Phases of combustion in a CI engine

In a CI engine, air is compressed to about 30 to 40 bar and 500 to 600 degrees C. Diesel is injected near TDC and burns by auto-ignition. Combustion occurs in four stages.

 P
 |            C
 |          /  \  D
 |        /B    \___
 |  A----/            .
 |  inj starts    TDC
 +------------------------ crank angle
   A-B delay  B-C rapid  C-D controlled  after D: after-burning

1. Ignition delay (A-B): from the start of injection to the first flame. It is made up of the physical delay (atomisation, vaporisation, mixing) and the chemical delay (pre-flame reactions). There is no pressure rise above the motoring curve.
2. Rapid (uncontrolled) combustion (B-C): the fuel injected during the delay burns almost at once, giving a steep pressure rise. This is the source of diesel noise.
3. Controlled (mixing-controlled) combustion (C-D): fuel injected now burns as it enters; the rate depends on injection and mixing. Peak temperature is reached in this stage.
4. After-burning (after D): the remaining fuel burns late in the expansion stroke. This is undesirable, since it raises the exhaust temperature and smoke.

Diesel knock and ignition delay

If the delay is long, a large amount of fuel accumulates and then burns suddenly. This causes a very high rate of pressure rise, shock waves and a metallic knocking sound called diesel knock.

Remedies: use a high-cetane fuel, a higher compression ratio, higher intake temperature/pressure, pilot injection, a small injection quantity at the start (pintle nozzle), and keep the engine warm.