What's The Diff?
It hardly seems possible that a differential could become the selling point for a $40,000 vehicle, but increasingly the driveline determines if a new car or truck is truly "geared" for success. All-wheel-drive (AWD) is the system of choice for today's luxury and crossover sport-utility vehicles, as well as wagons and sedans trying to distinguish themselves as technology leaders.
By our calculation, there are currently 17 different AWD or 4WD systems available on North American market cars and trucks. With such an array of technology from which product planners and engineers can choose, it's no wonder that the subtle nuances between these systems are driving marketing campaigns -- and even creating brands among themselves.
For example, Honda's Acura division is introducing its 2001 MDX luxury SUV this fall, and one of its key selling points is the vehicle's claimed superiority on slippery road conditions.
"A typical luxury SUV user needs snow and ice capability more than off-road capability," says Frank Paluch, principal engineer for the MDX. "We looked for a system that would give the customer the best possible control under those conditions, but still provide a sporty character and fuel efficiency. The system just didn't exist, so we had to work with a supplier to come up withone."
Enlisting the expertise of Borg-Warner (see sidebar, page 47), Honda fashioned an electronically controlled system that met its exact design parameters.
Interestingly, Honda engineers passed over the viscous-coupling setup used on the CRV, deciding it didn't have the right characteristics for an Acura. Other conventional drivelines were considered as well, but they would have been a tough fit for the MDX, which is based on the Odyssey minivan.
Tailoring To The Vehicle
"Packaging is a very important part of this business," says Ward Bowerman, vice president of engineering for Zexel Torsen Inc., based in Rochester, NY.
"We supply the Audi Quattro system and the new Volkswagen Passat Synchro (4Motion), which am longitudinal engine layouts. But the AudiTT has a transverse engine and it would have required a clean sheet of paper to package the Torsen. So Audi went with one of our competitors: Haldex. That's just the way it is. The companies try to utilize as many existing components as possible."
The Torsen system is a unique approach to AWD in that it does not rely on electrical input, clutches or wheel slippage to operate. Torque transfer is a function of the internal gearing, which generates frictional resistance to differentiation through the axial and radial thrust forces of the gears in the differential. The Torsen system is widely praised for its ability to improve handling characteristics.
VW of America Product Strategy Leader Stefan Krebsfanger openly acknowledges the strength of the Torsen brand name, which has gained customer recognition over the years.
"This is yet another way of enhancing the German driving experience," says Krebsfanger. "This will boost the image of the Passat in the marketplace and increase our share of the segment." He predicts that the 4Motion installation rate will account for 29 percent of all Passat sedan sales and nearly half of all wagon sales.
Like Audi, VW turns to Sweden's Haldex for transverse engine AWD vehicles such as the Golf, Bora (Jetta) and Sharan.
"We just signed a contract with a major European OE," says Joakim Soderstrom, product manager at Haldex's Stockholm headquarters. In addition we are engaged in four development projects in Europe, two in the U.S., and one in Japan."
Soderstrom points out that electronically-controlled systems are being developed around the world because they can overcome the physics of a purely mechanical system. In Japan, Toyoda Machine Works and Tochigi Fuji have new systems available. Toyoda, in fact, is supplying Dana with AWD hardware for the 2001 Ford Escape/Mazda Tribute (see p. 51). And Borg-Warner, GKN and Steyr also have prototypes for controllable AWD systems.
McLaren Traction Technologies' Gerodisc is a relative latecomer to the AWD business, but is quickly winning favor for its ease of packaging, light weight and power handling capability. McLaren, based in Santa Barbara, California and formerly known as ASHA Corp., recently announced that Steyr-Daimler-Puch would supply Gerodisc-based AWD technology for the Pontiac Aztek and Buick Rendezvous (see p. 41). The Gerodisc is also the basis for the limited-slip device on the Ford Cobra R, and is at the heart of Jeep Grand Cherokee's Vari-Lok axle (see AI June 1998, p. 83.)
The key to the next generation of designer drivelines is "to find proven, existing technology and try to figure out how to use it in a new way," says Frank Paluch of Honda. "The basic hardware is probably out there, but the trick is to define exactly what you want it to do -- and then make it do it on your platform. The days of using an off-the-shelf AWD system are over."
Choosing An All-Wheel Drive System
Mechanical: These systems, typified by the Torsen, have no electrical intervention, fluid shearing or pumps. Torque is differentiated through the frictional forces developed by the internal gearing.
