Transmission for an electric drive unit of a vehicle

Abstract

Transmission, an electric drive unit and an electric vehicle are disclosed. The transmission 100 comprises a transmission housing defining a liquid collecting sump 170, a gear 118 arranged in the tran

Description

TECHNICAL FIELD The present disclosure relates to a transmission for an electric drive unit of a vehicle. Aspects of the invention relate to a transmission, to an electric drive unit, and to a vehicle. BACKGROUND It is known to provide an electric drive unit (EDU) with a transmission or gearbox by which the output torque of the electric drive motor of the EDU is increased for delivery to the wheels and / or axle of a vehicle. Such transmissions generally include a transmission housing and at least one transmission gear wheel within the transmission housing. The transmission housing may contain lubricant or coolant liquid to reduce friction between moving components of the transmission and / or avoid overheating of transmission components. It is an aim of the present invention to address one or more disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a transmission, an electric drive unit, and a vehicle as claimed in the appended claims. According to an aspect of the present invention there is provided a transmission of a vehicle, the transmission comprising: a housing defining a liquid collection area for a liquid lubricant and / or coolant in a lower portion thereof; a gear arranged in the transmission housing; and a gear shroud at least partially located in the lower portion for separating the gear from the liquid collection area, the gear shroud having an open cross-section defined by first and second shroud side walls and a transverse wall extending therebetween to provide a gear volume in which the gear is at least partly received, wherein the gear shroud comprises: a first discrete shroud part defining at least part of the first shroud side wall; and a second discrete shroud part defining at least part of the second shroud side wall; and optionally a seal element provided at an interface between the first and second discrete shroud parts. According to an aspect of the present invention there is provided a transmission for an electric drive unit of a vehicle, the transmission comprising: a transmission housing defining a liquid collection area for a liquid lubricant and / or coolant in a lower portion thereof; a gear arranged in the transmission housing such that at least part of the gear extends into the lower portion; and a gear shroud for separating the gear from the liquid collection area, the gear shroud having an open cross-section defined by first and second shroud side walls and a transverse wall extending therebetween to provide a gear volume in which the gear is at least partly received, wherein the gear shroud comprises: a first discrete shroud part defining the first shroud side wall; and a second discrete shroud part defining the second shroud side wall; and optionally a seal element provided at an interface between the first and second discrete shroud parts. Advantageously, the provision of a gear shroud which defines a gear volume within which the gear is at least partly received enables the gear to be isolated from liquid in the liquid collection area. Additionally, the two side walls of the shroud can also provide a barrier between the gear and other sources of liquid within the transmission housing, such one or more bearings or lubricant jets. The gear shroud limits the amount of liquid swept up by the gear during use. This has been found to reduce efficiency losses associated with the transmission and the electric drive unit with which it is used, for example by reducing viscous losses and / or reducing the rotational inertia of the gear. By forming the side walls from discrete shroud parts which are sealed together by the seal element, the gear shroud can be assembled around the gear with the gear in situ while still providing an effective barrier to liquid in the liquid collection area. This can be particularly beneficial in transmissions for electric drive units in which packaging constraints are challenging, access may be limited, and efficiency losses can reduce vehicle range. Optionally, the transmission housing comprises a first transmission housing part and a second transmission housing part which are fastened together to define at least part of the transmission housing, wherein the first discrete shroud part is fixed to the first transmission housing part and the second discrete shroud part is fixed to the second transmission housing part. Advantageously, the first and second shroud parts can be assembled together to form the gear shroud during the process of fastening the first and second transmission housing parts together to form the transmission housing. In this manner, the gear shroud can be held together by the same fasteners by which the transmission housing is held together. This can simplify assembly of the gear shroud and result in a robust structure. Optionally, the