quattro (meaning
four in Italian) is the name
used by Audi AG to indicate that four-wheel drive (4WD) technologies or systems are used on specific
models of the Audi automobiles.
The word
"quattro" is a registered trademark of
Audi AG, a subsidiary of the larger German automotive conglomerate, Volkswagen Group.
Quattro was first introduced in 1980 on the permanent four wheel
drive
Audi Quattro (often referred to
as the
Ur-Quattro - "Ur-" being German for "original").
The term
quattro has since been applied to all
subsequent Audi 4WD models. Due to the
nomenclature rights derived from the trademark,
the word
quattro is now always spelled with a
lower case
"q", in honour of its former
namesake.
Other companies in the Volkswagen Group have used different
trademarks for their 4WD vehicles. While Audi has always used the
term "quattro",
Volkswagen-branded cars
initially used
"syncro", but more
recently, VW use
"4motion".
Škoda simply uses the nomenclature
"4x4" after the model name, whereas
SEAT uses merely
"4". None of the above
trademarks or nomenclatures defines the operation or
type
of 4WD system, as detailed below.
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 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
(
TorqueSen
sing) Type 2 ("T2")
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:
- With placement of the engine and transmission assembly in a
fore/aft position (longitudinally), the front axle is placed
rearwards behind the engine, leading to a common criticism of Audi
vehicles: being nose-heavy (although this is common in cars where
the engine is placed at the front of the vehicle). In other words,
the 'ideal' 50:50 weight distribution cherished by many driving
enthusiasts was not achieved.
- 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/hr. 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 2 or “T2”) Torsen supports a “static”
torque ratio of 50:50 i.e. input torque is supported equally across
both output shafts, the T2 has a Torque Bias Ratio (TBR) of 2:1
i.e. it allows twice the torque to be supplied to the most tractive
output shaft than that that is available on the least tractive
shaft (i.e. a torque split of between 33% to 67%). However, by
nature the T2 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 T2 Torsen in a centre
differential installation, rather than being 50:50, will mirror the
weight distribution (both static & 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 T2 with
2:1 TBR is more than sufficient in most conditions, Torsen T2
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 T2 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 tansmission and 2007-model for B7 for both transmission
types.
S4. as well as the S5 and Q7. It is
planned to be incorporated in all future quattro-equipped cars
which use longitudinal-mounted engine layout.
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.
Evolutions
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 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 does the system perform: When all differentials are unlocked
the car will not be able to move if just one wheel loses traction
(is on ice or raised in the air). When both center and rear
differential are locked, the car will not be able to move when one
front and both rear wheels lose traction altogether.
quattro generation II
Starting from 1988 on older generation
Audi
100 C3 platform and Audi Quattro turbo coupé until the end of
their production, and on new generation B3 platform (1989–1992)
Audi 80/90 quattro, B4 platform
(1992–1995)
Audi 80,
Audi Coupé quattro,
Audi S2,
Audi RS2
Avant, C4 platform (1991–1994)
Audi
100 quattro,
Audi S4, earlier C4
platform (1995)
Audi A6/S6.
System type: Permanent
four-wheel
drive.
Torsen centre
differential, 50:50
'default' split, automatically apportioning up to 75% 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 .
How does the system perform: When rear differential is manually
locked, the car will not be able to move if one front wheel and
both rear wheels lose traction altogether.
quattro generation III
Used on
Audi V8 starting from 1990.
System type: Permanent or semi-permanent
four-wheel drive.
V8 with automatic
transmission:
Planetary gear centre differential
with electronically-controlled multi-plate locking clutch
Torsen differential rear.
Open differential front.
V8 with manual
transmission:
Torsen centre differential.
Torsen 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. Torsen effect with one wheel in the air still
applicable to V8 with manual transmission, but will not happen on
V8 with automatic transmission because center differential on this
model offers 100% locking even when no torque is sensed on the
spinning wheel.
quattro generation IV
Starting from 1996 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; also the
Volkswagen Touareg where they use
separate transmissions,
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 the 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 T-2 centre differential, 50:50
'default' split, automatically apportioning up to 67% of torque
transfer to either front or rear axle.
Open rear differential, Electronic Differential Lock (EDL).
Open front differential, Electronic Differential Lock (EDL).
quattro generation V
Starting from 2006 on B7
Audi RS4 and the
2008 B7
Audi S4. Will become the standard
fitment on all future quattro Audis with longitudinal engine
layout.
System type: Permanent asymmetric
four-wheel drive.
