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Slick and quick
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01/02/2007
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Redesigning the mechatronic systems, including the hydraulic controller, allowed ZF to massively speed up the response time of its six-speed auto box as well as boosting the torque handling, writes Roger Bishop.
ZF has re-engineered its six-speed automatic transmission to reduce both response and shift-to-shift times by 50%, raise its torque transmitting capability and to allow multiple downshifts without interruption.
Now being fitted to BMW’s new 3 Series Coupé and the X3 – both experienced by European Automotive Design during a day of test drives – the shift quality is truly exceptional and the change strategy such that the vehicle never appears to be ‘stranded’ in the wrong gear. In addition, the lock-up clutch can be closed at engine speeds as low as 1,000rev/min, and across the whole working range of the transmission, although BMW chooses to have it open momentarily during shifts so that the engine revs up to a higher point on the torque curve.
Downshift times have been reduced from typically 400ms to below or around the ‘perception threshold’ of 200ms. The 6 to 5 shift has moved from 990ms to around 210ms, a massive performance increase.
Fuel savings delivered by the new gearbox are estimated at 3% for gasoline engines and up to 6% for diesels. Models are available for input torques up to 850Nm – in other words engines up to 12 cylinders.
Most of the gearbox’s improved performance – about 80%, according to ZF engineers – has been realised by redesigning the mechatronic systems, including the hydraulic control unit. Other major changes include a fourth gear being added to the planetary gearset, a 0.5mm increase in main shaft diameter increasing torque capacity by 18%, and a revised torsional damping system within the torque converter.
Within the mechatronic control unit, the hydraulic logic manifold has been redesigned so that the fluid channels are wider but shorter and with fewer sharp changes in direction. The piston that presses the lamellas together is also now much slimmer so that only about half the previous volume of oil needs to flow through the channels to actuate it. In addition, the piston now has a single lip seal rather than an O-ring seal, reducing both friction and hysteresis. All these measures contribute very significantly to the faster response speed, as does the fact that each of the five clutches and the torque converter clutch, as well as the system pressure controller, have their own individual pressure regulator. As a result no additional time is required for jumping a drive stage.
The control software uses data received on load conditions, the driving situation, and the speed with which the accelerator pedal is actuated, to decide whether downshifts are effected for just one single drive stage or several. This decision depends upon the swiftness for acceleration commanded by the driver.The response time remains the same whether the system shifts through one or several gears. In addition the gearbox can be switched from cruise into the sports mode in less than 1s.
The shift strategy for the torque converter lock-up clutch is to close the clutch as soon as soon as possible – directly after setting off. To overcome vibration engineers developed a new in-house torque converter: For gasoline-powered engines, a turbine torsional vibration damper is used but for diesel-powered vehicles a special twin torsional damper (Twin TD) has been developed. This combines an outer spring set with an inner spring set and transmission connection. ZF engineers say the Twin TD could also find applications in four-cylinder gasoline engines along with future DI applications.
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Author
Roger Bishop
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