stages of combustion in a CI engine

1. First stage : Ignition Delay Period

During the first stage some fuel has been admitted but has not yet been ignited. The ignition delay is counted from the start of injection to the

point where the pressure-time (crankshaft rotation) curve separates from the pure air compression curve. The delay period is a sort of preparatory phase.

2. Second stage: Rapid or uncontrolled combustion (premixed flame)

In the second stage, the pressure rise is rapid because during the delay period the fuel droplets have had time to speed themselves over a wide area and they have fresh air all around them. This period is counted from the end of delay period to the point of maximum pressure on the indicator diagram: About one-third of the heat is evolved during this period

3. Third stage: Controlled combustion (diffusion flame)

At the end of second stage the temperature and pressure are so high that the fuel droplets injected during the last stage burn almost as they enter and any further pressure rise can he controlled by the injection rate The period of controlled combustion is assumed to end at maximum cycle temperature About 70 to 80 percent of total heat of the fuel is supplied during this period.

4. Fourth stage : After burning Because of poor distribution of the fuel particles, combustion continues during part of the remainder of the expansion stroke. This after-burning can be called the fourth stage of combustion. The after-burning period is about 700 to 80° of crank angle from TDC. The total heat evolved during the end of entire combustion process is 95 to 97% and 3 to 5% of heat goes to unburnt fuel.