Driving force distribution control method and device for four wheel drive vehicle

This invention relates to a driving force distribution control method and device for a four wheel drive vehicle, which controls a distribution ratio of engine torque transmitted to a front wheel and a rear wheel.

In a four wheel drive vehicle, a mechanism including a torque distribution clutch, a generator and a motor is known as a mechanism that distributes a torque of an engine, which is transmitted to a main drive wheel, to a secondary drive wheel. In a four wheel drive vehicle having the torque distribution clutch, an engaging force of the clutch is determined on the basis of a rotation speed difference between a front wheel and a rear wheel. Further, in a related art, a correction coefficient is determined on the basis of a deviation between a target yaw rate and an actual yaw rate, and a torque distribution clutch is controlled on the basis of a corrected clutch engaging force determined by multiplying the clutch engaging force by the correction coefficient (see JP5-278490A).

However, when the vehicle turns on a slippery road surface, for example, both the rotation speed difference between the front and rear wheels and the deviation between the target yaw rate and the actual yaw rate vary. Hence, with the above technique, the pre-correction clutch engaging force, which serves as a base, and the correction coefficient both vary, and therefore hunting may occur during control of the secondary drive wheel.

It is therefore an object of this invention to suppress hunting occurring during control of a secondary drive wheel, even when a rotation speed difference between a main drive wheel and a secondary drive wheel varies.

In order to achieve the above object, this invention provides a driving force distribution method for a four wheel drive vehicle, in which a torque of an engine is distributed to a main drive wheel, to which the torque is transmitted during two wheel driving, and a secondary drive wheel, to which a part of the torque is transmitted during four wheel driving, comprising calculating a first torque to be distributed to the secondary drive wheel on the basis of the torque of the engine, detecting a yaw rate of the vehicle, calculating a target yaw rate of the vehicle, calculating a deviation between the calculated target yaw rate and the detected yaw rate, correcting the first torque on the basis of the yaw rate deviation, detecting a rotation speed of the main drive wheel, detecting a rotation speed of the secondary drive wheel, calculating a rotation speed difference between the rotation speed of the main drive wheel and the rotation speed of the secondary drive wheel, calculating a second torque to be distributed to the secondary drive wheel on the basis of the calculated rotation speed difference, and distributing a larger torque of the first torque and the second torque to the secondary drive wheel when an absolute value of the yaw rate deviation is smaller than a predetermined value, and distributing a corrected first torque corrected on the basis of the yaw rate deviation to the secondary drive wheel when the absolute value of the yaw rate deviation is equal to or larger than the predetermined value.

In order to achieve the above object, this invention further provides a driving force distribution control device for a four wheel drive vehicle having a mechanism that distributes a torque of an engine, which is transmitted to a main drive wheel, to a secondary drive wheel, comprising a first torque calculation unit that calculates a first torque to be distributed to the secondary drive wheel on the basis of the torque of the engine, a yaw rate detection unit that detects a yaw rate of the vehicle, a target yaw rate calculation unit that calculates a target yaw rate of the vehicle, a yaw rate deviation calculation unit that calculates a yaw rate deviation between the calculated target yaw rate and the detected yaw rate, a torque correction unit that corrects the first torque on the basis of the yaw rate deviation, a rotation speed difference calculation unit that calculates a rotation speed difference between the main drive wheel and the secondary drive wheel, a second torque calculation unit that calculates a second torque to be distributed to the secondary drive wheel by the mechanism on the basis of the rotation speed difference and a control unit that controls the mechanism so as to distribute a larger torque of the first torque and the second torque to the secondary drive wheel when an absolute value of the yaw rate deviation is smaller than a predetermined value, and controls the mechanism so as to distribute a corrected first torque corrected by the torque correction unit to the secondary drive wheel when the absolute value of the yaw rate deviation is equal to or larger than the predetermined value.

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.

FIG. 1 is an overall system diagram of a driving force distribution control device for a four wheel drive vehicle according to an embodiment.

FIG. 2 is a flowchart showing the content of processing performed by a 4WD controller.

FIG. 3 is a view showing a relationship between a yaw rate deviation and a correction gain Gunder.

FIG. 4 is a view showing a relationship between the yaw rate deviation and a correction gain Gover.

FIG. 5 is a view showing a relationship between a limit value Tlim 2 and a vehicle speed.

FIG. 6 is an overall system diagram of a driving force distribution control device for a rear wheel drive-based four wheel drive vehicle.

