Abstract:
A motor vehicle having: a prime mover; first and second groups of one or more wheels; and a driveline to connect the prime mover to the first and second groups of wheels such that the first group of one or more wheels is driven by the prime mover when the driveline is in a first mode of operation and the second group of one or more wheels is additionally driven by the prime mover when the driveline is in a second mode of operation, the driveline including an auxiliary driveline comprising releasable torque transmitting means operable to connect the second group of one or more wheels to the prime mover when the driveline transitions between the first mode and the second mode, wherein when in the first mode the driveline is operable to transition to the second mode responsive to an output of a reactive evaluator and a predictive evaluator.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a motor vehicle and to a method of controlling a motor vehicle. In particular but not exclusively the invention relates to motor vehicles such as all-terrain vehicles (ATVs) having a driveline that is operable change the number of wheels that provide torque to drive the vehicle. 
     BACKGROUND 
     It is known to provide a motor vehicle having a four wheel drive mode of operation in which motive power is supplied to each of two pairs of wheels of the vehicle. Motive power is supplied to the wheels by means of a driveline. 
     Some known vehicles are arranged such that motive power is permanently supplied to both pairs of wheels. Some other vehicles are arranged such that motive power is selectively supplied to either only one pair or to both pairs of wheels. A driver operable selector may be provided to allow the driver to select two wheel or four wheel operation. Some driveline systems require the vehicle to be stationary when transitioning between two and four wheel drive modes. Such systems may be referred to as static disconnect/reconnect systems. 
     GB2407804 discloses a dynamic driveline reconnect arrangement in which reconnection of two of the wheels to the driveline following disconnection of the wheels from the driveline may be undertaken when the vehicle is moving. Such a system may be referred to as a dynamic driveline reconnect system. The system disclosed in GB2407804 employs clutch arrangements to enable dynamic driveline reconnection. 
     It is desirable to provide a vehicle having improved response time in triggering the transition from the two wheel drive to four wheel drive modes. It is further desirable to provide a vehicle having improved noise, vibration and harshness (NVH) performance in respect of the transition between the two wheel drive and four wheel drive modes. 
     STATEMENT OF THE INVENTION 
     Embodiments of the invention may be understood by reference to the appended claims. 
     Aspects of the invention provide a system, a vehicle and a method. 
     In a further aspect of the invention for which protection is sought there is provided a motor vehicle having:
         prime mover means;   at least first and second groups of one or more wheels; and   a driveline operable to connect the prime mover means to the first and second groups of one or more wheels such that the first group and not the second group of one or more wheels is driven by the prime mover means when the driveline is in a first mode of operation and the first and second groups are driven by the prime mover means when the driveline is in a second mode of operation,   the driveline including an auxiliary portion comprising releasable torque transmitting means by means of which the driveline is operable to connect the second group of one or more wheels to the prime mover means when the driveline transitions from the first mode to the second mode,   wherein when in the first mode the driveline is operable to transition to the second mode responsive to an output of a reactive evaluator and a predictive evaluator,   the output of the reactive evaluator being responsive to a determination whether an amount of wheel slip has exceeded a first prescribed threshold,   the output of the predictive evaluator being responsive to a determination whether one or more conditions in respect of one or more vehicle operating parameters are met when the amount of wheel slip is below the first prescribed threshold.       

     Within this application, the term “responsive to” is not intended to be limiting and may be substituted, for example, by the term “dependent on” or other similar terms. 
     It is to be understood that reference herein to a group of one or more wheels includes reference to a group having a membership of only one wheel. 
     Embodiments of the present invention have the advantage that a vehicle may transition from the first mode to the second mode either as soon as a condition requiring a transition arises (responsive to the output of the reactive evaluator) or, in some cases, in advance of the existence of one or more conditions that would trigger the reactive evaluator to trigger the transition. In other words, the predictive evaluator may trigger a transition to the second mode in advance of conditions occurring that would cause the reactive evaluator to trigger a transition to the second mode. 
     Thus it is to be understood that in some situations, by the time conditions actually occur that would normally trigger a transition to the second mode (responsive to the output of the reactive evaluator) the vehicle may already have made the transition to the second mode responsive to the output of the predictive evaluator. 
     In some situations, by performing a transition to the second mode in anticipation of conditions requiring the transition to the second mode actually occurring, occurrence of the conditions requiring the transition to the second mode may be averted. For example, if the predictive evaluator determines that a risk of wheel slip exists the vehicle may transition to the second mode in advance of wheel slip occurring thereby preventing wheel slip. 