Torsen T-3 centre differential, 40:60 'default' split front-rear,
automatically apportioning up to 80% of torque transfer to the
front axle or up to 100% torque to the rear axle.
Open rear differential, Electronic Differential Lock (EDL).
Open front differential, Electronic Differential Lock (EDL).
Vectoring quattro system (quattro generation VI ?)
Audi's new
vectoring quattro system, which will
allow the dynamic allocation of torque to all four wheels will
debut in the B8 S4. This will still use the 40:60 asymmetric Torsen
centre differential, but will use an electronically controlled
"Active Sport Differential" in the rear axle (instead of the
conventional "open" differential with EDL). The front axle will
still rely on an open differential with EDL.
The torque vectoring system Vector Drive rear axle drive
manufactured by
ZF is being
offered on Audi A4, A5, A6, Q7 and Q5. The Vector Drive system
distributes torque individually to the rear axle wheels generating
a yaw moment, which improves handling and stabilizes the vehicle
when it oversteers or understeers, increasing safety. It is also
currently offered as standard in the BMW X6. .
BorgWarner
The
Audi Q7, 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.
Transverse systems
Since
Volkswagen Group's first
mainstream
transverse engined vehicle in 1974,
four-wheel drive (4WD) has also been
considered for their
A-platform family of
cars. It was not until the
second generation of this
platform that 4WD finally appeared on the market. The mid-1980s
Mk2 Golf syncro, with its
transverse engine and
transmission positioning, still had
most of its torque sent primarily to the front axle. Vehicles using
this configuration therefore cannot be said to have a "permanent",
or "full-time" four 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 feeds
torque through itself to the front axle. At the rear
axle, torque was first sent through a viscous coupling before
reaching the final drive gearset. This coupling contained friction
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
A4-platform, the viscous
coupling has been dropped in favour of a
Haldex Traction electro-hydraulic
limited-slip "coupler" (LSC) or
clutch. The
Haldex Traction LSC unit is not a differential and therefore cannot
perform in the true sense like a
differential. A Haldex Traction
unit may divert up to a maximum 100% of the torque to the rear axle
as conditions warrant. Many people are confused with the torque
distribution on Haldex-based systems. Under normal operating
conditions the Haldex clutch operates a rate of 5% torque
transmission. Under adverse conditions where the cars road wheel
speed sensors have determined that both front wheels have lost
traction the Haldex clutch can lock at 100% clamping force, meaning
all torque is transersed to the rear axle. The torque split between
left and right wheels is achieved with a conventional open
differential. If one side of the driven axle loses grip, then the
Electronic Differential Lock (EDL) component of the ESP controls
this. EDL brakes a single spinning wheel, therefore, the torque
gets transferred across the axle to the opposite wheel via the open
differential. On all transverse engine cars with the Haldex-based
four wheel drive system, the EDL only controls front wheels, and
not the rear.
The main advantages of the Haldex Traction LSC system over the
Torsen-based system include: a slight gain in fuel economy (due to
the decoupling of the rear axle when not needed, thereby reducing
driveline losses due to friction), and the ability to maintain a
short engine bay and larger passenger compartment due to the
transverse engine layout. A further advantage of the Haldex, when
compared to just front wheel drive variants of the same model, is a
more balanced front-rear weight distribution (due to the location
of the Haldex center "differential" next to the rear axle).
Disadvantages of the Haldex Traction 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 LSC unit also requires additional maintenance, in the
form of an oil and filter change every (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 (and rolling radii), due to the
Haldex requiring data from all four road wheel speed sensors. A
final significant disadvantage is the reduction in luggage capacity
in the boot (trunk), due to the bulky Haldex LSC unit necessitating
a raised boot floor by some three inches.
Viscous Coupling
Important note: This 4WD system was used only on
Volkswagen branded vehicles, and was never used
on any
Audi cars.
The aforementioned viscous coupling 4WD system was found in the Mk2
generation of transverse-engined
A2-platform vehicles,
including the
Volkswagen Golf
Mk2 and
Jetta. It was also
found on the
Volkswagen Type
2 (Vanagon in the US), Mk3 generation of
Golf and Jetta, 3rd generation of
Volkswagen Passat B3 (which was
based on a heavily revised A-platform), and the
Volkswagen 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 axle near the final drive. This 4WD
system was known as
Syncro on all
vehicles.
What: Automatic four wheel drive (on demand).
A
viscous coupling installed
instead of a centre differential, with
freewheel mechanism to disconnect the driven axle
when braking.
