10.05.2006
After the 4WD schemes used in Toyota were examined in some detail in previous materials, it turned out that there is still an information vacuum with other brands ... Let's first take the four-wheel drive of Subaru cars, which many call "the most real, advanced and correct."
Mechanical boxes, by tradition, are of little interest to us. Moreover, everything is quite transparent with them - since the second half of the 90s, all Subaru on the mechanics have an honest all-wheel drive with three differentials (the center differential is blocked by a closed viscous coupling). Of the negative sides, it is worth mentioning an overly complicated design obtained by combining a longitudinally mounted engine and the original front-wheel drive. As well as the refusal of the Subarovites from the further mass use of such an undoubtedly useful thing as a downshift. On single "sports" versions of the Impreza STi, there is also an advanced manual transmission with an "electronically controlled" center differential (DCCD), where the driver can change the degree of its blocking on the go ...
But let's not digress. There are two main types of 4WD used in automatic transmissions currently operated by Subaru.
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1.1. Active AWD / Active Torque Split AWD |
Permanent front-wheel drive, without center differential, connection of the rear wheels with an electronically controlled hydromechanical clutch
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1 - torque converter lock-up damper, 2 - torque converter clutch, 3 - input shaft, 4 - oil pump drive shaft, 5 - torque converter clutch housing, 6 - oil pump, 7 - oil pump housing, 8 - transmission housing, 9 - speed sensor turbine wheel, 10 - 4th clutch, 11 - reverse clutch, 12 - 2-4 brake, 13 - front planetary gear set, 14 - 1st clutch, 15 - rear planetary gear set, 16 - 1st brake gear and reverse, 17 - transmission output shaft, 18 - "P" mode gear, 19 - front drive gear, 20 - rear output shaft speed sensor, 21 - rear output shaft, 22 - shank, 23 - clutch A- AWD, 24 - front drive driven gear, 25 - freewheel, 26 - valve block, 27 - sump, 28 - front output shaft, 29 - hypoid gear, 30 - impeller, 31 - stator, 32 - turbine. |
E this option has long been installed on the vast majority of Subaru (with automatic transmission type TZ1) and is widely known from the Legacy model of 89. In fact, this four-wheel drive is as “honest” as the fresh Toyota Active Torque Control - the same rear-wheel drive and the same TOD (Torque on Demand) principle. There is no center differential, and the rear-wheel drive is activated by a hydromechanical clutch (friction package) in the transfer case.
The Subar scheme has some advantages in the working algorithm over other types of plug-in 4WD (especially the simplest ones, like the primitive V-Flex). Albeit small, but the moment during A-AWD operation is constantly transmitted back (unless the system is forcibly turned off), and not only when the front wheels slip - this is more useful and efficient. Thanks to hydromechanics, the force can be redistributed a little more accurately than in an electromechanical ATC. In addition, A-AWD is structurally more durable. For cars with a viscous coupling for connecting the rear wheels, there is a danger of a sharp spontaneous “appearance” of the rear drive in a turn, followed by an uncontrolled “flight”, but in A-AWD this probability, although not completely excluded, is significantly reduced. However, with age, as wear and tear, the predictability and smoothness of the connection of the rear wheels decreases significantly.
The algorithm of the system remains the same throughout the entire release period, only slightly corrected.
1) Under normal conditions, with the accelerator pedal fully released, the torque distribution between the front and rear wheels is 95/5..90/10.
2) As you press on the gas, the pressure supplied to the clutch package begins to increase, the discs gradually tighten and the torque distribution begins to shift towards 80/20 ... 70/30 ... etc. The relationship between gas and line pressure is by no means linear, but rather looks like a parabola - so that a significant redistribution occurs only when the pedal is pressed hard. With a fully recessed pedal, the friction clutches are pressed with maximum effort and the distribution reaches 60/40 ... 55/45. Literally, "50/50" is not achieved in this scheme - this is not a hard lock.
3) In addition, the speed sensors of the front and rear output shafts installed on the box make it possible to determine the slip of the front wheels, after which the maximum part of the moment is taken back regardless of the degree of gas supply (except for the case of a fully released accelerator). This function is active at low speeds, up to about 60 km/h.
