Drivetrain: AWD System and Differentials
by Chris Wood
This section is intended to be a FAQ on the operation of AWD systems in general and the ways in which some components are different from other components. Please let me know if there is any specific information you would like to see added, or anything here you would like me to further expand upon. I hope to supplement this with diagrams and fill it out a bit more in time to come.
--Multiades 08:09, 18 March 2006 (EST)
What is an AWD/4WD system exactly?
All-Wheel Drive (AWD) has entered the vernacular as describing a system that operates seamlessly on paved-road conditions where no slip is expected most of the time. This means that some sort of slip must be introduced by the mechanism that supplies driving torque to a pair of wheels, so that the car may turn without dragging whichever wheel must rotate slower. An AWD car typically has three "differentials" -- the front, which allows "slip" between the front pair, the rear which does so between the rear pair, and the "center" differential which mediates between the front and rear pairs. Not all methods of allowing slip fit under the heading of "differential". A "diff" may not be used at all as the front-rear mediator in an AWD system; sometimes a torque coupling attaches the primary drive wheels (direct-driven from engine) to a secondary set of wheels. Many exotic AWD sportscars employ such a design.
Four-Wheel Drive (4WD) systems are generally part-time and found on trucks which may see duty on sand or slippery surfaces. The characteristic difference between this and the above-mentioned AWD is that typically one pair of wheels (almost always rear) is driven all of the time, and the other pair can be locked to the driven pair by a lever within the driver's reach. There is a front differential and a rear differential, but no differentiation allowed between them front-rear when the system is engaged. The engagement lever usually controls a separate secondary transmission with a choice between "low" gears and "high" gears. The terminology difference between "AWD" and "4WD" is primarily that of implied or accepted usage, obviously both systems technically drive "all" or "four" wheels.
Why do we need differential action?
Differential action, or rotational differentiation, is necessary because in any sort of turn the four corners of the car are moving along slightly different paths -- each outside wheel rotates faster than the inside wheel opposite it, and the average speed of the front wheels is faster than that of the rears. In its most generic form, a "differential" is a mechanical device that accepts rotational input which it splits between two outputs. The sum of the torques output must be equal to the total torque input. The sum of the absolute rotational speeds of the outputs is twice the speed of the input. The total power output is (obviously) never more than the total power input.
- False: A welded/locked differential provides 50:50 torque split.
- False: An "open" differential "burns up" all of the supplied torque on whichever wheel is spinning.
- False: A planetary differential splits 50:50 except during wheelspin.
The above I have heard from many different people, some of whom are passing on what they've heard elsewhere, as well as many who have put thought into the workings of each system. I feel the misunderstanding often comes from the tendancy to look at speed, power, and torque as the same thing. If the front wheels cannot differentiate with respect to the rear wheels in the case of AWD/4WD with a solid coupling, mustn't torque be equal between the pairs? Here I describe why not.
What sort of differentials are available?
"Open" Differentials do not practically restrict the rate of rotation of one output relative to the other, save for unavoidable internal friction of parts. The two major types of differentials that may be "open" are the common bevel-gear differential, which has been in near-universal use almost since the beginning of the automobile, and the spur-gear or planetary differential. Open differentials interfere least with steering feel and are often used at the front of FWD or AWD vehicles. Traditional bevel-gear differentials have a 50%-50% torque split (they operate at a permanent 1:1 ratio) at all times, while planetary gearsets can have any virtually any fixed ratio of output torques based on their gear ratios. Often this characteristic of planetary differentials is used to give uneven front-rear torque split in production AWD systems -- for example the 3/S planetary center differential provides a static front-rear torque split of 45:55, while the WRX STi employs one designed around a 35:65 ratio. Because these two examples also have limited-slip components (described below), torque distribution will vary in response to slip, but the basic non-slip characteristics (static torque split) established are intrinsic to all "differentials".
Limited-Slip Differentials (LSDs) are generically a bevel-gear or planetary differential with additional components that restrict the amount of differentiation that will occur. The simplest and most widely available systems use simple friction to bind both outputs together in response to different rates of slip between the outputs.
