All-Wheel Drive System

Abstract

To provide an all-wheel drive system which can vary a longitudinal torque distribution ratio from front wheel eccentricity to rear wheel eccentricity and can suppress heat damage of a clutch varying the longitudinal torque distribution ratio and reduce loss of an oil pump etc.SOLUTION: An all-wheel drive system 1 comprises: a center differential 40 which distributes inputted torque into front wheel side and rear wheel side; and a differential limit clutch 407 which limits the differential of the center differential 40 and varies a longitudinal torque distribution ratio of the front wheel side and the rear wheel side. The center differential 40 is set so that the longitudinal torque distribution ratio of initial is unequal (for example, front wheel eccentricity) and is configured so as to make it possible to change the setting. In addition, a reduction gear ratio of a front wheel side torque transmission system which transfers torque between the center differential 40 and front wheels 10FL, 10FR is set so as to become smaller than a reduction gear ratio of a rear wheel side torque transmission system which transfers torque between the center differential 40 and rear wheels 10RL, 10RR.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to all-wheel drive systems. [Background technology]

[0002] Conventionally, for example, all-wheel drive (AWD) vehicles (or four-wheel drive (4WD) vehicles) have been put into practical use. The vehicles have drive wheels that are directly driven by an engine and driven wheels that are connected to the engine via a transfer clutch, and are configured so that torque distribution to the driven wheels can be adjusted by controlling the fastening force of the transfer clutch according to the driving conditions, etc.

[0003] In such coupling-type all-wheel drive vehicles using a transfer clutch (torque-split part-time all-wheel drive vehicles), the torque distribution ratio between the drive wheels and the driven wheels can be varied between 100:0 and 50:50 (drive wheels) by adjusting (releasing and engaging) the engagement pressure (engagement force) of the transfer clutch. For example, in the case of a front-wheel-drive (FF) vehicle, the front-to-rear torque distribution ratio can be varied between 100:0 and 50:50 (front): (rear).

[0004] In contrast to this, Patent Document 1 discloses a four-wheel drive vehicle equipped with a control coupling (transfer clutch) in a transmission path that transmits the driving force from the driving wheels to the driven wheels, which controls the transmission rate of the driving force transmitted to the driven wheels.The four-wheel drive vehicle is configured so that the gear ratio of the driven wheels is set larger than the gear ratio of the driving wheels, and the reduction ratio of the driven wheels is set larger than the reduction ratio of the driving wheels, thereby making it possible to change the ratio between the driving force on the driven wheels and the driving force on the driven wheels (front / rear torque distribution ratio) over a range wider than 0:1 to 1:1.

[0005] More specifically, in this four-wheel drive vehicle, the output / input gear ratio in the front differential is set to 1, and the output / input gear ratio in the rear differential is set to 1.03 to 1.17. As a result, in this four-wheel drive vehicle, the driving state can be changed over a wide range, from a front-wheel drive driving state (front-wheel bias) to a driving state close to rear-wheel drive (rear-wheel bias). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 11-28942 Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, the all-wheel drive vehicle (AWD vehicle) of Patent Document 1 sets the gear ratio of the driven wheels (e.g., rear wheels) higher than the gear ratio of the driven wheels (e.g., front wheels) and sets the reduction ratio of the driven wheels higher than the reduction ratio of the driven wheels, making it possible to change the ratio of driven wheel torque to drive wheel torque (front-rear torque distribution ratio) over a range wider than (rear) 0:(front) 1 to 1:1. In other words, the all-wheel drive vehicle (AWD vehicle) of Patent Document 1 sets the reduction ratio (gear ratio) of the front wheels and the reduction ratio (gear ratio) of the rear wheels differently and increases the engagement pressure of the transfer clutch, so that, for example, when the front wheels are driven, the front-rear torque distribution ratio can be varied from an initial front-wheel bias to a rear-wheel bias.

[0008] However, in the all-wheel drive vehicle described in Patent Document 1, the reduction ratio (gear ratio) on the front wheels is different from the reduction ratio (gear ratio) on the rear wheels, which causes the transfer clutch to slip constantly, resulting in problems with heat damage, strength, durability, etc. of the transfer clutch. Also, for example, since it is necessary to generate a larger clutch engagement pressure (engagement force) to shift the weight to the rear wheels, loss in the oil pump that generates the hydraulic pressure becomes a problem.

[0009] The present invention has been made to solve the above problems, and aims to provide an all-wheel drive system that can change the front / rear torque distribution ratio from front-wheel bias to rear-wheel bias, and that can suppress thermal damage to the clutch that changes the front / rear torque distribution ratio and reduce losses in the oil pump, etc. [Means for solving the problem]

[0010] The all-wheel drive system of the present invention comprises a center differential that distributes input torque to the front and rear wheels, a limited slip differential clutch that limits the differential of the center differential according to the engagement pressure and varies the front / rear torque distribution ratio between the front and rear wheels, a front wheel torque transmission system that transmits torque between the center differential and the front wheels, a rear wheel torque transmission system that transmits torque between the center differential and the rear wheels, and a control unit that adjusts the engagement pressure of the limited slip differential clutch based on the driving state of the vehicle, and is characterized in that the reduction ratio of the front wheel torque transmission system and the reduction ratio of the rear wheel torque transmission system are set to be different, and the initial front / rear torque distribution ratio of the center differential is set unequal and is configured to be changeable.

