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Title: Basic of mechanical Engineering
Description: Here you can learn the Basic of Mechanical Engineering Notes
Description: Here you can learn the Basic of Mechanical Engineering Notes
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Metal casting
Manufacturing of a machine part by melting (heating a metal or alloy above its melting point )
and pouring the liquid metal/alloy in a cavity approximately of same shape and size as the
machine part is called casting process
...
The place, where
castings are made is called foundry
...
, (this operation is called ‘fettling’),
(g) Inspection of casting
...
Sand is chemically SiO2 (silicon dioxide) in
granular form
...
This sand, after
suitable treatment, is used for mould making
...
(ii) Permeability : ability to allow gases, water vapour and air to pass through it
...
(iv) Good flowability : when it is packed around a pattern in a moulding box, it should be able
to fill all nooks and corners, otherwise the impression of pattern in mould would not be sharp
and clear
...
It is particularly important in case of core making
...
Without cohesiveness, the moulds
will lack strength
...
If the moulding sand does not stick
to the walls of moulding box, the whole mould will slip through the box
...
Clay is a natural binder
...
Generally fresh moulding sand prepared in the foundry has the following composition:
Silica 75% (approx
...
A core, being surrounded on all sides by molten metal, should
be able to withstand high temperature
...
Cores are made with the help of core boxes
...
The sand is mixed and filled in the core
boxes
...
A core box is made in two halves, each half contains half impression
of core
...
The reinforcements are
in the shape of wire or nails, which can be extracted from the hole in the casting along with
core sand
...
Blow-holes: They appear as small holes in the casting
...
They are caused due to entrapped bubbles of gases
...
2
...
Shrinkage cavity is
totally internal
...
Remedy is to use either a chill or relocation of
risers
...
Misrun: This denotes incomplete filling of mould cavity
...
4
...
Low pouring temperature may be the primary cause of
this defect
...
Mismatch: This defect takes place when the mould impression in the cope and drag do not
sit exactly on one another but are shifted a little bit
...
6
...
Loose sand
inadequately rammed or lack of binder may cause this defect
...
Scab: This defect occurs when a portion of the face of a mould lifts or breaks down and the
recess is filled up by molten metal
...
Hot tear: These cracks are caused in thin long sections of the casting, if the part of the casting
cannot shrink freely on cooling due to intervening sand being too tightly packed, offers
resistance to such shrinking
...
Reason may be
excessively tight ramming of sand
...
Other defects include scars, blisters, sponginess (due to a mass of pin holes at one location)
and slag inclusions etc
...
Parts of complex shape can be made easily
...
Extremely large as well as extremely small objects can be made
...
The only way to making parts from cast iron is casting
4
...
Disadvantages
1
...
2
...
Time taking process
...
(without
pattern mould cannot be made
...
Die is essentially a metal mould and
can be used repeatedly
...
One portion is fixed and the
other is movable
...
After clamping or
locking the two halves of the dies together molten metal is introduced into the dies
...
On the other hand, if the metal is forced into the dies under pressure (e
...
, a piston in a cylinder
pushes the material through cylinder nozzle), the process is called “pressure die casting”
...
A great number of die castings are made of alloys of zinc, tin and lead,
and of alloys of aluminium, magnesium and copper
...
The dies are usually water or air cooled
...
Therefore, in the design of dies, some
arrangement for extraction of casting is incorporated
...
Close and lock the two halves of a die after coating the mould cavity surfaces with a mould
wash, if specified:
2
...
3
...
4
...
5
...
6
...
Types of pressure die casting methods:
1
...
In this process the chamber, in which molten metal is stored before being pressure
injected into the die, is kept heated
...
Cold chamber process: In this process, pressures as high as 150 MPa are used
...
This process is used mainly for metals and alloys having relatively higher
melting point e
...
, aluminium, magnesium and their alloys
...
It is used for mass production of castings of small and medium size
...
g
...
2
...
It is a disadvantage
...
This process produces high quality, defect free castings
...
The castings produced by this process are of good surface finish and have good dimensional
control and may not require much machining
...
5
...
It is a disadvantage
...
Castings with very complex shape or with many cores are difficult to produce by die casting
...
In case of mass production, castings can be produced cheaply
...
The process does not require use of sand and requires much less space as compared to a
conventional foundry using sand moulds
...
It is usually done hot (hot forging);
although sometimes forging is done at room temperature (cold forging)
...
Components produced by forging are bolts, spanners, crane hooks, crankshaft etc
Open-die forging
Open-die forging is also known as smith forging
...
Open-die forging gets its name from the fact that the dies do not enclose the work piece,
allowing it to flow except where contacted by the dies
...
The dies are usually flat in shape, but some have a specially shaped surface for specialized
operations
...
Closed die forging
In closed-die forging metal is placed in a die resembling a mold, which is attached to the anvil
...
The hammer is then dropped on the work piece,
causing the metal to flow and fill the die cavities
...
Excess metal is squeezed out of the die cavities, forming what is referred to
as flash
...
This also forces the metal to
completely fill the die cavity
...