4) When forced into 1st gear (selector), the clutches are immediately pressed with the maximum possible pressure - thus, as it were, "difficult all-terrain conditions" are determined and the drive remains the most "permanently full".
5) When the "FWD" fuse is plugged into the connector, no overpressure is supplied to the clutch and the drive is constantly carried out only on the front wheels (distribution "100/0").
6) With the development of automotive electronics, it has become more convenient to control slippage using standard ABS sensors and reduce the degree of clutch blocking when cornering or ABS is triggered.
It should be noted that all passport distributions of moments are given only in statics - during acceleration / deceleration, the weight distribution along the axes changes, so the real moments on the axes are different (sometimes "very different"), just like with different coefficients of wheel adhesion to the road.
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1.2. VTD AWD |
Permanent four-wheel drive, with center differential, electronically controlled hydromechanical clutch lock
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1 - torque converter lock-up damper, 2 - torque converter clutch, 3 - input shaft, 4 - oil pump drive shaft, 5 - torque converter clutch housing, 6 - oil pump, 7 - oil pump housing, 8 - transmission housing, 9 - speed sensor turbine wheel, 10 - 4th clutch, 11 - reverse clutch, 12 - 2-4 brake, 13 - front planetary gear set, 14 - 1st clutch, 15 - rear planetary gear set, 16 - 1st brake gear and reverse, 17 - countershaft, 18 - "P" mode gear, 19 - front drive gear, 20 - rear output shaft speed sensor, 21 - rear output shaft, 22 - shank, 23 - center differential, 24 - center differential lock clutch, 25 - front drive driven gear, 26 - overrunning clutch, 27 - valve block, 28 - sump, 29 - front output shaft, 30 - hypoid gear, 31 - impeller, 32 - stator, 33 - turbine . |
The VTD (Variable Torque Distribution) scheme is used on less mass-produced versions with automatic transmissions such as TV1 (and TZ102Y, in the case of the Impreza WRX GF8) - as a rule, the most powerful in the range. Here, everything is in order with "honesty" - the all-wheel drive is really permanent, with an asymmetric center differential (45:55), which is blocked by an electronically controlled hydromechanical clutch. By the way, since the mid-80s, Toyota 4WD has been working on the same principle on the A241H and A540H boxes, but now, alas, it has remained only on the original rear-wheel drive models (FullTime-H or i-Four all-wheel drive).
Subaru usually attaches a fairly advanced VDC (Vehicle Dynamic Control) system to the VTD, in our opinion - a system of exchange rate stability or stabilization. At the start, its component, TCS (Traction Control System), slows down the slipping wheel and slightly strangles the engine (firstly, by the ignition timing, and secondly, even by turning off part of the nozzles). Classic dynamic stabilization works on the go. Well, thanks to the ability to arbitrarily slow down any of the wheels, VDC emulates (simulates) a cross-axle differential lock. Of course, this is great, but you should not seriously rely on the capabilities of such a system - so far, none of the automakers has even managed to bring the "electronic lock" closer to traditional mechanics in terms of reliability and, most importantly, efficiency.
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1.3. "V Flex" |
Permanent front-wheel drive, no center differential, viscous coupling for rear wheels
Probably worth mentioning is 4WD, which is used on small models with CVTs (like the Vivio and Pleo). Here the scheme is even simpler - a permanent front-wheel drive and a rear axle "connected" by a viscous coupling when the front wheels slip.
We have already said that in English under the concept of LSD everyone gets self-locking differentials, but in our tradition this is usually called a system with a viscous coupling. But Subaru used a whole range of LSD differentials in different designs on their cars ...
2.1. Old style viscous LSD
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In the LSD differential, the right and left side gears are "connected" through a viscous coupling - the right splined shaft passes through the cup and engages with the clutch hub (the differential satellites are mounted cantilevered). The clutch housing is one piece with the gear of the left axle shaft. In a cavity filled with silicone fluid and air, there are discs on the splines of the hub and body - the outer ones are held in place by spacer rings, the inner ones are able to move slightly along the axis (for the possibility of obtaining a "hump effect"). The clutch works directly on the difference in speed between the right and left axle shafts.
