Laboratory Manual for Basic Mechanical Engineering by Dr. Anil Singh Yadav & Prof. Sachin Kumar Nikam

Four Stroke Engine Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

4. Exhaust Stroke During this stroke, the piston moves from the bottom
dead centre to the top dead centre, exhaust valve opens and the inlet valve
remains closed [Fig 7.2 (d)]. Burnt gases of the previous stroke are
expelled out from the cylinder by upward movement of the piston. At the
end of the power stroke the pressure of the gas is about 4-5 bar which is
higher than the exhaust manifold pressure.
At the end of the exhaust stroke the inlet valve opens, and the cylinder is
ready to receive the fresh charge to begin a new cycle.
It is obvious from the above discussions that the crankshaft makes two
revolutions during the four strokes, and in these four strokes, there is only
one power stroke. This means, that for every two revelations of the
crankshaft, there is only one power strike.

Theoretical indicator diagram (P-V Diagram)
In the above operation, the following assumption were made-

(i) Suction and exhaust take place at atmospheric pressure.
(ii) Suction and exhaust take place at 180o rotation of crank.
(iii) Compression and expansion also take place at 180o rotation of

crank.
(iv) Compression and expansion are isentropic.
(v) The combustion takes place instantaneously at constant

volume at the end of compression stroke.
(vi) Pressure suddenly falls to the atmospheric pressure at end of

expansion stroke.

With these assumptions the working of 4-Stroke Otto cycle engine on p-V
diagram as shown in Fig. 7.3.
Process 0-1 (Suction Stroke): In this process fresh air and fuel mixture
i.e., charge is passed inside the cylinder. The piston moves from top dead
centre to bottom dead centre. This comprises the first stroke of the
engine.

Figure 7.3 Theoretical indicator diagram (P-V Diagram) of a Four
Stroke Petrol Engine

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Process 1-2 (Compression Stroke): In this process the charge is
compressed and piston is moved to top dead centre. This comprises the
second stroke of the engine.
Process 2-3 (Instantaneous-Combustion): In this process spark plug
ignites the spark and the fuel is burnt. This process is of constant volume
and increase in pressure.
Process 3-4 (Expansion Stroke): In this process the burnt fuel expands
itself and exerts pressure on the piston. The piston moves from top dead
centre to bottom dead centre. This comprises the third stroke of the
engine and the power stroke.
Process 4-1(Sudden Fall in Pressure): In this process the burnt gas is
exhausted out and the pressure decreases with constant volume.
Process 1-0 (Exhaust Stroke): In this process the burnt gas is completely
moved out of the cylinder by the action of piston. Piston moves from
bottom dead centre to top dead centre. This comprises the fourth stroke of
the engine.

Actual indicator diagram
In the theoretical indicator diagram, all the ideal conditions are assumed
but in practice, the actual conditions differ from the ideal as described
below (Fig. 7.4.).
(i) The suction of mixture in the cylinder is possible only if the pressure
inside the cylinder is below atmospheric pressure.
(ii) The burnt gases can be pushed out into the atmosphere only if
thepressure of the exhaust gases is above atmospheric pressure.
(iii) The combustion and expansion do not follow the isentropic law, as
there will be heat exchange during process.
(iv) Sudden pressure rise is not possible after ignition as combustion take
sometimes for completion and actual pressure rise is less than theoretical
considered. The pressure increase takes place through some crank
rotation, or increase in volume.
(v) Sudden pressure release after the opening of expansion valve is not
possible and also takes place through some crank rotation.

Figure 7.4 Actual indicator diagram (P-V Diagram) of a Four Stroke
Petrol Engine

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The suction stroke by the line 0-1, which lies below the atmospheric
pressure line. It is this pressure difference, which makes the fuel air
mixture to flow into the engine cylinder. The inlet valve offers some
resistance to the incoming charge. That is why, the charge cannot enter
suddenly into the engine cylinder. As a result of this, pressure inside the
cylinder remains somewhat below the atmospheric pressure during the
suction stroke. The compression stroke is shown by the line 1-2, which
shows that the inlet valve close (I.V.C) a little beyond 1 (i.e. BDC). At
the end of this stroke, there is an increase in the pressure inside the engine
cylinder. Shortly before the end of compression stroke (i.e. TDC). the
charge is ignited (IGN) with the help of spark plug as shown in the figure.
The sparking suddenly increases pressure and temperature of the products
of combustion. But the valve practically, remains constant as shown by
the line 2-3. The expansion stroke is shown by the line 3-4, in which the
exit valve opens (EVO) a little before 4 (i.e. B.D.C). Now the burnt gases
are exhausted into the atmosphere through the exit valve. The exhaust
stroke is shown by the line 4-0, which lies above the atmospheric pressure
line. It is this pressure difference, which makes the burnt gases to flow
out of the engine cylinder. As a result of this. pressure inside the cylinder
remains somewhat above the atmospheric pressure line during the exhaust
stroke.
Valve Timing Diagram
Theoretically, in a four-stroke cycle engine, the inlet and exhaust valves
open and close at dead centres as shown in Fig. 7.5.
A typical actual valve-timing diagram for a four-stroke petrol engine is
shown in Fig. 7.6. The angular positions in terms of crank angle with
respect to TDC and BDC position of piston are quoted on the diagram.
When the inlet valve and exhaust valve remain open simultaneously, it is
called a valve overlap.

Figure 7.5 Theoretical Valve-Timing Diagram for Four-stroke cycle
engine

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Figure 7.6 Actual Valve-Timing Diagram for Four-Stroke Petrol
Engine

Applications
These engines are mostly used on automobiles, motor cycles, cars, buses,
trucks, aeroplanes, small pumping sets, mobile electric generators, etc.
Nowadays, the four-stroke petrol engines have been replaced by four-
stroke Diesel engines for most applications.
FOUR STROKE COMPRESSION IGNITION (CI)/DIESEL
ENGINE:
Constructional Details
A four-stroke Diesel engine (compression ignition engine) contains a fuel
injector, fuel pump, cylinder, cylinder head, inlet and exhaust valves,
piston attached with piston rings, connecting rod, crank shaft, cams,
camshaft, etc., as shown in Fig. 7.7. In a four-stroke engine, valves are
used instead of ports. There are inlet and exhaust valves. These valves are
operated by cams attached on a separate shaft, called a cam shaft. It is
rotated at half the speed of a crank shaft.
Operations
The travel of the piston from one dead centre to another is called piston
stroke and a four-stroke cycle consists of four strokes as suction,
compression, expansion and exhaust strokes. The sequence of four
strokes is as follows:

1. Intake or suction stroke,
2. Compression stroke,
3. Working or expansion or power stroke and,
4. Exhaust stroke.

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Figure 7.7 Four Stroke Diesel Engine
1. Suction Stroke Suppose that the piston is very near to the top dead
centre position (T.D.C.) [Fig 7.8 (a)]. During suction stroke the inlet
valve is opened and the discharge valve is closed and the piston moves
down (i.e. outward) due to rotation of the crankshaft either getting energy
from the flywheel or a motor starter. As the piston move, vacuum is
created between the piston and cylinder and the pressure in the cylinder
drops below atmospheric pressure. The piston moves from the top dead
centre to the bottom dead centre, only air rushes inside the cylinder
through inlet manifold and valve which is cam operated. The suction
process continues till the piston reaches the bottom dead centre position
(B.D.C.).

