Science & Technology in Action

1st Edition

The Science of Petrol


The fractional distillation of crude oil to produce petrol and other fuels is described in this lesson. The concept of the Octane Number is defined. The effects of incomplete combustion and the function of additives to improve performance are also covered.
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Lesson excerpt

What is petrol?
Petrol is one of the most common fuels in the world, used to fuel automobiles and other mechanical engines. In this lesson we will look at how petrol is produced and additives are used to make it more efficient and environmentally friendly. 

Petrol is extracted by the fractional distillation of crude oil(petroleum). Fractional distillation is a process used to separate a mixture of different liquids, based on their different boiling points. The different components are called fractions. Crude oil is heated in a furnace and passed through a fractioning column. The first gases to be released are small hydrocarbons. These have the lowest boiling points and would therefore evaporate first. The next fraction to come off is petrol. Petrol is a mixture of about one hundred different hydrocarbons. Diesel is the fifth fraction, released after naphtha and kerosene. Diesel is harder to vaporise than petrol. This is why the two are not interchangeable in car engines.

What is an octane number?
In a petrol engine, combustion is triggered by a spark. However, under certain temperature and pressure conditions the fuel can ignite spontaneously. When this happens there is a very rapid reaction, of some or all of the fuel-air mixture in an engine. The flame speed is many times greater than that which follows normal spark ignition. This is called knocking, auto ignition (or pinking). The noise associated with it is called knock or tinkling. The octane number of a fuel is a measure of its ability to resist knocking. A fuel with a high octane number has a greater resistance to spontaneous combustion. Obviously then, the lower the octane number the higher the chance of auto ignition. The octane number is also known as the anti anti knock index (AKI). 

How is octane number calculated?
2,2,4 trimethylpentane (iso-octane) is given the highest value of 100 while straight chain heptane is given the lowest value of 0. Good petrol has an octane number of 97 or more. Petrol with an octane number of 98 has the same anti-knocking characteristics as a mixture of 98% iso-octane and 2% heptane.

True or False?

  1. Diesel is one of the most common fuels in the world true
  2. Petrol is formed by evaporation of crude oil false
  3. In fractional distillation the first gases to be released are small hydrocarbons true
  4. Petrol and diesel are interchangeable false
  5. Isomerisation increases the octane number of a fuel true
  6. Dehydrocyclisation decreases the octane number of a fuel true
  7. Methane is an example of an oxygenate false
  8. Waste gases from fuel include carbon monoxide and nitrogen oxides true
  9. Catalytic converters increase the amount of pollutants released from car engines false
  10. Alkanes have greater octane numbers than aromatic compounds false
  11. Isomerisation is the breaking of long chain hydrocarbons false