Lubricant delivery system

Abstract

A lubricant delivery system 202 is suitable for an electric drive unit (20, 30, Fig. 2) and comprises a pump 230 configured to deliver lubricant at a predetermined supply pressure P0 and a conduit arr

Description

TECHNICAL FIELD The present disclosure relates to a lubricant delivery system for an electric drive unit. Aspects of the invention relate to a lubricant delivery system, to an electric drive unit comprising said lubricant delivery system, to an electric vehicle comprising said electric drive unit, to a method of delivering lubricant and to a method of designing a lubricant delivery system. BACKGROUND Lubricant circulation systems for modern electric vehicles typically comprise one or more lubricant conduits or passageways designed to supply lubricant to components of the vehicle, for example components of a drive unit of the vehicle. The intention of such lubricant delivery is to prevent components of the vehicle (such as the vehicle drive unit) from overheating and / or from wearing prematurely. The components of the vehicle drive unit may have different lubrication requirements depending, for example, on factors such as rotational speed of the component and load on the component. It is therefore difficult to supply lubricant to components at optimal flow rates and velocities for efficient lubricant of all components of the vehicle drive unit. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a lubricant delivery system, an electronic drive unit, a vehicle, a method of delivering lubricant and a method of configuring a lubricant delivery system as claimed in the appended claims. According to an aspect of the present invention there is provided a lubricant delivery system for an electric drive unit comprising: a pump configured to deliver lubricant at a predetermined supply pressure; and a conduit arrangement arranged to convey lubricant supplied by the pump to at least two lubricant outlets, wherein the conduit arrangement comprises: a first branch arranged to supply lubricant to a first lubricant outlet at a first pressure substantially equal to the supply pressure; and a second branch arranged to supply lubricant to a second lubricant outlet at a second pressure; wherein the second pressure is less than the supply pressure. The first branch may be arranged to supply an unrestricted flow of lubricant from the pump to a first lubricant outlet. Any difference in pressure between lubricant supplied by the pump and lubricant supplied to the first lubricant outlet may be due to pressure losses within the first branch. The second branch may be arranged to supply a restricted flow of lubricant from the pump to a second lubricant outlet. The difference in pressure between lubricant supplied by the pump and lubricant supplied to the second lubricant outlet may therefore be greater than system pressure losses expected within the second branch. Different lubricant outlets require lubricant to be supplied at different pressures depending on the components to be lubricated. In some cases, the highest pressure requirement of a single lubricant outlet may be substantially greater than the pressure requirements of all other lubricant outlets. Here, it is beneficial to provide a pump arranged to supply lubricant at a pressure equal to the highest pressure requirement of a single lubricant outlet. That is to say that the lubricant flows unrestricted through the conduit arrangement to the lubricant delivery point. The conduit arrangement comprises a plurality of branches such that further lubricant outlets requiring lubricant to be delivered at a lower pressure may be supplied by a different branch. Lubricant flow to the lower pressure lubricant outlets may be restricted such that the pressure of the lubricant supplied to the lubricant outlets is less than the pressure of the lubricant supplied by the pump. The first lubricant outlet may comprise a nozzle configured to disperse lubricant in a predetermined pattern. The second lubricant outlet may comprise a nozzle configured to disperse lubricant in a predetermined pattern. A nozzle may provide a convenient outlet feature to direct and / or disperse lubricant onto a target component. Each nozzle may be designed to disperse lubricant at a predetermined velocity and towards a predetermined target location. The second branch may be fluidically coupled to the pump via the first branch. Coupling the second branch to the pump via the first branch may provide a convenient system arrangement where only a single conduit (i.e. the first branch) is coupled directly to an outlet of the pump. The lubricant delivery system may comprise a passive system configured to effect a pressure drop in lubricant flowing through the second branch. A passive system is here defined as a system requiring no actuation and / or monitoring. The second branch may comprise a flow restrictor configured to effect a pressure drop across the flow restrictor in a lubricant flowing in the second branch. The flow restrictor may be passive as defined above. The flow restrictor may comprise a separate component such as an annular body inserted into the first and / or second branch. Alternatively, the flow restrictor may be integrally formed as part of the first and / or second branch. A flow restrictor may be used to reduce the pressure of lubricant flowing through the flow restrictor such that lubricant downstream of the flow restrictor is at a lower pressure than lubricant upstream of the flow restrictor. This may be achieved for example by using a flow restrictor to reduce the cross sectional area of the branch (optionally using an annular body inserted into the branch) so as