[0011] In the all-wheel drive system according to the present invention, the reduction ratio of the front-wheel torque transmission system and the reduction ratio of the rear-wheel torque transmission system are set to be different, and the initial front-rear torque distribution ratio of the center differential is set to be unequal and variable. In other words, because the initial front-rear torque distribution ratio is unequal and variable, compared to a coupling-type all-wheel drive vehicle with different front-rear reduction ratios (gear ratios), the torque distribution to the driven wheels (e.g., rear wheels) is set to be higher initially, thereby reducing the range of change in the front-rear torque distribution ratio caused by the limited slip differential clutch, and thereby reducing the engagement pressure (engagement force) of the limited slip differential clutch. This reduces the amount of heat generated by the limited slip differential clutch, thereby suppressing (reducing) thermal damage to the limited slip differential clutch. Furthermore, reducing the engagement pressure (engagement force) of the limited slip differential clutch can reduce losses, for example, in an oil pump that generates (boosts) hydraulic pressure. [Effects of the Invention]

[0012] According to the present invention, the front / rear torque distribution ratio can be varied from a front-wheel bias to a rear-wheel bias, and it is possible to suppress thermal damage to the clutch that varies the front / rear torque distribution ratio and reduce losses in the oil pump, etc. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a diagram showing the overall configuration of an all-wheel drive system according to an embodiment; [Figure 2] 5 is a flowchart showing a procedure for suppressing (preventing) heat damage to the limited slip differential clutch by the all-wheel drive system according to the embodiment. [Figure 3] FIG. 4 is a diagram showing the relationship between input torque, steering angle, and clutch engagement force. [Figure 4] FIG. 10 is a diagram showing the relationship between input torque, steering speed, and clutch engagement force. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts will be designated by the same reference numerals. In addition, the same elements will be designated by the same reference numerals in each drawing, and redundant explanations will be omitted.

[0015] First, the configuration of an all-wheel drive system 1 according to an embodiment will be described using Figure 1. Figure 1 is a diagram showing the overall configuration of the all-wheel drive system 1 and an all-wheel drive (AWD) vehicle 3 on which the all-wheel drive system 1 is mounted.

[0016] The engine 20 may be of any type, but may be, for example, a horizontally opposed, direct-injection, four-cylinder gasoline engine. In the engine 20, air drawn in through an air cleaner (not shown) is throttled by an electronically controlled throttle valve (hereinafter simply referred to as the "throttle valve") 85 provided in the intake pipe, passes through an intake manifold, and is drawn into each cylinder of the engine 20. The amount of air drawn in through the air cleaner is detected by an air flow meter. Furthermore, the throttle valve 85 is provided with a throttle opening sensor 83 that detects the opening of the throttle valve 85. Each cylinder is equipped with an injector that injects fuel. Each cylinder is also equipped with a spark plug that ignites the air-fuel mixture and a built-in igniter coil that applies high voltage to the spark plug. In each cylinder of the engine 20, the air-fuel mixture, consisting of the drawn air and the fuel injected by the injector, is ignited by the spark plug and combusted. The resulting exhaust gas is discharged through an exhaust pipe.

[0017] In addition to the air flow meter and throttle opening sensor 83 described above, a cam angle sensor 81 for identifying the cylinders of the engine 20 is attached near the camshaft of the engine 20. Also, a crank angle sensor 82 for detecting the position of the crankshaft is attached near the crankshaft of the engine 20. These sensors are connected to an engine control unit (hereinafter referred to as "ECU") 80, which will be described later. Also connected to the ECU 80 are various sensors, such as an accelerator opening sensor 84 that detects the amount of depression of the accelerator pedal, i.e., the opening of the accelerator pedal, and a water temperature sensor that detects the temperature of the cooling water for the engine 20.

[0018] A continuously variable transmission 30 that converts and outputs torque from the engine 20 is connected to the crankshaft 21 of the engine 20 via a torque converter 22 that has a clutch function and a torque amplification function, and a forward / reverse switching mechanism 27.

[0019] The torque converter 22 is mainly composed of a pump impeller 23, a turbine runner 24, and a stator 25. The pump impeller 23 connected to the crankshaft 21 generates a flow of oil, and the turbine runner 24, which is disposed opposite the pump impeller 23, receives power from the engine 20 via the oil to drive a turbine shaft 31. The stator 25, located between the two, rectifies the discharge flow (return) from the turbine runner 24 and returns it to the pump impeller 23, thereby generating torque amplification.

[0020] The torque converter 22 also has a lock-up clutch 26 that directly couples the input and output. When the lock-up clutch 26 is not engaged (in a non-lock-up state), the torque converter 22 amplifies the torque of the engine 20 and transmits it to the continuously variable transmission 30, and when the lock-up clutch 26 is engaged (in a lock-up state), the torque converter 22 directly transmits the torque of the engine 20 to the continuously variable transmission 30. The rotation speed (turbine rotation speed) of the turbine runner 24 that constitutes the torque converter 22 is detected by a turbine rotation sensor 94. The detected turbine rotation speed is output to a transmission control unit (hereinafter referred to as "TCU") 70, which will be described later.

[0021] The forward / reverse switching mechanism 27 switches between forward and reverse rotation (forward and reverse movement of the vehicle) of the drive wheels 10 (left front wheel 10FL, right front wheel 10FR, left rear wheel 10RL, right rear wheel 10RR). The forward / reverse switching mechanism 27 mainly includes a double-pinion planetary gear train, a forward clutch (forward clutch) 28, and a reverse brake (reverse brake) 29. The forward / reverse switching mechanism 27 is configured to be able to switch the transmission path of the engine torque by controlling the respective states of the forward clutch 28 and the reverse brake 29.

[0022] More specifically, when the D (drive) range is selected, the rotation of the turbine shaft 31 is transmitted directly to the primary shaft 32 (described later) by engaging the forward clutch 28 and disengaging the reverse brake 29, allowing the vehicle to travel forward. On the other hand, when the R (reverse) range is selected, the planetary gear train is operated by disengaging the forward clutch 28 and engaging the reverse brake 29, thereby reversing the rotation direction of the primary shaft 32 and allowing the vehicle to travel backward.

