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Old 07-29-2021, 05:25 AM
Luce Luce is offline
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I know better than to get lost in the weeds trying to read a SAE-J spec (or spend the $80 they want to download it from SAE), and the allowed flyback voltage may be an industry standard (that can easily be held constant by the injector driver electronics), but why I was suggesting the backyard hack method of using the various injector spray modes was because it would bake in that variable as well. I also had a fuel pressure gauge on my regulator, and at just above idle, the needle would violently bang on the pegs on both sides until it hammered the needle clean off of the stem. That may have been more spring mass resonance of just the gauge excited by the injectors, but it did make me think there was possibly resonances in the rails as well.

This is coming strictly from an electronics geek direction, but as you can see by the voltage offsets, the voltage is a very critical number in calculating opening time. Unless held constant, allowed flyback voltage is at least as big of a variable. If I don't recall the exact number but I think it was around -20V to -30V in the MS drivers. For example, if it was -24V flyback volts, the magnectic feild that opens the injector could collapse twice as fast as it opens. This doesn't necessarily mean the fuel will stop twice as fast. That would be determined for closing by the spring/mass of the pintle as well possibly fuel flow, and opening, by the force of the feild/mass and maybe fighting fuel pressure.
But something must be done to clamp the flybackvoltage or the resulting spark would wreck the transistor switching it. Since it's not mentioned, I'll have to assume the flyback voltage is constant or high enough to become irrelevant.

But wow. There is a bit more minutia in the specs than I was aware. I know MS has some rudimentary voltage compensation, but I think it's more limited in usefulness to starting conditions where battery voltage can sag to 6-9V. I think they rely more on a stout and well regulated alternator. And also, I haven't really felt a need with my applications to progress beyond MS2. There's a lot more just going to MS3 that was released 6 or 8 years back.

And I've been limited to relying on vendors claims of "matched sets" and with MS 2, you only have 2 driver circuits, so no individual cylinder trims. But even so, it's still an order of magnitude more accurate than a carb and able to compensate for weather and altitude.

If anyone is interested in what all of my yammerings about flyback is, check this out. I didn't find anything about a controlled flyback circuit, but the MS driver circuit does something similar to the frewheeling diode, but more complex so it can allow 20-30V instead of the .7-1.5 of just the diode in the article.

Add- Here's the MS schematic. It is set at -36V by the zeneer diode and TIP-42 transistor. it's about hald way down, title block says output injector Megasquirt 1 - V3.0 board schematics MS 2 and 3 use the same MS1 board but add newer and more powerful CPUs. The board just has driver circuits and basic sensor reading circuits.

Last edited by Luce; 07-29-2021 at 05:27 AM..
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