I called it an octane sensor, you called it an ethanol sensor. You claimed that it can't measure octane and only measures ethanol content. I in turn told you that they can measure changes in octane because of the way they work. They measure conductivity and that changes as octane changes, so they can detect differences in octane. That's simply all I said.
Again, it doesn't matter what you call it, its essentially telling the ECU how to adapt to the fuel coming in. I really don't know why you want to argue terms.
You are talking about two different but sometimes related things:
- Ethanol (flex fuel) sensors detect the % of ethanol in a gasoline mix. Since the octane of ethanol is known, and the octane of domestically blended gasoline can be estimated conservatively, the octane of an ethanol mix can be conservatively estimated based on the percentage of ethanol detected by the sensor. An engine can then be tuned to alter parameters based on the fuel mix and calculated octane. It's important to note that this only works for ethanol blends (I suppose it could also be calibrated to work with other alcohol blends that share similar properties to ethanol mixtures). There is a big difference between determining that the fuel is E0, E10, E70, E85, or E98 and comparing it to a table of known octane values (and applying the correct map) and actually detecting the octane of the fuel. It does have plenty of practical applications when your fuel type is known.
- Ethanol is not the only way to increase the octane rating of gasoline (i.e. 87 and 93 E0 gasoline are two unique blends, and there are higher octane blends that use different hydrocarbons with different resistances to knock to raise the octane).
- An ethanol (flex fuel) sensor is NOT the same thing as a laboratory octane sensor. Octane sensors exist, but they are not currently used in real time automotive applications that I'm aware of (please chime in if you have an example).
- Most EFI vehicles are NOT equipped wit an ethanol sensor. They do not actively determine the ethanol percentage (and thus the calculated octane) of the fuel they run on. They have a "normal" high octane tune that works with the fuel they were designed and tuned for (generally 91 octane or lower... sometimes 93 but very conservatively in the US because it's not available everywhere).
- If you use ethanol free 100+ octane race gas, you aren't going to get any better performance or fuel economy. However, if you use lower octane gas, the engine will pull timing to compensate after it detects knock events (the amount is based on the summation of knock over a time period). This is why the guys above referred to the knock sensor as the octane sensor (it effectively can tell when octane is not high enough but doesn't quantify the octane rating). When an engine pulls timing you will lose power and fuel economy. The only way you gain power with higher octane blends (like race gas) is to have your car specifically tuned for it.
- If you use ethanol free 100+ octane race gas in a flex fuel vehicle it won't detect the increased octane and treat it like E85. It'll run it like the highest octane gasoline map that it has (likely 91 or lower as above). It'll be just like you filled up with 91E0.
- As you said, ethanol was added for emissions reasons (and I'm not getting into the politics).
- In closed loop operations (ecu is using feedback from a AFR/lambda sensor) E0 gasoline will result in slightly more negative fuel trims as the narrow band afr/lambda sensor in your car detects your burn AFR and tries to compensate. Negative fuel trims are the result of a slightly rich condition detected in your exhaust stream (thus the fuel will be subtracted via reducing injector pulse width at a given duty cycle). E10 will result in slightly more positive fuel trims and more fuel will be injected to maintain stoich. Generally modern ECUs can compensate +/-25% or so as far as fuel delivery is concerned. This is plenty of range to deal with mixing between E0 and E10 which is why flex fuel sensors are not used and needed to deal with E10 gas blends and most vehicles don't have them.
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This is a fairly comprehensive tuning guide for the Evolution X (the platform that I'm most familiar with because it's what I own). It's long, but a great read and actually will show you what the fuel, timing, intake/exhaust cam tables look like and might help you get a better mental picture of what is going on. Note that the Evo community is actively working on incorporating flex fuel (ethanol) sensors into the factory ECU (which is pretty damn impressive in it's own right).
http://thefrost.net/randomfiles/tuning/merlin/Merlins Ralliart + EvoX TUNING GUIDE_V2-0.pdf
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Lets keep this as a polite discussion. I certainly don't profess to know everything and would like to learn just like the next guy.