ZF showcasing second-generation 8HP 8-speed transmission at NAIAS; additional 3% boost in fuel savings

At the North American International Auto Show (NAIAS) in Detroit, ZF is presentingits broad technology portfolio including the second generation of the 8HP 8-speed transmission. The new generation offers further developments ranging from lower drag torques and higher spread to an improved torsional vibration absorption.

Compared to the already highly energy-efficient first generation of the 8HP (earlier post), the second-generation unit achieves an additional three percent of fuel savings. The first generation of the eight speed 8HP saved up to 6% more fuel—and its stop-start variant up to 11% more—that the second-generation ZF 6-speed automatic transmission. With its flexible modular construction system, ZF’s 8HP covers a broad torque range between 300 and 1,000 N·m (221 to 738 lb-ft) and can be adapted to fit hybrid or all-wheel drive applications.

The transmission comprises 8 gears, four gear sets and five shift elements. ZF has further optimized the new transmission generation for the requirements of modern engine technology. 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.

The reduction of the speeds in particular demands transmissions with a higher spread; accordingly, 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.

—Dr. Jürgen Greiner, head of Passenger Car Transmission Development at ZF Friedrichshafen AG

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.

—Dr. Greiner

Meanwhile, a further innovative function minimizes the creeping torques: A clutch is now fully opened during deceleration and when the vehicle is stationary; it is no longer necessary to brake against the drive, explained 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 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.

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 (99 mph). 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.

ZF did not neglect dynamics; among 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.

The new 8HP entered volume production at the start of July 2014 in the BMW 520d as the 8HP50 version that is designed for torques of up to 500 N·m. The 8HP75 will be part of the ZF portfolio for drives with even higher torque of up to 750 N·m.

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 8-speed automatic transmission.