In the future, a hydraulic impulse storage can be integrated in the new generation of ZF's 8-speed automatic transmissions. 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%.
Technical innovations which allow for fuel savings without a loss in comfort: This requirement is met by the 8-speed automatic transmission, also as a micro hybrid version. If, for example, the engine is switched off at red traffic lights, start and further driving must take place with the same dynamics as with a vehicle that does not feature a start/stop function. One problem in this case is the oil supply of the transmission because, when the engine is off, the transmission hydraulics are not supplied with pressure. In contrast to vehicles which stop with a running engine, the automatic transmission's shift elements, that are required for setting off, cannot be activated during standstill.
This is where the hydraulic impulse oil storage from ZF comes into play. It is a spring piston accumulator which fills with oil and tensions the spring during operation. When the engine starts up, this "reserve" of around 100 milliliters is supplied to the hydraulics - in a flash - to supply oil to the shift elements in the transmission which are needed for setting off. Thus, the vehicle is ready to move already 350 milliseconds after starting up the engine. Without the hydraulic impulse oil storage, this would take approximately 800 milliseconds, leading to a loss in driving dynamics which the driver can notice.
The component presented by ZF at the Vienna Motor Symposium is approximately 19 centimeters long and has a diameter of five centimeters; it can be installed behind the hydraulic control unit in the standard installation space of the 8-speed automatic transmission.
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.
