Introduction-Internal Combustion Engines

7/28/2019 Introduction-Internal Combustion Engines

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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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C. Arrangement of Cylinder :

1. Horizontal Engine. 2. Vertical Engine

3. V type Engine 4. Radial Engine


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D. Uses :

1. Automobile Engine. 2. Marine Engine

3. Stationary Engine 4. Portable Engine


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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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G. Method of Ignition :

2. Compression Ignition (C.I.) Engine1. Spark Ignition (S.I.) Engine.


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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




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Fig.

IN LET VALVE


CLOSE POSITION

FOUR STROKE ENGINES




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Fig.

IN LET VALVE


CLOSE POSITION

FOUR STROKE ENGINES




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Fig.

IN LET VALVE


CLOSE POSITION

FOUR STROKE ENGINES




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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




plug.


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Fig.

IN LET VALVE

CLOSE POSITION

EXHAUST VALVE

CLOSE POSITION




plug.


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Fig.

IN LET VALVE

CLOSE POSITION

EXHAUST VALVE

CLOSE POSITION




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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transfer port.

Inlet port

exhaust port

piston

Transfer port


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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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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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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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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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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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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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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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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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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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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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Documents

Introduction-Internal Combustion Engines