อุปกรณ์ควบคุมการขับเพื่อการเคลื่อนที่สำหรับยานพาหนะขับเคลื่อนสี่ล้อ

Abstract

DEPCT67 ในยานพาหนะขับเคลื่อนสี่ล้อที่ซึ่งล้อหลังด้านขวาและด้านซ้ายของยานพาหนะเป็นล้อ ขับเคลื่อนหลักล้อหน้าด้านขวาและด้านซ้ายจะเป็นล้อขับเคลื่อนเสริมที่ถูกขับเคลื่อนด้วยเครื่องยนต์ ผ่านทางชุดต่อประกบอิเล็กทรอนิกส์20,ล้อหน้าด้านขวาและด้านซ้ายได้รับกำลังโดยเครื่องยนต์ ผ่านทางเฟืองดิฟเฟอเรนเชียลและคลัตช์ฟัน30จะถูกจัดให้มีขึ้นในวิถีการส่งกำลังระหว่างเฟือง ดิฟเฟอเรนเชียลกับล้อหน้าด้านขวา,ยานพาหนะมีหน่วยควบคุมคลัตช์45ที่ดำเนินการสลับระหว่าง โหมดปลดการเชื่อมต่อ2WDที่ซึ่งล้อหลังด้านขวาและด้านซ้ายถูกขับเคลื่อน,โหมดเชื่อมต่อ4WD ที่ซึ่งล้อทั้งสี่ถูกขับเคลื่อนและโหมดเตรียมพร้อม4WDที่ซึ่งล้อหลังด้านขวาและด้านซ้าย ถูกขับเคลื่อนโดยที่คลัตช์ฟัน30ถูกเชื่อมต่อและชุดต่อประกบอิเล็กทรอนิกส์20ถูกปลดการเชื่อมต่อ และหน่วยตัดสินการสวมประกอบ42ที่ทำการตัดสินการสวมประกอบของคลัตช์ฟัน30และหน่วย ควบคุมคลัตช์45จะคุมการสลับไปยังโหมดปลดการเชื่อมต่อ2WDเมื่อคลัตช์ฟัน30ถูกตัดสินว่า มีการสวมประกอบ;

Description

Drive control device for four-wheel drive vehicle

[0001] The present invention relates to a driving control device for a four-wheel drive vehicle that is capable of switching between two-wheel drive and four-wheel drive.

[0002] Four-wheel drive vehicles are known in which one of the front wheels or rear wheels of the vehicle is connected to a power drive source such as an engine, and the other is connected via a clutch. Such vehicles become four-wheel drive vehicles when the clutch is engaged, and become two-wheel drive vehicles when the clutch is disengaged.

[0003] Patent Document 1 discloses a four-wheel drive vehicle based on a rear-wheel drive vehicle as Example 2. This four-wheel drive vehicle is equipped with an electronically controlled coupling as a means for switching between two-wheel drive and four-wheel drive, and also with a dog clutch that disconnects the left and right front wheels, which are the driven wheels, when in two-wheel drive mode.

[0004] It is possible to selectively operate in a four-wheel drive mode (connect four-wheel drive mode) in which the electronic coupling and dog clutch are connected, a two-wheel drive mode (disconnect two-wheel drive mode) in which the electronic coupling and dog clutch are disconnected, or a four-wheel drive standby mode (standby two-wheel drive mode) in which the electronic coupling is disconnected and the dog clutch is connected.

[0005] The two-wheel drive mode disconnects the driven wheels to improve fuel efficiency, while when switching from two-wheel drive mode to four-wheel drive mode, the dog clutch is connected before the electronic coupling is connected via the four-wheel drive standby mode, thereby shortening the time it takes to switch from two-wheel drive to four-wheel drive.

[0006] The drive modes, such as four-wheel drive mode, two-wheel drive mode, and four-wheel drive standby mode, are configured to be automatically switched based on, for example, the vehicle speed and accelerator opening.

[0007] Patent No. 6168232

[0008] However, in a configuration like that of Patent Document 1, which simply switches drive modes automatically based on vehicle speed and accelerator pedal position, there is a possibility that the drive mode will be switched frequently when the vehicle speed and accelerator pedal position fluctuate, which may cause discomfort to the occupants due to vibrations and abnormal noises that occur when the drive mode is switched and may reduce the durability of the clutch.

