Under the skin: the evolution of an automatic transmission

ZF 9HP gearbox cutaway

ZF automatic transmissions for 9 horsepower have both evolved far beyond a box to replace cogs. They are now part of super-efficient integrated powertrains.

From early three-speed pioneers to today's silky-smooth nine-speed setup, the automatic transmission has become a truly advanced piece of kit.

Since the turn of the millennium and rapidly evolving emissions legislation, automatic transmission technology has moved on at a pace. To perform economically, engines must be run at the most efficient part of their duty cycle, the 'sweet spot', as often as possible.

To do this, they need a transmission with a sufficient number of gear ratios. Cars increased from three to four gears in the 1980s and from four to five in the 1990s, but to make the jump to six took some figuring out, due to the extra floor space needed. Planetary (aka planetary) gearboxes are the building block for traditional automatic transmissions and consist of a sun gear surrounded by three or more planet gears inside a ring gear. German transmission manufacturer ZF was one of those that managed to make a huge leap in automatic gearbox design in 2002 by incorporating the compact Lepelletier gearbox concept with planetary gearsets. This allowed more gear ratios to fit in the same or smaller space.

The new ZF gearbox for 6HP, which (not surprisingly) replaced the 5HP and made its first appearance in the fourth generation of the BMW 7 Series. It carried the game a long way, with a torque converter lock-up to prevent "slippage" in the fluid clutch, not just in top gear, but at all speeds. It can decouple the engine from the torque converter to save fuel too, and the new 'ASIS' adaptive strategy has a shift. It contains only 470 components instead of 5 liters. With. The 660 is shorter (thanks to a new compact gearbox), weighs 13% less, accelerates faster and uses 7% less fuel. It also has an integrated brain, the so-called mechatronic modules, marking care from the engine and gearbox to work independently in the new philosophy of 'integrated transmission', where the engine and gearbox communicate with each other. Now the two can work together to achieve better fuel economy and emissions. The 6HP has been replaced by the 8HP, adding two more ratios and further refinements to make it even more efficient.

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Another major step, also from ZF, was the introduction of 9 horsepower in 2013. Designed for transverse engines, 9 horsepower was quite different from other automatic transmissions. It supports a stop/start system that uses the new design of internal dog engagement paws, rather than conventional clutch packs to save space, the recommended nested gearboxes, which also helped make it shorter and, as a result, was also lighter than its predecessors.

The shift response was stated to be “below the threshold”, the torque converter locks up at low speeds, it's a 'curve' mode to stop it shifting into corners and other features that make it run better and be as efficient as possible.

Some of the latest transmissions have hybrid-ready features, such as an integrated electric pump to create the hydraulic pressure needed to keep them running even when the engine has been turned off. Torque-converter-based automations continue to evolve with new frills and features, and today they play almost as big a role in engine fuel efficiency.

Hybrid capabilities included

In the latest hybrid automatic transmissions, electric motors perform the job of fluid-based torque converters and serve as the basis for a bolt-on hybrid solution for automakers. This neat, eight-speed ZF gearbox can produce up to 160bhp and the power electronics for control are built into the body for the first time.


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