01-27-2011, 07:35 PM #1
Hooking and Unhooking......The Physics Involved
These pages got buried in my build thread and I wanted to put them in a specific thread so they're easier to access and, thereby, help people be better able to understand the extreme loads and pressures and RPM overshoots occurring in our skis, particulary (but not limited to) when the "the going gets rough"!
Motec Pete has taken his time to explain what is happening during pump unhooks and the resulting RPM overshoots.
and the physics of this......and the piston speeds....and the stresses placed on the rods/pistons/crank.....incredible. I knew the engine was zinging (my leet technical terminology), but i had no idea the rates of accel/decel being encountered. WHAT I WANTED TO ADD THAT PREVIOUSLY WASNT DISCUSSED IN THE PREVIOUS PAGES WAS THE EFFECT ALL THIS HAS ON THE SUPERCHARGER, IT'S SHAFT, BEARINGS, AND MOST IMPORTANTLY....THE DREADED CLUTCH. I had no idea the rates of acceleration were so high!!!
I believe this to be important information when evaluating problems/issues we are having with durability with our skis. It should also better help us evaluate new parts and the performance gains when we apply this extreme dynamic to our evaluation process. LET THE BENCH RACING BEGIN!!!!!
......here s a paste of his response of what happens when a ski leaves the water:
This is actually a big subject and I have done quite a bit of research into it as I noticed the same thing happening some time ago.
I will try to explain how it all works, hang on to your hats
Example is FZR that runs 8300 rpm in clean calm water.
RPM limit set to 8600
Rider is flat out at 8300 when he encounters a wave that completely unloads the jet unit, the engine then immediately accelerates due to no load on the crank.
We have measured this acceleration rate and on average it is between 30 to 40 thousand RPM per second. yes true
Now the ECU measures rpm in a very accurate way.
To do this it times between teeth on the crank but it cant do it between 2 neighboring teeth as the engine speeds up and slows down as each piston fires.
So the only way to get super accurate rpm is to measure at the same time in the cycle for each cylinder.
So that means that it updates its RPM calibration twice per revolution.
Sounds fast enough but at 30 thousand RPM per second acceleration rate the rpm is usually well on its way over the rpm limit before the ECU measures that its time to turn the coils off.
Ok, so lets assume that the ECU has now detected that the rpm is over 8600, it now has to shut off the coils. trouble with this is that if a coil has started to dwell (charge up ready to fire) the ECU has to let it charge completely before switching it off, the reason for this is that if it turns off the charging coil early it will fire its spark, the spark would then occur early and likely cause detonation (knock)
So the result of all that is that you will almost always get 1 firing coil after the engine has exceeded the limit.
The jet unit by this time is empty of water and you get 1 firing at close to full throttle.
Thats at least a 70 HP bang that will whip the crank around a few more revolutions.
Thats the simple version believe it or not and hopefully will allow you to understand why it happens.
Now some ECU's will start cutting power before the RPM limit to prevent that happening but thats certainly no good if racing is your game.
You can lower your RPM limit and that will help but the closer it is to your clean water rpm the worse your recovery time will be when landing in the water again.
RPM limit tuning is very important for Jet Skis and can make a big difference to speed in the chop.
You should have your RPM limit set to # 5 ign only and have your power recovery rate set to zero.
Hope thats been some help
01-27-2011, 07:35 PM #2
In Pete's previous post he explained what happens when the engine leaves the water, this time he is explaining what happens when it lands in the water.
The ski is flying through the air and the rpm has peaked at 8950, the ski then lands back in the water, the rpm falls at a huge rate of close to 70,000 rpm/second. yep
So this time you have the opposite problem.
The ECU is now looking for when the rpm goes below our limit setting of 8600, its calcultating in real terms at a very fast speed but the rpm is falling lightning fast.
The reality is that the rpm is below 8600 before the ECU starts to turn every thing back on.
At that point the ECU needs to wait for the correct time to start dwelling the coil, at that point it turns the coil on to charge it up and then once charging is complete, (approx 2 milliseconds later) the coil fires and off you go. The idea of having no fuel cut is that when the first coil fires we want pleanty of fuel there so that there is no further delay.
