(1980)
Volkswagen Group has been developing
four-wheel drive (4WD) systems almost since its inception during the
Second World War. The
Volkswagen Kübelwagen,
Volkswagen Schwimmwagen, and
Volkswagen Kommandeurswagen were all
military vehicles which required all four road wheels to be "driven", the latter being a 4WD
Volkswagen Beetle. Their military and four-wheel drive experiences later aided them in designing the
Volkswagen Iltis for the
German military (Bundeswehr) in the 1970s. The Iltis utilized an early form of 4WD, which would later become synonymous with "quattro".
Locking centre differential In that original quattro system, later found in road-going passenger cars, the
engine and
transmission are situated in a
longitudinal position.
Torque is sent through the transmission to a mechanical centre
differential (commonly abbreviated to "diff") which apportions (distributes) the torque between front and rear driven
axles. 4WD was permanently active.
The Torsen T1 centre differential After 1987, Audi replaced a manually locking centre differential with the
Torsen (torque sensing) Type 1 ("T1") centre differential. This allowed engine torque to be automatically directed to individual axles as driving conditions and grip warranted. Under 'normal' conditions (where grip in both front and rear
axles is equal), torque is split between front and rear with a 'default" 50:50 distribution in many, though not all, versions. In adverse conditions (i.e., when there is variation in grip between front and rear), a maximum of 90-100% (depending on the transmission, or model of Torsen diff) of the engine's torque can be directed to the front or rear axles. The fully automated mechanical nature of the Torsen centre differential helps prevent
wheel slippage from occurring, by diverting torque instantly, without any discernible notice to the vehicle occupants, to the axle which has more grip. This method of operation can be described as proactive. Furthermore, unlike the various types of electronically operated differentials, Torsen has no requirement for electronic data from sources such as
road wheel speed sensors; it, therefore, has an element of "
fail-safe", unlike designs such as
Haldex Traction, should one of the wheel speed sensors develop a fault. In comparison,
viscous coupling and electronically controlled centre differentials that are used in other four-wheel drive systems are reactive, since they only redirect torque
after wheel slippage has occurred. The advantage is felt under hard
acceleration, including whilst
cornering, since the torque transfer between axles is seamless, thus maintaining stable vehicle dynamics, and considerably reducing the chance of losing control of the vehicle. quattro
Torsen centre differential The Torsen-based quattro system also offers an advantage in the opposite function of distributing torque to the road wheels, namely
engine braking. When engine braking is used to slow the car down, with Torsen-based systems, the resulting "reverse-torque" loads on the front and rear axle are equally stabilized, in exactly the same way that engine "propulsion" torque is apportioned fully mechanically autonomously. This allows the spreading of the engine braking effect to all four wheels and tyres. The Torsen-based quattro-equipped vehicle is able to execute a more stable high-speed turn under deceleration, with less risk of losing control due to loss of grip in the front or rear axles. This configuration of the quattro system, however, does have some limitations: • With placement of the engine and transmission assembly in a fore/aft position (longitudinally), the front axle is placed rearwards behind the engine, which leads to the criticism of some Audi vehicles as being nose heavy. This results in a weight distribution of 55:45 (F:R). • The nature of the Torsen is akin to that of a limited slip differential in that, rather than actively allocating torque (as a computer controlled clutch can do), it supports a torque difference across the differential (the torque bias ratio/TBR), from the side with the least grip to the side with the most. Hence by nature the Torsen is limited in the amount of torque that can be supplied to the axle with the
most grip by the torque available at the axle with the
least amount of grip. Therefore, if one axle has no grip, regardless of the TBR, the other axle will not be supplied substantial torque. In the extreme, for a centre differential implementation, complete loss of traction on a single wheel will result in very limited torque to the other three wheels. Audi responded to this limitation for the first Torsen-equipped cars by adding a manually locking rear differential and then later replaced this feature with Electronic Differential Lock (EDL), which is the ability to use the individual wheel brakes (monitored by the ABS sensors) to limit individual wheel spin. EDL was implemented across both front and rear (open) differentials to operate at speeds less than 80 km/h. This has the effect of increasing torque from a single low-traction wheel hence allowing more torque to be passed by the Torsen to the remaining high-traction wheels. • While the standard (Type 1 or
T1) Torsen supports a static torque ratio of 50:50; i.e., input torque is supported equally across both output shafts, the T1 has a Torque Bias Ratio (TBR) of 2.7–4:1; i.e., it allows about 3 to 4 times the torque to be supplied to the most tractive output shaft than that is available on the least tractive shaft or, a torque split of between 25% and 75%. However, by nature the T1 Torsen is locked under most circumstances (output shafts locked together). Only when the TBR is reached (i.e., there is a greater torque difference across the output shafts than can be supported by the TBR) do the output shafts turn relative to each other, and the differential unlocks. This characteristic results in a relatively free torque movement between both outputs of the (centre) differential, within the limits of the TBR. Thus the static torque distribution of the T1 Torsen in a centre differential installation, rather than being 50:50, will mirror the weight distribution (both static and dynamic) of the vehicle due to the traction available at either (front:rear) output shaft. In a standard car, this is desirable from the perspective of stability, acceleration and traction, but can be undesirable in terms of handling (understeer). While the standard quattro Torsen T1 with 2.7:1 TBR is more than sufficient in most conditions, Torsen T1 differentials with higher TBRs (4:1) are available and can further limit understeer by supporting a wider torque split. A better solution, however, is to apportion a torque split directly between both output shafts (front & rear) and for this reason Audi has adopted the Type 3 (
T3) Torsen design in the latest generations of quattro.
