17 April 2017

Air fuel mixture

An engine is generally operated at different loads and different speeds. For this, the proper air-fuel mixture should be supplied to the engine is necessary. Fuel and air are mixed to form three different types of mixtures. The three types of mixtures are following below :
  1. Chemically correct mixture 
  2. Rich mixture
  3. Lean mixture
Chemically correct mixture :

It is one in which there is just enough air for complete combustion of the fuel. Now one question arise abruptly in your mind that what is complete combustion?
Complete combustion is all carbon in the fuel is converted to CO2 and all hydrogen is converted to H2O.
It is also called a stoichiometric mixture.

Now we can understand that via one simple example so you can get more idea about what is a stoichiometric mixture or stoichiometric air-fuel ratio.

To burn 1 kg of octane ( C8H18 ) completely 15.12 kg of air required. 
Hence, chemically correct A/F ratio for that is 15.12:1 or we can say that 15:1.

Rich mixture :

A mixture which contains less air than the stoichiometric requirement is called rich mixture.
For that approx A/F ratio is about 12:1 or 10:1.

Lean mixture :

A mixture which contains more air than the stoichiometric requirements is called lean mixture.
For that approx A/F ratio is about 17:1 or 20:1. 

The carburettor should provide an A/F ratio in accordance with the engine operating requirements. This will be under the combustible range.

The carburettor can be done this process of mixing and make a correct mixture for different engine speeds and loads and this process is called carburetion.