Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.

C Engines, Boilers

Introduction to Manufacturing Systems

Manufacturing can be defined as the process of converting raw materials (and information
such as specifications) into a usable form of products.

1.0 CASTING PROCESS

Casting is the process of producing metal parts by pouring molten metal into the mould cavity of
the required shape and allowing the metal to solidify. The solidified metal piece is called as
"casting".

Casting Terms
Flask or molding box: A frame made of metal or wood or plastic, in which the mold is formed.
Drag: Lower molding flask
Cope: Upper molding flask
Pattern: The replica of the object to be cast is known as pattern. The cavity in the mould is created
with the help of the pattern.

Parting line: The dividing line between the two molding boxes that makes up the mold.

Molding sand: Sand, which is sued for making the mould is called as molding sand. It is a mixture
of silica sand, clay, and moisture in appropriate proportions.
Facing sand: In order to give a better surface finish to the casting, a small amount of fine
carbonaceous material, known as facing sand.
Core: The part of mold, made of sand, used to create openings and various shaped cavities in the
castings.

Pouring basin: A funnel shaped cavity at the top of the mold into which the molten metal is poured.
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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

Sprue: The passage through which the molten metal flows from the pouring basin, and reaches the
mold cavity. It controls the flow of metal into the mold.

Runner: The channel through which the molten metal is carried from the sprue to the gate.
Gate: A passageway through which the molten metal enters the mold cavity.
Riser: The shapes of the Risers are like a sprue; it is used to store additional metal to prevent
shrinkage.

Vent: Small opening in the mold to facilitate escape of air and gases.

Fig: Sand casting Terminology

1.1 Steps involved in Making Sand Casting

1. Pattern making
2. Core making

3. Moulding
4. Melting and Pouring
5. Cleaning

6. Inspection
Pattern making: The pattern is a physical body of the casting used to make moulds. The mould
is made by packing moulding sand around the pattern.

Core making: Cores are forms, usually made of sand, which are placed into a mold cavity to
form the interior surfaces of castings.

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

Moulding: Moulding is nothing but the mould preparation activities for receiving molten metal.
Moulding usually involves:

i. Ramming sand around the pattern placed in flask,

ii. Removing the pattern,
iii. Setting cores in place, and
iv. Creating the gating/feeding system to direct the metal into the mould cavity
v. Finishing and closing the mould

Figure: Steps involved in Making Sand Casting

Melting and Pouring: The preparation of molten metal for casting is referred to simply as melting.
The molten metal is transferred to the pouring area where the moulds are filled
Cleaning: Cleaning involves removal of adhering sand, scale, and excess metal from the casting.
Excess metal, in the form of wires, parting line fins, risers gates and other foreign material is
removed.

Inspection: Inspection follows, to check for defects in the casting as well as to ensure that the
casting has the dimensions specified on the drawing and/or specifications.

1.2 Advantages of Casting

 Molten material can flow into very small sections so that intricate shapes can be made by
this process.
 It is possible to cast practically any material that is ferrous or non-ferrous.
 The necessary tools required for casting moulds are very simple and inexpensive.
 There are certain parts (like turbine blades) made from metals and alloys that can only be
processed this way. Turbine blades: Fully casting + last machining
 Size and weight of the product is not a limitation for the casting process.
 Wastage of raw material is less
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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

1.2 Disadvantages of Casting

 Poor dimensional accuracy and surface finish for some processes; e.g., sand casting
 Safety hazards to workers due to hot molten metals
 Metal casting is a labour-intensive process
 Limitations on mechanical properties

2.0 METAL FORMING PROCESSES

Forming is the process of obtaining the required shape and size on the raw material by subjecting
the material to plastic deformation through the application of tensile force, compressive force,
bending or shear force or combinations of these forces.

2.1 Rolling:

Rolling is the most rapid method of forming metal into desired shapes by plastic deformation
through compressive stresses using two or more than two rolls. It is one of the most widely used
of all the metal working processes. The main objective of rolling is to convert larger sections such
as ingots into smaller sections

2.2 Forging:

Forging is a process in which material is shaped by the application of localized compressive forces
exerted manually or with power hammers, presses or special forging machines. Typical forged
parts include spanners, axles, Crankshafts, rivets, bolts, crane hooks, connecting rods, gears,

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

turbine shafts, hand tools, railroads, and a variety of structural components used to manufacture
machinery.

