The ZF Eight-Speed Automatic Transmission
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The eight-speed transmission is not new to the automotive world. Toyota has had one in its Lexus LS and IS F sedans for a few years. But now, transmission giant ZF is bringing eight speeds to the masses with promises of improved fuel economy.
It’s known by the folks in its hometown of Friedrichshafen, Germany, as the “8HP kit,” and this transmission offers myriad options for automakers. It is already in the BMW 760Li and 5-series GT. The new Rolls-Royce Ghost, Bentley Mulsanne, and Audi A8 will all be equipped with the 8HP when they roll out of factories in the coming year.
Four planetary gearsets combine for eight gears in all. These gearsets are engaged and disengaged by five shifting elements. Toyota’s eight-speed gearbox also has four planetary gearsets, but it uses seven shifting elements to create eight ratios—the ZF is a simpler design.
ZF uses the term “kit” because 8HP is not a single transmission but rather one transmission case with various internal, customer-selectable options. It is designed for longitudinal, or “north-south,” applications. It can be set up for rear- or all-wheel drive and facilitates hybridization by providing the option of swapping the conventional torque converter for a 47-hp electric motor, increasing fuel economy by a claimed 25 percent when compared with ZF’s old six-speed (6HP). To save weight, the electric motor can also double as the starter. Expect to see a hybrid using the 8HP by the end of 2011.
A third coupling device is available for the most-performance-driven applications: a wet multiplate clutch pack developed by ZF, which is currently found in the Mercedes-Benz E63 and SL63.
ZF designed the 8HP to replace the 6HP—of which over 5 million have been produced—and, in doing so, made the new transmission identical not only in dimension but also in weight (200 pounds). This is an important consideration, as the six-speed is widely employed in the luxury-sport ranks. The 6HP will be out of production by 2014.
ZF says the 8HP is six percent more efficient than the outgoing sixer and nearly as efficient as a dual-clutch automated manual, which ZF also produces. However, the eight-speed is roughly 20 percent cheaper and retains the smooth, torque-converter starts to which U.S. buyers have become accustomed. Furthermore, the 8HP has a system to maintain hydraulic pressure even when the engine isn’t running and is therefore the first automatic that’s compatible with stop-start operation—good for 11 percent better fuel efficiency, says ZF. In the CAFE-conscious U.S. market, every percentage point is going to count.
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