Abstract:
An apparatus and method of operating an adaptive drive system of a motor vehicle which reduces drive line wear, improves safety margins and permits weight reduction in drive line components activates a clutch between a primary and secondary drive line when the vehicle is determined to be heavily loaded. The method steps include sensing the position of a throttle position sensor, sensing the instantaneous speed of a motor vehicle and computing instantaneous acceleration, determining whether the ratio of vehicle acceleration to throttle position is less than predetermined threshold value and engaging a transfer case clutch to transfer drive torque from a primary drive line to a secondary drive line. Operation of this method is transparent to the driver inasmuch as the clutch is activated when the vehicle is heavily loaded as determined by the throttle position to acceleration ratio.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to an operating strategy for motor vehicles having part time or adaptive drive systems and more particularly to an operating strategy for motor vehicles having part-time or adaptive drive systems which activates the drive system clutch in certain operating conditions when the vehicle is heavily loaded. 
     The design of motor vehicles and motor vehicle drive line components routinely addresses worst case scenarios. Operation and handling of the vehicle when it is loaded to the maximum, operation and stability of the vehicle at maximum operating speeds, operation and cooling of the vehicle at a maximum design ambient temperature and operation and performance of the vehicle under maximum braking conditions are all familiar concerns of motor vehicle design and test engineers. For example, rear axles, rear differential gearing, particularly the hypoid gears of a differential, and the rear prop shaft in an adaptive four-wheel drive vehicle must all be designed to withstand maximum engine torque since the vehicle will most generally be operated in two-wheel drive. 
     While the vehicle must perform competently, satisfy numerous operating parameters under these extreme conditions and component parts must be designed to survive them, it is acknowledged that few vehicles are subjected to such operating maximums and fewer still for repeated events or extended periods of time. This observation suggests that operational modes may be developed which are activated or engage only during extreme operating conditions which may then reduce loading, fatigue and wear on parts subjected to such operating extremes, thereby permitting designs which are smaller and lighter but which still provide the appropriate load carrying capability and safety margins for extreme operating conditions. 
     The present invention is directed to a drive line operating strategy which provides improved vehicle operation and durability while allowing reductions in the size and hence weight of certain drive line components. 
     SUMMARY OF THE INVENTION 
     An apparatus and method of operating an adaptive drive system of a motor vehicle which reduces drive line wear, improves safety margins and permits weight reduction in drive line components, activates a clutch between a primary and secondary drive line in certain operating conditions when the vehicle is determined to be heavily loaded. Heavy vehicle loading is determined through data manipulation from sensors typically already available in a vehicle. The operating method may be added to programs or subroutines in an adaptive system controller and may operate automatically, i.e., without driver intervention. 
     The steps of the method include sensing the position of a throttle position sensor, sensing instantaneous speeds of a motor vehicle and computing instantaneous acceleration, determining whether the ratio of vehicle acceleration to throttle position is less than a predetermined threshold and engaging a transfer case clutch to transfer drive torque from a primary drive line to a secondary drive line. 
     Operation of this method is transparent to the driver inasmuch as the clutch is activated only when the vehicle is heavily loaded as determined by the acceleration to throttle position ratio. Once activated, the clutch preferably remains activated for the duration of an ignition cycle. 
     Operation of the motor vehicle drive line according to this method may be accompanied by a reduction in the size of various primary drive line components such as the differential hypoid gears, the primary drive shaft and the primary axles due to their reduced maximum torque loading thereby not only lowering their cost but also reducing the weight of the vehicle. 
     Thus it is an object of the present invention to provide an operating strategy for an adaptive drive system of a motor vehicle. 
     It is a further object of the present invention to provide an operating strategy for a transfer case clutch of adaptive drive system of a four-wheel drive motor vehicle. 
     It is a still further object of the present invention to provide an operating strategy for an adaptive drive system of a motor vehicle wherein acceleration and throttle position are detected and utilized to control engagement of an adaptive drive system clutch. 
     It is a still further object of the present invention to provide an operating strategy for an adaptive drive system of a motor vehicle through which heavy passenger and/or cargo loading of a motor vehicle may be determined by sensing acceleration and throttle position. 
     It is a still further object of the present invention to provide an operating strategy for an adaptive drive system of a motor vehicle which engages a drive line clutch during certain operating conditions upon a determination that the vehicle is heavily loaded. 
