BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a drive force control method for a four-wheel drive vehicle. 
   2. Description of the Related Art 
   In the case of turning a corner having a small turning radius in a four-wheel drive mode of a four-wheel drive vehicle in a low to medium vehicle speed range, a difference in rotational speed due to a difference in turning radius is generated between the front and rear wheels of the vehicle, causing a tight corner braking phenomenon. As the prior art for eliminating such a tight corner braking phenomenon, front and rear wheels driving devices are disclosed in Japanese Patent Publication Nos. 7-61779 and 7-64219. 
   The front and rear wheels driving devices disclosed in these publications have such a structure that a speed increasing device is provided between main drive wheels and auxiliary drive wheels to thereby adjust an average rotational speed of the auxiliary drive wheels to an average rotational speed of the main drive wheels. This speed increasing device includes a lockup clutch and a speed increasing clutch, which are selectively switched between ON and OFF states to thereby obtain a lockup condition or same speed condition where the average rotational speed of the main drive wheels and the average rotational speed of the auxiliary drive wheels are substantially equal to each other or an increasing speed condition where the average rotational speed of the auxiliary drive wheels is greater than the average rotational speed of the main drive wheels. 
   Particularly in the front and rear wheels driving device disclosed in Japanese Patent Publication No. 7-61779, a torque distribution ratio between right and left rear wheels are controlled according to a vehicle speed and a steering angle so that the rear wheel torque is larger than the front wheel torque and the turning outer wheel torque is larger than the turning inner wheel torque. In this front and rear wheels driving device, the auxiliary drive wheels are increased in rotational speed by the speed increasing device in turning a corner having a small turning radius in the four-wheel drive mode, thereby preventing the tight corner braking phenomenon. 
   The object of distributing a drive force between right and left wheels in turning is to give a larger drive force to the outer wheel as compared with the inner wheel, thereby generating a turning moment to suppress understeer occurring at acceleration during turning. However, the expectable effect of the drive force distribution between right and left wheels may vary according to a vehicle speed. For example, there is a tendency to desire stabilization of a vehicle body rather than improvement in maneuverability by giving a large turning moment at high vehicle speeds. However, minute control of the drive force distribution between right and left wheels according to a vehicle speed is not sufficiently disclosed in the above publications. 
   By distributing a driving/braking force generated by an engine in a four-wheel drive vehicle between front wheels and rear wheels, the load on each wheel can be controlled to maximize the utilization of tire performance. Particularly at acceleration of a two-wheel drive vehicle, a drive force so large as to cause skid of the wheels is generated. It is therefore very effective to make the two-wheel drive vehicle into a four-wheel drive vehicle. 
   In also taking a turning performance into consideration, the performance can be improved by changing the drive force distribution ratio between front and rear wheels so that the drive force distributed to the rear wheels is greater than that to the front wheels. However, when a vehicle speed is increased, a drive force and an engine brake force generated by a vehicle body are decreased in general, so that the effect of changing the drive force distribution ratio between front and rear wheels is small. Changing the torque distribution ratio of rear wheels to front wheels according to a vehicle speed, accelerator opening, transmission shift position, etc. is not sufficiently described in the above publications. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide a drive force control method for a four-wheel drive vehicle which can accurately control a drive force distribution ratio between right and left front wheels or between right and left rear wheels. 
   It is another object of the present invention to provide a drive force control method for a four-wheel drive vehicle which can accurately control a drive force distribution ratio between front wheels and rear wheels according to a vehicle speed, accelerator opening, transmission shift position, etc. 
   In accordance with a first aspect of the present invention, there is provided a drive force control method for a four-wheel drive vehicle having main drive wheels connected to a driving source, auxiliary drive wheels whose drive torque is adjustable, and a mechanism capable of adjusting a torque distribution ratio between said main drive wheels and said auxiliary drive wheels so that the torque distribution ratio of said auxiliary drive wheels to said main drive wheels is increased in turning and also capable of adjusting a torque distribution ratio between said right and left auxiliary drive wheels in turning, said drive force control method including the steps of detecting a vehicle speed; and gradually decreasing the torque distribution ratio of a turning outer wheel as one of said right and left auxiliary drive wheels to a turning inner wheel as the other with an increase in said vehicle speed. 
   The object of distributing a drive force between right and left wheels in turning is to give a larger drive force to the outer wheel as compared with the inner wheel, thereby generating a turning moment to suppress understeer occurring at acceleration during turning. However, the expectable effect of the drive force distribution between right and left wheels may vary according to a vehicle speed. For example, there is a tendency to desire stabilization of a vehicle body rather than improvement in maneuverability by giving a large turning moment at high vehicle speeds. 
   According to the first aspect of the present invention, the torque distribution ratio of the turning outer wheel to the turning inner wheel is gradually decreased with an increase in vehicle speed. Accordingly, torque steps or the like can be eliminated to reduce vibrations, noise, shock, etc. 
   In accordance with a second aspect of the present invention, said drive force control method further includes the step of decreasing the torque distribution ratio of said turning outer wheel to said turning inner wheel in a low-speed position or a high-speed position as a transmission shift position. 
   When the transmission shift position is a low-speed position or a high-speed position, the necessity to improve the turning performance is lowered and constant control is therefore sufficient. According to the second aspect of the present invention, the torque distribution ratio of the turning outer wheel to the turning inner wheel is decreased when the transmission shift position is a low-speed position or a high-speed position. Accordingly, undue changes in input to the differential device for the auxiliary drive wheels can be reduced to thereby suppress the generation of noise, shock, and vibrations. 
   In general, the maneuverability of a vehicle as in forward running is not desired in reverse running. Therefore, drive force distribution control between right and left wheels similar to that in forward running is not desired. In particular, it is not necessary to improve the turning performance in reverse running, and constant control is therefore sufficient. 
   In accordance with a third aspect of the present invention, said drive force control method further comprises the step of decreasing the torque distribution ratio of said turning outer wheel to said turning inner wheel in reverse running. 
   Since the torque distribution ratio of the turning outer wheel to the turning inner wheel is decreased in reverse running, undue changes in input to the differential device for the auxiliary drive wheels can be reduced to thereby suppress the generation of noise, shock, and vibrations. 
   In accordance with a fourth aspect of the present invention, said drive force control method further includes the step of decreasing the torque distribution ratio of said turning outer wheel to said turning inner wheel with a decrease in temperature of hydraulic fluid for a differential device for the auxiliary drive wheels. 
