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ME 2155/ BASIC CIVIL AND MECHANICAL

ENGINEEERING

UNIT IV

IC ENGINES AND BOILERS

PART A

1. Define the following terms:

i) Cylinder bore

ii) Stroke or stroke lengthiii) TDC

iv) BDC

v) Compression ratio

vi) Stroke volume or swept volume or displacement volume

vii)Clearance volumeviii)Air-fuel ratio

ix) IHPx) BHP

xi)FHP

xii)Thermal effiencyxiii) Mechanical efficiency

1. Cylinder Bore: The inside diameter of the engine cylinder is known as Cylinder bore.

2. Stroke or Stroke Length: It is the linear distance through which the piston movesinside the cylinder during one stroke. In other words, stroke length is the distance

between the extreme upper and lower positions of the piston. Numerically, the strokelength is equal to twice the crank radius.

3. Top Dead Center (TDC) or Inner Dead Center (IDC): Top Dead Center (TDC) in

the vertical engine is the extreme position of the piston on the top of the cylinder (head

side). At this position, piston motion reverses direction and the cylinder volume is at aminimum.

In the case of horizontal engine, this position is known as Inner Dead Center (IDC). At

TDC or IDC, the crank angle is zero.

4. Bottom Dead Center (BDC) or Outer Dead Center (ODC): Bottom Dead Center

(BDC) in vertical engine indicates the extreme position at the bottom of the cylinder. At

this position, the piston motion reverses direction and the cylinder volume is at a maxim

urn.In the case of horizontal engine, this position is known Outer Dead Center (ODC). At

BDC or ODC, the crank angle is 180.

5. Compression Ratio: It is a ratio of the volume when the piston is at bottom deadcenter to the volume when the piston is at top dead center.

Compression Ratio = Maximum cylinder volume/ Minimum cylinder volume

6. Stroke Volume or Swept Volume or Displacement Volume: It is the volumegenerated by piston movement in one stroke from one dead center to other.

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Swept Volume= pie* D*D/4 x L ( D = cylinder bore and L = piston stroke length )

7. Clearance Volume: The volume contained in the cylinder above the top of the piston,

when the piston is at TDC is called Clearance Volume.Thus, when the piston is at BDC, total volume = Swept Volume + Clearance Volume.

8. Air-Fuel Ratio: This is expressed as a ratio of the mass of air to the mass of the fuel.

9. Indicated Horse Power (I.H.P.): It is the power produced within the engine cylinder.It is called indicated power as it can be measured with the help of an Indicator. Indicator

is an instrument that draws pressure-volume diagram for the engine.

10. Brake Horse Power (B.H.P.): This is the net output of an engine. It is called BrakePower, since it can be measured by absorbing the power with a brake system. (The brake

system consists of a brake pulley mounted on the engine shaft.)

11.. Friction Horse Power (F.H.P.): It is the difference between I.H.P. and B.H.P. This

is the power absorbed by the moving parts of the engine (piston bearings, etc.,).

12. Thermal Efficiency: It is the ratio of work done inside the engine cylinder (I.H.P.) to

the fuel energy supplied to the engine.

13. Mechanical Efficiency: Mechanical Efficiency of an IC. engine is defined as the

ratio of power delivered (B.H.P.) to the power provided to the piston (I.H.P.).

2. Name some of the BOILER MOUNTINGS?

For the safe operation, satisfactory functioning, efficient working and easy maintenance

of the boilers, Boiler Mountings are provided as per the Indian Boiler Act. These include:

1. Water Gauge (Water Level Indicator)2. Pressure Gauge

3. Safety Valves

(A) Spring Loaded Safety Valve(B) High Steam Low Water Safety Valve

4. Fusible Plug

5. Feed Check Valve6. Stop Valve

7. Blow-off Cock

3. Define the function of the following engine parts

i) Engine cylinder

ii) Cylinder head

iii) Pistoniv) Piston rings

v) Piston pin

vi) Connecting rodvii) Valves

viii) Crank and crank shaft

ix) crank casex) Flywheel

1. Engine Cylinder: The heart of the engine is the cylinder in which fuel is burnt and

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power is developed. The cylinder allows the piston to move to and fro. Combustion of

fuel takes place inside the cylinder. The cylinder has to withstand a high pressure (more

than 500 N / sq.cm.) and temperature (around 1500C to 2000C).

2. Cylinder Head: The cylinder is closed by the cylinder head at one end and the other

end is covered by the moving piston. The cylinder head contains inlet and exhaust valves

for admitting fresh charge and for exhausting the burnt gases.In petrol engines, the cylinder head also contains a spark plug for igniting the fuel

mixture. But in diesel engines, the cylinder head contains nozzle for injecting the into the

cylinder.

3. Piston: Piston is sliding within the cylinder. This sliding movement changes volume of

the cylinder and provides the combustion space. The space formed bet the cylinder head

and top of the piston during the process of combustion is known as Combustion

Chamber. Piston transmits the force exerted by the burning of the charge the connectingrod.

4. Piston Rings: Piston rings are circular rings used to maintain a pressure tight seal

between the moving piston and the cylinder wall.

5. Piston Pin (also known as Gudgeon Pin): A Piston Pin (Gudgeon Pin ) connectspiston to the small end of the connecting rod.

6. Connecting Rod: As the name suggests, the connecting rod connects the piston withthe crank. It is attached to the piston by piston pin. It converts the up and down motion

(reciprocating motion) of the piston to a rotary motion of the crankshaft.

7. Valves: Valves are needed to let the air and fuel into the cylinder (Intake Valve) and

also to let out the burnt or spent gases after they have done their work (Exhaust Valve).Valves are operated by cams, rotated by a camshaft, driven by the crankshaft. [However,

two stroke cycle engines have only ports at the cylinder walls and have no valves.]