Open rear differential (mechanical differential lock optional on
Vanagon).
Open front differential (mechanical differential lock optional on
Vanagon).
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 50% of torque is automatically transferred to rear axle (front
in Vanagon). In on-road conditions the car will not move if one
front wheel and one rear wheel lose traction.
The
freewheel segment, installed inside
the rear differential, lets rear wheels rotate faster than front
wheels without locking the viscous coupling and preventing
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. For
four-wheel drive to work when
reversing, a
vacuum-actuated "throttle control element" is installed on the
differential case. This device locks the freewheel mechanism when
in reverse gear. The freewheel mechanism unlocks when the gear
shift lever is pushed to the right pass the 3rd gear. The freewheel
is not unlocked immediately after leaving reverse gear on purpose -
this is to prevent the freewheel from cycling from locked to
unlocked if the car is stuck and driver is trying to "rock" the car
by changing from 1st to reverse and back.
Disadvantages of this four wheel drive system are related to
actuation time of the viscous coupling. 1: When cornering under
acceleration on a slippery surface, rear axle is engaged with delay
causing sudden change in the car's behaviour (from understeer to
oversteer). 2: When starting on a sandy surface, front wheels can
dig into the sand before all wheel drive is engaged.
Haldex
Starting
from 1998, the Swedish Haldex Traction LSC unit replaced the
viscous coupling. Haldex is used by
Audi
on the quattro versions of the
Audi A3 and
Audi S3, and the
Audi
TT. It is also used by
Volkswagen in
the
4motion versions of the Mk4 and Mk5
generations of
Volkswagen Golf,
Volkswagen Jetta, and the Golf R32,
Volkswagen Sharan,
6th
generation VW Passat (also based on the A-platform) and
Transporter T5. On the Audis,
the trademark holds, and are still referred to as quattro, whereas
the Volkswagens receive the 4motion name. The
Škoda Octavia 4x4 and
SEAT Léon 4 and
SEAT
Alhambra 4 also used Haldex LSC, being based on
Volkswagen Group models. Curiously, the
Bugatti Veyron also utilizes Haldex,
though with separate transmission, PTU and front and rear
axles.
What: Automatic four wheel drive (on demand).
Haldex Traction LSC multi-plate
clutch with ECU electronic control, acting as a pseudo center
differential.
Open rear differential, no EDL.
Open front differential, EDL.
How: Normally
front-wheel drive
vehicle. A Haldex Traction LSC unit may divert up to a maximum 100%
of the
torque to the rear
axle as conditions warrant. Many people are confused
with the torque distribution on Haldex Traction systems. Under
normal operating conditions, the Haldex LSC clutch operates at 5%
(divide 5% between front and rear, and 97.5% torque goes to the
front, and 2.5% goes to the rear). Under adverse conditions where
both front wheels lose traction, the Haldex clutch can lock at 100%
clamping force. This means, that since there is no torque
tranferred to the front axle, all torque (minus losses) must be
transferred to the rear axle. The torque split between left and
right wheels is achieved with a conventional open differential. If
one side of the driven axle loses grip, then the Electronic
Differential Lock (EDL) controls this. EDL brakes a single spinning
wheel, therefore torque gets transferred to the opposite wheel via
the open differential. On all transverse engine cars with the
Haldex Traction LSC
four-wheel
drive system, the EDL only controls front wheels, and not the
rear.
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 Traction system is more
reactive
than preventative, in that the front axle must lose traction and
start to spin before the Haldex operates and sends torque to the
rear axle. The Torsen's permanent 'full-time' even torque split
under non-slipping conditions makes slipping less likely to
start.
The Haldex Electronic Control Unit (ECU) disengages the Haldex
clutch in the centre coupling as soon as brakes are applied to
allow
ABS work properly.
When performing tight low-speed turns (e.g. parking) the clutch is
disengaged by Electronic Control Unit to avoid "wind-up" in
transmission.
Haldex aftermarket applications
The
Haldex Traction LSC centre
coupling is often used as upgrade to an aftermarket
four-wheel drive conversion on older
front-wheel drive 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
Volkswagen Corrado or
Volkswagen Golf), and then fabricating a
custom bracket to hold the Haldex rear coupling.
Enthusiasts often either use the OEM ECU and engine management from
a newer
Volkswagen Group car to
control the Haldex centre clutch using the standard ABS road wheel
speed 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 sensor (TPS)
calculations.
See also
References
External links