Types of Forging

Open-Die Forging: Compression of work-part between two flat dies. Similar to compression test
when work-part has cylindrical cross section and is compressed along its axis. Deformation
operation reduces height and increases diameter of work.

Figure: Open-Die Forging

Closed-Die Forging: Compression of workpart by dies with inverse of desired part shape. Flash
is formed by metal that flows beyond die cavity into small gap between die plates. Flash must be
later trimmed, but it serves an important function during compression

Figure: Closed-Die Forging

2.3 Extrusion:
Extrusion is a compression process in which the work metal is forced to flow through a die opening
to produce a desired cross-sectional shape. The process can be likened to squeezing toothpaste out
of a toothpaste tube.
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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

Types of Extrusion
Direct Extrusion: Flow of metal through the die is in the same direction as the movement of ram.
It is also called forward extrusion. In forward extrusion, the problem of friction is prevalent
because of the relative motion between the heated metal billet and the cylinder walls.

Figure: Direct extrusion

Indirect Extrusion: Flow of metal through the die is in the opposite direction as the movement of
ram. It is also called backward extrusion. Since, there is no friction force between the billet and
the container wall, therefore, less force is required by this method.

Figure: Indirect extrusion

3.0 WELDING PROCESS

Welding is a joining process of two similar or dissimilar metals by fusion, with or without the
application of pressure and with or without the use of filler metal. The fusion of metal takes place
by means of heat. The heat may be generated either from combustion of gases, electric arc, electric
resistance or by chemical reaction.

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

Advantages of welding process:

1. Welding is more economical and is much faster process as compared to other processes
(riveting, bolting, casting etc.)
2. Welding, if properly controlled results permanent joints having strength equal or
sometimes more than base metal.
3. Large number of metals and alloys both similar and dissimilar can be joined by welding.
4. General welding equipment is not very costly.
5. Portable welding equipments can be easily made available.
6. Welding permits considerable freedom in design.
7. Welding can join welding jobs through spots, as continuous pressure tight seams, end-to-
end and in a number of other configurations.
8. Welding can also be mechanized.
Disadvantages of welding process:

1. Welding gives harmful light radiations and fumes.

2. Required skilled labour
3. Jigs, and fixtures may also be needed to hold and position the parts to be welded
4. Change in metallurgical properties of base metal
5. Edges preparation of the welding jobs are required before welding
6. It results in residual stresses and distortion of the workpieces.
7. High standards of testing and inspection

Applications of welding process:

 Welding is widely used in fabrication of pressure vessels, bridges, building structures,

aircraft and space crafts, railway coaches and general applications.
 It is also being used in shipbuilding, automobile, electrical, electronic and defence
industries.
 Welding is vastly being used for construction of transport tankers for transporting oil, milk
and fabrication of welded tubes and pipes, chains, LPG cylinders and other items.
 Steel furniture, gates, doors and door frames, body and other parts of a number of items
such as refrigerators, washing machines, microwave ovens etc are fabricated by welding.

3.1 CLASSIFICATION OF WELDING PROCESSES:

I. Fusion Welding: The welding process that uses heat to join (or fuse) two or more materials by
heating them to their melting point is known as fusion welding.

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

1. Gas welding
2. Electric arc welding

(i) Shielded metal arc welding (ii) Carbon arc welding (iii) Gas metal arc welding (iv) Gas tungsten
arc welding (v) Submerged arc welding

II. Pressure Welding: The welding process in which two workpieces are joined under a pressure
providing an intimate contact between them and at a temperature essentially below the melting
point of the base materials is known as solid state welding.

(i) Spot welding (ii) Seam welding (iii) Projection welding (iv) Thermit welding (v) Friction
welding (vi) Forge welding
3.2 Oxy-Acetylene Gas Welding
In this process, acetylene is mixed with oxygen in correct proportions in the welding torch
and ignited. The flame resulting at the tip of the torch is sufficiently hot to melt and join the parent
metal. The oxy-acetylene flame reaches a temperature of about 3300°C and thus can melt most of
the ferrous and non-ferrous metals in common use. A filler metal rod or welding rod is generally
added to the molten metal pool to build up the seam slightly for greater strength. In oxy-acetylene
welding, flame is the most important means to control the welding joint and the welding process.
The correct type of flame is essential for the production of satisfactory welds. An arrangement of
oxy acetylene welding set up is shown in Fig.

Figure: Oxy acetylene welding set up

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

1. Neutral welding flame (Acetylene and oxygen in equal proportions) commonly used for the
welding of mild steel, stainless steel, cast Iron, copper, and aluminium.

