Patent Publication Number: US-11642959-B2

Title: Transfer for four wheel drive vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2021-115577 filed on Jul. 13, 2021, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The technology relates to a transfer for a four wheel drive vehicle. Japanese Unexamined Patent Application Publication (JP-A) No. H01-182128 discloses a four wheel drive vehicle including a propeller shaft that transmits power between a front wheel drive axle and a rear wheel drive axle. 
     SUMMARY 
     An aspect of the technology provides a transfer for a vehicle. The vehicle is a wheel drive vehicle and includes a first driving wheel, a second driving wheel, a power source, a first transmission mechanism, and a second transmission mechanism. The first driving wheel and the second driving wheel are arranged in longitudinally spaced relation. The first transmission mechanism is configured to transmit power to the first driving wheel. The second transmission mechanism is configured to transmit power to the second driving wheel. The transfer includes a first distribution mechanism and a second distribution mechanism. The first distribution mechanism includes a first clutch and is configured to distribute a portion of power from the power source to the second transmission mechanism through the first clutch. The first clutch is configured to be controlled to a half-engaged state between engagement and disengagement. The second distribution mechanism includes a second clutch and is configured to distribute a portion of power from the power source to the second transmission mechanism through the second clutch. The second clutch is configured to be controlled to the half-engaged state. A first difference is different from a second difference. The first difference is a difference between a gear ratio from an input shaft of the first clutch to the first driving wheel and a gear ratio from an output shaft of the first clutch to the second driving wheel. The second difference is a difference between a gear ratio from an input shaft of the second clutch to the first driving wheel and a gear ratio from an output shaft of the second clutch to the second driving wheel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the technology and are incorporated in and constitute a part of this specification. The drawings illustrate example embodiments and, together with the specification, serve to explain the principles of the technology. 
         FIG.  1 A  is a schematic diagram of a drive mechanism of a vehicle including a transfer according to a first embodiment of the technology. 
         FIG.  1 B  is a schematic diagram of a drive mechanism of a vehicle including a transfer according to a modification example of the first embodiment. 
         FIG.  2    is a block diagram of a configuration of a control system of the vehicle according to the first embodiment. 
         FIG.  3 A  is a graph that illustrates relation between a degree of engagement of a first clutch and power to be distributed to front wheels. 
         FIG.  3 B  is a graph that illustrates relation between a degree of engagement of a second clutch and the power to be distributed to the front wheels. 
         FIG.  4    is a time chart that illustrates a control example of the transfer according to the first embodiment. 
         FIG.  5 A  illustrates a power distribution example in a period of straight travel in  FIG.  4   . 
         FIG.  5 B  illustrates a power distribution example in a period of a start of cornering in  FIG.  4   . 
         FIG.  6    is a flowchart of an example of control processing to be carried out by a processor. 
         FIG.  7    is a schematic diagram of a configuration of a drive system of a vehicle including a transfer according to a second embodiment of the technology. 
         FIG.  8    is a schematic diagram of an example where the transfer according to the second embodiment is mounted on a vehicle including a transverse power source. 
     
    
    
     DETAILED DESCRIPTION 
     In the four wheel drive vehicle disclosed in JP-A No. H01-182128, a lock-up clutch, a speed increasing clutch, and a speed increasing mechanism are provided along the propeller shaft. One of the speed increasing clutch or the lock-up clutch is controlled to engagement while the other is controlled to disengagement. Switching which clutch to engage allows for two-stage switching of a power distribution ratio between front wheels and rear wheels. 
     It is desirable to provide a transfer for a four wheel drive vehicle that makes it possible to control a power distribution ratio in a wide range both on straight travel and when cornering. 
     In the following, some example embodiments of the technology are described in detail with reference to the accompanying drawings. Note that the following description is directed to illustrative examples of the disclosure and not to be construed as limiting to the technology. Factors including, without limitation, numerical values, shapes, materials, components, positions of the components, and how the components are coupled to each other are illustrative only and not to be construed as limiting to the technology. Further, elements in the following example embodiments which are not recited in a most-generic independent claim of the disclosure are optional and may be provided on an as-needed basis. The drawings are schematic and are not intended to be drawn to scale. Throughout the present specification and the drawings, elements having substantially the same function and configuration are denoted with the same reference numerals to avoid any redundant description. In addition, elements that are not directly related to any embodiment of the technology are unillustrated in the drawings. 
     In this specification, in the notation “a gear ratio from A to B”, the gear ratio means “a rotation speed of A divided by a rotation speed of B (rotation speed of A/rotation speed of B)”. That is, in a case where rotary motion is transmitted from A to B, the gear ratio as mentioned above corresponds to a speed reducing ratio. In the notation “a gear ratio to front wheels  2 ”, the gear ratio means a gear ratio on straight travel with the left and right front wheels  2  rotating at the same speed. Likewise, in the notation “a gear ratio to rear wheels  3 ”, the gear ratio means a gear ratio on straight travel with the left and right rear wheels  3  rotating at the same speed. In the description of the embodiments, the notation “a gear is fitted to a shaft” means that the gear and the shaft rotate unitarily. 
     &lt;First Embodiment&gt; 
       FIG.  1 A  is a schematic diagram of a drive mechanism of a vehicle including a transfer according to a first embodiment of the technology. A vehicle  1  in  FIG.  1 A  includes a four wheel drive vehicle. The vehicle  1  may include, without limitation, the two front wheels  2  and the two rear wheels  3 . The two front wheels  2  include the left and right front wheels  2 . The two rear wheels  3  include the left and right rear wheels  3 . In one embodiment of the technology, the front wheels  2  may each serve as a “a first driving wheel” and the rear wheels  3  may each serve as a “second driving wheel”. In the vehicle  1  including four wheels, the four wheel drive vehicle is also referred to as an all wheel drive vehicle. In a case with a vehicle including six or more wheels, the four wheel drive vehicle may mean a vehicle in which solely four wheels in longitudinally and laterally spaced arrangement serve as driving wheels, or alternatively, the four wheel drive vehicle may mean a vehicle in which six or more wheels serve as driving wheels. 
     As illustrated in  FIG.  1 A , the vehicle  1  may include, without limitation, a power source  11 , a torque converter  12 , and a transmission  13 . The torque converter  12  may receive an output of the power source  11  to amplify torque. The transmission  13  may perform shifting of an output of the torque converter  12 . The power source  11  may include, without limitation, an engine, or an internal combustion engine. Alternatively, the power source  11  may include, without limitation, an electric motor. The transmission  13  may include, without limitation, a forward and reverse switching mechanism  13   a  and a CVT (Continuously Variable Transmission)  13   b.  The transmission  13  may transmit power to a front wheel transmission shaft  142  through a gear g 13 . 
     The vehicle  1  may further include, without limitation, a first transmission mechanism  14 , a second transmission mechanism  17 , and a transfer  20 . The first transmission mechanism  14  may transmit power to the front wheels  2 . The second transmission mechanism  17  may transmit power to the rear wheels  3 . The transfer  20  may distribute power between the first transmission mechanism  14  and the second transmission mechanism  17 . 
     The first transmission mechanism  14  may include, without limitation, an input gear g 141 , a front wheel transmission shaft  142 , and a differential gear  143 . The input gear g 141  may receive power from the transmission  13 . The front wheel transmission shaft  142  may extend in a longitudinal direction of the vehicle  1 . The differential gear  143  may distribute rotation of the front wheel transmission shaft  142  to the left front wheel  2  and the right front wheel  2 . The input gear g 141  may be fitted to the front wheel transmission shaft  142 . 
