JPH0653467B2 - Transmission torque control device for four-wheel drive vehicle - Google Patents

Description

TECHNICAL FIELD The present invention relates to a four-wheel drive vehicle that controls torque transmitted from a power plant to front wheels and rear wheels via a torque transmission path in a four-wheel drive vehicle. The present invention relates to a transmission torque control device.

(Prior Art) In the field of four-wheel drive vehicles, depending on the running state of the vehicle, four
A so-called part-time four-wheel drive vehicle, which can selectively take a two-wheel drive state and a two-wheel drive state, has been put into practical use.

In such a part-time four-wheel drive vehicle, for the purpose of smoothly switching from the four-wheel drive state to the two-wheel drive state or from the two-wheel drive state to the four-wheel drive state, for example,
As disclosed in Japanese Utility Model Laid-Open No. 56-122630, a wet clutch is interposed in either one of the front-wheel side and rear-wheel side torque transmission paths for transmitting the torque generated by the power plant to the front wheels and the rear wheels. It has been known.

However, in the conventional part-time four-wheel drive vehicle as described above, from the four-wheel drive state to the two-wheel drive state,
Alternatively, switching from the two-wheel drive state to the four-wheel drive state is performed by an occupant operating switches or the like provided on a shift lever or the like in accordance with the running state of the vehicle, road conditions, or the like. Therefore, the operation is bothersome, and when the difference in rotation speed between the front wheels and the rear wheels is large, the four-wheel drive state is changed to the two-wheel drive state or the two-wheel drive state is changed to four. There is a problem that a switching shock is likely to occur at the time of switching to the wheel drive state and it is difficult to set the switching timing.

Therefore, in order to automatically switch between the four-wheel drive state and the two-wheel drive state, one of the front wheel side and the rear wheel side torque transmission paths is provided with the input side rotation speed and the output side rotation speed. A viscous fluid clutch that changes the transmission torque according to the difference with the rotation speed is interposed, and the front wheel or the rear wheel is changed according to the difference between the front wheel rotation speed and the rear wheel rotation speed (the average rotation speed of the left and right wheels). A transmission torque control device for a four-wheel drive vehicle has been proposed in which the transmission torque to the wheels is changed.

However, as described above, the transmission torque control device for a four-wheel drive vehicle in which the viscous fluid clutch is interposed in the torque transmission path to change the transmission torque to the front wheels or the rear wheels according to the difference in the rotational speeds of the front and rear wheels. However, there are the following problems.

That is, for example, the case where the front wheel and the rear wheel have the same diameter and the viscous fluid clutch is interposed in the rear wheel side torque transmission path that transmits the torque from the power plant to the rear wheel is taken into consideration. In Fig. 4, the transmission torque characteristic of is the difference obtained by subtracting the output side rotation speed (rear wheel rotation speed) from the input side rotation speed (front wheel rotation speed) of the viscous fluid clutch on the horizontal axis (hereinafter, rotation speed difference). Called Δn),
When the transmission torque T is plotted on the vertical axis and the rotational speed difference Δn is positive, positive torque (hereinafter referred to as drive torque) is transmitted to the rear wheels as indicated by the solid line. When the rotational speed difference Δn is negative, negative torque (hereinafter referred to as braking torque) is transmitted to the rear wheels as indicated by the alternate long and short dash line. In this way, when the braking torque is transmitted to the rear wheels, the rear wheels become a running resistance, which causes a disadvantage that fuel consumption and durability of each part are deteriorated.

As one means for eliminating such inconvenience, a clutch mechanism is arranged in series with the viscous fluid clutch in a torque transmission path in which a viscous fluid clutch is interposed, and this clutch mechanism is used for front wheels and rear wheels. It is conceivable to perform intermittent control based on the rotational speed difference such that the running state is set to the connected state when the running resistance does not occur on the rear wheels and the connection state is set to the cutting state when the running resistance occurs on the rear wheels.