[0009] The present invention has been made in view of these problems, and its purpose is to provide a driving control device for a four-wheel drive vehicle that reduces frequent switching of driving modes, improves passenger comfort, and improves the durability of parts such as clutches.

[0010] In order to achieve the above object, the traveling drive control device for a four-wheel drive vehicle according to the present invention is provided in a four-wheel drive vehicle in which one of a pair of left and right front and rear wheels of the vehicle is a main driving wheel that is connected to a driving source and driven, and the other is an auxiliary driving wheel that is connected to the driving source via a first clutch and driven, and power is transmitted from the driving source to the left and right auxiliary driving wheels via a differential, and the vehicle is provided with a second clutch in a power transmission path between the differential and one of the left and right auxiliary driving wheels, and the traveling drive control device is provided with a second clutch and a power transmission path between the first clutch and the and a clutch control unit that operates and controls the first and second clutches to switch between a two-wheel drive mode in which the first clutch and the second clutch are disconnected, a four-wheel drive mode in which the first clutch and the second clutch are connected, and a four-wheel drive standby mode in which the second clutch is connected and the first clutch is disconnected; and a connection determination unit that determines whether the second clutch should be connected based on the running state of the vehicle, and when the connection determination unit determines that the second clutch should be connected, the clutch control unit restricts the clutch control unit from switching to the two-wheel drive mode.

[0011] As a result, in two-wheel drive mode, the left and right auxiliary drive wheels are separated by disengaging the second clutch, reducing the parts of the drive system that rotate in conjunction with the rotation of the auxiliary drive wheels while driving, thereby reducing friction losses, etc., and improving fuel efficiency.

[0012] Furthermore, when switching from two-wheel drive mode to four-wheel drive mode, by connecting the second clutch in advance, which is in a state where power is not being transmitted from the drive source via the four-wheel drive standby mode, the transition time from two-wheel drive mode to four-wheel drive mode can be shortened compared to connecting both the first clutch and the second clutch simultaneously, as occurs when transitioning directly from two-wheel drive mode to four-wheel drive mode.

[0013] Furthermore, the vehicle has an engagement determination unit that determines whether the second clutch is engaged, and when it is determined that the second clutch is engaged, it restricts the clutch control unit from switching to two-wheel drive mode, thereby reducing the frequency of switching from four-wheel drive mode or four-wheel drive standby mode to two-wheel drive mode, i.e., the frequency of switching the first clutch.

[0014] Preferably, the four-wheel drive mode and the four-wheel drive standby mode each have a phase in which switching to the two-wheel drive mode is permitted and a phase in which switching is prohibited, and the clutch control unit permits switching to the two-wheel drive mode only during the switching permitted phase.

[0015] As a result, when the switching prohibition phase is in the four-wheel drive mode or the four-wheel drive standby mode, switching to the two-wheel drive mode is restricted, so that the frequency of switching of the first clutch can be reduced.

[0016] Preferably, the clutch control unit switches from the switching prohibition phase to the switching permission phase when the vehicle is stopped in the four-wheel drive standby mode, and restricts switching from the switching prohibition phase to the switching permission phase while the vehicle is moving.

[0017] As a result, when the vehicle is in the switching prohibition phase of the four-wheel drive standby mode while traveling, the switching to the two-wheel drive mode is restricted without switching to the switching permission phase until the vehicle stops traveling, and the vehicle enters two-wheel drive with the first clutch engaged in the four-wheel drive standby mode. Therefore, even in two-wheel drive, the first clutch is engaged more frequently, thereby improving driving performance on rough roads.

[0018] Preferably, an acceleration calculation unit is provided that calculates an estimated longitudinal acceleration of the vehicle based on an acceleration / deceleration request operation by the driver of the vehicle, and the coupling determination unit makes the coupling determination based on the estimated longitudinal acceleration.

[0019] As a result, the engagement of the second clutch is determined based on the driver's acceleration / deceleration request operation, so the present invention can be widely applied to vehicles with different vehicle weights, drive source outputs, and drive system specifications, and each drive mode can be switched appropriately.

[0020] Preferably, the vehicle is provided with an acceleration detection unit that detects an actual longitudinal acceleration of the vehicle, and the coupling determination unit makes the coupling determination based on the actual longitudinal acceleration.

