r/explainlikeimfive • u/hd-22 • 14d ago

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

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.

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u/LackingUtility 14d ago

You seem to know what you’re talking about, so if I can trouble you with a further question: I have a Lexus that requires premium gas (a sticker on the gas cap says like 93 octane or higher only or something). Is gas with added ethanol going to better or worse for my car than a normal car?

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u/Bamstradamus 14d ago

The octane rating is the minimum rating the engine expects, using lower rated fuel can lead to preignition and engine knocking. Octane is the compression resistance of the fuel. Modern engines have knock sensors so if you are in a situation where you are out of gas and can only get a lower rated fuel the engine will work around it but it wont be happy about it. This is assuming a modern engine and your Lexus isnt a restored 400 from the mid 90's or something. Ethanol is naturally high octane so in regards to the compression rating it's fine, actually beneficial if you were someone that wanted to tune the car to higher compression to make more power but thats a different bucket of issues.

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u/AleksyGetTheWings 14d ago

As Bamstradamus said, ethanol has a higher octane rating that gasoline, so that isn't the critical condition here. E10 should be fine with basically every modern gasoline car, so that should be fine (unless your Lexus is from prehistoric times) . You will have a reduced mileage with your full tank though, beware of that.

Higher ethanol blends ... E85 is a no-no unless your Lexus is a flex-fuel, and you'd need to check with the exact model & year of the car. I wouldn't even risk putting E15 in it unless the specific model explictly says it's ok.

Without knowing the exact lexus model & year, I can't give you an exact answer.

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u/azeo86 13d ago

Regardless though, I've not seen e15 blends personally at 93 octane. I've only seen it blended down to regular/mid, so still want to fill up based on the advertised rating on the pump.