8. Crank and Crankshaft: Crank is a lever. It is connected to the end of the connectingrod by a pin joint. Its other end is connected to a shaft called Crankshaft. It is the rotating

member of the engine. Its function is to convert the reciprocating motion of the piston

into a rotary motion with the help of the connecting rod.

9. Crank Case: The main body of the engine which contains the crank and crankshaft is

known as Crank Case. It serves as sump for the lubricating oil.

10. Flywheel: It is a heavy wheel, mounted on the crankshaft. Its function is to store theexcess energy during power stroke of the engine and help the movement of the piston

during the remaining idle strokes, thus maintaining uniform rotation (speed) of the

crankshaft.

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High Pressure Boilers. Water tube boilers are generally preferred for high pressure and

high output. But, fire tube boilers are used for low pressure and low output.

PART B

1. Explain the working of a four stroke petrol engine with the help of PV diagram.

Petrol Engine is also known as Spark Ignition (SI.) Engine. Four Stroke PetrolEngine requires four strokes of the piston to complete one cycle of operation in the

engine cylinder.

See Fig. 2. It consists of a cylinder. Its one end is fitted with a cover and the other

end left open. The cover is provided with inlet and exhaust apertures. These apertures areopened and closed by inlet and exhaust valves. A spark plug initiates the ignition of the

fuel. The piston reciprocates inside the cylinder. The connecting rod and crank convert

the reciprocating motion of the piston into rotary motion.

The petrol engine works on the principle of Otto Cycle, also known as Constant

Volume Cycle. Fig. 3 shows the Pressure Velocity Diagram of Theoretical Otto Cycle.

1. Suction Stroke: Fig. 2(a)

During suction stroke, the Inlet valve (1) opens and air and fuel (petrol) mixture

(charge) is sucked into the cylinder. The piston moves downward from Top Dead Center(TDC) till it reaches Bottom Dead Center (BDC). During suction stroke the Exhaust

value (E) is closed.

See Fig. 3. Suction stroke is theoretically represented by the horizontal line 1-2 in the PVDiagram. The drawal of air-fuel mixture is taking place at atmospheric pressure.

2. Compression Stroke: Fig. 2(b)

During this stroke, both the inlet and exhaust valves are closed. The air-fuelmixture is compressed as the piston moves upwards from BDC to TDC. The compression

ratio in petrol engines varies from 7 to 10. As a result of compression, pressure and

temperature of the charge are increased to 15-20 bar and 400C respectively. See Fig. 3.The process of compression is theoretically represented by the curve 2-3 in the PV

Diagram.

Shortly before the piston reaches TDC, the charge is ignited by means of a Spark

Plug. It suddenly increases the pressure and temperature of the products of combustion,the volume remains constant.

During the burning process, the chemical energy of the fuel is converted into heat

energy, producing a temperature rise of about 2000 C. See Fig. 3. This constant volumecombustion process is theoretically represented by the vertical line 3-4 in the PV

Diagram.

3. Expansion or Power or Working Stroke: Fig. 2(c)

During this stroke, both the valves remain closed. Due to the rise in pressure,

piston is pushed down with a great force. The hot burnt gases expand pushing the piston

from TDC to BDC. It is also called Working Stroke as work is done by the expansion of

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hot gases. See Fig. 3. The expansion stroke is theoretically represented by the curve 4-5

in PV Diagram.

At or near the end of the expansion the exhaust valve opens to release thegases to the atmosphere. This suddenly brings down the Cylinder to atmospheric

pressure.

This drop in pressure at constant volume is theoretically represented by thevertical line 5-2 in the PV Diagram as shown in Fig. 3.

Exhaust Stroke: Fig. 2(d)

During this stroke, the exhaust valve opens, as moves from BDC to TDC. This

movement of the piston pushes out the exhaust gases from the cylinder. The exhaust

gases are exhausted through the exhaust valve into the atmosphere.

See Fig. 3. The exhaust stroke is theoretically represented by the horizontalline 2-l in the PV Diagram.

Uses: Four stroke petrol engines have higher load carrying capacities than two stroke

petrol engines. Hence, they are used in high power high speed motor cycles and

passenger cars.

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2. Explain the working of a four stroke diesel engine with the help of PV diagram.

The basic construction of a four stroke cycle diesel engine is the same as that

of four stroke cycle petrol engine, except that instead of a spark plug, a fuel injector is

mounted in its place. A fuel pump supplies diesel to the injector at higher pressure.Dr. Rudalf Diesel invented the Diesel Engine. It is also known as

Compression Ignition (CI.) Engine, since ignition takes place due to the high temperature

produced during the compression of air in engine cylinder. Liquid fuel, i.e., diesel, which

cannot be vapourized, is injected into the cylinder in the form of fine spray using fuelpump and injector.

Diesel engine works on the principle of Theoretical Diesel Cycle, also known

as Constant Pressure Heat Addition Cycle. Fig. 5 shows the Pressure Velocity Diagramof the same. The ideal sequence of operation for the four stroke C.1. engine is explained

as follows:

1. Suction Stroke: Fig. 4(a)

During suction stroke, inlet valve (I) opens and exhaust valve (E) remains

closed. The piston travels downwards from TDC. Air is drawn in, from outside to fill the

cylinder through inlet valve till the piston reaches BDC. The air taken in is at atmosphericpressure. Suction stroke is theoretically represented by the horizontal line AB in the PV

Diagram in Fig. 5.

2. Compression Stroke: Fig. 4(b)

At the end of the suction stroke, both the inlet and the exhaust valves remain

closed. The piston moves upwards from BDC to TDC. The air sucked in during suctionstroke is compressed to a high pressure (35- 40 bar) and temperature with a decrease involume. These two strokes, viz., suction stroke and compression stroke complete one

revolution of the crankshaft. The compression stroke is theoretically represented by the

curve BC in Fig. 5.

