Info on ZF's 8HP automatic transmission

ZF's 8HP automatic transmission is fitted in BMW 7 Series and 5 Series Gran Turismo, while due to its flexible modular design, BMW ActiveHybrid models (earlier post) are using it as well. The all-new Bentley Mulsanne and the Rolls-Royce Ghost also come equipped with the 8HP transmission.

When compared to the second-generation 6-speed automatic transmission, ZF�s new 8-speed automatic transmission not only has a higher gear ratio of 7.05, but also provides considerably improved performance and fuel economy. ZF�s standard 8-speed transmission can be used with different drive and output models�from a mild hybrid to a full hybrid system and integrated all-wheel drive transmissions. Fuel savings can range from about 6% (standard transmission) to 25% (full hybrid).

The transmission is based on an innovative gear set design with 5 shift elements and 4 gear sets, of which only 2 are open in each gear, designed to significantly reduce internal drag losses, while the modern torsional damper systems in the torque converter improve fuel economy and reduce CO2 emissions.

Just like the second-generation 6-speed automatic transmission, the new 8-speed automatic transmission offers extremely short response and shift times (it takes 200 milliseconds to carry out a gear shift). This means that double shifts and direct multiple gearshifts are also possible - up to extreme shifts, namely from the 8th to the 2nd gear.

8HP transmission comes in four different configurations. This broad range makes it possible to install the transmission in different vehicle classes. While the transmission is adaptable for multiple drivetrain configurations (torque range from 300 to 1,000 Nm), each of these models is equipped with the same hydraulic control unit and the same electronic control unit to make the model management system as efficient as possible.

The 8HP automatic transmission is also designed for hybridizing the driveline. ZF is able to offer both a mild hybrid with crankshaft starter generator and a full hybrid as a parallel hybrid on the basis of the new 8-speed automatic transmission, requiring the same installation space as a conventional version.

The electric drive element, called DynaStart, is produced by the ZF Powertrain and Suspension Components division and has a power capacity between 10 and 100 kW and with a maximum torque of 100 to 1,000 Nm. For full hybrid applications, ZF also supplies the electric motor as a hybrid module with an integrated separating clutch for decoupling the electric motor and the combustion engine. The first example for DynaStart production application in cars is the Mercedes-Benz S400 Hybrid.

The 8-speed automatic transmission is the first multi-ratio transmission that can be equipped with a stop/start function, thus achieving fuel savings of 11% compared to the 6-speed automatic transmission from the second generation. After switching the engine off, the vehicle can be started up again in only 350 milliseconds thanks to the integrated hydraulic impulse oil storage.

The flexible modular design of the automatic 8HP transmission means current and future technologies, see below, can be applied without having to alter the required installation space in the vehicle.

• Hydrodynamically Cooled Clutch (allowing a rapid start-up and even faster gear changes)
• Integrated starting clutch (resulting in compact transmission design suitable for small and compact vehicles with low engine power)
• Crankshaft starter generator
• Integrated hybrid
• All-wheel Torsen integrated
• All-wheel ToD (Torque on Demand) integrated
• All-wheel Hang-On
  

Source: Green Car Congress 
Related links: ZF Friedrichshafen AG | 8-speed automatic transmission