Figure 7.8 Operations of a Four Stroke Diesel Engine
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2. Compression Stroke During a compression stroke [Fig 7.8 (b)], both
the valves are closed and the piston moves from bottom to top dead centre
position. The air is compressed up to a compression ratio which depends
upon the type and need of the engine. Since both the valves are closed
and the piston moves inwards, there is a reduction in volume of the air
which results in an increase of pressure and temperate of the cylinder
contents. For Diesel engine the compression ratio is 16 to 22 and the
pressure and temperature at the end of compression are 28 to 59 bar, and
600°C to 700°C respectively.

3. Expansion Stroke During power stroke [Fig 7.8 (c)], both the valves
are closed. The power stroke includes combustion of fuel and expansion
of the products of combustion. The combustion starts at the end of the
compression stroke when the piston is approaching the T.D.C. position.
In C.I engine, one or more jets of fuel, compressed to a pressure of 105-
210 bar by an injector pump are injected into the combustion chamber by
a fuel nozzle at the end of the compression stroke. The injected fuel is
vaporized in the combustion chamber, when the fuel vapor is raised to
self-ignition temperature, the combustion then starts automatically and
the pressure of the gas however remains constant during combustion.
In modern diesel engine (dual cycle), the injected fuel is vaporized in the
combustion chamber, when the fuel vapor is raised to self-ignition
temperature, the combustion then starts automatically and there is a
sudden rise of pressure at approximately constant volume. However, the
latter part of combustion occurs at almost constant pressure.

The high pressure and the high temperature of the products of
combustion, thus obtained, pushes the piston outward from T.D.C. to
B.D.C. position for expansion stroke. This reciprocating motion of the
piston is converted into rotary motion by the crankshaft, connecting rod
and crank mechanism.

4. Exhaust Stroke During this stroke, the piston moves from the bottom
dead centre to the top dead centre, exhaust valve opens and the inlet valve
remains closed [Fig 7.8 (d)]. Burnt gases of the previous stroke are
expelled out from the cylinder by upward movement of the piston. At the
end of the power stroke the pressure of the gas is about 4-5 bar which is
higher than the exhaust manifold pressure.

At the end of the exhaust stroke the inlet valve opens, and the cylinder is
ready to receive the fresh air to begin a new cycle.
It is obvious from the above discussions that the crankshaft makes two
revolutions during the four strokes, and in these four strokes, there is only
one power stroke. This means, that for every two revelations of the
crankshaft, there is only one power strike.

Theoretical Indicator Diagram (P-V Diagram)
The working of 4-Stroke Diesel cycle engine on p-V diagram as shown in
Fig. 7.9.

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Process 0-1: In this process fresh air is passed inside the cylinder. The
piston moves from top dead centre to bottom dead centre. This comprises
the first stroke of the engine.
Process 1-2: In this process the air is compressed and piston is moved to
top dead center. This comprises the second stroke of the engine.
Process 2-3: In this process diesel is pumped into the cylinder through
fuel pump and the fuel is burnt. This process is of constant volume and
increase in pressure.
Process 3-4: In this process the burnt fuel expands itself and exerts
pressure on the piston. The piston moves from top dead centre to bottom
dead centre. This comprises the third stroke of the engine and the power
stroke.
Process 4-1: In this process the burnt gas is exhausted out and the
pressure decreases with constant volume.
Process 1-0: In this process the burnt gas is completely moved out of the
cylinder by the action of piston. Piston moves from bottom dead centre to
top dead centre. This comprises the fourth stroke of the engine.

Figure 7.9 P-V Diagram of a Four Stroke Diesel Engine
Actual indicator diagram
In the theoretical indicator diagram, all the ideal conditions are assumed
but in practice, the actual conditions differ from the ideal as described
below (Fig. 7.10.).
The suction stroke is shown by the line 0-1 which lies below the
atmospheric pressure line. It is this pressure difference, which makes the
fresh air to flow into the engine cylinder.

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The inlet valve offers some resistance to the incoming air. That is why,
the air cannot enter suddenly into the engine cylinder. As a result of this
pressure inside the cylinder remains somewhat below the atmospheric
pressure during the suction stroke. The compression stroke is shown by
the line 1-2 which shows that the inlet valve close (IVC) a little beyond 3
(i.e. BDS). At the end of this stroke, there is an increase of pressure inside
the engine cylinder shortly before the end of compression stroke (i.e.
TDC) the fuel valve opens (FVO) and the fuel is injected into engine
cylinder. The fuel is ignited by high temperature of the compressed air.
The ignition suddenly increases volume and temperature of the products
of combustion. But the pressure practically remains constant as shown by
the line 2-3. The expansion stroke is shown by the line 3-4 in which the
exist valve opens a little before 4 (i.e. BDC). Now the burnt gases are
exhausted into the atmosphere through the exit valve. The exhaust stroke
is shown by the line 4-0, which lies above the atmospheric pressure line.
It is this pressure difference which makes the burnt gases to flow out of
the engine cylinder. The exist valve offers some resistance to the
outgoing burnt gases. That is why the burnt gases cannot escape suddenly
from the engine cylinder. As a result of this pressure inside the cylinder
remains somewhat above the atmospheric pressure line during the exhaust
stroke.

Figure 7.10 Actual indicator diagram (P-V Diagram) of a Four Stroke
Diesel Engine

Valve Timing Diagram
Theoretically, in a four-stroke cycle engine, the inlet and exhaust valves
open and close at dead centres as shown in Fig. 7.11.
A typical actual valve-timing diagram for a four-stroke diesel engine is
shown in Fig. 7.12. The angular positions in terms of crank angle with
respect to TDC and BDC position of piston are quoted on the diagram.
When the inlet valve and exhaust valve remain open simultaneously, it is
called a valve overlap.

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Figure 7.11 Theoretical Valve-Timing Diagram for Four-stroke Diesel
cycle engine

Figure 7.12 Actual Valve-Timing Diagram for Four-Stroke Diesel
Engine

Applications
These engines are mostly used in automobiles, small pumping sets for
agriculture, construction machinery, air compressor and drilling jigs,
tractors, jeeps, cars, taxies, buses, trucks, diesel-electric locomotives,
small power plants, mobile electric generating plants, boats and ships,
power saws, bulldozers, tanks, etc.

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Questions for Viva-Voce Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

REVIEW QUESTIONS
Q1. Draw the neat sketch of sequence of operations of four stroke

petrol engine.
A1.

Q2. Draw the neat sketch of sequence of operations of four stroke
diesel engine.

A2.

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Questions for Viva-Voce Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

Q3. Draw the neat sketch of theoretical indicator diagram (P-V
diagram) of four stroke petrol engine.

