unit-i introduction to i.c. engines

1

Unit-I

Introduction to Internal Combustion Engines

2

Historical and Modern Development

Nomenclature

Classification and Comparison of SI and CI engines

4 stroke & 2 stroke engines

First Law analysis, Energy Balance.

Outline of the Unit-I

2

Introduction

Heat engine:

It can be defined as any engine that converts thermal energy

to mechanical work output.

Examples of heat engines include: steam engine, diesel

engine, and gasoline (petrol) engine.

On the basis of how thermal energy is being delivered to

working fluid of the heat engine, heat engine can be

classified as an internal combustion engine and external

combustion engine.

3

In an Internal combustion engine, combustion takes place

within working fluid of the engine, thus fluid gets contaminated with

combustion products.

Petrol engine is an example of internal combustion engine,

where the working fluid is a mixture of air and fuel .

In an External combustion engine, working fluid gets energy

using boilers by burning fossil fuels or any other fuel, thus the

working fluid does not come in contact with combustion products.

Steam engine is an example of external combustion

engine, where the working fluid is steam.

4

Introduction Contd

Internal combustion engines may be classified as :

Spark Ignition engines.

Compression Ignition engines.

Spark ignition engine (SI engine): An engine in which the

combustion process in each cycle is started by use of an external

spark.

Compression ignition engine (CI engine): An engine in which

the combustion process starts when the air-fuel mixture self

ignites due to high temperature in the combustion chamber

caused by high compression.

5

Introduction Contd

Spark ignition and Compression Ignition engine operate

on either a four stroke cycle or a two stroke cycle.

Four stroke cycle: It has four piston strokes over two

revolutions for each cycle.

Two stroke cycle: It has two piston strokes over one

revolution for each cycle.

We will be dealing with Spark Ignition engine and

Compression Ignition engine operating on a four stroke

as well as two stroke cycle.

6

Introduction Contd

Fig.1 : Engine components [W. W. Pulkrabek] 7

Introduction Contd

8

Introduction Contd

Fig 2: Engine Parts 9

Introduction Contd

Internal combustion Engine Components

I.C. Engine components shown in fig.1 and fig. 2 are defined as

follows:

1. Block: Body of the engine containing cylinders, made of

cast iron or Aluminium.

2. Cylinder: The circular cylinders in the engine block in which

the pistons reciprocate back and forth.

3. Head: The piece which closes the end of the cylinders,

usually containing part of the clearance volume of the

combustion chamber.

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Internal combustion Engine Components Contd

4. Combustion chamber: The end of the cylinder between the

head and the piston face where combustion occurs.

The size of combustion chamber continuously changes

from minimum volume when the piston is at TDC to a

maximum volume when the piston at BDC.

5. Crankshaft: Rotating shaft through which engine work output

is supplied to external systems.

The crankshaft is connected to the engine block with

the main bearings.

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It is rotated by the reciprocating pistons through the

connecting rods connected to the crankshaft, offset from the

axis of rotation. This offset is sometimes called crank throw

or crank radius.

6. Connecting rod: Rod connecting the piston with the rotating

crankshaft, usually made of steel or alloy forging in most

engines but may be aluminum in some small engines.

7. Piston rings: Metal rings that fit into circumferential grooves

around the piston and form a sliding surface against the cylinder

walls.

12


8. Camshaft: Rotating shaft used to push open valves at the

proper time in the engine cycle, either directly or through

mechanical or hydraulic linkage (push rods, rocker arms,

tappets) .

9. Push rods: The mechanical linkage between the camshaft and

valves on overhead valve engines with the camshaft in the

crankcase.

10.Crankcase: Part of the engine block surrounding the

crankshaft. In many engines the oil pan makes up part of the

crankcase housing.

13


11. Exhaust manifold: Piping system which carries exhaust gases

away from the engine cylinders, usually made of cast iron.

12. Intake manifold: Piping system which delivers incoming air to

the cylinders, usually made of cast metal, plastic, or composite

material.

In most SI engines, fuel is added to the air in the intake

manifold system either by fuel injectors or with a

carburetor.

The individual pipe to a single cylinder is called runner.

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13.Carburetor: A device which meters the proper amount of

fuel into the air flow by means of pressure differential.

For many decades it was the basic fuel metering system

on all automobile (and other) engines.

14.Spark plug: Electrical device used to initiate combustion

in an SI engine by creating high voltage discharge across

an electrode gap.

