New technological development promises fuel savings without loss in comfort

New technologies are evolving around hybrid propulsion technology as auto manufacturers race towards meeting or exceeding the tighter emission norms. Most hybrids include an auto transmission that is hooked to the process of start-stop as the source of propulsion varies between an internal combustion engine and an electric drive motor. In a new development, ZF's new generation 8-speed auto transmission enables the integration of hydraulic impulse oil storage in micro hybrids.

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 centiliters 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 within 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 impulse oil storage component 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 best of the equation is arguably the fact that 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 with the new development. Further more, 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.