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During rectilinear motion, the right and left wheels rotate at the same speed, the differential cup and side gears move together, and the moment is equally divided between the axle shafts. When there is a difference in the frequency of rotation of the wheels, the housing and the hub with the disks fixed to them move relative to each other, which causes the appearance of a friction force in the silicone fluid. Due to this, in theory (only in theory), there should be a redistribution of torque between the wheels.
2.2. New viscous LSD
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- Impreza WRX manual transmission until 1997
- Forester SF, SG (except FullTime VTD + VDC versions)
- Legacy 2.0T, 2.5 (except FullTime VTD + VDC versions)
Working fluid - transmission oil class API GL-5, viscosity according to SAE 75W-90, capacity ~0.8 / 1.1 l.
2.3. Friction LSD
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The next in line of appearance is the friction mechanical differential, used on most versions of the Impreza STi since the mid-90s. The principle of its operation is even simpler - side gears have a minimum axial play, a set of washers is installed between them and the differential housing. When there is a difference in the speed between the wheels, the differential works like any free one. The satellites begin to rotate, while there is a load on the gears of the axle shafts, the axial component of which presses the pack of washers and the differential is partially blocked.
The cam-type friction differential was first used by Subaru in 1996 on turbo Imprezas, then it appeared on the Forester STi versions. The principle of its operation is well known to the majority from our classic trucks, shishigs and UAZs.
There is actually no rigid connection between the drive gear of the differential and the semi-axes, the difference in the angular velocity of rotation is provided by slipping of one semi-axis relative to the other. The separator rotates together with the differential case, the keys (or "crackers") fixed on the separator can move in the transverse direction. The protrusions and cavities of the cam shafts, together with the keys, form a transmission of rotation, like a chain.
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Scope (on domestic market models):
- Impreza WRX after 1996
- Forester STi
The working fluid is an ordinary gear oil of API GL-5 class, viscosity according to SAE 75W-90, capacity ~ 0.8 l.
Eugene
Moscow
[email protected] website
Legion-Autodata
Information on car maintenance and repair can be found in the book (books):
The question is interesting, especially since last year the Japanese brand celebrated the 40th anniversary of the moment the first four-wheel drive car, the Subaru Leone Estate Van 4WD, rolled off the assembly line of the enterprise. A little statistics - for forty years, Subaru has produced more than 11 million copies of cars with all-wheel drive. To this day, all-wheel drive from Subaru is considered one of the most efficient transmissions in the world. The secret of the success of this system is that Japanese engineers use a symmetrical torque distribution system between the axles and between the wheels, which allows the machines on which this type of transmission is installed to effectively cope with off-road conditions (Forester, Tribeca, XV crossovers), so and feel confident on sports tracks (Impreza WRX STI). Of course, the system's effect would not be complete without the company's signature Boxer horizontally-opposed engine, which sits symmetrically along the car's longitudinal axis while the all-wheel drive system is pushed back toward the wheelbase. This arrangement of the units provides Subaru vehicles with stability on the road due to low body roll - since the horizontally opposed engine provides a low center of gravity, and the car does not experience oversteer or understeer when cornering at speed. And constant traction control on all four drive wheels allows you to have excellent grip on the road surface of almost any quality.
I note that the symmetrical all-wheel drive system is just a common name, and Subaru has four systems themselves.
I will briefly point out the features of each of them. The first, commonly referred to as sports all-wheel drive, is the VTD system. Its feature is to improve the turning characteristics of the car, which is achieved through the use of an interaxle planetary differential and a multi-plate hydraulic locking clutch, which is controlled electronically. The basic distribution of torque along the axles is expressed as 45:55, but at the slightest deterioration in the condition of the road surface, the system automatically equalizes the torque between both axles. This type of drive is equipped with models Legacy GT, Forester S-Edition, Impreza WRX STI with automatic transmission and others.
The second type of symmetrical all-wheel drive, used on the Forester with automatic transmission, Impreza, Outback and XV with Lineatronic transmission, is called ACT. Its peculiarity is that its design uses a special multi-plate clutch that corrects the distribution of torque between the axles depending on the condition of the road surface. By default, the moment in this system is distributed in a ratio of 60:40.
The third type of all-wheel drive transmission from Subaru is the CDG, which uses an interaxle self-locking differential and a viscous coupling. This system is designed for manual transmission models (Legacy, Impreza, Forester, XV). The torque distribution ratio between the axles in a normal situation for this type of drive is 50:50.
Finally, the fourth type of all-wheel drive in Subaru is the DCCD system. It is installed on the Impreza WRX STI with "mechanics", distributes the torque between the front and rear axles in a ratio of 41:59 using a multi-mode center differential, which is controlled electrically and mechanically. It is the combination of mechanical, when the driver himself can choose the moment of locking the differential, and electronic locks that makes this system flexible and suitable for use in racing under extreme conditions.
Mechanical boxes, by tradition, are of little interest to us. Moreover, everything is quite transparent with them - since the second half of the 90s, all Subaru on the mechanics have an honest all-wheel drive with three differentials (the center differential is blocked by a closed viscous coupling). Of the negative sides, it is worth mentioning an overly complicated design obtained by combining a longitudinally mounted engine and the original front-wheel drive. As well as the refusal of the Subarovites from the further mass use of such an undoubtedly useful thing as a downshift. On single "sports" versions of the Impreza STi, there is also an advanced manual transmission with an "electronically controlled" center differential (DCCD), where the driver can change the degree of its blocking on the go ...
But let's not digress. There are two main types of 4WD used in automatic transmissions currently operated by Subaru.
|
1. Active AWD / Active Torque Split AWD |
|
1 - torque converter lock-up damper, 2 - torque converter clutch, 3 - input shaft, 4 - oil pump drive shaft, 5 - torque converter clutch housing, 6 - oil pump, 7 - oil pump housing, 8 - transmission housing, 9 - speed sensor turbine wheel, 10 - 4th clutch, 11 - reverse clutch, 12 - 2-4 brake, 13 - front planetary gear set, 14 - 1st clutch, 15 - rear planetary gear set, 16 - 1st brake gear and reverse, 17 - transmission output shaft, 18 - "P" mode gear, 19 - front drive gear, 20 - rear output shaft speed sensor, 21 - rear output shaft, 22 - shank, 23 - clutch A- AWD, 24 - front drive driven gear, 25 - freewheel, 26 - valve block, 27 - sump, 28 - front output shaft, 29 - hypoid gear, 30 - impeller, 31 - stator, 32 - turbine. |
This option has long been installed on the vast majority of Subaru (with automatic transmission type TZ1) and is widely known from the '89 Legacy model. In fact, this four-wheel drive is as “honest” as the fresh Toyota Active Torque Control - the same rear-wheel drive and the same TOD (Torque on Demand) principle. There is no center differential, and the rear-wheel drive is activated by a hydromechanical clutch (friction package) in the transfer case.
The Subar scheme has some advantages in the working algorithm over other types of plug-in 4WD (especially the simplest ones, like the primitive V-Flex). Albeit small, but the moment during A-AWD operation is constantly transmitted back (unless the system is forcibly turned off), and not only when the front wheels slip - this is more useful and efficient. Thanks to hydromechanics, the force can be redistributed a little more accurately than in an electromechanical ATC. In addition, A-AWD is structurally more durable and not prone to overheating. For cars with a viscous coupling for connecting the rear wheels, there is a danger of a sharp spontaneous “appearance” of the rear drive in a turn, followed by an uncontrolled “flight”, but in A-AWD this probability, although not completely excluded, is significantly reduced. However, with age, as wear and tear, the predictability and smoothness of the connection of the rear wheels decreases significantly.
The algorithm of the system remains the same throughout the entire release period, only slightly corrected.
1) Under normal conditions, with the accelerator pedal fully released, the torque distribution between the front and rear wheels is 95/5..90/10.
