Octane rating

Overview
The octane rating is a measure of the autoignition resistance of gasoline and other fuels used in spark-ignition internal combustion engines. It is a measure of anti-detonation of a gasoline or fuel.

Octane number is the number which gives the percentage, by volume, of iso-octane in a mixture of iso-octane and normal heptane, that would have the same anti-knocking capacity as the fuel which is under consideration. 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.

Definition of octane rating
The octane rating of a spark ignition engine fuel is the knock 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 proportions. It simply means that it has the same autoignition resistance as the described mixture.

A high tendency to autoignite, or low octane rating, is undesirable in a spark ignition engine but desirable in a diesel engine. The standard for the combustion quality of diesel fuel is the cetane number. A diesel fuel with a high cetane number has a high tendency to autoignite, as is preferred.

It should be noted that octane rating does not relate to the energy content of the fuel (see heating value), nor the speed at which the flame initiated by the spark plug propagates across the cylinder. It is only a measure of the fuel's resistance to autoignition. It is for this reason that one highly branched form, or isomer, of octane (2,2,4-trimethylpentane) has (by definition) an octane rating of 100, whereas n-octane (see octane), which has a linear arrangement of the 8 carbon atoms, has an octane rating of -10, even though the two fuels have exactly the same chemical formula and virtually identical heating values and flame speeds.

Measurement methods
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 these results with those for mixtures of iso-octane and n-heptane.

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. 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.

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 even deliver 98 (RON) or 100 (RON).

The octane rating may also be a "trade name", with the actual figure being higher than the nominal rating.

It is possible for a fuel to have a RON greater than 100, because iso-octane is not the most knock-resistant substance available. Racing fuels, straight ethanol, AvGas and liquified petroleum gas (LPG) typically have octane ratings of 110 or significantly higher - ethanol's RON is 129 (MON 102, AKI 116) reference. Typical "octane booster" additives include tetra-ethyl lead, MTBE and toluene. Tetra-ethyl lead is easily decomposed to its component radicals, which react with the radicals from the fuel and oxygen that would start the combustion, thereby delaying ignition. This is why leaded gasoline has a higher octane rating than unleaded.

Examples of octane ratings
The octane ratings of n-heptane and iso-octane are respectively exactly 0 and 100, by definition. For some other hydrocarbons, the following table gives the 'AKI' ratings.

*Hydrogen represents a paradox. As a fuel outright, it has low knock resistance, due to its low ignition energy (primarily due to its low dissociation energy) and extremely high flame speed. However, as a minor blending component (i.e., 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. These traits are highly desirable in rocket engines, but undesirable in Otto-cycle engines.

Effects of octane rating
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 knocking. (Note that it is the absolute pressure (compression) in the combustion chamber which is important &mdash; not the compression ratio. The compression ratio only governs the maximum compression that can be achieved).

Octane rating has no direct impact on the deflagration (burn) of the air/fuel mixture in the combustion chamber. Other properties of gasoline and engine design account for the manner at which deflagration takes place. In other words, the flame speed of a normally ignited mixture is not directly connected to octane rating. Deflagration is the type of combustion that constitutes the normal burn. Detonation is a different type of combustion and this is to be avoided in spark ignited gasoline engines. Octane rating is a measure of detonation resistance, not deflagration characteristics.

It might seem odd that fuels with higher octane ratings explode less easily and are therefore more powerful. One simple explanation for the effect is that various fuels can provide different heat (therefore energy) at different compression levels. As the compression level increases on many fuels so does the heat (energy) per unit of measure of fuel. Fuels burned in normal sea level pressure produce less energy than ones burned at the point of pre-ignition. The best energy pressure (compression ratio) for a fuel is at the point of where the engine "pings". Each fuel with its own resistance to pre-ignition requires its own ideal compression ratio. This is not always what emission levels require however. A motor must be constructed to work within a fuels compression ratio and emission levels.

Another simple explanation is that carbon-carbon bonds contain more energy than carbon-hydrogen bonds. Hence a fuel with a greater number of carbon bonds will carry more energy regardless of the octane rating. A premium motor fuel will often be formulated to have both higher octane as well as more energy. A counter example to this rule is that ethanol blend fuels have a higher octane rating, but carry a lower energy content by volume (per litre or per gallon). This is because ethanol is a partially oxidized hydrocarbon which can be seen by noting the presence of oxygen in the chemical formula: C2H5OH. Note the substitution of the OH hydroxyl group for a H hydrogen which transforms the gas ethane (C2H6) into ethanol. To a certain extent a fuel with a higher carbon ratio will be more dense than a fuel with a lower carbon ratio. Thus it is possible to formulate high octane fuels that carry less energy per liter than lower octane fuels. This is certainly true of ethanol blend fuels (gasohol), however fuels with no ethanol and indeed no oxygen are also possible.