The Torsen type "C" (T3) The Torsen T3 centre differential combines a planetary gear set with a Torsen differential in a compact package developed for centre differential installations. Unlike the T1 Torsen where the torque split is a nominal 50:50, in the T3 Torsen the torque split, due to the use of the planetary gear set, is an asymmetric 40:60 front-rear torque split (i.e., when grip is equal on both front and rear axles, 40% of torque is sent to the front axle, and 60% to the rear). As with the T1 Torsen, torque will be distributed dynamically depending on tractive conditions, but with an actual (rather than nominal) static bias. The T3 allows handling characteristics and vehicle dynamics more akin to
rear-wheel drive cars. This asymmetric Torsen was first introduced in the highly acclaimed 2006-model (B7)
Audi RS4. The Type 3 torsen was used in the Audi S4 and RS4 B7 manual transmissions from 2006 to 2008, as well as the S6, S8, and Q7 models from 2007. The torque split across axles and between left and right wheels has been achieved through the various evolutions of the quattro system, through a driver-selectable manually locking differential (rear axle only), and eventually through open differentials with Electronic Differential Lock (EDL). EDL is an electronic system, utilising the existing
anti-lock braking system (ABS), part of the
Electronic Stability Programme (ESP), which brakes just the one spinning wheel on an axle, therefore allowing the transfer of torque across the axle to the wheel which does have traction.
Crown-wheel centre differential Audi debuted a new generation of quattro in the 2010 RS5. The key change is the replacement of the Torsen Type "C" centre differential with an Audi-developed "Crown Gear" differential. Whilst this is superficially the same as a normal open differential, adapted for a centre application, it has some key differences: • The central carrier and associated spider gears interface directly to two crown wheels connected to the front and rear drive shafts • The two crown wheels interface to the spider gears at different diameters, and so produce different torque when turned by the spider gears. This is engineered to produce a 40:60 static torque split front and rear. • Each crown wheel interfaces to the respective output shaft directly, whilst the spider carrier interfaces to each output shaft using a clutch pack which gives the unit the ability to control torque distribution over and above the static torque distribution. If one axle loses grip, different rotational speeds arise inside the differential which result in an increase in axial forces that force the clutch plates to close. Once closed, the output shaft is locked resulting in the diversion of the majority of the torque to the axle achieving better traction. In the Crown Gear differential up to 85% of torque can flow to the rear, and up to 70% of torque can be diverted to the front axle. The features of the Crown Gear differential provide the following benefits over the Torsen Type "C" • The ability to set up a more stable torque distribution, with full locking whereas the Torsen can only provide a torque distribution up to the Torque Bias Ratio; i.e., the Crown Gear differential can lock fully, regardless of bias ratio. Unlike the Torsen, the Crown Gear differential does not operate like a limited slip differential and can operate, fully locked, with no traction on one output shaft. • Easier integration into control electronics allowing four-wheel electronic torque vectoring with or without the active rear sport differential • Considerable reduction in size and weight (at 4.8 kg, some 2 kg lighter than the Torsen Type C) The net result of this advance in quattro is the ability of the vehicle electronics to fully manage the vehicle dynamics in all traction situations, whether in cornering, acceleration or braking or in any combination of these.
Evolutions Audi has never officially debuted quattro in specific generations. Changes to quattro technology have generally been debuted with a specific range or model in the range and then brought into other models during appropriate points in the model cycle. The exception to the above was the debut of the 2010 RS5 which was, amongst other things, heralded by Audi as the debut of a "new generation of quattro".
quattro generation I Used from 1981 to 1987 in
Audi Quattro turbo coupé,
Audi 80 B2 platform (1978–1987, Audi 4000 in
North American market),
Audi Coupé quattro B2 platform (1984–1988),
Audi 100 C3 platform (1983–1987, Audi 5000 in North American market). Also, starting from 1984, used on the Volkswagen
VW Passat B2 platform (
VW Quantum in the US market) where it was known as
Syncro. System type: Permanent
four-wheel drive. Open centre
differential, manually lockable via switch on centre console.¹ Open rear differential, manually lockable via switch on centre console.¹ Open front differential, no lock. ¹
ABS disabled when locked. How the system performs: When all differentials are unlocked, the car will not be able to move if one wheel (front or rear) loses traction (is on ice or raised in the air). When the center differential is locked with the rear differential unlocked, the car will not be able to move if one front wheel and one rear wheel lose traction. When the rear differential is locked with the center unlocked, the car will not be able to move if both rears or one front loses traction. When both center and rear differentials are locked, the car will not be able to move if both rears and one front lose traction.
