Why are more automakers turning to all-wheel drive (AWD)?
The new Jaguar X-type uses a viscous coupling to gain a 40% front/60% rear torque distribution to enhance handling (see DN 10/1/2001, p. 62). And now Volvo has introduced an AWD option for its S60 small, sporty sedan, but more as a safety enhancement.
In the Haldex electronically controlled
coupling, differential front-to-rear axle wheel speed causes the clutch
hub (linked to the front wheels) to rotate relative to the clutch housing
(joined to the rear). Rollers on the hub's cam surface move the pump
rings, generating hydraulic pressure -- which pushes the pump reaction
ring against the pressure plate to engage the outer and inner friction
plates, transferring torque.
Volvo turned to the Swedish supplier Haldex for an electronically controlled AWD system that uses a wet multiplate clutch (see figure). Under normal conditions, the car is essentially front-wheel drive, having a 95% front/5% rear torque distribution. But if there is a difference in front and rear wheel speed, as when the front wheels start to slip on ice, oil, or dirt, or during aquaplaning, the clutch transfers torque to the rear wheels to maintain traction. Electronic control allows engineers to program the clutch for different driving situations.
An electronic module control, using CANbus relayed sensor information (such as wheel speed, engine data, and gas pedal position), drives a stepper motor-driven relief or "throttle" valve within tens of milliseconds to govern the coupling hydraulic pressure, and thus the percentage of torque transferred to the rear wheels. All this takes place typically within 15° of wheel rotation along the road, says Henrik Eldh, S60 AWD-system integration engineer. Torque will also be distributed to the rear wheels if the throttle is lifted, to facilitate engine braking. The controller disables the transfer function (full valve open) under conditions such as ABS activation and low-speed parking maneuvers, where expected wheel speed differences do not require activation.
The rear axle mounted controller uses a Siemens 16-bit microprocessor. The long, constantly rotating input shaft and rear mounting of coupling and controller was necessitated by space limitations up front.
Volvo developed the controller software in conjunction with Haldex for the specific vehicle response desired. Basic software from Haldex governs internal controller functions, such as an algorithm to compensate for temperature variations in hydraulic oil viscosity, ensuring consistent performance.
By the time you read this, dynamic stability (skid) control will also be available on the S60 AWD. Volvo is also planning to eventually introduce the Haldex AWD system on its Cross Country AWD wagon, not so much to enhance driving dynamics safety, as on the S60, but to improve rough road characteristics.
S60 specs. To accommodate driving the rear wheels, Volvo engineers have raised the S60 ground clearance height by 16 mm (0.63 inch) to 147 mm, and changed spring rates, compared to the front-wheel drive version of the car. The $1,750 AWD option adds 140 lbs to the weight of the car (3,529 lbs total). The S60 AWD comes only with a 2.4-liter, 5-cylinder, light-pressure turbocharged engine, producing 197 hp and 210 ft-lb of torque, with a 5-speed automatic trans-mission. To better compete with BMW and Jaguar, Volvo will debut a sport version next year. The car is designed to handle unimproved roads and wintry weather, and "typically equipped" will set you back $36,500.
For more information about clutches from Haldex: Enter 533