Combustion in spark ignition engine

combustion is a rapid chemical combination of hydrogen and carbon in the fuel with the oxygen in the air resulting energy in the form of heat.
Normal combustion: towards the end of the compression stroke, a spark is fired, and as it passes from one electrode to other it leaves a thin thread of flame. Combustion spread to the envelope of the mixture containing the threads at a rate depending primarily on the temperature of flame front and secondarily on the temperature and density of the surrounding envelope. As the flame front expands it travels across the chamber until finally the whole of the mixture is engulfed. Depending on the degree of the turbulence in the cylinder, the flame front wrinkles and perhaps breaks into eddies which speed up the process.
In typical indicator diagram taken in the cylinder, there are to stages in the combustion process. The first stage (AB) corresponds to the time for the formation of the self-propagation nucleus of the flame, and the second stage (BC) to the propagation of flame through the combustion chamber.
The