WO2006046018A1

WO2006046018A1 - All wheel drive system

Info

Publication number: WO2006046018A1
Application number: PCT/GB2005/004103
Authority: WO
Inventors: Damian Harty
Original assignee: Prodrive 2000 Limited
Priority date: 2004-10-25
Filing date: 2005-10-24
Publication date: 2006-05-04
Also published as: GB0423677D0; EP1805054A1; JP2008517819A; AU2005298489A1

Abstract

The present invention relates to an all wheel drive system for a motor vehicle (10). The vehicle (10) includes an engine (12), a front pair of driven wheels (16,18) having drive shafts (24,26) driven through a front axle unit having differential gear arrangement (36), a rear pair of driven wheels (20,22) having drive shafts (32,34) driven through a rear axle unit having a differential gear arrangement (42), a centre differential gear arrangement (38) provided between the front and rear axle units operable to transmit torque from the engine (12) to the front axle and rear axle units via respective front and rear propeller shafts (39,28) extending from the centre differential gear arrangement (38) to the front axle and rear axle units, wherein the front axle unit final drive gear has a different ratio to the rear axle unit final drive gear, and the system further comprises a synchronisation means associated with the centre differential operable to alter the rotational speed of one of or both the front and rear propeller shafts (28,39).

Description

ALL WHEEL DRIVE SYSTEM

The present invention relates to an all wheel drive system for a motor vehicle and particularly, though not exclusively, to a four wheel drive vehicle.

A current trend in the development of all wheel drive systems is the provision of a means to influence the dynamic behaviour of a vehicle by the distribution of drive torques around the vehicle. Such systems monitor the dynamic behaviour of the vehicle and intervene to modify or influence the dynamic behaviour thereof if it sensed that the vehicle behaviour is outside predetermined limits

According the a first aspect of the present invention there is provided an all wheel drive system for a motor vehicle having an engine, a front pair of driven wheels having drive shafts driven through a front axle unit having differential gear arrangement, a rear pair of driven wheels having drive shafts driven through a rear axle unit having a differential gear arrangement, a centre differential gear arrangement provided between the front axle and final drive units operable to transmit torque from the engine to the front axle and rear axle units via respective front and rear propeller shafts extending from the centre differential to the front axle and rear axle units, wherein the front axle unit final drive gear has a different ratio to the rear axle unit final drive gear, and the system further comprises a synchronisation means associated with the centre differential operable to alter the rotational speed of one of or both the front and rear propeller shafts.

By providing front axle and rear axle units having different ratios then it will be appreciated that where the front and rear wheels of the vehicle are of equal radius and rotate at equal speed, then the front propeller shaft will rotate at a different speed to that of the rear propeller shaft. For a four wheel drive vehicle having a front wheel drive bias the ratio of the front axle unit is higher than the ratio of the rear axle unit. Conversely for a vehicle having a rear wheel drive bias the ratio of the front axle unit is lower than the ration of the rear axle unit. In a preferred embodiment the difference in effective ratios of the front axle unit and final drive unit is achieved by the provision of front axle drive unit having a different final drive gear ratio to that of the final drive unit. In an alternative embodiment the difference in effective ratios may be achieved by the provision of an intermediate gearing arrangement intermediate the centre differential and one of the final drive unit and front axle unit. In such an embodiment the intermediate gearing arrangement may comprise a drop gear arrangement provided between the centre differential and one of the front axle and final drive unit. The drop gear may be provided between the centre differential and one of the front propeller shaft and rear propeller shaft.

The difference in the ratios of the front axle unit and final drive unit may be in the region of 3 to 10 percent

The synchronisation means may include a clutch mechanism operable to alter the rotational speed of one of or both the front and rear propeller shafts. The clutch mechanism is preferably operable between a first state whereupon the front and rear propeller shafts are freely able to rotate relative to one another and a second state whereupon the front and rear propeller shafts are rotationally fixed and hence unable to rotate relative to one another. The clutch mechanism may be operated such that the rotational speed of the rear propeller shaft is increased towards that of the front propeller shaft. Alternatively the clutch mechanism may be operated such that the rotational speed of the front propeller shaft is reduced towards that of the rear propeller shaft. Alternatively still the clutch mechanism may be operable so as to alter the speeds of both propeller shafts.

