Octane no. & Cetane no, Knocking, Anti-knocking agent

Octane and cetane number are standard measure of performance of a fuel.

Octane number is related to petrol and cetane number gives quality of diesel.

Higher the octane number, better the quality of petrol, and hence the vehicle gives higher efficiency.

The octane no. of iso-octane is 100, while that of n heptante is zero(0). Similarly, higher cetane number implies good quality diesel.

In the octane number scale, high values represent fuels that resist spontaneous ignition and, therefore, have less tendency to knock in a gasoline engine. Octane number is actually a measure of a fuel’s autoignition (prematurely) delay. In a particular internal combustion engine, higher octane fuels will have larger ignition delay periods than lower octane fuels.

High cetane number fuels have low octane numbers, and vice versa. This is one reason why we can’t use petrol in diesel engine and vice versa.

In India Current BIS specification of Anti-Knocking Index of petrol is 84 (unleaded regular) and 88 (unleaded premium), And cetane index of diesel is 45-50..

In Cetane number scale, high value represents fuels(diesel) that ignites readily and therefore will have less chance of detonation in a diesel engine. Cetane number is actually a measure of a fuel’s ignition delay. In a particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower cetane fuels.Fuels with lower cetane number have longer ignition delays, providing more time for the fuel combustion process to be completed.

The cetane number is calculated by determining which mixture of cetane (hexadecane) and isocetane (2,2,4,4,6,8,8-heptamethylnonane) will result in the same ignition delay.

Hexadecane (also called cetane) ignites very easily under compression; for this reason, it is assigned a cetane number of 100, and serves as a reference for other fuel mixtures.It has one of the lowest Octane ratings, at <−30.


Knocking is the result of the rapid combustion of the end-gas ahead of the flame front which undergoes chemical reactions prior to normal combustion. This cause high frequency pressure oscillations inside the cylinder that produce the sharp metallic noise called knock.

Two theories have been postulated to explain the origin of knock :

1. The Auto ignition theory which states that when the air/fuel mixture in the end- gas region is compressed to sufficiently high pressure and temperature the fuel oxidation process – starting with the preflame chemistry and ending with rapid energy release – can occur spontaneously in parts or all of the end-gas region.

2.The Detonation theory which holds that,under knocking conditions, the advancing flame front accelerates to sonic velocities and consumes the end-gas at a rate much faster than would occur with normal flame speeds.

In CI Engine(diesel engine, correctly known as a compression-ignition):

  • There is both physical delay (time taken for atomisation, vaporization of fuel and intermixing of fuel and air) and chemical delay (i.e; time taken to complete chemical reaction). As these delay period increases more and more fuel is accumulated and as the pressure and temperature reaches the optimum value for combustion a sudden blast occurs which is not favorable (this is called ‘diesel knock’)

In SI Engine(A spark-ignition engine is an internal combustion engine, generally a petrol engine)

  • There is only chemical delay , if this delay is more then chances of knocking are less as flame front will reach end gases before chemical reaction happens.

An antiknock agent is a chemical that, when added to gasoline, raises the octane value of the gasoline which, in turn, raises the temperature and pressure at which gasoline will auto-ignite. It allows the gasoline/air mixture to wait until the spark plug ignites the fuel, reducing pre-detonation which can be harmful to the engine. Anti Knocking agents are the additives which improve the octane value of the fuel used in Internal combustion engines & prevents or reduces knocking of the engines. . These help the fuel to ignite at the correct time during the combustion stage there by increasing the efficiency of the engine.

Alcohol, Toluene, Tetraethyllead(TEL) are typical anti knocking agents.



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