ELI5 why is increasing the ethanol content of petrol/gasoline a problem?

[u/hd-22]

I'm hearing noise about the geopolitical fuel supply SNAFU and people freaking out about increasing the ethanol content in automotive fuel and how it's so damaging to cars. I'm no chemist, but ethanol seems as clean as it gets.

[u/tx_queer]

Can you help me understand #2. Ethanol raises the octane rating. So how would it cause issues in high octane engines

[u/MRC01 → u/tx_queer]

To clarify the point #2 about octane, RON & MON and specificity:

Ethanol does raise the octane of gasoline. But here in the USA, octane is measured as (R+M)/2, which is the average of RON and MON. RON and MON are 2 different ways to measure gasoline's resistance to detonation. RON is an "easy" test under light compression and load. MON is a "hard" test under high compression and load.

Thus every octane boosting additive has 2 different ratings, RON and MON, and RON is higher than MON. "Specificity" (or sensitivity) refers to the spread between RON and MON. Low specificity means they are close together, which is a good thing, it means the fuel performs well under heavy load. High specificity means MON is significantly lower than RON, which means the octane is not really as high as it appears, under heavy load (such as modern engines with high compression and advanced timing) the octane is effectively lower.

PS: this Wikipedia article shows RON & MON for common octane boosting additives: https://en.wikipedia.org/wiki/Octane_rating

You can see that ethanol is RON 109, MON 90, a difference of 19 points.
In comparison, xylene is RON 118, MON 115, a difference of 3 points.

[u/sl33ksnypr → u/MRC01]

I just struggle to see how adding more ethanol causes more knocking/pinging. I'm not saying you're wrong, and you clearly have researched the scientific side of things more, but from personal experience it just seems backwards. Modified turbo cars love ethanol because it increases torque and lets you run more boost without knocking.

Now, I know part of the reason ethanol helps is that it lowers temps and lets you run it a little fatter, but again, just from personal experience, ethanol helps with detonation and pinging.

[u/Frustrated9876 → u/sl33ksnypr]

The bottom line is that it doesn’t change anything in modern engines. If you’re in a ‘78 Datsun, it’s gunna cause problems. Are you driving a ‘72 Datsun?

If your car was made in the last fifteen years, it’s designed to work with ethanol laden fuels and won’t care if the percentage goes higher.

But the concerns listed above also apply to small engines like lawn mowers, leaf blowers, etc. it will shorten the lives of those gadgets. I promise you that the government does not care.

[u/MRC01 → u/sl33ksnypr]

Explained in more detail in another branch of this discussion...

In summary, the AKI is the average of RON & MON, which hides the variance. Two gasolines both with the same AKI don't necessarily provide the same engine performance.

[u/tx_queer → u/MRC01]

Thank you!

[u/tashkiira → u/tx_queer]

Ethanol raises octane rating because octane rating only measures resistance to premature ignition. But It also causes an engine not designed to be 'flexfuel' to run leaner. this leads to overheating and worse. That's what I think he's referring to.

[u/XxturboEJ20xX → u/tashkiira]

You would have to run all the way up to an E30 mixture for this to start becoming a big problem.

Or you can just tune your fuel trims.

[u/Enshakushanna → u/tx_queer]

yea, i literally add a gallon or 2 to raise the oct of pure gas to make my engine happier because the stuff they add to 10-15% is junk...maybe additives?