Higher savings potential in terms of consumption and CO₂ combined with increased ride comfort and dynamics: The successful eight-speed automatic transmission (8HP) has been characterized by these advantages since the start of production in 2009. All of the advantages are now even more pronounced in the second generation that has been in volume production since July 2014 in the BMW 520d. The new 8HP offers comprehensive further developments that range from lower drag torques and higher spread through to improved torsional vibration absorption. Gradually, the new 8HPs from ZF are being deployed in further models and with many automotive manufacturers worldwide.
"Our longstanding customer BMW is impressively demonstrating how to successfully make vehicles more efficient yet more dynamic at the same time," stated Dr. Jürgen Greiner, head of Passenger Car Transmission Development at ZF Friedrichshafen AG. "Therefore, we are particularly delighted that the premium manufacturer is the first OEM to rely on the second generation of our eight-speed automatic transmission that, when it comes to these criteria, is also groundbreaking. Through many further developments, we have made the 8HP significantly more efficient compared to the predecessor transmission – up to three percent in total.”
The new 8HP entered volume production at the start of July in the BMW 520d as the 8HP50 version that is designed for torques of up to 500 Nm. In the future, the 8HP75 will be part of the ZF portfolio for drives with even higher torque of up to 750 Nm.
Gradually, all further automotive manufacturers to whom ZF has already supplied the 8HP in more than 1,000 vehicle applications will utilize the latest generation of the 8HP.
Optimized for downsizing and downspeeding
The new 8HP combines many technological advances: Above all, ZF has further optimized the new transmission generation for the requirements of modern engine technology. This is due to the fact that, in order to meet the forthcoming CO₂ emission specifications without losses in terms of torque and performance, there are two central trends when it comes to combustion units: Downsizing, which means turbocharged engines with fewer cylinders, and less engine displacement and downspeeding, which is the reduction of the engine speeds combined with a maximum torque that is applied at a very early stage.
It is the reduction of the speeds in particular that demands transmissions with a higher spread which is 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," explained 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," emphasized 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 benefit 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.