It is process in which the metal is subjected to forces above recrystallisation temperature to give it a desired shape. Above recrystallisation temperature, the metal becomes plastic and causes of growth grains. By hot working the grains broke up and forms small new crystals which is called refinement of grains.
The various methods of hot working are:
a) Hot rolling b) Forging c) Extrusion d) Piercing e) Drawing f) Spinning
The function of the reciprocating engine is to convert the reciprocating motion into rotary motion. It is a simplest from of single slider crank chain mechanism. In reciprocating engine there is one sliding pair and three turning pairs. In figure there are three turning pairs and the sliding pair is formed between the cross head and the guides. Link 1 is fixed (frame). Link 2 will act as a crank, which rotates. Link 3 is a connecting rod and link 4 is a crosshead.
In cold rolling the metal is subjected to forces below its recrystallisation temperature to give it a desired shape. In cold rolling the deformation of metals is brought about by the process of slip of planes. Cold working results in better surface finish and close tolerances. It increases the strength and hardness of a metal.
The various methods of cold working are:
Shearing: a) Blanking b) Punching c) Trimming d) Broaching e) Burnishing etc.
Drawing: a) Blank drawing b) Wire drawing c) Spinning d) Embossing e) Bulging
Squeezing: a) Cold rolling b) Sizing c) Stamping d) Coining e) Rivetting
Bending: a) Roll forming b) Seaming c) Angle bending
It states that when two systems have equality of temperature to a third system, they in turn have equality of temperature with each other.
e.g. Suppose three are three bodies A, B and C. If body A has the same temperature as that of B and C then bodies B and C will also have the same temperature. It means that all the three bodies are in thermal equilibrium.
On the basis of their physical properties, the materials can be classified into following four types.
Elastic Material: When we apply some external forces on a body, it undergoes deformation. If the material regain its shape on the removal of forces, it is known as elastic material. e.g. Steel, Rubber etc.
Plastic Material: If the material does not regain its original shape on removal of external forces then it is known as plastic material. e.g. lead
Ductile Material: The ability of a material to be drawn into wires is known as ductility of a material. e.g. gold, silver etc.
Brittle Material: When we apply some external force on a material, it does not undergo deformation. It fails by rupture. e.g. glass
t states that when the material is loaded within the elastic limit the stress is directly proportional to strain.
i.e. Stress α strain
or Stress = Constant x Strain
or Stress = Modulus of elasticity x Strain
or Stress/Strain = Modulus of elasticity
In the figure the material obeys the hooke's law up to point A.
Octane number is defined as the percentage by volume of iso-octane (2, 2, 4-trimethylpentane) in a mixture on n-heptane and iso-octane. Octane number is a measure resistance of gasoline (petrol) to knocking in spark ignition internal combustion engines.
Example-If a gasoline contains 80% of iso-octane and 20% of n-heptane then the octane rating of the fuel is 80.
The engines having high compression ratio require high octane rating fuel to resist the knocking.
Pascal law states that if pressure is applied at any point in a static fluid in a container then there is an equal increase in pressure at every point in the container.
i.e. P1 = P2
F1/A1 = F2 /A2
It is defined as the percentage by volume of normal-cetane in a mixture of normal-cetane and alpha-methyl naphthalene. It is used for diesel fuels. Normal-cetane is assigned a cetane number of 100 and alpha-methyl naphthalene is assigned cetane number of zero.
Cetane number gives the time period between the start of injection and ignition of fuel. Higher the cetane number, shorter will be the time period between the start of injection and ignition of fuel.
Kirchoff’s First law or Current Law: In current carrying coils, the algebraic sum of all currents meeting at a junction is zero. The incoming currents are considered as positive and the outgoing currents are considered as negative.
According to Kirchoff’s law
I1 + I2 - I3 + I4 + I5 - I6 + I7 = 0
Kirchoff’s Second Law or Voltage law: In a closed circuit, the algebraic sum of emfs acting in that circuit is equal to the algebraic sum of the products of the currents and resistances of each part of the circuit.
