Control device for four-wheel drive vehicle

Abstract

To provide a vehicle control device for a four-wheel drive vehicle which can optimally adjust a standby engaging degree or a distribution ratio of power transmission to a main drive wheel side and sub drive wheel side in preparation for occurrence of slip while minimizing influence of deceleration force caused by a tight corner braking phenomenon to travelling.SOLUTION: A control device 100 comprises: a running state grasping part 102 which grasps a running state of a vehicle 10; and a first clutch control part 104 which controls an engagement degree of a first clutch transmitting through a front wheel drive clutch 46. The control device 100 can control, when drive control is performed by a standby control method, for occurrence of slip, the front wheel drive clutch 46 to be waiting in an engagement state of a prescribed standby engagement degree S. The control device 100 stepwisely or continuously changes the standby engagement degree S according to a vehicle speed V of the vehicle 10.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a control device for a four-wheel drive vehicle. [Background technology]

[0002] Conventionally, a control system for an all-wheel drive vehicle, such as that disclosed in Patent Document 1 below, has been provided as a control device for a four-wheel drive vehicle. The control system of Patent Document 1 controls the connection between a first axle that is temporarily driven by a prime mover, a second axle that is constantly connected to the prime mover via a propeller shaft, and a sub-shaft that drivingly connects the propeller shaft and the first axle. This control system includes a first clutch that disconnectably connects the propeller shaft and the sub-shaft, a second clutch that disconnectably connects the sub-shaft and the first axle, a synchronization determination means that determines whether the second clutch is connectable, and an engagement determination means that determines whether the second clutch is engaged. By configuring this control system, Patent Document 1 aims to enable rapid switching between 2WD mode and AWD mode while preventing shock or vibration fluctuations in transmission torque. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-032773 Summary of the Invention [Problem to be solved by the invention]

[0004] Here, the inventors have studied the problems that can be expected when a control system is used in a four-wheel drive vehicle equipped with a first clutch that selectively disconnects or connects the power transmission path between the driving force source and the power transmission member, and a second clutch that selectively disconnects or connects the power transmission path between the power transmission member and the auxiliary drive wheels, to detect the occurrence of slippage and increase the distribution of driving force to the auxiliary drive wheels.

[0005] As a result, the researchers discovered that to suppress vehicle slippage using this control method, it is effective to engage the first clutch in advance at a predetermined engagement level (hereinafter also referred to as the "standby engagement level"), assuming a response delay of the first clutch. For example, if the first clutch is hydraulically operated, it is effective to apply a predetermined hydraulic pressure (hereinafter also referred to as the "standby pressure") to the first clutch in advance. This shortens the time required from when slippage is detected until driving force is distributed to the auxiliary drive wheels via the first clutch, thereby suppressing the amount of vehicle slippage. Furthermore, the researchers discovered that by waiting with standby pressure applied to the first clutch in the low vehicle speed range, it is possible to avoid tight corner braking, and when slippage occurs, it is possible to quickly increase the engagement level of the first clutch and minimize the occurrence of slippage.

[0006] On the other hand, if the standby engagement degree (standby pressure) of the first clutch is set high, the occurrence of slippage can be suppressed, but there is a problem in that tight corner braking occurs, causing a sense of discomfort to the user.

[0007] Therefore, the present invention aims to provide a control device for a four-wheel drive vehicle that can adjust the standby engagement level and the power transmission distribution ratio to the main drive wheel side and the auxiliary drive wheel side to an optimal state in preparation for the occurrence of slippage, while minimizing the effect on driving performance of the deceleration force caused by the tight corner braking phenomenon. [Means for solving the problem]

[0008] Here, the inventors conducted extensive research and came to the realization that, by focusing on the phenomenon in which the deceleration force associated with tight corner braking decreases as vehicle speed increases, it is possible to optimize the standby engagement degree and the drive distribution ratio for power transmission to the main drive wheel side and the auxiliary drive wheel side (hereinafter also referred to as the "drive distribution ratio") in accordance with vehicle speed, while minimizing the impact of tight corner braking on driving performance, and to optimize the standby engagement degree and drive distribution ratio in an optimal state in preparation for the occurrence of slippage.

