Abstract:
A vehicle powertrain includes a transaxle configured to drive front wheels and a power take-off unit configured to drive rear wheels through a driveshaft. The power take-off unit includes a disconnect clutch such that the power flow path to the rear wheels can be disconnected to reduce fuel consumption and reconnected when needed for traction enhancement. Although the disconnect clutch is physically located within the power take-off unit, it is actuated by fluid from the transaxle valve body. The disconnect clutch actuator includes a piston that slides within a chamber in a housing and a solenoid controlled valve that fluidly connects the chamber either to a pressure source or to the transaxle sump.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 61/859,402 filed Jul. 29, 2013, the disclosure of which is hereby incorporated in its entirety by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    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 
       [0003]    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. 
         [0004]    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. 
         [0005]    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 
       [0006]    A vehicle includes a transmission, a power take-off unit (PTU), and an auxiliary actuator. The transmission and the PTU each have housings which contain separate fluids. The PTU includes a disconnect clutch, actuated by the auxiliary actuator, to selectively establish or release a power flow path between the transmission output and a driveshaft. The transmission includes a valve body that, in addition to distributing transmission fluid to shift elements within the transmission, also distributes fluid to the auxiliary actuator. The actuator may include a piston configured to slide within a chamber, a tube from the transmission valve body, and a solenoid controlled valve that alternately connects the chamber to the tube of the transmission sump. 
         [0007]    A transaxle includes a gearbox within a housing, an auxiliary actuator, and a valve body. The auxiliary actuator moves a sleeve that is supported outside the housing in response to fluid pressure supplied by the valve body. The transaxle may also include a differential that distributes power to a first axle and a second axle. The sleeve may move parallel to the axis of the axle shafts. The actuator may include a piston configured to slide within a chamber, a tube from the transmission valve body, and a solenoid controlled valve that alternately connects the chamber to the tube of the transmission sump. The tube may run either inside the transmission housing or it may be external to the transmission housing. 
         [0008]    A transmission auxiliary actuator includes an actuator housing, a piston configured to slide within the housing, and a solenoid controlled valve that alternately connects a chamber within the housing to either a source of pressurized fluid or to a transmission sump. The source of pressurized fluid may be a transmission valve body. The actuator housing is adapted for fixation to an exterior surface of a transmission housing. The piston is adapted to actuate a clutch outside the transmission housing. The clutch may be a disconnect clutch within a power take-off unit fixed to the transmission housing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic representation of a vehicle powertrain. 
           [0010]      FIG. 2  is a cross sectional view of a PTU disconnect clutch. 
           [0011]      FIG. 3  is a cross sectional view of a first portion of a hydraulic actuation mechanism suitable for use with the PTU disconnect clutch of  FIG. 2 . 
           [0012]      FIG. 4  is a cross sectional view of a second portion of the hydraulic actuation mechanism of  FIG. 3 . 
           [0013]      FIG. 5  is a cut-away end view of a transmission case and pump body assembly adapted for use in conjunction with the hydraulic actuator mechanism. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    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. 
         [0015]      FIG. 1  schematically illustrates a FWD-based AWD vehicle. A transversely mounted engine  10  and transaxle  12  drive front wheels  14  via a front differential  16 . The front differential transmits approximately equal torque from transmission output  22  to each front wheel while allowing slight speed differences when the vehicle turns a corner. Specifically, output  22  rotates at a median speed between speeds of the left and right axle shafts. In order to also drive the rear wheels, a power transfer unit (PTU)  18  is mounted to the transaxle to drive a longitudinal driveshaft  20 . PTU input shaft  31  is coupled to transmission output shaft  22 . When disconnect clutch  32  is engaged, bevel gear  34  is driveably connected to PTU input shaft  31 . Bevel gear  34  meshes with bevel gear  35  to change the axis of rotation approximately  90  degrees. The driveshaft then drives the rear wheels  24  through a rear drive unit (RDU)  26  having a rear differential  28 . The RDU includes an actively controlled clutch  30  that selectively couples the driveshaft to the rear differential when loss of traction on the front wheels is detected or anticipated and decouples them in other conditions. 
         [0016]    Although the AWD system only transmits power to the rear wheels when the RDU clutch is engaged, a number of components, including the driveshaft, rotate at a speed proportional to vehicle speed whenever disconnect clutch  32  is engaged. Rotation of these components results in parasitic drag that increases the load on the engine and increases fuel consumption. The impact of the parasitic losses tends to be more severe at higher vehicle speeds. The adverse impact of this parasitic drag can be reduced if some of the components are disconnected during periods when traction enhancement is not needed. Disengaging disconnect clutch  32  permits these components to stop rotating which reduces fuel consumption. 