to reduce the mass flow rate through the flow restrictor. This in turn will decrease the velocity and pressure of lubricant flowing downstream of the flow restrictor. The flow restrictor may be positioned upstream of the second lubricant outlet relative to the direction of flow of lubricant through the second branch such that the pressure in the flow of lubricant supplied to the second lubricant outlet is substantially stable. It may be preferable to position the flow restrictor a predetermined distance upstream of the nozzle such that any pressure fluctuations in the flow downstream of the nozzle may be allowed to stabilise prior to the lubricant being supplied to the nozzle. The first branch may be arranged to supply lubricant to a plurality of first lubricant outlets at a first pressure equal to the supply pressure. The second branch may be arranged to supply lubricant to a plurality of second lubricant outlets at a second pressure, wherein the second pressure is less than the supply pressure. Several lubricant outlets may require lubricant to be supplied at the same or substantially the same pressure. In this case a single branch and optionally restrictor may be used to supply a plurality of nozzles with lubricant at the same pressure. This may reduce complexity and cost of the lubricant delivery system by reducing the number of branches and / or restrictors required to supply multiple outlets with lubricant at a required pressure. The conduit arrangement may comprise a third branch arranged to supply lubricant to a third lubricant outlet at a third pressure, wherein the third pressure is less than the second pressure. More complex lubricant delivery systems may comprise lubricant outlets each requiring lubricant supplied at a different pressure less than the supply pressure. Providing a third branch therefore provides a system for supplying a third lubricant outlet with lubricant at a pressure less than the second pressure. The third branch may comprise a third lubricant outlet at the third lubricant outlet, optionally wherein the third lubricant outlet comprises a nozzle configured to disperse lubricant in a predetermined pattern. Providing a lubricant outlet allows lubricant supplied to the third lubricant outlet to leave the lubricant delivery system and lubricate adjacent components. A nozzle may provide a convenient outlet feature to direct and / or disperse lubricant onto a target component. Each nozzle may be designed to disperse lubricant at a predetermined velocity and towards a predetermined target location. The third branch may be fluidically coupled to the pump via the first branch. Coupling the third branch to the pump via the first branch may provide a convenient system arrangement where only a single conduit (i.e. the first branch) is coupled directly to an outlet of the pump. The third branch may comprise a second flow restrictor used to effect a pressure drop across the second flow restrictor in a lubricant flowing in the conduit arrangement, optionally wherein the second flow restrictor is positioned upstream of the third lubricant outlet relative to the direction of flow of lubricant through the third branch such that the pressure in the flow of lubricant adjacent the third lubricant outlet is substantially stable. The second flow restrictor may comprise a separate component such as an annular body inserted into the first and / or second branch. Alternatively, the second flow restrictor may be integrally formed as part of the first and / or second branch. A flow restrictor may be used to reduce the pressure of lubricant flowing through the flow restrictor such that lubricant downstream of the flow restrictor is at a lower pressure than lubricant upstream of the flow restrictor. This may be achieved for example by using a flow restrictor to reduce the cross sectional area of the branch (optionally using an annular body inserted into the branch) so as to reduce the mass flow rate through the flow restrictor. This in turn will decrease the velocity and pressure of lubricant flowing downstream of the flow restrictor. It may be preferable to position the flow restrictor a predetermined distance upstream of the nozzle such that any pressure fluctuations in the flow downstream of the nozzle may be allowed to stabilise priorto the lubricant being supplied to the nozzle. The second and third branches may each be supplied with lubricant at the supply pressure. It may be preferable to provide second and third branches arranged in a branched configuration rather than an in-line configuration such that each branch is supplied with lubricant at a pressure equal to the supply pressure. This may be preferable over for example supplying the third branch with lubricant from the second branch at the second pressure (which may be referred to as an in-line configuration). A further aspect of the invention provides an electric drive unit comprising the lubricant delivery system of the aspect of the invention. A still further aspect of the invention provides an electric vehicle comprising an electric drive unit of the further aspect of the invention. Another aspect of the invention provides a method of delivering lubricant via a lubricant delivery system, the lubricant delivery system comprising a pump configured to deliver lubricant at a predetermined supply pressure; and a conduit arrangement arranged to convey lubricant supplied by the pump to at least two lubricant outlets via a conduit arrangement, the method comprising: operating the pump to supply a first lubricant outlet with lubricant at a first pressure equal to the supply pressure; and supplying a second lubricant outlet with lubricant at a second pressure