Couplings can be used either to join the outputs of a differential (bevel-gear or planetary) together so that when one wheel slips the opposite receives more torque, or simply to transfer torque from a driven wheelset in order to drive some other wheelset in turn. In the latter configuration there is no torque transfer to the wheelset controlled by the coupling until slip occurs, because coupling torque occurs primarily in proportion to difference in rate of rotation. In systems that use a coupling in place of a gear-differential, (such as Porsche AWD) manufacturers will often change the gear ratio of the coupled drive components to cause the coupling to friction at all times and transfer some torque.
Some limited-slip differentials use interlocking friction surfaces that attach to each output, and are compressed together by opposing plates. The clutch discs are immersed in the same gear oil that lubricates the gears to mitigate heat and wear. These plates usually have clamping preload and sometimes have a mechanism that further compresses them as additional torque is applied to the input (the "Salisbury" type). Clutches limit slip in a manner basically linear to difference in rotating velocity.
Viscous Coupling Units (VCU) depend on the fluid friction of exotic liquids in a sealed housing of interlocking perforated discs. The fluid is typically a non-Newtonian "dilatant" fluid, which is shear-thickening (apparent viscosity increases with shear), and so the coupling torque increases at a non-linear rate to speed of rotation.
Automatic Torque Biasing (ATB) or Torque Sensing (Torsen) differentials can be considered a separate class of differential from either the "open" or basic friction LSD. They are commonly sold by the trade name Zexel/Gleason Torsen, Quaife, or as the featured component in Audi "Quattro" drivetrains. This differential utilizes friction to limit slip also, but relies on friction dictated by the contact angles of carefully-engineered interlocking gears. It is not necessary to understand the mechanism to understand the characteristics of this differential; the unit differentiates with almost no resistance when power is not being applied, but can distribute supplied torque based on a "bias ratio" in response to available traction. If the bias ratio of a Torsen is 3:1, a common value, torque will be unevenly distributed by as much much as 3x the torque of the output with least traction -- in other words it can apply torque anywhere from 75%-25% to 25%-75% based on available traction and independant of slip. Because it operates like an open differential, a Torsen is generally unintrusive and can be found in high-end applications where drivability is paramount, such as the 3rd-generation RX-7, the 1996-present Miata, and the current Lancer Evolution (front-only). Its ability to function like an open differential makes it ideal for the front-drive components of an AWD system. The caveat is that in extreme circumstances where one wheel has ZERO resistance to spinning (say, it's off the ground), the opposite wheel will receive ZERO torque (3 * 0 = 0). This characteristic can be mitigated by applying the brakes to increase resistance, so that the differential may multiply the brake torque of the airborne wheel times the bias ratio to the opposite output, a technique taught to military operators of the HMMWV, which has front and rear Torsen differentials.
A "spool" is not a differential at all -- this term is merely a fancy way of saying that two output driveshafts are permanently locked together in rate of rotation. This component has no place on a proper AWD system, and even rally cars avoid them despite being designed to run on loose, slippery surfaces. The characteristic of a spool is to provide more torque to whichever side is rotating slower. This also means that if one side is on ice (or airborne, for whatever reason), the other side is assured of getting basically 100% of the available torque. Because neither output can slip and "run away" from the other output, and in straight-line acceleration where no differentiation is necessary, each output receives exactly as much torque as there is traction on that axle. This means that a spool will distribute torque 100:0 to 0:100 based solely on demand when the controlled wheels (or pairs) are traveling the same distance paths. In a straight line a spool will apply torque wherever traction is found, and in exactly the right proportion necessary to use all of it. If you have a welded center differential and can "launch" hard enough to pull the front wheels off the ground, guess what? You're RWD for that period of time, assuming nothing blows up. If you tried the same thing with an open differential based on the characteristic output ratio of unity (1:1), you would apply zero torque to the opposite pair of wheels (1 * 0 = 0). Technically you would never be able to apply enough rear torque to lift the front, because the front would slip at some lower value.
The above covers all common passive differentials. Later I may add a section on the many fascinating, creative, or just plain cheap active configurations employed by various manufacturers.