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j*******n
发帖数: 10868 | 1 http://legacygt.com/forums/showthread.php/explanation-continuou
There is a lot of confusion about the differences between the various Subaru
AWD systems:
Continuous (for 5MT)
Active (for 4EAT)
VTD (for 5EAT)
availability or non-availability of Rear LSD
So heres an attempt to explain the differences....
1. Continuous AWD System:
The manual transmission’s all-wheel drive is referred to as a continuous
all-wheel drive system. It uses a center differential located inside the
transmission case that is controlled by a viscous coupling device. In effect
, the center differential is a limited-slip differential.
In normal operation, power is distributed equally to the front and rear
wheels. Plates are alternately attached to the front and rear output shafts
inside the viscous coupling. When a rotational difference occurs between the
front and back wheels, the plates inside the viscous housing shear inside
the contained fluid (a type of silicone) heating it and causing the fluid to
thicken. The thickened fluid causes the plates to transfer torque from
those that rotate faster (the slipping wheels) to the plates that rotate
slower (the wheels with the best traction).
This no-maintenance system is simple, compact and virtually invisible in its
operation. The system can distribute torque from a 50:50 torque split for
maximum traction to mostly front or rear wheel drive.
(Source: http://www.autoworld.com/news/Subaru..._All-Wheel.htm)
A. Continuous AWD with Rear LSD: Available with 5MT on WRX, Legacy GT,
Outback 2.5i, Outback XT and with 6MT on Spec.B
B. Continuous AWD without rear LSD: Available with 5MT on Impreza 2.5i,
Outback Sport and Legacy 2.5i
2. Active AWD System:
Active all-wheel drive is a term coined by Subaru to differentiate the all-
wheel drive system in the automatic transmission (4EAT) from other "reactive
" all-wheel drive systems on the market today. What makes this all-wheel
drive system so special is its ability to anticipate traction needs and act
before a wheel slips.
The mechanism that transfers torque fore and aft is contained within the
transmission’s tailshaft. To the casual observer it looks just like a
typical hydraulic clutch found in any automatic. The key difference in this
clutch pack is its operation. It’s designed to slip according to how much
all-wheel drive is needed. When an automatic’s clutch slips, it is due to a
malfunction and will eventually burn up. But the multi-plate transfer (MPT)
clutch uses a special friction material that easily withstands the friction
loads generated during torque transfer. (Also referred to as VTC = Variable
Transfer Clutch)
The MPT’s operation is controlled by the Transmission Control Unit (or TCU)
and constantly changes dependent on how the vehicle is being driven. To get
more all-wheel drive, the TCU increases the hydraulic pressure to the
clutch for less slippage. Less all-wheel drive calls for more slip and the
TCU reduces the hydraulic pressure to the clutch.
Under normal, dry pavement operation torque split is about 90% front and 10%
rear. This distribution helps to compensate for the car’s weight
distribution and resultant smaller effective rolling diameter of the front
tires. As weight transfers to the rear of the vehicle, (i.e., under
acceleration), the TCU shifts the torque split more toward the rear wheels.
Under hard braking, torque is directed forward. Torque distribution is
changed based upon how the vehicle is being driven. Throttle position,
gearshift lever position, current gear and other factors combine to
influence the TCU and it, in turn, selects a software map that determines
how aggressively torque split will be adjusted.
Two speed sensors are used by the TCU to detect wheel slippage. One sensor
monitors the front axle set, the other the rear axle set. Pre-programmed
variables help the TCU differentiate between slipping wheels and normal
wheel speed differentials as what occurs when cornering. A speed
differential (front-to-rear) of up to 20% signals the TCU that the vehicle
is cornering and torque is distributed to the front wheels to help increase
traction during the turn. Anything above 20%, however, indicates to the TCU
that wheel slippage is occurring and torque is then distributed to the rear
wheels.
Another feature of the all-wheel drive system is its interaction with the
anti-lock brake system. When ABS is engaged, the transmission selects third
gear, reducing the unpredictability of engine braking and, thus, reducing
the possibility of wheel lock-up. But all four wheels are still connected to
the engine through the AWD system and are brought back up to overall
vehicle speed quicker and can, therefore, be controlled again sooner. In a
two-wheel drive system if the locking wheel isn’t a drive wheel, it can
only be brought back up to overall wheel speed by whatever traction exists
between it and the road. The quicker a wheel is controlled the better the
stopping performance
(Source: http://www.autoworld.com/news/Subaru..._All-Wheel.htm)
A. Active AWD with Rear LSD: Available with Outback 2.5i
B. Active AWD without Rear LSD: Available with 4EAT on Impreza 2.5i, Legacy
2.5i, Legacy 2.5i Ltd
3. Variable Torque Distribution (VTD):
It has the MPT (multi plate transfer) clutch aka VTC as found in the Active
AWD system but it also has a planetary-type center differential and a Rear
LSD. The center differential provides the ability to have a default torque
split of 45/55 front/rear (as against the 90/10 split in Active AWD). In
every other aspect it is similar to the Active AWD in that it anticipates
wheel slippage instead of reacting to it as in the case of a MT. We can say
the the VTD is an advanced Active AWD system
Available with 4EAT on Impreza WRX
Available with 5EAT on Outback XT, Outback 3.0R, Legacy GT
4. Driver Controlled Center Differential (DCCD):
Uses an electronically managed multiplate transfer clutch and a mechanical
limited-slip differential in conjunction with a planetary gear-type center
differential to control power distribution between the front and rear wheels
. Normally, DCCD splits power 41% front and 59% rear. Sensors monitor
parameters such as wheel slippage, steering angle, throttle position and
braking to help determine torque distribution and direct it to the wheels
with optimum traction. DCCD also features a helical-type limited-slip front
differential and a Torsen® limited-slip rear differential.