2. Carburizing welding flame or reducing (excess of acetylene, used for welding high carbon steel).

3. Oxidizing welding flame (excess of oxygen) used for copper-base metals and zinc-base metals

3.3 Electric Arc Welding

The process, in which an electric arc between an electrode and a work is utilized to weld
base metals, is called an arc welding process. The basic principle of arc welding is shown in Fig.
In this process, the heat is generated by an electric arc between base metal and a consumable
electrode. In this process electrode movement is manually controlled hence it is termed as manual
metal arc welding. To avoid contamination of the molten weld metal from atmospheric gases
present in and around the welding arc, protective environment must be provided

Figure: Electric arc welding set up

4.0 MACHINING PROCESS:
Machining is “the process of removing unwanted material in the form of chips, from a
block of metal, using cutting tool”
4.1 Turning: Turning is used to generate a cylindrical shape. In this process, the work piece is
rotated and cutting tool removes the unwanted material in the form of chips. The cutting tool has
single cutting edge. The speed motion is provided by the rotating work piece, and the feed motion
is achieved by the cutting tool. Additionally, the turning process works great for machining the
interior or exterior part of a material. Turning performed on the material exterior part is known as
facing, while that done on the inside is known as boring.

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

Fig: Turning

4.2 Drilling: Drilling is used to create a round hole. In this process, the cutting tool is rotated and
feed against the work piece fixed in a holding device. The cutting tool typically has two or more
cutting edges. The tool is fed in a direction parallel to its axis of rotation into the work piece to
form the round hole.

Fig: Drilling
4.3 Milling: Milling is used to remove a layer of material from the work surface. The milling
process is used to produce a plane or straight surface. The cutting tool used has multiple cutting
edges. The speed motion is provided by the rotating milling cutter. The direction of the feed motion
is perpendicular to the tool’s axis of rotation.

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

4.4 Grinding: Grinding is one of the types of machining process ideal for improving the finish
on a machined part’s surface and tightening its tolerance. Furthermore, the process produces
parts with identical shapes, finishes, and sizes.

5.0 Computer Numerical Control (CNC)

 Computer numerical control (CNC) is the numerical control system in which a dedicated
computer is built into the control to perform basic and advanced NC functions.
 CNC controls are also referred to as softwired NC systems because most of their control
functions are implemented by the control software programs.
 CNC is a computer assisted process to control general purpose machines from instructions
generated by a processor and stored in a memory system.
Components of modern CNC systems

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

6.0 3D PRINTING

3D printing is a process of making three dimensional solid objects from a digital file. The
creation of a 3D printed object is achieved using additive processes.

6.1 Steps in 3D printing

1. Modelling: A person creates a 3D image of an item using a CAD software program. It

takes virtual blueprints from modeling software and “slices” them into digital cross-
sections.
2. Printing: To perform a print, the machine reads the design from an .stl file and lays
down successive layers of liquid, powder, paper or sheet material to build the model from
a series of cross section.

3. Finishing: The printer forms the item by depositing the material in layers. In some
cases, light or lasers are used to harden the material.

Methods 3D printing
1.Selective Laser Sintering (SLS)
2.Stereolithography
3.Fused Deposition Modeling (FDM)

6.2 Selective Laser Sintering (SLS) Technique

It is an additive manufacturing technique that uses a high-power laser (for eg. CO2 laser) to fuse
small particles of plastic, metal, ceramic or glass powders into a mass that has a desired 3D shape.

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

Figure: Selective Laser Sintering

6.3 Materials used in 3D printing: 3D printing supports a wide range of materials, including
plastics, metals, ceramics, resins, and composites. The choice of material depends on the
intended application and the specific properties required.

6.4 Applications of 3D printing:

1. Prototyping: 3D printing is widely used for rapid prototyping, allowing designers

and engineers to quickly iterate and test their designs.

2. Customization: It enables the production of customized products, such as

personalized medical implants, orthopedic devices, and consumer goods.
3. Manufacturing: 3D printing is increasingly used for small-batch and on-
demand manufacturing of complex parts and components.
4. Dental and Medical: The technology is employed to produce dental crowns,
prosthetics, and anatomical models for surgical implants.
5. Aerospace: Aerospace industries use 3D printing for lightweight and
complex components, reducing material waste.
6. Automotive: Automotive manufacturers utilize 3D printing for prototyping, tooling,
and producing certain components.