     The second transmission mechanism  17  may include, without limitation, a rear wheel transmission shaft  171  and a differential gear  172 . The rear wheel transmission shaft  171  may extend in the longitudinal direction of the vehicle  1 . The differential gear  172  may distribute rotation of the rear wheel transmission shaft  171  to the left rear wheel  3  and the right rear wheel  3 . 
     The transfer  20  may include, without limitation, a first distribution mechanism  201  and a second distribution mechanism  202 . The transfer  20  is configured to distribute power from the front wheel transmission shaft  142  to the rear wheel transmission shaft  171  by two paths through the two distribution mechanisms  201  and  202 . The first distribution mechanism  201  may include, without limitation, a first driven gear g 21  and a first clutch c 23 . The first driven gear g 21  may be fitted to an input shaft of the first distribution mechanism  201 . The first clutch c 23  may be interposed between the input shaft and the rear wheel transmission shaft  171 . The second distribution mechanism  202  may include, without limitation, a second driven gear g 22  and a second clutch c 24 . The second driven gear g 22  may be fitted to an input shaft of the second transfer mechanism  202 . The second clutch c 24  may be interposed between the input shaft and the rear wheel transmission shaft  171 . A driving gear g 15  may be fitted to the front wheel transmission shaft  142 . The driving gear g 15  may be engaged with the first driven gear g 21  and the second driven gear g 22 . The transfer  20  may introduce power from the front wheel transmission shaft  142  through the first driving gear g 15 . 
     In one embodiment of the technology, out of the constituent elements mentioned above, a combination of the driving gear g 15  and the first driven gear g 21  may serve as a “first gear set”. In one embodiment of the technology, a combination of the driving gear g 15  and the second driven gear g 22  may serve as a “second gear set”. 
     The first clutch c 23  may include, without limitation, an input shaft i 23 , an output shaft o 23 , a driven clutch plate, a drive clutch plate, a clutch housing, and a clutch hub. The second clutch c 24  may include, without limitation, an input shaft i 24 , an output shaft o 24 , a driven clutch plate, a drive clutch plate, a clutch housing, and a clutch hub. The first clutch c 23  may include, for example, a hydraulically controlled multi-disk clutch. The first clutch c 23  may be brought to a half-engaged state as an intermediate state between engagement and disengagement by a hydraulic control. The engagement means a state of about 100% torque transmission from the input shaft i 23  to the output shaft o 23 . The disengagement means about 0% torque transmission from the input shaft i 23  to the output shaft o 23 . The first clutch c 23  is configured to change continuously a torque transmission rate in a predetermined range of, for example, 0% to 1 00 % by changing pressure of hydraulic oil to be supplied. The second clutch c 24  may be configured similarly to the first clutch c 23 . It is to be noted that the first clutch c 23  and the second clutch c 24  may have a configuration in which the torque transmission rate is continuously controlled by power of an electric actuator. 
     The first clutch c 23  and the second clutch c 24  may be disposed in adjacency to each other in an axial direction, i.e., a direction along a rotation axis. Furthermore, the driven clutch plate and the drive clutch plate of the first clutch c 23 , and the driven clutch plate and the driven clutch plate of the second clutch c 24  may have the same respective sizes. Moreover, the clutch housing of the first clutch c 23  and the clutch housing of the second clutch c 24  may be unitized. The clutch housing may be fitted to the driven clutch plate of the first clutch c 23  and rotate unitarily therewith, and be fitted to the driven clutch plate of the second clutch c 24  and rotate unitarily therewith. The clutch housing may be linked to the output shafts o 23  and o 24 , and to the rear wheel transmission shaft  171 , to transmit power to the rear wheel transmission shaft  171 . 
     The number of teeth of the first driven gear g 21  may be different from the number of teeth of the second driven gear g 22 . In one example, the number of teeth of the first driven gear g 21  may match the number of teeth of the driving gear g 15 , and the number of teeth of the second driven gear g 22  may be greater than the number of teeth of the driving gear g 15 . 
     On a power transmission path between the input shaft i 23  of the first clutch c 23  and the front wheels  2 , the gear set of the first driven gear g 21  and the driving gear g 15 , and the differential gear  143  may be interposed. Accordingly, a gear ratio of the gear set and a gear ratio of the differential gear  143  determine an overall gear ratio from the input shaft i 23  to the front wheels  2 . The overall gear ratio from the input shaft i 23  to the front wheels  2  equals to a rotation speed of the input shaft i 23  divided by a rotation speed of the front wheels  2 . The gear ratio is referred to as a “gear ratio frontward of the first clutch c 23 ”. 
     On a power transmission path between the output shaft o 23  of the first clutch c 23  and the rear wheels  3 , the differential gear  172  may be interposed. Accordingly, a gear ratio of the differential gear  172  determines an overall gear ratio from the output shaft o 23  to the rear wheels  3 . The overall gear ratio from the output shaft o 23  to the rear wheels  3  equals to a rotation speed of the output shaft o 23  divided by a rotation speed of the rear wheels  3 . The gear ratio is referred to as a “gear ratio rearward of the first clutch c 23 ”. 
     On a power transmission path between the input shaft i 24  of the second clutch c 24  and the front wheels  2 , the gear set of the second driven gear g 22  and the driving gear g 15 , and the differential gear  143  may be interposed. Accordingly, a gear ratio of the gear set and the gear ratio of the differential gear  143  determine an overall gear ratio from the input shaft i 24  to the front wheels  2 . The overall gear ratio from the input shaft i 24  to the front wheels  2  equals to a rotation speed of the input shaft i 24  divided by the rotation speed of the front wheels  2 . The gear ratio is referred to as a “gear ratio frontward of the second clutch c 24 ”. 
     On a power transmission path between the output shaft o 24  of the second clutch c 24  and the rear wheels  3 , the differential gear  172  may be interposed. Accordingly, the gear ratio of the differential gear  172  determines an overall gear ratio from the output shaft o 24  to the rear wheels  3 . The overall gear ratio from the output shaft o 24  to the rear wheels  3  equals to a rotation speed of the output shaft o 24  divided by the rotation speed of the rear wheels  3 . The gear ratio is referred to as a “gear ratio rearward of the second clutch c 24 ”. 
     On the power transmission path from the first clutch c 23  or the second clutch c 24  to the front wheels  2 , or on the power transmission path from the first clutch c 23  or the second clutch c 24  to the rear wheels  3 , a configuration having another gear ratio may be interposed. In this case, the overall gear ratio as mentioned above may be determined from gear ratios of a plurality of configurations including the gear ratio. 
     A first difference is different from a second difference. The first difference is a difference between the gear ratio frontward of the first clutch c 23  and the gear ratio rearward of the first clutch c 23 . The second difference is a difference between the gear ratio frontward of the second clutch c 24  and the gear ratio rearward of the second clutch c 24 . 
     In one example, the gear ratio frontward of the second clutch c 24  may be greater than the gear ratio rearward of the second clutch c 24 . As to the difference between the gear ratios, the rear ratio rearward may be greater than the gear ratio frontward, by about 0.5% to 3%. By the difference between the gear ratios, the input shaft i 24  of the second clutch c 24  rotates at a higher speed than the output shaft o 24  of the second clutch c 24  on straight travel with the front wheels  2  and the rear wheels  3  rotating at the same speed. Accordingly, bringing the second clutch c 24  to the half-engaged state causes transmission of a motive force from the input shaft i 24  to the output shaft o 24 , and transmission of a braking force, i.e., internal circulation torque, from the output shaft o 24  to the input shaft i 24 . 