wherein the second pressure is less than the supply pressure. Supplying the second lubrication outlet with lubricant at a second pressure may comprise restricting the pressure of the lubricant supplied to the second lubricant outlet. A final aspect of the invention provides a method of configuring a lubricant delivery system comprising a pump and a conduit arrangement arranged to convey lubricant supplied by the pump to at least two lubricant outlets, the method comprising: determining the lubricant outlet pressure requirement at each lubricant outlet; determining the maximum lubricant outlet pressure requirement; selecting a pump to supply a flow of lubricant at a pressure equal to the maximum lubricant outlet pressure requirement; determining a pressure restriction requirement for each of the remaining lubricant outlets based on each lubricant outlet pressure requirement and the maximum lubricant outlet pressure requirement; and manufacturing the lubricant delivery system in accordance with the determined criteria. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in anyway and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a vehicle in accordance with an embodiment of the invention; Figure 2 schematically shows functional units and a control system of the vehicle; Figure 3 shows a controller for use in the vehicle of Figure 2; Figure 4 shows a cross-sectional view of an Electric Drive Unit (EDU) assembly of the vehicle of Figures 1 and 2; Figure 5 shows a view of an underside of the EDU assembly of Figure 4; Figure 6 shows a schematic representation of a lubricant delivery system in accordance with an embodiment of the invention; Figure 7 shows a schematic representation of a second lubricant delivery system in accordance with an embodiment of the invention; Figure 8 shows a schematic representation of a third lubricant delivery system in accordance with an embodiment of the invention; and Figure 9 shows a method of configuring a lubricant delivery system in accordance with the invention. DETAILED DESCRIPTION Figures 1 and 2 show an example of an electric vehicle (EV) 10. The electric vehicle 10 comprises a battery or battery pack 40. The battery 40 may be recharged from an external electrical source. The electric vehicle 10 comprises a pair of front wheels 12 at a front axle 28 and a pair of rear wheels 14 at a rear axle 38. The vehicle has at least one electric drive unit (EDU) by which one or more of the wheels are driven. In the illustrated embodiment, the vehicle comprises two electric drive units, each associated with one of the pairs of wheels. In other embodiments, the vehicle may have a dedicated EDU for each of the front wheels 12 and / or a dedicated EDU for each of the rear wheels 14. In the illustrated embodiment, the front wheels 12 are driven by a first electric drive unit (EDU) 20. The first EDU 20 comprises a first motor 22, a front transmission 24 and power electronics 26. The rear wheels 14 are driven by a second electrical drive unit (EDU) 30. The second EDU 30 comprises a second motor 32, a rear transmission 34 and power electronics 36. The first EDU 20 and the second EDU 30 each receive a DC supply from battery 40. The first EDU 20 can be called a first propulsion unit and the second EDU 30 can be called a second propulsion unit. As used herein, the term “transmission” may refer to a device with a plurality of gears through which torque can be transmitted from the drive unit to one or more of the wheels. For example, this may refer to a differential, transaxle, and / or a gearbox. The electric vehicle 10 has a control system with a controller 50 which controls operation of the first EDU 20 and the second EDU 30. In operation, the controller 50 controls the power output of each of the EDUs 20, 30 to supply torque to the wheels 12,14. Power electronics 26 comprise an inverter which converts the DC supply from battery 40 to an AC supply to drive the first motor 22. The first motor 22 drives the front transmission 24 which, in turn, drives the front axle 28 to apply torque to the front wheels 12. Power electronics 36 comprise an inverter which converts the DC supply from battery 40 to an AC supply to drive the second motor 32. The second motor 32 drives the rear transmission 34 which, in turn, drives the rear axle 38 to apply torque to the rear wheels 14. One or both of the front and rear axles 28, 38 may be a continuous shaft extending through their respective EDU, or a pair of half shafts which extend from their respective EDU. Figure 3 schematically shows the control system. The control system comprises one controller 50, although it will be appreciated that this is merely illustrative. The controller 50 comprises at least one processor 56 which may be any type of processor for executing instructions to control the operation of the system. The processor 56 is electrically connected to other components of the controller via one or more buses 57. Processorexecutable instructions 48 may be provided using any data storage device or computer-readable media, such as memory 58. The processor-executable instructions 48 comprise instructions for implementing the functionality of the described methods. The storage / memory 58 is of any suitable type such as non-volatile memory, a magnetic or optical storage device. The processor 56 is configured to access the memory 58 and execute the stored instructions 48. Memory 58, or a separate memory / storage stores data 60 used by the processor 56. Data 60 may comprise data which defines a plurality of operating points of the first motor 22. Instructions 48 may comprise rules for selecting between plurality of operating points of the first motor 22. The