Abstract:
An engine drive torque is distributed to an extra wheel at a predetermined distribution ratio. When the vehicle makes a turn at low speed and when the torque distribution ratio to the extra drive wheel is high, a torsion is produced in the vehicle body, suspension and tires, etc., due to a rotation radius difference between the main and extra drive wheels. After the vehicle has stopped, and the engine is stopped, torque is no longer distributed to the extra drive wheel, the torsion is suddenly released and the vehicle vibrates. According to this invention, the release of torsion is performed gradually by decreasing the torque distribution ratio when the vehicle has stopped, with the engine still running in order to prevent this vibration.

Description:
FIELD OF THE INVENTION 
     This invention relates to distribution control in a torque distributor mechanism which distributes an engine drive torque between main drive wheels and extra drive wheels according to driving conditions. 
     BACKGROUND OF THE INVENTION 
     A device which varies the distribution ratio of a drive torque between the main drive wheels and extra drive wheels of a four-wheel drive vehicle is disclosed for example in Tokkai Hei 2-270641 published in 1990 by Japanese Patent Office. 
     In this device, the main drive wheels are directly driven, while the extra drive wheels and engine drive shaft are connected via a wet type multi-plate friction clutch. 
     The torque distribution ratio to the extra drive wheels is varied by varying the grip force of the clutch according to a control signal. The grip force of the clutch is increased the larger the difference between the rotation speeds of the front and rear wheels, i.e. the larger the slip of the rear wheels which are the main drive wheels. As a result, the distribution ratio of the drive force to the front wheels, which are the extra drive wheels, is increased, and the slip of the rear wheels is promptly terminated. 
     In such a four-wheel drive vehicle, when the vehicle makes a turn during four-wheel drive, the difference in the rotation radius of the front and rear wheels causes a torsion in the suspension, tires and vehicle body. 
     If the driver stops the vehicle in the turn and turns off the engine, the grip force of the clutch falls to zero because the clutch grip force comes from an oil pump driven by the engine, and as the front wheels suddenly become free, the torsion in the suspension, tyres and vehicle body is immediately released. The release of this torsion causes the vehicle body to vibrate and oscillate in a top/bottom and front/back direction, and this gives an uncomfortable feeling to the driver and passengers of the vehicle. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to prevent a vehicle body from vibrating and oscillating when an engine stopped while the vehicle is making a turn. 
     In order to achieve the above object, this invention provides a drive torque distribution control system for distributing engine power between a main drive wheel and an extra drive wheel of a vehicle. The system comprises a torque distribution mechanism for transmitting an engine torque to the extra drive wheel at a predetermined distribution ratio, a mechanism for detecting a vehicle speed, a mechanism for setting a first value to the predetermined ratio at a high vehicle speed, and setting a second value higher than the first value to the predetermined ratio at a low vehicle speed, a mechanism for detecting a specific operation by a driver that is performed preceding to turn off the engine, and a mechanism for setting a third value less than the second value to the predetermined ratio when the operation is detected. 
     It is preferable that the third value is set larger than the first value. 
     It is also preferable that the vehicle speed detecting mechanism comprises a mechanism for detecting a rotation speed of the extra drive wheel. 
     It is also preferable that the specific operation detecting mechanism comprises a mechanism for detecting an operation of a parking brake with which the vehicle is provided. 
     This invention also provides a drive torque distribution control system for a vehicle which is provided with an automatic transmission. This system comprises a torque distribution mechanism for transmitting an output torque of the transmission to the extra drive wheel at a predetermined distribution ratio, a mechanism for detecting a vehicle speed, a mechanism for setting a first value to the predetermined ratio at a high vehicle speed, and setting a second value higher than the first value to the predetermined ratio at a low vehicle speed, a mechanism for detecting that the automatic transmission is in a speed change position corresponding to the stopping of the vehicle, and a mechanism for setting a third value less than the second value to the predetermined ratio when the automatic transmission is changed over to a speed change position corresponding to the stopping of the vehicle. 
     In this system it is preferable that the mechanism for detecting that the automatic transmission is in a speed change position corresponding to the stopping of the vehicle comprises an inhibitor switch for generating a signal according to a position of a shift lever of the transmission. 
     It is also preferable that the third value is set larger than the first value. 
     This invention also provides a vehicle drive torque distribution control system comprising a torque distribution mechanism for transmitting an engine torque to the extra drive wheel at a predetermined distribution ratio, a vehicle speed detector, a detector for detecting that the vehicle has stopped, and a microprocessor programmed to set a first value to the predetermined ratio at a high vehicle speed, set a second value higher than the first value to the predetermined ratio at a low vehicle speed, and set a third value less than the second value to the predetermined ratio when a specific operation is performed by a driver preceding to turn off the engine. 
