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Comparison of petrol engine and diesel engine:

S.No Parameters Petrol engine (SI) Diesel engine (CI)

1. Thermodynamic cycle Otto cycle Diesel cycle for slow speed

Dual cycle for high speed engines

2. Intake Fuel (petrol) and air is admitted

into the cylinder during suction

stroke

Air alone is admitted into the cylinder

during suction stroke

3. Fuel used Petrol Diesel

4. Air­fuel ratio 10:1 to 20:1 18:1 to 100:1

5. Compression ratio Upto 11

Average value 7 to 9

12 to 24

Average value 15 to 18

6. Fuel supply By carburettor By injection

7. Ignition system Spark ignition Compression ignition

8. Operating pressure

Compression

pressure

Maximum

pressure

7 bar to 15 bar

45 bar to 60 bar

30 bar to 50 bar

60 bar to 120 bar

9. Operating speed High speed :2000 to 6000 r.p.m Low speed : 400 r.p.m

Medium speed: 400 to 1200 r.p.m.

High speed : 1200 to 3500 r.p.m.

10. Thermal efficiency Less due to lower compression

ratio

More due to higher compression ratio

11. Starting of the engine Easy to start Difficult to start the engine since more

cranking effort is required

12. Weight Lighter Heavier

13. Control of power Quantity governing Quality governing

14. Calorific value 44 MJ/kg 42 MJ/kg

15. Initial cost Less More

16. Cost of running High Low

17. Maintenance cost Minor maintenance required Major overhaul required but less

frequently

18. Super charging Limited by detonation. Used only

in aircraft engines

Limited by blower power and

mechanical and thermal stresses. Widely

used.

19. High powers No Yes

20. Vibration & noise Very less More, due to high operation pressure

21. Engine life Less than 60,000 km More than 1,50,000 km

22. Space Less space More space

23. Uses Light duty vehicles

Ex; Mopeds, scooters, motor

cycles, passenger cars, air­crafts

etc.

Heavy duty vehicles

Ex; Buses, trucks, locomotive, tractors,

earth moving machinery and stationary

generating plants

COMPARISON OF FOUR STROKE AND TWO STROKE ENGINES

S.No Parameters Four­stroke engine Two­stroke engine

1. Number of strokes per

cycle

Four strokes per cycle Two strokes per cycle

2. Number of revolutions per cycle

Two One

3. Number of cycles per n=N/2 n=N

min (n)

N=Engine speed

4. Power stroke and power

output

One power stroke for two

revolutions. Hence, power output for

the same size of engine is less.

One power stroke per revolution.

Hence, power output for the same

size of engine is more.

5. Torque Not uniform uniform

6. Direction of rotation of

the crankshaft

The crankshaft rotates only in one

direction

The crankshaft can rotate in either

directions ( reversible engine)

7. Valves /ports Inlet and exhaust valves are

required

Valves are operated by means

of cam using valve­gear

mechanism

Inlet, transfer and exhaust

ports are made the cylinder

walls

Piston movement itself covers

and uncovers the ports

8. Flywheel Heavier flywheel is required because

of non­uniform torque on the

crankshaft

Lighter flywheel is sufficient of uniform

torque on the crankshaft

9. Admission to the charge The charge is directly admitted into

the engine cylinder during the suction

stroke

The charge is first admitted into

crankcase and then transferred to

engine cylinder

10. Exhaust of products of

combustion

Due to upward movement of the

piston during exhaust stroke

Due to scavenging

11. Fuel consumption Less More

12. Mechanical efficiency Low High

13. Noise Less More

14. Engine size Heavy and bully More compact

15. Initial cost High Low

16. Cooling and lubrication

requirements

Required less Severely required

17. Starting of the engine Fairly difficult Easy

18. Rate of wear and tear Lesser Greater

19. Uses Used in high power heavy vehicles

such as buses, lorries, trucks, cars, etc

Used in low­power light­vehicles

as motor cycles, scooters, mopeds,

etc.