controller 50 comprises an input interface 54. The input interface 54 is configured to receive one or more input signals 53 (e.g., a demand for acceleration or a demand for speed). The controller 50 comprises an output interface 55. The output interface 55 is configured to output outputs, such as the control signal 51 to control the first motor 22 (sent to power electronics 26) and the control signal 52 to control the second motor 32 (sent to power electronics 36). The controller 50 is configured to use one or more of the input signals 53 and stored data 60, to generate output signals 51,52. Optionally, the controller 50 may operate the vehicle in the following ways: (i) Rear-wheel drive (RWD). Torque is only supplied to the rear wheels 14 by operating the second motor 32 to drive the rear wheels 14. The front wheels 12 are not driven by the first motor 22; (ii) All-wheel drive (AWD). Torque is supplied to the rear wheels 14 and to the front wheels 12 by operating the second motor 32 to drive the rear wheels 14 and operating the first motor 22 to drive the front wheels 12; (iii) Front-wheel drive (FWD). Torque is only supplied to the front wheels 12 by operating the first motor 22 to drive the front wheels 12. The rear wheels 14 are not driven by the second motor 32. In some vehicles, the controller 50 may only operate according to options (i) and (ii), and may not operate according to option (iii). In other embodiments, the electric vehicle may include only one EDU 20, 30. For example, the second EDU 30 may be omitted in embodiments where the electric vehicle is a “front-wheel drive” vehicle, or the first EDU 20 may be omitted in embodiments where the electric vehicle is a “rear-wheel drive” vehicle. In further embodiments, one or more of the wheels may be driven individually by a dedicated EDU. For example, the front wheels 12 may each be connected to one of a pair of front EDUs. Referring now to Figures 4 and 5, an EDU assembly is indicated at 100. The EDU assembly 100 includes a motor 102 (shown schematically in Figure 4), a transmission 112 (shown schematically in Figure 4), and a housing 150 for the motor 102 and the transmission 112. It will be understood that the EDU assembly 100 illustrated in Figures 4 and 5 could be, or form part of, the first EDU 20 and / or the second EDU 30 illustrated schematically in Figure 2, along with the additional power electronics 26, 36 described above (not illustrated in the EDU assembly 100 of Figures 4 and 5). The motor 102 may be an induction motor (IM). An induction motor can also be called an induction machine, as it is capable of operating as a motor and as a generator. Alternatively, the motor 102 may be a permanent magnet (PM) synchronous motor. In either case, the motor 102 includes a rotor 104 and a stator 106 with electrical windings 108. The motor 102 is operated by suppling an AC supply to the stator windings 108 which causes movement of the rotor 104 about a rotational axis R. IM and PM motors are known and will therefore not be described in more detail. In some embodiments, the motor 102 is of a different kind, such as a DC motor, a universal motor, ora non-electrical motor such as a hydraulic motor. The rotor 104 is coupled to a motor output shaft 110. In other words, the rotor 104 and output shaft 110 are configured for co-rotation about the rotational axis R. In this way, as the rotor 104 is rotated by the AC supply to the stator windings 108, the output shaft 110 is also rotated. The output shaft 110 is supported for rotation relative to the housing 150 by an output shaft bearing arrangement 116 which, in this embodiment, includes a first bearing 116A (shown schematically in Figure 4) on a first side of the output shaft 110 and a second bearing 116B (shown schematically in Figure 4) on a second side of the output shaft 110. The firstand second bearings 116A, 116B may be ball bearings, roller bearings or any other suitable bearing. The transmission 112 is responsible for transmitting power from the output shaft 110 of the motor 102 to the front or rear wheels 12, 14 of the electric vehicle 10. Optionally, the transmission 112 includes a differential 118 (shown schematically in Figure 4) which allows half shafts (not shown in Figure 4 or 5) of the respective axle 28, 38 to be rotated at different speeds while receiving power from the motor 102. In some embodiments, the transmission 112 may comprise a gearbox between the output shaft 110 of the motor 102 and the differential 118. The differential 118 may be of any suitable configuration. The differential 118 may include one or more components which are supported for rotation relative to the housing 150 by a differential bearing arrangement (not shown) including one or more bearings. Although not illustrated in Figures 4 and 5, the half shafts of the respective axle 28, 38 may be coupled to the differential 118 via any suitable means (e.g., by engaging external splines on the half shafts with internal splines on a component of the differential 118). The half shafts may be supported for rotation relative to the housing by one or more half shaft bearings 130, which may be ball bearings, roller bearings or any other suitable bearing. The illustrated EDU assembly 100 includes a lubricant recirculation system 200, which supplies lubricant to one or more rotating components of the EDU (e.g., motor 110, bearings 116A, 116B, 130, and / or the differential 118 outlined above). The lubricant, such as oil, may both lubricate and cool those rotating components. The lubricant recirculation system 200 includes a sump 152 and a lubricant pump 230 which supplies lubricant along a flow path between the sump 152 and the interior of