Latest Transmission Titbit – ZF 9HP

The ZF 9HP is a 9-speed automatic transmission for transverse engine layout, which is used on the Jeep Cherokee and the upcoming Acura TLX. While there are plenty of technical analysis on this new ZF product, here I am not going to feed you with some PhD-thesis-type content. Instead I have some interesting facts you may find interesting.

First off, don’t be frightened by the below ZF 9HP internal structure image, it is for your reference only, you don’t try and memorize it all in one go. It takes time to learn.

1. The ZF 9HP has 4 planetary gear sets (a planetary gear has 1 sun gear, 1 ring gear and 3 planetary pinions), plus 6 shifting elements (2 clutches and 4 brakes). Let Si denote the tooth number of the i-th sun gear, and Ri denote the tooth number of the i-th ring gear. For 9HP:

S1 = 42, R1 = 110;
S2 = 42, R2 = 110; (gear #2 is geometrically identical to gear #1)
S3 = 91, R3 = 133;
S4 = 42, R4 = 86

2. During the upshift from 4th gear to 5th gear, due to the design of the shifting elements (the transmission will be in complete Neutral for a short time), it will lead to a brief interruption of torque delivery. The engine ECU and transmission control unit will cooperate to make this time gap as short as possible, however this torque interruption will be noticeable to drivers;

3. Downshift from 8th to 5th, 8th to 4th and from 9th to 5th will be quite challenging for this design. This is because lots of clutch/brake actions are involved for these processes, so it will take as long as 0.5 to 1 second to complete. There will be no torque delivered during this time frame. As an example, let’s show you what the actions are for shifting from 8th gear to 5th when your engine is rotating at 1,700 rpm (now let’s use the above image to identify each parts mentioned):

– Brake D disengaged (~ 0.1 sec)
– Engine ECU increase throttle and increase engine speed to 2350 rpm (~ 0.4 sec)
– Engine ECU cut power
– Dog clutch A engaged (~ 0.1 sec)

At this stage 0.6 seconds passed, and there is no torque transmitted at all. Next, there is one more step to achieve the 5th gear:

– Clutch to clutch frictional lock need another 0.2 seconds to fully lock-into the 5th gear ratio. During this step there is only partial torque delivered (because of sliding friction exist between shifting elements)

In the above example the total shifting time is around 0.8 seconds, under the assumption that initial engine speed is 1,700 rpm. Consider these shifts usually happened during high speed passing, the shift time lag will give the driver a feeling of much longer than the actual time cost.

So in summary, the 9HP will achieve a better fuel economy, with the cost of potential small annoyance in some certain use cases.

Feel free to leave a question or share an opinion in the comment box below.

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