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steve_taylor | on: Unlocking any car from X brand in most of the world
4WD: Distribution of power to the wheels is static. AWD: Distribution is dynamic.

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.

sandworm101 | parent | next
There are plenty of AWDs that operate as FWD 90% of the time, only engaging the rear as needed. (I am told this is for fuel efficiency but the logic escapes me.) But I have yet to ever see a 4WD that could be "switched" to FWD.

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.

throw0101d | root | parent
> There are plenty of AWDs that operate as FWD 90% of the time, only engaging the rear as needed. (I am told this is for fuel efficiency but the logic escapes me.)

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.

sandworm101 | root | parent
>> the extra mechanics (and their friction loses) to the rear wheels an 'OpEx' that you don't necessarily have to pay

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.

pge | root | parent
It depends. With conventional AWD (like all Subarus), you are correct that there is significant energy loss due to the AWD because there is a drivetrain to the rear wheels, even if it is not delivering power. Many hybrids, however, have AWD and they work by having a second, small electric motor that powers the rear wheels only when needed. That means no physical drivetrain to the rear wheels, so no energy loss. That's why a hybrid Toyota RAV4 AWD gets about 15% better gas mileage than the smaller and lighter hybrid Subaru Crosstrek AWD. The disadvantage of the electric motor approach is that it cannot deliver as much power to the rear wheels as a conventional physical AWD drivetrain can. That said, probably only an issue in deep mud or snow.
sandworm101 | root | parent
Well, no physical drivetrain to the front half of the car would make a difference. But even with electric motors in the rear there is a drivetrain between the wheel and the motor. If the idle rear electric motors are is still spinning then the total mass of spinning parts might well approach the mass of a Subaru drivetrain. Driveshafts are very light.

It sounds like the savings are more in construction costs than fuel savings.

cucumber3732842 | parent
It's all marketing wank with overlapping venn diagrams. Nobody for whom the difference matters engages in hair splitting over language.