The new 8-speed automatic transmission emerged from a systematic search on the part of ZF developers for the “ideal” planetary gear set. When it came to construction costs, efficiency, and load capacity, the engineers considered every possible application for this new development. The resulting 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, 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 transmission is based on an innovative gear set design with four planetary gear sets and five shift elements designed to significantly reduce internal drag losses, while the modern torsional damper systems in the torque converter improve fuel economy and reduce CO2 emissions. Also, the higher gear ratio of 7.05 compared to the previous model with 6.05 optimizes fuel consumption by lowering the speed level.

Just like the second-generation 6-speed automatic transmission, the new 8-speed automatic transmission offers extremely short response and shift times below the threshold of perception. This means that double shifts and direct multiple gearshifts are also possible.

The electric motor, clutch, torsional vibration damper, and hydraulics are all part of the full hybrid design and have been integrated into ZF’s 8-speed automatic transmission. Click to enlarge.

Hybrid functionalities. Installing a full hybrid version of the 8-speed automatic transmission requires no changes or modifications to the standard transmission design. The 8-speed hybrid transmission is designed for all current hybrid functionalities. The electric motor, clutch, torsional vibration damper, and hydraulics for a full hybrid concept have been fitted in the transmission in a space-saving and efficient manner.

The 8-speed automatic transmission as a full hybrid thus requires the same installation space as a conventional version. Even a mild hybrid with a starter generator can be easily installed in the transmission regardless of installation space.

The electric drive element—called DynaStart—is produced by the ZF Powertrain and Suspension Components division for use in passenger cars and commercial vehicles with a power capacity between 10 and 100 kW and with a maximum torque of 100 to 1,000 N·m. 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 S-400 HYBRID.

The 8-speed automatic transmission is the first multi-ratio transmission that can be equipped with a stop/start function. After shutting the engine off, the vehicle can be started up again in only 350 milliseconds thanks to the integrated hydraulic impulse oil storage, HIS. (Earlier post.) The spring piston accumulator supplies the hydraulic oil to the shift elements in the transmission. Due to this stop/start function, the 8-speed automatic transmission can achieve fuel savings of 11% compared to the already highly economical 6-speed automatic transmission from the second generation.

The hydrodynamically cooled clutch HCC combines the comfort of a torque-converter transmission with the sportiness of a dual clutch transmission. Click to enlarge.

Special starting clutch and all-wheel drive. Several starting elements and all-wheel drive variants can be added to this transmission. The Hydrodynamically Cooled Clutch (HCC) from ZF Sachs, the Powertrain and Suspension Components division of ZF, allows for a rapid start-up and fast gear changes. The wet starting clutch is installed in the clutch bell housing of the 8-speed automatic transmission instead of the torque converter. Compared to the converter, a lower mass moment of inertia makes this starting element particularly attractive for high-torque sporty motorizations.

Moreover, the 8-speed transmission system offers the option of eliminating the external starting element. In this case, one of the multi-disk brakes in the interior of the transmission meets the special requirements of a wet starting system. It is rated for the appropriate thermal load. With a closed-loop starting control unit, it can be used as an integrated starting device (IAE). This solution results in a very compact transmission design that is suitable for installation in small and compact vehicles with low engine power.

The hybrid version of the new automatic transmission has also been designed in such a way that it can be combined with various all-wheel-drive systems. Click to enlarge.

Adaptable all-wheel systems. On the output side, the new ZF automatic transmission is optimally equipped because it can be easily configured for different all-wheel-drive concepts. First, as a classic hang-on system with separate transfer case, typically used in SUVs.

Secondly, it can be used as a Torque-on-Demand system with integrated transfer case for on-demand control of the front axle. This system offers installation space, weight, and efficiency advantages compared to the hang-on system and can be used in cars and SUVs.

And finally, it can be used as an integrated all-wheel system in which the axle drive, along with the centered transfer case, is part of the automatic transmission and is used in cars with longitudinal front or rear-mounted engines.

Initial production start-ups in summer 2009. The ZF 8-speed was introduced as a standard component in the BMW 760i and the 5 Series Gran Turismo. BMW recently announced it would include the hybrid version on the upcoming 7-Series Hybrid. Since the summer of 2009, the new Bentley Mulsanne and the Rolls-Royce Ghost also come with the transmission.