An engine which receives energy from the combustion of fuels and converts it into mechanical work is known as Heat engine.
Heat engine can be divided into two main categories.
1. External Combustion Engine
2. Internal Combustion Engine
External Combustion Engine: In this type of engine the combustion of fuel takes place outside the cylinder. These types of engines are used to locomotives, ships etc. In locomotive steam is produced by the combustion of fuel and this steam is used to move a piston in a cylinder.
It can be further divided into the following types.
I. Reciprocating type
a) Reciprocating steam engine
i. Simple
ii. Compound
iii. Uniflow
II. Rotary type
a) Steam Turbines
i. Radial flow
ii. Axial flow
Internal Combustion Engine: In this type of engine the combustion of fuel takes place inside the cylinder. Examples of these types of engines are gas engines, petrol engines and diesel engines. In these engines the mixtures of gases and air or fuel and air enters the cylinder.
It can be further divided into the following types.
I. Reciprocating Engine
a) Spark Ignition (Petrol and Gas Engine)
i. 2 stroke
ii. 4 stroke
b) Compressed Ignition (Diesel Engine)
i. 2 stroke
ii. 4 stroke
II. Rotary Engine
a) Gas Turbines
It states that for a steady and ideal flow of an incompressible fluid, the sum of all the energies of a fluid is same at any two points.
i.e. Pressure energy + Kinetic energy + Datum energy = Constant
(Bernoulli's equation)
The engine in which the cycle of operations is completed in two revolutions (720º) of the crank shaft or four strokes of the piston is known as the four stroke engine. One stroke is completed when the piston moves from Top dead centre to Bottom Dead Centre or when the crank rotates through 180º. In four stroke CI engine the combustion of fuel-air mixture takes place with compression. The engine operates at a high compression ratio of the order of 16 to 20. Due to high compression ratio the mixtures reaches its ignition temperature and the combustion takes place.
The major components of a four stroke compressed Ignition engine are.
Cylinder: It is a cylindrical vessel in which a piston makes up and down motion.
Piston: It is a cylindrical component making up and down movement in the cylinder.
Combustion Chamber: It is the portion above the cylinder in which the combustion of the Fuel-air mixture takes place.
Inlet and Exhaust valves: The inlet valves allow the fresh fuel-air mixture to enter the combustion chamber and the exhaust valve discharges the products of combustion.
Crank Shaft: It is a shaft which converts the reciprocating motion of piston into the rotary motion.
Connecting Rod: The connecting rod connects the Piston with the crankshaft.
Cam shaft: The cam shaft controls the opening and closing of inlet and Exhaust valves.
Fuel Injector: It is located at the top of head to inject the fuel into the combustion chamber.
A four stroke CI engine consists of the following four strokes.
1. Suction or Intake stroke
2. Compression Stroke
3. Expansion or power stroke
4. Exhaust stroke
1. Suction Stroke: This stroke starts when the piston is at the top dead centre. When it moves downwards it will create suction and only air enters the cylinder. The inlet valve is open at this time and exhaust valve is closed. When the piston reaches at the bottom dead centre the inlet valve closes and the suction stroke ends. It all takes place in 180º of the crankshaft rotation.
2. Compression stroke: In this stroke the piston starts moving upward. During this stroke both the inlet and exhaust valves are closed. The air is compressed by the upward movement of the piston. At the end of the compression stroke the fuel is injected into the combustion chamber. An injector is provided to inject the fuel. At the end of compression stroke the temperature is sufficient to ignite the fuel and the combustion of fuel-air mixture takes place.
3. Expansion or Power Stroke: Due to the high pressure of the burnt gases the piston moves towards bottom dead centre. Both the inlet and exhaust valve remains closed during the stroke.
4. Exhaust stroke: When the piston is at the bottom dead centre the exhaust valve opens. As the pressure falls to atmospheric level. The piston moves from Top dead centre to bottom dead centre and sweeps the products of discharge out at nearly atmospheric pressure. The exhaust valve closes at the end of exhaust stroke. The gases are not fully exhausted. Some of the burnt gases stills remains in the clearance volume.
