There are four stages of combustion in CI engine as follows:

1. Ignition delay- During this stage there is a physical delay period which is the time from beginning of injection to the attainment of chemical reaction conditions. The fuel is atomized and mixed with air and its temperature is raised. This period is followed by a chemical delay period in which preflame reactions start and accelerate until local ignition takes place.
2. Rapid or uncontrolled combustion- This is second stage in which pressure rise is rapid since during delay period the fuel droplets have had time to spread themselves over a wide area and have fresh air around them. This phase extends from end of delay period to point of maximum pressure.
3. Controlled combustion- The very high temperature and pressure at end of second stage cause the fuel droplets injected during last stage to burn instantly and any further pressure rise can be controlled by purely mechanical means that is injection rate. This period ends at maximum cycle temperature. The heat evolved by end of this phase is 70 to 80 percent of total heat of fuel supplied.
4. After burning- This fourth stage may not be present in all cases but due to poor distribution of fuel particles combustion may continue in the expansion stroke. Its duration may be 70 to 80 degrees of crank travel from TDC.