Combustion In CI Engine

First Stage Of Combustion:

  • Less air is allowed to enter the fuel spray during the first stage of combustion. Hence the fuel supply is also less.
  • The air cross the fuel spray before the chemical action begins. Hence the air is physically in contact with fuel for some time.
  • The time taken for the physical contact is known as the physical lag
  • The temperature of the combustion at the start of first stage of combustion is equal to or slightly above the self-ignition temperature.
  • Ignition lag = physical lag+ chemical lag
  • During the chemical action, after physical lag heat is transferred from the air to the fuel to maintain the temperature. The air particles in the surroundings supplies heat to the above air hence the chemical lag is longer.
  • Combustion is first stage is known as delay period.

Second Stage Of Combustion:

  • Temperature at the end of the second stage combustion is much above the self-ignition temperature
  • Plenty of air undergoes combustion in the second stage. Hence fuel supply also is also in large quantity
  • Due to the large particles of air circulated, the air circulated crosses the fuel spray in different batches.
  • Combustion takes place for the air fuel mixture in batch after batch.
  • If combustion takes place for the different batches one after another till the last batch, then the combustion in second stage is send to be normal or smooth.
  • For some reasons on the other hand, if the temperature of particular batch is less. There is a delay in the combustion of this batch of air fuel mixture.
  • More batches of air fuel mixture surround this batch of air fuel mixture. Hence combustion will takes place at some point or the other for all the batches releasing enormous amount of energy, which the piston cannot handle and here the knocking takes place. The second stage of combustion is also known as uncontrolled combustion.

Third stage of combustion:

  • The temperature of the combustion chamber is very high above the self-ignition temperature of the fuel.
  • Large mass of air is circulated during third stage of combustion. Hence fuel supply is also in large quantity.
  • The air particles cross the fuel supply in different batches
  • Due to presence of very high temperature of the combustion, chamber combustion takes place for the air fuel mixture which is immediately after crossing the fuel spray.
  • Detonation does not take place in this stage.
  • Combustion in this third stage known as controlled combustion
  • At the end of the third stage a major quantity of fuel is burnt completely.
  • CI Engine

Fourth stage:

  • Very small quantity of fuel is left for the combustion of 4th Hence the fuel spray also be less.
  • Combustion in the 4th stage takes place after burning of a major quality of fuel particles in the earlier stages. Hence the 4th stage of combustion is known as after burning.

Factors effecting knocking in CI engine:

Compression ratio:

  • The compression ratio will results in the higher temperature of the charge. Hence the knocking tendency will decrease. Due to this reason compression ratio for the diesel engine ranges between ‘16 to 20’.
  • The effect will be the same when the inlet valve temperature is higher or when a super charger is used.


  • For higher speeds more fuel enters the first stage of combustion. The diesel engine should not run at very high speeds.

Increasing the load:

  • Higher the loads or power more fuel will be supplied. Thus more energy will be released and the temperature of charge will be higher.
  • Knocking in CI engine will decreases, hence for minimum knocking in diesel engine, the ideal combustion will be maximum load and less speeds.


  • For better atomization the fuel spray will spread for distance. The physical lag will be more. At air will carry more fuel with it more energy will be liberated in the second stage of combustion will be smooth and knocking will be less.
  • It is to be noted that for a good atomization of fuel, the injection pressure must be very high.
  • For good atomization another requirement is at the cylinder pressure must have a valve near to the compression ratio 16

Quality of fuel:

For better quality of the fuel cetane number 2 should be higher, here by the anti- knocking quality of the fuel will increased.

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