ZF has developed a modification to its upcoming eight-speed automatic transmission that will allow it to be used with stop-start systems. This is claimed to reduce fuel consumption by 5%.

So called hydraulic impulse storage 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 after 350 milliseconds after starting the engine - as required by the start/stop function.

ZF said the extra saving is on top of the 11% saving the newly developed eight-speeder offers compared with its six-speed transmission.

With stop/start the restart and further driving must take place with the same speed and smoothness as with a vehicle which does not feature a start/stop function. The problem overcome is the oil supply to 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 unit 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 centilitres 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 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 would notice.

The new component is approximately 19 cm long and 5cm wide is installed behind the hydraulic control unit in the standard installation space of the eight-speed 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.

Provision was made for it in the basic design of the gearbox, ZF said.