four-wheel drive systems

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There are almost as many different types of four-wheel-drive systems as there are four-wheel-drive vehicles. It seems that every manufacturer has several different schemes for providing power to all of the wheels. The language used by the different carmakers can sometimes be a little confusing.

4×4/4WD/AWD systems were developed in many different markets and used in many different vehicle platforms. There is no universally accepted set of terminology to describe the various architectures and functions. The terms used by various manufactures often reflect marketing rather than engineering considerations or significant technical differences between systems

How it works
The most common layout for modern road cars includes an engine mounted in the front of the car powering just the front wheels. The engine’s power is sent through the gearbox and onto the differential. The torque (turning force) applied to the differential is sent along two drive shafts, which are joined to each wheel, causing them to rotate in the direction of travel.
In a permanent all-wheel-drive car, power is sent through the gearbox to a centre differential. This splits the power to the front and back axles where it meets differentials at the front and rear. These distribute power between the rear wheels as well as the fronts, hence ‘all-wheel drive’.

Four-by-four (4×4) refers to the general class of vehicles. The first figure represents the total wheels (more precisely, axle ends), and the second, the number that are powered. Syntactically, 4×2 means a four-wheel vehicle that transmits engine torque to only two axle-ends: the front two in front-wheel drive or the rear two in rear-wheel drive. Alternatively, a 6×4 vehicle has three axles, two of which provide torque to two wheel ends each.

Four-Wheel drive (4WD)
In 4WD system power goes from the engine, to the transmission, to what is known as a transfer case. This system splits power between the front and rear axles so that torque is evenly applied to each wheel. This process is nothing new, and is still used in modern Jeeps to tackle just about any off-road obstacle. It isn’t perfect, though. When the transfer case splits power evenly, it ensures that each wheel turns at the same speed. This is deeply problematic when doing things like turning. You see, for a car to make a turn, the inside wheel has to turn more slowly than the outside wheel, which is covering more ground. If the vehicle can’t do this, the inside wheel loses traction and spins freely. This, as you might be able to guess, isn’t great for maintaining momentum.
All-wheel drive (AWD)
These systems are sometimes called full-time four-wheel drive. All-wheel-drive systems are designed to function on all types of surfaces, both on- and off-road, and most of them cannot be switched off. Part-time and full-time four-wheel-drive systems can be evaluated using the same criteria. The best system will send exactly the right amount of torque to each wheel, which is the maximum torque that won’t cause that tire to slip.

why isn’t every car Four-wheel drive
There are two main reasons: cost and efficiency. Including extra drive shafts, differentials and electrical systems to keep everything under control adds weight, while the losses through rotating these extra masses makes the system less efficient, with the result that you burn more fuel for a given distance travelled.