Sounds like the ECU is a bit slow, well I can tell you we have one of the fastest processors in the industry, its just the the acell and decell rpm rates in the jetski world are amazingly fast.
How can you make it better if you are at the WF and want to win.
1. If legal, fit a CDI to the engine, CDI's pre charge their capacitor so there is virtualy no delay from when the ECU wants to fire.
2. Lift your RPM limit further away from your clean water rpm, if you never hit the rpm limit then the coils never get turned off, when the ski lands back in the water there is no delay, this is obviously mechanically dangerous for the engine and in surf conditions where wave frequency is much slower you could get big RPM numbers, if you have good internals fitted to your engine then no problem.
01-27-2011, 07:36 PM #3
So the engines r accelerating at 30-40 thousand RPM/second during a pump unhook and decelerating at approx 70 thousand rpm/second during the landing/loading of pump....wow
That's simply amazing to me.....the physics of that......and the piston speeds....and the stresses placed on the rods/pistons/crank.....incredible. I knew the engine was zinging (my leet technical terminology), but i had no idea the rates of accel/decel being encountered. WHAT I WANTED TO ADD THAT PREVIOUSLY WASNT DISCUSSED IN THE PREVIOUS PAGES WAS THE EFFECT ALL THIS HAS ON THE SUPERCHARGER, IT'S SHAFT, BEARINGS, AND MOST IMPORTANTLY....THE DREADED CLUTCH.
This has been a great topic of conversation. This definitely explains the RPM overshoots and also the high attrition rates encountered in the high-hp offSHOre ski races. The beating they take goes beyond what I imagined.
I definitely have newfound respect for my motor and what my ski/ECU has to process for me to ride offSHOre or to do rough/rapid transitions.
I'm starting to see the synergistic benefit of the BOV/surge ports/Water-cooled SC housing/triple-bearing SC's/heavier couplers/etc.
I think this goes a long way in describing why the SC clutches wont last and why the engines take such a beating. I guess I'm wondering if anyone else is amazed as I am at the beating our skis and their mechanicals take everytime we ride?
01-27-2011, 08:12 PM #4
- Join Date
- Oct 2007
- Brisbane, Australia
Thanks for the thread Rip.
I too have just re-read the notes from Pete and the more I understand, the less I know
Coldspace and I also asked about the "predictive cut" function on the motec and didn't really get any confirmed result. I wouldlike to know what others think of this particular?
This feature is the oppose of what most would want for racing as it would actually slow you down in choppy water or unhooks. But it would be much more kind to the internals of the poor motor.
The other thing that help with hooking is the setback props. I have a 2" setback on the magnum pump on BigPig which is supposed to help "collect" more water for it's busness. Yet to test side by side with anyone in chop. Idea's on this ascept would be appreicated?
Thirdly the R&D "flywheel" that Hitman champions would also be helpfull but a direct measuremnet of how much it helps?? Maybe he can chime in on this as well.
Last question............what other things can be done to help with Hook/ Unhook?
01-27-2011, 08:42 PM #5
This thread can spawn numerous other threads.....evaluating not only specific parts and the dynamic they're being subjected to (for example the supercharger clutch)........ but also when evaluating the ski/platform as a whole in specific conditions (be it SHO or FZR, in smooth/rough waters, during sprints/endurance, etc).
01-27-2011, 10:03 PM #6
My God thats alot of reading. Lol.
01-27-2011, 10:10 PM #7
It's enough to make me alter my riding style/throttle control and guide me to what parts I think will have the most beneficial effect on my ski.
01-27-2011, 11:21 PM #8
Glad I don't have to worry about any of this stuff.
01-28-2011, 12:28 AM #9
Dont worry, I'll be walking on crutches next to you
01-28-2011, 01:50 AM #10
My post was more or less referring to the absence of the overrev issue when running a stock or reflashed ECU. There was a ton of info about this in your thread that didn't make it into this one. Could be beneficial to a lot of guys who want maximum reliability out of their skis.
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