         [0017]    Disconnect clutch  32  is shown in more detail in  FIG. 2 . Although the disconnect clutch is axisymmetric, for convenience the top half of the figure shows the clutch in the disengaged position while the bottom half shows the clutch in the engaged position. A first set of dog teeth  36  is fixed to PTU input shaft  31 . A second set of dog teeth  38  is splined to rotate with bevel gear  34  but permitted to slide axially. Engagement spring  40  pushes dog teeth  38  axially into engagement with dog teeth  36  to couple PTU input shaft  31  to bevel gear  34 , as shown on the bottom of  FIG. 2 . Sleeve  42  slides axially with respect to RDU bevel gear  34 . When sleeve  42  slides to the right as shown on the top of  FIG. 2 , it pushes dog teeth  38  out of engagement with dog teeth  36 , decoupling PTU input  31  from bevel gear  34 . 
         [0018]      FIG. 3  illustrates the actuation mechanism for disconnect clutch  32 . Disconnect clutch  32  is contained within PTU housing  44 , which is fixed to transaxle housing  46  during vehicle assembly. Piston housing  48  is fixed to the exterior of transaxle housing  46 . When pressurized fluid is routed into chamber  50 , disconnect piston  52  is forced to the right. Disconnect fork  54  is supported by PTU housing  44  to slide axially with respect to transaxle  12  and PTU  18 . As disconnect piston  52  moves to the right, it forces disconnect actuation fork  54  to slide to the right. Disconnect fork  54 , in turn, pushes sleeve  42  to the right disengaging clutch  32 . Although sleeve  42  rotates with bevel gear  34  and disconnect fork  54  does not rotate, any combination of relative speed and force are of short duration. Once the dog clutch is disengaged, drag on the driveshaft causes the bevel gear to stop rotating. When the fluid pressure in cavity  50  is relieved, return spring  56  pushes disconnect piston  52  and disconnect fork  54  to the left. Engagement spring  40  pushes sleeve  42  to the left. In an alternative embodiment, spring  52  is eliminated and engagement spring  40  pushes sleeve  42 , disengagement fork  54 , and disconnect piston  52  to the left. In another alternative embodiment, a spring between the transmission case  46  and disconnect piston  50  pushes disconnect piston  54  to the left. RDU clutch  30  may be used to synchronize the speeds of PTU input  31  and bevel gear  34  before attempting engagement. 
         [0019]    Many automatic transaxles utilize pressurized fluid to engage various clutches and brakes to establish the various gear ratios. Therefore, such a transaxle already has a source of pressurized fluid. Integrating the actuator of the disconnect clutch with the transaxle eliminates the necessity to provide an independent source of fluid pressure. As shown in  FIG. 4 , solenoid  58 , which is mounted to the exterior of transaxle case  46 , regulates the pressure to cavity  50 . Tube  60  conveys pressurized fluid from the transaxle valve body to solenoid  58 . When electrical current is supplied to solenoid  58 , tube  60  is fluidly connected to cavity  50  via tube  62 . When electrical current is not supplied to solenoid  58 , cavity  46  is fluidly connected to an exhaust tube  64  allowing fluid to drain into the interior of transaxle case  46 . Alternatively, the solenoid may be configured to connect cavity  50  to exhaust tube  64  when current is supplied and to pressure supply tube  60  when no current is supplied. 
         [0020]    In an alternative embodiment, fluid may be routed to solenoid  58  through an interior passageway.  FIG. 5  shows an end cutaway view of transmission case  46 . Gearbox input shaft  70  passes through the case from the converter housing  72 . PTU input  31  and the passenger side half shaft pass through transmission case  12  at  74 . A pump housing  76  may be bolted to the inside of the transmission case. Pump  78 , which is enclosed in pump housing  76 , provides pressurized fluid to the transmission valve body which may be partially or completely integrated with the pump housing. An extension  80  of the pump housing is opposite solenoid  58 . Pressurized fluid may be routed from pump  78 , through extension  80 , then through a drilled hole in transmission case  46  to solenoid  58 . 
         [0021]    The mechanical interface between the transaxle and the PTU as described above provides advantages for manufacturing, testing, and assembly. The fluid used by the transaxle for actuation and lubrication is independent of the fluid used for lubrication within the PTU. The transaxle and PTU may be manufactured and tested separately and bolted together just before installation into the vehicle. 
         [0022]    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.