ItaliaspeedTV | January 19, 2011
ItaliaspeedTV - ZF 8HP70 8-Speed Automatic Transmission (Chrysler 300/Lancia Thema)
ZF 8HP70 8-speed automatic transmission
ZF engineers had set the bar high to produce a new benchmark for automatic car transmissions. The second generation of the ZF 6HP 6-speed transmission entered production only in 2006 included reaction times faster than human perception, direct engine linkage by early-stage torque converter lock-up and intelligent, adaptive control software, that almost reads the driver's intentions from his foot. But the new transmission is capable of even more - it saves (even more) fuel. It also guarantees ultimate driving enjoyment, as well as the variability needed to be able to use future technologies.
Several innovations have been introduced in the new 8HP to reduce fuel consumption: the completely new transmission concept with four gear sets and only two shift elements open, a higher overall gear ratio, a variable oil pump, new torque converter, and optimized hydraulic and transmission control. A stop-start function is offered as an optional function. Idling at a standstill, the engine automatically switches off. When engaged again, the engine automatically starts up. Even at standstill times of just 10 seconds, switching off the engine cuts average consumption noticeably -- and also CO2 emissions. The start/stop function is enabled by the development of the hydraulic impulse oil storage (HIS). It supplies the hydraulic oil that the transmission's shift elements need for starting. When the engine is switched off, it allows for a quick start -- as it is required with the start/stop function. Already 350 milliseconds after starting the engine, the vehicle is ready for setting off. With the start/stop function of the hydraulic impulse oil storage, it is possible to reduce fuel consumption by another 5%. Compared with the worldwide most efficient 6-speed automatic transmission by ZF, the newly developed 8-speed automatic transmission saves another 11%.
Benefits of HIS
The hydraulic impulse oil storage makes more complex solutions obsolete, such as a more powerful oil pump in the transmission or an electric hydraulic pump. As a larger dimensioned transmission oil pump would considerably neutralize the fuel savings in continuous operation, an electric pump is an additional burden on the main power supply, has disadvantages in terms of noise, and leads to a considerably higher integration effort. In contrast, with the hydraulic impulse oil storage, ZF engineers have already considered the lowest possible system costs and easy installation of the unit. Other expensive adaptations of the transmission are not required; after all, with the new development of the transmission, ZF engineers have already considered the integration of the hydraulic impulse oil storage by ensuring a favorable design of the components and the respective routing.
Farewell to conventional gear set concepts
The ZF engineers went on the lookout for a completely new transmission system and discarded all previous established gear set concepts. Armed with a comprehensive list of criteria, the development engineers systematically appraised thousands of possible epicyclic gearing systems and their arrangement variants. The outcome was an 8-speed automatic transmission system leaving just four simple gear sets and five shift elements.
Lower drag losses and higher gear meshing efficiency
The clever thing, though is the shift plan. There are only 5 shift elements - multi-disc clutches and brakes in the heart of the transmission - and only 2 are open in each gear. The fewer open shift elements there are, the fewer transmission parts there are rotating relative to one another. On the bottom line, this results in a significant reduction of drag losses in the transmission. The development engineers were also able to increase the gear meshing efficiency with the new transmission concept. Energy is lost in some gears when power is transmitted through gear wheels. In the new automatic transmission, these losses are below 2% throughout - a further factor helping to reduce consumption.