why it’s common to have 87-octane gasoline in the US but it’s almost always 95-octane in Europe?
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Mostly because the USA and Europe don’t measure octane the same way.
Europe uses RON or the “Research Octane Number” which is measured by running an engine under controlled conditions.
In contrast the US uses the AKI or “Anti-Knock Index” which is an average of the RON and MON, the “Motor Octane Number” which uses a similar test engine to RON, but operates it under different standardized conditions.
Roughly speaking:
87 AKI = 91 RON
91 AKI = 93 RON
93 AKI = 98 RON
100 AKI = 104 RON
104 AKI = 108 RON
A similar kind of misconception occurs when people think that the UK has more fuel efficient vehicles because they get more miles or kilometers per gallon. The truth is that the US gallon is 128 fluid ounces and the UK it is 160 fluid ounces, and the vehicles generally have equivalent fuel economy.
There are two different tests used to measure octane level, RON and MON (they test using different conditions). MON is usually about 10 lower than RON. European pumps show the RON number, US pumps show AKI which is the average of the two so it will be lower for the same fuel
https://en.wikipedia.org/wiki/Octane_rating
RON vs MON as the reference tables for octane rating.
RON is about 4 points higher than MON in the gas/petrol range of combustion resistance.
You’re comparing apples to oranges. The octane ratings aren’t calculated the same way so you can’t make a direct comparison. The way it’s calculated in Europe leads to a higher number on paper, but it’s the same old gas used in N/A.
It’s like asking ‘why does Canada get 4L jugs of milk but in the US we just have 1 gallon?’ It’s more or less the same, just a different measuring system.
Chemical engineer here working in refining and specifically in gasoline production.
Some of the other comments about Europe using Ron vs. Mon are correct, however, there is additional info:
European vehicles trend toward smaller displacement, higher compression ratios in their vehicles. Why? Because high compression ratios make a more efficient vehicle. What is a compression ratio? It is how much pressure you apply to the air/fuel mixture before setting off the spark.
Ok, so why is octane important here? All fluids heat up when compressed, if you heat up an air/fuel mixture beyond it’s “flash point”, it will auto ignite causing an explosion without the piston being in the right position for said explosion. This is what’s called “knock” and is very hard on your engine.
So, higher octane means you can compress the air/fuel mixture more without knock, which leads to higher engine efficiency
Edit: I want to make a comment about a common misconception here. A higher octane does not mean there is more energy per gallon of fuel in any meaningful way. The engine must be designed to fully utilize the properties of the higher octane fuel. If you just shove some high octane in your 87min Honda Accord, you won’t get any better fuel economy. In some cases, because of the fuels resistance to ignition, you can actually end up causing some carbon deposits to occur in the engine.
Can someone ELI5 this thread please? I am absolutely baffled as to what I am reading. Honestly.
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