Patent Publication Number: US-2022224202-A1

Title: Systems and methods for cooling electric motor

Description:
TECHNICAL FIELD 
     The present description relates generally to systems and methods for cooling electric motors, and particularly for cooling an electric motor fluidically coupled to a gearbox. 
     BACKGROUND AND SUMMARY 
     An electric motor may include heat sinks or other components configured to maintain an operating temperature of the electric motor within a desirable range. In some examples, the electric motor may be configured to receive a coolant, where heat produced by the electric motor may be transferred from the electric motor to the coolant to reduce the operating temperature. In such examples, the coolant may be delivered across various surfaces of the electric motor (e.g., surfaces which are commonly subjected to relatively high heat during electric motor operation) such that excess heat may be absorbed from the various surfaces. 
     In examples wherein the electric motor is fluidically coupled to a gearbox (e.g., to reduce packaging size and cost in an electric or hybrid electric vehicle), the coolant may desirably be shared between the electric motor and the gearbox. In one example, during electric motor operation, a fixed, passive coolant transport system (e.g., one or more baffles, channels, etc.) may be employed to deliver the coolant from the gearbox to the electric motor. However, the fixed, passive coolant transport system alone may be unable to efficiently deliver the coolant to the various surfaces of the electric motor which may benefit from cooling during electric motor operation (such as surfaces on an opposite end of the electric motor from the gearbox). Another option, alone or in combination with the fixed, passive coolant transport system, may include implementing an active coolant pump. However, such pumps may result in increased packaging space, complexity, maintenance, and cost. 
     In one example, the issues described above may be addressed by a system including a gearbox comprising a first sump, an electric motor fluidically coupled to the gearbox, an outer shaft extending through the electric motor and into the first sump, the first sump being fluidically coupled to the electric motor via the outer shaft, an inner shaft coaxial with the outer shaft, the inner shaft extending through each of the first sump and the outer shaft, and an external screw thread disposed along a portion of the inner shaft extending within the outer shaft. Upon rotation of the inner shaft, a coolant in the first sump may be drawn through the outer shaft from the first sump into the electric motor. Specifically, rotation of the external screw thread may induce flow of the coolant along the outer shaft. In some examples, rotation of the inner shaft may be responsive to operation of the electric motor, such that the coolant may flow to the electric motor whenever the electric motor is operated. In some examples, a plurality of openings or outlets may be provided on a portion of the outer shaft extending within the electric motor. Accordingly, the coolant may be distributed substantially evenly throughout the electric motor, or the coolant may be directed to surfaces specifically prone to excess heat. 
     In some examples, the electric motor may include one or more channels or grooves set in an internal surface of a housing of the electric motor, the one or more channels or grooves configured to direct the coolant back to a second sump of the gearbox. During operation of the electric motor, rotation of the inner shaft may rotate at least one gear partially submersed in the coolant collected in the second sump. As a result, the coolant may be splashed against a housing of the gearbox, wherefrom the splashed coolant may be directed back into the first sump (e.g., via one or more baffles or vanes affixed to the housing of the gearbox). In this way, during operation of the electric motor, the coolant may be passively cycled between the gearbox and the electric motor without significant increases in packaging space, complexity, maintenance, and cost, or significant losses in overall system efficiency. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  schematically shows an exemplary vehicle including an electric motor. 
         FIG. 2  shows a perspective view of an electric motor, such as the electric motor of  FIG. 1 . 
         FIG. 3  shows a schematic cross-sectional view of a passive cooling system for an electric motor, such as the electric motor of  FIG. 1 or 2 , fluidically coupled to a gearbox. 
         FIG. 4  shows a flow chart illustrating a method for passively cooling an electric motor fluidically coupled to a gearbox. 
     
    
    
     DETAILED DESCRIPTION 
     The following description relates to systems and methods for cooling an electric motor fluidically coupled to a gearbox by passively (e.g., without actively controlling) cycling a coolant between the electric motor and the gearbox. Specifically, a coolant pump may extend between the electric motor and the gearbox, the coolant pump being formed from an outer shaft enclosing at least a portion of an inner shaft. The inner shaft may include one or more surface features configured such that, during rotation of the inner shaft, the coolant may be drawn from a shaft sump in the gearbox to an internal volume of the electric motor. For example, the coolant pump may be a screw pump or an axial-flow pump, the one or more surface features being respectively configured as an external helical threading or a propeller, and the outer shaft providing a pump housing. The inner shaft may rotate during operation of the electric motor, such that the coolant may be directed along the outer shaft from the gearbox to the electric motor. A plurality of openings or outlets in the outer shaft may be configured to distribute the coolant to desirable locations within the electric motor, such as end windings of a stator of the electric motor. 
     Following distribution of the coolant within the electric motor, the coolant may be directed back into the gearbox via gravity drainage. For example, the coolant may collect in one or more channels or grooves set into one or more internal surfaces of the electric motor, the one or more channels or grooves being fluidically coupled to a gearbox sump, e.g., integrally formed within the gearbox, and configured to direct the coolant thereto. From the gearbox sump, the coolant may be directed back into the shaft sump via splashing of the coolant, thus completing passive cycling of the coolant. For example, at least one gear extending into the gearbox sump may be fixedly coupled to the inner shaft, such that rotation of the inner shaft during operation of the electric motor may splash the coolant against an internal surface of the gearbox. In some examples, one or more baffles or vanes may be provided at the internal surface of the gearbox to collect the splashed coolant and direct the splashed coolant into the shaft sump. In this way, a coolant pump may be provided which passively cycles and distributes coolant throughout an electric motor and a gearbox fluidically coupled thereto while substantially maintaining an overall system efficiency and mitigating increases in packaging size, complexity, maintenance, and cost (“substantially” may be used herein as a qualifier meaning “effectively” or “practically”). 