the housing 150. The flow path is at least in part defined by one or more conduits. The lubricant which is supplied into the housing 150 is then drained into the sump 152 at a lower end of the housing 150. A lubricant pick-up pipe 154 is provided adjacent to or in the sump 152. The lubricant pick-up pipe 154 has a lubricant inlet aperture 156 through which lubricant may be drawn into the pick-up pipe 154 by means of the lubricant pump 230. In this way, lubricant is recirculated by the lubricant pump 230 through the housing 150. The lubricant recirculation system 200 may also include a lubricant filter 204. In the illustrated embodiment, the lubricant filter 204 is located along the flow path downstream of the lubricant pump 230. In the illustrated embodiment, the housing 150 has a first portion 150A which houses the motor 102, and a second portion 150B which houses the transmission 112. In the illustrated embodiment, the first and second housing portions 150A, 150B are discrete components which are coupled together (e.g., via a bolting arrangement). The housing 150 may also include a cover 150C (shown schematically in Figure 4) which closes an end of the first housing portion 150A opposite to the second housing portion 150B. The cover 150C may be a discrete component which is coupled to the first housing portion 150A (e.g. via a bolting arrangement), or may instead be integrally formed with the first housing portion 150A. In alternative embodiments, any other suitable housing configuration may be used. The housing 150 may have one or more mounting features 132 for mounting the EDU assembly 100 to a subframe of the electric vehicle 10 and / or for reacting torsional forces generated by the EDU assembly. The illustrated EDU assembly 100 also includes seal assemblies 300 at opposite ends of the housing 150, for sealing against the half shafts of the respective axle 28, 38. Such seal assemblies 300 provide a sealed system inside the housing 150 and thereby inhibit ingress of contaminants (e.g., dirt,, debris, water, etc.) inside the housing 150. Figure 6 shows a schematic representation of a portion of the lubricant recirculation system 200, which may be referred to hereinafter as a lubricant delivery system 202. The lubricant delivery system 202 connects a lubricant source (not shown) to a plurality of lubricant outlets where lubricant is dispersed to lubricate components and / or systems. Lubricant exits the lubricant delivery system 202 at each lubricant outlet, for example through an aperture or a nozzle. It will be appreciated that lubricant exiting the lubricant delivery system 202 may still be contained within the overall lubricant recirculation system 200, for example lubricant may be dispersed into the housing 150 where it is drained into the sump 152 as described previously. Different components and / or systems have different lubrication requirements dependent on for example the speed at which the component is moving or rotating, how the component interacts with and / or engages with other components and where the component is situated within the EDU assembly 100. It is preferable to provide a nozzle designed to disperse lubricant in a predetermined pattern dependent on the lubrication requirements of a component adjacent the nozzle. A nozzle may be designed to disperse a predetermined flow rate of lubricant towards a component and / or to disperse lubricant towards a predetermined location or area of a component. These design features may be realised by the shape of the nozzle, such as for example the diameter of a bore through the nozzle and / or the shape of an aperture in the nozzle through which lubricant exits. These design features result in nozzles and hence lubricant outlets requiring lubricant supplied at a predetermined pressure, which may vary between different outlets. The pressure at which a lubricant outlet requires lubricant to be delivered at may be referred to hereinafter as a nozzle pressure requirement or more generally a lubricant outlet pressure requirement. The lubricant delivery system 202 comprises the pump 230 arranged to deliver lubricant at a predetermined supply pressure P0. That is to say that lubricant is supplied from an outlet of the pump 230 at a pressure referred to as a supply pressure P0. The lubricant delivery system 202 further comprises a conduit arrangement 210 arranged to convey lubricant from the pump 230 to a plurality of lubricant outlets. The conduit arrangement 210 may for example comprise a system of pipes fluidicaIly connected to an outlet of the pump 230. As shown in Figure 6, the conduit arrangement 210 comprises a first branch 212 arranged to supply lubricant to a first lubricant outlet 213 and a second branch 214 arranged to supply lubricant to a second lubricant outlet 215. Although shown as comprising two branches, it will be appreciated that the conduit arrangement 210 may comprise any number of a plurality of branches. The first branch 212 is arranged to supply lubricant to the first lubricant outlet 213 at a first pressure P1 that is substantially equal to the supply pressure PO. That is to say that an unrestricted flow of lubricant is supplied from the pump 230 to the first lubricant outlet 213 by the first branch 212. Therefore, any difference between the supply pressure PO and the first pressure P1 is due to system pressure losses only, such as for example frictional losses along the walls of the first branch 212 and / or dynamic losses in fittings within the first branch 212. By way of example, the first pressure P1 may be in the range 0.3 MPa to 0.5 MPa, for example in the range 0.38 MPa to 0.42 MPa The second branch 214 is arranged to supply lubricant