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Automatic transmission technology: More efficiency and comfort

Hans-Dieter Seufert
Automatic transmission technology
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Z wipe the engine and drive wheels, automotive technology has set the gears. Of course, this is not due to a whim of the engineers, but on the one hand to the rather unfavorable torque characteristics of the combustion engine and on the other hand to the extremely different operating conditions of a car. It has always been important to combine both with the transmission so that the engine can operate in its most efficient operating range as often as possible.

By five to eight gears

Up to the year 2000, five gears were considered the optimum for automatic car transmissions. In 2001, ZF brought a six-speed automatic transmission onto the market with the 6HP generation. With this, the specialists from Friedrichshafen gained more than just one tooth. The main advantage was the significantly wider spread of the gear ratios - i.e. the factor between first and last gear - to the peak value of 6.04 at the time. This advance benefits both acceleration and fuel economy. The fact that the technicians managed to reduce the number of individual parts by around 30 percent was thanks to advances in calculation methods and test technology - and the first use of the so-called Lepelletier wheel system, a further development of the Ravigneaux wheel set.

The technicians understand this as an extended variant of transmission gears (so-called sun and planet gears) to form a planetary gear set. Advantage of the Lepelletier wheel system: It makes it much easier to increase the number of gears.

After Mercedes had increased the number of gears with its 7G-Tronic with seven levels in 2003, it already dictated the logic of further development that ZF took a further step with the 8HP in 2009. Although the installation dimensions remained unchanged compared to the predecessor, and despite the additional gear ratios, the weight of the new automatic transmission fell by at least three percent.

ZF-8HP transmission in the BMW 5 Series since July

Technically, a completely new gear set concept marks the progress of the eight-speed automatic. With only four sets of wheels, it can be implemented with just five shift elements - namely three multi-plate clutches and two so-called brakes. And because fewer switching elements also fewerThat means operating effort and less drag losses, the overall efficiency can also be significantly improved. ZF speaks of a possible reduction in fuel consumption of around six percent - not least a consequence of the spread widened to 7.0, which enables the gear ratio to be adapted even more sensitively to the engine characteristics.

Due to the limit values ​​for In the meantime, ZF has further developed the 8HP transmission for CO2 emissions and thus consumption values. The second generation has been used in the BMW 520d since July. All other automakers that ZF has been supplying with the 8HP so far will gradually switch to this generation.

In this case, further development means a wealth of less spectacular, but definitely effective refinements in detail, which ultimately lead to further improvements in fuel consumption promise three percent. In order to reduce transmission losses even further, the second generation comes up with a further increased spread to 7.8, which lowers the speed across all gears by an average of 50 revolutions per minute. In addition, the oil pump works with a system pressure that has been reduced from five bar to 3.5 bar and therefore requires less energy. A new lamella separation also has a positive effect: Additional springs integrated in the laminae packs of the shifting elements ensure that the friction shifting elements are almost completely opened and thus cause less drag torque. The power loss is reduced by over two thirds compared to the first 8HP generation.

ZF tends to 'downspeed'

With this transmission concept, ZF is also taking into account the now strongly pronounced trend towards lowering engine speeds - so-called 'downspeeding'. The automobile manufacturers see this as a key factor on the way to even higher efficiency with conventional drive trains.

In addition, the new eight-speed automatic transmission offers further optimized economy functions: By decoupling the transmission and at the same time temporarily switching off the engine, the So-called sailing mode can now tease out further savings potential up to a speed of 160 km /h.

At the other end of the speed scale, ZF has further improved the optional start-stop function of the 8HP made possible by the hydraulic pulse memory: After When the vehicle coasts to a stop and comes to a standstill, it now switches the engine off without any noticeable delay - instead of only after 1.5 seconds as before.


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