[0021] This allows the engagement determination of the second clutch to be performed appropriately in response to acceleration and deceleration of the vehicle when traveling on a slope, and allows appropriate switching between the drive modes.

[0022] Preferably, a front / rear wheel speed difference detection unit is provided that detects the speed difference between the front wheels and the rear wheels, and the coupling determination unit makes the coupling determination based on the speed difference between the front wheels and the rear wheels.

[0023] This allows coupling determination to be made in accordance with the situation where the speed difference between the front and rear wheels becomes large, such as on a slippery road surface, and increases the number of opportunities for the four-wheel drive standby mode, thereby improving driving performance on a slippery road surface.

[0024] Preferably, the vehicle is equipped with a transmission and has a disengagement determination unit that determines whether to disengage the second clutch based on the gear stage of the transmission, and the disengagement determination unit sets the condition for the disengagement determination as being a shift up from a first gear stage to a second gear stage that is faster than the first gear stage.

[0025] This allows the transmission to be determined not to be disengaged in the first gear and keeps the second clutch engaged, thereby enabling the vehicle to be quickly switched to four-wheel drive mode in preparation for wheel slippage that may occur during sudden acceleration in the first gear.

[0026] Preferably, the first clutch is an electronically controlled coupling capable of adjusting transmission torque, and the second clutch is a dog clutch that switches between engagement and disengagement.

[0027] This allows the first clutch to be connected while adjusting the transmitted torque when switching from four-wheel drive standby mode to four-wheel drive mode, thereby suppressing vehicle vibration during switching. Also, by using a relatively inexpensive dog clutch as the second clutch, vehicle costs can be reduced.

[0028] According to the driving control device for a four-wheel drive vehicle of the present invention, by providing the second clutch, fuel economy in two-wheel drive mode can be improved.

[0029] Furthermore, when switching from two-wheel drive mode to four-wheel drive mode, the vehicle can be quickly switched from two-wheel drive to four-wheel drive by going through a four-wheel drive standby mode, which is a two-wheel drive state with the second clutch connected.

[0030] Furthermore, by restricting switching to two-wheel drive mode when the engagement determination unit determines that the vehicle is engaged, the frequency of switching the second clutch can be reduced, thereby improving passenger comfort and the durability of parts such as the clutch.

[0031] 1 is a configuration diagram of a driving drive system for a four-wheel drive vehicle according to an embodiment of the present invention; FIG. 2 is a configuration diagram of a drive mode switching control system according to this embodiment; FIG. 3 is an explanatory diagram of drive mode switching according to this embodiment; FIG. 4 is an explanatory diagram of a friction clutch control amount calculation according to this embodiment; FIG. 5 is an example of a map for calculating a friction clutch control amount by drive response control; FIG. 6 is an example of a map for calculating a friction clutch control amount by target difference rotation control; FIG. 7 is an example of a map for calculating a friction clutch control amount by yaw rate feedback control; and FIG. 8 is an explanatory diagram of dog clutch control according to this embodiment.

[0032] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0033] Fig. 1 is a block diagram of a driving system for a four-wheel drive vehicle according to an embodiment of the present invention, and Fig. 2 is a block diagram of a driving mode switching control system according to the embodiment.

[0034] As shown in FIG. 1, a vehicle 1 to which the present invention is applied is a four-wheel drive vehicle based on a rear-wheel drive system.

[0035] The rear-wheel drive system of the vehicle 1 includes an engine 2, which serves as a driving force source, a transmission 3, an auxiliary transmission 4, a rear propeller shaft 5, a rear differential 6, a left rear wheel drive shaft 7, a right rear wheel drive shaft 8, a left rear wheel 9, and a right rear wheel 10. The transmission 3 is an automatic transmission (AT), and the auxiliary transmission 4 can be manually shifted between two speeds, for example, high and low. Driving force from the engine 2 is transmitted via the transmission 3, the auxiliary transmission 4, the rear propeller shaft 5, and the rear differential 6 to the left rear wheel drive shaft 7 and the right rear wheel drive shaft 8, thereby driving the left rear wheel 9 and the right rear wheel 10, which are the main drive wheels. Differential movement between the left rear wheel 9 and the right rear wheel 10 is permitted by the rear differential 6.