15


15.Exhaust System: Flow system for removing exhaust gases

from the cylinders, treating them, and exhausting them to the

surroundings.

It consists of an exhaust manifold which carries the exhaust

gases away from the engine, a thermal or catalytic converter to

reduce emissions, a muffler to reduce engine noise, and a

tailpipe to carry the exhaust gases away from the passenger

compartment.

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16.Flywheel: Rotating mass with a large moment of inertia

connected to the crank shaft of the engine.

The purpose of the flywheel is to store energy and furnish

large angular momentum that keeps the engine rotating

between power strokes and smooths out engine operation.

17.Fuel injector: A pressurized nozzle that sprays fuel into the

incoming air (SI engines )or into the cylinder (CI engines).

18.Fuel pump: Electrically or mechanically driven pump to supply

fuel from the fuel tank (reservoir) to the engine.

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19.Glow plug: Small electrical resistance heater mounted

inside the combustion chamber of many CI engines, used

to preheat the chamber enough so that combustion will

occur when first starting a cold engine.

The glow plug is turn off after the engine is started.

20.Starter: Several methods are used to start IC engines.

Most are started by use of an electric motor (starter)

geared to the engine flywheel. Energy is supplied from an

electric battery.

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Fig. 3: Engine Terminology 19

Engine Terminology

20

Engine Terminology

Fig. 4: Engine Terminology

The engine terminology are explained as follows:

1. Top Dead Center (TDC):

Position of the piston when it stops at the furthest point

away from the crankshaft.

Top because this position is at the top of the engines

(not always), and dead because the piston stops as this

point. Because in some engines TDC is not at the top of

the engines(e.g.: horizontally opposed engines, radial

engines etc.).

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Engine Terminology

Some sources call this position Head End Dead Center

(HEDC).

Some source call this point TOP Center (TC).

When the piston is at TDC, the volume in the cylinder is a

minimum called the clearance volume.

2. Bottom Dead Center (BDC):

Position of the piston when it stops at the point closest to the

crankshaft.

Some sources call this Crank End Dead Center (CEDC)

because it is not always at the bottom of the engine. Some

source call this point Bottom Center (BC).

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Engine Terminology

3. Stroke: Distance traveled by the piston from one extreme position

to the other : TDC to BDC or BDC to TDC.

4. Bore: It is defined as cylinder diameter or piston face diameter;

piston face diameter is same as cylinder diameter( minus small

clearance).

5. Swept volume/Displacement volume

Volume displaced by the piston as it travels through one

stroke.

Swept volume is defined as stroke times bore.

Displacement can be given for one cylinder or entire engine

(one cylinder times number of cylinders).

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Engine Terminology

6. Clearance volume

It is the minimum volume of the cylinder available for the

charge (air or air fuel mixture) when the piston reaches at

its outermost point (top dead center or outer dead center)

during compression stroke of the cycle.

Minimum volume of combustion chamber with piston at

TDC.

24

Engine Terminology

7. Compression ratio

The ratio of total volume to clearance volume of the cylinder

is the compression ratio of the engine.

Typically compression ratio for SI engines varies form 8 to

12 and for CI engines it varies from 12 to 24.

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Engine Terminology

SI Engine & Ideal Otto Cycle

We will be dealing with four stroke SI engine.

The following figure shows the PV diagram of

Ideal Otto cycle.

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Fig. 4: Suction stroke 30

Working Principle of S.I. Engine

1. Suction/Intake Stroke:

Intake of air fuel mixture in cylinder through intake manifold.

The piston travel from TDC to BDC with the intake valve

open and exhaust valve closed.

This creates an increasing volume in the combustion

chamber, which in turns creates a vacuum.

31

Working Principle of S.I. Engine Contd

The resulting pressure differential through the intake system

from atmospheric pressure on the outside to the vacuum on

the inside causes air to be pushed into the cylinder.

As the air passes through the intake system fuel is added to it

in the desired amount by means of fuel injectors or a

carburetor.

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Fig.5: Compression Stroke 33


2. Compression stroke:

When the piston reaches BDC, the intake valve closes and the

piston travels back to TDC with all valves closed.

This compresses air fuel mixture, raising both the pressure

and temperature in the cylinder.

Near the end of the compression stroke the spark plug is

fired and the combustion is initiated.

