Active differential 4WD is the most sophisticated one available today. It was first used in Porsche 959 – supercar of 80′s. Until today, Porsche 959′s system is still the only one which make use of variable torque split for maximum traction under normal conditions. In most of the time, torque split between front and rear is 40:60, that is, the same as the car’s weight distribution. In hard acceleration, weight transfer to the rear axle increases traction in the rear tyres while reduces traction in the front.

Porsche PSK uses a multi-plate clutch instead of center differential. You may call it a “differential clutch” as well. The multi-plate clutch has 6 pairs of frictional plate, each pair is independently controlled by computer and actuated by hydraulic pressure. This simply equals to 6 independent clutches.

To make this system work, the front and rear driveshafts must run at different speed in normal condition. (so 959 adopted a pair of front tyres with 1% larger diameter than the rear’s) Because of the speed difference between front and rear driveshafts, the 2 frictional plates of each independent clutch are rotating relatively to each other. When apply hydraulic pressure to the first clutch, a small amount of torque will transfer to the front axle. But note that the two driveshafts cannot be fully locked up unless all 6 clutches are locked simultaneously.

Now, you may see how it works: lock up 2 clutches, 3 clutches … and the torque to front wheels will be increased, subsequently, torque split could be 50:50 if all the clutches are fully engaged. Of course, all these action is controlled by computer.
This is only for “normal” condition. Like other 4WD systems, when tyre slip occurs, most of the torque could be sent to either axle.

What about energy loss and wear due to the slipping clutches? As the speed difference is very small, Porsche claimed energy loss is no greater than 0.4% of the power developed by the engine. As for wear, the clutch is dimensioned that it was negligible and caused no problem during the whole life span

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