(Problems to be solved by the invention) However, as described above, a clutch mechanism is arranged in the torque transmission path in which the viscous fluid clutch is interposed so as to connect and disconnect the clutch, and the clutch mechanism is connected to the front wheels and the rear wheels. In a transmission torque control device for a four-wheel drive vehicle that is controlled to be intermittent based on the difference in the number of revolutions, for example, when the vehicle is changed from a straight traveling state to a turning traveling state, the turning radius of the front wheels is Since it becomes larger than the turning radius of the rear wheels, the rotational speed difference Δn becomes positive as shown in FIG. 4 described above. If the value of the rotational speed difference Δn at this time is Δn _c and the torque generated by the power plant is T _A , the driving torques transmitted to the front wheels and the rear wheels are indicated by Tf and Tr in FIG. 4, respectively. It will be as it is done. In such a state, when the torque generated by the power plant is reduced and becomes, for example, T _A ′ smaller than Tr, the difference Tf ′ between T _A ′ and Tr is transmitted to the front wheels as a braking torque. Become. As described above, when the braking torque is transmitted to the front wheels during turning of the vehicle, the torque generated by the power plant is wastefully consumed, which not only lowers the driving efficiency but also causes the turning radius of the vehicle. Is likely to be large, which may hinder the ease of driving. Such inconvenience similarly occurs when the viscous fluid clutch is interposed in the front wheel side torque transmission path for transmitting the torque from the power plant to the front wheels.

In view of such a point, the present invention provides a viscous fluid clutch or the like that changes the transmission torque according to the input / output rotational speed difference to either one of the torque transmission paths that transmits the torque from the power plant to the front wheels or the rear wheels. And a clutch mechanism is arranged in series with the fluid transmission means in the torque transmission path in which the fluid transmission means is interposed, and the clutch mechanism is controlled to be intermittent. Thus, the running resistance when the vehicle is running straight can be reduced under the condition that the four-wheel drive state is switched to the two-wheel drive state or the two-wheel drive state is switched to the four-wheel drive state. Even when the torque generated by the power plant is reduced when the vehicle is turning, it is possible to prevent the braking torque from being transmitted to the front wheels. And an object to provide a transmission torque control device has been four-wheel drive vehicle to be able to make a small can facilitate the operation of the vehicle as much as possible the turning radius of the vehicle.

(Means for Solving Problems) In order to achieve the above-mentioned object, a transmission torque control device for a four-wheel drive vehicle according to the present invention sets a final reduction ratio in a rear wheel side torque transmission path to a front wheel side torque transmission path. The clutch mechanism is made to be larger than the final reduction ratio, and the clutch mechanism is interposed in series in the torque transmission path in which the fluid type transmission means is interposed, so that the clutch mechanism is intermittently controlled based on the difference in the rotational speeds of the front and rear wheels. To be done.

Then, specifically, the torque generated by the power plant is interposed in either one of the front wheel side and rear wheel side torque transmission paths for transmitting the torque to the front wheels and the rear wheels, and the number of input side circuits and the number of output side circuits According to the difference between the hydraulic transmission means and the hydraulic transmission means for changing the transmission torque, and the front-wheel side or rear-wheel side torque transmission path in which the hydraulic transmission means is interposed, the hydraulic transmission means is connected in series to the hydraulic transmission means. The clutch mechanism, the front wheel side speed reduction mechanism that sets the final reduction ratio in the front wheel side torque transmission path, and the final reduction ratio in the rear wheel side torque transmission path that is set to be greater than the final reduction ratio in the front wheel side torque transmission path. The signals obtained from the first and second rotational speed detecting means for detecting the input shaft rotational speeds of the wheel side speed reducing mechanism, the front wheel side and the rear wheel side speed reducing mechanism, respectively, and the signals obtained from the first and second rotational speed detecting means. Then, when it is detected that the difference between the input shaft rotation speed of the front wheel side reduction gear mechanism and the input shaft rotation speed of the rear wheel side reduction gear mechanism has a negative value, the control means for disengaging the clutch mechanism. And is configured.

(Operation) In the transmission torque control device for a four-wheel drive vehicle according to the present invention configured as described above, the final reduction ratio in the rear wheel side torque transmission path is higher than the final reduction ratio in the front wheel side torque transmission path. Therefore, when the vehicle travels straight ahead, the input shaft speed of the rear wheel speed reduction mechanism becomes higher than the input shaft speed of the front wheel speed reduction mechanism, and when the clutch mechanism is engaged, Rotational speed difference Δn in FIG.
Is, for example, a negative value Δn _b . Therefore, the control means sets the clutch mechanism to the disengaged state to bring the vehicle into the two-wheel drive state. As a result, the running resistance of the vehicle is reduced, the driving efficiency is improved when the vehicle is running straight, and fuel efficiency and durability of each part are prevented from being lowered.

