Abstract:
A transfer case and transmission are designed to permit the transmission hydraulic control system to control a range selection coupler and a torque on demand clutch in the transfer case. Two pressure circuits are transmitted from the transmission to the transfer case: a high range circuit and a low range circuit. The low range circuit is pressurized to engage low range while the range circuit is pressurized to engage high range. The torque on demand clutch is controlled by whichever of these circuits has the higher pressure. Lubrication is provided to a front section of the transfer case via the transmission output shaft, with the fluid returning to the transmission sump through a drainback passageway. The rear portion of the transfer case has a segregated sump. A control strategy is employed to partially fill front section of the transfer case with fluid in preparation for vehicle towing.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to the field of vehicle transfer cases and associated hydraulic controls. More particularly, the disclosure pertains to a transfer case and an automatic transmission that share a common hydraulic control system. 
       BACKGROUND 
       [0002]    In a typical rear wheel drive powertrain, an internal combustion engine converts chemical energy into mechanical energy to rotate a shaft and a transmission adapts the speed and torque of the shaft to suit vehicle requirements. At slow vehicle speed, the transmission reduces the speed and multiplies the torque to improve acceleration. At cruising speeds, the transmission increases the speed allowing the engine to operate at a fuel efficient operating speed. Power is transferred from the transmission output to the vehicle wheels via a rear driveshaft, a rear differential, and rear axle shafts. The transmission may be an automatic transmission which establishes one of a fixed number of available power flow paths by engaging particular friction clutches. The clutches may be engaged by supplying pressurized fluid through a hydraulic control system. 
         [0003]    To improve traction, it is advantageous to be able to transmit power to all four vehicle wheels. To accomplish this, a transfer case mounted to the transmission may distribute power from the transmission output to the rear driveshaft and also to a front driveshaft that drives the front wheels via a front differential and front axle shafts. Many transfer cases include a torque on demand (TOD) clutch that selectively transfers power to the front driveshaft. Typically, control of the TOD clutch is independent of the transmission clutches. 
         [0004]    Many transfer cases also include a low range and a high range to provide added vehicle functionality. Control of the coupler that selects the desired range is also typically independent of the transmission clutches. Some transfer cases are also capable of selecting a neutral position in which the front and rear driveshafts are not coupled to the transmission output. This is useful for towing the vehicle because movement of the vehicle results in rotation of the front and rear driveshafts. However, since some transfer case components still rotate, proper lubrication of those components is still required during towing. 
       SUMMARY OF THE DISCLOSURE 
       [0005]    A vehicle powertrain includes an automatic transmission and a transfer case. The automatic transmission transmits power from a transmission input shaft to a transmission output shaft at a variety of speed ratios. The transmission has a hydraulic control system having a high range circuit and a low range circuit. The transfer case is mounted to the transmission and transmits power from the transmission output shaft to a rear driveshaft. The transfer case operates in high range in response to fluid pressure in the high range circuit and operates in low range in response to fluid pressure in the low range circuit. The transfer case may also include a torque on demand clutch that selectively transfers power from the transmission output shaft to a front driveshaft in response in the high range circuit or low range circuit, whichever is greater. The hydraulic control system may also include a lubrication circuit routed from the transmission to the transfer case and a drainback passageway from a transfer case front sump to the transmission sump. A drainback valve may selectively block the drainback passageway. 
         [0006]    A transmission hydraulic control system includes a high range circuit, a low range circuit, a pressure control valve, and a switch valve. The high range circuit and low range circuit are each adapted to transmit fluid across an interface to a transfer case. The pressure control valve adjusts a pressure in a controlled pressure circuit based on a first electrical current. The switch valve alternately connects the controlled pressure circuit to either the high range circuit or the low range circuit. 
         [0007]    A transfer case includes a high range circuit, a low range circuit, and a coupler. The high range circuit and low range circuit are each adapted to receive fluid across an interface from a transmission. The coupler establishes an underdrive speed relationship between a transmission output shaft and a driveshaft in response to fluid pressure in the low range circuit and a direct drive speed relationship in response to fluid pressure in the high range circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic diagram of a vehicle powertrain. 
