I only have four wheels. If all the wheels are driving, and I only have four wheels, then...
Again, I do know what the conventional difference is. But it's just that: convention. Unless you memorized the convention beforehand, there's really no way to decipher why 4WD/AWD has any meaningful distinction, nor which of the two specific names maps to which mechanism.
4WD vehicles are often part-time 4WD, meaning they can be switched between 4WD and RWD or FWD.
AWD vehicles are full-time AWD because part-time AWD is unnecessary given that the drivetrain delivers power where it's most needed.
The clearer distinction is that a 4WD has a second gearbox (aka a transfer case) that either manually or automatically engages the front wheels, the Jeep Wrangler /TJ being the architype. That means the extra wheels cannot be triggered seamlessly. A 4FW has distinct driving modes whereas an AWD uses clutches to bounce between modes as needed.
Bluey has a couple lines about this distinction. In Australia, even small children have opinions on 4WD v AWD.
AIUI, you lose energy because the power/torque has to go through an extra differential, transfer case, etc, before reaching the rear wheels.
Going from the engine through the transmission to the front wheels is already a 'fixed cost', but the extra mechanics (and their friction loses) to the rear wheels an 'OpEx' that you don't necessarily have to pay if you don't need to.
True for a FWD v RWD car decisions, but in an AWD are not the rear differential and driveshaft still turning whenever the rear wheels are turning? Even the clutch packs should still be rotating even if disengaged. The fact that there is no power through them doesn't mean they aren't generating losses.
It sounds like the savings are more in construction costs than fuel savings.
AWD - clutch packs. Best for nasty conditions on road, IMHO.
You might have something like a Series Landrover transfer box where in 4WD mode the front and rear axles are locked together and in 2WD mode only the rear axle is driven.
But does that make a Defender, RRC, or Disco with LT230 transfer box which has a lockable diff AWD or 4WD? The centre diff supplies power to both front and rear all the time, but can be locked in slippy conditions otherwise if one wheel spins *none* of the wheels get drive - the axle and centre diff will both spin.
Then what about the later Borg-Warner transfer box in later Range Rovers, where you still have a centre diff and epicyclic gearbox to set high and low range, but there's no diff lock - there's a massive viscous coupling that turns it into a ridiculously stiff LSD.
But in modern times, technology has most definitely blurred the distinction. Higher-end Jeeps also have a “full-time 4WD” that sounds an awful lot like an AWD system that can also do 4WD stuff.
So, eh, it’s probably not a good idea to argue too much about broad categories anymore. Individual systems must be compared
I'm guessing that maybe a distinction between offroad capable cars and non-capable cars is useful for liability reasons. Or something else.
Some AWD systems might intelligently change power to different wheels (or front and back) and that might vary per brand and model. 4WD could also do this. Usually with 4WD you might also have a 4 lo and 4 hi gear as well.
Then there's also various types of differentials and locks and whatnot which is a whole more detailed discussion.
Think of it more like it's going to be 2WD most of the time, but when there is low traction (such as ice) the 4WD will engage as power is always going to all wheels.
4WD is a permanent thing, or at least always on when you engage it. The system locks all the wheels to the same spin speed.
4WD is for off-road use. All tires rotate at the same speed.
At this point, the distinction is gone, because car bards marketing.
(AI lies)
Re: the 3 locking mechanisms comment, in typical 4WD configurations the transfer case in 4H/4L is essentially locked (which is what makes the above sum front == sum rear condition). AWD transfer cases can be more flexible there; some essentially act as a 3rd differential and result in all four wheel speeds being essentially decoupled (for better or worse, easier to get stuck) while others actively control the front/back locking with clutches or hydraulics or whatever.
The "locking mechanism" known as traction works pretty well most of the time and has the nice side effect that it only wears an easily replaceable consumable when you break it.
https://en.wikipedia.org/wiki/Six-wheel_drive
So nwheels x n driven wheels.
But 6x4 notation is handy when you have six wheels, sure.