     The gear ratio frontward of the first clutch c 23  may be equal to the gear ratio rearward of the first clutch c 23 . The term “equal” is not limited to exact coincidence, but includes coincidence including a negligible error as compared with the difference between the gear ratios frontward and rearward of the second clutch c 24 , e.g., 0.5% to 3%. 
       FIG.  1 B  is a schematic diagram of a modification example of the transfer according to the first embodiment. As illustrated in  FIG.  1 B , the transfer  20  may include two driving gears g 15   a  and g 15   b  that differ in the number of teeth, as driving gears that transmit power from the front wheel transmission shaft  142 . In the modification example, the driving gear g 15   a  may be engaged with the first driven gear g 21 , and the other driving gear g 15   b  may be engaged with the second driven gear g 22 . Such a configuration involves an increase in the number of gears. However, selecting the number of teeth for each of the gears g 15   a,  g 15   b , g 21 , and g 22  makes it possible to easily provide target gear ratios on the transmission path through the first driven gear g 21  and the transmission path through the second driven gear g 22 . 
     In one embodiment of the technology, out of the constituent elements mentioned above, a combination of the driving gear g 15   a  and the first driven gear g 21  may serve as the “first gear set”. In one embodiment of the technology, a combination of the driving gear g 15   b  and the second driven gear g 22  may serve as the “second gear set”. 
       FIG.  2    is a block diagram of a configuration of a control system of the vehicle according to the first embodiment. The vehicle  1  may further include, without limitation, auxiliaries  31 , a driving operation unit  32 , a travel control processor  33 , and sensors  34 . The auxiliaries  31  is provided for driving the power source  11 . The driving operation unit  32  may be operated by a driver who drives the vehicle  1 . The travel control processor  33  may control the auxiliaries  31  in accordance with an operation on the driving operation unit  32 . The sensors  34  may detect a traveling situation. Moreover, the transfer  20  in the first embodiment may include, for example, hydraulic cylinders  37   a  and  37   b,  and a processor  35 . The hydraulic cylinders  37   a  and  37   b  may change a degree of engagement of each of the first clutch c 23  and the second clutch c 24 . The processor  35  may control the hydraulic cylinders  37   a  and  37   b  through control valves  36   a  and  36   b.  The hydraulic cylinders  37   a  and  37   b  may be replaced with electric actuators. 
     The driving operation unit  32  may include, without limitation, an accelerator operation part  32   a,  a brake operation part  32   b,  and a steering part  32   c.  The accelerator operation part  32   a  is provided for accelerating the vehicle  1 . The brake operation part  32   b  is provided for decelerating the vehicle  1 . The steering part  32   c  is provided for controlling a direction of travel of the vehicle  1 . A signal indicating an amount of operation of the accelerator operation part  32   a  may be transmitted to the travel control processor  33  and the processor  35 . A signal indicating amounts of operations of the brake operation part  32   b  and the steering part  32   c  may be transmitted to the processor  35 . 
     The sensors  34  is provided for detecting the traveling situation. The sensors  34  may include, without limitation, a vehicle speed sensor  34   a,  a yaw rate sensor  34   b,  and a travel path sensor  34   c.  The yaw rate sensor  34   b  may measure a change in a yaw angle of the vehicle  1 . The travel path sensor  34   c  may detect entry of the vehicle  1  into a curve. The travel path sensor  34   c  may be configured to cooperate with, for example, a navigation system and predict the entry of the vehicle into a curve on the basis of a preset planned travel path and a present position of the vehicle  1 . Each detection result of the sensors  34  may be transmitted to the processor  35 . 
     The travel control processor  33  may include, without limitation, an ECU (Electronic Control Unit). The travel control processor  33  may include one ECU, or alternatively, the travel control processor  33  may include a plurality of ECUs that communicate with one another and operate in cooperation with one another. 
     The travel control processor  33  may calculate requested torque corresponding to the operation of the accelerator operation part  32   a  and calculate target torque by placing a predetermined restriction on the requested torque. The requested torque means torque requested to be outputted on the basis of the driving operation. The target torque means torque to be controlled to be actually outputted from the power source  11 . Non-limiting examples of the restriction to be placed on the requested torque may include to limit a rate of change in the torque to an upper limit or lower to avoid a sudden change in the torque. After calculating the target torque, the travel control processor  33  may operate the auxiliaries  31  to allow the target torque to be outputted from the power source  11 . In a case where the vehicle  1  comprises an automatic driving control device configured to automatically control an acceleration of the vehicle  1 , the travel control processor  33  may calculate the requested torque and the target torque based on signals output from the automatic driving control device, instead of based on the operation of the accelerator operation part  32   a  by the driver. 
     The travel control processor  33  and the processor  35  are configured to transmit an instruction to increase the torque, from the processor  35  to the travel control processor  33 . Upon receiving the instruction to increase the torque, the travel control processor  33  may add an amount of increase in the torque to the target torque calculated on the basis of the requested torque, to update the target torque. 
     The processor  35  may include, without limitation, an ECU. The processor  35  may include one ECU, or alternatively, the processor  35  may include a plurality of ECUs that communicate with one another and operate in cooperation with one another. A portion or all of the processor  35  may share an ECU or ECUs with the travel control processor  33 . 
     The processor  35  may make a control of the degrees of engagement of the first clutch c 23  and the second clutch c 24  of the transfer  20  on the basis of the signals from the driving operation unit  32  and signals from the sensors  34 . The control of the degrees of engagement makes it possible to provide a control of a power distribution ratio between the front wheels  2  and the rear wheels  3  in accordance with the traveling situation of the vehicle  1 . 
     &lt;Workings of Transfer  20 &gt; 
       FIG.  3 A  is a graph illustrating relation between the degree of engagement of the first clutch c 23  and power to be distributed to the front wheels  2 . The graph illustrates the relation as mentioned above, in a case where a steering angle of the vehicle  1  is zero, i.e., the vehicle  1  is on straight travel, the second clutch c 24  is disengaged, and constant power is being outputted from the power source  11 . 
     In a state in which both the first clutch c 23  and the second clutch c 24  are disengaged, power of the front wheel transmission shaft  142  is not distributed to the rear wheel transmission shaft  171 . Accordingly, power to be outputted to the front wheels  2  becomes greater. This corresponds to a state at a point p 0  of the graph in  FIG.  3 A . 
     In a state in which the first clutch c 23  is engaged and the second clutch c 24  is disengaged, power of the front wheel transmission shaft  142  is transmitted to the rear wheel transmission shaft  171  through the first clutch c 23 . By setting the gear ratios frontward and rearward of the first clutch c 23  as mentioned above, almost half of power is transmitted to the rear wheel transmission shaft  171  because a rotation speed ratio between the input shaft i 23  and the output shaft o 23  of the first clutch c 23  is 1:1. Thus, power to be outputted to the front wheels  2  is reduced by about half. This corresponds to a state at a point p 1  of the graph in  FIG.  3 A . 
     In a state in which the second clutch c 24  is disengaged and the degree of engagement of the first clutch c 23  changes from 0% to 100%, as illustrated in  FIG.  3 A , power to be outputted to the front wheels  2  continuously decreases from a value at the point p 0  to a value at the point p 1 . 
     To the rear wheels  3 , power to be obtained by subtracting the power to be distributed to the front wheels  2  and a loss from the output of the power source  11  is outputted. Accordingly, in a case where the steering angle is zero and the second clutch c 24  is disengaged, changing the degree of engagement of the first clutch c 23  makes it possible to change the power distribution ratio between the front wheels  2  and the rear wheels  3  in a range of about 100:0 to 50:50. 
