Editing Octane rating (section)

==Effects==
{{Unreferenced section|date=September 2018}}
Higher octane ratings correlate to higher [[activation energy|activation energies]]: the amount of applied energy required to initiate combustion. Since higher octane fuels have higher activation energy requirements, it is less likely that a given compression will cause uncontrolled ignition, otherwise known as autoignition, self-ignition, pre-ignition, detonation, or knocking. 

Because octane is a measured and/or calculated rating of the fuel's ability to resist autoignition, the higher the octane of the fuel, the harder that fuel is to ignite and the more heat is required to ignite it. The result is that a hotter ignition spark is required for ignition. Creating a hotter spark requires more energy from the ignition system, which in turn increases the parasitic electrical load on the engine. The spark also must begin earlier in order to generate sufficient heat at the proper time for precise ignition. As octane, ignition spark energy, and the need for precise timing increase, the engine becomes more difficult to "tune" and keep "in tune". The resulting sub-optimal spark energy and timing can cause major engine problems, from a simple "miss" to uncontrolled detonation and catastrophic engine failure.

Mechanically within the cylinder, stability can be visualized as having a flame wave initiate at the spark plug and then "travel in a fairly uniform manner across the combustion chamber"<ref>{{Cite journal |last=Sturgis |first=B.M. |date=1954 |title=Some Concepts of Knock and Antiknock Action |url=https://www.jstor.org/stable/44468563 |journal=Society of Automotive Engineers |volume=63 |pages=253–264 |jstor=44468563 }}</ref> with the expanding gas mix pushing the piston throughout the entirety of the power stroke. A stable gasoline and air mix will combust when the flame wave reaches the molecules, adding heat at the interface. [[Engine knocking|Knock]] occurs when a secondary flame wave forms from instability and then travels against the path of the primary flame wave, thus depriving the power stroke of its uniformity and causing issues including power loss and heat buildup.<ref>{{Cite journal |last=Zhi |first=Wang |date=2017 |title=Knocking combustion in spark-ignition engines |journal=Progress in Energy and Combustion Science |volume=61 |pages=78–112 |doi=10.1016/j.pecs.2017.03.004 |bibcode=2017PECS...61...78W |doi-access=free }}</ref>

The other rarely-discussed reality with high-octane fuels associated with "high performance" is that as octane increases, the [[specific gravity]] and energy content of the fuel per unit of weight are reduced. The net result is that to make a given amount of [[Power (physics)|power]], more high-octane fuel must be burned in the engine. Lighter and "thinner" fuel also has a lower [[Heat Capacity|specific heat]], so the practice of running an engine "rich" to use excess fuel to aid in cooling requires richer and richer mixtures as octane increases.

Higher-octane, lower-energy-dense "thinner" fuels often contain [[Alcohol (chemistry)|alcohol]] compounds incompatible with the stock fuel system components, which also makes them [[hygroscopic]]. They also evaporate away much more easily than heavier, lower-octane fuel which leads to more accumulated contaminants in the fuel system. It is typically the {{Citation needed span|text=hydrochloric acids that form due to that water|date=September 2018|reason=HCl can't form from plain water unless chlorine atoms are somehow involved (petrol has almost no Cl so where do these come from?)}} and the compounds in the fuel that have the most detrimental effects on the engine fuel system components, as such acids corrode many metals used in gasoline fuel systems.

During the compression stroke of an internal combustion engine, the temperature of the air-fuel mix rises as it is compressed, in accordance with the [[ideal gas law]]. Higher compression ratios necessarily add parasitic load to the engine, and are only necessary if the engine is being specifically designed to run on high-octane fuel. Aircraft engines run at relatively low speeds and are "[[undersquare]]". They run best on lower-octane, slower-burning fuels that require less heat and a lower compression ratio for optimum vaporization and uniform fuel-air mixing, with the ignition spark coming as late as possible in order to extend the production of cylinder pressure and torque as far down the power stroke as possible. The main reason for using high-octane fuel in air-cooled engines is that it is more easily vaporized in a cold carburetor and engine and absorbs less intake air heat which greatly reduces the tendency for [[carburetor icing]] to occur.

With their reduced densities and weight per volume of fuel, the other obvious benefit is that an aircraft with any given volume of fuel in the tanks is automatically lighter. And since many airplanes are flown only occasionally and may sit unused for weeks or months, the lighter fuels tend to evaporate away and leave behind fewer deposits such as "varnish" (gasoline components, particularly alkenes and oxygenates slowly polymerize into solids).{{clarify|date=December 2020}} Aircraft also typically have dual "redundant" ignition systems which are nearly impossible to tune and time to produce identical ignition timing, so using a lighter fuel that's less prone to autoignition is a wise "insurance policy". For the same reasons, those lighter fuels which are better solvents are much less likely to cause any "varnish" or other fouling on the "backup" spark plugs.{{citation needed|date=December 2020}}

In almost all general aviation piston engines, the [[air–fuel ratio|fuel mixture]] is directly controlled by the pilot, via a knob and cable or lever similar to (and next to) the [[throttle]] control. Leaning{{snd}}reducing the mixture from its maximum amount – must be done with knowledge, as some combinations of fuel mixture and throttle position (that produce the highest ) can cause [[Engine knocking|detonation]] and/or [[pre-ignition]], in the worst case destroying the engine within seconds.{{citation needed|date=December 2020}} Pilots are taught in primary training to avoid settings that produce the highest exhaust gas temperatures, and run the engine either "rich of peak [[Exhaust gas temperature|EGT]]" (more fuel than can be burned with the available air) or "lean of peak" (less fuel, leaving some oxygen in the exhaust) as either will keep the fuel-air mixture from detonating prematurely.<ref>{{cite web|url=https://www.avweb.com/features/avweb-classics/pelicans-perch/pelicans-perch-63where-should-i-run-my-engine-part-1/|title=Pelican's Perch #63:Where Should I Run My Engine? (Part 1)|date=13 December 2002}}</ref> Because of the high cost of unleaded, high-octane [[avgas]], and possible increased range before refueling, some general aviation pilots attempt to save money by tuning their fuel-air mixtures and ignition timing to run "lean of peak". Additionally, the decreased air density at higher altitudes (such as Colorado) and temperatures (as in summer) requires leaning (reduction in amount of fuel per volume or mass of air) for the peak EGT and power (crucial for takeoff).