     The specific operation may comprises an operation of a parking brake with which the vehicle is provided. 
     This invention also provides a drive torque distribution control system for a vehicle which is provided with an automatic transmission. The system comprises a torque distribution mechanism for transmitting an engine torque to the extra drive wheel at a predetermined distribution ratio, a vehicle speed detector, a detector for detecting that the vehicle has stopped, and a microprocessor programmed to set a first value to the predetermined ratio at a high vehicle speed, set a second value higher than the first value to the predetermined ratio at a low vehicle speed, and set a third value less than the second value to the predetermined ratio when the automatic transmission is changed over to a speed change position corresponding to the stopping of the vehicle. 
     The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a drive torque distribution controller according to this invention. 
     FIG. 2 is a block diagram showing connections between a control unit and sensors according to this invention. 
     FIG. 3 is a flowchart describing a torque distribution control process performed by the control unit. 
     FIGS. 4A-4D are timing charts describing changes of a clutch grip torque and vehicle speed with reference to a state of an ignition switch and inhibitor switch under the torque distribution control by the controller. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 of the drawings, a drive torque of an engine  1  of a four-wheel drive vehicle is input to a transfer  11  via an automatic transmission  2 . The output torque of the transmission  2  is directly transmitted to rear wheels  6 , which are the main drive wheels, via a transfer input shaft  3 , the transfer  11 , a rear propeller shaft  4  and rear differential  5 . 
     The drive torque distributed to an output shaft  7  of the transfer  11  is transmitted to front wheels  10 , which are the extra drive wheels, via a front propeller shaft  8  and front differential  9 . 
     A wet type multi-plate friction clutch  11 A, which is a torque distribution mechanism, is housed in the transfer  11  between the transfer input shaft  3  and transfer output shaft  4 . 
     The clutch force of the wet type multi-plate friction clutch  11 A is applied by a control hydraulic pressure Pc supplied by a control hydraulic pressure generator  20 . The value of the control hydraulic pressure Pc is controlled according to a solenoid drive current i output to a solenoid valve  28  of the control hydraulic pressure generator  20  from a control unit  40 . 
     The control hydraulic pressure generator  20 , which is activated when a relief switch  21  is turned on, comprises a stop motor  22  and a hydraulic pump  24  which pressurizes hydraulic fluid in a reservoir tank  23  due to the running of this motor  22 . 
     An accumulator  26  is connected to a discharge port of the hydraulic pump  24  via a check valve  25 . The discharge pressure of the hydraulic pump  24  is accumulated as a primary pressure by the accumulator  26 , and the cumulative pressure of the accumulator  26  is supplied to the solenoid valve  28  as a secondary pressure or line pressure. The relief switch  21  turns on and off according to this line pressure. 
     The control hydraulic pressure Pc is supplied to the wet type multi-plate friction clutch  11 A of the transfer  11  via a pipe  29 . The wet type multi-plate friction clutch  11 A updates the grip force of the clutch plates according to the control hydraulic pressure Pc. The distribution ratio of the output torque of the transmission  2  to the transfer output shaft  7  varies according to this grip force. 
     The control unit  40 , which comprises a microcomputer, outputs the solenoid drive current i based on signals input from sensors  30 , an inhibitor switch  50  and an ignition switch  51 . 
     The sensors  30  comprise a left front wheel rotation sensor  30 A, right front wheel rotation sensor  30 B, left rear wheel rotation sensor  30 C, right rear wheel rotation sensor  30 D, first lateral acceleration sensor  30 E, second lateral acceleration sensor  30 F and accelerator opening sensor  30 G as shown in FIG. 2, and signals are input to the control unit  40  from these sensors. The first lateral acceleration sensor  30 E detects acceleration of a front wheel axis as a first lateral acceleration Y G1  and the second lateral acceleration sensor  30 F detects acceleration of a rear wheel axis as a second lateral acceleration Y G2 . 
     The inhibitor switch  50  outputs a signal according to the position of a shift lever of the automatic transmission  2 . 
     When the shift lever is in a parking position “P” or a neutral position “N”, this signal is an ON signal and when the shift lever is in a drive range “D” or reverse position “R”, this signal is an OFF signal. 
     When the engine is running, the ignition switch  51  outputs an ON signal, and when the engine  1  stops, the ignition switch  51  outputs an OFF signal. 
     Based on these input signals, the control unit  40  outputs the control current i to the solenoid valve  28 , and varies the grip force of the wet type multi-plate friction clutch  11 A via the control hydraulic pressure Pc supplied thereto. 
