Turbine housing

Abstract

A turbine housing for a turbocharger comprises: an inlet portion; a second portion for housing a turbine wheel; and an outlet portion. The inlet portion defines a first inlet and a second inlet. The inlet portion and the second portion define: a first volute which defines a first flow passage between the first inlet and a first volute outlet, and which extends around an axis of the turbine housing; and a second volute which defines a second flow passage between the second inlet and a second volute outlet, and which extends around the axis of the turbine housing. The first volute and the second volute are separated, in an axial direction, by a dividing wall. For at least a portion of the dividing wall upstream of a tongue of the first volute and a tongue of the second volute a thickness of the dividing wall varies in a first direction that is generally perpendicular to a second direction that, in use, gas will flow along through the first and second volutes such that the thickness increases in a direction generally away from the second portion of the turbine housing.

Description

[0001] Turbine housingField of the inventionThe present invention relates to a new design of turbine housing. In particular, it relates to anew design of a turbine housing which defines two volutes. The turbine housing may haveparticular application as part of a turbocharger. The present invention also relates to a turbine comprising the new design of turbine housing. The present invention also relates to an assembly comprising the new design of turbine housing. The assembly may comprise at least one wastegate valve. Additionally or alternatively, the assembly may comprise a turbine wheel, a shaft supporting the turbine wheel and a bearing housing supporting the shaft. The present invention also relates to a turbocharger comprising the new design of turbine housing. The present invention also relates to a new method for designing a turbine housing. Background of the invention Turbomachines are machines that transfer energy between a rotor and a fluid. For example,a turbomachine may transfer energy from a fluid to a rotor or may transfer energy from a rotorto a fluid. Two examples of turbomachines are a power turbine, which uses the rotationalenergy of a rotor driven by a fluid to do useful work, for example, generating electrical power;and a compressor which uses the rotational energy of the rotor to compress a fluid.Turbochargers are known turbomachines for supplying air to an inlet of an internal combustionengine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. 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 an engine inlet manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected betweenthe turbine and compressor wheel housings. After driving the turbine wheel the exhaust gasexists the turbine through a gas outlet which is at the opposite end of the rotational axis of the turbine wheel from the bearing housing.

[0002] 69777365-1 It is known for the multiple cylinders of an internal combustion engine to be portioned into two groups of cylinders, and for the exhaust gas emitted by the respective groups of cylinders to be transmitted into two respective gas inlets of a turbocharger. The gas inlets are in fluid communication with a chamber of the housing containing the turbine wheel via respectivevolutes. The volutes may be spaced from each other along the rotational axis of the turbinewheel, such that a first of the volutes is closer to the bearing housing (the “bearing housing side volute”), and the other is closer to the turbine outlet side (the “turbine outlet side volute”).It may be desirable to provide a new turbine housing and / or a new method for designing aturbine housing for turbochargers that at least partially addresses one or more problemsassociated with prior art methods, whether identified here on not. Summary of the invention According to a first aspect of the present disclosure there is provided a turbine housing for a turbocharger, the turbine housing comprising: an inlet portion; a second portion for housing a turbine wheel; and an outlet portion; wherein the inlet portion defines a first inlet and a second inlet; wherein the inlet portion and the second portion define: a first volute which defines a first flow passage between the first inlet and a first volute outlet, and which extends around an axis of the turbine housing; and a second volute which defines a second flow passage between the second inlet and a second volute outlet, and which extends around the axis of the turbine housing; wherein the first volute and the second volute are separated, in an axial direction, by a dividing wall; and wherein for at least a portion of the dividing wall upstream of a tongue of the first volute and a tongue of the second volute a thickness of the dividing wall varies in a first direction that is generally perpendicular to a second direction that, in use, gas will flow along through the first and second volutes such that the thickness increases in a direction generally away from the second portion of the turbine housing. The turbine housing according to the first aspect of the present disclosure is advantageous, as now discussed. In use, the turbine housing may form part of a turbocharger and a turbine wheel may be disposed at least partially within the housing, downstream of the first and second volutes. A turbocharger is a known device for extracting energy from the exhaust gas from an engine and using this energy to deliver air at increased pressure to the inlet of the engine (so as to increase the power of the engine). Exhaust gas admitted into the inlet is accelerated as it

[0003] 69777365-1 passes through the volutes. This increase in gas velocity imparts more momentum to the turbine wheel thus increasing the power generated by the turbine wheel. The inventors of the present invention have found that, with some designs of turbine housing, there is a low thermomechanical failure (TMF) lifetime at the outboard region of the turbinehousing inlet portion. As used here the outboard region is intended to mean the region of theturbine housing distal the second portion or, equivalently, a generally radially outer portion of the turbine housing. One way to address such a problem is to increase the thickness of the dividing wall by adding a uniform pad to the dividing wall. However, the inventors have found that with some designs this does not address the problem. The turbine housing according to the first aspect of the present disclosure effectively uses anon-uniform thickness pad which is thinner at the (radially) inboard region of the inlet portion(proximate the second portion) and thicker at the (radially) outboard region (distal the second portion). As will be appreciated by the skilled person, when altering the thickness of the dividing wall, care should be taken that this does not affect the performance of the turbine. There is a limit to how much material can be added to the dividing wall without adversely affecting the performance of the turbine. By providing a non-uniform thickness dividing wall, more material can be provided at the outboard region (distal the second portion), which has been found to increase the TMF lifetime of the turbine housing. Furthermore, it does so without making theentire dividing wall thicker, which would involve the addition of more material and would havea greater impact on the performance of the turbine. Advantageously, the inventors have realized that, in principle, a thickness distribution of the dividing wall in the first direction (that, in use, is perpendicular to the gas flow) can beoptimized, in dependence on the shape and dimensions of the flange, so as to maximize theTMF life of the turbine housing. For example, an amount by which the thickness increases in the first direction (and over what extent in the first direction) may be selected to maximize TMF life of the turbine housing. Furthermore, a turbine housing according to the first aspect of the present disclosure potentially represents a relatively small change to the design of the turbine housing and socan be implemented on existing turbine housings with a relatively small (and inexpensive)change to a tool used for casting the turbine housing.

[0004] 69777365-1 The increased thickness of the dividing wall distal the second portion of the turbine housing can provide additional strength to a flange of the turbine housing. In particular, the increased thickness of the dividing wall distal the second portion of the turbine housing can provide additional strength to a radially outboard portion of the flange of the turbine housing, as now discussed. It will be appreciated that the (radially) inboard region of the inlet portion is proximate the second portion and that therefore both the (radially) inboard region of the inlet portion and the second portion will provide support to a radially inboard portion of the flange. In contrast, the radially outboard portion of the flange is only supported by the outboard region of the inlet portion (distal the second portion). Therefore, advantageously, by increasing a thickness of the dividing wall distal the second portion (where it joins the outboard region of the inlet portion) additional strength can be provided to the radially outboard portion of the flange. It will be appreciated that, unless stated to the contrary, any reference herein to an axial direction is intended to mean a direction that is along, or parallel to, the axis of the housing. In addition, unless stated to the contrary, any reference herein to a radial direction is intended to mean a direction that is generally perpendicular to, extends through, the axis of the housing. In addition, unless stated to the contrary, any reference herein to a circumferential or tangential direction is intended to mean a direction that is generally perpendicular to the axial and radial directions. At least a portion of the first and second volutes defined by the inlet portion of the housing may extend generally linearly in a circumferential or tangential direction. The inlet portion may extend from a flange of the housing to the tongues of the first and second volutes. The second portion of the housing (which is for housing a turbine wheel) may extend around the axis of the turbine housing. The second portion of the housing may define parts of the first and second volutes that are downstream of their respective tongues. A critical plane of each volute may be defined as a plane that contains the axis of the housing and which intersects a tip of a tongue that partially defines that volute. Put differently, the critical plane is the plane at which the known A / R ratio of a turbocharger comprising the turbine housing would be calculated. In general, the critical plane of a volute is a plane that contains the axis of the housing where the cross-sectional area of that volute is at a local minimum. That is, in general, as a function of distance through the volute the area decreases between the inlet and the critical area and then increases immediately downstream of the critical area

[0005] 69777365-1 (as the gas flow clears the tongue of the volute). Further downstream still, the area then typically decreases down to the exit of the volute. The outlet portion may extend generally axially. It will be appreciated that, in general, the dividing wall has some extent in three dimensions. A thickness of the dividing wall may be the smallest dimension of the dividing wall. The thickness is generally in an axial direction of the turbine housing. The dividing wall may be said to extend in a plane that is generally perpendicular to the axial direction. The turbine housing according to the first aspect of the present disclosure relates to an arrangement wherein the thickness of the dividing wall is not uniform across the plane in which it extends. As used herein a direction of gas flow through the first and second volutes during use may be referred to as the second direction. In the inlet portion of the turbine housing, the seconddirection is generally circumferential or tangential with respect to the axis of the turbine. Adirection that is generally perpendicular the second direction may be referred to as a first direction. The first direction has at least a component that is generally radial. It will be appreciated that, at least in an inlet portion, at both ends in the first direction the dividing wall will meet another portion or wall of the turbine housing. Equivalently, in the inlet portion, both an inner and outer radial portion of the dividing wall will merge into another wall of the turbine housing. In the second portion of the turbine housing (downstream of the tongue(s) of the volutes), a radially inner portion of the volutes may be open and so the dividing wall may only merge into the radially outer (or outboard) wall of the turbine housing. Furthermore, the volutes will typically be defined by a smooth, curved inner surface of the turbine housing. Therefore, where the dividing wall meets another wall (at the radially inner and outer ends of the dividing wall), a thickness of material will increase. For example, in a plane perpendicular to a flow direction of the gas, the dividing wall may comprise: a central portion, which may be referred to as a main portion of the dividing wall; and two end portions which comprise curved fillets where the dividing wall meets the external walls of the turbine housing. The main portion of the dividing wall may, for example, have an extent in the first direction that is of the order of 60% of a maximum extent of the volutes in the first direction. Each of the two end portions may, for example, have an extent in the first direction that is of the order of 20% of a maximum extent of the volutes in the first direction.

[0006] 69777365-1 In general, at least a part of the central portion of the dividing wall may increase in thickness in the first direction. A circumferential outlet portion of each of the first and second volutes may be defined by one or more tongues. In some embodiments, the first and second volutes may both be partially defined by a common tongue. For such embodiments, the first and second volutes may share a common critical plane. In some embodiments, the first volute may be partially defined by a first tongue and the second volute may be partially defined by a second tongue. The first and second tongues may be circumferentially spaced apart. In some embodiments, the portions of the first and second volutes between the inlet and the respective tongues of the first and second volutes may be generally linear (in contrast, a portion of each volute downstream of the tongue(s) may spiral around the axis of the housing). In some embodiments, for at least a portion of the dividing wall in the second direction, the thickness of the dividing wall in the first direction may increase in the direction generally away from the second portion of the turbine housing for a central portion of the dividing wall in the first direction. In some embodiments, for at least a portion of the dividing wall in the second direction, the thickness of the dividing wall in the first direction may increase in the direction generally awayfrom the second portion of the turbine housing for a portion of the dividing wall having anextent in the first direction that is at least 30% of a maximum extent of the volutes in the first direction. In some embodiments, the portion of the dividing wall may have an extent in the first direction that is at least 40% of a maximum extent of the volutes in the first direction. In someembodiments, the portion of the dividing wall may have an extent in the first direction that isat least 50% of a maximum extent of the volutes in the first direction. In some embodiments, the portion of the dividing wall may have an extent in the first direction that is at least 60% of a maximum extent of the volutes in the first direction. In some embodiments, the portion of the dividing wall may have an extent in the first direction that is at least 70% of a maximum extent of the volutes in the first direction. In some embodiments, the portion of the dividing wall may have an extent in the first direction that is at least 80% of a maximum extent of the volutes in the first direction.

[0007] 69777365-1 In some embodiments, for at least a portion of the dividing wall in the second direction, the thickness of the dividing wall in the first direction may increase linearly for at least a portion of the dividing wall in the direction generally away from the second portion of the turbine housing. In some embodiments, for at least a portion of the dividing wall in the second direction, the thickness of the dividing wall in the first direction may increase in the direction generally away from the second portion of the turbine housing with a rate of increase of thickness in the first direction in a range of 0.02 mm / mm to 0.06 mm / mm. In some embodiments, for at least a portion of the dividing wall in the second direction, the thickness of the dividing wall in the first direction may increases in the direction generally away from the second portion of the turbine housing with a rate of increase of thickness in the first direction in a range of 0.03 mm / mm to 0.05 mm / mm. In some embodiments, for at least a portion of the dividing wall in the second direction, the thickness of the dividing wall in the first direction may increases in the direction generally away from the second portion of the turbine housing with a rate of increase of thickness in the first direction of approximately 0.04 mm / mm. For a dividing wall of uniform thickness it will be appreciated that two opposite sides of the dividing wall will be mutually parallel. In contrast, for a dividing wall with non-uniform thickness, the two opposite sides of the dividing wall will be at non-zero angle to each other. The angle between two opposite sides of the dividing wall may be referred to as a taper angle of the dividing wall. In some embodiments, for at least a portion of the dividing wall in the second direction, the thickness of the dividing wall in the first direction may increase in the direction generally away from the second portion of the turbine housing with a taper angle in a range of 1° to 5°. In some embodiments, for at least a portion of the dividing wall in the second direction, the thickness of the dividing wall in the first direction may increase in the direction generally away from the second portion of the turbine housing with a taper angle in a range of 2° to 4°. In some embodiments, for at least a portion of the dividing wall in the second direction, the thickness of the dividing wall in the first direction may increase in the direction generally away from the second portion of the turbine housing with a taper angle of approximately 3°.

[0008] 69777365-1 In some embodiments, for at least a portion of the dividing wall in the second direction, the thickness of the dividing wall in the first direction may increase in the direction generally away from the second portion of the turbine housing with a taper angle of less than 10°. Note that it may be generally desirable to keep the taper angle of the dividing wall relatively low so as to reduce the impact that the taper of the dividing wall 516 has on the efficiency of a turbine stage that comprises the new turbine housing. In some embodiments of the turbine housing 500 the taper angle may be less than 9°. In some embodiments of the turbine housing 500 the taper angle may be less than 8°. In some embodiments of the turbine housing 500 the taper angle may be less than 7°. In some embodiments of the turbine housing 500 the taper angle may be less than 6°. In some embodiments, for at least a portion of the dividing wall in the second direction, the thickness of the dividing wall in the first direction may increase in the direction generally away from the second portion of the turbine housing with a taper angle of less than 5°. In some embodiments, for at least a portion of the dividing wall in the second direction, the thickness of the dividing wall in the first direction may monotonically increase in the direction generally away from the second portion of the turbine housing for at least a portion of the dividing wall in the first direction. It will be appreciated that the thickness of the dividing wall in the first direction monotonically increasing in the direction generally away from the second portion of the turbine housing for at least a portion of the dividing wall in the first direction means that for that portion of the dividing wall the thickness does not decrease in the direction generally away from the second portion of the turbine housing. In some embodiments, for at least a portion of the dividing wall in the second direction, the thickness of the dividing wall in the first direction may increase in the direction generally away from the second portion of the turbine housing for a radially outboard portion of the dividing wall that has an extent in the first direction that is at least 70% of a maximum extent of the volutes in the first direction.In some embodiments, the at least a portion of the dividing wall upstream of the tongue of thefirst volute and the tongue of the second volute may have a thickness that is greater than athickness of the dividing wall at the first inlet and the second inlet.

[0009] 69777365-1 That is, between the inlets (defined at the flange) and the tongues of the first and second volutes there is a portion of the dividing wall which has an enlarged thickness which is greater than the dividing wall thickness at the inlets (defined at the flange). Advantageously, such an enlarged thickness may improve a thermomechanical performance of the turbine housing whilst still meeting the boundary condition of a specific flange design. The enlarged portion of the dividing wall may, for example, be referred to as a pad. In some existing arrangements, in an inlet portion of the turbine housing (between the first and second inlets and the tongues of the first and second volutes), an enlarged thickness portion may be provided. However, typically the enlarged thickness portion would have a uniform thickness (excluding internal fillets or scalloping that may be provided at each end in the first direction). In some embodiments, for at least a portion of the dividing wall in the second direction, thethickness of the dividing wall in the first direction may be asymmetric about a center of thedividing wall in the first direction such that it increases from the center in the direction generally away from the second portion of the turbine housing by a greater amount than it increases from the center in the direction generally towards the second portion of the turbine housing. That is, there is an asymmetry such that there is a net increase in the thickness of the dividing wall towards the outboard direction even accounting for internal fillets and / or scalloping of the dividing wall.In some embodiments the turbine housing may further define at least one wastegate passagevia which at least one of the first and second volutes is in fluid communication with the outlet portion. For example, at least one wastegate passage may connect each of the first and second volutes to the outlet portion. The or each at least one wastegate passage may connect to a portion of at least one of the first and second volutes upstream of a tongue of that volute to the outlet portion.According to a second aspect of the present disclosure there is provided a turbine comprising:the turbine housing according to the first aspect of the present disclosure; and a turbine wheel,the turbine wheel received in the turbine housing.

