full-time all wheel driveNext
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 each wheel 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 automatic lock, a Torsen differential, viscous coupling, multi-plate hydraulic clutch, or similar traction control device is used in conjunction with the center differential. When a wheel slips on one of the axles (one driveshaft rotates faster than the other) the device locks the center differential and 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 slipping wheels and 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.
automatic all wheel driveNext
This is an "on-demand" all wheel drive system. Under normal driving conditions, only one axle is powered. When wheel slipping occurs (the driving driveshaft rotates faster than the driven driveshaft), a multiplate hydraulic clutch, viscous coupling, or other similar traction control device locks and engages another axle. Torque gets transferred to another axle. As soon as difference in front and rear axle speeds is eliminated, the device unlocks and the vehicle goes back into two-wheel drive mode.
The difference between the traction devices that are used in full-time all wheel drive and automatic all wheel drive systems is that the device used in automatic all wheel drive system replaces the center differential.
Advanced electronically controlled all wheel drive systems can be proactive and lock the traction control device even before wheels start to slip - the need for all wheel drive is determined in real time, based on the information that is collected from various sensors (i.e. g-force sensor, accelerator pedal position, etc.).
Some vehicles allow the driver to lock the multiplate hydraulic clutch manually when the driver feels that he needs all wheel drive engaged permanently, for example, to drive off road. In the Nissan X-Trail, for example, this is accomplished by pressing a button on the dashboard console. In Subaru Legacy, the clutch is locked when the automatic transmission gear shift lever is at the position "1".
A known problem with multiplate k hydraulic clutches, used in automatic all wheel drive systems, is traction device overheating. For example, this can occur with some compact SUVs when they are driven off-road, through a thick layer of snow, etc. ECU detects overheating, disengages the clutch, and puts the vehicle into 2WD mode. A warning light is displayed on the instrument panel.
selectable all wheel driveTop
In this category falls the Mitsubishi Pajero(Montero) with its Super Select transmission, Jeep Grand Cherokee with SelecTrac transmission, and a few other off-road vehicles. Mitsubishi, for example, has transmission that is similar to the one used in full-time all wheel drive vehicles, but with two-wheel drive possibility. In Mitsubishi, the driver can choose between the 2wd mode, 4wd mode with automatic distribution of torque via viscous coupling (vehicle behaves just like the one with full-time all wheel drive), 4wd with locked differential (behaves like part-time all wheel drive) and 4wd with low gearing (low range part-time all wheel drive).