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Octane rating

Overview

Octane rating or octane number is a standard measure of the anti-knock

Engine knocking

Knocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.The...

properties (i.e. the performance) of a motor or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating. In broad terms, fuels with a higher octane rating are used in high-compression engines that generally have higher performance.

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As you increase altitude, due you require more or less octane in a designed engine.

Encyclopedia

Octane rating or octane number is a standard measure of the anti-knock

Engine knocking

Principles

Octane

Octane is a hydrocarbon and an alkane with the chemical formula C8H18, and the condensed structural formula CH36CH3. Octane has many structural isomers that differ by the amount and location of branching in the carbon chain...

is a hydrocarbon liquid that is used as a reference standard to describe the tendency of gasoline, petrol, or benzin

Gasoline

Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...

fuels to self ignite during compression prior to the desired position of the piston in the cylinder as appropriate for valve and ignition timing. The problem of premature ignition is referred to as pre-ignition and also as engine knock

Engine knocking

, which is a sound that is made when the fuel ignites too early in the compression stroke.

Severe knock causes severe engine damage, such as broken connecting rods, melted pistons, melted or broken valves and other components. The octane rating is a measure of how likely a gasoline or liquid petroleum fuel is to self ignite. The higher the number, the less likely an engine is to pre-ignite and suffer damage.
The most typically used engine management systems found in automobiles today monitor the level of knock that is being produced by the fuel being used. In modern computer controlled engines, the timing of the ignition will be automatically altered by the fuel management system to reduce the pre-ignition to an acceptable level.

The octane rating of gasoline

Gasoline

is measured in a test engine and is defined by comparison with the mixture of 2,2,4-trimethylpentane

2,2,4-Trimethylpentane

2,2,4-Trimethylpentane, also known as isooctane, iso-octane, is an organic compound with the formula 3CCH2CH2. It is one of several isomers of octane . This particular isomer is the standard for 100 point on the octane rating scale...

(iso-octane) and heptane

Heptane

n-Heptane is the straight-chain alkane with the chemical formula H3C5CH3 or C7H16. When used as a test fuel component in anti-knock test engines, a 100% heptane fuel is the zero point of the octane rating scale...

that would have the same anti-knocking

Engine knocking

capacity 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, petrol with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90. A rating of 90 does not mean that the petrol contains just iso-octane and heptane in these proportions, but that it has the same detonation resistance properties. Because some fuels are more knock-resistant than iso-octane, the definition has been extended to allow for octane numbers higher than 100.

Octane rating does not relate to the energy content of the fuel (see heating value). It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner. Where the octane number is raised by blending in ethanol, energy content per volume is reduced.

It is possible for a fuel to have a Research Octane Number (RON) greater than 100, because iso-octane is not the most knock-resistant substance available. Racing fuels, avgas

Avgas

Avgas is an aviation fuel used to power piston-engine aircraft. Avgas is distinguished from mogas , which is the everyday gasoline used in cars and some non-commercial light aircraft...

, liquefied petroleum gas (LPG), and alcohol fuel

Alcohol fuel

Although fossil fuels have become the dominant energy resource for the modern world, alcohol has been used as a fuel throughout history. The first four aliphatic alcohols are of interest as fuels because they can be synthesized chemically or biologically, and they have characteristics which allow...

s such as methanol

Methanol

Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

may have octane ratings of 110 or significantly higher. Typical "octane booster" gasoline additive

Gasoline additive

Gasoline additives increase gasoline's octane rating or act as corrosion inhibitors or lubricants, thus allowing the use of higher compression ratios for greater efficiency and power, however some carry heavy environmental risks...

s include MTBE, ETBE

ETBE

Ethyl tert-butyl ether is commonly used as an oxygenate gasoline additive in the production of gasoline from crude oil. ETBE offers equal or greater air quality benefits than ethanol, while being technically and logistically less challenging...

, isooctane and toluene

Toluene

Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...