[0009] (1) Based on this knowledge, the present invention provides a control device for a four-wheel drive vehicle, which includes a first clutch that selectively connects or disconnects a power transmission path between a driving force source and a power transmission member, and a second clutch that selectively connects or disconnects a power transmission path between the power transmission member and auxiliary drive wheels, and which provides a two-wheel drive state in which driving force is transmitted from the driving force source to left and right main drive wheels by disengaging at least one of the first clutch and the second clutch, a four-wheel drive state in which driving force is also transmitted from the driving force source to the left and right auxiliary drive wheels by engaging the first clutch and the second clutch, and a four-wheel drive state in which driving force is transmitted from the driving force source to the left and right auxiliary drive wheels by engaging the second clutch while engaging the first clutch, provided that occurrence of slippage in the four-wheel drive vehicle is detected. This control system is used in a four-wheel drive vehicle that can switch between a variable drive distribution state in which drive control is performed using a standby control method that increases the distribution of drive force transmitted to the auxiliary drive wheels relative to the drive force transmitted to the main drive wheels by changing the degree of engagement, and a variable drive distribution state in which drive control is performed using a standby control method that increases the distribution of drive force transmitted to the auxiliary drive wheels relative to the drive force transmitted to the main drive wheels by changing the degree of engagement. This control system is characterized in that, provided that drive control using the standby control method is being performed, the first clutch can be controlled to wait in an engaged state at a predetermined standby degree of engagement in preparation for the occurrence of slippage, and the higher the vehicle speed of the four-wheel drive vehicle, the more the standby engagement degree or the distribution ratio of power transmission to the auxiliary drive wheels relative to the power transmission to the main drive wheels is adjusted to increase stepwise or continuously.

[0010] Based on the above findings, the control device for a four-wheel drive vehicle of the present invention performs power transmission control such that the standby engagement degree of the first clutch or the distribution ratio of power transmission to the auxiliary drive wheels relative to the power transmission to the main drive wheels (drive distribution ratio) increases stepwise or continuously as the vehicle speed of the four-wheel drive vehicle increases. As a result, the control device for a four-wheel drive vehicle of the present invention can optimally adjust the standby engagement degree and the distribution ratio of power transmission to the main drive wheels and the auxiliary drive wheels so as to minimize the tight corner braking phenomenon in the low vehicle speed range while suppressing the occurrence of slip in the high vehicle speed range.

[0011] Here, the power transmission control as described above may be configured not to be performed in a situation where slippage is unlikely to occur, for example.

[0012] (2) Based on this knowledge, it is preferable that the above-mentioned control device for a four-wheel drive vehicle of the present invention is such that the power transmission control is not performed under the condition that at least one of the following is satisfied: (a) the outside air temperature is higher than a predetermined temperature, (b) the estimated road surface μ value is higher than a predetermined value, (c) there is no history of slippage after the four-wheel drive vehicle has started to move, and (d) there is no slippage when starting off.

[0013] The control device for a four-wheel drive vehicle of the present invention does not perform power transmission control when it is assumed that slip is unlikely to occur, as in (a) to (d) above. This eliminates the need to increase the standby engagement degree or drive distribution ratio as vehicle speed increases when slip is unlikely to occur, thereby simplifying control accordingly. Furthermore, by adopting the configuration described in (2) above, the control device for a four-wheel drive vehicle of the present invention reduces the frequency with which the standby engagement degree or drive distribution ratio is increased, thereby contributing to improved fuel economy of the four-wheel drive vehicle compared to when the configuration described in (2) above is not adopted.

[0014] (3) In the above-described control device for a four-wheel drive vehicle of the present invention, the power transmission control may be performed until the four-wheel drive vehicle reaches a predetermined upper limit vehicle speed.

[0015] By configuring the control device for a four-wheel drive vehicle of the present invention in this way, it is possible to minimize the tight corner braking phenomenon at speeds lower than the upper limit vehicle speed at which the tight corner braking phenomenon is expected to affect driving performance, and to suppress the occurrence of slippage at high vehicle speeds.

[0016] (4) The above-mentioned control device for a four-wheel drive vehicle of the present invention may be configured such that, provided that the vehicle speed is below a predetermined lower limit vehicle speed, the standby engagement degree is a predetermined minimum standby engagement degree, or the distribution ratio of power transmission to the auxiliary drive wheels relative to the power transmission to the main drive wheels is a minimum distribution ratio, and the power transmission control is performed in a vehicle speed range in which the four-wheel drive vehicle is at or above a predetermined lower limit vehicle speed.

[0017] With this configuration, the control device for a four-wheel drive vehicle of the present invention can keep the first clutch engaged so as to achieve the minimum standby engagement degree or the minimum distribution ratio even at vehicle speeds below the minimum vehicle speed limit. As a result, the control device for a four-wheel drive vehicle of the present invention can keep the first clutch engaged in preparation for slip even at low vehicle speeds below the minimum vehicle speed limit.

[0018] Here, the inventors have conducted extensive research and have found that the above-mentioned power transmission control works effectively, particularly when the four-wheel drive vehicle is a front-engine, rear-wheel drive vehicle.

