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Why are Octane Ratings important?

Have you ever wondered about how petrol actually powers your car's engine? Or how safe it is? Here is a closer look at the chemistry behind octane and how its rating works.

How car engines work

The engine used in your car is called an 'internal combustion engine'. That is because it burns fuel inside the engine (a steam engine burns fuel outside to heat the steam which drives the engine).

A few drops of fuel are mixed with air and injected from the tank into the 'combustion chamber' of your car's engine. The car battery then provides an electric spark. The spark causes the fuel to catch fire, making the chamber hot. The heat makes the air expand, and that in turn drives the piston which turns the wheels. A little complex isn't it?

It's important that the fuel catch fire safely, and not explode. If the fuel drops exploded when catching fire, that would cause 'knocking' in the engine. This is dangerous, for it can damage the piston and cause leakage of hot fuel into other parts of the engine. That's why the octane rating (also called octane number) is important.

What petrol is made of

Before we understand octane number, let's see what petrol is made of. It is a mixture of a number of organic chemicals called hydrocarbons. A hydrocarbon is a chemical compound made only of carbon and hydrogen atoms.

Iso-octane is a hydrocarbon made of eight carbon atoms and eighteen hydrogen atoms. It burns quite safely. Heptane is a hydrocarbon of seven carbon atoms, and it burns very explosively. Scientists have created a scale that measures how safely fuel burns, by comparing them with these hydrocarbons. Iso-octane is the safest (100 points) while heptane is the most dangerous (0 points).

Octane rating

Experts have made different mixtures of isooctane and heptane and burned them to create the octane scale. For example, 9 parts of iso-octane and 1 part heptane burns with a rating of 90. If a sample of petrol burns just like this mix, it will get a rating of 90. The closer it is to 100, the safer it is.

However, it is difficult and expensive to create petrol that has only iso-octane in it, because heptane is quite hard to remove. Instead, fuel experts sometimes add other chemicals that prevent the fuel mixture from exploding. These are called 'anti-knocking agents.' For a long time, tetra-ethyl lead was used as an additive, but it is banned now because it causes lead poisoning. Nowadays, safer anti-knock agents are used.

So next time you go to a petrol pump, do remember to ask for the octane rating of the fuel, and buy only the fuel that has a high rating.

Tags :     Everyday Chemistry     Octane     Petrol    


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