3. Expansion or Power or Working Stroke: Fig. 4(c)

Just before the beginning of this stroke, fuel (diesel) is Injected in the form of

fine spray in the cylinder through the Fuel Injector. At this moment, the fuel is ignited by

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temperature of the hot compressed air and it starts burning at constant pressure.Due to the

high compression ratio of 16 to 20, the temperature at the end compression stroke is more

than 550C. This temperature is sufficient to ignite the fuel injected into the combustionchamber. The fuel is continuously injected for 20% of expansion stroke.

The ignited air-fuel mixture expands and forces the piston downwards from

TDC to BDC. During this constant pressure expansion stroke, both the valves remainclosed.

See Fig. 5. This constant pressure expansion with simultaneous combustion

theoretically represented by the horizontal line CD in the PV Diagram.The piston is forced further during the remaining part of the expansion stroke

due the expansion of the burnt gases. [The linear motion of the piston causes the piston

produce the mechanical work during this stroke.]

As the piston moves, the pressure of the hot gases gradually decreases. Theexpansion of the burnt gases is theoretically represented by the curve DE in the PV

Diagram as in Fig. 5.

At the end of the outstroke, the exhaust valve opens. Some of the burnt gases

escape into the atmosphere from the cylinder through the exhaust outlet at constantvolume. This is theoretically represented by the vertical line EB.

4. Exhaust Stroke: Fig. 4(d)During the exhaust stroke, the inlet valve is closed and the exhaust valve is

opened. The piston is on its upstroke from BDC to TDC, forcing the burnt gases out of

the cylinder through the exhaust valve.

See Fig. 5. The exhaust stroke is theoretically represented by the horizontalline BA. Expansion and exhaust stroke complete one revolution of the crankshaft. This

completes the cycle and the engine cylinder is ready to suck the fresh air once again.

Uses: They are used in heavy-duty transport vehicles such as trucks, tractors, bulldozers.etc., power generation, industrial and marine applications.

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3. Draw and explain the sequence of operations of a two stroke cycle petrol engine.

Scavenging

In the two-stroke petrol engine, the exhaust gases are removed from the cylinder with the

help of fresh compressed charge. This process of removing exhaust gases is calledScavenging. A specific shape is given to the piston, called deflector. The deflector helps

to prevent the loss of incoming charge and helps for exhausting the hot gases effectively.

It will be easier to describe the cycle beginning at the point when the piston reaches TDC

at the end of the compression stroke. In the two-stroke petrol engine, the draw of petrol-air mixture into the cylinder will not take place in a separate stroke. Therefore, the

method of draw of petrol-air mixture should be understood properly.

First Stroke

Fig. 7(a) shows the position of the piston at the end of compression. The spark

is produced by the spark plug as the piston reaches TDC. The pressure and temperature ofthe gases are increased and hence the gases push the piston downwards producing the

power stroke.

Refer Fig. 7(b). When the piston opens the exhaust port during the downward

stroke, burnt gases leave the cylinder through the exhaust port.Refer Fig. 7(c). A little later, the piston uncovers the transfer port and the

crank case is directly connected to the cylinder through the transfer port. The downward

stroke of the piston compresses the charge in the crank case by the underside of the

piston.Scavenging: In the above position of the piston, the compressed mixture of petrol and air

is transferred through the transfer port to the upper part of the cylinder. The exhaust gasesare pushed out, with the help of compressed charge. This is known as Scavenging.

Scavenging is continued until the piston reaches BDC.

Second Stroke

As the piston moves upwards, it covers the transfer port. Hence, flow of charge

into the cylinder is stopped. The upward motion of the piston lowers the pressure in the

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crank case below atmosphere and fresh air is induced in the crank case through the inlet

port as it is uncovered. A little later, the piston covers the exhaust port and actual

compression of the charge starts as shown in Fig. 7(d).The compression is continued until the piston reaches TDC. The ratio of

compression ranges from 1:7 to 1:10. The cycle is thus completed within two strokes.

Note: The two-stroke engine requires only two strokes of the piston to complete onecycle of operation. The crankshaft makes only one revolution to complete one cycle.

Power is developed in every revolution of the crankshaft.

Uses: Two-stroke petrol engines are used in mopeds, scooters, motorcycles, because theyat high speeds with moderate power outputs.

4. Draw and explain the sequence of operations of a two stroke cycle diesel engine.

Refer Fig. 8. In two-stroke diesel engine, the draw of pure air into the cylinder will not

take place in a separate stroke.

First Stroke

Fig. (a) shows the position of the piston at the end of compression. Diesel is

injected using fuel injector just before completing the compression. It starts burning. Thehigh pressure, high temperature gases push the piston downwards, producing the power

stroke. As & piston moves little down, the supply of diesel stops.

Refer Fig. (b). In this, the piston uncovers the exhaust port during the

downward stroke. Hence the burnt gases leave the cylinder through the exhaust port.Refer Fig. (c). A little later, the piston uncovers the transfer port as shown.

Now the crank case is directly connected to the cylinder through the transfer port. Air in

crank case is compressed by underside of the piston and the compressed air is transferredto the cylinder through transfer port. The exhaust gases are pushed out with the help of

fresh air until the piston reaches BDC (similar to two stroke petrol engine).

Second Stroke

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The piston moves upwards. It first covers the transfer port and stops the flow of air into

cylinder. A little later, the piston covers the exhaust port as shown in Fig. (d), and actual

compression of air starts. The upward motion of the piston lowers the pressure in thecrank case below atmosphere and fresh air is induced in the crank case through the inlet

port as it is uncovered. The compression of air is continued until the piston reaches TDC.

The fuel supply starts just before the piston reaches TDC and the cycle iscompleted.

5. Compare and contrast four stroke cycle engines with two stroke cycle engines.

Sl.No DETAILS FOUR STROKE

ENGINE

TWO STROKE

ENGINE

1. No. of Strokes / Cycle Four strokes per

cycle

Two strokes per

cycle

2. No. of Revolutions / Cycle Two One

3 Power Stroke and Power Output One power stroke

for two revolutions.