           [0009]      FIG. 2  is a schematic diagram of a transmission hydraulic control system suitable for use in the powertrain of  FIG. 1 . 
           [0010]      FIG. 3  is a partial cross section of a transfer case suitable for use in the powertrain of  FIG. 1 . 
           [0011]      FIG. 4  is a schematic diagram of a hydraulic control system suitable for use in the transfer case of  FIG. 3 . 
           [0012]      FIG. 5  is a schematic diagram of a switch valve, shown in a high range position, suitable for use in the hydraulic control system of  FIG. 2 . 
           [0013]      FIG. 6  is a schematic diagram of the switch valve of  FIG. 5 , shown in a low range position. 
           [0014]      FIG. 7  is a schematic diagram of a supplemental pump system suitable for use in conjunction with the hydraulic control system of  FIG. 2 . 
           [0015]      FIG. 8  is a flow chart illustrating a method of preparing the powertrain of  FIG. 1  for towing to ensure proper lubrication. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    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. 
         [0017]      FIG. 1  schematically illustrates a four wheel drive vehicle powertrain. Solid lines indicate shafts capable of transferring torque and power. Engine  10  converts chemical energy in the fuel into mechanical power which is delivered to transmission input shaft  12 . Transmission  14  modifies the speed and torque to suit vehicle requirements and delivers the power to transmission output shaft  16 . Transfer case  18  drives rear driveshaft  20  and front driveshaft  22 . The transfer case alternately operates in a high range mode in which the front and rear driveshafts are driven at the same speed as the transmission output shaft or in a low range mode in the which the front and rear driveshafts are driven at a speed substantially slower than the transmission output shaft. Bold dotted line  24  indicates a flow of hydraulic fluid at various pressures between transmission  14  and transfer case  18 . Rear differential  26  distributes power from the rear driveshaft  20  to a left rear wheel  28  and a right rear wheel  30 . The differential provides approximately equal torque to each wheel while permitting slight speed differences as the vehicle turns a corner. Rear differential  26  may include a hypoid gear which changes the axis of rotation and reduces the speed by a final drive ratio. Similarly, front differential  32  distributes power from the front driveshaft  22  to a left front wheel  34  and a right front wheel  36 . 
         [0018]    Powertrain controller  38  adjusts the power produced by engine  10  and the state of transmission  14  and transfer case  18  based on signals from various sensors. The sensors may include a gear selector (PRNDL), a transfer case range selector, a brake pedal, and an accelerator pedal that are manipulated by the driver. Powertrain controller  38  may also use signals from other types of sensors such as speed sensors, torque sensors, pressure sensors, temperature sensors, etc. As discussed in detail below, the state of transfer case  18  is manipulated by sending electrical signals to transmission  14  which produce changes in the hydraulic pressures in hydraulic circuits  24 . The powertrain controller may be a single microprocessor or may be a network of communicating microprocessors. 
         [0019]      FIG. 2  schematically illustrates an integrated transmission and transfer case hydraulic control system. The flow of mechanical power is indicated by heavy solid lines. Flow of hydraulic fluid is shown by medium dashed lines. Narrow dashed lines indicate the flow of electrical signals. The engine crankshaft  12  drives a torque converter  40 . Torque converter  40  drives turbine shaft  42  which provides power to gearbox  44 . Gearbox  44 , in turn, drives transmission output shaft  16 . Torque converter  40  includes an impeller fixed to transmission input shaft  12 , a turbine fixed to turbine shaft  42 , and a stator. The torque applied to turbine shaft  42  and the resistance torque applied to transmission input shaft  12  both depend upon the relative speeds of the two shafts. The torque converter may also include a bypass clutch that couples the transmission input shaft to the turbine shaft providing more efficient power transfer. 