Available with 6MT on Impreza WRX Sti
Does my Subaru have a Rear LSD?
Another point of confusion is which Subarus have Rear Limited Slip
Differentials (Rear LSD) and which ones do not. For this you can refer to
the list of Subaru AWDs above where I have also listed the Subaru models
which have that type of AWD. But to make things even clearer heres a list of
Subarus with and without Rear LSD:
Subarus without Rear LSD:
Impreza 2.5i (5MT and 4EAT)
Outback Sport (5MT and 4EAT)
Legacy 2.5i (5MT and 4EAT)
Legacy 2.5i Ltd. (4EAT)
Subarus with Rear LSD:
Impreza WRX (5MT and 4EAT)
Outback 2.5i (5MT and 4EAT)
Outback XT (5MT and 5EAT)
Outback 3.0R (5EAT)
Legacy GT (5MT and 5EAT)
Spec.B (6MT)
WRX Sti (also has Front LSD)
As we all know that the center differential can split the torque in between
the front and the rear axles depending on the front/rear wheel slippage. But
its the Front and the Rear LSDs which can split the power between left and
right wheels. So it is very advantageous to have at least the Rear LSD if
not the Front LSD. If the left rear wheel slips the Rear LSD can transfer
the power to the right rear wheel.
Subaru does not offer Rear LSDs in its lower models except in Outback 2.5i!!
which is understandable since it is designed for off-road conditions. (This
might explain the mpg difference in Outback 2.5i and Legacy 2.5i)
Which is Better?
Now that brings us to the next point of debate as to which one is better?
So here's an excerpt from an article I found:
(Source: http://www.cars.com/carsapp/cars/?sr...all_wheel.tmpl)
"Simpler AWD systems “bias” the power to the front or the rear in this way
and react to slippage when it occurs. Subaru's “continuous” AWD system is
this type. More advanced systems are designed to be proactive rather than
reactive. For example, Subaru’s “active” AWD is claimed to anticipate and
prevent slippage for a seamless driving experience. Audi's quattro and
related Volkswagen 4MOTION AWD systems route power based on vehicle dynamics
the rear wheels receive more of the engine’s power. In turns, the outside
front wheel has the most grip and thus gets the most power, followed by both
rear wheels.
With advanced AWD like this available on cars such as the Porsche 911
Carrera 4, it’s clear that four driven wheels are no longer the province of
high-ridin’ mud buggies. For surefooted handling during acceleration —
even on bone-dry roads — AWD is tough to beat."
As per the author of this article the Continuous AWD system found on Subarus
with 5MT and 6MT is a simpler system in that it is more reactive than
proactive. Whereas the Active AWD systems (with Variable Transfer Clutch)
found on 4EAT Subarus and the VTD (which is an advanced Active AWD system)
found on 5EAT Subarus are proactive in predicting wheel slippage conditions
and hence react faster. But obviously the Continuous AWD systems have the
advantages of being less complex and hence less prone to failures and low
repair costs.
So again which system is the best? I would say the order could be as follows:
DCCD with Front and Rear LSDs - WRX Sti
VTD - 4EAT on WRX and 5EAT on Outback XT, Outback 3.0R and Legacy GT
Active AWD with Rear LSD: 4EAT on Outback 2.5i
Tie. Depends on what you need more? proactive AWD system or the Rear LSD (if
offroading)
Active AWD without Rear LSD: 4EAT on Impreza 2.5i, Legacy 2.5i and Legacy 2.
5i Ltd
Continuous AWD with Rear LSD: 5MT on WRX, Legacy GT, Outback 2.5i, Outback
XT and 6MT on spec.B
Continuous AWD without Rear LSD: 5MT on Impreza 2.5i and Legacy 2.5i
Subaru saves its best AWD system for the Impreza WRX Sti which has the DCCD
system with front and rear LSDs.
I wasnt aware that the Outback 2.5i has the Rear LSD. Thats an advantage
over the Legacy 2.5i.
Disclaimer: These are my deductions from what I have read on the internet
and limited by the capability of my 3lbs. So please no one should get
offended . Also, if you feel any of this information is incorrect and needs
to be updated then please feel free to enlighten all of us. | g*****i
发帖数: 2162 | | n****e
发帖数: 6292 | 3 评价四驱的标准就两个，一是怎么把动力送过去，二是怎么限滑 | k*****a
发帖数: 7389 | 4 傻八路 may be a good car, but...... |
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