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.



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One comment to Latest Transmission Titbit – ZF 9HP

  • Ray  says:

    My new Cherokee never enters gear 8 or 9. I thought it may be locked out until the 300 mile break in period but I have 315 now and it never goes beyond 7. My trip mileage was excellent on a ride up country on a warm day in Florida. I drove easily enjoying the drive of 90 miles from 205 to 295 on the odometer and full throttled from 40 mph 2 times over the entire trip as it states in the user guide to do this several times for a good break in. I reached 70 in about 3 seconds and let off and still saw only 7th gear. I can drive in 7th at about any easy driven speed on a flat and level road. My mileage for the trip was posted at 34.4 average on the cars center meter. I purchased gas the following day and it was even higher than stated by the meter average. I assume the meter average is quite honest. I have no idea why it never got to 8 at least since I drive gently. My a/c was off and all 4 windows wide open since I love the Florida heat after living up in Jersey! I purchased the 4 cylinder latitude for driving in Florida and am never in need of more power. I love this motor car for sure. It is a lovely thing to own.

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