What is the difference between Full time Four Wheel Drive (4WD) All Wheel Drive (AWD) and automatic All Wheel Drive (auto AWD)?
Full time 4WD, also called permanent 4WD, (not to be confused with: part time 4WD ) is a system that powers all four wheels at all times and it 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 not available (only part time 4WD offers that option). Each tire creates about 25% of the available torque when the ground is level with a uniform surface. Driver has a choice of a "4-high" - 4H (that's your every day setting) and "4-low" - 4L.
Full time 4WD vehicles work very well on-road and are very capable off-road.
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 at slower speeds and that can be detrimental when the ground is slippery. Slipping tires are more likely in "low" than in "high"! On snowy, icy roads "low" would be a bad choice - some really deep snow, however, puts up so much resistance that "low" is needed to push forward. You see, using 4WD is not an easy task.
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 almost the same thing as full time 4WD - it is a system that powers all four wheels of a vehicle at all times as well. It can be used full time on all surfaces including pavement like full time 4WD. 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.
Automatic AWD system
is the newest kid on the block. PR agency generated names like "Real
Time 4WD", "intelligent AWD" or "active AWD" are hiding the fact that
automatic AWD is essentially a sophisticated 2WD system. Automatic AWD is NOT powering all 4 wheels all the time. Only on rare occasions all 4 wheels will be powered very briefly. Since AWD is only engaged on rare occasions, sometimes automatic AWD is falsely called "part time 4WD". Engineers refer to this system as an "on demand system". Since it is based on an existing 2WD steup, it is also referred to as a "hang on" solution.
Here is how they work: Under normal conditions one axle gets 100% of the torque - meaning you are driving in 2WD. During traction loss at the driven axle (could be front or rear) a fully automatic system (hydraulic, mechanical or electronic) makes some of the torque to the axle with traction available. This means you have to lose traction in 2WD on your driven axle first and then the other axle will be added and try to keep the car moving and stable. Once the primary driven axle regains traction and both axles rotate at the same speed again, the system reverts back to 2WD. So, for a brief moment you had AWD.
Automatic AWD is much less capable in off-road settings than full time AWD systems and inferior to full time 4WD. In fact, automatic AWD is almost useless beyond pavement. However, automatic AWD is becoming more and more sophisticated and offers pretty much everything consumers expect for everyday (pavement) driving.
Examples of automatic AWD: Honda CRV, (newer) Toyota RAV4, LandRover Freelander, Isuzu Trooper (TOD), Volvo V70, 1999 and later Jeep Grand Cherokee (in high range).
Recently some confused magazine writers have called the automatic AWD system "part time 4WD", since it offers AWD only part of the time. They have a point - however, the term "part time 4WD" has been used since WW II for cars like the Willys and Jeep Wrangler and their part time 4WD . A manual system where the driver had to select 2WD or 4WD. The name coming from the fact that 4WD was designed to be used only part of the time (when off-road), most of the time it had to be operated in 2WD (on-road).
Confusing but true: some vehicles have a combination of part time and
full time 4WD systems, or even a combination of 2WD, automatic
asymmetric AWD and part time 4WD (low range). Impossible to give those
custom mixes a name.. more...