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New Hydraulic Oil Element in ZF 8-Speed Automatic to Better Support Microhybrid Start-Stop

ZF’s hydraulic impulse oil storage is a spring piston accumulator which directly supplies transmission oil to the shift elements which are essential for startup. Click to enlarge.

In a presentation at the Vienna Motor Symposium last week, ZF introduced a new hydraulic impulse oil storage element that can be integrated in the new generation of its 8-speed automatic transmissions to better support start-stop microhybrid systems.

The system supplies the hydraulic oil that the transmission’s shift elements need for starting. When the engine is switched off, it allows for a quick restart within 350 milliseconds, as required with the start/stop function.

The combination of the 8-speed transmission with start-stop function can reduce fuel consumption up to 11% compared to a 6-speed. Click to enlarge.

Compared with the most efficient 6-speed automatic transmission from ZF, the newly developed 8-speed automatic transmission reduces fuel consumption by another 6%. The start/stop function supported by the hydraulic impulse oil storage delivers an additional reduction in fuel consumption of up to another 5%, for an 11% overall reduction.

When an engine stops under start-stop control, 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.

The hydraulic impulse oil storage system 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 rapidly supplied to the hydraulics to supply oil to the shift elements in the transmission which are needed for setting off. The vehicle is thus ready to move 350 milliseconds after starting up the engine. Without the hydraulic impulse oil storage, this could take approximately 800 milliseconds, leading to a loss in driving dynamics which the driver can notice.

The hydraulic impulse oil storage is installed behind the transmission hydraulics with neutral installation space requirements. Click to enlarge.

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 is low-cost and easily installed. Other expensive adaptations of the transmission are not required.

The hydraulic impulse oil storage eliminates the need for a more powerful oil pump in the transmission or an electric hydraulic pump. A larger dimensioned transmission oil pump would neutralize the fuel savings in continuous operation, and 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.



I wouldn't mind paying about a thousand dollars for a start-stop system that's retrofitable. I get so frustrated that I'm wasting gas sitting at a stop light. Some of them have bad sensors and just sit there with no cars going through the intersection.


There was an estimate made of how much fuel is wasted at stop lights that are not programmed correctly. It was astounding. That would be a definite gain to be made, but the cities do not seem to get their act together to do it.

Harvey D

Intelligent stop lights, while synchronized, could give priority to the directions with most trafic while keeping waiting periods reasonable for the other directions. Intelligently controlled stop lights don't cost much more to install, would accellerate trafic and reduce GHG as a side benefit.

On the other hand, it would negate some of the advantages of this new 8-speed system.


I live next to traffic lights and I would love to see more stop/start and hybrid cars.
As it is, there are more and more diesel cars around, and it does not smell too good in the morning.

It would be good to legislate for a minimum of stop/start systems for all cars in say 5 years - enough time to get the costs down and give manufacturers time to get ready.

It would give both air quality, fuel use and GHG reduction - at a manageable increase in purchase cost.

If it was done EU wide, it would have a major effect.


this is for class 8 trucks, right...
what do we need 8 speed transmissions on cars/light trucks? this thing must weight a ton, and adds mechanical complexity and cost to a vehicle.
we should aim to improve max load rating for cvt transmissions. why limit yourself to several fixed ratios- especially when you apparently need at least 8 of them, even if you spend most of your time in a city under 40mph.



I agree with you regarding CVTs.

This transmission is for LDVs (I believe its first application will be in BMW's new 7 series)

Just because it has 8 speeds doesn't inherently mean it weighs more and has more parts. I recall reading that when Lexus introduced its 8 speed transmission, it actually weighed 50lbs less than the outgoing 6 speed and had 20% fewer parts. How? I don't know.

Brian P

CVT's that have been developed to date, all have higher mechanical losses than gear-to-gear transmissions. The potential improvement by running the engine at precisely the best-efficiency point for a given load setting, rather than somewhere close to it as with a multi-speed gear-to-gear transmission, is offset by the mechanical losses - particularly when you have a lot of ratios to choose from in a gear-to-gear multispeed transmission.

CVT's already in production, example Nissan's, don't have miraculously good fuel consumption figures. Better than a normal automatic gearbox, yes, but not better than the manual versions of the same car.


well I agree with this but at the same time there must be some human thought of saving the fuel by switching off the engine when required. This will bring drastic reduction in fuel consumption iff it is properly followed by every citizen


One thing follows another. When you have start/stop, now you need something for the transmission. As things progress, you need other things to make it better. At some point engineers stop and try to think of a totally better way than just going down one path.

Eliminating the need for a transmission altogether is one way of seeing things differently. Hub motors do not require a transmission nor differential. I am not a real fan of them at this point, but it illustrates the idea.


The oil reservoir cann't be 100 centiliter (= 1 liter), as the whole device is only 19 cm in length and 5 cm in diameter (373 cm^3).
100 milliliter would make much more sense.

Besides, if you need 1 liter of high-pressure oil to actuate the transmission, i'd think that would give plenty room for improvement... :)


...isn´t there already a Lexus 8-speed auto and a real hybrid drive?


I do not trust the starting ability of certain manufacturers vehicles and worry that start stop systems may clog intersections with non starters.I would prefer a little control over the system so that I could choose not to have the car stop running say if I knew the fuel had a water issue(something that happens here in the northeast occasionally)or if I was being chased by paparazzi.


about using of the hydraulic oil pump reservoir in car

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