     Further objects and advantages of the present invention will become apparent by reference to the following description of the preferred embodiment and appended drawings wherein like reference numbers refer to the same component, element or feature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view of an adaptive four-wheel drive motor vehicle power train having a transfer case and controller according to the present invention; 
     FIG. 2 is a full, sectional view of a transfer case according to the present invention; 
     FIG. 3 is a flat pattern development of the ball ramp operator of a transfer case incorporating the present invention taken along line  4 — 4  of FIG. 2; 
     FIG. 4 is a software or computer program flow chart setting forth the operating steps according to the present invention; and 
     FIG. 5 is a graph presenting a qualitative relationship between throttle position presented in the Y-axis and acceleration presented on the X-axis for three different representative conditions of vehicle loading. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a four-wheel vehicle drive train is diagrammatically illustrated and designated by the reference number  10 . The four-wheel vehicle drive train  10  includes a prime mover  12  which is coupled to and drives a transmission  14 . The output of the transmission  14  directly drives a transfer case assembly  16  which provides motive power to a rear or primary drive line  20  comprising a rear or primary prop shaft  22 , a rear or primary differential  24 , a pair of live rear or primary axles  26  and a respective pair of rear or primary tire and wheel assemblies  28 . 
     The transfer case assembly  16  also selectively provides motive power to a front or secondary drive line  30  comprising a front or secondary prop shaft  32 , a front or secondary differential assembly  34 , a pair of live front or secondary axles  36  and a respective pair of front or secondary tire and wheel assemblies  38 . The front tire and wheel assemblies  38  may be directly coupled to a respective one of the secondary axles  36 , as noted, or, if desired, a pair of manually or remotely activatable locking hubs  42  may be operably disposed between the secondary axles  36  and a respective one of the tire and wheel assemblies  38  to selectively connect same. Alternatively, center axle disconnects (not illustrated) may be disposed in the secondary differential assembly  34 . Finally, both the primary drive line  20  and the secondary drive line  30  may include suitable and appropriately disposed universal joints  44  which function in conventional fashion to accommodate static and dynamic offsets and misalignments between the various shafts and components. 
     A control assembly  50  having a plurality of push buttons  52  which correspond to the various driver selectable operating modes of the transfer case assembly  16  such as high, neutral and low may be mounted within the passenger cabin in a location proximate the driver of the motor vehicle. If desired, the push buttons  52  may be replaced by a rotary switch or other analogous driver selectable input device. The control assembly  50  includes a microprocessor or microcontroller  54  which includes input devices which receive signals, condition them, undertake computations and provide control outputs and logic decisions based upon feedback or signals provided by components of the transfer case assembly  16 . Typically, such conditioning, computation and logic decisions will be performed by software stored in non-volatile memory devices. A throttle position sensor  56  provides either an analog or digital signal to the microcontroller  54  in a data line  58  representative of the instantaneous position of the throttle. 
     The foregoing and following description relates to a vehicle wherein the rear drive line  20  functions as the primary drive line, i.e., it is engaged and operates substantially all the time and, correspondingly, the front drive line  30  functions as the secondary drive line, i.e., it is engaged and operates only part-time or in a secondary or supplemental fashion, such a vehicle commonly being referred to as a primary rear wheel drive vehicle. 
     These designations “primary” and “secondary” are utilized herein rather than “front” and “rear” inasmuch as the invention herein disclosed and claimed may be readily utilized with transmissions  14  and transfer cases  16  wherein the primary drive line  20  is disposed at the front of the vehicle and the secondary drive line  30  is disposed at the rear of the vehicle, the designations “primary” and “secondary” thus broadly and properly characterizing the function of the individual drive lines rather than their specific locations. 
     Referring now to FIGS. 1 and 2, the transfer case assembly  16  includes a multiple piece housing assembly  60  having mating sealing surfaces, openings for shafts and bearings and various recesses, shoulders, counterbores and the like to receive various components or assemblies of the transfer case  16 . An input shaft  62  includes female or internal splines or gear teeth  64  or other suitable coupling structures which drivingly couple the output of the transmission  14  illustrated in FIG. 1 to the input shaft  62 . The input shaft  62  is rotatably supported at one end by an anti-friction bearing such as a ball bearing assembly  66  and at its opposite end by an internal anti-friction bearing such as a roller bearing assembly  68 . The roller bearing assembly  68  is disposed upon a portion of a stepped output shaft  70 . A suitable oil seal  72 , positioned between the input shaft  62  and the housing assembly  60 , provides an appropriate fluid tight seal therebetween. The opposite end of the output shaft  72  is supported by an anti-friction bearing such as a ball bearing assembly  74  and include a flange  76  which may be a portion of a universal joint  44  or may be secured to associated drive line components such as the primary prop shaft  22 . A suitable oil seal  78 , disposed between the flange  76  and the housing assembly  60  provides an appropriate fluid tight seal therebetween. 