A3.

Q4. Draw the neat sketch of actual indicator diagram (P-V diagram) of
four stroke petrol engine.

A4.

Q5. Draw the neat sketch of theoretical indicator diagram (P-V
diagram) of four stroke diesel engine.

A5.

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Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

Q6. Draw the neat sketch of actual indicator diagram (P-V diagram) of
four stroke diesel engine.

A6.

Questions for Viva-Voce Q7. What is the function of piston rings?
Q8. What is the main basic difference between petrol and diesel
engine?
Q9. What are the functions of flywheel?
Q10. What is indicator diagram?

WRITE YOUR ANSWERS HERE

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Questions for Viva-Voce Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

MULTIPLE CHOICE QUESTIONS (✔)

Q1. Which of the following is not used in four stroke compression-
ignition (CI) engines?
a) Fuel pump
b) Spark plug
c) Fuel injector
d) Inlet and outlet Valves

Q2. In compression ignition (CI) engine, the compression ratio is
a) Cylinder volume/ Clearance volume
b) Swept Volume/ Cylinder Volume
c) Clearance volume/ Cylinder volume
d) Cylinder volume/ Swept volume

Q3. In four stroke engine there is one power stroke in …….. of
crankshaft rotation.
a) 180°
b) 360°
c) 540°
d) 720°

Q4. The ascending order of strokes in four stroke engine is
a) Suction stroke–Expansion stroke–Compression stroke–
Exhaust stroke
b) Suction stroke–Compression stroke–Expansion stroke–
Exhaust stroke
c) Suction stroke–Expansion stroke–Exhaust stroke–
Compression stroke
d) Suction stroke–Compression stroke–Exhaust stroke–
Expansion stroke

Q5. In four stroke engine which of the following is also known as
power stroke
a) Suction stroke
b) Compression stroke
c) Expansion stroke
d) Exhaust stroke

Q6. In a four-stroke cycle petrol engine, the compression
a) Starts at 40° after bottom dead centre and ends at 30° before
top dead centre
b) Starts at 40° before bottom dead centre and ends at 30° after
bottom dead centre
c) Starts at bottom dead centre and ends at top dead centre
d) May start and end anywhere

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Q7. In a four-stroke cycle diesel engine, the exhaust valve
a) Opens at 30° before bottom dead centre and closes at 10° after
top dead centre
b) Opens at 30° after bottom dead centre and closes at 10° before
top dead centre
c) Opens at bottom dead centre and closes at top dead centre
d) May open and close anywhere

Questions for Viva-Voce Q8. Which of the following is the lightest and most volatile liquid
fuel?
a) Diesel
b) Kerosene
c) Fuel oil
d) Gasoline

Q9. Which of the following medium is compressed in a Diesel engine
cylinder?
a) Air alone
b) Air and fuel
c) Air and lub oil
d) Fuel alone

Q10. Which of the following is not an internnal combustion engine?
a) 2-stroke petrol engine
b) 4-stroke petrol engine
c) Diesel engine
d) Steam turbine

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Two Stroke Engine

Experiment# 8

Two Stroke Engine Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

8 EXPERIMENT NO. 8

Two Stroke Engine

AIM
To study of two stroke engines.

APPARATUS
Models of two stroke petrol and diesel engine.

THEORY
In a two-stroke engine, the working cycle is completed in two stroke of
the piston or one revolution of crankshaft, whereas in a 4-strokes engine it
is completed in 4-strokes or two revolutions of the crankshaft. In the two–
stroke engine the intake and compression processes are completed during
the inward stroke and expansion and exhaust processes during the
outward stroke. The two stroke cycle engines can be either spark ignition
or a compression ignition engine. For spark ignition, a carburettor and a
spark plug are used while for a C.I. engine, a pump and an injector are
used. In a four-stroke engine, the admission of fresh charge and the
removed of exhaust gases are controlled by intake and exhaust valves,
generally located in the cylinder head. But in two –stroke engine these
operations are performed by ports located in the cylinder walls, which are
opened or closed by the piston.
The term scavenging refers to the process of removal of the burnt gases
from the cylinders at the end of exhaust stroke, by deflecting the fresh
charge across the cylinder. The purpose of scavenging is to reduce the
dilution of the fresh charge with the residual gases.
The projection on the piston is called the deflector. The function of the
deflector is to deflect the fresh charge to the top before flowing to the
exhaust post.

TWO STROKE PETROL ENGINE

Constructional Details
A two-stroke petrol engine is shown in Fig. 8.1. It consists of a cylinder,
cylinder head, piston, piston rings, connecting rod, crank, crank case,
crank shaft, etc., The charge (air-fuel mixture) is prepared outside the
cylinder in the carburettor.

In the simplest type of two-stroke engine, the ports are provided for
charge inlet and exhaust outlet, which are uncovered and closed by the
moving piston. The suction port S with a reed-type valve is used for
induction of charge into the crank case, the transfer port T is used for
transfer of charge from the crank case to the cylinder and the exhaust port
E serves the purpose of discharging the burnt gases from the cylinder,
The spark plug is located in the cylinder head.

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Figure 8.1 Crankcase Scavenged Petrol Engine.
Operation
In a two-stroke engine, the inlet, transfer and exhaust ports are covered
and uncovered by a moving piston. The following operations take place
in a two-stroke or in one cycle of the engine.
1. Charge Transfer and Scavenging When the piston is nearer to the
crank case (bottom dead center), the transfer port and exhaust port are
uncovered by the piston as shown in Fig. 8.2(a). A mixture of air and fuel
as a charge, slightly compressed in the crank case, enters through the
transfer port T and drives out the burnt gases of the previous cycle
through the exhaust port E.
In a two-stroke engine, the piston top is made deflected. Therefore, the
incoming charge is directed upward, and aids in sweeping of the burnt
gases out of the cylinder. This operation is known as scavenging (a gas-
exchange process).
As the piston moves upward, the fresh charge passes into the cylinder for
1/6th of the revolution and the exhaust port remains open a little longer
than the transfer port.
2. Compression and Suction As the piston moves upward, both the
transfer port and exhaust port are covered by the piston and the charge
trapped in the cylinder is compressed by the piston's upward movement
as shown in Fig. 8.2(b). At the same time, a partial vacuum is created into
the crank case, the suction port S opens by moving the crank and the
fresh charge enters the crank case [Fig. 8.2(c)].
3. Combustion When the piston reaches at its end of stroke nearer to the
cylinder head or at the top dead center, a high-intensity spark from the
spark plug ignites the charge and initiates the combustion in the cylinder.
The burning of the charge generates the pressure in the cylinder.

