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INTERNAL COMBUSTION ENGINES-II Combustion
University of Petroleum & Energy Studies
Dehradun
I C ENGINES - II
Air Motion & Combustion
• Fuel - Air Mixing and Combustion :
-The air motion so organized inside the combustion
chamber and the turbulence of the air passes across
the fuel jet tears it into fine particles.
- A mixture of air and fuel forms at some location in
the spray envelope and oxidation starts
- The liquid fuel droplets initially evaporate and then
get heated up , and A/F ratio ( local ) within
combustible range , autoignite.
I C ENGINES - II
Air Motion & 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
Air Motion & Combustion
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
Air Motion & Combustion
• Combustion in CI Engines : Ignition Delay
I C ENGINES - II
Air Motion & Combustion
Stages/ Phases of combustion
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
Air Motion & Combustion
Stages/ Phases of combustion
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
Air Motion & Combustion
Stages/ Phases of combustion
2. Rapid combustion period / rapid pressure rise
- This is the phase of combustion when the rate of
pressure rise is maximum and is also called
uncontrolled combustion phase.The rate of heat
release is maximum during this phase.
- 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
Air Motion & Combustion
Stages/ Phases of combustion
completed so that the layers of fuel vapours burn as
fast as they can find fresh oxygen to continue the
combustion process
- 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
Air Motion & Combustion
- Stages/ Phases of combustion
3. Controlled combustion period / mechanically
controlled period
- This is the phase of combustion when very steep
rate of pressure rise has changed to much slower
rate of pressure rise.
- process ; This is the second phase of fuel injection
when injected fuel finds more difficult to seek out
and react with fresh oxygen in the remaining
unburnt air mass.
- period ; from the point of maximum pressure to the
I C ENGINES - II
Air Motion & Combustion
- Stages/ Phases of combustion
point of maximum cycle temperature.
- 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.
Prolongation of this variable period is determined
by the amount of additional fuel supplied above that
needed for no- load running.
I C ENGINES - II
Air Motion & Combustion 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
Air Motion & Combustion
I C ENGINES - II
Air Motion & Combustion
• 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
Air Motion & Combustion
• Factors Affecting the Delay Period :
- 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
Air Motion & Combustion
• Factors Affecting the Delay Period :
- It can be seen that the comp. temp. increases and
the auto ignition temp.decreases due to increased
density of air resulting in closer contact between
molecules of air and fuel, thereby reducing the time
of reaction and consequently the delay period.
- The peak pressure is only marginally affected due
to lower rate of pressure rise whereas mechanical
efficiency decreases.
- Optimum CR required considering cold starting &
light loads at high speeds.
I C ENGINES - II
Air Motion & Combustion
• Factors Affecting the Delay Period :
- Engine speed :
The effect of engine speed on ID ( time ) is shown
below
I C ENGINES - II
Air Motion & Combustion
• Factors Affecting the Delay Period :
- It can be seen that delay period in milliseconds
decreases with increase in speed. With increase in
speed, the heat loss during compression decreases
resulting in rise in temp. & press. and therefore
reducing the delay period.
- However, in ºCA the delay period increases as
engine operate at higher rpm. The amount of fuel
injected during delay period increases (as fuel pump
is geared to engine) resulting in high rate of press.
rise during second stage of combustion.
I C ENGINES - II
Air Motion & Combustion
• Factors Affecting the Delay Period :
- 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.
- The rate of pressure rise is unaffected but the peak
pressure may be high.
I C ENGINES - II
Air Motion & Combustion
• Factors Affecting the Delay Period :
- 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
Air Motion & Combustion
• Factors Affecting the Delay Period :
higher pressure rise during second stage of
combustion.
- On the other hand, lower injection pressure giving
larger droplets give lower pressure rise and soother
running.
- An optimum group mean diameter of the droplet
should be attempted with appropriate fuel delivery
characteristics.
I C ENGINES - II
Air Motion & Combustion
• Factors Affecting the Delay Period :
- 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
Air Motion & Combustion
• Factors Affecting the Delay Period :
- 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
Air Motion & Combustion
• Factors Affecting the Delay Period :
- 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
Air Motion & Combustion
• Factors Affecting the Delay Period :
- 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
Air Motion & Combustion
I C ENGINES - II
Air Motion & Combustion
I C ENGINES - II
Air Motion & Combustion
I C ENGINES - II
Air Motion & Combustion
• 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
Air Motion & Combustion
• Knock in CI Engines :
- 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
Air Motion & Combustion
• Knock in CI Engines :
- 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
Air Motion & Combustion
• Knock in CI Engines :
- 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
Air Motion & Combustion
• Knock in CI Engines :
- 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)
I C ENGINES - II Air Motion & Combustion
Combustion Chambers for C.I. Engines
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
I C ENGINES - II Air Motion & Combustion
Combustion Chambers for C.I. Engines
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
IC ENGINES-II Air Motion & Combustion
Course Outlines ADEG-222 LTP-3 1 1
I: BASIC THEORY
II: FUEL INJECTION SYSTEM
III: AIR MOTION, COBUSTION & COMBUSTION
CHAMBERS
IV: SUPERCHARGING and TUBOCHARGING
V: DIESEL ENGINE TESTING &
PERFORMAMCE
VI: DIESEL ENGINE Emissions AND their
Control Technology