Combustion process in SI engines is divided mainly into three phases:

1. Ignition lag or delay period- This first phase is the duration between the occurrence of spark at spark plug and the deviation of combustion curve from motor curve (AB in figure). This period tends to be very nearly constant in time. Factors influencing this phase are:

   i. Fuel- Higher the self-ignition temperature of fuel, longer is the ignition lag.

   ii. Mixture ratio- Ignition lag increases as the relative fuel air ratio increases or decreases from 1.2

   iii. Initial temperature- Ignition lag increases rapidly with increase in temperature of the air-fuel mixture.

   iv. Flame temperature- As the temperature of the flame between the spark plug electrodes increases, the reaction time decreases and so the ignition lag decreases.

2. Rapid combustion phase- This second phase (BC) starts when combustion curve deviates from the motor curve and extends till the maximum pressure is reached. In this phase mean temperature of gases in the engine cylinder continues to increase beyond the maximum pressure point. The time required mainly depends on the intensity of turbulence or state of agitation of the air-fuel mixture. The duration of the phase is approximately constant in terms of the crank-angle movement.

3. Afterburning- This third and final phase (CD) occurs between points of maximum pressure and maximum temperature. After the flame front ahs reached the cylinder walls about 25% of the charge is still not completely burnt. But at this stage it is difficult for the remaining oxygen in the charge to react with the petrol vapours which slows down the rate of combustion. Simultaneously heat is liberated due to chemical interaction caused by reassociation of the combustion products throughout the expansion stroke.

However it has to be noted that no clear distinction can be made between these three phases as no abrupt change is noticed.