[0019] (5) Based on this knowledge, the above-described control device for a four-wheel drive vehicle of the present invention can be suitably used when the four-wheel drive vehicle is a front-engine, rear-drive vehicle. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide a control device for a four-wheel drive vehicle that can minimize the tight corner braking phenomenon while optimally adjusting the standby engagement degree and the distribution ratio of power transmission to the main drive wheel side and the auxiliary drive wheel side in preparation for the occurrence of slippage. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is an explanatory diagram showing a control device according to an embodiment of the present invention and a vehicle equipped with the control device; [Figure 2] 3 is a flowchart showing a control flow performed when drive control is performed by a standby control method in the vehicle shown in FIG. [Figure 3] 3 is a timing chart corresponding to the control flow according to FIG. 2. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, a four-wheel drive vehicle control device (control device 100) according to an embodiment of the present invention will be described with reference to the drawings, taking a four-wheel drive vehicle (vehicle 10) that employs the same as an example. In the following description, before describing the specific configuration of the control device 100 and the control performed by the control device 100, a general configuration of the vehicle 10 will be described.

[0023] FIG. 1 is a diagram illustrating the schematic configuration of a vehicle 10 to which the present invention is applied. The vehicle 10 employs a front-engine, rear-wheel-drive drive system. As shown in FIG. 1, the vehicle 10 includes a drive power source 12, a pair of left and right front wheels 14L, 14R, a pair of left and right rear wheels 16L, 16R, a power transmission device 18, and a control device 100. The vehicle 10 is a four-wheel drive vehicle that can be switched between a two-wheel drive state in which drive power is transmitted to the rear wheels 16L, 16R, and a four-wheel drive state in which drive power is transmitted to the front wheels 14L, 14R in addition to the rear wheels 16L, 16R.

[0024] The driving force source 12 generates driving force for the vehicle 10. The driving force source 12 can be configured, for example, by an engine or a motor. The front wheels 14L, 14R constitute auxiliary driving wheels of the vehicle 10. The front wheels 14L, 14R function as driving wheels in four-wheel drive mode and as driven wheels in two-wheel drive mode. The rear wheels 16L, 16R constitute main driving wheels of the vehicle 10. The rear wheels 16L, 16R function as driving wheels in both four-wheel drive mode and two-wheel drive mode.

[0025] As shown in FIG. 1, the power transmission device 18 includes a speed change device 20, a transfer case 22, a front propeller shaft 24 (power transmission member), a rear propeller shaft 26, a front wheel differential gear device 28, a rear wheel differential gear device 30, a pair of left and right front wheel axles 32L, 32R, and a pair of left and right rear wheel axles 34L, 34R.

[0026] The power transmission device 18 can transmit the power generated in the driving force source 12 to the rear wheels 16L, 16R via a power transmission path that sequentially passes from the transmission 20 to the transfer 22, the rear propeller shaft 26, the rear wheel differential gear device 30, and the rear axles 34L, 34R, etc., before reaching the rear wheels 16L, 16R. The power transmission device 18 can also configure a power transmission path that distributes and transmits a portion of the driving force transmitted from the driving force source 12 to the transfer 22 to the front wheels 14L, 14R. That is, by adjusting the engagement state of the front wheel drive clutch 46, which will be described in detail later, the power transmission device 18 can transmit a portion of the power generated in the driving force source 12 to the front wheels 14L, 14R via a power transmission path that sequentially passes from the transmission 20 to the transfer 22, the front propeller shaft 24, the front wheel differential gear device 28, the front axles 32L, 32R, etc., before reaching the front wheels 14L, 14R.

[0027] The transmission 20 operates by receiving output from the driving force source 12, and is configured by, for example, a conventionally known MT (manual transmission), AT (automatic transmission), CVT (continuously variable transmission), or the like.

[0028] The transfer case 36 houses an input shaft 38, a rear-wheel-side output shaft 40, a front-wheel-drive drive sprocket 42, and a front-wheel-drive clutch 46 (first clutch) around a first rotational axis C1. The transfer case 36 also houses a front-wheel-side output shaft 48 and a front-wheel-drive driven sprocket 50 around a second rotational axis C2 extending in a direction along the first rotational axis C1 (substantially parallel to the first rotational axis C1 in this embodiment). The transfer case 22 also houses a front-wheel-drive chain 52 wound around the front-wheel-drive drive sprocket 42 and the front-wheel-drive driven sprocket 50.

[0029] The input shaft 38 is connected to the transmission 20. This allows the input shaft 38 to receive power transmitted from the driving power source 12. The rear-wheel output shaft 40 is connected to the rear propeller shaft 26 so as to be capable of transmitting power. A front-wheel drive drive sprocket 42 is supported on the rear-wheel output shaft 40 so as to be rotatable relative to the rear-wheel output shaft 40.

