Quattro (four-wheel-drive System) - Longitudinal Systems

Longitudinal Systems

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 Kommandeurwagen 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".

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.

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 67-80% (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.

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 stabilised, 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:

1. 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, though the system still leads to better weight distributions than transverse mounted engine packages as in Mitsubishis and similar cars. This allows for a better weight distribution of 55:45 (F:R).
2. 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 < 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.
3. 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% to 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 T3 centre differential combines a planetary gear set with a Torsen differential in a compact package developed for centre differential installations. Unlike the T2 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 actual 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. It was then used in the 2006 manual transmission and 2007-model for B7 for both transmission types.S4. as well as the S5 and Q7. It was used in longitudinal-engined quattro-equipped cars (A4, A6, A8, Q7) until replaced for some applications by the "Crown-Gear" centre differential.

The torque split across axles, 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.

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:

1. The central carrier and associated spider gears interface directly to two crown wheels connected to the front and rear drive shafts
2. 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.
3. 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"

1. 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 doesn't operate like a limited slip differential and can operate, fully locked, with no traction on one output shaft.
2. Easier integration into control electronics allowing four-wheel electronic torque vectoring with or without the active rear sport differential
3. 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.