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Octane Number : Responsible for knocking?

Octane Number : Responsible For Knocking?

Whenever you go to trip or your office, you frequently notice the words like speed, power, octane, Xtra Premium, special etc. at the fuel stations. Do you ever think what this is? At fuel station you were asked to normal or special. How it may affect your engine performance? Did you think what is special behind these “special”? If yes then well or if not then go through the article. Let’s find out.



Diesel Engine

Today in automobile world company are using four stroke engines, whether it is car, bike, truck, bus etc. it doesn’t matter. If you are from technical background then it is easy for you to understand and if you are from automobile, mechanical or aeronautical background then I am sure you know this. In order to get power to wheel, it is required that power should be produced. How it is made in engine is a very big topic and we will not discuss it here. Let take a brief look.

Four - Stroke Engine 


Normal Combustion vs. Premature Combustion


In first stroke (suction stroke) fuel is injected to the combustion chamber of the engine and in this process intake valves remains in open condition and exhaust valves in closed condition. After the suction stroke the fuel is compressed to very small volume which approaches clearance volume (volume between cylinder head and top dead center), this compression results in high pressure and temperature. This second stroke is called compression stroke. Do you hear about compression ratio? It is defined for this compression stroke. After the second stroke third stroke starts. In this stroke compressed fuel is ignited. Ignition took place due to spark in case of spark ignition engine and due to hot compressed air in case of compression ignition engine. This burning of fuel results in expansion of burnt gases inside the combustion chamber. This burnt gas pushes the engine piston hardly which transmits the power to wheel via connecting rod, camshaft, crankshaft, flywheel, clutch gearbox, clutch, etc. (This is a complex mechanism that how power is transmitted to wheel but for simplicity intro is provided). This third stroke is called power stroke or combustion stroke. After this stroke burnt gases are thrown out via exhaust valves.

In my opinion above explanation is sufficient for a non-techie. 


What is Knocking?

So in actual practice it is always desirable to burn fuel uniformly but in some high performance engines operating conditions are different, this engines deals with high pressure, high temperature and high rpm than a normal engine. And in these conditions it frequently happen that fuel ignites prematurely i.e. fuel ignites before standard set points and this prematurely burning of fuel results in violent reactions that causes series of mini explosion inside the combustion chamber and this can be felt and heard as knock. Hence this is termed as knocking. This knocking results in damage of engine parts/combustion chamber. If this occurs frequently then it should be figured out.

We saw knocking is result of premature combustion, so it depends on the “quality of fuel” up to a major extent. Quality of fuel depends on various parameters and one parameter is “tendency to burn”. This tendency to burn for a fuel is defined by octane number.

As the Octane Number increases, the detonating tendency of the fuel decreases. The Octane Number affects the activation energy of the fuel. In simple terms, an increase in the octane number will make the fuel require a higher amount of energy before it starts to combust. So even if the side conditions like pressure, temperature are higher, the increased activation energy will make sure that the fuel will only catch fire because of the spark plug and not because of the surrounding conditions.


Sometime you frequently encountered the word – detonation or a question, what is difference between knocking and detonation. If read the article completely then I think you know the answer.

The premature combustion of fuel due to surrounding conditions is called detonation and the result which includes mini explosions is called knocking.

More precisely detonation is cause and knocking is result.


Hope you like the article.


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