Combustion in S.I Engine

Three Stage of Combustion:
According to Ricardo, there are three stages of combustion in SI Engine as shown
1. Ignition lag stage
2. Flame propagation stage
3. After burning stage

1. Ignition lag stage:
There is a certain time interval between instant of spark and instant where there is
a noticeable rise in pressure due to combustion. This time lag is called 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. The ignition lag is generally expressed in terms of crank
angle theta 1. The period of ignition lag is shown by path (a-b). Ignition lag is very small
and lies between 0.00015 to 0.0002 seconds. An ignition lag of 0.002 seconds
corresponds to 35 deg crank rotation when the engine is running at 3000 RPM. Angle of
advance increase with the speed. This is a chemical process depending upon the nature
of fuel, temperature and pressure, proportions of exhaust gas and rate of oxidation or
burning.

2. Flame propagation stage:
Once the flame is formed at “b”, it should be self -sustained and must be able to
propagate through the mixture. This is possible when the rate of heat generation by
burning is greater than heat lost by flame to surrounding. After the point “b”, the flame
propagation is abnormally low at the beginning as heat lost is more than heat generated.
Therefore, pressure rise is also slow as mass of mixture burned is small. Therefore, it is
necessary to provide angle of advance (30-35) degrees, if the peak pressure to be
attained (5-10) degrees after TDC. The time required for crank to rotate through an
angle theta 2 is known as combustion period during which propagation of flame takes,
place.
3. After burning:
Combustion will not stop at point “c” but continue after attaining peak pressure and
combustion is known as after burning. This generally happens when the rich mixture is
supplied to engine.