Octane rating

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A US gas station pump showing the five different AKI octane ratings offered

The octane rating is a measure of the resistance of gasoline and other fuels to detonation (engine knocking) in spark-ignition internal combustion engines. High-performance engines typically have higher compression ratios and are therefore more prone to detonation, so they require higher octane fuel. A lower-performance engine will not generally perform better with high-octane fuel, since the compression ratio is fixed by the engine design.

The octane number of a fuel is measured in a test engine, and is defined by comparison with the mixture of iso-octane and normal heptane which would have the same anti-knocking capacity as the fuel under test: the percentage, by volume, of iso-octane 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.^[1] Because some fuels are more knock-resistant than iso-octane, the definition has been extended to allow for octane numbers higher than 100.

Definition of octane rating

Octane rating of a spark ignition engine fuel is the detonation resistance (anti-knock rating) compared to a mixture of iso-octane (2,2,4-trimethylpentane, an isomer of octane) and n-heptane. By definition, iso-octane is assigned an octane rating of 100, and heptane is assigned an octane rating of zero. An 87-octane gasoline, for example, possesses the same anti-knock rating of a mixture of 87% (by volume) iso-octane, and 13% (by volume) n-heptane. This does not mean, however, that the gasoline actually contains these hydrocarbons in these actual proportions. It simply means that it has the same detonation resistance properties as the described 'standard' defined mixture.

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.

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, liquefied petroleum gas (LPG), and alcohol fuels such as methanol or ethanol may have octane ratings of 110 or significantly higher — ethanol's RON is 129 (102 MON, 116 AKI). Typical "octane booster" gasoline additives include tetra-ethyl lead, MTBE, and toluene. Tetra-ethyl lead (the additive used in leaded gasoline) is easily decomposed to its component radicals, which react with the radicals from the fuel and oxygen that start the combustion, thereby delaying ignition and leading to an increased octane number. However, tetra-ethyl lead and its byproducts are poisonous, and the use of tetra-ethyl lead creates an environmental hazard. Since the 1970s, its use in the United States, and most of the industrialised world has been restricted. Its use is now generally currently limited to being an additive 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 with a variable compression ratio 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 done at 900rpm instead of the 600 rpm of the RON^[2][3]. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, a 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 gasoline will be about 8 to 10 points lower than the RON. Normally, fuel specifications require both a minimum RON and a minimum MON.^{[citation needed]}

Anti-Knock Index (AKI)

In most countries, including all of those of Europe, and Australia, the "headline" octane rating shown on the pump is the RON - but in the United States, Canada, and some other countries,^[which?] the headline number is the average of the RON and the MON, called the Anti-Knock Index (AKI). It may also sometimes be called the Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2.

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. For example, 87 AKI octane fuel, the "regular" gasoline in the US and Canada, is 91–92 RON in Europe. 93 AKI octane fuel, the "premium" gasoline, is 97-98 RON in Europe.

However most European pumps deliver 95 RON as "EuroSuper" or "EuroPremium" (equivalent to 90–91 AKI). In Germany, Great Britain and some other countries, 98 RON as "SuperPlus" (93-94 AKI) is available almost everywhere. Even 100 RON (95-96 AKI), is widely sold, and BP sell 102 RON, marketed as "BP Ultimate 102", at some very select few filling stations.^[4]

Examples of octane ratings

The octane ratings of n-heptane and iso-octane are exactly 0 and 100, by definition. For some other hydrocarbons, the following table^[5][6] gives the 'AKI' ratings.

*Hydrogen does not fit well into the normal definitions of octane number. It has a very high RON and a low MON,^[7] so that it has low knock resistance in practice,^[8] 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 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 by contributing its high thermal conductivity^{[citation needed]}

Effects of octane rating

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Higher octane ratings correlate to higher activation energies. Activation energy is the amount of energy necessary to start a chemical reaction. Since higher octane fuels have higher activation energies, it is less likely that a given compression will cause detonation.

It might seem odd that fuels with higher octane ratings are used in more powerful engines, since such fuels explode less easily. However, an explosion is not desired in an internal combustion engine. An explosion will cause the pressure in the cylinder to rise far beyond the cylinder's design limits, before the force of the expanding gases can be absorbed by the piston traveling downward. This actually reduces power output, because much of the energy of combustion is absorbed as strain and heat in parts of the engine,^{[citation needed]} rather than being converted to torque at the crankshaft.

A fuel with a higher octane rating can be run at a higher compression ratio without detonating. Compression is directly related to power (see engine tuning), so engines that require higher octane usually deliver more motive power. Engine power is a function of the fuel, as well as the engine design, and is related to octane rating of the fuel. Power is limited by the maximum amount of fuel-air mixture that can be forced into the combustion chamber. When the throttle is partially open, only a small fraction of the total available power is produced because the manifold is operating at pressures far below atmospheric. In this case, the octane requirement is far lower than when the throttle is opened fully and the manifold pressure increases to atmospheric pressure, or higher in the case of supercharged or turbocharged engines.

