BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a powertrain for a motor vehicle, and, more particularly, to a powertrain having multiple power sources including an electric motor for driving a set of vehicle wheels. 
         [0003]    2. Description of the Prior Art 
         [0004]    In a powertrain for a hybrid electric vehicle (HEV), inertial masses and drag losses offset the fuel economy, performance, and dynamic response gains of the hybrid system. These offsets are greater when the hybrid drive components are not substituted for standard powertrain components. 
         [0005]    Electric rear axle drive units for front wheel drive vehicles add additional components and thus inertias and drag losses in both electric all wheel drive and shaft-driven mechanical all wheel drive systems. Electric front axle or rear axle drives for rear wheel drive vehicles added on top of the existing longitudinal driveline present the same problems. 
         [0006]    The inertial masses include the added motor/generator rotor, gear assemblies, and shaft and hub assemblies. The drag losses include the additional gear and bearing losses of the drive components and electromagnetic drag losses in motor assemblies. The various drag losses can be reduced by design detail, but cannot be eliminated. 
         [0007]    These inertias and drag torques are further multiplied by the gear ratios often present between the motor/generator and their mechanical outputs to the drivetrain. 
         [0008]    It would be desirable to reduce the effects of inertial masses and drag torques, especially when the electric drive is not in operation. A need exists for a technique to connect and disconnect an electric motor for a front axle drive or rear axle drive motor from the driveline or the respective axle so that inertia and drag losses can be reduced. Controlled hydraulically-actuated friction clutches for this purpose increase the complexity of a hydraulic system and fluidic drag losses. 
       SUMMARY OF THE INVENTION 
       [0009]    A drive unit for transmitting power to the wheels of a motor vehicle includes an input driveably connectable to a first power source, a final drive gear set driveably connectable to the wheels, a motor/generator including a stator and a rotor arranged about an axis, the rotor being able to rotate about the axis and to move along the axis relative to the stator, a gear unit arranged about the axis and driveably connected to the gear set for driving the gear set at a speed that is less than a speed of the rotor, and a coupler secured to the rotor for alternately coupling the rotor and the gear unit mutually and transmitting power therebetween and decoupling the rotor and the gear unit mutually. 
         [0010]    The drive unit improves fuel efficiency, performance, and dynamics of a HEV with an electric front axle drive unit or a rear axle drive unit by connecting and disconnecting an electric motor from the driveline or the respective axle thereby reducing inertia and drag losses. 
         [0011]    The drive coupling and decoupling can be used in a powertrain that includes a modular hybrid transmission (MHT) and an integrated starter generator (ISG). In a MHT system, the ISG motor is coupled or decoupled to the engine and transmission depending on the relative magnitudes of motoring, electric power generating and engine starting or braking desired. 
         [0012]    The motor provides both axial displacement and rotation in one unit. Rotation and displacement can be controlled separately, providing two fully independent degrees of freedom. 
         [0013]    The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0015]      FIG. 1  is a schematic diagram of a powertrain for a hybrid electric vehicle, whose rear axle shafts are driven by an electric rear axle drive (ERAD) unit; 
           [0016]      FIG. 2  is a schematic diagram of a drive unit that includes a motor having two degrees of freedom; 
           [0017]      FIG. 3  is a schematic diagram of the drive unit of  FIG. 2 , in which the motor is decoupled from the output; 
           [0018]      FIG. 4  is a schematic diagram of a second embodiment of a drive unit that includes a motor having two degrees of freedom; 
           [0019]      FIG. 5  is a schematic diagram of the drive unit of  FIG. 4 , in which the motor is decoupled from the prop shaft and engine input; and 
           [0020]      FIG. 6  is a schematic diagram of a third embodiment of a drive unit, in which the motor is decoupled from both the output and the prop shaft and engine input. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    The powertrain  10  for a hybrid electric motor vehicle illustrated in  FIG. 1  includes an IC engine  12 , a transmission  14 , which drives a front final drive unit  16  connected to a pair of front wheels  18 ,  19  by front drive shafts  20 ,  21 . Transmission  14  may be a manual gearbox or any type of automatic transmission. The front final drive unit  16  also drives a rear drive take-off unit  22 , which is connected to an electric rear drive unit  24  by a longitudinal prop shaft  26 . Drive unit  24  is driveably connected to a pair of rear wheels  28 ,  29  by rear drive shafts  30 ,  31 . Drive unit  24  includes a casing  32 , which is prevented from rotating by being secured to the vehicle chassis, contains the inboard ends of the rear drive shafts  30 ,  31 . 
         [0022]      FIG. 2  shows an electric machine, such as a motor/generator  34 , arranged longitudinally in a drive unit  16 ,  24  and having two degrees of freedom including rotation of rotor  36  about axis  37  and displacement of the rotor along the axis. 