[0023] Furthermore, when the N (neutral) range or P (parking) range is selected, the forward clutch 28 and the reverse brake 29 are released, thereby separating the turbine shaft 31 from the primary shaft 32 (the transmission of engine torque is interrupted), and the forward / reverse switching mechanism 27 is in a neutral state in which it does not transmit power to the primary shaft 32. The operations of the forward clutch 28 and the reverse brake 29 are controlled by the TCU 70 and the control valve (valve body) 60.

[0024] The transmission mechanism (variator) 33 of the continuously variable transmission 30 has a primary shaft 32 connected to the turbine shaft 31 of the torque converter 22 via the forward / reverse switching mechanism 27, and a secondary shaft 37 arranged parallel to the primary shaft 32.

[0025] The continuously variable transmission 30 has a primary shaft 32 connected to the turbine shaft 31 of the torque converter 22 via the forward / reverse switching mechanism 27, and a secondary shaft 37 disposed parallel to the primary shaft 32. A primary pulley 34 is provided on the primary shaft 32. The primary pulley 34 has a fixed sheave 34a joined to the primary shaft 32 and a movable sheave 34b mounted opposite the fixed sheave 34a and slidable in the axial direction of the primary shaft 32, and is configured to change the cone face spacing between the sheaves 34a and 34b, i.e., the pulley groove width. Meanwhile, a secondary pulley 35 is provided on the secondary shaft 37. The secondary pulley 35 has a fixed sheave 35a joined to the secondary shaft 37 and a movable sheave 35b mounted opposite the fixed sheave 35a and slidable in the axial direction of the secondary shaft 37, and is configured to change the pulley groove width.

[0026] A chain 36 that transmits torque is wound around the primary pulley 34 and the secondary pulley 35. The gear ratio is continuously changed by changing the groove width of the primary pulley 34 and the secondary pulley 35 to change the ratio of the winding diameter of the chain 36 to each of the pulleys 34, 35 (pulley ratio). Here, if the winding diameter of the chain 36 around the primary pulley 34 is Rp and the winding diameter around the secondary pulley 35 is Rs, the gear ratio i is expressed as i = Rs / Rp. Therefore, the gear ratio i can be calculated by dividing the primary pulley rotation speed Np by the secondary pulley rotation speed Ns (i = Np / Ns).

[0027] Here, a hydraulic chamber 34c is formed in the primary pulley 34 (movable sheave 34b). On the other hand, a hydraulic chamber 35c is formed in the secondary pulley 35 (movable sheave 35b). The groove widths of the primary pulley 34 and the secondary pulley 35 are set or changed by adjusting the primary hydraulic pressure introduced into the hydraulic chamber 34c of the primary pulley 34 and the secondary hydraulic pressure introduced into the hydraulic chamber 35c of the secondary pulley 35.

[0028] The secondary shaft 37 of the continuously variable transmission 30 is connected to a center differential 40 (described in detail below) that distributes input torque to the front and rear wheels. The torque converted by the continuously variable transmission 30 is distributed by the center differential 40 to the front and rear wheels at a predetermined torque distribution ratio (described in detail below). The torque distributed from the center differential 40 to the front wheels is transmitted to a front differential (hereinafter also referred to as the "front differential") 44 via a transfer gear 401 (transfer drive gear 401a, transfer driven gear 401b) and a front drive shaft 43. The front differential 44 is, for example, a bevel gear-type differential device. The torque from the front differential 44 is transmitted to a left front wheel 10FL via a left front wheel drive shaft 45L and to a right front wheel 10FR via a right front wheel drive shaft 45R. Here, the transfer gear 401 (transfer drive gear 401a, transfer driven gear 401b), front drive shaft 43, front differential 44, left front wheel drive shaft 45L, and right front wheel drive shaft 45R correspond to the "front wheel torque transmission system that transmits torque between the center differential and the front wheels" described in the claims.

[0029] Torque distributed from the center differential 40 to the rear wheels is transmitted to a rear differential 47 (hereinafter also referred to as "rear differential") via a propeller shaft 46 extending rearward of the vehicle. A left rear wheel drive shaft 48L and a right rear wheel drive shaft 48R are connected to the rear differential 47. Torque from the rear differential 47 is transmitted to the left rear wheel 10RL via the left rear wheel drive shaft 48L and to the right rear wheel 10RR via the right rear wheel drive shaft 48R. Here, the propeller shaft 46, rear differential 47, left rear wheel drive shaft 48L, and right rear wheel drive shaft 48R correspond to a "rear wheel torque transmission system that transmits torque between the center differential and the rear wheels" as defined in the claims.

[0030] Here, the reduction ratio (total gear ratio) of the front-wheel torque transmission system and the reduction ratio (total gear ratio) of the rear-wheel torque transmission system are set to be different. More specifically, in this embodiment, the gear ratios of transfer gear 401, front differential 44, and rear differential 47 are set so that the reduction ratio (total gear ratio) of the front-wheel torque transmission system is smaller than the reduction ratio (total gear ratio) of the rear-wheel torque transmission system. That is, in this embodiment, the gear ratios are set so that "gear ratio of transfer gear 401 × gear ratio of front differential 44 < gear ratio of rear differential 47". Therefore, "rotational speed of rear wheels 10RL, 10RR < rotational speed of front wheels 10FL, 10FR" holds.

[0031] In the center differential 40, the secondary shaft 37 is rotatably inserted from the front of a rotatably housed carrier 402. On the other hand, a propeller shaft 46 is rotatably inserted from the rear.