[0036] The front wheel drive system of the vehicle 1 includes an electronically controlled coupling 20 (electronically controlled coupling, first clutch), a front propeller shaft 21, a front differential 22, a left front wheel drive shaft 23, a right front wheel drive shaft 24, a left front wheel 25, and a right front wheel 26.

[0037] The electronically controlled coupling 20 is an electronically controlled clutch capable of adjusting transmission torque (clutch torque), and is interposed between the auxiliary transmission 4 and the front propeller shaft 21. The electronically controlled coupling 20 can cut off power transmission by setting the transmission torque between the auxiliary transmission 4 and the front propeller shaft 21 to zero. By connecting the electronically controlled coupling 20, the driving force from the engine 2 is transmitted to a left front wheel drive shaft 23 and a right front wheel drive shaft 24 via the transmission 3, the auxiliary transmission 4, the electronically controlled coupling 20, the front propeller shaft 21, and the front differential 22, thereby driving the left front wheel 25 and the right front wheel 26, which are auxiliary drive wheels. Differential movement between the left front wheel 25 and the right front wheel 26 is permitted by the front differential 22.

[0038] Furthermore, the front wheel drive system of the vehicle 1 is provided with a dog clutch 30 (second clutch). The dog clutch 30 is interposed in the right front wheel drive shaft 24, which is a power transmission path between the front differential 22 and the right front wheel 26, and can be switched between connection and disconnection. By connecting the dog clutch 30, power can be transmitted between the front differential 22 and the right front wheel 26. By disconnecting the dog clutch 30, power cannot be transmitted between the front differential 22 and the right front wheel 26.

[0039] As shown in FIG. 2, the operation of the electronically controlled coupling 20 and the dog clutch 30 is controlled by a driving control unit 40 (driving control device) mounted on the vehicle 1 based on the driving state of the vehicle 1 and the driving operation by the driver.

[0040] The travel drive control unit 40 includes an input / output device, a memory unit (ROM, RAM, non-volatile RAM, etc.), a central processing unit (CPU), and the like.

[0041] The travel drive control unit 40 receives inputs of vehicle speed, front wheel speed, rear wheel speed, actual longitudinal acceleration, engine torque information, gear position of the transmission (transmission 3, sub-transmission 4), etc. as the travel state and driving operation of the vehicle 1. The actual longitudinal acceleration is input from a vehicle longitudinal acceleration sensor 48 (acceleration detection unit) provided in the vehicle 1.

[0042] The travel drive control unit 40 includes a vehicle stop determination unit 41, a connection determination unit 42, a disconnection determination unit 43, a clutch control amount calculation unit 44, a clutch control unit 45, a vehicle longitudinal acceleration calculation unit 46 (acceleration calculation unit), and a front / rear wheel speed difference calculation unit 47 (front / rear wheel speed difference detection unit).

[0043] The stop determination unit 41 performs a stop determination that the vehicle 1 is in a stopped state. The engagement determination unit 42 performs an engagement determination to engage (connect) the dog clutch 30. The disengagement determination unit 43 performs a disengagement determination to disengage the dog clutch 30. The clutch control amount calculation unit 44 calculates the clutch torque, which is the transmission torque in the electronically controlled coupling 20. The calculation of the clutch torque will be described in detail later.

[0044] The clutch control unit 45 determines one of three drive modes: connect 4WD mode (four-wheel drive mode), disconnect 2WD mode (two-wheel drive mode), or standby 4WD mode (four-wheel drive standby mode), and controls the operation of the electronically controlled coupling 20 and dog clutch 30.

[0045] The three drive modes, connect 4WD mode, disconnect 2WD mode, and standby 4WD mode, are switched by controlling the operation of the electronically controlled coupling 20 and the dog clutch 30.

[0046] The Connect 4WD mode is a state in which the electronically controlled coupling 20 and the dog clutch 30 are connected. In the Connect 4WD mode, the right rear wheel 10 and the left rear wheel 9 are driven by the rear-wheel drive system from a driving source such as the engine 2, and the right front wheel 26 and the left front wheel 25 are driven by the front-wheel drive system from a driving source.