         [0023]    The rotor  36  of electric machine  34  is a hollow rotor, which is connected by a sleeve shaft  38  to a speed reduction planetary gear unit  40 . The stator  42  of electric machine  34  is secured to casing  32 . The drive unit input, prop shaft  26 , is driveably connected to a shaft  44 , which is secured to a final drive gear set that includes bevel pinion  46 . A bevel gear  48  meshing with bevel pinion  46  is secured to a ring gear of a differential mechanism  50 , which drives the axle shafts  30 ,  31  and wheels  28 ,  29 . 
         [0024]    Differential  50  may be of the type comprising a ring gear that rotates about the laterally directed axis of drive shafts  30 ,  31 , a spindle driven by the ring gear and revolving about the lateral axis, bevel pinions secured to the spindle for revolution about the lateral axis and rotation about the axis of the spindle, and side bevel gears meshing with the bevel pinions, each side bevel gear being secured to one of the drive shafts  30 ,  31 . 
         [0025]    Under low vehicle speed driving conditions, the electric motor/generator  34  is used to drive the vehicle with the engine  12  stopped, in which case the rear wheels  28 ,  29  are driven through the speed reduction planetary gear unit  40  and the differential mechanism  50 . Under heavier load at low vehicle speed, the motor/generator  34  can be used to supplement power produced by the engine  12 . At higher vehicle speed, engine  12  is the primary power source for driving wheels  28 ,  29  through prop shaft  26 , shaft  44 , bevel pinion  46 , bevel gear  48 , and differential mechanism  50 . 
         [0026]    The motor/generator  34  is controlled by an electronic control unit (ECU)  52 . Electric power and rotating power are generated by the motor/generator  34  and by a starter/generator  54 , which alternately drives and is driven by the engine  12 . Both the motor/generator  34  and the starter/generator  54  alternately draw electric current from and supply electric current to a traction battery  64  and an auxiliary battery  66 . The traction battery  64  is a high voltage unit. The auxiliary battery  66  is a 12V unit for the supply and control of the vehicle electrical systems. 
         [0027]    The engine  12  drives the front wheels  18 ,  19  through transmission  14 , the front final drive unit  16  and the front drive shafts  20 ,  21 , while also driving the rear wheels  28 ,  29  through the rear take-off unit  22 , prop shaft  26 , drive unit  24  and the rear drive shafts  30 ,  31 . 
         [0028]    The speed reduction planetary gear unit  40  includes a sun gear  70 , ring gear  72 , a carrier  74  secured to shaft  44 , and a set of planet pinions  76 , supported for rotation on carrier  74  and meshing with ring gear  72  and sun gear  70 . Ring gear  72  is grounded on casing  32 . Sun gear is connected by a spline  78  to shaft  38 , which is secured to rotor  36 . The angular velocity of rotor  36 , shaft  38  and sun gear  70  is preferably about three times greater than that of carrier  98 , shaft  44  and bevel pinion  46 , although a greater speed reduction can be provided by gear unit  40  between rotor and pinion  46 . 
         [0029]      FIG. 3  shows rotor  36  displaced leftward from the position of  FIG. 2  along the axis of prop shaft  26  while supported by bearings  80 ,  82 , located between the prop shaft and rotor. The spline  84  on the end of shaft  38  is formed with axially-directed teeth that disengage the axially-directed teeth of the spline  78  that is formed on sun gear. 
         [0030]    In operation, when rotor  36  is in the position shown in  FIG. 2 , engine  12  and rotor  36  are driveably connected to sun gear  70 . When electric power is provided to motor/generator  34  and engine  12  is operating, they transmit power to axle shafts  30 ,  31  through differential  50 . Carrier  74  drives shaft  44  at a reduced speed compared to that of rotor  36  and sun gear  70 , and the bevel pair  46 ,  48  produces an additional speed reduction at the input of differential  50 . 
         [0031]    When rotor  36  is in the position shown in  FIG. 3 , motor/generator  34  is driveably disconnected from axle shafts  30 ,  31 , which are driven by engine  12  through bevel pinion  46 , bevel gear  48  and differential  50 . The rotor  36  of motor/generator  34 , therefore, has two degree of freedom: rotation about the axis of prop shaft  26  and axial displacement along the prop shaft. Such motors are often referred to as “helical” or “X-theta” (X-θ) motors. Alternatively, a standard rotary motor/generator can be used as a replacement for motor/generator  34  to drive the axle shafts  30 ,  31  and for regenerative braking, and a separate linear mechanism, such as an actuated shift rail, alternately connects its rotor to shaft  44  and disconnects its rotor from shaft  44 . 
