Quality of gasoline

 

This is the notes of  Quality of gasoline.

Quality of gasoline: Octane number 

Gasoline (Petrol) is a fraction of petroleum which includes mainly a mixture of several alkanes having carbon number C7 -C11 and is used in petrol engines as fuel. 

In petrol engine, a mixture of petrol vapour an air is compressed into the cylinder. The fuel burns just before the completion of compression. 

If the quality of fuel is not good, combustion takes place during compression before the sparking plug ignites and piston is pushed violently producing a metallic noise which is called knocking of the engine. Knocking reduces the life span and efficiency of the engine.

The quality of fuel is measured on the basis of anti-knocking property and is expressed in terms of octane number.

Iso-octane has low knocking property (good fuel) so arbitrarily assigned octane number of 100. While, n-heptane has high knocking property (bad fuel) and arbitrarily assigned zero.

The octane number to any fuel is given by comparing its anti-knocking property with the mixture of iso-octane and n-heptane. 

If the anti-knocking property of the given fuel is identical with the antiknocking property of a mixture of 75% iso-octane and 25% nheptane, then its octane number is given as 75.

The greater the octane number, the better is the fuel. 

 Q(for you). A fuel is assigned octane number 80. What does this mean? 

Gasoline additives/anti-knocking agents 

The compounds which are added to gasoline to improve its octane number are called Gasoline additives/anti-knocking agents. 

• One way of increasing the octane number of the fuel is to increase the % of the branched chain alkanes or to add aromatic hydrocarbons. 
• Another method is to add small quantity (about 0.01%) of tetraethyl lead (TEL) in the gasoline. Gasoline containing TEL is called leaded gasoline or leaded petrol. TEL reduces the knocking property of un-branched alkanes and is known as gasoline additive. 

 TEL dissociates to give ethyl free radicals which convert straight chain hydrocarbons into branched chain hydrocarbons and improve octane number. 

   (C2H5 )4Pb → Pb + C2H5

Q(for you). What are gasoline additives ? Give example.

Hope this will help you a lot.

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