     In an exemplary embodiment, an electric or hybrid-electric vehicle, such as the vehicle of  FIG. 1 , may include an electric motor, such as the electric motor of  FIG. 2 , fluidically coupled to a gearbox. A passive cooling system, such as the passive cooling system of  FIG. 3 , may be included within the electric motor and the gearbox, the passive cooling system configured to distribute a coolant throughout internal volumes of the electric motor and the gearbox. An exemplary method for passively cooling the electric motor via the passive cooling system is depicted at  FIG. 4 . 
     Referring to  FIG. 1 , a vehicle  100  is shown schematically. The vehicle  100  may include a first shaft  102  and a second shaft  112 . The first shaft  102  may be configured to drive a first set of wheels  104  of the vehicle  100  and the second shaft  112  may be configured to drive a second set of wheels  114  of the vehicle  100 . In one example, the first shaft  102  may be arranged nearer to a front of the vehicle  100  than the second shaft  112  such that the second shaft  112  is arranged nearer to a rear of the vehicle  100  than the first shaft  102 . 
     The vehicle  100  may include an engine  110  coupled to a first gearbox  111  via a third shaft  122  (e.g., a first output shaft  122 ). In some examples, the vehicle  100  may further include an electric motor  120  coupled to a second gearbox  121  via a fourth shaft  132  (e.g., a second output shaft  132 ). Each of the first gearbox  111  and the second gearbox  121  may transfer power to a first differential  103  arranged on the first shaft  102  and a second differential  113  arranged on the second shaft  112 . Accordingly, in one example, the vehicle  100  may include a coaxial motor-gearbox configuration, wherein the second gearbox  121  includes an internal differential (not shown at  FIG. 1 ) having at least one output shaft (e.g., the fourth shaft  132 ) extending therefrom through the electric motor  120  and a side of the second gearbox  121  opposite the electric motor  120 , the at least one output shaft coupling output of the electric motor  120  to the first and second sets of wheels  104 ,  114  via the first and second shafts  102 ,  112 , respectively. 
     In one example, the engine  110  and the electric motor  120  may be arranged in a power-series hybrid configuration. However, it will be appreciated by those of ordinary skill in the art that the hybrid configuration of the vehicle  100  may be in any form without departing from the scope of the present disclosure. As one example, the vehicle  100  may be adjustable to a plurality of different modes. In one example mode, the vehicle  100  may be propelled via torque provided to the wheels (e.g., the first set of wheels  104  and the second set of wheels  114 ) by only the engine  110 . In another example mode (e.g., an all-electric mode of a hybrid configuration) or another example configuration (e.g., an all-electric configuration), the vehicle  100  may be propelled via torque provided to the wheels (e.g., the first set of wheels  104  and the second set of wheels  114 ) by only the electric motor  120 . In yet another example, the vehicle  100  may be propelled via torque provided to the wheels (e.g., the first set of wheels  104  and the second set of wheels  114 ) by both of the engine  110  and the electric motor  120 . In some examples, both of the engine  110  and the electric motor  120  may be coupled to the first gearbox  111  and/or the second gearbox  121 . 
     The electric motor  120  may be configured to receive energy (e.g., electrical energy) from a power source  130 . The power source  130  may be a battery or a battery pack, as one example. In some examples, each of the engine  110  and the electric motor  120  may be fluidically coupled to a common cooling system  140 . In additional or alternative examples, each of the electric motor  120  and the second gearbox  121  may be fluidically coupled to a common cooling system  170  (such as the passive cooling system described in detail below with reference to  FIG. 3 ). 
     The cooling system  140  and/or the cooling system  170  may include a cooling media, such as air, water, glycol, oil, a phase change material, a conductive solid, or the like. In examples wherein the cooling media includes a cooling fluid or coolant, the cooling system  140  may flow the coolant through coolant passages of the engine  110  and the electric motor  120  and/or the cooling system  170  may flow the coolant through coolant passages of the electric motor and the second gearbox  121 . In some examples, one or both of the engine  110  and the electric motor  120  may be fluidly coupled to separate cooling systems. For example, the electric motor  120  may be fluidly coupled to a dedicated electric motor cooling system, such as the cooling system  170  including a passive coolant pump  172  (e.g., a coolant pump not actively controlled during vehicle operation) and a plurality of coolant flow paths, where the plurality of coolant flow paths may be configured to receive the coolant (e.g., oil) flowing from a coolant outlet of the electric motor  120  and flow the coolant to a coolant inlet of the electric motor  120  via the passive coolant pump  172  (e.g., after cycling the coolant through the second gearbox  121 ). 
     As described above, the vehicle  100  may include the electric motor  120  configured to deliver torque to the wheels of the vehicle  100  to propel the vehicle  100 . In some examples, the vehicle  100  may further include the electric motor  152  configured to provide torque to power one or more other devices onboard the vehicle  100 . For example, the electric motor  152  may be configured to power (e.g., deliver torque to) a cooling fan, compressor, or other device of the vehicle  100 . As shown, and similar to the electric motor  120 , the electric motor  152  may be configured to receive energy (e.g., electrical energy) from the power source  130 . The electric motor  152  and the electric motor  120  may further be configured to receive the coolant (e.g., oil) from a same cooling system, in some examples (e.g., the cooling system  140  or the cooling system  170 ). 
     The vehicle  100  may additionally include an electronic controller  150 . The controller  150  may receive signals (e.g., input) from the various sensors of  FIG. 1  and employs the various actuators of  FIG. 1  to adjust engine operation based on the received signals and instructions stored on non-transitory memory of the controller  150 . As non-limiting examples, the sensors of the vehicle  100  may include various temperature sensors (e.g., a temperature sensor  160  configured to measure a temperature of the electric motor  120 , a temperature sensor  162  configured to measure a temperature of the electric motor  152 , etc.), pressure sensors, speed sensors, throttle sensors, battery charge sensors, air-fuel ratio sensors, etc. As non-limiting examples, the actuators of the vehicle  100  may include various valves, throttles, fuel injectors, etc. The types of sensors and actuators listed herein are for illustrative purposes and any type of sensors and/or actuators may be included without departing from the scope of this disclosure. 