6.5 Advantages of 3D printing:

 Rapid Prototyping: Allows for quick iteration and testing of designs.

 Customization: Enables the production of personalized and unique products.
 Complex Geometries: Capable of creating intricate and complex shapes that may
be challenging with traditional manufacturing methods.
 Reduced Material Waste: Material is added only where needed, minimizing waste.
 On-Demand Manufacturing: Suitable for small-batch and on-demand production.

6.6 Limitations of 3D printing:

 Material Limitations: Some materials suitable for traditional manufacturing may not
yet be available for 3D printing. 13
 Speed: Printing large and complex objects can be time-consuming.
Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

 Surface Finish: Achieving a smooth surface finish may require

 Health hazards
7.0 SMART MANUFACTURING:
Smart manufacturing is a broad category of manufacturing that employs computer-
integrated manufacturing, high levels of adaptability and rapid design changes, digital
information technology, and more flexible technical workforce training. Other goals
sometimes include fast changes in production levels based on demand, optimization of
the supply chain, efficient production and recyclability. In this concept, as smart
factory has interoperable systems, multi-scale dynamic modelling and
simulation, intelligent automation, strong cyber security, and networked sensors.

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

THERMAL ENGINEERING
BOILER:
A boiler is defined as a closed vessel which is used to heat liquid usually water or to generate
vapour or steam or any of such combination under pressure for external use by combustion of fossil
fuels.

Working Principle of Boiler

Understanding the working of the boiler is very simple. The boiler is a closed vessel in which the
water is stored. Hot gases are produced by burning fuel in the furnace. These hot gases are made
to come in contact with the water vessel where the heat transfer takes place between the water and
the steam. Therefore, the basic principle of the boiler is to convert water into steam by using heat
energy. There are different types of boilers used for different purposes.

Types of Boilers:

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

Fire Tube Boiler

Fire Tube Boilers are used widely in small and medium -sized industries. They are
factory-made and come wit h pumps and control systems, all set to start operations as
soon as connected to a fuel and water supply. They are often called the packaged
boilers. These kinds of boilers are befitt ing for low to medium pressure steam
requirements. Here, the combust ion gases flow outside the water pipes.

Water Tube Boiler

The Water Tube Boiler is mult ifaceted. They are significant ly different from the fire
tube boilers. The two types differ in the ways they funct ion. Water Tube boilers are
usually used in large manufacturing factories where there is a need for high -pressure
requirement. There is a circulat ion of water between the mud and the steam drum,
ensuring more control over the circulat ion by densit y differences.

DIESEL CYCLE

The Diesel cycle is a thermodynamic process that is commonly used in diesel engines for internal
combustion. It operates on the principle of constant pressure combustion and consists of four
distinct processes: intake, compression, combustion, and exhaust.

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

1-2 In this process suction takes place

2-3 (Adiabatic process) In this process compression takes place. Both the inlet and exhaust
valves are closed and the compression takes place which is much higher than that of an otto cycle.
This increases the pressure and temperature.

3-4 (Isobaric process) In this process, fuel is added, and combustion occurs due to high
temperature, while maintaining a constant pressure because the volume is also increasing.

4-5 (Adiabatic process) In this process expansion takes place, due to combustion the piston moves
from TDC to BDC and power is generated.

5-2 (Isochoric process) In this process, heat rejection is taking place at constant volume.

Compression ratio is 14 to 22.

Efficiency of diesel cycle is

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OTTO CYCLE:
Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

The Otto cycle, also referred to as the spark-ignition cycle, is the fundamental thermodynamic
cycle used in petrol engines. It operates on the principle of constant volume combustion and
consists of four processes: intake, compression, combustion, and exhaust.

1-2 (Adiabatic process): In this process compression takes place, as the piston moves from
BDC to TDC increasing its temperature

2-3 (Isochoric process): In this process, ignition is taking place, combustion happens when
the piston is at TDC and pressure increases at a constant volume.

3-4 (Adiabatic process): In this process expansion is taking place, the heat produced due to
the combustion pushes the piston down which rotates the crankshaft.

4-1 (Isochoric process): In this process, heat rejection is taking place at constant volume.

The compression ratio of the otto cycle is 8 to 12.

The efficiency of otto cycle is

Refrigeration and Air Conditioning Cycle:

The purpose of a refrigeration cycle is to absorb and reject heat. The four basic components of a
basic cycle are the compressor, condenser, expansion device, and evaporator. Let us explore them
individually.

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

Compressor

o The first stage in the refrigeration cycle is compression.

o A compressor is the component of the system that boosts the pressure of the refrigerant
fluid.
o The refrigeration fluid enters the compressor as a low-pressure, low-temperature gas and
exits as a high-pressure, high-temperature gas.
o Compression can be accomplished through a variety of mechanical processes, and as a
result, several compressed air designs are used in HVAC and cooling systems today.
o Here are a few popular ones: Reciprocate compressors, Scroll compressors, Rotary
compressors, and so on.

Condenser

o In a basic refrigeration loop, the condenser is also known as the condenser coil.
o It consists of a series of tubes of external fins situated at the back of the refrigerator.
o This component aids in the conversion of the gaseous refrigerant to liquid form.
o This component receives high-temperature, high-pressure vaporised refrigerant from the
compressor.
o The condenser extracts heat from hot refrigerant vapour gas vapour until it cools into a
concentrated liquid state, also known as condensation.
o The coolant is a high-pressure, low-temperature fluid after condensing, and it is routed to
the loop’s expansion device.

Expansion device 19
Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

o The expansion valve regulates the flow of coolant into the evaporator, also known as the
cooling coil.
o Flow control valves are another name for expansion valves.
o It is a delicate simple tool that aids in sensing refrigerant temperature changes. However,
regardless of setup, the purpose of a system’s expansion device is the same: to create a
pressure drop after the refrigerant exits the condenser.
o Because of the pressure drop, some of the refrigerants will quickly boil, resulting in a two-
phase mixture.
o This rapid phase change is known as flashing, and it assists the evaporator, the next part of
hardware in the circuit, in performing its intended function.
o These components are available in a variety of styles including fixed orifices, thermostatic
valves or thermal expansion valves, and more modern automation expansion valves are all
popular configurations.

Evaporator

o The evaporator is another heat exchanger in a typical refrigeration circuit, and it, like the
condenser, is named after its primary function.
o Because it accomplishes what we predict air conditioning to do is absorb heat.
o It serves as the end of a refrigeration cycle.
o It is the main component of the cooler that assists in keeping the device and its contents
cool at all times.
o It has high thermal conductivity tubes that aid in the absorption of heat rejected by the
system’s fan or coil.
o This occurs when coolant enters the exchanger as a low-temperature fluid at low pressure,
and a blower forces air across the evaporator’s fins, cooling the atmosphere by absorbing
heat from the space in question.

Internal combustion engine

In internal combustion engine, the combustion of fuel takes place inside the engine cylinder and
heat is generated within the cylinder. This heat is added to the air inside the cylinder and thus the
pressure of the air is increased tremendously. This high pressure air moves the piston which rotates
the crank shaft and thus mechanical work is done. Most of the internal combustion engines are
reciprocating engines with a piston that reciprocate back and forth in the cylinder.

Two stroke SI (Petrol) engine:

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

1. FIRST STROKE ( SUCTION AND COMPRESSION ) :

The first stroke consists of the suction and compression processes. During the first stroke , the
piston moves upward from BDC to TDC. When the piston is at BDC, the partially compressed air
fuel mixture from crank case enters into the cylinder through transfer port. The piston moves
upward and compress air contained in it till the piston reaches TDC. At the end of compression
stroke, the spark plug produces spark, it will ignite the compressed high pressure fuel air mixture.
]

2. SECOND STROKE ( EXPANSION OR POWER AND EXHAUST STROKE ):

When air fuel mixture is ignited, both the pressure and temperature of the products of combustion
will suddenly increase . Therefore, the piston receives power impulse from the expanded gas and
it pushes the piston downward and also produces the power stroke. During the expansion stroke,
some of the heat energy produced is converted into mechanical work.

During downward stroke of piston, already entered air fuel mixture in the crank case is partially
compressed by the underside of the piston. This pre compression process is called crank case
compression. At the end of the power stroke, the exhaust port opens and burnt gases are sent out
of the engine through this port.

At the same time, all the burnt gases are not exhausted. Some portion of it will remain in the
cylinder. when the piston moves to BDC, The fresh air fuel mixture from crank case enters into
the cylinder to sweep out the burnt gases. The process of sweeping out the exhaust gases with the
help of fresh air fuel mixture is known as scavenging. The scavenging helps to remove the burnt
gases from the cylinder.