     The transfer case assembly  16  also includes a two-speed planetary gear drive assembly  80  disposed about the input shaft  62 . The planetary drive assembly  80  includes a sun gear  82  having a plurality of female or internal splines or gear teeth  84  which engage a complementary plurality of male splines or gear teeth  86  on the input shaft  62 . The sun gear  82  is thus coupled to the input shaft  62  and rotates therewith. The sun gear  82  includes external or male gear teeth  88  about its periphery. Radially aligned with the sun gear  82  and its teeth  84  is a ring gear  90  having inwardly directed gear teeth  92 . The ring gear  90  is retained within the housing assembly  60  by a cooperating circumferential groove and snap ring assembly  94 . A plurality of pinion gears  96  are rotatably received upon a like plurality of stub shafts  98  which are mounted within and secured to a planet carrier  100 . The planet carrier  100  includes a plurality of female or internal splines or gear teeth  102  disposed generally adjacent the male splines or gear teeth  86  on the input shaft  62 . The planetary gear assembly  80  is more fully described in co-owned U.S. Pat. No. 4,440,042 which is herein incorporated by reference. 
     The planetary drive assembly  80  also include a dog clutch or clutch collar  104  defining female or internal splines or gear teeth  106  which are axially aligned with and, in all respects, complementary to the male splines or gear teeth  86  on the input shaft  62 . The clutch collar  104  and its internal splines or gear teeth  106  are slidably received upon a complementary plurality of male or external splines or gear teeth  108  on the stepped output shaft  70 . The clutch collar  104  thus rotates with the output shaft  70  but may translate bi-directionally along it. The clutch collar  104  also includes male or external splines or gear teeth  110  on one end which are in all respects complementary to the female splines or gear teeth  102  on the planet carrier  100 . 
     Finally, the dog clutch or clutch collar  104  includes a pair of radially extending, spaced-apart flanges  114  on its end opposite the splines or gear teeth  110  which define a circumferential channel  116 . The channel  116  receives a complementarily configured semi-circular throat or yoke  118  of a shift fork  120 . The shift fork  120  includes a through passageway defining female or internal threads  122  which engage complementarily configured male or external threads  124  on a rotatable shift rail  126 . The shift rail  126  is received within suitable journal bearings or bushings  128  and is coupled to and driven by a rotary electric, pneumatic or hydraulic motor  130 . The rotary motor  130  is provided with energy through a line  132 . 
     The end of the shift fork  120  opposite the semi-circular yoke  118  includes a cam  136  having a recess flanked by two projections. A three position sensor  140  having a roller or ball actuator includes proximity or position sensors such as Hall effect sensors which provide outputs in a preferably multiple conductor cable  146  defining a first signal indicating that the shift fork  120  and associated clutch collar  104  is in the neutral position illustrated in FIG. 2; that the shift fork  120  has moved to the left from the position illustrated such that the sensor  140  provides a signal indicating that the clutch collar  104  is in a position which selects high gear or direct drive, effectively by-passing the planetary gear assembly  80 , or, conversely, that the shift fork  120  has moved to the right from the position illustrated in FIG. 2 such that the sensor  140  indicates that the shift fork  120  has translated to select the low speed output or speed range of the planetary drive assembly  80 . Such translation is achieved by selective bi-directional operation of the drive motor  130  which rotates the shift rail  126  and bi-directionally translates the shift fork  120  along the male threads  124  of the shift rail  126 . 
     The transfer case assembly  16  also includes an electromagnetically actuated disc pack type clutch assembly  150 . The clutch assembly  150  is disposed about the output shaft  70  and includes a circular drive member  152  coupled to the output shaft  70  through a splined interconnection  154 . The circular drive member  152  includes a plurality of circumferentially spaced apart recesses  156  in the shape of an oblique section of a helical torus, as illustrated in FIG.  3 . Each of the recesses  156  receives one of a like plurality of load transferring balls  158 . 
