Hydraulic Impulse Oil Storage - How ZF Automatic Transmissions Contribute to CO₂ Reduction

Authors

Dipl.-Ing.(FH) M. Bek, Dipl.-Ing.(FH) P. Schiele, ZF Getriebe GmbH, Friedrichshafen

Year

2008

Print Info

Fortschritt-Berichte VDI, Reihe 12, Nr. 672

Summary

Due to EU legislation and public discussion the automotive industry is facing enormous pressure to meet the requested CO2 targets. This causes great demands on vehicle and driveline design engineers. By developing an automatic 8-speed transmission family ZF makes a significant contribution to consumption reduction and thus also to CO2 reduction /1/. Besides the changes inside the transmission, the 8-speed transmission family also offers the possibility of an engine-start-stop function which turns off the engine when the vehicle stands still and thus avoids consumption at idling speed.


During the engine-stop phases the oil pump of the automatic transmission is not driven anymore, thus, there is no oil pressure supply during these phases, the shift elements open and the transmission goes into Neutral. To meet a spontaneous driver's start request after an engine-stop phase it does not suffice to have a quickly starting engine, this also requires an automatic transmission with shift elements which can be closed so quickly that the engine start-up alone governs the starting process. The oil volume required for filling the shift elements quickly must be made available in short time. During the starting process, however, the oil quantity available is insufficient to fill and close the shift elements in the given time due to the speed start-up of the combustion engine and transmission oil pump. The hydraulic impulse oil storage which fills the shift elements very quickly during the engine starting process by means of a stored oil volume smartly solves this problem.


The hydraulic impulse oil storage consists of a spring-type storage, which is charged by the oil pressure present during normal transmission operation and locked by a detent in charged condition. When the engine is started, the locking opens and the oil volume are released. By means of the stored oil volume the shift elements required for the starting process can be filled so quickly that the driver will not notice any starting delay. Thanks to the design version of the hydraulic impulse oil storage electromagnetic locking, the current which is required for the locking process is very low, which means that the stress on the battery in the engine-stop phase is also very low.


The 8HP transmission by ZF already brings about a consumption reduction of approx. 6% in comparison with the 2nd generation of the 6HP transmission family, the possibility of using the engine-start-stop function leads to a further reduction of approx. 5% in the NEDC (New European Driving Cycle).

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