[0010] 69777365-1 The turbine may be a fixed geometry turbine. The turbine may be a variable geometry turbine.The turbine may further comprise: a shaft supporting the turbine wheel; and a bearing housingsupporting the shaft for rotation about a rotation axis. The rotation axis may be colinear with the axis of the turbine housing.The turbine may further comprise a nozzle member disposed between the exits of each of thefirst and second volutes and the turbine wheel.According to a third aspect of the present disclosure there is provided a turbochargercomprising: the turbine housing according to the first aspect of the present disclosure; aturbine wheel, the turbine wheel received in the turbine housing; a bearing housing configuredto support a shaft for rotation about a rotation axis; and a compressor, the compressorcomprising a compressor housing and a compressor wheel; wherein the turbine wheel andcompressor wheel are in power communication via the shaft. The turbocharger may be a fixed geometry turbocharger. The turbocharger may be a variable geometry turbocharger. The turbocharger may form part of an engine arrangement e.g. for a vehicle or a pump. The bearing housing may support the shaft for rotation by one or more bearing assemblies. The bearing assemblies may comprise journal and / or thrust bearings. The turbine wheel and compressor wheel being in power communication with one another is intended to mean that rotation of one of the turbine wheel or compressor wheel drives rotation of the other of the compressor wheel and the turbine wheel. In operation, when the turbocharger is being used, exhaust gas drives the turbine wheel which, in turn, drives the compressor wheel to provide a boost pressure.According to a fourth aspect of the present disclosure there is provided a method for designinga turbine housing, the turbine housing defining: an inlet portion; a second portion for housing a turbine wheel; and an outlet portion; wherein the inlet portion and the second portion define: first and second volutes separated by a dividing wall, first and second volutes respectively defining first and second flow passages between the inlet portion and the outlet portion, themethod comprising: for at least a portion of the dividing wall upstream of a tongue of the firstvolute and a tongue of the second volute, selecting a thickness distribution of the dividing wall 69777365-1 in a first direction that is generally perpendicular to a second direction that, in use, gas will flow along through the first and second volutes such that the thickness increases in a direction generally away from the second portion of the turbine housing.In some embodiments of the method, the thickness distribution may be selected so as tooptimize a thermomechanical performance of the turbine housing.In some embodiments of the method, selecting the thickness distribution of the dividing wallin the first direction may comprise selecting an extent of a portion of the dividing wall over which the thickness increases. For example, the extent of a portion of the dividing wall over which the thickness increases may be selected to be a fraction or percentage of a maximum extent of the volutes in the first direction.In some embodiments of the method, selecting the thickness distribution of the dividing wallin the first direction may comprise selecting a rate of increase of thickness in the first direction of a portion of the dividing wall over which the thickness increases. In some embodiments of the method, selecting the thickness distribution of the dividing wall in the first direction may comprise selecting a taper angle of a portion of the dividing wall over which the thickness increases.According to a fifth aspect of the present disclosure there is provided a turbine housing, theturbine housing defining: an inlet; an outlet; first and second volutes which respectively define first and second flow passages between the inlet and the outlet which respectively define firstand second volute exits distal the inlet; and an axis of the housing, the first and second volutesbeing defined around the axis; wherein each of the first and second volutes defines a criticalplane; wherein the first and second volutes are separated by a dividing wall; and wherein athickness distribution of the dividing wall as a function of distance from the inlet is such that at least a portion of the dividing wall upstream of the critical planes of the first and second volutes has a thickness that is greater than the thickness of the dividing wall at said critical plane(s) by at least a first factor and wherein the first factor is 1.3 or greater. A thickness of the dividing wall may be dependent on a distance from the inlet. A thickness of the dividing wall may vary with a distance from the inlet. The thickness distribution of the 69777365-1 dividing wall as a function of distance from the inlet describes this dependence / variation in the thickness of the dividing wall.The turbine housing according to the fifth aspect of the present disclosure is advantageous,as now discussed. In use, the turbine housing may form part of a turbocharger and a turbine wheel may be disposed at least partially within the housing, downstream of the first and second volute exits.As stated above, a turbocharger is a known device for extracting energy from the exhaust gasfrom an engine and using this energy to deliver air at increased pressure to the inlet of the engine (so as to increase the power of the engine). Exhaust gas admitted into the inlet is accelerated as it passes through the volutes. This increase in gas velocity imparts more momentum to the turbine wheel thus increasing the power generated by the turbine wheel. However, one known drawback of turbochargers in general is that the turbocharger creates backpressure, which can negatively impact the efficiency of the engine to which it is connected. Furthermore, the performance of fixed geometry turbochargers is strongly dependent on the flow conditions, which in turn is dependent on operating conditions of the engine to which it is connected. Fixed geometry turbochargers are typically designed so as to be optimized for a particular set point of engine running conditions. The critical plane of the volute is a plane that contains the axis of the housing and which intersects a tip of a tongue that partially defines that volute. Put differently, the critical plane is the plane at which the known A / R ratio of a turbocharger comprising the turbine housing would be calculated. In general, the critical plane of a volute is a plane that contains the axis of the housing where the cross-sectional area of that volute is at a local minimum. That is, in general, as a function of distance through the volute the area decreases between the inlet and the critical area and then increases immediately downstream of the critical area (as the gas flow clears the tongue of the volute). Further downstream still, the area then typically decreases down to the exit of the volute. The flow characteristics through the turbine stage of a turbocharger are dependent on: the flow area and radius (of the centroid of the flow area from the axis) of the volutes at the criticalplane(s); and the radius of the turbine wheel. Therefore, for a given turbine wheel the flowcharacteristics of the turbine stage are dependent on the flow area and radius (of the centroid of the flow area from the axis) of the volutes at the critical plane(s). As explained above, these are selected so as to achieve optimal performance for a particular set point of engine running conditions. 69777365-1 Typically, the thickness of the dividing wall between the inlet and the critical plane(s) is generally uniform. In particular, there is a prejudice against rapidly increasing the thickness of the dividing wall proximate to (and upstream of) the critical planes as any increase in thickness may result in decrease of the cross sectional areas of the first and second flow passages upstream of the critical planes. However, note that the critical planes are the planes at which the cross-sectional area of the volutes are at a local minimum upstream of an exit of the volute. Therefore, such a change cannot be tolerated since this may change the location and size of the critical plane(s) and, as explained above, this has been carefully selected to match a desired set point of engine conditions. However, the inventors have realized that it is possible for the thickness distribution of the dividing wall as a function of distance from the inlet to be such that at least a portion of the dividing wall upstream of the critical planes of the first and second volutes has a thickness that is greater than the thickness of the dividing wall at said critical plane(s) by at least a first factor and wherein the first factor is 1.3 or greater. For example, this may be achieved by also changing the other walls of the turbine housing that define the volutes so that the critical planes remain unchanged. Furthermore, it has been found that such an arrangement can result in a surprising increase in the lifetime of the turbine housing. For example, it has been found by the inventors (from thermomechanical analysis) that such turbine housings tend to fail in a region of the outer (circumferential) surface of the turbine housing in the vicinity of the dividing wall (typically between the inlet and the critical planes). Furthermore, it has been found that by adjusting the dividing wall thickness the lifetime of the turbine housing can be improved by at least a factor of two. It is particularly surprising that such a variation in the thickness of the dividing wall can have such an impact on the lifetime of the turbine housing since the turbine housings tend to fail on an outer wall. Therefore, it would seem more obvious to consider varying a thickness of the outer wall(s) of the housing. It will be appreciated that upstream of the critical planes of the first and second volutes means a position between the inlet and the critical planes of the first and second volutes. A circumferential outlet portion of each of the first and second volutes may be defined by one or more tongues. In some embodiments, the first and second volutes may both be partially defined by a common tongue. For such embodiments, the first and second volutes may share a common critical plane. In some embodiments, the first volute may be partially defined by a first tongue and 69777365-1 the second volute may be partially defined by a second tongue. The first and second tongues may be circumferentially spaced apart. In general, the first factor may be significantly greater than, potentially significantly greater than 1.3. In some embodiments the first factor may be 1.4 or greater. In some embodiments the first factor may be 1.5 or greater. In some embodiments the first factor may be 1.6 or greater. In some embodiments the first factor may be 1.7 or greater. In some embodiments the first factor may be 1.8 or greater. In some embodiments the first factor may be 1.9 or greater. In some embodiments the first factor may be 2.0 or greater. In some embodiments the first factor may be 2.1 or greater. In some embodiments the first factor may be 2.2 or greater. In some embodiments the first factor may be 2.3 or greater. In some embodiments the first factor may be 2.4 or greater. In some embodiments the first factor may be 2.5 or greater. In some embodiments, the portions of the first and second volutes between the inlet and the respective first and second critical planes may be generally linear (in contrast, a portion of each volute downstream of the critical planes may spiral around the axis of the housing). The portions of the first and second volutes between the inlet and the respective first and second critical planes extend generally in a first direction parallel to the direction of gas flow through the volutes during use. The first direction may be referred to as the z-direction. The portions of the first and second volutes between the inlet and the respective first and second critical planes also extend generally in a second direction (which is generally perpendicular to the direction of gas flow through the volutes during use). The second direction may be referred to as the y-direction. The thickness distribution may be a thickness in a third direction. The third direction may be referred to as the x-direction. Note that although, as noted above, the portions of the first and second volutes between the inlet and the respective first and second critical planes may be generally linear, in general the first, second and third directions may vary with distance from the inlet. Note that, in general, if the cross-sectional shape of the volutes changes between the inlet and the critical planes (which it typically does) then the thickness distribution in the third 69777365-1 direction (x-direction) will, in general, be different at different positions in the second position (y-direction). As used herein (unless stated otherwise) the thickness distribution of the dividing wall as a function of distance from the inlet is the thickness distribution in the first direction (x-direction) at a position in the second direction (y-direction) that corresponds to a midpoint of the first and second volutes in the second direction. In general, the increase in thickness of the dividing wall may be occur as close to the critical planes as possible without altering the flow characteristics of the turbine housing (i.e. without changing the position or cross-sectional area of the critical planes). As discussed above, it may be difficult to increase the thickness of the dividing wall immediately upstream of the critical planes (as it may be difficult to make compensating changes in the other walls of the turbine housing in this region). Therefore, typically, the thickness of the dividing wall immediately upstream of the critical planes may decrease slightly. At least a portion of the dividing wall upstream of the critical planes of the first and second volutes and less than a first percentage of the distance between the inlet and the closestcritical plane from the closest critical plane may have a thickness that is greater than thethickness of the dividing wall at said critical plane(s), wherein the first percentage is less than 30 %. It will be appreciated that as used here the closest critical plane is intended to mean the critical plane that is closest to the inlet. At least a portion of the dividing wall upstream of the critical planes of the first and second volutes and less than a second percentage of the distance between the inlet and the closestcritical plane from the closest critical plane may have a thickness that is greater than thethickness of the dividing wall at said critical plane(s) by at least the first factor, wherein the second percentage is less than 40 %.The turbine housing may further define at least one wastegate passage that connects at leastone of the first and second volutes, at a position upstream of the critical plane of the volute, to the outlet.At least one wastegate passage may connect each of the first and second volutes, at a positionupstream of the critical plane of the volute, to the outlet. 69777365-1 The thickness distribution of the dividing wall as a function of distance from the inlet may be such that at least a portion of the dividing wall upstream of the critical planes of the first and second volutes has a thickness that is greater than the thickness of the dividing wall at the inlet by at least a second factor and wherein the second factor is 1.3 or greater. In some embodiments, the thickness of the dividing wall at the critical plane(s) may be substantially the same as the thickness of the dividing wall at the inlet. For such embodiments, the second factor may be substantially equal to the first factor. In general, the second factor may be significantly greater than, potentially significantly greater than 1.3. In some embodiments the second factor may be 1.4 or greater. In some embodiments the second factor may be 1.5 or greater. In some embodiments the second factor may be 1.6 or greater. In some embodiments the second factor may be 1.7 or greater. In some embodiments the second factor may be 1.8 or greater. In some embodiments the second factor may be 1.9 or greater. In some embodiments the second factor may be 2.0 or greater. In some embodiments the second factor may be 2.1 or greater. In some embodiments the second factor may be 2.2 or greater. In some embodiments the second factor may be 2.3 or greater. In some embodiments the second factor may be 2.4 or greater. In some embodiments the second factor may be 2.5 or greater.According to a sixth aspect of the present disclosure there is provided an assemblycomprising: the turbine housing according to the fifth aspect of the present disclosure; and atleast one wastegate valve member. The at least one wastegate valve may be configurable in at least: a first configuration in which the at least one wastegate passageway is in fluid communication with the at least one volute; and a second configuration in which the at least one wastegate passageway is not in fluid communication with the at least one volute. For embodiments wherein a single wastegate passage is defined by the turbine housing that connects only one of the first and second volutes to the outlet, the assembly according to the second aspect may comprise a single wastegate valve member. 69777365-1 For embodiments wherein two wastegate passages are defined by the turbine housing that connect each of the first and second volutes to the outlet, the assembly according to the second aspect may comprise two wastegate valve members. For example, a first valve member may be associated with a first wastegate passage and a second valve member may be associated with a second wastegate passage. Alternatively, for embodiments wherein two wastegate passages are defined by the turbine housing that connect each of the first and second volutes to the outlet, the assembly according to the second aspect may comprise a single wastegate valve member that can be used to selectively open or close both of the wastegate passages.The assembly may further comprise a support for the or each wastegate valve member. Theor each support may comprise: a shaft extending through a through a bore in a wall of the turbine housing such that a first portion of the shaft is disposed in a wastegate passageway and a second portion of the shaft is disposed on an exterior of the turbine housing; and aportion extending at least partially radially from the first portion of the shaft. The or eachwastegate valve member may be supported by the portion extending at least partially radiallyfrom the first portion of the shaft of a corresponding support. With such an arrangement the wastegate valve member is supported eccentrically by the shaft. As the shaft rotates the wastegate valve member moves around an axis of the shaft. It will be appreciated that the turbine housing may be formed using a casting process. Once so formed one or more through bores may be machined in the casting for receipt of a shaft of a support for a wastegate valve member. A bushing may be inserted into the machined bore. Finally, a shaft of a support for a wastegate valve member may be inserted into the bushing.The assembly may further comprise an actuator for the or each support, the actuator beingoperable to rotate the second portion of the shaft. The actuator may be attached to an outer surface of the turbine housing. The assembly may further comprise: a turbine wheel; a shaft supporting the turbine wheel; and a bearing housing supporting the shaft for rotation about an axis. The turbine wheel may be disposed between the exits of each of the first and second volutes and the outlet.According to a seventh aspect of the present disclosure there is provided an assemblycomprising: the turbine housing according to the fifth aspect of the present disclosure; a69777365-1 turbine wheel; a shaft supporting the turbine wheel; and a bearing housing supporting the shaftfor rotation about an axis; wherein the turbine wheel is disposed between the exits of each ofthe first and second volutes and the outlet.The assembly according to the third aspect may further comprise a nozzle member disposedbetween the exits of each of the first and second volutes and the turbine wheel. The nozzle member may define a plurality of vanes, which may be circumferentially spaced around an axis of the assembly. Each pair of adjacent vanes may define between them a vane passage. The vane passages may define a nozzle of the assembly.According to a eighth aspect of the present disclosure there is provided a turbochargercomprising the turbine housing according to the fifth aspect of the present disclosure or theassembly according to the second or third aspects of the present disclosure.According to a ninth aspect of the present disclosure there is provided a method for designinga turbine housing, the turbine housing defining: an inlet; an outlet; and first and second volutes separated by a dividing wall, first and second volutes respectively defining first and second flow passages between the inlet and the outlet which respectively define first and second volute exits distal the inlet, the method comprising: selecting a thickness distribution of the dividing wall upstream of critical planes of the first and second volutes in dependence on a thermomechanical performance of the turbine housing. The method according to the fifth aspect is advantageous, as now discussed. Typically, the thickness of the dividing wall between the inlet and the critical plane(s) is generally uniform. However, the inventors have realized that by optimizing the thickness distribution of the dividing wall as a function of distance from the inlet can result in a surprising increase in the lifetime of the turbine housing. For example, it has been found by the inventors (from thermomechanical analysis) that such turbine housings tend to fail in a region of the outer (circumferential) surface of the turbine housing in the vicinity of the dividing wall (typically between the inlet and the critical planes). Furthermore, it has been found that by adjusting the dividing wall thickness the lifetime of the turbine housing can be improved by at least a factor of two. The turbine housing may define: an axis of the housing and the first and second volutes may be defined around the axis. 69777365-1The thickness distribution may be selected so as to optimize a thermomechanical performanceof the turbine housing. It will be appreciated that as used herein a thickness distribution selected so as to optimize a thermomechanical performance of the turbine housing is not intended to mean a perfect thickness distribution which has the best possible thermomechanical performance. The skilled person will recognize that such a solution is, in practice, not generally achievable. Rather, a thickness distribution selected so as to optimize a thermomechanical performance of the turbine housing is intended to mean a thickness distribution which has a better thermomechanical performance than one or more other thickness distributions. In practice, a finite number of possibilities may be considered and the best one of these may be selected. Such an optimized thickness distribution may be close to, but not exactly at, a region of parameter space occupied by a perfect thickness distribution which has the best possible thermomechanical performance. It will be appreciated that this optimization may be performed using various merit functions. In some embodiments, the merit function is based on the thermomechanical performance of at least a portion of the turbine housing. For example, the at least a portion may include an outer circumferential portion of the turbine housing. Furthermore, the optimization may be performed using any type of optimization process (for example to minimize a merit function) as desired or required. Examples of such optimization methods may include, for example, any of the following: trial and error; genetic optimization algorithms; Newton’s method in optimization (also known as the Newton-Raphson method); simulated annealing; a least squares method; and / or a quadratic programming active set method.The may comprise: generating a plurality of different candidate thickness distributions of thedividing wall; assessing a thermomechanical performance of each of the plurality of differentcandidate thickness distributions; and selecting the thickness distribution of the dividing wallto be one of the plurality of different candidate thickness distributions that has the best thermomechanical performance.A selection of a thickness distribution that optimizes a thermomechanical performance of theturbine housing may be made using finite element analysis.Thermomechanical performance may be assessed based on a magnitude of thermallyinduced stress in the, or a part of, the housing. 