Cooling system for I.C EnginesThe temperature of the burning gases in the engine cylinder is about 2000 to 2500 ºC.

www.vidyarthiplus.comThis heat is absorbed by the engine components like cylinder head, cylinder walls, piston

and valves. The heating of these parts may cause the following.

The strength of piston and cylinder body may be reduced. The engine valves may twist due to overheating. The lubricating oil may decompose and become gummy. It also gives carbon deposits. It causes thermal stresses in the engine parts, which may lead to their distortion. Pre­ignition may occur due to the overheating of spark plug. It reduces the volumetric efficiency of the engine.

In order to avoid the effects of overheating, It is necessary to provide a cooling system. However, a cooling system should remove only about 30% of the heat generated by the combustion of fuel.

The following two methods of cooling are generally used in internal combustion engine.

Air cooling or direct cooling Water cooling or indirect cooling

Air cooling

Air cooling is used in small engines like, air craft engines, engines used in scooters, motor cycles, etc. In air cooling system, the air is allowed to circulate around the cylinder block and cylinder head. Radiating fins are provided on the outer surface of the cylinder block and cylinder head. This increases the heat radiating surface. As more air comes in contact with the cylinder, the heat is removed efficiently. The high velocity of air required for cooling is obtained by the forward

motion of the engine itself. Air circulating fan is provided in stationary engines.

Advantages: The design of air cooling system is much simpler. Lighter in weight. The system is free from leakage and freezing of water. Less space is sufficient. This is very much useful in water scarcity areas.

Disadvantages: This system is limited to small engines. The engine parts are not uniformly cooled.

www.vidyarthiplus.com Air cooled engine produces loud noise. Separate circulating fan is needed in stationary engines.

Water cooling system:

Water cooling is used in light and heavy vehicles such as automobiles, buses, lorries, trucks, etc. In water cooling system, this water is allowed to circulate around the cylinder block and cylinder head to carry the heat. It consists of radiator, fan, water pump, water jacket, thermostat valve, radiator shutters, etc.

The water pump driven by the engine draws cold water from the radiator. The cold water is forced to circulate through water jackets of cylinder block and cylinder head. Thus, the circulating water cools the engine parts by absorbing heat. Them the water enters the radiator top and flows from top to bottom. While flowing through radiator tubes, the heat of water is transmitted to the air drawn by a fan through radiator. Then the water is cooled and re circulated.However, the temperature of the cooling water should not fall 75 ºC. A thermostat valve is provided

between the engine and the radiator top to control the temperature of the cooling water. The valve remains open under normal operating temperatures. When the temperature falls below normal, the valve is closed to bypass the water. Thus cooling of water is automatically ceased. When the heat of the engine is more, the radiator shutters are opened to allow more air to flow through the radiator. Thus, the rate of cooling is increased. A drain tap is provided at the bottom of the radiator to remove the water periodically.

Advantages More efficient cooling can be obtained. Hence, efficiency of the engine is more. Higher compression ratio is permitted. The engine parts are uniformly cooled. Water cooled engines can be installed anywhere in the vehicle

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Disadvantages Water cooling system is heavier in weight. More maintenance is required. A slight leakage of water may stop the engine. Freezing of water causes trouble during cold weather. Water circulating pump and radiator pump consume power.

Comparison of air cooling and water cooling systemSl.No. Air cooling system Water cooling system1. Engine design is simple Engine design is not much simple2. Weight is less Weight is more3. No leakage problems Leakage of water may occurs4. No risks of freezing of water in cold climates Freezing of water in cold climate may cause

trouble5. Less maintenance is sufficient More maintenance is required6. Require less space More space is required7. No danger occurs due to the damage in cooling

system.A small damage in the cooling system may cause danger.