     Based on received input from the sensors, the controller  150  may send control signals to the actuators, the actuators being communicably coupled to the electric motor  120 , the electric motor  152 , the engine  110 , and/or other components of the vehicle  100 . For example, adjusting an amount of coolant flowing to the electric motor  120  and/or the electric motor  152  may include adjusting an amount of energization and/or energization timing of a coolant pump (e.g., a coolant pump  141  included in the cooling system  140 ) configured to pump the coolant to the electric motor  120  and/or the electric motor  152  based on received input from the temperature sensor  160  and/or the temperature sensor  162 , respectively (however, it will be appreciated that other embodiments herein may include no actively controlled coolant pump in either the cooling system  140  or the cooling system  170 ). 
     The controller  150  may be a microcomputer electrically coupled to the power source  130 , the microcomputer including a microprocessor unit, input/output ports, and an electronic storage medium for executable programs and calibration values. The controller  150  may include a non-transitory computer readable medium (memory) in which programming instructions are stored, and may be programmed with computer readable data representing instructions executable to perform various methods, such as the method described in detail below with reference to  FIG. 4 , as well as other variants that are anticipated but not specifically listed. Memory as referenced herein may include volatile and non-volatile or removable and non-removable media for storage of electronic-formatted information such as computer readable instructions or modules of computer readable instructions, data, etc. Examples of memory may include, but are not limited to, random-access memory (RAM), read-only memory (ROM), electrically erasable programmable ROM (EEPROM), flash memory, or any other medium which may be used to store a desired electronic format of information and which may be accessed by a processor or processors or at least a portion of a computing device (e.g., the controller  150  or an external computing device). 
     Referring now to  FIG. 2 , a perspective view of an electric motor  200  is shown. In some examples, the electric motor  200  may be similar to, or the same as, the electric motor  120  or the electric motor  152  as described in detail above with reference to  FIG. 1 . For example, the electric motor  200  may be configured to provide torque to one or more wheels of a vehicle, such as the vehicle  100  as described in detail above with reference to  FIG. 1 . As another example, the electric motor  200  may be configured to drive one or more devices of the vehicle, such as one or more fans, compressors, etc. (e.g., similar to the electric motor  152  described above). 
     The electric motor  200  may include a housing  202 . In some examples, the housing  202  may be physically coupled to the electric motor  200  via welds, fusions, adhesives, fasteners, or other similar coupling elements. The housing  202  may house a plurality of components of the electric motor  200 . For example, the electric motor  200  may include a rotor, a stator, windings, and the like disposed within the housing  202 . Energization of the electric motor  200  may include energizing the rotor and the stator via a power source coupled to the electric motor  200  (e.g., the power source  130  as described in detail above with reference to  FIG. 1 ). During conditions in which the electric motor  200  is energized, an output shaft  218  (e.g., similar to, or the same as, the fourth shaft  132  as described in detail above with reference to  FIG. 1 ) may be driven to rotate around a central axis  298  of the electric motor  200  by an electromechanical coupling between the rotor and the stator (e.g., to provide torque to wheels of the vehicle, drive one or more vehicle devices, etc.). In some examples, the electric motor  200  may include a coolant jacket arranged between the stator and the housing  202 , the coolant jacket ensheathing the stator. In other examples, the electric motor  200  may include no coolant jacket therein. 
     In some examples, the housing  202  may include a coolant inlet  206  and a coolant outlet  208 . A coolant (e.g., oil) may flow into an internal volume of the electric motor  200  via the coolant inlet  206 , and the coolant may flow out of the electric motor  200  via the coolant outlet  208 . In some examples, the coolant inlet  206  and the coolant outlet  208  may instead be fluidically coupled to the coolant jacket ensheathing the stator. In one example, the coolant inlet  206  and coolant outlet  208  may be arranged at opposing ends of the housing  202 . One such configuration of the coolant inlet  206  and the coolant outlet  208  is shown (however, it will be appreciated that other configurations, such as arranging the coolant inlet  206  and the coolant outlet  208  on opposite ends of the same radial outer surface of the housing  202 , are within the scope of the present disclosure). In another example, and as described in detail below with reference to  FIG. 3 , the coolant may enter the housing  202  via a flow path included within the output shaft  218 , from which the coolant may flow into the internal volume of the electric motor  200  via a plurality of openings or outlets disposed in the output shaft  218  within the housing  202  (for reference, an exemplary cutline  210  is shown to contextualize the electric motor  200  within the schematic cross-sectional view of  FIG. 3 ). 
     Referring now to  FIG. 3 , a schematic cross-sectional view depicting a passive cooling system  300  for an electric motor  302  fluidically coupled to a gearbox  352  is shown. In some examples, the gearbox  352  may be similar to, or the same as, the second gearbox  121  as described in detail above with reference to  FIG. 1 . Similarly, in some examples, the electric motor  302  may be similar to, or the same as, the electric motor  120  or the electric motor  152  as described in detail above with reference to  FIG. 1 . In other examples, the electric motor  302  may be similar to, or the same as, the electric motor  200  as described in detail above with reference to  FIG. 2 . For example, the schematic cross-sectional view may be taken along the cutline  210  of  FIG. 2 . It will be appreciated, however, that the electric motor  302  of  FIG. 3  may not match the relative dimensions and proportions of the electric motor of  FIG. 2 . A set of reference axes  301  is provided for describing relative positioning of the components shown, the axes  301  indicating an x-axis, a y-axis, and a z-axis. In one example, the z-axis may be parallel with a direction of gravity (e.g., in a negative direction along the z-axis) and a vertical direction (e.g., in a positive direction along the z-axis). 