. Lead

Lead

Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

in the form of tetra-ethyl lead

Tetra-ethyl lead

Tetraethyllead , abbreviated TEL, is an organolead compound with the formula 4Pb. An inexpensive additive, its addition to gasoline from the 1920's allowed octane ratings and thus engine compression to be boosted significantly, increasing power and fuel economy...

was once a common additive, but since the 1970s, its use in most of the industrialised world has been restricted, and its use is currently limited mostly to aviation gasoline.

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

Engine

An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...

with a variable compression ratio

Compression ratio

The 'compression ratio' of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity...

under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.

Motor Octane Number (MON)

There is another type of octane rating, called Motor Octane Number (MON), or the aviation lean octane rating, which is a better measure of how the fuel behaves when under load, as it is determined at 900 rpm engine speed, instead of the 600 rpm for RON. 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

Ignition system

An ignition system is a system for igniting a fuel-air mixture. Ignition systems are well known in the field of internal combustion engines such as those used in petrol engines used to power the majority of motor vehicles, but they are also used in many other applications such as in oil-fired and...

to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON, however there is no direct link between RON and MON. Normally, fuel specifications require both a minimum RON and a minimum MON.

Anti-Knock Index (AKI)

In most countries, including Australia and all of those in Europe, the "headline" octane rating shown on the pump is the RON, but in Canada, the United States and some other countries, like Brazil, the headline number is the average of the RON and the MON, called the Anti-Knock Index (AKI, and often written on pumps as (R+M)/2). It may also sometimes be called the Pump Octane Number (PON).

Difference between RON and AKI

Because of the 8 to 10 point difference noted above, the octane rating shown in the United States is 4 to 5 points lower than the rating shown elsewhere in the world for the same fuel. See the table in the following section for a comparison.

Observed Road Octane Number (RdON)

The final type of octane rating, called Observed Road Octane Number (RdON), is derived from testing gasolines in real world multi-cylinder engines, normally at wide open throttle. It was developed in the 1920s and is still reliable today. The original testing was done in cars on the road but as technology developed the testing was moved to chassis dynamometers with environmental controls to improve consistency.

Examples of octane ratings

The RON/MON values of n-heptane

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.

