The present invention relates to a power transfer system for controlling the distribution of drive torque between the front and rear wheels of a four-wheel drive vehicle as a function of various system and operator-initiated inputs.
In view of increased consumer popularity in four-wheel drive vehicles, a plethora of power transfer systems are currently being utilized in vehicular driveline applications for selectively directing power (i.e., drive torque) to all four wheels of the vehicle. In many power transfer systems, a part-time transfer case is incorporated into the driveline and is normally operable in a two-wheel drive mode for delivering drive torque to the driven wheels. In addition, such part-time transfer cases also include a mechanical "mode" shift mechanism which can be selectively actuated by the vehicle operator for rigidly coupling the non-driven wheels to the driven wheels for establishing a part-time four-wheel drive mode. As will be appreciated, a motor vehicle equipped with a part-time transfer case offers the vehicle operator the option of selectively shifting between the two-wheel drive mode during normal road conditions and the part-time four-wheel drive mode for operation under adverse road conditions. Alternatively, it is known to use "on-demand" power transfer systems for automatically directing power to the non-driven wheels, without any input or action on the part of the vehicle operator, when traction is lost at the driven wheels. Modernly, the "on-demand" feature is incorporated into the transfer case by replacing the mechanically-actuated "mode" shift mechanism with a clutch assembly that is interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the clutch assembly is maintained in a non-actuated condition such that drive torque is only delivered to the driven wheels. However, when the sensors detect a low traction condition at the driven wheels, the clutch assembly is automatically actuated to deliver drive torque "on-demand" to the non-driven wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the non-driven wheels can be varied as a function of specific vehicle dynamics, as detected by the sensor arrangement. One example of such an "on-demand" power transfer system is disclosed in U.S. Pat. No. 4,773,500 to Naito, et al wherein a hydraulically-actuated clutch assembly is operable for automatically controlling the amount of drive torque transferred to the non-driven wheels as a function of the wheel speed difference (i.e., the wheel slip) between the front and rear wheels.
As an alternative to part-time four-wheel drive systems, some vehicles are equipped with full-time transfer cases having a center (i.e., inter-axle) differential for constantly splitting the drive torque between the front and rear drivelines. To minimize loss of traction due to wheel slippage, most full-time transfer cases are also equipped with a slip limiting device for locking the inter-axle differential to prevent excessive speed differentiation and establish a part-time four-wheel drive mode. Examples of manually-actuated differential lock-up arrangements are disclosed in commonly owned U.S. Pat. No. 3,848,691 to Dolan and U.S. Pat. No. 4,677,873 to Eastman. An automatic differential lock-up arrangement is disclosed in commonly owned U.S. Pat. No. 3,845,671 to Sharp et al. wherein an electrically-controlled clutch is actuated to lock-up the inter-axle differential when speed differentiation across the inter-axle differential due to a wheel slip condition is detected as exceeding a predetermined value. In addition, torque-biasing differential lock-up arrangements are disclosed in commonly owned U.S. Pat. No. 4,031,780 to Dolan et al. and U.S. Pat. No. 5,046,998 to Frost, which both utilize a viscous coupling to progressively modify the torque distribution in proportion to the magnitude of the speed differentiation across the inter-axle differential. Finally, electronically-controlled full-time transfer cases are disclosed in U.S. Pat. No. 4,718,303 to Fogelberg and U.S. Pat. No. 4,860,612 to Dick et al. wherein an electromagnetic biasing clutch is provided across the inter-axle differential to controllably bias the torque delivered to the front and rear drivelines in response to wheel slip.