This process was developed to minimize cost
and waste associated with post forging operations
...
Cost savings are gained from the use of less material, and thus less scrap, the overall decrease
in energy used, and the reduction or elimination of machining
...
The metal is initially pushed into the space between two rolls, thereafter once the roll
grips the edge of the material, the material gets pulled in by the friction between the surfaces
of the rolls and the material
...
This is a process to deal with material in bulk in which the cross-section of material
is reduced and its length increased
TWO ROLL PROCESS:
It comprises of two heavy rolls placed one over the other
...
The rolls rotate in opposite directions and are driven by powerful electrical motors
...
Since transporting material (which is in red hot condition) from one side to another is
difficult and time consuming (material may cool in the meantime), a ‘‘two high reversing mill’’
has been developed in which the direction of rotation of rolls can be changed
...
The direction of rotation
of the first and second rolls are opposite as in the case of two high mill
...
The advantage of this mill is that the work material can be fed in one direction between the
first and second roll and the return pass can be provided in between the second and third rolls
...
Extrusion
Extrusion is a process in which the metal is subjected to plastic flow by enclosing the metal in
a closed chamber in which the only opening provided is through a die
...
In the process the metal comes out as a long strip with the
same cross-section as the die-opening
...
However, some steel products are also made by extrusion
...
(ii) Backward or Indirect extrusion
...
(ii) Hydrostatic extrusion
...
(iv) Cold extrusion forging
...
It is heated to requisite temperature and
then transferred to a chamber
...
In the front portion of
the chamber, a die with an opening in the shape of the cross-section of the extruded product,
is fitted
...
As the ram moves forward, pressure develops and metal plastically deforms
...
Backward or indirect extrusion:
The block of heated metal is inserted into the container/chamber
...
As the
ram presses backwards, the material has to flow forwards through the opening in the die
...
This
process is called backward extrusion process as the flow of material is in a direction opposite to
the movement of the ram
...
Hydrostatic extrusion:
It is a type of cold extrusion process
...
The
fluids commonly used are glycerin, ethyl glycol, mineral oils, castor oil mixed with alcohol
etc
...
This is a direct extrusion process
...
The advantages of Hydrostatic extrusion process include:
•
No friction between the container and the billet reduces force requirements
...
•
Usually the ductility of the material increases when high pressures are applied
...
•
Large billets and large cross-sections can be extruded
...
The disadvantages are
•
The billets must be prepared by tapering one end to match the die entry angle
...
•
Handling the fluid under high pressures can be difficult
...
The
welding process is subdivided into two main classes
...
Fusion welding: which involves heating the ends of metal pieces to be joined to a
temperature high enough to cause them to melt or fuse and then allowing the joint to
cool
...
2
...
This results in the pieces welding together to produce a
strong joint
Based on the sources of heat, fusion welding is again classified to different type
Electric arc welding: electric arc is the source of heat
Gas welding: A burning gas is producing the heat
...
Electric resistance welding: heat produced from the electric resistance of
material
Thermite welding: chemical reaction is the source of heat
...
SMAW (shielded metal arc welding)
it is a manual arc welding process that uses a consumable electrode coated with flux
...
As the weld is laid, the flux coating of the electrode disintegrates,
giving off vapors that serve as a shielding gas and providing a layer of slag, both of which
protect the weld area from atmospheric contamination
Oxy fuel welding
It is a welding process in which required heat is obtained by a combustion of a fuel gas
...
Several gas mixtures are using but mainly acetylene is mostly used for
welding as it produces high temp
...
Acetylene gas is obtained by mixing
calcium carbide with water
CaC₂+2H₂O=Ca(OH)₂+C₂H₂
C₂H₂+O₂=4CO+H₂
4CO+ 2H₂+3 O₂=4CO₂+2H₂O
Acetylene and oxygen are stored separately in different cylinders as shown in fig
...
Tube from oxygen cylinder
and acetylene cylinder are connected to respective valves
...
Hydraulic turbines
Hydraulic turbines are the machines which use the energy of water and convert it into
mechanical energy
...
According to the type of energy at the inlet or action of water flowing through the turbine
runners, turbines classified as
1
...
Reaction turbine
Also turbines are classified according to the direction of fluid flow or flow path
1
...
Axial flow turbine
3
...
Penstock is the pipe
which carries water from the dam to power station
...
The runner is a circular frame with series of
buckets
...
The buckets are made up of
cast iron , steel or bronze
...
Working
The water from the reservoir enters the nozzle through penstock
...
Water leaves the nozzle in the form of jet and impinges the buckets
of runner, thus causing it to revolve
...
As the water
flows through the runner the change from pressure energy to K
...
takes place gradually
...
Modern
Francis Turbine is a mixed flow type turbine (i
...
Water enters the runner of the turbine in the
radial direction and leaves the runner in the axial direction)
...
In
Francis Turbine the water flows from outwards to inwards through the runner (Inward Flow
Radial Turbine)
...
CONSTRUCTION: The main parts of Francis Turbine are:
CASING
The runner is completely enclosed in an air-tight spiral casing
...
