Four-wheel drive


A four-wheel drive, also called 4×4 or 4WD, is a two-axled vehicle drivetrain capable of providing torque to all of its wheels simultaneously. It may be full-time or on-demand, and is typically linked via a transfer case providing an additional output drive shaft and, in many instances, additional gear ranges.
A four-wheel drive vehicle with torque supplied to both axles is described as "all-wheel drive". However, "four-wheel drive" typically refers to a set of specific components and functions, and intended off-road application, which generally complies with modern use of the terminology.

Definitions

Four-wheel-drive systems were developed in many different markets and used in many different vehicle platforms. There is no universally accepted set of terminology that describes the various architectures and functions. The terms used by various manufacturers often reflect marketing rather than engineering considerations or significant technical differences between systems. SAE International's standard J1952 recommends only the term "all-wheel drive" with additional subclassifications that cover all types of AWD/4WD/4x4 systems found on production vehicles.

4×4

"Four-by-four" or "4×4" is frequently used to refer to a class of vehicles in general. Syntactically, the first figure indicates the total number of axle ends and the second indicates the number of axle ends that are powered. Accordingly, 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. Similarly, a 6×4 vehicle has three axles, two of which provide torque to two axle ends each. If this vehicle were a truck with dual rear wheels on two rear axles, so actually having ten wheels, its configuration would still be formulated as 6x4. During World War II, the U.S. military would typically use spaces and a capital 'X' – as "4 X 2" or "6 X 4".

4WD

Four-wheel drive refers to vehicles with two axles providing torque to four axle ends. In the North American market, the term generally refers to a system optimized for off-road driving conditions. The term "4WD" is typically designated for vehicles equipped with a transfer case that switches between 2WD and 4WD operating modes, either manually or automatically.

AWD

All-wheel drive was historically synonymous with "four-wheel drive" on four-wheeled vehicles, and six-wheel drive on 6×6s, and so on, being used in that fashion at least as early as the 1920s. Today in North America, the term is applied to both heavy vehicles and light passenger vehicles. When referring to heavy vehicles, the term is increasingly applied to mean "permanent multiple-wheel drive" on 2×2, 4×4, 6×6, or 8×8 drive-train systems that include a differential between the front and rear drive shafts. This is often coupled with some sort of antislip technology, increasingly hydraulics-based, that allows differentials to spin at different speeds, but still be capable of transferring the torque from a wheel with poor traction to one with better. Typical AWD systems work well on all surfaces, but are not intended for more extreme off-road use. When used to describe AWD systems in light passenger vehicles, it refers to a system that applies torque to all four wheels or is targeted at improving on-road traction and performance, rather than for off-road applications.
Some all-wheel drive electric vehicles use one motor for each axle, thereby eliminating a mechanical differential between the front and rear axles. An example of this is the dual-motor variant of the Tesla Model S, which controls the torque distribution between its two motors electronically.

SAE recommended practices

According to the SAE International standard J1952, AWD is the preferred term for all the systems described above. The standard subdivides AWD systems into three categories.
Part-time AWD systems require driver intervention to couple and decouple the secondary axle from the primarily driven axle, and these systems do not have a center differential. The definition notes that part-time systems may have a low range.
Full-time AWD systems drive both front and rear axles at all times via a center differential. The torque split of that differential may be fixed or variable depending on the type of center differential. This system can be used on any surface at any speed. The definition does not address the inclusion or exclusion of a low-range gear.
On-demand AWD systems drive the secondary axle via an active or passive coupling device or "by an independently powered drive system". The standard notes that in some cases, the secondary drive system may also provide the primary vehicle propulsion. An example is a hybrid AWD vehicle where the primary axle is driven by an internal combustion engine and the secondary axle is driven by an electric motor. When the internal combustion engine is shut off, the secondary, electrically driven axle is the only driven axle. On-demand systems function primarily with only one powered axle until torque is required by the second axle. At that point, either a passive or active coupling sends torque to the secondary axle.
In addition to the above primary classifications, the J1952 standard notes secondary classifications resulting in a total of eight systems, designated as:
  • Part-time nonsynchro
  • Part-time synchro
  • Full-time fixed torque
  • Full-time variable-torque passive
  • Full-time variable-torque active
  • On-demand synchro variable-torque passive
  • On-demand synchro variable-torque active
  • On-demand independently powered variable-torque active