     The electric motor  302  may include a housing  304  defining an internal volume or interior  302   a  of the electric motor  302 . Various components of the electric motor  302  may be disposed within the internal volume  302   a  (e.g., enclosed or housed within the housing  304 ). For example, a rotor  306  ensheathed by a stator  308  including a plurality of end windings  310  may be positioned within the housing  304  such that an axis of rotation of the rotor  306  is coaxial with a central axis of the electric motor  302  along the x-axis. In some examples, the electric motor  302  may include a cooling jacket  312  arranged between the housing  304  and the stator  308 , such that the stator  308  may be ensheathed by the cooling jacket  312 . In one example, and as discussed below, the cooling jacket  312  may supplant or supplement the passive cooling system  300  during select operation of the electric motor  302 . In another example, however, the electric motor  302  may include no cooling jacket therein. 
     The gearbox  352  may include a housing  354  defining an internal volume or interior  352   a  of the gearbox  352 . Various components of the gearbox  352  may be disposed within the internal volume  352   a  (e.g., enclosed or housed within the housing  354 ). For example, a differential  356  and a plurality of gears (including gears  362 ,  364 ,  366 , and  368 ) may be positioned within the housing  354 , the differential  356  and each of the plurality of gears being independently coupled to at least one rotating shaft and configured to deliver torque from the electric motor  302  to one or more outputs (e.g., wheels of a vehicle). In one example, the differential  356  may be configured to independently rotate a port shaft  330  (also referred to herein as an inner shaft  330 , as discussed below) and a starboard shaft  390  to deliver outputs along a negative direction of the x-axis and a positive direction of the x-axis, respectively. 
     Each of a shaft sump  370  and a gearbox sump  372  may further be disposed within the housing  354 , each of the shaft sump  370  and the gearbox sump  372  including at least some of a coolant  380  during operation of the electric motor  302 . Each of the shaft sump  370  and the gearbox sump  372  may be entirely enclosed within the housing  354  of the gearbox  352  and entirely external to the housing  304  of the electric motor  302 . In some examples, the shaft sump  370  may be open and positioned above the gearbox sump  372  along the z-axis such that, when the shaft sump  370  is overfilled, the coolant  380  may spill over from the shaft sump  370  into the gearbox sump  372 . In some examples, and as shown, the gearbox sump  372  may be positioned below the shaft sump  370  and integrally formed with the housing  354 . 
     The electric motor  302  may be fluidically coupled to the gearbox  352  at least via an internal volume or interior  320   a  of an outer shaft  320  extending through the electric motor  302  and into the gearbox  352 . Specifically, the outer shaft  320  may extend from the electric motor  302  and through the shaft sump  370 , such that the shaft sump  370  may be fluidically coupled to the electric motor  302  via the outer shaft  320 . Accordingly, the outer shaft  320  may be at least partially enclosed within each of the housing  304  of the electric motor  302  and the housing  354  of the gearbox  352 . 
     The outer shaft  320  may be fixedly coupled to the rotor  306 , such that rotation of the rotor  306  during operation of the electric motor  302  may rotate the outer shaft  320  therewith. Respective seals  342  may be provided at interfaces of the outer shaft  320  with the housing  304  and the shaft sump  370  so as to prevent leakage of the coolant  380  thereat. However, in some examples, such as when the shaft sump  370  continuously receives an amount of the coolant  380  equal to or greater than an amount of the coolant  380  lost during operation of the electric motor  302 , no seal may be provided at the interface of the outer shaft  320  with the shaft sump  370  such that the coolant  380  may leak therethrough and flow to the gearbox sump  372 . 
     The inner shaft  330  may be coaxial with and disposed within the outer shaft  320 , the inner shaft  330  extending through each of the outer shaft  320  and the shaft sump  370 . Similar to the outer shaft  320 , respective seals  342  may be provided at interfaces of the inner shaft  330  with the housing  304 , the shaft sump  370 , and the housing  354  so as to prevent leakage of the coolant  380  or lubricant thereat. However, in some examples, such as when the shaft sump  370  continuously receives an amount of the coolant  380  equal to or greater than an amount of the coolant  380  lost during operation of the electric motor  302 , no seal may be provided at the interface of the inner shaft  330  with the shaft sump  370  such that the coolant  380  may leak therethrough and flow to the gearbox sump  372 . In some examples, and as shown, the seal  342  provided at the interface of the inner shaft  330  with the housing  304  may be the same seal  342  provided at the interface of the outer shaft  320  with the housing  304 . 
     Rotation of the outer shaft  320  may drive rotation of the inner shaft  330  via rotation of the plurality of gears and at least one lay shaft  360  (e.g., a single lay shaft  360 , as shown at  FIG. 3 , or a plurality of lay shafts  360 , such as in a multi-stage gearbox). Specifically, and as shown at  FIG. 3 , the outer shaft  320  may rotate the gear  368  via a fixed coupling therewith, the gear  368  meshingly engaging the gear  366 . The (rotating) gear  366  may be fixedly coupled to the lay shaft  360  at one end thereof, such that the gear  364 , being fixedly coupled to the lay shaft  360  at an opposite end, may be rotated. The (rotating) gear  364  may meshingly engage the gear  362 , which may be a ring gear of the differential  356  fixedly coupled to a housing  357  thereof. Upon engagement of the housing  357  of the differential  356  via the (rotating) gear  362 , the differential  356  may drive independent rotation of the inner (port) shaft  330  and the starboard shaft  390  via a differential mechanism. In this way, the rotation of the plurality of gears may be coupled to the rotation of the inner shaft  330 . A direction of rotation of each of the rotor  306 , the outer shaft  320 , and the inner shaft  330  during forward operation of the electric motor  302  is indicated by an arrow  332 . As shown, a plurality of bearings  340  may be coupled at various locations within and without the electric motor  302  and the gearbox  352  to the outer shaft  320 , the inner shaft  330 , the lay shaft  360 , and the starboard shaft  390  to facilitate respective rotations thereof. Additionally or alternatively, a disconnect mechanism may be provided at the differential  356  such that the inner shaft  330  may be reversibly decoupled from rotation of the rotor  306  and the outer shaft  320  during select modes of operation (e.g., when relatively rapid warmup of components of the electric motor  302  is desired). 