69777365-1 It will be appreciated that, as used here, thermally induced stress is intended to comprise compressive and / or tensile stress. Thermomechanical performance may be assessed based on a time of thermomechanical failure of the, or a part of, the housing.The method may comprise generating at least one candidate thickness distribution. Thegeneration of the or each candidate thickness distribution may comprise: for each of the first and second volutes: selecting a cross-section of that volute at at least two positions along thedividing wall at or upstream of the critical plane of that volute; and interpolating between thecross-sections at said at least two positions so as to define a shape of that volute for a rangeof positions corresponding to said at least two positions; and determining the candidatethickness distribution from said defined shapes of the first and second volutes. As used here “cross section” is intended to mean the size, shape and position of a cross section of a volute. Such a cross section may be generally perpendicular to a flow direction of gas through the volute in use. It will be appreciated that any suitable interpolation method may be used. For example, the interpolation may be performed within a computer-aided drafting (CAD) model or design. For example, the interpolation may comprise a sweeping function within a CAD program. It will also be appreciated that the cross-sections of the volute(s) at various positions along the dividing wall can be selected in various ways. For example, at least some of the cross sections may be imposed by other considerations (i.e. they may be of the form of boundary conditions). For example, the turbine housing may form part of a turbocharger and, in use, exhaust gas may be provided to the two (exhaust gas inlet) volutes from an exhaust manifold (also referred to as an outlet manifold) of an engine to which the turbocharger is attached. The turbinehousing may be designed for use with a specific engine and, therefore, the inlet of the turbinehousing may be matched to a corresponding outlet of the exhaust manifold (also referred to as an outlet manifold) of that engine. For example, the turbine housing may define a flange for connection to an engine. At least one of the cross sections of the first and second volutes that is selected (and input into the interpolation) may be a cross section at the inlet of the 69777365-1 turbine housing and may be selected (or input into the design method as a boundary condition) so as to match an outlet cross section of an engine. As explained above, the flow characteristics of through the turbine stage of a turbocharger are dependent on: the flow area and radius (of the centroid of the flow area from the axis) of the volutes at the critical plane(s); and the radius of the turbine wheel. Therefore, for a given turbine wheel the flow characteristics of the turbine stage are dependent on the flow area and radius (of the centroid of the flow area from the axis) of the volutes at the critical plane(s). As explained above, these are selected so as to achieve optimal performance for a particular set point of engine running conditions. Therefore, at least one of the cross sections of the first and second volutes that is selected (and input into the interpolation) may be a cross section at each of the critical planes of the first and second volutes so as to achieve optimized performance for a desired set point of engine running conditions. Typically, the thickness of the dividing wall between the inlet and the critical plane(s) is generally uniform. In particular, there is a prejudice against rapidly increasing the thickness of the dividing wall proximate to (and upstream of) the critical planes as any increase inthickness may result in decrease of the cross sectional areas of the first and second flowpassages upstream of the critical planes. However, note that the critical planes are the planes at which the cross-sectional area of the volutes are at a local minimum upstream of an exit of the volute. Therefore, such a change cannot be tolerated since this may change the location and size of the critical plane(s) and, as explained above, this has been carefully selected to match a desired set point of engine conditions. Additionally or alternatively, at least some of the cross sections may be selected, for example manually, by a designer / engineer. For the generation of the or each candidate thickness distribution, for each of the first and second volutes, the at least two positions along the dividing wall may comprise: a cross section at the inlet; a cross section at a critical plane of that volute; and a cross section at at least one position intermediate the inlet and the critical plane of that volute. A position intermediate the inlet and the critical plane of a volute may be referred to as an intermediate position of that volute. The cross section(s) at the least one intermediate position may be selected, for example manually, by a designer / engineer. 69777365-1 Such an arrangement is particularly advantageous since it allows an designer or engineer to manipulate the thickness distribution of the dividing wall so as to influence (for example improve or optimize) a thermomechanical performance of the turbine housing. Rather than simply interpolating between just the two boundary conditions, such embodiments allow for additional constraints to be input before the interpolation is performed. This may allow, for example, for a thickness of the dividing wall intermediate the inlet to be increased to improve a thermomechanical performance of the turbine housing.The method may comprise generating a plurality of such candidate thickness distributions.Any two of the plurality of candidate thickness distributions may be generated from two different sets of selected cross-sections of the first and second volutes. It will be appreciated that in order for any two of the plurality of candidate thickness distributions to be generated from two different sets of selected cross-sections of the first and second volutes, at least one member from one of the two sets of selected cross-sections of the first and second volutes should be different from all of the members of the other one of the two sets of selected cross-sections of the first and second volutes. It will also be appreciated that, in general, there may be some cross-sections of the first and second volutes that are common to two different sets of selected cross-sections of the first and second volutes. For example, in some embodiments, each candidate thickness distribution may be generated from a set of cross sections comprising: for each of the first and second volutes: a cross section at the inlet; cross section at a critical plane of that volute; and a cross section at at least one position intermediate the inlet and the critical plane of that volute. Two different candidate thickness distributions may be generated using the same cross sections at the inlet and the same cross sections at the critical planes of that volutes. However, for any two candidate thickness distributions, at least one of different intermediate cross section may be used. In some embodiments, the portions of the first and second volutes between the inlet and the respective first and second critical planes may be generally linear (in contrast, a portion of each volute downstream of the critical planes may spiral around the axis of the housing). The portions of the first and second volutes between the inlet and the respective first and second critical planes extend generally in a first direction parallel to the direction of gas flow through the volutes during use. The first direction may be referred to as the z-direction. 69777365-1 The portions of the first and second volutes between the inlet and the respective first and second critical planes also extend generally in a second direction (which is generally perpendicular to the direction of gas flow through the volutes during use). The second direction may be referred to as the y-direction. The thickness distribution may be a thickness in a third direction. The third direction may be referred to as the x-direction. In general, the positions of the cross sections used for the interpolation (in a direction in which the first and second volutes are spaced apart from each other) may vary across different sets of cross sections. A position in a direction in which the first and second volutes are spaced apart from each other may be referred to as the x-direction herein. A position in a direction in which the first and second volutes are spaced apart from each other is a position in the third direction (i.e. the direction in which the thickness distribution is defined) and may be referred to as the x-direction herein.At least one of the plurality of candidate thickness distributions may be generated such that,for at least one of the first and second volutes, a position of at least one of the selected cross sections in a direction in which the first and second volutes are spaced apart from each other is different to the position in the direction in which the first and second volutes are spaced apart from each other of all of the selected cross sections for that volute for at least one other of the plurality of candidate thickness distributions. In general, the different sets of cross sections may vary in position(s) (of at least one of the cross sections) along the dividing wall. Position along the dividing wall may be referred to as the z-direction herein. At least one of the plurality of candidate thickness distributions may be generated such that, for at least one of the first and second volutes, a position along the dividing wall of at least one of the selected cross sections is different to the positions of all of the selected cross sections for that volute for at least one other of the plurality of candidate thickness distributions. In general, the sizes of the cross sections may vary across different sets of cross sections.At least one of the plurality of candidate thickness distributions may be generated such that,for at least one of the first and second volutes, a size of at least one of the selected cross 69777365-1 sections is different to the size of all of the selected cross sections for that volute for at least one other of the plurality of candidate thickness distributions. In general, the shapes of the cross sections may vary across different sets of cross sections. At least one of the plurality of candidate thickness distributions may be generated such that, for at least one of the first and second volutes, a shape of at least one of the selected cross sections is different to the shape of all of the selected cross sections for that volute for at least one other of the plurality of candidate thickness distributions.The method may comprise generating a plurality of such candidate thickness distributions andmay further comprise selecting the thickness distribution of the dividing wall to be one of saidplurality of different candidate thickness distributions that has the best thermomechanical performance. Each of the first and second volutes may define a critical plane. At least one candidatethickness distribution may be generated at least partially from a cross section of the first voluteand a cross section of the second volute at a position along the dividing wall upstream of the critical planes of the first and second volutes that correspond to a thickness of the dividing wall that is greater than the thickness of the dividing wall at said critical planes by at least a first factor and wherein the first factor is 1.3 or greater.Each of the first and second volutes may define a critical plane. The selected thicknessdistribution of the dividing wall as a function of distance from the inlet may be such that at least a portion of the dividing wall upstream of the critical planes of the first and second volutes has a thickness that is greater than the thickness of the dividing wall at said critical plane(s) by at least a first factor and wherein the first factor is 1.3 or greater. This may, for example, be achieved by appropriate selection of one or more intermediate cross sections (i.e. cross sections between the inlet and the critical plane) of the first and second volutes. As explained above, despite prejudice in the art against rapidly increasing the thickness of the dividing wall proximate to (and upstream of) the critical planes, the inventors have realized that it is possible for the thickness distribution of the dividing wall as a function of distance from the inlet to be such that at least a portion of the dividing wall upstream of the critical planes of the first and second volutes has a thickness that is greater than the thickness of the 69777365-1 dividing wall at said critical plane(s) by at least a first factor and wherein the first factor is 1.3 or greater. For example, this may be achieved by also changing the other walls of the turbine housing that define the volutes so that the critical planes remain unchanged. Furthermore, it has been found that such an arrangement can result in a surprising increase in the lifetime of the turbine housing. For example, it has been found by the inventors (from thermomechanical analysis) that such turbine housings tend to fail in a region of the outer (circumferential) surface of the turbine housing in the vicinity of the dividing wall (typically between the inlet and the critical planes). Furthermore, it has been found that by adjusting the dividing wall thickness the lifetime of the turbine housing can be improved by at least a factor of two. It is particularly surprising that such a variation in the thickness of the dividing wall can have such an impact on the lifetime of the turbine housing since the turbine housings tend to fail on an outer wall. Therefore, it would seem more obvious to consider varying a thickness of the outer wall(s) of the housing. In general, the first factor may be significantly greater than, potentially significantly greater than 1.3. In some embodiments the first factor may be 1.4 or greater. In some embodiments the first factor may be 1.5 or greater. In some embodiments the first factor may be 1.6 or greater. In some embodiments the first factor may be 1.7 or greater. In some embodiments the first factor may be 1.8 or greater. In some embodiments the first factor may be 1.9 or greater. In some embodiments the first factor may be 2.0 or greater. In some embodiments the first factor may be 2.1 or greater. In some embodiments the first factor may be 2.2 or greater. In some embodiments the first factor may be 2.3 or greater. In some embodiments the first factor may be 2.4 or greater. In some embodiments the first factor may be 2.5 or greater. Each of the first and second volutes may define a critical plane. At least one candidate thickness distribution may be generated at least partially from a cross section of the first volute and a cross section of the second volute at a position along the dividing wall upstream of the critical planes of the first and second volutes that correspond to a thickness of the dividing wall that is greater than the thickness of the dividing wall at the inlet by at least a second factor and wherein the second factor is 1.3 or greater.Each of the first and second volutes may define a critical plane. The selected thicknessdistribution of the dividing wall as a function of distance from the inlet may be such that at leasta portion of the dividing wall upstream of the critical planes of the first and second volutes has 69777365-1 a thickness that is greater than the thickness of the dividing wall at the inlet by at least a second factor and wherein the second factor is 1.3 or greater. In some embodiments, the thickness of the dividing wall at the critical plane(s) may be substantially the same as the thickness of the dividing wall at the inlet. For such embodiments, the second factor may be substantially equal to the first factor. In general, the second factor may be significantly greater than, potentially significantly greater than 1.3. In some embodiments the second factor may be 1.4 or greater. In some embodiments the second factor may be 1.5 or greater. In some embodiments the second factor may be 1.6 or greater. In some embodiments the second factor may be 1.7 or greater. In some embodiments the second factor may be 1.8 or greater. In some embodiments the second factor may be 1.9 or greater. In some embodiments the second factor may be 2.0 or greater. In some embodiments the second factor may be 2.1 or greater. In some embodiments the second factor may be 2.2 or greater. In some embodiments the second factor may be 2.3 or greater. In some embodiments the second factor may be 2.4 or greater. In some embodiments the second factor may be 2.5 or greater.The thickness distribution may be selected so as to maximize a lifetime of the turbine housing.The method may comprise: parameterizing the thickness distribution; and selecting a set of parameters. Brief description of the drawings A the invention will now be described, for the sake of example only, with reference to the following figures, in which:Figure 1 is a cross-sectional drawing of a known turbocharger;Figures 2A shows a first perspective view of a new turbine housing according to anembodiment of the present disclosure;Figure 2B shows a second, different perspective view of the new turbine housing shownin Figure 2A;Figure 2C shows a first cross sectional view of the new turbine housing shown in Figures2A and 2B in a first plane that is perpendicular to an axis of the turbine housingand which passes through a first volute of the turbine housing; 69777365-1Figure 2D shows a second cross sectional view of the new turbine housing shown inFigures 2A to 2C in a second plane that is perpendicular to the axis of theturbine housing and which passes through a second volute of the turbine housing;Figure 2E shows a third cross sectional view of the new turbine housing shown in Figures2A to 2D in a third plane that is parallel to, and offset from, the axis of the turbinehousing and which passes through first and second inlet apertures of the turbine housing;Figure 2F shows a fourth cross sectional view of the new turbine housing shown inFigures 2A to 2E in a plane that passes through an inlet portion of the turbinehousing and which is perpendicular to a direction of gas flow through the inlet portion;Figure 2G shows a side view of the new turbine housing shown in Figures 2A to 2F;Figure 2H shows a partial cross sectional view of an inlet portion of the new turbinehousing shown in Figures 2A to 2G;Figure 3A shows a fourth cross sectional view of a turbine housing comprising a uniformthickness, in a plane that passes through an inlet portion of the turbine housingand which is perpendicular to a direction of gas flow through the inlet portion;Figure 3B shows a partial cross sectional view of an inlet portion of the turbine housingshown in Figure 3A;Figure 4 schematically shows an embodiment of a new method for designing a turbinehousing in dependence on a thermomechanical performance of the turbine housing;Figure 5A shows a first different perspective view of a second new turbine housingaccording to an embodiment of the present disclosure;Figure 5B shows a second different perspective view of the new turbine housing shownin Figure 5A;Figure 5C shows a first cross sectional view of the new turbine housing shown in Figures5A and 5B in a first plane that is perpendicular to an axis of the turbine housingand which passes through a first volute of the turbine housing;Figure 5D shows a second cross sectional view of the new turbine housing shown inFigures 5A to 5C in a second plane that is perpendicular to the axis of theturbine housing and which passes through a second volute of the turbine housing;Figure 5E shows a third cross sectional view of the new turbine housing shown in Figures5A to 5D in a third plane that is parallel to, and offset from, the axis of the turbine69777365-1 housing and which passes through first and second inlet apertures of the turbine housing;Figure 5F shows a fourth cross sectional view of the new turbine housing shown inFigures 5A to 5D in a fourth plane that contains the axis of the turbine housing;Figure 5G shows a fifth cross sectional view of the new turbine housing shown in Figures5A to 5F in a critical plane of a first volute of the turbine housing;Figure 5H shows a sixth cross sectional view of the new turbine housing shown in Figures5A to 5G in a critical plane of a second volute of the turbine housing; Figure 6 shows a gas flow volume defined by the new turbine housing shown in Figures 5A to 5H, the gas flow volume being the void, or region with no material, defined by the housing; Figure 7 shows a plurality of (eight) cross sections of the turbine housing shown in Figures 5A to 5H, each in a different plane perpendicular to the z-direction, the planes are labelled S2-S9 and are shown in Figure 6;Figure 8 shows the thickness distribution of the dividing wall of the turbine housingshown in Figures 5A to 5H as function of distance from the inlet of the volutes;Figure 9A shows a gas flow volume defined by a turbine housing having a more typical dividing wall thickness distribution; Figure 9B shows a cross sectional view of the gas flow volume as shown in Figure 9A through line A-A.Figure 10A shows the gas flow volume defined by the new turbine housing as shown inFigures 5A to 5H; Figure 10B shows a cross sectional view of the gas flow volume as shown in Figure 10A through line B-B;Figure 11A shows the cross sectional view of the gas flow volume through line B-B that isalso shown in Figure 10B.Figure 11B shows a cross sectional view of a first modified gas flow volumes through thesame line B-B (see Figures 10A and 10B);Figure 11C shows a cross sectional view of a second modified gas flow volumes throughthe same line B-B (see Figures 10A and 10B);Figure 11D shows a cross sectional view of a third modified gas flow volumes through thesame line B-B (see Figures 10A and 10B); Figure 12 shows a gas flow volume defined by a third new turbine housing according to an embodiment of the present disclosure; Figure 13 shows a plurality of (six) cross sections of the third turbine housing (cf. Figure 12) each in a different plane perpendicular to the z-direction, the planes are labelled S1-S6 and are shown in Figure 12; 69777365-1Figure 14 shows the thickness distribution of the dividing wall of the third turbine housing(cf. Figures 12 and 13) as function of distance from the inlet of the volutes;Figure 15 shows a thickness distribution of the dividing wall of a fourth turbine housingaccording to an embodiment of the present disclosure as function of distance from the inlet of the volutes;Figure 16A shows a partially cut-away perspective view of a new assembly according tothe present disclosure, the new assembly comprising the new turbine housing shown in Figures 5A to 5H and a wastegate valve member;Figure 16B is a cross sectional view of the new assembly shown in Figure 16A;Figure 17 schematically shows an embodiment of a new method for designing a turbinehousing in dependence on a thermomechanical performance of the turbine housing; andFigure 18 schematically shows an embodiment of a method for generating a candidatethickness distribution which may, for example, form part of the method shown in Figure 17. Detailed description of the embodimentsFigure 1 shows a schematic cross-section through a known turbocharger 1. The turbocharger1 comprises a turbine 11 joined to a compressor 12 via a central bearing housing 13. The turbine 11 comprises a turbine wheel 14 for rotation within a turbine housing 15. The turbinewheel 14 has a rotational axis 2 (in the plane of the diagram) and blades 9. Similarly, thecompressor 12 comprises a compressor wheel 16 (or “impeller”) which can rotate within a compressor housing 17. The compressor housing 17 defines a compressor chamber 38 whichis largely filled by the compressor wheel 16, and within which the compressor wheel 16 canrotate. The turbine wheel 14 and compressor wheel 16 are mounted on opposite ends of acommon turbocharger shaft 18 which extends through the central bearing housing 13. Theturbocharger shaft 18 is rotatably supported by a bearing assembly in the bearing housing 13 which comprises two journal bearings 34 and 35 housed towards the turbine end and compressor end respectively of the bearing housing 13. The bearing assembly further includes a thrust bearing 36. The turbine housing 15 has two exhaust gas inlet volutes 19a, 19b located annularly aroundthe turbine wheel 14, and an axial exhaust gas outlet 10. The axial exhaust gas outlet 10 isdefined by a generally cylindrical exit portion 30 of the turbine housing 15. The volutes 19a,19b are symmetrical with respect to each other in a mirror plane perpendicular to the axial direction (note that in other known turbine housings the volutes are not symmetrical; 69777365-1furthermore in some known turbines the volutes are circumferentially spaced, for example by180 degrees, about the rotational axis 2 of the turbine). The compressor housing 17 has an axial air intake passage 31 and a volute 32 arranged annularly around the compressor chamber 38. The volute 32 is in gas flow communication with a compressor outlet 33. The compressor chamber 38 is connected to the volute 32 by a radially-extending diffuser space 39 (also referred to here as a “diffuser”) which is a gap between a radially-extending shroud surface 25 of the housing 17, and a radially extending hub surface 26 of the bearing housing 13. The diffuser 39 is rotationally symmetric about the rotational axis 2 of the shaft 18. In use, exhaust gas is provided to the two exhaust gas inlet volutes 19a, 19b from an exhaust manifold (also referred to as an outlet manifold) of the engine (not shown) to which theturbocharger is attached. The turbine housing 15 