Hence, power

output for the samesize of the engine is

less.

One power stroke

per revolution.

Hence, power output

for the same size ofthe engine is more.

4. Torque Not uniform Uniform

5. Weight of the Engine Weight of theengine is more for

the same power

output.

Weight of theengine is less for the

same power output.

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6. Operating temperature Less More

7. Lubrication and Cooling

Requirement

Consumption of

lubricating oil is

less. Coolingrequirement is less.

Lubricating oil

consumption is

more. Special pistoncooling is necessary.

8. Fuel Consumption Less More9. Thermal Efficiency High thermal

efficiency. Part load

efficiency betterthan two-stroke

engine,

Low thermal

efficiency. Part load

efficiency less thanfour-stroke engine.

10. Valves /Ports Contains valves Containns ports

11. InitialCost High, due to heavyweight and

complication of

valve mechanism.

Low, due tolightweight and due

to the absence of

valve mechanism.

12. Wear and Tear Less More13. Noise Less More

14 Uses Used for slow speed

high powerapplications, i.e., in

cars, buses, tractors,

aeroplanes, powergeneration, etc.

Used for high speed

low powerapplications, i.e.,

scooters,

motorcycles, lawnmowers, etc.

6. Discuss the merits and demerits of diesel engine over petrol engine.

Sl.No. Details Petrol (S.I.) Engine Diesel (C.I.) Engine

1. Fuel Petrol Diesel oil

2. Charge drawn in suctionstroke

Air-fuel mixture isadmitted.

Air alone is admitted.

3. Fuel admission Through carburettor. Through fuel pumpand fuel injector.

4. Mixing of air and fuel Air and fuel mixed

externally in thecarburetor.

Mixing of air and fuel

takes place inside thecylinder.

5. Fuel Ignition Requires an IgnitionSystem with spark plug

(Spark- Ignition)

Self-Ignition due tohigh temperature

caused by high

compression of air(Compression-

Ignition).

6. Compression ratio Low (7 to 10) High (16 to 20)

7. Power Output Less due to low More due to high

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compression ratio. compression ratio.

8. Cycle of operation Otto Cycle (Constant

Volume Cycle)

Diesel Cycle (Constant

Pressure Cycle)

9. Engine speed High speed (3000 rpm) Low speed (400-1500

rpm)

10. Engine starting in coldweather

Easy Difficult due to highcompression ratio.

11. Engine cost Less More

12. Fuel consumption More Less

13. Fuel cost More Less

14. Maintenance Cost Less Slightly higher

15. Thermal Efficiency Less(about 20%) due to

low compression ratio

More (about 30%) due

to high compressionratio.

16. Weight Light Heavy

17. Uses Automobiles & aero-

planes.

Buses, tractors, trucks,

etc.18. Vibration and Noise Almost nil More due to high

operating pressure

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BOILERS

1. Explain, with a neat sketch, the working of a Cochran boiler.

Description [Fig. 11

Cochran Boiler is a vertical, multi-tubular, internally fired, fire tube boiler having anumber of horizontal fire tubes. Maximum evaporative capacity is 4000 kg of steam per

hour. Maximum pressure of steam is 10 bar. Cochran boiler consists of:

1. Cylindrical Shell2. Grate and Ashpit

3. Fire Box (Furnace)

4. Flue Pipe

5. Fire Tubes or Flue Tubes6. Combustion Chamber

7. Chimney

8. Manhole

9. Damper

1. Cylindrical Shell: The Cylindrical Shell is vertical. It is hemispherical on the top,which forms the steam space.

2. Grate and Ashpit: Grate is placed at the bottom of the furnace where coal is burnt.

Ashpit is provided below the grate for the collection of ash.

3. Fire Box (Furnace): It is also dome-shaped like the shell so that the gases can bedeflected back till they are passed out through the flue pipe to the combustion chamber.

4. Flue Pipe: It is a short passage connecting the fire box with the combustion chamber.

5. Fire Tubes or Flue Tubes (F): A number of horizontal fire tubes are provided,thereby the heating surface is increased.

6. Combustion Chamber: It is lined with fire bricks to prevent overheating of the boiler.

Hot gases enter the fire tubes from the flue pipe through the combustion chamber.

7. Chimney: Chimney is provided for the exit of the flue gases to the atmosphere.

8. Manhole: It is provided for inspection and repair of the interior of the boiler shell.

9. Damper: Damper is provided in the chimney.

Working

The boiler is filled with water to the specified level. This level is maintained by supplying

with make-up water using a feed pump as and when the water level drops below thespecified level. The fuel (coal) is fed into the grate through the fire door and burnt. Ash

formed is collected in the ashpit and then it is removed manually.

The hot gases from the furnace along with the un-burnt volatile matter pass through theflue pipe to the combustion chamber. The un-burnt volatile matter leaving the furnace

along with the hot gases are burnt in the combustion chamber. The fire tubes are

completely submerged in water. The hot gases from the combustion chamber flowthrough the horizontal fire tubes and transfer the heat to the water by convection. Water is

also heated by the furnace directly. The steam generated accumulates in the steam space.

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6. Grate: Grate is placed at the bottom of the fire box where coal is burnt.

7. Steam Dome: It is fitted at the top of the steam barrel, where the steam will be

collected.8. Headers: Headers are rectangular boxes. There are two headers, viz., Superheated

Steam Header and Wet Steam Header.

9. Chimney: The hot gases from the smoke box are discharged to the atmosphere througha short chimney. The height of the chimney is kept low so that when the locomotive is

passing under a bridge, it does not hit against the top.

10. Damper: Function of the damper is to control the quantity of air entering the fire box.

Working

See Fig. 2. Water is filled to three-fourth of the barrel so as to submerge the fire tubes and

fire box. Fuel, i.e., coal is introduced into the boiler furnace through the fire hole. Thegrate is fitted in an inclined position for charging the coal into the furnace.