     Some embodiments have the advantage that wear or one or more components of the vehicle may be reduced. For example if wheel slip is prevented or reduced, wear of one or more tyres may be reduced as well as wear of one or more other components of the driveline. 
     Thus it is to be understood that the reactive evaluator is arranged to provide a feedback signal indicating that a transition to the second mode is required responsive to conditions that currently exist. 
     In some arrangements the predictive evaluator may be considered to provide a feed-forward signal indicating that a transition to the second mode may soon be triggered by the reactive evaluator unless the vehicle pre-empts the reactive evaluator by assuming the second mode immediately. Thus in some circumstances the predictive evaluator may trigger a transition to the second mode ahead of the reactive evaluator. 
     In an embodiment the output of the predictive evaluator may be further responsive to a determination that one or more conditions in respect of one or more vehicle operating parameters are met indicating that the amount of wheel slip may be about to exceed a second prescribed threshold. 
     Optionally the output of the predictive evaluator may be further responsive to a determination that one or more conditions in respect of one or more vehicle operating parameters are met indicating that a passenger discomfort parameter may be about to exceed a prescribed threshold. 
     The passenger discomfort parameter may be a parameter indicative of a level of discomfort suffered by a passenger. The higher the value, the greater the level of discomfort suffered. The discomfort parameter may be responsive to an amount of lateral and/or longitudinal acceleration of a vehicle, an amount of yaw rate error. The discomfort parameter may be responsive to one or more other parameters in addition or instead. 
     The second threshold may be substantially equal to the first threshold. 
     Alternatively the second threshold may be less than the first threshold. 
     In an embodiment the amount of wheel slip includes the amount of lateral wheel slip. 
     Optionally the amount of wheel slip includes the amount of longitudinal wheel slip. 
     In an embodiment the amount of wheel slip is determined according to a combination of the amount of lateral wheel slip and the amount of longitudinal wheel slip. 
     Optionally the predictive evaluator is arranged to determine a likelihood that a transition from the first mode to the second mode will be required within a prescribed period of time based on the value of the one or more vehicle operating parameters. 
     The predictive evaluator may be responsive to one or more historical values of the one or more parameters. 
     Thus the predictive evaluator may take into account historical values of one or more operating parameters such as parameters responsive to suspension activity, lateral acceleration and/or any other suitable vehicle parameters in determining a likelihood of a transition from the first mode to the second mode being required. 
     In an embodiment when a transition from the first mode to the second mode is made responsive to the output of the predictive evaluator the releasable torque transmitting means is arranged to connect the second group of wheels to the prime mover means at a rate responsive to the value of the one or more vehicle operating parameters responsive to which the predictive evaluator has triggered the transition. 
     It is to be understood that the rate of connection of the second group of one or more wheels to the torque transmission path from the prime mover means by the auxiliary portion of the driveline may be considered to relate to a time period within which a connect operation is performed. The higher the rate of connection, the smaller the time period over which the driveline completes an operation in which driveline transitions from the first mode to the second mode. Thus reference to a connect rate may be alternatively considered to be reference to a time period within which a connect operation is completed, i.e. a time period within which a transition from the first mode to the second mode may be completed. By completed is meant that the releasable torque transmitting means has completed an operation in which the second group of one or more wheels is connected to the prime mover means. In the case where the releasable torque transmission means comprise clutch means, completion is understood to mean that the clutch means is fully (and not partially) closed. 
     Thus, the control means may be operable to control the driveline to transition from the first mode to the second mode over a time period the length of which is responsive to the value of the one or more vehicle operating parameters. 
     In some embodiments where the releasable torque transmitting means comprises a clutch or clutch means, reference to a connect rate may be understood as reference to a rate of closure of the clutch or clutch means. In some embodiments reference to a connect rate may be understood as reference to a total time taken for a plurality of clutch means to fully close; for example a time delay between closure of one clutch means and closure of another clutch means may be changed, resulting in a different connect rate. In some arrangements it may be advantageous to close clutch means of one of the first and second releasable torque transmitting means before closing the clutch means of the other of the first and second releasable torque transmitting means in order to spin up the prop shaft before the other clutch means is closed. This can reduce noise, vibration and/or harshness (NVH) associated with driveline connect operations. However where a connect operation is urgent, the clutch means of the first and second releasable torque transmitting means may be closed substantially simultaneously or one immediately after the other with little or no delay between the respective closure operations. 