FIG. 7 is a view showing a relationship between a yaw rate deviation and a correction gain Gunder in the case where the invention is applied to the rear wheel drive-based four wheel drive vehicle.

FIG. 8 is a view showing a relationship between the yaw rate deviation and a correction gain Gover in the case where the invention is applied to the rear wheel drive-based four wheel drive vehicle.

FIG. 1 is an overall system diagram of a driving force distribution control device for a four wheel drive vehicle according to an embodiment. A four wheel drive vehicle to which this driving force distribution control device is applied is a front wheel drive-based four wheel drive vehicle. In other words, the front wheel serves as a main drive wheel, and the rear wheel serves as a secondary drive wheel.

The driving force of an engine 1 is transmitted directly to left and right front wheels 4 , 5 via an automatic transmission 2 and a front differential 9 . Further, the driving force of the engine 1 is transmitted to left and right rear wheels 6 , 7 via a propeller shaft 10 , a torque distribution clutch 3 , a rear differential 15 , and a rear wheel drive shaft 8 .

In this front wheel drive-based four wheel drive vehicle, when a clutch engaging force of the torque distribution clutch 3 increases, the torque distribution ratio of the rear wheels increases. For example, when the torque distribution clutch 3 is disengaged, the front wheel-rear wheel torque distribution ratio equals 100:0, but when the torque distribution clutch 3 is engaged by at least ½ the engine torque, an equal torque distribution ratio of front wheel:rear wheel=50:50 is obtained. In other words, the engaging torque of the torque distribution clutch 3 is controlled such that the distribution ratio of the torque transmitted to the front wheels 4 , 5 and rear wheels 6 , 7 stays within a range of front wheel:rear wheel=100 to 50:0 to 50. It should be noted that the sum total of the distribution ratios of the front wheels 4 , 5 and rear wheels 6 , 7 is always set at 100.

A VDC controller 11 , an engine controller 12 , and a 4WD controller 13 are all connected to each other and are capable of exchanging various types of information through CAN communication.

Wheel speed sensors 24 to 27 output wheel speed signals corresponding respectively to the wheel speeds of the respective wheels 4 to 7 to the VDC controller 11 . A steering angle sensor 18 outputs a steering angle signal corresponding to a steering angle of a steering wheel to the VDC controller 11 .

On the basis of the wheel speed signals input from the wheel speed sensors 24 to 27 , the VDC controller 11 calculates the wheel speeds of the respective wheels 4 to 7 , and on the basis of the steering angle signal input from the steering angle sensor 18 , the VDC controller 11 calculates the steering angle.

The engine controller 12 determines the engine torque on the basis of an accelerator opening signal input from an accelerator opening sensor 21 and an engine rotation speed signal input from an engine rotation speed sensor 22 . A known method may be used as the method of determining the engine torque on the basis of the accelerator opening and the engine rotation speed.

A yaw rate sensor 16 detects a yaw rate of the vehicle and outputs the detected yaw rate to the respective controllers 11 to 13 . A front-rear acceleration sensor 17 detects acceleration in a front-rear direction of the vehicle and outputs the detected acceleration to the respective controllers 11 to 13 .

A 4WD switch 14 is a switch that is operated by a driver to switch between 2WD and 4WD.

The 4WD controller 13 determines a clutch engaging force of the torque distribution clutch 3 on the basis of a signal input from the 4WD switch 14 , a signal input from the VDC controller 11 , a signal input from the engine controller 12 , a signal input from the yaw rate sensor 16 , and a signal input from the front-rear acceleration sensor 17 . The 4WD controller 13 outputs a clutch control signal corresponding to the determined clutch engaging force to the torque distribution clutch 3 . Clutch control of the torque distribution clutch 3 is performed on the basis of the clutch control signal.

FIG. 2 is a flowchart showing the content of processing performed by the 4WD controller 13 . When the signal input from the 4WD switch 14 following activation of the vehicle indicates a 4WD mode, the 4WD controller 13 starts the processing of a step S 10 .

In the step S 10 , the steering angle is read via the VDC controller 11 , whereupon the routine advances to a step S 20 . In the step S 20 , a target yaw rate of the vehicle is calculated on the basis of the steering angle read in the step S 10 using a known method. Once the target yaw rate of the vehicle has been calculated, the routine advances to a step S 30 .

