Combustion in a SI engine involves three stages: 1. Flame development stage where the spark ignites the fuel-air mixture and a flame nucleus forms. 2. Flame propagation stage where the flame spreads through the combustion chamber. The flame propagation speed affects combustion efficiency. 3. Flame termination stage where combustion continues after peak pressure is reached if a rich fuel mixture is supplied.

1. COMBUSTION PROCESS IN SI ENGINE
HARSH PARASHAR
131501013 | PRE-FINAL YEAR
DEPARTMENT OF MECHANICAL ENGINEERING

2. COMBUSTION
 Combustion may be defined as a relatively rapid chemical combination of hydrogen and
carbon in fuel with oxygen in air resulting in liberation of energy in the form of heat.
 Following conditions are necessary for combustion to take place
1. The presence of combustible mixture
2. Some means to initiate mixture
3. Stabilization and propagation of flame in Combustion Chamber
 Ignition Limits : Ignition of charge is only possible within certain limits of fuel-air ratio.
Ignition limits correspond approximately to those mixture ratios, at lean and rich ends of
scale, where heat released by spark is no longer sufficient to initiate combustion in
neighbouring unburnt mixture.

3. COMBUSTION IN SI ENGINES
 In SI engine uniform or homogeneous air-fuel mixture is supplied during combustion
process.
 In SI Engines, carburettor supplies a combustible mixture of petrol and air and spark plug
initiates combustion
 From ‘intake valve closing’ to till start of the combustion is compression process then
combustion starts and it continues until peak pressure is reached after that pressure
decreases and expansion occurs as shown in the diagram.
 In ideal case entire pressure rise occurs at constant volume but in actual cycles it does not
happen.

4. COMBUSTION IN SI ENGINE
Pressure variation along with crank
angle in SI engine
Three stages of combustion in SI engine:
1. Flame Development Stage
2. Flame propagation Stage
3. Flame Termination Stage
Image Source : www.ques10.com

5. COMBUSTION IN SI ENGINE
1. Flame Development Stage
 Ignition delay / Ignition lag: The time
interval between instant of spark and
instant where there is a noticeable rise in
pressure due to combustion.
 In this time interval molecules get heated
up to self ignition temperature, get ignited
and pa self propagating nucleus of flame.
 It depends on the nature of the fuel,
temperature and pressure, proportions of
exhaust gas and rate of burning.
2. Flame Propagation Stage
 Once the flame is formed at the end of
ignition delay, it should be self sustained
and must be able to propagate through the
mixture.
 The flame will propagate only if heat
generated by burning is greater than the
heat lost by the flame to the surrounding.
 Initially heat lost to the surroundings is
more than the heat generated by burning
therefore initially pressure increases very
slowly

6. COMBUSTION IN SI ENGINE
3. Flame Termination Stage
 Combustion does not stop when the peak
pressure is attained but continues, it
happens if rich fuel mixture is supplied.
Factors affecting Flame Propagation :
 Rate of flame propagation affects the
combustion process in SI engines. At higher
flame propagation speed higher
combustion efficiency and fuel economy
can be achieved.
 Factors which affect the flame propagation
are:
1. Air fuel ratio
2. Compression ratio
3. Load on engine
4. Turbulence, engine speed and engine size