Abstract:
A method for controlling a vehicle drivetrain includes de-energizing a clutch that connects a differential output to a wheel, rotating said component through a sump by pulsing the clutch when a speed of a differential component is less than a reference speed, and cyclically pulsing the clutch while a speed of said component exceeds the reference speed and a count of a timer, started when the clutch is de-energized, exceeds a reference count.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to protecting components of a motor vehicle&#39;s all-wheel-drive (AWD) system, particularly its differential and halfshafts, when operating with its disconnect clutch de-engergized. 
     2. Description of the Prior Art 
     When an AWD drivetrain operates in disconnect mode, the hypoid ring and pinion gears and one of the output shafts of the Rear Drive Unit (RDU) are disconnected from the drivetrain by low-drag torque clutches, thereby preventing transmission of rotary motion and torque from the engine and vehicle wheels to the hypoid ring and pinion gears and driveshaft. A first RDU output shaft, which is connected to its respective differential side gear, rotates at a speed corresponding to vehicle speed. A second RDU output shaft, which is disconnected from the drivetrain, rotates at the same speed as that of the first output shaft but in the opposite direction. The relative speed across the differential assembly, which is referred to as differential speed, is two times the rotational speed of the first output shaft. 
     A purpose of an AWD disconnect system is to minimize parasitic losses of the AWD system when not engaged, thus significantly improving fuel economy during most driving conditions. When in AWD disconnect mode, the differential speed at maximum vehicle speed can be extremely high. If the hypoid ring and pinion gears are stationary, the differential housing does not rotate. Therefore, the differential pinions contained within the differential housing, which rotate at high differential speeds, can remain above the sump oil level for an extended period of time. This condition reduces ability to reject heat. 
     The previous solutions involved the addition of special surface treatments, extra parts such as bearings, and more elaborate lubrication methods, all of which increase the cost of the RDU. A need exists for a technique to prevent damage to the RDU differential and output shafts due to inadequate heat rejection at the interfaces experiencing high relative speed while in AWD disconnect mode. 
     SUMMARY OF THE INVENTION 
     A method for controlling a vehicle drivetrain includes de-energizing a clutch that connects a differential output to a wheel, rotating a differential component through a sump by pulsing the clutch when a speed of said differential component is less than a reference speed, and cyclically pulsing the clutch while a speed of said component exceeds the reference speed and a count of a timer, started when the clutch is de-energized, exceeds a reference count. 
     The method minimizes parasitic losses while protecting the hardware by pulsing the clutch as a function of AWD state, vehicle speed and RDU fluid sump temperature to reduce differential speed and rotate the differential housing to prevent damage to the hardware, without objectionable noise. 
     The method uses a calibrated frequency of clutch pulses to rotate the differential housing to ensure that the structural limitations of the hardware are not exceeded. This enables the differential pinions to rotate and to sweep through the lubrication sump at a predetermined frequency. The frequency and duration of the clutch pulses are optimized to protect the hardware with minimal parasitic losses. 
     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 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram showing an AWD powertrain and a related control system; 
         FIGS. 2A and 2B  are schematic diagrams showing a RDU differential with AWD disconnected and connected, respectively; 
         FIG. 3  is a cross-sectional view of the RDU showing the elevation of gears of the differential relative to a level of lubricant in a sump; 
         FIG. 4  is a logic flow diagram for controlling the AWD clutch; and 
         FIG. 5  is a cross section taken through a rear portion of a transfer case. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIGS. 1 ,  2 A and  2 B, the powertrain  10  of a motor vehicle includes an engine  12 , such as an internal combustion engine; a transaxle  13  for producing multiple forward drive speed ratios and reverse drive; halfshafts  14 ,  16  for transmitting rotating power between the transaxle&#39;s output and the front driven wheels  18 ,  20 ; an RDU  22 ; a driveshaft  24 ; a power take-off unit (PTU)  26  for transmitting rotating power between the transaxle&#39;s output and the driveshaft; and an AWD clutch  28  for alternately driveably connecting and disconnecting the output of the RDU and the rear driven wheels  30 ,  32 . 
     The RDU  22  includes a differential assembly  34 , which includes a differential side gear  36 , connected by a shaft  40  to wheel  30 , and a differential side gear  38 , connected by a shaft  42  to an AWD clutch  28 . Shaft  44  connects rear wheel  32  to the output of AWD clutch  28 . 
       FIG. 2B  shows the AWD powertrain  10  in connect mode, wherein the AWD clutch  28  is energized, thereby transmitting power from the RDU  22  to the rear wheels  30 ,  32 .  FIG. 2A  shows the AWD powertrain  10  in disconnect mode, wherein the AWD clutch  28  is de-energized, whereby shaft  42  rotates opposite the rotary direction of shaft  40  and at the same speed. 
       FIG. 3  is a cross section taken at a diametric plane through the RDU housing showing the differential mechanism  34 , which includes a ring gear  52 , connected to differential housing  50  and connected by a pin  54  to a differential shaft  56 , which revolves about a lateral axis substantially parallel to the axis of the shafts  40 ,  42 ,  44 ; differential pinions  58 ,  60 , which revolve with the differential shaft and are supported on the differential shaft to rotate about the axis of the differential shaft; and differential side gears  36 ,  38 , which mesh continually with the differential pinions  58 ,  60 . 
     When ring gear  52  and differential housing  50  are stationary, line  62  represents the upper surface of hydraulic lubricant in the sump  64  of the housing. 
     The RDU&#39;s ring gear  52  may not rotate at all when the powertrain  10  is in AWD disconnect mode due to clutch  28  being a low drag clutch. Therefore, the differential pinions  58 ,  60  can remain above the oil sump level for an extended period or indefinitely if the ring gear  52  is stationary. 
     Referring to the logic flow diagram of  FIG. 4 , at step  70  a test is made to determine whether the AWD system is operating in Connect Mode or Disconnect Mode. If the result of test  70  is logically false, step  70  is re-executed. If the result of test  70  is logically true, vehicle parameters are determined, preferably by measurement. At step  72  the speed of vehicle  10  is determined, either measured or inferred; at step  74  the speed of differential housing  50  is determined, either measured or inferred; at step  76  the temperature of the RDU sump  64  is determined, either measured or inferred. 
     At step  78  the reference speed of differential housing  50  is determined, preferably from a look-up table similar to Table 1, as a function of vehicle speed and the temperature of the RDU sump  64 . 
     Similarly at step  80  the number of revolutions of housing  50  that occur after the disconnect mode is entered and its entry is verified at step  70  is determined as a function of vehicle speed the temperature of the RDU sump  64 , preferably from a look-up table similar to Table 1. Housing  50  may rotate slowly due to clutch drag even during AWD disconnect mode. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Reference Speed 
                   
