Idle RPM Timing Correction Clobbers Crank-To-Run Taper Time
#1
Idle RPM Timing Correction Clobbers Crank-To-Run Taper Time
Idle RPM Timing Correction (IRTC) is great, most people including myself use it for idle. There is one nagging issue I have noticed though: It clobbers crank-to-run taper time. During very cold starts I like to jump up to 1500 RPM after start and taper down to CL target RPM (1200-1300 RPM when very cold). Almost all modern vehicles do this from the factory, and crank-to-run taper time is designed to allow for this in MegaSquirt. However, having IRTC enabled with even mildly aggressive timing correction when RPM is higher than target essentially fights the crank-to-run taper time by pulling timing. The end result is that the IRTC pulls a bunch of timing right as the IACV duty is tapering down to it's normal run position and this combination causes a major undershoot in target RPM which is occasionally bad enough to cause a stall. My current solution has, reluctantly, been to set IRTC so it does not modify timing when idle RPM is higher than target.
The question then is: Why in the world is IRTC active during the crank-to-run taper time? Taper time is obviously a transient condition wherein CL idle controls (like IRTC) should be disabled. Later firmwares have even added the crank-to-run taper time setting to the closed loop idle page, so maybe they are on the right track.
The question then is: Why in the world is IRTC active during the crank-to-run taper time? Taper time is obviously a transient condition wherein CL idle controls (like IRTC) should be disabled. Later firmwares have even added the crank-to-run taper time setting to the closed loop idle page, so maybe they are on the right track.
#2
Tweaking Enginerd
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Hmmm, sounds like you may not be understanding/using the features appropriately. You want to us CLT based rpm target, IIRC crank-to-run is primarily a fuel based feature with a short duration.
#3
Crank-to-run taper time is a taper for IACV duty, it tapers from cranking duty to whatever your CL initial value table has over the specified number of seconds. And I don’t want a general coolant based RPM target in this case, I already have that set up. I want to idle higher than my CL target for a few seconds after startup, which is what crank to run taper is meant to do from everything I can find
#5
I have thought about it every which way and it just doesn't add up -- IRTC should never activate until the crank-to-run taper time is over after startup to prevent it from pulling/adding a heap of timing while the engine is settling into idle RPM. Going to try and pursue this further on msextra but am doubtful on how far I will get.
#6
Tweaking Enginerd
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So am I to understand that PID idle activation triggers during crank-to-run? (I haven't looked at this, or if i did it was years ago and have forgotten)
Why do you care about Idle Advance in this otherwise open-loop condition?
Why do you care about Idle Advance in this otherwise open-loop condition?
#7
I don't care all that much about Idle Advance during this short crank-to-run period, but I do care about having it after the car is idling normally, regardless of temperature.
#8
Tweaking Enginerd
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ahh, well that is a pickle. At least it sounds like you have a straight-forward ask for the devs ... PID idle activation should not be triggering during crank-to-run. This is easy enough to implement in code.
#11
Retired Mech Design Engr
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@intenseapple I note that my idle advance setting doesn't kick in until about 5 seconds after start.
One lingering issue that many people have, that may be your issue, is that; under Closed-Loop Idle Settings, PID delay needs to be set to "3". If less, then you will get an occasional move into PID prematurely, even at start-up.
I have that set, and then Crank to Run set at 5.
In Idle Advance Settings, I have Use PID Activation set.
I note that my Idle Advance does make a step change when it comes on, not just the RPM timing correction, so that, too, could be an issue if your Idle Advance is much different from your normal advance settings in the idle region (as mine is). Idle Advance and Correction both turn out at the same time after the 5 second programmed delay.
DNM
One lingering issue that many people have, that may be your issue, is that; under Closed-Loop Idle Settings, PID delay needs to be set to "3". If less, then you will get an occasional move into PID prematurely, even at start-up.
I have that set, and then Crank to Run set at 5.
In Idle Advance Settings, I have Use PID Activation set.
I note that my Idle Advance does make a step change when it comes on, not just the RPM timing correction, so that, too, could be an issue if your Idle Advance is much different from your normal advance settings in the idle region (as mine is). Idle Advance and Correction both turn out at the same time after the 5 second programmed delay.
DNM
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