AWD Cars - All Wheel Drive - Explained

How does all wheel drive work?This video explains how All Wheel Drive works and how it differs from 4WD. It also explains automatic AWD, meaning the car usually only uses 2 wheels to drive, until slipping is detected, and the other two wheels will automatically be powered.

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[EngineeringExplained]

Well it depends on throttle position, but to answer your question in a simple manner, yes. You put a constant A/F ratio, but that doesn't mean that the amount of air that comes in is constant. Increase the amount of air (by designing for optimum flow, or adding forced induction) and you must then increase the amount of fuel burned.

[EngineeringExplained]

My next car will likely be an STI actually, so I guess that answers your question. I love hatchbacks, I think the STI sedan looks boring. From a performance basis, the EVO can probably put down better track times, but it's not hatch and loses so much practicality. Also, the next gen STI is supposedly going to shed some weight, and possibly come in 2 door. I'm a big fan of 2 doors.

[DTechnologyservices]

Why even read a dam book when you got intelligent people like this to spoil the fun on youtube!!! thanks so much great work!!!

[EngineeringExplained]

Oh I don't know. It's all about the application, they all work. But do you want full engagement (good for off-road) or do you want to allow slip (great for on road)? Or do you want partial slip with torque transfer (viscous coupling)? All just depends really, they each have a purpose for sure.

[EngineeringExplained]

If you haven't watched my video on transfer cases that's a great resource to check out. After that I'd look to any documents relating to your vehicle, to see what the actual set-up is. Glad you like the videos!

[EngineeringExplained]

Some sort of coupling, yes. Though I suppose it would be possible to eliminate one if it was driven directly from the transmission.

[EngineeringExplained]

One can be placed near the bevel gears at the front. I did this for simplicity so it would be easy to understand power going to front and back.

[harrelchris]

I would love to see a video where you discuss AWD vs FWD in terms of traction and safety to help decide if it's actually worth the additional cost when purchasing a vehicle.

[EngineeringExplained]

It allows for some difference in speed between the front and back. When the speed of one exceeds the speed of another it transfers torque, but this does require a difference in speed, which makes cornering on pavement doable. Kinda the same principle as a torque converter, when you step on the gas, your engine is spinning faster than your transmission, but they are linked.

[EngineeringExplained]

Well, it's symmetrical. Unlike the one I have shown, the transmission is placed longitudinally, so all of the components are balanced as far as weight. And a viscous coupling is used to transfer torque to the tires with more grip.

[normaspirated]

Honda's full time sh-awd uses multi electromagnetic clutch packs that are integrated into the rear differential. When taking a fast turn, the clutches that are associated with the rear outside wheel tighten up. This inevitably causes 100% of the torque/power to transfer to that one rear wheel, and no power to any of the other 3 wheels. Kind of causes a "peg leg" or row boat effect (rowing outside oar only). Pretty cool and sophisticated system.

[EngineeringExplained]

Some sort of differential yes, you wouldn't want a locked mechanism that wouldn't allow for separate speeds. I was just saying it's possible to have a direct drive.

[EngineeringExplained]

Yes, you could see gains by reducing the amount of driving wheels. It is better in that all of the weight of the car is on the driving wheels, so you can accelerate quicker if you are traction limited without AWD.

[EngineeringExplained]

Power would transfer through any 3 (or combination of any) of the center items, whether it be a clutch pack, center diff, or viscous coupling.

[EngineeringExplained]

Yes, the back would get 5 lbft UNTIL the viscous coupling/locked diff began to spin. If the differential was locked, and the front tires are spinning, that would mean the rear tires are also spinning; so you'd either be moving, or burning all four tires.

[EngineeringExplained]

I guess it would still depend on how much power they have, how much grip, and where the weight is located. AWD will allow for a faster start, but it means more losses from the engine, so it could still lose over a 1/4 mile if they all have the same engine. RWD could be second after AWD, as weight transfers to the back upon acceleration, meaning it could put down more power, but FWD typically has the least amount of drivetrain loss. Any of them could win depending on differing characteristics.

[EngineeringExplained]

I'm fairly sure power is always split on the GTR, there's never a time when it's just sent to one axle.

[JoeJ-]

Yeah, this drawing of this kind of setup would ONLY apply to the FWD biased "auto" 4WD systems that use either the viscous coupling or the electronic clutch pack to send power to the rear, but NOT to the one with a center differential, because if you have a center differential, (whether with or without an attached viscous coupling, which only makes it a limited slip diff.), then in that case your drawing would have to be changed completely, because in order for a diff to work properly you have to feed the diff. first from the transmission, (and not the front wheels directly), and then and only then, the torque would transfer through the front and rear outputs of the diff to the respective front and rear diffs and axles... As in how Subaru and a few other of the more "symmetrical AWD" systems work... You should make another video explaining this major difference between the two major types of systems, (i.e. FWD biased like this and ones with a center diff)!

[EngineeringExplained]

Yes, but it's purpose is to get them to rotate at close speeds, to eliminate the disadvantage of an open differential (one set of tires spinning, the others stationary).

[EngineeringExplained]

Well, for one it's actually symmetrical, as in when you look at it from above the vehicle, shafts and diffs and the whole system is basically mirrored if you cut the vehicle in half. I don't believe this is very common, most are offset one way or another.