A fire brick arch is fitted to the furnace above the grate. This arch deflects the

flue gases causing them to come in contact more thoroughly with the whole heating

surface of the furnace.The flue gases from the furnace pass through the flue tubes to the smoke box.

Flue gases from the smoke box are led to the atmosphere through the chimney. The pathof the flue gases is shown by arrows in the Figure.

Due to the continuous flow of hot gases from the fire box to fire .tubes, water

surrounding the tubes becomes more and more heated and wet steam is produced. The

wet steam enters the wet steam header.To remove the moisture in the wet steam and thereby, to increase the

temperature of steam, it is superheated. From the wet steam header, steam flows through

superheater tube, provided in the big fire tube. Note that the flue tube into which superheater tube is accommodated, is

comparatively larger in diameter than the flue tubes which do not contain superheater

tube.The superheated steam is accumulated in superheated steam header. It is then

led to the engine cylinder. Inside the steam dome, there is a steam stop valve in the

regulator. This valve is regulated by a regulating rod to allow the required quantity ofsteam to pass. Regulator is operated by the driver from the cabin by a hand wheel.

A blast pipe is provided at the bottom of the smoke box. The exhaust waste

steam from the engine cylinder enters the blast pipe and flows out, expanding with a high

velocity. Due to the expansion of waste steam, a partial vacuum is created within thesmoke box. This vacuum improves the movement of waste flue gases and rapidly

removes the waste flue gases from within the smoke box through the chimney. This

vacuum also draws in atmospheric air through the fuel in the furnace.The boiler is fitted with a pressure gauge, safety valve, water level indicator,

whistle, fusible plug, blow-off cock and manhole.

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3. Explain the working principle of Babcock and Wilcox boiler with neat sketch.

Babcock and Wilcox boiler is a water tube boiler. In this, water is circulated inside the

tubes and hot gases flow over the tubes.

Description [Fig. 3]

1. Water and Steam Drum (Boiler Shell): One half of the drum is filled with water Steam

remains in the top half of the drum. It is about 8 m in length and 2 m in diameter

2. Water Tubes: Water tubes are placed between the drum and the furnace in an incline position (at an angle of 12 to 15) to promote water circulation. These tubes at

connected at right angles to the uptake-header and the down-take header as shown.3. Uptake Header and Down-take Header: Drum is connected at one end to uptake headerby short tubes and at the other end to down-take header by long tubes.

4. Grate and Furnace: Grate and Furnace are provided below the uptake-header. Coal fed

to the grate through the fire door.5. Baffles: The fire-brick baffles, two in number, are provided to deflect the hot flue

gases

6. Superheater: The boiler is fitted with a superheater tube of U-shape. It is placed justunder the drum and above the water tubes. Its upper box is connected to a vertical tub the

top of which is situated in the steam space. Its lower box is connected to stop valve.

7. Mud Box: Mud box is provided at the bottom end of the down-corner.

8. Two Inspection Doors: These are provided for cleaning and inspection of the boiler.Working

A constant water level is maintained in the boiler drum. Coal is fed to the grate through

the fire door and is burnt. There is a slow moving chain grate on which the coal is fedfrom the hopper. By using the moving grate, the rate of fuel burning is controlled by

changing thickness of the coal bed and also by changing the speed of the moving grate.

Flow of flue gases: The hot flue gases from the furnace rise upward and pass across theleft-side portion of the water tubes. The baffles deflect the flue gases. Hence, the flow

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gases travel in a zig-zag manner (i.e., the hot gases are deflected by the baffles to move

upward direction, then downward and again in the upward direction) over the water tubes

and along the superheater. The flue gases finally escape to the atmosphere through thechimney . A damper is fitted as shown to regulate the flue gas outlet.

Water circulation: Water descends into the down-take header. It flows upward inclined

water tubes, then in the uptake headers and finally to the drum.That portion of the water tubes which is just above the furnace is

comparatively at a higher temperature than the rest of it. Water, its density being d rises

into the drum through the uptake-header. Here, the steam and water are separated in thedrum. Steam being lighter is collected in the upper part of the drum. The water from the

drum comes down through the down-take header into the water tubes.

A continuous circulation of water from the drum to the water tubes and water

Tubes to the drum is thus maintained. The circulation of water is known as NaturalCirculation.

The mud or sediment in the water is collected in the mud box. It is blown-off

time to time by means of a blow-off cock.

Superheating: Steam is taken from the steam space of the drum through a tube to thesuperheater. Steam is superheated in the superheater, as it receives additional heat. From

the superheater, the superheated steam goes to the stop valve and finally to the turbine.

Boiler Mountings

Pressure gauge and water level indicator are mounted on the boiler at its left end Steam

safety valve and stop valve are mounted on the top of the drum.

Salient Features

1. Its overall efficiency is higher than a fire tube boiler.2. The defective tubes can be replaced easily.

3. All the components are accessible for inspection even during the operation.

4. The draught loss is minimum compared with other boilers.5. Steam generation capacity and operating pressure are high compared with fire tube

boilers.

6. Normally, the furnace is provided with a moving chain grate. By changing the speed ofthe moving Chain grate, the rate of fuel burning can easily be controlled.

7. The water tubes are kept inclined at an angle of 10deg to 15 deg to promote water

circulation.

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3. Side Flues (SF) and Bottom Flue (BF): There are two side flues and one bottom flue

outside the shell. These flues are made of ordinary brick lines with fire bricks on their

inner faces. Side Flues are connected at their rear end to a common rear passage, which isconnected to the chimney flue.

Internal flue tubes are connected to the Bottom Central Flue. Bottom flue, in turn, is

connected to the side flues.4. Grate (G): The grate is provided at the front end of the internal flue tubes.

5. Furnace Door (FD): Coal is fed on to the grate through the furnace door.

6. Fire Bridge (FB): It is made of brick work. It is built at the end of the grate. Itspurpose is to prevent the flow of coal and ash particles into the interior of the flue tubes.

