Article: Regular Vs. Premium

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The octane rating is the most important characteristic of gasoline (petrol) and other fuels used in spark-ignition internal combustion engines. It is a measure of how resistant the fuel is to premature detonation (knocking). It is measured relative to a mixture of 2,2,4-trimethylpentane (an isomer of octane) and n-heptane. So an 87-octane gasoline has the same knock resistance as a mixture of 87% isooctane and 13% n-heptane. A complementary standard for diesel fuel is the cetane number.

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), 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 "normal" gasoline in the US and Canada, would be 91 in Europe.

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 LPG typically have octane ratings of 110 or significantly higher. Typical "octane booster" additives include tetra-ethyl lead and toluene.

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 ratio will cause knocking.

It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful. Using a fuel with a higher octane lets an engine be run at a higher compression ratio without having problems with knock. Compression is directly related to power, so engines that require higher octane usually deliver more power. Some high-performance engines are designed to operate with a compression ratio associated with high octane numbers, and thus demand high-octane gasoline. It should be noted that the power output of an engine also depends on the energy content of its fuel, which bears no simple relationship to the octane rating. Some people believe that adding a higher octane fuel to their engine will increase its performance or lessen its fuel consumption; this is mostly false.engines perform best when using fuel with the octane rating they were designed for and any increase in performance is minimal.

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.

Some people believe that there is almost no differernce between regular and premium, so you can run regular fuel in any car. But you should check with your dealer first before running fuel that is not rated. There have been reports of people being denied warranty repair because the cars computer logged improper fuel usage. The higher octane gas mainly will eliminate pinging (knocking) but sometimes, if you run a lower octane than the car was rated for, the car's computer will try to compensate, and this will reduce HP.

ARTICLE DISCLAIMER: To learn more about this and related topics, please visit www.Wikipedia.com. This article was adapted from information in Wikipedia, with permission granted by the GNU license. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. If you publish this article or any work derived from it, then you must also display this GNU License information, and you must grant everyone else the right to republish and edit your work as well. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Octane Rating". Finally, there is no guarantee that the information in this article is accurate; for more information on disclaimers, please see Wikipedia General Disclaimer.

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