     That is, the releasable torque transmitting means may be arranged to perform to completion a connect operation over a period of time the length of which is responsive to the output of the predictive evaluator. 
     It is to be understood that faster rates of transition from the first mode to the second mode (resulting in reduced required time periods to complete a connect operation) may result in increased wear of components of the vehicle and increased NVH (i.e. a reduced NVH performance). By selecting a connect rate that is responsive to the value of the one or more operating parameters triggering the transition, slower rates may be used in some circumstances and faster rates in others. Thus some embodiments of the invention have the advantage that a rate of wear of components of the vehicle may be reduced and/or NVH performance improved. 
     Optionally the output of the predictive evaluator is responsive to a plurality of vehicle operating parameters. 
     Further optionally the releasable torque transmitting means is arranged to connect the second group of wheels to the prime mover means at a rate responsive to the identity of the one or more operating parameters responsive to which the output of the predictive evaluator triggered the vehicle to transition from the first mode to the second mode. 
     The predictive evaluator may be configured to provide an output corresponding to a required mode of operation of the driveline. 
     In an embodiment when the predictive evaluator provides an output indicating that the second mode of the driveline should be assumed the predictive evaluator is further configured to provide an output corresponding to the identity of the one or more operating parameters responsive to which the predictive evaluator has provided the output indicating that the second mode should be assumed. 
     This has the advantage that the transition to the second mode may be made at a rate that is responsive to the identity of the triggering parameter. This allows a faster rate to be employed where the identity of the triggering parameter is such that the transition should be performed more urgently. 
     Optionally when the predictive evaluator determines that the second mode should be assumed the predictive evaluator is arranged to provide an output indicative of a rate at which the transition from the first mode to the second mode should be made. 
     This has the advantage that the controller  40  does not need to make a separate determination of the required rate of transition from the first mode to the second mode. Rather, the controller  40  controls the vehicle to transition from the first mode to the second mode responsive to the output of the predictive evaluator. 
     This has the advantage that a speed at which the vehicle can begin the transition from the first mode to the second mode may be increased in some embodiments. This in turn has the advantage of reducing the probability that the vehicle will be found in the first mode when vehicle operating parameters are such that vehicle operation in the second mode is demanded by the reactive evaluator. 
     The predictive evaluator may be configured to provide an output corresponding to a likelihood that a transition from the first mode to the second mode will be required. 
     The output may comprise a probability value being a value corresponding to a probability that a transition from the second mode to the first mode will be required within a prescribed time period. 
     In an embodiment the vehicle is arranged to perform the transition from the first mode to the second mode at a slower rate when the transition to the second mode is made responsive to the output of the predictive evaluator compared with a transition made responsive to the output of the reactive evaluator. 
     This feature has the advantage that an amount of wear of components of the driveline may be reduced compared with a vehicle not having a predictive evaluator. 
     As noted above, if a transition to the second mode is made responsive to the output of a predictive evaluator when conditions otherwise demanding a transition to the second mode do not exist, the transition may be made at a rate slower than in the case that the conditions do exist. 
     This is because even if a transition to the second mode is made responsive to the output of the predictive evaluator at a slower rate, in some circumstances it is likely that the vehicle will be in the second mode at a time earlier than that which would be the case if the transition to the second mode were made at a higher rate responsive to the output of the reactive evaluator. 
     The transition to the second mode may therefore be made earlier in some circumstances than it would otherwise have been made, reducing wear of the vehicle and reducing a risk that NVH is compromised by performing the transition at a higher rate. 
     In an embodiment the reactive evaluator is responsive to at least one vehicle operating parameter selected from amongst: 
     (a) an amount of wheel slip, 
     (b) a yaw rate error, 
     (c) a determination whether an antilock braking system (ABS) is active; 
     (d) a determination whether a dynamic stability control (DSC) system is active; and 
     (e) a determination whether a traction control system (TCS) is active. 