Otherwise, the coal and ash particles carried with gases may settle down as deposits on

the interior of the flue tubes. Thus, these deposits reduce the heat transfer from the flue

tubes to the water surrounding the tubes.

7. Dampers (D1 and D2): Dampers are in the form of sliding doors. These are provided

at the end of the side flues. Their function is to regulate the flow of gases from the side

flues to the chimney flue. Thus, the dampers regulate the combustion rate and thereby the

quantity of steam generated.NOTE : For examination purpose, only sectional front view and sectional side view need

be drawn.

Working

Water Circulation: Boiler shell is filled with water to three-fourth of its volume so as to

submerge both the internal flue tubes. The remaining space above water surface is the

steam space.

Path of hot flue gases

See Fig. 4. The fuel (coal) is fed through the fire door on to the grate and is burnt. The

hot flue gases leaving the grate move along the length of the internal flue tubes from thefront end upto the rear end of the shell.

As these hot gases pass through the flue tubes, heat transfer takes place from

the hot gases to the water through the walls of the flue tubes. Note that the hot gasesemerge from both the flue tubes into the respective rear enclosed chambers.

Then, from the rear enclosed chambers, the hot gases flow downwards to the

bottom flue from its rear end to the front end. This results in the heat transfer from the hotgases to the water through the bottom portion of the boiler shell which is exposed to the

bottom flue.

Then, the flue gases divide into two streams at the front end of the shell and

pass I the side flues. Thus, the two sides of the boiler shell exposed to the side flues areheated.

Passing along the two side flues, the hot gases travel upto the rear end of the

boiler 1 the chimney. Then, the flue gases are discharged into the atmosphere through thechimney.

The above arrangement of the flow passages of the hot flue gases increases the

heating surface of the boiler to a large extent. The path of the flow of the flue gases hasbeen shown by arrows in the Figure.

Dampers (D1 and D2): Dampers control the flow of hot flue gases and regulate the

combustion rate of the fuel as well as the steam generation rate of the boiler. Dampers

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control the draught. The object of draught is to ensure complete combustion of coal by

supplying sufficient quantity of air through the fuel.

Boiler Mountings

The boiler is fitted with necessary mountings. Water level indicator and pressure gauge

fitted at the front. Steam stop valve, Safety valve, High steam and low water safety valve

and Manhole are fitted on the top of the shell.High Steam and Low Water Safety Valve: It is mounted over the low water alai

apparatus. It is a combination of two valves. One valve blows off steam when t working

pressure of steam exceeds the design value. The other valve blows off ste when the levelof water falls below the normal level.

Blow-off Cock: It is placed below the front end of the shell for the removal of mud and

sediments. It is also used to empty water in the boiler during inspection.

Fusible Plug: It is placed at the top of the internal flues just above the grate. It preventsoverheating of the boiler tubes by extinguishing the fire when the water le falls below the

required level.

Manhole: It is provided at the top of the shell for the purpose of periodical inspect and

repair.Salient Features The arrangement of flow of flue gases in Lancashire boiler increases the heat surface ofthe shell to a large extent.

It has a very good steaming capacity.

Coal of inferior quality can be used in this boiler without any operational defects.

Superheater can be easily incorporated into the system at the end of the internal tubes.Thus, overall efficiency of the boiler can be increased.

Feed water used does not require strict treatment before use in the boiler shell.

Low initial cost. Its maintenance is easy. Due to moderate working pressure and slow evaporation rate, it is excellent to supply

steam to process industries like paper mills, sugar mills, chemical industries, etc.,

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7. What are the advantages and disadvantages of water tube boilers over fire tube

boilers?

Advantages

1. Steam can be generated at very high pressures in water tube boilers.2. In water tube boilers, water is contained in a large number of tubes. Hence, heatingsurface is more than that of fire tube boilers. Thus, evaporation rate increases.

3. Circulation of water is more positive in water tube boilers. Steam can be raised quickly

than is possible with a fire tube boiler. Hence, it can be used for variations of load.4. The hot gases flow almost at right angles to the direction of water flow. Hence,

maximum amount of heat is transferred to water.

5. A good and rapid circulation of water can be made.6. Bursting of one or two water tubes does not affect the boiler very much with regard to

its working. Hence, water tube boilers are sometimes called as Safety Boilers. But,

bursting of flue tubes in a fire tube boiler causes serious problems.

7. The different parts of a water tube boiler can be separated. Hence, it is easy totransport.

8. For a given power, water tube boiler occupies less space than that of fire tube boiler.

9. It is suitable for use in thermal power plants (because of various advantages listedabove).

Disadvantages

1.. Water tube boiler is not suitable for impure and sedimentary water, as a small depositof scale may cause the overheating and bursting of tubes. Hence, water treatment is very

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essential for water tube boilers.

2. Failure in feed water supply even for a short period is liable to make the boiler

overheated. Hence, the water level must be watched very carefully during operation of awater tube boiler.

3. Maintenance cost is high.

4. Initial cost of water tube boiler is more than that of fire tube boiler.5. Water tube boilers are not suited for mobile purpose.

8. what are the advantages of high pressure boilers?

1. Method of Water Circulation: Water circulation through the boiler may be either

natural circulation due to the density difference of water and steam or by forced

circulation. In high pressure boilers, water circulation is made with the help of a

centrifugal pump which forces water through the boiler tubes. This is called ForcedCirculation of Water. Forced circulation increases the rate of heat transfer and hence

increases the steam generating capacity of boilers.

2. Size of Drum: The high pressure boilers are characterized by the use of very small

steam separating drum or by the complete absence of any drum.3. Type and Arrangement of Tubes: The heat of combustion is utilized more efficiently

by the use of small diameter and light weight tubes in large numbers. To avoid largefrictional resistance to the flow of water, the high pressure boilers have a parallel set of

arrangement of tubes.