     The process of controlling the distributed torque to the extra drive wheels  10  by the control unit  40  will next be explained referring to the flowchart of FIG.  3 . 
     This flowchart is executed at a fixed interval. 
     In a step S 1 , a right front wheel speed V WFR , left front wheel speed V WFL , right rear wheel speed V WRF , left rear wheel speed V WRL , first lateral acceleration Y G1 , second lateral acceleration Y G2  and accelerator opening q are read from the sensors  30 . Signals are read also from the inhibitor switch  50  and ignition switch  51 . 
     In a step S 2 , the average value Y G  of the first lateral acceleration Y G1  and second lateral acceleration Y G2  is calculated by the following equation.          Y   G     =         Y   G1     +     Y   G2       2                            
     In a step S 3 , a rotation speed V WR  of the rear wheels  6  is calculated by averaging the left-hand wheel speed V WRL  and right-hand wheel speed V WRR.            V   WR     =         V   WRL     +     V   WRR       2                            
     In a step S 4 , a rotation speed V WF  of the front wheels  10  is calculated by averaging the left front wheel speed V WFL  and right front wheel speed V WFR . Herein, the rotation speed of the front wheels  10  is considered as a value corresponding to the vehicle speed VSP.          V   WF     =         V   WFL     +     F   WFR       2                            
     In a step S 5 , a rotation speed difference ΔV W  between the rotation speed V WR  of the rear wheels  6  and the rotation speed V WF  of the front wheels  10  is computed. 
      ΔV W =V WR −V WF   
     In a step S 6 , a grip force TΔV of the wet type multi-plate friction clutch  11 A is found for example by looking up a table built into the controller  40 , based on the rotation speed difference ΔV W  and lateral acceleration Y G . According to this table, the grip force TΔV increases and the distribution ratio of the drive torque to the front wheels  10  increases the larger the rotation speed difference ΔV W . Also according to the table, the distribution ratio of drive torque to the front wheels  10  decreases the larger the lateral acceleration Y G . 
     This is in order to avoid a so-called tight comer braking phenomenon on roads with a high frictional coefficient when Y G  is large. Tight corner braking is a phenomenon which occurs when a large drive torque is supplied to the front wheels when the vehicle is making a turn. In this situation, it is difficult to turn due to the difference of rotation radius of the front and rear wheels, and the driver feels as if braking is applied. 
     In a step S 7 , it is determined whether or not the vehicle speed VSP (=V WF ) is higher than a predetermined speed. 
     Herein, it is desirable to set this predetermined speed to the minimum detectable speed, e.g. if the minimum speed that can be detected by the front wheel rotation sensors  30 A and  30 B is 4 Km/hr, the predetermined speed is set to 4 km/hr. 
     When the vehicle speed VSP is equal to or higher than the predetermined speed, a second minimum value TV of the grip force of the wet type multi-plate friction clutch  11 A is for example set to 4 kgm in a step S 11 . 
     On the other hand, when the vehicle speed VSP is less than 4 Km/hr, the routine proceeds to a step S 8 , and a first or third minimum value of the grip force is set according to the state of the inhibitor switch  50 . 
     In the step S 8 , it is determined whether or not the vehicle is running. 
     When the inhibitor switch  50  is ON, i.e. when the shift lever is at “P” or “N”, it is determined that the vehicle is not running and the routine proceeds to a step S 9 . 
     In the step S 9 , the third value of the grip force of the torque distribution clutch  11 A is set to, for example 5 kgm. 
     When the inhibitor switch  50  is OFF, it is determined that the vehicle is running at low speed, the routine proceeds to a step S 10 , and the first minimum value of the grip force is set to for example 10 kgm to maintain starting stability on a slippery road surface as in the aforesaid prior art. 
     As a result, up to a vehicle speed of 4 Km/hr after the vehicle starts, the minimum value TV of grip force is set at a comparatively high value, and when the vehicle speed VSP exceeds the predetermined speed of 4 Km/hr, the minimum value TV falls to a lower value. 
     Next, in a step S 12 , the larger of the grip force TΔV found in the step S 6  and the minimum value TV set in the steps S 9 -S 11 , is set as a target grip force T 1 . 
     In a step S 13 , this target grip force T 1  is converted into the solenoid current i, and is output to the solenoid valve  28 . 
     The solenoid valve  28  increases or decreases the control hydraulic pressure Pc according to this solenoid current i so as to obtain a target grip force. 
     Next, the variation of this grip force under this control process will be described with reference to FIGS. 4A-4D. 
     The engine  1  is started at a time t 0  when the vehicle is not running, the vehicle starts running at a time t 1  and stops running at a time t 4 , and the engine  1  stops at a time t 5 . 