[0030] By engaging the front-wheel drive clutch 46, the front-wheel drive sprocket 42 can rotate integrally with the rear-wheel output shaft 40, and can transmit power to the front-wheel output shaft 48 via the front-wheel drive chain 52. Therefore, by engaging the front-wheel drive clutch 46, a portion of the driving force transmitted from the driving power source 12 to the rear propeller shaft 26 via the rear-wheel output shaft 40 can be distributed and transmitted to the front-wheel output shaft 48 via the front-wheel drive drive sprocket 42 and the front-wheel drive chain 52. On the other hand, by disengaging the front-wheel drive clutch 46, the driving force transmitted from the driving power source 12 to the rear-wheel output shaft 40 can be transmitted to the rear propeller shaft 26 without being transmitted (distributed) to the front-wheel drive drive sprocket 42.

[0031] The front-wheel drive clutch 46 is configured as a wet multi-plate clutch. The front-wheel drive clutch 46 is capable of adjusting the degree of engagement (degree of engagement) to adjust the transmission torque transmitted from the rear-wheel output shaft 40 to the front-wheel drive drive sprocket 42. In other words, the front-wheel drive clutch 46 functions as a clutch (first clutch) for selectively disconnecting or connecting the power transmission path between the driving power source 12 and the front propeller shaft 24, which functions as a power transmission member to the front wheels 14L, 14R. The front-wheel drive clutch 46 is actuated by the action of hydraulic pressure, and the degree of engagement can be adjusted by controlling the magnitude of the hydraulic pressure.

[0032] The front-wheel-side output shaft 48 is connected to the front propeller shaft 24 so as to be able to transmit power. The front-wheel-drive driven sprocket 50 is provided so as to be able to rotate integrally with the front-wheel-side output shaft 48. In addition, a front-wheel-drive chain 52 is wound around the front-wheel-drive drive sprocket 42 and the front-wheel-drive driven sprocket 50, so that power can be transmitted between the two sprockets.

[0033] The front wheel differential gear device 28 includes a differential case 80, a pinion shaft 82, a pair of side gears 84L, 84R, a pair of pinions 86a, 86b, and a ring gear 90. The pinions 86a, 86b are attached to the differential case 80, with the pinions 86a, 86b disposed at both ends of the pinion shaft 82. The side gears 84L, 84R are disposed opposite each other within the differential case 80 and mesh with the pinions 86a, 86b, respectively. The side gears 84L, 84R are connected to the front wheels 14L, 14R via the front axles 32L, 32R. The ring gear 90 is attached integrally to the differential case 80. A front drive pinion 25 connected to the front propeller shaft 24 meshes with the ring gear 90. The front wheel differential gear device 28 also includes a mesh clutch 94 (second clutch). The mesh clutch 94 can be engaged by applying pressure (negative pressure). The mesh clutch 94 functions as a second clutch that selectively connects or disconnects the power transmission path between the front propeller shaft 24, which functions as a power transmission member to the front wheels 14L, 14R, and the front axles 32L, 32R, which are auxiliary drive wheels.

[0034] The rear wheel differential gear device 30 includes a differential case 120, a pinion shaft 122, a pair of side gears 124L and 124R, a pair of pinions 126a and 126b, and a ring gear 130. The pinions 126a and 126b are attached to the differential case 120, with the pinions 126a and 126b disposed at both ends of the pinion shaft 122. The side gears 124L and 124R are disposed opposite each other within the differential case 120 and mesh with the pinions 126a and 126b, respectively. The side gears 124L and 124R are connected to the rear wheels 16L and 16R via rear wheel axles 34L and 34R. The ring gear 130 is attached integrally to the differential case 120. A rear drive pinion 27 connected to the rear propeller shaft 26 meshes with the ring gear 130.

[0035] Because the vehicle 10 is configured as described above, when both the front-wheel drive clutch 46 and the dog clutch 94 are connected (engaged) to transmit torque, the vehicle 10 is in a state (four-wheel drive state) in which power generated in the driving force source 12 can be transmitted not only to the rear wheels 16L, 16R but also to the front wheels 14L, 14R. On the other hand, when at least one of the front-wheel drive clutch 46 and the dog clutch 94 is disconnected (disengaged), the power transmission path to the front wheels 14L, 14R is cut off, and the vehicle 10 is in a state in which torque transmission is disabled. As a result, the vehicle 10 is in a state (two-wheel drive state) in which power generated in the driving force source 12 can be transmitted to the rear wheels 16L, 16R but cannot be transmitted to the front wheels 14L, 14R.