       FIG.  3 B  is a graph illustrating relation between the degree of engagement of the second clutch c 24  and power to be distributed to the front wheels  2 . The graph illustrates the relation as mentioned above, in a case where the steering angle of the vehicle  1  is zero, i.e., the vehicle  1  is on the straight travel, the first clutch c 23  is disengaged, and constant power is being outputted from the power source  11 . 
     In a state in which both the first clutch c 23  and the second clutch c 24  are disengaged, power of the front wheel transmission shaft  142  is not distributed to the rear wheel transmission shaft  171 . Accordingly, the power to be outputted to the front wheels  2  becomes greater. This corresponds to a state at a point p 10  of the graph in  FIG.  3 B . 
     In a state in which both the front wheels  2  and the rear wheels  3  are traveling on the grip, by setting the gear ratios frontward and rearward of the second clutch c 24  mentioned above, the rotation speed of the input shaft i 24  of the second clutch c 24  becomes slightly higher than the rotation speed of the output shaft o 24 . Accordingly, in a case where the first clutch c 23  is disengaged and the second clutch c 24  is brought to the half-engaged state close to the engagement, power is transmitted from the input shaft i 24  of the second clutch c 24  to the output shaft o 24 . In other words, power is transmitted from the front wheel transmission shaft  142  to the rear wheel transmission shaft  171  through the second clutch c 24 . Thus, traction in a driving direction is applied to the rear wheels  3 . Furthermore, internal circulation torque is generated between the front wheels  2  and the rear wheels  3 , causing traction in a braking direction to be applied to the front wheels  2 . Accordingly, the power to be outputted to the front wheels  2  becomes a total value of the power to be transmitted from the power source  11  and a braking force by the internal circulation torque. This corresponds to a state at a point p 11  of the graph in  FIG.  3 B . 
     Furthermore, in a state in which the degree of engagement of the second clutch c 24  changes from 0% to nearly 100% while the first clutch c 23  is disengaged, as illustrated in  FIG.  3 B , the power to be outputted to the front wheels  2  decreases from a value at the point p 10  to a value at the point p 11 . At the degree of engagement in the middle, the power to be outputted to the front wheels  2  becomes substantially zero. This corresponds to a state at the point p 12  of the graph in  FIG.  3 B . 
     To the rear wheels  3 , power to be obtained by subtracting the power to be distributed to the front wheels  2  and the loss from the output of the power source  11  is outputted. Accordingly, in a case where the steering angle is zero, changing the degree of engagement of the second clutch c 24  from 0% at the point p 10  to the degree of engagement at the point p 12  causes the power distribution ratio to change in a range of front: rear=100:0 to 0:100. However, in a case where the degree of engagement of the second clutch c 24  becomes higher, the internal circulation torque becomes greater, causing an increase in the loss. Accordingly, with the output of the power source  11  being constant, power to be outputted to the front wheels  2  and the rear wheels  3  is reduced. 
     Characteristics of the transfer  20  illustrated in  FIGS.  3 A and  3 B  are those in the case where the steering angle of the vehicle  1  is zero, i.e., the vehicle  1  is on the straight travel. When the vehicle  1  corners, a change occurs in the internal circulation torque to be generated between the front wheels  2  and the rear wheels  3 , in accordance with a radius of bend. This is because a difference occurs between a length of a locus of the front wheel  2  and a length of a locus of the rear wheel  3 . Thus, when cornering, even in a case where the degrees of engagement of the first clutch c 23  and the second clutch c 24  are fixed, a change occurs in the power distribution ratio between the front wheels  2  and the rear wheels  3  in accordance with the radius of bend. 
     &lt;Composite Workings of First Distribution Mechanism  201  and Second Distribution Mechanism  202 &gt; 
     In a case where both the first clutch c 23  and the second clutch c 24  are engaged, the first driven gear g 21  and the second driven gear g 22  do not move. 
     In a case where the first clutch c 23 , the second clutch c 24 , or both are in the half-engaged state, composite power of the power described with reference to  FIG.  3 A  and the power described with reference to  FIG.  3 B  is outputted to the front wheels  2  and the rear wheels  3 . This composition of the power of the power source  11  and the power by the internal circulation torque makes it possible to control the power distribution ratio between the front wheels  2  and rear wheels  3  in a wide range from front: rear≈100:0 to front: rear≈0:100. This is because the power by the internal circulation torque acts negatively. 
     Furthermore, when the vehicle  1  corners, whatever radius of bend is, the internal circulation torque occurring between the front wheels  2  and the rear wheels  3  does not become zero in the first distribution mechanism  201 , the second distribution mechanism  202 , or both. Alternatively, even in a case where the internal circulation torque becomes too large in one of the first transfer mechanism  201  or the second transfer mechanism  202 , the internal circulation torque in the other of the first transfer mechanism  201  or the second transfer mechanism  202  may be suppressed to moderate magnitude. Accordingly, when cornering, whatever the radius of bend is, the composition of the power to be distributed by the first distribution mechanism  201  and the power to be distributed by the second distribution mechanism  202  makes it possible to allow the power to include negative power by the internal circulation torque of the moderate magnitude. Hence, it is possible to control the power distribution ratio between the front wheels  2  and the rear wheels  3  in the wide range from, for example, front: rear≈100:0 to front: rear≈0:100. 
     The internal circulation torque described above is torque to be transmitted from the front wheel  2  to the rear wheel  3  or vice versa on the power transmission path between the front wheel  2  and the rear wheel  3 . The internal circulation torque means torque to be absorbed by, for example, sliding of the first clutch c 23  or the second clutch c 24  and torsion of each mechanism on the power transmission path. 
     It is to be noted that, as for the power distribution ratio controllable by the transfer  20 , the power to be distributed to the front wheels  2  does not have to become zero. For example, the power distribution ratio may range from front: rear=100:0 to 30:70, or alternatively, the power distribution ratio may range from front: rear=100:0 to 20:80. 
     &lt;Control Example of Transfer  20 &gt; 
       FIG.  4    is a time chart illustrating a control example of the transfer according to the first embodiment.  FIG.  5 A  illustrates a power distribution example in a period T 1  of the straight travel in  FIG.  4   .  FIG.  5 B  illustrates a power distribution example in a period T 2  of a start of cornering in  FIG.  4   . In  FIGS.  5 A and  5 B , the composite power is illustrated beside the vehicle  1 . 
       FIG.  4    illustrates a travel example in which the vehicle  1  makes sequential transitions from the straight travel to the cornering, and from the cornering to the straight travel. A line A in  FIG.  4    represents the power distribution ratio to the rear wheels  3  in a case where the control according to the embodiment is made. A line B in  FIG.  4    represents the power distribution ratio to the rear wheels  3  in a case where the first clutch c 23  is engaged. A line C in  FIG.  4    represents the power distribution ratio to the rear wheels  3  in a case where the second clutch c 24  is engaged. 
     As denoted by the lines B and C, in a case where the relation of the gear ratios between the front wheels  2  and the rear wheels  3  is fixed, the power distribution ratio to the rear wheels  3  becomes higher when cornering than on the straight travel. Furthermore, when cornering, the smaller the radius of bend, the higher the power distribution ratio to the rear wheels  3 . This is because the smaller the radius of bend, the longer the locus of the front wheel  2  than the locus of the rear wheel  3 . 