first and second discrete shroud parts each comprise at least one mounting aperture by which the first and second discrete shroud parts are fixed to their respective transmission housing part using fasteners. This can provide a simple yet secure means of fixing the shroud parts to transmission housing. The at least one mounting aperture may be defined in the shroud side wall of one or both of the first and second discrete shroud parts. The at least one mounting aperture may be provided in a mounting boss defined in the shroud side wall of one or both of the first and second discrete shroud parts. Optionally, the first and second discrete shroud parts each comprise at least one alignment pin received in a corresponding alignment recess in a wall of their respective transmission housing part. This can facilitate correct alignment of the shroud parts with respect to the gear and the transmission housing. This can minimise assembly delays due to misalignment and ensure correct functioning of the shroud. The first discrete shroud part may have a transverse portion which defines at least part of the transverse wall. Optionally, the second discrete shroud part has a transverse portion which defines at least part of the transverse wall, and wherein the seal element forms a seal against the transverse portion. The seal element may form a seal against an end face of the transverse portion. With this arrangement, the seal element is compressed between the shroud parts by the same forces by which the shroud parts are held together in the transverse direction. This can result in higher contact pressure on the seal element than if the seal element is compressed in a vertical direction. The seal element may be mounted on the transverse portion of the second discrete shroud part so that the seal is abuts against the first discrete shroud part when the first and second shroud parts are brought together. The first discrete shroud part may also comprise a transverse portion which forms part of the transverse wall. In such embodiments, the seal element may be mounted on the transverse portion of one of the discrete shroud parts and abut against the end face of the transverse portion of the other discrete shroud part. Optionally, the seal element abuts against an end face of the transverse portion and / or the first shroud side wall. With this arrangement, the first shroud side wall can provide a large area against which the seal element may form an effective seal. This can reduce the sensitivity of the seal to slight misalignments between the first and second shroud parts and thereby facilitate the formation of an effective seal. Optionally, the first and second shroud side walls extend upwardly along only a part of the circumference of the gear, such that the gear extends above the distal ends of the first and second shroud side walls. With this arrangement, the gear shroud can form a barrier where required without needing to extend upwardly around the entire gear. This can reduce weight and simplify assembly of the transmission. It can also enable liquid which is flung outwardly from the gear to be ejected from the gear volume above the distal edge of the gear shroud, rather than collecting on the inside of the gear shroud and staying inside the gear volume. Optionally, an inwardly extending flange is provided along a distal edge of at least one of the first and second discrete shroud parts. With this arrangement, the flange can form a barrier to liquid which is projected towards the gear above the distal edge of the gear shroud. This can further reduce the amount of liquid swept up by the gear. Optionally, the angle formed between the flange and an inside surface of the side wall of the shroud part from which it extends, is obtuse. Thus, the flange extends toward the gear in an upward direction from the side wall. With this arrangement, the flange can define a funnel on the outer surface of the shroud side wall by which liquid “caught” by the flange is caused to flow down the flange along the outer surface of the shroud wall to be collected in the liquid collection area of the transmission housing, rather than in the gear volume. Optionally, the transverse wall extends circumferentially around and substantially parallel to the gear, along part of a circumference of the gear. The shroud may have a part-circular shape. The shroud may conform to the shape of the gear. Advantageously, this can minimise the space envelope required by the gear shroud within the housing. Optionally, the liquid collection area has a cross-sectional shape which conforms to the cross-sectional shape of the gear shroud around at least part of the width of the gear shroud. This can minimise the space envelope required by the gear shroud within the housing. Optionally, the gear shroud comprises a plurality of stiffening ribs on its outer surface. This can provide the gear shroud with a more robust structure. Optionally, the first and second discrete shroud parts are formed from a polymer, for example an injection moulded polymer. Optionally, the first and second discrete shroud parts are formed from Polyamide 6. This has been found to provide a light-weight structure which can be formed into complex shapes suitable for the gear shroud. According to an aspect of the present invention there is provided an electric drive unit comprising the transmission of the above aspect. According to an aspect of the present invention there is provided a vehicle comprising the transmission and / or the electric drive unit of any of the above aspects. 