   When the temperature of hydraulic fluid for the differential device for the auxiliary drive wheels is low, there arises a problem such that an actual output from the mechanism in response to a control command may delay because of an increase in viscosity of the fluid. According to the fourth aspect of the present invention, the torque distribution ratio of the turning outer wheel to the turning inner wheel is decreased with a decrease in temperature of hydraulic fluid for the differential device for the auxiliary drive wheels. Accordingly, a degradation in performance due to a decrease in temperature of the hydraulic fluid can be minimized. 
   In accordance with a 5th aspect of the present invention, there is provided a drive force control method for a four-wheel drive vehicle having main drive wheels connected to a driving source, auxiliary drive wheels whose drive torque is adjustable, and a mechanism capable of adjusting a torque distribution ratio between said main drive wheels and said auxiliary drive wheels so that the torque distribution ratio of said auxiliary drive wheels to said main drive wheels is increased in turning, said drive force control method including the steps of detecting a vehicle speed; and gradually decreasing the torque distribution ratio of said auxiliary drive wheels to said main drive wheels with an increase in said vehicle speed. 
   When a vehicle speed is increased, a drive force and an engine brake force generated by a vehicle body are decreased in general, so that the effect of changing the drive force distribution ratio between main drive wheels and auxiliary drive wheels is small. According to the 5th aspect of the present invention, the torque distribution ratio of the auxiliary drive wheels to the main drive wheels is gradually decreased with an increase in vehicle speed. Accordingly, torque steps or the like can be eliminated to reduce vibrations, noise, and shock. 
   In accordance with a 6 th  aspect of the present invention, said drive force control method further includes the step of increasing the torque distribution ratio of said auxiliary drive wheels to said main drive wheels with an increase in accelerator opening. 
   At acceleration, the vertical load on the auxiliary drive wheels is increased by the influence of a longitudinal acceleration, causing an increase in performance of the auxiliary drive wheels. According to the 6th aspect of the present invention, the torque distribution ratio of the auxiliary drive wheels to the main drive wheels is effectively increased at acceleration. The operator&#39;s intention to accelerate the vehicle can be detected earliest by detecting an accelerator opening. Accordingly, by changing the torque distribution ratio between the main drive wheels and the auxiliary drive wheels according to an accelerator opening, quick-response control can be performed. 
   In accordance with a 7th aspect of the present the invention, said drive force control method further includes the step of decreasing the torque distribution ratio of said auxiliary drive wheels to said main drive wheels in a low-speed position or a high-speed position as a transmission shift position. 
   When the transmission shift position is a low-speed position or a high-speed position, the necessity to improve the turning performance is lowered and constant control is therefore sufficient. According to the 7th aspect of the present invention, the torque distribution ratio of the auxiliary drive wheels to the main drive wheels is decreased when the transmission shift position is a low-speed position or a high-speed position. Accordingly, undue changes in input to the differential device for the auxiliary drive wheels can be reduced to thereby reduce noise, shock, and vibrations. 
   In accordance with an 8th aspect of the present the invention, said drive force control method further includes the step of decreasing the torque distribution ratio of said auxiliary drive wheels to said main drive wheels in reverse running. 
   In general, the maneuverability of a vehicle as in forward running is not desired in reverse running. Therefore, drive force distribution control between main drive wheels and auxiliary drive wheels similar to that in forward running is not desired. In particular, it is not necessary to improve the turning performance in reverse running, and constant control is therefore sufficient. According to the 8 th  aspect of the present invention, the torque distribution ratio of the auxiliary drive wheels to the main drive wheels is decreased in reverse running. Accordingly, undue changes in input to the differential device for the auxiliary drive wheels can be reduced to thereby suppress the generation of noise, shock, and vibrations. 
   In accordance with a 9th aspect of the present the invention, said drive force control method further includes the step of decreasing the torque distribution ratio of said auxiliary drive wheels to said main drive wheels with a decrease in temperature of hydraulic fluid for a differential device for auxiliary drive wheels. 
   When the temperature of hydraulic fluid for the differential device for the auxiliary drive wheels is low, there arises a problem such that an actual output from the mechanism in response to a control command may delay because of an increase in viscosity of the fluid. According to the 9th aspect of the present invention, the torque distribution ratio of the auxiliary drive wheels to the main drive wheels is decreased with a decrease in temperature of hydraulic fluid for the differential device for the auxiliary drive wheels. Accordingly, a degradation in performance due to a decrease in temperature of the hydraulic fluid can be minimized. 
   The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram showing a power transmitting system for a four-wheel drive vehicle to which the drive force control method of the present invention is applicable; 
       FIG. 2  is a sectional view of a speed increasing device (speed changing device) and a rear differential device; 
       FIG. 3  is a diagram showing the locus of each wheel during turning of the vehicle; 
       FIG. 4A  is a diagram showing the transmission of power to the rear wheels during straight running at acceleration; 
       FIG. 4B  is a diagram showing the transmission of power to the rear wheels during turning at acceleration; 
       FIG. 5  is a block diagram of a control system according to a preferred embodiment of the present invention; 
       FIG. 6  is a graph showing the relation between lateral G and torque distribution ratios of the outer wheel and the rear wheels; 
       FIG. 7  is a flowchart showing the processing of calculating a drive force distribution ratio between the front and rear wheels and a drive force distribution ratio between the right and left rear wheels in the preferred embodiment of the present invention; 
       FIG. 8  is a graph showing the relation between estimated slip angle and torque reducing amounts to the outer wheel and the rear wheels; 
       FIG. 9  is a flowchart showing the detection of a running condition; 
       FIG. 10  is a flowchart showing the calculation of a target rear wheel torque; 
       FIG. 11  is a flowchart showing 4 WD control according to the target rear wheel torque; 
       FIG. 12  is a graph showing the relation between vehicle speed and torque distribution to the rear wheels; 
       FIG. 13  is a graph showing the relation between accelerator opening and torque distribution to the rear wheels; 
       FIG. 14  is a graph showing the relation between shift position and torque distribution to the rear wheels; 
       FIG. 15  is a graph showing the relation between rear differential oil temperature and torque distribution to the rear wheels; 
       FIG. 16  is a flowchart showing the processing of calculating a target rear outer wheel torque; 
       FIG. 17  is a graph showing the relation between vehicle speed and torque distribution to the rear outer wheel; 
       FIG. 18  is a graph showing the relation between shift position and torque distribution to the rear outer wheel; 
       FIG. 19  is a graph showing the relation between rear differential oil temperature and torque distribution to the rear outer wheel; 
       FIG. 20  is a flowchart showing the processing of controlling the change from a lockup condition to a speed increase condition; 
       FIG. 21  is a flowchart showing the processing of controlling the change from a speed increase condition to a lockup condition; 
       FIG. 22  is a flowchart showing the processing of stabilizing the behavior of the vehicle in an unstable condition of the vehicle; 
       FIG. 23  is a graph showing the relation between shift position and permission/inhibition of the speed increase; 
       FIG. 24  is a flowchart showing the control in an engine brake condition; 
       FIG. 25  is a flowchart showing the control during braking; and 