8. The cost for cooling system is less. The cost for cooling system is more.9. Cooling is not so efficient Cooling is more efficient10. Engine parts are not uniformly cooled Engine parts are uniformly cooled11. Higher compression ratio is not permitted Higher compression ratio is permitted12. It is used in light duty vehicles It is used in heavy duty vehicles

Lubrication system for I.C.Engine

In an internal combustion engine, moving parts and rotating parts rub against each other and frictional force is developed. Due to this frictional force, heat is generated and the engine parts wear

quickly. Also, power is lost due to friction. Lubricants are introduced between moving and rotating parts to reduce heat loss wear and tear.

Purpose of lubrication or functions of lubrication in I.C Engines To reduce friction between moving parts To reduce wear and tear of the moving parts To reduce the power loss due to friction To dissipate the heat generated from the moving parts To provide cushioning effect against the shocks of the engines To clean the moving parts by dissolving the impurities during its circulation To provide and effective seal against high pressure gases in the cylinder from leaking out. To reduce the noise while the moving parts rub against each other. To reduce the corrosion and erosion of moving parts.

Methods of lubrication Petroil lubrication system or mist lubrication system Wet sump lubrication system

www.vidyarthiplus.com Gravity lubrication system Splash lubrication system Pressure lubrication Semi­pressure lubrication system

Dry sump lubrication systemSplash lubrication system

This lubrication system is employed in stationary internal combustion engines. In this system, the lubricating oil is filled in the sump at the bottom of the crank case. Scoops are attached to the big end of the connecting rod.When the piston is at the bottom of its stroke, the big end of the connecting rod, crank pin and scoop dip into oil. As the crank shaft rotates, the scoop picks up the lubricating oil from the sump and the surplus oil is splashed over the engine parts in the form of droplets by centrifugal action. Splashed oil droplets settle on the surface of the piston, cylinder walls, cam shaft and crank shaft bearing, etc. and lubricate these parts. The splashed oil is drained back to the sump after lubricating.

Pressure lubrication system or forced lubrication system

This system is used in light engines like automobile engines. In this method, the lubricating oil is forced under pressure for efficient lubrication. It consists of oil sump, oil pump, oil gallery, oil filter; pressure relief valve, pressure gauges and oil dip stick.

The oil pump (gear pump) is submerged in the lubricating oil and it is driven by the cam shaft. The gear pump supplies oil to the oil gallery with high pressure through filters. From the oil gallery, the oil is distributed under pressure to the different parts of the engine through oil tubes. Separate oil tubes carry oil for lubricating big end bearings, timing gears, crank shaft, cam shaft, valve assembly, etc.Oil from the gallery is supplied to the crank shaft bearing. The oil flows to the connecting rod big end bearing through the diagonally drilled hole in the crank shaft. A through hole is drilled at the centre of the connecting rod

The oil from the big end bearings flows to the small end bearings through the hole and lubricate it. The excess oil is drained back into the oil sump. The delivery pressure of the oil is controlled by relief valve. A pressure gauge indicates the oil pressure in the system. An oil dip stick is provided to measure the oil level in the sump.

Requirements of Good Lubricating Oil

High viscosity index (if the change of viscosity with temperature is less, the oil is rated to have a high viscosity index).

High flash point and low pour point temperatures. Non­corrosive. Good detergent quality to keep the rubbing surfaces clean High film strength (ability to maintain a thin film of oil even at high load). The quality of lubricating oil is improved by adding different types of additives such as the

viscosity index improvers, the corrosion inhibitors, the detergent additives, and the film strength additives

Lubricating oils are graded in SAE number according to the viscosity. For example, the viscosity of oil SAE 40 is higher than that of oil SAE 30. For different engines or machines, we should use the oil of correct grade as specified by the manufacturer. In I.e. engines, oil should

www.vidyarthiplus.combe filled in the crank case to the specified level, as indicated by the dip stick.

