ZF Takes Evolutionary Approach to 6-Speed Transmission

After introducing a family of 6-speed automatic transmissions in 2001—6 HP 19, 6 HP 26 and 6 HP 32—ZF Friedrichshafen AG (www.zf.com) is turning another page via the debut of its latest iteration, the 6 HP 28, which provides improved fuel economy and quicker shift times. The 6 HP 28 can be found under the sheet metal of the BMW 3-Series coupe and X5 SUV. “There is a lot of bang for the buck for automakers in terms of efficiency improvements in the area of transmission technology,” said Tim Miskotten, account manager for ZF Industries.

While some automakers have decided to up the ante in terms of overall gears—Lexus’ 8-speed automatic taking the cake—ZF took a different tack. The new transmission features improved torque converter technology aimed at reducing slippage while helping raise fuel economy 3% in gasoline and 6% in diesel applications. Engineers focused on speeding up the lock up of the converter clutch, which posed some challenges in terms of NVH characteristics. The solution was the development of a turbine torsion vibration damper for gasoline-powered applications and a twin-torsional damper for diesel applications, both of which help to eliminate vibrations during hard driving. The new converter itself helps to produce the majority of the fuel consumption improvements. On the performance side, engineers worked to reduce shift times to as little as 200 milliseconds, compared with as long as 800 milliseconds on the previous iteration, by optimizing hydraulics—through direct control of the clutch via a separate pressure regulator—and pre-filling the five clutch packs. This configuration enables the transmission to better identify its target gear quickly, based on load condition, vehicle speed and driving situation. “The driver’s level of perception is about 200 milliseconds, and we have kept our shifts at that threshold mostly through changes to the controls and the pre-fill of the clutches,” Miskotten added. Other notable mechanical changes are modifications to the shaft and carriers to handle increased torque loads, along with changes to the Lepelletier gear sets to enhance their life expectancy. The use of a planetary gear set as well as a Ravigneaux gear set with five shift elements provide additional weight savings.

OEMs can tailor the behavior of the transmission to each individual brand’s DNA through software modifications. Each upgraded 6-speed will feature the same gear sets and kinematics, although how the transmission performs will be decided on by the OEMs engineering department. “Fundamentally, BMW will have the same gearbox as, say, Jaguar would, but each will calibrate and setup the transmission based on how they want it to act. The hardware is basically the same,” Miskotten said.

ZF plans to bring out its third-generation 6-speed automatic in 2010. The 6 HP 34 will come with a 5% improvement in fuel economy over the latest generation. Also on the drawing board is a pair of dual-clutch 7-speed gearboxes—7 DCT 50 and 7 DCT 70. Insiders are mum on whether a 7-speed fully-automatic unit will follow, or whether there is an 8-speed in ZF’s future.—KMK