     In the period T 1  of the straight travel, the processor  35  may disengage the second clutch c 24  and control the first clutch c 23  in the engagement or the half-engaged state. By this control, as denoted by the line A, the power distribution ratio ranges from front: rear=50:50 to 70:30. This leads to stability on the straight travel. That is, even in a case where a yaw moment is applied to the vehicle  1  due to a disturbance, an anti-yaw moment acts to restore the straight travel.  FIG.  5 A  illustrates a state in which the second clutch c 24  is disengaged, the first clutch c 23  is in an 80% engaged state, and the power distribution ratio is front: rear=60:40. 
     In the period T 2  of the start of the cornering, the processor  35  may disengage the first clutch c 23  and raise the degree of engagement of the second clutch c 24 . By this control, as denoted by the line A, the power distribution ratio to the rear wheels  3  becomes higher. Furthermore, at this occasion, as illustrated in  FIG.  5 B , the internal circulation torque occurs between the front wheels  2  and the rear wheels  3 . This causes a braking force F 2  due to the internal circulation torque to be applied to the power F 1  to be transmitted from the power source  11  to the front wheel  2 . Thus, the composite power F 3  to be distributed to the front wheels  2  is reduced. The reduction in the composite power F 3  to be distributed to the front wheels  2  causes reduction in a ratio of a grip force to be used in a direction of propulsion among grip forces on the front wheels  2 , while causing an increase in a ratio of a grip force to be used in a lateral direction. That is, a lateral force limit of the front wheel  2  becomes large. The lateral force limit is a limit lateral force that suppresses a side-slip. This leads to enhanced cornering performance of the vehicle  1 . Furthermore, even in a case where the composite power F 3  on the front wheel  2  becomes smaller, the high power distribution ratio to the rear wheels  3  makes it possible to obtain a propulsive force of the vehicle  1  from the rear wheels  3 . With such workings, the vehicle  1  makes the stable transition to the cornering as intended by the driver. 
     In a period T 3  of a middle stage of the cornering, the processor  35  may bring the first clutch c 23  to the half-engaged state, and bring the second clutch c 24  to the half-engaged state. Furthermore, as the radius of bend becomes smaller, the processor  35  may gradually change the first clutch c 23  from the disengagement to the engagement, and gradually change the second clutch c 24  from the engagement to the disengagement. By this control, as denoted by the line A, the radius of bend becomes smaller. This suppresses the power distribution ratio between the front wheels  2  and the rear wheels  3  from changing greatly, and maintains the power distribution ratio between the front wheels  2  and the rear wheels  3  at an appropriate value. Accordingly, as illustrated in  FIG.  5 B , it is possible to maintain the cornering of the vehicle  1  in a state in which the composite power F 3  on the front wheels  2  is appropriately small, and the lateral force limit of the front wheels  2  is large. Moreover, the high power distribution ratio to the rear wheels  3  inhibits reduction in the propulsive force of the vehicle  1 . This makes it possible to realize the stable cornering of the vehicle  1  as intended by the driver. 
     Afterwards, in a period T 4  of steady circle cornering, a period T 5  of a final stage of the cornering, and a period T 6  of the straight travel after the end of the cornering, the processor  35  may maintain the first clutch c 23  at a high degree of engagement, and disengage the second clutch c 24 . By this control and the change in the radius of bend, the power distribution ratio between the front wheels  2  and the rear wheels  3  is gradually restored to the ratio for the straight travel, toward the end of the cornering. Thus, the restoration of the distribution ratio to the ratio for the straight travel causes a gradual decrease in the yaw moment applied to the vehicle  1  when cornering. In the period T 6  of the straight travel, stable straight travel is restored. 
     For example, in the period T 3  of the middle stage of the cornering, both the first clutch c 23  and the second clutch c 24  are controlled to the half-engaged state. On this occasion, the internal circulation torque is generated between the first clutch c 23  and the second clutch c 24 , separately from the internal circulation torque between the front wheels  2  and the rear wheels  3 . Thus, a loss because of clutch sliding occurs. The processor  35  may calculate loss energy caused by the internal circulation torque described above, on the basis of the rotation speeds of the front wheels  2  and the rear wheels  3 , and the degrees of engagement of the first clutch c 23  and the second clutch c 24 . The processor  35  may output the instruction to increase the torque to the travel control processor  33 . To the instruction to increase the torque, data indicating an amount of increase in the torque based on the calculated loss energy described above may be added. 
     Upon receiving the instruction to increase the torque, the travel control processor  33  may add the torque for the loss as mentioned above to the target torque corresponding to the driving operation, to update the target torque. The update of the target torque causes an increase in output torque of the power source  11 . Such a control provides compensation of the loss of the torque caused by bringing both the first clutch c 23  and the second clutch c 24  to the half-engaged state. This suppresses an actual propulsive force of the vehicle  1  from deviating greatly from a propulsive force corresponding to the driving operation by the driver. The internal circulation torque to be generated between the first clutch c 23  and the second clutch c 24  described above is torque to be transmitted between the first clutch c 23  and the second clutch c 24 . The internal circulation torque to be generated between the first clutch c 23  and the second clutch c 24  means torque to be absorbed by, for example, the sliding of the first clutch c 23  or the second clutch c 24 , and the torsion of the mechanisms between the first clutch c 23  and the second clutch c 24 . 
     It is to be noted that, in the period T 2  of the start of the cornering, in a case where a side-slip of a tire may possibly occur, the processor  35  may estimate slip rates of the front wheels  2  and the rear wheels  3  on the basis of the outputs of the vehicle speed sensor  34   a  and the yaw rate sensor  34   b.  The processor  35  may calculate the power distribution ratio between the front wheels  2  and the rear wheels  3  in accordance with the radius of bend, the vehicle speed, and the estimated slip rates. The processor  35  may calculate the degree of engagement of the first clutch c 23  and the degree of engagement of the second clutch c 24 , to provide the distribution ratio in accordance with the radius of bend. In this case, typically, as for both the degree of engagement of the first clutch c 23  and the degree of engagement of the second clutch c 24 , the degrees of engagement in the half-engaged state are obtained as a calculation result. The processor  35  may adjust the degrees of engagement of the first clutch c 23  and the second clutch c 24  to the respective values of the degrees of engagement obtained by the calculation, and thereby make the power distribution between the front wheels  2  and the rear wheels  3  corresponding to the situation described above. With such a control, it is possible to allow the vehicle  1  to corner as intended by the driver. 
     Furthermore, when traveling on a slippery low μ road, the processor  35  may make a feed-forward control of a rotation speed ratio between the front wheels  2  and the rear wheels  3  by changing the power distribution ratio between the front wheels  2  and the rear wheels  3  on the basis of a time change of the steering angle appearing in an operation signal of the steering part  32   c.  In addition, the processor  35  may make a feedback control of the power distribution to the front wheels  2  and the rear wheels  3  on the basis of on the rotation speed ratio between the front wheels  2  and the rear wheels  3  detected by the vehicle speed sensor  34   a . In the feedback feed-forward control described above on the low μ road, it is desirable to smoothly change the power distribution ratio between the front wheels  2  and the rear wheels  3  in a situation where the steering angle varies variously. This is because an abrupt change in the power distribution ratio on the low μ road hinders stable travel. The processor  35  is configured to control the degree of engagement of the first clutch c 23  and the degree of engagement of the second clutch c 24 , and thereby change smoothly the power distribution ratio between the front wheels  2  and the rear wheels  3  even in the situation where the steering angle varies variously. This leads to the stable travel of the vehicle  1  on the low μ road. 