Further, when the difference (rotational speed difference Δn) between the input shaft rotation speed of the front wheel side reduction gear mechanism and the input wheel rotation speed of the rear wheel side reduction gear mechanism takes a zero or positive value, that is, the front wheels slip to some extent. In this case, the clutch mechanism is brought into the connected state by the control means, so that the torque generated by the power plant is transmitted to the rear wheels by the amount corresponding to the slip amount of the front wheels via the hydraulic transmission means. As a result, the vehicle is brought into the four-wheel drive state and has great running performance.

On the other hand, when the vehicle is in the turning traveling state, the rotation speed of the front wheels becomes higher than the rotation speed of the rear wheels, and the input shaft rotation speed of the front wheel side reduction gear mechanism becomes higher than the input shaft rotation speed of the rear wheel side reduction gear mechanism. The control means brings the clutch mechanism into the connected state. In this case, since the final reduction ratio in the rear wheel side torque transmission path is set to be higher than the final reduction ratio in the front wheel side torque transmission path, as shown in FIG.
n is a value Δn smaller than a value Δn _c when the final reduction ratios in the rear wheel side and front wheel side torque transmission paths are the same.
_c ′ Therefore, when the torque generated by the power plant is reduced from T _A to T _A ′, the driving torque Tr ′ is transmitted to the rear wheels and the driving torque is transmitted to the front wheels without transmitting the braking torque. Tf ″ is transmitted. As a result, the driving efficiency during turning of the vehicle is improved and the expansion of the turning radius is suppressed.

(Example) Hereinafter, the Example of this invention is described with reference to drawings.

FIG. 1 schematically shows an example of a transmission torque control device for a four-wheel drive vehicle according to the present invention, together with a vehicle to which the transmission torque control device is applied. In FIG. 1, the torque generated by the power plant 10 including the engine and the transmission is transmitted to the pair of left and right front wheels 12 via the front wheel side torque transmission path indicated by the one-dot chain line arrow F and the one-dot chain line arrow R. Through the rear wheel side torque transmission path indicated by
It is transmitted to a pair of left and right rear wheels 14 having the same diameter as 2.

The front wheel-side torque transmission path F has a pair of gears 1 having the same number of teeth.
6A and 16B and a final gear unit 18 including a front wheel differential gear and the like are interposed. Here, comprised of a final gear unit 18 and the like, the final reduction ratio of the final reduction mechanism in the front wheel torque transmission path F is set to i _f. On the other hand, in the rear wheel side torque transmission path R, the input side rotary shaft 19 and the output side rotary shaft 2 are provided.
1, a viscous fluid clutch 20 serving as a fluid type transmission means for changing its transmission torque, an electromagnetic clutch 22 arranged in series with this viscous fluid clutch 20, and a rear wheel differential gear. And a final gear unit 24 composed of the same.
Here, the final gear unit 24, etc.,
The final reduction ratio of the final reduction mechanism in the rear wheel side torque transmission route R is the final reduction ratio i in the front wheel side torque transmission route F.
It is set to i _r with a value greater than _f .

Further, in this example, the control unit 100 is provided to automatically perform the above-mentioned on / off control of the electromagnetic clutch 22. The control unit 100 includes a front wheel rotation speed sensor 40 for detecting the rotation speed of the input shaft 26 of the final gear unit 18 and the input shaft 2 of the final gear unit 24.
Rotation detection signals Sf and S, which are obtained from a rear wheel side rotation speed sensor 50 that detects the rotation speed of seven wheels, according to the rotation speeds of the front wheel 12 and the rear wheel 14 (both are the average rotation speeds of the left and right wheels).
r is supplied.

The control unit 100 uses the rotation detection signals Sf and S described above.
The input shaft 26 corresponding to the rotation speed of the front wheel 12 based on r
When the value obtained by subtracting the rotation speed of the input shaft 27 corresponding to the rotation speed of the rear wheel 14 from zero is greater than or equal to zero, an operation signal Cc for connecting the electromagnetic clutch 22 is generated. Then, this is supplied to the electromagnetic clutch 22. As a result, the solenoid of the electromagnetic clutch 22 is excited to bring the electromagnetic clutch 22 into the connected state, the output side rotation shaft 21 and the input shaft 27 are connected, and the viscous fluid clutch 2 is connected.
The output side of 0 and the input side of the final gear unit 24 are connected. On the other hand, the control unit 100 supplies the operation signal Cc when the value obtained by subtracting the rotational speed of the input shaft 27 on the rear wheel side from the rotational speed of the input shaft 26 on the front wheel side is less than or equal to zero or a value in the vicinity thereof. Stop. As a result, the solenoid of the electromagnetic clutch 22 is demagnetized and the electromagnetic clutch 2
2 is cut off, and the connection between the output side of the viscous fluid clutch 20 and the input side of the final gear unit 24 is cut off.