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 any way 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 shows an electric drive unit (EDU) in accordance with an embodiment of the invention; Figure 3 shows a longitudinal cross-sectional view of a lower portion of a transmission in accordance with an embodiment of the invention; Figure 4 shows a transverse cross-sectional view of the lower portion of the transmission of Figure 3; Figure 5 shows a perspective view of a first example gear shroud for the transmission of Figures 3 and 4; Figure 6 shows an exploded perspective view of the gear shroud of Figure 5; Figure 7 shows an inboard side view of the gear shroud of Figure 5; Figure 8 shows an outboard side view of the gear shroud of Figure 5; Figure 9 shows a front view of the gear shroud of Figure 5; Figure 10 shows a bottom view of the gear shroud of Figure 5; and Figure 11 shows a cross-sectional view of the gear shroud of Figure 5 through line XI-XI in Figure 10. DETAILED DESCRIPTION A transmission in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figures 1 to 11. The Figures illustrate a vehicle 1, an electric drive unit (EDU) 10, a transmission 100, and a gear shroud 200. As shown in Figures 1 and 2, the transmission 100 forms part of an EDU 10 installed in a vehicle 1. Figure 1 illustrates a vehicle 1 to provide context forthis invention. The vehicle 1 may be a fully electric vehicle having only an electric machine for propulsion and no internal combustion engine or may be a hybrid vehicle having both an electric machine and an internal combustion engine arranged to propel the vehicle. The vehicle 1 may be a plug-in hybrid electric vehicle (PHEV) or a mild hybrid electric vehicle (MHEV). The vehicle 1 has at least one vehicle drive unit 10 containing a power source such as an electric machine or an internal combustion engine and one or more drivetrain components such as a gear or a differential arranged to transfer torque from the power source to the wheels of the vehicle. The vehicle drive unit 10 may be an electric drive unit (EDU) that contains an electric machine and one or more drivetrain components. In the illustrated embodiment, the vehicle 1 has a first vehicle drive unit 10-1 for driving one or more front wheels of the vehicle 1 and a second vehicle drive unit 10-2 for driving one or more rear wheels of the vehicle 1. In other embodiments the vehicle 1 may comprise only a single vehicle drive unit arranged or configured to drive one or more front wheels of the vehicle 1 and / or one or more rear wheels of the vehicle 1. At a vehicle axle the drive unit may be arranged to drive both wheels, either directly or through other transmission components. In other arrangements there may be more than one vehicle drive unit arranged to provide torque to a single vehicle axle, for example, to provide torque vectoring functionality for the vehicle 1. Other arrangements may have one drive unit for each wheel of the vehicle 1. Figure 2 is a schematic illustration of a vehicle drive unit for the vehicle 1 in the form of an electronic drive unit (EDU) 10. The drive unit 10 includes a motor portion 50 and a transmission 100. The motor portion 50 includes an electric machine 51 for propelling the vehicle. The electric machine 51 has a stator 52 and a rotor 54, the stator 52 being arranged to receive electrical power from a battery or generator and to impart a magnetic field to the rotor 54 to rotate the rotor and power the vehicle. The rotor 54 is coupled to a rotor shaft 56 which is arranged to transfer torque to downstream components of the electronic drive unit. The rotor 54 rotates about a rotor axis 55 which extends in the longitudinal direction of the EDU 10. The rotor shaft 56 has a geared end portion 57 that is arranged to engage with and to transfer torque to the transmission 100. The motor portion 50 comprises a motor housing 60 within which the components of the motor portion 50 are housed. The transmission 100 comprises a transmission housing 160 within which the components of the transmission 100 are housed. The motor housing 60 and the transmission housing 160 are fastened together, for example by bolting, to form an EDU housing. The transmission housing 160 may be defined in part by an extension of the motor housing 60. The transmission housing 160 may comprise a first transmission housing part 161 and a second transmission housing part 162 which are fastened together to define at least part of the transmission housing 160. In the illustrated embodiment, the