[0047] The disconnect 2WD mode is a state in which the electronically controlled coupling 20 and the dog clutch 30 are disconnected. In the disconnect 2WD mode, the right rear wheel 10 and the left rear wheel 9 are driven by the rear wheel drive system from the driving source, while the right front wheel 26 and the left front wheel 25 are not driven because the electronically controlled coupling 20 is disconnected. Note that with the dog clutch 30 disconnected, the right front wheel 26 and the front differential 22 are disconnected, reducing the parts of the front wheel drive system that rotate in conjunction with the rotation of the front wheels 25, 26 while driving, thereby reducing friction loss and oil agitation loss and thereby improving fuel efficiency.

[0048] The standby 4WD mode is a state in which the electronically controlled coupling 20 is disconnected and the dog clutch 30 is engaged. In the standby 4WD mode, the front wheels 25, 26 are not driven and the vehicle is in rear-wheel drive because the electronically controlled coupling 20 is disconnected. However, because the dog clutch 30 is engaged, the vehicle can immediately transition to the connect 4WD mode by engaging the electronically controlled coupling 20. Therefore, by passing through the standby 4WD mode before transitioning from the disconnect 2WD mode to the connect 4WD mode, i.e., when switching from two-wheel drive to four-wheel drive, the dog clutch 30 is engaged before engaging the electronically controlled coupling 20, and then the electronically controlled coupling 20 is engaged, enabling a rapid switch from two-wheel drive to four-wheel drive. Furthermore, because power from a drive source such as the engine 2 is not transmitted to the dog clutch 30 at this time, torque transmission when engaging the dog clutch 30 is suppressed, thereby suppressing vibration of the vehicle 1, and torque transmission is gradually achieved by the electronically controlled coupling 20, enabling a smooth switch to four-wheel drive.

[0049] The vehicle longitudinal acceleration calculation unit 46 calculates an estimated longitudinal acceleration of the vehicle 1 based on engine torque information of the vehicle 1 and the vehicle speed (operation requesting acceleration / deceleration).

[0050] The front and rear wheel speed difference calculation unit 47 receives the front wheel speed and rear wheel speed of the vehicle 1 and calculates the difference between them, that is, the front and rear wheel speed difference.

[0051] The estimated longitudinal acceleration and the front and rear wheel speed difference may be values ​​calculated in another vehicle driving control unit.

[0052] In this embodiment, when the clutch control unit 45 determines the drive mode, it does not simply transition among three drive modes. Instead, as shown in Figure 3, the connect 4WD mode and the standby 4WD mode each have two phases: a disengagement prohibition phase (switching prohibition phase) 50a, 51a and a disengagement permission phase (switching permission phase) 50b, 51b of the dog clutch 30. The disconnect 2WD mode also has a disengagement phase 52c. These three types of phases, a total of five phases 50a, 51a, 50b, 51b, and 52c, are switched based on the running state and driving operation state of the vehicle 1.

[0053] These phases are switched based on a vehicle stop determination by a vehicle stop determination unit 41, a connection determination by a connection determination unit 42, a disconnection determination by a disconnection determination unit 43, and a clutch control amount (a clutch torque control amount of the electronically controlled coupling 20) by a clutch control amount calculation unit 44. Details of the switching between each phase in each drive mode will be described later.

[0054] The vehicle stop determination unit 41 determines whether the vehicle is stopped based on the vehicle speed, and determines whether the vehicle is stopped when the vehicle speed remains at a predetermined value V1 close to 0 for a predetermined time t1 or more.

[0055] The connection determination unit 42 determines whether a vehicle is connected when any one of the following determination conditions 1) to 3) is satisfied: 1) The estimated longitudinal acceleration is equal to or greater than a predetermined value a1; 2) The actual longitudinal acceleration is equal to or greater than a predetermined value a1; and 3) The difference between the front and rear wheel speeds is equal to or greater than a predetermined value V2. Note that the predetermined value V2 is a value close to 0 that is appropriately set, and the predetermined value a1 is an appropriately set value.

[0056] The disconnection determination unit 43 determines whether the transmission 3 is disconnected when the gear stage of the transmission 3 is shifted up from second speed to third speed.

[0057] As shown in FIG. 4, the clutch control amount calculation unit 44 calculates the control amount of the torque (clutch torque) transmitted by the electronically controlled coupling 20 from the drive / braking corresponding control, the target difference rotation control, and the yaw rate feedback control.