Viscous couplings: When two axles rotate at different speeds, such as when a wheel loses traction and spins, the viscous coupling locks up through the shearing action of a viscous fluid contained within. This system serves as the differential lock on manual transmission Subarus.
Pump Type: As with the viscous coupling, a difference in axle rotational speeds provides "pumping" action, but pressure generated is used to engage a clutch. This system is typified by McLaren's Gerotor.
Electronic/Hydraulic: Typified by the Haldex differential, these systems are electronically controlled, but use hydraulic pressure to engage a clutch mechanism.
Electronic: Differential clutches are activated by electromagnetic force. No fluid is sheared and operation is fully controlled by an ECU. Typified by the Borg-Warner system.
Honda and Borg-Warner Develop VTM-4 -- The "Smart" AWD System
Acura's new MDX sport-utility features a simpler-is-better AWD system. At its heart is a single-speed, permanently engaged transfer case. It turns a two-piece prop shaft that runs to a unique, electronically controlled rear differential.
Inside the diff, the hypoid ring-and-pinion gear is connected to the left and right half shafts via an electromagnetic clutch on each side. When the ECU determines that torque should be sent to the rear wheels, current flows to the clutch magnets, activating a ball cam assembly. The ball cam activates pressure plates that engage a wet clutch, transmitting the torque to the half shafts.
"We worked on this system with Borg-Warner," says Frank Paluch, principal engineer on MDX. "This same electromagnetic clutch is used by Ford in a rear-drive application, only it is located in the center transfer case. We packaged it in the rear axle."
The clutches are designed to vary torque to the rear wheels anywhere from zero to a maximum of 53 percent. Because the clutch actuation is fully electronic, it doesn't rely on wheel slip to be activated like viscous-based units. In fact, the algorithm controlling the clutch allows it to anticipate slippage based on throttle input, road condition, vehicle speed, etc.
There are four primary modes in the VTM-4, the main mode being "automatic." When you accelerate, torque is automatically proportioned between the front and rear axles, for best vehicle dynamics. Even without wheel slip, there is torque split to the rear. When cruising or coasting, the unit goes to the second mode, which cuts torque to the rear for the best feel. If the unit senses slip at the front wheels, it goes into the third mode -- reacting to slippage by sending torque to the rear.
The fourth mode is a driver-selectable "lock" setting, which only operates in first, second and reverse gears. This would typically be used if the vehicle is stuck or in deep snow.
"The fact that this unit is fully electronic sets it apart," says Roger Wood, vice president of business development for Borg-Warner TorqTransfer systems.
"Other systems on the market are activated electronically, but then mechanically or hydraulically controlled. Ours is electronic through the full range of operation. We also control torque left to right, and not just front to rear.
"I think the differentiators for future AWD systems will be their ability to interact with the rest of the vehicle," Wood notes. "That means electronics. We will see an evolution of AWD systems similar to what we have seen on ABS. They were introduced as a niche product, but today they are mainstream." -- GK
Haldex T-2 AWD System
Audi TT's transverse engine made a better business case for the Haldex AWD system. The Haldex unit is comprised of an hydraulic pump driven by the slip between the axles, a wet clutch and a controllable throttle valve and electronics. Reaction to rotational differences is instantaneous, engaging a clutch pack that synchronizes axle speeds. The electronic controls allow the torque transfer characteristics to be fine-tuned, and allows rapid disengagement of the system to accommodate ABS functions. Electronics also compensate for changes in oil viscosity for consistent operation at all temperatures. Under steady cruise the vehicle is basically front-wheel driven.
The McLaren Gerodisc is a versatile technology that can be used as the basis for a variety of drivetrain applications. The Gerodisc provides the limited slip characteristics in Ford's new rear-drive Mustang Cobra R (shown), the four-wheel-drive function in the Jeep Grand Cherokee and the all-wheel-drive mode for Pontiac's 2001 Aztek.
VW' Passat's 4Motion AWD system by Torsen
VW's 4Motion AWD is supplied by Torsen and is identical to the Quattro system on all but Audi's A3 and TT. The all-mechanical system transfers torque via the friction developed within the internal gearing, and does not require wheel slip to function. Because it is fully engaged at all times there is no harsh lockup and torque transfer is instant. The Torsen system differentiates freely under light loads, or no load. This makes it compatible with ABS without the need for a clutch to disengage it. Through design of the gearing, the system is tunable to adjust the torque-biasing effect. Under steady cruise conditions, torque is split roughly 50/50 front/rear.