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What is ‘octane’ in fuel and how does it make your vehicle preform better or worse?

It’s important to recognise the difference between the chemicals called “octane” and what the “octane rating” of a fuel is.
Octanes are chemicals which contain carbon and hydrogen in the ratio 8 carbon to 18 hydrogen. These are widely found in refined petroleum products. There are many different octanes, with slightly different properties. When it comes to determining “octane rating” of a fuel is, a specific octane called “2,4,4 trimethylpentane” is used.

The octane rating of a fuel is a measure of how easily a fuel ignites in a gasoline engine. In a gasoline engine, you need the fuel to be difficult to ignite. When an gasoline engine works, air and gasoline are sucked in, then the mixture is compressed, after it is fully compressed it is ignited, and the heat and pressure from the burn makes the engine go.
Because the process of compressing the air/fuel mixture creates a lot of heat, it is important that gasoline does not ignite during this process. The mixture must only ignite when a very hot electrical spark is generated – at the same time, when the spark goes, the fuel must ignite gently, and not just explode – because the force of the explosion can damage the engine.

Octane rating measures how resistant the fuel is to pre-ignition (self-igniting during compression) and detonation (exploding when hit with a spark). An “octane rating” of 100, means that the fuel has the same resistance to ignition as pure “2,2,4 trimethylpentane”. An “octane rating” of 0 means that the fuel has the same properties as pure “n-heptane”. An “octane rating” of 90, means that the fuel burns similarly to a 90:10 mixture of 2,2,4 TMP and n-heptane. It is possible to have ratings higher than 100, for fuels that are even more difficult to ignite than the reference “octane”.

There are lots of additives which were used to improve “octane rating” – most called “organometallic” compounds. The most notorious was tetraethyl lead (usually just called lead) which is a very strong fuel ignition stabiliser. This was banned because of toxicity. Many oil companies replaced it with a different organometallic called methylcyclopendienyl manganese tricabonyl (MMT) – but this is now mostly banned, because it damages catalytic converters. These days high octane fuel is made by adding lots of “aromatic” hydrocarbons produced from advanced refining methods, and/or adding ethanol.
Gasoline engines are designed to take fuel of a specific octane rating. The amount and speed with which an engine compresses a fuel-air mixture is fixed in the design, and nothing can change that. This means that if you have an engine which uses very strong compression (e.g. a highly tuned sports car engine), and put low octane fuel into it, the fuel may “detonate” during compression – causing the engine to “knock” or “ping/pink”. The knocking noise is the sound of the fuel exploding – as the engine is not designed to take that pressure, it will suffer severe damage.

There is also the issue of detonation when the spark hits. If the spark comes soon after compression, the fuel is very hot and more likely to detonate – but igniting early gives the best burn time, and therefore best engine power and best fuel consumption. Igniting late allows the compressed fuel to cool down, but wastes burn time and produces less power. Older cars would be manually tuned to a specific “ignition timing” by a garage – so you could tune a car to work better on higher octane rating fuel by adjusting the timing.
Modern cars use electronic ignition and timing control. The car has a microphone bolted to the engine which listens for the sound of the burn. The engine computer analyses that signal and uses it to adjust the ignition timing. If it hears detonation starting to occur, then it delays the timing – if everything seems to be going OK, then it’ll try advancing the timing. The computer tends to be programmed with limits based upon typical fuel ratings in the country the car was sold in. So, if you put 110 octane race fuel into a corolla, the computer won’t super tune the ignition timing, it’ll move it up to where the manufacturer expects premium fuel to be, but go no further.

If you put low octane fuel in, the computer will try to delay the ignition timing – but that can only do so much – and if the problem is that the fuel is igniting during compression, there is nothing the computer can do, except put on the check engine light and hope that you stop before you blow the engine.


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