[0036] The vehicle 10 is equipped with a selector switch 140 for switching the drive state. The selector switch 140 can be set to any one of three drive states: "2WD," "4WD LOCK," and "4WD AUTO." When the selector switch 140 is set to "2WD," one or both of the front-wheel drive clutch 46 and the dog clutch 94 are disengaged. This puts the vehicle 10 in a state where it drives in two-wheel drive mode. When the selector switch 140 is set to "4WD LOCK," the dog clutch 94 is engaged and the front-wheel drive clutch 46 is fully engaged. This puts the vehicle 10 in a state where it drives in four-wheel drive mode.

[0037] Furthermore, when the selector switch 140 is set to "4WD AUTO," the drive control is enabled by a standby control method (a drive distribution variable state) in which the drive distribution distribution of the drive force transmitted to the front wheels 14L, 14R relative to the drive force transmitted to the rear wheels 16L, 16R is increased by changing the engagement degree of the front-wheel drive clutch 46 while the mesh clutch 94 is engaged, provided that slippage is detected in the vehicle 10. Specifically, when the selector switch 140 is set to "4WD AUTO," the mesh clutch 94 is engaged, and the front-wheel drive clutch 46 is engaged at a predetermined standby engagement degree S in preparation for slippage. When the selector switch 140 is set to "4WD AUTO," if slippage occurs in the rear wheels 16L, 16R, which are the main drive wheels, the drive state of the vehicle 10 is switched by fully engaging the front-wheel drive clutch 46 while the mesh clutch 94 is maintained engaged.

[0038] The control device 100 is capable of controlling the connection state (engagement state) of the front-wheel drive clutch 46 and the dog clutch 94. The control device 100 is configured to include, for example, a microcomputer equipped with a CPU, RAM, ROM, an input / output interface, etc. The control device 100 includes a driving state detection unit 102, a first clutch control unit 104 (first clutch control unit), and a second clutch control unit 106.

[0039] The traveling state grasping unit 102 grasps the traveling state of the vehicle 10. In this embodiment, the traveling state grasping unit 102 can grasp the vehicle speed based on information output from a sensor or the like provided in the vehicle 10.

[0040] The first clutch control unit 104 controls the transmission torque (degree of engagement of the first clutch) transmitted to the front propeller shaft 24 via the front-wheel drive clutch 46. As described above, in this embodiment, the front-wheel drive clutch 46 is a wet multi-plate clutch whose degree of engagement (engagement pressure) can be adjusted by controlling the magnitude of hydraulic pressure. Therefore, the first clutch control unit 104 controls the magnitude of the torque transmitted to the front propeller shaft 24 by controlling the magnitude of hydraulic pressure acting on the front-wheel drive clutch 46.

[0041] The second clutch control unit 106 controls the operation of the dog clutch 94 to control the torque transmission from the front-wheel drive clutch 46 to the front-wheel differential gear unit 28. As described above, the dog clutch 94 can be engaged by applying pressure (negative pressure). Therefore, the second clutch control unit 106 can switch the dog clutch 94 between an engaged state and a disengaged state by controlling the magnitude of the pressure acting on the dog clutch 94.

[0042] The above-described control device 100 is characterized by a method for controlling the standby engagement degree S of the front-wheel drive clutch 46, i.e., the distribution ratio (drive distribution ratio) of power transmission to the front wheels 14L, 14R, which are auxiliary drive wheels, relative to power transmission to the rear wheels 16L, 16R, which are primary drive wheels, when the selector switch 140 is set to "4WD AUTO" and drive control is performed using the above-described standby control method. The control device 100 is characterized by performing power transmission control that adjusts the standby engagement degree S (drive distribution ratio) of the front-wheel drive clutch 46 so that it increases stepwise or continuously as the vehicle speed V of the vehicle 10 increases. The control performed by the control device 100 when the selector switch 140 is set to "4WD AUTO" will be described in detail below in accordance with the flowchart of FIG. 2 and with reference to the timing chart of FIG. 3.

[0043] (Step 1) In step 1, the control device 100 checks whether the vehicle 10 is in a state where drive control is performed using the standby control method by checking whether the selector switch 140 is set to "4WD AUTO." If it is confirmed that the vehicle 10 is set to "4WD AUTO," the control flow proceeds to step 2 to optimize the standby engagement degree S of the front-wheel drive clutch 46.

[0044] (Step 2) In step 2, the control device 100 engages the dog clutch 94 under the control of the second clutch control unit 106. As a result, when the dog clutch 94 is engaged, the control flow proceeds to step 3.

[0045] (Step 3) In step 3, the control device 100 determines whether or not slippage has occurred in the vehicle 10 using the running state determination unit 102. If it is confirmed that slippage has not occurred, the control flow proceeds to step 4. On the other hand, if slippage has occurred, the control flow proceeds to step 10, which will be described later.