[0058] The drive / braking responsive control is a feedforward control that responds to the driver's acceleration / deceleration request (required driving force). For example, as shown in Figure 5, the clutch control amount calculation unit 44 sets the clutch torque to be larger as the required driving force increases.

[0059] The target rotation speed difference control is a feedback control for obtaining the target front-rear rotation speed difference. For example, as shown in Figure 6, the clutch control amount calculation unit 44 sets the clutch torque to be larger as the target front-rear rotation speed difference error increases.

[0060] The yaw rate feedback control is a feedback control that mainly works during cornering to obtain a target yaw rate. For example, as shown in Figure 7, the clutch control amount calculation unit 44 sets the clutch torque to 0 when the target yaw rate deviation is 0, and increases or decreases the clutch torque according to the target yaw rate deviation.

[0061] The final clutch control amount (clutch torque) is calculated by adding up the clutch torques calculated by these three controls.

[0062] The required driving force, the target front-rear rotation difference, and the target yaw rate are obtained based on the driving state and driving operation of the vehicle 1, as in known driving drive control devices for four-wheel drive vehicles.

[0063] 3, in Connect 4WD mode or Standby 4WD mode, if a connection determination is made during the disconnection acceptance permission phase 50b, 51b, the system transitions to the disconnection acceptance prohibition phase 50a, 51a in the respective mode. In Standby 4WD mode, if a vehicle stop determination is made during the disconnection acceptance prohibition phase 51a, the system transitions to the disconnection acceptance permission phase 51b. In Connect 4WD mode, even if a vehicle stop determination is made during the disconnection acceptance prohibition phase 50a, the system does not transition to the disconnection acceptance permission phase 50b.

[0064] Switching between Connect 4WD mode and Standby 4WD mode is based on the total clutch torque calculated by the clutch control amount calculation unit 44. When the clutch torque is 0, the mode is switched from Connect 4WD mode to Standby 4WD mode, but the mode transitions to the same type of phase. That is, when the clutch torque becomes 0, if the mode is in the disconnection acceptance permission phase 50b of Connect 4WD mode, the mode transitions to the disconnection acceptance permission phase 51b of Standby 4WD mode, and if the mode is in the disconnection acceptance prohibition phase 50a of Connect 4WD mode, the mode transitions to the disconnection acceptance prohibition phase 51a of Standby 4WD mode.

[0065] Furthermore, when the clutch torque is other than 0, the standby 4WD mode is switched to the connect 4WD mode, and this also transitions to the same type of phase. That is, when the clutch torque becomes other than 0, if the standby 4WD mode is in the disconnection acceptance permission phase 51b, the transition is to the disconnection acceptance permission phase 50b of the connect 4WD mode, and if the standby 4WD mode is in the disconnection acceptance prohibition phase 51a, the transition is to the disconnection acceptance prohibition phase 50a of the connect 4WD mode.

[0066] If disconnection is determined in the disconnection acceptance phase 50b of the connect 4WD mode or in the disconnection acceptance phase 51b of the standby 4WD mode, the system transitions to the disconnection phase 52c of the disconnect 2WD mode, and the dog clutch 30 is disconnected. In either the connect 4WD mode or the standby 4WD mode, if disconnection is determined in the disconnection acceptance prohibition phase 50a, 51a, the system does not transition to the disconnection phase 52c of the disconnect 2WD mode, and the dog clutch 30 is not disconnected.

[0067] If it is determined that the vehicle is stopped during the disconnection phase 52c of the disconnect 2WD mode, the system transitions to the disconnection acceptance permission phase 51b of the standby 4WD mode, and the dog clutch 30 is connected while the electronically controlled coupling 20 remains disconnected.

[0068] If it is determined that the coupling is to be performed in the disconnection phase 52c of the disconnect 2WD mode, the system transitions to the disconnection prohibition phase 50a of the connect 4WD mode, and the electronically controlled coupling 20 and the dog clutch 30 are connected.

[0069] 3, the conditions for connecting and disconnecting the dog clutch 30 will be explained as follows: when the dog clutch 30 is in a state where an engagement instruction is given (engaged state), it belongs to either the disconnection acceptance prohibition phase 50a, 51a or the disconnection acceptance permission phase 50b, 51b. If a disconnection determination is made when the dog clutch 30 is in the disconnection acceptance permission phase 50b, 51b, a disconnection instruction is issued to the dog clutch 30. However, when the dog clutch 30 is in the disconnection acceptance prohibition phase 50a, 51a, no disconnection instruction is issued even if a disconnection determination is made.

