Full-Time 4WD – This system allows 4WD (high or low) to be used on any road surface by using a center differential (diff). It also has a 2WD mode.
Permanent 4WD - Has a center diff but does not have a 2WD mode.
Part-time 4WD is a system that does not have a center diff and can only be used on low traction road surfaces. The front and rear diffs turn at the same speed (in 4WD mode).
The difference between 4WD and AWD is AWD does not have a transfer case that provides 4wd low range.
The best (most versatile) system is; full-time 4WD with a locking/limited slip center diff. This allows 4WD to be used all the time under any road conditions and will divert torque to the diff/wheel with the most traction. It also can be used as 2WD or permanent 4WD (by locking the center diff). If you want the best off-road system you would add lockers to the front and rear diffs. This locks the two wheels on the same axle (like it is a solid axle).
The system that is in my TV has full-time 4WD with a Torsen locking/limit slip center diff. The Torsen can be locked to provide a 50/50 split of torque (same as permanent 4WD) to the front and rear diffs or when unlocked it will automatically divert torque between the front and rear diffs depending on available traction. It will divert up to 70% to the rear and up to 60% to the front diff. It also uses a system called ATRAC that pulses the brakes on the wheel that loses traction and diverts the torque to the wheel with traction, on the same axle (computer controlled). It also has crawl control, which is a low speed cruise control (can only be used in 4WD low) that will maintain a set speed (1 to 5 mph) on low traction inclines or descents (including reverse). It uses computers to control the brakes (can pulse one or more wheels) and throttle to keep the vehicle in a straight line under severe traction conditions (all you do is steer).
Koz
|