Fuel	RON	MON	AKI
hexadecane Hexadecane Hexadecane is an alkane hydrocarbon with the chemical formula C16H34. Hexadecane consists of a chain of 16 carbon atoms, with three hydrogen atoms bonded to the two end carbon atoms, and two hydrogens bonded to each of the 14 other carbon atoms.Cetane is often used as a short-hand for cetane...	< -30
n-octane Octane Octane is a hydrocarbon and an alkane with the chemical formula C8H18, and the condensed structural formula CH36CH3. Octane has many structural isomers that differ by the amount and location of branching in the carbon chain...	-10
n-heptane Heptane n-Heptane is the straight-chain alkane with the chemical formula H3C5CH3 or C7H16. When used as a test fuel component in anti-knock test engines, a 100% heptane fuel is the zero point of the octane rating scale... (RON and MON 0 by definition)	0	0	0
diesel fuel	15–25
2-methylheptane 2-Methylheptane 2-Methylheptane is a branched alkane isomeric to octane. Its structural formula is 2CH4CH3.-External links:*...	23	23.8
n-hexane Hexane Hexane is a hydrocarbon with the chemical formula C6H14; that is, an alkane with six carbon atoms.The term may refer to any of four other structural isomers with that formula, or to a mixture of them. In the IUPAC nomenclature, however, hexane is the unbranched isomer ; the other four structures...	25	26.0	26
1-pentene Pentene Pentene refers to all the alkenes with chemical formula . Each contains one double bond within its molecular structure. There are a total of five different compounds in this class, differing from each other by whether the carbon atoms are attached linearly or in a branched structure, and whether...	34
2-methylhexane 2-Methylhexane 2-Methylhexane is an isomer of heptane. It is structurally a hexane molecule with a methyl group attached to its second carbon atom...	44	46.4
3-methylhexane 3-Methylhexane 3-Methylhexane is a branched hydrocarbon. It is one of the isomers of heptane....		55.0
1-heptene Heptene Heptene is a higher olefin, or alkene with the formula C7H14.The commercial product is a liquid that is a mixture of isomers. It is used as an additive in lubricants, as a catalyst, and as a surfactant. This chemical is also known as heptylene....	60
n-pentane Pentane Pentane is an organic compound with the formula C5H12 — that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the n-pentane isomer; the other two being called...	62	61.9
requirement for a typical two-stroke outboard engine	69	65	67
Pertamina "Premium" gasoline in Indonesia Indonesia Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...	88
n-butanol N-Butanol n-Butanol or n-butyl alcohol or normal butanol is a primary alcohol with a 4-carbon structure and the molecular formula C4H9OH. Its isomers include isobutanol, 2-butanol, and tert-butanol...	92	71	83
2,2-dimethylpropane		80.2
"regular" gasoline in Australia, New Zealand, Canada and the US	91–92	82–83	87
Pertamina "Pertamax" gasoline in Indonesia Indonesia Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...	92
Shell "Super" in Indonesia Indonesia Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...	92
n-butane Butane Butane is a gas with the formula C4H10 that is an alkane with four carbon atoms. The term may refer to any of two structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, butane refers only to the unbranched n-butane isomer; the other one being called "methylpropane" or...	94	90.1
2-methylbutane		90.3
Pertamina "Pertamax Plus" gasoline in Indonesia Indonesia Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...	95
Shell "Super Extra" in Indonesia Indonesia Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...	95
Shell "FuelSave " in Malaysia	95
"EuroSuper" or "EuroPremium" or "Regular unleaded" in Europe, "SP95" in France	95	85–86	90–91
Shell "V-Power 97" in Malaysia	97
"SuperPlus" in Germany, Great Britain, Slovenia and Spain, "SP98" in France	98	89–90	93–94
Shell V-Power in Italy Italy Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and... and Germany Germany Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...	100
Eni(or Agip) Blu Super +(or Tech) in Italy	100
IP Plus 100 in Italy	100
Tamoil WR 100 in Italy	100
San Marco Petroli F-101 in Italy(northern Italy only, just a few gas stations)	101
Aral Ultimate 102 in Germany Germany Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...	102
IES 98 Plus in Italy	98
2,2-dimethylbutane 2,2-Dimethylbutane 2,2-Dimethylbutane, trivially known as neohexane, is an organic compound with formula C6H14 or 3-C-CH2-CH3. It is therefore an alkane, indeed the most compact and branched of the hexane isomers — the only one with a quaternary carbon and a butane backbone.-See also:*methylbutane...		93.4
2,3-dimethylbutane 2,3-Dimethylbutane 2,3-Dimethylbutane, also known as diisopropyl, is an isomer of hexane. It has the chemical formula 2CHCH2 It is a colorless liquid which boils at 57.9 °C....		94.4
ExxonMobil Avgas Avgas Avgas is an aviation fuel used to power piston-engine aircraft. Avgas is distinguished from mogas , which is the everyday gasoline used in cars and some non-commercial light aircraft... 100		99.5 (min)
Shell "V-Power Racing" in Australia Australia Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area... - discontinued July 2008	100
"isooctane" 2,2,4-Trimethylpentane 2,2,4-Trimethylpentane, also known as isooctane, iso-octane, is an organic compound with the formula 3CCH2CH2. It is one of several isomers of octane . This particular isomer is the standard for 100 point on the octane rating scale... (RON and MON 100 by definition)	100	100	100
benzene Benzene Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....	101
i-butane Isobutane Isobutane, also known as methylpropane, is an isomer of butane. It is the simplest alkane with a tertiary carbon. Concerns with depletion of the ozone layer by freon gases have led to increased use of isobutane as a gas for refrigeration systems, especially in domestic refrigerators and freezers,...	102	97.6
"BP Ultimate 102 - now discontinued"	102	93–94	97–98
t-butanol	103	91	97
2,3,3-trimethylpentane 2,3,3-Trimethylpentane 2,3,3-Trimethylpentane is a chemical compound in the family of hydrocarbons which has a formula of C8H18. It is an isomer of octane....	106.1	99.4	103
ethane Ethane Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....	108
2,2,3-trimethylpentane	109.6	99.9	105
toluene Toluene Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...	111	95	103
E85 gasoline E85 E85 is an abbreviation for an ethanol fuel blend of up to 85% denatured ethanol fuel and gasoline or other hydrocarbon by volume. E85 is commonly used by flex-fuel vehicles in the US, Canada, and Europe. Some of the benefits of E85 over conventional gasoline powered vehicles include the potential...			94-96
propane Propane Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...	112	97
2,2,3-trimethylbutane	112.1	101.3	106
xylene Xylene Xylene encompasses three isomers of dimethylbenzene. The isomers are distinguished by the designations ortho- , meta- , and para- , which specify to which carbon atoms the two methyl groups are attached...	117
isopropanol	118	98	108
methanol Methanol Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...	108.7	88.6	98.65
ethanol Ethanol Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...	108.6	89.7	99.15
2,5-Dimethylfuran 2,5-Dimethylfuran 2,5-Dimethylfuran is a heterocyclic compound with the formula 2C4H2O. Although often abbreviated DMF, it should not be confused with dimethylformamide. A derivative of furan, this simple compound is a potential biofuel, being derivable from cellulose.-Production:Fructose can be converted into...	119
methane Methane Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...	120	120	120
hydrogen*	> 130	very low

