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Internal combustion engines
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Heat Engines
Any type of engine or machine which derives Heat Energy from the combustion of the
fuel or any other source and converts this energy into Mechanical Work is known as a
Heat Engine.
Classification :
1. External Combustion Engine (E. C. Engine) :
Combustion of fuel takes place outside the cylinder.
e.g. Steam Turbine, Gas Turbine Steam Engine, etc.
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2. Internal Combustion Engine (I.C. Engine) :
Combustion of fuel occurs inside the cylinder.
Heat Engines
e.g. Automobiles, Marine, etc.
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Heat Engines
Advantages ofExternal Combustion Enginesover Internal Combustion Engines :
1. Starting Torque is generally high.
2. Due to external combustion, cheaper fuels can be used (even solid fuels !).
3. Due to external combustion, flexibility in arrangement is possible .
4. Self Starting units.Internal Combustion Engines require additional unitfor starting the engine !
Advantages ofInternal Combustion Enginesover External Combustion Engines :
1. Overall efficiency is high.
2. Greater mechanical simplicity.
3. Weight to Power ratio is low.
4. Easy Starting in cold conditions.
5. Compact and require less space.
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Classification of I. C. Engines
A. Cycle of Operation :
B. Cycle of Combustion :
2. Four Stroke Engine1. Two Stroke Engine.
1. Otto Cycle (Combustion at Constant Volume).
2. Diesel Cycle (Combustion at Constant Pressure).
3. Dual Cycle (Combustion partly at Constant Volume + Constant Pressure).
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Classification of I. C. Engines
C. Arrangement of Cylinder :
1. Horizontal Engine. 2. Vertical Engine
3. V type Engine 4. Radial Engine
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Classification of I. C. Engines
D. Uses :
1. Automobile Engine. 2. Marine Engine
3. Stationary Engine 4. Portable Engine
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Classification of I. C. Engines
E. Fuel used :
1. Oil Engine. 2. Petrol Engine
3. Gas Engine 4. Kerosene Engine
F. Speed of Engine :
1. High Speed 2. Low Speed
G. Method of Cooling :
1. Air Cooled Engine. 2. Water Cooled Engine
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Classification of I. C. Engines
G. Method of Ignition :
2. Compression Ignition (C.I.) Engine1. Spark Ignition (S.I.) Engine.
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Classification of I. C. Engines
I. No. of cylinders :
1. Single Cylinder Engine. 2. Multi - Cylinder Engine
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Application of I. C. Engines
APPLICATIONS
Road vehicles.Aircrafts.
Locomotives.
Construction
EquipmentsPumping Sets
Generators for Hospitals,
Cinema Hall, and Public Places.
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Diesel Cycle
In S. I. Engines, max. compression ratio (r) is limited by self ignition of the fuel.
This can be released ifair and fuel are compressed separatelyand brought together
at the time of combustion.
i.e. Fuel can be injectedinto the cylinder with compressed air at high temperature.
i.e. Fuel ignites on its own and no special device for ignition is required.
This is known as Compression Ignition (C. I.) Engine.
Ideal Cycle corresponding to this process is known as Diesel Cycle.
Main Difference :
Otto Cycle Heat Addition at Constant Volume.
Diesel Cycle Heat Addition at Constant Pressure.
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Dual Cycle
Combustion process is neither Constant Volume nor Constant Pressure Process.
Real engine requires :
1. Finite time for chemical reaction during combustion process.
Combustion can nottake place at Constant Volume.
2. Rapid uncontrolled combustion at the end.
Combustion can nottake place at Constant Pressure.
Hence, a blend / mixture of both the processes are proposed as a compromise.
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Four Stroke / Compression Ignition (C.I.) Engine
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Four Stroke / Compression Ignition (C.I.) Engine
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FOUR STROKE ENGINES
FIRST STROKESUCTION STROKE
While the inlet valve is open ,the descending piston draws freshpetrol and air mixture into the cylinder.