In the step S 30 , the actual yaw rate of the vehicle detected by the yaw rate sensor 16 is read, whereupon the routine advances to a step S 40 . In the step S 40 , a yaw rate deviation, i.e. a deviation between the target yaw rate calculated in the step S 20 and the actual yaw rate read in the step S 30 , is calculated. Once the yaw rate deviation has been calculated, the routine advances to a step S 50 .

It should be noted that in the following description, it is assumed that when the vehicle is in an understeer condition, the yaw rate deviation is calculated as a positive value, and when the vehicle is in an oversteer condition, the yaw rate deviation is calculated as a negative value, regardless of whether leftward steering or rightward steering is underway. In other words, the yaw rate deviation is calculated during steering by subtracting the actual yaw rate from the target yaw rate.

In the step S 50 , a determination is made as to whether or not the yaw rate deviation calculated in the step S 40 is equal to or larger than a predetermined threshold Th 1 . The predetermined threshold Th 1 (Th 1 >0) is a threshold for determining whether or not the vehicle is in an understeer condition, which is set in advance to an appropriate value through experiment or the like. When it is determined that the yaw rate deviation is equal to or larger than the predetermined threshold Th 1 , the routine advances to a step S 60 .

In the step S 60 , the vehicle is determined to be in an understeer condition, and a flag F_under indicating that the vehicle is in an understeer condition is set at unity, whereupon the routine advances to a step S 70 . In the step S 70 , a correction control operation flag Fc is set at unity, whereupon the routine advances to a step S 140 .

Strictly speaking, a case in which the yaw rate deviation is greater than zero and lower than the predetermined threshold Th 1 may also be referred to as an understeer condition. In this specification, however, the vehicle is defined as being in an understeer condition, such that the flag F_under is set at unity, when the yaw rate deviation is equal to or greater than the predetermined threshold Th 1 .

When the yaw rate deviation is determined to be lower than the predetermined threshold Th 1 in the step S 50 , on the other hand, the routine advances to a step S 80 . In the step S 80 , the flag F_under indicating that the vehicle is in an understeer condition is set at zero, whereupon the routine advances to a step S 90 .

In the step S 90 , a determination is made as to whether or not the yaw rate deviation calculated in the step S 40 is equal to or smaller than a predetermined threshold Th 2 . The predetermined threshold Th 2 (Th 2 <0) is a threshold for determining whether or not the vehicle is in an oversteer condition, which is set in advance to an appropriate value through experiment or the like. When it is determined that the yaw rate deviation is equal to or smaller than the predetermined threshold Th 2 , the routine advances to a step S 100 .

In the step S 100 , the vehicle is determined to be in an oversteer condition, and a flag F_over indicating that the vehicle is in an oversteer condition is set at unity, whereupon the routine advances to a step S 110 . In the step S 110 , the correction control operation flag Fc is set at unity, whereupon the routine advances to the step S 140 .

Strictly speaking, a case in which the yaw rate deviation is greater than the predetermined threshold Th 2 and lower than zero may also be referred to as an oversteer condition. In this specification, however, the vehicle is defined as being in an oversteer condition, such that the flag F_over is set at unity, when the yaw rate deviation is equal to or lower than the predetermined threshold Th 2 .

When the yaw rate deviation is determined to be greater than the predetermined threshold Th 2 in the step S 90 , on the other hand, the routine advances to a step S 120 . In the step S 120 , the flag F_over indicating that the vehicle is in an oversteer condition is set at zero, whereupon the routine advances to a step S 130 . In this case, the yaw rate deviation is greater than the predetermined threshold Th 2 (Th 2 <0) and smaller than the predetermined threshold Th 1 (Th 1 >0), and therefore the behavior of the vehicle is normal. Hence, in the step S 130 , the correction control operation flag Fc is set at zero, whereupon the routine advances to the step S 140 .

In the step S 140 , a determination is made as to whether or not the correction control operation flag Fc is set at unity. When it is determined that the correction control operation flag Fc is set at unity, the routine advances to a step S 150 .

In the step S 150 , a reference engaging torque T 1 of the torque distribution clutch 3 is determined. A method of calculating the reference engaging torque T 1 will be described below.

First, a transfer output torque Tout is determined on the basis of the engine torque determined by the engine controller 12 using a known method. Next, a basic distribution torque Teng is determined by multiplying (rear load/total vehicle weight) by the determined output torque Tout. The basic distribution torque Teng is equal to a rear torque when the transfer output torque Tout is distributed in an identical distribution ratio to a wheel load distribution ratio of the front and rear wheels.