GUIDE MECHANISM/GUIDE VANE
It consists of a circular wheel all round the runner of the turbine
...
The guide vanes allow the water to strike the vanes fixed on the
runner without shock at inlet
...
RUNNER
It is a circular wheel on which a series of Radial Curved Vanes are fixed
...
DRAFT TUBE
The pressure at the exit of the runner of Reaction Turbine is generally less than atmospheric
pressure
...
A tube or pipe of
gradually increasing area is used for discharging water from the exit of turbine to the tail race
...
One end of the tube is connected to the outlet
of runner while the other end is sub-merged below the level of water in the tail-race
...
A radial flow turbine may be
either inward radial flow or outward radial flow type
...
Eg: propeller turbine, Kaplan Turbine
Mixed flow turbine
Water enters the runner at the outer periphery in radial direction and leaves the turbine at the
centre in the direction parallel to the axis of rotation
...
Thereby
increasing the energy of flowing fluid
...
Pumps broadly classified into two
1
...
Rotodynamic pumps
Positive displacement pumps are those pumps in which liquid is sucked and then it is pushed or
displaced due to the thrust exerted on it, by a moving member
...
During this motion its angular momentum changes, due to which the pre
...
here pump does not push the liquid as in the case of positive displacement pump
...
The cylinder is connected to suction and delivery tube each of which provide with a
non return valve called suction valve and delivery valve
...
Crank rotated by an engine or motor
...
) a partial vacuum created inside the
cylinder
...
This operation is known as suction stroke
...
In this stroke crank rotates θ=0˚ to θ=180˚
...
When the crank rotates from θ=180˚ to θ=360˚ piston moves inwardly from position right to
left
...
the liquid is then foced up through delivery pipe
...
Now the pump has completed one cycle
...
Work done by reciprocating pump
The volume of liquid pumped is known as discharge
...
so Qth
ALN
60
A= area of cross section of piston
L= stroke length ( distance between P1 and P2)
N=no of revolutions per minute
Normally actual discharge found to be less than theoretical discharge
...
Qth
...
g
Hs=suction head,
Power P
Hd= delivery head
w( ALN)(H s
60
Coefficient of discharge =
Hd )
Actual disch arg e
theoretical dischrge
Centrifugal pumps
Qa
Qt
The basic principle on which a centrifugal pump work is that when a certain mass of liquid is
made to rotate by an external force
...
Since in these pumps the lifting of the liquid is due to the centrifugal action, these pumps
are called centrifugal pumps
...
it is mounted on a shaft which is coupled to an external source of energy(electric
motor)
Suction pipe: it is a pipe connects its upper end to the inlet of pipe and lower end dips into
water
Delivery pipe: pipe which is connected at its lower end to the outlet of the pump and it
delivers liquid to required height
Working
The first step is priming
...
If there is any
air pocket, it result in no delivery of liquid from pump
...
After the pump is primed, electric motor started to rotate the impeller
...
Then the
liquid starts to flow in an outward radial direction therby leaving the vanes of impeller
...
efficiency
wQ ( Hs Hd )
power given to the shaft
q
disch arg e
w specific weight
Hs
suction head
Hd
delivery head
Multi stage pumps
g
Normally a pump with a single impeller can be used to deliver the required discharge
against a maximunhead of about 100m
...
This arrangement can
be replaced by a multi stage pump
...
All impellers are connected in series so that liquid
discharged with increased pressure
...
Vane pump
A rotary vane pump is a positive-displacement pump that consists of vanes mounted on
a rotor that rotates inside of a cavity
...
This combination of actions creates a seal on the interior of
the cavity, and effectively forms a series of small chambers within the larger chamber
...
Essentially, there is atmospheric pressure on the intake side of the pump that
helps to suck in the liquid, while the pressure created by the rotating action help to move and
discharge the collected liquid from the outtake or discharge side of the pump
...
Advantages
Handles thin liquids at relatively higher pressures
Compensates for wear through vane extension
Sometimes preferred for solvents, LPG
Can run dry for short periods
Can have one seal or stuffing box
Develops good vacuum
Disadvantages
Can have two stuffing boxes
Complex housing and many parts
Not suitable for high pressures
Not suitable for high viscosity
Not good with abrasives
Applications
Aviation Service - Fuel Transfer
Auto Industry – pumping of Fuels, Lubes, Refrigeration Coolants
Bulk Transfer of LPG and NH3
LPG Cylinder Filling
Refrigeration – pumping of Freons, Ammonia
In distilleries and chemical industries
Gear pump
External gear pumps are similar in pumping action to internal gear pumps in that two gears
come into and out of mesh to produce flow
...
Each gear is supported by a shaft with bearings on both sides of the gear
...
As the gears come out of mesh, they create expanding volume on the inlet side of the pump
...
2
...
3
...
Because the gears are supported on both sides, external gear pumps are quiet-running and are
routinely used for high-pressure applications such as hydraulic applications
...