defines a flange 40 for connection to anengine. The flange 40 defines an inlet 41 of the turbine housing 15. Gas flows via the inlet 41 into the two exhaust gas inlet volutes 19a, 19b. The inlet volutes 19a, 19b are divided by a divider wall 20 which extends radially inwardly from the radially outer wall 21 of the turbine housing 15, to a tip 22. The exhaust gas exits volute 19a through a gap 27a between the divider wall 20 and a first shroud surface 23 of the turbine11. The exhaust gas exits volute 19b through a gap 27b between the divider wall 20 and asecond shroud surface 24 of the turbine 11. In variants, the second shroud 24 surface may be provided as a surface of the bearing housing or some other component, instead of being asurface of the turbine housing 15. Each of the gaps 27a, 27b between the divider wall 20 andthe first and second shroud surfaces 23, 24 of the turbine 11 defines an outlet passage of oneof the volutes 19a, 19b (also referred to as an exit of the volutes 19a, 19b).The turbine blades 9 extend generally radially outwards from a radially inner hub of the turbinewheel 14. An outer radial edge of the turbine blades 9 comprises a generally flat, first portion, which is adjacent to the bearing housing 13 and which defines a turbine wheel tip 28. A second portion 29 of the outer radial edge of the turbine blades 9, which is distal from thebearing housing 13, is curved and has a shape that is generally complimentary to an innersurface of exit portion of the turbine housing 15. In use, the second portion 29 of the outerradial edge of the turbine blades 9 sweeps across said an inner surface of exit portion of theturbine housing 15 with as small a clearance between these components as possible.Exhaust gasses flowing out of the gas inlet volutes 19a, 19b via gaps 27a, 27b flows thought a gap defined between the first shroud surface 23 and the second shroud surface 24 of the turbine 11. In use, the turbine wheel tip 28 sweeps over the gap 37. Thus, the exhaust gas 69777365-1 passes from the exhaust gas inlet volutes 19a, 19b, via gaps 27a, 28b and gap onto turbine wheel tip 28. The exhaust gas flows over the turbine wheel 14 (between the blades 9) to the exhaust gas outlet 10. The turbine blades 9 are shaped so that this flow of exhaust gas acts on the turbine wheel 15 and causes it to rotate about rotational axis 2. The turbine wheel 14 in turn rotates the compressor wheel 16 which thereby draws intake air through the compressor inlet 31 and delivers boost air to an inlet manifold of the engine via the diffuser 39, the volute 32 and then the outlet 33. It will be appreciated that, in use, each of the two volutes 19a, 19b may be connected to a different set of cylinders of an internal combustion engine such that pulses of exhaust gas generated by the internal combustion engine are directed alternately to volute 19a and volute 19b. Depending on the operating conditions of the engine and turbocharger 1, as a pulse of exhaust gas passes through one volute 19a it may affect the flow of gas through the other volute 19b. A range of operating conditions may exist. In one extreme, the pressure differentials across the two volutes 19a, 19b may be such that as a pulse of exhaust gas passes through one volute 19a the flow of gas through the other volute 19b is unaffected. This is known as equal admission. At another extreme, the pressure differentials across the two volutes 19a, 19b may be such that as a pulse of exhaust gas passes through one volute 19a the flow of gas through the other volute 19b ceases until the next pulse passes through the other volute 19b (at which time the flow through volute 19a ceases). This is known as single admission. In general, the turbine may operate somewhere in between these two extreme conditions. As gas passes through the turbine stage it undergoes an expansion process from a firstpressure and temperature to a second (lower) pressure and temperature. The efficiency ofthe turbine stage may be defined as the ratio between the actual power produced by the realmachine under given conditions of massflow, temperature and pressure to the power thatwould be produced by an ideal or perfect machine under the same operating conditions. Itwill be appreciated that the expansion process for the ideal machine is an isentropic process (i.e. there is no change in entropy) and hence is without losses. It will be appreciated that the efficiency of the turbine stage will, in general, be dependent on a wide range of parameters of the turbine wheel 14 and the turbine housing 15, which characterize the size and shape of the turbine wheel 14 and the turbine housing 15. As used here, the term turbine stage is intended to refer to the combination of the turbine housing 15 and the turbine wheel 14. 69777365-1It will be further appreciated that many of these parameters may have been selected basedupon one or more design constraints. For example, the geometry of the volutes 19a, 19b andthe critical area(s) of the volutes 19, 19a may have been selected based upon one or moredesign constraints or desired set points. As used here, the critical area of one of the volutes19a, 19b is the cross-sectional area of that volute 19a, 19b measured at a plane that intersectsa tip of a tongue that partially defines the volute 19a, 19b. Furthermore, the parameters of the turbine wheel 14 may have been selected, for example using known matching techniques, to achieve a desired set point. These parameters include, for example, the critical area and of the turbine wheel 14 and the width of the turbine wheel tip 28. The critical area and of the turbine wheel 14 is defined as the smallest area along the meridional length of the wheel (that is, along the flow path defined between the blades) transverse to the flow path. This is usually located in the exducer of the wheel. The critical area and of the turbine wheel 14 may alternatively be referred to as the wheel throat area.Some embodiments of the present disclosure relate to new designs of turbine housing havingtwo volutes (i.e. of the general type shown in Figure 1). Figures 2A to 2H show a new turbine housing 500 according to an embodiment of the present disclosure. The turbine housing 500 is generally of the form of the turbine housing 15 shown in Figure 1 and described above. As described further below, a consistent right-handed set of Cartesian axes are shown used in Figures 2A to 2H. Figures 2A and 2B each show a different perspective view of the new turbine housing 500.Figure 2C shows a first cross sectional view of the new turbine housing 500 in a first planethat is perpendicular to an axis 514 of the turbine housing 500 and which passes through afirst volute 510a of the turbine housing 500. Figure 2D shows a second cross sectional viewof the new turbine housing 500 in a second plane that is perpendicular to the axis 514 of theturbine housing 500 and which passes through a second volute 510b of the turbine housing 500.Figure 2E shows a third cross sectional view of the new turbine housing 500 in a third planethat is parallel to, and offset from, the axis 514 of the turbine housing 500 and which passesthrough first and second inlets 508a, 508b of the turbine housing 500. Figure 2F shows afourth cross sectional view of the new turbine housing 500 in a fourth plane that passes69777365-1 through the inlet portion 502 of the turbine housing 500 and which is perpendicular to adirection of gas flow through the inlet portion 502.Figure 2G shows a side view of the new turbine housing 500.Figure 2H shows a partial cross sectional view of the inlet portion 502 of the new turbinehousing 600. The turbine housing 500 is for use in a turbocharger, for example of the form of the turbocharger 1 shown in Figure 1 and described above. The turbine housing 500 comprising:an inlet portion 502; a second portion 504 for housing a turbine wheel (not shown); and anoutlet portion 506.The inlet portion 502 defines a first inlet 508a and a second inlet 508b. The inlet portion 502and the second portion 504 define: a first volute 510a; and a second volute 510b. The firstvolute 510a defines a first flow passage between the first inlet 508a and a first volute outlet512a. The first volute 510a extends around an axis 514 of the turbine housing 500. The second volute 510b defines a second flow passage between the second inlet 508b and asecond volute outlet 512b. The second volute 510b extends around the axis 514 of the turbinehousing 500.The first volute 510a and the second volute 510b are separated, in an axial direction, by a dividing wall 516. It will be appreciated that, unless stated to the contrary, any reference herein to an axial direction is intended to mean a direction that is along, or parallel to, the axis 514 of the turbine housing 500. In addition, unless stated to the contrary, any reference herein to a radial direction is intended to mean a direction that is generally perpendicular to, extends through, the axis 514 of the turbine housing 500. In addition, unless stated to the contrary, any reference herein to a circumferential or tangential direction is intended to mean a direction that is generally perpendicular to the axial and radial directions. The outlet portion 506 may extend generally axially. At least a portion of the first and second volutes 510a, 510b defined by the inlet portion 502 of the housing 500 may extend generally linearly in a circumferential or tangential direction. 69777365-1 A circumferential outlet portion of each of the first and second volutes 510a, 510b may be defined by one or more tongues 518a, 518b. A critical plane 520a, 520b of each volute 510a, 510b may be defined as a plane that contains the axis 514 of the housing 500 and which intersects a tip of a tongue 518a, 518b that partially defines that volute 510a, 510b. Put differently, the critical plane 520a, 520b is the plane at which the known A / R ratio of a turbocharger comprising the turbine housing 500 would becalculated. In general, the critical plane 520a, 520b of a volute 510a, 510b is a plane thatcontains the axis 514 of the housing 500 where the cross-sectional area of that volute 510a, 510b is at a local minimum. That is, in general, as a function of distance through the volute 510a, 510b the area decreases between the inlet 508a, 508b and the critical area 520a, 520b and then increases immediately downstream of the critical area 520a, 520b (as the gas flow clears the tongue 518a, 518b of the volute 520a, 520b). Further downstream still, the area then typically decreases down to the exit of the volute 512a, 512b. In some embodiments, the first and second volutes 510a, 510b may both be partially defined by a common tongue. For such embodiments, the first and second volutes 510a, 510b mayshare a common critical plane. In some embodiments, the first volute 510a may be partiallydefined by a first tongue and the second volute 510b may be partially defined by a second tongue. The first and second tongues may be circumferentially spaced apart. In some embodiments, the portions of the first and second volutes 510a, 510b between the inlet 502 and the respective tongues 518a, 518b of the first and second volutes 510a, 510b may be generally linear (in contrast, a portion of each volute 510a, 510b downstream of the tongue(s) 518a, 518b may spiral around the axis 514 of the housing 500). The inlet portion 502 may extend from a flange 522 of the housing 500 to the tongues 518a, 518b of the first and second volutes 510a, 510b. The second portion 504 of the housing 500 (which is for housing a turbine wheel) may extend around the axis 514 of the turbine housing 500. The second portion 504 of the housing 500 may define part of the first and second volutes 510a, 510b that is downstream of their respective tongues 518a, 518b.A flow path through the first volute 510a is schematically shown by dashed line 511a and aflow path through the second volute 510b is schematically shown by dashed line 511b. 69777365-1 For at least a portion of the dividing wall 516 upstream of a tongue 518a of the first volute 510a and a tongue 518b of the second volute 510b a thickness of the dividing wall 516 varies in a first direction that is generally perpendicular to a second direction that, in use, gas will flow along through the first and second volutes 510a, 510b such that the thickness increases in a direction generally away from the second portion 504 of the turbine housing. It will be appreciated that, in general, the dividing wall 516 has some extent in three dimensions. A thickness of the dividing wall 516 may be the smallest dimension of the dividing wall 516. The thickness is generally in an axial direction of the turbine housing 500 (the x- direction in Figures 2A to 2H). The dividing wall 516 may be said to extend in a plane that is generally perpendicular to the axial direction (i.e. the y-z plane in Figures 2A to 2H). The turbine housing 500 shown in Figures 2A to 2H relates to an arrangement wherein thethickness of the dividing wall 514 is not uniform across the plane in which it extends.As used herein a direction of gas flow through the first and second volutes during use may be referred to as the second direction. In the inlet portion 502 of the turbine housing 500, the second direction is generally circumferential or tangential with respect to the axis 514 of the turbine 500. In particular, in the inlet portion 502 of the turbine housing 500, the second direction is generally along the z-direction in Figures 2A to 2H. A direction that is generally perpendicular the second direction may be referred to as a first direction (this is the y-direction in Figures 2A to 2H). The first direction has at least a component that is generally radial. The new turbine housing 500 shown in Figures 2A to 2H is advantageous, as now discussed. In use, the turbine housing 500 may form part of a turbocharger (for example a turbocharger 1 of the type shown in Figure 1) and a turbine wheel (for example a turbine wheel 14 of the type shown in Figure 1) may be disposed at least partially within the housing 500, downstreamof the first and second volutes 510a, 510b. A turbocharger is a known device for extractingenergy from the exhaust gas from an engine and using this energy to deliver air at increased pressure to the inlet of the engine (so as to increase the power of the engine). Exhaust gas admitted into the inlet is accelerated as it passes through the volutes. This increase in gas velocity imparts more momentum to the turbine wheel thus increasing the power generated by the turbine wheel. The inventors of the present invention have found that, with some designs of turbine housing, there is a low thermomechanical failure (TMF) lifetime at the outboard region 524 of the turbinehousing inlet portion 502 (near the flange 522). As used here the outboard region is intended69777365-1 to mean the region of the turbine housing 500 distal the second portion 504 or, equivalently, a generally radially outer portion of the turbine housing 500. The outboard region of the turbinehousing inlet portion 502 may be referred to as the radially outboard region of the turbinehousing inlet portion 502. One way to address such a problem is to increase the thickness ofthe dividing wall 516 by adding a uniform pad to the dividing wall 516. Such an arrangement is shown in Figures 3A and 3B. Figure 3A shows a cross sectional view of such a turbine housing 600 with a uniform thickness pad in a plane that passes through an inlet portion of the turbine housing and which is perpendicular to a direction of gas flow through the inlet portion (i.e. an equivalent cross section to Figure 2F); Figure 3B shows a partial cross sectional view of an inlet portion of the turbine housing 600 with a uniform thickness (i.e. an equivalent cross section to Figure 2H).Features of the turbine housing 600 with a uniform thickness have reference numerals thatare incremented by 100 relative to generally equivalent to features of the new turbine housing 500 described above. However, the inventors have found that with some designs the solution of a uniform thicknesspad 626, as shown schematically in Figures 3A and 3B, does not address the problem.The turbine housing 500 shown in Figures 2A to 2H effectively uses a non-uniform thicknesspad or dividing wall 516 which is thinner at the (radially) inboard region 528 of the inlet portion502 (proximate the second portion 504) and thicker at the outboard region 524 (distal the second portion 504). As will be appreciated by the skilled person, when altering the thickness of the dividing wall 516, care should be taken that this does not affect the performance of the turbine. There is a limit to how much material can be added to the dividing wall 516 without adversely affecting the performance of the turbine. By providing a non-uniform thickness dividing wall, more material can be provided at the outboard region 524 (distal the second portion 504), which has been found to increase the TMF lifetime of the turbine housing 500. Furthermore, it does so without making the entire dividing wall 516 thicker, which would involve the addition of more material and would have a greater impact on the performance of the turbine. Advantageously, the inventors have realized that, in principle, a thickness distribution of the dividing wall 516 in the first direction (that, in use, is perpendicular to the gas flow; the y- direction in Figures 2A to 2H) can be optimized, in dependence on the shape and dimensions of the flange 516, so as to maximize the TMF life of the turbine housing 500. For example, an 69777365-1 amount by which the thickness increases in the first direction (and over what extent in the first direction) may be selected to maximize TMF life of the turbine housing 500. Furthermore, a turbine housing 500 shown in Figures 2A to 2H potentially represents a relatively small change to the design of the turbine housing and so can be implemented on existing turbine housings with a relatively small (and inexpensive) change to a tool used for casting the turbine housing. The increased thickness of the dividing wall 516 distal the second portion 504 of the turbine housing 500 can provide additional strength to a flange 522 of the turbine housing 500. In particular, the increased thickness of the dividing wall 516 distal the second portion 504 of the turbine housing 500 can provide additional strength to a radially outboard portion 522a of the flange 522 of the turbine housing 500, as now discussed. It will be appreciated that the(radially) inboard region 528 of the inlet portion 502 is proximate the second portion 504 andthat therefore both the (radially) inboard region 528 of the inlet portion 502 and the secondportion 504 will provide support to a radially inboard portion 522b of the flange 522. In contrast,the radially outboard portion 522a of the flange 522 is only supported by the outboard region524 of the inlet portion 502 (distal the second portion 504). Therefore, advantageously, byincreasing a thickness of the dividing wall 516 distal the second portion 504 (where it joins theoutboard region 524 of the inlet portion 502) additional strength can be provided to the radiallyoutboard portion 522a of the flange 522. It will be appreciated that by the skilled person that changing a thickness of a wall within a turbine housing will change: (a) a rate of temperature change of that wall; (b) a rate of thermal expansion of that wall; and (c) a strength of that wall. To ensure good TMF lifetime thesethree aspects are balanced to achieve a lower stress level in the material of the housing duringthermal cycling. It will be appreciated that if a part takes longer to heat up than surroundingareas then that part may undergo less thermal distortion that the surrounding areas, leadingto higher stresses. The skilled person will appreciate that in general it is desirable for all partsof the turbine housing to undergo a similar thermal expansion over a typical range of operatingtemperatures. It will be appreciated that, at least in the inlet portion 502, at both ends in the first direction (the y-direction in Figures 2A to 2H) the dividing wall 516 will meet another portion or wall of the turbine housing 500. Equivalently, in the inlet portion 502, both an inner and outer radial portion of the dividing wall 516 will merge into another wall of the turbine housing 500. In the second portion 504 of the turbine housing 500 (downstream of the tongue(s) 518a, 518b of 69777365-1 the volutes 510a, 510b), a radially inner portion of the volutes 510a, 510b may be open and so the dividing wall 516 may only merge into the radially outer (or outboard) wall of the turbine housing 500. Furthermore, the volutes 510a, 510b will typically be defined by a smooth, curved inner surface of the turbine housing 500. Therefore, where the dividing wall 516 meets another wall (at the radially inner and outer ends of the dividing wall 516), a thickness of material will increase. For example, as shown in Figure 2F, in a plane perpendicular to a flow direction of the gas (the x-y plane for the inlet portion 502 shown in Figures 2A to 2H), the dividing wall may comprise: a central portion 530, which may be referred to as a main portion 530 of the dividing wall 516; and two end portions 532, 534 which comprise curved fillets where the dividing wall516 meets the external walls of the turbine housing 500. The main portion 530 of the dividingwall 516 may have an extent 530a in the first direction. The extent 530a of the dividing wall516 in the first direction may be of the order of 60% of a maximum extent 536 of the volutes510a, 510b in the first direction. Each of the two end portions 532, 534 of the dividing wall 516 may have an extent 532a, 534a in the first direction. The extent 532a, 534a of the twoend portions 532, 534 of the dividing wall 516 in the first direction may be of the order of 20%of the maximum extent 536 of the volutes 510a, 510b in the first direction.Note that, in general, the extent of the two end portions may be selected based on a desiredfillet radius and may be similar for different sized turbine housings whereas the central portion will typically have a larger extent for larger turbine housings (and vice versa). Therefore, the skilled person will be aware than the extent of the central portion and the two end portions may be dependent on a size of the turbine housing. In general, at least a part of the central portion 530 of the dividing wall 516 may increase in thickness in the first direction. Note that it may be generally more advantageous for the increase in thickness of the dividingwall to occur over substantially the entire central portion 530 of the dividing wall 516 in the firstdirection rather than, for example, the increase occurring over part of the central portion and part of the central portion having a uniform thickness in the first direction. In the embodiment shown in Figures 2A to 2H, for at least a portion of the dividing wall 516 inthe second direction (z-direction), the thickness of the dividing wall 516 in the first direction (y-69777365-1 direction) increases in the direction generally away from the second portion 504 of the turbinehousing 500 for a central portion 530 of the dividing wall in the first direction.In the embodiment shown in Figures 2A to 2H, for at least a portion of the dividing wall 516 inthe second direction (z-direction), the thickness of the dividing wall 516 in the first direction (y-direction) increases in the direction generally away from the second portion 504 of the turbine housing 500 for a portion 530 of the dividing wall 516 having an extent 530a in the first directionthat is around 64% of a maximum extent 536 of the volutes 510a, 510b in the first direction.In some embodiments, the portion 530 of the dividing wall 516 may have an extent 530a in the first direction (y-direction) that is at least 30% of a maximum extent 536 of the volutes 510a, 510b in the first direction (y-direction). In some embodiments, the portion 530 of the dividing wall 516 may have an extent 530a in the first direction (y-direction) that is at least 40% of a maximum extent 536 of the volutes 510a, 510b in the first direction (y-direction). In some embodiments, the portion 530 of the dividing wall 516 may have an extent 530a in the first direction (y-direction) that is at least 50% of a maximum extent 536 of the volutes 510a, 510b in the first direction (y-direction). In some embodiments, the portion 530 of the dividing wall516 may have an extent 530a in the first direction (y-direction) that is at least 60% of amaximum extent 536 of the volutes 510a, 510b in the first direction (y-direction). In some embodiments, the portion 530 of the dividing wall 516 may have an extent 530a in the first direction (y-direction) that is at least 70% of a maximum extent 536 of the volutes 510a, 510b in the first direction (y-direction). In some embodiments, the portion 530 of the dividing wall516 may have an extent 530a in the first direction (y-direction) that is at least 80% of amaximum extent 536 of the volutes 510a, 510b in the first direction (y-direction). In the embodiment shown in Figures 2A to 2H, for at least a portion of the dividing wall 516 inthe second direction (z-direction), the thickness of the dividing wall 516 in the first direction (y-direction) increases linearly for at least a portion 530 of the dividing wall 516 in the directiongenerally away from the second portion 504 of the turbine housing 500.In the embodiment shown in Figures 2A to 2H, for at least a portion of the dividing wall 516 in the second direction (z-direction), the thickness of the dividing wall 516 in the first direction (y- direction) increases in the direction generally away from the second portion 504 of the turbinehousing 500 with a rate of increase of thickness of 0.04 mm / mm in the first direction. It will beappreciated that the rate of increase of thickness of in the first direction may be dt / dx, where t is the thickness and x is the first direction. 