quattro generation II Starting from 1986 on new generation B3 platform (1986–1991)
Audi 80/90 quattro, on older generation
Audi 100 C3 platform and
Audi Quattro until the end of their production, and B4 platform (1991–1995)
Audi 80,
Audi S2,
Audi RS2 Avant, C4 platform (1991–1994)
Audi 100 quattro,
Audi S4. System type: Permanent
four-wheel drive.
Torsen centre
differential, 50:50 'default' split, automatically apportioning up to 71.5% of
torque transfer to either axle. Open rear differential, manually lockable via switch on centre console located next to handbrake.¹ Open front differential, no lock. ¹
ABS disabled when locked, automatically unlocks if speed exceeds .
quattro generation III Used only on the
Audi V8 starting from 1988 to 1994. System type: Permanent/ Part-time
four-wheel drive. '
V8 with automatic
transmission:'
Planetary gear centre differential with electronically controlled multi-plate locking clutch
Torsen type 1 differential rear. Open differential front. '
V8 with manual
transmission:'
Torsen type 1 centre differential. Torsen type 1 rear differential. Open front differential. How does the system perform: In on-road conditions the car will not be able to move if one front and both rear wheels lose traction altogether.
quattro generation IV Starting from 1995 on
Audi A4/
S4/
RS4 (B5 platform),
Audi A6/
S6/
allroad/
RS6,
Audi A8/
S8 with both manual and automatic transmissions. Also on
VW Passat B5, where it was initially referred to as
syncro, but by the time it reached US soil, it had been re-christened
4motion. Also used on the
Volkswagen Phaeton and
Volkswagen Group D platform sister vehicles. The
Volkswagen Touareg used 4Xmotion with a separate transmission,
PTUs and front axles. The manually locking rear differential from the earlier generations was replaced with a conventional open differential, with Electronic Differential Lock (EDL) (which detects wheelspin via ABS road wheel speed sensors, and applies brakes to one spinning wheel, thus transferring torque via open differential to the opposite wheel which has more traction). EDL works at speeds up to , on all quattro models (on non-quattro models: up to ). System type: Permanent
four-wheel drive.
Torsen type 1 centre differential, 50:50 'default' split, automatically apportioning up to 75% of torque transfer to either front or rear axle. Open rear differential, Electronic Differential Lock (EDL). and is being offered on Audi A4, A5, A6 and their derivatives (including
RS models). The
Sport Differential selectively distributes torque to the rear axle wheels thereby generating a yaw moment, which improves handling and also stabilizes the vehicle when it oversteers or understeers, thereby increasing safety. The sport differential operates by using two superposition ("step up") gears at the differential, which are operated via multi-plate clutches each side of the differential crown wheel. When required by the software (using lateral and longitudinal yaw sensors, the ABS wheel sensors, and a steering wheel sensor), the control software (located in a control unit close to the rear differential), actuates the relevant clutch pack. This has the effect of taking the output shaft drive through the step-up gear to the attached wheel, while the other shaft continues to drive its wheel directly (i.e., the clutch pack not actuated). The higher speed output shaft produces increased torque to the wheel, producing a yaw (turning) moment. In normal operation increased torque is delivered to the wheel on the outside of the turn thereby increasing the vehicles turning moment, in other words, its willingness to turn in the direction pointed by the steering wheel.
quattro generation VI Audi debuted the 6th generation of quattro in the 2010 RS5. The key change in generation VI is the replacement of the Torsen Type "C" centre differential with an Audi-developed "Crown Gear" differential. With the new "Crown Gear" center differential, up to 70% of the torque can be applied to the front wheels while up to 85% can be applied to the rear wheels if necessary. The net result of this advance in quattro is the ability of the vehicle electronics to fully manage the vehicle dynamics in all traction situations, whether in cornering, acceleration, braking, snow or in any combination of these. This system was later adopted by the A7, latest generation of the A6 and A8.
BorgWarner The
Audi Q7 (First generation), the platform-mate of the
Volkswagen Touareg and
Porsche Cayenne, does not use the same underpinnings of either previous model.
BorgWarner instead provides the 4WD system for this more off-road appropriate SUV. A Torsen Type 3 (T3) differential is used.
Ultra Audi announced "Audi Quattro with Ultra Technology" in February 2016, it is a front-wheel drive biased system for use on platforms with longitudinally mounted engines. ==Transverse systems==