     Moreover, on the low μ road, sliding of the front wheels  2  and the rear wheels  3  may sometimes cause difficulty in controlling behavior of the vehicle  1 . In such a case, in a configuration including either the first clutch c 23  or the second clutch c 24 , it is difficult to control the power distribution ratio between the front wheels  2  and the rear wheels  3  to a desired value depending on the steering angle and the slip rates of the front wheels  2  and the rear wheels  3 . This results in a compulsive stand-by without making any control until the sliding of the front wheels  2  and the rear wheels  3  is eliminated by, for example, resistance of a road surface. In contrast, in this embodiment, the processor  35  is configured to control the first clutch c 23  and the second clutch c 24 , and thereby control the power distribution ratio between the front wheels  2  and the rear wheels  3  to a desired value in many situations in which the steering angle and the slip rates of the front wheels  2  and the rear wheels  3  differ variously. Hence, it is possible to reduce possible uncontrollability of the behavior of the vehicle  1 . 
     &lt;Control Processing by Processor  35 &gt; 
       FIG.  6    is a flowchart illustrating control processing to be carried out by the processor  35 . The control processing in  FIG.  6    may be carried out by the processor  35  while the vehicle  1  is traveling. 
     At a start of the control processing, the processor  35  may determine whether or not it is start timing of a predetermined control cycle (step S 1 ). In a case where it is the start timing, the processor  35  may start the processing. The control cycle is set to a short period of time. 
     As the processing progresses, the processor  35  may acquire the detection result of the traveling path sensor  34   c  and the signal of the steering part  32   c  (step S 2 ), and estimate the traveling state of the vehicle  1  (step S 3 ). 
     Thereafter, the processor  35  may determine whether or not the traveling state is at the start of the cornering (step S 4 ). In negation, the processor  35  may determine whether or not the traveling state is at the middle stage of the cornering (step S 6 ). In negation, the processor  35  may determine whether or not the traveling state is at the final stage of the cornering (step S 13 ). In negation in steps S 4 , S 6  and S 13 , the processor  35  may cause the flow to return to step S 1 , and wait for the start of the next control cycle. The cornering to be determined in steps S 4 , S 6 , and S 13  may be limited to cornering of a predetermined size, e.g., cornering in which a centrifugal force of predetermined magnitude acts on the vehicle  1 . Small cornering may be excluded from the cornering to be determined in steps S 4 , S 6 , and S 13 . 
     In step S 4 , in a case with a determination that the traveling state is at the start of the cornering, the processor  35  may control the first clutch c 23  to the disengagement and control the second clutch c 24  to the engagement (step S 5 ). Thereafter, the processor  35  may cause the flow to return to step S 1 . 
     In step S 6 , in a case with a determination that the traveling state is at the middle stage of the cornering, the processor  35  may, first, calculate the slip rates of the left front wheel  2 , the right front wheel  2 , the left rear wheel  3 , and the right rear wheel  3  on the basis of the outputs of the sensors  34  (step S 7 ). Thereafter, the processor  35  may calculate the power distribution ratio between the front wheels  2  and the rear wheels  3  suitable for the traveling state, on the basis of the radius of bend, the vehicle speed, the yaw moment, and the slip rates (step S 8 ). The processor  35  may calculate the degrees of engagement of the first clutch c 23  and the second clutch c 24  to provide the distribution ratio in step S 8  (step S 9 ). Furthermore, the processor  35  may calculate the loss caused by the internal circulation torque to be generated between the first clutch c 23  and the second clutch c 24  at the degrees of engagement (step S 10 ). The processor  35  may output the instruction to increase the torque to compensate the loss, to the travel control unit  33  (step S 11 ). The processor  35  may control the first clutch c 23  and the second clutch c 24  to provide the degrees of engagement calculated in step S 9  (step S 12 ). Thereafter, the processor  35  may cause the flow to return to step S 1 . 
     In a storage of the processor  35 , map data may be held in advance. The map data represents relation between arguments and results in the calculation of respective steps S 8 , S 9 , and S 10 . The processor  35  may carry out the calculation of respective steps S 8 , S 9 , and S 10  with the use of the map data. The map data may be created by running simulation or running tests of the vehicle  1 . 
     In a case where steps S 4 , S 6 , and S 13  result in a determination that the traveling state is at the final stage of the cornering, the processor  35  may control the first clutch c 23  to the engagement and control the second clutch c 24  to the disengagement (step S 14 ). Thereafter, the processor  35  may cause the flow to return to step S 1 . 
     By the control processing described above, states of the engagement of the first clutch c 23  and the second clutch c 24  of the transfer  20  are switched, to make the power distribution to the front wheels  2  and the rear wheels  3  in accordance with the traveling state of the vehicle  1 . 
     In the control processing described above, an example is given in which the processor  35  carries out the control of steps S 7  to S 12  at the middle stage of the cornering. In one alternative, the processor  35  may carry out the control of steps S 7  to S 12  in various traveling situations. 
     The program of the control processing described above may be held in a non-transitory computer readable medium such as a ROM (Read Only Memory) in the processor  35 . The processor  35  may be configured to read the program held in a portable non-transitory computer readable medium and execute the program. The portable non-transitory computer readable medium as mentioned above may hold the program of the control processing described above. 
     &lt;Second Embodiment&gt; 
       FIG.  7    illustrates a configuration of a drive system of a vehicle including a transfer according to a second embodiment of the technology. A transfer  20 A according to the second embodiment differs from the first embodiment in a gear structure and a clutch structure. Similar configurations to those of the first embodiment are denoted by the same reference characters, and detailed description thereof is omitted. 
     The transfer  20 A may include, without limitation, a planetary gear set  25  and a driven gear g 21 A. The planetary gear set  25  may include an input shaft  25   in  and an output shaft  25   out . The driven gear g 21 A may be fitted to the input shaft  25   in . Furthermore, the transfer  20 A may include a first clutch c 23 A and a second clutch c 24 A. The first clutch c 23 A may be interposed between the input shaft  25   in  and the rear wheel transmission shaft  171 . The second clutch c 24 A may be interposed between the output shaft  25   out  of the planetary gear set  25  and the rear wheel transmission shaft  171 . The first clutch c 23 A may include, without limitation, the input shaft i 23  and the output shaft o 23 . The second clutch c 24 A may include, without limitation, the input shaft i 24  and the output shaft o 24 . To the front wheel transmission shaft  142 , the driving gear g 15  may be fitted. The driving gear g 15  may be engaged with the driven gear g 21 A. The transfer  20 A may introduce power from the front wheel transmission shaft  142  through the driving gear g 15 . 
     In one embodiment of the technology, out of the constituent elements mentioned above, a configuration of a combination of the driven gear g 21 A and the first clutch c 23 A may serve as the “first distribution mechanism”. In one embodiment of the technology, a configuration of a combination of the driven gear g 21 A, the planetary gear set  25 , and the second clutch c 24 A may serve as the “second distribution mechanism”. 
     The planetary gear set  25  may include, without limitation, a sun gear  25   s,  a pinion gear  25   p,  two ring gears  25   ra  and  25   rb , an input shaft  25   in , and an output shaft  25   out . The sun gear  25   s  is fixed. The pinion gear  25   p  is engaged with the sun gear  25   s.  The two ring gears  25   ra  and  25   rb  are engaged with the pinion gear  25   p.  The input shaft  25   in  is coupled to the ring gear  25   ra  and rotates unitarily with the ring gear  25   ra . The output shaft  25   out  is coupled to the ring gear  25   rb  and rotates unitarily with the ring gear  25   rb . The two ring gears  25   ra  and  25   b  differ in the number of teeth, causing the rotary motion inputted to the planetary gear set  25  through the input shaft  25   in  to be shifted and outputted to the output shaft  25   out.    
