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AFR bounce during instant wot


jetape
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So this is on a jetski, no launch control stuff you just press the throttle as fast as you can.

AFR is correct if I accelerate smoothly to wot but if I just hit it the AFR drops to 9.x then bounces up into 17.x before recovering, all this happens very quickly, within one second. 

I noticed that  boost numbers during this kind of a launch are way higher at any given RPM versus smooth acceleration so any fuel table tuning I've done does not account for these areas so far. Things happen so fast I'm not sure how accurate the AFR is for the given RPM during this single second.  Should I simply go by the log and add/remove fuel at the cells the log is reporting or is there a better approach?

 

Thanks

 

 

launchlog.PNG

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You probably need to set up acceleration enrichment. This applies an extra burst of fuel during throttle changes to account for exactly this. I need to see a log or pclr file to be certain but the fact the AFR initially drops suggests that you have this enabled already but just need to configure it to hold that extra fuel for a bit longer.

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In the logs the effect of this is called Accel Fuel. You can see it kick in for a couple data points then drop off right when you hit the throttle. There is another thing that might be relevant too and its hard to tell them apart over such a short time frame as depending on the distance between your engine and the lambda sensor it can take a few data points for inputs to affect lambda. Your RPM hits the RPM limit for a brief second, and because of this, you see a 8degree ignition cut which drops your RPM by about 1000rpm. I'm assuming at some point about the same time the water pressure against the impeller starts to slow the engine down too, but it looks like either that drop in MAP+RPM moving you to a lower cell in the fuel table or maybe a fuel cut RPM limit causes the injector pulse to drop from just over 10ms to about 6ms immediately before the lean spike. It then recovers up to about 8ms at stead state high rpm.

Maybe you can try reducing the harshness of your RPM limiter to prevent this sudden drop in load+fueling? Have a read of the RPM limit parameters in the help file. In a lot of cases the limit will begin to apply earlier than you would think - eg the various window parameters mean it starts pulling fuel/spark that many RPM before the specified limit.

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Ah nice catch, I didn't even notice it hit the limiter since max RPM I saw in the log was under 8900 and my limiter table is at 9300 with 200 cut range which I expected to start cutting above 9100.

Is my cut range too large? The RPM limiter is mostly to keep engine from free spinning out of control when losing water in the jet pump when ski gets airborne for split second.

 

rpmlimit.PNG

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You could try switching to spark cut limiter. Some engines dont take too kindly to this if used repeatedly but it should give you a good test. 

I've never had to set up a limiter on a boat where the force acting against the engine will change rapidly at high RPM like this so cant give you any numbers. I dont think you're hitting the hard limit as the highest fuel cut logged is 63%. This suggests that when you hit the start cut level, its having such a big impact that you dont get any further into the limit range. The data isnt that granular though so its hard to tell if this is correct. Assuming it is, try dropping the "start cut TP100" number to 30 and the start cut TPLow number to maybe 15-20. You could also try making the limit range a bit bigger at 300 or 400rpm. The idea here to let the fuel cut come in more gradually so it doesnt have such a kick to it. You could also try bumping up the exit decay rate but its already pretty high. This would still put you well outside RPM limit at 8300 cruise RPM.

Is this turbo or supercharged? Reason I ask is figuring out whether to be worried about turbo spool in the way the limiter works.

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