Knocking

A petrol/diseal engine basically works on the combustion of oil in the presence of

Air which produces enough thrust by spark and burst to push a piston

Forcibly.

The piston in turn rotates the gears of engine in circular manner as shown.

In petrol engine

The saturated long chains of hydrocarbons (petrol oil) is too good for combustion/burning.

It disturbs the harmony in which the piston moves up and down.

This in turn produces crackling noise (just like rattling of hammer).

This is called knocking of a petrol engine.

In diesel engine 

In a similar but exactly opposite way knocking is observed in Diesel engine.

Diesel contains compounds which are highly branched or aromatic which delays the combustion of the oil in the engine.

This delay disturbs the harmony in which the piston moves up and down and causes knocking of diesel engine.

Disadvantages or drawbacks of knocking

*  Causes harm to engine.

*  Wastage of fuel as well energy produced by it.

*  Harms piston and cylinder.

*  Increases maintenance cost.

Measurement of knocking

In petrol engine

The knocking in petrol engine is measured by Octane number.

Octane number is the percentage of iso-octane present in mixture of

iso-octane and n-heptane.

iso-octane is a compound which has very good, calm and controlled combustion properties.

n-heptane detonates and burns readily before it is expected to burn.

This causes knocking of petrol engine.

Thus, for Reference and convention

Octane number of iso-octane is taken as 100

Octane number of n-heptane is taken as 0

In diesel engine

Cetane number is used to measure knocking in diesel engine.

Cetane number is percentage of n-hexadecane present in mixture of n-hexadecane

and methylnapthalene.

In order to appreciate the diesal engine one needs to know how it works.

Working of diesel engine

A diesel engine demands more arrangements as compared to its peer petrol engine.

1. Firstly, air is pumped and compressed in the engine to attain 500C-700C temperatures.

2. Then oil is injected inside.

3. Atomization of oil begins at these elevated temperatures and carbon of diesel combines with oxygen of air

    to form CO2 (other gases are also produced because of impurities).

4. The steps 2 and 3 have to be as fast as possible (1/500 th of a second).

The presence of aromatic and branches hydrocarbons produces delayed combustion

And ignition lag causing Knocking of engine.

So, for reference and convection

n-hexadecane ignites readily

                Thus, it has Cetane number 100

whrereas methylnapthalene delays combustion

                Thus, it has Cetane number 0