To form combustible mixture are using fuel and air. These components are part compound of combustible mixture so they must to be cleaning from other mechanical and other admixture.
Combustible mixture – is forming mixture in carburetor include the fuel vapours and an air. The combustible mixture enters to the cylinder of engine and mix with burnt gases, as a result the combustible mixture is ready. Compound of combustible mixture are characterized by certain ratio fuel mass and air.
What is the mass of air required to burn 1kg of petrol? There is 14.9 kg mass of air required to burn 1 kg of fuel. However, the amount of air in combustible mixture compound, can be different from the theoretical. So the compound of combustible mixture usually characterizes coefficient of air excess. So, the compound of combustible mixture usually characterizes coefficient of air excess referred to as α.
The coefficient α is an attitude of a really amount of air needs for burn petrol to theoretically necessary.
If the burn of 1 kg of petrol need 15 kg of air, so α=15/15=1 (this combustible mixture is called normal).
For example, if the coefficient α<1 this is fuel-rich mixture, because it has less amount of air then theoretically need. If the coefficient α>1 this combustible mixture is called lean mixture, because it has more amount of air then theoretically need.
To accurately determine the degree of fuel rich or lean of the combustible mixture, there are following mixtures: fuel-rich mixture (α=0,70-0,85); enriched mixture (α=0,85-0,95); diluted mixture (α=1,05-1,15); lean mixture (α=1,15-1,20).
To be sure, the fully rich or fully lean mixture loses the ability to ignite. At first case, it happens because lack of air, and the second shows by excess of air.