3. • A engine or Motor is a machine designed to convert one
form of energy into mechanical energy.
• Types of Engine are –
1. External Combustion engine
2. Internal Combustion Engine – 2 stroke
4 stroke
Engines
4. • Combustion is defined as ‘the relatively rapid chemical
combination of hydrogen and carbon in fuel with oxygen
in the air resulting liberation of energy in the form of
heat.
• Conditions necessary for combustion are –
1. The presence of combustible mixture.
2. Some means of initiating combustion
3. Stabilization and propagation of flame in the
combustion chamber
Combustion
5. • In S.I. engine, ignition of charge is possible within certain
limits of air-fuel ratio.
• The flame will propagate only if temperature of the burnt
gases exceeds 1500 K in the case of HC mixture
• For HC fuel, the stoichiometric air fuel ratio is 14.5:1
• Theoretical limit of combustion are 7:1 and 20:1
• Practical limits are 9:1 to 20:1
Mixture requirement in Petrol engine
6. Practical Limit of Combustion
Theoretical Ignition Limit
Stoichiometric
A/FRatio
Too
rich
Too
lean
0 7 9 14.5 20 30 A/F ratio
Excess fuel Excess air
Ignition limits of Hydrocarbon
7. • In C.I. engine the supply of air is constant and fuel is injected
according to load in engine.
• The average air to fuel ratio for C.I. engine varies from 100:1 at
no load to 20:1 at full load.
• The HC fuel will burn only between air fuel ratios 8:1 to 30:1
• But in C.I. engines air-fuel ratio is much higher than 30:1
So, combustion takes places as follows –
The mixture here is heterogeneous
The variation of A/F ratio is varying at different places in
combustion chamber.
But at some places the A/F ratio may be within combustible
limits and combustion is initiated at such a point
Due to this combustion A/F ratio at other points reaches the
flammability limit and combustion starts at those points also.
Mixture requirement in Diesel engine
8. • In S.I. engine sufficiently homogenous mixture of
vaporized fuel, air and residual gases is ignited by a
single spark produced between electrodes of spark plug
in the for of thin thread of flame.
• From this thin thread of flame, combustion spreads to
envelop mixture immediately and burn completely.
• These are three stages of combustion –
1. Ignition Lag
2. Propagation of flame
3. After Burning
Stages of combustion in S.I. engine
9. 60 40 20 8 TDC 20 40 60
1 32
Pressure
Spark
Crank angle
A
B
C
10. • Diesel engine handles the entire process of vaporizing,
mixing and igniting fuel inside the cylinder in extremely
short period of time.
• Heat required for vaporization of fuel is taken from
compressed air therefore air is compressed sufficiently high
pressure to get required temperature after compression.
• In combustion of Diesel engine there are
4 stages of combustion –
1. Ignition delay period
2. Period of uncontrolled combustion
3. Period of controlled Combustion
4. Period of after burning
Stages of combustion in C.I. engine
