cross-sectional area at each point measured perpendicular to the longitudinal axis, which is a non-decreasing function of the distance of the point along the longitudinal axis from the upstream end to the downstream end.

14. A method of manufacture according to any preceding claim wherein the mould cavity further comprises a portion having the shape of the outer wall, whereby the step of providing the molten metal in the mould cavity and solidifying the molten metal forms the outer wall of the turbine diffuser.

15. A method of manufacture according to any of preceding claim wherein the mould cavity is further defined by at least one die that is arranged with the one or more cores to define the mould cavity.

16. A method of manufacture according to claim 15 when dependent on claim 14, wherein the die forms a radially outward wall of the portion of the mould cavity having the shape of the outer wall.

17. A method of manufacture according to any preceding claim in which, following said step of defining the mould cavity, the one or more first core members are supported using the at least one aperture.

18. A method of manufacture according to any preceding claim in which, following said step of defining the mould cavity, the one or more first core members extend through the at least one aperture, and are supported externally of the core cavity.

19. A turbine diffuser comprising an annular outer wall and an inner wall disposed radially inwardly of the outer wall, the inner wall encircling a longitudinal axis which extends from an upstream end to a downstream end, with a flowpath defined between a radially outer surface of the inner wall and a radially inner surface of the outer wall, the diffuser further comprising a support assembly that attaches the radially outer surface of the inner wall to the radially inner surface of the outer wall, the inner wall defining at least one aperture which is spaced from the support assembly, the inner wall and support assembly being a single moulded one-piece unit.

20. A turbine diffuser according to claim 19 in which the at least one aperture is spaced axially from the support assembly.

21. A turbine diffuser according to claim 19 or 20 in which the inner wall defines respective apertures at opposite ends of the longitudinal axis.

22. A turbine diffuser according to claim 19, 20 or 21 in which the support structure is formed as substantially solid struts between the inner and outer walls.

22. A turbine diffuser according to any of claims 19 to 21 in which, for successive points along the longitudinal axis, the inner wall has a respective cross-sectional area at each point measured perpendicular to the longitudinal axis, which is a nondecreasing function of the distance of the point along the longitudinal axis from the upstream end to the downstream end.

22. A turbine assembly comprising: a turbine comprising an inlet and an outlet, a turbine chamber disposed between the inlet and the outlet, with a turbine wheel rotatably mounted in the turbine chamber, wherein the turbine outlet is in gas communication with the inlet of a turbine diffuser according to any of claims 18 to 21.

23. A turbocharger comprising a turbine assembly according to claim 22 and a compressor, the compressor comprising a housing defining an inlet and an outlet and an impeller wheel chamber provided between the inlet and the outlet, with an impeller wheel rotatably mounted within the impeller wheel chamber, wherein the turbine wheel is coupled to the impeller wheel such that rotation of the turbine wheel rotates the impeller wheel.

24. A method of manufacture substantially as described herein, with reference to the accompanying drawings.

25. A turbine diffuser substantially as described herein, with reference to the accompanying drawings.

26. A turbine assembly substantially as described herein, with reference to the accompanying drawings.

27. A turbocharger substantially as described herein, with reference to the accompanying drawings.