69777365-1 In some embodiments of the turbine housing 500, for at least a portion of the dividing wall 516 in the second direction (z-direction), the thickness of the dividing wall 516 in the first direction (y-direction) may increases in the direction generally away from the second portion 504 of theturbine housing 500 with a rate of increase of thickness in the first direction in the range 0.02mm / mm to 0.06 mm / mm, for example in the range 0.03 mm / mm to 0.04 mm / mm. For a dividing wall of uniform thickness it will be appreciated that two opposite sides of the dividing wall will be mutually parallel. In contrast, for a dividing wall 516 with non-uniform thickness, the two opposite sides of the dividing wall 516 will be at non-zero angle to each other. The angle between two opposite sides of the dividing wall 516 may be referred to as a taper angle of the dividing wall 516. In the embodiment shown in Figures 2A to 2H, for at least a portion of the dividing wall 516 in the second direction (z-direction), the thickness of the dividing wall 516 in the first direction (y- direction) increases in the direction generally away from the second portion 504 of the turbine housing 500 with a taper angle of approximately 3°. In some embodiments of the turbine housing 500, for at least a portion of the dividing wall 516 in the second direction (z-direction), the thickness of the dividing wall 516 in the first direction (y-direction) increases in the direction generally away from the second portion 504 of theturbine housing 500 with a taper angle of in a range of 1° to 5°, for example in a range of 2°to 4°. For the at least a portion of the dividing wall 516 in the second direction (z-direction) where the thickness of the dividing wall 516 in the first direction (y-direction) increases in the direction generally away from the second portion 504 of the turbine housing 500, the taper angle maybe θ°. Note that it may be generally desirable to keep the taper angle θ of the dividing wall516 relatively low so as to reduce the impact that the taper of the dividing wall 516 has on the efficiency of a turbine stage that comprises the new turbine housing 500. For example, insome embodiments of the turbine housing 500 the taper angle may be less than 10°. In someembodiments of the turbine housing 500 the taper angle may be less than 9°. In some embodiments of the turbine housing 500 the taper angle may be less than 8°. In some embodiments of the turbine housing 500 the taper angle may be less than 7°. In some embodiments of the turbine housing 500 the taper angle may be less than 6°. In some embodiments of the turbine housing 500 the taper angle may be less than 5°. 69777365-1 In the embodiment shown in Figures 2A to 2H, for at least a portion of the dividing wall in the second direction (z-direction), the thickness of the dividing wall 516 in the first direction (y- direction) may monotonically increase in the direction generally away from the second portion 504 of the turbine housing 500 for at least a portion 530, 532 of the dividing wall 516 in the first direction. It will be appreciated that the thickness of the dividing wall 516 in the first direction monotonically increasing in the direction generally away from the second portion 504 of the turbine housing 500 for at least a portion 530, 532 of the dividing wall 516 in the first direction means that for that portion 530, 532 of the dividing wall the thickness does not decrease in the direction generally away from the second portion 504 of the turbine housing 500. In the embodiment shown in Figures 2A to 2H, for at least a portion of the dividing wall 516 in the second direction (z-direction), the thickness of the dividing wall 516 in the first direction (y- direction) increases in the direction generally away from the second portion 504 of the turbine housing 500 for a radially outboard portion 530, 532 of the dividing wall 516 that has an extentin the first direction that is at least 70% of a maximum extent 536 of the volutes 510a, 510b inthe first direction. In some embodiments of the new turbine housing 500, at least a portion of the dividing wall 516 upstream of the tongue 518a of the first volute 510a and the tongue 518b of the second volute 510b may have a thickness that is greater than a thickness of the dividing wall 516 at the first inlet 508a and the second inlet 508b. That is, between the inlets 508a, 508b (defined at the flange 522) and the tongues 518a, 518b of the first and second volutes 510a, 510b there is a portion of the dividing wall which has an enlarged thickness which is greater than the dividing wall 516 thickness at the inlets 508a, 508b (defined at the flange 522). Advantageously, such an enlarged thickness may improve a thermomechanical performance of the turbine housing 500 whilst still meeting the boundary condition of a specific flange 522 design. The enlarged portion of the dividing wall 516 may, for example, be referred to as a pad. In some existing arrangements, in an inlet portion of the turbine housing 600 (between the first and second inlets and the tongues of the first and second volutes 610a, 610b), an enlarged thickness portion 626 may be provided (see Figures 3A and 3B, as discussed above). However, typically the enlarged thickness portion 626 would have a uniform thickness (excluding internal fillets or scalloping that may be provided at each end in the first direction). 69777365-1 In the embodiment shown in Figures 2A to 2H, for at least a portion of the dividing wall in the second direction (z-direction), the thickness of the dividing wall 516 in the first direction (y- direction) is asymmetric about a center 538 of the dividing wall 516 in the first direction (y- direction). In particular, the thickness of the dividing wall 516 increases from the center 538 in the direction generally away from the second portion 504 of the turbine housing 500 (i.e. positive y-direction) by a greater amount than the thickness of the dividing wall 516 increases from the center 538 in the direction generally towards the second portion 504 of the turbine housing (i.e. negative y-direction). That is, there is an asymmetry such that there is a net increase in the thickness of the dividing wall 516 towards the outboard direction even accounting for internal fillets and / or scalloping of the dividing wall 516. In some embodiments, the turbine housing 500 may further define at least one wastegatepassage 540 (see Figure 2E) via which at least one of the first and second volutes 510a, 510bis in fluid communication with the outlet portion 506. For example, at least one wastegate passage may connect each of the first and second volutes 510a, 510b to the outlet portion 506. The or each at least one wastegate passage may connect to a portion of at least one of the first and second volutes 510a, 510b upstream of a tongue 518a, 518b of that volute 510a, 510b to the outlet portion 506.Some embodiments of the present disclosure relate to a turbine comprising: the turbinehousing 500 shown in Figures 2A to 2H; and a turbine wheel (not shown), the turbine wheel received in the turbine housing 500. The turbine may be a fixed geometry turbine. The turbine may be a variable geometry turbine. The turbine may further comprise: a shaft supporting the turbine wheel; and a bearing housing supporting the shaft for rotation about a rotation axis. The rotation axis may be colinear with the axis 514 of the turbine housing 500. The turbine may further comprise a nozzle member disposed between the exits 512a, 512b of each of the first and second volutes 510a, 510b and the turbine wheel.Some embodiments of the present disclosure relate to a turbocharger comprising: the turbinehousing 500 shown in Figures 2A to 2H. The turbocharger may, for example, be generally of the form of, and comprise any combination of features of, the turbocharger 1 shown in Figure 1 and described above. In particular, the turbocharger may further comprise: a turbine wheel received in the turbine housing 500; a bearing housing configured to support a shaft for rotation about a rotation axis; and a compressor, the compressor comprising a compressor housing and a compressor wheel. The turbine wheel and the compressor wheel may be in power communication via the shaft. 69777365-1 The turbocharger may be a fixed geometry turbocharger. The turbocharger may be a variable geometry turbocharger. The turbocharger may form part of an engine arrangement e.g. for a vehicle or a pump. The bearing housing may support the shaft for rotation by one or more bearing assemblies. The bearing assemblies may comprise journal and / or thrust bearings. The turbine wheel and compressor wheel being in power communication with one another is intended to mean that rotation of one of the turbine wheel or compressor wheel drives rotation of the other of the compressor wheel and the turbine wheel. In operation, when the turbocharger is being used, exhaust gas drives the turbine wheel which, in turn, drives the compressor wheel to provide a boost pressure. Some embodiments of the present disclosure relate to a method for designing a turbine housing, for example a turbine housing 500 of the form shown in Figures 2A to 2H and as described above. Such a method 700 for designing a turbine housing 500 is shown schematically in Figure 4. In general, the turbine housing defines: an inlet portion 502; a second portion 504 for housing a turbine wheel; and an outlet portion 506 and the inlet portion 502 and the second portion 504 define: first and second volutes 510a, 510b separated by a dividing wall 516. The first and second volutes 510a, 510b respectively define first and second flow passages between the inlet portion 502 and the outlet portion 506. The method 700 comprises a step 710 of, for at least a portion of the dividing wall 516 upstream of a tongue 518a of the first volute 510a and a tongue 518b of the second volute 51b, selecting a thickness distribution of the dividing wall in a first direction (y-direction in Figures 2A to 2H) that is generally perpendicular to a second direction (z-direction in Figures 2A to 2H) that, in use, gas will flow along through the first and second volutes 510a, 510b such that the thickness increases in a direction generally away from the second portion 504 of the turbine housing 500. In some embodiments, at step 710 the thickness distribution is selected so as to optimize a thermomechanical performance of the turbine housing 500. In some embodiments of the method 700, the step 710 of selecting the thickness distribution of the dividing wall 516 in the first direction may comprise a step 712 of selecting an extent 530a of a portion 530 of the dividing wall 516 over which the thickness increases. For example, the extent 530a of a portion 530 of the dividing wall 516 over which the thickness 69777365-1 increases may be selected to be a fraction or percentage of a maximum extent of the volutes in the first direction, for example which optimizes a thermomechanical performance of the turbine housing 500. It will be appreciated by the skilled person that as used here optimizing a thermomechanical performance of the turbine housing may be carried out either using simulations and / or modelling. For example, the method 700 may be carried out using an iterative analysis. Additionally or alternatively, the method 700 may be carried out using physical methods,In some embodiments of the method 700, the step 710 selecting the thickness distribution ofthe dividing wall 516 in the first direction may comprise a step 714 of selecting a rate of increase of thickness in the first direction of a portion 530 of the dividing wall 516 over which the thickness increases.In some embodiments of the method 700, the step 710 of selecting the thickness distributionof the dividing wall 516 in the first direction may comprise a step 716 of selecting a taper angle of a portion 730 of the dividing wall 716 over which the thickness increases.In some embodiments of the method 700, any combination of steps 712, 714, 716 may becarried out sequentially or in parallel to optimize a thermomechanical performance of the turbine housing 500. For example, in some embodiments, (a) an extent 530a of a portion 530 of the dividing wall 516 over which the thickness increases; (b) a rate of increase of thickness in the first direction of a portion 530 of the dividing wall 516 over which the thickness increases; and / or (c) a taper angle of a portion 730 of the dividing wall 716 over which the thickness increases may be co-optimized to optimize a thermomechanical performance of the turbine housing 500. Figures 5A to 5H show a second new turbine housing 100 according to an embodiment of the present disclosure. The turbine housing 100 is generally of the form of the turbine housing 15shown in Figure 1 and described above. As described further below, a consistent right-handedset of Cartesian axes are shown used in Figures 5A to 5H. Figures 5A and 5B each show a different perspective view of the new turbine housing 100.Figure 5C shows a first cross sectional view of the new turbine housing 100 in a first planethat is perpendicular to an axis 108 of the turbine housing 100 and which passes through afirst volute 106a of the turbine housing 100. Figure 5D shows a second cross sectional view69777365-1of the new turbine housing 100 in a second plane that is perpendicular to the axis 108 of theturbine housing 100 and which passes through a second volute 106b of the turbine housing 100.Figure 5E shows a third cross sectional view of the new turbine housing 100 in a third planethat is parallel to, and offset from, the axis 108 of the turbine housing 100 and which passesthrough first and second inlet apertures 102a, 102b of the turbine housing 100. Figure 5Fshows a fourth cross sectional view of the new turbine housing 100 in a fourth plane thatcontains the axis of the turbine housing 100.Figure 5G shows a fifth cross sectional view of the new turbine housing 100 in a critical plane116a of a first volute 106a of the turbine housing 100. Figure 5H shows a sixth cross sectionalview of the new turbine housing 100 in a critical plane 116b of a second volute 106b of theturbine housing 100.The turbine housing 100 defines: an inlet 102; an outlet 104; a first volute 106a; a secondvolute 106b; and an axis 108.The turbine housing 100 defines a flange 110 for connection to an engine. The flange 110 defines the inlet 102 of the turbine housing 100 as two apertures 102a, 102b. Gas flows via the inlet 102a, 102b into the two exhaust gas inlet volutes 106a, 106b. The first volute 106 and the second volute 106b respectively define first and second volute exits 112a, 112b which are distal the inlet 102. The first and second volute exits 112a, 112b are generally equivalent to the gaps 27a, 27b shown in Figure 1. The first volute 106 and the second volute 106b respectively define first and second flow passages between the inlet 102 and the outlet 104. As can be seen in Figures 5C and 5D,the first volute 106a and the second volute 106b are respectively partially defined by a tongue114a, 114b. The first and second volutes 106a, 106b are defined around the axis 108 andgenerally spiral around the axis 108. Again, as can be seen in Figures 5C and 5D, each of the first and second volutes 106a, 106bdefines a critical plane 116a, 116b. The critical plane 116a, 116b of the volute 106a, 106b isa plane that contains the axis 108 of the housing 100 and which intersects a tip of the tongue114a that partially defines that volute 106a, 106b. Put differently, the critical plane 116a, 116bis the plane at which the known A / R ratio of a turbocharger comprising the turbine housing 69777365-1100 would be calculated. In general, the critical plane 116a, 116b of a volute 106a, 106b is aplane that contains the axis 108 of the housing 100 where the cross-sectional area of that volute 106a, 106b is at a local minimum. That is, in general, as a function of distance throughthe volute (following a path schematically shown by dashed line 118a, 118b) the areadecreases between the inlet 102a, 102b and the critical area 120a, 120b and then increasesimmediately downstream of the critical area 120a, 120b (as the gas flow clears the tongue114a, 114b of the volute 106a, 106b). Further downstream still, the area then typicallydecreases down to the exit 112a, 112b of the volute 106a, 106b.Figure 5C shows a u-axis which is parallel to the critical plane 116a of the first volute 106a. That is, as shown in Figure 5G, the critical plane 116a of the first volute 106a is the u-x plane. Figure 5D shows a v-axis which is parallel to the critical plane 116b of the second volute 106b. That is, as shown in Figure 5H, the critical plane 116b of the second volute 106b is the v-x plane.As used here, the critical area 120a, 120b is the intersection of the portion of the volute 106a,106b which is radially outboard of the tongue 114a, 114b and the critical plane 116a, 116b.As can best be seen in Figures 5E and 5F, the first and second volutes 106a, 106b areseparated by a dividing wall 122. Furthermore, as can be best seen in Figure 5E, a thicknessof the dividing wall 122 is dependent on a distance from the inlet 102a, 102b. That is, a thickness of the dividing wall 122 varies with a distance from the inlet 102a, 102b.Figure 6 shows a gas flow volume 124 defined by the new turbine housing 100. The gas flowvolume 124 is the void, or region with no material, defined by the housing 100. The first and second volutes 106a, 106b, their inlets 102a, 102b and the outlet 104 are labelled in Figure 6. In this embodiment, the turbine housing 100 further defines a wastegate passage 126 that connects the second volute 106b, at a position upstream of the critical plane 116b of the volute 106b, to the outlet 104. The flow of gas through the wastegate passage 126 is controlled by a wastegate valve (not shown). In this embodiment, the portions of the first and second volutes 106a, 106b between the inlet 102a, 102b and the respective first and second critical planes 116a, 116b is generally linear (see Figure 5E). In contrast, a portion of each volute 106a, 106b downstream of the criticalplanes 116a, 116b spirals around the axis 108 of the housing 100 (see, for example, Figures5C and 5D). 69777365-1 The portions of the first and second volutes 106a, 106b between the inlet 102a, 102b and the respective first and second critical planes 116a, 116b extend generally in a first direction parallel to the direction of gas flow through the volutes during use. The first direction may be referred to as the z-direction and is labelled as the z-direction in Figures 5A to 7.The portions of the first and second volutes 106a, 106b between the inlet 102a, 102b and therespective first and second critical planes 116a, 116b also extend generally in a second direction (which is generally perpendicular to the direction of gas flow through the volutes during use). The second direction may be referred to as the y-direction and is labelled as the y-direction in Figures 5A to 7.A thickness distribution of the dividing wall 122 referred to herein may be a thickness in a thirddirection. The third direction may be referred to as the x-direction and is labelled as the x-direction in Figures 5A to 7. With these co-ordinates, the flange 110 lies generally in an x-y plane. Figure 7 shows a plurality of (eight) cross sections of the turbine housing 100, each in a different plane perpendicular to the z-direction. The planes are labelled S2-S9 and are shown in Figure 6.This shows how the thickness of the dividing wall 122 varies from the inlets 102a, 102b (atplane S9) towards the critical areas 120a, 120b.Figure 8 shows the thickness distribution 128 of the dividing wall 122 as function of distancefrom the inlet 102 of the volutes 106a, 106b. The thickness distribution of the dividing wall122 as a function of distance from the inlet 102 describes the variation in the thickness of thedividing wall 122.Note that although, as noted above, the portions of the first and second volutes 106a, 106bbetween the inlet 102a, 102b and the respective first and second critical planes 116a, 116b may be generally linear, in general the first, second and third directions may vary with distancefrom the inlet 102 downstream of the critical areas 120a, 120b. Note that, in general, if thecross-sectional shape of the volutes 106a, 106b changes between the inlet 102a, 102b and the critical planes 120a, 120b (which it typically does) then the thickness distribution in the third direction (x-direction) will, in general, be different at different positions in the second position (y-direction). 69777365-1As used herein (unless stated otherwise) the thickness distribution of the dividing wall 122 asa function of distance from the inlet 120a, 102b is the thickness distribution in the first direction (x-direction) at a position in the second direction (y-direction) that corresponds to a midpoint of the first and second volutes 106a, 106b in the second direction.Figure 8 also shows a more typical thickness distribution 130 of the dividing wall 122 asfunction of distance from the inlet 102 of the volutes 106a, 106b. This more typical type of thickness distribution 130 is relatively flat with little variation in the thickness of the dividing wall 122 as a function of distance from the inlet 102. Figure 9A shows a gas flow volume 132 defined by a turbine housing having this more typical dividing wall thickness distribution.Again, the gas flow volume 132 is the void, or region with no material, defined by such ahousing. The first and second volutes 106a’, 106b’, their inlets 102a’, 102b’ and the outlet104’ of such a more typical housing are also labelled in Figure 9A. Figure 9B shows a crosssectional view of the gas flow volume 132 through line A-A.For comparison, the gas flow volume 124 defined by the new turbine housing 100 is shown inFigure 10A and a cross sectional view of the gas flow volume 132 through line B-B is shownin Figure 10B. Referring to Figure 8, a thickness distribution of the dividing wall 122 as a function of distance from the inlet 102 is such that at least a portion of the dividing wall 122 upstream of the critical planes 116a, 116b of the first and second volutes 106a, 106b has a thickness that is greaterthan the thickness of the dividing wall 122 at said critical plane(s) 116a, 116b by at least a firstfactor. The first factor is 1.3 or greater. A line 134 is shown in Figure 8 corresponding to 1.3times the thickness of the dividing wall 122 at the critical areas 120a, 120b (which is approximately 6 mm). As can also be seen from Figure 8 there are portions of the dividingwall 122 with a thickness is that larger than the thickness of the dividing wall 122 at the criticalareas 120a, 120b by a significantly greater factor. For example, at a peak thickness, thedividing wall 122 has a thickness that is larger than the thickness of the dividing wall 122 atthe critical areas 120a, 120b by a factor of around 2.5. At least a portion of the dividing wall 122 upstream of the critical planes 116a, 116b of the firstand second volutes 106a, 106b less than 13% of the distance between the inlet 102 and theclosest critical plane 116a from the closest critical plane 116a has a thickness that is greaterthan the thickness of the dividing wall 122 at the critical planes 116a, 116b. It will beappreciated that as used here the closest critical plane is intended to mean the critical plane 116a, 116b that is closest to the inlet 102. 69777365-1 Put differently, when moving from the closest critical plane 120a towards the inlet within 13%of the total distance to the inlet 102 the thickness of the dividing wall 122 has increased suchthat it is greater than the thickness of the dividing wall 122 at the closest critical plane 120a. Put differently, length 136 in Figure 8 is less than 13% of length 138. At least a portion of the dividing wall 122 upstream of the critical planes 116a, 116b of the firstand second volutes 106a, 106b and less than 37% of the distance between the inlet 102 andthe closest critical plane 116a from the closest critical plane 116a has a thickness that isgreater than the thickness of the dividing wall 122 at said critical planes 116a, 116b by at leasta factor of 1.3.Put differently, when moving from the closest critical plane 120a towards the inlet within 37% of the total distance to the inlet 102 the thickness of the dividing wall 122 has increased such that it is greater than the thickness of the dividing wall 122 at the closest critical plane 120a by a factor of 1.3. Put differently, length 140 in Figure 8 is less than 37% of length 138.The new turbine housing 100 is advantageous over typical designs (for example relative to adesign as shown in Figures 9A and 9B), as now discussed. In use, the turbine housing 100 may form part of a turbocharger (for example of the type of turbocharger 1 shown in Figure 1) and a turbine wheel may be disposed at least partially withinthe housing 100, downstream of the first and second volute exits 112a, 112b. As stated above,a turbocharger is a known device for extracting energy from the exhaust gas from an engine and using this energy to deliver air at increased pressure to the inlet of the engine (so as to increase the power of the engine). Exhaust gas admitted into the inlet is accelerated as it passes through the volutes. This increase in gas velocity imparts more momentum to the turbine wheel thus increasing the power generated by the turbine wheel. However, one known drawback of turbochargers in general is that the turbocharger creates backpressure, which can negatively impact the efficiency of the engine to which it is connected. Furthermore, the performance of fixed geometry turbochargers is strongly dependent on the flow conditions, which in turn is dependent on operating conditions of the engine to which it is connected. Under some conditions the turbocharger may increase the efficiency of the engine to which it is connected whereas under some conditions the turbocharger may decrease the efficiency of the engine to which it is connected. Fixed geometry turbochargers are typically designed so as to be optimized for a particular set point of engine running conditions. 