This distinction (although I know it) has never made logical sense to me.
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.
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.
> 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.
>> 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.
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.
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.
^ this. 4wd is used on offroads and locks all wheels together, transfering equal power to all wheels. AWD varies power using different technologies, from a simple Haldex that transfers power back when front wheels slip to complex sensor controlled individual wheels. Some are front-biased (generally but not exclusively cheaper ones), some are rear-biased (mainly in sports cars) and some are true AWD, depending on the needs and expected pricing of the vehicle.
Not quite, no. It only locks all the wheels together if you also have axle lockers.
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.
It's more of an industry wide thing:AWD means all wheel drive even when the vehicle has more than 4 wheels. So in the case of a vehicle with more than 4 wheels, 4 wheel drive would indicate four of the wheels are driven and the remainder are idlers. For instance 6x4 would indicate six wheels with four driven wheels.
You seem to be talking about something different. The conversation is about vehicles with four wheels and the differences between 4WD and AWD in that context. Perhaps you're saying that there is no difference?
But 6x4 notation is handy when you have six wheels, sure.
I don't know, but both Gemini and Wikipedia in my primary language seem to agree that AWD refers to 4WD with a center decoupling mechanism to run front and back in an asynchronous manner, with emphasis on Wikipedia only that this is strictly American definition of it and both describe both style of systems in the rest of the world.
I'm guessing that maybe a distinction between offroad capable cars and non-capable cars is useful for liability reasons. Or something else.
I don't think most people here have it right. From my understanding the main difference is that 4WD can be manually turned off (meaning going to 2WD mode). AWD is always on.
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.
The RAV4 dynamically engages the back wheels with no mechanical coupling to the front wheels. It uses a separate electric motor in the back. Toyota calls it AWD.
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.
I'm assuming he means AWD.