     A circular driven member  162  is disposed adjacent the circular drive member  152  and includes a like plurality of opposed recesses  166  defining the same shape as the recesses  156 . The oblique side walls of the recesses  156  and  166  function as ramps or cams and cooperate with the balls  158  to drive the circular members  152  and  162  apart in response to relative rotation therebetween. It will be appreciated that the recesses  156  and  158  and the load transferring balls  158  may be replaced with other analogous mechanical elements which cause axial displacement of the circular members  152  and  162  in response to relative rotation therebetween. For example, tapered rollers disposed in complementarily configured conical helices may be utilized. 
     The circular driven member  162  extends radially outwardly and is secured to a soft iron rotor  170 . The rotor  170  is disposed in opposed, facing relationship with an armature  176 . The rotor  170  is U-shaped and surrounds a housing  178  containing an electromagnetic coil  180 . A single conductor wire  182  provides electrical energy to the electromagnetic coil  180 . 
     Providing electrical energy to the electromagnetic coil  180  through the wire  182  causes magnetic attraction of the armature  176  to the rotor  170 . This magnetic attraction results in frictional contact of the armature  176  with the rotor  170 . When the output shaft  70  is turning at a different speed than the armature  176  which turns at the same rotational speed as a secondary output shaft  184 , this frictional contact results in a frictional torque being transferred from the output shaft  70 , through the circular drive member  152 , through the load transferring balls  158  and to the circular driven member  162 . The resulting frictional torque causes the balls  158  to ride up the ramps of the recesses  156  and  166  and axially displaces the circular drive member  152 . Axial displacement of the circular drive member  152  translates an apply plate  186  axially toward a disc pack clutch assembly  188 . A plurality of compression springs  190  provides a restoring force which biases the  15  circular drive member  152  toward the circular driven member  162  and returns the load transferring balls  158  to center positions in the circular recesses  156  and  166  to provide maximum clearance and minimum friction between the components of the electromagnetic clutch assembly  150  when it is deactivated. 
     The disc pack clutch assembly  188  includes a plurality of interleaved friction plates or discs  192 A and  192 B. A first plurality of discs  192 A are coupled by interengaging splines  194  to a clutch hub  196  which is, in turn, coupled to the output shaft  70  for rotation therewith. A second plurality of discs  192 B are coupled to an annular housing  198  by interengaging splines  202  for rotation therewith. 
     An important design consideration of the components of the electromagnetic clutch assembly  150  is that their geometry, such as the ramp angles of the recesses  156  and  166 , the spring rate of the compression springs  190  and the clearances in the disc pack clutch assembly  188  ensure that the electromagnetic clutch assembly  150  is neither self-engaging nor self-locking. The electromagnetic clutch assembly  150  must not self-engage but rather must be capable of controlled, proportional engagement of the clutch discs  192 A and  192 B and torque transfer in direct, proportional response to the control input. 
     The annular housing  198  is disposed for free rotation about the output shaft  70  and is rotationally coupled to a chain drive sprocket  204  by a plurality of interengaging lugs and recesses  206 . The drive sprocket  204  is also freely rotatably disposed on the output shaft  70 . A drive chain  208  is received upon the teeth of the chain drive sprocket  204  and engages and transfers rotational energy to a driven chain sprocket  212 . The driven sprocket  212  is coupled to the secondary output shaft  184  of the transfer case assembly  16  by interengaging splines  214 . 
     The transfer case assembly  16  also includes a first Hall effect sensor  220  which is disposed in proximate, sensing relationship with a plurality of teeth  222  on a tone wheel  224 . The tone wheel  224  is coupled to and rotates with the primary output shaft  70 . A second Hall effect sensor  226  is disposed in proximate, sensing relationship with a plurality of teeth  228  of a tone wheel  230  disposed on the secondary output shaft  184 . Preferably, the number of teeth  222  on the tone wheel  224  is identical to the number of teeth  228  on the tone wheel  230  so that identical shaft speeds result in the same number of pulses per unit time from the Hall effect sensors  220  and  226 . This simplifies computations and improves the accuracy of all decisions based on such data. As to the actual number of teeth  222  on the tone wheel  224  and teeth  228  on the tone wheel  230 , it may vary from thirty to forty teeth or more or fewer depending upon rotational speeds and sensor construction. The use of thirty-five teeth on the tone wheels  224  and  230  has provided good results with the Hall effect sensors  220  and  226 . 