         [0020]    Gearbox  44  may include gears and clutches configured to establish a variety of power flow paths between turbine shaft  42  and transmission output shaft  16 . The different power flow paths establish different speed ratios. Which power flow path is established depends upon which clutches are engaged. The set of clutches in gearbox  44  may include hydraulically actuated friction clutches. A hydraulically actuated friction clutch is engaged by supplying pressurized fluid to a piston apply chamber. The torque capacity of the clutch is linearly related to the fluid pressure. When the pressure is reduced, the clutch releases. 
         [0021]    Transmission sump  46 , typically located at the lowest point of the transmission, contains a supply of transmission fluid at ambient pressure. Pump  48  draws fluid from sump  46  and delivers it to line pressure circuit  50  at elevated pressure. Pump  48  may be a positive displacement pump that transfers a fixed quantity of fluid per revolution of the transmission input shaft  12 . In some embodiments, the pump displacement may be fixed while in other embodiments the displacement may vary in response to commands from the controller. Regulator valve  52  controls the pressure of line pressure circuit  50  by exhausting a fraction of the flow from pump  48  to exhaust circuit  54  which circulates the fluid back to the pump inlet. The regulator valve accomplishes this by adjusting the size of a valve opening leading to the exhaust circuit such that the pressure in the line pressure circuit matches a commanded line pressure from powertrain controller  38 . A set of clutch control valves  56  establish pressures between the line pressure and ambient pressure in a number of clutch apply circuits  58  and a bypass clutch apply circuit  60  according to commands from powertrain controller  38 . There is one clutch apply circuit for each hydraulically actuated friction clutch in gearbox  44 . In some embodiments, there may be one clutch control valve for each clutch apply circuit. In other embodiments, a network of hydraulic switches may direct flow from a smaller number of clutch control valves to particular clutch apply circuits while directing either line pressure or exhaust pressure to the others. Some embodiments may include a manual valve that is mechanically linked to a gear selector and which may inhibit the supply of line pressure to certain clutch apply circuits depending on the position of gear selector to avoid possible error states. For example, when the gear selector is in reverse, the manual valve may preclude applying clutches that would result in forward transmission output torque. 
         [0022]    Pump  48  also supplies fluid to fill torque converter  40  and to lubricate gearbox components. When the fluid is cool, fluid exiting torque converter  40  is routed into lube circuit  62  by thermal bypass valve  64 . In addition to providing lubrication, this fluid absorbs heat that is generated by friction between transmission gears and heat that is dissipated by slipping friction clutches. After flowing past gearbox components, the fluid drains back to sump  46 . The lube circuit  62  extends from the gearbox into the transfer case. After flowing past transfer case components, the fluid drains back into transmission sump  46  via a drainback passageway  63 . Since the fluid absorbs heat from many processes in the transmission and transfer case, it gradually gets warm. When a predetermined temperature is reached, thermal bypass valve  64  diverts the flow exiting the torque converter through a heat exchanger  66  before routing the fluid to lube circuit  62 . 
         [0023]    The state of the transfer case is controlled by adjusting the pressures of high range circuit  68  and low range circuit  70 . Transfer case control valve  72  adjust the pressure in controlled pressure circuit  74  to a value less than line pressure and proportional to an electrical current from powertrain controller  38 . When an electrical current from powertrain controller  38  is present, switch valve  76  directs the controlled pressure  74  to the high range circuit  68  and vents the low range circuit  70  to the exhaust circuit  54 . When the electrical current is not present, switch valve  76  reverses these connections, directing the controlled pressure  74  to the low range circuit  70  and venting the high range circuit  68 . Transfer case control valve  72  and switch valve  76  are physically part of the transmission valve body. Therefore, the hydraulic connections between the transmission and the transfer case include i) lube circuit  62 , ii) fluid drainback passageway  63 , iii) the high range circuit  68 , and iv) the low range circuit  70 . 
