Octane Number and Octane Rating

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.

For all you burgeoning chemists, gasoline is basically octane and heptane, or hydrocarbon chains that are 8 (octane) or 7 (heptane) carbons long. Octane simply can be compressed well (i.e. without the exploding part, at least at the same levels of pressure) than heptane. An octane rating of 87 means it is 87% octane.

The octane rating is a measure of the auto ignition resistance of gasoline (petrol) and other fuels used in spark-ignition internal combustion engines. 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 knock resistance 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 auto ignition resistance as the described mixture.

A high tendency to auto ignite, 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 octane number. A diesel fuel with a high octane number has a high tendency to auto ignite, as is preferred.

Octane rating is also method for describing antiknock properties of gasoline. Knocking is a pinging sound produced by internal combustion engines when fuel ignites prematurely during the engine's compression cycle.

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 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.

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 liquefied 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.

Effect 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).

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 turbo charged engines) that the full octane requirement is achieved.

Now does higher octane fuel contains more energy? 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 (i.e. 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 denser 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 the 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 of energy, where ethanol run at its preferred max power mixture of 6.5:1 will liberate approximately 24,400 BTU, and Methanol at a 4.5:1 AFR liberates about 27,650 BTU.

To account for these differences, a measure called the fuels 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 energy numbers are as follows.

Fuel net Energy Units

Gasoline 2.92 MJ/kg

Ethanol 3.00 MJ/kg

Methanol 3.08 MJ/kg

Do you need higher Octane fuel?

The higher octane the more it can be compressed Also the larger the explosion. Combustion engines are simple; Fuel is injected through the fuel rail into the cylinders and mixed with oxygen. The More oxygen the larger the explosion, higher the octane rating, the higher the explosion. The higher the gasoline/air mixture is compressed the higher the displacement. The knocking sound in cars requiring premium gasoline is because for example; in a V4 one cylinder fires and displaces, this causes the other cylinder to close and compress the gasoline and oxygen before it combusts. If the ECU is trying to compress it too much it will explode in the chamber before it is completely compressed and it has to change velocity before its ready causing the chamber that was exploding to have to shut again before it is ready. That’s bad for your engine.

High-octane gas is especially important for some older vehicles and high-performance engines as well as many boats, snowmobiles, motorcycles and other small engines. Octane determines to what extent the fuel in the engine's cylinders can be compressed before it ignites. When fuel ignites from compression, rather than from a spark plug, it causes knocking. Over time, persistent knocking can burn holes in the tops of pistons and cause other damage to engines, Repairs can cost thousands of dollars. Higher octane premium is not a “myth” for these vehicles it is required unless you modify the motor.

Most new cars have anti-knock sensors that detect knocking and actually retard ignition timing to prevent it from re-occurring. As a result, knocking damage to newer vehicles is unlikely. However, problems still arise if lower octane is used in engines designed for high octane, said Dave Buchko, product communication manager for BMW, which manufactures all its engines for use with a minimum 91 octane rating.

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.

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-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 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.

Why is higher Octane fuel more expensive?

The reason why higher octane fuel is more expensive is because it’s harder to refine the gasoline so that it contains more octane. Now let’s add another wrinkle… the octane is actually an octane rating, or it behaves as gasoline with that percentage octane would behave but might not actually have that much octane in it. While that doesn’t really matter, it does mean that the gasoline you use could have a mix of other things in it (still real gas though) to give it properties of a higher octane without actually having more octane. Does it matter? Remember, higher octane fuel is expensive but sometimes it might not cost you extra due to the improvement in mileage.

To understand the impact of price vs. gas mileage, here is a good example:

MINI Cooper, a car manufactured by BMW, comes with the “Premium Gas Only” sticker Of course, the MINI will still run with lower octane fuel. In fact, it will run without knocking because it has an anti-knock sensor that will automatically adapt if lower octane fuel is used. MINI owners who have used or do use lower octane fuels have reported some or all of the following: lower fuel mileage (lower enough that the gas savings on lower grade isn’t worth it), slight to major loss in horsepower, increased emissions, and occasional stalls when starting from a stop.

We ran some numbers on the cost and mileage, On a 30mpg car the cost per mile was approximately 7-8 cents. General loss is mileage(From 93 to 91 octane gas) was reported to be around 5 mpg., So that extra 5 MPG is worth approximately 35-40 cents on a 30 mpg car and you will probably not going to see 35-40 cents price difference between 91 and 93 octane gas at the pump.