Fig.
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Fig.
IN LET VALVE
OPEN POSITIONEXHAUST VALVE
CLOSE POSITION
FOUR STROKE ENGINES
FIRST STROKESUCTION STROKE
While the inlet valve is open ,the descending piston draws freshpetrol and air mixture into the cylinder.
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Fig.
IN LET VALVE
OPEN POSITIONEXHAUST VALVE
CLOSE POSITION
FOUR STROKE ENGINES
FIRST STROKESUCTION STROKE
While the inlet valve is open ,the descending piston draws freshpetrol and air mixture into the cylinder.
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Fig.
IN LET VALVE
OPEN POSITIONEXHAUST VALVE
CLOSE POSITION
FOUR STROKE ENGINES
FIRST STROKESUCTION STROKE
While the inlet valve is open ,the descending piston draws freshpetrol and air mixture into the cylinder.
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Fig.
IN LET VALVE
OPEN POSITIONEXHAUST VALVE
CLOSE POSITION
FOUR STROKE ENGINES
FIRST STROKESUCTION STROKE
While the inlet valve is open ,the descending piston draws freshpetrol and air mixture into the cylinder.
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SECOND STROKE-COMPRESSION STROKE
While the valves are closed,the rising piston compresses the mixture
to a pressure about 7-8atm; the mixture is then ignited by the spark
plug.
Fig.
IN LET VALVE
CLOSE POSITION
EXHAUST VALVE
CLOSE POSITION
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Fig.
IN LET VALVE
CLOSE POSITION
EXHAUST VALVE
CLOSE POSITION
SECOND STROKE-COMPRESSION STROKE
While the valves are closed,the rising piston compresses the mixture
to a pressure about 7-8atm; the mixture is then ignited by the spark
plug.
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Fig.
IN LET VALVE
CLOSE POSITION
EXHAUST VALVE
CLOSE POSITION
SECOND STROKE-COMPRESSION STROKE
While the valves are closed,the rising piston compresses the mixture
to a pressure about 7-8atm; the mixture is then ignited by the spark
plug.
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Fig.
IN LET VALVE
CLOSE POSITION
EXHAUST VALVE
CLOSE POSITION
SECOND STROKE-COMPRESSION STROKE
While the valves are closed,the rising piston compresses the mixture
to a pressure about 7-8atm; the mixture is then ignited by the spark
plug.
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THIRD STROKE-POWER STROKE
While the valves are closed the pressure of the burned gases of the
combustion forces push the piston downwards.
Fig.
IN LET VALVE
CLOSE POSITION
EXHAUST VALVE
OPEN POSITION
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Two Stroke / Spark Ignition (S.I.) Engine
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Two Stroke / Spark Ignition (S.I.) Engine
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When the piston moves from T.D.C to B.D.C the inlet port is closed, the
mixture is compressed and transferred the into the cylinder through
transfer port.
Inlet port
exhaust port
piston
Transfer port
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When the piston moves from T.D.C to B.D.C the inlet port is closed, the
mixture is compressed and transferred the into the cylinder through
transfer port.
Inlet port
exhaust port
piston
Transfer port
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When the piston moves from T.D.C to B.D.C the inlet port is closed, the
mixture is compressed and transferred the into the cylinder through
transfer port.
Inlet port
exhaust port
piston
Transfer port
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When the piston is moving upward ,the mixture is compressed.
At the same time,air and fuel mixture is coming into the crankcase.
Inlet port
exhaust port
Transfer port
NO FUEL MIXTURE AVAILABLE
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Compressed mixture
When the piston is moving upward ,the mixture is compressed.
At the same time,air and fuel mixture is coming into the crankcase.
Inlet port
exhaust port
Transfer port
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At the end of the compression stroke, a spark is given by a spark
plug.The fuel mixture expands rapidly.A high power is produced.