transmission housing 160 is defined by a first transmission housing part 161 which is integral with an end portion of the motor housing 60 and a second transmission housing part 162 in the form of a transmission cover which is fastened to the first transmission housing part 161. The transmission 100 includes transmission gear 110 arranged in the transmission housing 160 and configured to transfer torque from the rotor shaft 56 to one or more wheels of the vehicle 1. The transmission may comprise a differential gear (not shown in Figure 2). The EDU 10 may comprise a liquid circulation system for circulating lubricant and / or cooling liquids around certain regions of the motor portion 50 and / or the transmission 100. The transmission housing 160 defines a liquid collection area 170 in a lower portion thereof fora lubricant and / or coolant liquid. The transmission 100 may comprise one or more spray jets 140 arranged to project lubricant or coolant liquid, for example oil, onto high temperature regions of the transmission and / or regions of the transmission in which moving parts of the transmission interface, for example shaft bearings orthe interface between the rotor shaft and one or more gears of the transmission. In Figure 2, the rotor axis 55 is illustrated vertically, but the EDU is orientated in use such that the liquid collection area 170 is at its lower end and the rotor axis 55 extends generally horizontally in a width-wise direction of the vehicle. Figures 3 and 4 illustrate a lower portion of a transmission 100 in accordance with an embodiment. Figure 3 shows a longitudinal cross-sectional view of the lower portion of the transmission 100 taken through a plane which is generally parallel to the rotor axis of the electric machine. Figure 4 shows a transverse cross-sectional view of the lower portion of the transmission 100 taken through line IV-IV in Figure 3 which represents a plane which is generally perpendicular to the rotor axis of the electric machine. The transmission 100 includes a transmission gear 110 by which torque from the electric machine is transferred to the drive train. The transmission gear 110 includes a transmission shaft 112 mounted for rotation within the transmission housing 160 by a first bearing 114 and a second bearing 116 towards either end of the transmission shaft 112. The transmission gear 110 also includes an input gearwheel 118 which is rotationally fixed to the transmission shaft 112 for rotation therewith. As can be seen in Figure 4, the gearwheel 118 is meshed with a rotor output gear 58 fixed to the geared end portion of the rotor shaft 56. The transmission gear 110 also includes an output gear wheel 120 which is rotationally fixed to the transmission shaft 112 for rotation therewith and which provides torque to an EDU output drive shaft 70. The transmission shaft 112, input gearwheel 118, and output gearwheel 120 all rotate about transmission axis 113. In this example, the transmission 100 is a two-stage and single-speed transmission in which the input gear wheel 118 is a first stage gear and the output gear wheel 120 is a second stage gear wheel. However, it will be understood that transmission may be a multispeed transmission with two, three or more stages. It will also be understood that the transmission shaft 112 could also define the EDU output drive shaft or be directly connected to the EDU output drive shaft rather than connected via one or more gears. The bearings 114 and 116 are supported by the walls of the transmission housing 160. The bearings 114 and 116 are supplied with lubricant and / or coolant liquid by one or more spray jets (not shown) of the liquid circulation system of the EDU. The second stage gear 120 is supplied with lubricant and / or coolant liquid by a second stage spray jet (not shown). The lubricant and / or coolant liquid drains from the first and second bearings 114 and 116 and from the second stage gear 120 along drain paths A, B and C to the liquid collection area 170 defined at a lower portion of the transmission housing 160. The collected liquid is denoted in Figures 3 and 4 by a dashed line (feature L in Figure 4) which illustrates the typical maximum liquid level in the liquid collection area 170. The lubricant and / or coolant liquid may be temporarily stored in the liquid collection area 170 before being drained, for example to a sump (not shown) in the EDU housing from which it may be recirculated. The first stage gear 118 extends into the lower portion of the transmission housing 160. This is primarily due to the large diameter of the first stage gear 118 and the need to minimise the overall size of the transmission housing 160 due to packaging constraints. The transmission 100 also includes a gear shroud 200 for separating the first stage gear 118 from the liquid collection area 170. The gear shroud 200 has an open cross-section defined by first and second shroud side walls 202 and 204 and a transverse wall 206 (see Figures 5 and 11) extending therebetween to provide a gear volume 208 in which