COMBUSTION IN S I & C I ENGINES

COMBUSTION IN S.I & C.I
ENGINES

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
In SI Engines, carburetor supplies a combustible mixture of petrol and air
and spark plug initiates combustion.

THREE STAGE OF
COMBUSTION1. Ignition lag stage(ab)
2. Flame propagation stage(bc)
3. After burning stage(cd)
 ab is called as ignition lag or preparation
face in which the growth & development of
flame takes place. This is chemical process.
 bc is concerned with spreading of flame
through out the combustion chamber.
 cd is after burning where certain local
pockets of air fuel particles undergone
combustion.

FACTORS AFFCTING THE
FLAME PROPAGATION
Rate of flame propagation affects the
combustion process in SI engines. Higher
combustion efficiency and fuel economy can be
achieved by higher flame propagation velocities.
The factors which affect the flame propagations
are
1. Air fuel ratio
2. Compression ratio
3. Load on engine
4. Turbulence and engine speed

PHENOMENON OF KNOCKING IN SI ENGINE

AUTO IGINITION
A mixture of fuel and air can react
spontaneously and produce heat by chemical
reaction in the absence of flame to initiate the
combustion or self-ignition. This type of self-
ignition in the absence of flame is known as
Auto-Ignition. The temperature at which the
self-ignition takes place is known as self-
igniting temperature.

PRE -IGINITION
Pre-ignition is the ignition of the homogeneous
mixture of charge as it comes in contact with hot
surfaces, in the absence of spark .
Auto ignition may overheat the spark plug and
exhaust valve and it remains so hot that its
temperature is sufficient to ignite the charge in
next cycle during the compression stroke before
spark occurs and this causes the pre-ignition of
the charge.
Pre-ignition is initiated by some overheated
projecting part such as the sparking plug
electrodes, exhaust valve head, metal corners
in the combustion chamber, and carbon
deposits.

EFFECT OF DETONATION
 Noise and Roughness.
 Vibration
 Mechanical Damage.
 Decrease in power output and efficiency.
 Pre-ignition.

Factors affecting Detonation or
Knocking in SI engine
1.Compression ratio:
The pressure and temperature at the
end of compression increases with
increase in compression ratio. This in
turn increase the maximum pressure
during the combustion and creates a
tendency to knock

2.Supercharging:
It is the process of allowing the charge at a
pressure higher than atmospheric pressure.
Because of supercharging the density and
temperature increases there by knocking
tendency increases.
Super charging is generally done on CI
engines
3.Increasing inlet temperature:
It results in increase in knocking.

4.Increase in load:
increase in load result in increase in
temperature of the cylinder, there by rising
the temperature and hence knocking
tendency increasing at high loads.
5.Advancing the spark:
when the spark is advance the burning gas
is compressed by rising piston and therefore
the increase in temperature will be much
more and hence the knocking tendency
increases.
To avoid detonation spark must be retarded.

6.Flame travel distance:
Flame travel distance should be small to
avoid detonation.
7.Spark plug location:
spark plug which is located centrally has
minimum tendency for knocking because the
flame travel distance decreases.
8.Engine size:
For larger engine size the flame travel
distance will be longer and hence knocking
chances are high, because of this reason SI
engine sizes are generally limited.

9.Location of exhaust valve:
There are two hot spot zone in SI engines. Ie
spark plug & exhaust valve. If the exhaust
valve is very far from the spark plug the
temperature around exhaust valve region will
be high. If the spark plug is very far it will
take much time and during this time the
charge near exhaust valve would autoignitied
due to high temperature and hence exhaust
valve must be located as close to spark plug
as possible. So the flame travel distance is
minimized.

10. Turbulence:
With increase in turbulence the flame speed
increases and hence the chances of
detonation would decreases.
11.Engine speed:
Increase in engine speed increases
turbulence & hence knocking tendency
decreases.
12. Octane rating of fuel:
For better antiknock properties
octane number must be high.

STAGES OF COMBUSTION IN
CI ENGINE
• Ignition Delay period /Pre-flame
combustion
i. Physical delay
ii. Chemical delay
• Uncontrolled combustion
• Controlled combustion
• After burning

1.Ignition Delay period
The fuel does not ignite
immediately upon injection into the
combustion chamber. There is a definite
period of inactivity between the time of
injection and the actual burning this
period is known as the ignition delay
period.
The ignition delay period can be
divided into two parts, the physical delay
and the chemical delay.

 When the fuel is injected into the
combustion chamber the fuel mix with
air. The time spend in this process is
known as physical delay.
 Certain pre flame reactions starts and
after some time fuel burns automatically
and the time consuming during this
process is known as chemical delay.

