Gradually ZF’s new 8-speed automatic transmissions are being used by other automotive manufacturers worldwide. Dr. Jürgen Greiner, head of Passenger Car Transmission Development at ZF Friedrichshafen AG said BMW demonstrates how to make vehicles more efficient and dynamic at the same time.
BMW is the first OEM to opt for the new transmission, which is upto 3% more efficient than its predecessor. The 8HP50 version is designed for torques of up to 500 Nm. A 8HP75 in the lines will be used for rives with even higher torque of up to 750 Nm.
ZF has optimized the new transmission for requirements of modern engine technology. This is in conforming with forthcoming CO2 emission specifications without torque and performance. Effectively a solution comes through downsizing with turbocharged engines with fewer cylinders and less engine displacement and downspeeding (reduction of engine speeds combined with max torque applied at a very early stage).
– Further savings potential of up to three percent
– Focus of developers: Reducing drag torques and engine speed
– Modular transmission construction kit predestined for downsizing engines
Higher savings potential in terms of consumption and CO2 combined with increased ride comfort and dynamics
It is the reduction of the speeds in particular that demands transmissions with a higher spread which is the reason why ZF has increased it from 7.0 to 7.8 on the new 8HP by means of optimized gearsets. “This measure alone reduces the speed across all gears by 50 revolutions per minute on average and reduces fuel consumption by almost one percent,” explains Jürgen Greiner. Furthermore, the ZF developers have succeeded in once again reducing the internal transmission losses. This is primarily ensured by the new multidisk separation: Additional springs integrated into the multidisk packages of the shift elements ensure that the friction shift elements are almost fully opened and, consequently, cause less drag torque. “Compared to the first 8HP model range, we have reduced the power losses by more than two-thirds with the new generation,” emphasizes Greiner.
Meanwhile, a further innovative function minimizes the creeping torques: A clutch is now fully opened during deceleration and when the vehicle is stationary. “Consequently, it is no longer necessary to ‘brake’ against the drive,” explains Jürgen Greiner. Another positive effect is that the oil pump belonging to the ZF automatic transmission works with a system pressure that has been reduced from 5 to 3.5 bar and, as a result, requires less energy overall.
Furthermore, the objective was to take the changing vibration behavior of the increasingly economical yet, at the same time, more powerful downsizing units into account. Therefore, all 8HP automatic transmissions feature particularly advanced torsional vibration dampers: These dampers eliminate the vibrations that occur on the engine side in a particularly effective manner so that they are unable to move further along and into the driveline and the body. The newly developed torque converter contributes to downspeeding because the hydrodynamic transmission of power can be bridged even more quickly – this enables traveling with a closed lock-up clutch at an early stage and at an extremely low engine speed.
Optimized start/stop and coasting function
The tailored, fuel-efficient uncoupling of the transmission in connection with a temporary engine stop, the so-called coasting function, is now possible at speeds up to 160 km/h. At the other end of the speed scale, ZF has further optimized the optional start/stop function of the 8HP: After the vehicle has come to a stop, it stops the engine without any discernible delay – instead of after 1.5 seconds as was previously the case.
However, dynamics is not neglected despite the great fuel-saving potential: Amongst other things, the new transmission control unit enables nested multiple downshifts which permits the 8HP to respond in an even more spontaneous and dynamic manner where necessary.