       FIG. 26  is a flowchart showing the processing of permitting the speed increase condition after low-speed running. 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , there is shown a schematic diagram of a power transmitting system for a four-wheel drive vehicle having a speed increasing device (speed changing device)  10  based on a front-engine front-drive (FF) vehicle. As shown in  FIG. 1 , the power transmitting system for the four-wheel drive vehicle mainly includes a front differential device  6  to which the power of an engine  2  located at a front portion of the vehicle is transmitted from an output shaft  4   a  of a transmission  4 , a speed increasing device (speed changing device)  10  to which the power from the front differential device  6  is transmitted through a propeller shaft  8 , and a rear differential device  12  to which the power from the speed increasing device  10  is transmitted. 
   The front differential device  6  has a structure well known in the art, and the power from the output shaft  4   a  of the transmission  4  is transmitted through a plurality of gears  14  and output shafts  16  and  18  in a differential case  6   a  to left and right front wheel drive shafts  20  and  22 , thereby driving front wheels. As will be hereinafter described, the rear differential device  12  includes a pair of planetary gear sets and a pair of electromagnetic actuators for controlling the engagement of multiplate clutch mechanisms. The electromagnetic actuators are controlled to transmit the power to left and right rear wheel drive shafts  24  and  26 , thereby driving rear wheels. 
     FIG. 2  is a sectional view of the speed increasing device  10  and the rear differential device  12  located downstream of the speed increasing device  10 . The speed increasing device  10  includes an input shaft  30  rotatably mounted in a casing  28  and an output shaft (hypoid pinion shaft)  32 . The speed increasing device  10  further includes an oil pump subassembly  34 , a planetary carrier subassembly  38 , a lockup clutch  40 , and a speed increasing clutch (speed increasing brake)  42 . 
   When the lockup clutch  40  is engaged, the rotation of the input shaft  30  is directly transmitted to the output shaft  32  without changes in rotational speed. On the other hand, when the lockup clutch  40  is disengaged and the speed increasing clutch  42  is engaged, the rotation of the input shaft  30  is transmitted to the output shaft  32  with the rotational speed being increased by a predetermined amount. A detailed structure of the speed increasing device  10  is disclosed in Japanese Patent Application NO. 2002-278836 previously filed by the present applicant. The rear differential device  12  located downstream of the speed increasing device  10  has a hypoid pinion gear  44  formed at the rear end of the hypoid pinion shaft  32 . The hypoid pinion gear  44  is in mesh with a hypoid ring gear  48 , and the power from the hypoid ring gear  48  is input to the ring gears of a pair of left and right planetary gear sets  50 A and  50 B. 
   The sun gears of the planetary gear sets  50 A and  50 B are rotatably mounted on a left rear axle  24  and a right rear axle  26 , respectively. The planetary carriers of the planetary gear sets  50 A and  50 B are fixed to the left rear axle  24  and the right rear axle  26 , respectively. In each of the planetary gear sets  50 A and  50 B, the planetary gear carried by the planetary carrier is in mesh with the sun gear and the ring gear. The left and right planetary gear sets  50 A and  50 B are connected to a pair of left and right clutch mechanism (brake mechanisms)  51  provided to variably control the torque of the respective sun gears. Each clutch mechanism  51  includes a wet multiplate clutch (brake)  52  and an electromagnetic actuator  56  for operating the multiplate clutch  52 . 
   The clutch plates of each wet multiplate clutch  52  are fixed to a casing  54 , and the clutch discs of each wet multiplate clutch  52  are fixed to the sun gear of each of the planetary gear sets  50 A and  50 B. Each electromagnetic actuator  56  is composed of a core (yoke)  58 , an exciting coil  60  inserted in the core  58 , an armature  62 , and a piston  64  connected to the armature  62 . When a current is passed through the exciting coil  60 , the armature  62  is attracted to the core  58  by the coil  60  to thereby generate a thrust. Accordingly, the piston  64  integrally connected to the armature  62  pushes the multiplate clutch  52  to thereby generate a clutch torque. 
   Accordingly, the sun gears of the planetary gear sets  50 A and  50 B are fixed to the casing  54 , and the drive force of the hypoid pinion shaft  32  is transmitted through the ring gears, the planet gears, and the planetary carriers of the planetary gear sets  50 A and  50 B to the left and right rear axles  24  and  26 . By making variable the currents to be passed through the left and right exciting coils  60 , the output torques to the left and right rear axles  24  and  26  can be variably controlled. 
   When the lockup clutch  40  of the speed increasing device  10  is engaged and the left and right exciting coils  60  of the rear differential device  12  are off, the left and right clutch mechanisms  51  are disengaged and the sun gears of the planetary gear sets  50 A and  50 B therefore idly rotate about the left and right rear axles  24  and  26 . Accordingly, the drive force (torque) of the hypoid pinion shaft  32  is not transmitted to the left and right rear axles  24  and  26 . In this case, the rear wheels idly rotate and the drive force from the engine is fully transmitted to the front wheels, so that this four-wheel drive vehicle runs in a two-wheel drive mode. 
   When predetermined amounts of currents are passed through the left and right exciting coils  60  to completely engage the left and right multiplate clutches  52  through the pistons  64 , the sun gears of the planetary gear sets  50 A and  50 B are fixed to the casing  54 . Accordingly, the drive force of the input shaft  30  is uniformly divided by the planetary gear sets  50 A and  50 B and transmitted to the left and right rear axles  24  and  26 . As a result, this four-wheel drive vehicle runs in a four-wheel drive mode. 
   In the case of turning a corner having a small turning radius in the four-wheel drive mode in a medium vehicle speed range, the lockup clutch  40  is disengaged and the speed increasing clutch  42  is engaged. Accordingly, the rotational speed of the output shaft  32  is increased over that of the input shaft  30 . The speed increasing rate is about 5%, for example. In such a case that the vehicle is turned in the condition where the rotational speed of the output shaft  32  is increased over that of the input shaft  30 , the rear wheel on the turning outside can be rotated faster than the front wheel on the same side, so that the drive force can be transmitted to the rear wheel on the turning outside, and the turning performance in the medium vehicle speed range can be improved. 
   The loci of the front wheels and the rear wheels during turning of the vehicle will now be described with reference to  FIG. 3 . Reference numeral  66  denotes the center of turning, reference numerals  68 L and  68 R denote the left and right front wheels, respectively, and reference numerals  70 L and  70 R denote the left and right rear wheels, respectively. It is assumed that the vehicle is turned counterclockwise about the center  66 . Reference numeral  72  denotes the locus of the front inner wheel  68 L, reference numeral  74  denotes the locus of the front outer wheel  68 R, and reference numeral  76  denotes the average locus of the front wheels. Reference numeral  78  denotes the average locus of the rear wheels in the engaged condition of the lockup clutch  40 , and reference numeral  80  denotes the locus of the rear outer wheel  70 R in the engaged condition of the lockup clutch  40 . 
   In the case of turning at high lateral G as shown in  FIG. 3 , the slip angle of the rear wheels becomes larger (the cornering force becomes larger), so that the locus  80  of the rear outer wheel  70 R is larger in radius than the average locus of the rear wheels  78  in the engaged condition of the lockup clutch  40 , and the drive force (torque) is not transmitted to the rear outer wheel  70 R. In the four-wheel drive vehicle according to the present invention, the speed increasing clutch  42  of the speed increasing device  10  is engaged in this case, thereby increasing the rotational speed of the output shaft  32  by about 5% over the rotational speed of the input shaft  30 . Accordingly, the drive force (torque) can be transmitted to the rear outer wheel  70 R. Reference numeral  82  denotes the locus of the rear outer wheel  70 R in the engaged condition of the speed increasing clutch  42 . 
   Operation modes of the drive force control method according to the present invention are shown in Tables 1A and 1B. 
   
