Combustion & Combustion Chamber

Combustion:

The ignition of fuel in the presence of air producing carbon dioxide, water and heat energy.

The topics like Calorific value, Net Calorific Value and Gross Calorific Value is defined by Calorific Value.

Homogeneous Mixture:

It is the mixture of two components such that you have the same proportion of components in each sample.

• It is used in SI engines.
• Combustion starts at the end of compression stroke.
• When A/F mixture is ignited, flame front appear and starts to spread.
• Propagation of flame is due to heat transfer and diffusion.
• Velocity with which the flame front moves is called Normal Flame Velocity.
• If equivalence ratio is in between 1-1.2, the flame velocity is maximum.
• If heat due to combustion zone becomes equal to the heat due to adjacent zone then flame gets extinguished.

Heterogeneous Mixture:

It is the mixture of two components such that you have the different proportion of components in each sample.

• It is used in CI engines.
• When ignition starts, flame front appear which burn fuel in adjacent zone where mixture is leaner.

Combustion Stages in SI Engines:

There are three stages of combustion SI engines which are discussed ahead:

1. Growth of Flame/ Ignition Lag:

In this stage flame front appear die to ignition.

2. Flame Propagation:

Flame propagates due to heat transfer and diffusion. In this stage flame velocity remains constant.

Another factors of flame front propagation are:

• Reaction Rate  ↠  rate at which flame eats unburned particles.
• Transportation Rate  ↠  physical movement of flame front relative to the cylinder walls.

3. After Burning:

This stage starts from when peak pressure released on indicator diagram which means expansion process.

Factors Affecting Flame Velocity:

Following are the factors which affect the flame velocity:

• Turbulence  ↠  As the turbulence of air in the combustion chamber increases, flame velocity increases.
• F/A Ratio  ↠  if the equivalence ratio is 1.0-1.2, the flame velocity increases.
• Compression Ratio  ↠  As the compression ratio increases, pressure and temperature increases which increases the flame speed.
• Engine Speed  ↠  it increases with the increase in engine speed.

Knocking in SI Engines:

• Fuel is combusted in the form of layers.
• When 1st layer gets combusted, it increases the pressure and temperature of 2nd layer, which is combusted to further increase in temperature and pressure.
• A time will come when temperature in the combustion chamber reach the self ignition temperature of fuel.
• It results in generation of secondary flame which apply pressure in opposite direction which produce noise.

Factors Affecting Knocking of Engine:

There are two types of knocking factor namely:

1. Density Factor:

• Compression Ratio  ↠  As the compression ratio increases ,pressure and temperature also increase which decrease ignition lag and increase knocking tendency.
• Intake Mass  ↠  if mass intake is greater, temperature and pressure increase due to rapid combustion which decrease ignition lag and increase Knocking tendency.
• Temperature of C/C Walls  ↠  combustion chamber walls temperature should be lower to reduce knocking.
• Power Output  ↠  if output decreases, temperature and pressure decrease which reduce knocking tendency.

2. Time Factor:

• Engine Speed  ↠  if speed is greater, turbulency increases, flame speed increases which reduce knocking.
• Flame Distance  ↠  knocking tendency is reduced of flame distance is shorter.
• Location of Spark Plug  ↠  if plug is centrally located, distance travelled by flame is shorter which reduce knocking.

Combustion in CI Engines:

• Air is compressed in combustion chamber.
• Then fuel is injected which have layers.
• The air in C/C takes the fuel molecules on the jet periphery.
• Due to compression, temperature in the C/C reaches to its self-ignition temperature and then ignites.

Combustion Stages in CI Engines:

There are 4 stages of combustion in CI engines which are discussed ahead:

1. Ignition Delay Period:

In this stage fuel is injected but not yet burnt. There are two types of delay:

• Physical Delay  ↠  Due to atomization or vaporization.
• Chemical Delay  ↠  Due to gain of activation energy.

2. Period of Uncontrolled Combustion:

This stage starts from combustion to max. Point of pressure on indicator diagram. If delay period is greater, combustion is rapid.

3. Period of Controlled Combustion:

Fuel is injected with low delay period and ignites. Combustion is controlled by controlling fuel.

 4. Period of After-Burning:

Burning of un-burnt or left particles. This period is called After-Burning.

Factors Affecting Delay Period:

Following are the factors which affect delay period which are discussed ahead:

• Compression Ratio  ➠  If compression ratio is greater, compression temperature increases which reduce auto-ignition temperature which results in the reduction of delay period.
• Intake Temperature & Pressure  ➠  Delay period decreases with the increase in intake temperature and pressure.
• Fuel Quality  ➠  Higher cetane number gives lower delay period.
• Speed  ➠  If speed is greater, delay period is reduced.

References:

• University notes + my knowledge.
• Videos from YouTube.
• Images from Google Images.
• Book: IC Engines by V Ganesan (4th Edition).
• Class Lectures.