Advantages
High speed
High pressure
No overhung bearing loads
Relatively quiet operation
Design accommodates wide variety of materials
Disadvantages
Four bushings in liquid area
No solids allowed
Fixed End Clearances
Applications
Pumping of various fuel oils, kerosene and lube oils
Pumping of Chemicals and polymers
For Chemical mixing and blending
Industrial and mobile hydraulic applications
Pumping Acids and paints etc
For Low volume transfer
Jet pump
A jet pump is a device that uses the venturi effect of a converging-diverging nozzle to convert
the pressure energy of a fluid to velocity energy which creates a low pressure zone that draws
in and entrains a suction fluid
...
The operating principle of a jet pump is as follows: Upon starting up, the fluid is entering
through the suction tube and delivered through the delivery pipe
...
Refer fig
...
Due to
this low pressure more fluid will enter to the suction pipe
...
For use in producing a vacuum pressure in steam jet cooling systems
...
The construction industry uses them for pumping turbid water and slurries
...
belt drive
2
...
gear drive
Belt drives
A belt is a loop of flexible material used to link two or more rotating shafts mechanically
...
In a two pulley system, the belt can either drive the pulleys in the same
direction, or the belt may be crossed, so that the direction of the shafts is opposite
...
Types of belts
Based on arrangement of shafts and belt
Open belt drive: in this the direction of rotation is same for both driver and driven shaft
...
Above
...
hence the tension on the former side will be greater than the later side
...
Cross belt drive: in this driver and driven pulley have different direction of rotation
...
In order to reduce this shaft should be
placed at a minimum distance of 20 d, where ‘d’ is the width of belt
...
Flat belts are again classifie as open belt drive and cross belt drive
Vee/ V- belt- these are used to transmit large amout of power between two shafts for a short
distance
Circular belt/rope- these belts are used to transmit large amount of power for large
distance(>8m)
Flat-belt drives are simple and convenient
...
However, they
are bulky in design and low in strength
...
Round-belt drives are now rare and are used only in mechanisms of low power, such as
those in sewing machines
...
It can be used with very high speed drives
...
The disadvantages include the
short lifetime of the belts, relatively large size, heavy stress on the shafts and bearings, and
variation in the tension ratio caused by the inevitable slipping of the belt
...
Belt
drives are widely used in agricultural machines, electric generators, certain machine tools, and
textile machines
...
Power transmitted by belt drive
Here the driving pulley pulls the belt from the lower side to the upper side
...
The upper side is called the
slack side and lower side is the tight side
...
Therefore the force causing the rotation is the difference between the two tensions the
belt exerts a force on the pulley
...
N/60
...
Let the tight side tension be T1 and
slack side tension T2
...
Let T be tension at M and (T + δT) be the tension at N
...
Suppose the belt is in equilibrium
...
Here x direction is
horizontal(radial direction) and y direction is tangential at point P
...
… (1)
and resolving the force is radial reaction
...
δθ / 2 is neglected)
T δθ=R
...
A gear is a rotating machine part having cut teeth,
which mesh with another toothed part in order to transmit torque
...
It is having less maintenance cost
...
Gears may be classified according to the relative position of
the axis of revolution
...
2
...
4
...
spur,
helical
bevel gear
worm and worm wheel
Rack and pinion
...
They are characterized by teeth
which are perpendicular to the face of the gear or teeth are parallel to the axis of rotation
...
The basic
descriptive geometry for a spur gear is shown in the figure
...
Advantages: Spur gears are easy to find, inexpensive, and efficient
...
The resulting teeth are longer than the teeth on a spur gear of equivalent
pitch diameter
...
Tooth strength is greater because the teeth are longer,
2
...
The longer surface of contact reduces the efficiency of a helical gear relative to a spur
gear
Helical gears may be used to mesh two shafts that are parallel
...
Limitations: Helical gears have the major disadvantage that they are expensive
...
Bevel Gears
Bevel gears are primarily used to transfer power between intersecting shafts
...
Standard bevel gears have teeth which are cut
straight and are all parallel to the line pointing the apex of the cone on which the teeth are
based
...
Cannot be used for parallel shafts
...
Advantages: Excellent choice for intersecting shaft systems
...
Worm gears, like helical gears, allow two non-intersecting , non parallel shafts
to mesh
...
A worm gear is equivalent
to a V-type screw thread
...
Worm gears are normally used when a high gear ratio is desired, or
again when the shafts are perpendicular to each other
...
The resulting mesh is
'self locking', and is useful in ratcheting mechanisms
...
The worm drives the drive gear primarily with slipping motion, thus
there are high friction losses
...
Meshes are self locking (which can
be either an advantage or a disadvantage)
...
(They are in theory a gear with an infinite pitch diameter)
...
Limitations: Limited usefulness
...
Advantages: The only gearing component that converts rotational motion to translational
motion
...
Generally offers better precision than other conversion
methods
...
Root (or dedendum) circle: The circle drawn through the bottom of the teeth
...
Dedendum: The radial distance between the pitch circle and the dedendum circle
...
Face of a tooth: That part of the tooth surface lying outside the pitch surface
...
Circular thickness (also called the tooth thickness) : The thickness of the tooth
measured on the pitch circle
...