69777365-1 The flow characteristics through the turbine stage of a turbocharger are dependent on: the flow area and radius (of the centroid of the flow area from the axis) of the volutes at the critical plane(s); and the radius of the turbine wheel. Therefore, for a given turbine wheel the flowcharacteristics of the turbine stage are dependent on the flow area and radius (of the centroidof the flow area from the axis) of the volutes at the critical plane(s). As explained above, theseare selected so as to achieve optimal performance for a particular set point of engine running conditions. Typically, the thickness of the dividing wall between the inlet and the critical plane(s) isgenerally uniform (see Figures 8 and 9A). In particular, there is a prejudice against rapidlyincreasing the thickness of the dividing wall 122 proximate to (and upstream of) the critical planes 116a, 116b as any increase in thickness may result in decrease of the cross sectionalareas of the first and second flow passages upstream of the critical planes 116a, 116b.However, note that the critical planes 116a, 116b are the planes at which the cross-sectional area of the volutes 106a, 106b are at a local minimum upstream of an exit 112a, 112b of thevolute 106a, 106b. Therefore, such a change cannot be tolerated since this may change thelocation and size of the critical planes 112a, 112b and, as explained above, this has beencarefully selected to match a desired set point of engine conditions. However, the inventors have realized that it is possible for the thickness distribution of thedividing wall 122 as a function of distance from the inlet 102 to be such that at least a portionof the dividing wall upstream of the critical planes of the first and second volutes has a thickness that is greater than the thickness of the dividing wall at said critical plane(s) by at least a first factor and wherein the first factor is 1.3 or greater. For example, this may be achieved by also changing the other walls of the turbine housing 100 that define the volutes 106a, 106b so that the critical planes remain unchanged. Furthermore, it has been found that such an arrangement can result in a surprising increase in the lifetime of the turbine housing 100. For example, it has been found by the inventors (from thermomechanical analysis) that such turbine housings tend to fail in a region of the outer (circumferential) surface of the turbine housing 100 in the vicinity of the dividing wall 122 (typically between the inlet 102 and thecritical planes 116a, 116b). Furthermore, it has been found that by adjusting the dividing wall122 thickness the lifetime of the turbine housing 100 can be improved by at least a factor oftwo. It is particularly surprising that such a variation in the thickness of the dividing wall 122can have such an impact on the lifetime of the turbine housing 100 since the turbine housings 100 tend to fail on an outer wall. Therefore, it would seem more obvious to consider varyinga thickness of the outer wall(s) of the housing 100.69777365-1 It will be appreciated that upstream of the critical planes 116a, 116b of the first and second volutes 106a, 106b means a position between the inlet 102 and the critical planes 116a, 116bof the first and second volutes 106a, 106b.A circumferential outlet portion of each of the first and second volutes 106a, 106b may be defined by one or more tongues. In this embodiment, the first volute 106a is partially defined by a first tongue 114a and thesecond volute 106b is partially defined by a second tongue 114b (the first and second tongues114a, 114b being circumferentially spaced apart). However, it will be appreciated that in somealternative embodiments, the first and second volutes may both be partially defined by acommon tongue. For such embodiments, the first and second volutes may share a common critical plane. As can be best seen in Figure 7, in the y-direction, in this embodiment of a new turbine housing 100 the thickness of the dividing wall 122 is generally uniform (for a given z position). In particular, for a given z position, the dividing wall 122 comprises a central portion which is of generally uniform thickness and two end portions which comprise curved fillets where the dividing wall 122 meets the external walls of the turbine housing 100. In alternative embodiments, the thickness of the dividing wall 122 (for a given z position) may vary in the y-direction. In particular, the central portion of the dividing wall may have a non- uniform thickness distribution as a function of y-position. Some examples of such embodiments are shown in Figures 11B to 11D. Figure 11A shows the cross sectional view of the gas flow volume 132 through line B-B (see Figures 10A and 10B) that is also shown in Figure 10B. Figures 11B to 11C show, respectively, a cross sectional view of three modified gas flow volumes 132a, 132b, 132cthrough the same line B-B (see Figures 10A and 10B). It will be appreciated that when a non-uniform thickness distribution in the y-direction is used, the optimum thickness distribution in the z-direction may be slightly different to that shown in Figure 8. A third embodiment of a new turbine housing having two volutes 106a, 106b is now described with reference to Figures 12 to 14. Features of the third embodiment of a new turbine housing that are similar to corresponding features of the second new turbine housing 100 sharecommon reference numerals therewith. The following description will focus on the differences69777365-1 between the third embodiment of a new turbine housing and the second new turbine housing 100. Figure 12 shows a gas flow volume 142 defined by the third new turbine housing. The gas flow volume 142 is the void, or region with no material, defined by the housing. The first and second volutes 106a, 106b, their inlets 102a, 102b and the outlet 104 are labelled in Figure 12.In this embodiment, the turbine housing also defines a wastegate passage 126 that connectsthe second volute 106b, at a position upstream of the critical plane 116b of the volute 106b, to the outlet 104. In addition, in this embodiment, the turbine housing also defines a second wastegate passage 126a that connects the first volute 106a, at a position upstream of the critical plane 116a of the first volute 106a, to the outlet 104. The flow of gas through thewastegate passages 126, 126a is controlled by one or more wastegate valves (not shown).Figure 13 shows a plurality of (six) cross sections of the turbine housing 100, each in a differentplane perpendicular to the z-direction. The planes are labelled S1-S6 and are shown in Figure12. This shows how the thickness of the dividing wall 122 varies from the inlets 102a, 102b(at plane S6) towards the critical areas 120a, 120b.Figure 14 shows the thickness distribution 144 of the dividing wall 122 as function of distancefrom the inlet 102 of the volutes 106a, 106b. The thickness distribution 144 of the dividingwall 122 as a function of distance from the inlet 102 describes the variation in the thickness ofthe dividing wall 122.In this embodiment, the first volute 106a and the second volute 106b are both partially definedby a common tongue (not shown). Therefore, the first and second volutes 106a, 106b sharea common critical area120 (also shown in Figure 14).Figure 14 also shows a more typical thickness distribution 146 of the dividing wall 122 asfunction of distance from the inlet 102 of the volutes 106a, 106b. This more typical type ofthickness distribution 146 is relatively flat with little variation in the thickness of the dividingwall 122 as a function of distance from the inlet 102. In fact, in this embodiment, the more typical type of thickness distribution 146 is such that the thickness of the dividing wall decreases upstream of the critical plane 120. 69777365-1Referring to Figure 14, a thickness distribution 144 of the dividing wall 122 as a function ofdistance from the inlet 102 is such that at least a portion of the dividing wall 122 upstream ofthe critical area 120 of the first and second volutes 106a, 106b has a thickness that is greaterthan the thickness of the dividing wall 122 at said critical area 116a, 116b by at least a firstfactor. The first factor is 1.3 or greater. A line 134 is shown in Figure 14 corresponding to 1.3times the thickness of the dividing wall 122 at the critical area 120 (which is approximately 6mm). As can also be seen from Figure 14 there are portions of the dividing wall 122 with a thickness is that larger than the thickness of the dividing wall 122 at the critical area by aneven greater factor. For example, at a peak thickness, the dividing wall 122 has a thicknessthat is larger than the thickness of the dividing wall 122 at the critical areas 120a, 120b by a factor of around 1.9.At least a portion of the dividing wall 122 upstream of the critical area 120 of the first andsecond volutes 106a, 106b less than 27% of the distance between the inlet 102 and the closest critical plane from the critical area 120 has a thickness that is greater than the thickness of the dividing wall 122 at the critical area 120. Put differently, when moving from the closest criticalarea 120 towards the inlet within 27% of the total distance to the inlet 102 the thickness of thedividing wall 122 has increased such that it is greater than the thickness of the dividing wall122 at the critical area 120. Put differently, length 148 in Figure 14 is less than 27% of length150. At least a portion of the dividing wall 122 upstream of the critical area 120 of the first and second volutes 106a, 106b and less than 37% of the distance between the inlet 102 and the critical area 120 from the critical area 120 has a thickness that is greater than the thickness ofthe dividing wall 122 at said critical area 120 by at least a factor of 1.3.Put differently, when moving from the critical area 120 towards the inlet within 37% of the total distance to the inlet 102 the thickness of the dividing wall 122 has increased such that it isgreater than the thickness of the dividing wall 122 at the critical area 120 by a factor of 1.3.Put differently, length 152 in Figure 14 is less than 37% of length 150.The third new turbine housing (discussed with reference to Figures 12 to 14) is advantageousover typical designs for the reasons discussed above with reference to the second new turbinehousing 100. A fourth embodiment of a new turbine housing having two volutes 106a, 106b is now described with reference to Figure 15. Features of the fourth embodiment of a new turbine housing that 69777365-1 are similar to corresponding features of the second new turbine housing 100 share common reference numerals therewith. The following description will focus on the differences between the fourth embodiment of a new turbine housing and the second new turbine housing 100.Figure 15 shows the thickness distribution 154 of the dividing wall 122 as function of distancefrom the inlet 102 of the volutes 106a, 106b. The thickness distribution 154 of the dividingwall 122 as a function of distance from the inlet 102 describes the variation in the thickness ofthe dividing wall 122.In this embodiment, the first volute 106a and the second volute 106b are both partially definedby a common tongue (not shown). Therefore, the first and second volutes 106a, 106b sharea common critical area 120 (also shown in Figure 15).Figure 15 also shows a more typical thickness distribution 156 of the dividing wall 122 asfunction of distance from the inlet 102 of the volutes 106a, 106b. This more typical type of thickness distribution 156 is relatively flat with little variation in the thickness of the dividing wall 122 as a function of distance from the inlet 102.Referring to Figure 15, a thickness distribution 154 of the dividing wall 122 as a function ofdistance from the inlet 102 is such that at least a portion of the dividing wall 122 upstream ofthe critical area 120 of the first and second volutes 106a, 106b has a thickness that is greaterthan the thickness of the dividing wall 122 at said critical area 116a, 116b by at least a firstfactor. The first factor is 1.3 or greater. A line 134 is shown in Figure 15 corresponding to 1.3times the thickness of the dividing wall 122 at the critical area 120 (which is approximately 6 mm). As can also be seen from Figure 15 there are portions of the dividing wall 122 with a thickness is that larger than the thickness of the dividing wall 122 at the critical area by aneven greater factor. For example, at a peak thickness, the dividing wall 122 has a thicknessthat is larger than the thickness of the dividing wall 122 at the critical areas 120a, 120b by a factor of around 2.3.At least a portion of the dividing wall 122 upstream of the critical area 120 of the first andsecond volutes 106a, 106b less than 20% of the distance between the inlet 102 and the closest critical plane from the critical area 120 has a thickness that is greater than the thickness of the dividing wall 122 at the critical area 120. Put differently, when moving from the closest critical area 120 towards the inlet within 20% of the total distance to the inlet 102 the thickness of the dividing wall 122 has increased such that it is greater than the thickness of the dividing wall 69777365-1 122 at the critical area 120. Put differently, length 158 in Figure 15 is less than 20% of length 160. At least a portion of the dividing wall 122 upstream of the critical area 120 of the first and second volutes 106a, 106b and less than 27% of the distance between the inlet 102 and the critical area 120 from the critical area 120 has a thickness that is greater than the thickness of the dividing wall 122 at said critical area 120 by at least a factor of 1.3. Put differently, when moving from the critical area 120 towards the inlet within 27% of the total distance to the inlet 102 the thickness of the dividing wall 122 has increased such that it is greater than the thickness of the dividing wall 122 at the critical area 120 by a factor of 1.3. Put differently, length 162 in Figure 15 is less than 27% of length 160.The fourth new turbine housing (discussed with reference to Figure 15) is advantageous overtypical designs for the reasons discussed above with reference to the second new turbinehousing 100. The thickness distributions 128, 144, 154 of the dividing wall 122 as a function of distancefrom the inlet 102 shown in Figures 8, 14 and 15 are all such that at least a portion of thedividing wall 122 upstream of the critical area 120 of the first and second volutes 106a, 106b has a thickness that is greater than the thickness of the dividing wall 122 at said critical area 116a, 116b by at least a first factor. In general, the first factor is greater than 1.3. For example, for the thickness distribution 128 of the dividing wall 122 as a function of distance from theinlet 102 shown in Figure 8 the first factor may be as large as 2.5; for the thickness distribution144 of the dividing wall 122 as a function of distance from the inlet 102 shown in Figure 14 thefirst factor may be as large as 1.85; and for the thickness distribution 154 of the dividing wall122 as a function of distance from the inlet 102 shown in Figure 15 the first factor may be aslarge as 2.3. In general, in embodiments of the present disclosure, the first factor may be significantly greater than, potentially significantly greater than 1.3. In some embodiments the first factor may be 1.4 or greater. In some embodiments the firstfactor may be 1.5 or greater. In some embodiments the first factor may be 1.6 or greater. Insome embodiments the first factor may be 1.7 or greater. In some embodiments the first factormay be 1.8 or greater. In some embodiments the first factor may be 1.9 or greater. In someembodiments the first factor may be 2.0 or greater. In some embodiments the first factor may69777365-1be 2.1 or greater. In some embodiments the first factor may be 2.2 or greater. In someembodiments the first factor may be 2.3 or greater. In some embodiments the first factor maybe 2.4 or greater. In some embodiments the first factor may be 2.5 or greater.In general, the increase in thickness of the dividing wall 122 may be occur as close to the critical planes as possible without altering the flow characteristics of the turbine housing 100(i.e. without changing the position or cross-sectional area of the critical planes 116a, 116b).As discussed above, it may be difficult to increase the thickness of the dividing wall 122immediately upstream of the critical planes 116a, 116b (as it may be difficult to make compensating changes in the other walls of the turbine housing 100 in this region). Therefore, typically, the thickness of the dividing wall 122 immediately upstream of the critical planes maydecrease slightly (as can be seen in Figures 8, 14 and 15).The thickness distributions 128, 144, 154 of the dividing wall 122 as a function of distancefrom the inlet 102 shown in Figures 8, 14 and 15 are all such that at least a portion of thedividing wall 122 upstream of the critical area 120 of the first and second volutes 106a, 106b has a thickness that is greater than the thickness of the dividing wall 122 at the inlet 102 by at least a second factor. In general, the second factor is greater than 1.3. For example, for the thickness distribution 128 of the dividing wall 122 as a function of distance from the inlet 102shown in Figure 8 the second factor may be as large as 2.5; for the thickness distribution 144of the dividing wall 122 as a function of distance from the inlet 102 shown in Figure 14 thesecond factor may be as large as 2.9; and for the thickness distribution 154 of the dividing wall122 as a function of distance from the inlet 102 shown in Figure 15 the second factor may beas large as 2.25. In some embodiments, the thickness of the dividing wall at the critical plane(s) may be substantially the same as the thickness of the dividing wall at the inlet. For such embodiments, the second factor may be substantially equal to the first factor. In general, the second factor may be significantly greater than, potentially significantly greater than 1.3. In some embodiments the second factor may be 1.4 or greater. In some embodiments thesecond factor may be 1.5 or greater. In some embodiments the second factor may be 1.6 orgreater. In some embodiments the second factor may be 1.7 or greater. In someembodiments the second factor may be 1.8 or greater. In some embodiments the secondfactor may be 1.9 or greater. In some embodiments the second factor may be 2.0 or greater.69777365-1In some embodiments the second factor may be 2.1 or greater. In some embodiments thesecond factor may be 2.2 or greater. In some embodiments the second factor may be 2.3 orgreater. In some embodiments the second factor may be 2.4 or greater. In someembodiments the second factor may be 2.5 or greater.Some embodiments of the present disclosure relate to an assembly comprising one of thenew turbine housings 100 described above. In some embodiments, the assembly comprising one of the new turbine housings 100described above may further comprise at least one wastegate valve member. Such anassembly 200 is shown in Figures 16A and 16B, which comprises the new turbine housing100 shown in Figures 5A to 5H and a wastegate valve member 202The wastegate valve 202 is configurable in at least: a first configuration in which the wastegatepassageway 126 defined by the new turbine housing 100 is in fluid communication with thesecond volute 106b; and a second configuration (as shown in Figures 16A and 16B) in whichthe wastegate passageway 126 is not in fluid communication with the second volute 106b.In this embodiment the turbine housing 100 defines a single wastegate passage 126 thatconnects only second volute 106b to the outlet 104, and the assembly 200 comprises a singlewastegate valve member 202.For embodiments wherein two wastegate passages are defined by the turbine housing 100that connect each of the first and second volutes 106a, 106b to the outlet 104, the assemblymay comprise two wastegate valve members. For example, a first valve member may beassociated with a first wastegate passage 126 and a second valve member may be associatedwith a second wastegate passage (not shown). Alternatively, for embodiments wherein twowastegate passages are defined by the turbine housing 100 that connect each of the first andsecond volutes 106a, 106b to the outlet 104, the assembly may comprise a single wastegatevalve member that can be used to selectively open or close both of the wastegate passages.The assembly 200 further comprises a support for the wastegate valve member 202. Thesupport comprises: (a) a shaft 204 extending through a through a bore 170 (see Figure 5A) ina wall of the turbine housing 100 such that a first portion of the shaft is disposed in a wastegate passageway 126 and a second portion of the shaft is disposed on an exterior of the turbinehousing 100; and (b) a portion 206 extending at least partially radially from the first portion of69777365-1the shaft 204. The wastegate valve member 202 is supported by the portion 206 extendingat least partially radially from the first portion of the shaft 204. With such an arrangement the wastegate valve member is supported eccentrically by the shaft. As the shaft rotates the wastegate valve member moves around an axis of the shaft. It will be appreciated that the turbine housing may 100 be formed using a casting process. Once so formed one or more through bores may be machined in the casting for receipt of a shaft of a support for a wastegate valve member. A bushing may be inserted into the machined bore. Finally, a shaft of a support for a wastegate valve member may be inserted into the bushing.The assembly may further comprise an actuator for the or each support, the actuator beingoperable to rotate the second portion of the shaft. The actuator may be attached to an outersurface of the turbine housing 100.The assembly may further comprise: (i) a turbine wheel; (ii) a shaft supporting the turbinewheel; and (iii) a bearing housing supporting the shaft for rotation about an axis. The turbinewheel may be disposed between the exits 112a, 112b of each of the first and second volutes106a, 106b and the outlet 104.In some embodiments, the assembly comprising one of the new turbine housings 100described above may further comprise: (i) a turbine wheel; (ii) a shaft supporting the turbinewheel; and (iii) a bearing housing supporting the shaft for rotation about an axis. The turbinewheel may be disposed between the exits 112a, 112b of each of the first and second volutes106a, 106b and the outlet 104.In some embodiments, the assembly comprising one of the new turbine housings 100 described above may further comprise a nozzle member disposed between the exits 112a, 112b of each of the first and second volutes 106a, 106b and the turbine wheel. The nozzle member may define a plurality of vanes, which may be circumferentially spaced around an axis of the assembly. Each pair of adjacent vanes may define between them a vane passage. The vane passages may define a nozzle of the assembly. It will be appreciated that as used in this context, the term nozzle refers to a position in a passage which represents a local minimum in cross sectional area of a flow path of the exhaust gases through the turbine stage. 