09-11-2012, 03:29 PM
Four-wheel drive
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Terminology
The term four-wheel drive describes truck-like vehicles that require the driver to manually switch between two-wheel drive mode for streets and four-wheel drive mode for low traction conditions such as ice, mud, snow, slippery surfaces, or loose gravel.[2]
All-wheel drive (AWD) is often used to describe a "full time" 4WD that may be used on dry pavement without destroying the drivetrain, although the term may be abused when marketing a vehicle.[3] AWD can be used on dry pavement because it employs a center differential, which allows each tire to rotate at a different speed. ("Full-Time" 4WD can be disengaged and the center differential can be locked, essentially turning it into regular 4WD. On the other hand, AWD cannot be disengaged and the center differential cannot be locked.[4][5]) This eliminates driveline binding, wheel hop, and other driveline issues associated with the use of 4WD on dry pavement. With vehicles with more than four wheels, AWD means all wheels drive the vehicle, to varying degrees of engagement, while 4WD means only four of the wheels drive the vehicle continuously.
Identical drivetrain systems are commonly marketed under different names for upmarket and downmarket branding and, conversely, different drivetrain systems are commonly marketed under the same name for brand uniformity. Audi's quattro, DaimlerChrysler's 4Matic used on Mercedes-Benz products, BMW with the xDrive, and Volkswagen's 4motion, for example, can mean either an automatically-engaging "on-demand" system with a Haldex Traction clutch, or a continuously-operating permanent 4WD system with a Torsen (torque-sensing) differential.
[edit] Design
Differentials
The Lamborghini Murciélago is a 4WD/AWD that powers the front via a viscous coupling unit if the rear slips.
The HMMWV is a 4WD/AWD that powers all wheels evenly (continuously) via a manually lockable center differential, with Torsen differentials for both front and rear.
A Subaru Impreza rally car uses AWD for traction on loose dirt.
When powering two wheels simultaneously the wheels must be allowed to rotate at different speeds as the vehicle goes around curves. The problem is even more complicated when driving all four wheels. A design that fails to account for this will cause the vehicle to handle poorly on turns, fighting the driver as the tires slip and skid from the mismatched speeds.
A differential allows one input shaft to drive two output shafts with different speeds. The differential distributes torque (angular force) evenly, while distributing angular velocity (turning speed) such that the average for the two output shafts is equal to that of the input shaft. Each powered axle requires a differential to distribute power between the left and the right sides. When all four wheels are driven, a third differential can be used to distribute power between the front and the rear axles.
Such a design handles well. It distributes power evenly and smoothly, and makes 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 will spin faster than the other wheels due to the lower traction at that wheel. Although the amount of torque applied to each wheel will be identical, the amount of traction at each driven wheel will be limited to that of the wheel with the least traction (at least one wheel on ice in this case). 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 to lose traction, increasing the likelihood that it will happen. 4WD vehicles may also be more likely to be driven on surfaces with reduced traction.
Limiting slippage
Traction control was invented to solve this problem for 2WD vehicles. When one wheel spins out of control the brake is automatically applied to that wheel. By preventing one wheel from spinning freely power is divided between the pavement for the non-slipping wheel and the brake for the slipping wheel. This is an effective solution, although it causes additional brake wear and may cause a sudden jolt that affects handling. By extending traction control to act on all four wheels the simple three-differential 4WD design will see limited wheel spin. This design is commonly seen on luxury crossover SUVs.
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 multi-plate clutch or viscous coupling unit to join the shafts, while other differentials 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 which hastens torque transfer.
A third approach to limiting slippage is the Torsen differential. A Torsen differential allows the output shafts to receive different amounts of torque. This design does not provide for traction when one wheel is spinning freely, where there is no torque. It 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 lower tractive side.
Finally, many lower-cost vehicles entirely eliminate the center differential. These vehicles behave as 2WD vehicles under normal conditions. When the drive wheels begin to slip, one of the locking mechanisms discussed above will join the front and rear axles. Such systems distribute power unevenly under normal conditions and thus do not help prevent the loss of traction, instead only enabling recovery once traction is lost. Most minivan 4WD/AWD systems are of this type, usually with the front wheels powered during normal driving conditions and the rear wheels served via a viscous coupling unit. Such systems may be described as having a 95/5 or 90/10 power split.
[edit] History
The 1903 Spyker 60 H.P. 4WD
Center transfer case sending power from the transmission to two output shafts: to the rear axle (visible on the right) as well as to the front axle (on the left side).
Selection lever
The true inventor of four-wheel drive is not really known; the history of such was not well recorded. In 1893, before the establishment of a modern automotive industry in Britain, English engineer Joeseph Bramah Diplock patented a four wheel drive system for a traction engine, including four-wheel steering and three differentials, which was subsequently built. The development also incorporated Bramagh's Pedrail wheel system in what was one of the first four-wheel drive automobiles to display an intentional ability to travel on challenging road surfaces. It stemmed from Bramagh's previous idea of developing an engine that would reduce the amount of damage to public roads.