     In an embodiment the predictive evaluator is responsive to at least one vehicle operating parameter selected from amongst: 
     (a) an instantaneous amount of torque being delivered through the driveline to one or more wheels, 
     (b) a steering wheel angle, 
     (c) a steerable road wheel angle, 
     (d) a rate of change of steerable road wheel angle, 
     (e) a rate of change of steering wheel, 
     (f) a lateral acceleration of the vehicle, 
     (g) a rate of change of lateral acceleration, 
     (h) a throttle or accelerator pedal position, 
     (i) a rate of change of throttle or accelerator pedal position, 
     (j) a driver demanded torque, 
     (k) a prime mover means torque, 
     (l) an amount of wheel slip, 
     (m) a vehicle acceleration, 
     (n) a vehicle deceleration, 
     (o) a yaw rate, 
     (p) a yaw rate error, 
     (q) a gear shift position value of a manual or automatic gear shift control, 
     (r) an operating temperature, 
     (s) an ambient temperature, 
     (t) a temperature of a vehicle component, 
     (u) a temperature of a vehicle fluid, 
     (v) a temperature of a component of the auxiliary driveline, 
     (w) a temperature of a fluid of the auxiliary driveline, 
     (x) a speed of the vehicle, 
     (y) the identity of a vehicle operating program being executed by the vehicle, 
     (z) a roughness of a driving surface over which the vehicle is moving, 
     (a1) a current or target gear of an automatic transmission is set to one of a first set of one of one or more gear values, 
     (b1) a brake pedal position value is greater than a first prescribed brake pedal position threshold, and 
     (c1) a brake pressure value is greater than a first prescribed brake pressure threshold. 
     Optionally the auxiliary portion of the driveline comprises a prop shaft and the releasable torque transmitting means comprises first releasable torque transmitting means operable to connect the prop shaft to a torque transmission path from the prime mover means and the second releasable torque transmitting means is operable to connect the prop shaft to the second group of one or more wheels. 
     Optionally the vehicle further comprises control means operable to switch the vehicle between the first and second modes of operation such that in the first mode the first and second releasable torque transmission means disconnect the prop shaft from both the torque transmission path to the prime mover means and the second group of wheels. 
     It is to be understood that when the prop shaft is disconnected from both the torque transmission path from the prime mover means and said second group of one or more wheels the prop shaft may be substantially stationary even when the vehicle is moving. This has the advantage that energy losses associated with prop shaft rotation may be reduced or eliminated in some embodiments. 
     Optionally the first group of wheels comprises a first pair of wheels and the first mode corresponds to a two wheel drive mode of operation wherein the first pair of wheels are driven by the prime mover means. 
     Further optionally the second group of wheels comprise a second pair of wheels and the second mode of operation corresponds to a four wheel drive mode of operation. 
     Still further optionally the second group of wheels comprises a second and a third pair of wheels and the second mode of operation corresponds to a six wheel drive mode of operation. 
     In an embodiment at least one of the first and second groups of wheels comprises a plurality of pairs of wheels. 
     Optionally the releasable torque transmitting means comprises one or more actuators arranged to perform the connection operation, the connection operation rate being dependent on a rate of movement of the one or more actuators. 
     In an embodiment the releasable torque transmitting means comprises a plurality of actuators arranged to perform the connection operation. 
     In an embodiment the prime mover means comprises at least one selected from amongst an internal combustion engine and an electric propulsion motor. 
     In a further aspect of the invention there is provided a method of controlling a motor vehicle having a driveline comprising:
         in a first mode of operation of the driveline, coupling a first group of one or more wheels and not a second group to prime mover means by means of the driveline,   in a second mode of operation of the driveline, coupling the second group of one or more wheels to the prime mover means in addition to the first group by means of an auxiliary portion of the driveline,   when the driveline is in the first mode the method comprising controlling the driveline to transition to the second mode responsive to an output of a reactive evaluator and a predictive evaluator,   the method comprising determining by means of the reactive evaluator whether an amount of wheel slip has exceeded a first prescribed threshold and controlling the driveline to assume the second mode if the amount of wheel slip has exceeded the first prescribed threshold, and   determining by means of the predictive evaluator whether one or more conditions in respect of one or more vehicle operating parameters are met when the amount of wheel slip is below the first prescribed threshold.       

     In an embodiment the step of determining by means of the predictive evaluator whether one or more conditions in respect of one or more vehicle operating parameters are met when the amount of wheel slip is below the first prescribed threshold may comprise determining by means of the predictive evaluator whether one or more conditions in respect of one or more vehicle operating parameters are met indicating that the amount of wheel slip may be about to exceed a second prescribed threshold when the amount of wheel slip is below the first prescribed threshold 
     Optionally the step of determining by means of the predictive evaluator whether one or more conditions in respect of one or more vehicle operating parameters are met when the amount of wheel slip is below the first prescribed threshold may comprise determining by means of the predictive evaluator whether one or more conditions in respect of one or more vehicle operating parameters are met indicating that a passenger discomfort parameter may be about to exceed a prescribed threshold when the amount of wheel slip is below the first prescribed threshold. 
