Abstract:
A front wheel drive transmission is adapted for all-wheel drive by the addition of a selectively engageable power take-off shaft. When a disconnect clutch is engaged, power may be transferred to rear wheels via a power take-off unit and a rear drive unit to improve vehicle mobility. When the disconnect clutch is disengaged, various components of the all-wheel drive system do not rotate, reducing parasitic losses and improving fuel economy. To provide packaging space for the disconnect clutch, the differential is moved to the left (driver side) of the driven transfer gear. A planetary differential, such as a double pinion planetary differential, is suitable for this location.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application Ser. No. 61/824,670 filed May 17, 2013, the disclosure of which is hereby incorporated in its entirety by reference herein. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to the field of automotive transmissions. More particularly, the disclosure pertains to a front wheel drive transmission with a power transfer shaft configured to selectively transfer power to rear wheels. 
     BACKGROUND 
     Two vehicle powertrain configurations predominate the modern passenger vehicle market, rear wheel drive (RWD) and front wheel drive (FWD). With additional hardware, both of these configurations can be configured to direct power to all four wheels. Because traction at any particular wheel may be limited at certain times, the ability to direct power to all four vehicle improves mobility. However, the additional hardware introduces additional parasitic losses which increase fuel consumption even in conditions that do not require the additional capability. 
     In a typical RWD configuration, the engine is oriented longitudinally in the vehicle such that the crankshaft axis is aligned with the direction of vehicle movement. A transmission mounted to the engine drives a rear driveshaft at a speed which may be less than or greater than the speed of the engine crankshaft according to current vehicle requirements. The rear driveshaft is connected to a rear axle that changes the axis of rotation, reduces the rotational speed, and drives left and right rear axles while permitting slight speed differences between the axles as the vehicle turns a corner. A RWD configuration is adapted to also drive the front wheels by adding a transfer case between the transmission and the rear driveshaft. In addition to driving the rear driveshaft, the transfer case drives a front driveshaft that, in turn, drives a front axle. Some transfer cases include a planetary gear set that divides the torque between front and rear driveshafts while allowing slight speed differences. Other transfer cases have an actively controlled torque on demand (TOD) clutch that only drives the front driveshaft in certain conditions, such as when a controller senses loss of traction of the rear wheels. 
     In a typical FWD configuration, the engine is oriented transversely in the vehicle such that the crankshaft axis is aligned with the axis of wheel rotation. A transmission mounted to the engine drives a front differential at a speed suitable for current vehicle requirements. The front differential is typically integrated into a common housing with the transmission gearbox. The front differential drives left and right front axles while permitting slight speed differences between the axles as the vehicle turns a corner. A FWD configuration is adapted to also drive the rear wheels by adding a power take off unit (PTU) that drives a rear driveshaft at a speed proportional to the speed of the front differential. A rear drive unit (RDU) typically includes a TOD clutch that, when engaged drives a rear differential that, in turn, drives left and right rear axles. 
     SUMMARY 
     A vehicle powertrain includes an engine, a multiple ratio gearbox, a transfer shaft, a differential, and a disconnect clutch. A gearbox input shaft extending from the right side of the multiple ratio gearbox is driven by a crankshaft of the engine. For example, the input shaft may be driven via a torque converter having an impeller fixed to the crankshaft and a turbine fixed to the gearbox input shaft. A gearbox output shaft is supported for rotation about the gearbox input shaft and meshes with a driven transfer gear fixed to the transfer shaft. A driving transfer gear on the transfer shaft meshes with a final drive gear. The differential, axially located to the left of the driven transfer gear, transfers power from the final drive gear to left and right axle shafts. The differential may be a planetary differential with relatively short axially length. For example, the differential may be a double pinion planetary gear set with the ring gear fixed to the final drive gear, the sun gear fixed to the one front axle shaft, and the carrier fixed to the other front axle shaft. The disconnect clutch, axially located to the right of the driven transfer gear, selectively transfers power from the final drive gear to a hollow power take-off shaft supported for rotation about the right axle shaft. The disconnect clutch may be a dog clutch. The disconnect clutch may be either normally engaged or normally disengaged. The disconnect clutch may be hydraulically actuated, electro-magnetically actuated, or actuated by other means. The vehicle may further include a power take-off unit configured to transfer power from the power take-off shaft to a longitudinal driveshaft. A rear drive unit may include a torque-on-demand clutch to selectively transfer power from the driveshaft to left and right rear axles in response to loss of traction on the front wheels. 