[0046] (Step 4) In step 4, the control device 100 acquires information relating to the vehicle speed V of the vehicle 10 from a sensor or the like provided on the vehicle 10. Thereafter, the control device 100 advances the control flow to step 5.

[0047] (Step 5) In step 5, the control device 100 checks whether the vehicle speed Vs is equal to or greater than a predetermined lower limit vehicle speed Vmin. The lower limit vehicle speed Vmin is the lower limit value of the vehicle speed V at which power transmission control is performed, and can be determined as appropriate taking into consideration the characteristics of the vehicle 10, etc. The lower limit vehicle speed Vmin can be determined to be, for example, 20 km / h. Here, if the vehicle speed V is less than the lower limit speed Vmin, the control flow proceeds to step 6. On the other hand, if the vehicle speed V is equal to or greater than the lower limit speed Vmin, the control flow proceeds to step 7.

[0048] (Step 6) When the control flow proceeds from step 5 to step 6, the first clutch control unit 104 of the control device 100 controls the operation of the front-wheel drive clutch 46 so that the front-wheel drive clutch 46 is engaged at the minimum standby engagement degree Smin in preparation for the occurrence of slippage of the vehicle 10. In this embodiment, the front-wheel drive clutch 46 is hydraulically driven, and therefore a hydraulic pressure (minimum standby pressure Pmin) required to engage the front-wheel drive clutch 46 at the minimum standby engagement degree Smin is applied to the front-wheel drive clutch 46 (see FIG. 3).

[0049] Here, the minimum standby engagement degree Smin is the engagement degree defined as the minimum value of the engagement degree of the front wheel drive clutch 46 when the vehicle 10 is driven and controlled using the standby control method, and in the low vehicle speed region where the vehicle speed V is less than the lower limit vehicle speed Vmin, the engagement degree of the front wheel drive clutch 46 is maintained at the minimum standby engagement degree Smin.

[0050] (Step 7) On the other hand, when the control flow proceeds from step 5 to step 7, the control device 100 checks whether the vehicle speed V is equal to or less than the upper limit vehicle speed Vmax. The upper limit vehicle speed Vmax is the upper limit value of the vehicle speed V at which power transmission control is performed, and can be determined as appropriate taking into consideration the characteristics of the vehicle 10, etc. The upper limit vehicle speed Vmax can be determined to be, for example, 50 km / h. If the vehicle speed V is greater than the upper limit vehicle speed Vmax, the control flow proceeds to step 8. On the other hand, if the vehicle speed V is equal to or less than the upper limit vehicle speed Vmax, the control flow proceeds to step 9.

[0051] (Step 8) When the control flow proceeds from step 7 to step 8, the first clutch control unit 104 of the control device 100 controls the operation of the front-wheel drive clutch 46 so that the front-wheel drive clutch 46 is engaged at the maximum standby engagement degree Smax in preparation for the occurrence of slippage of the vehicle 10. In this embodiment, a hydraulic pressure (maximum standby pressure Pmax) required to engage the front-wheel drive clutch 46 at the maximum standby engagement degree Smax is applied to the front-wheel drive clutch 46 (see FIG. 3). When the engagement degree S of the front-wheel drive clutch 46 is set to the maximum standby engagement degree Smax, the distribution ratio (drive distribution ratio) of power transmission to the front wheels 14L, 14R, which are auxiliary drive wheels, relative to power transmission to the rear wheels 16L, 16R, which are primary drive wheels, becomes 1:1.

[0052] (Step 9) When the process proceeds from step 7 to step 9 described above, the vehicle speed V of the vehicle 10 is within a range of not less than the lower limit speed Vmin and not more than the upper limit vehicle speed Vmax. In this state, control (power transmission control) is performed to adjust the engagement degree of the front-wheel drive clutch 46 so that the front-wheel drive clutch 46 is engaged at an optimum standby engagement degree S according to the vehicle speed V of the vehicle 10. In this embodiment, the standby engagement degree S is set by adding an additional engagement degree Spls, which increases proportionally according to the speed V, to the minimum standby engagement degree Smin. In step 7, operation control (hydraulic control in this embodiment) of the front-wheel drive clutch 46 is performed so that the front-wheel drive clutch 46 is engaged at the standby engagement degree S set in this manner (see FIG. 3).

[0053] (Step 10) When the control flow proceeds from step 3 to step 8 described above, the control device 100 puts the vehicle 10 into a four-wheel drive state. Specifically, the control device 100 increases the engagement degree of the front-wheel drive clutch 46 from the standby engagement degree S, so that the front-wheel drive clutch 46 is fully engaged. This puts the vehicle 10 into a state where it can be driven in four-wheel drive mode.