     
       
             
             
           
             
             
             
             
             
           
             
             
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 1A 
             
           
           
             
                 
                 
             
             
                 
               Forward 
             
           
        
         
             
                 
                 
               Straight 
               Left turn 
               Left turn 
             
             
                 
               Straight 
               (LSD) 
               (lockup) 
               (speed increase) 
             
           
        
         
             
                 
                 
               Accelera- 
               Decelera- 
               Accelera- 
               Decelera- 
               Accelera- 
               Decelera- 
               Accelera- 
               Decelera- 
             
             
               Element 
               Mode 
               tion 
               tion 
               tion 
               tion 
               tion 
               tion 
               tion 
               tion 
             
             
                 
             
           
        
         
             
               1 
               Speed 
               — 
               — 
               — 
               — 
               — 
               — 
               on 
               on 
             
             
                 
               increasing 
                 
             
             
                 
               clutch 
                 
             
             
               2 
               Lockup 
               on 
               on 
               on 
               on 
               on 
               on 
               — 
               — 
             
             
                 
               clutch 
                 
             
             
               3 
               Left 
               Medium 
               Small 
               Large 
               Small 
               Small 
               Small 
               Small 
               Small 
             
             
                 
               clutch 
                 
             
             
               4 
               Right 
               Medium 
               Small 
               Large 
               Small 
               Large 
               Small 
               Large 
               Small 
             
             
                 
               clutch 
                 
             
             
                 
             
           
        
       
     
   
   
     
       
             
             
           
             
             
             
           
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
                 
               TABLE 1B 
             
           
           
             
                 
                 
             
             
                 
               Reverse 
             
           
        
         
             
                 
                 
               Straight 
             
             
                 
               Straight 
               (LSD) 
             
           
        
         
             
                 
                 
               Accelera- 
               Decelera- 
               Accelera- 
               Decelera- 
             
             
               Element 
               Mode 
               tion 
               tion 
               tion 
               tion 
             
             
                 
             
           
        
         
             
               1 
               Speed 
               — 
               — 
               — 
               — 
             
             
                 
               increasing 
                 
             
             
                 
               clutch 
                 
             
             
               2 
               Lockup 
               on 
               on 
               on 
               on 
             
             
                 
               clutch 
                 
             
             
               3 
               Left 
               Medium 
               Small 
               Large 
               Small 
             
             
                 
               clutch 
                 
             
             
               4 
               Right 
               Medium 
               Small 
               Large 
               Small 
             
             
                 
               clutch 
                 
             
             
                 
             
           
        
       
     
       
       
         
           In the case of right turn, the magnitudes in the element (3) and the magnitudes in the element (4) are interchanged. 
           Conditions for turning (lockup):
           The vehicle speed is less than 30 km/h or greater than 120 km/h.   The lateral G is less than 0.075 G.   
         
         
       
     
  
   Conditions for turning (speed increase):
         The vehicle speed is 30 to 120 km/h, and the lateral G is not less than 0.075 G.       

