The innovation comes from an addition called a 'hydraulic impulse oil storage unit'. The unit supplies the necessary oil to the transmission, which stops flowing when the engine is not operating. That oil must be circulating when the gears begin turning, however, for the car to be able to set off again after stopping.
A reservoir filled with hydraulic oil is fitted with a spring-loaded piston that is retracted while the engine is running and shut off, but once it needs to be restarted, the piston is triggered and it sends the reservoir's oil into the transmission within 350 milliseconds, reports JustAuto. This allows for a seamless transition from stopped, engine off to moving, engine on.
Without the system, the delay to circulate the oil within the transmission would take at least 800 milliseconds (0.8 seconds), causing a noticeable lag in the system.
ZF's eight-speed transmission was developed to save fuel - the company claims it can reduce fuel consumption by up to 6% compared to a six-speed automatic. The addition of stop-start technology will improve that figure further, though the actual numbers will depend on the application.
Some of the earliest automatic transmissions had only two gears - GM's Powerglide, for example. Buick's Dynaflow managed four forward gears, but both were soon obsoleted by the rise of torque converter-based three-speeds, which dominated the market through the 1980s with the simple addition of overdrive gearing. Since then, however, the world of automatic transmissions has been a steady progression of more gears, clutches, and computer-controlled complexity.
This latest development was no surprise to ZF, however. The company designed the gearbox for just such a feature, though it did not then exist.