DETROIT - Chrysler has introduced an all-wheel-drive system on its new Jeep Grand Cherokee that takes a more mechanical approach to transferring power than many competitive systems.
'About 15 percent of our customers go off-road,' says Don Schmanski, a Chrysler product engineer who worked on the Quadra-Drive.
'With other manufacturers, that is about 4 percent,' he says. 'We needed a robustness and durability (in the drivetrain) that you don't get with some other systems.'
Quadra-Drive is a full-time, four-wheel-drive system for distributing power among the wheels. It replaces a system called Quadra-Trac.
Quadra-Trac sends most engine torque to the rear axle until those wheels slip. When the axles turn at different speeds, the transfer case that links them reacts. A silicone fluid within heats up and thickens because of the forces caused by the plates turning at different speeds. As it thickens, the fluid progressively locks the clutch plates together, sending engine torque from the rear to the front.
Viscous couplings are common in many all-wheel-drive vehicles, including the Subaru Outback and Lexus RX 300.
For Quadra-Drive, Jeep engineers used a new gerodisc coupling invented by supplier ASHA Corp. of California.
The gerodisc coupling has clutch plates like a viscous coupling, but they are squeezed together by a pump that squirts a precise amount of fluid with each stroke - like an engine's oil pump.
As long as both axles turn at the same speed, the pump is idle. As axle speeds vary with slippage, a rotor is activated and begins pumping oil against a large, flat piston. As it moves, the piston squeezes together the friction plates joining the front and rear axles.
In this way, the gerodisc coupling sends torque from the back to the front axles.
The gerodisc proved so effective at switching torque forward and backward that Jeep engineers put couplings in each axle for transferring torque from side to side. The gerodisc couplings replace the base Quadra-Trac's conventional differentials.
Now all the torque can be focused on a single tire if necessary, says Schmanski.
The ASHA invention was adapted for the Grand Cherokee by Dana Corp. of Toledo, Ohio, and New Venture Gear, the Chrysler-General Motors joint venture.
A spokesman for ASHA says the company is conducting development work on 34 new platforms. Other licensees include Magna Steyr-Daimler-Puch of Graz, Austria, which builds vehicles for Chrysler and Daimler-Benz.
The gerodisc is more efficient than ordinary differentials and more seamless than viscous couplings. It is tuneable by engineers so that the torque transfer is consistent and predictable.
Schmanski says Jeep buyers will approve of the simple mechanics of the gerodisc.
Some competing drivetrains depend heavily on electronics. Control Trac, used by the Ford Expedition, uses electromechanical clutches driven by a complex computer algorithm. And 4ETS, used on the Mercedes-Benz M-class, is an electronic traction-control system that senses wheel slip and automatically applies brakes.
The benefits come at a cost. Quadra-Drive is cheaper than most electronic systems, but more expensive than the viscous coupling it replaced.