69777365-1Some embodiments of the present invention relate to a turbocharger comprising one of thenew turbine housings 100 described above. The turbocharger may, for example, have any of the features of the turbocharger 1 shown in Figure 1 and described above.Some embodiments of the present disclosure relate to a method for designing a turbinehousing 100, the turbine housing 100 defining: an inlet 102; an outlet 104; and first and secondvolutes 106a, 106b separated by a dividing wall 122. The first and second volutes 106a, 106b respectively defining first and second flow passages between the inlet 102 and the outlet 104 which respectively define first and second volute exits 112a, 112b distal the inlet 102. In general, the method comprises: selecting a thickness distribution of the dividing wall 122 upstream of critical planes 116a, 116b of the first and second volutes 106a, 106b in dependence on a thermomechanical performance of the turbine housing 100. As explained above, typically, the thickness of the dividing wall 122 between the inlet 102 and the critical plane(s) 116a, 116b is generally uniform. However, the inventors have realized that by optimizing the thickness distribution of the dividing wall 122 as a function of distance from the inlet 102 can result in a surprising increase in the lifetime of the turbine housing. For example, it has been found by the inventors (from thermomechanical analysis) that such turbine housings tend to fail in a region of the outer (circumferential) surface of the turbine housing in the vicinity of the dividing wall (typically between the inlet and the critical planes). Furthermore, it has been found that by adjusting the dividing wall 122 thickness the lifetime of the turbine housing 100 can be improved by at least a factor of two. The turbine housing 100 may have any of the features of the turbine housings 100 described above. The turbine housing 100 may define: an axis 108 of the housing 100 and the first and second volutes 106a, 106b may be defined around the axis 108.In some embodiments of the method, the thickness distribution is selected so as to optimizea thermomechanical performance of the turbine housing. It will be appreciated that as used herein a thickness distribution selected so as to optimize a thermomechanical performance of the turbine housing 100 is not intended to mean a perfect thickness distribution which has the best possible thermomechanical performance. The skilled person will recognize that such a solution is, in practice, not generally achievable. Rather, a thickness distribution selected so as to optimize a thermomechanical performance of the turbine housing 100 is intended to mean a thickness distribution which has a better 69777365-1 thermomechanical performance than one or more other thickness distributions. In practice, a finite number of possibilities may be considered and the best one of these may be selected. Such an optimized thickness distribution may be close to, but not exactly at, a region ofparameter space occupied by a perfect thickness distribution which has the best possiblethermomechanical performance. It will be appreciated that this optimization may be performed using various merit functions. In some embodiments, the merit function is based on the thermomechanical performance of atleast a portion of the turbine housing 100. For example, the at least a portion may include anouter circumferential portion of the turbine housing 100. Furthermore, the optimization may be performed using any type of optimization process (for example to minimize a merit function) as desired or required. Examples of such optimization methods may include, for example, any of the following: trial and error; genetic optimization algorithms; Newton’s method in optimization (also known as the Newton-Raphson method); simulated annealing; a least squares method; and / or a quadratic programming active set method. An embodiment of the method 300 is shown schematically in Figure 17. This embodiment ofthe method 300 comprises: a step 310 of generating a plurality of different candidate thicknessdistributions of the dividing wall. This embodiment of the method 300 further comprises: a step 320 of assessing a thermomechanical performance of each of the plurality of different candidate thickness distributions. This embodiment of the method 300 further comprises: a step 330 of selecting the thickness distribution of the dividing wall to be one of the plurality of different candidate thickness distributions that has the best thermomechanical performance.A selection of a thickness distribution that optimizes a thermomechanical performance of theturbine housing 100 may be made using finite element analysis.In some embodiments of the method, thermomechanical performance may be assessed (forexample at step 320) based on a magnitude of thermally induced stress in the, or a part of,the housing 100. It will be appreciated that, as used here, thermally induced stress is intendedto comprise compressive and / or tensile stress. In some embodiments of the method, thermomechanical performance may be assessed (for example at step 320) based on a time of thermomechanical failure of the, or a part of, thehousing 100.69777365-1 In some embodiments of the method, the method comprises generating at least one candidatethickness distribution (for example at step 310).Figure 18 schematically shows an embodiment of a method 400 for generating a candidatethickness distribution (which may, for example, form part of step 310). As schematically shown in Figure 18, in an embodiment of the method, the generation 400 of the or each candidatethickness distribution may comprise: a step 410 of selecting a cross-section of the first volute106a at at least two positions along the dividing wall 122 at or upstream of the critical plane116a of that volute 106a; and a step 420 of interpolating between the cross-sections at saidat least two positions so as to define a shape of that volute 106a for a range of positions corresponding to said at least two positions. The generation 400 of the or each candidate thickness distribution may further comprise: a step 430 of selecting a cross-section of the second volute 106b at at least two positions along the dividing wall 122 at or upstream of the critical plane 116b of that volute 106b; and a step 440 of interpolating between the cross-sections at said at least two positions so as to define a shape of that volute 106a for a range of positions corresponding to said at least two positions. As schematically shown in Figure 18, in an embodiment of the method, the generation 400 of the or each candidate thickness distribution may further comprise: a step 450 of determining the candidate thickness distribution from said defined shapes of the first and second volutes (as defined at steps 420 and 440).As used here (for example, in steps 410 and 430) “cross section” is intended to mean the size,shape and position of a cross section of a volute 106a, 106b. Such a cross section may begenerally perpendicular to a flow direction of gas through the volute 106a, 106b in use. It will be appreciated that any suitable interpolation method may be used. For example, theinterpolation (performed at steps 420 and 440) may be performed within a computer-aideddrafting (CAD) model or design. For example, the interpolation may comprise a sweeping function within a CAD program. Note that the turbine housing 100 shown in Figures 5A-5H may have been generated using a method 400 of the form shown in Figure 18. For example, the (eight) cross sections of theturbine housing 100 shown in Figure 7 (in planes S2-S9) may have been selected and thenan interpolation may have been used to generate the a gas flow volume (as shown in Figure 6) and the turbine housing 100 (shown in Figures 5A to 5H). Similarly, the second new turbine 69777365-1housing represented by the gas flow volume 142 shown in Figure 12 may have beengenerated using a method 400 of the form shown in Figure 18. For example, the (six) crosssections of the second new turbine housing shown in Figure 13 (in planes S1-S6) may havebeen selected and then an interpolation may have been used to generate the a gas flow volume 142 as shown in Figure 12. It will also be appreciated that the cross-sections of the volute(s) 106a, 106b at various positions along the dividing wall 122 can be selected in various ways. For example, at least some of the cross sections may be imposed by other considerations (i.e. they may be of the form of boundary conditions). For example, the turbine housing 100 may form part of a turbocharger and, in use, exhaust gas may be provided to the two (exhaust gas inlet) volutes 106a, 106b from an exhaust manifold (also referred to as an outlet manifold) of an engine to which the turbocharger is attached. The turbine housing 100 may be designed for use with a specific engine and, therefore, the inlet 102 of the turbine housing 100 may be matched to a corresponding outlet of the exhaust manifold (also referred to as an outlet manifold) of that engine. For example, the turbine housing 100 may define a flange 110 for connection to an engine. At least one of the cross sections of the first and second volutes 106a, 106b that is selected (and input into the interpolation) may be a cross section at the inlet 102 of the turbine housing 100 and may be selected (or input into the design method as a boundary condition) so as to match an outlet cross section of an engine. As explained above, the flow characteristics of through the turbine stage of a turbocharger aredependent on: the flow area and radius (of the centroid of the flow area from the axis) of thevolutes 106a, 106b at the critical planes 116a, 116b; and the radius of the turbine wheel.Therefore, for a given turbine wheel the flow characteristics of the turbine stage are dependenton the flow area and radius (of the centroid of the flow area from the axis) of the volutes 106a,106b at the critical planes 116a, 116b. As explained above, these are selected so as toachieve optimal performance for a particular set point of engine running conditions. Therefore, at least one of the cross sections of the first and second volutes 106a, 106b that is selected (and input into the interpolation) may be a cross section at each of the critical planes 116a, 116b of the first and second volutes 106a, 106b so as to achieve optimized performance for a desired set point of engine running conditions. 69777365-1 Typically, the thickness of the dividing wall 122 between the inlet 102 and the critical plane(s)116a, 116b is generally uniform. In particular, there is a prejudice against rapidly increasingthe thickness of the dividing wall 122 proximate to (and upstream of) the critical planes 116a, 116b as any increase in thickness may result in decrease of the cross sectional areas of thefirst and second flow passages upstream of the critical planes 116a, 116b. However, notethat the critical planes 116a, 116b are the planes at which the cross-sectional area of thevolutes 106a, 106b are at a local minimum upstream of an exit 112a, 112b of the volute 106a,106b. Therefore, such a change cannot be tolerated since this may change the location andsize of the critical plane(s) 116a, 116b and, as explained above, this has been carefullyselected to match a desired set point of engine conditions. Additionally or alternatively, at least some of the cross sections may be selected, for example manually, by a designer / engineer. In some embodiments, in the generation of the or each candidate thickness distribution, at step 410, the cross sections of the first volute 106a at least two positions along the dividingwall may comprise: a cross section at the inlet 102; a cross section at a critical plane 116a ofthat volute 106a; a cross section at at least one position intermediate the inlet 102 and thecritical plane 116a of that volute 106a. Similarly, in some embodiments, in the generation ofthe or each candidate thickness distribution, at step 430, the cross sections of the second volute 106b at least two positions along the dividing wall may comprise: a cross section at the inlet 102; a cross section at a critical plane 116b of that volute 106b; a cross section at at least one position intermediate the inlet 102 and the critical plane 116b of that volute 106b. A position intermediate the inlet 102 and the critical plane 116a, 116b of a volute 106a, 106bmay be referred to as an intermediate position of that volute 106a, 106b. The cross section(s)at the least one intermediate position may be selected, for example manually, by a designer / engineer. Such an arrangement is particularly advantageous since it allows an designer or engineer to manipulate the thickness distribution of the dividing wall 122 so as to influence (for example improve or optimize) a thermomechanical performance of the turbine housing. Rather than simply interpolating between just the two boundary conditions, such embodiments allow foradditional constraints to be input before the interpolation is performed. This may allow, forexample, for a thickness of the dividing wall 122 intermediate the inlet to be increased to improve a thermomechanical performance of the turbine housing. 69777365-1In some embodiments, the new method may comprise generating a plurality of such candidatethickness distributions. For example, the method 400 for generating at least one candidate thickness distribution, as shown in Figure 18, may form part of the step 310 of the method 300 shown in Figure 17. Any two of the plurality of candidate thickness distributions may be generated from two different sets of selected cross-sections of the first and second volutes 106a, 106b. It will be appreciated that in order for any two of the plurality of candidate thickness distributions to be generated from two different sets of selected cross-sections of the first andsecond volutes 106a, 106b, at least one member from one of the two sets of selected cross-sections of the first and second volutes 106a, 106b should be different from all of the membersof the other one of the two sets of selected cross-sections of the first and second volutes 106a,106b. It will also be appreciated that, in general, there may be some cross-sections of the first and second volutes 106a, 106b that are common to two different sets of selected cross-sections of the first and second volutes 106a, 106b.For example, in some embodiments, each candidate thickness distribution may be generatedfrom a set of cross sections comprising: for each of the first and second volutes 106a, 106b:a cross section at the inlet 102; cross section at a critical plane 116a, 116b of that volute 106a,106b; and a cross section at at least one position intermediate the inlet 102 and the criticalplane 116a, 116b of that volute 106a, 106b. Two different candidate thickness distributionsmay be generated using the same cross sections at the inlet 102 and the same cross sectionsat the critical planes 116a, 116b of that volutes 106a, 106b. However, for any two candidatethickness distributions, at least one of different intermediate cross section may be used. As explained above, in some embodiments, the portions of the first and second volutes 106a, 106b between the inlet 102 and the respective first and second critical planes 116a, 116b may be generally linear (in contrast, a portion of each volute 106a, 106b downstream of the criticalplanes 116a, 116b may spiral around the axis 108 of the housing 100).The portions of the first and second volutes 106a, 106b between the inlet 102 and the respective first and second critical planes 116a, 116b extend generally in a first direction parallel to the direction of gas flow through the volutes 106a, 106b during use. The first direction may be referred to as the z-direction. The portions of the first and second volutes 106a, 106b between the inlet 102 and the respective first and second critical planes 116a, 116b also extend generally in a second 69777365-1 direction (which is generally perpendicular to the direction of gas flow through the volutes 106a, 106b during use). The second direction may be referred to as the y-direction. The thickness distribution may be a thickness in a third direction. The third direction may be referred to as the x-direction. In general, the positions of the cross sections used for the interpolation (in a direction in which the first and second volutes 106a, 106b are spaced apart from each other) may vary acrossdifferent sets of cross sections (i.e. for different thickness distribution candidates). A positionin a direction in which the first and second volutes 106a, 106b are spaced apart from each other may be referred to as the x-direction herein. A position in a direction in which the first and second volutes 106a, 106b are spaced apart from each other is a position in the third direction (i.e. the direction in which the thickness distribution is defined) and may be referred to as the x-direction herein. In some embodiments, at least one of the plurality of candidate thickness distributions may begenerated such that, for at least one of the first and second volutes 106a, 106b, a position ofat least one of the selected cross sections in a direction in which the first and second volutes 106a, 106b are spaced apart from each other (i.e. the x-direction) is different to the position in the direction in which the first and second volutes 106a, 106b are spaced apart from each other of all of the selected cross sections for that volute 106a, 106b for at least one other of the plurality of candidate thickness distributions. In general, the different sets of cross sections may vary in position(s) (of at least one of thecross sections) along the dividing wall 122. Position along the dividing wall may be referredto as the z-direction herein.In some embodiments, at least one of the plurality of candidate thickness distributions may begenerated such that, for at least one of the first and second volutes 106a, 106b, a positionalong the dividing wall 122 (i.e. position in the z-direction) of at least one of the selected cross sections is different to the positions of all of the selected cross sections for that volute 106a, 106b for at least one other of the plurality of candidate thickness distributions. In general, the sizes of the cross sections may vary across different sets of cross sections. In some embodiments, at least one of the plurality of candidate thickness distributions may begenerated such that, for at least one of the first and second volutes 106a, 106b, a size of at69777365-1 least one of the selected cross sections is different to the size of all of the selected cross sections for that volute 106a, 106b for at least one other of the plurality of candidate thickness distributions. In general, the shapes of the cross sections may vary across different sets of cross sections. In some embodiments, at least one of the plurality of candidate thickness distributions may begenerated such that, for at least one of the first and second volutes 106a, 106b, a shape of atleast one of the selected cross sections is different to the shape of all of the selected cross sections for that volute 106a, 106b for at least one other of the plurality of candidate thickness distributions. In some embodiments, the method comprises generating a plurality of such candidate thickness distributions and further comprises selecting the thickness distribution of the dividing wall 122 to be one of said plurality of different candidate thickness distributions that has the best thermomechanical performance. In some embodiments, at least one candidate thickness distribution is generated at least partially from a cross section of the first volute 106a and a cross section of the second volute 106b at a position along the dividing wall 122 upstream of the critical planes 116a, 116b of the first and second volutes 106a, 106b that correspond to a thickness of the dividing wall 122 that is greater than the thickness of the dividing wall 122 at said critical planes 116a, 116b by at least a first factor and wherein the first factor is 1.3 or greater. In some embodiments, each of the first and second volutes 106a, 106b defines a critical plane116a, 116b and the selected thickness distribution of the dividing wall 122 as a function ofdistance from the inlet 102 is such that at least a portion of the dividing wall 122 upstream of the critical planes 116a, 116b of the first and second volutes 106a, 106b has a thickness that is greater than the thickness of the dividing wall 122 at said critical plane(s) 116a, 116b by at least a first factor and wherein the first factor is 1.3 or greater. This may, for example, be achieved by appropriate selection of one or more intermediate cross sections (i.e. cross sections between the inlet and the critical plane) of the first and secondvolutes 106a, 106b.As explained above, despite prejudice in the art against rapidly increasing the thickness of thedividing wall 122 proximate to (and upstream of) the critical planes 116a, 116b, the inventors69777365-1 have realized that it is possible for the thickness distribution of the dividing wall 122 as a function of distance from the inlet 102 to be such that at least a portion of the dividing wall 122 upstream of the critical planes 116a, 116b of the first and second volutes 106a, 106b has a thickness that is greater than the thickness of the dividing wall 122 at said critical plane(s) 106a, 106b by at least a first factor and wherein the first factor is 1.3 or greater. For example, this may be achieved by also changing the other walls of the turbine housing 100 that define the volutes 106a, 106b so that the critical planes 116a, 116b remain unchanged. Furthermore, it has been found that such an arrangement can result in a surprising increase in the lifetimeof the turbine housing 100. For example, it has been found by the inventors (fromthermomechanical analysis) that such turbine housings 100 tend to fail in a region of the outer (circumferential) surface of the turbine housing 100 in the vicinity of the dividing wall 122(typically between the inlet 102 and the critical planes 116a, 116b). Furthermore, it has beenfound that by adjusting the dividing wall thickness 122 the lifetime of the turbine housing 100can be improved by at least a factor of two. It is particularly surprising that such a variation in the thickness of the dividing wall 122 can have such an impact on the lifetime of the turbine housing 100 since the turbine housings 100 tend to fail on an outer wall. Therefore, it wouldseem more obvious to consider varying a thickness of the outer wall(s) of the housing 100.In general, in embodiments of the present disclosure, the first factor may be significantly greater than, potentially significantly greater than 1.3. In some embodiments the first factor may be 1.4 or greater. In some embodiments the first factor may be 1.5 or greater. In some embodiments the first factor may be 1.6 or greater. In some embodiments the first factor may be 1.7 or greater. In some embodiments the first factor may be 1.8 or greater. In some embodiments the first factor may be 1.9 or greater. In some embodiments the first factor may be 2.0 or greater. In some embodiments the first factor may be 2.1 or greater. In some embodiments the first factor may be 2.2 or greater. In some embodiments the first factor may be 2.3 or greater. In some embodiments the first factor may be 2.4 or greater. In some embodiments the first factor may be 2.5 or greater.In some embodiments, at least one candidate thickness distribution may be generated at leastpartially from a cross section of the first volute 106a and a cross section of the second volute106b at a position along the dividing wall 122 upstream of the critical planes 116a, 116b of thefirst and second volutes 106a, 106b that correspond to a thickness of the dividing wall 122 that is greater than the thickness of the dividing wall 122 at the inlet 102 by at least a second factor and wherein the second factor is 1.3 or greater. 69777365-1 In some embodiments, each of the first and second volutes 106a, 106b defines a critical plane 116a, 116b and the selected thickness distribution of the dividing wall 122 as a function of distance from the inlet 102 may be such that at least a portion of the dividing wall 122 upstream of the critical planes 116a, 116b of the first and second volutes 106a, 106b has a thickness that is greater than the thickness of the dividing wall 122 at the inlet 102 by at least a second factor and wherein the second factor is 1.3 or greater.In some embodiments, the thickness of the dividing wall 122 at the critical plane(s) 106a, 106bmay be substantially the same as the thickness of the dividing wall 122 at the inlet 102. Forsuch embodiments, the second factor may be substantially equal to the first factor. In general, the second factor may be significantly greater than, potentially significantly greater than 1.3. In some embodiments the second factor may be 1.4 or greater. In some embodiments thesecond factor may be 1.5 or greater. In some embodiments the second factor may be 1.6 orgreater. In some embodiments the second factor may be 1.7 or greater. In some embodiments the second factor may be 1.8 or greater. In some embodiments the second factor may be 1.9 or greater. In some embodiments the second factor may be 2.0 or greater. In some embodiments the second factor may be 2.1 or greater. In some embodiments the second factor may be 2.2 or greater. In some embodiments the second factor may be 2.3 or greater. In some embodiments the second factor may be 2.4 or greater. In some embodiments the second factor may be 2.5 or greater. In some embodiments of the method, the thickness distribution may be selected so as to maximize a lifetime of the turbine housing 100. In some embodiments, the method may comprises: parameterizing the thickness distribution; and selecting a set of parameters. Although the invention has been described in relation to a turbine which may form part of a turbocharger, in other embodiments the turbine may form part of any appropriate turbomachine. While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that 69777365-1 modifications may be made to the invention as described without departing from the scope of the claims set out below 69777365-1