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Figure 8.2 Operations of a two-stroke petrol engine.
4. Power and Exhaust The burning gases apply pressure on the top of the
piston, and the piston is forced downward as a result of pressure
generated.
As the piston descends through about 80% of the expansion stroke, the
exhaust port E is uncovered by the piston, and the combustion gases leave
the cylinder by pressure difference and at the same time, the underside of
the piston causes compression of charge taken into crank case as shown
in Fig. 8.2(d).
5. Charging The slightly compressed charge in the crank case passes
through the transfer port and enters the cylinder as soon as it is uncovered
by the descending piston and when it approaches the bottom dead center,
the cycle is completed.
Theoretical indicator diagram
The theoretical indicator diagram for a two-stroke cycle petrol engine is
shown in Fig. 8.3.
Process 1-2: isentropic compression of the charge in the cylinder. The
charge inside the cylinder is compressed and the piston moves from
bottom dead center to top dead center. This comprises first stroke of the
engine.
Process 2-3: combustion at constant volume. This process takes place as
process of constant volume and increase in pressure. In this process spark
is ignited by spark plug inside the cylinder and fuel is burnt.

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Figure 8.3 Theoretical Indicator Diagram (P-V Diagram) of Two
Stroke Petrol Engine.

Process 3-4: isentropic expansion. The burnt fuel exerts pressure and
moves the piston to bottom dead center. The gas expands in this process.
This comprises the second stroke and the power stroke of the engine.
Process 4-1: Sudden release of burnt gases to the atmosphere as the
exhaust port opens. This process takes place as process of constant
volume and decrease in pressure. In this process, the burnt gas is
exhausted outside the cylinder.
Process 1-6& 6-1: Sweeping out the exhaust gases to the atmosphere at
the atmospheric pressure and charging the cylinder with charge.
Process 5-6 & 6-5: Charging the cylinder with charge through the
transfer ports and scavenging through exhaust ports.
The point “5” indicates the opening of the inlet or transfer ports.
Also, the compression of the charge starts from point “1” instead of point
“6” Therefore, the effective stroke (compression stroke) is less than the
actual stroke.

Actual indicator diagram
The actual indicator diagram for a two-stroke cycle petrol engine is
shown in Fig. 8.4. The suction is shown by the line 1-2-3 (from the
instant transfer port opens (TPO) and transfer port closes). During the
suction stage the exhaust port is also open. In the first half of suction
stage, the volume of fuel-air mixture and burnt gases increases. This
happens as the piston moves from 1 to 2. In the second half of suction
stage, the volume of charge and burnt gases decreases. This happens as
the piston moves upwards from 2 to 3. A little beyond 3, the exhaust port
closes (EPC) at 4. Now the charge inside the engine cylinder is
compressed which is shown by the line 4-5. At the end of compression,
there is an increase in pressure inside the engine cylinder. Shortly before
the end of compression, the charge is ignited with the help of spark plug.

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The sparking suddenly increases temperature and pressure of the product
of combustion. But the volume, practically, remains constant as shown by
the line 5-6. The expansion is shown by the line 6-7. Now the exhaust
port opens at 7, and the burnt gases are exhausted into the atmosphere
through the exhaust port. It reduces pressure. As the piston moving
towards BDC, therefore volume of burnt gases increases from 7 to 1. At 1
the transfer port opens (TPN) and suction starts.

Figure 8.4 Actual Indicator Diagram of Two Stroke Petrol Engine.
Port-Timing Diagram
A typical port-timing diagram for a two-stroke petrol engine is shown in
Fig. 8.5.

Figure 8.5 Port-Timing Diagram for a Two-Stroke Petrol Engine.
Applications
Two-stroke gasoline engines are used where simplicity and low cost are
main considerations. These engines have a little higher specific fuel
consumption. These engines are used in scooters, motor cycles, mopeds,
lawn movers, non-gear vehicles, small electric generator sets, pumping
sets and motor boats.

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TWO STROKE DIESEL ENGINE
Constructional Details
All engines using diesel as a fuel operate on the Diesel cycle. They work
similar to a petrol engine except they take in only air as charge during
suction, and fuel is injected at the end of the compression stroke. The
Diesel engines have a fuel injector instead of a spark plug in the cylinder
head as shown in Fig. 3.6. The diesel engines use a high compression
ratio in the range of 14 to 21. The temperature of intake air reaches quite
a high value at the end of compression. Therefore, the injected fuel is
self-ignited. The Diesel engines use a heterogeneous air-fuel mixture,
ratio ranging from 20 to 60.

Figure 8.6 Crankcase Scavenged Diesel Engine.
Operation
Both inlet and exhaust take place through the cylinder ports which are
covered and uncovered by the movement of piston.
1. Charge Transfer and Scavenging When the piston is nearer to the
crank case (bottom dead centre), the transfer port and exhaust port are
uncovered by the piston and the slightly compressed air enters into the
cylinder through the transfer port and helps to scavenge the remaining
burnt gases from the cylinder as shown in Fig. 8.7(a). The charge transfer
and scavenging continue till the piston completes its downward stroke
and further, it moves upward and covers the transfer port.
2. Compression and Suction After covering the transfer port, the exhaust
port is also covered by the upward moving piston. As both ports are
covered by the piston in Fig. 8.7(b), the air trapped in the cylinder is
compressed during the forward stroke of the piston. As the piston moves
towards the cylinder head, a partial vacuum is created in the crank case,
the inlet port opens and fresh air enters the crank case, Fig. 8.7(c).

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 96

Two Stroke Engine Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

Figure 8.7 Operations of a Two Stroke Diesel Engine.
3. Combustion and Power Near the end of the compression stroke, the
fuel is injected at a very high pressure with the help of the fuel pump and
injector. The injected fuel is self-ignited in the presence of hot air and
combustion starts. The piston is forced downward by very high pressure
of burnt gases and power is transmitted to the crank shaft.
4. Exhaust Near the end of the power stroke, the exhaust port is
uncovered first by the piston and the products of combustion start leaving
the cylinder as a result of pressure difference as shown in Fig. 8.7(d).
5. Charging The slightly compressed air in the crank case passes through
the transfer port and enters the cylinder as soon as it is uncovered by the
descending piston and when it approaches the bottom dead centres, the
cycle is completed.
Theoretical indicator diagram
The theoretical indicator diagram for a two-stroke cycle diesel engine is
shown in Fig. 8.8.
Process 1-2: Isentropic compression of the air in the cylinder. The air
inside the cylinder is compressed and the piston moves from bottom dead
centre to top dead centre. This comprises the first stroke of the engine.
Process 2-3: Combustion at constant Pressure. In this process diesel is
pumped inside the cylinder by the help of fuel pump and fuel is burnt.
Process 3-4: Isentropic Expansion of the burnt fuel in the cylinder. The
burnt fuel exerts pressure and moves the piston to bottom dead centre.
The gas expands in this process. This comprises the second stroke and the
power stroke of the engine.

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 97

Two Stroke Engine Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

Process 4-1: Sudden release of burnt gases to the atmosphere as the
exhaust port opens. This process takes place as process of constant
volume and decrease in pressure. In this process the burnt gas is
exhausted outside the cylinder.
Process 1-6 & 6-1: Sweeping out the exhaust gases to the atmosphere.
Process 5-6 & 6-5: Charging the cylinder with fresh charge through the
transfer ports and scavenging action takes place.