the first stage gear 118 is at least partly received. In this manner, the gear shroud 200 isolates the gear 118 from liquid stored in the liquid collection area 170. The side walls of the gear shroud 200 also form a barrier between the gear 118 and liquid returning to the liquid collection area 170 along drain paths A, B and C. This can be particularly beneficial for separating the gear 118 from liquid flowing along drain path A from the first bearing 114 which would otherwise drain onto the side face of the gearwheel 118 to be swept up by the gear as it rotates and / or collect in the bottom of the gear volume 208. Separating the gear 118 from the volume of liquid in the liquid collection area 170 allows the gear 118 to move through air rather than through the lubricant and / or coolant liquid, thus reducing the drag acting on the gear 118 due to the viscosity of the liquid. Reducing the amount of liquid on the gear 118 as it rotates can also reduce the “spin losses” caused by liquid swept up by the gear wheel 118 as it rotates. These spin losses would otherwise result in a drop in rpm or an increase in the power required to rotate the gear 118 at a given rpm. These “spin losses” can reduce the efficiency of the EDU. Consequently, the shroud 200 can help to increase EDU efficiency. Furthermore, preventing oil / lubricant from being spun and agitated by the rotating gear reduces aeration of the oil which is undesirable. The gear shroud 200 comprises a first discrete shroud part 210 defining the first shroud side wall 202, second discrete shroud part 220 defining the second shroud side wall 204 and at least part of the transverse wall 206, and a seal element 230 provided at an interface between the first and second discrete shroud parts 210 and 220. The gear shroud 200 is discussed below in more detail in relation to Figures 5 to 11. Optionally, the transmission housing 160 comprises a first transmission housing part 161 and a second transmission housing part 162 which are fastened together to define at least part of the transmission housing 160. In such embodiments, the first discrete shroud part 210 may be fixed to the first transmission housing part 161 and the second discrete shroud part 220 may be fixed to the second transmission housing part 162. In this manner, the two parts of the gear shroud 200 can be fixed to their respective transmission housing parts and assembled together when the transmission housing 160 is assembled. This can avoid the need for any separate gear shroud installation steps following the assembly of the transmission housing 160. This can be beneficial due to the limited or lack of access to the interior of the transmission housing 160 once it has been assembled. As best seen in Figure 4, the transverse wall 206 extends circumferentially around and substantially parallel to the gear 218 along part of the circumference of the gear. In this manner, the gear shroud 200 can conform to the shape of the gear to minimise the space envelope required by the gear shroud within the housing. This can reduce the overall size of the transmission housing and / or increase the volume of the liquid collection area 170 available to the lubricant and / or coolant liquid. The liquid collection area 170 may have any suitable shape. The liquid collection area 170 may have a cross-sectional shape which conforms to the cross-sectional shape of the gear shroud 200 along at least part of the width and / or length of the gear shroud 200. In the illustrated embodiment, the liquid collection area 170 has a longitudinal cross-sectional shape which conforms to the shape of the gear shroud 200 along substantially the entire width of the gear shroud 200 (see Figure 3) and has a part-circular lateral cross-sectional shape which conforms to the part-circular shape of the gear shroud 200 along the majority of the length of the gear shroud 200 (see Figure 4). In this manner, the transmission housing can most closely conform to the shape of the gear shroud to reduce the size of the housing. The transverse wall 206 may be arranged at a substantially constant distance from the transmission axis 113 and / or from the pitch circle diameter (PCD) of the gear 118. For example, the inner surface of the transverse wall 206 may be arranged at a distance of less than 0.75 x PCD, optionally less than 0.6 x PCD, optionally less than 0.55 x PCD from the transmission axis 113. The transverse wall 206 may be arranged at a distance of less than 10 millimetres, optionally less than 7 millimetres, optionally less than 5 millimetres from the radially outermost surface of the gear 118. Arranging the transverse wall 206 relatively close to the gear 118 may reduce the overall size of the gear shroud 200 and transmission housing 160. The first and second shroud side walls 202 and 204 extend upwardly along only a part of the circumference of the gear 118 such that the gear 118 extends above the distal ends of the first and second shroud side walls 202 and 204. Thus, the gear shroud 200 forms a barrier to lubricant and / or coolant liquid in the liquid collection area 170 and draining from the bearings without needing to extend around the entire gear. This can reduce weight and simplify assembly of the transmission. It can also enable liquid which is present on the gear 118 to be spray outwardly and be ejected from the gear volume above the distal edge of the gear shroud, rather than collecting on the inside of the gear shroud and staying inside the gear volume. The gear shroud 200 may extend upwardly along only a part of the circumference of the gear 118 such that the transverse wall 206 extends around between 15% of the circumference of the gear profile (to subtend an angle of 54 degrees) and 50% of the circumference of the gear profile (to subtend an angle of 180 degrees), optionally around between 25% of the circumference of the gear profile (to subtend an angle of 90 degrees) and 35% of the circumference of the gear profile (to subtend an angle of 126 degrees). Figures 5 to 11 show an example of the gear shroud 200. The gear shroud 200 comprises the first discrete shroud part 210 defining the first shroud side wall 202, second discrete shroud part 220 defining the second shroud side wall 204, and the seal element 230 provided at an interface between the first and second discrete shroud parts 210 and 220. Each shroud wall 122, 124 may be a relatively thin wall, optionally between 1 millimetre and 10 millimetres thick, for example between 2 millimetres and 5 millimetres thick. The transverse wall 206 may be a discrete component. In the illustrated embodiment, the transverse wall 206 is unitary with the second shroud side wall 204. The transverse wall 206 may extend from the second shroud side wall 204 in a substantially perpendicular direction when viewed in cross-section. Generally, the transverse wall 206 extends from the second shroud side wall 204 at an angle of between 60 and 120 degrees, for example at an angle of between 85 and 100 degrees. The transverse wall 206 may have a constant radius of curvature as viewed from the side (see Figures 7 and 8). It may be beneficial to provide a gear shroud 200 having a curved transverse shroud wall 206 to reduce the distance between the gear shroud 200 and the gear 118 such that the overall EDU may be more compact. The first and second discrete shroud parts 210 and 220 may be formed from any suitable material. In some embodiments, the first and second discrete shroud parts 210 and 220 are formed from a polymer, for example an injection moulded polymer such as Polyamide 6. The first and second discrete shroud parts 210 and 220 each comprise at least one mounting aperture 212 by which the first and second discrete shroud parts 210 and 220 are fixed to their respective transmission housing part 161 and 162 using fasteners, such as bolts (not shown). The mounting apertures 212 may be defined in a mounting boss 214 formed in the first and second shroud side walls 202 and 204, as best seen in Figure 9. The first and second discrete shroud parts 210 and 220 each comprise at least one alignment pin 216 received in a corresponding alignment recess (not shown) in a wall of their respective transmission housing part 161 and 162. In the illustrated embodiment, each discrete shroud part comprises a single alignment pin 216, as best seen in Figure 9. The alignment pins assist in the correct locating of the shroud parts during installation and prevent misalignment of the gear shroud 200 with respect to the gear 118 and the transmission housing 160 due to the rotation of one or both shroud parts about their respective mounting aperture. The first and second discrete shroud parts 210 and 220 may also comprise one or more stiffening structures. For example, the first and second discrete shroud parts 210 and 220 may each comprise one or more transverse deformations 222 in the first and second shroud side walls 202 and 204 and / or a plurality of stiffening ribs 224 extending transversely along an outer surface of the transverse wall 206. The stiffening structures increase the rigidity of the walls of the gear shroud. This reduces the amount of deformation of the gear shroud 200. It may also be beneficial for Noise, vibration, and harshness (NVH) purposes by reducing the risk or amplitude of vibrations of the walls of the gear shroud 200. The mounting apertures 212 are optionally located adjacent to or coincident with a transverse deformation in their respective shroud side wall, as shown in Figures 7 and 8. As best seen in Figures 6 and 11, the seal element 230 comprises a mounting portion 232 by which the seal element 230 is attached to the second discrete shroud part 220 and a flexible portion 234 by which the seal element 230 forms a seal against the first discrete shroud part 210. The second discrete shroud part 220 has a transverse portion 221 which extends transversely from the second shroud side wall 204 to define the majority of the transverse length of the transverse wall 206. The mounting portion 232 is attached to the transverse portion 221 to form a seal