When the power plant 10 is operated under the configuration as described above, the torque generated by the power plant 10 becomes
It is transmitted to the front wheels 12 via the front wheel side torque transmission path F,
The vehicle is put into a running state. In this case, when the vehicle is traveling straight ahead, the final reduction ratio i _r in the rear wheel side torque transmission route R is equal to the final reduction ratio i in the front wheel side torque transmission route F.
_Since the rotation speed of the input shaft 26 on the front wheel side is lower than the rotation speed of the input shaft 27 on the rear wheel side, the control unit 100 stops supplying the operation signal Cc to the electromagnetic clutch 22. Then, the electromagnetic clutch 22 is turned off.
As a result, when the vehicle is traveling straight ahead, the vehicle is in the two-wheel drive state, and the traveling resistance is reduced. As a result, the driving efficiency of the vehicle is improved, and the fuel efficiency and the durability of each part are prevented from being lowered.

Further, when the front wheels 12 slip to some extent while the power plant 10 is operating, the rotational speed difference Δn in FIG. 4 described above has a positive value. Therefore, the control unit 100 causes the electromagnetic clutch 2 to
The operation signal Cc is supplied to 2 to bring it into the connected state.
As a result, the torque generated by the power plant 10 is transmitted to the rear wheels 14 via the viscous fluid clutch 20 by an amount corresponding to the slip amount of the front wheels 12. As a result, the vehicle is brought into the four-wheel drive state and has great running performance.

On the other hand, when the vehicle is turning, since the turning radius of the front wheels 12 is larger than the turning radius of the rear wheels 14, the rotation speed of the input shaft 26 on the front wheel side is higher than that of the input shaft 27 on the rear wheel side. Become. Therefore, the control unit 100 supplies the actuation signal Cc to the electromagnetic clutch 22 to bring it into the connected state.
In this case, the final reduction ratio final drive ratio i _r at the rear wheel side torque transmission path R is the front wheel torque transmission path F i _r
Therefore, the rotational speed difference Δn becomes
As shown in the drawing, the value Δn _c ′ is smaller than the value Δn _c when the final reduction ratios _if and i _r in the front wheel side torque transmission path F and the rear wheel side torque transmission path R are the same. Accordingly, at the time of cornering of the vehicle, even if the torque power plant 10 is generated is made to decrease the T _A 'from T _A, without the braking torque to the front wheels 12 is transmitted, driving torque Tf "is transmitted As a result, when the vehicle is turning, the driving efficiency is improved, the turning radius is suppressed from increasing, and the driving control of the vehicle is facilitated.

As described above, in the present example, the control unit 100 performs the on-off control of the electromagnetic clutch 22 so that the four-wheel drive state changes to the two-wheel drive state or the four-wheel drive state changes to the four-wheel drive state. Switching is automatically performed, but depending on the timing of this switching, switching shock may occur due to the operating time of the electromagnetic clutch 22. Therefore, in this example, in order to prevent such a switching shock, the control unit 100 takes the operating time (time lag) of the electromagnetic clutch 22 into consideration so as to change the electromagnetic clutch 22 from the connected state to the disconnected state or the disconnected state. In order to switch from the state to the connected state, the supply timing and the stop timing of the actuation signal Cc to the electromagnetic clutch 22 are controlled.

Assuming that the rotational speed difference Δn changes with time as shown in FIG. 2, the target timing for switching the electromagnetic clutch 22 from the disengaged state to the connected state is that the rotational speed difference Δn is greater than zero. Time t ₂ at which the above-mentioned value Δn _D is reached from a negative value whose absolute value is made larger than the slightly smaller value Δn _D
The target time to switch the electromagnetic clutch 22 from the connected state to the disconnected state is a positive value whose absolute value is larger than the value Δn _D ′ at which the rotational speed difference Δn is slightly larger than zero. Is a time t _{5 at} which the value Δn _D ′ of In this way, the disengagement timing of the electromagnetic clutch 22 is set to the rotational speed difference Δ.
By n is to hasten slightly from the time t ₃ and t ₆ becomes zero, the portion between the value [Delta] n _D 'in the value [Delta] n _D and time t ₅ at time t ₂ is the so-called, it is a dead zone of control Thus, when the rotational speed difference Δn slightly fluctuates in this portion, a situation in which the electromagnetic clutch 22 is frequently disengaged is avoided.