[0054] The control device 100 mounted on the vehicle 10 described above has the following characteristic configurations (A) to (D), and therefore can achieve characteristic effects. Furthermore, the control device 100 can also achieve characteristic effects that could not be achieved by conventional techniques by adopting the following configuration (E).

[0055] (A) The control device 100 of this embodiment includes a front-wheel drive clutch 46 (first clutch) that selectively connects or disconnects the power transmission path between the drive source 12 and the front propeller shaft 24 (power transmission member), and a dog clutch 94 (second clutch) that selectively connects or disconnects the power transmission path between the front propeller shaft 24 and the front wheels 14L, 14R (auxiliary drive wheels). By disengaging the front-wheel drive clutch 46 and the dog clutch 94, the power transmission path between the drive source 12 and the left and right rear wheels 16L, 14R is selectively connected or disconnected. The control device 100 is switchable between a two-wheel drive state in which driving force is transmitted to the front wheels 14L and 14R (main drive wheels), and a four-wheel drive state in which driving force is also transmitted from the drive wheel 12 to the left and right front wheels 14L and 14R by engaging the front-wheel drive clutch 46 and the dog clutch 94, respectively, and is used in a vehicle 10 capable of drive control using a standby control method in which, if slippage is detected in the vehicle 10 while in the two-wheel drive state, the allocation of driving force transmitted to the front wheels 14L and 14R is increased to switch to the four-wheel drive state. The control device 100 is capable of controlling the front-wheel drive clutch 46 to wait in a state where it is engaged at a predetermined standby engagement level in preparation for the occurrence of slippage, provided that drive control using the standby control method is being performed on the vehicle 10. The control device 100 is configured to perform power transmission control such that the standby engagement degree or the distribution ratio of power transmission to the front wheels 14L, 14R relative to power transmission to the rear wheels 16L, 16R increases stepwise or continuously as the vehicle speed of the vehicle 10 increases.

[0056] As described above, the control device 100 of this embodiment performs power transmission control to adjust the standby engagement degree of the front-wheel drive clutch 46 or the distribution ratio (drive distribution ratio) of power transmission to the front wheels 14L, 14R relative to power transmission to the rear wheels 16L, 16R so that the higher the vehicle speed of the vehicle 10, the higher the standby engagement degree or the distribution ratio of power transmission to the rear wheels 16L, 16R and the front wheels 14L, 14R. This allows the control device 100 to optimally adjust the standby engagement degree and the distribution ratio of power transmission to the rear wheels 16L, 16R and the front wheels 14L, 14R so as to minimize the tight corner braking phenomenon in the low vehicle speed range while suppressing the occurrence of slippage in the high vehicle speed range.

[0057] (B) As described above, the control device 100 of this embodiment is designed to perform power transmission control until the vehicle 10 reaches a predetermined upper limit vehicle speed Vmax.

[0058] Because the control device 100 of this embodiment is configured in this manner, it is possible to minimize the tight corner braking phenomenon in areas lower than the upper limit vehicle speed Vmax, where the tight corner braking phenomenon is expected to affect driving performance, and to suppress the occurrence of slippage in high vehicle speed areas.

[0059] In this embodiment, an example is shown in which power transmission control is performed in a vehicle speed range up to when the vehicle 10 reaches a predetermined upper limit vehicle speed Vmax. However, the present invention is not limited to this, and it is possible to configure the vehicle in such a way that the upper limit vehicle speed Vmax, which is the upper limit of the vehicle speed range in which power transmission control is performed, is not specified.

[0060] (C) The control device 100 of the above-described embodiment is configured to perform power transmission control in a vehicle speed range where the four-wheel drive vehicle is at or above a predetermined lower limit vehicle speed Vmin, provided that the vehicle speed V is below a predetermined lower limit vehicle speed Vmin, the standby engagement degree S is set to a predetermined minimum standby engagement degree Smin, or the distribution ratio of power transmission to the auxiliary drive wheels relative to the main drive wheels is set to a minimum distribution ratio.

[0061] Because the control device 100 of this embodiment is configured in this manner, it can keep the front-wheel drive clutch 46 engaged so as to achieve the minimum standby engagement degree Smin or the minimum distribution ratio even at vehicle speeds V that are less than the lower limit vehicle speed Vmin. This allows the control device 100 of this embodiment to keep the front-wheel drive clutch 46 engaged in preparation for slippage even in a low vehicle speed range that is less than the lower limit vehicle speed Vmin.

[0062] In this embodiment, an example is shown in which the vehicle 10 performs power transmission control in a vehicle speed range equal to or greater than a predetermined lower limit vehicle speed Vmin, but the present invention is not limited to this, and it is possible to configure the vehicle 10 so that, for example, the lower limit vehicle speed Vmin, which is the lower limit of the vehicle speed range in which power transmission control is performed, is not specified.