     Referring now to FIG. 4, the method of operating the transfer case  16  in accordance with information received from the throttle position sensor  56  and one or both of the Hall effect sensors  220  and  226  includes a software subroutine or program  240  stored in memory in the microcontroller  54 . The operating method embodied in the program  240  commences at a start point  242  and immediately proceeds to a decision point  244  wherein it is determined whether a new ignition cycle, i.e., a new engine start-up or vehicle use cycle, has begun since the last iteration of the program  240 . If it is a new ignition cycle, the decision point  244  is exited at YES and the program  240  proceeds to a process step  246  which initializes the system, erases all previously stored data and sets all temporary memory and counters to zero. If it is not a new ignition cycle, the decision point  244  is exited at NO. The program  240  then moves to a decision point  248  where the electromagnetic disc pack clutch assembly  150  or its electronic driver circuitry (not illustrated) is interrogated to determine if it is energized. If it is, the decision point  248  is exited at YES. If it is not, the decision point  248  is exited at NO and the program  240  moves to a process step  250 . 
     At the process step  250 , the instantaneous position of the throttle is read by the throttle position sensor  56  and this information, in either digital or analog form, is provided to the microcontroller  54  through the data line  58 . If the throttle position sensor  56  is an analog output device, proper conditioning and conversion of its analog output to a digital signal occurs. The instantaneous position of the throttle position sensor  56  which may take the form of a number from zero to one hundred and thus numerically represent the percent of activation of the throttle is stored in a temporary or volatile memory and the program  240  moves to another process step  252  which reads a first instantaneous speed of the vehicle (VS 1 ). The first instantaneous vehicle speed (VS 1 ) is preferably read from either the first Hall effect sensor  220  associated with the primary drive line  20  or the second Hall effect sensor  226  associate with the secondary drive line  30 . Alternatively, the distinct signals from the Hall effect sensors  220  and  226  may be averaged together to provide vehicle speed information, if desired. The first instantaneous speed data (VS 1 ) is then stored in a temporary or volatile memory. 
     The program  240  then moves to a process step  254  and executes a dwell or hold (ΔT) for a sufficient period of time, preferably on the order of 20 to 100 milliseconds, to ensure an accurate subsequent computation after which a second instantaneous vehicle speed will be read. It will be appreciated that relatively shorter dwell times increase iteration speed of the program  240  but may provide less accurate acceleration data due to the limited time over which the change in vehicle speed is measured. On the other hand, relatively longer dwell times decrease iteration speed of the program  240  but will generally provide more accurate acceleration data due to the greater time over which the change in vehicle speed is measured. 
     As will be appreciated, the dwell or base line time (ΔT) over which acceleration is computed is not critical and may be selected to conform to other sampling and time intervals in the microcontroller  54 . If, for example, the entire program  240  is commenced every 100 or 200 milliseconds, the denominator of the fraction            VS   2     -     VS   1           T   2     -     T   1                              
     may be, as noted, from 20 to 100 milliseconds and preferably is between 30 and 80 milliseconds. The dwell or hold period of the process step  254  may be accomplished by a conventional programmed timer or subroutine. 
     After the dwell period of the process step  254  is elapsed, a process step  256  in which a second instantaneous vehicle speed (VS 2 )is determined and stored is undertaken. The instantaneous vehicle speed information (VS 1  and VS 2 ) is then utilized in a process step  258  in which the acceleration (ΔVS) of the motor vehicle is computed. Alternatively, vehicle acceleration (ΔVS) may be determined by the use of an on-board accelerometer or other known detection and computation methods. 
     Referring briefly to FIG. 5, a graph qualitatively presenting various relationships between acceleration and throttle position for distinctly loaded vehicles is illustrated. The line L illustrates the performance of a motor vehicle which is lightly loaded, i.e., the acceleration of the vehicle is the greatest or most significant for a given throttle position whereas line H represents a heavily loaded vehicle and illustrates that acceleration is the slowest or lowest for a given throttle position. The line M disposed between the lines L and H represents a vehicle with a medium load and, of course, represents intermediate acceleration for a given throttle position. 
     The program  240  then moves to a decision point  260  wherein a determination is made whether the instantaneous vehicle acceleration divided by the position of the throttle as sensed by the throttle position sensor  56  is greater than a predetermined value X. The predetermined value X will vary widely based upon empirical and performance data as well as the aggressive or conservative performance goals of a given vehicle and control system. Test track or computer simulated trials of an unloaded or lightly loaded vehicle are undertaken to determine the preliminary value of the vehicle acceleration-throttle position ratio. The value X is preferably 80% (0.8 times) the value of this ratio. While a multiplier of 0.8 has been found to provide optimum operation in typical sport utility vehicles of average torque and horsepower, the multiplier (and thus the final value of X utilized in the decision point  260 ) may be adjusted up or down within the range of approximately 0.7 to approximately 0.9 to adjust the degree of aggressiveness with which the program  240  responds to throttle position and vehicle acceleration in accordance with design goals. 