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Combustion of the air-fuel mixture occurs in a very short but finite

length of time with the piston near TDC (i.e., nearly constant volume

combustion).

It starts near the end of the compression stroke slightly before

TDC and lasts into the power stroke slightly after TDC.

Combustion changes the composition of the gas mixture to

that of exhaust products and increases the temperature in the

cylinder to a high value.

This in turn increases the pressure in the cylinder to a high

value.

35


Fig. 6: Combustion followed by Expansion stroke. 36


3. Expansion stroke/Power stroke

With all valves closed the high pressure created by the combustion

process pushes the piston away from the TDC.

This is the stroke which produces work output of the engine

cycle.

As the piston travels from TDC to BDC, cylinder volume is

increased, causing pressure and temperature to drop.

37


Fig.7: Exhaust blowdown followed by Exhaust stroke 38


Exhaust Blowdown:

Late in the power stroke, the exhaust valve is opened and exhaust

blowdown occurs.

Pressure and temperature in the cylinder are still high relative

to the surroundings at this point, and a pressure differential is

created through the exhaust system which is open to

atmospheric pressure.

This pressure differential causes much of the hot exhaust gas

to be pushed out of the cylinder and through the exhaust

system when the piston is near BDC.

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This exhaust gas carries away a high amount of enthalpy,

which lowers the cycle thermal efficiency.

Opening the exhaust valve before BDC reduces the work

obtained but is required because of the finite time needed for

exhaust blowdown.

40


4. Exhaust stroke

By the time piston reaches BDC, exhaust blowdown is complete, but

the cylinder is still full of exhaust gases at approximately

atmospheric pressure.

With the exhaust valve remaining open, the piston travels from

BDC to TDC in the exhaust stroke.

This pushes most of the remaining exhaust gases out of the

cylinder into the exhaust system at about atmospheric

pressure, leaving only that trapped in the clearance volume

when the piston reaches TDC.

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Near the end of the exhaust stroke before TDC, the intake

valve starts to open, so that it is fully open by TDC when the

new intake stroke starts the next cycle.

Near TDC the exhaust valve starts to close and finally is fully

closed sometime after TDC.

This period when both the intake valve and exhaust valve

are open is called valve overlap, it can be clearly seen in

valve timing chart given below.

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43


Compression Ignition Engine

We will deal with Compression Ignition engine.

The ideal diesel cycle PV diagram is shown in following

fig. 8.

44

Fig.8: P-V Diagram 45

Ideal diesel cycle

Fig.9: Four strokes of Ideal Diesel Cycle. 46

Working Principle of C.I. Engine

Fig.10: Suction stroke 47

Working Principle of C.I. Engine Contd

Fig.11: Compression stroke 48


1. Intake/Suction Stroke

The same as the intake stroke in an SI engine with one major

difference : no fuel is added to the incoming air, refer fig. 10.

2. Compression Stroke

The same as in an SI engine except that only air is compressed and

compression is to higher pressures and temperature, refer fig.11.

Late in the compression stroke fuel is injected directly into the

combustion chamber, where it mixes with very hot air.

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This causes the fuel to evaporate and self ignite, causing

combustion to start.

Combustion is fully developed by TDC and continues at about

constant pressure until fuel injection is complete and the piston

starts towards BDC, refer fig.12.

50


Fig.12:Fuel injection and combustion followed by Expansion stroke 51


Fig.13: Exhaust blowdown followed by exhaust stroke 52


3. Expansion/Power stroke

The power stroke continues as combustion ends and the

piston travels towards BDC, refer fig. 12.

Exhaust blowdown same as with an SI engine.

4. Exhaust stroke

Same as with an SI engine, refer fig. 13.

53


3

54


3

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Comparison of SI and CI Engines

Description SI Engine CI Engine

Basic Cycle It works on Otto cycle or constant

volume heat addition cycle.

It works on Diesel cycle or constant

pressure heat cycle.

Fuel Gasoline, a highly volatile fuel. Self-

ignition temperature is high.

Diesel oil, a non-volatile fuel. Self-

ignition temperature is comparatively

low.

Introduction of fuel A mixture of air and fuel is

introduced during the piston's

suction stroke. A carburetor and an

ignition system are necessary.

Modern engines have gasoline

injection.

Fuel is injected directly into the

combustion chamber at high pressure at

the end of the compression stroke. A fuel

pump and injector are necessary.

