An octane rating, or octane number, is a standard measure of a fuel's ability to withstand compression in an internal combustion engine without undergoing pre-ignition. The higher the octane number, the more compression the fuel can withstand before detonating. 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 gasoline's resistance to detonating under pressure without a spark.

Whether or not a higher octane fuel improves or impairs an engine's performance depends on the design of the engine. In broad terms, fuels with a higher octane rating are used in higher-compression gasoline engines, which may yield higher power for these engines. 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.^[1]

In contrast, fuels with lower octane (but higher cetane numbers) are ideal for diesel engines because diesel engines (also called compression-ignition engines) do not compress the fuel, but rather compress only air, and then inject fuel into the air that was heated by compression. Gasoline engines rely on ignition of air and fuel compressed together as a mixture, which is ignited only near the end of the compression stroke by electric spark plugs. Therefore, being able to compress the air/fuel mixture without causing detonation is important mainly for gasoline engines. Using gasoline with lower octane than an engine is built for may cause engine knocking and/or pre-ignition.^[2]

The octane rating of aviation gasoline was extremely important in determining aero engine performance in the aircraft of World War II.^[3] The octane rating affected not only the performance of the gasoline, but also its versatility; the higher octane fuel allowed a wider range of lean to rich operating conditions.^[3]

Principles

The problem: detonation

In spark ignition internal combustion engines, knocking (also knock, detonation, spark knock, pinging, or pinking) occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but when one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel-air charge is meant to be ignited by the spark plug only, and at a precise point in the piston's stroke. Knock occurs when the peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive.

Knocking should not be confused with pre-ignition—they are two separate events with pre-ignition occurring before the combustion event.

Most engine management systems commonly found in automobiles today, typically electronic fuel injection (EFI), have a knock sensor that monitors if knock is being produced by the fuel being used. In modern computer-controlled engines, the ignition timing will be automatically altered by the engine management system to reduce the knock to an acceptable level.

Iso-octane as a reference standard

Octanes are a family of hydrocarbons that are typical components of gasoline. They are colorless liquids that boil around 125 °C (260 °F). 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.

The octane rating of gasoline is measured in a test engine and is defined by comparison with the mixture of 2,2,4-trimethylpentane (iso-octane) and normal heptane that would have the same anti-knocking capability as the fuel under test. The percentage, by volume, of 2,2,4-trimethylpentane in that mixture is the octane number of the fuel. For example, gasoline with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90.^[4] A rating of 90 does not mean that the gasoline contains just iso-octane and heptane in these proportions, but that it has the same detonation resistance properties (generally, gasoline sold for common use never consists solely of iso-octane and heptane; it is a mixture of many hydrocarbons and often other additives).

Octane ratings are not indicators of the energy content of fuels. (See Effects below and Heat of combustion). They are only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner.^[5]

Where the octane number is raised by blending in ethanol, energy content per volume is reduced. Ethanol energy density can be compared with gasoline in heat-of-combustion tables.

It is possible for a fuel to have a Research Octane Number (RON) more than 100, because iso-octane is not the most knock-resistant substance available today. Racing fuels, avgas, LPG and alcohol fuels such as methanol may have octane ratings of 110 or significantly higher. 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.^[6]

Measurement methods

Research Octane Number (RON)

The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.^[7] The compression ratio is varied during the test to challenge the fuel's antiknocking tendency, as an increase in the compression ratio will increase the chances of knocking.

Motor Octane Number (MON)

Another type of octane rating, called Motor Octane Number (MON), is determined at 900 rpm engine speed instead of the 600 rpm for RON.^[2] MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern pump gasoline will be about 8 to 12 lower than the RON,^{[citation needed]} but there is no direct link between RON and MON. See the table below.

Anti-Knock Index (AKI) or (R+M)/2

In most countries in Europe, and in Australia and New Zealand, the "headline" octane rating prominently displayed on the pump is the RON, but in Canada, the United States, and Mexico, the headline number is the simple mean or average of the RON and the MON, called the Anti-Knock Index (AKI), and often written on pumps as (R+M)/2. AKI is also sometimes called PON (Pump Octane Number).

Difference between RON, MON, and AKI

Because of the 8 to 12 octane number difference between RON and MON noted above, the AKI shown in Canada and the United States is 4 to 6 octane numbers lower than elsewhere in the world for the same fuel. This difference between RON and MON is known as the fuel's sensitivity,^[8] and is not typically published for those countries that use the Anti-Knock Index labelling system.

See the table in the following section for a comparison.

Observed Road Octane Number (RdON)

Another type of octane rating, called Observed Road Octane Number (RdON), is derived from testing the gasoline in ordinary multi-cylinder engines (rather than in a purpose-built test engine), normally at wide open throttle. This type of test was developed in the 1920s and is still reliable today. The original RdON tests were done in cars on the road, but as technology developed the testing was moved to chassis dynamometers with environmental controls to improve consistency.^[9]

Octane Index

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. The standard engine required for the test may not always be available, especially in out-of-the-way places or in small or mobile laboratories. These and other considerations led to the search for a rapid method for the evaluation of the anti-knock quality of gasoline. Such substitute methods include FTIR, near infrared on-line analyzers, and others. Deriving an equation that can be used to calculate ratings accurately enough would also serve the same purpose, with added advantages. The term Octane Index is often used to refer to the use of an equation to determine a theoretical rating, in contradistinction to the direct measurements required for research or motor octane numbers. An octane index can be of great service in the blending of gasoline. Motor gasoline, as marketed, is usually a blend of several types of refinery grades that are derived from different processes such as straight-run gasoline, reformate, cracked gasoline etc. These different grades are blended in amounts that will meet final product specifications. Most refiners produce and market more than one grade of motor gasoline, differing principally in their anti-knock quality. Being able to make sufficiently accurate estimates of the octane rating that will result from blending different refinery products is essential, something for which the calculated octane index is specially suited.^[10]

Aviation gasoline octane ratings

Aviation gasolines used in piston aircraft engines common in general aviation have a slightly different method of measuring the octane of the fuel. Similar to an AKI, it has two different ratings, although it is usually referred to only by the lower of the two. One is referred to as the "aviation lean" rating, which for ratings up to 100 is the same as the MON of the fuel.^[11] The second is the "aviation rich" rating and corresponds to the octane rating of a test engine under forced induction operation common in high-performance and military piston aircraft. This utilizes a supercharger, and uses a significantly richer fuel/air ratio for improved detonation resistance.^{[8][unreliable source?]}

The most common currently used fuel, 100LL, has an aviation lean rating of 100 octane, and an aviation rich rating of 130.^[12]

Examples

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The RON/MON values of n-heptane and iso-octane are exactly 0 and 100, respectively, by the definition of octane rating. The following table lists octane ratings for various other fuels.^[13][14]