8-speed battles DCT at BMW
ZF's new eight-speed planetary automatic transmission is making it hard for dual clutch transmissions at BMW. Increasingly, the company is seeing the DCT as the solution only for the more extreme sports models such as the 420 hp M3, while the full-automatic ZF will equip everything from the top-luxury 7 series to the volume-market 3 series.
While the new Z4 sports roadster, introduced at the Detroit show in January, features the Getrag-supplied M-DCT, the eight-speed ZF will appear first on an additional version of the big 7 series debuting at the Frankfurt show in September. The eight-speed will also be the principal transmission on production versions of the new 5 series GT, which was shown as a concept at the Geneva show in March.
Company product development director Dr Klaus Draeger explained that the eight-speed ZF is now able to shift as fast as a good DCT and, he told DCTFacts.com, offered other advantages too.
"If you want to shift quickly from 6th to 4th, or from 7th to 5th, you can do this in a single step with the eight-speed. On the DCT it takes longer as the shifts are sequential."
Sportier versions of the eight-speed will also be introduced, said Draeger, but he declined to say whether they would use a wet clutch launch device, as on the AMG Speedshift, which allows sharper shifts and higher engine revolutions.
"The torque converter is an excellent launch device," said Draeger. "It has many advantages: we close it out at just 1000 rpm, when the car has traveled only 2 to 3 meters from rest, so it's almost never involved. And besides, its performance stays exactly the same throughout the whole lifetime of the vehicle - we don't ever get warranty claims on torque converters."
The choice of a DCT would be influenced by the vehicle's architecture, added Draeger. "DCT is interesting for transverse applications," he said, noting that it could be hard to package for rear-drive installations, especially powerful ones.
"The DCT is like a manual: when you've got a lot of torque, you have to have bigger gears to handle that torque," he explained. "And when the gears are bigger the transmission shafts have to be further apart, so the packaging envelope becomes larger."
Draeger's argument that DCTs are well suited to transverse applications is an interesting one: the only car with this engine configuration in the BMW group is the Mini - the brand which recently displayed a concept version of a larger Crossover model.
Story Filed: 3/10/2009
By Tony Lewin, managing editor DCTfacts.com