Then, as described above, in order to switch the electromagnetic clutch 22 from the disengaged state to the connected state at the time point t ₂ , the operating time t _K when the electromagnetic clutch 22 switches from the disengaged state to the connected state is taken into consideration at the time point t 2. It is necessary to supply the operation signal Cc to the electromagnetic clutch 22 at the time point t _{1 which} is earlier than the operation time t _{K by 2} and, in order to switch the electromagnetic clutch 22 from the connected state to the disconnected state at the time point t ₅ , the electromagnetic clutch 22 the operating time t _K when switched to the cutoff state from the connected state 'in anticipation of operating time from the time t ₅ t _K'
It is necessary to stop the supply of the actuation signal Cc to the electromagnetic clutch 22 at time t ₄ , which is only as early as possible. Therefore, the rate of change d / dtΔn of the rotational speed difference Δn (in FIG. 2, the rate of change d / dtΔn at the times t ₁ and t ₄ is dN ₁
And dN ₄ and has been operating time t _K or a and are) indicated, 'by multiplying the operating time t _K or t _K' t _K calculates a variation amount of the rotation speed difference Δn in a period corresponding to, The operation time t _K is calculated by adding the calculated change amount of the rotational speed difference Δn to the rotational speed difference Δn at that time.
Alternatively, the rotational speed difference Δn after a period corresponding to t _K ′
Expected, when the expected rotational speed difference [Delta] n is equal to or greater than the value [Delta] n _D, or, when a value [Delta] n _D 'less a
The control unit 100 sends the operation signal C to the electromagnetic clutch 22.
The timing when c should be supplied, or the electromagnetic clutch 2
It is the timing at which the supply of the actuation signal Cc to 2 should be stopped.

The control as described above is mainly performed based on the operation of the microcomputer incorporated in the control unit 100, and an example of the program executed by this microcomputer is shown in a flowchart in FIG.

Hereinafter, the control operation of the control unit 100 will be described with reference to the flowchart of FIG.

The program shown in FIG. 3 starts, for example, when the power plant 10 is operated, and after the start, a rotation detection signal obtained from the front wheel side rotation speed sensor 40 and the rear wheel side rotation speed sensor 50 in process 101. Input Sf and Sr respectively and proceed to process 102. Process 1
In 02, based on the rotation detection signals Sf and Sr input in the process 101, the front wheel side input shaft speed Nf of the front wheel side input shaft 26 and the rear wheel side input shaft speed Nr of the rear wheel side input shaft 27. And the rotational speed difference Δn calculated in the process 102 in the subsequent decision 103 is zero or a negative value, that is, whether the front wheel side input shaft rotational speed Nf is the rear wheel side. Input shaft speed Nr
It is determined whether or not When it is determined that the rotation speed difference Δn is zero or a negative value, the process proceeds to the process 104, and conversely, when it is determined that the rotation speed difference Δn is a positive value. , Go to process 107.

In the process 104 to be executed when the decision 103 determines that the rotational speed difference Δn is zero or a negative value, the rotational speed difference Δn is added to the change rate d / dt of the rotational speed difference Δn.
Δn multiplied by the operating time t _K in, by adding the variation amount of the rotation speed difference Δn in a period corresponding to the operation time t _K, the value Δn expected rotational speed difference after a period corresponding to the operation time t _K
Calculate _P , and then proceed to decision 105. In decision 105 determines whether the value [Delta] n _P is the value [Delta] n _D above, when the value [Delta] n _P is the value [Delta] n _D above, in order to a connected state of the electromagnetic clutch 22 from the cutoff state, the process Proceeding to 106, the operation signal Cc is supplied to the electromagnetic clutch 22 and the process 101 is returned to. Further, when it is determined in the decision 105 that the value Δn _P is not greater than or equal to the value Δn _D , the process proceeds to the process 109 to stop the supply of the operation signal Cc to the electromagnetic clutch 22 in order to maintain the disengaged state of the electromagnetic clutch 22. The state that has been continued is continued and the process 101 is returned to.