This power forces the piston downwards.So the piston moves
from T.D.C to B.D.C
Burning the fuel mixture
Inlet port
exhaust port
Transfer port
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When the piston comes down, the exhaust port opens and exhaust gases
are going out.At the same time,the transfer port also opens and the fresh
mixture comes inside the cylinder
Thus the four strokes are completed in two strokes of the engine
Inlet port
exhaust port
Transfer port
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When the piston comes down, the exhaust port opens and exhaust gases
are going out.At the same time,the transfer port also opens and the fresh
mixture comes inside the cylinder
Thus the four strokes are completed in two strokes of the engine
Inlet port
exhaust port
Transfer port
exhaust port
Transfer port
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Comparison : Two Stroke Vs. Four Stroke
Sr.No. Particulars Four Stroke Cycle Two Stroke Cycle1. Cycle Completion 4 strokes
/ 2 revolutions
2 strokes
/ 1 revolution
2. Power Strokes 1 in 2 revolutions 1 per revolution
3. Volumetric Efficiency High Low
4. Thermal and
PartLoad EfficiencyHigh Low
5.
Power for same Engine Size
Small;
as 1 power stroke for
2 revolutions
Large;
as 1 power stroke
per revolutions
6. Flywheel Heavier Lighter
7. Cooling / Lubrication Lesser Greater
8. Valve Mechanism Required Not Required
9. Initial Cost Higher Lower
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Comparison : S.I. Vs. C.I. Engines
Sr.
No. Particulars S. I. Engine C. I. Engine1. Thermodynamic Cycle Otto Diesel
2. Fuel Used Gasoline Diesel
3. Air : Fuel Ratio 6 : 1 20 : 1 16 : 1 100 : 1
4. Compression Ratio Avg. 7
9 Avg. 15
18
5. Combustion Spark Ignition Compression Ignition
6. Fuel Supply Carburettor Fuel Injector
7. Operating Pressure 60 bar max. 120 bar max.
8. Operating Speed Up to 6000 RPM Up to 3500 RPM
9. Calorific Value 44 MJ/kg 42 MJ/kg
10. Running Cost High Low
11. Maintenance Cost Minor Major
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Comparison : Gasoline Vs. Diesel Engines
Sr. No. Gasoline Engine Diesel Engine1. Working : Otto Cycle Working : Diesel Cycle
2. Suction Stroke :
Air / Fuel mixture is taken in
Suction Stroke :
only Air is taken in
3. Spark Plug Fuel Injector
4. Spark Ignition generates Power Compression Ignition generates Power
5. Thermal Efficiency 35 % Thermal Efficiency 40 %
6. Compact Bulky
7. Running Cost High Running Cost Low
8. Light Weight Heavy Weight
9. Fuel : Costly Fuel : Cheaper
10. Gasoline : Volatile and Danger Diesel : Non-volatile and Safe.
11. Less Dependable More Dependable
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Performance of I.C. Engines
Engine Performance Indication of Degree of Success for the work assigned.
(i.e. Conversion ofChemical Energy to useful Mechanical Work)
Basic Performance Parameters :
1. Power & Mechanical Efficiency
3. Specific Output
5. Air : Fuel Ratio
7. Thermal Efficiency and Heat Balance
9. Specific Weight
2. Mean Effective Pressure & Torque
4. Volumetric Efficiency
6. Specific Fuel Consumption
8. Exhaust Emissions
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Performance of I.C. Engines
A. Power and Mechanical Efficiency :Indicated Power Total Power developed in the Combustion Chamber,
due to the combustion of fuel.
)(6010
)10(
.. 3
5
kW
NkALpn
PI
i
n = No. of Cylinders
Pmi= Indicated Mean Effective Pressure (bar)
L = Length of Stroke (m)
A = Area of Piston (m2)
k= for 4 Stroke Engine,
= 1 for 2 Stroke Engine
N = Speed of Engine (RPM)
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Performance of I.C. Engines
A. Power and Mechanical Efficiency :Brake Power Power developed by an engine at the output shaft.
)(1060
2..