Many high-performance engines are designed to operate with a high maximum compression, and thus demand high-octane premium gasoline. A common misconception is that power output or fuel mileage can be improved by burning higher octane fuel than a particular engine was designed for. The power output of an engine depends in part on the energy density of its fuel, but similar fuels with different octane ratings have similar density. Since switching to a higher octane fuel does not add any more hydrocarbon content or oxygen, the engine cannot produce more power.

However, burning fuel with a lower octane rating than required by the engine often reduces power output and efficiency one way or another. If the engine begins to detonate (knock), that reduces power and efficiency for the reasons stated above. Many modern car engines feature a knock sensor – a small piezoelectric microphone which detects knock, and then sends a signal to the engine control unit to retard the ignition timing. Retarding the ignition timing reduces the tendency to detonate, but also reduces power output and fuel efficiency.

Most fuel stations have two storage tanks (even those offering 3 or 4 octane levels), and you are given a mixture of the higher and lower octane fuel. Purchasing premium simply means more fuel from the higher octane tank. The detergents in the fuel are the same.

The octane rating was developed by chemist Russell Marker at the Ethyl Corporation c1926. The selection of n-heptane as the zero point of the scale was due to the availability of very high purity n-heptane, not mixed with other isomers of heptane or octane, distilled from the resin of the Jeffrey Pine. Other sources of heptane produced from crude oil contain a mixture of different isomers with greatly differing ratings, which would not give a precise zero point.

Regional variations

Octane ratings can vary greatly from region to region. For example, the minimum octane rating available in much of the United States is 87 AKI, and the highest is 93. However this does not mean that the fuel is different.

In the Rocky Mountain (high altitude) states, 85 AKI is the minimum octane, and 91 AKI is the maximum octane available in fuel. The reason for this is that in higher-altitude areas, a typical combustion engine draws in less air per cycle due to the reduced density of the atmosphere. This directly translates to reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill up a car with a carburetor 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 is available [1]. In parts of the Midwest (primarily Minnesota, Iowa, Illinois and Missouri) ethanol based E-85 fuel with 105 AKI is available [2]. 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 octane fuels, and at some stations, 100 AKI or higher octane, sold as racing fuel. Until Summer 2001 before the phase-out of methyl tert-butyl ether aka MTBE as an octane enhancer additive, 92 AKI was offered in lieu of 91.

Generally, octane ratings are higher in Europe than they are in North America and most other parts of the world. This is especially true when comparing the lowest available octane level in each country. In many parts of Europe, 95 RON (90-91 AKI) is the minimum available standard, with 97/98 RON being higher specification (being called Super Unleaded). In Germany, big suppliers like Shell or Aral offer 100 RON gasoline (Shell V-Power, Aral Ultimate) at almost every gas station. The United Kingdom also offers Shell V-Power, but in a 99 RON octane rating, and Tesco fuel stations also supply the Greenergy produced 99 RON "Tesco 99". In 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 from a small number of service stations, most of which are located in capital cities (stopped in August 2008). In Malaysia, the "regular" unleaded fuel is 92 RON, "premium" fuel is rated at 97 RON, and Shell's V-Power at 97 RON. In the Netherlands Shell V-Power is a 97 RON (labelled as 95 due to the legalities of only using 95 or 98 labelling), whereas in neighbouring Germany Shell V-Power consists of the regular 100 RON racing fuel. In other countries "regular" unleaded gasoline, when available, is sometimes as low as 85 RON (still with the more regular fuel, 95, and premium, around 98, available). In Russia and CIS countries 80 RON (76 MON) is the minimum available, the standard is 92 RON, however, the most used type is 95 RON.

In Italy, 95 RON is the regular gasoline offered (verde), and most gas stations offer 98 RON as the premium type (Super/Blu Super), many Shell stations close to the cities offer also V-Power Gasoline rated at 100 RON.

This higher rating seen in Europe is an artifact of a different underlying measuring procedure. In most countries (including all of Europe and Australia) the "headline" octane that would be shown on the pump is the RON, but in the United States, Canada and some other countries the headline number is the average of the RON and the MON, sometimes called the Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2. Because of the 8 to 10 point difference noted above, this means that the octane in the United States will be about 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "regular" gasoline in the US and Canada, would be 91-92 in Europe. However most European pumps deliver 95 (RON) as "regular", equivalent to 90–91 US (R+M)/2, and deliver 98 (RON), 99 or 100 (RON) labeled as Super Unleaded.

In the United Kingdom, 'regular' petrol has an octane rating of 95 RON, with 97 RON fuel being widely available as the Super Unleaded. Tesco and Shell both offer 99 RON fuel. BP is currently trialling the public selling of the super-high octane 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 is (as of October 2007) only available throughout the UK in 10 filling stations, and is priced at about two and half times more than their 97 RON fuel.

References

Khoo, Kenny K. Understanding Octane and its Related Components. Yellowknife: Smithsonian Press, 2006.

See also

• Avgas
• Cetane number

External links

Octane ratings of some hydrocarbons

• Petroleum and Coal
• Gasoline Refining and Testing

Information in general

• Gasoline FAQ
• How Octane Works at HowStuffWorks.com