     Within the scope of this application it is envisaged that the various aspects, embodiments, examples and alternatives, and in particular the features thereof, set out in the preceding paragraphs, in the claims and/or in the following description and drawings, may be taken independently or in any combination thereof. For example, features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures in which: 
         FIG. 1  is a schematic illustration of a vehicle according to an embodiment of the present invention; and 
         FIG. 2  is a schematic diagram of a controller of a motor vehicle according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic illustration of a motor vehicle  1  having a driveline  5  according to an embodiment of the present invention. The driveline  5  is connected to an internal combustion engine  11  and has a gear box  18 , a pair of front wheels  12 ,  13 , an auxiliary portion  10  and a pair of rear wheels  14 ,  15 . 
     The driveline  5  is arranged selectively to transmit power supplied to the gearbox  18  from the internal combustion engine  11  to the front wheels  12 ,  13  only in a first (or two wheel drive) mode of operation or to the front wheels  12 ,  13  and the rear wheels  14 ,  15  simultaneously in a second (or four wheel drive) mode of operation. 
     Power is transmitted to the front wheels  12 ,  13  via a pair of front drive shafts  19  of the driveline  5 . 
     Power is transmitted to the rear wheels  14 ,  15  by means of the auxiliary portion  10 . The auxiliary portion  10  has a power transfer unit (PTU)  24  having a power transfer clutch (PTC)  22  operable to connect a prop shaft  23  of the auxiliary portion  10  to the gearbox  18 . The prop shaft  23  is coupled in turn to a rear differential  30  operable to couple the prop shaft  23  to the rear drive shafts  26 . 
     The rear differential  30  has a pair of clutches  27  by means of which the rear differential  30  is operable to connect the prop shaft  23  to the rear drive shafts  26  when the four wheel drive mode of operation is required. 
     The driveline  5  has a controller  40  arranged to control operation of the PTC  22  and clutches  27 . When a four wheel drive mode of operation is required the controller  40  is arranged to close the PTC  22  and to close clutches  27  of the differential  30 . 
     In the embodiment of  FIG. 1  the PTC  22  and differential clutches  27  have respective actuators operable to close the respective clutches at a selected one of a plurality of different rates. This allows the transition from the two wheel drive mode of operation to the four wheel drive mode of operation to be made at one of a corresponding plurality of different rates. 
     It is to be understood that the rate at which the respective actuators actuate the respective clutch arrangements may affect a rate of wear of the clutches and potentially one or more other components of the driveline  5 . The rate of actuation may also impact the level of NVH (noise, vibration and harshness) experienced by a driver or passenger of the vehicle  1 . Thus, in the interests of driver comfort and vehicle longevity it may be preferable in some embodiments to perform a transition from the two wheel drive mode of operation to the four wheel drive mode of operation at a slower rate in order to reduce component wear and reduce vehicle NVH. 
     However in some situations it is preferable to perform the transition from the two wheel drive mode to the four wheel drive at a higher rate. For example, if it is detected that the vehicle is skidding it may be preferable to assume the four wheel drive mode quickly. 
     The vehicle  1  also has an antilock braking system (ABS) arranged to control a brake of one or more wheels of the vehicle to reduce an amount of braking action when required when braking is performed in order to prevent skidding. The vehicle  1  also has a dynamic stability control system (DSC)  60  arranged to control an amount of torque delivered to one or more wheels of the vehicle to prevent wheel slip. 
     Furthermore, the vehicle  1  has a traction control system (TCS)  70  arranged to monitor wheels of the vehicle  1  and to apply a brake to one or more wheels in the event that it is determined that the wheel is rotating at a speed that is higher than that required for substantially no wheel slip to occur. 
       FIG. 2  is a schematic illustration of a portion of the controller  40 . The controller  40  has a reactive evaluator  43  embodied in computer software code that is run by a computing device of the controller  40 . The reactive evaluator  43  is arranged to evaluate signals S 1 , S 2 , S 3 , S 4 , S 5  received by the controller  40 . The signals are responsive to: 
     (a) an amount of longitudinal wheel slip of one or more wheels, 
     (b) a yaw rate error, 
     (c) a determination whether an antilock braking system (ABS)  50  is active; 
     (d) a determination whether a dynamic stability control system (DSC)  60  is active; and 
     (e) a determination whether a traction control system (TCS)  70  is active. 
