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Stages of SI combustion and flame propagation

Short answer

SI combustion has three stages: ignition lag (from spark to start of pressure rise), flame propagation (rapid burning and pressure rise), and after-burning (slow oxidation during expansion). Flame speed is affected mainly by turbulence, air–fuel ratio (fastest slightly rich), compression ratio, charge temperature, residual gas, engine speed, and chamber shape.

Describe the stages of combustion in a spark-ignition engine with the help of a pressure-crank angle diagram. List the factors affecting the flame speed.

Combustion in an SI engine is a flame front that starts at the spark plug and spreads through the premixed charge. It occurs in three stages.

 p
 |                 C
 |               /   \
 |             /       \   firing
 |           /          '.....
 |         B  . . . . . . . . motoring
 |  . . . . .'
 |     A (spark)
 +--|------|-----|-----|-------- crank angle
    A      B    TDC    C

A to B: ignition lag; B to C: flame propagation; after C: after-burning.

Stages

  1. Ignition lag (A to B): from the spark to the point where the pressure line leaves the motoring curve. A small nucleus of flame forms and a self-propagating flame is established. Chemical reactions dominate and there is little pressure rise. The lag depends on the mixture, temperature and pressure, and is typically about 0.0015 s.
  2. Flame propagation (B to C): the turbulent flame travels through the charge. Most of the heat is released, the pressure rises rapidly, and the maximum pressure occurs about 10-15 degrees after TDC. Flame speed is 15-30 m/s.
  3. After-burning (beyond C): the piston is moving down and the last part of the charge burns slowly while the pressure falls. Combustion should be complete early in this stage, or work is lost.

Factors affecting flame speed

  • Turbulence: greatest effect; flame speed increases almost in proportion to turbulence, which rises with engine speed and with combustion chamber shape.
  • Air-fuel ratio: the maximum flame speed is at a slightly rich mixture (about 10% rich); lean and very rich mixtures burn slowly.
  • Compression ratio: higher ratio gives higher pressure and temperature, faster flame.
  • Initial temperature and pressure of the charge: a higher value gives faster burning.
  • Residual gas: more exhaust gas (exhaust gas recirculation, low load) slows the flame.
  • Engine speed: flame speed rises with speed, since turbulence rises; ignition lag in degrees increases but the combustion time in degrees stays almost constant.
  • Fuel type and the size and shape of the combustion chamber (short flame path helps).

✓ Answer checked by Claude Opus 5.5 (high effort)

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