Figure 8.8 Theoretical Indicator Diagram (P-V Diagram) of Two
Stroke Diesel Engine.

Actual indicator diagram
Fig. 8.9 shows the actual indicator diagram for a two-stroke cycle diesel
engine. The suction is shown by the line 1-2-3, from the instant transfer
ports opens and transfer port closes. During the suction stage, the exhaust
port is opens. In the first half of suction stage, the volume of air and burnt
gases increases. This happens as the piston moves from 1-2. In the second
half of the suction stage, the volume of air and burnt gases decreases.
This happens as the piston moves upwards from 2-3. A little beyond 3,
the exhaust port closes at 4. Now the air inside the engine cylinder is
compressed which is shown by the line 4-5. At the end of compression
there is an increase in pressure inside the engine cylinder, shortly before
the end of compression, the fuel valve opens and the fuel is injected into
the engine cylinder. The fuel is ignited by high temperature of the
compressed air. The ignition suddenly increases volume and temperature
of the products of combustion. But the pressure, practically, remains
constant as shown by the line 5-6. The expansion is shown by the line 6-
7. Now the exhaust port opens at 7, and the burnt gases are exhausted into
the atmosphere through the exhaust port. It reduces pressure.

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 98

Two Stroke Engine Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

As the piston moving towards BDC, therefore volume of burnt gases
increases from 7 to 1. At 1 the transfer port opens (TPO) and suction
starts.

Figure 8.9 Actual Indicator Diagram (P-V Diagram) of Two Stroke
Diesel Engine.

Port-Timing Diagram
A typical port-timing diagram for a two-stroke diesel engine is shown in
Fig. 8.10.

Figure 8.10 Port-Timing Diagram for a Two-Stroke Diesel Engine.
Applications
Two-stroke Diesel engines are nowadays most commonly seen in
applications demanding a large power output, such as ships and electric
generation plants or in generating power for building roads, mining,
aircraft ground support equipment, high-end marine pleasure craft, and in
most of the world's armies and navies.

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 99

Questions for Viva-Voce Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

REVIEW QUESTIONS
Q1. Draw the neat sketch of sequence of operations of two stroke

petrol engine.
A1.

Q2. Draw the neat sketch of sequence of operations of two stroke
diesel engine.

A2.

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 100

Questions for Viva-Voce Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

Q3. Draw the neat sketch of theoretical indicator diagram (P-V
diagram) of two stroke petrol engine.

A3.

Q4. Draw the neat sketch of theoretical indicator diagram (P-V
diagram) of two stroke diesel engine.

A4.

Q5. Draw the neat sketch of port-timing diagram of two stroke petrol
engine.

A5.

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 101

Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

Q6. Draw the neat sketch of port-timing diagram of two stroke diesel
engine.

A6.

Questions for Viva-Voce Q7. Write down the Difference between port and valve.
Q8. What is scavenging?
Q9. What are the advantages of a two-stroke engine?
Q10. What is carburettor?

WRITE YOUR ANSWERS HERE

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Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 102

Questions for Viva-Voce Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

MULTIPLE CHOICE QUESTIONS (✔)
Q1. In two stroke engine there is one power stroke in ………… of

crankshaft rotation.
a) 90°
b) 180°
c) 270
d) 360°

Q2. In two stroke engine, which of the following functions are
performed at the same time?
a) Compression and exhaust
b) Intake and Expansion
c) Intake and exhaust
d) Intake and compression

Q3. Which of the following is not true for two stroke engine as
compared to four stroke engine?
a) Less cooling is required
b) Greater lubrication is required
c) More uniform torque on crankshaft
d) Complete exhaust of products of combustion

Q4. Thermal efficiency of a two-stroke cycle engine is ………… a
four-stroke cycle engine.
a) Equal to
b) Less than
c) Greater than
d) None of these

Q5. In a two-stroke engine, the working cycle is completed in two
revolutions of the crankshaft.
a) True
b) False

Q6. A two stroke cycle engine gives…………the number of power
strokes as compared to the four-stroke cycle engine, at the same
engine speed.
a) Half
b) same
c) double
d) four times

Q7. A two stroke engine gives…………. mechanical efficiency than a
four-stroke cycle engine.
a) Higher
b) lower
c) equal
d) none of the mentioned

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 103

Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

Q8. The two-stroke cycle engine have lighter flywheel.
a) True
b) False

Q9. Thermal efficiency of a two-stroke cycle engine is …………a
four-stroke cycle engine.
a) equal to
b) less than
c) greater than
d) none of the mentioned

Questions for Viva-Voce Q10. A two stroke cycle engine occupies larger floor area than a four-
stroke cycle engine.
a) True
b) False

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 104

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mfieraes-tuuribneg orInwsatrtuemr-etunbt e tbooilemrewashuerree
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bpVareercsritsaiecrealamnbgaudoleialerfrrsommewaaesurheriegmhuescnehtd.romfoSiruinmea
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haanrdenedo,theprrecismioonbilegroumnda, chianneds,
stabilized.

Simple Vertical
Boiler

Experiment# 9

Simple Vertical Boiler Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

9 EXPERIMENT NO. 9

Simple Vertical Boiler

AIM
To study of simple vertical boiler.

APPARATUS
Models of simple vertical boiler

THEORY
A steam boiler is a closed vessel, strongly constructed of steel, in which
steam is generated from water by the application of heat. The steam
generated is used for producing power and for industrial work and heating
work. The steam boiler is also known as steam generator. The function of
a steam boiler or generator is to convert chemical energy of fuel by
combustion into heat and to transfer this heat to water and thus to produce
steam.

According to A.S.M.E. (American Society of Mechanical Engineers,
U.S.A.) code a boiler is defined as a combination of apparatus for
producing, furnishing or recovering heat together with the apparatus for
transferring the heat so made available to water which could be heated
and vaporised to steam form.

The boilers which produce steam at pressure below 80 bar are called low
pressure boilers. Simple Vertical, Cochran, Cornish, Lancashire,
Locomotive boilers, Babcock and Wilcox etc. are the example of low-
pressure boiler. The boilers which produce steam at pressures of 80 bar
and above are called high pressure boilers. Velox, Lamont, Benson
boilers etc. are the example of high-pressure boiler.

A fire-tube or smoke-tube boiler is one in which the hot products of
combustion (hot gases) flow through the inside of tubes, known as smoke
tubes, and water surrounding the tubes. The fire-tube boiler belongs to the
old class of boilers, but they have still a place of usefulness where the
steam pressure does not exceed about 10 bar and where a moderate
quantity of steam is required. They have the advantage of low cost and
compact design. Their evaporative capacities range from 200 to 3,000 kg
of water per hour. Their thermal efficiencies vary from 65 to 68 per cent
under normal conditions, and smaller size of these are easily portable.
Boilers belonging to this class are: Simple vertical boiler, Vertical boiler
with horizontal smoke tubes (Cochran boiler), Locomotive boiler, and
Scotch marine boiler. Fire-tube boilers are also termed as economical
boilers. These boilers have much less water capacity and so they require
less time to raise steam from cold water. Water-tube boilers have water
inside the tubes and hot gases surrounding the tubes.