     One or more surface features  334  may be affixed to the inner shaft  330 , the one or more surface features  334  disposed at least along a portion of the inner shaft  330  extending within the outer shaft  320 . In some examples, and as shown, the one or more surface features  334  may be configured as an external helical threading pattern (e.g., an external screw thread) extending from the inner shaft  330 . It will be apparent that the one or more surface features  334  as shown in  FIG. 3  have not been cut to illustrate the exemplary configuration of the external helical threading pattern. In other examples, the one or more surface features  334  may be configured as a propeller extending from the inner shaft  330 . The one or more surface features  334  may be clearance fit to an inner or internal surface  320   b  of the outer shaft  320  such that rotation of the inner shaft  330  may draw the coolant  380  through the outer shaft  320  from the shaft sump  370  to the electric motor  302  via the one or more surface features  334 . In certain examples, the coolant  380  may only be drawn into the electric motor  302  via rotation of the inner shaft  330  (e.g., flow of the coolant  380  along the outer shaft  320  from the shaft sump  370  to the internal volume  302   a  within the housing  304  may be induced only based on rotation of the inner shaft  330 ). In some examples, the outer shaft  320  may rotate independently from or concurrently with the inner shaft  330 , so as to substantially evenly distribute the coolant  380  on one or more surfaces (e.g., on each of the plurality of end windings  310 ) within the internal volume  302   a  of the electric motor  302  via one or more openings or outlets  322  disposed on the outer shaft  320  (as described in detail below). In this way, the outer shaft  320  and the inner shaft  330  may be configured as a passive coolant pump  336 , the outer shaft  320  providing a pump housing and the one or more surface features  334  inducing a flow of the coolant  380  from the gearbox  352  to the electric motor  302 . Accordingly, in examples wherein the one or more surface features  334  is configured as the external helical threading pattern, the passive coolant pump  336  may be correspondingly configured as a screw pump, and in examples wherein the one or more surface features  334  is configured as the propeller, the passive coolant pump  336  may be correspondingly configured as an axial-flow pump. 
     Because rotation of the outer shaft  320  and the inner shaft  330  are coupled, reverse operation of the electric motor  302  may result in the coolant  380  being siphoned from the electric motor  302  and back into the shaft sump  370  (e.g., via reversed rotation of the one or more surface features  334 ). Accordingly, the systems and methods described by embodiments of the present disclosure may not be readily apparent to those of ordinary skill in the art at least because less cooling may be provided to the electric motor  302  by the passive cooling system  300  during reverse operation of the electric motor  302 . However, as performance may be more limited during reverse operation of the electric motor  302  than during forward operation of the electric motor  302 , degradation to components of the electric motor  302  during reverse operation thereof may be substantially avoided even with less cooling. Nonetheless, in some examples, the cooling jacket  312  may be included in the electric motor  302  to supplant the passive cooling system  300  during reverse operation. Additionally or alternatively, the cooling jacket  312  may be included in the electric motor  302  to supplement the passive cooling system  300  during forward operation. 
     The one or more openings or outlets  322  may be disposed on a portion of the outer shaft  320  enclosed within the housing  304 . Accordingly, upon rotation of the inner shaft  330 , the coolant  380  may enter the housing  304  via the one or more openings  322  (as indicated by arrows  382 ). In some examples, a relative location of the one or more openings  322  may be selected based on a configuration or application of the electric motor  302 . As an example, the one or more openings  322  may be positioned to direct the coolant  380  to specific surfaces within the internal volume  302   a  of the electric motor  302 . For instance, the one or more openings  322  may be axially positioned beneath respective end windings  310  along the z-axis, such that the coolant  380  may be supplied to lubricate and/or cool surfaces of the plurality of end windings  310 . Additionally or alternatively, the one or more openings  322  may be positioned adjacent to respective bearings  340  within the internal volume  302   a  of the electric motor  302 , such that the coolant  380  may be supplied to lubricate and/or cool surfaces of such bearings. As another example, the one or more openings  322  may be positioned to distribute the coolant  380  substantially evenly throughout the internal volume  302   a  of the electric motor  302 . 
     The flow of the coolant  380  may be controlled via judicious selection of a plurality of design parameters during manufacture of the passive cooling system  300 . Specifically, a rotational speed of the outer shaft  320  and a rotational speed of the inner shaft  330  may be a function of the plurality of design parameters. In non-limiting examples, the plurality of design parameters may include a configuration (e.g., overall shape) of the one or more surface features  334  (e.g., as the external helical threading pattern, the propeller, etc.), dimensions (e.g., width along the x-axis or height along the y-axis or the z-axis) of the one or more surface features  334 , a number of the one or more surface features  334 , an angle of the one or more surface features  334  with respect to the inner shaft  330 , a tolerance of the clearance fit of the one or more surface features  334  with the outer shaft  320 , an overall shape of each of the one or more openings  322 , the relative location of the one or more openings  322 , a number of the one or more openings  322 , and an angle of the one or more openings  322  with respect to the outer shaft  320 . For example, the overall shape of the one or more openings  322  may be circular, elliptical, square, rectangular, diamond-shaped, substantially annular, etc. 