The synchronisation means is preferably provided with a controller which is responsive to the dynamic state of the vehicle. The controller is provided with a number of sensors positioned so as to sense operating characteristics of the vehicle. The sensors may include, for example, one or more wheel speed sensors, lateral acceleration sensors, longitudinal acceleration sensors, steering input sensors, engine torque demand sensors and vehicle yaw sensors. According to a further aspect of the present invention there is provided a method of influencing or modifying the dynamic behaviour of an all wheel drive vehicle by the distribution of drive torques around the vehicle, the method comprising the steps of:

providing an all wheel drive vehicle having an engine, a front pair of driven wheels having drive shafts driven through a front axle unit having differential gear arrangement, a rear pair of driven wheels having drive shafts driven through a rear axle unit having a differential gear arrangement, a centre differential gear arrangement provided between the front axle and final drive units operable to transmit torque from the engine to the front axle and rear axle units via respective front and rear propeller shafts extending from the centre differential to the front axle and rear axle units, wherein the front axle unit final drive gear has a different ratio to the rear axle unit final drive gear, and the system further comprises a synchronisation means associated with the centre differential operable to alter the rotational speed of one of or both the front and rear propeller shafts; providing the vehicle with a plurality of sensors operable to monitor the dynamic behaviour of the vehicle; providing a controller arranged to receive inputs from the sensors, said controller being provided with one or more dynamic behaviour parameters for the vehicle; sensing via one or more of the sensors that the dynamic behaviour of the vehicle has reached or exceeded one or more of the dynamic behaviour parameters; and operating the synchronisation means via the controller to alter the rotational speed of one of or both the front and rear propeller shafts.

An embodiment of the present invention will now be described with reference to figure 1 which shows a schematic representation of a vehicle 10 having a drive management system according to the present invention. The vehicle 10 includes an engine 12 having a transmission housing 14 attached thereto and four wheels 16,18,20,22. Front drive shafts 24,26 extend between the front wheels 16,18 and the transmission housing 14 . A propeller shaft 28 extends from the gearbox 14 to a rear axle unit 30 having a differential gear arrangement. Rear drive shafts 32,34 extend between the rear axle unit 30 and the rear wheels 20,22. It will be appreciated therefore that the vehicle 10 may be considered for the sake of simplicity to have notional front and rear axles to which the front and rear wheels 16,18,20,22 are mounted.

The transmission housing 14 includes a gearbox (not shown) having multiple forward gears and a reverse gear. The transmission housing 14 further includes a front axle unit 36 having a differential gear arrangement provided between the front drive shafts 24,26, and a centre, between axle differential 38. An internal drive shaft 39 extends between the front and centre differentials 36,38. Both the centre and rear differentials 38,30 are of the limited slip type and include a clutch mechanism 40,42 operable to modify the torque transmission characteristics across each differential 30,38.

The vehicle 10 further includes a number of sensors operable to determine certain operative states and positions of the vehicle 10 and its control systems. The sensors include a steering sensor 44 operable to measure steering input by the driver via the steering wheel 45, lateral and longitudinal acceleration sensors 46,48, and a yaw sensor 50. Each wheel 16,18,20,22 is provided with a wheel speed sensor 52 which advantageously doubles as a sensor for an anti lock braking system of the vehicle 10. There is further provided a mode switch 54 and an engine torque demand signal sensor 56, as well as controllers 58,60 which control the operation of the centre and rear differential clutch mechanisms 40,42. The mode switch 54 and sensors 44,46,48,50,52,56 are connected to a central control unit 62 which in turn is connected to the clutch mechanism controllers 58,60.

A feature of the present invention is the provision of front and rear differentials 36, 30 having differing ratios and more specifically the rear axle unit 30 having a lower ratio than that of the front axle unit 36. For example, the front axle unit 36 may have a ratio of 3.7:1 while the rear axle unit 30 has a ratio of 3.52:1.

Operation of the drive management system will now be described. Looking firstly at the operation of the vehicle 10 with the system inactive, the gearbox is arranged to split the torque supplied thereto between the axles in a predetermined manner. For example the torque split may be 35% to the front axle and 65% to the rear axle. It will be understood that other torque splits may be employed and the vehicle 10 may be arranged so as to vary the torque split between the axles in response to changes in road surface and driving conditions. Taking the example of the vehicle 10 travelling at a constant velocity it will be appreciated that both the front and rear wheels 16,18,20,22 of the vehicle 10 will rotate at notionally the same speed, assuming that the rolling radius of the front wheels 16,18 is the same as that of the rear wheels 20,22. As the respective ratios of the front and rear differentials 36,30 is different, it will be understood that the rotational speeds of the propeller shaft 28 and the internal drive shaft 39 are also different. This difference in shaft rotational speeds is accommodated by the presence of the centre differential 38. As the rear axle unit 30 has a lower ratio than the front axle unit 36 then, for a given constant engine speed, the internal drive shaft 39 rotates faster than the propeller shaft 28.