[0032] A small-diameter first sun gear 403 is axially attached to the rear end of the input side secondary shaft 37, and a large-diameter second sun gear 404 is axially attached to the front end of the propeller shaft 46 that outputs to the rear wheels 10RL and 10RR, and the first sun gear 403 and the second sun gear 404 are stored within the carrier 402.

[0033] First sun gear 403 meshes with large-diameter first pinion gear 405 to form a first gear train, and second sun gear 404 meshes with small-diameter second pinion gear 406 to form a second gear train. First pinion gear 405 and second pinion gear 406 are formed integrally, and multiple pairs (e.g., three pairs) of pinion gears are rotatably supported on carrier 402. Transfer drive gear 401a is connected to the front end of carrier 402, and power is output from carrier 402 to the front wheels.

[0034] That is, the center differential 40 is configured as a compound planetary gear type without a ring gear, in which torque from the secondary shaft 37 is transmitted to the first sun gear 403, output from the second sun gear 404 to the propeller shaft 46, and output from the carrier 402 to the front drive shaft 43 via the transfer drive gear 401a and the transfer driven gear 401b.

[0035] This composite planetary gear type center differential 40 has a differential function by appropriately setting the number of teeth of the first and second sun gears 403, 404 and the first and second pinion gears 405, 406, which are arranged in multiple numbers around these sun gears 403, 404.

[0036] Furthermore, by appropriately setting the meshing pitch circle radii (=torque ratios) between the first and second sun gears 403, 404 and the first and second pinion gears 405, 406, it is possible to set the reference torque distribution (initial) to an equal torque distribution of 50:50 between the front and rear, or an unequal torque distribution biased towards either the front or rear. In this embodiment, the front / rear torque distribution ratio in the initial state (initial state) of the center differential 40 is set to, for example, a front-to-rear bias of 80:20 (torque distribution on the front wheels > torque distribution on the rear wheels).

[0037] In other words, the center differential 40 is configured so that the front and rear torque distribution ratio in the initial state is set to be unequal, and this setting can be changed (i.e., by changing the meshing pitch circle radius between the first and second sun gears 403, 404 and the first and second pinion gears 405, 406).

[0038] In addition, a limited slip differential clutch (LSD clutch) 407 employing a hydraulic multi-plate clutch is provided between carrier 402 of center differential 40 and propeller shaft 46. This clutch limits the differential of center differential 40 and varies the front / rear torque distribution ratio (torque distribution between front and rear wheels). By controlling the engagement force of this limited slip differential clutch 407, it is possible to variably control the front / rear torque distribution ratio within a range of, for example, 80:20 to 20:80. The engagement pressure (engagement force) of limited slip differential clutch 407 is controlled by TCU 70 and control valve (valve body) 60.

[0039] With the torque transmission system of the powertrain configured as described above, for example, when the selector lever is operated to the D range, engine torque is input to the primary shaft 32 of the continuously variable transmission 30. The torque converted by the continuously variable transmission 30 is output from the secondary shaft 37 and distributed to the front and rear wheels by the center differential 40. Here, when clutch torque is applied to the limited slip differential clutch 407, torque distributed in accordance with the clutch torque is output to the front drive shaft 43 and the propeller shaft 46.

[0040] The torque distributed to the front wheels is transmitted to a front drive shaft 43. The torque is then distributed to the left and right by a front differential 44 and transmitted to the left and right front wheels 10FL, 10FR. On the other hand, the torque distributed to the rear wheels is transmitted to a propeller shaft 46. The torque is then transmitted to the rear wheels 10RL, 10RR via a rear differential 47.

[0041] Each of the wheels 10FL-10RR (hereinafter, all of the wheels 10FL-10RR may be collectively referred to as wheels 10) is fitted with a brake 11FL-11RR (hereinafter, all of the brakes 11FL-11RR may be collectively referred to as brake 11) that brakes the wheels 10FL-10RR. Also, each of the wheels 10FL-10RR is fitted with a wheel speed sensor 12FL-12RR (hereinafter, all of the wheel speed sensors 12FL-12RR may be collectively referred to as wheel speed sensor 12) that detects the wheel rotation speed.

[0042] In this embodiment, a disc brake is used as the brake 11. The brake 11 is configured with a brake disc attached to the wheel 10 of the AWD vehicle 3, and a brake caliper incorporating brake pads and wheel cylinders. During braking (braking), the brake pads are pressed against the brake disc by hydraulic pressure, and the wheel 10 connected to the brake disc is braked by frictional force. Note that although the brake 11 used in this embodiment is a disc brake, a drum brake that applies braking by pressing a friction material against the inner peripheral surface of a drum may also be used.

[0043] The wheel speed sensor 12 is a non-contact sensor that detects changes in the magnetic field caused by a rotor (gear rotor or magnetic rotor) that rotates together with the wheel 10, and is preferably a semiconductor type that detects rotor rotation using a Hall element or MR element, for example.

[0044] In addition, this AWD vehicle 3 is equipped with a VDC control unit (hereinafter referred to as "VDCU") 50 that prevents skidding and ensures excellent driving stability when, for example, entering a corner at an excessive speed or when the vehicle's posture (behavior) becomes unstable due to sudden steering.

