SUPER-HANDLING ALL-WHEEL-DRIVE SYSTEM™
Super-Handling All-Wheel Drive™ (SH-AWD™) is the first and only all-wheel-drive platform that distributes the optimum amount of torque not only between the front and rear wheels but also between the left and right rear wheels. SH-AWD goes a step beyond conventional all-wheel drive by actively controlling the torque delivered to each rear wheel during corning. The result is neutral, accurate steering when cornering under power that front-drive, rear-drive or conventional all-wheel-drive can't equal.
Torque splits are as follows:
- During straight-line cruising and moderate cornering below about half throttle, up to 70 percent of the torque is delivered to the front wheels
- In full-throttle straight line acceleration, up to 40 percent of the power is sent to the rear axle
- In hard cornering, up to 70 percent of available torque goes to the rear wheels for enhanced chassis balance. Up to 100 percent of this torque can be applied to the outside rear wheel if the situation dictates.
SH-AWD ingeniously varies the amount of torque to the left and right rear wheels. When cornering, a planetary gear set overdrives (or accelerates) the outer rear wheel faster than the average of the front wheels to dramatically enhance the cornering, steering feel, overall handling and stability of the RL. The result is class leading cornering precision as well as enhanced traction.
Direct Yaw Control System Theory
SH-AWD counters understeer under power with the Direct Yaw Control System. Spinning the outside rear wheel faster than the average speed of the two front wheels allows the system to use engine power to yaw the vehicle while turning. By relieving the front tires of some of the work of turning the car, the system reduces understeer and the vehicle stays balanced and controllable. In addition, with the cornering load more evenly distributed between the front and rear tires, the total cornering grip is increased. In conventional cars, cornering is created almost entirely by the steering angle of the front tires; In the RL, cornering is created by steering angle of front tires combined with the extra drive torque supplied by the outside rear tire.
This is a significant advance over conventional drive systems. To deal with high power output, front- or rear-drive systems generally use some type of limited-slip device to maintain traction under power. The linking effect of the inside and outside drive wheels in these systems resists turning, however. This is a factor that works against the front tires as they attempt to turn the car. Conventional AWD systems have a similar linking effect between the inboard and outboard tires and front and rear axles, causing a similar resistance to turning. This is part of the reason why traditional AWD systems typically lack the more nimble feel of the best two-wheel drive systems. By using drive torque to actually help turn the car, the RL can be more responsive, neutral and predictable, while simultaneously offering all of the usual benefits of all-wheel drive.
Electronic Controls and Parameters
The logic and control of SH-AWD is integrated with the RL Engine Electronic Control Unit (ECU), and Vehicle Stability Assist ECU. The Engine ECU provides engine rpm, intake manifold pressure, and transmission gear ratio data. The VSA ECU provides data on lateral g, yaw rate, wheel rotation speed and steering angle. The SH-AWD ECU monitors the status of the acceleration device and the right and left Direct Electromagnetic Clutch torque. Traction is calculated based on the information from the engine ECU. Then the acceleration situation, lateral g and steering angle are used to set the torque split between the right and left rear wheels. At the same time, this data is used to set the acceleration device.
SH-AWD System Layout
SH-AWD is a full-time all-wheel drive system that requires no driver interaction for operation. A torque transfer unit is bolted directly to the front-mounted transaxle. Attached to the front wheel differential's ring gear is a helical gear that provides input torque to the transfer unit. A short horizontal shaft and a hypoid gear set within the case turn the propeller shaft ninety degrees and move it to the vehicle center line. A lightweight carbon fiber reinforced composite propeller shaft carries power to the rear drive unit.
The rear drive unit of the RL, unlike the MDX, contains three planetary gear and clutch sets. Torque from the propeller shaft passes through the first clutch/planetary gearset, which is as a unit called the Acceleration device.
Output torque from the Acceleration device is carried a short distance rearward to a hypoid gear that turns the output 90-degrees and drives the rear axle shafts. A matched pair of Direct Electromagnetic Clutch systems, one on each side, send power to each rear wheel. These clutch systems can be controlled as a pair to alter the front/rear torque split; depending on the situation, the rear wheels receive between 30 and 70 percent of the total engine output. The right and left Direct Electromagnetic Clutch systems can also be controlled independently, to allow up to 100 percent of the total rear axle torque to go to only one side of the vehicle.
Positioned at the front of the RL rear drive unit, the Acceleration device typically passes torque rearward to the rear axle at very close to a one-to-one ratio. In cornering, however, the Acceleration device's output shaft spins faster than its input shaft.
The Acceleration assembly uses a compact planetary gearset to achieve its speed increase. Hydraulic actuators operate clutch packs that control the planetary gearset. When the input shaft is locked with the planetary gear carrier, there is no ratio change (this is the straight-line mode). During cornering, the carrier is coupled with the case, and the output shaft speed increases up to five percent. A speed sensor at the hypoid gear, downstream of the Acceleration device provides a feedback loop to the SH-AWD Electronic Control Unit to ensure that the system is working properly.
Direct Electromagnetic Clutch Systems
Located on either side of the hypoid gear that drives the rear axle, two identical Direct Electromagnetic Clutch systems control the amount of drive torque that reaches each rear wheel, and provide limited-slip differential function. An electric coil controls the pressure applied to a clutch, which slows the sun gear in a planetary gearset to modulate the torque that is sent to the wheel. The amount of torque transmitted to each rear wheel can vary continuously, between zero and 100 percent, depending on the conditions.
Under deceleration (throttle closed) while cornering, torque to the outside rear wheel is varied to change from an inward to an outward yaw moment, helping vehicle stability. A search coil sensor allows the ECU to estimate the clutch plate coefficient of friction (which changes with heat,) and then adjusts voltage sent to the electromagnetic coil that controls the clutch to compensate. To ensure that the amount of torque transmitted remains optimized as miles and wear accumulate, a coil provides a feedback loop that the ECU uses to adjust voltage to the electromagnetic clutches to compensate for potential clutch wear.