Octane rating is a measure of fuel resistance to detonation. The numerical value shows the percentage of octane in its mixture with heptane, which demonstrates detonation resistance identical to the fuel in question. Contrary to the popular belief, octane rating has nothing to do with the fuel burning speed. Yes, higher octane fuels usually burn a tad slower, but this does not define fuel’s resistance to detonation. To give you an analogy, it is like color of the car and its size – big cars and trucks tend to use darker colors, but those two characteristics are absolutely independent. In reality the temperature of air/fuel mixture auto-ignition under high pressure defines resistance to detonation.
There are two methods of determining octane rating – motor method and research method. Corresponding numbers are denoted as MON and RON, respectively. MON standard uses harsher ambient conditions for the test, and therefore shows slightly lower numbers than RON.
In the US the yellow sticker number on the pump is the average of those two numbers, i.e. (MON+RON)/2. In Russia, if nothing changed since I left, lower grade gasoline gets rated as MON, and higher grade – as RON. Don’t ask me why – I don’t know. I can only guess that RON number looks more impressive.
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