These remained gases mixed with the fresh fuel-air mixture entering the chamber.
The current flowing through a conductor is directly proportional to the potential difference across the ends of the conductor and inversely proportional to its resistance.
If V is the potential difference across the ends of a conductor, R is its resistance and I is the current passing through it then as per the Ohm’s law
I a V/R
or I = V/R
or V = IR
Where I is the current in Amperes, V potential difference is volts and R is the resistance of the conductor in ohms.
Limitations:
1. It can not be applied to the circuits containing transistors.
2. It is not applicable for the circuits containing non linear such as electric arc and powdered carbon etc.
According to Second law the engine based on the cyclic process is not possible in which we can convert all the heat into the work. There are a number of statements for the second law but the two most common statements are.
1. Kelvin plank statement
2. Clausius Statement
Kelvin plank statement: No process is possible whose sole result is the absorption of heat from a reservoir and the conversion of all of this heat into work.
Clausius Statement: No process is possible whose sole result is the transfer of heat from a colder to a hotter body.
The function of the carburetor is to supply the proper fuel-air ratio to the engine cylinder during suction created by the downward movement of the piston. As the piston moves downward a pressure difference is created between the atmosphere and the cylinder which leads to the suction of air in the cylinder. This sucked air will also carry with it some droplets of fuel discharged from a tube. The tube has an orifice called carburetor jet which is open to the path of sucked air. The rate at which fuel is discharged into the air will depend upon the pressure difference created. To ensure the atomization of fuel the suction effect must be strong and the fuel outlet should be small.
Working of Simple Carburetor: To increase the suction effect the passage of air is made narrow. It is made in the form of venturi. The opening of the fuel jet is placed at the venturi where the suction is greatest because the velocity of air will be maximum at that point.
The fig. shows a simple carburetor consists of float chamber, nozzle, a venturi, a choke valve and a throttle valve. The narrow passage is called venturi. The opening of the fuel is normally placed a little below the venturi section. The atomized fuel and air is mixed at this place and then supplied to the intake manifold of the cylinder. The fuel is supplied to the fuel jet from the float chamber and the supply of the fuel to the float chamber is regulated by the float pivot and supply valve. As the fuel level in the chamber decreases the float pivot will open the supply of the fuel from fuel tank.
As the air velocity of air passes through the venturi section will be maximum correspondingly the pressure will be minimum. Due to the pressure difference between the float chamber and the throat of the venturi, fuel is discharged from the jet to the air. To prevent the overflow of fuel from the jet, the level of fuel in the chamber is kept at a level slightly below the tip.
The quantity of the fuel supplied is governed by the opening of the butterfly valve situated after the venturi tube. As the opening of the valve is small, a less quantity of fuel-air mixture is supplied to the cylinder which results in reduced power output. If the opening of the valve is more then an increased quantity of fuel is supplied to the cylinder which results in greater output.
Types of Carburetors:
1. Solex Carburetor
2. Carter carburetor
3. S.U. Carburetor
Each resistant body in a machine which moves relative to another resistant body is called Kinematic link or element. A resistant body is which do not go under deformation while transmitting the force.
Kinematic links can be divided into three types.
1. Rigid link- In this type of link there is no deformation while transmitting the motion. Motion between the piston and crank can be considered as a rigid link.
2. Flexible link- In this type of link there is partial deformation while transmitting the motion. Belt drive is an example of flexible link.
3. Fluid link- In this type of link the motion is transmitted with the help of fluid pressure. Hydraulic brake is an example of fluid link.
1. Elasticity: The tendency of a material to regain its original dimensions (size and shape) upon the removal of load or force. Eg. Steel is more elastic than rubber. The ratio between tensile stress and tensile strain or Compressive stress and compressive strain is called young’s modulus of Elasticity.
Young’s Modulus of elasticity = E = Stress/Strain = σ / ε
Modulus of rigidity or shear modulus is the ratio of shear stress to the shear strain.
Modulus of Rigidity or Shear Modulus = Shear stress/shear strain
Bulk or Volume modulus of elasticity is the ratio of normal stress to the volumetric strain.
Bulk Modulus = Normal stress/Volumetric strain
2. Plasticity: The tendency of a material to permanently deform when subjected to external load beyond the elastic limit.
3. Toughness: The ability of a material to absorb energy in plastic deformation till the point of fracture is known as toughness. Toughness is indicated by the total area under the stress strain curve up to the fracture point. Eg. Copper has higher toughness than Cast iron.
Modulus of toughness = ½ (Ultimate Tensile strength + Yield strength) X elongationstrength) X elongation
4. Resilience: The ability of a material to absorb energy under elastic deformation and to recover this energy upon removal of load is termed as resilience. Resilience is indicated by the area under the stress strain curve till the point of elastic limit.
Modulus of Resilience = (Yield strength)2 / 2 Modulus of elasticity
5. Yield strength: The ability of a material to resist plastic deformation and represents the stress below which the deformation is entirely elastic in nature. The magnitude of yield strength for a metal is a measure of resistance to plastic deformation.
6. Tensile strength or Ultimate tensile strength: It is the ratio of maximum stress that a material can withstand without being fractured to the original area of cross section of the material. Ultimate tensile strength or tensile strength is the highest point in a stress-strain curve.
7. Impact strength: Ability of material to resist or absorb energy before it fractures during plastic deformation. It is closely associated with toughness with the difference that toughness takes into account both the strength and ductility of the material. Ductile materials have higher impact strength than brittle materials. Impact strength can be measured by two methods. (a). Izod Test (b). Charpy test
8. Ductility: The ability of a material to be drawn into wire is known as ductility. It is a tensile property and it is the capacity of a material to undergo deformation without being fractured. It cab be measured as the percent (%) elongation or percent area reduction.
% elongation = [(lf – l0) / l0] X 100
% area reduction = [(A0 – Af) / A0] X 100
Where lf is the length at the point of fracture
l0 is the original length
Af is the original cross section area
A0 is the cross sectional at the point of fracture
9. Malleability: The ability of a material to be formed into sheets by hammering or rolling is called malleability. It is a compressive property. e.g. Gold is the most malleable metal.
10. Brittleness: It is the tendency of a material to crack when it is subjected to deformation. It is opposite to ductility and malleability. e.g. Cast iron is a brittle material.
11. Creep: It is a deformation of a material due to the constant load for a long period of time. It is time and temperature dependent property of material. It takes place in three stages. i.e. primary, secondary and tertiary. a). First stage of creep known as primary creep occurs at relatively low temperature and the creep rate decreases with time. b). Second stage of creep known as secondary creep in the range of 0.4 to 0.7 Tm i.e. is the absolute melting temperature is a period of constant creep rate and hence referred to as steady state creep. It is the most important part of the creep curve for engineering applications. c). The third stage of creep known as tertiary creep occurs beyond 0.7 Tm has an accelerated rate and results in fracture of the material.
12. Fatigue: When a body is subjected to repeated and fluctuating load it tends to develop a characteristic behavior under which failure occurs which is referred to as fatigue.
13. Hardness: The resistance offered by a material to indentation is referred to as hardness of that material. It is the property by virtue of which it can resist abrasion, scratching and penetration. Hardness can be measured by the following tests.
a. Rockwell hardness test
b. Brinell Hardness test
c. Vicker’s hardness test.
14. Hardenability: It indicates the degree of hardness that can be imparted to a material by the process of hardening. It deals with the depth and distribution of hardness that can be induced in that particular material which can be increased by addition of alloying elements.
15. Wear Resistance: Wear is the unintentional removal of the material from the surface of the body. Wear is of two types. 1). Abrasive wear 2). Adhesive wear. Wear resistance is the ability of a metal to resist this unintentional removal.