   Small: 0 to 40 kgfm Medium: 40 to 80 kgfm Large: 80 to 110 kgfm 
   In Tables 1A and 1B, “Small”, “Medium”, and “Large” indicate the magnitudes of the engaging force of each clutch. “Small” means 0 to 40 kgfm, “Medium” means 40 to 80 kgfm, and “Large” means 80 to 110 kgfm. In the case that the vehicle speed is less than 30 km/h or greater than 120 km/h during turning, the lockup clutch  40  is engaged. Further, also in the case that the lateral G is less than 0.075 G, the lockup clutch  40  is engaged. 
   In the case that the vehicle speed is 30 to 120 km/h and the lateral G is not less than 0.075 G during turning, the speed increasing clutch  42  is engaged, so that torque transmission to the rear outer wheel is allowed. While the engaging forces of the left and right clutches  52  during left turning are shown in Table 1, the magnitudes of the engaging force of the left clutch  52  may be interchanged with the magnitudes of the engaging force of the right clutch  52  in the case of right turning. 
     FIG. 4A  shows the condition where the lockup clutch  40  is engaged at acceleration during straight running. In this condition, the torque is transmitted uniformly to the left and right rear axles  24  and  26 . In  FIGS. 4A and 4B , torque transmission paths are shown by bold lines.  FIG. 4B  shows the condition where the speed increasing clutch  42  is engaged at acceleration during left turning. In this condition, the engaging force of the right clutch  52  is controlled to become larger than the engaging force of the left clutch  52 , thereby increasing the torque distribution to the right rear axle  26 . 
   While the operational conditions shown in Tables 1A and 1B are the general outlines of the drive force control method according to the present invention, the drive force control method will now be described in detail. 
     FIG. 5  is a block diagram of a control system according to the present invention. This control system has a feed-forward control section  84 , a feedback control section  86 , and a speed increase control section  88 . Engine torque and transmission gear position are input into a block  90  in the feed-forward control section  84  to calculate a tire drive force. A vehicle speed detected by a vehicle speed sensor  92  and a steering angle detected by a steering angle sensor  94  are input into a block  96  to calculate an estimated lateral acceleration (estimated lateral G). 
   A lateral acceleration (lateral G) detected by a lateral acceleration sensor (lateral G sensor)  98  is input into a block  100  to determine a lateral acceleration (lateral G). The lateral G output from the block  100  is corrected by the estimated lateral G output from the block  96  to obtain a control lateral G signal. This correction is made by averaging the lateral G signal and the estimated lateral G signal, for example. The control lateral G signal is input into an outer wheel decision block  102  to determine which of the right and left rear wheels is an outer wheel. The control lateral G signal is also input into a block  104  to calculate a torque distribution ratio between the front and rear wheels, and is also input into a block  106  to calculate a torque distribution ratio between the right and left wheels. 
   An outer wheel signal from the outer wheel decision block  102 , a rear wheel distribution ratio signal from the block  104 , and a rear outer wheel distribution ratio signal from the block  106  are input into a block  108  to obtain a torque distribution ratio between the rear outer wheel and the rear inner wheel. The vehicle speed detected by the vehicle speed sensor  92 , the steering angle detected by the steering angle sensor  94 , the lateral G detected by the lateral G sensor  98 , and a yaw rate detected by a yaw rate sensor  110  are input into a vehicle model block  112  in the feedback control section  86  to calculate a slip angle of the vehicle. Further, a slip angle threshold is calculated by a block  114  according to the vehicle speed detected by the vehicle speed sensor  92  and the lateral G detected by the lateral G sensor  98 . 
   A rear wheel torque reducing amount is obtained by a block  116  according to a difference between the slip angle and the slip angle threshold, and an outer wheel torque reducing amount is obtained by a block  118  according to this difference. In other words, if the slip angle of the vehicle is greater than a predetermined value, it is determined that the vehicle is in an unstable condition, and the rear wheel distributed torque and the outer wheel distributed torque are reduced to eliminate this unstable condition. A left rear wheel torque command value is generated by a block  120  according to the drive torque calculated by the block  90 , the left rear wheel torque from the block  108 , the rear wheel torque reducing amount from the block  116 , and the outer wheel torque reducing amount from the block  118 , and the left electromagnetic actuator  56  is controlled by a left clutch control section  122  according to the left rear wheel torque command value generated above. 
   Similarly, a right rear wheel torque command value is generated by a block  124  according to the drive torque calculated by the block  90 , the right rear wheel torque from the block  108 , the rear wheel torque reducing amount from the block  116 , and the outer wheel torque reducing amount from the block  118 , and the right electromagnetic actuator  56  is controlled by a right clutch control section  126  according to the right rear wheel torque command value generated above. 
   A speed increase threshold is calculated by a block  128  in the speed increase control section  88  according to the vehicle speed detected by the vehicle speed sensor  92 . The estimated lateral G calculated by the block  96  and the speed increase threshold calculated by the block  128  are compared with each other, and it is determined by a block  130  that a speed increasing condition is to be provided when the estimated lateral G is greater than the speed increase threshold, whereas the lockup condition is to be provided when the estimated lateral G is not greater than the speed increase threshold. A speed increase signal or a lockup signal from the block  130  is input into a speed increasing device control section  132  to control the speed increase/lockup of the speed increasing device  10 . 
   The drive force control method of the present invention will now be described in detail. When the vehicle is accelerated during turning, the vertical loads on the inner wheels and the front wheels are reduced by the influence of lateral and longitudinal accelerations acting on the vehicle body. Further, since the front wheels are steered for turning, a lateral force acting on the front wheels is greater than that acting on the rear wheels. The greater the vertical load, the greater the drive force that can be generated by each tire. Therefore, the load on the tire of each front wheel is greater than the load on the tire of each rear wheel during turning at acceleration, and the load on the tire of each inner wheel is greater than the load on the tire of each outer wheel during turning at acceleration. 
   The load on each tire depends on the degree of turning (the magnitude of lateral G) and the magnitude of acceleration. Owing to this tendency, understeer occurs in the vehicle during turning at acceleration, and the running locus of the vehicle is deviated to the outside of turn. As a result, the acceleration performance during turning is limited. It is effective to make the load on each tire uniform in improving this acceleration performance. According to the drive force control method of the present invention, the torque distribution ratio between the front and rear wheels is controlled so that the rear wheel torque is increased with an increase in lateral acceleration (lateral G), and the torque distribution ratio between the right and left wheels is controlled so that the outer wheel torque is increased with an increase in lateral G as shown in  FIG. 6 . Thus, the rear wheel torque distribution ratio and the outer wheel torque distribution ratio are increased with an increase in lateral G. Accordingly, understeer occurring during turning at acceleration can be suppressed to thereby allow stable acceleration. 