FOUR STROKE PETROL ENGINES (SI):

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

Suction Stroke

 During the suction stroke, the piston is moved downward by the crankshaft, which is
revolved either by the momentum of the flywheel or by the power generated by the electric
starting motor.
 The inlet valve remains open and the exhaust valve is closed during this stroke.
 The downward movement of the piston sucks the air-fuel mixture in the cylinder from the
carburetor through the open inlet valve.
 Here the fuel is petrol mixed with air, broken up into a mist, and partially vaporized in the
carburetor.
Compression Stroke

 During the compression stroke, the piston moves upward, thus compressing the charge.
 Ignition and compression also take place during this stroke.
 The heat produced by the compression performs like a mixture of air and petrol inside the
cylinder.
 Heat makes petrol easier to burn, while the compression pushes it into closer combination
with the air.
 The mixture under compression is ignition by the spark produced by a spark plug,and the
combustion is about half-completed when the piston is at the top dead center.
 Both the inlet valves and exhaust valves closed during the compression stroke.
Power Stroke

 The expansion of the gases due to the heat of combustion exerts pressure on the cylinder
and piston.
 Under this impulse, the piston moves downward thus doing useful work. Both the valves
remain closed during this stroke.
Exhaust Stroke

 During this stroke, the inlet valve remains closed and an exhaust valve opens.
 The greater part of the burn gases escapes their expansion.
 The piston moves upward and pushes the remaining gases out of the open exhaust wall.
 Only a small quantity of exhaust gases remains in the clearance space which will dilute the
fresh incoming charge.
Thus, in this type of engine, four strokes of the piston are required to complete the cycle, and the
four-stroke makes two revolutions of the crankshaft. The processes are repeated over and over
again in running the engine. each alternative revolution of the crankshaft as one power stroke.

WORKING PRINCIPLE OF 2 STROKE DIESEL ENGINE

1. 1st Stroke – As the piston starts rising from its B.D.C. position, it closes the transfer and the
exhaust port. The air which is already there in the cylinder is compressed. At the same time with
the upward movement of the piston, vacuum is created in the crank case. As soon as the inlet port
is uncovered the fresh air is sucked in the crank case. The charging is continued until the crank
case and the space in the cylinder beneath the piston in filled with the air.
2. 2nd Stroke – Slightly before the completion of the compression stroke a very fine spray of diesel
22
is injected into the compressed air (which is at a very high temperature). The fuel ignites
spontaneously.
Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

Figure of Two stroke CI Engine

Pressure is exerted on the crown of the piston due to the combustion of the air and the piston is
pushed in the downward direction producing some useful power. The downward movement of the
piston will first close the inlet port and then it will compress the air already sucked in the crank
case.
Just at the end of power stroke, the piston uncovers the exhaust port and the transfer port
simultaneously. The expanded gases start escaping through the exhaust port and at the same time
the fresh air which is alredy compressed in the crank case, rushes into the cylinder through the
transfer port and thus the cycle is repeated again.
WORKING PRINCIPLE OF FOUR STROKE DIESEL ENGINE.
There are four strokes as:
1. Suction Stroke
2. Compression stroke
3. Expansion stroke
4. Exhaust stroke
1. Suction stroke: This stroke starts with the piston at top dead centre position. The inlet value is
opened and the exhaust value is closed. The downward movement of the piston creates vacuum in
the cylinder due to which air is drawn into the cylinder. The movement of the piston is obtained
either by the starter motor or by the momentum of the fly wheel.

2. Compression stroke: This stroke starts with the piston at B.D.C. position. Both the inlet and
exhaust values are closed.
The air sucked during the suction stroke is compressed as the piston moves in the upward direction.
A few degree before the completion of compression
23 stroke, a very fine spray of diesel is injected
into the compressed air. The fuel ignites spontaneously.
Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

Figure of CI Engine Cycle

3. Expansion stroke: Both the inlet and exhaust valves remain closed. The heat energy released
by the combustion of the fuel, results in the rise in pressure of the gases. This high pressure rise
drives the piston in the downward direction, thereby producing some useful work. This stroke is
called as power stroke.
4. Exhaust stroke: This stroke starts with the piston at the B.D.C. position. The inlet value remains
closed whereas the exhaust value is opened. The upward movement of the piston pushes the burnt
gases out of the cylinder through the exhaust valve. At the end of exhaust stroke, the exhaust valve
is also closed.
The four-strokes complete one cycle which may repeat again to produce power.

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Basic Mechanical Engineering UNIT 2: Manufacturing Process, I.C Engines, Boilers

COMPONENTS OF ELECTRICAL AND HYBRID VEHICAL

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