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

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BMW ActiveHybrid technology: How it works

 

 

 

 

Hybrid technology may be a fine technical achievement, but makes more sense for countries with no diesel infrastructure. The development of such technology has been mainly led by Japanese manufacturers (relevant post from Japanese Green Cars blog), but now every car manufacturer wants to add a hybrid model/version in its lineup. By 2015, Goldman Sachs expects the hybrid-vehicle market (including plug-in hybrids) to grow to 2.5 million, up from 500,000 in 2007, with Toyota and Honda in the lead.

BMW ActiveHybrid technology is the result of BMW's collaboration with GM and the former DaimlerChrysler, the Global Hybrid Cooperation. Parallel to the Global Hybrid Cooperation, the BMW Group has entered also into yet another joint venture with the Mercedes-Benz, developing components for hybrid drive in the luxury performance sedan.

Following EfficientDynamics technologies,  BMW sees ActiveHybrid technology as the next step.  A key component of the system is the two-mode active transmission which links the combustion engine with two high-performance electric motors. The two-mode active transmission uses planetary gear sets to variably split the power between the different units. The system switches between two operating modes according to need: one is used for moving off at high torque and low speed, the other for faster travelling speeds.

 

 

 

This ensures that power is optimally exploited depending on the particular driving situation. Fixed-ratio gears within the two operating modes furnish additional benefits, boosting efficiency and driving dynamics across the entire speed range. Here the two electric motors can be used both during acceleration and for the recovery of brake energy. This energy, which is subsequently on tap to drive the vehicle, is provided practically free of charge. Thus BMW ActiveHybrid cars can be driven either in full electric mode, solely on power from the combustion engine, or in a combination of both drive sources. 

ActiveHybrid models feature also Auto Start/Stop function, as well as an air conditioning and ventilation system which remains fully operational also with the engine switched off, power being supplied directly through the 120 volt on-board network operating on the lithium-ion battery. This unique concept of engine management allows, for the first time, truly effective climate control also at a standstill, cooling the interior to a significantly lower temperature right from the start, before the driver even fires the engine.

It's almost certain that Rolls-Royce will come up with a version of Ghost equipped with ActiveHybrid technology.

READ MORE - BMW ActiveHybrid technology: How it works