Abstract:
One vehicle wheel is disconnected from a differential while the vehicle is in a front wheel drive mode. A controller checks for a malfunction of the differential while the vehicle is in the front wheel drive mode and in response to a request to enter an all wheel drive mode. If speeds of the other vehicle wheel and the differential input indicate that the malfunction is present, the all wheel drive mode is disabled and the driver is informed. If fluid temperature indicates a risk of the malfunction, all wheel drive mode is temporarily disabled and the driver is informed. If the temperature condition continues to be present for a predetermined duration, the all wheel drive mode is disabled.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to the field of vehicle driveline controls. More particularly, the disclosure pertains to a method of controlling an all wheel drive powertrain with a wheel end disconnect. 
       BACKGROUND 
       [0002]    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. 
         [0003]    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. 
         [0004]    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 OF THE DISCLOSURE 
       [0005]    A vehicle includes a differential, first and second wheels, and a controller. The first wheel is fixedly coupled to a first axle shaft. The second wheel is selectively coupled to a second axle shaft by a clutch. To reduce parasitic losses, the controller disengages the clutch to decouple the second wheel from the second axle shaft while the vehicle is in a front wheel drive mode. In response to a request to enter an all wheel drive mode, the controller engages the clutch if a ratio of a speed of a differential input to a speed of the first axle shaft is less than a first threshold. If the ratio exceeds the first threshold, the controller inhibits engagement of the clutch. The differential may also include a temperature sensor. The controller may inhibit engagement of the clutch if the temperature sensor indicates a temperature exceeding a second threshold. If the temperature then decreases to less than the second threshold, the controller may engage the clutch. If the temperature remains above the second threshold for more than a predetermined duration, the controller may not engage the clutch even when the temperature does decrease to less than the second threshold. The controller may display a message to the driver when it inhibits engagement of the clutch, whether due to the ratio exceeding the first threshold or due to the temperature exceeding the second threshold. The controller may also inhibit engagement of a power-take-off unit. 
         [0006]    A method of operating a vehicle includes disengaging a clutch in front wheel drive mode and engaging the clutch in response to a request to enter all wheel drive mode only under specified conditions. Disengaging the clutch decouples a rear wheel from a first rear axle. Engagement of the clutch is inhibited if a ratio of a rear differential input speed to a speed of a second rear axle exceeds a first threshold. Engagement of the clutch is also inhibited if a differential fluid temperature exceeds a threshold. The controller may also display a message to a driver. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic diagram of a vehicle powertrain. 
           [0008]      FIG. 2  is a schematic cross section of a rear differential suitable for use in the powertrain of  FIG. 1 . 
           [0009]      FIG. 3  is a flow chart indicating state transitions for operating the vehicle powertrain of  FIG. 1 . 
           [0010]      FIG. 4  is a flow chart illustrating a process for completing the safety check in the flow chart of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    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. 
         [0012]      FIG. 1  schematically illustrates an all-wheel drive vehicle powertrain. The heavy lines indicate the flow of mechanical power, whereas the dashed lines indicate the flow of information. An internal combustion engine  10  generates mechanical power by converting stored chemical energy in a fuel source. Transaxle  12  adapts the speed and torque of the mechanical power produced by the engine to suit the current needs of the vehicle. Transaxle  12  includes a multiple-ratio gearbox and also a front differential that provides roughly equal torque to left and right front wheels  14  and  16  while accommodating slight differences in speed such as when the vehicle turns a corner. A Power Take-off Unit (PTU)  18  selectively driveably connects the output of the multiple-ratio gearbox of transaxle  12  to driveshaft  20 . Rear Drive Unit (RDU)  22  selectively transmits power from driveshaft  20  to rear differential  24 . Rear differential  24  transmits roughly equal torque to left and right rear wheels  26  and  28  while accommodating slight differences in speed. Wheel-end disconnect  30  includes a controlled clutch that selectively couples or decouples right rear wheel  28  from the corresponding side of rear differential  24 . 
         [0013]    Wheel-end disconnect  30  and the PTU disconnect may be synchronizers that include friction clutches with sufficient torque capacity to bring the components to the same speed before engaging a positive engagement dog clutch. Alternatively, one of them may be only a dog clutch and the RDU clutch may be used to synchronize the speeds before engagement. Alternatively, engagement may be limited to times when the vehicle is stationary. 