               
               
                 Vehicle Speed 
                 RDU Sump 64 
                 of Housing 50 
                 Reference Count 
               
               
                 (mph) 
                 Temp. (deg C.) 
                 (rpm) 
                 (msec) 
               
               
                   
               
             
             
               
                 xx 
                 xx 
                 xx 
                 xx 
               
               
                   
               
             
          
         
       
     
     At step  82  a test is made to determine whether the speed of differential housing  50 , either measured or inferred, is greater than the reference speed of housing  50  determined in step  78 . 
     If the result of test  82  is logically true, at step  84  a timer is incremented and its count is monitored. 
     At step  86  a test is made to determine whether the timer, which preferably increases a count of clock pulses following step  84 , contains a count of clock pulses that is less than the reference count determined at step  80 . 
     If the result of test  86  is true, indicating that insufficient time has elapsed, control returns to step  70 . 
     If the result of test  86  is false, indicating that sufficient time has elapsed, at step  88  a magnitude of electric current is to be applied to a device that actuates clutch  28  into engagement, and the length of the period during which the current is applied to the device, i.e., its duration, are determined as a function of vehicle speed the temperature of the RDU sump  64 , preferably from a look-up table similar to Table 3. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Vehicle Speed 
                 RDU Sump 64 
                 Clutch 88 Coil 
                 Duration 
               
               
                   
                 (mph) 
                 Temp. (deg C.) 
                 Current (ADC) 
                 (msec) 
               
               
                   
                   
               
             
             
               
                   
                 xx 
                 xx 
                 xx 
                 xx 
               
               
                   
                   
               
             
          
         
       
