Combustion may be defined as a relatively rapid chemical combination of hydrogen and carbon in fuel with oxygen in air resulting in liberation of energy in the form of heat. A spark-ignition engine (SI engine) is an internal combustion engine, generally a petrol engine, where the combustion process of the air-fuel mixture is ignited by a spark from a spark plug.

Phase of Combustion in SI Engine

In an ideal engine the entire pressure rise during combustion takes place at constant volume. However, in an actual SI engine, the combustion process being completed in three stages.

  • v  Ignition Lag
  • v  Flame Propagation
  • v  Afterburning

Fig - 1: Phase of Combustion in SI Engine

Ignition Lag

Ignition lag is the time interval in the process of chemical reaction during which molecules get heated up to self-ignition temperature, get ignited and produce a self-propagating nucleus of flame. In Fig – 1, the region between A and B denotes the ignition lag.

The duration of ignition lag depends on the following factors:

Fuel – Chemical nature of the fuel. Higher the SIT longer the ignition lags

Mixture Ratio – Higher the max. temperature of the mixture shorter the ignition lag.

Initial Temperature and Pressure – Ignition lag decreases with increase in temperature pressure of the gas.

It also depends on density and the proportion of residual gases in the mixture.

Flame Propagation

Once the self-sustaining flame appears at point B, the flame travels outwards and burns the fuel in air. Initially the rate of burning of fuel and flame speeds are low with small rate of pressure rise. However, as the combustion proceeds, the pressure and temperature keep on rising with heat energy release which is transferred from burned to unburned charge, the flame propagates across the combustion chamber at high speeds. The start of flame propagation is considered at point B on diagram (Fig-1) and this phase ends when the peak pressure is attained in the cylinder i.e at Point C. The slope of curve BC represents the rate of pressure rise.

Flame speed is affected by the following factors: 

Fuel-air ratio – The max. flame velocities occur when mixture strength is 110% of stochiometric. When the mixture is made leaner or richer velocity of flame diminishes.

Turbulence – the flame speed is very low in non-turbulent mixtures


Actually, combustion is not completed at point C though it represents the completion of flame travel. It is due to the fact the burning continues due to left over fuel and the reassociation of dissociated gases existing in the combustion chamber. This combustion beyond point C continues during the expansion stroke and it is called as after burning representing the third stage of combustion up to point D.

Afterburning occurs due to the following:

The whole heat of the fuel has not been liberated at the point C.

For the passage of the flame even after point C.

Some further chemical adjustments due to re-association.

Abnormal Combustion

The combustion is normal when 2 conditions are respected:

  1. The ignition of combustible mixture is controlled by the spark plug, with a present timing.
  2. After the ignition, the flame propagates regularly to the whole mixture, without any sudden increase of velocity.

When the 2 conditions of normal combustion do not occur, the combustion process is abnormal, causing engine damages or simply performance worsening and noise.

The important abnormal combustions are:

Detonation or Knock – Combustion takes place at different position or the end charge auto ignites before the flame front reaches it, caused due to unburned charges reaches to critical temperature or SIT.

Preignition – Ignition occurs before the passage of the spark, caused by an overheated spot.

Run-on – If the engine continues to fire when ignition system is switched off.  


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