therewith. The mounting portion 232 may be attached to the transverse portion 221 in any suitable manner, for example using adhesive. In the illustrated embodiment, the mounting portion 232 comprises a plurality of flexible tabs 236 which extend from an upwardly extending wall 238 of the seal element 230 and are received in corresponding seal recesses 225 in the transverse portion 221. The flexible tabs 234 each have a retention projection (not shown) extending from its underside which is received in a corresponding retention cavity 227 in its respective seal recess 225. It is to be noted that the section in Figure 11 does not pass through a flexible tab 236 and its respective seal recess 225 and retention cavity 227, but rather extends through a region between such tabs. The seal element 230 may be formed from any suitable material. By way of non-limiting example, the seal element 230 may be a deformable rubber sealing element arranged to form an impermeable connection with both discrete shroud parts. The mounting portion 232 comprises a seal surface 233 which abuts against a corresponding seal surface 223 of the transverse portion 221. In the illustrated embodiment, the seal surface 233 is defined by the upwardly extending wall 238 but it will be appreciated that the seal surface 233 could be provided by any one or more of the surfaces of the seal element 230 and the second discrete shroud part 220 which are in contact. The flexible portion 234 defines a seal lip which forms a seal against an end face surface of the first discrete shroud part 210 to form a seal therewith. When the gear shroud 200 is assembled, the first and second discrete parts are spaced from each other in the transverse direction by a transverse clearance. The seal element 230 has a transverse dimension which is greater than the transverse clearance such that the seal element 230 is compressed in the transverse direction between the first and second discrete parts when the gear shroud 200 is assembled. The stiffening structures on the first and second discrete shroud parts 210 and 220 can help to increase the contact pressure between the seal lip and the first discrete shroud part 210. In this manner, the seal lip of the flexible portion 234 forms a seal between the first shroud part 210 and the seal element 230 and the seal surfaces 223 and 233 form a seal between the second shroud part 220 and the seal element 230 such that the gear shroud 200 forms a barrier between the gear volume 208 and the liquid collection area to separate the gear from the liquid collection area 170. Of course, it is to be noted that such a perfect seal is not essential but is preferable to retain a small amount of oil in the volume 208 when the motor is stationary. Then, oil is immediately available to lubricate the gears 58, 218 when they start to rotate. In the illustrated embodiment, the transverse portion 221 and the seal element 230 together define the transverse wall 206. It will be appreciated that the transverse portion could be defined instead by the first discrete shroud part 210, or the first discrete shroud part 210 could define a further transverse portion which defines the transverse wall 206 together with the transverse portion 221 and the seal element 230. Further, although the seal element 230 is shown as being attached to the transverse portion 221 and compressed against an end face of the first discrete shroud part, the seal element 230 could be attached to the first discrete shroud part 210 and compressed against the transverse portion 221 and / or against an end face of the second discrete shroud part 220. As best seen in Figure 11, an inwardly extending flange 219 may be provided along a distal edge of at least one of the first and second discrete shroud parts 210 and 220. The inwardly extending flange 219 may extend at any suitable angle relative to the inside surface of the side wall of the shroud part from which it extends, for example at an obtuse angle or at an angle of around 90 degrees. In the illustrated embodiment, the inwardly extending flange 219 is provided along a distal edge of the first discrete shroud part 210 such that the angle (180-a)° formed between the flange 219 and an inside surface of the first shroud side wall 202 of the first discrete shroud part 210 is obtuse. With this arrangement, the flange 219 can define a funnel on the outer surface of the shroud side wall 202 by which liquid “caught” by the flange 219 is caused to flow down the flange 219 along the outer surface of the shroud side wall 202 to be collected in the liquid collection area of the transmission housing, rather than in the gear volume 208. This prevents excess oil entering and collecting in the gear volume 208 (that would inhibit rotation of the gear and cause excessive aeration and churning of the oil) but does not prevent oil on the teeth and sides of the gear 218 dropping into the gear volume to maintain some depth of oil in the gear volume when the motor is stationary. 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.