to the second lubricant outlet 215 at a second pressure P2 that is less than the supply pressure P0. That is to say that a restricted flow of lubricant is supplied from the pump 230 to the second lubricant outlet 215 by the second branch 214. A flow restrictor 240 may be used to effect a pressure drop in lubricant flowing through the second branch 214. In other words, lubricant may be supplied to an inlet of the flow restrictor 240 at a pressure substantially equal to the supply pressure P0 and lubricant may exit an outlet of the flow restrictor at a second pressure P2 less than the supply pressure. The pressure drop of lubricant flowing in the second branch 214 across the flow restrictor 240 may be in the range 0.01 MPa to 0.4 MPa, for example in the range 0.05 MPa to 0.3MPa. Any difference between the supply pressure P0 and the pressure at the inlet of the flow restrictor 240 is due to system pressure losses only, such as for example frictional losses along the walls of the second branch 214 and / or dynamic losses in fittings within the second branch 214. The flow restrictor 240 may be a separate component to the conduit system 210 and fitted to the conduit system. In alternative embodiments, the flow restrictor 240 may be formed integrally with the conduit system 210. In order to effect the pressure drop therethrough, the flow restrictor 240 is configured to reduce an internal diameter of the second branch 214 from a first internal diameterto a second internal diameter. The first internal diameter is a diameter of the second branch 214 immediately upstream of the flow restrictor 240. The second internal diameter is a diameter of an aperture extending through the flow restrictor 240. It shall be appreciated that after lubricant has flowed through the flow restrictor 240, the diameter of the second branch 214 increases to a third internal diameter. The third internal diameter is equal to an internal diameter of the second branch 214 immediately upstream of the flow restrictor 240. The third internal diameter may be equal to the first internal diameter, or the third internal diameter may be greater than or less than the first internal diameter. The second internal diameter may be in the range 0.4mm to 2mm, for example in the range 0.8mm to 1,6mm, or 1.2 mm to 1.6mm. It shall be appreciated that the internal diameter and / or an axial length of the restrictor 240 may be varied in orderto increase or decrease the pressure dropthereacross. As such, the restrictor240 is a simple component capable of delivering a range of pressures to the second lubricant outlet 215 through simple modifications to dimensions of the flow restrictor 240. It is preferable to position the flow restrictor 240 a distance upstream of the second lubricant outlet 215 relative to the direction of flow of lubricant through the second branch 214. The distance between the outlet of the flow restrictor 240 and the second lubricant outlet 215 should be greater than a predetermined threshold distance. The predetermined threshold distance may be in the range of 5 mm to 50 mm, for example in the range of 10 mm to 40 mm. As lubricant flows through the flow restrictor 240 turbulence may be introduced, for example as a result in a change in diameter of the second branch 214. It is preferable to allow any turbulence or more generally pressure instabilities to stabilise prior to the lubricant being supplied to the second lubricant outlet 215. In this way, the second pressure P2 may be substantially stable or substantially constant. Supplying lubricant having a varying or unstable pressure to a lubricant outlet (particularly a nozzle) may result in poor or unpredictable performance of the nozzle, which in turn may cause reduced efficiency of the lubricant recirculation system 200 and hence any components to be lubricated. It may be convenient to fluidically couple the second branch 214 to the pump 230 via the first branch 212 as shown in Figure 6. This may allow a single conduit to be coupled to an outlet of the pump 230 as is common practise in fluid system design. Figure 7 shows a second lubricant delivery system 302. The second lubricant delivery system 302 comprises a pump 230 and a conduit arrangement 210 comprising a first branch 212 and a second branch 214 similar to those described with reference to Figure 6. The second branch 214 comprises a flow restrictor 240 identical to the flow restrictor described with reference to Figure 6. These features will therefore not be discussed again in detail. The first branch 212 of the second lubricant delivery system 302 is arranged to supply lubricant to three first lubricant outlets 213A, 213B, 213C. Each first lubricant outlet is supplied with lubricant at a first pressure P1 substantially equal to the supply pressure P0 of the pump 230. It will be appreciated that a first pressure P1 substantially equal to the supply pressure P0 here refers to an unrestricted flow of lubricant supplied from the pump 230 to each of the first lubricant outlets 213A, 213B, 213C. Substantially equal pressure may be defined as described previously in relation to Figure 6 and the lubricant delivered to a first outlet 213. This description will not be repeated here for brevity. Although shown as supplying lubricant to three first lubricant outlets 213A, 213B, 213C, it will be appreciated that the first branch 212 may be arranged to supply lubricant to any number of a plurality of first lubricant outlets. Similarly, the second branch 214 of the lubricant delivery system 302 is arranged to supply lubricant to three second lubricant outlets 215A, 215B, 215C. Each second lubricant outlet is supplied with lubricant at a second pressure P2 