In the “non-car” world of people, there still is a significant amount of confusion between the term “All-wheel Drive” and “Four-Wheel drive”. The non petrol-heads of the world have a bad history of interchanging these two terms and perhaps believing that they mean exactly the same thing, but THEY DON’T! There is a very clear and important difference between these two types of drivetrains and both have their own customary advantages and disadvantages. Lets begin with the more mechanical side of things..
Well 4WD is usually associated with those heavy pickups or those lifted Jeeps and only them. I can guarantee you that you will not find any stock sedan or people’s car that will have the “4 X 4” sticker on it’s side. The simple reason is that 4WD is just very impractical for daily use, it is primarily engineered for extreme off-road situations.
The diagram on the left shows you the overall layout of a typical 4WD vehicle, more specifically a Part-time 4WD system. This means that your car will have the ability to switch between low and high gear and also between 4WD and 2WD (powering only the rear axle). Low gear means using smaller and larger gears to increase torque and reduce speed, high gear is using identically sized gears to allow for speed and probably less torque for the wheels.
The torque from the engine is fed through the usual transmission (blue box) and then to a, 4WD unique, Transfer Case (green). This case is what does the magic of switching between the aforementioned settings.
The power from the transmission is coming from through the red shaft and powered through the orange gear set where, using a selector fork, the shaft can be geared up higher or lower using a lever in the cabin. From there, the torqued up or high gearing is fed (yellow) to a gear that is connected to a chain (green) and another gear on a transaxle, that is attached to the front axle. The yellow shaft is connected to the rear axle. To switch between rear-drive and 4WD, another gear selector ring is used (not shown) at the green gear to disengage and engage based on the drivers lever.
The 1:1 gear ratio’s of the green gears means a complete 50-50 torque split between the front and rear axle.
The two axle’s will also usually have locked differential’s, that lock the two tires together and can allow for better torque transfer when one tire loses traction.
Some of the disadvantages of 4WD systems and why they are not used in many standard cars is that they force all four wheels to spin with same amount of speed/torque. But when you turn a car, the front axle spins faster than the rear and hence you would have scuffing going on (Ackerman steering concept)
All wheel drive is the more driver-friendly of the two and arguably the more complex and heavier one due to it’s components and high computer dependence. It is the safer and smarter choice which has the ability to, depending on conditions, to vary wheel speed and torque to allow for maximum traction and efficiency.
Many different companies can come up with different combinations to fit their own needs
For example: Mercedes-Benz use 4-matic, Audi use Quattro and BMW use X-drive
For AWD cars you can three different devices to move power to rear axle (these devices replace the transfer box in a 4WD):
1) Standard Open Diff.— An open differential that can allow the two axles to rotate at different speeds.
2)A Multi-plate clutch pack— Typically used in part-time AWD, it uses friction between two or more plates to engage or cut off power to one axle.
3)A Viscous Coupling— uses high-viscosity liquid inside a clutch-type housing to transfer torque between axles. Very helpful when one axle loses traction.
Various computers used by cars measure wheel speeds to check if a wheel spins and accordingly engage clutches or start cutting power to efficiently provide power to gripping wheels and allow them to spin at different speeds.
A Viscous coupling–>
<–A Multi plate clutch