4. Compactness: High head required for natural circulation is eliminated by using forced

circulation. The space required is less and the arrangement is compact.

5. Foundation Cost: Due to the light weight tubes and small size drum required and the

arrangement being compact, the cost of foundation is reduced.

6. Efficiency: Overall efficiency of the power plant is increased upto 40%, by using highpressure superheated steam. Also, steam can be raised quickly after the boiler is fired.

7. Cost of Electricity: The cost of electricity production is less.

8. Overheating: All the parts are uniformly heated. Therefore, the danger of overheatingis reduced. Also, thermal stress problem is avoided.

9. Scale Formation: The tendency of scale formation is eliminated due to the high

velocity of flow of water through the boiler tubes.

10. Forced Draught using Blower: The flow of flue gases through the boiler furnace,

economizer, pre-heater and chimney require a difference of pressure equal to that

necessary to accelerate the hot gases. The purpose of draught is to supply required

quantity of air for combustion. Also, it removes the burnt gases from the system.Draught can be obtained by the use of chimney or blower. The former is called Natural

Draught and the later is Forced Draught. In the forced draught, a blower is located near

the base of the chimney and accelerates the flow of hot gases through the economizer,pre-heater and chimney, thus improving the efficiency of the system.

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9. Explain the working principle of La-Mont Boiler

A Forced Circulation Boiler was first introduced by La-Mont in the year 1925 which is

used in power plants. This is a modern high pressure water tube type steam boilerworking on forced circulation system.

Description [Fig. 9]

Fig. 9 shows the flow-circuit of La-Mont boiler.1. Furnace: In the furnace, water wall pipes are used to provide a large heating surface,

thereby to increase the capacity of the boiler and also to cool the furnace wall. Water

Wall Pipes are vertical or inclined pipes in the interior walls of the furnace. These pipesare connected at the top and bottom to the other parts of the boiler so that there is

continuous rapid circulation of water through the water wall.

2. Steam Separator Drum (Boiler): Steam Separator Drum is placed wholly outside the

boiler setting.

3. Circulating Pump: It is a centrifugal pump used for forced circulation of water.

Forced circulation of water prevents the tubes from being overheated.

4. Evaporator Tubes: These are provided above the furnace.

5. Convection Superheater: The wet steam should not b used in the steam turbine. Thepresence of moisture in it will cause corrosion of turbine blades. To raise the temperature

of steam and thereby to increase the turbine efficiency, wet steam is passed into theSuperheater.

6. Economiser: The feed pump supplies feed water to the economiser.

7. Air Preheater: A blower draws atmospheric air and supplies compressed high

pressure air (forced draught) to the air preheater.

Working Principle

Water from the circulating pump is circulated through the evaporator tubes.Hot gases from the furnace heat the water and evaporate into steam. Wet steam from the

evaporator enters the steam space in the steam separator drum.

In the convection superheater, the moisture in the wet steam is removed andsuperheated steam is produced. The principle of convection superheater is similar to

steam generating tubes of the boiler. The hot flue gases at high temperature sweep over

convection superheater tubes and raise the steam temperature, producing superheatedsteam. The superheater, thus, receives heat from the flue gases flowing from the furnace,

entirely by convective heat transfer. Such a superheater is conveniently located, since it is

not necessary for it to see the furnace.

Feed water is supplied by the feed pump to the economiser. Economiser isused to preheat the feed water using the waste hot gases before going to the chimney.

Thus, some of the heat in the hot gases, which otherwise gets wasted, is used to preheat

the feed water. This results in an increase in the boiler thermal efficiency.The heat of the exit gases cannot be fully extracted through the economizer.

These exit gases preheat the air from the blower in the Air Preheater. The preheated air is

supplied to the furnace for combustion.

Capacity

Due to forced circulation of water, the rate of heat transfer and the steam capacity of the

boiler are increased. The capacity of La-Mont boiler is about 50 Tonnes/hr of superheated

steam at a pressure of 170 bar and at a temperature of 500C.

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Boiler Mountings

This boiler is fitted with mountings, viz., water gauge, pressure gauge and block-off cock.

Also, three safety valves are fitted as per Indian Boiler Act. The design, manufacture anderection of these boilers are very difficult. It requires skilled personnel and huge

investment.

10. Explain the working principle of BENSON BOILER

Principle: The presence of steam bubbles in contact with the surface of water tubes

seriously impairs heat transmission from the flue gases to water. By raising the boilerpressure to the critical pressure of steam (225 kgf/sq.cm.), this difficulty is overcome as

suggested by Mark Benson in 1922. At the critical pressure, water and steam have the

same density and no bubbles form.The first modern high pressure drumless boiler developed by Benson was put into

operation in 1927 in West Germany power station.

Absence of Drum: Benson boiler is a water tube type, forced circulation, high pressure

boiler. The unique characteristic of this boiler is that it does not use any drum at all. Theentire process of heating, steam generation and superheating is done in a single

continuous tube. Hence, it is also known as Once-Through Boiler. It withstands very high

pressure, even higher than the critical pressure of steam.

Description [Fig. 10]

Fig. 10 shows the flow-circuit of Benson boiler. Feed pump is connected to the

Economiser. Radiant Evaporator is placed just above the furnace. It is connected to theeconomiser at one end and to the convection evaporator at the other end. Convection

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Evaporator is connected to the radiant evaporator at one end and to the convection

superheater at the other end. Convection Superheater is connected to the convection

evaporator at one end and to the steam turbine at the other end.A Blower draws atmospheric air and supplies compressed high pressure air (forced

draught) to the Air Preheater.

Working The feed water from the feed pump is circulated through the Economiser

Tubes. Hot flue gases pass over the economiser tubes and the feed water is preheated.

Economiser is used to preheat the feed water using the waste hot gases before going tothe chimney. Thus, some of the heat in the hot gases, which otherwise gets wasted, is

used to preheat the feed water. This results in an increase in the boiler thermal efficiency.