*Hydrogen does not fit well into the normal definitions of octane number. It has a very high RON and a low MON, so that it has low knock resistance in practice, due to its low ignition energy (primarily due to its low dissociation energy) and extremely high flame speed. These traits are highly desirable in rocket engines, but undesirable in Otto-cycle

Otto cycle

An Otto cycle is an idealized thermodynamic cycle which describes the functioning of a typical reciprocating piston engine, the thermodynamic cycle most commonly found in automobile engines....

engines. However, as a minor blending component (e.g. in a bi-fuel vehicle), hydrogen raises overall knock resistance. Flame speed is limited by the rest of the component species; hydrogen may reduce knock because of its high thermal conductivity.

Effects of octane rating

Higher octane ratings correlate to higher activation energies

Activation energy

In chemistry, activation energy is a term introduced in 1889 by the Swedish scientist Svante Arrhenius that is defined as the energy that must be overcome in order for a chemical reaction to occur. Activation energy may also be defined as the minimum energy required to start a chemical reaction...

: This being 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 or detonation.

It might seem odd that fuels with higher octane ratings are used in more powerful engines, since such fuels ignite less easily. However, detonation is undesirable in a spark ignition engine, and is signified by audible "pinging" or in more extreme cases "knock".

A fuel with a higher octane rating can be burnt in an engine with a high compression ratio

Compression ratio

without causing detonation, as such fuels are less prone to detonation. Compression is directly related to power and to thermodynamic efficiency (see engine tuning

Engine tuning

Engine tuning is the adjustment, modification or design of internal combustion engines to yield optimal performance, to increase an engine's power output, economy, or durability....

), so engines that require a higher octane fuel usually develop more motive power

Motive power

In thermodynamics, motive power is an agency, as water or steam, used to impart motion. Generally, motive power is defined as a natural agent, as water, steam, wind, electricity, etc., used to impart motion to machinery; a motor; a mover. The term may also define something, as a locomotive or a...

and therefore do more work in relation to the calorific value of the fuel (BTU) being used. Power output is a function of the properties of the fuel used, as well as the design of the engine itself, and is related to octane rating of the fuel. Power is limited by the maximum amount of fuel-air mixture that can be brought into the combustion chamber

Combustion chamber

A combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...