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 105

Simple Vertical Boiler Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

These boilers are used extensively because they can be built for high
pressures and large evaporative capacities. They are safe, quick steaming,
and flexible in construction and operation. Babcock and Wilcox Water-
tube Boiler is the example of water tube boiler.

SIMPLE VERTICAL BOILER

A vertical boiler is a type of fire-tube or water-tube boiler where the
boiler barrel is oriented vertically instead of the more common horizontal
orientation. Vertical boilers were used for a variety of steam-powered
vehicles and other mobile machines, including early steam locomotives.
Vertical boilers are also used in small factories, small water supply
plants, cranes, excavators, which engines in mines and on sites of
temporary nature such as construction work. For similar capacity the
advantages of this type of boiler are that it requires the minimum floor
space, does not require elaborate foundations (any firm ground is
suitable), and is portable. These boilers are cheap at first cost and are
easy to start. The main disadvantage of these boilers is that they are
inefficient due to large wastage of fuel and less heating surface. It is
stationary, vertical, fire tube/water tube, internally fixed and natural
circulation boiler. Two types of simple vertical boilers have been used:

1. Simple vertical cross-tube (water tubes) boiler
2. Simple vertical multitubular (fire-tube) boiler

SIMPLE VERTICAL CROSS-TUBE (WATER TUBES) BOILER

It consists of vertical steel cylindrical shell. The major portion of which
is full of water and surrounds the fire box. The remaining space is for
steam collection. The grate is provided below the firebox. The one or
more inclined cross tubes are fitted in fire box. The water is circulated
through inclined cross tubes. At the end of each cross tubes, hand holes
are provided to give access to clean these inclined tubes. The cross tubes
also help to increase the heating surface and circulation of water. At the
bottom of fire box is grate on which fuel is burnt and the ash from it falls
in the ash pit. Fig. 9.1 and 9.2 shows the schematic diagram of imple
vertical cross-tube (water tubes) boiler.

Feeding of fuel is from fire door and combustion takes place on the grate.
The flue gases thus produced collects in combustion chamber. The heat
of flue gases is transferred to water i.e. circulating in cross tubes and also
to the water come in contact to the circular walls of firebox. The water is
thus heated and evaporated in boiler shell. The generated steam collects
in steam space and can be taken out through steam stop valve. Constant
level of water is maintained in the boiler shell. The level of water is seen
in the glass tubes of water level indicator fitted in the boiler shell.

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 106

Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

Simple Vertical Boiler Figure 9.1 Simple Vertical Cross-Tube (Water Tubes)

Particulars of boiler are as follows:

Diameter of boiler shell 1 meter to 1.25 metre
Height of boiler shell 2.75 metre to 3 metre
Diameter of cross tube 22 to 25 cm
Steam generation capacity 2500 kg/hr
Pressure of steam 7.5 bars to 15 bars

Various necessary mountings fitted with the boiler are
1. Feed check valve
2. Water level indicator
3. Pressure gauge
4. Safety valve
5. Steam stop valve
6. Fusible Plug
7. Blow off cock
8. Manhole

The description of each is as follows:

Feed check valve: The feed check valve is fitted in feed water supply
line. The feed water is supplied to the boiler through this valve. It is
unidirectional flow valve, ii opens only when the feed water pressure is
higher than the pressure inside the boiler and feed water will flow into the
boiler shell. If the feed water pressure is less than the boiler steam
pressure, then this valve remains closed and no back flow of steam/water
through the supply line (through the valve) takes place. Hence it checks
the back flow of steam/ water from boiler.

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 107

Lakshmi Narain College of Technology (LNCT) Bhopal (MP)Figure 9.2 Simple Vertical Cross-Tube
(Water Tubes) Boiler
Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 108

Simple Vertical Boiler Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

Pressure gauge: It is fitted on top of the boiler shell and indicates the
pressure of steam inside the boiler when boiler is working.

Safety valve: The safety valve is fitted on top of the crown of the boiler
shell. The function of safety valve is to maintain a desired pressure in the
boiler shell. If the pressure in the boiler shell increases then extra steam is
released to atmosphere through opening of safety valve.

Steam stop valve: The steam stop valve is fitted on top of crown of the
boiler shell i.e. in the steam space. The stop valve is used to regulate the
supply of generated steam in the boiler shell. The steam generated in the
boiler and stored in the dome of the boiler shell, first enters to the
antipriming pipe where the water particles associated in the steam are
removed and then only steam is required to flow through steam stop
valve. The steam coming out through steam stop valve is used for various
purposes.

Fusible Plug: The fusible plug is mounted on the boiler shell above the
furnace and below the water level in the shell. This plug under normal
condition is immersed in water in the boiler which keeps the temperature
of fusible metal in the fusible plug below its melting point. When the
water level in the shell falls below the fusible plug level, the fusible metal
of the fusible plug melts by the heat of the flue gases in the furnace. Thus,
the plug drops out and the high-pressure steam/water rushes out to the
furnace and puts off the fire in the furnace. Thus, boiler is protected from
overheating and mishappening.

Blow off cock: During annual maintenance and also other such occasions
when it is necessary to empty the boiler, so the boiler shell can be
cleaned, inspected and repaired instantly. Similarly, it is also a common
practice to remove periodically the sediments and other deposited
material from the boiler shell. The same can be removed by opening the
blow off cock. It is mounted in the boiler at the lowest position of the
shell to remove all sediments and water by gravity flow.

Manhole: A man hole usually elliptical or oval shaped and is provided at
the top of the boiler shell. The man hole is provided for the entry of
person for repair, cleaning and inspection purposes during annual
maintenance. The man hole and other opening are made steam tight with
the help of asbestos sheet or asbestos board, which are used as a packing
material between shell and manhole. Oval shaped manhole is usually 40
cm by 30 cm in sizes.

The following are the advantages of vertical boilers:
• Minimum floor area is required.
• Boilers are self-contained and hence require no brick-work
setting.
• Cost of construction is low.
• Easy to maintain and simple to operate

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 109

Simple Vertical Boiler Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

• It is semi-portable. It can be moved and set up readily in different
locations.

The following are disadvantages of the vertical boilers:
• Capacity and pressure are limited. The maximum practical
capacity of the vertical boiler is 2500 kg/hr and the maximum
usual pressure is 14 bars.
• The furnace cannot be altered to meet requirement for the change
in fuel.
• These boilers require a high head room.

VERTICAL MULTITUBULAR (FIRE TUBES) BOILER:
A vertical fire-tube boiler or vertical multitubular boiler is a vertical
boiler where the heating surface is composed of multiple small fire-tubes,
arranged vertically (Fig. 9.3). Multi-tubular vertical boilers are
constructed with a cylindrical shell but contain a fire-box and a
combustion chamber designed for arrangements of vertical smoke tubes
of small diameter.

Figure 9.3 Simple Vertical Multitubular (Fire-Tube) Boiler
These boilers were not common, owing to drawbacks with excessive wear
in service. The more common form of vertical boiler, which was very
similar in external appearance, instead used a single flue and water-filled
cross-tubes. Another form used horizontal fire-tubes, even where this
added complexity, such as the Cochran boiler.
Where a sustained high evaporative capacity (i.e. power) was required,
vertical tubes were used, but rarely. These cases were mostly for
locomotives, either railway locomotives or road steam wagons.

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 110

Questions for Viva-Voce Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

REVIEW QUESTIONS
Q1. Draw a neat sketch of simple vertical cross-tube (water tubes)

boiler.
A1.

Q2. What is low pressure boiler?
Q3. What is high pressure boiler?
Q4. What is fire-tube or smoke-tube boiler?
Q5. What is water-tube boiler?
Q6. What type of boiler is simple vertical cross-tube (water tubes)

boiler?
Q7. What are the basic specifications of simple vertical cross-tube

(water tubes) boiler?
Q8. What are the advantages of vertical boilers?

WRITE YOUR ANSWERS HERE
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Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 111

Questions for Viva-Voce Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

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Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 112

Questions for Viva-Voce Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

MULTIPLE CHOICE QUESTIONS (✔)

Q1. In fire tube boilers, pressure is limited to
a) 16 bars
b) 32 bars
c) 48 bars
d) 64 bars

Q2. The following is a boiler mounting
a) Feed pump
b) Water level gauge
c) Economizer
d) Superheater

Q3. The following is a boiler mounting
a) Feed pump
b) Fusible Plug
c) Economizer
d) Superheater

Q4. The diameter of boiler shell in simple vertical boiler is
a) 4 meters to 4.25 metres
b) 3 meters to 3.25 metres
c) 2 meters to 2.25 metres
d) 1 meter to 1.25 metres

Q5. Which of the following is a low-pressure boiler?
a) Benson
b) Velox
c) Lamont
d) Simple vertical

Q6. Simple vertical boiler is a
a) Horizontal fire tube boiler
b) Horizontal water tube boiler
c) Vertical water tube boiler
d) Vertical fire tube boiler

Q7. Blow off cock in a boiler is used to
a) Control the flow of steam from the boiler to the main pipe and
to shut off the steam completely when required
b) Empty the boiler when required and to discharge the mud,
scale or sediments which are accumulated at the bottom of the
boiler
c) Put off fire in the furnace of the boiler when the level of water
in the boiler falls to an unsafe limit
d) Increase the temperature of saturated steam without raising its
pressure

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 113

Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

Q8. In simple vertical boiler, maximum steam pressure is limited to
a) 1 bar
b) 50 bars
c) 100 bars
d) 15 bars

Q9. The diameter of cross tube in simple vertical boiler is
a) 22 to 25 cm
b) 25 to 50 cm
c) 50 to 75 cm
d) 75 to 100 cm

Questions for Viva-Voce Q10. The height of boiler shell in simple vertical boiler is
a) 2.75 metre to 3 metre
b) 3.75 metre to 4 metre
c) 4.75 metre to 5 metre
d) 5.75 metre to 6 metre

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 114

SAinveertBicaarl biosilear wPirtehchisoiornizonAtnagl ufilraer-
mtuebaesusri(nCgochIrnastnrubmoeinletr) tios amteyapseuroef
apsnimegcaleelsl aantvdertatoiclaorlecqautebiroetihdleerw,anogrukleseodrvJeorbyto
agcecnuerraatetley sttoeammacfhoirnisnmga. lIltmisacuhseinderiny.
cTohnejunbcotiiolenr wiisthaslcipylginadugriecabllovckerstficoarl
pbwarearcstiesareredamrnugamudlearfwroimtmheaasauhriehgmehmecnihts.rpohmeSriuiincmeal
cdoormroesidontopr.esIisttainst chsaterealc, terainsedd bisy
hhaarvdiennged, mapnreycisinoanrrowgroufnirde, -tuabneds,
srtuabninliiznegd. horizontally. Boilers like
this have been widely used on ships
as either auxiliary or donkey boilers.

Cochran Boiler

Experiment# 10

Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

10 EXPERIMENT NO. 10

Cochran Boiler

Cochran Boiler AIM
To study of Cochran boiler.

APPARATUS
Model of Cochran boiler.

THEORY
A steam boiler is a closed vessel, strongly constructed of steel, in which
steam is generated from water by the application of heat. The steam
generated is used for producing power and for industrial work and heating
work. The steam boiler is also known as steam generator. The function of
a steam boiler or generator is to convert chemical energy of fuel by
combustion into heat and to transfer this heat to water and thus to produce
steam.

According to A.S.M.E. (American Society of Mechanical Engineers,
U.S.A.) code a boiler is defined as a combination of apparatus for
producing, furnishing or recovering heat together with the apparatus for
transferring the heat so made available to water which could be heated
and vaporised to steam form.

The boilers which produce steam at pressure below 80 bar are called low
pressure boilers. Simple Vertical, Cochran, Cornish, Lancashire,
Locomotive boilers, Babcock and Wilcox etc. are the example of low-
pressure boiler. The boilers which produce steam at pressures of 80 bar
and above are called high pressure boilers. Velox, Lamont, Benson
boilers etc. are the example of high-pressure boiler.

A fire-tube or smoke-tube boiler is one in which the hot products of
combustion (hot gases) flow through the inside of tubes, known as smoke
tubes, and water surrounding the tubes. The fire-tube boiler belongs to the
old class of boilers, but they have still a place of usefulness where the
steam pressure does not exceed about 10 bar and where a moderate
quantity of steam is required. They have the advantage of low cost and
compact design. Their evaporative capacities range from 200 to 3,000 kg
of water per hour. Their thermal efficiencies vary from 65 to 68 per cent
under normal conditions, and smaller size of these are easily portable.
Boilers belonging to this class are: Simple vertical boiler, Vertical boiler
with horizontal smoke tubes (Cochran boiler), Locomotive boiler, and
Scotch marine boiler. Fire-tube boilers are also termed as economical
boilers. These boilers have much less water capacity and so they require
less time to raise steam from cold water. Water-tube boilers have water
inside the tubes and hot gases surrounding the tubes.

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 115

Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

These boilers are used extensively because they can be built for high
pressures and large evaporative capacities. They are safe, quick steaming,
and flexible in construction and operation. Babcock and Wilcox Water-
tube Boiler is the example of water tube boiler.

COCHRAN BOILER
It is vertical, stationary, multi tubular, horizontal fire tube, natural
circulation and internally fired boiler. They are made in various sizes with
maximum required steam generating capacities up to 3000 Kgs per hour
and maximum pressure up to 20 bars.

