Octane rating

Octane rating does not relate directly to the power output or the energy content of the fuel per unit mass or volume, but simply indicates the resistance to detonating under pressure without a spark.

The added power in such cases comes from the way the engine is designed to compress the air/fuel mixture, and not directly from the rating of the gasoline.

[2] The octane rating of aviation gasoline was extremely important in determining aero engine performance in the aircraft of World War II.

The fuel-air charge is meant to be ignited by the spark plug only, and at a precise point in the piston's stroke.

Effects of engine knocking range from inconsequential (incremental heating plus power loss) to completely destructive (detonation while one of the valves is still open).

One member of the octane family, 2,2,4-Trimethylpentane (iso-octane), is used as a reference standard to benchmark the tendency of gasoline or LPG fuels to resist self-ignition.

Typical "octane booster" gasoline additives include MTBE, ETBE, iso-octane and toluene.

Lead in the form of tetraethyllead was once a common additive, but concerns about its toxicity have led to its use for fuels for road vehicles being progressively phased out worldwide beginning in the 1970s.

RON is determined by running the fuel in a test engine at 600 rpm with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.

In Canada, The United States, and Mexico, the advertised octane rating is the average of the RON and the MON, called the Anti-Knock Index (AKI).

This difference between RON and MON is known as the fuel's sensitivity,[9] and is not typically published for those countries that use the Anti-Knock Index labelling system.

[10] The evaluation of the octane number by either of the two laboratory methods requires a special engine built to match the tests' rigid standards, and the procedure can be both expensive and time-consuming.

Most refiners produce and market more than one grade of motor gasoline, differing principally in their anti-knock quality.

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.

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"[39] 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.

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.

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).

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] Pilots are taught in primary training to avoid settings that produce the highest exhaust gas temperatures, and run the engine either "rich of peak 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.

[41] 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".

In reality, the octane rating is defined as a number describing the stability and ability of a fuel to prevent an engine from unwanted combustions[83] that occur spontaneously in the other regions within a cylinder (i.e., delocalized explosions from the spark plug).

This is a misleading term, because the octane rating of gasoline is not directly related to the power output of an engine.