   The torque distribution between the front and rear wheels and the torque distribution between the right and left rear wheels will now be described in detail with reference to the flowchart shown in  FIG. 7 . In step  10  (shown by “S 10 ” in  FIG. 7 ), the lateral G signal from the lateral G sensor  98  is detected. In step  11 , the estimated lateral G is calculated according to the steering angle detected by the steering angle sensor  94  and the vehicle speed detected by the vehicle speed sensor  92 . In step  12 , the lateral G signal is corrected by the estimated lateral G signal to calculate the control lateral G. This correction is performed by averaging the lateral G signal and the estimated lateral G signal, for example. 
   The use of an output signal from a lateral G sensor as the lateral G signal is most general. However, it is known that the output from the lateral G sensor delays from a turning operation by the operator. Further, an actuator for performing the torque distribution generally has delay characteristics. Accordingly, if only the output signal from the lateral G sensor is used, control delay occurs. To suppress such control delay, the estimated lateral G is calculated according to the steering angle and the vehicle speed detected and the output signal from the lateral G sensor is corrected by the estimated lateral G signal obtained above according to this preferred embodiment. Since the steering angle is a turning operation itself by the operator, the estimated lateral G signal can be generated earlier than the output signal from the lateral G sensor. As a result, a control command can be early output to thereby allow quick-response control. 
   After calculating the control lateral G in step  12 , the program proceeds to step  13  to calculate the rear wheel torque and the outer wheel torque according to the control lateral G. In step  14 , it is determined whether or not the vehicle is in an unstable condition. For example, in the case that the slip angle of the vehicle is greater than a predetermined value or the change rate of the slip angle is greater than a predetermined value, it is determined that the vehicle is in an unstable condition. These predetermined values may be changed according to the condition of a road surface. For example, the smaller the coefficient of friction (μ) between a road surface and each tire, the smaller the predetermined values to be set. Accordingly, the unstable condition can be detected earlier and more accurately. 
   If the unstable condition of the vehicle is detected, the program proceeds to step  15  to obtain a rear wheel torque reducing amount and an outer wheel torque reducing amount and to correct the rear wheel torque and the outer wheel torque according to these reducing amounts, respectively. The rear wheel torque reducing amount and the outer wheel torque reducing amount are increased with an increase in estimated slip angle as shown in  FIG. 8 . In other words, the unstable condition of the vehicle is corrected in step  15  by making the torque distribution ratio between the front and rear wheels greater on the front wheel side and making the torque distribution ratio between the right and left wheels smaller on the outer wheel side. 
   If the unstable condition of the vehicle is not determined in step  14  or after the rear wheel torque and the outer wheel torque are corrected in the unstable condition of the vehicle in step  15 , the program proceeds to step  16  to calculate an actuator control value according to the rear wheel torque and the outer wheel torque. This actuator control value includes control values for the right and left electromagnetic actuators  56  and control values for the lockup clutch  40  and the speed increasing clutch  42  of the speed increasing device  10 . In step  17 , the right and left electromagnetic actuators  56  are controlled and whether the speed increasing device  10  is to become a lockup condition or a speed increasing condition is controlled according to the above control values. The degree of this speed increase is set so that the rotational speed of the output shaft  32  becomes greater by about 5% than the rotational speed of the input shaft  30 , for example. 
   A control method for drive force (torque) distribution between the front and rear wheels of the four-wheel drive vehicle will now be described with reference to the flowcharts shown in  FIGS. 9 to 11 . Running condition detection processing will now be described with reference to the flowchart shown in  FIG. 9 . In step  20 , a turning condition is detected. More specifically, the lateral G signal detected by the lateral G sensor  98  is corrected by the estimated lateral G calculated according to the vehicle speed and the steering angle to calculate the control lateral G. 
   In step  21 , a vehicle speed is detected from the signal from the vehicle speed sensor  92 . In step  22 , an accelerator opening is detected. In step  23 , a transmission shift position is detected. In step  24 , a transmission reverse range is detected. In step  25 , a  4 WD oil temperature, or an oil temperature of the rear differential device  12  is detected. Target rear wheel torque calculation processing will now be described with reference to the flowchart shown in  FIG. 10 . In step  30 , a rear wheel torque according to the turning condition is calculated. In step  31 , a rear wheel torque correction amount K 1  according to the vehicle speed is calculated. In this preferred embodiment, the torque distribution to the rear wheels is decreased with an increase in the vehicle speed by using the correction amount K 1  as shown in  FIG. 12 . 
   In step  32 , a rear wheel torque correction amount K 2  according to the accelerator opening is calculated. In this preferred embodiment, the torque distribution to the rear wheels is increased with an increase in the accelerator opening by using the correction amount K 2  as shown in  FIG. 13 . In step  33 , a rear wheel torque correction amount K 3  according to the transmission shift position is calculated. In this preferred embodiment, the torque distribution to the rear wheels is decreased by using the correction amount K 3  in the case that the transmission shift position is a low-speed position or a high-speed position as shown in  FIG. 14 . 
   In step  34 , a rear wheel torque correction amount K 4  according to the reverse range is calculated. In this preferred embodiment, the torque distribution to the rear wheels is decreased by using the correction amount K 4  in the case of reverse running. In step  35 , a rear wheel torque correction amount K 5  according to the 4 WD oil temperature, or the oil temperature of the rear differential device  12  is calculated. In this preferred embodiment, the torque distribution to the rear wheels is decreased with a decrease in the oil temperature of the rear differential device  12  by using the correction amount K 5  as shown in  FIG. 15 . 