Load control Throttle controls the quantity of fuel-

air mixture to control the load.

The quantity of fuel is regulated to

control the load. Air quantity is not

controlled.

3

Description SI Engine CI Engine

Ignition Requires an ignition system with spark

plug in the cylinder head of the engine.

The voltage is provided to the spark plug

either from the battery or from the

magneto.

When air is compressed to high pressures,

its temperature also increases beyond the

self-ignition temperature of the fuel. Hence,

the ignition of fuel occurs due to

compression of the air and there is no need

for spark plugs.

Compression ratio 6 to 10. Upper limit is fixed by antiknock

quality of the fuel.

16 to 20. Upper limit is fixed by weight

increase of the engine.

Thermal efficiency The lower compression ratio reduces their

potential to achieve higher thermal

efficiency.

The value of compression ratio is higher;

hence these engines have the potential to

achieve higher thermal efficiency.

Speed SI engines are lightweight, and the fuel is

homogeneously burned, hence achieving

very high speeds.

CI engines are heavier and the fuel is

burned heterogeneously, hence producing

lower speeds.

Weight Light weight Heavier in comparison to SI engines.

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Comparison of SI and CI Engines

Two Stroke

Internal Combustion Engines

57

58

Two Stroke Engines

Everything a 4 stroke

engine does in 2

revolutions a 2 stroke

engine does in 1

revolution of the

crankshaft.

Applications of Two Stroke Engines

This type of engine is commonly found in applications such

as:

1. Lawn and garden equipment

2. Old motorbikes

3. Diesel engines in ships etc.

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Introduction to the Two Stroke Engine

Two stroke engines have:

Simplified construction (no valves).

Fire once every revolution for a significant power

boost.

Great power to weight ratio.

60

61

Two Stroke Engines part names

Piston

Cylinder

Crankshaft Connecting

Rod

Still uses a flywheel

(not shown) Combustion

chamber

Intake port

Exhaust port

Reed valve

Transfer port

Crankcase

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Working Principle of Two Stroke Engine

The two stroke engine ignites every revolution of the

crankshaft.

360 degrees rotation of crankshaft completes the cycle.

In a two stroke engine we have only:

Compression

Combustion Thus, Two Strokes.

63

Working Principle of Two Stroke Engine Contd

64


65

TDC BDC

Piston moves from

BDC to TDC

Air/Fuel/Oil mixture is sucked into crankcase

Reed Valve

Is sucked

open


1. Intake stroke

The fuel/air mixture is first

drawn into the crankcase by

the vacuum created during the

upward stroke of the piston

through the reed valve.

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67

Two Stroke

Piston BDC to TDC.

A/F/O mixture sucked into crankcase.

Four Stroke

Piston TDC to BDC.

A/F mixture sucked into cylinder.


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TDC BDC

Piston gets to

TDC

Air/Fuel/Oil mixture is now trapped in crankcase.

Reed Valve

Shuts


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TDC BDC

Piston moves back

To BDC

Air/Fuel/Oil mixture is now pressurized in crankcase called primary compression.

Reed Valve

sealing


70

Crankcase Compression

Is only a few pounds of pressure per square inch (psi) [very

weak].

Cylinder compression in a four stroke engine was several psi

[very strong].

The crankcase in a two stroke engine has to be very small so

we can build some pressure when the piston is moving to

BDC.


71

Crankcase

Compression

What is going to happen when the top of piston gets to here?


72

A/F/O mixture squirts into cylinder because of crankcase compression


73

Piston reaches BDC and Cylinder fills with A/F/O mixture.


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Another A/F/O mixture is sucked into crankcase while First one is compressed in cylinder.


2. Compression stroke

The piston then rises, driven by

flywheel momentum, and

compresses the fuel

mixture. (At the same time,

another intake stroke is

happening beneath the piston).

75


76

TDC BDC

Piston gets to

TDC

Air/Fuel/Oil mixture is ignited in cylinder.

Piston is pounded

Down the cylinder


3. Power stroke

At the top of the stroke the

spark plug ignites the fuel-air

mixture.

The burning fuel expands,

driving the piston downward.

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4. Exhaust/Transfer stroke

Toward the end of the stroke, the

piston exposes the intake port,

allowing the compressed fuel/air

mixture in the crankcase to escape

around the piston into the main

cylinder.

This expels the exhaust gasses out

the exhaust port, usually located on

the opposite side of the cylinder.

