Layman's Guide to Four Wheel Drive Systems
Four-wheel-drive (4WD) systems used to be reserved for trucks. They were designed mainly for rugged off-road use when standard 2-wheel-drive (2WD) powertrains would simply lead to a stuck truck. While early 4WD systems were rather crude in operation (the driver had to manually engage the front wheels on most part-time systems), they have come a long way. What was once a simple 2- or 4-wheel-drive question now requires a bit more insight as to the operation-and purpose-of today's 4WD systems.
Today, there are four basic systems that send engine power to all four wheels of the vehicle: all-wheel drive (AWD), part-time AWD, part-time 4WD, and full-time 4WD.
Full-Time 4-Wheel Drive
Full-time 4WD is the original system that dates back nearly a century. Today, its use is mostly limited to vehicles that are designed for serious off-road duty. This drivetrain configuration traditionally sends engine power equally to all four wheels-but some modern variants are able to split the engine's twisting power (called "torque") to the wheels that need it the most.
Differentiating true 4WD systems from nearly all of the other types is the fact that they come equipped with a special secondary "low" gearing ratio for driving very slowly in rugged terrain. Many also feature locking differentials (these systems prevent one wheel from slipping if another wheel has traction). While the 4WD is extremely capable off-road and during inclement weather, the weight of these systems (which negatively affects fuel economy) and the fact that they are always engaged means that these types of systems aren't for everyone.
Part-Time 4-Wheel Drive
Part-time 4WD addresses some of the fuel-economy penalties found with true 4WD systems. While the original 4WD system sends power to all wheels at all times, part-time 4WD systems remain in rear-wheel-drive mode until additional traction is needed. This reduces unnecessary drag on the driveline and helps to minimize fuel economy penalties.
The change from 2- to 4-wheel drive is done via electronic, mechanical, or hydraulic switching-often by the driver from a switch or lever in the cabin. Nearly all part-time 4WD systems are fitted with the same "low" gearing ratio and locking differentials as dedicated 4WD systems. However, most part-time 4WD systems cannot be engaged on dry pavement due to concerns about damaging the mechanical components. The part-time 4WD system is more fuel efficient, but that benefit does come at the price of flexibility.
Part-Time All-Wheel Drive
Part-time AWD systems work just like front-wheel-drive powertrains until traction is lost-then the system automatically sends power to the rear wheels for additional traction. This type of system must wait until the front wheels begin to slip before electronics (or hydraulics) automatically switch to AWD mode. This can be a slight disadvantage when starting from a standstill. The benefits of part-time AWD include low cost, less weight, and increased efficiency, making it ideal for entry-level or compact vehicles. As expected, part-time AWD systems are not designed for heavy off-road use (they do not come equipped with "low" gearing). Regardless, part-time AWD systems are good for use during inclement weather or to have in reserve for an emergency.
All-wheel drive systems are among the most sophisticated drivelines today. Engine power is sent to all four wheels based on demand-as determined by electronic or hydraulic sensors. While this does add cost to a vehicle, automakers prefer AWD systems as they allow the engineers to "tune" the handling of the vehicle through the powertrain-more power can be sent rearward to mimic the sporty handling attributes of a rear-wheel-drive vehicle, if desired.
Many of today's sophisticated AWD systems are even able to send all of the engine's power to one wheel if that's where the traction is-meaning an AWD vehicle can outperform a 4WD vehicle under some conditions. As expected, there are a few drawbacks to AWD. These include the aforementioned additional weight over a 2WD powertrain, and the fact that they are primarily only suited for on-road travel. In addition, increased mechanical complexity and potential for additional maintenance as the vehicle ages are also factors.
Unlike many vehicle systems that evolve and disappear over the years as new technology makes them obsolete (tungsten headlights, bias-ply tires, etc.), the basic age-old 4WD system still lives on today. However, modern electronics have allowed engineers to develop more efficient systems-with greatly improved traction-without the penalties and shortcomings found in traditional systems. Quite simply, there is no "best" four- or all-wheel-drive system-each is suited to its own individual role, based on the vehicle's intended use and design.