Combustion in Internal Combustion engines

Combustion in Internal Combustion engines

Keywords: Ignition delay- Detonation- Knocking – Dopes


Combustion is rapid burning of the air fuel mixture (carbon + Oxygen and hydrogen + oxygen) to liberate heat energy. Combustion takes place differently in spark ignition engines and diesel engines

Combustion in spark ignition engines (Petrol engines or gasoline engines) takes place by igniting the compressed air fuel mixture by a spark produced by a spark plug.

Combustion in compression ignition engines (diesel engines) takes place by self ignition (no spark is available) of the air fuel mixture. It is obtained by first compressing of air and then injecting of fuel in the compressed high temperature air. The mixture is ignited because of high temperature of air being higher than the self ignition temperature of the fuel.

During combustion of the air fuel mixture, there is a flame. A flame is conversion of an air fuel mixture into a luminous phase by the rapid release of chemical energy in the form of heat energy.

Mathematically the combustion process is as given below:

C8H18 + 12.5 O2 + 125 x 3.76 N2 = 8CO2+ 9H2O+ 125 x 3.76 N2 + large amount of heat energy

Phases of combustion

Initiation of combustion phase—During this phase chain carriers, H, O, and OH are formed.

H2 = 2H

O2 = 2O

H2O = OH+H

Propagation phase—during propagation phase, combustion takes place with the release of heat energy

OH+H2 = H2O +H

H+O2 = OH+O

O+H2 = OH+O

Termination phase— Cold combustion chamber absorb the chain carriers and soot is formed on the combustion chamber walls.

Ultimately combustion process is controlled by the followings:

  1. Rate of formation of chain carriers
  2. Rate of destruction of chain carriers
  3. Temperature, pressure and density of the air fuel explosive mixture

Combustion stages in SI engines

(a) Period of Ignition delay

(b) Period of flame propagation

© Period of after burning


Factors affecting ignition delay in petrol engines

  1. Higher pressure and higher temperature
  2. Slightly rich mixture
  3. Less amount of residual gases after exhaust
  4. Electrode gap

Factors which increase the speed of flame propagation

  1. Higher compression ratio
  2. Higher input pressure temperature
  3. Slightly rich mixture say by about 10 %
  4. Increasing the engine speed

Combustion stages in diesel engines

  • Period of ignition delay
  • Period of uncontrolled combustion
  • Period of controlled combustion
  • Period of after burning

Factors affecting ignition delay in CI engines

  1. Compression ratio
  2. Inlet pressure
  3. Inlet temperature
  4. Engine speed
  5. Jacket water temperature
  6. Load on the engine
  7. Injection pressure
  8. Injection timing
  9. Engine size

Detonation in petrol engines

Loud sound produced by the abnormal combustion of end fuel charge.

Knocking in diesel engines

It is the loud sound produced due to sudden rise 0of pressure caused by the combustion of the initial part of the fuel mixture

Detonation and knocking can melt the piston, piston ring; can cause the failure of an engine part, overheating of spark plug and loss of power as well as efficiency.

Dopes in petrol engines


Chemicals like Tetra Ethyl Lead, Tetra Methyl Lead are added in petrol engines to reduce detonation. These substances which reduce detonation are called dopes.

Dopes in diesel engines

Chemicals like Amyl Nitrate and Amyl Nitrite are added in diesel engines to reduce knocking. These substances which reduce knocking are called dopes.