     First, as the shift lever is in the parking position “P” or neutral position “N” when the vehicle is not running, the inhibitor switch is ON when the engine  1  starts at the time t 0 . 
     The minimum value TV of the grip force of the wet type multi-plate friction clutch  11 A is therefore set to 5 kgm in the process of steps S 8  and S 9 . When the main drive wheels  6  do not slip or slip only very little, the wet type multi-plate friction clutch  11 A is controlled by this minimum TV. 
     When the vehicle starts at the time t 1 , the shift lever is set to the drive position “D” or updating position “R”, and the inhibitor switch  50  switches OFF. Consequently, from when the vehicle starts to when the vehicle speed VSP reaches the predetermined value of 4 Km/h, the minimum value TV of the grip force of the wet type multi-plate friction clutch  11  A is set to 10 kgm, which is the first minimum value, by the process of the steps S 8  and S 10 . 
     As the minimum value TV of the wet type multi-plate friction clutch  11 A is set to the first minimum value, which is the largest among the minimum values, a sufficiently large drive torque is distributed to the front wheels  10 , which are the extra drive wheels, when the vehicle starts. Therefore, racing of the rear wheels  6  which are the main drive wheels can be prevented even if the road surface is slippery. 
     At a time t 2 , the vehicle speed VSP reaches the predetermined value of 4 Km/h. Subsequently, the minimum value TV of the grip force decreases to 4 kgm, which is the second minimum value, in the process of steps S 7  and S 11 . 
     As the minimum value of the grip force is suppressed, excessive drive torque is not distributed to the front wheels  10  in this running state as far as the drive wheels do not suffer from slipping. 
     When a large drive torque is supplied to the front wheels  10  when the vehicle is making a turn, the aforesaid tight corner braking occurs, but, according to this drive torque distribution controller, as the minimum value TV of the grip force when the vehicle is traveling at a speed of 4 km/hr or more is set as low as 4 kgm, tight corner braking is not likely to occur. 
     When the vehicle is stopped, firstly at a time t 3  when the vehicle speed VSP is less than 4 Km/h, the minimum value TV of grip force is updated to the first minimum value of 10 kgm in the steps S 7 , S 8  and S 10 . As a result, the distribution ratio of drive torque to the front wheels  10  increases. 
     At the time t 4 , the vehicle speed VSP=0 again, the shift lever changes to the parking position “P” or the neutral position “N”, and the signal from the inhibitor switch  50  is ON. Therefore the minimum value TV of grip force decreases to 5 kgm, which is the third minimum value, in the process of steps S 7 -S 9 . 
     At the time t 5 , the ignition switch is OFF, and the engine  1  stops. As the hydraulic pressure supplied by the control hydraulic pressure generator  20  also stops, the grip force is 0, and the grip of the wet type multi-plate friction clutch  11 A is released. Due to this operation, the drive mode changes from a four-wheel drive to a two-wheel drive by the rear wheels  6  alone. 
     When the vehicle is stopped during a turn and the engine is stopped, a torsion is produced in the suspension, tires or vehicle body due to a difference of rotation radius of the inner and outer wheels or of the front and rear wheels in the turn. When, due to the engine stopping, the pressure generated by the control hydraulic pressure generator  20  becomes zero, this torsion is released. However according to this drive torque distribution controller, the minimum value of the grip force of the wet type multi-plate friction clutch  11 A first decreases to 5 kgm which is the third minimum value, due to the shift of the shift lever to the stop position, and it then decreases to zero when the engine stops. The release of torsion in the vehicle body is therefore gradual, compared to the case where the minimum value of the grip force suddenly falls to zero from the first minimum value of 10 kgm. 
     As a result, even when the vehicle stops during a turn, vibration or oscillation of the vehicle body due to sudden release of this torsion is suppressed, and the pleasant feeling of traveling in the vehicle is not impaired. 
     According to this embodiment, the third minimum value was set at 5 kgm, but if this value is set even lower, the torsion in the vehicle body may be released more rapidly after the vehicle turns and stops. If however this third minimum value is set lower than 4 kgm which is the second minimum value required to prevent tight corner braking, the distribution ratio of drive torque to the front wheels  10  decreases when the vehicle restarts after stopping. As a result, the starting stability of the vehicle on a slippery road surface may be adversely affected. 
     In order to ensure starting stability when the vehicle restarts after stopping, and to prevent vibration of the vehicle body after a turn and a stop, it is therefore desirable to set the third minimum value less than the first minimum value of 10 kgm and equal to or greater than the second minimum value of 4 kgm. 
     According to the above embodiment, the stopping of the vehicle was detected from the ON state of the inhibitor switch  50 , however it may also be detected from the state of the side brake, etc. 
     The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows: