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Lab Manual

B.Tech 1st Year

Basic Mechanical Engineering Lab

Dev Bhoomi Institute of Technology Dehradun www.dbit.ac.in

Affiliated to

Uttrakhand Technical University, Dehradun www.uktech.in

http://www.dbit.ac.in/

http://www.uktech.in/

Experiment No-1

Aim: To study of the constructional details and working of vapour compression refrigeration unit/ refrigerator.

Equipment/Apparatus: Model of any commercial / industrial refrigeration unit or an old / new refrigerator.

Theory: A vapour compression refrigeration system is an improved type of air refrigeration system in which a suitable

working substance, termed as refrigerant is used. It condenses and evaporates at temperatures and pressures close to

the atmospheric conditions. The refrigerant used does not leave the system but is circulated throughout the system

alternately condensing and evaporating. The vapour compression refrigeration system is now days used for all-purpose

refrigeration. It is used for all industrial purpose from a small domestic refrigerator to a big air conditioning plant.

The components used are:

1. Evaporator

2. Compressor

3. Condenser and receiver

4. Throttling device

Fig. 1.1 Components of vapour compression refrigeration system

The vapour compression refrigeration cycle is based on the following factor: Refrigerant flow rate, Type of refrigerant

used, Kind of application viz air-conditioning, refrigeration, dehumidification etc. The system equipments/

components proposed to be used in the system. The vapour compression refrigeration cycle is based on a circulating

fluid media, viz, a refrigerant having special properties of vaporizing at temperatures lower than the ambient and

condensing back to the liquid form, at slightly higher than ambient conditions by controlling the saturation temperature

and pressure. Thus, when the refrigerant evaporates or boils at temperatures lower than ambient, it extracts or removes

Dev Bhoomi Institute Of Technology Department of Mechanical Engineering

LABORATORY MANUAL

PRACTICAL INSTRUCTION SHEET

EXPERIMENT NO. 1 ISSUE NO. : ISSUE DATE:

REV. NO. : REV. DATE :

PAGE: 2-3

LABORATORY Name & Code: Basic Mechanical Engineering Lab (PME-101/201) SEMESTER: First & Second

heat from the load and lower the temperature consequently providing cooling. The super-heated vapour pressure is

increased to a level by the compressor to reach a saturation pressure so that heat added to vapour is dissipated/ rejected

into the atmosphere, using operational ambient conditions, with cooling medias the liquid from and recycled again to

form the refrigeration cycle.

Fig. 1.2 Schematics of a refrigerator

The refrigeration cycle can be explained schematically in the two diagrams i.e. Pressure enthalpy diagram Temperature

entropy diagram The working of vapour compression refrigeration cycle and function of each above component is

given below. Figure: Components of vapour refrigeration system

(a) Evaporator: The liquid refrigerant from the condenser at high pressure is fed through a throttling device to an

evaporator at a low pressure. On absorbing the heat to be extracted from Media to be cooled, the liquid refrigerant

boils actively in the evaporator and changes state. The refrigerant gains latent heat to vaporizes at saturation

temperature/ pressure and further absorbs sensible heat from media to be cooled and gets fully vaporized and super

heated. The “temperature-pressure relation chart” table can determine the pressure and temperature in the evaporator.

(b) Compressor: The low temperature, pressure, superheated vapour from the evaporator is conveyed through suction

line and compressed by the compressor to a high pressure, without any change of gaseous state and the same is

discharge into condenser. During this process heat is added to the refrigerant and known as heat of compression ratio

to raise the pressure of refrigerant to such a level that the saturation temperature of the discharge refrigerant is higher

than the temperature of the available cooling medium, to enable the super heated refrigerant to condense at normal

ambient condition.

Different types of compressors are reciprocating, rotary and centrifugal and are used for different applications.

(c) Condenser: The heat added in the evaporator and compressor to the refrigerant is rejected in condenser at high

temperature/ high pressure. This super heated refrigerant vapour enters the condenser to dissipate its heat in three

stages. First on entry the refrigerant loses its super heat, it then loses its latent heat at which the refrigerant is liquefied

at saturation temperature pressure. This liquid loses its sensible heat, further and the refrigerant leaves the condenser as

a sub cooled liquid. The heat transfer from refrigerant to cooling medium (air or water) takes place in the condenser.

The sub-cooled liquid from condenser is collected in a receiver (wherever provided) and is then fed through the

throttling device by liquid line to the evaporator. There are several methods of dissipating the rejected heat into the

atmosphere by condenser. These are water-cooled, air cooled or evaporative cooled condensers. In the water-cooled

condenser there are several types viz. Shell and tube, shell and coil, tube in tube etc. In Evaporative cooled condenser,

both air and water are used. Air-cooled condensers are prime surface type, finned type or plate type. The selecting of

the type depends upon the application and availability of soft water.

(d) Throttling device: The high-pressure liquid from the condenser is fed to evaporator through device, which should

be designed to pass maximum possible liquid refrigerant to obtain a good refrigeration effect. The liquid line should be

properly sized to have minimum pressure drop. The throttling device is a pressure-reducing device and a regulator for

controlling the refrigerant flow. It also reduces the pressure from the discharge pressure to the evaporator pressure

without any change of state of the pressure refrigerant. The types of throttling devices are: Capillary tubes

thermostatic expansion valves, hand expansion valves. The most commonly used throttling device is the capillary tube

for application upto approx. 10 refrigeration tons. The capillary is a copper tube having a small dia-orifice and is

selected, based on the system design, the refrigerant flow rate, the operating parameters (such as suction and discharge

pressures), type of refrigerant, capable of compensating any variations/ fluctuations in load by allowing only liquid

refrigerant to flow to the evaporator.

Application of Refrigeration:

a. ice making

b. for cooling of chamber in which perishable food, drinks and medicines and stored.

c. Processing food product.

d. Manufacturing and treatment of metal.

e. Oil refining and manufacturing of synthetic rubber.

f. For liquefying gases and vapour in chemicals and pharmaceutical industrials.

Result: Various components of the vapour compression system have been studied.

Viva Questions:

1. What is the basic difference between air conditioning and refrigeration?

2. What are applications of the refrigeration?

3. Differences between heat pump and refrigerator.

4. What is the theoretical basis of all practical refrigeration systems?

Aim: To study the constructional details and working of window air-conditioner.

Equipment/Apparatus: Working model or prototype of a window air-conditioner.

Theory: Window air conditioner is sometimes referred to as room air conditioner as well. It is the simplest form of an

air conditioning system and is mounted on windows or walls. It is a single unit that is assembled in a casing where all

the components are located. This refrigeration unit has a double shaft fan motor with fans mounted on both sides of the

motor, one at the evaporator side and the other at the condenser side. The evaporator side is located facing the room for

cooling of the space and the condenser side outdoor for heat rejection. There is an insulated partition separating this

two sides within the same casing. Front Panel The front panel is the one that is seen by the user from inside the room

where it is installed and has a user interfaced control be it electronically or mechanically. Older unit usually are of

mechanical control type with rotary knobs to control the temperature and fan speed of the air conditioner. The newer

units come with electronic control system where the functions are controlled using remote control and touch panel with

digital display. The front panel has adjustable horizontal and vertical (some models) louvers where the direction of air

flow are adjustable to suit the comfort of the users. The fresh intake of air called VENT (ventilation) is provided at the

panel in the event that user would like to have a certain amount of fresh air from the outside.

Fig. 2.1 Components of window air-conditioner system

Indoor Side Components: The indoor parts of a window air conditioner include:

Cooling Coil with a air filter mounted on it. The cooling coil is where the heat exchange happens between the

refrigerant in the system and the air in the room.

Fan Blower is a centrifugal evaporator blower to discharge the cool air to the room.

Capillary Tube is used as an expansion device. It can be noisy during operation if installed too near the

evaporator.


LABORATORY MANUAL




PAGE: 4-5


Operation Panel is used to control the temperature and speed of the blower fan. A thermostat is used to sense

the return air temperature and another one to monitor the temperature of the coil. Type of control can be

mechanical or electronic type.

Filter Drier is used to remove the moisture from the refrigerant.

Drain Pan is used to contain the water that condensate from the cooling coil and is discharged out to the

outdoor by gravity.

Outdoor Side Components: The outdoor side parts include:

Compressor is used to compress the refrigerant.

Condenser Coil is used to reject heat from the refrigerant to the outside air.

Propeller Fan is used in air-cooled condenser to help move the air molecules over the surface of the

condensing coil.

Fan Motor is located here. It has a double shaft where the indoor blower and outdoor propeller fan are

connected together.

Operations: During operation, a thermostat is mounted on the return air of the unit. This temperature is used to control

the on or off of the compressor. Once the room temperature has been achieved, the compressor cuts off. Usually, it has

to be off for at least 3 minutes before turning on again to prevent it from being damaged. For mechanical control type,

there is usually a caution to turn on the unit after the unit has turned off for at least 3 minutes. For electronic control,

there is usually a timer to automatically control the cut-in and cut-out of compressor. The evaporator blower fan will

suck the air from the room to be conditioned through the air filter and the cooling coil. Air that has been conditioned is

then discharge to deliver the cool and dehumidified air back to the room. This air mixes with the room air to bring

down the temperature and humidity level of the room. The introduction of fresh air from outside the room is done

through the damper which is then mixed with the return air from the room before passing it over the air filter and the

cooling coil. The air filter which is mounted in front of the evaporator acts as a filter to keep the cooling coil clean to

obtain good heat-transfer from the coil. Hence, regular washing and cleaning of the air filter is a good practice to

ensure efficient operation of the air conditioner.

Heat Pump Window Air Conditioner: In temperate countries, heating of the room is required. A heat pump window

air conditioner unit is able to cool the room during summer and heat the room during winter. A reversing valve (also

known as 4-Way-Valve) is used to accomplish this. During heating operation, it reverses the flow of the refrigerant

which results in the evaporator to act as a condenser and the condenser as evaporator.

Result: Various components of room air conditioner have been studied.

Viva Questions:

1. What do you mean conditioning of air?

2. Explain the working principle of air conditioning system?

3. What are different types of air conditioning systems?

4. What is the function of the blower?

5. What is the function of the filter in front of the evaporator coil?

Objective: To determine the moment of inertia of a flywheel about its axis of rotation.

Equipment/Apparatus: The apparatus consist of a flywheel a large heavy wheel, through the centre of which passes a

long cylindrical axle. The centre of gravity lies on its axis of rotation so that when it is mounted over bull

bearings. It comes to rest in any desired position. To increase the moment of inertia, it is usually made thick at the rim.

To count the number of revolutions made by wheel a line is marked on the circumference. A string is wound on the

axle and Carries a mass m.

Stop watch, meter scale. Theory: The flywheel consists of a heavy circular disc/massive wheel fitted with a strong axle projecting on either side. The axle is mounted on ball bearings on two fixed supports. There is a small peg on the axle. One end of a cord is loosely looped around the peg and its other end carries the weight-hanger.

Fig. 3.1 Flywheel

Let "m" be the mass of the weight hanger and hanging rings (weight assembly).When the mass "m" descends through a

height "h", the loss in potential energy is

𝑃𝑙𝑜𝑠𝑠 = 𝑚𝑔𝑕

The resulting gain of kinetic energy in the rotating flywheel assembly (flywheel and axle) is

Kflywheel =1

2Iω2

Where

I -moment of inertia of the flywheel assembly

ω-angular velocity at the instant the weight assembly touches the ground.


LABORATORY MANUAL




PAGE: 6-9


The gain of kinetic energy in the descending weight assembly is,

Kweight =1

2mv2

Where v is the velocity at the instant the weight assembly touches the ground.

The work done in overcoming the friction of the bearings supporting the flywheel assembly is

Wfriction = nWf

Where

n - number of times the cord is wrapped around the axle

Wf - work done to overcome the frictional torque in rotating the flywheel assembly completely once

Therefore from the law of conservation of energy we get

𝑃𝑙𝑜𝑠𝑠 = 𝐾𝑓𝑙𝑦𝑤 𝑕𝑒𝑒𝑙 + 𝐾𝑤𝑒𝑖𝑔 𝑕𝑡 + 𝑊𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛

On substituting the values we get

mgh =1

2Iω2 +

1

2mv2 + nWf

Now the kinetic energy of the flywheel assembly is expended in rotating N times against the same frictional torque.

Therefore

𝑁𝑊𝑓 =1

2𝐼𝜔2

Wf =1

2NIω2

and

If r is the radius of the axle, then velocity v of the weight assembly is related to r by the equation

𝑣 = 𝑟𝜔

Substituting the values of v and Wf we get:

𝑚𝑔𝑕 =1

2𝐼𝜔2 +

1

2𝑚𝑟2𝜔2 +

𝑛

𝑁×

1

2𝐼𝜔2

Now solving the above equation for I

𝐼 =𝑁𝑚

𝑁 + 𝑛

2𝑔𝑕

𝜔2− 𝑟2

Where, I = Moment of inertia of the flywheel assembly

N = Number of rotation of the flywheel before it stopped

m = mass of the rings

n = Number of windings of the string on the axle

g = Acceleration due to gravity of the environment.

h = Height of the weight assembly from the ground.

r = Radius of the axle.

Now we begin to count the number of rotations, N until the flywheel stops and also note the duration of time t for N

rotation. Therefore we can calculate the average angular velocity 𝜔𝑎𝑣𝑔 in radians per second.

𝜔𝑎𝑣𝑔 =2𝜋𝑁

𝑡

Applications: Flywheels can be used to store energy and used to produce very high electric power pulses for experiments, where drawing the power from the public electric network would produce unacceptable spikes. A small motor can accelerate the flywheel between the pulses.

The phenomenon of precession has to be considered when using flywheels in moving vehicles. However in one modern application, a momentum wheel is a type of flywheel useful in satellite pointing operations, in which the flywheels are used to point the satellite's instruments in the correct directions without the use of thrusters rockets.

Flywheels are used in punching machines and riveting machines. For internal combustion engine applications, the flywheel is a heavy wheel mounted on the crankshaft. The main function of a flywheel is to maintain a near constant angular velocity of the crankshaft.

Procedure:

Step1: Attach a mass m to one end of a thin thread and a loop is made at the other end, which is fastened to the peg.

Step2: The thread is wrapped evenly round the axle of the wheel.

Step3: Allow the mass to descend slowly and count the number of revolutions n1 during descent.

Step4: When the thread has unwound itself and detached from the axle after n1 turns.

Start the stopwatch. Count the number of revolutions n2 before the flywheel comes to rest and stop the

stopwatch. Thus n2 and t are known.

Step5: Repeat the above procedure for different masses.

Observations:

Total load

applied in

kg

Height h

in m

No. of

revolutions

of

flywheel

before the

mass

detached

No. of

revolution

of

flywheel

to come

to rest

after mass

detached

Time for N

revolution t

in sec

Angular

Velocity

M.O.I. of

flywheel

Mean

M.O.I.

n N

Result: Moment of inertia of the fly wheel =.........kgm2

Precautions:

There should be uniform winding on the axle.

The string loop should be loose.

Friction should be made small by greasing the ball bearings.

Mass must start with zero velocity.

Viva Questions:

1. Define moment of inertia.

2. What is its unit?

3. Is it constant for a body and if not what are the factors on which it depends.

4. What is radius of gyration? Is it a constant?

5. What is a flywheel and what are its uses?

6. How would you increase the moment of inertia of a flywheel?

Objective: Study of the working of a 2- stroke petrol / diesel engine.

Equipment/Apparatus: Working models of 2- stroke petrol and diesel engines.

General Theory/Information: In a two stroke system, the working cycle is completed in two strokes of the piston or

in one revolution of the crankshaft as against two crankshaft revolutions in a four stroke cycle engine. The preparatory

strokes (suction and exhaust) are combined with working strokes (compression and expansion). The following two

methods have been used to accomplish the desired objective.

1. Providing the separate pump outside the engine cylinder to compress the charge (air fuel mixture from

carburetor or air alone from atmosphere) before forcing it into the cylinder. The pump is an integral part of the

engine and gets its motive power from the engine itself. The arrangement is referred to as two channel system

and is used for large capacity multi cylinder engines.

2. Crank – case compression system where the crank – case works as an air pump as the piston moves up and

down. The charge (air fuel mixture or air alone) is compressed by the pumping action of the piston before

being supplied to the engine cylinder. The arrangement is referred to as three channel system and is commonly

used for single cylinder small power engines such as scooter and motor cycle engines.

Construction: Fig.4.1 shows the arrangement of the typical three – port engine employing crank case compression.

The piston which is closely fitted in the cylinder is connected to the crankshaft through connecting rod and crank. The

top of the piston is usually crown shaped and that assists in sweeping the spent up gases towards the exhaust port with

the help of fresh charge. The engine employs ports as against valves as provided in a four stroke system. These ports

are cut in the cylinder walls and are three in number: the transfer port, inlet or induction port and the exhaust port. The

inlet and exhaust ports are located on one side, and the transfer port is provided on the other side. The cylinder top is

provided with an electric spark plug in a petrol engine, or a nozzle for injecting the fuel in a diesel engine.

Fig. 4.1 2 Stroke engine


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PAGE: 110-15


Working: The charge is led to the crank case through the inlet port. The charge consists of a mixture of air and petrol

prepared by the carburetor in case of petrol engine. The diesel engine admits only fresh air through the atmosphere.

The transfer port takes the compressed charge from the crankcase to the engine cylinder. The spent – up gases are

discharged to the atmosphere through the exhaust port. The closing and opening of the ports is controlled by up and

down motion of the piston inside the cylinder. The piston crown helps to prevent the loss of incoming fresh charge

(charge being carried with the spent up gases) and uses its momentum for exhausting only the burnt gases through

exhaust port by deflecting fresh charge across the cylinder.

Ignition: The piston occupies the almost TDC position towards the end of compression stroke. The compressed charge

is being ignited by providing a spark, or fuel is being injected into the hot compressed air. The combustion of fuel

occurs and thermal energy is released. There occurs a rise both in the pressure and temperature of combustion

products. At the same time a partial vacuum (pressure lower than atmosphere) exists in the crank case and fresh charge

is being inducted into the crankcase through the inlet port which is uncovered by the piston.

Expansion & Compression: The high pressure gases push the piston down, expansion takes place and power is

developed. With downward movement of the piston, the charge in the crankcase gets compressed by the underside of

the underside of the piston to a pressure of about 1.4 bar absolute.

After completion of about 80% of expansion stroke, the piston uncovers the exhaust port. Some of the

combustion products which are still above atmospheric pressure escape to the atmosphere. On its further downward

motion, the piston uncovers the transfer port and allows the slightly compressed charge from the crankcase to be

admitted into the cylinder via the transfer port.

Exhaust: The piston lies at its bottom dead centre position. The expanded gases are escaping through the exhaust port

and simultaneously the slightly compressed charge from the crankcase is being forced into the engine cylinder through

the transfer port. The charge strikes the deflector on the piston crown, rises to the top of the cylinder and pushes out

most of the burnt gases. During the scavenging action, a part of fresh charge is likely to leave with the exhaust gases.

The cylinder is completely filled with the fresh charge, although it is somewhat diluted due to its mixing with the burnt

gases.

When the piston moves upward from its BDC position, it first covers the transfer port and stops the flow of fresh

charge into the cylinder. A little water, the exhaust port too gets covered and actual compression of the charge begins

and continues till the piston reaches TDC position. The cycle of the engine is thus completed within two strokes of the

piston (One up and one down) and one revolution of the crankshaft.

Merits

1. For the same power output, a two stroke engine is simple in design, easy to manufacture and operate.

2. A two stroke cycle engine gives one working stroke for each revolutions of the crankshaft. The four stroke

cycle engine gives one working stroke for every two revolutions of the crankshaft. As such, a two stroke

engine develops theoretically twice the power developed of four stroke engine for the same engine speed and

cylinder volume.

3. The number of working strokes is twice than in a four stroke engine. Consequently, the turning moment is

uniform and hence the need only for a lighter fly wheel.

4. Less friction loss due to the absence of suction and exhaust strokes: consequently high mechanical

efficiency. Absence of cams, cam shaft and rockers etc. also contributes toward high mechanical efficiency.

5. Simpler construction and mechanism because of no valve and valve mechanism. The ports are easy to

design and they are covered and uncovered by the movement of piston itself.

6. A two stroke engine occupies less space, needs lighter foundations and requires few spare parts.

7. The initial cost of a two stroke engine is less due to light weight and absence of complete valve mechanism.

8. The two stroke engines are much easier to start.

Demerits:

1. Scavenging (driving out of burnt gases) is not complete due to short time available for exhaust. This results

in the dilution of fresh charge.

2. Exhaust and inlet parts are uncovered (open) simultaneously during a certain period.

Some fresh charge is likely to escape without giving any work output.

3. Thermal efficiency of a two – stroke engine is likely to be lower due to some charge escaping without

burning, and poor scavenging. Consequently fresh charge is diluted which results not only decrease in

performance but also slow running, low combustion pressure and poor efficiency.

4. For the same stroke and clearance volume, the effective compression ratio is lower in a

Two stroke engine. This too lowers the engine efficiency.

5. More wear and tear of moving parts due to double the number of power strokes.

6. The piston gets overheated due to firing in each revolution of crankshaft. Higher temperatures make the

cooling and lubrication requirements quite severe.

7. Greater consumption of lubrication oil due to high operating temperatures.

8. Noisy exhaust due to sudden release of burnt gases.

Applications: Two stroke engines are generally used where low cost; compactness and light weight are the major

considerations such as in scooters, motor cycles, mopeds and other light vehicles.

Viva Questions:

1. What is the difference b/w 2 stroke and 4 stroke engine?

2. Describe the working principle of 2-Stroke petrol Engine?

3. Explain the air-fuel ratio

4. What are the functions of a fly wheel?

5. What are Internal Combustion (IC) Engines?

6. What is the main difference between petrol engine and diesel engine?

7. Why more lubricating oil is needed in two-stroke engines than four stroke engines?

8. Give the names of different Thermodynamic Processes.

Objective: Study of the working of a 4 – stroke petrol / diesel engine.

Equipment/Apparatus: Working mode of a 4 – stroke petrol and diesel engine.

General Tehory / Information: An internal combustion engine is a reciprocating heat engine in which fuel mixed

with correct amount of air is burnt inside a cylinder. The gaseous products of combustion form the working substance

which makes the engine crankshaft.

The I.C. engines are classified on the basis of following systems and their variations.

NUMBER OF STROKES REQUIRED FOR THE COMPLETION OF CYCLE:

1. Tow stroke engine in which the engine cycle is completed in two strokes of the piston, i.e., in the

revolution of the crankshaft.

2. Four stroke engines in which engine cycle is completed in four strokes of the piston, i.e., in two

revolutions of crankshafts.

THERMODYNAMIC CYCLE: The thermodynamic cycles commonly used are :

1. Constant volume combustion (Otto) cycle: Most of the petrol and gas engine work on the cycle.

2. Constant pressure combustion (Diesel1) cycle: Low speed diesel engines work on this cycle.

3. Mixed or limited pressure (Dual) cycle: The high speed diesel engines work on this cycle.

IGNITION SYSTEM: The following two methods are used for the ignition of furl:

1. Spark Ignition: Petrol engines use a spark for the ignition of compressed charge (mixture of air and

petrol) and the spark may be produced by the magneto or battery.

2. Compression Ignition: Diesel engines have a high compression ratio. The resulting high temperature

is utilized to burn the fuel.

KIND OF FUEL USED:

1. Light oil engines using kerosene or petrol.

2. Heavy oil or diesel oil engines: The oil used may be crude oil or mineral oil.

3. Gas Engines: The gas used may be coal gas, producer gas, blast furnace gas or coke oven gas.

4. By fuel engines: The gas is used as the main fuel and liquid fuel is used for starting purposes.

FUEL SUPPLY STSTEM:

1. Carburetor Engines: mixture of petrol and air is prepared in the carburetor and is supplied to the engine

during suction stroke.

2. Solid Injection or airless injection: a fuel pump is used to inject the in diesel. Engines.

3. Air Injection: fuel is supplied, under pressure, to the engine cylinder or diesel engines by using

compressed air.

COOLING SYSTEM :

1. Water cooled engines in which the heat from the cylinder walls is transferred to cooling water which is

kept circulating in the water jackets provided in the cylinder block. The water picks up heat and is

taken to the radiator where the heat is transferred to the surrounding air. The water is repeatedly returned

to the engine. After being cooled in the radiator. Medium and large sized engines and the automobile

engines use the water cooling system.

2. Air cooling in which the heat from the cylinder walls is directly transferred to surrounding air. Air cooling

is generally employed for small capacity engines like scooter and motorcycle engines.

LUBRICATING SYSTEM: Lubrication system refers to the act of reducing friction by introducing a

substance (called lubricant) between the mating parts of the engine.

1. Splash lubrication system suitable for small capacity engines with moderate speed and bearing, loads.

2. Pressure lubrication system used for heavy duty engines.

GOVERNING SYSTEM (SPEED CONTROL UNDER VARIABLE LOAD ):

1. Quality control engines in which composition of mixture (air – fuel ratio) is changed by admitting,

more or less fuel in accordance with variation in load on the engine. This method is used in diesel

engines.

2. Quantity control engines in which the fuel mixture has a constant composition. However, the quantity of

the mixture supplied is changed in accordance with load on the engine. This is used in petrol and gas

engines.

SPEED: Engines having speed above 900 rpm are called high speed engines, and less than 400 rpm are called

slow speed engines.

FIELD OF APPLICATION:

1. Stationery engines used for small and medium capacity electric power plants, concrete, mixers and

pumping units.

2. Mobile engines installed in motor vehicles airplanes and ships.

FOUR STROKE PETROL/DIESEL ENGINE: A cycle is a sequence of operations constantly repeated, and

„Four – Stroke‟ refers to the number of strokes of the piston required to complete to complete one cycle. Refer Fig.

4.2 for the arrangement of different parts of a four – stroke cycle system. The piston reciprocating inside a

cylinder is connected to the crankshaft through connecting rod and crank. The inlet (suction) and outlet (exhaust)

valves are housed in the cylinder head. The cylinder head is also provided with an electric spark plug (petrol engine) or

an injector (diesel engine).

All events of the cycle namely suction, compression, compression, combustion and expansion, and

exhaust are completed in two revolutions of the crankshaft.

The salient features of the four strokes in petrol/ diesel engines are:

1. INTAKE OR SUCTION STROKE: Initially the piston is a top dead centre (TDC) position, the inlet valve is

open and the outlet valve is closed. The piston moves downwards towards bottom dead centre (BDC) position and

the pressure inside the cylinder is reduced to a value below the atmospheric pressure. The vacuum thus created

causes the charge to rush in and fill the space vacated by the piston. The charge consists of a mixture of air and

petrol prepared by the carburetor (petrol engine) or only atmospheric air (diesel engine).

The suction continues till the piston reaches its BDC position. The piston has now made one stroke and the

crankshaft has turned through 180 – degreed, i.e., has made half the revolution.

2. COMPRESSION STROKE: Both the valves (inlet and outlet) are closed and the movement of the piston is from

BDC to TDC position. The charge inside the cylinder is compressed to the clearance volume: the volume

decreases and there is a continuous rise both in temperature and pressure of the charge. Majority of the petrol

engines use compression rations between 5 to 1 and 8 to 1. Towards the end of compression, the approximate

values of pressure and temperature are 6-12 bar and 250 - 3000 C respectively. Majority of the diesel engines use

compression ratios between 15:1 Towards the end of compression, the approximate values of pressure and

temperature are 60 bar and 6000 C respectively.

3. WORKING EXPANSION OR POWER STROKE: When the piston reaches TDC position, the charge is ignited

by causing an electric spark between the electrodes of a spark plug which is located in the cylinder head. During

combustion, the chemical energy of the fuel is released and there is rise both in pressure and temperature of the

gases at almost constant volume.

When the piston reaches TDC position in four – stroke diesel system, a fine spray of diesel is injected into

the combustion space containing the high temperature compressed air. The fuel vapors are raised to self

ignition temperature and combustion occurs at approximately constant pressure.

With both valves closed, the gases at increased pressure and temperature expand: push the piston down the

cylinder and work is done by the system. The reciprocating motion of the piston is subsequently converted into

rotary motion of the crankshaft by connecting rod and crank. It is the rotary motion which is required to make

wheels run, a cutting blade spin or a pulley rotate. During expansion, there is increase in volume of the gases

and pressure drops to as low as 3 bar.

4. EXHUST STROKE: The inlet valve remains closed but the exhaust valve opens when the piston reaches BDC

position towards the completion of power stroke, the pressure falls slightly above atmospheric pressure at constant

volume. The piston moves upwards from BDC to TDC and this upward movement of the piston pushes the spent

up gases into the atmosphere through exhaust valve and the exhaust manifold. Much of the noise associated with

automobile engines is due to high exhaust velocity. The exhaust stroke completes the cycle and engine

cylinder is ready to suck the fresh charge inside the cylinder once again and the cycle is repeated. Since the

beginning of suction stroke, the piston has made four strokes inside the cylinder: two up and two down. During the

same period, the crank has turned two revolutions. Thus, for a four stroke cycle, there is only one power stroke

for every two revolutions of the crankshaft.

Fig. 4.2 Working of 4 Stroke engine

Applications: The important applications of I.C. engines are:

1. Road vehicles, Locomotives, ships and aircraft. As such I.C. engines enable passengers and cargos

to cross lands, oceans and skies.

2. Portable stand by units for power generation in case of scarcity of electric power.

3. Extensively used in farm tractors, lawn movers, concrete mixing devices and motor boats.

4. Petrol engines are used in cars, scooters and motor cycles. Diesel engines are used in heavy duty

vehicles like trucks, buses and locomotive engines.

Viva Questions:

1. What is the difference between 2-stroke and 4-stroke IC engine?

2. What is the significance of clearance volume?

3. What is a stroke?

4. Difference between SI and CI?

5. What is the function of piston rings

6. Difference between four stroke and two stroke and why four stroke is mostly preferred?

7. What is the difference between Knocking phenomenon in CI and SI engines?

8. Why Diesel engines don‟t have spark plug?

Objective: To determine the impact strength of a given mild steal specimen in Izod Impact test and Charpy Impact

test.

Equipment/Apparatus: Impact testing Machine (Capacity – 300 Jules)

Theory: It is not enough to know only how strong a metal is in tensile strength, or how ductile it is; information as

to how it reacts under sudden impact also is of prime importance. This quality is known as toughness. The test determines the behavior of materials when subject when subjected to high rate of sudden loading usually in

bending tension or torsion. It measure the energy absorbed in breaking the specimen by a high blow or impact.

Construction: The machines consist of a rigid and no bust base on which a pillar is mounted. On the top of the pillar a

pendulum mounted on anti-traction bearing is suspended. The anvil for fixing and placing the test specimen for

conducting Izod and charpy is fitted on the base. The equipment has 2 readings, that of 0 – 300 Joules and 0 – 164

Joules is fixed to the bracket with a pointer. The readings against the pointer after the rupture of test specimen

denote the Impact energy of charpy or Izod. The release lever for releasing the pendulum is felted with on the

pendulum pipe. On releasing, the pendulum swings down to Fracture the test specimen and the energy thus absorbed is

doing so is measured as the difference between the height of drop before rupture and the height after rupture of the

test specimen and is read directly from the position on the dial specimen indicator.

Fig. 5.1 Impact Testing Machine

CHARPY TEST:

Material of test piece – Mild steal

Type of Notch – V Notch.

Dimension of test pieces – 10 x 10 x 55 (mm3)

Angle of drop of Pendulum - 1350


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PAGE: 176-19


Fig. 5.2 Test Specimen for Charpy Test

Fig. 5.3 Position of Test Specimen for Charpy Test

PROCEDURE:

1) Place the test specimen as simply supported Beam.

2) The specimen is placed on support or anvil. So that blow of hammer is opposite to notch.

3) Bring the striking hammer to its top most striking position unless it is already there and lock it at that

position.

4) Release the hammer. It will fall due to gravity and break the specimen through its momentum.

Test No. Energy absorbed Energy Left

1.

2.

3.

4.

Average value of Impact − Strength =Sum of all energy absorbed

n No. of readings

RESULT:

Charpy test: Average value of Impact strength

PRECAUTIONS:

1) Note down reading carefully.

2) Measure the dimension of the specimen carefully.

3) Hold the specimen firmly.

4) Proper safely instruction should be used.

IZOD TEST:

1) Material of test = Mild steal

2) Type of Notch - V notch

3) Dimension of test piece – 10 x 10 x 75 (mm3)

4) Angle of drop = 900

Fig. 5.4 Test Specimen for Izod Test

PROCEDURE:

1) With the striking hammer in safe rest position, firmly hold the steal specimen in Impact testing vice

in such a way that the notch faces the hammer and is half inside and half a bone the top surface of vice.

(A s a cantilever)

2) Bring the striking hammer to its top most striking position unless it is already there and lock it at that

position.

3) Release the hammer. It will fall due to gracing and break the specimen through its momentum.

No. of Test Energy Observed Energy Left

1.

2.

3.

4.

Average value of Impact − Strength =Sum of all energy absorbed

n No. of readings

RESULT:

Izod test: Average value of Impact strength

Viva Questions:

1. What is the necessity of making a notch in impact test specimen?

2. What is resilience? How is it different from proof resilience and toughness?

3. In what way the values of impact energy will be influenced if the impact tests are conducted on two

specimens, one having smooth surface and the other having scratches on the surface?

4. If the sharpness of V-notch is more in one specimen than the other, what will be its effect on the test

result?

Objective: To conduct tensile test on a mild steel specimen with the help of universal testing machine (UTM) and

determine:-

a. Ultimate strength

b. Percentage elongation

c. Percentage reduction in area.

d. To conduct compressive test on a given specimen and determine its ultimate compressive strength.

Equipment/Apparatus: Universal testing machine, Micrometer, Vernier Caliper, Centre Punch, Test Specimen.

NEED OF TENSILE / COMPRESSIVE TEST: Prior to design of machine component here is need to

determine the Mechanical Properties which describe the behavior of material when in use. These properties

include:

1. STRENGTH – It is the ability of the material to resist stress without failure and it is prescribed by

ultimate stress.

2. ELASTICITY - It is the property by virtue of which the entire strain produced in the material by a

stress disappears when the stress is removed. It is prescribed by the modules of elasticity which is the

ration of stress to strain within the range of elastic deformation.

3. DUCTILITY – The material allows it to be drawn out by tension to smaller section. For example a wire

is made by drawing out metal through a hole.

The lack of ductility is called brittleness.

Fig. 6.1 Test specimen

UTM – CONSTRUCTION & WORKING:

The tensile test for determining the ultimate tensile strength, percentage elongation and percentage reduction in area of a

given specimen is conducted on UTM. This is hydraulically operated machine essentially consist of two main units

mainly the loading (straining) unit and control panel. The control panel is located on the right and comprises an oil sump,

a positive displacement pump run by the electric motor, load dial indicator and control buttons. A zero adjustment knob

is provided to set the pointer on load dial to zero valves. The right side control button operates the flow control value

while left one regulates the return value. The rate of load can be adjusted by the flow control valve. The loading unit,

located on left, has upper, middle and lower cross heads. The control panel and the loading unit are connected by pipes

through which oil flows under pressure to the hydraulic cylinder of the loading unit, when in operation the


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PAGE: 20-23


hydraulic system draws down the central cross head. Obviously when a specimen gripped between the central and upper

cross head is loaded in tension. For compression test, the specimen is placed on the bottom crosshead while the middle

one is made to move down.

A typical UTM is specified by following parameters:

a. Load Capacity

b. Overall dimension (length x breadth x height)

c. Ram

d. Power Supply.

Fig. 6.2 Universal Testing Machine

STRESS – STRAIN DIAGRAM: The test specimen is gripped firmly between the jaws of the machine and the applied

load gradually increased in suitable steps till failure of the specimen occurs. Value of load and the elongation in the

specified gauge length are observed simultaneously load dial and for the very small extensions involved in the initial

stages, the elongation is recorded by an extensometer. After failure, the length of broken specimen is measured and

diameter at section of failure is taken. These readings are plotted on a graph sheet with the ordinate representing the

stress and the abscissa representing the strain. Stress is calculated by dividing the load by the original cross – sectional

area of the test specimen. Strain is calculated by dividing the extension of the gauge length by the original unstrained

strength.

Fig. 6.3 Stress Strain diagram

Fig. 6.3 shows the typical behavior of stress strain curve for mild steel specimen. Its salient features are:

PROPORTIONAL LIMIT: When the load is increased gradually in the initial stages, the elongation and hence the

strain is proportional to load and hence to stress. This proportionality, called Hook‟s Law, extends up to point A and

this point is called proportional unit.

a. ELASTIC LIMIT: Beyond proportional limit, the stress and strain depart from straight line relationship. The

material, however, still remains elastic up to point B in the sense that it is able to return to its original form

upon removal of load. The condition at point B is referred to as the elastic limit.

b. YIELD POINT: Beyond elastic limit, the material shows considerable strain even through there is no increase in

loan or stress. The material becomes inelastic (strain not recoverable) and this onset of plastic deformation is

called the yielding of material. Yielding pertains to the region C –D and there is drop in load at the point D. The

point C is called the upper yield point and point D is the lower yield point.

c. TENSILE ( ULTIMATE ) AND BREAKING STRENGYH: After yielding, the material becomes strain

hardened (strength of specimen increases) and an increase in load is required to take the material to its maximum

stress at point E. the stress at this point is known as ultimate or the tensile stress of the material.

d. In the portion EF, there is falling off the load (stress) from the maximum until fracture takes place at F. The

point F is referred to as the fracture or breaking point and the corresponding stress is called the breaking stress.

PROCEDURE: The sequence of operations in the performance of tensile test is outlined below:

a. Mark the gauge length on the test specimen and measure its length.

b. Measure the diameter of the gauge length portion by means of a micrometer at least at three places and

determine its mean value.

c. Fix extensometer (dial gauge) firmly to the test piece and adjust it to read zero.

d. Set the load point at zero by adjusting the initial setting knob.

e. Grip the specimen vertically and firmly between the upper and middle crosshead jaws of the machine.

f. Switch on the machine by pressing the appropriate button and apply the load gradually by turning the load

valve.

g. Record the elongation corresponding to each increment of load. Continue loading until yield point is reached.

This is indicated by high values of extension with not much increase in load. At this stage, remove the diameter

and there after note the changes in length by a linear scale.

h. Continue loading and load down the elongation until the specimen breaks. Note down the breaking load.

i. Unload the machine by turning the load valve in the opposite direction.

j. Remove broken pieces from the jaws of machine. Join these pieces together and measure the extended final

length between the gauge points and the reduced diameters at the broken ends, i.e., where the neck is formed.

Observe and sketch the nature of fracture, i.e., the shape of fracture ends.

k. Draw the stress strain diagram after making the necessary calculations.

OBSERVATIONS:

a) Original Dimensions:

Gauge length of specimen (L1) = 5.65 √ A1

A1 = original cross section area

𝐴1 =𝜋

4𝑑1

2

Where d1 is = diameter of specimen.

b) Final Dimension

Gauge length of the specimen (L2) =

Diameter of specimen (d2) =

𝐴2 =𝜋

4𝑑2

2

S.

No.

Ultimate Load

(KN)

Extension

(L2-L1)

Ultimate Strength =

Ultimate Load / Area (A1)

(L2-L1)x 100

% Elongation= ---------------

L1

% Reduction Area

(A1 – A2)x 100

=--------------------

A1

RESULT:

1. Ultimate Strength = ----------------------------KN

2. % Elongation = ----------------------------- %

3. % Reduction in Area = ----------------------------- %

PRECAUTIONS:

1. The specimen should be prepared in uniform cross sectional area.

2. Load should be applied gradually.

3. Take reading more frequently.

4. The test specimen should be free from stress noisier like scratch marks in the gauge

length portion.

5. While fixing the test specimen axial load must be ensured. If loading is not perfectly

axial, the load value shown will be on higher side.

Viva Questions:

1. Hooke‟s law holds good up to

(a) yield point (b) limit of proportionality (c) breaking point (d) elastic limit (e) plastic limit.

2. In a tensile test on mild steel specimen, the breaking stress as compared to ultimate tensile stress is

(a) more (b) less (c) same (d) more/less depending on composition (e) may have any value.

3. Which of the following materials is most elastic

(a) rubber (b) plastic (c) brass (d) steel (e) glass.

4. Poisson‟s ratio is defined as the ratio of

(a) longitudinal stress and longitudinal strain (b) longitudinal stress and lateral stress (c) lateral stress and

longitudinal stress (d) lateral stress and lateral strain (e) none of the above.

5. The ratio of direct stress to volumetric strain in case of a body subjected to three mutually perpendicular

stresses of equal intensity, is equal to

(a) Young‟s modulus (b) bulk modulus (c) modulus of rigidity (d) modulus of elasticity (e) Poisson‟s ratio.

6. What is difference between force and load?

7. What is the purpose of UTM?

8. What is the purpose of graph paper?

9. What is the relation between E, K & G?

Objective: To verify principle of moments using Bell Crank Lever Apparatus.

Equipment/Apparatus: Bell Crank lever apparatus, weights.

THEORY: The algebraic sum of the moments of all the forces about a point in a body is zero for equilibrium. In other

works sum of the clock wise moment about a points is equal to the sum of the anticlockwise moments about the

same point for equilibrium.

Bell Crank is a lever bent at 900 angle forming a forming a fulcrum between its two arms. One arm is

longer and other is shorter. This works on the principle of moments. Longer arm is horizontal while shorter arm is

vertical. The lever arm is engraved with a scale. On the free end of longer arm, load is applied and on the shorter arm

effort is measured from the spring balance. If the algebric sum of the moment of load and effort about the fulcrum is

zero, then the principle of moments is provided.

SUGGESTED EXPERIMENTAL WORK:

Step1: Put hook (hanger) at a distance x from fulcrum on horizontal long arm of the Bell Crank Lever

Apparatus.

Step2: Measure the distance x accurately.

Step3: Take initial reading of spring balance. Let it be T1.

Step4: Put weight on the hanger. Note it. Let be W.

Step5: Now horizontal arm will be bend down. Again adjust the adjustable screw to bring horizontal arm

in horizontal position.

Step6: Take final reading of spring balance let it be T2.

Step7: Measure distance y on vertical short leg from fulcrum to the point of application of effort P.

Step8: Repeat above steps by changing the position of weight.

Fig. 7.1 Bell Crank Lever Apparatus

RESULT:

Check the W.x = T.y. equation satisfies. If not. Than find the percentage error.


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PAGE: 24-25


SAMPLE DATA SHEET:

Sr.

No.

Weight

W

(kg)

Distance

X from

Fulcrum

(cm)

W.x

(kgcm)

Reading of

spring

Balance

Effort

T

Distance

Y

(cm)

T.y Wx-Ty

%

error

Initial

T1

Final

T2

PECAUTIONS:

Reading of the spring balance for measurement of effort be taken accurately.

Distance x and y should be taken accurately.

Reading should be taken in horizontal position only.

Weight should be hang freely and accurately.

Objective: To study the constructional and operational details of a simple steam engine.

Equipment/Apparatus: Working model / prototype of a simple steam Engine.

General Theory/Information: Steam engine is an external combustion engine in which steam is used as the working

substance to convert heat into work. The first steam engine was developed by James Watt in 1763 and that led to

the era of industrial revolution.

The steam engines are generally classified as:

Class of service :

1. Stationary

2. Locomotive

3. Marine

Rotational Speed.

1. Slow speed engine (up to 100 to 250 rpm)

2. Medium speed engine (between 100 to 250 rpm)

3. High speed (above 250 rpm)

Arrangement of cylinder:

1. Horizontal steam engine – the axis of the cylinder is horizontal

2. Vertical steam engine – the axis of the cylinder is vertical

The horizontal engine is more easily accessible than a vertical engine but it occupies more

floor area.

Acting steam on piston:

1. Single acting – steam acts on one side of the piston and the other side of the piston is open to

atmosphere.

2. Double acting – steam acts alternately on both sides of the piston

A double acting steam engine will theoretically develop twice the power of a single acting

engine. All steam engine in practice are double acting engines.

