COMBUSTION IN INTERNAL COMBUSTION ENGINES CLASS NOTES

COMBUSTION IN INTERNAL COMBUSTION

ENGINES CLASS NOTES

Fig. Internal Combustion Petrol Engine

1. Combustion is rapid burning of the

air fuel mixture  to liberate heat energy.

2. Process of combustion takes place

differently in spark ignition and diesel

engines. In spark ignition engines,

combustion takes place by igniting the

compressed air fuel mixture by a spark

produced by a spark plug. Spark ignition

engines are (Petrol engines or gasoline

engines).

3. Combustion in compression ignition

engines takes place by self ignition of the

air fuel mixture. First compress the air.

Then fuel enters the cylinder. The

temperature of the compressed air is

higher than the fuel ignition temperature.

Thus the fuel ignites.

4. A flame comes during combustion of the

air fuel mixture.  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

INITIATION PHASE

During this phase, H, O, and OH form.

H2 = 2H

O2 = 2O

H2O = OH+H

PROPAGATION PHASE

In this 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 absorbs the chain carriers.  Soot forms on the combustion chamber walls. Ultimately combustion process controls 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

  1. Period of Ignition delay

  2.  Flame propagation period

  3.  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. High input pressure temperature

  3. Slightly rich mixture say by about 10 %

  4. Increasing the engine speed

COMBUSTION STAGES IN DIESEL ENGINES

  1. Period of ignition delay

  2.  Uncontrolled combustion period

  3. Period of controlled combustion

  4. After burning period

 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

 Initial combustion of the fuel mixture causes sudden rise of pressure and loud noise. 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 & efficiency.

DOPES IN PETROL ENGINES

Add chemicals like Tetra Ethyl Lead, Tetra Methyl Lead in petrol engines to reduce detonation. Dopes  reduce detonation.

DOPES IN DIESEL ENGINES

Add chemicals like Amyl Nitrate and Amyl Nitrite in diesel engines to reduce knocking. Dopes reduce knocking in diesel engines.

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