     
     An electronic controller  96  includes a microcomputer  98  and electronic memory  100 , the microcomputer being accessible to the control algorithm of  FIG. 4  expressed in computer-readable coded format. The microcomputer  98  includes a clock or timer  102 , which maintains a count of the number of pulses produced by the clock between the time when the count is started and ended. As a result of executing the control algorithm, controller  96  produces output signals  104 , which causes pulses of electric current to be applied to the actuation device of clutch  28  for the reference duration during each repetitive execution of the algorithm. The electric current pulses alternately energize and de-energize the clutch  28 . When clutch  28  is energized by a current pulse the differential pinions  58 ,  60  rotate into the oil sump  64  and are lubricated, thereby preventing damage to RDU  22  and the components of the rear differential  34 . 
     At step  90  the timer  102  is reset and control returns to step  70 . 
     If the result of test  82  is false, indicating that the rotational speed of differential housing  50  is relatively low, at step  92  a magnitude of electric current to be applied to a device that actuates clutch  28  into engagement, and the length of the period during which the current is applied to the actuation device, i.e., its duration, are determined as a function of vehicle speed and the temperature of the RDU sump  64 , preferably from a look-up table similar to Table 2. Control advances to step  90  after step  92  is executed 
     Although Tables 2 and 3 appear similar, the magnitudes of the electric current to be applied to a device that actuates clutch  28  into engagement and the length of the period may be mutually different. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Vehicle Speed 
                 RDU Sump 64 
                 Clutch 88 Coil 
                 Duration 
               
               
                   
                 (mph) 
                 Temp. (deg C.) 
                 Current (ADC) 
                 (msec) 
               
               
                   
                   
               
             
             
               
                   
                 xx 
                 xx 
                 xx 
                 xx 
               
               
                   
                   
               
             
          
         
       
     
     The controller  96  is supplied with electronic signal sensors, whose signals represent wheel speed, housing speed and sump temperature. 
     The critical interfaces of the driveline  10  that are lubricated due to pulsing clutch  28  include, without limitation, (i) an interface between the central bore of each differential pinion  58 ,  60  and the spindle  56 ; (ii) an interface between a thrust face of each differential pinion  58 ,  60  and a respective thrust washer; (iii) an interface between a thrust face of each differential side gear  36 ,  38  and a respective thrust washer; (iv) the meshing teeth of differential pinions  58 ,  60  and side gears  36 ,  38 : and (v) an interface between each output shaft  40 ,  42  and a respective journal bore  80 ,  82  in the differential housing  50 . 
       FIG. 5  is a cross section taken through a rear portion of a transfer case  110 , in which a sealed sump chamber  112  contains a drive chain  114  engaged with sprocket wheels  116 ,  118 . Sump chamber  112  is enclosed by a rear casing  120 , forward casing  122  and a disc seal  124 . Bearings  126 ,  127  support a driveshaft  128 , which transmits rotating power to the front axle shafts of a motor vehicle. A bearing  130  support a driveshaft  132 , which transmits rotating power to the rear axle shafts of the vehicle. Seals  134 ,  136 ,  138 , and  144  seal sump  112  against the flow of lubricant, such as automatic transmission fluid (ATF), from sump chamber  112 . The forward side of bearing  130  is sealed against ATF flow from a rear output cavity  140 . The upper surface of ATF in cavity  140  is represented by a drain port  142 , which limits the flow of ATF from cavity  140  into sump chamber  112 . 
     ATF in rear output cavity  140  lubricates bearing  130 , bushing  146 , and seal  144 . When sprockets  116 ,  118  rotate, chain  114  moves on the sprockets through the ATF in chamber  112  and slings ATF into the rear output cavity  140 . The chain  114 , however, remains motionless on the sprockets in certain operating modes of transfer case  110 , during which time driveshaft  132  continues to rotate. In order to maintain lubricant on the surfaces of bearing  130 , bushing  146 , and seal  144 , cavity  140  must remain full of lubricant. To replenish lubricant in cavity  140  which may splash out on rough roads or drain out on inclines, the algorithm of  FIG. 4  periodically actuates a clutch, which functions similarly to clutch  28 , to transmit torque to front driveshaft  132 , causing the sprockets  116 ,  118  to rotate and chain  114  to move through the lubricant in sump chamber  112 . 
     The algorithm of  FIG. 4  may rely on road slope (either measured or inferred) and road surface roughness in addition to vehicle speed and temperature of lubricant in chamber  112 , when the control strategy for pulsing clutch  28  is applied to a transfer case  110 . 
     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.