The drive management system may be activated by the mode switch. In an alternative embodiment the drive management system may automatically be active at all times during operation of the vehicle. Once activated the central control unit 62 is supplied with information related to the dynamic state of the vehicle 10 from the sensors 44,46,48,50,52,56. The central control unit 62 is able to evaluate the dynamic state of the vehicle as determined by the sensors with pre-set parameters and is thus able to determine whether or not any intervention is required.

A common characteristic of performance all wheel drive vehicles is the tendency to understeer when accelerating through a corner, an occurrence commonly termed power understeer. The drive management system of the present invention is operable to mitigate the effects of such power understeer. In the event that the central control unit 62 determines that the vehicle 10 is understeering then a command is sent to clutch controller 58 for the centre differential clutch mechanism 40. The controller 58 causes the clutch mechanism 40 to operate and thereby alter the rotational speed of the propeller shaft 28. The clutch mechanism 40 may be fully engaged so as to match the speed of the propeller shaft 28 to that of the internal drive shaft 39 or partially engaged so as to drive the propeller shaft 28 at a desired speed between that of the initial speed of the propeller shaft 28 prior to activation of the clutch mechanism 40 and the speed of the internal drive shaft 39.

Given the different ratios of the front and rear differentials 36,30, it will be understood that operation of the clutch mechanism 40 results in the speeding up of the propeller shaft 28. This in turn results in an increase in the rotational speed of the rear wheels

20,22 with the result that the rear wheels 20,22 are driven harder than the front wheels

16,18. The tyres of the rear wheels 20,22 in being driven harder than the tyres of the front wheels 16,18 have their lateral load capacity decreased and hence a net yaw moment is imposed on the vehicle 10.

The above described example relates to the control of a vehicle experiencing power understeer. It will be appreciated that the drive management system of the present invention may intervene in other situations including, but not limited to, instances of lift off oversteer, sharp steering inputs to avoid an obstacle, and changes in throttle position during cornering.

The sensors 44,46,48,50,52,56 continue to monitor the dynamic state of the vehicle 10 and the central control unit 62 is thus able to determine the effect driving the rear wheels 20,22 harder than the front wheels 16,18 is having on the dynamic state of the vehicle 10. The central control unit 62 is thus able to modulate the operation of the centre differential clutch 40 to ensure that the vehicle 10 takes into account changes in the dynamic state of the vehicle as a result of the intervention of the centre differential clutch as well as, for example, changes in the nature of the road surface and changes in the steering input applied by the driver to the steering wheel 45.

As described above, the rear axle unit 30 is provided with a clutch mechanism 42 operable via instructions sent from the central control unit 62. The clutch mechanism may be operated to modify drive shaft torque distribution through the rear axle unit 30 to each of the rear wheels 20,22. Operation of the clutch mechanism 42 may be either separate or supplemental to the drive management system described above. The system described above refers to the speeding up of the propeller shaft 28 so as to drive the rear wheels 20, 22 harder and thereby reduce the lateral load capacity of the rear tyres. It will be appreciated that the operation of the system may be modified so that the speed of the internal drive shaft 39 is altered to more closely match the rotational speed of the propeller shaft 28. In such an embodiment it will be understood that the rotational speed of the internal drive shaft 39 will be reduced with the effect that there is a reduction in drive to the front wheels 16, 18. This results in an increase in the lateral load capacity of the front tyres which may mitigate any understeer being experienced by the vehicle.

The above described embodiment also utilises front and rear differentials 36, 30 having differing ratios with the rear axle unit 30 having a lower ratio than the front differential. It will be appreciated that the same effect may be realised by means other than having front and rear differentials 36, 30 of differing ratios. For example an intermediate drop gear may be provided in the vehicle gearbox between the centre differential 38 and the propeller shaft 28. Typically such a drop gear has a ratio of 1:1, however by changing the ratio of the drop gear to, for example, 1.08:1 then the net effect is to increase the ratio of the rear axle unit 30.