         [0024]    A cross section of transfer case  18  is shown in  FIG. 3 . The transfer case includes a front housing  80  bolted to the transmission housing  82  and a rear housing  84  bolted to the front housing  80 . The transmission output shaft  16  extends into the transfer case front housing. Rear driveshaft  20  is supported by the rear housing  84  via ball bearings and by the front housing  80  via roller bearings. Rear driveshaft  20  interfaces with transmission output shaft  16  such that lube circuit  62  flows from transmission output shaft  16  into rear driveshaft  20 . Sun gear  86  is splined to transmission output shaft  16 . Ring gear  88  is splined to front housing  80 . Carrier  90  is supported for rotation about the rear driveshaft. A number of planet gears are supported for rotation with respect to carrier  90  and mesh with both sun gear  86  and with ring gear  88 . The speed of carrier  90  is a fixed fraction of the speed of transmission output shaft  16  based on the relative number of teeth on sun gear  86  and sun gear  88 . 
         [0025]    The top half of  FIG. 3  is drawn with components positioned as they would be with high range selected while the bottom half corresponds to low range. Dog  92  rotates with rear driveshaft  20  but slides axially. When dog  92  is in its most forward position, as shown on the top half of  FIG. 3 , it engages with sun gear  86  causing the rear driveshaft to rotate at the same speed as transmission output shaft  16 . When dog  92  is in its most rearward position, as shown on the bottom half of  FIG. 3 , it engages with carrier  90  causing the rear driveshaft to rotate slower than transmission output shaft  16 . High range is engaged by supplying pressurized fluid through high range circuit  68  to the rear side of piston  94  pushing it forward. Rings  96  and  98  are separated by spring  100  and constrain the relative position of piston  94  and dog  92 . When piston  94  moves forward, it pushes ring  98  forward compressing spring  100 . Spring  100  exerts forward force on ring  96  which exerts forward force on dog  92 . If the teeth on dog  92  are lined up with gaps between corresponding teeth on sun gear  86 , dog  92  immediately slides into the position shown on the top half of  FIG. 3  and high range is engaged. If the teeth are not properly aligned for engagement, the force is sustained until slight relative movement between the shafts allows engagement and then engagement occurs. Once piston  94  is in the position corresponding to high range, detent  102  holds it in that position. Similarly, low range is engaged by supplying pressurized fluid through low range circuit  70  to the front side of piston  94  pushing it rearward. When piston  94  moves rearward, it pushes ring  96  rearward compressing spring  100 . Spring  100  exerts rearward force on ring  98  which exerts rearward force on dog  92 . When the teeth on dog  92  are lined up with gaps between corresponding teeth on carrier  90 , dog  92  slides into the position shown on the bottom half of  FIG. 3  and low range is engaged. Once piston  94  is in the position corresponding to low range, detent  102  holds it in that position. Detent  102  also holds piston  94  in a middle position causing dog  92  to disengage from both sun gear  86  and carrier  90 . In this position, no speed relationship is imposed between rear driveshaft  20  and transmission output shaft  16 . 
         [0026]    Sprocket  104  is supported for rotation about rear driveshaft  20 . Chain  106  engages sprocket  104  and a corresponding sprocket fixed to front driveshaft  22 . When controller  38  senses or anticipated wheel slip, it routes hydraulic fluid to clutch apply circuit  108 . The fluid pressure pushes piston  110  rearward. Acting through bearings, piston  110 , which does not rotate, pushes pressure plate  112 , which rotates with rear driveshaft  20 . Clutch pack  114  includes friction plates splined with sprocket  104  interleaved with separator plates splined with rear driveshaft  20 . When pressure plate  112  compressed clutch pack  114 , friction causes the speeds of rear driveshaft  20  and sprocket  104  to equalize. This has the effect of transferring torque from wheels that have lost traction to wheels that retain traction. When the pressure in clutch apply circuit  108  is removed, return spring  116  pushes piston  110  forward. In an alternative embodiment, piston  110  could rotate and slide within a rotating housing. In that case, fluid at low pressure from lube circuit  62  could be routed to the opposite side of piston  110  to cancel the effects of centrifugal forces. This type of clutch is known as a torque on demand (TOD) clutch. In other types of transfer cases, the front and rear driveshafts  18  and  22  may be driven via a center differential that divides the torque while allowing some speed differences. Such transfer cases may include a torque on demand clutch that locks the center differential in response to loss of traction on either front or rear wheels in order to provide all of the torque to the wheels with traction. 
