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I have an Acura TL, which specifies "premium" fuel. However, I know that the engine has anti-knock sensors that retard timing to prevent detonation if somebody accidentally fills it with lower-octane fuel.

How does the ECU decide when to try returning to its preferred timing after detecting knock? E.g., does it wait until the fuel door is opened? Does it try every time the engine is started? With some frequency even during the same drive? When it detects a change in incoming air mass? And how does this strategy vary among manufacturers and engine model years?

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The ECU(or at least the better ones) can indeed actively 'search' for the point of pinging as you said. The pinging sensor listens to the combustion, based on that the ECU determines how close it is to knocking. Real, actual knocking is audible even to the untrained human ear, but knock sensors can also detect near knocking situations. And that is the most useful functionality.

In case this gives better performance, and that is a clear requirement, they can advance the timing to the point of near pinging. Obviously not when the power isn't needed, since more spark advance increases NOx emissions due to increased peak pressures and heat. A PID loop continually adjusts the timing to make sure the engine doesn't actually ping, but advances as much as is possible if needed. Thus, the target value of the PID changes according to the demand of the driver, limited by emission regulations.

So effectively, it continually measures the effect of the fuel on pinging. Not the octane rating of the fuel itself. That's not what you ultimately want to know, but pinging is. I doubt measuring octane rating is even possible with a sensor.

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  • Thanks for the excellent answer! Sounds like you might know enough to provide information on this related question. – feetwet Nov 23 '16 at 14:51
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The only sensors used to determine fuel quality are the knock and mixture (Oxygen) sensors. The adjustments are continuous whenever the engine is running. The vehicles I have experience with, this includes your Acura, all use this method. This method is accurate and not especially challenging from an engineering perspective.

Attempts in the past to determine alcohol content with in tank sensors by some OEM's resulted in prompt modifications due to severe driveabilty problems. This was done for flex fuel models. A far more challenging problem than changes in fuel octane rating.

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    Can you elaborate on the "continuous" adjustment methodology? E.g., engine detects knock, so it retards until it does not detect knock. Then does it immediately begin advancing timing again until it detects knock? So it somehow continuously teaters on the brink of detonation without damaging the engine? – feetwet Nov 9 '15 at 3:12
  • The exact methods are in the proprietary engine management software that will vary greatly given the wide variety of engines and engineering approaches, regulatory requirements, etc. It will also vary with engine speed load and temperature. – Fred Wilson Nov 9 '15 at 7:15
  • I'm also interested in this, could you provide an example? When does the ECU decide to advance timing again? – I have no idea what I'm doing Nov 10 '15 at 9:23
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    @Bart - that should be its own answer! Especially if you can elaborate more on the PID implementation in ECUs. E.g., are the PID parameters hard-coded? Or does the ECU run a parameter tuning cycle with some frequency? – feetwet Nov 23 '16 at 3:43
  • @feetwet Deleted my former comment and elaborated in my answer. I don't think the ECU tunes the PID's parameters. You'd gain minimal profit, while developing software for that would be expensive. Lower range ECU's don't even bother to search for the point of pinging. – Bart Nov 23 '16 at 12:11

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