     After the coolant  380  is drawn into the electric motor  302  via the passive coolant pump  336 , the coolant  380  may collect in one or more channels or grooves  314  set into an inner or internal surface  304   a  (e.g., of the housing  304 ) of the electric motor  302 . The one or more channels  314  may be configured to drain (e.g., via gravity drainage, induced or gravity-assisted drainage, etc.) the collected coolant  380  into the gearbox sump  372  through an opening  316  fluidically coupling the electric motor  302  to the gearbox sump  372 . 
     At least one gear of the plurality of gears may extend into the gearbox sump  372 , the at least one gear being at least partially submerse in the coolant  380  during operation of the electric motor  302 . For example, and as shown, the at least one gear may be the gear  362  fixedly coupled to the housing  357  of the differential  356 . Upon rotation of the plurality of gears, the coolant  380  may be supplied from the gearbox sump  372  to the shaft sump  370 . Specifically, the gear  362  may splash the coolant  380  directly into the shaft sump  370  (as indicated by arrows  384  and  386 ) or indirectly via first splashing the coolant  380  against an inner or internal surface  354   a  (e.g., of the housing  354 ) of the gearbox  352  opposite to the gearbox sump  372  (e.g., following the arrows  384 ), wherefrom the splashed coolant  380  may flow into the shaft sump  370  (e.g., following the arrow  386 ). In certain examples, the shaft sump  370  may only receive the coolant  380  via the splashing of the coolant  380 , e.g., directly into the shaft sump  370 , first against the inner surface  354   a  of the gearbox  352  and then to the shaft sump  370 , or both. 
     In some examples, one or more baffles or vanes  358  may be disposed on and affixed to the inner surface  354   a  of the gearbox  352  opposite to the gearbox sump  372 , the one or more baffles  358  configured to receive the splashed coolant  380  from rotation of the plurality of gears and direct the received coolant  380  to the shaft sump  370 . Any configuration of the one or more baffles  358  which may direct the splashed coolant  380  to the shaft sump  370  may be considered within the scope of the present disclosure. For example, varying configurations of the one or more baffles  358  may include an overall shape of each of the one or more baffles  358 , a number of the one or more baffles  358 , and an angle of the one or more baffles  358  with respect to the inner surface  354   a  of the gearbox  352 . In this way, the passive cooling system  300  may cool the electric motor  302  during operation thereof via passively cycling of the coolant  380  through the gearbox  352 . 
     Referring now to  FIG. 4 , a flow chart depicting a method  400  for passively cooling an electric motor fluidically coupled to a gearbox is shown. Specifically, the electric motor may be cooled by passively cycling a coolant along a flow path extending through each of the electric motor and the gearbox. In some examples, the gearbox may be similar to, or the same as, the second gearbox  121  as described in detail above with reference to  FIG. 1  or the gearbox  352  as described in detail above with reference to  FIG. 3 . Similarly, in some examples, the electric motor may be similar to, or the same as, the electric motor  120  or the electric motor  152  as described in detail above with reference to  FIG. 1 , the electric motor  200  as described in detail above with reference to  FIG. 2 , or the electric motor  302  as described in detail above with reference to  FIG. 3 . In examples wherein the electric motor is the electric motor  302  and the gearbox is the gearbox  352 , method  400  may be physically carried out via various components of the passive cooling system  300 . Accordingly, in some examples, the various components employed to execute method  400  as described below may be similarly named to and the same as the various components of the passive cooling system  300 , respectively. 
     Further, at least some steps (e.g.,  402 ,  404 ,  414 ) of method  400  may be implemented as executable instructions in non-transitory memory of a vehicle controller (e.g., the controller  150  of  FIG. 1 ). For example, the instructions may be executed by the vehicle controller according to the at least some steps of method  400  in conjunction with signals received from sensors or requests received from an operator of a vehicle (e.g., the vehicle  100  of  FIG. 1 ) including the vehicle controller. Further, the vehicle controller may employ actuators of the vehicle to adjust vehicle operation according to the at least some steps of method  400 . The instructions for carrying out the at least some steps of method  400  and commands to the actuators may be executed automatically by the vehicle controller (following an operator command and/or under preset conditions). 
     At  402 , method  400  may include receiving a vehicle startup request. For example, an operator of the vehicle may generate the vehicle startup request via actuation at the vehicle (e.g., by turning a key, depressing a mechanical button, actuating light, movement, and/or weight sensors, etc.) while the vehicle speed is zero. Additionally or alternatively, the vehicle startup request may be received autonomously (e.g., without operator input). Responsive to receiving the vehicle startup request, the vehicle may startup and operation of the electric motor may commence. 
     At  404 , method  400  may include rotating a second shaft (e.g., the inner shaft  330  of  FIG. 3 ) via operation of the electric motor. Specifically, the second shaft may extend through the electric motor, the second shaft being coaxial with and extending through a first shaft (e.g., the outer shaft  320  of  FIG. 3 ). The first shaft may fluidically couple a first sump (e.g., the shaft sump  370  of  FIG. 3 ) of the gearbox with the electric motor, the first sump configured to contain a coolant. 
     At  406 , method  400  may include delivering the coolant from a second sump (e.g., the gearbox sump  372  of  FIG. 3 ) of the gearbox to the first sump via splashing of the coolant directly into the first sump or indirectly via first splashing the coolant against a housing of the gearbox followed by flowing the coolant therefrom into the first sump. Specifically, at least one gear (e.g., the gear  362  of  FIG. 3 ) fixedly coupled to the second shaft may extend into the second sump, such that the at least one gear may splash the coolant towards a portion of the housing of the gearbox opposite to the second sump during rotation of the second shaft, at least some of the coolant being splashed directly into the first sump in some examples and at least some of the coolant first splashing against the portion of the housing of the gearbox and flowing therefrom into the first sump in additional or alternative examples. In some examples, one or more baffles may be affixed to the portion of the housing of the gearbox opposite to the second sump, the one or more baffles configured to receive and flow the splashed coolant into the first sump. In certain examples, the first sump may only receive the coolant via the splashing of the coolant, e.g., directly into the first sump, first against the housing of the gearbox and then into the first sump, or both. 
