Explanation About Air and Fuel Mixture In Carburetor .....

The ratio by weight of air and fuel in the mixture being sent into the cylinders of a reciprocating engine.
Theoretically correct mixture of air and petrol is 15:1 (approximately).Thus, the uniform supply of such mixture would be result in burning without leaving excess of air of fuel.But it is difficult to get such a mixture in actual practice. Besides the ideal (stoichiometric) ratio, the air-fuel mixture may be rich or lean depending on the quantity of fuel in the mixture.When there is too little air, some of the fuels goes un brunt or simply changed to carbon.When there is too much air in mixture, it burns slowly and erratically and there is a loss of power.

There is however a range of mixture between which combustion will take place.
*The 'Lower Limit' is approximately 7:1 to 10:1. This mixture is barely explosive.
*The 'Upper Limit' is approximately 20:1 to. This mixture burns irregularly.

The above limits will also vary with the characteristics of the fuel , the shape of the combustion space and the temperature and pressure in the combustion space.

Mixture Requirement Of Automotive Engines :-

*For "average cruising speeds" the air-fuel ratio is approximately from 15:1 to 7:1.
*In order to obtain "maximum power" to be able to accelerate the engine quickly, a richer mixture of the ratio of about 12:1 is desirable.This is also called maximum power ratio.To start the engine from cold even a mixture richer than this but above the lower limit is obtained.
*For "maximum economy", i.e,, less fuel consumption for unit power, the mixture ratio may be approximately 16:1 to 17:1.The ratio, however , entails loss of power.


Hence, it is essential that the carburetor should be so designed that proper proportions of air and fuel are obtained to meet these varying operating conditions.