         [0027]    Plate  118  separates the transfer case cavity into a front cavity and a rear cavity. Seals prevent the flow of fluid between these cavities. The rear cavity contains a quantity of fluid that provides lubrication to the chain and sprockets. This fluid is distributed by splashing. Components in the front cavity are lubricated by fluid from lube circuit  62 . This fluid never enters the rear cavity. After lubricating the components, the fluid from lube circuit  62  drains by gravity to the bottom of the front housing and from there drains back to the transmission via fluid drainback passageway  63 . 
         [0028]      FIG. 4  illustrates the portions of the hydraulic network within the transfer case. The circuits associated with lubrication are shown on the left. Lube circuit  62  flows into the transfer case and then flows past the gearing and the friction surfaces to the transfer case front sump. If the torque on demand clutch is a rotating clutch with a balance chamber, the lube circuit would also be routed to the balance chamber. From the front sump, the fluid drains back by gravity to the transmission sump through drainback passageway  63 . Some embodiments may include a drainback valve  120  configured to block the drainback passageway in preparation for vehicle towing as described below. The drainback valve may be part of the transmission or may be part of the transfer case. 
         [0029]    The circuits associated with control of range selection and actuation of the torque on demand clutch are shown on the right. High range circuit  68  flows into the high range chamber and low range circuit flows into the low range chamber  70 . Check ball  122  routes flow from either the high range circuit  68  or the low range circuit  70  to the clutch apply circuit  108 . Specifically, when high range circuit  68  is at a higher pressure than the low range circuit  70 , the ball moves to the position shown, blocking flow from the low range circuit  70  and allowing fluid from high range circuit  68  to flow into the clutch apply circuit  108 . When the low range circuit  70  is at higher pressure, the ball moves to the other end and the low range circuit is connected to the clutch apply circuit  108 . 
         [0030]      FIG. 5  shows switch valve  76  in the state corresponding to high range. Controller  38  adjusts the switch valve state by commanding an electrical current  130  to a solenoid valve  132 . Solenoid valve  132  is connected to the line pressure circuit  50  and the exhaust circuit  54  and controls the pressure in switch circuit  138  to a pressure less than line pressure in response to the electrical signal  130 . Solenoid valve  132  may be, for example, a Mini Direct Acting (MDA) solenoid valve. To place the switch valve  76  in the position shown in  FIG. 5 , the controller adjust the current such that the pressure in switch circuit  138  is relatively low. A spool  144  moves within a bore. Bore lands  146 ,  148 ,  150 ,  152 ,  154 , and  156  define first through seventh ports. The first port is connected to the switch circuit; the second, sixth, and seventh ports are vented to exhaust circuit  54 ; the third port is connected to low range circuit  70 ; the fourth port is connected to controlled pressure circuit  74 ; and the fifth port is connected to high range circuit  68 . In the high range state shown in  FIG. 5 , since the pressure in switch circuit  138  is relatively low, return spring  158  pushes the second spool to the left. In this position, the low range circuit  70  is connected to the exhaust circuit  54  between first spool land  160  and second spool land  162  and the high range circuit is connected to the controlled pressure circuit  74  between second spool land  162  and third spool land  164 . 
         [0031]      FIG. 6  shows switch valve  76  in the state corresponding to low range. Controller  38  places the switch valve into this state by setting the electrical current  130  to solenoid valve  132  such that the pressure in switch circuit  138  is relatively high. The relatively high switch pressure  138  pushes the spool to the right, compressing return spring  158 . In this position, the low range circuit  70  is connected to the controlled pressure circuit  74  between first spool land  160  and second spool land  162  and the high range circuit is connected to the exhaust circuit  54  between second spool land  162  and third spool land  164 . 
