Full-time all wheel drive
This is a permanent all wheel drive or permanently engaged all wheel drive system. All wheels are powered at all times. The vehicles with full-time all wheel drive are equipped with a center differential that lets all wheels travel different distances while turning. This type of all wheel drive can be used both on and off road. In slippery conditions, the center differential can be locked, whether manually or automatically, depending on the vehicle.
When a manual center differential lock (available on off-road vehicles and some SUVs) is engaged, the transmission's behavior is similar to part-time all wheel drive, i.e. the front and rear driveshafts rotate at the same speed. The use of full-time all wheel drive with locked center differential is limited to surfaces with low traction.
In case of an automatic lock, a Torsen differential, viscous coupling, multi-plate hydraulic clutch, or similar traction device is used in conjunction with the center differential. When a wheel slip occurs (one driveshaft rotates faster than the other) the device locks the center differential and the torque is transferred from the axle that slips to the other axle that has traction. As soon as the wheel slip is eliminated, the device unlocks.
Some vehicles (Land Rover Discovery II, pre-xDrive BMW X5) do not have a locking center differential, but are equipped with an electronic traction control system (known as Electronic Differential Lock - EDL) on all four wheels. This electronic system detects slipping wheels by reading ABS sensors, then it applies brakes to the slipping wheels and the torque gets transferred to the wheels that have traction. While it performs well on slippery roads, the system cannot compete with a real mechanically locking differential when driving off-road.
It should be noted that the tech and name might be different, but the ideas are the same: keep all wheels torqued to achieve a better maneuverability on all terrains.
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