   In step  36 , the rear wheel torque calculated in step  30  is corrected according to the correction amounts K 1 , K 2 , K 3 , K 4 , and K 5  to thereby calculate a target rear wheel torque. In step  40  of the flowchart showing 4 WD control in  FIG. 11 , an actuator control value is calculated according to the target rear wheel torque. In step  41 , the actuator is controlled according to the actuator control value calculated above. More specifically, the degree of engagement of the right and left electromagnetic actuators  56  is controlled according to the control value to thereby control the torque distribution ratio between the front and rear wheels. 
   Target rear outer wheel torque calculation processing will now be described with reference to the flowchart shown in  FIG. 16 . In step  50 , a rear outer wheel torque according to the turning condition is calculated. This turning condition is determined according to the lateral G. In step  51 , a rear outer wheel torque correction amount K 6  according to the vehicle speed is calculated. In this preferred embodiment, the torque distribution to the rear outer wheel is decreased with an increase in the vehicle speed by using the correction amount K 6  as shown in  FIG. 17 . 
   In step  52 , a rear outer wheel torque correction amount K 7  according to the transmission shift position is calculated. In this preferred embodiment, the torque distribution to the rear outer wheel is decreased by using the correction amount K 7  in the case that the transmission shift position is a low-speed position or a high-speed position as shown in  FIG. 18 . In step  53 , a rear outer wheel torque correction amount K 8  according to the reverse range is calculated. In this preferred embodiment, the torque distribution to the rear outer wheel is decreased by using the correction amount K 8  in the case of reverse running. 
   In step  54 , a rear outer wheel torque correction amount K 9  according to the 4 WD oil temperature, or the oil temperature of the rear differential device  12  is calculated. In this preferred embodiment, the torque distribution to the rear outer wheel is decreased with a decrease in temperature of hydraulic fluid for the rear differential device  12  by using the correction amount K 9  as shown in  FIG. 19 . In step  55 , the rear outer wheel torque calculated in step  50  is corrected according to the correction amounts K 6 , K 7 , K 8 , and K 9  to thereby calculate a target rear outer wheel torque. 
   Further, as in step  40  of the flowchart shown in  FIG. 11 , an actuator control value is next calculated according to the target rear outer wheel torque calculated above, and as in step  41  in  FIG. 11 , the degree of engagement of the right and left electromagnetic actuators  56  are controlled according to the control value calculated above. The lockup/speed increase control for the speed increasing device  10  will now be described. The object of the lockup/speed increase control for the speed increasing device  10  is to operate the speed increasing device  10  so that the outer wheel can be driven during turning. 
   Accordingly, the lateral G signal is used to quickly and accurately determine the turning condition. In a straight running condition of the vehicle, the lateral G is zero. Accordingly, by using a small value as a lateral G threshold, the speed increasing device  10  can be controlled to an increasing speed condition immediately after the vehicle starts turning. For example, when the lateral G signal for the vehicle exceeds the lateral G threshold according to the vehicle speed, the lockup condition or same speed condition of the speed increasing device  10  is changed to the increasing speed condition. As a result, the speed increasing can be performed before largely driving the outer wheel to thereby ensure a condition where the outer wheel can be driven. Accordingly, a larger drive force can be applied to the outer wheel as compared with the inner wheel, thereby improving the turning performance. 
   Further, by using the estimated lateral G signal calculated according to the steering angle and the vehicle speed as the lateral G signal, the lateral G signal can be obtained more quickly during the process of transition from the straight running condition to the turning condition. The steering angle is an input itself from the operator, and a delay of motion of the vehicle is added to the actual generation of lateral G. In compensating for the drawbacks of a lateral G sensor, it is also effective to partially correct the output signal from the lateral G sensor by using the estimated lateral G signal or to use the average of the lateral G signal and the estimated lateral G signal. 
   A speed increase command is generated after the decision of turning. If the speed increasing device  10  is operated immediately according to the speed increase command, the controller is influenced by the noise included in the signal, and a speed increase stop command is generated every time the turning direction changes as in slalom running, causing an increase in frequency of operation of the speed increasing device  10 . In order to minimize the noise, shock, etc. due to the operation of the speed increasing device and reduce the frequency of operation of the speed increasing device with a reduced size and weight, the speed increasing device  10  is controlled so that the command to the speed increasing device  10  is not immediately executed, but the command is continued for about one second, for example, prior to performing the actual operation of the device  10 . 
   This control will now be described with reference to the flowcharts shown in  FIGS. 20 and 21 .  FIG. 20  shows the flowchart of change control from the lockup condition (same speed condition) to the increasing speed condition. In step  60 , it is determined whether or not a speed increase command is ON. If the speed increase command is ON, the program proceeds to step  61  to start time measurement by a timer. In step  62 , it is determined whether or not the measured time T is greater than a predetermined value T 0 . 
   If T&gt;T 0  in step  62 , the program proceeds to step  64  to determine the speed increasing operation. Then, the lockup clutch  40  of the speed increasing device  10  is disengaged and the speed increasing clutch  42  is engaged. If the measured time T is less than or equal to the predetermined value T 0  in step  62 , the program proceeds to step  63  to determine whether or not the speed increase command is OFF. If the speed increase command is not OFF, the determination of step  62  is executed again, whereas if the speed increase command is OFF, the determination of step  60  is executed again. 
   Change control from the increasing speed condition to the lockup condition will now be described with reference to the flowchart shown in  FIG. 21 . In step  70 , it is determined whether or not a lockup command is ON. If the lockup command is ON, the program proceeds to step  71  to start time measurement by a timer. In step  72 , it is determined whether or not the measured time T is greater than a predetermined value T 0 . If T&gt;T 0  in step  72 , the program proceeds to step  74  to determine the lockup operation. Then, the speed increasing clutch  42  of the speed increasing device  10  is disengaged, and the lockup clutch  40  is engaged. 
   If the measured time T is less than or equal to the predetermined value T 0  in step  72 , the program proceeds to step  73  to determine whether or not the lockup command is OFF. If the lockup command is not OFF, the determination of step  72  is executed again, whereas if the lockup command is OFF, the determination of step  70  is executed again. The object of this speed increase control is to improve the maneuverability of the vehicle by driving the outer wheel more than the inner wheel. When the vehicle becomes an unstable condition, there is a case that any particular improvement in the maneuverability is not desired under any circumstances such as counter steer running. 