BOILERS:Boiler is also known as steam generator. It is a closed metallic vessel. In the boiler, water is converted into steam above atmospheric pressure by the application of heat. Fossil fuels are the source of heat energy.The boiler transfers heat produced by the combustion of fuel (solid, liquid or gas) to water and generates steam above atmospheric pressure. The steam generated by the boiler is used for power generation, process heating and space heating purposes.Definition and function:Boiler is defined as a closed metallic vessel in which the water is heated beyond the boiling state by the application of heat liberated by the combustion of fuels to convert into steam.Function: The function of a boiler is to supply the steam at the required constant pressure. The quality of steam supplied may be either wet or dry or superheated.The steam generated by boilers may be used for the following purposes.

Steam is used for power generation by expanding it in a steam engine or turbine. Steam is used in winter air conditioning of residential and industrial building. Steam is used to heat water for hot water supply. Steam is used in textile industries for some processes like sizing, bleaching, etc. Steam is used in sugar mills and chemical industries for heating and in modern rice mills for

boiling paddy, etc.

CLASSIFICATION OF BOILERS

The steam boilers are classified according to the following conditions

1. According to the Circulation of water and hot gases

a) Fire tube boiler [Cochran Boiler]

The hot gases pass through the tubes and water surrounds these tubes.

Ex: Cochran, Locomotive, Lancashire and Cornish boilers

b) Water tube boiler [Babcock and Wilcox Boiler]

The water is inside the tubes and hot gases surround the tubes.

Ex: Bobcock and Wilcox, Lamont, Benson and Loeffler boilers.

2. According to the axis of the boiler shell

a) Vertical boiler [Cochran Boiler]

b) Horizontal boiler [Lancashire Boiler]

3. According to the location of furnace

a) Internally fired Boiler [all fire tube boilers] [Cochran Boiler]

when the furnace is located inside the boiler shell, they are called internally fired.

Ex: Cochran, locomotive, and Lancashire boilers

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b) Externally fired Boiler [all water tube boilers] [Babcock and Wilcox Boiler]

In these boilers, the furnace is located outside the boiler shell.

Ex: water tube boilers such as Bobcock and Wilcox, Lamont, Benson boilers.

4. According to Pressure of steam produced

a) Low pressure boilers ­ pressure < 30 atm.

b) Medium pressure boilers ­ pressure = 30 to 70 atm.

c) High pressure boilers ­ pressure Upto 150 atm.

d) Super pressure boilers ­150 to 190 atm.

e) Super critical boiler ­ pressure> 225 atm.

5. According to the method of water circulation

a) Natural circulation [all low pressure boilers] [Cochran Boiler]

Water is circulated by natural convection current that are set up due to the temperature difference.

Ex: Most of the low pressure and low capacity boilers such as Lancashire, Locomotive and

Bobcock and Wilcox boilers are natural circulation boilers.

b) Forced circulation [all high pressure boilers] [Lamont Boiler]

The water is circulated by a centrifugal pump driven by a motor

Ex: Most of the high pressure and high capacity boilers such as Lamont, Loeffler, and Benson

boilers are forced circulation boilers.

6. According to the use of the Boiler

a) Stationary Boiler [Cochran Boiler]

These boilers do not move from one place to another.

Ex: Cochran and Benson.

b) Mobile or Portable Boiler [Locomotive boiler]

These boilers are fitted on vehicles that can move from one place to another.

7. According to the number of tubes

a) single tube boilers

As the name implies, these boilers have only one ( fire/water ) tube.

Ex: simplex vertical boiler and Cornish boiler.

b) Multiple tubular boilers

In multiple tubular boilers, there are more than one fire tubes or water tube.

Ex: Lancashire, Locomotive and Bobcock & Wilcox boilers.

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8. According to Capacity

a) Low Capacity boilers ­ capacity < 20 tonnes of steam per hour.

b) Medium capacity boilers ­ capacity = 20 to 75 tonnes /hr.

c) High capacity boilers ­ capacity> 100 tonnes / hr.