         [0032]      FIG. 4  illustrates a motor/generator  34 , whose rotor  36  rotates about axis  37  and moves along the axis. A prop shaft  84 , functionally similar to prop shaft  26 , is formed with a spline  86  having axial teeth, which alternately engage and disengage the axial spline teeth  88  formed on rotor shaft  90 . The opposite end of rotor shaft  90  is a spline  92  having axial teeth, which continually engage the long axial spline teeth  94  formed on shaft  96 , which is secured to sun gear  78 . 
         [0033]      FIG. 5  illustrates the rotor  36  and rotor shaft  90  displaced axially rearward such that rotor shaft  90  is disconnected from prop shaft  84  and remains connected to sun gear  78 . 
         [0034]    In operation, when rotor  36  is in the position shown in  FIG. 4 , rotor  36  and prop shaft  84  are driveably connected to sun gear  70 . When electric power is provided to motor/generator  34 , it transmits power to gear unit  40 . When engine  12  is operating, the engine transmits power to gear unit  40 . Carrier  74  drives shaft  44  at a reduced speed compared to that of rotor  36 , and the bevel pair  46 ,  48  produces an additional speed reduction at the input of differential  50 . Axle shafts  30 ,  31  are driven through differential  50 . 
         [0035]    When rotor  36  is in the position shown in  FIG. 5 , the engine  12  and prop shaft  84  are disconnected from gear unit  40  and the axle shafts  30 ,  31 . When electric power is provided to motor/generator  34 , rotor  36  drives gear unit  40 , which transmits power to axle shafts  30 ,  31  through differential  50 . 
         [0036]      FIG. 6  illustrates a motor/generator  34 , whose rotor  36  rotates about axis  37  and moves along the axis. The prop shaft  84  is formed with a spline  86  having axial teeth, which alternately engage and disengage the axial spline teeth  88  formed on rotor shaft  90 . The opposite end of rotor shaft  90  is formed with a spline having axial teeth  92 , which alternately engage and disengage the long axial spline teeth  98  formed on shaft  100 , which is secured to sun gear  78 . 
         [0037]    In operation, when the power unit of  FIG. 6  is in the position shown in  FIG. 4 , rotor  36  and prop shaft  84  are driveably connected to sun gear  70 . When electric power is provided to motor/generator  34 , it transmits power to gear unit  40 . When engine  12  is operating, the engine transmits power to gear unit  40 . Carrier  74  drives shaft  44  at a reduced speed compared to that of rotor  36 , and the bevel pair  46 ,  48  produces an additional speed reduction at the input of differential  50 . Axle shafts  30 ,  31  are driven through differential  50 . 
         [0038]    When the power unit is in the position shown in  FIG. 6 , both the engine  12  and rotor  36  are disconnected from gear unit  40  and the axle shafts  30 ,  31 . 
         [0039]    Although  FIG. 1  illustrates a drive unit  24  and axle shafts  30 ,  31  located at the rear of the vehicle, the drive unit illustrated in  FIGS. 2-6  may also be used to drive the front shafts  20 ,  21 , in which case the drive unit is located in the front final drive unit  16 . 
         [0040]    The drive units of  FIGS. 1-6  can be driven by a permanent magnet motor, induction motor, switched reluctance motor, variable reluctance motor, Halbach array, stepper, Sawyer, or other motor types. The drive unit may include one or more of the following: one moving translational rotor shaft assembly with both a one degree-of-freedom rotary stator and a one degree-of-freedom linear stator core; multiple standard rotary stator cores used to add a thrust on one rotor shaft; multiple standard linear stator cores used to add rotation on one rotor shaft; and a standard inside stator and outside stator with one moving rotor shaft assembly. Using Halbach array motors for inside and outside system designs can add higher strength and efficiency and easier field decoupling. 
         [0041]    A suitable multiple degree-of-freedom motor system could include dual, helically wound, in-line cores with helical flux that can be controlled by variable frequency for independent thrust and rotation. HEV use would tend towards angles set primarily in the rotational direction, with field harmonics, such as in the lower frequencies, controlling axial thrust and position. 
         [0042]    In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.
A drive unit for transmitting power to wheels of a motor vehicle includes an input driveably connectable to a first power source, a final drive gear set driveably connectable to the wheels, a motor/generator including a stator and a rotor arranged about an axis, the rotor being able to rotate about the axis and to move along the axis relative to the stator, a gear unit arranged about the axis and driveably connected to the gear set for driving the gear set at a speed that is less than a speed of the rotor, and a coupler secured to the rotor for alternately coupling the rotor and the gear unit mutually and transmitting power therebetween and decoupling the rotor and the gear unit mutually.