   For example, when the slip angle of the vehicle body becomes greater than a predetermined value or when counter steer such that the steering angle and the lateral G are different in sign is detected, the speed increase control is inhibited. Accordingly, outer wheel driving that may invite a further degradation in behavior can be avoided to thereby allow the stabilization of behavior. Such behavior stabilization control will now be described with reference to the flowchart shown in  FIG. 22 . In step  80 , it is determined whether or not counter steer is detected. If the counter steer is detected, the program proceeds to step  82  to generate a lockup command, thereby engaging the lockup clutch  40  of the speed increasing device  10 . 
   If the counter steer is not detected in step  80 , the program proceeds to step  81  to determine whether or not the slip angle β of the vehicle body is greater than a slip angle threshold β 0 . If the slip angle β is greater than the threshold β 0 , it is determined that the behavior of the vehicle is unstable, and the program proceeds to step  82  to generate the lockup command, thereby engaging the lockup clutch  40  of the speed increasing device  10  to stabilize the behavior. In such circumstances that an improvement in driving stability is not desired or that a large effect cannot be obtained by the outer wheel driving as control, the speed increase control is inhibited to thereby allow a reduction in torque to be input into the speed increasing device  10  and a reduction in frequency of operation of the device  10 . Accordingly, this is effective in reducing the weight of the device  10  and in improving the durability of the device  10 . 
   For example, when the shift position is a first-speed position or a fifth-speed position, the speed increase control is inhibited as shown in  FIG. 23 . That is, when the shift position is a first-speed position, a very large torque is generated. However, since the vehicle speed at the first-speed position is low, the effect by the outer wheel driving cannot be so obtained. Conversely, when the shift position is a fifth-speed position, the vehicle speed is too high and there is a danger that the vehicle is excessively turned. Therefore, the speed increase control is inhibited also in this case. In addition, when the shift position is in a reverse position, an improvement in driving stability cannot be expected and the speed increase control is therefore inhibited. 
   Further, in an engine brake condition or during braking where the drive force cannot be transmitted to the outer wheel, the speed increase control is also inhibited to thereby allow a reduction in torque to be input into the speed increasing device  10  and a reduction in frequency of operation of the device  10 . Accordingly, the weight of the device  10  can be reduced and the durability of the device  10  can be improved. Further, by controlling the speed increasing device  10  into the lockup condition in the engine brake condition or during braking, a braking force can be applied to the outer wheel, and this is effective also in suppressing oversteer occurring in braking during turning. 
   Such control in the engine brake condition or during braking will now be described with reference to the flowcharts shown in  FIGS. 24 and 25 .  FIG. 24  shows the flowchart of control in the engine brake condition. In step  90 , it is determined whether or not the drive torque is negative, that is, whether or not the vehicle is in the engine brake condition. If the vehicle is in the engine brake condition, the program proceeds to step  91  to generate a lockup command, thereby disengaging the speed increasing clutch  42  of the speed increasing device  10  and engaging the lockup clutch  40 . 
     FIG. 25  shows the flowchart of control during braking. In step  100 , it is determined whether or not the vehicle is being braked by the operator. If the vehicle is being braked by the operator, the program proceeds to step  101  to generate a lockup command, thereby disengaging the speed increasing clutch  42  of the speed increasing device  10  and engaging the lockup clutch  40 . In the case that the operation of the speed increasing device  10  is relied on the oil pressure of a pump driven by an axle, there is a possibility that an oil pressure required for the speed increasing cannot be obtained at certain low vehicle speeds. If the control is relied on only the lateral G threshold, a speed increase command is undesirably generated in the stage where a sufficient oil pressure is not obtained, causing a possibility of adverse effects on the speed increasing clutch  42 . 
   Further, when the vehicle speed becomes a value at which a sufficient oil pressure can be obtained, the lockup condition is shifted to the increasing speed condition. Accordingly, even during turning at this vehicle speed or higher, the lockup condition is changed to the increasing speed condition. To avoid possible instability of the behavior of the vehicle because of the above control, the change to the increasing speed condition is inhibited until the vehicle runs straight at a given vehicle speed (V 1 ) or more during low-speed running at a given vehicle speed (V 0 ) or less. Accordingly, the speed increase control at the vehicle speed V 0  or less can be avoided. Further, a rapid change to the increasing speed condition during turning can also be prevented. 
   This control will now be described with reference to the flowchart shown in  FIG. 26 . In step  110 , it is determined whether or not the vehicle speed V is less than the given vehicle speed V 0 . If the vehicle speed V is less than the given vehicle speed V 0 , the program proceeds to step  111  to inhibit the change to the increasing speed condition. Thereafter, the vehicle continues to run. In step  112 , it is determined whether or not the vehicle speed V is greater than V 1  which is greater than V 0  and the lateral G is less than G 0 . If the answer in step  112  is YES, the program proceeds to step  113  to permit the change to the increasing speed condition. The value G 0  in step  112  is set to about 0.1 G. Further, the determination in step  112  is to determine whether or not the vehicle is running straight at a vehicle speed greater than V 1 . 
   While the present invention is applied to a four-wheel drive vehicle based on a FF vehicle in the above preferred embodiment, the control method of the present invention is also applicable to a vehicle such that the power from a driving power source such as an engine is directly transmitted to the rear wheels, that the transmission of the power to the right and left rear wheels can be controlled by a clutch or the like, and that the power can also be transmitted to the front wheels by a clutch or the like. Further, the vehicle may be of such a type that the rear wheels are normally increased in rotational speed.
A drive force control method for a four-wheel drive vehicle having front wheels as main drive wheels always connected to a driving source, rear wheels as auxiliary drive wheels whose drive torque is adjustable, and a mechanism capable of adjusting a torque distribution ratio between the front wheels and the rear wheels so that the torque distribution ratio of the rear wheels to the front wheels is increased in turning and also capable of adjusting a torque distribution ratio between the right and left rear wheels in turning. The drive force control method includes the steps of detecting a vehicle speed, and gently decreasing the torque distribution ratio of a turning outer wheel to a turning inner wheel with an increase in the vehicle speed. When the transmission shift position is a low-speed position or a high-speed position or the vehicle is in reverse running, the torque distribution ratio of the turning outer wheel to the turning inner wheel is decreased.