     When the vehicle  1  is in the two wheel drive mode of operation and one or more values of the signals S 1 , S 2 , S 3 , S 4 , S 5  meet a set of one or more prescribed reactive evaluator conditions, the reactive evaluator  43  is arranged to trigger a transition from the two wheel drive mode to the four wheel drive mode. 
     In order to trigger the transition from the two wheel drive mode to the four wheel drive mode the reactive evaluator  43  provides an output signal R 1  to a control module  47  that controls the state of the driveline  5 , i.e. whether the driveline  5  is in the two wheel drive mode or the four wheel drive mode. 
     In the embodiment of  FIG. 2  the output signal R 1  is provided to the control module  47  via a logical OR gate  46 . 
     The reactive evaluator  43  is arranged to determine when conditions actually exist that require the four wheel drive mode to be assumed immediately. 
     In the present embodiment, when the reactive evaluator  43  determines that the amount of longitudinal wheel slip exceeds a prescribed threshold, or the amount of yaw rate error (indicting lateral wheel slip) exceeds a prescribed threshold the vehicle is arranged to assume the four wheel drive mode immediately and at a relatively rapid rate. 
     Similarly, if the reactive evaluator  43  determines that any one of ABS system  50 , DSC system  60  and TCS system  70  is active the reactive evaluator  43  controls the vehicle to assume the four wheel drive mode at a relatively rapid rate. In some embodiments the reactive evaluator  43  is arranged to determine a severity of the response of the ABS system  50 , DSC system  60  and/or TCS system  70  before deciding whether to trigger a transition to the second mode. 
     The reactive evaluator  43  provides a control signal R 2  to a control module  47  configured to control the vehicle to transition between the two wheel drive and four wheel drive modes. The control signal R 2  is responsive to the rate at which the transition from the two wheel drive mode to the four wheel drive mode should be made. Thus the control module  47  is able to determine when and at what rate a transition to the four wheel drive mode is required. 
     The controller  40  also has a predictive evaluator  45  embodied in computer software code that is run by a computing device of the controller  40 . In the embodiment shown in  FIG. 2  the computing device is the same computing device as the device running the software embodying the reactive evaluator  43 . Alternatively the computing device may be a different computing device. 
     Like the reactive evaluator  43 , the predictive evaluator  45  is also arranged to evaluate signals (signals S 6 , . . . , SN) received by the controller  40  corresponding to the values of different respective vehicle operating parameters. 
     However, rather than determining when conditions actually exist for which four wheel drive operation is required as in the case of the reactive evaluator  43 , the predictive evaluator  45  is arranged to determine when conditions exist that indicate that a transition from the two wheel drive mode to the four wheel drive mode is likely to be required within a prescribed period of time based on the values of one or more operating parameters, which may be: 
     (a) an instantaneous amount of torque being delivered through the driveline to one or more wheels, 
     (b) a steering wheel angle, 
     (c) a steerable road wheel angle, 
     (d) a rate of change of steerable road wheel angle, 
     (e) a rate of change of steering wheel, 
     (f) a lateral acceleration of the vehicle, 
     (g) a rate of change of lateral acceleration, 
     (h) a throttle or accelerator pedal position, 
     (i) a rate of change of throttle or accelerator pedal position, 
     (j) a driver demanded torque, 
     (k) a prime mover means torque, 
     (l) an amount of wheel slip, 
     (m) a vehicle acceleration, 
     (n) a vehicle deceleration, 
     (o) a yaw rate, 
     (p) a yaw rate error, 
     (q) a gear shift position value of a manual or automatic gear shift control, 
     (r) an operating temperature, 
     (s) an ambient temperature, 
     (t) a temperature of a vehicle component, 
     (u) a temperature of a vehicle fluid, 
     (v) a temperature of a component of the auxiliary driveline, 
     (w) a temperature of a fluid of the auxiliary driveline, 
     (x) a speed of the vehicle, 
     (y) the identity of a vehicle operating program being executed by the vehicle, 
     (z) a roughness of a driving surface over which the vehicle is moving, 
     (a1) a current or target gear of an automatic transmission, 
     (b1) a brake pedal position value is greater than a first prescribed brake pedal position threshold, and 
     (c1) a brake pressure value is greater than a first prescribed brake pressure threshold. 
