ZF Leads Multi-speed Transmission Race
Media reports prompted by a ZF press announcement at January’s Detroit auto show have hailed the beginning of a race toward increased ratios in automatic transmissions.
At the show, ZF outlined its development of what is claimed to be the world's first nine-speed automatic transmission for vehicles equipped with transversely mounted engines. Press reports, including one in the SAE’s Engineering International Online, indicate that the unit will be supplied to Chrysler Group LLC's minivans starting in 2013 − the first application of a nine-speed gearbox for a mass-market vehicle.
Production will be at the new facility near Greenville, South Carolina, which was first announced in 2010. This facility will also manufacture an eight-speed automatic transmission for longitudinal fitment for Chrysler. At the 2009 Frankfurt show, ZF had displayed a new transverse, front-drive automatic without giving any technical details, such as torque handling capacity, ratio spread or the number of gear ratios.
Compared to conventional, six-speed automatic transmissions for front-drive platforms, ZF asserts that its new nine-speed automatic transmission enhances driving performance and fuel economy. An advanced shock absorber system in the torque converter allows rapid lock-up of the converter clutch, enabling greater fuel economy and lower CO2 emissions. Sophisticated electronic controls select the right gear for the driving conditions, eliminating unnecessary "stepping" – or constant shifting. ZF also says that, in a similar fashion to its eight-speed 8HP automatic transmission used in rear-drive platforms, the new nine-speed delivers extremely short response and shifting times that are below the threshold of perception.
Requests by DCTfacts.com to ZF for further information on the new transmission were declined; the company stated that full details would only be given at a more formal launch in June of this year.
Automotive Engineering Online quotes industry sources as saying the unit has a ratio spread of 6.4 and a torque handling capacity of 400 Nm. This would make it suitable for premium diesel models, as well as mid-market vehicles.
However, it is plausible that the internal architecture of the new unit could be informed by a conceptual study carried out by ZF and reported on at the CTI Transmission Symposium in Berlin in December 2010. In the study, ZF compared the theoretical merits of a number of multi-speed stepped transmission formats, including both DCTs and planetary automatics. For its planetary model, ZF started from the basis of its established eight-speed in-line, rear-drive 8HP. To improve the suitability of this unit, with its four planetary gearsets, for the transverse packaging where axial length is critical, ZF rearranged two of the gearsets radially, so that the ring gear of the inner gearset also formed the sun gear of the outer gearset. This gave the design the axial length of a three-gearset transmission to allow it to fit within the required envelope for front drive.
Another advantage of this transmission, said ZF in the simulation study, was the low number of open shift elements in most gears, helping to keep drag torque to a minimum.
Story Filed: 3/15/2011
By Tony Lewin, managing editor DCTfacts.com