Quattro (all wheel drive system)
From Wikipedia, the free encyclopedia
- The correct title of this article is quattro. The initial letter is shown capitalized due to technical restrictions.
quattro is a permanent all wheel drive (AWD) system used on Audi brand automobiles. The quattro system was first introduced in 1980 on the Audi Quattro and has since been deployed to most of the models that Audi sells. The name is also a registered trademark of Audi AG.
Longitudinal quattro Systems
In the original quattro system, the engine and transmission are situated in a longitudinal position. Power is sent through the transmission to a mechanical center differential which splits it between the front and rear axles.
After 1987, the inclusion of a Torsen (torque-sensing) center differential, instead of a manually-locking one, allowed engine power to be redirected to individual axles as driving conditions warranted. Under ideal conditions (i.e., grip is equal in both front and rear axles) power is split 50/50 front to rear. In extreme conditions (i.e., when grip varies from front to rear), a maximum of 66-80% (depending on the transmission or model) of the engine's power can be directed to the front or rear axles. The mechanical nature of the Torsen centre differential helps prevent wheel slippage from ocurring by diverting power to the axle that has more grip. In comparison, viscous coupling and electronically controlled differentials that are used in other all-wheel drive systems are reactive since they only redirect power once wheel slippage has occurred. The advantage is felt under acceleration while turning since the power transfer between axles is less sudden, reducing the chance of spinning.
The Torsen-based quattro system also offers an advantage in the opposite function of distributing power to the wheels: engine-braking. When engine-braking is used to slow the car down, the resulting loads on the front and rear axle are stabilized by the Torsen system in exactly the same way that engine power is diverted - mechanically. This allows the spreading of the engine-braking load on the tires among the four wheels. The Torsen-based quattro-equipped vehicle is able to execute a more stable high-speed turn under deceleration with less risk of spinning due to loss of grip in the front or rear axles.
Additionally, this configuration greatly reduces torque-steer due to the equal length drive-shafts on the front axle. However this configuration of the quattro system does have some limitations. With placement of the engine-transmission assembly in a fore/aft position (longitudinally) there arises the need to place the front axle well behind the engine, leading to a common criticism of Audi vehicles: being nose-heavy. In other words, the ideal 50/50 weight distribution cherished by many driving enthusiasts is not possible. Recently the centre differential has been adapted to a 40/60 front/rear static split (i.e. when grip is equal on both front and rear axles), allowing more rear-wheel-drive-like handling characteristics. This was first introduced in the 2006-model RS4 and shortly thereafter in the Q7 SUV. It is planned to be incorporated in all future quattro-equipped cars which use longtitudinal-mount engines.
The power split between left and right wheels has progressed in the quattro system through various combinations of open, limited slip, and eventually electronically locking differentials. See quatto generations below.
Transverse quattro Systems
In some recent quattro-equipped models using a transverse engine and transmission positioning (i.e., TT Coupe and Roadster, and A3), power is sent primarily to the front axle. As a result, quattro badged cars using this configuration cannot be said to have a "permanent" all-wheel drive system. A Haldex electrohydraulic center limited-slip "coupler" (i.e. not a differential in the true sense), may divert up to 50% of the power to the rear axle as conditions warrant. The power split between left and right wheels is achieved through limited slip differentials with electronic locking. This system is also available in A-platform Volkswagen, Seat, and Skoda vehicles, such as the Golf R32, under the name 4motion, on which the subjected Audi models are based. The exception among Volkswagen models would be the B5 Passat, which uses the aforementioned Torsen-based system.
An advantage of the Haldex-based quattro system is slightly increased fuel economy due to the decoupling of the rear axle when not needed, which reduces driveline friction. Also, the location of the centre "differential" in the rear axle moves the front/rear weight distribution closer to 50/50.
A disadvantage of this system is that engine-braking is only effected by the front wheels, reducing stability when turning and decelerating. As well, due to the reactive nature of the Haldex-based quattro, slightly more slippage is allowed before the power transfer to the rear axle occurs.
Contents |
[edit] quattro generations
This article describes quattro all wheel drive systems used in mass production V.A.G. vehicles since 1981.
[edit] quattro generation I
Where: Used from 1981 to 1987 in Audi Quattro turbo coupe, Audi 80 B2 platform (1978-1987), Audi 100 C3 platform (1983-1992), Audi 4000, Audi 5000 in the American market.
What: Permanent all wheel drive.
Open center differential, manually lockable via switch on center console. ¹
Open rear differential, manually lockable via switch on center console. ¹
Open front differential, no lock...
¹ - ABS disabled when locked.
How: When two differentials are locked, up to 100% of torque can be transferred to either axle. The car will not move if one front wheel and both rear wheels lose traction.
Pros: A true solid 4x4, very good for off-road.
Cons: not good for everyday use, needs driver intervention.
[edit] quattro generation II
Where: Starting from 1988 on older generation Audi C3 platform and Audi Quattro turbo coupe until the end of their production and on new generation B3 platform (1989-1992) Audi 80/90 Quattro, B4 platform (1992-1995) Audi 80, Coupe Quattro, Audi S2, Audi RS2, C4 platform (1991-1997) Audi 100 Quattro, Audi S4, earlier C4 platform (1995-1998) Audi A6/S6.
What: Permanent all wheel drive.
Torsen center differential, 50/50 split, automatically locking with up to 80% of torque transfer to either axle.
Open rear differential, manually lockable via switch on center console located next to handbrake. ¹
Open front differential, no lock.
¹ - ABS disabled when locked, automatically unlocks if speed exceeds 25 km/h (15 mph)
How: When rear differential is manually locked the car will not move if one front wheel and both rear wheels lose traction, but this is valid if all wheels are on the ground. Note that due to the constructive feature of the Torsen (TORque SENsing) differential if no torque is sensed on one axle the Torsen works as open differential e.g. if one front wheel is raised in the air the Torsen differential will not lock, all power is transferred to the wheel in the air and the car will not move.
Pros: All weather all wheel drive with automatic torque distribution front/rear, manual locking of rear differential is required only in very severe conditions.
Cons: If one front wheel happens to be in the air spinning freely, the all wheel drive will not work.
[edit] quattro generation III
Where: Used on Audi V8 starting from 1990.
What: Permanent all wheel drive
V8 with automatic transmission:
Planetary gear center with electronically-controlled multi-plate locking clutch
Torsen differential rear.
Open differential front.
V8 with manual transmission:
Torsen differential middle.
Torsen differential rear.
Open differential front.
How: The car will not be able to move if one front and both rear wheels lose traction. The Torsen effect with a front wheel raised in the air will not happen on V8 with automatic transmission. If front axle loses traction the multi-plate clutch is automatically locked transferring up to 100% of torque to the rear axle.(?)
Pros: All weather full-time all wheel drive, no driver intervention.
Cons: no major flaws.
[edit] quattro generation IV
Where: Starting from 1996 on Audi A4/S4, Audi A6/S6, Audi A8/S8 with both manual and automatic transmissions. Also on VW Passat B5 4motion. Manually locking rear differential replaced by Electronic Differential Lock (Difflock imitation, detects wheelspin via ABS sensors and applies brakes to spinning wheels thus transferring torque via open differential to another wheel which has more traction). EDL works at speeds up to 40 km/h (25 mph), on more powerful versions such as S4/S6/S8 - up to 80 km/h (50 mph). In addition to EDL, Audi A8/S8 limits engine rpm when excessive wheelspin occurs.
What: Permanent all wheel drive.
Torsen center differential, 50/50 split, automatically locking with up to 80% of torque transfer to either axle.
Open rear differential, Electronic Differential Lock.
Open front differential, Electronic Differential Lock.
How: This is a superb all wheel drive system for on-road car. In on-road conditions the car will not move if all four wheels lose traction. Torsen effect with one wheel in the air will not happen on quattro IV because Electronic Differential Lock will apply brakes to spinning wheel and Torsen differential will transfer torque to rear axle.
In off-road conditions (wheels in the air and an obstacle restricting the vehicle from moving forward) the car will not move further when one front and one rear wheels lose traction. The reason of this behavior is that Electronic Differential Lock is not a replacement for mechanical differential lock and it is not able to transfer enough torque to another wheel. The car will end up spinning one front and one rear wheel with crackling EDL trying to stop them from spinning.
Pros: Superb all weather full-time all wheel drive, no driver intervention.
Cons: no major flaws.
[edit] quattro which is not quattro any more: Haldex AWD.
 |
The neutrality of this section is disputed. Please see the discussion on the talk page. |
Where: Starting from 1998 quattro has become just a trade mark and it does not indicate that all wheel drive system with Torsen differential is used on the vehicle any more. Vehicles with transverse engine mounting (Audi A3/S3 quattro, Audi TT quattro) are equipped with Haldex automatic all wheel drive. The same type of automatic all wheel drive is used on VW Golf/Bora/Jetta 4motion (Mark IV 1998-2005, Mark V 2003-), VW Sharan 4motion (2001-) and other Volkswagen AG vehicles with transverse engine mounting – Seat and Skoda.
What: Automatic all wheel drive.
Haldex multiplate clutch with electronic control acting as a center differential.
Open rear differential, EDL on some models (not proved).
Open front differential, EDL.
How: Normally front wheel drive vehicle. In normal driving conditions 100% of torque transferred to front axle. When slipping occurs, Haldex multiplate clutch locks and up to 100% of torque is automatically transferred to rear axle. Haldex clutch offers instant activation – it locks as soon as one driveshaft travels just 1/8 of a turn further than the other. Front wheels do just 1/4 of a turn before AWD is activated. Accompanied by EDL on front wheels, in on-road conditions the car will not move if both front and one rear wheels lose traction.
Again, due to limitations of Electronic Differential Lock (see quattro IV description above), if off-road conditions it is enough for one front and one rear wheel to lose traction and the car will not move.
The Haldex system is more reactive than preventative; the Torsen's even power split under non-slipping conditions makes slipping less likely to start, while the Haldex's greater torque split under slipping conditions is helpful for recovery.
Haldex Electronic Control Unit disengages the clutch as soon as brakes are applied to allow ABS work properly; it also disengages the clutch when handbrake is pulled to allow “handbrake turns”. When performing tight low-speed turns (e.g. parking) the clutch is disengaged by Electronic Control Unit to avoid wind-up in transmission.
Pros: Claimed better fuel economy when compared to full-time all wheel drive. Instant activation with no driver intervention.
Cons: Full-time all wheel drive is better. Some say Haldex system has less predictable behavior when cornering in snow than permanent all wheel drive systems. Other say there is nothing wrong with it; it is just a little different.
[edit] See also
4MATIC - a four wheel drive system from Mercedes-Benz.
All-trac - a four wheel drive system from Toyota.