[0045] The hydraulic pressure for shifting the continuously variable transmission 30, i.e., the primary hydraulic pressure and secondary hydraulic pressure described above, are controlled by a control valve (valve body) 60. The valve body 60 adjusts the hydraulic pressure discharged from the oil pump 62 by opening and closing an oil passage formed therein using a spool valve and a solenoid valve (electromagnetic valve) that operates the spool valve, and supplies the oil to the hydraulic chamber 34c of the primary pulley 34 and the hydraulic chamber 35c of the secondary pulley 35. Similarly, the control valve 60 adjusts the hydraulic pressure discharged from the oil pump 62 by opening and closing an oil passage formed therein using a spool valve and a solenoid valve (electromagnetic valve) that operates the spool valve, and supplies the hydraulic pressure for adjusting the engagement pressure (engagement force) of the differential limiting clutch 407. Here, the solenoid valve that adjusts the hydraulic pressure supplied to the differential limiting clutch 407 may, for example, be a duty solenoid that can control the drive amount according to the duty ratio of an applied voltage.

[0046] The speed change control of the continuously variable transmission 30 is executed by the TCU 70. That is, the TCU 70 controls the operation of a solenoid valve (electromagnetic valve) constituting the valve body 60 described above, thereby adjusting the hydraulic pressure supplied to the hydraulic chamber 34c of the primary pulley 34 and the hydraulic chamber 35c of the secondary pulley 35, thereby changing the gear ratio of the continuously variable transmission 30. Similarly, the TCU 70 controls the operation of a solenoid valve constituting the control valve 60 described above, thereby adjusting the hydraulic pressure supplied to the limited slip differential clutch 407, thereby adjusting the distribution ratio (front / rear torque distribution ratio) of the torque transmitted to the front wheels 10FL, 10FR and the torque transmitted to the rear wheels 10RL, 10RR.

[0047] A shift lever (selector lever) 75 is provided on the floor (center console) of the vehicle or the like, allowing the driver to selectively switch the operating state (range) of the continuously variable transmission 30. A range switch 93 is attached to the shift lever 75 and is connected to move in conjunction with the shift lever 75 to detect the selected position of the shift lever 75. The range switch 93 is connected to the TCU 70, and the detected selected position of the shift lever 75 is read into the TCU 70. The shift lever 75 can selectively switch between a drive range (D) and a manual range (M), as well as a parking range (P), a reverse range (R), and a neutral range (N). A switch-type selector mechanism may be used instead of the shift lever 75.

[0048] Here, when the shift lever 75 is operated to select the D range (forward driving range), oil is supplied to the hydraulic chamber of the forward clutch 28 and oil is discharged from the hydraulic chamber of the reverse brake 29. As a result, the forward clutch 28 is engaged and the reverse brake 29 is disengaged, allowing the vehicle to move forward. On the other hand, when the shift lever 75 is operated to select the R range (reverse driving range), oil is supplied to the hydraulic chamber of the reverse brake 29 and oil is discharged from the hydraulic chamber of the forward clutch 28. As a result, the reverse brake 29 is engaged and the forward clutch 28 is disengaged, allowing the vehicle to move backward. Note that when the shift lever 75 is operated to select the N range or P range, oil is discharged from each of the hydraulic chambers of the forward clutch 28 and the reverse brake 29. As a result, the forward clutch 28 and the reverse brake 29 are each disengaged (transmission of engine torque is interrupted), and the continuously variable transmission 30 is in neutral.

[0049] As described above, the TCU 70 performs the shift control of the continuously variable transmission 30, the engagement / disengagement control of the forward clutch 28 and the reverse brake 29 (forward / reverse switching control), and the engagement pressure control of the limited slip differential clutch 407 (front / rear torque distribution control). The TCU 70 is connected to the ECU 80, which comprehensively controls the engine 20, the VDCU 50, and the like, via a CAN (Controller Area Network) 100, for example, so as to be able to communicate with each other.

[0050] The TCU 70, ECU 80, and VDCU 50 are each configured to include a microprocessor that performs calculations, an EEPROM that stores programs and the like for causing the microprocessor to execute each process, a RAM that stores various data such as calculation results, a backup RAM whose stored contents are maintained by a battery, and an input / output I / F, etc.

[0051] The ECU 80 identifies the cylinder from the output of the cam angle sensor 81, and determines the engine speed from changes in the rotational position of the crankshaft detected by the output of the crank angle sensor 82. The ECU 80 also acquires various information such as the intake air amount, accelerator pedal opening, air-fuel ratio of the air-fuel mixture, and water temperature based on detection signals input from the various sensors described above. The ECU 80 then comprehensively controls the engine 20 by controlling the fuel injection amount, ignition timing, and various devices such as the throttle valve 85 based on the acquired information.

[0052] Furthermore, the ECU 80 transmits various information such as the engine water temperature (coolant temperature), the accelerator pedal opening, the engine speed, and the engine torque to the TCU 70 via the CAN 100.

[0053] The VDCU 50 is connected to four wheel speed sensors 12FL-12RR, a steering angle sensor 16, an acceleration (G) sensor 55, a pitch angle sensor 56, and a brake switch 57. As described above, the wheel speed sensors 12FL-12RR detect the rotation of gears attached to the centers of the wheels 10FL-10RR using a magnetic pickup or the like to detect the rotation of the wheels 10FL-10RR. The acceleration sensor 55 detects the longitudinal and lateral accelerations acting on the AWD vehicle 3. The pitch angle sensor 56 detects the rotation angle (road surface gradient) around an axis passing through the center of gravity of the AWD vehicle 3 and penetrating the vehicle body in the vehicle width direction. In addition, the steering angle sensor 16 detects the rotation angle of the front wheels 10FL, 10FR (i.e., the steering angle of the steering wheel 15) by detecting the rotation angle of the pinion shaft.