     A transmission includes a planetary differential, a clutch, and a transfer shaft. The differential is configured to transfer power from a final drive gear to left and right front axle shafts. For example, the differential may be a double pinion planetary gear set with the ring gear fixed to the final drive gear, the sun gear fixed to the one front axle shaft, and the carrier fixed to the other front axle shaft. The clutch selectively transfers power from the final drive gear to a power take-off shaft. The transfer shaft includes a driving transfer gear meshing with the final drive gear and a driven transfer gear that extends between the differential and the clutch. The transmission may also include a gearbox. A gearbox input shaft of the gearbox extends from the right side of the gearbox and a gearbox output gear rotates about the gearbox input shaft and meshes with the driven transfer gear. The transmission may also include a launch device such as a torque converter having an impeller and a turbine fixed to the input shaft. The disconnect clutch may be a dog clutch. The disconnect clutch may be either normally engaged or normally disengaged. The disconnect clutch may be hydraulically actuated, electro-magnetically actuated, or actuated by other means. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a vehicle powertrain. 
         FIG. 2  is a cross sectional view of a planetary differential suitable for use in the powertrain of  FIG. 1 . 
         FIG. 3  is a cross sectional view of a hydraulically actuated normally engaged disconnect clutch in the engaged position. 
         FIG. 4  is a cross sectional view of the hydraulically actuated disconnect clutch of  FIG. 3  in the disengaged position. 
         FIG. 5  is a cross sectional view of an electro-magnetically actuated normally disengaged disconnect clutch in the disengaged position. 
         FIG. 6  is a cross sectional view of the electro-magnetically actuated disconnect clutch of  FIG. 5  in the engaged position. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
       FIG. 1  is a schematic illustration of a FWD based all-wheel drive (AWD) powertrain configuration. Engine  10  generates power to rotate crankshaft  12 . Torque converter  14  transmits the power to gearbox input shaft  16 . Torque converter  14  includes an impeller fixed to crankshaft  12  and a turbine fixed to gearbox input shaft  16 . The torque converter serves as a launch device by transmitting power from the engine to the gearbox input shaft without requiring that the two shafts rotate at the same speed, such as when the vehicle is starting from a stationary position. Gearbox  18  transmits power from shaft  16  to output gear  20  at a speed ratio selected from among a set of available speed ratios based on vehicle speed and accelerator pedal position. Both the gearbox input shaft  16  and the output gear  20  extend from the right side of the gearbox. Output gear  20  is supported for rotation around gearbox input shaft  16 , although not necessarily supported by gearbox input shaft  16 . 
     Output gear  20  meshes with driven transfer gear  22  which is fixed to transfer shaft  24 . Driving transfer gear  26 , also fixed to transfer shaft  24 , meshes with final drive gear  28  which is fixed to shaft  30  for rotation about the front axle axis. Final drive gear  28  drives the ring gear  32  of a double pinion planetary differential. The double pinion planetary differential also includes a carrier  34  supporting a set of inner planet gears  36  and a set of outer planet gears  38 . Each outer planet gear  38  meshes with one of the inner planet gears  36  and with interior gear teeth of ring gear  32 . Each inner planet gear  36  also meshes with sun gear  40 . Carrier  34  drives left (driver side) front axle  42  and left front wheel  44 . Sun gear  40  drives right (passenger side) front axle  46  and right front wheel  48 . 
     Power take-off gear  50  is fixed to power take-off shaft  49  which is selectively coupled to shaft  30  by disconnect clutch  52 . Power take-off gear  50  meshes with gear  54  which drives beveled gear  56 . Beveled gear  56  meshes beveled gear  58  which is fixed to driveshaft  60 . Beveled gear  62  is selectively coupled to driveshaft  60  by TOD clutch  64 . Beveled gear  62  meshes with beveled gear  66  which drives rear differential  68 . Rear differential divides the power between left rear axle  70  and right rear axle  72  which drive left rear wheel  74  and right rear wheel  76  respectively. 