Finally, the reference engaging torque T 1 is determined by multiplying a front-rear acceleration sensitive gain KG by the basic distribution torque Teng. By multiplying the front-rear acceleration sensitive gain KG by the basic distribution torque Teng, a dynamic load distribution torque T 1 , i.e. a rear torque distribution in which a rear load increment is increased during uphill travel or the like, is determined.

Here, the front-rear acceleration sensitive gain KG takes a value corresponding to the front-rear acceleration detected by the front-rear acceleration sensor 17 , and increases as the front-rear acceleration increases. For example, data determining a relationship between the front-rear acceleration and the front-rear acceleration sensitive gain KG are prepared in advance, and by referring to these data, the front-rear acceleration sensitive gain KG is determined on the basis of the front-rear acceleration detected by the front-rear acceleration sensor 17 .

Also in the step S 150 , processing is performed to limit the determined reference engaging torque T 1 to a predetermined limit value Tlim 1 . More specifically, when the reference engaging torque T 1 is larger than the predetermined limit value Tlim 1 , processing is performed to replace the value of the reference engaging torque T 1 with the predetermined limit value Tlim 1 . The predetermined limit value Tlim 1 is a threshold for preventing calculation errors. Once the processing of the step S 150 is complete, the routine advances to a step S 160 .

In the step S 160 , a determination is made as to whether or not the flag F_under indicating that the vehicle is in an understeer condition is at unity. When the flag F_under is determined to be at unity, the routine advances to a step S 170 , and when the flag F_under is determined to be at zero, the routine advances to a step S 180 .

In the step S 170 , a corrected torque T 2 is determined by multiplying an understeer correction gain Gunder (to be referred to simply as the correction gain Gunder hereafter) by the reference engaging torque T 1 determined in the step S 150 .

FIG. 3 is a view showing a relationship between the yaw rate deviation and the correction gain Gunder. As shown in FIG. 3 , when the yaw rate deviation is equal to or smaller than the predetermined threshold Th 1 , the correction gain Gunder is set at 1.0, and as the yaw rate deviation increases beyond the predetermined threshold Th 1 , the value of the correction gain Gunder increases. Here, data determining the relationship shown in FIG. 3 between the yaw rate deviation and the correction gain Gunder are prepared in advance, and by referring to these data, the correction gain Gunder is determined on the basis of the yaw rate deviation determined in the step S 40 .

Meanwhile, in the step S 180 , the corrected torque T 2 is determined by multiplying an oversteer correction gain Gover (to be referred to simply as the correction gain Gover hereafter) by the reference engaging torque T 1 determined in the step S 150 .

FIG. 4 is a view showing a relationship between the yaw rate deviation and the correction gain Gover. As shown in FIG. 4 , when the yaw rate deviation is equal to or larger than the predetermined threshold Th 2 , the correction gain Gover is set at 1.0, and as the yaw rate deviation decreases below the predetermined threshold Th 2 , the value of the correction gain Gover also decreases. Here, data determining the relationship shown in FIG. 4 between the yaw rate deviation and the correction gain Gover are prepared in advance, and by referring to these data, the correction gain Gover is determined on the basis of the yaw rate deviation determined in the step S 40 .

Once the processing of the step S 170 or the step S 180 is complete, the routine advances to a step S 190 . In the step S 190 , processing is performed to limit the determined corrected torque T 2 to a predetermined limit value Tlim 2 . More specifically, when the corrected torque T 2 is greater than the predetermined limit value Tlim 2 , the value of the corrected torque T 2 is replaced by the predetermined limit value Tlim 2 .

FIG. 5 is a view showing a relationship between the limit value Tlim 2 and the vehicle speed. This relationship is set such that as the vehicle speed increases, the value of the limit value Tlim 2 decreases in accordance with reductions in a gear ratio of the transmission 2 .

The 4WD controller 13 outputs a clutch control signal corresponding to the limit processed torque T 2 to the torque distribution clutch 3 . As a result, a clutch force corresponding to the target torque T 2 is generated in the torque distribution clutch 3 , and thus the driving force distribution of the front and rear wheels is controlled.

On the other hand, when it is determined in the step S 140 that the correction control operation flag Fc is not set at unity, the routine advances to a step S 200 . In the step S 200 , a target torque T 3 of the torque distribution clutch 3 is determined. A method of calculating the target torque T 3 will be described below.