[0054] The VDCU 50 drives the brake actuator to brake the vehicle in accordance with the amount of brake pedal operation (depression amount), and detects vehicle behavior using various sensors (e.g., wheel speed sensor 12, steering angle sensor 16, acceleration sensor 55, pitch angle sensor 56, yaw rate sensor, etc.), and suppresses skidding and ensures vehicle stability during cornering by automatically pressurizing the brakes and controlling the torque of the engine 20. That is, the VDCU 50 prevents skidding and ensures excellent driving stability, for example, when the vehicle enters a corner at an excessive speed or when the vehicle's attitude (behavior) is disturbed by a sudden steering operation. More specifically, the VDCU 50 detects the vehicle's attitude (behavior) using the above sensors, etc., and if it determines that the vehicle is oversteering, it automatically applies the brakes to the front wheels 10FL and 10FR on the outside of the corner. Conversely, if it determines that the vehicle is understeering, it automatically reduces engine power and applies the brakes to the rear wheels 10RL and 10RR on the inside of the corner, depending on the driving situation. In addition to the VDC (Vehicle Dynamic Control) function, the VDCU50 also has ABS (Anti-lock Brakes) and TCS (Traction Control) functions.

[0055] The VDCU 50 transmits various types of information such as the detected wheel speed, steering angle, acceleration, pitch angle (road surface gradient), and braking information of each wheel 10 to the TCU 70 via the CAN 100.

[0056] In addition to the turbine rotation sensor 94 described above, the TCU 70 is connected to an oil temperature sensor 91 that detects the oil temperature of the continuously variable transmission 30 (i.e., the oil temperature of the differential limiting clutch 407), an output shaft rotation sensor 92 that detects the rotation speed of the secondary shaft 37 (the rotation speed of the secondary pulley 35), a range switch 93 that detects the selected position of the shift lever, and the like.

[0057] As described above, the TCU 70 receives various information such as the wheel speed, steering angle, acceleration, pitch angle (road surface gradient), and braking information of each wheel 10 from the VDCU 50 via the CAN 100, and also receives information such as the engine water temperature, accelerator pedal opening, engine speed, and engine torque from the ECU 80.

[0058] The TCU 70 automatically and steplessly changes the gear ratio in accordance with a shift map in accordance with the driving conditions (such as accelerator opening and vehicle speed) of the AWD vehicle 3. The shift map is stored in, for example, an EEPROM within the TCU 70.

[0059] The TCU 70 also executes limited slip clutch control (front / rear torque distribution control) based on various information acquired from the above-mentioned various sensors, etc. In other words, the TCU 70 adjusts the engagement force of the limited slip clutch 407 (i.e., the front / rear torque distribution ratio) based on the operating state (e.g., steering angle, pitch angle, etc.) of the AWD vehicle 3. In other words, the TCU 70 corresponds to a control unit recited in the claims.

[0060] In particular, the TCU 70 can change the front / rear torque distribution ratio from front-wheel bias to rear-wheel bias, and has the function of suppressing heat damage from the limited slip differential clutch 407 that changes the front / rear torque distribution ratio, and reducing losses in the oil pump 62, etc. In the TCU 70, these functions are realized by a microprocessor executing a program stored in an EEPROM or the like.

[0061] For example, during cornering acceleration or uphill acceleration, TCU 70 adjusts the engagement pressure (engagement force) of limited slip differential clutch 407 to vary the front / rear torque distribution ratio toward the rear wheel bias side. On the other hand, TCU 70 disengages limited slip differential clutch 407 in driving states other than cornering acceleration or uphill acceleration. In other words, the front / rear torque distribution ratio is biased toward the front wheel (initial).

[0062] More specifically, when the input torque (engine torque x gear ratio) of the center differential 40 is equal to or greater than a predetermined torque (e.g., 10 Nm) and the steering angle of the steering wheel 15 is equal to or greater than a predetermined angle (e.g., 1°), or when the input torque of the center differential 40 is equal to or greater than a predetermined torque (e.g., 10 Nm) and the pitch angle (road surface gradient) of the AWD vehicle 3 is equal to or greater than a predetermined angle (e.g., 1°), the TCU 70 increases the engagement force (engagement pressure) of the differential limiting clutch 407, thereby shifting the front / rear torque distribution ratio toward the rear wheels.

[0063] Here, for example, the EEPROM of the TCU 70 stores a map (target clutch torque map) that defines the relationship between the input torque of the center differential 40, the steering angle of the steering wheel 15, the pitch angle (gradient) of the AWD vehicle 3, and the target clutch torque of the differential limiting clutch 407, and the target clutch torque of the differential limiting clutch 407 is determined by searching this target clutch torque map based on the steering angle and pitch angle.

[0064] As shown in Fig. 3, the TCU 70 increases the engagement pressure (engagement force) of limited slip differential clutch 407 in proportion to the steering angle as the steering angle increases. However, applying a large clutch engagement force during low-speed cornering makes it difficult to turn (tight corner braking phenomenon), so it is preferable not to increase the clutch engagement force when steering slowly. Therefore, as shown in Fig. 4, the TCU 70 increases the engagement pressure (engagement force) of limited slip differential clutch 407 in proportion to the steering angular velocity as the steering angular velocity increases. Note that Fig. 3 is a diagram showing the relationship between input torque, steering angle, and clutch engagement force. Fig. 4 is a diagram showing the relationship between input torque, steering speed (steering angular velocity), and clutch engagement force.

[0065] Furthermore, the TCU 70 disengages the differential limiting clutch 407 when the oil temperature of the differential limiting clutch 407 (continuously variable transmission 30) is equal to or higher than a predetermined temperature (for example, 80° C.).

[0066] Next, the operation of the all-wheel drive system 1 will be described with reference to Figure 2. Figure 2 is a flowchart showing the procedure for processing to suppress (prevent) thermal damage to the limited slip differential clutch 407 by the all-wheel drive system 1. This processing is executed repeatedly at predetermined timings, mainly by the TCU 70.