that is less than the supply pressure P0. That is to say that a restricted flow of lubricant is supplied from the pump 230 to each second lubricant outlet 215A, 215B, 215C by the second branch 214. A single flow restrictor 240 may be used to effect a pressure drop in lubricant flowing through the second branch 214 and supplied to each second lubricant outlet 215A, 215B, 215C. In other words, lubricant may be supplied to an inlet of the flow restrictor 240 at a pressure substantially equal to the supply pressure P0 and lubricant may exit an outlet of the flow restrictor at a second pressure P2 less than the supply pressure to be supplied to each second lubricant outlet. Any difference between the supply pressure P0 and the pressure at the inlet of the flow restrictor 240 is due to system pressure losses only, such as for example frictional losses along the walls of the second branch 214 and / or dynamic losses in fittings within the second branch 214. Although shown as supplying lubricant to three second lubricant outlets 215A, 215B, 215C, it will be appreciated that the second branch 214 may be arranged to supply lubricant to any number of a plurality of second lubricant outlets. Figure 8 shows a third lubricant delivery system 402. The third lubricant delivery system 402 comprises a pump 230 and a conduit arrangement 210 comprising a first branch 212 and a second branch 214 identical to those described with reference to Figure 6. The second branch 214 comprises a flow restrictor 240 identical to the flow restrictor described with reference to Figure 6. These features will therefore not be discussed again in detail. The conduit arrangement 210 of the third lubricant delivery system 402 further comprises a third branch 216 arranged to supply lubricant to a third lubricant outlet 217 at a third pressure P3. The third pressure P3 is less than the second pressure P2. It may be beneficial to provide such an arrangement where a nozzle at the third lubricant outlet 217 has a lower nozzle pressure requirement than a nozzle at the second lubricant outlet 215. Although shown as supplying lubricant to a single third lubricant outlet 217 only, it will be appreciated that the third branch 216 may be arranged to supply lubricant to a plurality of third lubricant outlets (not shown). A second flow restrictor 242 may be used to effect a pressure drop in lubricant flowing through the third branch 216. In otherwords, lubricant may be supplied to an inlet of the second flow restrictor242 at a pressure greater than the third pressure P3 and lubricant may exit an outlet of the second flow restrictor 242 at a third pressure P3 less than the second pressure P2. The second flow restrictor 242 may be similar to the flow restrictor 240 described previously. Details of the second flow restrictor 242 will not be repeated here for brevity. It will be appreciated however that where the flow restrictor 240 and the second flow restrictor 242 are both configured to reduce an internal diameter of a branch from a first internal diameter to a second internal diameter, the second internal diameter of the flow restrictor 240 may be different to the second internal diameter of the second flow restrictor 242. The third branch 216 may be fluidically connected to the pump 230 via the first branch 212 as shown in Figure 8. More generally, the third branch 216 may be supplied with lubricant at the supply pressure P0. This may be referred to as a branched system arrangement, where each of the first branch 212, the second branch 214 and the third branch 216 are supplied with lubricant at a pressure equal to the supply pressure P0 of the pump 230. In an alternative embodiment (not shown), the third branch 216 may be fluidically connected to the pump 230 via the second branch 214, such that the third branch 216 is supplied with lubricant at a pressure equal to the second pressure P2. Where the second branch 214 comprises a flow restrictor 240, this may be achieved by fluidically coupling the third branch 216 to the second branch 214 downstream of the flow restrictor 240 relative to a direction of flow of lubricant through the second branch 214. This may be referred to as an in-line arrangement, wherein the pressure of the lubricant is reduced in a plurality of stages as it flows through the conduit arrangement 210. Figure 9 shows a method 500 comprising a plurality of steps for configuring any of the oil delivery systems 202, 302, 402 described herein. At step 502 the lubricant outlet pressure requirement for each lubricant outlet is determined. At step 504 the maximum lubricant outlet pressure requirement is determined based on the outcome of step 502. At step 506 a pump 230 is selected to supply a flow of lubricant at a pressure equal to the maximum lubricant outlet pressure requirement. At step 508 pressure restriction requirements are determined for the remaining lubricant outlets based on each lubricant outlet pressure requirement and the maximum lubricant outlet pressure requirement. Each restriction requirement may be determined based on 5 the difference between the lubricant outlet pressure requirement and the maximum lubricant outlet pressure requirement. It will be appreciated that step 506 and step 508 may be performed simultaneously based on the results of step 504. Finally, at step 510 an oil delivery system comprising the selected pump 230 and a conduit arrangement 210 arranged to supply lubricant to each lubricant outlet is manufactured. The conduit arrangement 210 is arranged to apply the pressure restriction requirements determined at step 508 to lubricant 10 supplied to each lubricant outlet. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. 15