Tooth space: The distance between adjacent teeth measured on the pitch circle
...
Circular pitch p: The width of a tooth and a space, measured on the pitch circle
...
A
toothed gear must have an integral number of teeth
...
The diametral pitch is,
by definition, the number of teeth divided by the pitch diameter
...
The pitch diameter is usually
specified in inches or millimeters; in the former case the module is the inverse of
D
diametral pitch
...
Velocity ratio: The ratio of the number of revolutions of the driving (or input) gear to
the number of revolutions of the driven (or output) gear, in a unit of time
...
Line of action: A line normal to a pair of mating tooth profiles at their point of contact
...
Pressure angle : The angle between the common normal at the point of tooth contact
and the common tangent to the pitch circles
...
Base circle :An imaginary circle used in involute gearing to generate the involutes that
form the tooth profiles
...
Figure a shows a simple ordinary gear train in which there is only
one gear for each axis
...
The
circular pinion engages teeth on a flat bar ( the rack)
...
The rack and pinion arrangement is commonly found in the steering mechanism of cars or other
wheeled, steered vehicles
...
Slider crank mechanism
A slider crank mechanism(see fig
...
Positions at which slider motion reverses are called dead centers
...
The conventional internal combustion engine employs a piston arrangement in which the
piston becomes the slider of the slider-crank mechanism
...
The driving force is applied at the crankpin so
that, at TDC, a much larger force is developed at the slider
...
In mechanical engineering, an eccentric is a circular disk (eccentric sheave) solidly fixed to a
rotating axle with its centre offset from that of the axle/shaft (hence the word "eccentric", out
of the centre)
It is most often employed in steam engines and used to convert rotary into linear reciprocating
motion in order to drive a sliding valve or a pump ram
...
Internal combustion engines
Introduction
A device which transforms one form of energy into another form is called an engine
...
Heat engines can
be broadly classified into two categories
(i)External combustion engines (EC engines)eg: steam engine
(ii)Internal combustion engines (IC engines)eg: petrol and diesel engine
The internal combustion engine is an engine in which the combustion of a fuel (normally
a fossil fuel) occurs with an oxidizer (usually air) in a combustion chamber inside the engine
...
It operates based on the theoretical air
cycle known as diesel cycle
...
Diesel cycle (Constant pressure cycle)
Atmospheric air is drawn into the engine cylinder during the suction stroke and is
compressed by the piston during the compression stroke to high pressure and temperature
...
Just before
the end of the compression stroke a metered quantity of fuel under pressure is injected in the
form of fine spray by means of a fuel injector
...
After doing work on the piston
the burnt gases escape from the engine cylinder
Two stroke diesel engine
In two stroke diesel engine, one cycle of operation is completed in two strokes of the
piston, (in one revolution of the crank shaft ) by eliminating separate suction and exhaust
strokes
...
Fig
...
The cylinder is connected to a closed
crankcase
...
At the
same time fresh air enters the crankcase through the air inlet port
...
(I)
Towards the end of this stroke fuel is introduced in the form of fine spray by the fuel injector
and due to the high pressure and temperature of the air, the fuel starts burning
...
The
transfer port is then uncovered (fig
...
The incoming fresh air helps to move the burnt gases from the engine cylinder
...
e
...
The various strokes of a four stroke diesel engine are detailed
below
...
1
...
The inlet valve opens and air at atmospheric pressure is drawn into the engine
cylinder
...
This operation is represented by the line 5-1 in
PV diagram
...
Compression stroke
In this stroke the piston moves towards TDC and compresses the enclosed air to
high temperature and pressure
...
Both the inlet and exhaust valves remain closed during this stroke
...
Expansion or working stroke
Towards the end of compression stroke a metered quantity of fuel is injected into
the hot compressed air in the form of fine spray by means of a fuel injector
...
This is shown
by line 2-3 in PV diagram
...
The high pressure gas in the
cylinder expand up to point 4, doing work on the piston
...
At the end of this stroke the exhaust valve opens
...
Exhaust stroke
The piston moves from BDC to TDC and the burnt gases escape through the
exhaust valve
...
This stroke is represented
by the line 1-5 in PV diagram
...
By this one cycle is completed
...
5 Working principle of petrol engines (Spark Ignition Engines)
Petrol engines operate on the so called Otto cycle
...
Otto cycle (Constant volume cycle)
In this cycle, heat is supplied at constant volume
...
A
carburettor provides a mixture of petrol and air in the
required proportion
...
The gas expands and moves the
piston downwards, during work
...
Refer PV diagram
I Suction stroke II Compression stroke III Working stroke IV Exhaust stroke
1 Inlet valve 2 Exhaust valve 3 Fuel injector 4 Piston 5 connecting rod 6 Crank
Fig: Working of four stroke petrol engine
1) Suction stroke
During this stroke the piston moves from top dead centre (TDC) to bottom dead centre
(BDC)
...
The
exhaust valve remains closed throughout this stroke
...
2) Compression Stroke
The air fuel mixture is compressed as the piston moves from BDC to TDC
...
Both the inlet and exhaust valves
remain closed throughout this stroke
...
This expansion process is shown by the line 3-4 in PV diagram
...
4) Exhaust stroke
The removal of the burnt gases is accomplished during this stroke
...
This operation is
represented by the line 1-5 in PV diagram
...
By this one cycle is completed
...
Here ports are provided in place of valves
1 Cylinder 2 Crank case 3 Piston 4 Air inlet port 5 Transfer port 6 Exhaust port 7 Spark plug
Fig 2
...
shows the working of a two stroke petrol engine
...
During the upward stroke of the piston, the air fuel mixture in the cylinder is
compressed
...
(Fig
...
The piston then travels downwards due to the expansion of the gases (fig
...
The transfer port is then uncovered (fig 2 III) and the compressed air fuel mixture
from the crankcase flow into the cylinder
...
Refer fig
...
In a two stroke petrol engine the operations are the same as that for a two stroke
diesel engine with some difference
...
A carburettor is used for mixing the fuel and air in the correct
proportion
...
In this case, combustion process is assumed to take place at
constant volume
...
Working cycle: The SI engine, in general, works based on Otto cycle, while the CI engine,
in general, works based on diesel cycle
...
Fuel: A highly volatile fuel such as petrol is used in SI engine while non-volatile fuel such
as diesel is used in CI engines
...
Method of fuel ignition: In most of SI engines, the fuel and air are introduced into the
engine cylinder as a gaseous mixture while in CI engines, the fuel is directly introduced into
the cylinder in the form of fine spray
...
4
...
5
...
6
...
The higher compression ratio of CI engines result in higher
thermal efficiency and hence a greater power output for the same amount of fuel
consumed
...
Weight: Because of the higher compression ratio and higher pressure, CI engines require
stronger engine parts and hence are heavier
8
...
9
...
Comparison of two stroke and four stroke cycle engines
1) In a two stroke engine, there is one working stroke for every revolution of the crank shaft
whereas in a four stroke engine there is only one power stroke for two revolutions of the crank
shaft
...
That is, in order to produce the same of power, a two stroke cycle engine
will be of less weight and occupies less space
...
3) As there is no valves in a two stroke engine the construction will be simple and hence low
initial cost
...
The mechanical efficiency will be
higher
...
Due to this, the fresh charge gets diluted with exhaust gases and the thermal efficiency
decreases
...
This will
increase the fuel consumption
...
6) In two stroke engine the power needed to operate suction and exhaust valves is saved
...
Carburation
Function of the fuel supply system is to store the fuel required for the engine in a tank and to
supply it to the cylinder for combustion
...
It is a device for atomizing and
vapourizing the volatile liquid fuel (petrol) and mixing it with air
...
In the S
...
engine, combustible petrol – air mixture
is prepared outside the engine cylinder
...
I
...
With less air, some portion of the fuel will remain unburnt due to the insufficient supply of
oxygen while with excess air, the rate of burning will be slower
...
For
this, the carburetor regulates the throttle valve using accelerator
...
Carburetor is one chamber where petrol and air was mixed in a fixed ratio and then sent to
cylinders to burn it to produce power
...
It was not very efficient in burning petrol, it will burn more petrol than needed
at times and will produce more pollution
...
It is the latest state-of-theart technology for diesel engines and suits passenger cars as well as commercial vehicles
...
A CRDI engine is based on direct injection technology and has common rails i
...
tubes
which inject pressurised fuel directly into the engine
...
The high pressure in the common rail ensures that
upon injection, the fuel atomises and mixes consistently with the air, thereby leaving minimal
unburnt fuel
...
The onboard computer makes sure that the fuel is injected at the precise
moment
...
Common rail direct fuel injection is a modern variant of direct fuel injection system for petrol
and diesel engines
...
Carburetors
were the predominant method used to mix fuel in petrol engines before the widespread use of
fuel injection
...
Multi-point fuel injection(MPFI)
Multi-point fuel injection injects fuel into the intake port just before the cylinder's intake valve,
in each cylinder, rather than a common point as in carburator
...
So in case of an MPFI engine, there is one fuel –injector installed near
each cylinder, that is why they call it Multi-point (more than one points) Fuel Injection
...
In MPFI system, each
cylinder has one injector (which makes it multi-point)
...
This computer is a small micro-processor, which keeps telling each
Injector about how much petrol and at what time it needs to inject near the cylinder so that
only the required amount of petrol goes into the cylinder at the right moment
...
But one major Key difference is that MPFI
is an intelligent system and Carburetor is not
...
It makes that decision based on the inputs it reads
...
Through
these sensors, the microprocessor knows the temperature of the Engine, the Speed of the
Engine, it knows the load on the Engine, it knows how hard you have pressed the accelerator, it
knows whether the Engine is idling at a traffic signal or it is actually running the car, it knows
the air-pressure near the cylinders, it knows the amount of oxygen coming out of the exhaust
pipe
...
Thus it makes it fuel efficient as it knows what amount of petrol should
go in
...