     The predictive evaluator  45  may also have as an input one or more of the signals fed to the reactive evaluator. 
     It is to be understood that in some embodiments the predictive evaluator  45  may be arranged to determine one or more of the above operating parameters (such as yaw rate error) based on the value of one or more other operating parameters rather than receiving an input signal specifically representing the value of that parameter. 
     When the predictive evaluator  45  determines that a transition from the first mode to the second mode is likely to be required the predictive evaluator  45  is arranged to provide an output signal P 1  to trigger a transition from the two wheel drive mode to the four wheel drive mode. 
     As shown in  FIG. 2 , the output signal P 1  of the predictive evaluator  45  is combined with the output signal R 1  of the reactive evaluator  43  in the logical OR operation performed by the logical OR operator  46 . The output of the logical OR operator  46  is fed to control module  47  which controls the vehicle to transition to the second mode as described above. 
     Thus the vehicle  1  may be arranged (by means of the predictive evaluator  45 ) to assume the four wheel drive mode in advance of conditions actually occurring that would cause the reactive evaluator  43  to trigger a transition to the four wheel drive mode. 
     In some embodiments the predictive evaluator  45  is arranged to determine a probability that a transition to the four wheel drive mode will be required and to generate an output responsive to the probability so determined. 
     It can be seen in  FIG. 2  that the predictive evaluator also provides an output signal P 2  to the control module  47 . The output signal P 2  is recognised by the control module  47  as indicating that the transition from the two wheel drive mode to the four wheel drive mode has been triggered by the predictive evaluator  45 . The control module  47  may therefore control the vehicle to transition to the four wheel drive mode at a corresponding rate. This rate may be lower than the rate required by the reactive evaluator  43 . 
     In some embodiments control line P 2  may also provide an indication as to the rate at which the transition to the four wheel drive mode should be made. 
     In some embodiments the control module  47  also checks control signal line R 2  to see if the reactive evaluator  43  has also triggered the vehicle  1  to assume the four wheel drive mode. If the value of R 2  indicates that a higher rate of connection is required than that which would otherwise be employed by the control module  47  responsive to P 2 , the control module  47  controls the vehicle  1  to assume the four wheel drive mode at the higher rate required by control signal R 2 . 
     Thus if the reactive and predictive evaluators both trigger a transition to the four wheel drive mode substantially simultaneously and prescribe different respective rates, the control module  47  is arranged to trigger the transition to the four wheel drive mode at the higher of the two rates. 
     In some embodiments the vehicle is configured such that if whilst a transition to the second mode is occurring at one rate, a control input is received requesting a transition at a higher rate, the vehicle continues the transition at the higher of the two rates. 
     Examples of conditions that may cause the predictive evaluator  45  to trigger a transition from the two wheel drive mode to the four wheel drive mode include (a) the condition that the amount of torque applied to a stationary wheel of the vehicle exceeds a prescribed value (suggesting the vehicle may be seeking to launch whilst on a hill with a heavy load and therefore an increased risk of skidding exists) and (b) that the speed of the vehicle exceeds a prescribed value for a given steerable road wheel angle (suggesting the vehicle is cornering at speed with an increased risk of skidding). 
     It is to be understood that the reactive evaluator  43  may not trigger a transition to the four wheel drive mode under condition (a) since the wheel is not actually slipping. It is to be further understood that the fact that the predictive evaluator  45  determines that a risk of wheel slip exists and triggers the transition to the four wheel drive mode of operation before wheel slip actually takes place reduces a risk that the vehicle  1  will subsequently experience wheel slip (longitudinal or lateral). 
     It is to be understood that the reactive evaluator  43  may also not trigger a transition to the four wheel drive mode under condition (b) above if the vehicle  1  is not actually skidding as it negotiates a corner. Again, the fact that the predictive evaluator  45  determines that a risk of skidding exists and triggers the transition to the four wheel drive mode of operation before a skid actually takes place reduces a risk that the vehicle  1  will experience wheel slip (longitudinal or lateral) as it negotiates the corner. 
     It is to be understood that the reactive evaluator  43  is required to process a substantial amount of information continually in order to determine whether or not conditions requiring an immediate transition from the two wheel drive mode to the four wheel drive mode exist. 
