In an SI engine, the combustion:

Concept:

Combustion: It is defined as the rapid chemical reaction of hydrogen and carbon in fuel with oxygen in the air. When the fuel and oxygen reacts, it releases energy in the form of heat.

Following conditions are necessary for combustion to take place:

• A combustible mixture
• Some means to initiates combustion
• Stabilization and propagation of flame in the combustion chamber

In SI engines, a carburettor generally supplies a combustible mixture and the electric spark from a spark plug initiates the combustion.

• The homogeneous mixture of vaporized fuel and air is compressed and ignited by the single intense, high temperature spark which initiates the flame kernel.
• Flame kernel generated by spark grows, and a turbulent flame propagates throughout the mixture until it reaches the combustion chamber walls.
• The flame speed is very low in non-turbulent (laminar) mixture. A turbulent motion of the mixture intensifies the processes of heat transfer and mixing of the burned and unburned portions in the flame front.