The embodiment of the invention described with reference to figure 1 relates to a four wheel drive vehicle having a front wheel drive bias, which is to say that during normal operation of the vehicle the engine torque is split such that a greater percentage is supplied to the front wheels than the rear wheels. It will be understood that the underlying principle of the present invention is equally applicable to vehicles having a rear wheel drive bias wherein a greater percentage of the engine torque is supplied to the rear wheels than the front wheels. In such an embodiment the front axle unit final drive gear is provided with a lower ratio than that of the rear axle unit final drive gear.

Claims

1. An all wheel drive system for a motor vehicle having an engine, a front pair of driven wheels having drive shafts driven through a front axle unit having differential gear arrangement, a rear pair of driven wheels having drive shafts driven through a rear axle drive unit having a differential gear arrangement, a centre differential gear arrangement provided between the front and rear axle units operable to transmit torque from the engine to the front axle and rear axle drive units via respective front and rear propeller shafts extending from the centre differential to the front axle and rear axle drive units, wherein the front axle unit final drive gear has a different ratio to the rear drive unit final drive gear, and the system further comprises a synchronisation means associated with the centre differential operable to alter the rotational speed of one of or both the front and rear propeller shafts.

2. A drive system as claimed in claim 1 wherein the difference in ratios of the front axle unit and rear axle unit is achieved by the provision of front axle drive unit having a different final drive gear ratio to that of the final drive unit.

3. A drive system as claim 1 wherein the difference in ratios of the front axle unit and final drive unit is achieved by the provision of an intermediate gearing arrangement intermediate the centre differential and one of the rear axle unit and front axle unit.

4. A drive system as claimed in claim 3 wherein the intermediate gearing arrangement includes a gear provided between the centre differential and one of the front axle unit and rear axle unit.

5. A drive system as claimed in claim 4 wherein the intermediate gearing arrangement includes a gear between the centre differential and one of the front propeller shaft and rear propeller shaft.

6. A drive system as claimed in any preceding claim wherein the difference in the respective ratios of the front axle unit and the rear axle unit is in the region of 3 to 10 percent

7. A drive system as claimed in any preceding claim wherein the synchronisation means includes a clutch mechanism operable to alter the rotational speed of one of or both the front and rear propeller shafts.

8. A drive system as claimed in claim 7 wherein the clutch mechanism is operable between a first state whereupon the front and rear propeller shafts are freely able to rotate relative to one another and a second state whereupon the front and rear propeller shafts are rotationally fixed and hence unable to rotate relative to one another.

9. A drive system as claimed in claim 7 or claim 8 wherein the clutch mechanism is operable such that the rotational speed of the rear propeller shaft is increased towards that of the front propeller shaft.

10. A drive system as claimed in claim 7 or claim 8 wherein the clutch mechanism is operable such that the rotational speed of the front propeller shaft is reduced towards that of the rear propeller shaft.

11. A drive system as claimed in claim 7 or claim 8 wherein the clutch mechanism is operable so as to alter the speeds of both propeller shafts.

12. A drive system as claimed in any preceding claim wherein the synchronisation means includes a controller which is responsive to the dynamic state of the vehicle.

13. A drive system as claimed in claim 12 wherein the controller is provided with a number of sensors positioned so as to sense operating characteristics of the vehicle.

14. A drive system as claimed in claim 12 wherein the sensors include one or more wheel speed sensors, lateral acceleration sensors, longitudinal acceleration sensors, steering input sensors, engine torque demand sensors and vehicle yaw sensors.

15. A a method of influencing or modifying the dynamic behaviour of an all wheel drive vehicle by the distribution of drive torques around the vehicle, the method comprising the steps of: providing an all wheel drive vehicle having an engine, a front pair of driven wheels having drive shafts driven through a front axle unit having differential gear arrangement, a rear pair of driven wheels having drive shafts driven through a rear axle unit having a differential gear arrangement, a centre differential gear arrangement provided between the front and rear axle units operable to transmit torque from the engine to the front axle and rear axle units via respective front and rear propeller shafts extending from the centre differential to the front axle and rear axle units, wherein the front axle unit final drive gear has a different ratio to the rear axle unit final drive gear, and the system further comprises a synchronisation means associated with the centre differential operable to alter the rotational speed of one of or both the front and rear propeller shafts; providing the vehicle with a plurality of sensors operable to monitor the dynamic behaviour of the vehicle; providing a controller arranged to receive inputs from the sensors, said controller being provided with one or more dynamic behaviour parameters for the vehicle; sensing via one or more of the sensors that the dynamic behaviour of the vehicle has reached or exceeded one or more of the dynamic behaviour parameters; and operating the synchronisation means via the controller to alter the rotational speed of one of or both the front and rear propeller shafts