Cochran Boiler It is a well-known design of a vertical multitubular fire tube boiler, and is
an improvement over the simple vertical boiler as it provides greater
heating surface with horizontal fire-tubes. The details of Cochran boiler
are shown in Fig. 10.1. The boiler consists of a cylindrical shell with its
crown having a hemispherical shape. Similarly, the furnace is also
hemispherical in shape (the hemispherical shape gives higher strength and
higher area/volume ratio. This shape also required less material in
construction. The coal is fed into the furnace through fire hole and
combustion takes place above the grate inside the furnace) The burnt
gases pass through multiple numbers of (165 to 170 nos) horizontal tubes
which are approximately 6.25 cm in external diameter. The flue gases
after passing through horizontal tubes will be collected in smoke box and
at the same time large quantity of heat will transfer to the water by
convection and thus water temperature increases. The flue gases finally
collected in the smoke box and will pass to atmosphere through vertical
chimney. The smoke box is provided with an inspection door.

The Cochran boiler may be arranged to burn almost any kind of fuel
including wood, paddy husk and oil fuel. If the coal is used as a fuel, the
combustion of coal will take place over the grate and ashes will collect in
the ash pit provided under the grate in bottom most portion of the furnace.
If the oil is used as a fuel, no grate is needed but the bottom of the furnace
is lined with firebricks. In such cases oil burners will be fitted at a
suitable point below the fire hole.

The air circulation through the grate is caused by means of draught which
is produced by the selection of suitable size of the chimney. Similarly, to
regulate the flow of flue gases from the chimney to atmosphere is done by
means of damper which will be provided at a suitable outlet of the
chimney

Particulars of boiler are as follows: 1 meter to 3 metre
Diameter of boiler shell 2 metre to 6 metre
Height of boiler shell 20 kg/hr to 3000 kg/hr
Steam generation capacity upto 20 bars
Pressure of steam 70 to 75 per cent
Efficiency about 10 to 25 times the
Total heating surface area
great area

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 116

Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

Figure 10.1 Cochran Boiler
Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 117

Cochran Boiler Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

The special features of Cochran boilers are:
• The spherical crown and spherical shape of a fire box are the
special features of this boiler. These shapes require least material
for a given volume.
• It is very compact and requires minimum floor area.
• Any type of fuel can bum in the boiler.
• It is well suited for small industries.
• It gives about 70% thermal efficiency with coal firing and about
75% with oil firing.
• It is used for small capacity steam generation purpose.
• The ratio of grate area to heating surface area varies from 10:1 to
25:1.
• it is self-contained and stronger from design point of view.
• As there are no seams (joints) in the furnace, this source of trouble
is eliminated.

The following necessary mountings provided for safe and easy working
of the boiler as an essential component:

Feed Check Valve

The feed check valve is fitted in feed water supply line. The feed water is
supplied to the boiler through this valve. It is unidirectional flow control
valve. It opens only when the feed water pressure is higher than the
pressure inside the boiler and feed water will flow into the boiler shell. If
the feed water pressure is less than the boiler steam pressure, then this
valve remains closed and no back flow of steam/water through the supply
line (through the valve) takes place. Hence it checks the back flow of
steam/ water from boiler.

Water level indicator

The water level indicator is fitted outside the boiler shell which indicates
the level of water inside the boiler with the help of glass tube. It helps to
maintain a particular level of water inside the boiler otherwise
overheating may take place which in turn may burn out the tubes. They
are two in numbers in a boiler.

Pressure Gauge

It is fitted on top of the boiler shell and indicates the pressure of steam
inside the boiler when boiler is working.

Safety Valve

The safety valve is fitted on top of the crown of the boiler shell. The
function of safety valve is to maintain a desired pressure in the boiler
shell. If the pressure in the boiler shell increases then extra steam is
released to atmosphere through opening of safety Valve.

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 118

Cochran Boiler Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

Fusible plug
The fusible plug is mounted on the boiler shell above the furnace and
below the water level in the shell. This plug under normal condition is
immersed in water in the boiler which keeps the temperature of fusible
metal in the fusible plug below its melting point. When the water level in
the shell falls below the fusible plug level, the fusible metal of the fusible
plug melts by the heat of the flue gases in the furnace. Thus, the plug
melts and drops cut and high-pressure steam/water rushes out to the
furnace and puts off the fire in the furnace. Thus, boiler is protected from
overheating and mishappening.
Blow off cock
During annual maintenance and also other such occasions when it is
necessary to empty the boiler, so the boiler shell can be cleaned,
inspected and repaired instantly. Similarly, it is also a common practice to
remove periodically the sediments and other deposited material from the
boiler shell. The same can be removed by opening the blow off cock. It is
mounted in the boiler at the lowest position of the shell to remove all
sediments and water by gravity flow.
Man hole
A man hole usually elliptical or oval shaped and is provided at the top of
the boiler shell. The man hole is provided for the entry of person for
repair, cleaning and inspection purposes during annual maintenance. The
man hole and other opening are made steam tight with the help of
asbestos sheet or asbestos board, which are used as a packing material
between shell and manhole. Oval shaped manhole is usually 40cm by
30cm in sizes. In Cochran and Lancashire boilers generally, circular
manhole is provided.

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 119

Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

REVIEW QUESTIONS
Q1. Draw a neat sketch of Cochran boiler.
A1.

Questions for Viva-Voce

Q2. What type of boiler is Cochran boiler?
Q3. What is the thermal efficiency of Cochran boiler?
Q4. What are the basic specifications of Cochran boiler?
Q5. What are the special features of Cochran boilers?
Q6. What is main difference between firetube simple vertical boiler
and Cochran boiler?
Q7. What is the function of feed check valve?
Q8. What is the function of water level indicator?
Q9. What is the function of pressure gauge?
Q10. What is the function of safety valve?

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 120

Questions for Viva-Voce Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

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Questions for Viva-Voce Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

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Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 122

Lakshmi Narain College of Technology (LNCT) Bhopal (MP)

MULTIPLE CHOICE QUESTIONS (✔)

Q1. Which of the following is a low-pressure boiler?
a) Benson
b) Velox
c) Lamont
d) Cochran

Questions for Viva-Voce Q2. Cochran boiler is a
a) Horizontal fire tube boiler
b) Horizontal water tube boiler
c) Vertical water tube boiler
d) Vertical fire tube boiler

Q3. The diameter of fire tubes in Cochran boiler is of the order of
a) 2 cm
b) 6 cm
c) 8 cm
d) 15 cm

Q4. The fire tubes in a Cochran boiler are
a) Horizontal
b) Vertical
c) Inclined

Q5. The number of fire tubes in a Cochran boiler are
a) 75
b) 115
c) 165
d) 225

Q6. The fire tubes in a Cochran boiler usually have………. diameter.
a) 6.25 mm
b) 62.5mm
c) 72.5mm
d) 92.5mm

Q7. The shell of the Cochran boiler is made hemispherical
a) to give maximum space and strength
b) to withstand the pressure of steam inside the boiler
c) both (a) & (b)
d) none of the above

Q8. Cochran boiler is the modification of
a) Pressure cooker
b) Simple vertical boiler
c) Lancashire boiler
d) Babcock and Wilcox boiler

)

Mechanical Engineering Department | Lab Manual | Basic Mechanical Engineering [BT-203] 123