     In the planetary gear set  25 , the input shaft  25   in  and the output shaft  25   out  may be disposed coaxially. Moreover, the driven gear g 21 A, the first clutch c 23 A, and the second clutch c 24 A may be disposed coaxially with the input shaft  25   in  and the output shaft  25   out  of the planetary gear set  25 . A clutch plate of the first clutch c 23 A and a clutch plate of the second clutch c 24 A may be disposed at the same position in a direction of a rotation axis, and disposed respectively inwardly and outwardly in a radial direction. The direction of the rotation axis is a direction along the rotation axis. The radial direction is a direction perpendicular to the rotation axis. Out of the first clutch c 23 A and the second clutch c 24 A, a member h 23  of the first clutch c 23 A, e.g., a housing, and a member h 24  of the second clutch c 24 A, e.g., a clutch hub, may be unitized. The member h 23  may be fitted to a driven clutch plate of the first clutch c 23 A and rotate unitarily therewith. The member h 24  may be fitted to a driven clutch plate of the second clutch c 24 A and rotate unitarily therewith. 
     The gear ratios between the planetary gear set  25 , the gear mechanism, i.e., the driving gear g 15  and the driven gear g 21 A, and the differential gears  143  and  172  may be set to satisfy the following conditions 1 and 2. 
     The condition 1: the gear ratio from the input shaft i 24  of the second clutch c 24 A to the front wheel  2  is greater than the gear ratio from the output shaft o 24  of the second clutch c 24 A to the rear wheel  3 . As for the difference between the gear ratios, the latter gear ratio may be greater than the former gear ratio by about 0.5% to 3%. 
     The condition 2: the gear ratio from the input shaft i 23  of the first clutch c 23 A to the front wheel  2  is equal to the gear ratio from the output shaft o 23  of the first clutch c 23 A to the rear wheel  3 . Here, the term “equal” is not limited to exact coincidence, but includes coincidence including a negligible error compared with the difference between the gear ratios on the condition 1, e.g., 0.5% to 3%. 
     As with the first embodiment, the processor  35  is configured to control both the first clutch c 23 A and the second clutch c 24 A to the half-engaged state. Furthermore, the processor  35  is configured to independently control the degree of engagement in the half-engaged state of the first clutch c 23 A and the degree of engagement in the half-engaged state of the second clutch c 24 A. As with the first embodiment, the processor  35  may control the transfer  20 A in accordance with the traveling state. 
     According to the transfer  20 A of the second embodiment, a configuration that introduces power is unified in the single driven gear g 21 A. Accordingly, the transfer  20 A of the second embodiment is easily applicable to a vehicle  1 B including the transverse power source  11 . The term “transverse” means an arrangement in which an axial direction of an output shaft from which power is outputted is substantially perpendicular to a longitudinal direction of the vehicle  1 . 
       FIG.  8    is a schematic diagram of an example where the transfer according to the second embodiment is mounted on a vehicle including a transverse power source. As illustrated in  FIG.  8   , in a case of application to the transverse power source  11 , the driven gear g 21 A configured to introduce power to the transfer  20 A is changed from a gear configured to transmit power between parallel shafts, e.g., a spur gear and a helical gear, to a gear configured to transmit power between intersecting shafts, e.g., a bevel gear. Similarly, the driving gear g 15  configured to output power to the transfer  20 A is changed from a gear configured to transmit power between parallel shafts to a gear configured to transmit power between intersecting shafts. In the case with the transverse power source  11 , the front wheel transmission shaft  142  extending in the longitudinal direction of the vehicle  1  may be omitted, and the driving gear g 15  may be fitted to an output shaft of the transmission  13 . Furthermore, a gear mechanism configured to adjust a gear ratio may be interposed between the driving gear g 15  and the transmission  13 . 
     In the configuration in  FIG.  8    as well, as for a plurality of gear configurations located on the power transmission path between the front wheels  2  and the rear wheels  3 , the gear ratios may be set to satisfy the following conditions 1 and 2. 
     The condition 1: the gear ratio from the input shaft i 24  of the second clutch c 24 A to the front wheel  2  is greater than the gear ratio from the output shaft o 24  of the second clutch c 24 A to the rear wheel  3 . As for the difference between the gear ratios, the latter gear ratio may be greater than the former gear ratio by about 0.5% to 3%. 
     The condition 2: the gear ratio from the input shaft i 23  of the first clutch c 23 A to the front wheel  2  is equal to the gear ratio from the output shaft o 23  of the first clutch c 23 A to the rear wheel  3 . The term “equal” is not limited to exact coincidence, but includes coincidence including a negligible error compared with the difference between the gear ratios on the condition 1, e.g., 0.5% to 3%. 
     As described, the transfer  20  according to the first embodiment includes the first distribution mechanism  201 . The first distribution mechanism  201  is configured to distribute a portion of power from the power source  11  to the rear wheel transmission shaft  171  through the first clutch c 23 . The first clutch c 23  is configured to be controlled to the half-engaged state. Furthermore, the transfer  20  includes the second distribution mechanism  202 . The second distribution mechanism  202  is configured to distribute a portion of power from the power source  11  to the rear wheel transmission shaft  171  though the second clutch c 24 . The second clutch c 24  is configured to be controlled to the half-engaged state. Accordingly, it is possible to control both the first clutch c 23  and the second clutch c 24  to the half-engaged state. With this control, it is possible to compose the power to be distributed to the front wheels  2  and the rear wheels  3  by the first distribution mechanism  201  and the power to be distributed to the front wheels  2  and the rear wheels  3  by the second distribution mechanism  202 . Furthermore, controlling the degrees of engagement of the first clutch c 23  or c 23 A, and the degrees of engagement of the second clutch c 24  or c 24 A makes it possible to continuously adjust the power distribution ratio through the first distribution mechanism  201  and the power distribution ratio through the second distribution mechanism  202 . 
     In addition, according to the transfer  20  of the first embodiment, the first difference is different from the second difference. The first difference is the difference between the gear ratio frontward of the first clutch c 23  and the gear ratio rearward of the first clutch c 23 . The second difference is the difference between the gear ratio frontward of the second clutch c 24  and the gear ratio rearward of the second clutch c 24 . Such setting of the gear ratios makes it possible to generate the internal circulation torque between the front wheels  2  and the rear wheels  3  in the first distribution mechanism  201 , the second distribution mechanism  202 , or both. The internal circulation torque acts as negative power. Accordingly, with the internal circulation torque, it is possible to distribute power at the ratio in the wider range than 50:50 to 100:0. Meanwhile, when cornering, the difference occurs between the length of the locus of the front wheel  2  and the length of the locus of the rear wheel  3 . Accordingly, the magnitude of the internal circulation torque as mentioned above changes in accordance with the radius of bend. However, the setting of the gear ratios as mentioned above keeps the internal circulation torque through the first distributing mechanism  201  and the internal circulation torque through the second distributing mechanism  202  from both becoming zero. Furthermore, even in the case where one of the internal circulation torque through the first distribution mechanism  201  or the internal circulation torque through the second distribution mechanism  202  becomes excessive, it is possible to suppress the other to moderate magnitude. Thus, composing the power to be distributed by the first distribution mechanism  201  and the power to be distributed by the second distribution mechanism  202  makes it possible to compose the power from the power source  11  and the power caused by the internal circulation torque of appropriate magnitude, regardless of the radius of bend when cornering. With the composition described above, it is possible to control the power distribution ratio in a wide range both on the straight travel and when cornering. 
