What is the difference between Full time Four Wheel Drive (4WD) All Wheel Drive (AWD) and automatic All Wheel Drive (auto AWD)?
Full time four wheel drive, also called permanent 4WD, (not to be confused with: part time 4WD ) is a system that powers all four wheels at all times and can be used full time on all surfaces including pavement. The additional feature of a differential incorporated into the transfer case makes it possible to use 4WD all the time.
2WD is no longer available. Each tire creates about 25% of the available torque when the ground is level with a consistant surface. Driver has a choice of a "4-high" (that's your every day setting) and "4-low".
When "4-low" is selected the wheels create substantially more torque (on a Grand Cherokee its 2.72 times more) than in "4-high" - at the same time the vehicle moves at substantially slower speeds (2.72 times slower on a Jeep Grand Cherokee).
Important: "4-low" does not create more traction - it creates more torque and that can be detrimental when traction is marginal. Slipping tires are more likely in "low" than in "high"!
The low setting is an advantage for drivers who need to tow and maneuver a heavy trailer etc. and for drivers who at one point or another may want to negotiate difficult off-road terrain, when more torque and/or slower speed is needed.
All wheel drive (AWD) is a system that powers all four wheels of a vehicle at all times as well. Full time symmetric AWD would be the best term to be used. Difference to full time 4WD is that a "4-low" setting is not available in AWD cars. Due to the lack of "low range" AWD vehicles are much less capable in off-road settings than full time 4WD vehicles, but work perfectly well on-road.
Recently some new "automatic" AWD systems have emerged. Fancy names like "Real Time 4WD" or "active AWD" are hiding the fact that they are essentially sophisticated 2WD systems. Automatic asymmetric AWD would be the best term for them. Unfortunately, since they offer AWD only part of the time, some magazines have now called it "part time 4WD" - but that term has been used since WW II for cars like the Willys and Jeep Wrangler and their part time 4WD - the name coming from the fact that 4WD can only be used part of the time (when off-road), most of the time they have to operate in 2WD (on-road). Automatic asymmetric AWD is much less capable in off-road settings than full time AWD systems and inferior to full time 4WD. However, automatic asymmetrical AWD is becoming more and more sophisticated and offers pretty much everything consumers expect for everyday (pavement) driving.
Here is how they work: During traction loss on the driven axle (could be front or rear) a fully automatic system (hydraulic, mechanical or electronic) routes torque to the axle with traction. This means you have to completely lose traction in 2WD on your driven axle first and then the other axle will take over and try to keep the car moving. So, for a moment you have 4WD (AWD). Doesn't mean much because only two wheels have traction. Once the primary driven axle regains traction and both axles rotate at the same speed again, the system reverts back to 2WD.
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Last edited by NHogan; 11-14-2006 at 01:24 AM.