Modern car’s computers have memory, which will remember your driving style and will behave
in a way so that you get the desired power output from engine based on your driving style
Power plants
Power plant is the place where electricity produced using some conventional or
nonconventional energy sources
...
See table
Sources of energy
Coal (Fossil Fuel)
Hydro energy
Nuclear Fuel (U215)
Natural gas
Diesel oil
Power plant
Thermal power plant
Hydel power plant
nuclear power plant
Gas turbine power plant
Diesel engine power plant
Prime mover
Steam turbine
Hydraulic turbine
Steam turbine
Gas turbine
I
...
Engine(C
...
Engine)
Thermal or steam power plants
The fuel used in thermal power plant is coal
...
Steam is utilised to drive the steam turbine which are coupled directly to the electric
generator
...
Coal is stored in the coal storage
yard from which we are transferring coal to boiler for producing steam
...
The steam from the boiler is passed through
turbine
...
Here steam is condensed to water
...
Cooling water is circulated
around the condenser for cooling
The flue gases(smoke and other gases) from the boiler is initially passed through an
economizer in which heat energy of flue gas utilised for preheat the water
...
After that flue gas is going out through chimney
...
Economiser is equipment used to
pre heat intake water to boiler using flue gas
...
The alternator in turn, converts
mechanical energy into electrical energy
...
Components of diesel power plant
Engine: it is the main component of power plant this engine basically an IC engine directly
coupled with an alternator for producing electricity
...
Air is
drawn through an air filter and supplied to the engine
...
A system provide for supplying compressed air which used to start the engine
...
After combustion smoke and
burned particles removed from engine through this exhaust system
...
This tank mainly located outside the power
plant
...
the fuel from day tank
flows under gravity to the engine
...
Cooling and lubrication: proper cooling is required to extend the life of the plant
...
But in large engine water or oil cooling system is employed
...
lubrication system includes oil pump, oil tank, filter, and
connecting tubes
Advantages and disadvantages of diesel power plant
It is more efficient than thermal power plant
...
It can start quickly
...
Here
fuel handling and waste removal is very simple
...
Also cooling water
requirement is less than thermal or nuclear plant
Disadvantages
Life of diesel plant is comparatively less
...
Lubrication
cost is higher
...
Gas turbine power plant
The three main sections of a Gas Turbine are the Compressor, Combustor/combustion chamber
and Turbine
...
During continuous operation the impurities
and dust in the air deposits on the compressor blades
...
The Air Filter in the Air Intake system prevents this
...
This
compressed air is enters to the combustion chamber
...
Burners arranged circumferentially on the combustion chamber control the
fuel entry to the chamber
...
The hot gases in
the range of 1400 to 1500 °C leave the chamber with high energy levels
...
The turbine does the main work of energy conversion
...
The exhaust gases then exit to atmosphere
...
The gas turbine shaft connects
to the generator to produce electric power
Nuclear power plant
Nuclear power is produced by controlled nuclear reactions (nuclear fission)
...
The fission process occurs in a vessel called reactor
...
They are pressurized water
reactor and boiling water reactor
...
And water in the boiler is boiled using the heat from
this pressurized water
...
pressurized water reactor
boiling water reactor
Advantages:
Nuclear power plant is more economical compared with thermal plants where coal field is far
away
...
Therefore cost of operation reduced
...
There is a danger of nuclear radiation
...
Fuels and their properties
Fuels are chemical substances which may be burned in oxygen to generate heat
...
By burning the fuel this energy can extract in the form of heat
...
Fuels broadly classified into
1
...
Liquid fuel: petrol, diesel, kerosene etc
3
...
Some Properties of fuels
Calorific value: The calorific value of a fuel is the quantity of heat produced by its combustion at constant pressure and under "normal" conditions
...
Calorific value of some
fuels are given in the table below
coal
17,000 23,250
Butane
49,510
Charcoal
29,600
Diesel
44,800
Ethanol
29,700
petrol
47,300
Methane
55,530
Octane number is the measure of the ignition quality of petrol
...
Knocking occurs when fuel combusts prematurely
or explodes in an engine, causing a distinctive noise which resembles knocking, rattling, or
pinging
...
Cetane number or CN is a measurement of the combustion quality of diesel fuel during
compression ignition
...
In a
particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower
cetane fuels
...
The flash point of a volatile liquid is the lowest temperature at which it can vaporize to form an
ignitable mixture in air
...
This temperature is required to supply the activation
energy needed for combustion
...
It is a rough indication of the lowest temperature at which oil/fuel is
readily pumpable
...
Coal
Coal is our most abundant fossil fuel resource
...
Coal is a combustible black or brownishblack sedimentary rock normally occurring in rock strata in layers
...
Sub-bituminous coal, whose properties range from those of lignite to those of bituminous
coal are used primarily as fuel for steam-electric power generation
...
LSHS (Low sulphur Heavy Stock)
During distillation of petroleum, fuels are separated from the petroleum
...
Low Sulphur Heavy Stock (LSHS) is a residual fuel processed from crude oil(petroleum)
...
The main advantage in the use of LSHS lies in its low sulphur content
...
Apart from that, it is also
advantageous from the environmental pollution point of view
...