     At  408 , method  400  may include delivering the coolant from the first sump to the electric motor via rotation of one or more surface features affixed to the second shaft. Specifically, the one or more surface features, being affixed to the second shaft and, in some examples, being clearance fit to an inner surface of the first shaft, may be rotated concurrently with the second shaft. Further, the one or more surface features may be configured (e.g., as an external helical threading, a propeller, etc.) such that the coolant may be drawn through the first shaft via rotation of the second shaft therewithin. In certain examples, the coolant may only be drawn through the first shaft from the first sump to the electric motor via rotation of the second shaft. In this way, the first and second shafts may be configured as a passive coolant pump during operation of the electric motor. 
     At  410 , method  400  may include delivering the coolant from the electric motor to the second sump via gravity drainage. Specifically, the coolant may be collected in one or more channels set into an inner surface of the electric motor, wherefrom the coolant may gravity drain into the second sump through an opening fluidically coupling the electric motor to the second sump. 
     At  412 , method  400  may include determining whether an electric motor shutdown request has been received. For example, an operator of the vehicle may generate the electric motor shutdown request via actuation at the vehicle (e.g., by turning a key, depressing a mechanical button, actuating light, movement, and/or weight sensors, etc.) while the vehicle speed is zero. Additionally or alternatively, the electric motor shutdown request may be received autonomously (e.g., without operator input). In some examples, the electric motor shutdown request may be accompanied by a vehicle shutdown request. In other examples, the electric motor shutdown request may be accompanied by an engine startup request. If the electric motor shutdown request has not been received, method  400  may proceed to  414 , where method  400  may include to continue operation of the electric motor. Method  400  may return to  404  and the coolant may continue to be passively cycled through the electric motor and the gearbox. 
     If the electric motor shutdown request has been received, method  400  may proceed to  416 , where method  400  may include shutting down the electric motor and passive cycling of the coolant may cease. In examples wherein the electric motor shutdown request is accompanied by the vehicle shutdown request, shutting down the electric motor may be executed in tandem with shutting down the vehicle. In examples wherein the electric motor shutdown request is accompanied by the engine startup request, shutting down the electric motor may be executed in tandem with starting up an engine (e.g., the engine  110  of  FIG. 1 ) of the vehicle. 
     In this way, systems and methods are provided for passively cycling and distributing a coolant throughout an electric motor fluidically coupled to a gearbox. In an exemplary embodiment, an inner shaft at least partially housed within an outer shaft may extend through each of the electric motor and a shaft sump of the gearbox, the shaft sump being substantially continuously supplied with the coolant during a given operation of the electric motor. The outer shaft may fluidically couple the electric motor to the shaft sump such that a flow path for the coolant may be provided. Upon rotation of the inner shaft during the given operation of the electric motor, one or more surface features extending therefrom (e.g., clearance fit to an internal surface of the outer shaft) may draw the coolant along the flow path from the shaft sump to the electric motor. A plurality of openings or outlets in a portion of the outer shaft housed within the electric motor may extend the flow path into an interior of the electric motor. In some examples, the plurality of openings or outlets may be positioned based on a desired configuration or application of the electric motor. In one example, the plurality of openings or outlets may be positioned to direct the coolant to specific surfaces within the interior of the electric motor or to distribute the coolant substantially evenly throughout the interior of the electric motor. 
     The coolant within the interior of the electric motor may be collected in one or more channels or grooves set into internal surfaces of the electric motor and fluidically coupled to a gearbox sump integrally formed within the gearbox, such that the one or more channels or grooves may extend the flow path of the coolant to the gearbox sump (e.g., via gravity drainage of the collected coolant). The gearbox may further include a gear extending into the gearbox sump, whereby the gear may be at least partially submerged in the coolant delivered to the gearbox sump via the one or more channels or grooves. The gear may be fixedly coupled to the inner shaft, such that the gear may splash the coolant from the gearbox sump against an internal surface of the gearbox upon rotation of the inner shaft during the given operation of the electric motor. The splashed coolant, guided by one or more baffles or vanes extending from the internal surface of the gearbox in some examples, may flow from the internal surface of the gearbox back into the shaft sump (e.g., continuously supplying the shaft sump with the coolant during the given operation of the electric motor). Thus, the flow path may be extended from the gearbox sump to the shaft sump, wherefrom the coolant may again be directed to the electric motor along the outer shaft. 
     A technical effect of configuring the flow path in this way is that the coolant may be sufficiently distributed throughout the electric motor via passive cycling of the coolant through the gearbox. Accordingly, a complexity and a cost of cooling the electric motor may not be significantly increased, contrary to examples wherein coolant flow is induced via active pumping. Such advantages of such passive cycling may not be readily apparent, as a reverse operation of the electric motor may result in the coolant being siphoned away from the electric motor (as rotation of the inner shaft may be reversed during the reverse operation). However, as performance may be limited for many applications during such reverse operation, less excess heat may be generated, and cooling operations may concomitantly be reduced with minimal practical consequence. Further, in some examples, an auxiliary cooling system such as a cooling jacket may be provided to supplant or supplement the passive cycling during such reverse operation (and/or to augment the passive cycling during forward operation, thereby further improving continuous performance). 