9) According to the type of fuel (heat source) used

a) Coal fired boilers

b) Oil fired boilers

c) Gas fired boilers

d) Nuclear energy boilers

Comparison of fire tube and water tube boilers:

Sl.No Particulars Fire tube boilers Water tube boilers

1. Position of water and hot

gases

Hot gases inside the tubes and

water outside the tubes

Water inside the tubes and hot

gases outside the tubes

2. Mode of firing Generally internally fired Externally fired

3. Operating pressure Operating pressure limited to

16 bar

Can work under as high

pressure as 100 bar.

4. Rate of steam production Lower Higher

5. Thermal efficiency Low High

6. Suitability Not suitable for large power

plants

Suitable for large power

plants

7. Risk on bursting Involves lesser risk on

explosion due to lower pressure

Involves more risk on bursting

due to high pressure.

8. Type of fuel Only coal is suitable as fuel Suitable for any type of fuel

9. Floor area For a given power it occupies

more floor area

For given power it occupies

less floor area

10. Construction Difficult Simple

11. Transportation Difficult Simple

12. Shell diameter Large for same power Small for same power

13. Chances of explosion Less More

14. Treatment of water Not so necessary More necessary

15. Initial and maintenance costs Low High

16. Maintenance Difficult Easy

17. Accessibility of various parts Various parts not so easily

accessible for cleaning, repair

and inspection

Various parts are more

accessible

18. Requirement of skill Require less skill for efficient

and economic working.

Require more skill and careful

attention.

BOILER TERMS

The details of the boilers are listed below:

1. Shell: The shell of a boiler consists of one or more steel plates bent into a cylinder form and

riveted or welded together. The shell ends are closed with the end plates. The shell of the boiler is

the main container usually of cylindrical shape, which contains water and steam. It is a pressure

vessel and is welded construction.

2. Setting: The primary function of setting is to confine heat to the boiler and form a passage for

gases. It is made of brick work and may form the wall of the furnace and combustion chamber. It

also provides support in some types of boilers (Lancashire boilers).

3. Grate: It is the platform in the furnace upon which fuel is burnt and it is made of CI bars. The

bars are arranged that air may pass on to the fuel for combustion. The area of the grate on which the

fire rests in a coal or wood fired boiler is called grate surface.

4. Furnace: A furnace is another important part of the boiler. This may be a grate to burn coal or a

burner to atomise and burn pulvarized coal or liquid fuel or gas.

5. Water Flow Path: Water flow path is the path followed by the water in the boiler (particularly in

water tube boilers) during the process of absorption of heat of hot gases should be transferred to the

water for steam generation. The boiler efficiency mainly depends upon the gas flow path.

6. Gas Flow Path: The hot gas flow path either in fire tubes or around water tubes should be

arranged in such a way that maximum heat of hot gases should be transferred to the water for steam

generation. The boiler efficiency mainly depends upon the gas flow path.

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7. Steam Path: In most of the boilers, the steam is taken out preferably at the top of the shell to

avoid water particles being carried with the steam. To reduce the water particles carried by the

steam, it is generally taken out through steam separators, in the case of large boilers.

8. Foaming: Formation of steam bubbles on the surface of boiler water due to high surface tension

of water.

9. Scale: A deposit of medium to extreme hardness occurring on water heating surfaces of boiler

because of an undesirable condition in the boiler water.

10. Blowing off: The removal of the mud and other impurities of water from the lowest part of the

boiler is termed as ‘blowing off’. This is accomplished with the help of a blow off cock or valve.

11. Lagging: Blocks of asbestos or magnesia insulation wrapped on the outside of a boiler shell or

steam piping.

12. Refractory: A heat insulation material, such as fire brick or plastic fire clay, used for such

purposes as lining combustion chambers.

13. Fittings: The valves and gauges which are necessary for the safety of the boiler are known as

mountings. Water level indicator, safety valve, blow­off cock and fusible plug are some of the

mountings.

14. Accessories: Some equipment like economizer, air preheater and super heater is attached to

the boiler to improve its overall efficiency. The economizer and air preheater are used to extract

maximum heat from the flue gases and superheater is used to increase the temperature of steam

above saturation temperature.