Why are there so many options for vehicular gasoline at the pumps and what’s even the difference between them?
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I’m going to start off by assuming you’re not asking about diesel or high ethanol fuels.
The octane number refers to the percentage of gasoline that is N-octane, which is a molecule composed of 8 carbon atoms in a straight line and saturated with hydrogens. The rest of the gasoline mixture is composed of other hydrocarbons that are similar in weight (such as other Octanes, septanes, nonanes, etc.) and thus difficult to separate during the refining process. These days a small amount of ethanol is often added as well, which accounts for much of the non N-Octane portion of the fuel.
N-Octane ignites at the appropriate time in the engine cycle (typically when a spark is introduced) however, some of the other molecules that are mixed in may ignite prematurely. If this happens it essentially causes the ignition of the fuel to occur before it would push the piston, and instead it either works against the rest of the engine or is simply wasted. This is called engine knocking, it reduces your fuel efficiency and can cause engine damage.
Gasoline mixtures with higher percentages of N-Octane are less likely to ignite prematurely because they have lower percentages of the other molecules. Thus they are less likely to cause engine knocking. Aside from this issue there’s little difference between the percentages because the other hydrocarbons that aren’t N-Octane will deliver roughly the same amount of energy per mass from a thermodynamics standpoint, so fuel efficiency isn’t really affected unless knocking is occurring.
Engine knocking, however, is far less common in newer vehicles due to better engine design, thus the higher octane fuels are typically less necessary these days and often don’t accomplish anything that the lower octane fuels wouldn’t.
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