Even as it introduces the concept of an eight-speed automatic transmission that it will put into production in two years, ZF has made it clear that its strategy is not about the number of gears, but what those gears can do. Since introducing the 6 HP 26 six-speed automatic transmission for passenger cars and light trucks in 2001, ZF has made what it calls “significant advancements” in the recent technical upgrade of the transmission in the form of the new 6 HP 28.

    Even as it introduces the concept of an eight-speed automatic transmission that it will put into production in two years, ZF has made it clear that its strategy is not about the number of gears, but what those gears can do. Since introducing the 6 HP 26 six-speed automatic transmission for passenger cars and light trucks in 2001, ZF has made what it calls “significant advancements” in the recent technical upgrade of the transmission in the form of the new 6 HP 28.BMW.

At about 95 kg (210 lb) and 700 N·m (520 lb·ft), the    second-generation 6 HP 28 was engineered for multiple downshifts without interruption.

    While the geartrain itself is fundamentally the same, the use of a new vibration- reducing torque converter, in combination with optimized software control and hydraulics, has resulted in reduced shift times by 50%, says ZF. A 3% fuel economy improvement is expected with gasoline engines via a turbine torsional vibration damper, with 6% improvement for diesel engine applications via a twintorsional damper (Twin TD). A diesel’s harsher NVH characteristic was the impetus behind the choice of different dampers for the gasoline and diesel engines.

    The new torque converter improves the connection between the driveline and the engine, allowing the driver to feel a more active and direct vehicle response when accelerating; the objective is to close the clutch completely as soon as possible.

    Downshifts are performed in up to four stages without any time losses or noticeable deceleration. The hydraulic system enables the clutch to be directly controlled by a separate pressure regulator so that no additional time is required for jumping a drive stage. To find the target gear, control electronics determine the optimum gear based on load conditions, driving situation, and vehicle speed. If a driver is cruising in sixth gear and wants to rapidly accelerate, the transmission needs just 100 ms to trigger the change from sixth to second gear. The response time for downshifts is approximately 200 ms, below the limit of human perception

    The new 6 HP 28 uses the Lepelletier gear set, which allows for six ratios with fewer components than previous fivespeed transmissions, and thus a lessweighty gear set. The system uses a planetary gear set as well as a Ravigneaux gear set with five shift elements. A plastic oilpan also contributes to weight reduction

    Manufactured in Saarbrücken, Germany, the new transmission is currently available in the BMW 3 Series Coupe, X3, and X5. It is designed for high-torque engines, including 12-cylinder applications.

   ZF produced nearly one million sixspeed automatic transmissions in 2006.

Torque requirements have certainly not come down as power has gone up, and the diesel versions of ZF’s new 6 HP 28 automatic transmission features a twintorsional damper that reduces vibrations in the driveline and contributes to a 6% increase in fuel economy.