[0067] In step S100, a determination is made as to whether the oil temperature of limited slip clutch 407 (continuously variable transmission 30) is equal to or higher than a predetermined temperature (e.g., 80°C). If the oil temperature of limited slip clutch 407 is equal to or higher than the predetermined temperature, the target clutch torque of limited slip clutch 407 is set to zero in step S110, and then the target oil pressure of limited slip clutch 407 is set to zero in step S112, and limited slip clutch 407 is disengaged. Thereafter, the process temporarily exits. On the other hand, if the oil temperature of limited slip clutch 407 is lower than the predetermined temperature, the process proceeds to step S102.

[0068] In step S102, it is determined whether the input torque of center differential 40 is equal to or greater than a predetermined torque (e.g., 10 Nm). If the input torque of center differential 40 is less than the predetermined torque, the target clutch torque of limited slip clutch 407 is set to zero in step S110, and then the target hydraulic pressure of limited slip clutch 407 is set to zero in step S112, and limited slip clutch 407 is disengaged. Thereafter, the process temporarily exits. On the other hand, if the input torque of center differential 40 is equal to or greater than the predetermined torque, the process proceeds to step S104.

[0069] In step S104, it is determined whether the steering angle of the steering wheel 15 is equal to or greater than a predetermined angle (for example, 1°). If the steering angle of the steering wheel 15 is less than the predetermined angle, the process proceeds to step S106. On the other hand, if the steering angle of the steering wheel 15 is equal to or greater than the predetermined angle, the process proceeds to step S108.

[0070] In step S106, a determination is made as to whether the pitch angle (road surface gradient) of the AWD vehicle 3 is equal to or greater than a predetermined angle (for example, 1°). If the pitch angle of the AWD vehicle 3 is less than the predetermined angle, the target clutch torque of the limited slip clutch 407 is set to zero in step S110, and then the target hydraulic pressure of the limited slip clutch 407 is set to zero in step S112, and the limited slip clutch 407 is disengaged. Thereafter, the process temporarily exits. On the other hand, if the pitch angle of the AWD vehicle 3 is equal to or greater than the predetermined angle, the process proceeds to step S108.

[0071] In step S108, a map (target clutch torque map) that defines the relationship between the input torque of the center differential 40, the steering angle of the steering wheel 15, the pitch angle (road surface gradient) of the AWD vehicle 3, and the target clutch torque of the differential limiting clutch 407 is searched, and the target clutch torque of the differential limiting clutch 407 is determined.

[0072] Next, in step S112, a target clutch oil pressure for the differential limiting clutch 407 is set in accordance with the target clutch torque for the differential limiting clutch 407 calculated in step S108. Then, the oil pressure (i.e., engagement pressure) of the differential limiting clutch 407 is controlled so that the actual oil pressure matches the target clutch oil pressure.

[0073] As described above in detail, according to this embodiment, the reduction ratio of the front-wheel torque transmission system and the reduction ratio of the rear-wheel torque transmission system are set to be different, and the initial front-rear torque distribution ratio of center differential 40 is set to be unequal and changeable. That is, because the initial front-rear torque distribution ratio is unequal and changeable, compared to a coupling-type all-wheel drive vehicle with different front-rear reduction ratios (gear ratios), by setting the torque distribution to the driven wheels (e.g., the rear wheels) to be larger initially, the range of change in the front-rear torque distribution ratio can be reduced, and the engagement pressure (engagement force) of limited slip differential clutch 407 can be correspondingly reduced. This reduces the amount of heat generated by limited slip differential clutch 407, thereby suppressing (reducing) thermal damage to limited slip differential clutch 407. Furthermore, because the engagement pressure (engagement force) of limited slip differential clutch 407 can be reduced, for example, loss in oil pump 62, which generates (boosts) hydraulic pressure, can be reduced. As a result, the front / rear torque distribution ratio can be varied from a front-wheel bias to a rear-wheel bias, and it is possible to suppress thermal damage to the differential limiting clutch 407, which varies the front / rear torque distribution ratio, and to reduce losses in the oil pump 62, etc.

[0074] According to this embodiment, the initial front-rear torque distribution ratio of center differential 40 is set to be front-wheel biased (front wheel torque distribution > rear wheel torque distribution), and the gear ratio of transfer gear 401, the gear ratio of front differential 44, and / or the gear ratio of rear differential 47 are set so that the reduction ratio of the front wheel torque transmission system is smaller than the reduction ratio of the rear wheel torque transmission system. Therefore, by increasing the engagement pressure of limited slip differential clutch 407, the front-rear torque distribution ratio can be varied from front wheel biased (initial) to rear wheel biased.

[0075] According to this embodiment, the engagement pressure (engagement force) of limited slip clutch 407 is adjusted during cornering acceleration or uphill acceleration, and limited slip clutch 407 is disengaged during driving states other than cornering acceleration or uphill acceleration. This makes it possible to limit the range in which limited slip clutch 407 is activated, and to more effectively suppress (reduce) heat damage to limited slip clutch 407.

[0076] According to this embodiment, when the input torque of the center differential 40 is equal to or greater than a predetermined torque and the steering angle of the steering wheel 15 is equal to or greater than a predetermined angle, or when the input torque of the center differential 40 is equal to or greater than a predetermined torque and the pitch angle (gradient) of the AWD vehicle 3 is equal to or greater than a predetermined angle, the engagement pressure (engagement force) of the limited slip differential clutch 407 is increased. Therefore, an appropriate front / rear torque distribution ratio can be achieved according to the driving state of the AWD vehicle 3 (as needed).

[0077] According to this embodiment, when the oil temperature of the differential limiting clutch 407 (continuously variable transmission 30) is equal to or higher than a predetermined temperature, the differential limiting clutch 407 is disengaged. Therefore, thermal damage to the differential limiting clutch 407 can be reliably prevented.