       FIGS. 2 and 3  show example configurations with relative positioning of the various components. If shown directly contacting each other, or directly coupled, then such elements may be referred to as directly contacting or directly coupled, respectively, at least in one example. Similarly, elements shown contiguous or adjacent to one another may be contiguous or adjacent to each other, respectively, at least in one example. As an example, components laying in face-sharing contact with each other may be referred to as in face-sharing contact. As another example, elements positioned apart from each other with only a space there-between and no other components may be referred to as such, in at least one example. As yet another example, elements shown above/below one another, at opposite sides to one another, or to the left/right of one another may be referred to as such, relative to one another. Further, as shown in the figures, a topmost element or point of element may be referred to as a “top” of the component and a bottommost element or point of the element may be referred to as a “bottom” of the component, in at least one example. As used herein, top/bottom, upper/lower, above/below, may be relative to a vertical axis of the figures and used to describe positioning of elements of the figures relative to one another. As such, elements shown above other elements are positioned vertically above the other elements, in one example. As yet another example, shapes of the elements depicted within the figures may be referred to as having those shapes (e.g., such as being circular, straight, planar, curved, rounded, chamfered, angled, or the like). Further, elements shown intersecting one another may be referred to as intersecting elements or intersecting one another, in at least one example. Further still, an element shown within another element or shown outside of another element may be referred as such, in one example. 
     In one example, a system, comprising: a gearbox comprising a first sump; an electric motor fluidically coupled to the gearbox; an outer shaft extending through the electric motor and into the first sump, the first sump being fluidically coupled to the electric motor via the outer shaft; an inner shaft coaxial with the outer shaft, the inner shaft extending through each of the first sump and the outer shaft; and an external screw thread disposed along a portion of the inner shaft extending within the outer shaft. A first example of the system further includes wherein a coolant is included in the first sump, and whereupon rotation of the inner shaft, the coolant is drawn through the outer shaft from the first sump into the electric motor. A second example of the system, optionally including the first example of the system, further includes wherein the gearbox further comprises a second sump and a plurality of gears, at least one of the plurality of gears extending into the second sump, and wherein rotation of the plurality of gears supplies the coolant from the second sump to the first sump. A third example of the system, optionally including one or more of the first and second examples of the system, further includes wherein the rotation of the plurality of gears is coupled to the rotation of the inner shaft. A fourth example of the system, optionally including one or more of the first through third examples of the system, further comprises one or more baffles disposed on an inner surface of the gearbox, the one or more baffles configured to receive the coolant from the rotation of the plurality of gears and direct the received coolant to the first sump. A fifth example of the system, optionally including one or more of the first through fourth examples of the system, further includes wherein the electric motor is fluidically coupled to the second sump, and wherein, after the coolant is drawn into the electric motor, the coolant collects on an inner surface of the electric motor and drains into the second sump via one or more channels set in the inner surface of the electric motor. A sixth example of the system, optionally including one or more of the first through fifth examples of the system, further includes wherein the first sump is not sealed where each of the outer shaft and the inner shaft extends therethrough. 
     In another example, an electric motor, comprising: a housing enclosing each of a stator and a rotor; a first shaft fixedly coupled to the rotor, the first shaft at least partially enclosed within the housing and fluidly coupled to a coolant sump external to the housing; and a second shaft extending through each of the first shaft and the coolant sump, a threading pattern helically extending from the second shaft, whereupon rotation of the second shaft, the threading pattern draws coolant from the coolant sump into the housing, the coolant only being drawn into the housing via rotation of the second shaft. A first example of the electric motor further includes wherein a plurality of openings is disposed on a portion of the first shaft enclosed within the housing, and whereupon the rotation of the second shaft, the coolant enters the housing via the plurality of openings. A second example of the electric motor, optionally including the first example of the electric motor, further includes wherein at least some openings of the plurality of openings are axially positioned beneath respective end windings of the stator. A third example of the electric motor, optionally including one or more of the first and second examples of the electric motor, further comprises a cooling jacket arranged between the housing and the stator. A fourth example of the electric motor, optionally including one or more of the first through third examples of the electric motor, further includes wherein the electric motor includes no cooling jacket therein. 
     In yet another example, a method, comprising: rotating an inner shaft relative to an outer shaft via operation of an electric motor, the inner shaft coaxial with and extending through the outer shaft; delivering a coolant from a gearbox sump to a shaft sump via splashing of the coolant during rotation of the inner shaft; delivering the coolant from the shaft sump to an internal volume of the electric motor via rotation of one or more surface features affixed to the inner shaft; and delivering the coolant from the internal volume of the electric motor to the gearbox sump via gravity drainage. A first example of the method further includes wherein the shaft sump only receives the coolant via the splashing of the coolant. A second example of the method, optionally including the first example of the method, further includes wherein a gear is fixedly coupled to the inner shaft and extends into the gearbox sump, and wherein the splashing of the coolant comprises the gear splashing the coolant directly into the first sump. A third example of the method, optionally including one or more of the first and second examples of the method, further includes wherein a gear is fixedly coupled to the inner shaft and extends into the gearbox sump, and wherein the splashing of the coolant comprises the gear splashing the coolant against a portion of a gearbox housing including one or more baffles affixed thereto, the one or more baffles configured to flow the splashed coolant into the shaft sump. A fourth example of the method, optionally including one or more of the first through third examples of the method, further includes wherein the gearbox sump is integrally formed in the gearbox housing opposite to the one or more baffles. A fifth example of the method, optionally including one or more of the first through fourth examples of the method, further includes wherein the delivery of the coolant from the shaft sump to the internal volume of the electric motor comprises inducing flow of the coolant along the outer shaft only based on the rotation of the inner shaft. A sixth example of the method, optionally including one or more of the first through fifth examples of the method, further includes wherein the one or more surface features is configured as an external helical threading. A seventh example of the method, optionally including one or more of the first through sixth examples of the method, further includes wherein the one or more surface features is configured as a propeller. 
     It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising,” “including,” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property. The terms “including” and “in which” are used as the plain-language equivalents of the respective terms “comprising” and “wherein.” Moreover, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements or a particular positional order on their objects. 
     The following claims particularly point out certain combinations and sub-combinations regarded as novel and non-obvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.