. When the throttle

Throttle

A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...

is partly open, only a small fraction of the total available power is produced because the manifold

Inlet manifold

In automotive engineering, an inlet manifold or intake manifold is the part of an engine that supplies the fuel/air mixture to the cylinders...

is operating at pressures far below that of the external atmosphere (depression). In this case, the octane requirement is far lower than when the throttle is opened fully and the manifold pressure increases to almost that of the external atmosphere, or higher in the case of forced induction engines (See supercharged

Supercharger

A supercharger is an air compressor used for forced induction of an internal combustion engine.The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally aspirated engine, which allows more fuel to be burned and more work to be done per cycle,...

or turbocharged

Turbocharger

A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...

engines).

Many high-performance engines are designed to operate with a high maximum compression, and thus demand fuels of higher octane. A common misconception is that power output or fuel efficiency can be improved by burning fuel of higher octane than that specified by the engine manufacturer. The power output of an engine depends in part on the energy density of the fuel being burnt. Fuels of different octane ratings may have similar densities, but because switching to a higher octane fuel does not add more hydrocarbon content or oxygen, the engine cannot develop more power.

However, burning fuel with a lower octane rating than that for which the engine is designed often results in a reduction of power output and efficiency. Many modern engines are equipped with a knock sensor (a small piezoelectric microphone), which sends a signal to the engine control unit

Engine control unit

An engine control unit is a type of electronic control unit that determines the amount of fuel, ignition timing and other parameters an internal combustion engine needs to keep running...

, which in turn retards the ignition timing

Ignition timing

Ignition timing, in a spark ignition internal combustion engine , is the process of setting the angle relative to piston position and crankshaft angular velocity that a spark will occur in the combustion chamber near the end of the compression stroke...

when detonation is detected. Retarding the ignition timing reduces the tendency of the fuel-air mixture to detonate, but also reduces power output and fuel efficiency. Because of this, under conditions of high load and high temperature, a given engine may have a more consistent power output with a higher octane fuel, as such fuels are less prone to detonation. Some modern high performance engines are actually optimized for higher than pump premium (93 AKI in the US). The 2001 - 2007 BMW M3 with the S54 engine is one such car. Car and Driver magazine tested a car using a dynamometer, and found that the power output increased as the AKI was increased up to approximately 96 AKI.

Most fuel filling stations have two storage tanks (even those offering 3 or 4 octane levels): those motorists who purchase intermediate grade fuels are given a mixture of higher and lower octane fuels. "Premium" grade is fuel of higher octane, and the minimum grade sold is fuel of lower octane. Purchasing 91 octane fuel (where offered) simply means that more fuel of higher octane is blended with commensurately less fuel of lower octane, than when purchasing a lower grade. The detergents and other additives in the fuel are often, but not always, identical.

The octane rating was developed by chemist Russell Marker

Russell Marker

Russell Earl Marker was an American chemist who invented the octane rating system when he was working at the Ethyl Corporation. Later in his career, he went on to found a steroid industry in Mexico when he successfully made synthetic progesterone from chemical constituents found in Mexican yams...

at the Ethyl Corporation

Ethyl Corporation

Ethyl Corporation is a fuel additive company headquartered in Richmond, Virginia. The company is a manufacturer, blender and distributor of fuel additives...

in 1926. The selection of n-heptane

Heptane

as the zero point of the scale was due to its availability in high purity. Other isomers of heptane produced from crude oil have greatly different ratings.

Regional variations

The selection of octane ratings available at the pump can vary greatly from region to region.

Australia: "regular" unleaded fuel is 91 RON, "premium" unleaded with 95 RON is widely available, and 98 RON fuel is also reasonably common. Shell used to sell 100 RON petrol (5% ethanol content) from a small number of service stations, most of which are located in major cities (stopped in August 2008). United Petroleum sells 100 RON unleaded fuel (10% ethanol content) at a small number of its service stations (originally only two, but it has now expanded to 19 outlets).

Bahrain: 90 and 95 (RON), standard in all petrol station in the country and advertised as (Jayyid) for Regular or 90 and (Mumtaz) for Premium or 95.