Claims

Claims1. A turbine housing for a turbocharger, the turbine housing comprising: an inlet portion;a second portion for housing a turbine wheel; and an outlet portion; wherein the inlet portion defines a first inlet and a second inlet; wherein the inlet portion and the second portion define: afirst volute which defines a first flow passage between the first inlet and a firstvolute outlet, and which extends around an axis of the turbine housing; anda second volute which defines a second flow passage between the second inlet and a second volute outlet, and which extends around the axis of the turbine housing; wherein the first volute and the second volute are separated, in an axial direction, by a dividing wall; and wherein for at least a portion of the dividing wall upstream of a tongue of the first volute and a tongue of the second volute a thickness of the dividing wall varies in a first direction thatis generally perpendicular to a second direction that, in use, gas will flow along through thefirst and second volutes such that the thickness increases in a direction generally away from the second portion of the turbine housing.

2. The turbine housing of claim 1 wherein, for at least a portion of the dividing wall in thesecond direction, the thickness of the dividing wall in the first direction increases in thedirection generally away from the second portion of the turbine housing for a central portion ofthe dividing wall in the first direction.

3. The turbine housing of claim 1 or claim 2 wherein, for at least a portion of the dividingwall in the second direction, the thickness of the dividing wall in the first direction increases inthe direction generally away from the second portion of the turbine housing for a portion of thedividing wall having an extent in the first direction that is at least 30% of a maximum extent ofthe volutes in the first direction.

4. The turbine housing of any preceding claim wherein, for at least a portion of the dividingwall in the second direction, the thickness of the dividing wall in the first direction increaseslinearly for at least a portion of the dividing wall in the direction generally away from the secondportion of the turbine housing.

5. The turbine housing of any preceding claim wherein, for at least a portion of the dividingwall in the second direction, the thickness of the dividing wall in the first direction increases in 69777365-1the direction generally away from the second portion of the turbine housing with a rate ofincrease of thickness in the first direction in a range of 0.02 mm / mm to 0.06 mm / mm.

6. The turbine housing of any preceding claim wherein, for at least a portion of the dividingwall in the second direction, the thickness of the dividing wall in the first direction increases inthe direction generally away from the second portion of the turbine housing with a taper anglein a range of 1° to 5°.

7. The turbine housing of any preceding claim wherein, for at least a portion of the dividingwall in the second direction, the thickness of the dividing wall in the first direction increases inthe direction generally away from the second portion of the turbine housing with a taper angleof less than 10°.

8. The turbine housing of any preceding claim wherein, for at least a portion of the dividingwall in the second direction, the thickness of the dividing wall in the first direction increases inthe direction generally away from the second portion of the turbine housing with a taper angleof less than 5°.

9. The turbine housing of any preceding claim wherein, for at least a portion of the dividingwall in the second direction, the thickness of the dividing wall in the first direction monotonicallyincreases in the direction generally away from the second portion of the turbine housing for atleast a portion of the dividing wall in the first direction.

10. The turbine housing of any preceding claim wherein, for at least a portion of the dividingwall in the second direction, the thickness of the dividing wall in the first direction increases inthe direction generally away from the second portion of the turbine housing for a radiallyoutboard portion of the dividing wall that has an extent in the first direction that is at least 70%of a maximum extent of the volutes in the first direction.

11. The turbine housing of any preceding claim wherein the at least a portion of the dividingwall upstream of the tongue of the first volute and the tongue of the second volute has a thickness that is greater than a thickness of the dividing wall at the first inlet and the second inlet.

12. The turbine housing of any preceding claim wherein, for at least a portion of the dividingwall in the second direction, the thickness of the dividing wall in the first direction is asymmetric about a center of the dividing wall in the first direction such that it increases from the center in 69777365-1the direction generally away from the second portion of the turbine housing by a greateramount than it increases from the center in the direction generally towards the second portionof the turbine housing.

13. The turbine housing of any preceding claim wherein the turbine housing further definesat least one wastegate passage via which at least one of the first and second volutes is in fluidcommunication with the outlet portion.

14. A turbine comprising:the turbine housing according to any preceding claim; and a turbine wheel, the turbine wheel received in the turbine housing.

15. The turbine of claim 14 further comprising:a shaft supporting the turbine wheel; and abearing housing supporting the shaft for rotation about a rotation axis.

16. The turbine of claims 14 or claim 15 further comprising a nozzle member disposedbetween the exits of each of the first and second volutes and the turbine wheel.

17. A turbocharger comprising:the turbine housing according to any one of claims 1 to 13; a turbine wheel, the turbine wheel received in the turbine housing; a bearing housing configured to support a shaft for rotation about a rotation axis; and a compressor, the compressor comprising a compressor housing and a compressor wheel; wherein the turbine wheel and compressor wheel are in power communication via the shaft.

18. A method for designing a turbine housing, the turbine housing defining: an inlet portion;a second portion for housing a turbine wheel; and an outlet portion; wherein the inlet portion and the second portion define: first and second volutes separated by a dividing wall, first and second volutes respectively defining first and second flow passages between the inlet portion and the outlet portion, the method comprising: for at least a portion of the dividing wall upstream of a tongue of the first volute and a tongue of the second volute, selecting a thickness distribution of the dividing wall in a first direction that is generally perpendicular to a second direction that, in use, gas will flow along 69777365-1through the first and second volutes such that the thickness increases in a direction generally away from the second portion of the turbine housing.

19. The method of claim 18 wherein the thickness distribution is selected so as to optimizea thermomechanical performance of the turbine housing.

20. The method of claim 18 or claim 19 wherein selecting the thickness distribution of thedividing wall in the first direction comprises selecting an extent of a portion of the dividing wall over which the thickness increases.

21. The method of any one of claims 18 to 20 wherein selecting the thickness distributionof the dividing wall in the first direction comprises selecting a rate of increase of thickness in the first direction of a portion of the dividing wall over which the thickness increases.

22. The method of any one of claims 18 to 21 wherein selecting the thickness distributionof the dividing wall in the first direction comprises selecting a taper angle of a portion of the dividing wall over which the thickness increases. 69777365-1

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