Alcohol fuels such as methanol and ethanol, are partially oxidized fuels and need to be run at much richer mixtures than gasoline. As a consequence, the total volume of fuel burned per cycle counterbalances the lower energy per unit volume, and the net energy released per cycle is higher. If gasoline is run at its preferred maximum power air/fuel mixture of 12.5:1, it will release approximately 20 MJ (about 19,000 BTU) of energy, where ethanol run at its preferred maximum power mixture of 6.5:1 will liberate approximately 25.7 MJ (24,400 BTU), and methanol at a 4.5:1 AFR liberates about 29.1 MJ (27,650 BTU). To account for these differences, a measure called the fuel's specific energy is sometimes used. It is defined as the energy released per air/fuel ratio.

Using a fuel with a higher octane lets an engine run at a higher compression ratio without having problems with knock. Actual compression in the combustion chamber is determined by the compression ratio as well as the amount of air restriction in the intake manifold (manifold vacuum) as well as the barometric pressure, which is a function of elevation and weather conditions.

Compression is directly related to power (see engine tuning), so engines that require higher octane usually deliver more power. Engine power is a function of the fuel as well as the engine design and is related to octane ratings of the fuel. Power is limited by the maximum amount of fuel-air mixture that can be forced into the combustion chamber. At partial load, 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 what is available. It is only when the throttle is opened fully and the manifold pressure increases to atmospheric (or higher in the case of supercharged or turbocharged engines) that the full octane requirement is achieved.

Many high-performance engines are designed to operate with a high maximum compression and thus need a high quality (high energy) fuel usually associated with high octane numbers and thus demand high-octane premium gasoline. Ethanol with an octane of 116 could be a high performance fuel if engines were designed with a 14 to 1 compression ratio, possibly improving the mileage to compete with gasoline. The Offenhauser engine had a 15 to 1 ratio and burned methanol. The power output of an engine depends on the energy content of its fuel, and this bears no simple relationship to the octane rating. A common understanding that may apply in only limited circumstances amongst petrol consumers is that adding a higher octane fuel to a vehicle's engine will increase its performance and/or lessen its fuel consumption; this may be false under most conditions &mdash; while engines perform best when using fuel with the octane rating for which they were designed and any increase in performance by using a fuel with a different octane rating is minimal or even imaginary, unless there are carbon hotspots, fuel injector clogging or other conditions that may cause a lean situation that can cause knocking that are more common in high mileage vehicles, which would cause modern cars to retard timing thus leading to a loss of both responsiveness and fuel economy. This also does not apply to turbocharged vehicles, which may be allowed to run greater advance in certain circumstances due to external temperatures.

Using high octane fuel for an engine makes a difference when the engine is producing its maximum power or when under a high load such as climbing a large hill or carrying excessive weight. This will occur when the intake manifold has no air restriction and is running at minimum vacuum. Depending on the engine design, this particular circumstance can be anywhere along the RPM range, but is usually easy to pinpoint if you can examine a printout of the power output (torque values) of an engine. On a typical high-revving motorcycle engine, for example, the maximum power occurs at a point where the movements of the intake and exhaust valves are timed in such a way to maximize the compression loading of the cylinder; although the piston is already rising at the time the intake valve closes, the forward speed of the charge coming into the cylinder is high enough to continue to load the air-fuel mixture in.

When this occurs, if a fuel with below recommended octane is used, the engine will knock. Modern engines have anti-knock provisions built into the control systems and this is usually achieved by dynamically de-tuning the engine while under load by increasing the fuel-air mixture and retarding the spark. Here is a link to a white paper that gives an example:. In this example, the engine maximum power is reduced by about 4% with a fuel switch from 93 to 91 octane (11 hp, from 291 to 280 hp). If the engine is being run below maximum load, the difference in octane will have even less effect. The example cited does not indicate at what elevation the test is being conducted or what the barometric pressure is. For each 1000 feet of altitude the atmospheric pressure will drop by a little less than 11 kPa/km (1 inHg). An engine that might require 93 octane at sea level may perform at maximum on a fuel rated at 91 octane if the elevation is over, say, 1000 feet. See also the APC article.

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 gas is different.

In the Rocky Mountain (high altitude) states, 85 octane is the minimum octane and 91 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. In parts of the Midwest (primarily Minnesota, Illinois and Missouri) ethanol based E-85 fuel with 105 AKI is available.

California fuel stations will offer 87, 89, and 91 octane fuels, and at some stations, 100 or higher octane, sold as racing fuel. Until 2003 or 2004, 92 octane 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 being higher specification (being called Super Unleaded). In Germany, big suppliers like Shell or Aral offer 100 octane gasoline (Shell V-Power, Aral Ultimate) at almost every gas station. In Australia, "regular" unleaded fuel is RON 91, "premium" unleaded with RON 95 is widely available, and RON 98 fuel is also reasonably common. Shell sells RON 100 petrol from a small number of service stations, most of which are located in capital cities. In Malaysia, the "regular" unleaded fuel is RON92 while "premium" fuel is rated at RON97. 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 AKI) is the minimum available and the standard.

It should be noted that 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 the oil company 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 RON 102. Although BP Ultimate Unleaded (with an octane rating of RON 97) 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.