[0063] (D) The control device 100 of this embodiment is used in a front-engine, rear-wheel-drive vehicle 10. The power transmission control described above works particularly effectively in a front-engine, rear-wheel-drive vehicle, and is therefore suitable for use in the vehicle 10 described above.

[0064] Although the control device 100 of this embodiment performs power transmission control only when the vehicle speed V of the vehicle 10 satisfies a predetermined condition, the present invention is not limited to this, and it is possible to add further conditions for performing power transmission control or to separately define conditions for not performing power transmission control. Specifically, as shown in (E) below, even if the vehicle speed V of the vehicle 10 satisfies the start condition for performing power transmission control, it is also possible to configure the control device 100 not to perform power transmission control, for example, on the condition that slippage is unlikely to occur.

[0065] (E) The control device 100 of the present embodiment described above can be configured not to perform power transmission control if at least one of the following conditions is satisfied: (a) the outside air temperature is higher than a predetermined temperature, (b) the estimated road surface μ value is higher than a predetermined value, (c) there is no history of slippage after the vehicle 10 starts moving, and (d) there is no slippage when starting.

[0066] The control device 100 of this embodiment can be configured not to perform power transmission control when it is assumed that slip is unlikely to occur, as in the above (a) to (d). This eliminates the need to increase the standby engagement degree S or the drive distribution ratio in response to an increase in vehicle speed V when slip is unlikely to occur, thereby simplifying control accordingly. Furthermore, by adopting the configuration as in (E) above, the control device 100 can reduce the frequency with which the standby engagement degree S or the drive distribution ratio is increased, thereby contributing to improved fuel economy of the vehicle 10 compared to when the configuration as in (E) above is not adopted.

[0067] The present invention is not limited to the above-described embodiments and variations thereof, and other embodiments may be possible within the scope of the claims. The components of the above-described embodiments may be arbitrarily selected and combined. Furthermore, any component of the embodiments may be arbitrarily combined with any component described in the Summary of the Invention or any component embodying any component described in the Summary of the Invention. The present invention intends to obtain rights to these as well through amendments to this application or divisional applications, etc. [Industrial Applicability]

[0068] The present invention can be suitably used in control devices for four-wheel drive vehicles in general, which are equipped with a first clutch that selectively disconnects or connects the power transmission path between a driving force source and a power transmission member, and a second clutch that selectively disconnects or connects the power transmission path between the power transmission member and an auxiliary drive wheel. [Explanation of symbols]

[0069] 10: Vehicle (four-wheel drive vehicle) 12: Driving force source 14L, 14R: Front wheels (auxiliary drive wheels) 16L, 16R: Rear wheels (main drive wheels) 18: Power transmission device 24: Front propeller shaft (power transmission member) 46: Front wheel drive clutch (first clutch) 94: Dog clutch (second clutch) 100: Control device 102: Driving condition detection unit 104: First clutch control unit

Claims

[Claim 1] a first clutch that selectively connects or disconnects a power transmission path between the driving power source and the power transmission member; a second clutch that selectively connects or disconnects a power transmission path between the power transmission member and the auxiliary drive wheels, a two-wheel drive state in which driving force is transmitted from the driving force source to the left and right main drive wheels by disengaging at least one of the first clutch and the second clutch; a four-wheel drive state in which the first clutch and the second clutch are engaged to transmit driving force from the driving force source to the left and right auxiliary drive wheels; a variable drive distribution state in which drive control is performed using a standby control method in which the degree of engagement of the first clutch is changed while the second clutch is engaged, on the condition that occurrence of slippage is detected, thereby increasing the distribution of drive force transmitted to the auxiliary drive wheels relative to the drive force transmitted to the main drive wheels; and A control device for a four-wheel drive vehicle used in a four-wheel drive vehicle capable of switching a drive state, On the condition that drive control by the standby control method is being performed, control can be performed to wait in a state in which the first clutch is engaged at a predetermined standby engagement degree in preparation for the occurrence of slippage, power transmission control is performed to adjust the standby engagement degree or the distribution ratio of power transmission to the auxiliary drive wheels relative to the power transmission to the main drive wheels so that the standby engagement degree or the distribution ratio of power transmission to the auxiliary drive wheels relative to the main drive wheels becomes higher in a stepwise or continuous manner as the vehicle speed of the four-wheel drive vehicle increases, A control device for a four-wheel drive vehicle, characterized in that the power transmission control is not performed when at least one of the following conditions is satisfied: (a) the outside air temperature is higher than a predetermined temperature, (b) the estimated road surface μ value is higher than a predetermined value, (c) there is no history of slippage after the four-wheel drive vehicle starts to travel, and (d) there is no slippage when starting.

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