2.Period of Rapid Combustion
During total delay period more fuel droplet
come from the injector into the combustion
chamber. This group of fuel droplets burn
together and this produces uncontrolled
combustion is known as knocking and this
occurs at the end of delay period or the
beginning of combustion.
The rate of heat-release is
maximum during this period. This is also known
as uncontrolled combustion phase, because it is
difficult to control the amount of burning/
injection during the process of burning.

3.Period of Controlled
Combustion
The rapid combustion period is
followed by the third stage, the
controlled combustion. The temperature
and pressure in the second stage are so
high that fuel droplets injected burn
almost as they enter and any further
pressure rise can be controlled by
injection rate. The period of controlled
combustion is assumed to end at
maximum cycle temperature.

4.Period of After-Burning
Combustion does not stop
with the completion of the injection
process. The unburnt and partially burnt
fuel particles left in the combustion
chamber start burning as soon as they
come into contact with the oxygen. This
process continues for a certain duration
called the after-burning period. This
burning may continue in expansion stroke
up to 70 to 80% of crank travel from TDC.

Parameters controlling
abnormal combustion in CI
engines
Abnormal combustion means knocking.
1.Compression ratio:
Greater the compression the
temperature in the cylinder are higher
and hence due to higher molecular
activity the ignition delay will be less and
the chances of abnormal combustion
are less at higher compression ratios.

2.cylinder wall temperature:
If the cylinder wall temperatures are high
the delay period will be less and hence
knocking tendency will be less.
3.Delay period:
The delay period must be small. If the
delay period is large more fuel will be
accumulated in the cylinder and may burn
suddenly.
4.Inlet temperature:
For smaller delay period the inlet
temperature must be high.

5.Self ignition temperature:
Self ignition temperature must be low, so
that less time is required to reach that
temperature. Therefore only small amount of
fuel burning in a controlled manner.
6.Ignition advance angle:
If the ignition angle is advanced the fuel
injection starts at low pressure and low
temperature and due to this low temperature
the delay period will be large and hence
more fuel is accumulated in the cylinder and
may burn in an uncontrolled manner.

7. engine speed:
The fuel injector is geared to
the engine and hence the injection remains
constant interms of crank angle. If the speed
is increases the duration of injection time
decreases and hence at higher speed more
fuel is accumulated in less time and hence in
CI engines the chances of abnormal
combustion are high at high speeds.
wt = Ѳ = constant
↑w X t↓ = constant

8.Using a better fuel:
Higher CN fuel has lower delay
period and reduces knocking tendency.
9.Knock reducing fuel injector :
This type of injector avoid the
sudden increase in pressure inside the
combustion chamber because of
accumulated fuel. This can be done by
arranging the injector so that only small
amount of fuel is injected first. This can be
achieved by using two or more injectors
arranging in out of phase.

COMPARISON OF KNOCKING IN
SI AND CI ENGINES
SI NO FACTORS AFFECTING
KNOCKING
SI ENGINE CI ENGINE
1 SIT HIGH LOW
2 DELAY PERIOD LONG SHORT
3 CR LOW HIGH
4 INLET TEMPERATURE LOW HIGH
5 INLET PRESSURE LOW HIGH
6 SPEED HIGH LOW
7 COMBUSTION CHAMBER
WALL TEMP.
LOW HIGH

1.In spark ignition engines, auto ignition of end
gas away from the spark plug, most likely near
the end of combustion causes knocking. But in
compression engines the auto ignition of charge
causing knocking is at the start of combustion.

2.In order to avoid knocking in SI engine, it is
necessary to prevent auto ignition of the end
gas to take place at all. In CI engine, the
earliest auto –ignition is necessary to avoid
knocking
3.The knocking in SI engine takes place in
homogeneous mixture, therefore , the rate of
pressure rise and maximum pressure is
considerably high. In case of CI engine, the
mixture is not homogenous and hence the
rate of pressure is lower than in SI engine.

4.In CI engine only air is compressed, therefore
there is no question of Pre-ignition in CI
engines as in SI engines.
5.SI fuels should have long delay period to
avoid knocking. CI fuels should have short
delay period to avoid knocking.

CETANE NUMBER:
The cetane number of a diesel fuel
is a measure of its ignition quality.
OCTANE NUMBER:
The octane number of a petrol fuel
is a measure of its ignition quality.

 Define & explain octane number & cetane
number. (10
Marks)
 Explain pollutants from SI & CI engines.
(10
Marks)
 Explain different types of combustion chambers
in SI engines with neat sketches.
(10
Marks)
 Explain different types of combustion chambers
in CI engines with neat sketches.
(10
Marks)

COMBUSTION IN S I & C I ENGINES

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COMBUSTION IN S I & C I ENGINES