China: 93 and 97 (RON) are commonly offered. In limited areas higher rating such as 99 RON is available. In some rural areas it can be difficult to find fuel with over 93 RON.

Chile: 93, 95 and 97 RON are standard at almost all gas stations thorough Chile. The three types are unleaded

Egypt: 80 RON is commonly used for all taxis and old cars and is the predominant rating in rural areas. 90 RON and 92 RON are available at almost all gas stations with a negligible price difference between them. 95 RON is becoming more common especially in the big cities and upscale suburbs. All fuels are unleaded.

Finland: 95 and 98 (RON), advertised as such, at almost all gas stations. Most cars run on 95, but 98 is available for vehicles that need higher octane fuel. Shell offers V-Power, advertised as "over 99 octane", instead of 98. In the beginning of 2011 95 RON was replaced by 95E10, and 98 RON will be advertised as 98E5. ST1 also offers RE85 on some stations, which is E85 made from waste from which the advertised name "ReFuel" comes from.

Germany: "Super E10" 95 RON and "Super Plus E5" 98 RON are available practically everywhere. Big suppliers like Shell
Royal Dutch Shell
Royal Dutch Shell plc , commonly known as Shell, is a global oil and gas company headquartered in The Hague, Netherlands and with its registered office in London, United Kingdom. It is the fifth-largest company in the world according to a composite measure by Forbes magazine and one of the six...

or Aral
Aral AG
Aral is a brand of automobile fuels and gas stations, present in Germany and Luxembourg, but formerly used in most countries of Western and Central Europe. The company behind the brand name, Aral AG , is owned by BP, but was established in 1898 as Westdeutsche Benzol-Verkaufs-Vereinigung GmbH...

offer 100 RON gasoline (Shell V-Power, Aral Ultimate) at almost every fuel station. "Normal" 91 RON is only rarely being offered, because lower production amounts make it more expensive than "Super" 95 RON, so it is often not offered any more. Due to a new European Union law, gas stations are being required to offer and sell a minimum rate of the new mixture of "Super" 95 RON with up to 10% Ethanol branded as "Super E10" since early 2011. Due to that process the big suppliers are discontinuing the average "Super E5" 95 RON with <5% Ethanol so cars without the capability for using E10 need to use 98 RON petrol instead.

Hong Kong: only 98 RON and 99 RON are available in the market. There have been calls to re-introduce 95 RON, but the calls have been rejected by all petrol station chains, citing that 95 RON was phased out because of market forces.

India: India's Ordinary And Premium Petrols are of 89–91 RON. The premium petrols are generally ordinary fuels with additives, that do not really change the octane value. Two variants, "Speed 93" and "Speed 97", were launched, with RON values of 93 and 97. India's economy-class vehicles usually have compression ratios under 10:1, thus enabling them to use lower-quality petrol without engine knocking.

Indonesia: Indonesia's "Premium" petrol rated at 88 RON and being subsidized it cost only about US$0.50/liter. Other options are "Pertamax" rated at 92 RON and the "Pertamax Plus" rated at RON 95, which is the highest octane available for automotive gasoline in Indonesia.

Ireland: 95 RON "unleaded" is the only petrol type available through stations, although E5 (99 RON) is becoming more commonplace.

Italy: 95 RON is the only compulsory gasoline offered (verde, "green"), only a few fuel stations (Agip, IP, IES, OMV) offer 98 RON as the premium type, many Shell and Tamoil stations close to the cities offer also V-Power Gasoline rated at 100 RON. Recently Agip introduced "Blu Super+", a 100 RON gasoline.

Israel: 95 RON & 98 RON are normally available at most petrol stations. 96 RON is also available at a large number of gas stations but 95 RON is more preferred because it's cheaper and performance differences aren't very wide and noticeable. "Regular" fuel is 95 RON. All variants are unleaded.

Latvia: 95 RON and 98 RON widely available.