First, a front-rear wheel speed difference ΔVw is determined on the basis of the speed of each wheel, determined on the basis of the output from the respective wheel speed sensors 24 to 27 . More specifically, the front-rear wheel speed difference ΔVw is determined by subtracting an average speed of a right rear wheel speed VwRR and a left rear wheel speed VwRL from an average speed of a right front wheel speed VwFR and a left front wheel speed VwFL. A torque TΔV corresponding to the front-rear wheel speed difference ΔVw is then determined by multiplying a gain KD by the front-rear wheel speed difference ΔVw. Similarly to the reference engaging torque T 1 , the torque TΔV is the torque that is distributed to the rear wheels. The gain KD is determined on the basis of the vehicle speed and the left-right wheel speed difference of the front wheels, for example.

Further, the reference engaging torque T 1 is determined on the basis of the engine torque output from the engine controller 12 . Processing to determine the reference engaging torque T 1 is identical to the processing of the step S 150 , and therefore detailed description thereof has been omitted.

Next, a target torque T 3 is determined by comparing the torque TΔV to the reference engaging torque T 1 , selecting the larger value of the two, and limiting the selected torque using the predetermined limit value Tlim 1 .

In a step S 210 that follows the step S 200 , a clutch control signal corresponding to the target torque T 3 is output to the torque distribution clutch 3 . As a result, a clutch force corresponding to the target torque T 3 is generated in the torque distribution clutch 3 , and thus the distribution of driving force to the front and rear wheels is controlled.

According to the driving force distribution control device for a four wheel drive vehicle of this embodiment, which comprises the torque distribution clutch 3 for distributing engine torque transmitted to the main drive wheel to the secondary drive wheel, the corrected torque T 2 is calculated by determining a first engaging torque T 1 of the torque distribution clutch 3 on the basis of the engine torque and correcting the determined reference engaging torque T 1 on the basis of the yaw rate deviation between the target yaw rate and the actual yaw rate of the vehicle. Further, a second engaging torque TΔV of the torque distribution clutch 3 is determined on the basis of the rotation speed difference between the main drive wheel and the secondary drive wheel. When the yaw rate deviation is smaller than a positive first predetermined value Th 1 or larger than a negative second predetermined value Th 2 , the torque distribution clutch 3 is controlled on the basis of the larger engaging torque of the first engaging torque T 1 and second engaging torque TΔV, and when the yaw rate deviation is equal to or larger than the positive first predetermined value Th 1 or equal to or smaller than the negative second predetermined value Th 2 , the torque distribution clutch 3 is controlled on the basis of the corrected torque T 2 . The pre-correction base torque T 1 is determined on the basis of the engine torque, and therefore the torque T 1 serving as a reference does not vary even when the rotation speed difference between the front and rear wheels varies during advancement on a slippery road surface or the like. As a result, variation in the torque distributed to the secondary drive wheels can be suppressed, and therefore hunting occurring during control of the secondary drive wheels can be suppressed.

When the yaw rate deviation is equal to or larger than the positive first predetermined value Th 1 or equal to or smaller than the negative second predetermined value Th 2 , the base torque T 1 is determined on the basis of the engine torque, and therefore hunting occurring during control of the secondary drive wheels can be suppressed even in a situation where the rotation speed difference between the front and rear wheels varies.

Further, when the yaw rate deviation is larger than the second predetermined value Th 2 and smaller than the first predetermined value Th 1 , the torque distribution clutch 3 is controlled on the basis of the larger torque of the first engaging torque T 1 and second engaging torque TΔV. Hence, a clutch engaging force that corresponds to the vehicle behavior can be obtained even when μ (the frictional coefficient) of the road surface varies, leading to variation in the rotation speed difference between the front and rear wheels, for example, and as a result, the behavior of the vehicle can be stabilized. When a yaw rate deviation occurs at this time, the target torque for controlling the torque distribution clutch 3 is not subjected to correction based on the yaw rate deviation. Accordingly, hunting caused by variation in the yaw rate deviation does not occur during control of the secondary drive wheels. It should be noted that since the yaw rate deviation is small, a problem does not occur even if the target torque for controlling the torque distribution clutch 3 is not corrected.

Also according to the driving force distribution control device for a four wheel drive vehicle of this embodiment, a correction is performed to increase the reference engaging torque T 1 when the yaw rate deviation is equal to or larger than the first predetermined value Th 1 , whereas a correction is performed to reduce the reference engaging torque T 1 when the yaw rate deviation is equal to or smaller than the second predetermined value Th 2 . As a result, an improvement in vehicle behavior stability with respect to a steering operation performed by the driver can be achieved.

