Octane Number or Octane Rating – What Is It, and Why You Should Bother About It

The word octane is often associated with the performance offered by gasoline fuel. It’s generally assumed that a higher octane rating means more performance, and enthusiasts often go for fuel with a higher octane rating for their machines. But what exactly does this much talked about term mean? Is a higher octane number always a good thing? Should you always be using fuel with a higher octane rating? Does a higher octane number have any downsides? Why does octane rating even matter? In this article, we’ll try and answer all your questions in as simple a manner as possible.

To understand the importance and use of octane rating, we first have to understand the qualities that a fuel must possess to be suitable for gasoline-powered internal combustion engines. In case of all four-stroke engines, the air-fuel mixture gets compressed, and is then ignited by a spark to create a controlled explosion, which in turn moves the piston. Now, if the fuel ignites prematurely owing to the heat inside the cylinder or the high compression before the spark kicks in, it would completely disturb the desired functioning of the engine. Such undesired, premature combustion instances are termed as knocking. Since they’re uncontrolled, and the engine parts aren’t designed for such uninhibited explosions, these may cause undesirable noise, bothersome vibrations, and may even damage engine components.

Now, such instances of uncontrolled, undesirable combustion of fuel are relatively less in low-compression engines. Since the temperature and pressure that the air-fuel mixture is subjected to in such engines is relatively less, the fuel generally combusts in a predictable fashion. However, high performance engines run a much higher compression, and instances of fuel combusting itself prematurely disturb the workings of such engines in a very undesirable manner. The engine, in such instances, will make a lot of noise, vibrate and may even sustain permanent damage.

And this is where octane rating of a fuel comes into the picture. In simple terms, octane rating is the resistance of the fuel to spontaneously combust even at relatively higher temperature and compression. The higher the octane-rating, the more the fuel will ‘behave’, and combust predictably.

The word octane is derived from iso-octane, which shows excellent combustion properties and is very resistance to spontaneous combustion. The octane rating of a fuel, thus, is a reference used after testing a fuel against the ideal high-quality fuel which is extremely resistant to undesired combustion.

When we look at the octane number scale, pure iso-octane isomer scores the standard 100 points. On the other hand, another compound, called the n-heptane scores a zero. In 1926, Graham Edgar first discovered that adding iso-octane (technically known as 2, 2, 4 – trimethylpentane) stopped, or greatly reduced knocking in internal combustion engines. He created test cases, where using iso-octane delivered a certain performance, which was standardized as ‘100 octane’. Similarly, the same test engines were run using n-heptane and the test results were standardized as ‘zero octane’. The results of these tests were documented for reference. All the blends of various compounds are graded on a scale of 0 to 100 against these two standards and are then assigned the respective octane number.

An important thing to note here is that the octane rating isn’t necessarily the percentage of iso-octane in the fuel blend. It is a rating of the fuel blend for its resistance to self-combust, or ‘knock’, as against the standard benchmark created using iso-octane. So a fuel blend may not have a high percentage of iso-octane. However, it’s resistance to combustion under compression is what would ultimately give it its octane rating.

A fuel with high octane rating, thus, allows an engine to run very high compression, and burns only when it’s supposed to. This delivers desired performance, and results in smooth, knock-free power delivery and high fuel efficiency as well.

Now, while a lower than recommended octane rating for a fuel may be detrimental to engine performance, smoothness and fuel efficiency; modern engines are designed to mitigate this to an extent. New age engines come equipped with advanced fuel injection systems, and sensors within the combustion chamber. These sensors can easily pick aberrations from the desired conditions in terms of temperature or pressure, and easily sense knock. As the sensors detect a knock, the fuel injection system alters various parameters such as ignition timing to mitigate the negative effect and reduce knocking. Having said that, the altered settings would still have some impact on the engine’s performance and fuel efficiency, and thus, it’s recommendable to always use fuel with manufacturer recommended octane rating. Carburetted engines which have gravity-fed fuel pumps, thus, are less efficient in dealing with low octane fuel than their fuel injected counterparts.

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Another important thing to note in terms of octane rating is that different octane rating parameters are used for various uses. For example, the octane rating used for aviation fuel is different from the one used for vehicles plying on the road. The octane rating used for road vehicles is known as Research Octane Number, or RON. The fuels available at various fuel stations, thus, carry RON ratings. There are a few other ratings known as Motor Octane Number (MON), Anti-Knock Index (AKI) and an average of RON and MON (R+M/2). RON, however, is the most commonly used rating worldwide, and for all practical purposes, the only one that you should know about.

So is it always beneficial to go for fuel with a higher octane rating? Well, every engine is designed to run on a certain octane rating, and complying with that fuel rating is important. India being a price sensitive market, most automobiles offer sedate to moderate performance and can run on the regular RON 87 fuel which is available as the standard fuel everywhere. Filling up higher octane rated fuel will cost you more money, and will not necessarily offer you any advantage over the standard fuel.

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If, however, you are running a high performance machine, and the maker has recommended a minimum RON rating, then by all means you should comply with the recommendation. Going below the recommended Octane rating will put your bike’s engine under stress and may even cause damage in the long term.

So as a general rule, comply with the octane number recommended by your two-wheeler maker, and never go below the recommended rating. If you have the means to afford it, you can opt for a higher (RON 91 or 97) rated fuel, though it may not bring any advantages in terms of performance or efficiency.

We hope this article was useful in helping you understand the importance and use of octane rating better. If you found this article useful, do share it with your octane addict friends, who would get to understand their healthy addiction even better after reading it. You can also ask us questions, or share your thoughts on this topic using the comments section below, or by giving us a buzz on any of the social networks.

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