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New Edition of a Classic

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Tags: ZeroEmissions, Emobility, Efficiency

In the future, internal combustion engines will continue to be widely used. With the fourth generation of its 8-speed automatic transmission, ZF presents a drivetrain that delivers an impressive performance – particularly in hybrid electric vehicles.
Johannes Winterhagen, September 10, 2019
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Johannes Winterhagen When things get really complicated, the journalist for energy and mobility topics is in his element. He prefers to meet engineers in the laboratory.
ZF’s 8-speed automatic transmission for passenger cars has enjoyed a fairytale career, with the number of units sold since its market launch in 2009 reaching the upper double-digit millions range. Since its launch, the ZF engineers have twice revised this torque converter transmission, which has received high praise from both motorists and automotive journalists, so that it can be installed lengthwise. With each revision, the transmission has become even easier to use and at the same time even more efficient. The fourth generation of the 8-speed transmission is now waiting in the wings.

Extraordinary: designed from the outset as a hybrid transmission

Extraordinary: designed from the outset as a hybrid transmission

Production of this transmission will begin in 2022. The only thing in common that the new drivetrain has with its predecessor models is that it still has eight gears; apart from that, the new model has a great many new features. “For the first time ever, we have designed an automatic transmission for the premium segment from the outset as a hybrid transmission,” said Stephan von Schuckmann, before adding: “This gave rise to a modular system comprising electrical, hydraulic, and mechanical modules, from which every customer can pick and choose to create the ideal drive for their vehicle.” Von Schuckmann heads the Car Powertrain Technology division at ZF.
The fourth generation of ZF’s 8-speed automatic transmission is completely suitable for use in all hybrid concepts.

New transmission as the answer to new legislative requirements

New transmission as the answer to new legislative requirements

Why has a modular concept been chosen? The reason behind this is the stricter CO2 emission limits set out by legislators, with which automotive manufacturers will have to comply over the next 10 years. As only the emissions generated by the vehicle itself are currently taken into consideration – and not, for example, the emissions generated in the production of the vehicle – almost every manufacturer is attempting to increase their sales volume for battery-electric vehicles. Electric vehicles, however, are not suitable for every customer or every application. “We are thus assuming that around 70 percent of new cars in 2030 will still have an internal combustion engine,” explained von Schuckmann. With the aim of reducing fuel consumption and in turn emissions in such vehicles, many different types of hybridization will become popular in the next decade.

Ideally equipped for every hybrid application

Ideally equipped for every hybrid application

The first concept worth mentioning here is the powerful plug-in hybrid drive. Vehicles equipped with these drives can reach speeds of up to 130 km/h and have a range of 80 to 100 kilometers, powered solely by electricity. At ZF, this type of drive is named EVplus – an electric drive supported by an internal combustion engine.
At the other end of the performance spectrum are the 48-volt mild hybrid systems. With these drives, the electric motor is used to provide additional power when the vehicle is first started and during acceleration, and also converts the energy generated in braking procedures to electricity, which is then stored in the battery. The new 8-speed automatic transmission is completely suitable for use in all of these hybrid concepts and can achieve top electrical performances between 24 and 160 kilowatts.
This explains why the amount of work that needed to be carried out on the new transmission was more than just technical fine-tuning. To meet the requirements of the new plug-in hybrid vehicle models, a powerful electric motor had to be integrated into the limited installation space within the transmission housing (torque converter bell housing).
“For the first time ever, we have designed an automatic transmission for the premium segment from the outset as a hybrid transmission.”
— Stephan von Schuckmann, heads the Car Powertrain Technology division at ZF

New efficient electric motor in use

New efficient electric motor in use

This electric motor is manufactured in an innovative process by the E-Mobility division at ZF. The performance of an electric motor depends on the “copper filling ratio” of the rotor, i.e. the ratio of conductive material to the total volume. To maximize this ratio, the ZF engineers are no longer utilizing windings of copper wire for the high-performance variants, but now use copper bars that are inserted and welded together.

Power electronics – now integrated in the transmission housing

Power electronics – now integrated in the transmission housing

Another new feature is the place in which the power electronics are housed.
In all series hybrid vehicles, these were previously installed in a separate box somewhere in the vehicle, as space in the engine compartment was in very short supply. As a result, cables measuring meters in length could often be found running from the power electronics to the transmission. This is no longer necessary with the new transmission, as the entire power electronics is integrated into the transmission housing. This proved to be a huge challenge for the team of developers headed by Dr. Michael Ebenhoch. “On the one hand, we had to free up space in the housing, while on the other, we had to design the power electronics to be as small as possible,” explained Head of Development from the Car Powertrain Technology division. The downscaling of the power electronics did set limits, however.

New approaches to cooling, switching, and the oil circuit

New approaches to cooling, switching, and the oil circuit

Although the power semiconductor itself is not very big, it switches high currents. As part of this process, the semiconductors generate many kilowatts of thermal output, which needs to be discharged. Instead of the power electronics being cooled by transmission oil, they are connected to the refrigerant circuit in the vehicle’s air-conditioning system. During their attempts to find sufficient space for the power electronics in the transmission housing, Ebenhoch’s team of engineers analyzed all of the components in the hydraulic actuators involved in the gear-changing process. Electromagnetic direct shift valves now carry out this work. In contrast to the electric pressure actuators that were previously used, these electromagnetic direct shift valves do not require additional pistons and bushes. By replacing these actuators, the amount of installation space needed for the hydraulic control unit fell from 3.1 liters to 1.8 liters.
All of the other components in the new transmission construction kit were also intelligently designed for hybrid operation. This is especially evident in the oil circuit. Previously, two oil pumps were used: a very efficient vane cell pump driven directly by the internal combustion engine and a second electric pump or pulse memory for electric operation. The new 8-speed automatic transmission comes equipped with one single power-split pump. When the internal combustion engine is switched off, the pump is driven by a small electric motor.

Mechanics improved once again for greater efficiency

Mechanics improved once again for greater efficiency

At first glance, no changes have been made to the mechanics of the fourth-generation transmission: two brakes and three clutches continue to switch the four planetary gearsets. “We have, however, checked every detail and most notably have lowered the friction loss considerably,” explained Ebenhoch. The reward for all their hard work: by optimizing the mechanics alone, the amount of CO2 emitted for every kilometer in which the internal combustion engine is in use has fallen by a gram.
Work on the fourth generation of the 8-speed automatic transmission is now at a very advanced stage. BMW and Fiat Chrysler have already decided to use this transmission in their vehicles. The first vehicle road tests will be carried out later this year and the ZF transmission plant in Saarbrücken is already prepping itself for industrialization.

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