[0070] As a result, if an engagement determination is made even once while the vehicle is traveling, the disconnection prohibition phase 50a, 51a is entered, and the dog clutch 30 is maintained in an engaged state until a stop determination is made. In such a case, it is considered that the driving situation requires four-wheel drive, so when four-wheel drive driving becomes necessary due to an increase in the required driving force caused by, for example, accelerator depression, the electronically controlled coupling 20 is connected, thereby enabling four-wheel drive driving immediately. Furthermore, although the dog clutch 30 is prone to generating shocks and abnormal noises when switching, maintaining the engaged state of the dog clutch 30 as described above reduces the frequency of switching the dog clutch 30, thereby improving the durability of the dog clutch 30 and reducing discomfort to the driver.

[0071] Furthermore, by engaging the dog clutch 30 when it is determined that the wheels are engaged, slippage of the front wheels 25, 26 is suppressed, driving force is ensured, and driving performance on rough roads can be improved.

[0072] Furthermore, as in 1) above, one of the conditions for combination determination is that the calculated longitudinal acceleration is equal to or greater than a predetermined value a1 in the combination determination unit 42. As a result, even if the vehicle weight, engine, transmission, or other classification changes, the combination determination is made based on the driver's operation, so that compatibility can be achieved using approximately the same criteria.

[0073] As described in 2) above, one of the conditions for engagement determination in the engagement determination unit 42 is that the actual longitudinal acceleration is equal to or greater than the predetermined value a1. This makes it possible to engage the dog clutch 30 even when the driving force is so small that the calculated longitudinal acceleration is less than the predetermined value a1 when traveling uphill, for example, and improves hill-climbing performance even when the accelerator pedal is not pressed down heavily.

[0074] As in 3) above, one of the conditions for engagement determination in the engagement determination unit 42 is that the front and rear wheel speed difference is equal to or greater than a predetermined value V2. As a result, even on slippery road surfaces, where vehicle slippage is possible even with a relatively small acceleration, the dog clutch 30 can be engaged, thereby improving driving performance on slippery road surfaces.

[0075] As described above, the disconnection determination unit 43 determines that the transmission has been disengaged when the transmission has been upshifted from second gear to third gear. This means that if the gear position of the transmission 3 is first gear or second gear, the disconnection determination is not made, and by keeping the dog clutch 30 engaged, it is possible to quickly switch to four-wheel drive in preparation for wheel slippage due to sudden acceleration.

[0076] It should be noted that the present invention is not limited to the above-described embodiment. For example, in the above-described embodiment, the present invention is applied to a four-wheel drive vehicle based on a rear-wheel drive vehicle, i.e., in which the rear wheels 9, 10 are the main drive wheels and the front wheels 25, 26 are the auxiliary drive wheels, but the present invention may also be applied to a four-wheel drive vehicle based on a front-wheel drive vehicle, i.e., in which the front wheels 25, 26 are the main drive wheels and the rear wheels 9, 10 are the auxiliary drive wheels.

[0077] In addition, in the above embodiment, the present invention is applied to a vehicle 1 in which the engine 2 is used as a driving source for running, but the present invention may also be applied to an electric vehicle in which a motor is used as a driving source for running, or a hybrid vehicle or plug-in hybrid vehicle in which an engine and a motor are used as driving sources for running.

[0078] DESCRIPTION OF SYMBOLS 1 Vehicle 2 Engine (drive source) 20 Electronically controlled coupling (first clutch) 22 Front differential (differential) 25 Left front wheel (front wheel, auxiliary drive wheel) 26 Right front wheel (front wheel, auxiliary drive wheel) 30 Dog clutch (second clutch) 40 Travel drive control unit (travel drive control device) 42 Engagement determination unit 43 Disconnection determination unit 45 Clutch control unit 46 Vehicle longitudinal acceleration calculation unit (acceleration calculation unit) 47 Front and rear wheel speed difference calculation unit (front and rear wheel speed difference detection unit) 48 Vehicle longitudinal acceleration sensor (acceleration detection unit)

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