Although the invention has been described above by reference to certain embodiment of the invention, the invention is not limited to the embodiment described above. Modifications and variations of the embodiment described above will occur to those skilled in the art, within the scope of the claims.

For example, in the above description, this invention is applied to a front wheel drive-based four wheel drive vehicle in which the torque distribution ratio of the rear wheels increases as the clutch engaging force of the torque distribution clutch 3 is increased. However, this invention may also be applied to a rear wheel drive-based four wheel drive vehicle in which the torque distribution ratio of the front wheels increases as the clutch engaging force of the torque distribution clutch 3 is increased.

FIG. 6 is an overall system diagram of a driving force distribution control device for a rear wheel drive-based four wheel drive vehicle. Elements which are the same as those of FIG. 1 are denoted by the same reference numerals and the detail explanation thereof is omitted.

The driving force of the engine 1 is transmitted to a transfer 3 A via the automatic transmission 2 , and is distributed to the front wheels 4 , 5 and the rear wheels 6 , 7 by the torque distribution clutch 3 in the transfer 3 A. To the front wheels 4 , 5 , the driving force of the engine 1 is transmitted via a front propeller shaft 10 a and the front differential 9 . To the rear wheels 6 , 7 , the driving force of the engine 1 is transmitted via a rear propeller shaft 10 b , the rear differential 15 and the rear wheel drive shaft 8 .

In a case where the invention is applied to a rear wheel drive-based four wheel drive vehicle, a correction to reduce the reference engaging torque T 1 may be performed when the yaw rate deviation is equal to or larger than the first predetermined value Th 1 , and a correction to increase the reference engaging torque T 1 may be performed when the yaw rate deviation is equal to or smaller than the second predetermined value Th 2 .

FIG. 7 is a view showing a relationship between the yaw rate deviation and the correction gain Gunder in the case where the invention is applied to the rear wheel drive-based four wheel drive vehicle. As shown in FIG. 7 , when the yaw rate deviation is equal to or smaller than the predetermined threshold Th 1 , the correction gain Gunder is set at 1.0, and as the yaw rate deviation increases beyond the predetermined threshold Th 1 , the value of the correction gain Gunder decreases. In the case where the invention is applied to the rear wheel drive-based four wheel drive vehicle, the correction gain Gunder is determined by using these data defining the relationship between the yaw rate deviation and the correction gain Gunder as shown in FIG. 7 .

FIG. 8 is a view showing a relationship between the yaw rate deviation and the correction gain Gover in the case where the invention is applied to the rear wheel drive-based four wheel drive vehicle. As shown in FIG. 8 , when the yaw rate deviation is equal to or larger than the predetermined threshold Th 2 , the correction gain Gover is set at 1.0, and as the yaw rate deviation decreases below the predetermined threshold Th 2 , the value of the correction gain Gover increases. In the case where the invention is applied to the rear wheel drive-based four wheel drive vehicle, the correction gain Gover is determined by using these data defining the relationship between the yaw rate deviation and the correction gain Gover as shown in FIG. 8 .

The positive first predetermined value Th 1 and an absolute value of the negative second predetermined value Th 2 may be equal or different. In a case where the first predetermined value Th 1 and the absolute value of the second predetermined value Th 2 are equal, the torque distribution clutch 3 is controlled on the basis of the larger torque of the first engaging torque T 1 and second engaging torque TΔV when an absolute value of the yaw rate deviation is smaller than the predetermined value Th 1 . When the absolute value of the yaw rate deviation is equal to or larger than the predetermined value, on the other hand, the torque distribution clutch 3 is controlled on the basis of the torque T 2 , which is obtained by correcting the first engaging torque T 1 on the basis of the yaw rate deviation.

The torque of the engine 1 , which is transmitted to the main drive wheel may be distributed to the secondary drive wheel by using a mechanism other than the torque distribution clutch 3 .

As a mechanism that distributes the torque of the engine 1 to the secondary drive wheel, a generator driven by the engine 1 and a motor driven by the power generated by the generator may be used. In this case, a torque of the motor is transmitted to the secondary drive wheel.

The entire contents of Japanese Patent Application P2007-205592 (filed on Aug. 7, 2007) are incorporated herein by reference.