Ci engine combustion chamber

INTERNAL COMBUSTION ENGINES-II
Combustion
ARUNKUMAR.A
AP/MECH

I C ENGINES - II
Stages/ Phases Of Combustion
• Combustion in CI Engines
- Stages/ Phases of combustion
1. Ignition delay period ;
Physical delay and chemical delay
2. Rapid combustion period / rapid pressure rise
3. Controlled combustion period /
mechanically controlled period
4. After-Burning period

I C ENGINES - II
Air Motion & Combustion
• Combustion in CI Engines

I C ENGINES - II
Stages/ Phases of combustion
1. Ignition delay period ;
-Preparatory phase:some fuel admitted,not ignited
-Period : start of injection to start of combustion
(p-t curve separates from motoring curve)
-Important : combustion, design, performance,
emissions
-Fuel does not ignite immediately and total delay
period can be explained as; Physical delay and
Chemical delay. Details are shown in the figure.

I C ENGINES - II
• Combustion in CI Engines : Ignition Delay

I C ENGINES - II
Physical delay :
- period ; from beginning of injection to attainment
of chemical reactions.
- process; atomization, vaporization, mixing of fuel
& air, raising to self-ignition temperature
- depends on; fuel,air motion,combustion chamber

I C ENGINES - II
Chemical delay :
- Period ; from beginning of chemical reactions to
auto-ignition
- process ; pre-reactions in the mixture taking place
up to auto-ignition that occurs at a local
excess-air factor of 0.5 < λ < 0.7
-depends ; chemical reactions are faster at higher
temperature of the surroundings and accordingly
the chemical delay becomes shorter.

I C ENGINES - II
2. Rapid combustion period / rapid pressure rise
- period ; from end of delay or beginning of
combustion to the point of maximum pressure on
indicator diagram
-process ; most of the fuel would have formed
combustible mixture with air and preflame reactions

I C ENGINES - II
- depends ;
It may be noted that the maximum pressure
reached during this phase depends upon the delay
period. Longer the delay the higher is the rate of
pressure rise as more fuel gets accumulated
during the delay period.

I C ENGINES - II
- Stages/ Phases of combustion
3. Controlled combustion period / mechanically
controlled period
-process ; This is the second phase of fuel injection
- period ; from the point of maximum pressure to
thepoint of maximum cycle temperature.

I C ENGINES - II
-depends ; crank-angle duration of fuel injection as
per load requirements (higher and full load power
out-put ).
-Therefore, this is mechanically controlled by the
injection pump out put characteristics.

I C ENGINES - II
4. After-Burning period : fuel with the air
- Combustion does not cease with completion of fuel
injection.The unburnt and partially burnt fuel particles
start burning as soon as they come in contact with
oxygen. This continue for a certain duration called
after- burning period.
- period ; starts from the point of max.cycle temp. and
continue over the part of expansion process.
- depends ; rate of after burning depends upon velocity
of diffusion and turbulent mixing of unburnt and
partially burnt fuel with the air

I C ENGINES - II

I C ENGINES - II
• Factors Affecting the Delay Period :
Many design and Operating factors affect the delay
period. Some of the significant factors are;
- Compression ratio
- Engine speed
- Output
- Atomization of fuel and duration of injection
- Injection timing
- Quality of fuel
- Intake temperature and pressure

I C ENGINES - II
- Compression ratio (CR )
The effect of CR on compression temperature of air
and the minimum auto ignition temperature of fuel
at the end of compression is shown below

I C ENGINES - II
- Engine speed :
The effect of engine speed on ID ( time ) is shown
below

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- Output :
With increase load or output, the amount of fuel
injected ( at constant rpm ) increases and therefore
the air-fuel decreases and results in increase in
temperature thereby reducing delay period.