     Furthermore, the transfer  20  according to the first embodiment may include the processor  35 . The processor  35  is configured to control both the first clutch c 23  and the second clutch c 24  to the half-engaged state in accordance with the traveling state of the vehicle  1 . Accordingly, with the control by the processor  35 , it is possible to control the power distribution ratio in a wide range both on the straight travel and when cornering. It is possible to provide a four wheel drive control suitable for the traveling state. 
     Furthermore, according to the transfer  20  of the first embodiment, in controlling both the first clutch c 23  and the second clutch c 24  to the half-engaged state, the processor  35  may output the instruction to increase the torque, to the travel control processor  33 . By the instruction to increase the torque, the loss caused by the internal circulation torque to be generated between the first clutch c 23  and the second clutch c 24  is compensated. This suppresses the actual propulsive force of the vehicle  1  from deviating greatly from the propulsive force corresponding to the driving operation by the driver. 
     Furthermore, according to the transfer  20  of the first embodiment, the gear ratio from the input shaft i 23  of the first clutch c 23  to the front wheel  2  may be equal to the gear ratio from the output shaft o 23  of the first clutch c 23  to the rear wheel  3 . Meanwhile, the gear ratio from the input shaft i 24  of the second clutch c 24  to the front wheel  2  may be greater than the gear ratio from the output shaft o 24  of the second clutch c 24  to the rear wheel  3 . With such setting, it is possible to easily make the power distribution to the front wheels  2  and the rear wheels  3  suitable for the straight travel and the cornering. 
     The workings and the effects to be obtained by the transfer  20  of the first embodiment described above may be similarly produced in the configurations illustrated in  FIGS.  1 B,  7 , and  8   . 
     In addition, according to the transfer  20  of the first embodiment, the first distribution mechanism  201  may include the gear set including the first driven gear g 21 . The second distribution mechanism  202  may include the gear set including the second driven gear g 22 . By the gear ratios of the two gear sets mentioned above, it is possible to provide the difference between the gear ratios described above. With such a configuration, even in a case where, for example, the gear ratios of the differential gears  143  and  172  are different, making the gear ratios of the gear sets correspond makes it possible to provide the difference between the gear ratios described above. Hence, it is possible to easily cope with modifications of the vehicle  1 . 
     Although some example embodiments of the technology have been described in the forgoing by way of example with reference to the accompanying drawings, the technology is by no means limited to the example embodiments described above. It should be appreciated that modifications and alterations may be made by persons skilled in the art without departing from the scope as defined by the appended claims. The use of the terms first, second, etc. does not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. The technology is intended to include such modifications and alterations in so far as they fall within the scope of the appended claims or the equivalents thereof. 
     For example, in the forgoing embodiments, a configuration is described in which power from the power source is transmitted to the front wheels without passing through the transfer, and to the rear wheels through the transfer. In an alternative configuration, power from the power source may be transmitted to the rear wheels without passing through the transfer, and to the front wheels through the transfer. 
     Moreover, in the forgoing embodiments, a configuration is described in which the two distribution mechanisms of the transfer are disposed coaxially and unitized. However, the two distribution mechanisms may be provided separately. 
     In the first embodiment, the modification example, and the second embodiment, an example is given in which the first difference becomes substantially zero. The first difference is the difference between the “gear ratio from the input shaft i 23  of the first clutch c 23  or c 23 A to the front wheel  2 ” and the “gear ratio from the output shaft o 23  of the first clutch c 23  or c 23 A to the rear wheel  3 ”. Moreover, an example is given in which the second difference takes a value greater than zero. The second difference is the difference between the “gear ratio from the input shaft i 24  of the second clutch c 24  or c 24 A to the front wheel  2 ” and the “gear ratio from the output shaft o 24  of the second clutch c 24  or c 24 A to the rear wheel  3 ”. However, the setting of these gear ratios is merely an example. The gear ratios may be set to allow both the first difference and the second difference to become greater than zero. Alternatively, the gear ratios may be set to allow both the first difference and the second difference to become smaller than zero. In another alternative, the gear ratios may be set to allow the first difference to become smaller than zero, and to allow the second difference to become greater than zero. In a case with a difference between the first difference and the second difference described above, it is possible to avoid the situation that both the internal circulation torque through the first distribution mechanism  201 , and the internal circulation torque through the second distribution mechanism  202  become zero, regardless of the radius of bend when cornering. Moreover, even in the case where one of the internal circulation torque through the first distribution mechanism  201  or the internal circulation torque through the second distribution mechanism  202  becomes excessive, it is possible to suppress the other to the moderate magnitude. 
     Other details described in the forgoing example embodiments may be changed as appropriate within the scope of the technology. 
     According to the aspects of the technology, controlling both a first clutch and a second clutch to a half-engaged state makes it possible to compose power to be distributed to a first driving wheel and a second driving wheel by a first distribution mechanism, and power to be distributed to the first driving wheel and the second driving wheel by a second distribution mechanism. Furthermore, controlling a degree of engagement of the first clutch and a degree of engagement of the second clutch makes it possible to adjust continuously a power distribution ratio through the first distribution mechanism and a power distribution ratio through the second distribution mechanism. In addition, setting of gear ratios makes it possible to generate internal circulation torque between the first driving wheel and the second driving wheel, in the first distribution mechanism, the second distribution mechanism, or both. The internal circulation torque acts as negative power. Accordingly, with the internal circulation torque, it is possible to provide the power distribution ratio in a wider range than 50:50 to 100:0. For example, it is possible to provide the power distribution ratio in a range of 0:100 to 100:0, or a range of 30:70 to 100:0. Meanwhile, when cornering, a difference occurs between a length of a locus of the first driving wheel and a length of a locus of the second driving wheel. This causes a change in the magnitude of the internal circulation torque as mentioned above in accordance with a radius of bend. However, the setting of the gear ratios keeps the internal circulation torque through the first distribution mechanism and the internal circulation torque through the second distribution mechanism from both becoming zero. Furthermore, in a case where one of the internal circulation torque through the first distribution mechanism or the internal circulation torque through the second distribution mechanism becomes excessive, it is possible to suppress the other to moderate magnitude. Thus, composing power to be distributed by the first distribution mechanism and power to be distributed by the second distribution mechanism makes it possible to compose power from a power source and power caused by the internal circulation torque of moderate magnitude, regardless of the radius of bend when cornering. Hence, with the composition described above, it is possible to control the power distribution ratio in a wide range both on straight travel and when cornering. 
     The processor  35  illustrated in  FIG.  2    is implementable by circuitry including at least one semiconductor integrated circuit such as at least one processor (e.g., a central processing unit (CPU)), at least one application specific integrated circuit (ASIC), and/or at least one field programmable gate array (FPGA). At least one processor is configurable, by reading instructions from at least one machine readable non-transitory tangible medium, to perform all or a part of functions of the processor  35  illustrated in  FIG.  2   . Such a medium may take many forms, including, but not limited to, any type of magnetic medium such as a hard disk, any type of optical medium such as a CD and a DVD, any type of semiconductor memory (i.e., semiconductor circuit) such as a volatile memory and a non-volatile memory. The volatile memory may include a DRAM and a SRAM, and the nonvolatile memory may include a ROM and a NVRAM. The ASIC is an integrated circuit (IC) customized to perform, and the FPGA is an integrated circuit designed to be configured after manufacturing in order to perform, all or a part of the functions of the processor  35  illustrated in  FIG.  2   .