[0078] Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments and various modifications are possible. For example, in the above embodiments, the front / rear torque distribution ratio in the initial state (initial state) of center differential 40 is set to 80:20 (front-wheel bias). However, the setting of the initial front / rear torque distribution ratio of center differential 40 is not limited to the above embodiment (80:20) and can be set arbitrarily according to requirements, etc. Furthermore, in the above embodiments, the front / rear torque distribution ratio can be varied within a range of 80:20 to 20:80 by controlling the engagement force of limited slip differential clutch 407. However, the range of change (variable range) of the front / rear torque distribution ratio is not limited to the above embodiment (80:20 to 20:80) and can be set arbitrarily according to requirements, etc.

[0079] In the above embodiment, the reduction ratio (total gear ratio) of the front wheel torque transmission system is set to be smaller than the reduction ratio (total gear ratio) of the rear wheel torque transmission system, but the reduction ratio (total gear ratio) of the front wheel torque transmission system can also be set to be larger than the reduction ratio (total gear ratio) of the rear wheel torque transmission system.

[0080] In the above embodiment, the engagement pressure (engagement force) of the differential limiting clutch 407 was adjusted according to the input torque of the center differential 40, the steering angle of the steering wheel 15, the pitch angle of the AWD vehicle 3, and the oil temperature of the differential limiting clutch 407 (continuously variable transmission 30), but the operating conditions of the differential limiting clutch 407 are not limited to those in the above embodiment.

[0081] In the above embodiment, a chain-type continuously variable transmission (CVT) has been described as an example of an automatic transmission, but instead of the chain-type continuously variable transmission, for example, a belt-type continuously variable transmission or a toroidal continuously variable transmission may be used. Also, instead of the continuously variable transmission, a stepped automatic transmission (AT), a manual transmission (MT), a DCT (Dual Clutch Transmission), etc. may be used.

[0082] Furthermore, the configuration of the torque transmission system (e.g., the configuration of the gear train, shafts, etc.) described above is an example and is not limited to the above embodiment. Furthermore, the configuration of the control system is not limited to the above embodiment (i.e., the configuration in which each ECU is connected via the CAN 100). [Explanation of symbols]

[0083] 1 All-wheel drive system 3 All-wheel drive vehicles (AWD vehicles) 10FL,10FR,10RL,10RR wheels 11FL, 11FR, 11RL, 11RR Brake 12FL, 12FR, 12RL, 12RR wheel speed sensor 15. Steering wheel 16 Steering angle sensor 20 Engine 22 Torque converter 24 Turbine runner 26 Lock-up clutch 27 Forward / reverse switching mechanism 28 Forward clutch 29 Reverse brake 30 Continuously variable transmission 32 Primary Axis 34 Primary pulley 35 Secondary pulley 36 Chain 37 Secondary Axis 40 Center differential 401 Transfer gear 401a Transfer drive gear 401b Transfer Driven Gear 402 Carrier 403 1st sun gear 404 Second sun gear 405 1st pinion gear 406 2nd pinion gear 407 Limited Differential Clutch (LSD Clutch) 44 Front differential 47 Rear differential 50 VDCU 56 Pitch angle sensor 60 Control valve (valve body) 70 TCU 80 ECU 81 Cam angle sensor 82 Crank angle sensor 83 Throttle opening sensor 84 Accelerator opening sensor 85 Electronically controlled throttle valve 91 Oil temperature sensor 92 Output shaft rotation sensor 93 Range switch 94 Turbine rotation sensor 100 CAN

Claims

1. a center differential that distributes input torque to the front and rear wheels; a differential limiting clutch that limits the differential of the center differential in accordance with an engagement pressure and varies the front / rear torque distribution ratio between the front wheel side and the rear wheel side; a front wheel torque transmission system that transmits torque between the center differential and the front wheels; a rear wheel torque transmission system that transmits torque between the center differential and rear wheels; a control unit that adjusts the engagement pressure of the differential limiting clutch based on the operating state of the vehicle, a reduction ratio of the front wheel side torque transmission system and a reduction ratio of the rear wheel side torque transmission system are set to be different from each other, The center differential is configured such that an initial front-rear torque distribution ratio is set to be unequal, and the setting is changeable, The control unit adjusts the engagement pressure of the limited slip differential clutch during cornering acceleration or uphill acceleration, and disengages the limited slip differential clutch during other than cornering acceleration or uphill acceleration.

2. The center differential has an initial front-to-rear torque distribution ratio set to be front-wheel-biased, 2. The all-wheel drive system according to claim 1, wherein the gear ratio of the transfer gear, the gear ratio of the front differential, and / or the gear ratio of the rear differential are set so that the reduction ratio of the front wheel torque transmission system is smaller than the reduction ratio of the rear wheel torque transmission system.

3. 3. The all-wheel drive system according to claim 1, wherein the control unit increases the engagement pressure of the limited slip differential clutch when the input torque of the center differential is equal to or greater than a predetermined torque and the steering angle of the steering wheel is equal to or greater than a predetermined angle, or when the input torque of the center differential is equal to or greater than a predetermined torque and the pitch angle of the vehicle is equal to or greater than a predetermined angle.

4. An all-wheel drive system as described in Claim 3, characterized in that the control unit increases the engagement pressure of the differential limiting clutch as the steering angle increases, and increases the engagement pressure of the differential limiting clutch as the steering angular velocity increases.

5. 5. The all-wheel drive system according to claim 3, wherein the control unit disengages the limited slip differential clutch when the oil temperature of the limited slip differential clutch is equal to or higher than a predetermined temperature.

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