         [0014]    Controller  32  adjusts the state if several components including PTU  18 , RDU  22 , and wheel-end disconnect  30 . Specifically, when the vehicle is in a Front Wheel Drive (FWD) operating mode, controller  32  signals PTU  18  to disconnect driveshaft  20  from transaxle  12  and signals wheel-end disconnect  30  to disconnect the right rear wheel from rear differential  24 . By so doing, the driveshaft can remain stationary as the vehicle moves, reducing the parasitic drag associated with driveshaft rotation and improving fuel economy. When the vehicle is in an All Wheel Drive (AWD) operating mode, the controller signals PTU  18  and wheel-end disconnect  30  to drivably connect the corresponding components. Then, controller  32  monitors vehicle traction and signals RDU  22  to transmit power from driveshaft  20  to rear differential  24  when it detects a low of traction at the front wheels. Controller  32  may also signal RDU  22  to complete this power flow path in advance of maneuvers that are likely to cause loss of front wheel traction. Controller  32  may receive input signals from other components such as PTU  18 , RDU  22 , and rear differential  24 . Controller  32  may be a stand-along driveline controller and may be integrates into another controller such as a transaxle controller or powertrain controller. 
         [0015]      FIG. 2  illustrates the structure of rear differential  24 . The differential is encased in a housing  40 . In vehicles with independent rear suspension, housing  40  is fixed to vehicle structure. In vehicles with a solid rear axle, housing  40  is suspended and moves with the axles. The housing does not rotate in either type of vehicle. Housing  40  is partially filled with fluid the provides lubrication to moving components and convects heat generated by friction between components to the housing, from which the heat is dissipated. Temperature sensor  42  sends an electronic signal to the controller indicating the temperature of the fluid. 
         [0016]    Power enters the rear differential from the RDU via stub shaft  44  which is fixed to drive pinion  46 . Drive pinion  46  is in continuous meshing contact with ring gear  48 . The axes of rotation of pinion  46  and ring gear  48  are offset by 90 degrees. They may also be vertically offset, in which case hypoid gear geometry is common. Ring gear  48  is fixed to differential carrier  50 . A number of planet shafts  52  are fixed to an interior of differential carrier  50 . A number of beveled planet gears are supported for rotation with respect to each of the planet shafts. Alternatively, the planet gears could be fixed to the planet shafts and the planet shafts could rotate with respect to the carrier. Each of the beveled planet gears  54  mesh with a left side gear  56  and a right side gear  58 . The left side gear is fixed to a left axle shaft  60  while the right side gear is fixed to a right axle shaft  62 . 
         [0017]    When the vehicle is traveling in a straight line with wheel-end disconnect  30  engaged, differential carrier  50 , left axles shaft  60 , and right axle shaft  62  rotate at the same speed and in the same direction. In this condition, there is no motion among the planet gears  54 , the side gears  56  and  58 , and the planet shafts  52 . Stub shaft  44  rotates at a speed proportional to the speed of differential carrier  50  with the ratio determined by the tooth counts of pinion gear  46  and ring gear  48 . When the vehicle turns to the left, left axle shaft  60  rotates slightly slower than differential carrier  50  and right axle shaft  62  rotates slightly faster than differential carrier  50 . Planet gears  54  rotate about planet shafts  52  at a speed proportional to the difference in speed between left axle  60  and right axle  62 . Speed sensors  64  and  66  send electronic signals to the controller indicating the rotational speeds of left axle shaft  60  and right axle shaft  62  respectively. Due to the speed relationships among components, two sensors are sufficient to calculate the speeds of all rotating components within the differential. Alternatively, one or both speed sensors could sense the speed of a different component and the controller could calculate the speed of each axle shaft based on speed relationships. 
         [0018]    When wheel-end disconnect  30  is disengaged, the speed of left axle shaft  60  is proportional to vehicle speed, but the speeds of other components are determined by parasitic drag. Generally, because the drag on components linked to differential carrier  50  is greater than the drag on components linked to right axle  62 , carrier  50  will be stationary. With differential carrier  50  stationary, right axle shaft will rotate at the same speed as left axle shaft but in the opposite direction. Planet gears  54  rotate with respect to planet shafts  52  at relative speeds much higher than during connected speed differentiation. Furthermore, this condition may be maintained for much longer periods of time than the duration of turning maneuvers. 