Lebanon: 95 RON and 98 RON are widely available.

Malaysia: Had RON 92 until 2009. Replaced with "regular" unleaded fuel Ron 95 RON, "premium" fuel is rated at 97 RON(but for Shell 97 RON is V-Power, and Shell's V-Power Racing is rated at 97 RON.)

Montenegro: 95 RON is sold as a "regular" fuel. As a "premium" fuel, 98 RON is sold. Both variants are unleaded.

Netherlands: 95 RON "Euro" and 98 RON "Super" are sold at practically every station. Shell V-Power is a 97 RON (labelled as 95 due to the legalities of only using 95 or 98 labelling), some independent test have shown that one year after introduction it was downgraded to 95 RON, whereas in neighboring Germany Shell V-Power consists of the regular 100 RON fuel.

New Zealand: 91 RON "Regular" and 95 RON "Premium" are both widely available. 98 RON is available instead of 95 RON at some service stations in larger urban areas.

Philippines: A brand of Petron
Petron Corporation
Petron Corporation is the largest oil refining and marketing company in the Philippines, supplying more than a third of the country’s oil requirements....

, Petron Blaze is rated at 100 RON (the only brand of gasoline in the Philippines without an ethanol blend). Other "super premium" brands like Petron XCS, Calex Gold, Shell V-Power are rated at 95-97 RON, while Petron Xtra Unleaded, Caltex Silver, and Shell Super Unleaded are rated at 93 RON.

Poland: Eurosuper 95 (RON 95) is sold in every gas station. Super Plus 98 (RON 98) is available in most stations, sometimes under brand (Orlen - Verva, BP - Ultimate, Shell - V-Power) and usually containing additives. Shell offers V-Power Racing fuel which is rated RON 100.

Russia and CIS
CIS
CIS usually refers to the Commonwealth of Independent States, a modern political entity consisting of eleven former Soviet Union republics.The acronym CIS may also refer to:-Organizations:...

countries: 80 RON (76 MON) is the minimum available, the standard is 92 RON and 95 RON. 98 RON is available on some stations but it's usually quite expensive compared to the lower octane rating fuels.

Saudi Arabia: Two types of fuel are available at all gas stations in Saudi Arabia. "Premium 91" (RON 91) where the pumps are coloured green, and "Super Premium 95" (RON 95) where the pumps are coloured red. While gas stations in Saudi Arabia are privatized, the prices are regulated by the authorities and have a fixed at S.A.R. 0.45 (U.S. $0.12) and S.A.R. 0.60 (U.S. $0.16) per litre respectively. Prior to 2006, only Super Premium RON 95 was available and the pumps weren't coloured in any specific order. The public didn't know what Octane rating was, therefore big educating campaigns were spread, telling the people to use the "red gas" only for high end cars, and save money on using the "green gas" for regular cars and trucks.

South Africa: "regular" unleaded fuel is 95 RON in coastal areas with most fuel stations optionally offering 97 RON. Inland (higher elevation) "regular" unleaded fuel is 93 RON; once again most fuel stations optionally offer 95 RON.

Spain: 95 RON "Euro" is sold in every station with 98 RON "Super" being offered in most stations. Many stations around cities and highways offer other high-octane "premium" brands.

Sri Lanka: In Ceypetco filling stations, 90 RON is the regular petrol and 95 RON is called 'Super Petrol', which comes at a premium price. In LIOC filling stations, 90 RON remains as regular petrol and 92 RON is available as 'Premium Petrol'. The cost of premium petrol is lower than the cost of super petrol.

Taiwan: 92 RON, 95 RON and 98 RON are widely available at gas stations in Taiwan.

Thailand: 91 RON and 95 RON are widely available.

Turkey: 95 RON and 98 RON are widely available in gas stations. 92 RON (Regular) has been dropped in 2006.

Ukraine: the standard gasoline is 95 RON, but 92 RON gasoline is also widely available and popular as a less expensive replacement for 95 RON gasoline. 80 RON gasoline is available for old cars and motorcycles.