    Design

Differentials

Two wheels fixed to the same axle need to turn at different speeds as a vehicle goes around a curve. The reason is that the wheel that is located on the inner side of the curve needs to travel less distance than the opposite wheel for the same duration of time. However, if both wheels are connected to the same axle driveshaft, they always have to spin at the same speed relative to each other. When going around a curve, this either forces one of the wheels to slip, if possible, to balance the apparent distance covered, or creates uncomfortable and mechanically stressful wheel hop. To prevent this, the wheels are allowed to turn at different speeds using a mechanical or hydraulic differential. This allows one driveshaft to independently drive two output shafts, axles that go from the differential to the wheel, at different speeds.
The differential does this by distributing angular force evenly, while distributing angular velocity such that the average for the two output shafts is equal to that of the differential ring gear. When powered, each axle requires a differential to distribute power between the left and right sides. When power is distributed to all four wheels, a third or 'center' differential can be used to distribute power between the front and rear axles.
The described system handles extremely well, as it is able to accommodate various forces of movement and distribute power evenly and smoothly, making slippage unlikely. Once it does slip, however, recovery is difficult. If the left front wheel of a 4WD vehicle slips on an icy patch of road, for instance, the slipping wheel spins faster than the other wheels due to the lower traction at that wheel. Since a differential applies equal torque to each half-shaft, power is reduced at the other wheels, even if they have good traction. This problem can happen in both 2WD and 4WD vehicles, whenever a driven wheel is placed on a surface with little traction or raised off the ground. The simplistic design works acceptably well for 2WD vehicles. It is much less acceptable for 4WD vehicles, because 4WD vehicles have twice as many wheels with which to lose traction, increasing the likelihood that it may happen. 4WD vehicles may also be more likely to drive on surfaces with reduced traction. However, since torque is divided between four wheels rather than two, each wheel receives roughly half the torque of a 2WD vehicle, reducing the potential for wheel slip.

Limiting slippage

Many differentials have no way of limiting the amount of engine power that gets sent to their attached output shafts. As a result, if a tire loses traction on acceleration, either because of a low-traction situation or the engine power overcomes available traction, the tire that is not slipping receives little or no power from the engine. In very low-traction situations, this can prevent the vehicle from moving at all. To overcome this, several designs of differentials can either limit the amount of slip or temporarily lock the two output shafts together to ensure that engine power reaches all driven wheels equally.
Locking differentials work by temporarily locking together a differential's output shafts, causing all wheels to turn at the same rate, providing torque in case of slippage. This is generally used for the center differential, which distributes power between the front and the rear axles. While a drivetrain that turns all wheels equally would normally fight the driver and cause handling problems, this is not a concern when wheels are slipping.
The two most common factory-installed locking differentials use either a computer-controlled multiplate clutch or viscous coupling unit to join the shafts, while other differentials are more commonly used on off-road vehicles generally use manually operated locking devices. In the multi-plate clutch, the vehicle's computer senses slippage and locks the shafts, causing a small jolt when it activates, which can disturb the driver or cause additional traction loss. In the viscous coupling differentials, the shear stress of high shaft speed differences causes a dilatant fluid in the differential to become solid, linking the two shafts. This design suffers from fluid degradation with age and from exponential locking behavior. Some designs use gearing to create a small rotational difference that hastens torque transfer.
A third approach to limiting slippage is taken by a Torsen differential, which allows the output shafts to receive different amounts of torque. This design does not provide for traction when one wheel is spinning freely, where no torque exists, but provides excellent handling in less extreme situations. A typical Torsen II differential can deliver up to twice as much torque to the high-traction side before traction is exceeded at the low-traction side.
A fairly recent innovation in automobiles is electronic traction control. It typically uses a vehicle's braking system to slow a spinning wheel. This forced slowing emulates the function of a limited-slip differential, and by using the brakes more aggressively to ensure wheels are being driven at the same speed, can also emulate a locking differential. This technique normally requires wheel sensors to detect when a wheel is slipping, and only activates when wheel slip is detected. Therefore, typically no mechanism exists to actively prevent wheel slip ; rather, the system is designed to expressly permit wheel slip to occur, and then to attempt to send torque to the wheels with the best traction. If preventing all-wheel slip is a requirement, this is a limiting design.