The preheated feed water from the economiser flows into the Radiant

Evaporator with radiant parallel tube sections. The radiant evaporator receives heat fromthe burning fuel in the furnace through radiation process. Thereby, the major part of

water is converted into steam in it.

The remaining water is evaporated in the Convection Evaporator, absorbing

the heat from the hot gases by convection. Thus, the saturated high pressure steam at apressure of 230 bar is produced. The saturated steam available from the convection

evaporator is passed through the Convection Superheater, where the saturated steam issuperheated to 650C.

Note that the radiant evaporator, the convection evaporator and the convection

superheater are all arranged in the path of the flue gases. The superheated steam is then

supplied to the steam turbine.The heat of the exit gases cannot be fully extracted through the economizer.

These exit gases preheat the air from the blower in the Air Preheater. The preheated air is

supplied to the furnace for combustion.

Capacity: Capacity of Benson boiler is about 150 Tonnes/hr at a pressure of 230 bar and

at a temperature of 650C. (Efficiency may be improved by running the boiler at a

pressure slightly lower than the critical pressure).

Salient Features

1. High Thermal Efficiency: No higher limit for higher steam pressure. Therefore,

highest steam pressure can be used to achieve high thermal efficiency.

2. Less Weight and Less Cost: As there are no drums, the total weight of Benson boiler

is 20% less than other boilers. This also reduces the cost of the boiler.

3. Load Fluctuations: Sudden fall of demand creates circulation problems due to bubble

formation. This never occurs in Benson boiler.

4. Easy Transportation: As no drums are required, the transfer of Benson parts is easy.

Majority of the parts may be carried to the site without pre-assembly.

5. Once-through Boiler: Since no drum is used, this is an once-through boiler. The feedwater entering at one end is discharged as superheated steam at the other end.

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REVIEW QUESTIONS

PART A1. What is a heat engine? How do you classify heat engines?

2. Define the terms: Stroke, Top Dead Center, Bottom Dead Center, Compression Ratio,

Air Fuel Ratio, Stroke Volume, Brake Horse Power and Thermal Efficiency.3. What do you understand by scavenging?

4. Why are two stroke engines preferred for two wheelers?

5. Compare and contrast four stroke cycle engines with two stroke cycle engines.6. Discuss the merits and demerits of diesel engine over petrol engine.

7. Defines the terms: External Combustion Engine and Internal Combustion Engine.

8. Define Compression Ratio of an I.C. engine.

9. What is the function of deflector in a two stroke engine?10. What is the function of the choke in a carburettor?

11. What is meant by carburetion?

12. What is the function of the float, float chamber and needle valve assembly in a single

jet carburettor?13. State any two limitations of a single jet carburettor.

14. How do you provide an extra-rich mixture to the petrol engine during starting in coldweather?

15. What is the function of choke in the carburettor?

16. Name the three ports provided at the cylinder walls in a two stroke engine.

17. Why is diesel engine called as compression ignition engine?18. How do the three ports in a two stroke engine function?

19. What is the function of a spark plug?

20. State any two advantages of LPG as SI engine fuel.21. Enumerate the various types of steam generators.

22. What are the two different uses of steam produced in the boiler?

23. Where do you use process system?24. State any two primary requirements of a boiler?

25. How do you classify boilers?

26. State any two salient features of Cochran boiler.27. What is the function of baffles in Babcock and Wilcox boiler?

28. What do you understand by natural circulation?

29. State the functions of:

i) Safety Valve iii) Fusible plugii) Stop Valve iv) Blow-off Cock

30. State any two advantages of high pressure boilers.

31. What do you understand by forced circulation?

PART B

1. Describe the principal parts and functions of a four stroke I.C. engine with a sketch.

2. Describe the principal parts and functions of a two stroke I.C. engine with a sketch.

3. Explain the working of a four stroke petrol engine with the help of PV diagram.

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4. Explain the working of a four stroke diesel engine with the help of PV diagram.

5. Draw and explain the sequence of operations of a two stroke cycle petrol engine.

6. Draw and explain the sequence of operations of a two stroke cycle diesel engine.7. Describe with a sketch the design of a single jet carburettor.

8. Sketch and explain the working of a diesel fuel pump.

9. Explain the working principle of the fuel injector with a neat diagram.10. Draw a schematic circuit diagram of a coil (battery) ignition system and label all the

components. State the functions of each component.

11. Discuss briefly the ignition system of SI engine.12. Explain the working principle of spark plug with a neat diagram.

13. Explain, with a neat sketch, the working of a Cochran boiler.

14. Sketch and describe the working principle of a locomotive boiler.

15. With the aid of neat sketches, explain the constructional features and functioning of aLancashire boiler.

16. Give an outline sketch showing the arrangement of water tubes and furnace of a

Babcock and Wilcox boiler. Indicate on it the path of the flue gases and water circulation.

Show the positions of superheater, fusible plug and blow-off cock. Mention the functionof each.

17. Explain, with the help of a diagram, the construction and working of a fire tubeboiler.

18. Distinguish between fire tube boilers and water tube boilers.

19. Name the important boiler mountings and briefly explain their functions.

20. Describe, with a neat sketch, a water level indicator. Explain how the flow of steamand water is automatically stopped when the glass tube breaks.

21. Explain how the Bourdon type pressure gauge works.

22. Explain why safety valves are needed in a boiler. Draw a neat sketch of a springloaded safety valve and explain its working.

23. What is the purpose of a fusible plug? Explain.

24. Define a High Pressure Boiler. Mention the advantages of high pressure boilers.25. Explain the working principle of La-Mont High Pressure Boiler with a neat sketch.

26. Discuss the working and the salient features of Benson Boiler with a neat sketch.

27. Discuss the concept of cogeneration. Explain topping cycle system.28. Explain the bottoming cycle system of cogeneration with a suitable sketch.

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