United Kingdom: 'regular' petrol has an octane rating of 95 RON, with 97 RON fuel being widely available as the Super Unleaded. Tesco
Tesco
Tesco plc is a global grocery and general merchandise retailer headquartered in Cheshunt, United Kingdom. It is the third-largest retailer in the world measured by revenues and the second-largest measured by profits...

and Shell
Royal Dutch Shell
Royal Dutch Shell plc , commonly known as Shell, is a global oil and gas company headquartered in The Hague, Netherlands and with its registered office in London, United Kingdom. It is the fifth-largest company in the world according to a composite measure by Forbes magazine and one of the six...

both offer 99 RON fuel. In April 2006, BP
BP
BP p.l.c. is a global oil and gas company headquartered in London, United Kingdom. It is the third-largest energy company and fourth-largest company in the world measured by revenues and one of the six oil and gas "supermajors"...

started a public trial of the super-high octane
Octane
Octane is a hydrocarbon and an alkane with the chemical formula C8H18, and the condensed structural formula CH36CH3. Octane has many structural isomers that differ by the amount and location of branching in the carbon chain...

petrol BP Ultimate Unleaded 102, which as the name suggests, has an octane rating of 102 RON. Although BP Ultimate Unleaded (with an octane rating of 97 RON) and BP Ultimate Diesel are both widely available throughout the UK, BP Ultimate Unleaded 102 was available throughout the UK in only 10 filling stations, and was priced at about two and half times more than their 97 RON fuel. In March 2010, BP stopped sales of Ultimate Unleaded 102, citing the closure of their specialty fuels manufacturing facility. Shell V-Power is also available, but in a 99 RON octane rating, and Tesco
Tesco
Tesco plc is a global grocery and general merchandise retailer headquartered in Cheshunt, United Kingdom. It is the third-largest retailer in the world measured by revenues and the second-largest measured by profits...

fuel stations also supply the Greenergy
Greenergy
Founded as a bedroom start-up in 1992, Greenergy has grown rapidly to become one of the UK’s largest road fuel suppliers, supplying about one fifth of the UK’s fuel....

produced 99 RON "Tesco 99".

United States: in the US octane rating is displayed in AKI. In the Rocky Mountain (high elevation) states, 85 AKI (90 RON) is the minimum octane, and 91 AKI (95 RON) is the maximum octane available in fuel . The reason for this is that in higher-elevation areas, a typical naturally aspirated engine draws in less air mass per cycle because of the reduced density of the atmosphere. This directly translates to less fuel and reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill a carbureted car that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, but at sea level the fuel may cause damage to the engine. A disadvantage to this strategy is that most turbocharged vehicles are unable to produce full power, even when using the "premium" 91 AKI fuel. In some east coast states, up to 94 AKI (98 RON) is available. In Colorado as well as parts of the Midwest (primarily Minnesota, Iowa, Illinois and Missouri) ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

-based E-85 fuel with 105 AKI is available. Often, filling stations near US racing tracks will offer higher octane levels such as 100 AKI . California fuel stations will offer 87, 89, and 91 AKI (91, 93 and 95 RON) octane fuels, and at some stations, 100 AKI or higher octane, sold as racing fuel.

Venezuela: 91 RON and 95 RON gasoline is available nationwide, in all PDV gas stations. 95 RON petrol is the most widely used in the country, although most cars in Venezuela would work with 91 RON gasoline. This is because petrol prices are heavily subsided by the government. All gasoline in Venezuela is unleaded.

Vietnam: 92 is in every gas station and 95 is in the urban areas.

External links

Octane ratings of some hydrocarbons

Information in general

Octane rating

Principles

Research Octane Number (RON)

Motor Octane Number (MON)

Anti-Knock Index (AKI)

Difference between RON and AKI

Observed Road Octane Number (RdON)

Examples of octane ratings

Effects of octane rating

Regional variations

Further reading

External links