Chemically correct air-fuel ratio by mass for combustion of petrol is approximately

Concept:

Stoichiometric ratio:

• An ideal fuel/air mixture in which both the fuel and the oxygen in the air are completely consumed is called the stoichiometric mixture. Such a mixture doesn’t cause knocking.
• The stoichiometric ratio is directly dependent on the number of hydrogen atoms per carbon molecule in a fuel.
• The exact chemical composition of petrol varies depending on its grade or octane rating, but generally speaking, it is a mixture of combustible hydrocarbons.
• This octane rating describes the quality of the fuel, so the octane itself is the highest quality of petrol which is C8H18.
• The stoichiometric ratio of the air-fuel ratio by mass for petrol (or gasoline) is 14.7: 1.
• The air-fuel ratio for CI engines varies from 18 : 1 to 80 : 1 from full load to no load.