     The powertrain of  FIG. 1  can be operated with disconnect clutch  52  engaged or disengaged. Power is transferred to the front wheels independent of the state of disconnect clutch  52 . When disconnect clutch  52  is engaged, the powertrain provides the advantages associated with a FWD based all-wheel drive powertrain configuration. Specifically, if a controller senses that the front wheels have lost traction, TOD clutch  64  is engaged to transfer power to the rear wheels. During a maneuvers that are likely to result in loss of traction of the front wheels, such as rapid acceleration, the TOD clutch may be engaged pre-emptively. 
     When disconnect clutch  52  is disengaged, many of the components no longer rotate. Specifically, power take-off gear  50 , bevel gear  56 , and driveshaft  60  no longer rotate. Any parasitic losses attributable to the rotation of these components is eliminated, improving fuel economy. Determination of whether to engage disconnect clutch  52  may be based on explicit driver or may be based on sensing of operating conditions such as temperature that are correlated with likelihood of loss of traction. 
       FIG. 2  shows the structure of the planetary differential in more detail. Transmission housing  80  supports shaft  30  via tapered roller bearings  82  and  84 . Transmission housing also supports left front axle  42  via roller bearings  86  and supports right front axle  46  via roller bearings  88 . Unlike a bevel gear differential, the axis of rotation of the planet gears of a planetary differential are parallel to the axle axis. The relatively short axial length of a planetary differential relative to a bevel gear differential permits packaging the differential to the left of driven transfer gear  22 , making the space on the right side of the driven transfer gears available for disconnect clutch  52 . This arrangement also accommodates a driven transfer gear with a relatively large diameter permitting a greater degree of speed reduction and torque multiplication. Although a double pinion planetary differential is illustrated, other types of planetary differential have sufficiently short axial length to package in this available space. 
       FIG. 3  shows a first embodiment of disconnect clutch  52  in an engaged position. Dog  90  is splined to power take-off shaft  49  at  92  such that dog  90  rotates with power take-off shaft  49  but may slide axially with respect to power take-off shaft  49 . In the axial position shown in  FIG. 3 , teeth of dog  90  engage with teeth of shaft  30  such that the two shafts are forced to rotate together. Member  96  is fixed to dog  90  by snap rings  98 . Spring  100  pushes dog  90  to the left towards the position shown. Thus, this embodiment of the disconnect clutch is biased toward the engaged state. To release the disconnect clutch, pressurized fluid is routed through channel  102  to push piston  104  toward the right. Piston  104  pushes member  96  to the right through thrust bearing  106 .  FIG. 4  shows this embodiment in the disengaged position. In this position, dog  90  is axially separated from shaft  30  such that the two shafts are free to rotate at different speeds. Since disconnect clutch  52  is integrated into the transmission, the same valve body that controls the flow of pressurized fluid to various clutches in gearbox  18  to select speed ratios can control the flow of hydraulic fluid to disconnect clutch  52 . 
       FIG. 5  shows a second embodiment of disconnect clutch  52  in a disengaged position. Dog  110 , made of a magnetically conductive material, is splined to shaft  30  at  112  such that dog  90  rotates with shaft  30  but may slide axially with respect to shaft  30 . Spring  114  pushes dog  110  to the left towards the position shown. In this position, dog  100  is axially separated from power take-off shaft  49  such that the two shafts are free to rotate at different speeds. Thus, this embodiment of the disconnect clutch is biased toward the disengaged state. Coil module  116  is fixed to transmission case  80 . To engage the disconnect clutch, electrical current is supplied to coils  118  creating a magnetic field to push dog  110  toward the right.  FIG. 6  shows this embodiment in the engaged state. In the axial position shown in  FIG. 6 , teeth of dog  110  engage with teeth of power take-off shaft  49  at  120  such that the power take-off shaft  49  and shaft  30  are forced to rotate together. 
     The clutches illustrated in  FIGS. 3-6  are not designed to be engaged in the presence of relative speed between shaft  30  and shaft  49 . In order to engage clutch  52  while the vehicle is moving, engaging TOD clutch  64  synchronizes the speed of shaft  30  and shaft  49  as long as the front and rear wheels are rotating at the same speed, as they would be if both have traction. After bringing the speeds close with the TOD clutch, the TOD clutch may be released while disconnect clutch  52  is engaged. If the speed difference is small, the disconnect clutch will be able to engage as long as vehicle inertia is not restraining the driveshaft from changing speed slightly. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications.