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quattro (all wheel drive system)
From Wikipedia, the free encyclopedia
quattro (four in Italian) is a registered trademark of Audi AG for all wheel drive (AWD) systems used on Audi brand automobiles. quattro was first introduced in 1980 on the permanent-AWD Audi Quattro and has since been applied to all AWD models that Audi sells. Due to the trademark, the name is always spelled with a lower case "q".
Audi parent Volkswagen has used different trademarks for their AWD vehicles, including syncro and 4motion. Sister company Skoda simply uses the nomenclature "4x4" after the model name, and SEAT uses "4." It is important to note that none of the above trademarks or nomenclatures define the type of AWD system, as discussed below.
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VWAG has been developing all wheel drive and four wheel drive (4WD) systems since nearly its inception during the Second World War. The Kubelwagen, Schwimmwagen and Kommandeurwagen were all military vehicles that required torque to all four wheels, the latter being a 4WD Beetle. Their military and all wheel drive experiences later aided them in designing the Iltis for the German military in the 1970s. The Iltis utilized an early form of AWD which would become synonymous with quattro.
In that original quattro® system, later found in passenger cars, the engine and transmission are situated in a longitudinal position. Torque is sent through the transmission to a mechanical center differential which splits it between the front and rear axles. AWD was turned on permanently.
After 1987, the inclusion of a Torsen (TorqueSensing) center differential, instead of a manually-locking one, allowed engine torque to be redirected to individual axles as driving conditions warranted. Under 'normal' conditions (i.e., grip is equal in both front and rear axles) torque is split between front and rear (50/50 in many, though not all, versions). In adverse conditions (i.e., when grip varies from front to rear), a maximum of 75-80% (depending on the transmission or model) 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 discernable notice to the vehicle occupants, 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 after 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 fully automonous. 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.
This configuration of the quattro system, however, 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. The ideal 50/50 weight distribution cherished by many driving enthusiasts, in other words, is not possible. More recently, the centre differential has been adapted to give an assymetric 40/60 front/rear 'default' torque 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 (B7) RS4. It is planned to be incorporated in all future quattro-equipped cars which use longitudinal-mounted engines.
The torque split between left and right wheels has been achieved through the various iterations of the quattro system through manually locking diffrential (rear axle only), and, eventually, through open differentials with electronic diffrential lock (EDL). EDL is an electronic system that brakes spinning wheels transferring torque to the wheels that have traction.
Since VWAG's first mainstream transverse engined vehicle in 1974, AWD has also been considered for their A-platform family of cars. It was not until the second generation of this platform that AWD finally appeared on the market. The mid-1980's Golf syncro, with its transverse engine and transmission positioning, still had most of its power sent primarily to the front axle. Vehicles using this configuration therefore cannot be said to have a "permanent" all wheel drive system. Attached to the transaxle is a power transfer unit (PTU), which is connected to a rear axle through a propeller shaft. The PTU also fed power through itself to the passenger-side front wheel. At the rear axle, power was first sent through a viscous coupling before reaching the final ratio gearset. This coupling was full of plates and an oil just viscous enough so that pressure affected how many plates were connected and active (and therefore, how much power was being delivered to the rear wheels).
Starting with the Mk4 generation A-platform, the viscous coupling has been dropped in favor of a Haldex electro-hydraulic limited-slip "coupler" or clutch (i.e. not a differential in the true sense). A Haldex unit may divert up to 100% of the torque to the rear axle as conditions warrant. The power split between left and right wheels is achieved with EDL (electronic diffrential lock). EDL brakes the spinning wheel and torque gets transferred to the other wheel via the open differential. On Mk4 VW Golf 4motion, EDL controls front wheels only.
Advantages of the Haldex system over the Torsen-based system include: a slight increase in fuel economy (due to the decoupling of the rear axle when not needed, thereby reducing driveline losses due to friction), and a more balanced front/rear weight distribution (due to the location of the Haldex center "differential" in the rear axle).
Disadvantages of the Haldex system include: the vehicle has inherent front-wheel drive handling characteristics (as when engine-braking, load is only applied on the front wheels, and due to the reactive nature of the Haldex system and slight lag time in the redistribution of engine power), and the Hadex unit also requires additional maintenance, in the form of an oil and filter change every 40,000 miles (whereas the Torsen is completely maintenance free). Another important disadvantage of the Haldex system, is the requirement for all four tyres to be of nigh-on identical wear levels.
Used from 1981 to 1987 in Audi Quattro turbo coupe, Audi 80 B2 platform (1978-1987), Audi 100 C3 platform (1983-1987), Audi 4000, Audi 5000 in the American market. Also, starting from 1984, used on the Volkswagen Passat B2 platform (Quantum in the US market) where it was known as Syncro.
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.
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 (1991-1994) Audi 100 Quattro, Audi S4, earlier C4 platform (1995) 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 torque is transferred to the wheel in the air and the car will not move.
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 the front axle loses traction the multi-plate clutch is automatically locked transferring up to 100% of torque to the rear axle.
Starting from 1996 on Audi A4/S4/RS4, Audi A6/S6/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 rechristened 4motion. Also used on the Volkswagen Phaeton and sister vehicles; also the Volkswagen Touareg where they use separate transmissions, PTU's and front axles.
Manually locking rear differential was 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)[1]. 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). Audi A8/S8 also 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: 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.
Starting from 2006 on Audi Q7 and B7 platform RS4.
What: Permanent all wheel drive.
Torsen center differential, 40/60 split front/rear, 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: 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.
Audi's new Vectoring quattro System, which will allow the dynamic allocation of torque to all four wheels[2] will debut in the B8 S4.
The Audi Q7, platform-mate of the Touareg and Cayenne, does not use the same underpinnings of either previous model. BorgWarner instead provides the AWD system for this more on-road-appropriate SUV.
Important: This AWD system was never used on Audi cars under the quattro badge.
The aforementioned viscous coupling AWD system was found in the Mk2 generation of transverse-engined A-platform vehicles, including the Golf and Jetta. It was also found on the Vanagon, Mk3 generation of Golf and Jetta, 3rd generation of Passat (which was based on the A-platform), and the Eurovan. Note that the Vanagon system was RWD-biased due to the vehicle being rear wheel drive by default; the engine and transaxle were in the rear, whereas the viscous coupling was found in the front near the final drive. This AWD system was known as Syncro on all vehicles.
What: Automatic all wheel drive (on demand).
A viscous coupling acting as a center "differential," with freewheel mechanism to disconnect the driven axle when braking.
Open rear differential.
Open front differential.
How: Normally a front wheel drive vehicle (except Vanagon, see above). In normal driving conditions 95% of torque transferred to front axle. Because viscous coupling is considered to be "slow" (some time is needed for silicone fluid to warm-up and solidify), 5% of torque is transferred to rear axle at all times to "pre-tension" the viscous coupling and reduce activation time. The coupling locks when slipping occurs and up to near 100% of torque is automatically transferred to rear axle. In on-road conditions the car will not move if one front wheel and one rear wheel lose traction.
Besides slow activation (which affects handling), another disadvantage of this all wheel drive system is that all wheel drive does not work when vehicle is moving backwards. This is because of a freewheel mechanism installed next to the viscous coupling so that locked viscous coupling would not prevent ABS from applying brakes to each wheel independently. Because of the freewheel, torque can be transferred to rear axle only when vehicle is moving forward.
Starting from 1998, the Swedish Haldex unit replaced the viscous coupling. Haldex is used on the Audi A3/S3, Audi TT, Mk4 and Mk5 generations of Golf, Jetta and R32, VW Sharan, 6th generation Passat (also based on the A-platform) and 5th generation Transporter (van). On the Audi's, the trademark holds true and is referred to as quattro, where the Volkswagens receive the 4motion name. The Škoda Octavia 4x4 and SEAT Léon 4 and Alhambra 4 also used Haldex, being based off of Volkswagen models. Curiously, the Bugatti Veyron also utilizes Haldex, though with separate transmission, PTU and front and rear axles.
What: Automatic all wheel drive (on demand).
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 near 100% of torque transferred to front axle. When slipping occurs, Haldex multiplate clutch locks and up to near 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), in 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.
The Haldex centre differential is often used as upgrade to an aftermarket AWD conversion on older FWD Volkswagens. It is said to be capable of withstanding larger power outputs than the also commonly used viscous coupling system from a syncro vehicle.
The conversion is carried out by way of a viscous coupling rear axle and associated live suspension system from a syncro vehicle being fitted to a suitable project car (i.e. A VW Corrado or VW Golf), and then fabricating a custom bracket to hold the Haldex rear differential.
Enthusiasts often either use the OEM ECU and engine management from a newer VWAG car to control the Haldex centre clutch using the standard yaw, steering angle and acceleration sensors - or can buy aftermarket controllers that supply the relevant Pulse Width Modulation to actuate the clutch and transfer drive to the rear wheels either via simple variable dial or based on Throttle Position (TPS) calculations
- ^ VAG four-wheel drive systems and brand names.
- ^ AudiWorld: Performance Preview: Exclusive Look at the B8 S4.
- 4MATIC - a four wheel drive system from Mercedes-Benz.
- All-trac - a four wheel drive system from Toyota.
- ATTESA-ETS - a four wheel drive system from Nissan.