Octane rating
The octane rating is a measure of the autoignition resistance of gasoline (petrol) and other fuels used in spark-ignition internal combustion engines. It's a measure of anti-detonation of a gasoline or fuel.
Knock resistance
Engine knocking or 'pinking (pinging)' is compression detonation of fuel in the power stroke of the engine. Knocking occurs when the air-fuel mixture autoignites all at once (or sometimes perhaps when the flame front goes supersonic because of early ignition timing), before the flame front from spark plug ignition can reach it. The explosive reaction causes combustion to stop before the optimum timing, causing a decrease in performance. A fuel such as ethanol, with a high autoignition temperature that burns reasonably fast and thus does not need early ignition timing, will most often have high practical value knock resistance.[citation needed]
Definition of octane rating
Octane is measured relative to a mixture of isooctane (2,2,4-trimethylpentane, an isomer of octane) and n-heptane. An 87-octane gasoline, for example, has the same octane rating as a mixture of 87 vol-% isooctane and 13 vol-% n-heptane. This does not mean, however, that the gasoline actually should contain these chemicals 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 gasoline 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.
Measurement methods
The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel through a specific test engine with a variable compression ratio under controlled conditions, and comparing these results with those for mixtures of isooctane 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 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 91-95 (regular) in Europe.
The octane rating may also be a "trade name", with the actual figure being higher than the nominal rating.[citation needed]
It is possible for a fuel to have a RON greater than 100, because isooctane 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). Typical "octane booster" additives include tetra-ethyl lead 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.
Octane ratings above 100
By definition, the maximum octane number is 100, i.e. 100% of the solution. For substances with higher shock resistance, the octane performance (note: not octane number) is an extrapolation of the octane performance chart. Some sources mention adoption of the performance number (PN) scale in US in 1943. Anecdotal evidence suggests that before adoption of such widely accepted standard many producers assigned arbitrary numbers for everything above 100.
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 gives the road octane numbers as stated in [1]. See references for another source.
| 2-methylheptane | 23 |
| n-hexane | 25 |
| 2-methylhexane | 44 |
| 1-heptene | 60 |
| n-pentane | 62 |
| 1-pentene | 84 |
| n-butane | 91 |
| cyclohexane | 97 |
| benzene | 101 |
| toluene | 112 |
More data can be found here [2]
Note: The octane rating of cyclohexane significantly varies form source to source. See for example [3]
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 - not the compression ratio. The compression ratio only governs the maximum compression that can be achieved).
It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful. The misunderstanding is caused by confusing the ability of the fuel to resist compression detonation (pre-ignition = engine knock) as opposed to the ability of the fuel to burn (combustion). However, premium grades of petrol often contain more energy per litre due to the composition of the fuel as well as increased octane.[citation needed]
A simple explanation is the carbon bonds contain more energy than 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 on a volume basis (per liter or per gallon). The reason for this is that 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 radical for a H hydrogen which transforms the gas ethane (C2H6) (which is an alkane) into ethanol (which is an alcohol). Note that 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.
In the case of alcohol fuels, like Methanol and Ethanol, since they are partially oxidized fuels they need to be run at much richer mixtures than gasoline. As a consequence the total amount of fuel burned per cycle counter balances the lower energy per unit volume, and the net energy released per cycle is higher. If gasoline is run at its preferred max power air fuel mixture of 12.5:1, it will release approximately 19,000 BTU (about 20 MJ) of energy, where ethanol run at its preferred max power mixture of 6.5:1 will liberate approximately 24,400 BTU (25.7 MJ), and Methanol at a 4.5:1 AFR liberates about 27,650 BTU (29.1 MJ).
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. For the case of gasoline compared to the alcohol fuels the specific energys are as follows.
Using a fuel with a higher octane lets an engine run at a higher compression 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.
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 myth 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 is false—engines perform best when using fuel with the octane rating they were designed for and any increase in performance by using a fuel with a different octane rating is minimal or even imaginary.
Using high octane fuel for an engine makes a difference when the engine is producing its maximum power. 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 pin-point if you can examine a print-out of the power-output (torque values) of an engine. On a typical high-rev'ving 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 cylinder 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, then 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 white paper that gives an example: [4] . 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 then 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 1 inHg (11 kPa/km). 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 the chemist Russell Marker. 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.
References
Octane ratings of some hydrocarbons
Information in general
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Khoo, Kenny K. Understanding Octane and its Related Components.Yellowknife: Smithsonian Press, 2006.