2) As you press on the gas, the pressure supplied to the clutch package begins to increase, the discs gradually tighten and the torque distribution begins to shift towards 80/20 ... 70/30 ... etc. The relationship between gas and line pressure is by no means linear, but rather looks like a parabola - so that a significant redistribution occurs only when the pedal is pressed hard. With a fully recessed pedal, the friction clutches are pressed with maximum effort and the distribution reaches 60/40 ... 55/45. Literally, "50/50" is not achieved in this scheme - this is not a hard lock.
3) In addition, the speed sensors of the front and rear output shafts installed on the box make it possible to determine the slip of the front wheels, after which the maximum part of the moment is taken back regardless of the degree of gas supply (except for the case of a fully released accelerator). This function is active at low speeds, up to about 60 km/h.
4) When forced into 1st gear (selector), the clutches are immediately pressed with the maximum possible pressure - thus, as it were, "difficult all-terrain conditions" are determined and the drive remains the most "permanently full".
5) When the "FWD" fuse is plugged into the connector, no overpressure is supplied to the clutch and the drive is constantly carried out only on the front wheels (distribution "100/0").
6) With the development of automotive electronics, it has become more convenient to control slippage using standard ABS sensors and reduce the degree of clutch blocking when cornering or ABS is triggered.
It should be noted that all passport distributions of moments are given only in conditional statics - during acceleration / deceleration, the weight distribution along the axes changes, so the real moments on the axes are different (sometimes "very different"), just like with different coefficients of adhesion of wheels to the road .
|
2. VTD AWD |
|
1 - torque converter lock-up damper, 2 - torque converter clutch, 3 - input shaft, 4 - oil pump drive shaft, 5 - torque converter clutch housing, 6 - oil pump, 7 - oil pump housing, 8 - transmission housing, 9 - speed sensor turbine wheel, 10 - 4th clutch, 11 - reverse clutch, 12 - 2-4 brake, 13 - front planetary gear set, 14 - 1st clutch, 15 - rear planetary gear set, 16 - 1st brake gear and reverse, 17 - countershaft, 18 - "P" mode gear, 19 - front drive gear, 20 - rear output shaft speed sensor, 21 - rear output shaft, 22 - shank, 23 - center differential, 24 - center differential lock clutch, 25 - front drive driven gear, 26 - overrunning clutch, 27 - valve block, 28 - sump, 29 - front output shaft, 30 - hypoid gear, 31 - impeller, 32 - stator, 33 - turbine . |
The VTD (Variable Torque Distribution) scheme is used on less massive versions with automatic transmissions such as TV1, TG (and TZ102Y, in the case of the Impreza WRX GF8) - as a rule, the most powerful in the range. Here, everything is in order with "honesty" - the all-wheel drive is really permanent, with an asymmetric center differential (45:55), which is blocked by an electronically controlled hydromechanical clutch.
By the way, since the second half of the 1980s, Toyota 4WD has been working on the same principle on A241H and A540H boxes, but after 2002, alas, it remained only on the original rear-wheel drive models (FullTime-H or i-Four all-wheel drive for Mark/Crown families).
Subaru usually attaches a fairly advanced VDC (Vehicle Dynamic Control) system to the VTD, in our opinion - a system of exchange rate stability or stabilization. When starting, its component, TCS (Traction Control System), slows down the slipping wheel and slightly strangles the engine (firstly, by the ignition timing, and secondly, by turning off part of the nozzles). Classic dynamic stabilization works on the go. Well, thanks to the ability to arbitrarily slow down any of the wheels, VDC emulates (simulates) a cross-axle differential lock. Of course, one should not seriously rely on the capabilities of such a system - so far, none of the automakers has been able to bring the "electronic lock" closer to traditional mechanics in terms of reliability and, most importantly, efficiency.
|
3. "V-Flex" |
Probably worth mentioning is 4WD, which is used on small models with CVTs (like the Vivio and Pleo). Here the scheme is even simpler - a permanent front-wheel drive and a rear axle "connected" by a viscous coupling when the front wheels slip.
March 2006
autodata.ru