Claims

1. A transmission for an electric drive unit of a vehicle, the transmission comprising:a transmission housing defining a liquid collection area for a liquid lubricant and / or coolant in a lower portion thereof;a gear arranged in the transmission housing such that at least part of the gear extends into the lower portion; anda gear shroud for separating the gear from the liquid collection area, the gear shroud having an open cross-section defined by first and second shroud side walls and a transverse wall extending therebetween to provide a gear volume in which the gear is at least partly received, wherein the gear shroud comprises:a first discrete shroud part defining the first shroud side wall; and a second discrete shroud part defining the second shroud side wall.

2. The transmission of claim 1, wherein the transmission housing comprises a first transmission housing part and a second transmission housing part which are fastened together to define at least part of the transmission housing, wherein the first discrete shroud part is fixed to the first transmission housing part and the second discrete shroud part is fixed to the second transmission housing part.

3. The transmission of claim 2, wherein the first and second discrete shroud parts each comprise at least one mounting aperture by which the first and second discrete shroud parts are fixed to their respective transmission housing part using fasteners.

4. The transmission of claim 3, wherein the first and second discrete shroud parts each comprise at least one alignment pin received in a corresponding alignment recess in a wall of their respective transmission housing part.

5. The transmission of any preceding claim, further comprising a seal element provided at an interface between the first and second discrete shroud parts, and optionally wherein the second discrete shroud part has a transverse portion which defines at least part of the transverse wall, and wherein the seal element forms a seal against the transverse portion.

6. The transmission of claim 5, wherein the seal element abuts against an end face of the transverse portion and / or the first shroud side wall.

7. The transmission of any preceding claim, wherein the first and second shroud side walls extend upwardly along only a part of the circumference of the gear, such that the gear extends above the distal ends of the first and second shroud side walls.

8. The transmission of claim 7, wherein an inwardly extending flange is provided along a distal edge of at least one of the first and second discrete shroud parts.

9. The transmission of claim 8, wherein the angle formed between the flange and an inside surface of the side wall of the shroud part from which it extends, is obtuse.

10. The transmission of any preceding claim, wherein the transverse wall extends circumferentially around and substantially parallel to the gear, along part of a circumference of the gear.

11. The transmission of any preceding claim, wherein the liquid collection area has a cross-sectional shape which conforms to the cross-sectional shape of the gear shroud around at least part of the width of the gear shroud.

12. The transmission of any preceding claim, wherein the gear shroud comprises a plurality of stiffening ribs on its outer surface.

13. The transmission of any preceding claim, wherein the first and second discrete shroud parts are formed from a polymer.

14. An electric drive unit comprising the transmission of any preceding claim.

15. A vehicle comprising the transmission of any of claims 1 to 13 or the electric drive unit of claim 14.14

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