         [0032]      FIG. 7  illustrates a supplemental electric pump system that may be incorporated with the transmission hydraulic control system of  FIG. 2 . In some embodiments, the components of the supplemental electric pump system may be physically integrated with the transfer case control valve  72  and switch valve  76  in a supplemental valve body. The supplemental valve body may be included only in transmissions that will be mated to a transfer case. The supplemental electric pump system includes a second pump  170  driven by an electric motor  172 . Electric motor  172  rotates in response to commands from powertrain controller  38 . For example, powertrain controller  38  may command electric pump to rotate when pressurized fluid is needed and engine  10  is not on. The second pump  170 , like the primary pump  48 , may be a positive displacement pump of either fixed or variable displacement. Pump  170  draws fluid from the transmission sump  46  and provides the fluid to line pressure circuit  50 . From line pressure circuit  50 , the fluid may be routed to other circuits as required. Check ball  174  prevents fluid from the line pressure circuit from back-feeding the electric pump  170  when the electric pump is not in operation. 
         [0033]    When a vehicle with the powertrain of  FIG. 1  is towed with the rear wheels  28  and  30  on the ground, rotation of the rear wheels causes the rear driveshaft  20  to rotate. If front wheels  34  and  36  are also on the ground, front driveshaft  22  also rotates. Whenever parts rotate, it is important that the parts have proper lubrication to avoid excessive wear. Rear differential  26  and front differential  32  are filled with fluid which is distributed to the moving parts by splashing. Similarly, components in the rear portion of transfer case  18  are lubricated by splashed fluid from the rear transfer case sump. However, under normal circumstances, the components in transmission  12  and the front portion of transfer case  18  rely upon fluid pumped through lube circuit  62  for lubrication. Mechanical pump  48  only operates when the engine is running. Operating the engine during towing uses fuel and causes additional wear on the engine. Optional electric pump  170  can provide fluid to the lubrication circuit as long as battery power is available. However, with the engine off, the battery may become discharged. 
         [0034]      FIG. 8  illustrates a method of preparing a vehicle for towing. The method is initiated in response to a flat tow mode being activated. The flat tow mode may be activated, for example, by selecting a corresponding position with a range selection knob or by moving a shift selector through a sequence that is not likely to occur during normal driving. At  180 , a pump is operated to provide pressurized fluid to line pressure circuit  50  and to provide flow through lube circuit  62 . For a vehicle equipped with electric pump  170 , this may be accomplished by commanding electric motor  172  to rotate. Alternatively, this may be accomplished by running engine  10  to drive mechanical pump  48 . At  182 , the transfer case is shifted to neutral. If the transfer case is in high range, it is shifted into neutral by commanding switch valve  76  to the low range position of  FIG. 6  and then commanding transfer case control valve  72  to generate pressure in circuits  74  and  70  for long enough to move piston  94  to the middle detent, but not long enough to move it all of the way to the low range position. Similarly, if the transfer case is in low range, it is shifted to neutral by commanding switch valve  76  to the high range position of  FIG. 5  and then commanding transfer case control valve  72  to generate pressure for an appropriate interval. Once the transfer case is in neutral, rotation of rear driveshaft  20  does not result in rotation of transmission output shaft  16 . Therefore, there is no longer any need to provide lubrication to components in the transmission during towing. 
         [0035]    At  184 , drainback valve  120  is commanded to the closed position. Once drainback valve  120  is closed, fluid flowing into the front portion of the transfer case via lube circuit  62  accumulates in the transfer case front sump. Once a sufficient amount of fluid has accumulated, operation of the pump stops at  186  to avoid over-filling and the method is completed. During towing, components in the front portion of the transfer case will be lubricated by splashing fluid from the transfer case front sump. 
         [0036]    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. 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.