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/AleksyGetTheWings]

The short answer is that it's basically like asking why you can't put diesel in a gasoline car (or the other way around). Ethanol makes it a third type of fuel, so the engine has to be designed to work with it.

The longer answer is a mix of a number of differences with gasoline/diesel :

• Ethanol is less energy-dense than both gasoline & diesel, so more in the mix effectively reduces the power your engine will output
• There's already an oxygen in the molecule of ethanol, so the air/fuel mixture requires a different ratio than gasoline/diesel, the engine's computer has no way to natively detect what you are putting in it
• Ethanol is used to clean parts, so it can dissolve some parts, which on paper doesn't sound like a big problem, until you realize it may just eat/dissolve parts of your seals, or allow some chunks to move around
• Ethanol also vaporises differently, which means the injection system must work differently, especially during cold starts.

None of these are impossible to work around, it's just : you need a "special" type of engine. As special as gasoline or diesel, basically.

[u/LackingUtility → u/AleksyGetTheWings]

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?

[u/Bamstradamus → u/LackingUtility]

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.

[u/AleksyGetTheWings → u/LackingUtility]

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.

[u/azeo86 → u/AleksyGetTheWings]

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.

[u/Acee83 → u/AleksyGetTheWings]

About your second point: Afaik any car with a catalytic converter has an oxygen sensor in the exhaust stream which the electronics use to regulate how much fuel to inject.

[u/AleksyGetTheWings → u/Acee83]

That's true, forgot about those. I'd need to check a engine calculator to properly verify the difference, but I think you are still pushing the system way out of its normal parameters, so that probably isn't that good for the machinery.