Type of exhaust

1. Non condensing – the steam exhausts into atmosphere

2. Condensing – the steam exhausts into a condenser where the pressure maintained is much

lower than atmosphere. Steam is condensed here and the condensate is sent back to boiler by

means of feed pump.

Range of steam expansion ( number of cylinders ) :

1. Simple steam engine – the cylinder receives steam direct from boiler and exhausts to the

atmosphere or the condenser.


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PAGE: 26-29


2. Compound steam engine – the steam from the boiler partly expands in one cylinder, called

high pressure (HP) cylinder. The partially expanded steam then exhausts into a large cylinder

called low pressure (LP) cylinder where expansion is completed. The steam engine may also

be classified on the type of governing (throttle or cut off), type of valves and valve gear,

position of crank and on nature of expansion (expansive or non – expansive). If the steam is

supplied throughout the engine is said to be non – expansive. If the steam is supplied

during part of the stroke and then allowed to expand, it is said to be expansive engine.

CONSTRUCTIONAL DETAILS: Refer fig. 8.1 which shows the schematic arrangement of a single cylinder: double

acting, non condensing horizontal steam engine fitted with a slide valve.

Fig. 8.1 Simple steam engine

The various parts of a simple steam engine are:

1. Frame: The frame supports and holds all the stationary and moving parts of the engine in position. It is

essentially a heavy casting that rests upon a foundation.

2. Cylinder: The engine cylinder is a hollow cylindrical vessel made of cast iron and bored out perfectly

true. The piston has a reciprocating motion inside it under the pressure of steam. The cylinder is bolted

to the frame at one end and its ends are closed by separate covers. The end on left of the piston is

called crank end and the other end through which the piston rod passes is known as crank end.

3. Steam chest: It is a reservoir of steam from which the steam is admitted to the engine cylinder. It is

always cast integral with the cylinder and is closed by a cover known as steam chest cover which may

be of circular or rectangular in shape depending on the type of a valve used.

4. Piston, piston rings and piston rod: The piston is a cylindrical body made of cast iron, cast steel or

forged steel. Under the action of steam pressure, it has a reciprocating motion from end to end of the

cylinder. The piston must form a steam tight division between the two ends of the cylinder. This

is accomplished by fitting at least two piston rings in the circumferential grooves cut on the piston.

The piston rings not only prevent the leakage of steam past the piston but also check any wear and tear

taking place in the cylinder. The piston rod is circular rod made of mild steel. It transmits the

mechanical force which is developed at the piston to the crosshead.

5. Crosshead and guides: It is a link between a piston rod and connecting rod. It guides the motion of

piston rod and prevents it from bending.

6. Connecting rod: The connecting rod serves to convert the reciprocating motion of piston into rotary

motion of crank. It‟s one end, called the small end, is connected to the crosshead by means of a pin

called the gudgeon pin or wrist pin. The other end, called the big end, is connected to the crank by

means of a pin known as crank pin. The connecting rod is made of steel forging of rectangular, I or H

– section.

7. Crank and crankshaft: The crank converts the reciprocating motion of piston into rotary motion of

crankshaft. The crankshaft is supported on the main bearings and is free to rotate in them. The fly

wheel and the eccentric are mounted on the crankshaft.

8. Slide valve: The slide valve is situated in the steam chest and its function is to admit the steam from

the steam chest to the cylinder and exhaust it from the cylinder at the correct moment.

9. Eccentric, eccentric rod and valve rod: The eccentric is fitted to the crankshaft and functions to covert

the rotary motion of crank shaft into reciprocating motion which is transmitted to the D – slide valve

through eccentric rod and valve rod.

10. Flywheel and governor: The flywheel is a heavy cast iron or cast steel wheel mounted on the crank

shaft. It absorbs energy when the supply is greater than demands and gives it back when the demand

exceeds the supply. Thus it prevents fluctuations in engine speed and jerks to the crankshaft. Governor

is a device which maintains the engine speed more or less constant at all load conditions. It is done

either by controlling the quantity or the pressure of steam supplied to the engine.

11. Stuffing Box: The engine has stuffing boxes placed at the cylinder and the steam chest end. They serve

to allow the reciprocating motion of the piston rod and slide valve rod without a leakage.

12. Main bearing: Bearings are attached to the frame at the ends opposite to the cylinder. Bearings support

the crankshaft. They are generally made of cast iron and are lined with babbit metal to form the

bearing surface.

WORKING: With reference to Fig. 8.1, the piston is on the crank end side and ready for inward stroke. The

port P1 is open and steam from the steam chest enters the cylinder on the right side of the piston. The

high pressure steam pushes the piston inward and motion of the piston of the piston moves the crank,

the Crankshaft and the eccentric. Admission of steam continues till the eccentric moves the D – slide

valve to a position that closes the port – P1. At this instant, the cut off takes place and the supply of

steam to the cylinder is stopped. Subsequently the steam expands, its volume increases with

corresponding fall in pressure, and work is done. Towards the end of outward stroke, the D slide valve

opens the port P1 and release of steam occurs to atmosphere. Thereafter the port P1 is closed to

exhaust, the outward movement of piston continues and the steam entrapped between piston and crank

end is compressed till the piston reaches the extreme location on crank end side. That completes the

cycle and the system gets ready for the next cycle.

A similar cycle is repeated on the other end of the cylinder. When admission and expansion take place

on cover end side, exhaust and compression take place on the crank end side.

Indicator diagram: Various events taking place during the operating of a steam engine can be shown

on indicator diagram. In this p – v – plot, the length of the diagram shows the stroke length and the

height represents the pressure acting on the piston at any point of stroke.

Fig. 8.2 Theoretical & actual indicator diagram of a steam engine

The hypothetical indicator diagram

ABCDE shown in fig. 8.2 presumes that:

(i) steam is admitted in the cylinder at constant pressure

(ii) cut off takes place instantaneously.

(iii) the expansion is hyperbolic

(iv) release takes place instantaneously

(v) exhaust takes place at constant pressure

(vi) there is no compression when the deviation from these aspects are considered, the

actual indicator diagram A‟B‟C‟D‟E‟ is obtained. The area of actual indicator diagram

is less than that of hypothetical indicator diagram, and the ratio is known as diagram

factor (K).

Area of actual indicator diagram

K = -----------------------------------------------

Area of theoretical indicator diagram

The diagram factor varies from engine to engine, its average value lies between 0.7 to 0.8.

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