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EME - 505: Internal Combustion Engines

In this article, we shall discuss about burning of fuel inside a SI or spark ignition type internal combustion engine. From our common experience, we can say inside an IC engine fuel burns in presence of air and liberates large amount of heat or thermal energy.

Burning of fuel air mixture inside the engine is a very complex process. The process of burning and releasing large amount of energy is called "Combustion Process."

Therefore, we can say combustion is a burning process where several oxidation processes take place and being exothermic reactions by nature, they release large amount of thermal energy as the heat of combustion, which may be glowing or produce a hot flame.

Like, in hydrocarbon fuels, combustion is basically the exothermic oxidation processes of Carbon and Hydrogen forming Co2 and H20 together with little amount of Sulfur and Nitrogen forming So2 and NOx.

Combustion is very complex by nature and a lot of reactions and intermediate products take part in the process of combustion and making it one of the most difficult mechanisms which we could not yet understand fully. The important roles played by the intermediate products make it such difficult that it still remains one of the least understood natural processes. Friction remains the other common process that is still unexplored about the exact mechanism of the phenomenon.

The Requirement Conditions of a Combustion Process:

(i) A combustible mixture of fuel and air within the cylinder or combustion chamber supplied by carburettor or other means.

(ii) Some means of initiating the combustion either by a high temperature high voltage spark or auto ignition at the ignition temperature.

(iii) Stabilization and Propagation of the flame inside the combustion chamber.


(i) The fuel-air mixture is prepared and supplied into the combustion chamber by carburettor in SI Engines.

(ii) Initiation of combustion is done by an electric spark of very high voltage produced by spark plug. This is also known as Ignition.

(iii) Stabilization and propagation of flame is rapid in SI Engines.

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