I C ENGINES - II
-Atomization of fuel and duration of injection :
Higher fuel-injection press. increases the degree
(fineness) of atomization which reduces ignition
delay due to higher surface to volume ratio.
-Smaller droplets have low depth of penetration due
to less momentum. Such droplets have less velocity
relative to air and shorter fuel spray path, where it
has to find oxygen after vaporization affecting air
utilization factor. Also the aggregate area of
inflammation after ignition will increase resulting in

I C ENGINES - II
- Injection timing :
The effect of injection advance timings ( 9º,18º,27º )
for constant injected quantity of fuel on cylinder
pressure is shown below :

I C ENGINES - II
-As pressure and temperature at the begning of
injection are lower for higher injection advance, the
delay period increases with increase in injection
advance.
-The optimum angle of advance depends on many
factors. Generally it is about 20º bTDC.

I C ENGINES - II
-Quality of fuel
Self ignition temperature is the most important
property of fuel which affects the delay period. A
lower self-ignition temperature results in lower
delay period.
-Fuels with higher cetane number give lower delay.
Other properties of fuel which affect delay period
include volatility, latent heat, viscosity and surface
tension.

I C ENGINES - II
-Intake temperature and pressure :
Increase in intake temp. increases compressed air
temp. resulting in reduced delay period. Preheating
may not be desirable as it reduces density of air
affecting volumetric efficiency and power.
-Increase in intake pressure reduces the auto -
ignition temp. and hence the delay period. The peak
pressure will be higher as compression pressure
will be higher with higher intake pressure.

I C ENGINES - II
• Knock in CI Engines :
-Fuel Injection takes definite interval
- Injection & Combustion take place simultaneously
-Initial droplets while undergoing ignition delay, additional
droplets are being injected
- If ignition delay of fuel is short,
initial few droplets will commence
actual burning
- As a result, mass rate of mixture
burned produce rate of press. rise
for soother combustion as shown

I C ENGINES - II
- As ignition delay is longer , the actual burning of
first few droplets is delayed and accumulation of
greater quantity of fuel take place. When actual
burning take place under such conditions the rate of
pressure rise increases as shown in fig.

I C ENGINES - II
- If ignition delay is quite large and the actual
burning is substantially delayed and the
accumulation of fuel is high when actual burning
take place the rate of pressure rise is almost
instantaneous the knocking begins as shown in fig.

I C ENGINES - II
-Knocking is characterized by extreme pressure
differentials and violent gas vibrations evidenced by
audible knock.
-In CI engines knocking occurs in the beginning of
combustion.
-In order to decrease knocking the actual burning
should start as early as possible after the fuel
injection begins.
-It has been found that provided the rate of press.
rise does not exceed 3 bar per ºCA combustion is

I C ENGINES - II
-smooth. Between 3 to 4 bar there is tendency to
knock, above this rate of pressure rise the diesel
knock will be prominent.
-Normally audible knock is always present in CI
engines, when it becomes sever and cause heavy
vibrations in the engine , it is said to be knocking. It
is, therefore, a matter of judgment.
-Low ignition temp., ignition delay & speed reduce
knock as also higher CR, Inlet Temp. & press.,
comb. wall temp. cylinder size

I C ENGINES - II Air Motion & Combustion
Combustion Chambers for C.I. Engines
 CI combustion chamber :
- provide proper mixing of fuel and air in short time
- An organized air movement ;
swirl, turbulence,squish
- produce high relative velocity between fuel
droplets and air
- Shape of combustion chamber control air motion
- CI engines classified as ;
Direct- Injection (DI) & Indirect- Injection (IDI)

 CI combustion chamber :
Direct- Injection (DI)
- entire volume of combustion chamber located
in main cylinder where fuel is injected. Also
called open combustion chamber.
Indirect- Injection (IDI)
- Combustion space divided into two parts ;
one part main cyl. & other part in cylinder head

 Indirect- Injection (IDI)
- Classified as :
1. Swirl chamber ;
compression swirl generated
2. Precombustion chamber ;
combustion swirl induced
3. Air cell chamber ;
compression & combustion swirl induced

Ci engine combustion chamber

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