     If the acceleration divided by the throttle position calculation is less than the predetermined value X the decision point  260  is exited at NO and the program  240  enters a process step  262  which activates engagement of the electromagnetic disc pack type clutch assembly  150 . The engagement level as a percentage of full (100%) clutch actuation is also a value preferably determined by empirical and actual performance testing of a specific vehicle. One of the more aggressive and most practical schemes of activation of the electromagnetic disc pack type clutch assembly  150  is activation to a level commensurate with the activation of the throttle as sensed by the throttle position sensor  56 . That is, if the throttle is activated or depressed to 50% of full travel, the electromagnetic clutch assembly  150  is engaged or activated to 50%. If the throttle is depressed to 90% of full travel, the electromagnetic clutch assembly  150  is engaged to 90% of full engagement. Upon activation of the electromagnetic clutch assembly  150 , the process step  262  is exited and the program ends at a step  264  and returns to its executive system or main program. 
     Returning to the decision point  260 , if the instantaneous vehicle acceleration divided by the throttle position is greater than the predetermined value X, the decision point  260  is exited at YES and the program  240  ends at the step  264 . 
     The program  240 , of course, is capable of updating and increasing the level of engagement of the electromagnetic clutch assembly  150 . This feature is provided by the series of process steps in the program  240  commencing with a YES response to the interrogation undertaken in decision point  248  wherein the status of the electromagnetic clutch assembly  150  is determined. If the electromagnetic clutch assembly  150  is energized, the decision point  248  is exited at YES and the program  240  enters a process step  272  wherein a second throttle position (TP 2 ) is read and stored. The program  240  then moves to a decision point  274  wherein a determination is made as to whether the second throttle position (TP 2 ) is greater than the first throttle position (TP 1 ). If it is not, the decision point  274  is exited at NO and the program  240  ends at the step  264 . If the second throttle position (TP 2 ) is greater than the first throttle position (TP 1 ), the decision point  274  is exited at YES and the program  240  enters a process step  276  wherein the electromagnetic disc pack type clutch assembly  150  is incremented to the higher level of engagement corresponding to the percentage of throttle advance of the second throttle position (TP 2 ). When the electromagnetic clutch assembly  150  has been so incremented, the program  240  ends at the step  264  and returns to the executive system or main program. 
     It should be noted that the program  240  does not include a provision for decrementing, i.e., deactivating the electromagnetic clutch assembly  150 . This functional consideration is based upon the fact that once the vehicle has been determined to be heavily loaded, this heavily loaded condition will, under normal circumstances, not change during an ignition, i.e., use, cycle. That is, if it is determined that the vehicle is heavily loaded or towing a trailer during the beginning of an ignition cycle, it will remain so at least until the vehicle is stopped and possibly unloaded. 
     An alternate design philosophy wherein, for example, fuel economy is of great importance, suggests the benefit of disengaging or deactivating the electromagnetic disc pack clutch assembly  150  if, for example, the vehicle has reached highway speed and no significant acceleration events such as might occur from a stop have occurred for a period of time. In this case, the program  240  may be modified to include a timer which times out a predetermined period of time and a decision point which determines whether the vehicle has recently or is stopped. If the predetermined time has elapsed and if the vehicle has not stopped, the electromagnetic clutch assembly  150  may be disengaged. An accelerative event would, however, re-engage the clutch assembly  150  according to the program  240 . 
     It will be appreciated that the operating method of the present invention is intended to be and is fully capable of adaption and integration into vehicles and control systems which provide on demand or adaptive torque distribution between primary and secondary drive lines when, for example, slip of the primary drive wheels due to loss of traction is detected. 
     The foregoing disclosure is the best mode devised by the inventor for practicing this invention. It is apparent, however, that apparatus and methods incorporating modifications and variations will be obvious to one skilled in the art of motor vehicle operating strategies. Inasmuch as the foregoing disclosure presents the best mode contemplated by the inventor for carrying out the invention and is intended to enable any person skilled in the pertinent art to practice this invention, it should not be construed to be limited thereby but should be construed to include such aforementioned obvious variations and be limited only by the spirit and scope of the following claims.