Deschid topicul asta ca nu am gasit altul in care sa scriu informatiile astea generice.
De cand am luat Outback CVT-ul meu am tot cautat pe net si am gasit multe variante de explicatii, dar asta mi se pare cea mai pertinenta:
"VTD = variable torque distribution. This is a marketing term used by Subaru for the AWD system used on the 5-speed automatic transmission, which is used with the 3.6 engine. It uses a planetary differential to direct power to the front and rear drive shafts. Because of the way the gears in the differential are cut, if the car were to be put on a dynamometer, it would indicate slightly more power at the rear wheels than at the front. Mathematically, based on the actual number of teeth on the gears, the proportion is 45 front, 55 rear.
However, this applies only when all four wheels have good traction and are turning at the same speed. Like a regular "open" differential, as soon as one wheel turns at a different speed, the transfer of power (if I might use the term, which isn't strictly correct) is changed. If a wheel loses traction and begins to spin, the planetary differential will not do anything to prevent this.
What does prevent the wheels from spinning, notwithstanding the VDC system, is a multi-plate transfer (MPT) clutch. The clutch, in effect, is between the front and rear drive shafts, and the AWD control system adjusts the clutch hydraulic pressure to ensure that the two are turning at the same speed. When driving on clear pavement and all wheels are turning at the same speed, the driveshafts are turning together, and the clutch is engaged, but not fully. It's important to realize that at this stage, there is no clutch slip because the two sides are turning at the same rate.
But if one wheel starts to turn faster, the AWD system will detect this, apply more clutch pressure, and this causes the two driveshafts to turn at the same speed. In other words, the driveshaft to the wheel that loses traction will be slowed back down to the speed of the other driveshaft of the wheels that have traction. This eliminates the wheel spin, and ensures that engine power is being maintained to the wheels with traction. This action overrides what the planetary differential is doing, so the 45/55 split is irrelevant.
How does this differ from the AWD system in the CVT? Functionally, very little. The CVT doesn't have a differential. Instead, the front drive shaft is connected, via permanently engaged gears, to the transmission output shaft. The output shaft is also connected to a multi-plate clutch, which is essentially the same as the one in the VTD system. In the CVT case, power to the rear, instead of being transferred via a planetary differential, is connected through the clutch.
As soon as the engine starts, the control system applies hydraulic pressure to the clutch. It's not a lot, but then again, the car isn't even in gear yet, so there really isn't any need. When the gear selector is set to D or R, the hydraulic pressure is immediately increased, perhaps to around 50% of maximum. Then, as the throttle is advanced, this is further increased proportionally. So the rear wheels are receiving power through the clutch (just as in a manual transmission vehicle with rear drive). As long as all four wheels have traction and are turning the same speed, all four wheels are "pulling" the car forward.
Unlike the VTD, where the number of teeth on the differential gears determines how the power is split under good traction conditions, there is no similar calculable number with the CVT's multi-plate clutch. There is always some power to the rear, but it is constantly responding to multiple factors including throttle position, vehicle speed, engine torque, transmission gear ratio etc. But if it were possible to simulate a smooth cruising condition, with nothing changing, on a dynamometer, it would probably come out with a slight bias toward the front. Some sources indicate 60/40, but this is an imaginary situation.
With the CVT, if a wheel starts to spin, the AWD system reacts exactly the same as in the VTD. It increases pressure at the clutch, effectively locking the front and rear driveshafts, thereby slowing the one that has speeded up, and making sure power is sent to the wheels with traction.
If the two systems are working properly, there should be no difference in how they performs from an AWD control perspective. The only perceptable difference might be when driving on clear pavement, and only under specific conditions. The rear bias of the VTD might give the rear a bit more tendency to move out when accelerating on sharp curves, like a RWD vehicle, whereas the CVT-equipped cars will tend to react, if at all, more like a FWD.
I haven't taken into account the effect of the VDC system, but would note that the impact of the VDC is the same in both cases. It can complement or override the AWD in situations requiring VDC implementation.
I hope I've provided an explanation that helps you, and perhaps others, better understand the two AWD systems. "
Daca aveti si alte informatii, ar fi interesant sa le afle si altii aici.
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