     By providing a predictive evaluator  45  in addition to the reactive evaluator  43 , the reactive evaluator  43  may continue rapidly to process vehicle operating condition information in respect of whether an immediate transition from the two wheel drive mode to the four wheel drive mode is required whilst the predictive evaluator  45  performs potentially more sophisticated calculations in respect of the values of vehicle operating parameters in order to determine whether conditions are likely to exist in the future that will require a transition to the four wheel drive mode. 
     It is to be understood that wheel slip is undesirable at least in part because excessive wear of tyres or one or more other components of the driveline  5  may occur as well as damage to the driving surface. A vehicle may become immobilised on a driving surface following a wheel spin event in which the surface is modified sufficiently to prevent traction. 
     It is to be understood that the predictive evaluator  45  may perform more sophisticated calculations taking a longer period of time to complete than those of the reactive evaluator  43  without compromising a response time of the vehicle  1  in assuming the four wheel drive mode responsive to the existence of wheel slip because the reactive evaluator  43  continues to function in parallel with the predictive evaluator  45 . 
     In some embodiments the predictive evaluator  45  is arranged to determine whether a transition to the four wheel drive mode may be required responsive to a history of a driving style of a driver of the vehicle  1 . Thus if the predictive evaluator  45  determines that wheel slip is occurring frequently when the vehicle  1  first launches from a stationary condition, the predictive evaluator  45  may be arranged to control the vehicle automatically to assume the four wheel drive mode of operation when the vehicle is stationary. The predictive evaluator  45  may be arranged to consider the history of the driving style of the user over a prescribed historical time period or a prescribed number of previous drivecycles. Other arrangements and configurations are also useful. 
     Similarly, the predictive evaluator  45  may determine that an amount of lateral acceleration of the vehicle  1  is fluctuating repeatedly between relatively high values and relatively low values. Such fluctuations may suggest for example that the vehicle  1  is negotiating a winding road. The predictive evaluator  45  may therefore control the vehicle  1  to assume the four wheel drive mode on the basis that an increased probability exists that the reactive evaluator  43  will trigger a transition to the four wheel drive mode of operation. 
     The historical time period over which evaluation takes place may be a period of the order of a few seconds, a few minutes, a few tens of minutes, a few hours, a few days, a few weeks or any other suitable period of time. The length of the historical time period employed may also be responsive to the particular operating parameter(s) under consideration. 
     In some embodiments, the reactive evaluator  43  is arranged only to output a signal requesting a transition to the second mode at a relatively high rate when it determines that a transition to the second mode is required. In contrast, the predictive evaluator  45  may be arranged to output a signal requesting a transition to the second mode at either a relatively high rate or a relatively low rate when the predictive evaluator  45  determines that a transition to the second mode is required. 
     The relatively high rate instructed by the reactive evaluator  43  may be substantially the same as the relatively high rate instructed by the predictive evaluator  45 . Alternatively it may be higher or greater. 
     In some embodiments the predictive evaluator  45  is arranged to output a signal requesting a relatively fast transition if the predictive evaluator  43  determines that the transition to the second mode should be made and prescribed conditions are met in respect of one or more of the accelerator pedal position, the engine speed and the currently selected gear. 
     If the prescribed conditions are not met, the predictive evaluator  45  is arranged to output a signal requesting a relatively slow transition if the predictive evaluator  45  determines that the transition to the second mode should be made. 
     In some arrangements, if the accelerator pedal position exceeds a prescribed position when the vehicle  1  is in a prescribed gear and the engine speed is above a prescribed speed the predictive evaluator  45  is arranged to request a transition at a relative high rate. For example, if the engine speed exceeds 3000 revolutions per minute (rpm) or any other suitable number. 
     The critical throttle position may vary with selected gear. For example the critical position may be 70% depression when in first gear, 80% depression when in second gear and 95% depression when in third gear. Other depression values are also useful. 
     Other arrangements are also useful in respect of triggering by the reactive evaluator  43  or the predictive evaluator  45  a transition from the first mode to the second mode. 
     It is to be understood that reference to a ‘relatively high’ (or ‘HIGH) rate is to be understood to be reference to a rate that is higher than a ‘relatively low’ (or ‘LOW’) rate, and reference to a ‘relatively low’ (LOW) rate is to be understood to be reference to a rate that is lower than a ‘relatively high’ (HIGH) rate. 
     Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps. 
     Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
     Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. 
     This application claims priority from UK patent application no. GB1102825.5 filed 18 Feb. 2011, the entire contents of which are expressly incorporated by reference herein.