Calorific Value - The gross calorific value of LSHS is more than that of Furnace oil
...
Viscosity - LSHS is a low viscosity fuel oil at handling temperature when compared with Furnace
oil
...
FO(Fuel Oil)
Fuel oil is a derivative from petroleum
...
Often, some distillate is added to residual fuel oil to get a desired viscosity
...
Distillate fuel oils are similar to diesel oil
...
It is found mainly in coal beds
...
This gas collected and stored in liquid state is known as liquefied natural gas
...
Liquefied natural gas, or
LNG, is natural gas that has been super cooled to minus 162 degrees Celsius
...
When in liquid form, natural gas takes up to
600 times less space than in its gaseous state, which makes it feasible to transport over long
distances
...
Its weight is less than onehalf that of water
...
Compressed natural gas (CNG) is often confused with liquefied natural gas (LNG)
...
e
...
Gas turbine requiring distillate fuels
normally make use of HSD as fuel
...
bio mass energy is an indirect form of
solar energy
...
Wood
and agricultural residue etc are examples for biomass
...
Biodiesel cannot be made from any
other kinds of oil (such as used engine oil)
...
The biodiesel process uses a catalyst (KOH or NAOH) to break off the glycerine molecule and
combine each of the three fatty-acid chains with a molecule of methanol, creating mono-alkyl
esters, or Fatty Acid Methyl Esters (FAME) this FAME is the biodiesel
...
This process is called transesterification
...
Bio gas
Biogas typically refers to a gas produced by the biological breakdown of organic matter in the
absence of oxygen
...
This type of biogas comprises primarily methane and carbon
dioxide
...
The anaerobic digestion takes place in the digester tank (see fig)
...
The remaining things containg nitrogen, phosphorous and potassium etc which can be used as
a fertiliser
...
also used for domestic
lighting and power generation
Solar power plants
Solar power plants converting solar light into electricity
...
Solar photovoltaic power stations using the silicon photovoltaic cells for converting solar energy
into electricity
...
Solar cells are made up of semi-coductors that generate elelctricity on absorbing light
...
the electricity produced in the solar panel will be DC
...
From this battery we will get DC out put
...
Components of Solar photo voltaic Power Plant
1
...
2
...
Solar thermal power stations
In this power plant, solar collector is a highly polished metallic surface of parabolic
shape is used
...
At
the focal point a tube is kept so that when the fluid is passed through a temperature rise
from 250-500 degree Celsius is obtained
...
In many applications instead of parabolic collector, a flat plate collector is used
...
Wind power plant
wind turbine is a device that converts kinetic energy from the wind into mechanical energy
...
If the mechanical energy is used to drive machinery, such as for
grinding grain or pumping water, the device is called a windmill or wind pump
...
these rotors or blades having thin cross
section
...
This mechanical energy transferred to the generator through a gear box
...
Key
advantages of this arrangement are that the turbine does not need to be pointed into the wind
to be effective
...
VAWT are of two type: Savonnius type
and Darrieus type
...
In this 3 blades are curved
and attached to a hub on the vertical shaft
...
it avoids fuel and its
transportation
...
it is available both during day and night
...
the disadvantages of wind energy is fluctuating and available in small quantity
...
The cost of installation is high
...
When the water level of sea above the mean sea level it is called high tide
...
The difference in the levels of water during the high and low tide are utilized in operating
hydraulic turbine
...
) hydraulic turbine fixed in the dam as shown in fig
...
During high tide water will flow through the turbine from sea to tidal
basin
...
During low tide period water will flow from tidal basin to sea through the same water
turbine
...
The turbine used here is of reversible type means it can be rotated in both cock wise and anti
clockwise direction
Advantages
It is independent of rainfall
...
It has the capacity to meet
the peak power demand
...
OTEC
OTEC, or ocean thermal energy conversion, is an energy technology that converts solar
radiation to electric power
...
In ocean surface water temperature is about 25 degree celcius and 1 km directly
below, the temperature is about 4 degree celcius
...
There are 2 different types of OTEC
...
In open cycle, sea
water itself is used as working fluid
...
using this low pressure steam a turbine runs, from which
power extracts
...
see
the fig
...
This ammonia vapour send to the turbine
for extracting energy from it
...
then it is pumped back to the vaporator
Advantages of OTEC
Energy is freely available from the sea throughout the year
...
5% as compared to 30-40% efficiency of conventional plants
...
Energy is present as heat in the earth’s
crust
...
This water gets heated by magma and become less dense
...
This fluid can taken out through a vent or tube
...
The system utilizing the steam is
known as vapou dominated system and system using the hot fluid is known as liquid dominated
system
...
Advantages : it is cheaper than other energies
...
Energy is available throughout the year
...
The steam and hot water coming out may contain
other poisonous gases like CO2, H2S, NH3 etc
...
Overall
efficiency of geothermal power plant is low
Title: Basic of mechanical Engineering
Description: Here you can learn the Basic of Mechanical Engineering Notes
Description: Here you can learn the Basic of Mechanical Engineering Notes