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Stages of CI combustion and diesel knock

Short answer

CI combustion has four stages: ignition delay, rapid uncontrolled burning, controlled combustion, and after-burning. Diesel knock results from a long delay causing abrupt pressure rise early in combustion, unlike SI knock which is end-gas auto-ignition late in combustion. Reducing diesel knock involves high-cetane fuel, high compression ratio, intake heating, pilot injection, and increased turbulence.

Explain the stages of combustion in a CI engine. How does diesel knock differ from knock in an SI engine, and how can diesel knock be reduced?

In a CI engine fuel is injected into the hot compressed air near TDC. Combustion has four stages.

 p
 |                C-D
 |              /     \
 |          B /         \ . . .
 |   . . . . '  (motoring)
 |  A (injection starts)
 +--|-----|-----|-----|------- crank angle
    A     B    TDC    D

A to B: ignition delay; B to C: rapid combustion; C to D: controlled combustion; after D: after-burning.

Stages

  1. Ignition delay (A to B): from the start of injection to the start of pressure rise. Fuel is atomised, vaporised, mixed with air and pre-flame reactions occur. Physical delay (atomisation, vaporisation) and chemical delay (pre-reactions) are included. It is about 0.001 s (10-15 degrees).
  2. Rapid or uncontrolled combustion (B to C): the fuel injected during the delay burns suddenly. The pressure rises rapidly, and the rate of rise (bar per degree) determines the roughness.
  3. Controlled combustion (C to D): the fuel injected afterwards burns as it is injected, at a rate set by the injection and mixing. The peak temperature occurs here.
  4. After-burning: unburnt fuel burns on the expansion stroke, with a fall of pressure. Too much of it gives smoke and loss.

Diesel knock versus SI knock

BasisDiesel knockSI knock
CauseLong delay, a large amount of fuel burns at onceAuto-ignition of end gas
WhereBeginning of combustionEnd of combustion
MixtureHeterogeneous (in-cylinder spray)Homogeneous
RemedyShort delay: high cetane, high compression ratioLong delay: high octane, low compression ratio

The remedies are opposite.

Reducing diesel knock

  • Use a high-cetane fuel and a high compression ratio.
  • Increase the intake air temperature and pressure (turbocharging).
  • Use a pilot injection or injection with a small initial quantity; correct injection timing.
  • Increase swirl and turbulence, use fine atomisation and high injection pressure.
  • Keep the jacket water temperature high.

✓ Answer checked by Claude Opus 5.5 (high effort)

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