The graph shows the variations in cylinder pressure plotted on a continuous crank angle base.
Phase 1: Ignition delay period. This is the time taken (or angle turned by the crank) between the start of injection to the commencement of the pressure rise. During this important period, the injected fuel particles are being heated by the hot air to the temperature required for the fuel to self-ignite.
Phase 2: Flame spread causes a sharp pressure rise due to the sudden combustion of the fuel that was injected during the first phase. The rate of pressure rise governs the extent of the combustion knock. This is commonly called 'diesel knock' and is considered to be the main disadvantage of the CI engine.
Phase 3: Direct burning of the fuel as it enters the chamber gives a more gradual pressure rise. When the engine is operating at less than full load, this phase does not exist.
