The AWD (all-wheel-drive) story begins in 1893 when the idea was first patented, 10 years before actually being implemented by a couple of Dutch gents from Amsterdam and a solid century before you thought the Evo was cool for having it. Audi and the now defunct AMC were among the first companies to mass produce full-time AWD passenger cars, beginning in the early 1980s, but it's auto makers like Subaru, Mitsubishi, and Nissan that have taken the concept and made it something that you actually care about.
You don't have to be smart to know that an AWD layout means better traction. Modern AWD systems have come a long way; they're flexible and have the ability to go unnoticed when they're not needed but provide additional grip when they are. Aside from the extra weight, marginal power losses through the drivetrain, and typically higher cost, there aren't many bad things you can say about it.
Common sense says they are, but 4WD and AWD aren't synonymous with one another. The differences span beyond the fact that one denotes lifted bro-trucks and the other continually distributes torque to all four wheels. 4WD applications do something similar, but only part-time, by means of separate, low-range gears that allow for more useable low-end torque when needed, which makes them ideal for commercial vehicles, SUVs, and, of course, bro-trucks.
Modern AWD vehicles have the ability to direct varying amounts of engine torque to each wheel as needed. As a given wheel's grip is compromised, the system distributes more or less torque where needed. For example, if the left front wheel loses traction, engine torque might be distributed away from the front, toward the rear. The best part is that most AWD systems are passive, which means the chances of you bungling things up are slim.
The locking center differential, housed inside of the transfer case, makes all of this possible in combination with a viscous coupling, computer-controlled, multi-plate hydraulic clutch, or Torsen differential (more on those later). A differential is really just a fancy gear that has the ability to transfer power from a transmission to its half shafts. Essentially, it allows a single input shaft to drive two output shafts, independently, at different speeds. On a FWD car, it's what allows the outside wheel to rotate faster than the inside wheel while turning to prevent skidding, binding or tire scrub. Instead of limiting left-to-right wheel slip, though, an AWD center differential limits wheel slip front to rear, allowing its front and rear wheels to rotate at different speeds. The type of differential an AWD vehicle has can have a significant impact on how it behaves.
Conventional open differentials allow for unequal wheel rotation, but won't do much for traction. When grip's compromised, a locking, or limited-slip, differential has the ability to temporarily lock its output shafts together, causing two opposing wheels to spin at equal rates, whether left to right or front to rear. Almost every AWD vehicle has some sort of locking center differential. Acting upon the center differential is either a viscous coupling, a multi-plate hydraulic clutch or a Torsen differential. Each has the ability to lock drivetrain components together for increased traction.
A viscous coupling is really just a sealed housing full of goop and made up of a series of plates, holes and slots, all of which connects to the transfer case's output shafts. Viscous couplings are typically integrated into the center differential although they can be independent. Under normal conditions, both plates spin at equal speeds, but when one set of wheels begin to spin faster than the other, the viscous goop reacts, forming a semi-solid state, linking both output shafts together, causing the plate connected to the slower output shaft to speed up, essentially transferring torque to that end via the car's driveshaft. Viscous couplings aren't fast-reacting, but are still commonly used.
Multi-plate hydraulic clutch systems accomplish similar results but through a more complex, limited-slip mechanism that electronically controls clutch engagement. Once the system senses wheel slippage, the clutches engage, locking the output shafts, and transferring torque appropriately and quickly. Multi-plate hydraulic clutch systems are among the most effective and most expensive.
Seldom-used Torsen differential systems normally behave as an open differential would but allow each output shaft to receive a different amount of torque when needed. The downside is that if one wheel loses traction completely, the opposing wheel won't receive the appropriate torque transfer, at which point, you'd be hosed.
The fairly unexciting remainder of an AWD system consists of a FWD-biased transaxle or a RWD-biased transmission, each with their own differentials, and front or rear differential housings, depending on the application. AWD purists would have you believe that any given AWD vehicle was designed from the ground up with nary a thought of any other drive type, but the truth is, AWD vehicles are almost always based upon FWD or RWD layouts.
AWD vehicles like Mitsubishi's Evo and its early Diamond Star trio that feature transversely mounted engines up front are inherently more FWD than not. Like the FWD platform, Mitsubishi's AWD system is based upon a transaxle, which, not surprisingly, houses the front differential. As you'd expect, a transfer case with a gear-type differential is connected to the transaxle and to the rear differential via a driveshaft.
Unlike most AWD platforms, the Evo's center differential is a non-locking type. This would normally mean bad news were it not for Mitsubishi's ACD (Active Center Differential). The ACD system's multi-plate clutch limits the extent to which the differential may react, essentially locking the front and rear output shafts together when needed and to varying degrees. A series of sensors and predetermined, driver-selected programs makes it hard to screw things up. When compared to viscous coupling units, multi-plate clutch systems can provide up to three times more force.
Vehicles like Nissan's Skyline, with its longitudinally mounted engine up front that's mated to a conventional, RWD-type transmission, essentially behave like a RWD vehicle until hoonery begins to ensue. Nissan accomplishes all of this with ATTESA (Advanced Total Traction Engineering System for All-Terrain). The system's based off of a conventional RWD gearbox that drives the rear differential via a standard tailshaft. At the end of the transmission sits the AWD transfer case of which a short driveshaft transverses back to the front wheels via another differential. Inside the transfer case, a multi-plate clutch pack distributes torque. Information like G-force, boost pressure, throttle position and individual wheel speed is fed into the computer. If traction loss is sensed, the clutches intervene, engaging and sending torque to the appropriate wheels.
Nissan's latest GT-R, the R35, is slightly different. The R35 features two, full-size driveshafts: one that spans from the engine to the transmission, which is peculiarly located at the rear of the vehicle, and another that spans from the transmission back to the front. While older GT-R center differentials make due with mechanical feedback, the R35 relies on a series of electrical sensors and hydraulically actuated clutches.
Subarus symmetrical approach to AWD evenly distributes its engine and drivetrain component
If there were any vehicle that could claim to be designed from the ground up based upon the AWD platform, the Subaru is it. Perhaps that's because Subaru, until recently, made nothing but AWD vehicles.
The beauty of symmetrical AWD, although similar to FWD systems, is its balanced distribution of weight. Symmetrical layouts position the engine and drivetrain laterally and proportionally even, resulting in equal axle lengths and even weight distribution. Typically, symmetrical AWD vehicles also spread drivetrain weight evenly, front to rear. Unlike other AWD configurations, symmetrical applications' center differentials are often integrated into the transaxle. This helps retain optimum weight balance since there are no extraneous cases or shafts protruding from the side of the gearbox. Not unlike FWD-based systems, a driveshaft exits the transaxle and drives the rear differential.
Subaru implements more than one type of center differential, including a viscous coupling locking type and, more famously, its DCCD (Driver-Controllable Center Differential) system. DCCD is made up of two differentials: a planetary gear-type unit and an electronically controlled limited-slip type. The system allows for center differential adjustment from inside the cabin for driver-specific control. For example, the center differential can be tightened for increased traction on slippery pavement, a feature that the bro-truck has yet to adopt.