         [0019]    During this high planet gear relative speed operation, some heat is generated by friction between the planet gears and the planet shafts. If fluid is flowing past this interface, the quantity of heat generated in substantially reduced due to the lubrication effect and that heat effectively removed from the area by movement of the fluid. However, if fluid is not flowing past this interface for some reason, a large amount of heat is generated making the components in the vicinity of the interface extremely hot. When the relative motion ceases, for example because the vehicle momentarily stops, the planet pinions may become effectively welded to the planet shafts. Fluid flow may be interrupted for a couple of reasons. If there is a leak in housing  40 , the quantity of fluid decreases. The differential carrier  50  may stop in a position in which one of the planet shafts and corresponding planet pinion is above the fluid level. 
         [0020]    If this welding phenomenon occurs, then differential carrier  50 , left axle shaft  60 , and right axle shaft  62  are constrained to rotate at the same speed. When wheel-end disconnect  30  is disengaged, this results in a slight increase in parasitic drag which is not a serious detriment to vehicle operation. However, if wheel-end disconnect  30  is engaged, then left rear wheel  26  is constrained to rotate at the same speed as right rear wheel  28 . In this condition, one or both wheels are likely to lose traction when the vehicle turns, which could result in under-steer or over-steer. 
         [0021]      FIGS. 3 and 4  illustrate a method of avoiding the vehicle handling effects if the differential fails to differentiate. When the driver selects front wheel drive mode, the vehicle operates in FWD mode at  70 . The driveshaft is disconnected at both the PTU and the wheel-end to reduce parasitic drag. The RDU clutch is disengaged. While operating in this mode, a timer is set to expire at regular intervals, such as every 90 seconds. When the timer expired, or when the driver requests activation of the all wheel drive feature, the controller performs a safety check at  72 . The details of performing this safety check are discussed below. Three outcomes of the safety check are possible. If the differential appears to be differentiating properly but the fluid temperature indicates a risk of failure, the safety check return Warning and the controller remains in the FWD mode. If the safety check indicates that the differential is Unsafe, a message is displayed to the driver at  74  and the controller enters a Disabled mode at  76 . In the Disabled mode, the driveshaft disconnects and RDU continue to operate to provide front wheel drive only. The controller does not exit Disabled mode until service has been performed and the controller is manually reset. If the safety check indicates that the differential is operating normally, the controller checks at  78  whether AWD is selected. If AWD is not selected, implying the safety check was triggered by the timer, then the controller remains in FWD mode. The safety check may not properly detect an unsafe condition when the vehicle is stationary. Running the safety check periodically while the vehicle is moving ensures that any unsafe condition is detected even if the driver requests AWD while the vehicle is stationary. Also, running the safety check periodically provides an opportunity to repeat the safety check if a previous safety check returned Warning. If AWD is selected, then the controller engages the disconnects at  80  and transitions to AWD mode at  82 . In AWD mode, the RDU is engaged when front wheel slip is detected to transfer some of the engine power to the rear wheels. The RDU may also be engaged pre-emptively in conditions that are likely to cause front wheel slip, such as a rapid increase in driver demanded power. 
         [0022]      FIG. 4  indicates how the safety check is performed. At  90 , the controller computes the speed of the differential carrier and wheel  26 . At  92 , the controller computes the ratio of wheel speed divided by carrier speed. In normal operation, the carrier speed would be zero, so this ratio should be zero. If the differential is failing to differentiate at all, then the ratio will be equal to one. At  94 , the controller compares the ratio to a calibratable threshold. If the ratio is less than the calibratable threshold, the safety check returns Unsafe. If the ratio is greater than the threshold, the controller proceeds to check the fluid temperature at  96 . If the temperature is below a second calibratable threshold, the controller resets a counter to zero at  98  and returns Safe. If the temperature is above the second threshold, the controller increments the counter at  100 . Then, if the counter is less than a third calibratable threshold at  102 , the controller return Warning. If the counter is equal to the third threshold, indicating that the fluid temperature has remained above the second threshold for an extended time, the controller returns Unsafe. 
         [0023]    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.