On the other hand, in the decision 103 described above, the rotational speed difference Δn
Process 10 to proceed if is determined to be greater than zero
7, the rotational speed difference Δn is the same as in the process 104.
, The operation time rate of change d / dtΔn rotational speed difference Δn (<0) 'multiplied by the operating time t _K' t _K by adding the variation amount of the rotation speed difference [Delta] n in the period corresponding to the operating time The expected rotational speed difference value Δn _P ′ after the period corresponding to t _K ′ is calculated and the process proceeds to decision 108. In decision 108, if the value [Delta] n _P 'is the value [Delta] n _D' is equal to or less than a value [Delta] n _P 'is the value [Delta] n _D' is less,
In order to change the electromagnetic clutch 22 from the connected state to the disconnected state, the process 109 is proceeded to, and the operation signal C to the electromagnetic clutch 22 is sent.
The supply of c is stopped and the process 101 is returned to. If the decision 108 determines that the value Δn _P ′ is not less than or equal to the value Δn _D ′, the process proceeds to process 106 to maintain the connection state of the electromagnetic clutch 22, and the operation signal Cc to the electromagnetic clutch 22 is transmitted. Continue supply and process 10
Return to 1.

In the above example, the case where the viscous fluid clutch 20 and the electromagnetic clutch 22 are interposed in the rear wheel torque transmission path R has been described, but the present invention is not limited to this, and the front wheel torque transmission path F is not limited to this. Alternatively, the viscous fluid clutch 20 and the electromagnetic clutch 22 may be interposed.

(Effects of the Invention) As is apparent from the above description, according to the transmission torque control device for a four-wheel drive vehicle of the present invention, the hydraulic transmission is provided to either one of the front wheel side and rear wheel side torque transmission paths. Means is interposed, a clutch mechanism is arranged in series therewith, the final reduction ratio in the rear wheel side torque transmission path is set to be higher than the final reduction ratio in the front wheel side torque transmission path, and the clutch mechanism is a control means. Since the on-off control is performed at a predetermined timing by the vehicle, when the vehicle is in a straight running state, a two-wheel drive state with less running resistance is obtained, and when the front wheels of the running vehicle slip to some extent, It is possible to obtain a four-wheel drive state with great flexibility. For this reason, the four-wheel drive state and the two-wheel drive state are appropriately selected according to the traveling state of the vehicle, etc., and drive efficiency can be improved without complicated operations, and fuel consumption and fuel consumption can be improved. It is possible to prevent deterioration of durability of each part.

Further, since the final reduction ratio in the rear wheel side torque transmission path is set to be larger than the final reduction ratio in the front wheel side torque transmission path, even if the torque generated by the power plant during turning of the vehicle is reduced, Since the braking torque is prevented from being transmitted, the turning radius of the vehicle can be made as small as possible to facilitate the driving control of the vehicle and improve the driving efficiency.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an example of a transmission torque control device for a four-wheel drive vehicle according to the present invention together with a vehicle to which it is applied, and FIG. 2 is provided for explaining the operation of the example shown in FIG. FIG. 3 is a characteristic diagram, FIG. 3 is a flowchart showing an example of an operation program of a microcomputer in an example of the control unit used in the example shown in FIG. 1, and FIG. 4 is a characteristic diagram used for explaining the action of the viscous fluid clutch. Is. In the figure, 10 is a power plant, 12 is a front wheel, 14 is a rear wheel, 18 and 24 are final gear units, 20 is a viscous fluid clutch, 22 is an electromagnetic clutch, 40 is a front wheel side rotation speed sensor, and 50 is a rear wheel side rotation. A number sensor, 100 is a control unit.

Claims (1)

[Claims] 1. A difference between an input side rotational speed and an output side rotational speed of the power plant is interposed in either one of a front wheel side and a rear wheel side torque transmission path for transmitting a torque generated by a power plant to a front wheel and a rear wheel. Fluid transmission means for changing the transmission torque in accordance with the above, and a front and rear wheel side torque transmission path in which the fluid transmission means is interposed to connect and disconnect the fluid transmission means in series with the fluid transmission means. A clutch mechanism, a front wheel speed reduction mechanism that sets a final reduction ratio in the front wheel torque transmission path, and a final speed reduction ratio in the rear wheel side torque transmission path set to be larger than a final speed reduction ratio in the front wheel side torque transmission path. Rear wheel speed reducing mechanism, first and second speed detecting means for detecting the input shaft speeds of the front wheel side and rear wheel speed reducing mechanisms, respectively, and the first and second speed detecting means. When it is detected that the difference between the input shaft rotational speed of the front wheel side reduction mechanism and the input shaft rotational speed of the rear wheel side reduction mechanism becomes a negative value based on the signal obtained from A transmission torque control device for a four-wheel drive vehicle, comprising: a control unit that brings the mechanism into a cutoff state.