3kW
X
TNPB
N = Speed of Engine (RPM)
T= Torque (N m)
Frictional Power (F. P.) = I. P. B. P.
..
..
PI
PBmech
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Performance of I.C. Engines
B. Mean Effective Pressure :Mean Effective Pressure Hypothetical Pressure which is thought to be
acting on the Piston throughout Power Stroke.
Fmep= Imep
Bmep
Imep MEP based on I.P.
Bmep MEP based on B.P.
Fmep MEP based on F.P.
Power and Torque are dependent on Engine Size.
Thermodynamically incorrect way to judge the performance w.r.t. Power / Torque.
MEP is the correct wayto compare the performance of various engines.
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Performance of I.C. Engines
C. Specific Output :Specific Output Brake Output per unit Piston Displacement.
LXA
PBOutputSp
...
D. Volumetric Efficiency :
Volumetric Efficiency Ratio of Actual Vol. (reduced to N.T.P.) of the Chargedrawn in during the suction stroke, to the Swept Vol. of
the Piston.
Avg. Vol. Efficiency = 70 80 %
Supercharged Engine 100 %
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Performance of I.C. Engines
E. Fuel : Air Ratio :
Fuel : Air Ratio Ratio of Mass of Fuel to that of Air, in the mixture.
Rel. Fuel : Air Ratio Ratio ofActual Fuel : Air Ratio to that of
Schoichiometric Fuel : Air Ratio.
F. Sp. Fuel Consumption :
Sp. Fuel Consumption Mass of Fuel consumed per kW Power.
)./(..
.. hrkWkgPB
mcfs
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Performance of I.C. Engines
G. Thermal Efficiency :
Thermal Efficiency Ratio of Indicated Work done, to the Energy Supplied by the fuel.
..
.., .).(
VCXm
PIEfficiencyThermalIndicated
f
PIth
)/(..
sec)/(
kgMJfuelofValueCalorificVC
kgusedfuelofmassmf
..
.., .).(
VCXm
PBEfficiencyThermalBrake
f
PBth
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Performance of I.C. Engines
H. Heat Balance :
Heat Balance Indicator for Performance of the Engine.
Procedure :
1. Engine run at Const. Load condition.
2. Indicator Diagram obtained with help of the Indicator.
3. Quantity of Fuel used in given time and itsCalorific Value are measured.
4. Inlet and Outlet Temperatures for Cooling Waterare measured.
5. Inlet and Outlet Temperaturesfor Exhaust Gasesare measured.
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Performance of I.C. Engines
H. Heat Balance :
)(.. kJVCXmf
Heat Supplied by Fuel =
)(60.. kJXPIHeat equivalent of I.P. =
)(12 kJTTXCXm ww Heat taken away by Cooling Water =
mw= Mass of Cooling Water used (kg/min)
Cw= Sp. Heat of Water (kJ/kg.C)
T1= Initial Temp. of Cooling Water (C)
T2= Final Temp. of Cooling Water (C)
)(kJTTXCXm rePge Heat taken away by Exhaust Gases =me = Mass of Exhaust Gases (kg/min)
CPg = Sp. Heat of Exhaust Gases @ Const. Pr. (kJ/kg.C)
Te= Temp. of Exhaust Gases (C)
Tr= Room Temperature (C)
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Performance of I.C. Engines
Sr.
No.Input
Amount
(kJ)
Per cent
(%)Output
Amount
(kJ)
Per cent
(%)
1.Heat Supplied
by FuelA 100
Heat equivalent to
I.P.B
2. Heat taken byCooling Water
C
3.Heat taken by
Exhaust GasesD
4.Heat Unaccounted
E =A(B+C+D)
E
Total A 100 Total A 100
H. Heat Balance :
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