78


79


1. Two-stroke engines do not have valves, which simplifies their

construction and lowers their weight.

2. Hence less maintenance problems.

3. Two-stroke engines fire once every revolution, while four-

stroke engines fire once every other revolution. This gives

two-stroke engines a significant power boost which is twice

theoretically as compared to four stroke engines.

4. The work required to overcome the friction of the exhaust and

suction strokes is saved.

80

Advantage of Two Stroke Engines

Advantage of Two Stroke Engines

5. More uniform turning moment is obtained on the crankshaft

and hence a lighter flywheel is required.

6. For the same output, two strokes engines occupy lesser

space.

7. In case of two-stroke engines, burnt gases do not remain in

clearance space as in case of four strokes engines.

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Disadvantages of a Two Stroke Engines

1. The engines do not last as long as four stroke does due to

poor lubrication.

2. You have to mix two cycle engine oil with gasoline.

3. The engines do not use fuel efficiently.

4. These engines produce a lot of pollution.

5. With heavy loads, two stroke engines get heated due to

excessive heat produced.

6. Consumption of lubrication is more in two stroke engines.

7. Effective compression is less in two stroke engines. 82

3

83

Comparison of Four Stroke and Two Stroke Engines

Four Stroke Engine Two Stroke Engines

One power stroke for every two

revolutions of the crankshaft.

One power stroke for each revolution of

the crankshaft.

There are inlet and exhaust valves in

the engine.

There are inlet and exhaust ports instead

of valves.

Crankcase is not fully closed and air

tight.

Crankcase is fully closed and air tight.

Top of the piston compresses the

charge.

Both sides of the piston compress the

Charge.

Size of the flywheel is comparatively

larger.

Size of the flywheel is comparatively

smaller.

3

Four Stroke Engine Two Stroke Engines

Thermal efficiency is higher. Part

load efficiency is better.

Thermal efficiency is comparatively

low. Part load efficiency is poor.

For a given weight, engine would

give only half the power of two

stroke.

For same weight, two stroke engine

gives twice the power that of four

stroke engine.

Two stroke engines are great for the power to weight ratio and

their simple design, however, due to there pollution concerns

these engines will be harder to find.

84

Comparison of Four Stroke and Two Stroke Engines

85 Fig. 15. Energy flow through reciprocating engine

The First Law Analysis of Engine Cycle

86 Fig. 16. Reciprocating engine as an open system


87

The fuel is fed into the combustion chamber where it burns in

air converting chemical energy of fuel into heat.

Some part of heat is lost through the engine exhaust, to the

coolant and due to radiation.

The heat energy which is converted to power on the top of the

piston is called the indicated power ip.

A part of energy is lost due to bearing friction, pumping losses

etc. and a part of energy available is utilized in driving the

auxiliary devices like feed pump, valve mechanisms, ignition

system etc.



88

The sum of all these losses, expressed in units of power

is termed as frictional power fp.

The remaining energy is the useful mechanical energy

and is termed as the brake power, bp.

Internal Combustion engine fundamentals by John B. Heywood

Engineering Fundamentals of the Internal combustion Engine by

Willard W. Pulkrabek

Internal combustion engines by V Ganesan

http://content.answers.com/main/content/img/BritannicaConcise/i

mages/72180.jpg

http://www.howcarswork.co.uk/modules/articles/index.php?cat_id

=1

http://www.howcarswork.co.uk/modules/content/index.php?id=23

References

89

http://www.small-engines.com/4cycleth.html

http://en.wikipedia.org/wiki/Engine_displacement

http://en.wikipedia.org/wiki/Stroke_(engine)

http://en.wikipedia.org/wiki/Internal_combustion_engine

http://www.howstuffworks.com/diesel-two-stroke.htm

http://www.mustangmonthly.com/techarticles/97278_how_engines

_work/index.html

http://www.kruse-ltc.com/Otto/otto_cycle.php

http://www.kruse-ltc.com/Diesel/diesel_cycle.php

References

90

http://www.answers.com/topic/internal-combustion-engine

http://www.britannica.com/EBchecked/topic/162716/diesel-engine

http://content.answers.com/main/content/img/BritannicaConcise/i

mages/72180.jpg

http://www.howcarswork.co.uk/modules/articles/index.php?cat_id

=1

References

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Thank You

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unit-i introduction to i.c. engines

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