Combustion in SI Engine

First stage of combustion

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Separately first stage of combustion:

  • When the spark contacts some areas of the fuel then the chemical action begins in those particular areas.
  • The heat generated in this area spreads to the other area of the fuel, where the chemical actions begin in the other area.
  • There is propagation of nucleus of the area in which chemical action takes place.
  • There is rise in temperature of the fuel.
  • By this different amount of heat gets collected in the different areas of the fluid.
  • If the temperatures of the fuel particle are at rest, then the self-ignition of temperature takes place at ends of the chemical reaction.
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  • The fuel remains as an unburned high carbon which is in circulation with the hot gases.
  • If the temperature of the fuel is equal to or higher than the self-ignition temperature, then there is distribution of heat until equal amount of heat is stored in the equal different areas of the fuel.
  • There is no rise in temperature during the heat distribution. Ignition takes place for the fuel.
  • The time is taken to beginning the chemical action
  • The time taken from the beginning of the chemical action to beginning of the ignition is known as chemical ignition or chemical delay
  • For petrol engine chemical lag is known as ignition lag
  • The first sage of combustion is also known as preparatory phase.

Normal and Abnormal combustion in SI Engine:

Flame front:

  • Due to sparking the nearby fuel particles get ignited.
  • The line joining the ignited fuel particles is known as flame front.
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Propagation of flame front:

  • After the formation of the flame front, heat from this flame front travels to nearby fuel particle.
  • Another front flame is formed in the meantime.
  • The first flame is varnished.
  • It appears as the first flame fronts, released the new position.
  • This movement of the flame front is known as the flame front propagation.

End charge:

  • The air fuel mixture at the opposite end of the flame front is known as the end charge.
  • During propagation of the flame front, the following events take place for the end charge.
  • During the flame front propagation, the air particles near the flame front expands and compresses the end charge temperature as the end charge increases.
  • Due to increases in the temperature of the end charge chemical action begins, and the heat generated increases the temperature of the end charger further
  • There is transfer of heat from the flame front to the end charge temperature continuously for it to increase.

Due to the above reasons if the temperature of the end charges equals the self-ignition temperature. A separate flame front is fronted at the end charge. That moves opposite to the other flame front, later collides. This collision is defined as the knocking or detonation.

SI Engine

The following are the disadvantages of detonation:

  • A separate high energy wave is focused due to the detonation which travels at the speed of 2500 to nearly 5000 cycles / sec.
  • Since a stroke to the cylinder and piston.
  • A crank may be developed in the piston or cylinder due to the detonation.
  • Mechanical vibrations are developed in the engine due to knocking
  • Due to knocking a high metal is produced which is known as pinging.
  • Hence knocking is known as pinging or knock or detonation or spark knock or pinking.
  • If the flame front forms the spark plug then it reaches the end charge.
  • Before another flame front is formed then the combustion is said to be normal or smooth

Chemical action of the fuel

Chemical action starts at point A, as the time passes there is raise in temperature. The graph is seen as a straight line in this condition.  If the temperature of the fuel is below the self-ignition temperature then it leads to the end of chemical action. This fuel particle swill remains unburned and will be in circulation along with the hot gases. During this circulation the temperature of this unburned high carbon keep decreasing until it is exhausted.

Two fuel particles  and undergo chemical action for the fuel. The temperature at the end of the chemical action is equal to the self-ignition temperature for the fuel. This is above the self-ignition temperature. At the end of the chemical action heat distribution is shown at the constant temperature.

It may be seen that the chemical land for the fuel particles whose temperature is above the self-ignition temperature is shorter.

Thus for petrol engine temperature is higher at the end change. Lesser the ignition lag, more the change in detonation is observed. Thus for SI engines the temperature of the end charge should to be very close to the lag ignition temperature to see minimum or no detonation.

Factors affecting the detonation in SI engine:

Compression ratio:

  • High compression ratio will result in higher temperature of the charge, this the detonation tendency will increase. Hence for the compression ratio for SI engine are limited to 6 and 10.
  • The effect will be the same when the inlet valve temperature is higher or when a super charge is used.
  • On using a super charger an inter cooler must be provided between the super charger and the cylinder engine. It is to be noted that for SI engines, the super charger should be placed before the carburetor.

Increasing the load:

  • For higher load or power, more fuel needs to be supplied. The temperature of change is in higher tendency to knock, which increases.
  • Hence for lag knocking SI engine must run at part loading conditions only.

Advancing the spark:

  • Advancing the spark will result in more sparking. It means that additional energy is received by the change in temperature when it is higher.
  • Detonation increases, for the lesser detonation spark which is to be restored.

 Distance of flame travel:

  • The time taken for the flame travel is more than the chemical lag of end changer.
  • Then a flame front is formed at the end charges, when the detonation takes place.
  • Hence diameter of SI engine cylinder is limited to a maximum of 15 cm.

Location of spark plug:

The spark plug must be located nearest to center and away from the inlet valve.

Engine speed:

When the speed of the engine increases, the propagation of the flame becomes turbulent. It occupies more space and reaches the charger faster tendency for the knock to decrease.

Octane number:

  • Higher the octane number better will be the anti-knock quality of the fuel.
  • If the air fuel ratio is such that the equivalent ratio is between 1:1 and 1:4, then both temperature and velocity of the charge are higher.
  • If the increase in temperature is more effective than the knocking will increase.
  • On the other hand if the velocity rises of the flame front which is more effective then knocking will decrease.
  • At present the combustion chambers are so designed, where the velocity of the flame front is more effective.
  • Hence the knocking in present days SI engine is maximum.

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