Claims

1. A lubricant delivery system for an electric drive unit comprising:a pump configured to deliver lubricant at a predetermined supply pressure; anda conduit arrangement arranged to convey lubricant supplied by the pump to at least two lubricant outlets, wherein the conduit arrangement comprises:a first branch arranged to supply lubricant to a first lubricant outlet at a first pressure substantially equal to the supply pressure; anda second branch arranged to supply lubricant to a second lubricant outlet at a second pressure; wherein the second pressure is less than the supply pressure.

2. A lubricant delivery system according to claim 1, wherein the first lubricant outlet comprises a nozzle configured to disperse lubricant in a predetermined pattern.

3. A lubricant delivery system according to claim 1 or 2, wherein the second lubricant outlet comprises a nozzle configured to disperse lubricant in a predetermined pattern.

4. A lubricant delivery system according to any preceding claim, wherein the second branch is fluidically coupled to the pump via the first branch.

5. A lubricant delivery system according to any preceding claim, wherein the second branch comprises a flow restrictor configured to effect a pressure drop across the flow restrictor in a lubricant flowing in the second branch.

6. A lubricant delivery system according to claim 5, wherein the flow restrictor is positioned upstream of the second lubricant outlet relative to the direction of flow of lubricant through the second branch such that the pressure in the flow of lubricant supplied to the second lubricant outlet is substantially stable.

7. A lubricant delivery system according to any preceding claim, wherein the first branch is arranged to supply lubricant to a plurality of first lubricant outlets at a first pressure substantially equal to the supply pressure and / or wherein the second branch is arranged to supply lubricant to a plurality of second lubricant outlets at a second pressure, wherein the second pressure is less than the supply pressure.

8. A lubricant delivery system according to any preceding claim, wherein the conduit arrangement comprises a third branch arranged to supply lubricant to a third lubricant outlet at a third pressure, wherein the third pressure is less than the second pressure.

9. A lubricant delivery system according to claim 8, wherein the third lubricant outlet comprises a nozzle configured to disperse lubricant in a predetermined pattern.

10. A lubricant delivery system according to claim 8 or 9, wherein the third branch is fluidically coupled to the pump via the first branch.

11. A lubricant delivery system according to any of claims 8 to 10, wherein the third branch comprises a second flow restrictor configured to effect a pressure drop across the second flow restrictor in a lubricant flowing in the third branch, optionally wherein the second flow restrictor is positioned upstream of the third lubricant outlet relative to the direction of flow of lubricant through the third branch such that the pressure in the flow of lubricant supplied to the third lubricant outlet is substantially stable,12. A lubricant delivery system according to any of claims 8 to 11, wherein the second and third branchesare each supplied with lubricant at a pressure substantially equal to the supply pressure.

13. An electric drive unit comprising the lubricant delivery system of any preceding claim.

14. An electric vehicle comprising an electric drive unit of claim 13.

15. A method of delivering lubricant via a lubricant delivery system, the lubricant delivery systemcomprising a pump configured to deliver lubricant at a predetermined supply pressure; and a conduit arrangement arranged to convey lubricant supplied by the pump to at least two lubricant outlets via a conduit arrangement, the method comprising:operating the pump to supply a first lubricant outlet with lubricant at a first pressure substantially equal to the supply pressure; andsupplying a second lubricant outlet with lubricant at a second pressure wherein the second pressure is less than the supply pressure.

16. A method of delivering lubricant according to claim 15, wherein supplying the second lubricant outlet with lubricant at a second pressure comprises restricting the pressure of lubricant supplied to the second lubricant outlet.

17. A method of configuring a lubricant delivery system comprising a pump and a conduit arrangement arranged to convey lubricant supplied by the pump to at least two lubricant outlets, the method comprising: determining the lubricant outlet pressure requirement at each lubricant outlet;determining the maximum lubricant outlet pressure requirement;selecting a pump to supply a flow of lubricant at a pressure substantially equal to the maximum lubricant outlet pressure requirement;determining a pressure restriction requirement for each of the remaining lubricant outlets based on each lubricant outlet pressure requirement and the maximum lubricant outlet pressure requirement; andmanufacturing the lubricant delivery system in accordance with the determined criteria.16

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