All-Wheel Drive Torque Vectoring System

Abstract

The present invention relates to an all-wheel drive system for a vehicle having an engine and a drive shaft, and where the all-wheel drive system is arranged between the drive shaft and front and rear propellable shafts. These shafts are interconnected by first and second clutches resp., and the propellable shafts are coupled to front and rear wheels, for transferring torque from the drive shaft to the front and/or rear wheels. The all-wheel drive system further comprises at least first and second actuators that are coupled to at least first and second clutches, where at least said first clutch is engaged by a spring and at least said first actuator is coupled for disengaging said first clutch when the first actuator is operated. The second actuator is coupled for engaging said second clutch when the second actuator is operated.

Description

FIELD OF THE INVENTION

The present invention relates to an all-wheel drive system for vectoring the motive torque between a front and rear axle.

BACKGROUND OF THE INVENTION

The patent publications EP-A-O.352.994 and U.S. Pat. No. 5,358,084 disclose a distributing system which comprises two separate output axles with active couplings/clutches that independently will distribute torque to the front and rear axles. The two couplings of the described system are active, which means that no transfer or distribution of torque is possible if there is loss of electrical power to the couplings. This will make it impossible to drive the vehicle. Moreover, two separate couplings lead to a higher cost compared to a more common system with one coupling parallel to a center differential.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an all-wheel drive system with vectoring of the motive torque that overcomes the drawbacks of the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described with references to the attached drawings, wherein:

FIG. 1 is a schematical view of a hydraulic circuit according to a first embodiment of the invention,

FIG. 2 is a cross-sectional view of the embodiment shown in FIG. 1,

FIG. 3 is a schematical view of a hydraulic circuit according to a second embodiment of the invention,

FIG. 4 is a cross-sectional view of the embodiment shown in FIG. 3,

FIG. 5 is a diagram showing the amount of torque that each clutch can deliver, depending on the applied pressure, for the second embodiment, and

FIG. 6 is a diagram showing the amount of torque that is transferred by each clutch at a part load condition for the second embodiment.

DETAILED DESCRIPTION OF THE INVENTION

An all-wheel drive system with torque vectoring will be described in more detail below with references to the attached drawings.

The all-wheel drive system with torque vectoring can distribute a motive torque between a front axle and a rear axle, depending on the current situation and conditions. The system comprises two parallel disc clutches DC1, DC2 mounted in a housing H, as can be seen in FIGS. 1-4. The torque from a gearbox (not shown) enters the system through the housing H via an input axle IA. The distributed torque leaves the system through the housing H via a front OAF and a rear output axle OAR. One of the clutches DC1 is activated by a spring S1, preferably a disc spring, and is deactivated by an actuator A1. The second clutch DC2 is activated by an actuator A2. The spring-activated clutch DC1 can be coupled either to the front OAF or rear output axle OAR. The actuators A1, A2 may be electromechanical or hydraulic.

In a first embodiment, as can be seen in FIGS. 1 and 2, the clutches DC1, DC2 are each coupled to an actuator A1, A2. One of the actuators A2 is arranged to activate the clutch DC2 and one of the actuators A1 is arranged to deactivate the clutch DC1 against the spring action of the spring S1. The actuators are powered by hydraulic fluid supplied from an electric pump EP via the directional control valves DCV1 and DCV2. The system may further comprise an accumulator ACC and a pressure relief valve PRV, set to open at a predetermined maximum pressure, e.g. 40 bar.

Another embodiment can be seen in FIGS. 3 and 4, where only one hydraulic source is provided, between the two pistons P1, P2 of the actuators A1, A2. The hydraulic fluid forces the pistons in opposite directions with the same force, and an almost zero axial reaction force will result within the transmission housing. Hydraulic power is still supplied by the electric pump EP, which may be coupled to an accumulator ACC and a pressure relief valve PRV in the same way as for the first embodiment. An electromechanical solution could also be introduced in this embodiment, with one or two ball ramps (not shown) activating the clutches DC1, DC2.

The above-mentioned systems have an advantage over prior-art systems, in that they comprise a spring-activated clutch that will ensure that traction is always transferred to an output axle, even when an actuator (or all of them) is out of order. The double-clutch arrangement is further more compact and may utilise the same power source. If only one hydraulic source (one valve) is used, or if an electromechanical actuator is used, the design will be simple and robust, and the mechanism for distributing the torque between the front and rear axles will be more secure. When the clutches are powered by the same hydraulic source, their operation will be simultaneous and the clutches will never be open in parallel with consequent loss of power transfer.

The function of the two clutches can be seen in FIGS. 5 and 6, where the maximum transferable torque for each clutch is plotted against the hydraulic pressure in the corresponding actuator. When the hydraulic pressure increases, see e.g. FIG. 5, one of the clutches is being

partially engaged, indicated by the dashed line, and the other clutch is being disengaged partially, indicated by the solid line. At full hydraulic pressure, the initially unengaged clutch is fully engaged and the initially engaged clutch is fully disengaged.

When the input axle IA provides a lower torque, the behaviour is slightly different, see FIG. 6. At zero hydraulic pressure, only one DC1 of the two clutches DC1, DC2 is engaged and locked and the other clutch DC2 becomes engaged gradually as the hydraulic pressure increases, but is subjected to slip. As the transferred torques from the two clutches are equal, at p_hi, both clutches become locked. As the hydraulic pressure increases above a certain value p_h2, the initially locked clutch DC1 starts to lose its engagement and is subjected to slip. The torque is instead transferred by the other clutch DC2 which is still locked.

The diagrams shown in FIGS. 5 and 6 are normalized, and the transferable torques actually depend on the size of each clutch as well as other parameters, such as road conditions. The forces on the clutches, in the diagrams, are given from the hydraulic pressure, but can just as well be supplied by a strictly mechanical actuator.

Claims

1. An all-wheel drive system for a vehicle having an engine and a drive shaft, the all-wheel drive system being arranged between the drive shaft and front and rear propellable shafts, being interconnected by first and second clutches respectively, and the propellable shafts being coupled to front and rear wheels, for transferring torque from the drive shaft to the front and/or rear wheels, characterized in that the all-wheel drive system comprises at least first and second actuators being coupled to at least first and second clutches, where at least said first clutch is engaged by a spring and at least said first actuator is coupled for disengaging said first clutch when the first actuator is operated, and where at least said second actuator is coupled for engaging said second clutch when the second actuator is operated.

2. An all-wheel drive system according to claim 1, wherein the actuators are operated synchronously.

3. An all-wheel drive system according to claim 1, wherein the actuators are hydraulically driven.

4. An all-wheel drive system according to claim 2, wherein the actuators are driven by a common source of pressurized hydraulic fluid.

5. An all-wheel drive system according to claim 2, wherein the actuators are arranged so that they operate in opposite directions and are substantially coaxial so that a substantially zero net axial reaction force is transferred to an accommodating structure.

6. An all-wheel drive system according to claim 3, wherein the actuators are driven by a common source of pressurized hydraulic fluid.