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Latching current is the minimum current which is required to flow from the anode to the cathode to switch "ON" the SCR.

Holding current is the minimum current which needs to keep flowing to keep the SCR in the 'ON' state.

The Latching current will be greater than the Holding current for an SCR.

Dynamic displacement compressors work by increasing the velocity of the incoming gas and then restricting the air flow.  This restriction in the airflow causes the pressure to increase.

Dynamic displacement compressors are also known as oil free or oil-less compressors.  There are no seals and valves in dynamic displacement compressors.

Centrifugal compressors are the most common type of dynamic displacement compressors.  The operation is similar to that of a centrifugal pump.

Air is drawn into the centre of an impeller and then pushed out of the perimeter at high velocity.  Centrifugal compressors operate at high speeds of the order of 75000 rpm.  They are driven by gears.
They have higher efficiencies as compared to dynamic displacement compressors.  Changes in temperature and humidity will have a significant impact on  the operating characteristics of dynamic displacement compressors.

Pneumatic tools are tools which are driven by air.  In a Pneumatic tool, compressed air from a cylinder is made to pass through a control valve which is operated by the user.  The air is then passed through a small turbine which generates rotary motion.  This rotary motion is used to drive a shaft.

This rotary motion can be used for drilling or grinding purposes.  The air is then drained through an outlet.

Advantages
Compressed air is easily available
Tools can be smaller than electric tools.
Pneumatic tools develop high torque
They can be used in environments where there is a risk of fire
They are not affected by temperature
They are clean and do not leave any residue
Compressed air can be guided easily through small tubes wherever required.

Disadvantages
They are noisy
Leakages occur frequently
Air should be dried properly otherwise there can be condensation.

Compressed air has many uses in the industry.  Compressing air is an effective means of storing energy and transporting it over large distances.

Some of the applications of Compressed Air are


  1. As a source of energy to drive machines and pneumatic tools.  Pneumatic tools can be smaller than electrically driven tools.
  2. In Shot blasting applications
  3. In Spray painting
  4. In control systems. Compressed air is widely used in the petrochemical industry where electric power cannot be used due to the risk of fire.
  5. In Braking systems in trains.
  6. In Refrigeration
  7. In spray cans such as those used in perfumes and other sprays.
  8. In cleaning

In single acting compressors, air is drawn and compressed on only one side of the piston.  When the piston moves down, the suction valve opens and air enters the cylinder.  When the piston moves up, at the end of the compression, the discharge valve opens and the compressed gas is released.

The other side of the piston is connected to the crankshaft. 

In a double acting compressor, air is drawn and compressed on both sides of the piston.  The piston is operated by a connecting rod through an airtight seal.  There are compression chambers on both sides of the piston and a set of suction and discharge valves.  .  

Double acting compressors have higher efficiency.  However, they are expensive to manufacture. They are usually water cooled.  


Positive displacement compressors suck a specific quantity of air into a cylinder.  The piston then compresses it.  The output of a positive displacement compressor is always.

Examples of positive displacement compressors are Reciprocating Piston compressors, Rotary Screw compressors, Scroll compressors and Rotary Vane compressors.

Positive displacement compressors can build very high pressures.  The downsides are high noise, lesser efficiency, leakages from the seals and frequent maintenance.


The mean effective pressure is related to the operation of a reciprocating engine and is a valuable measure of an engine's capacity to do work which is independent of the amount of engine displacement.

Mean effective pressure is another way to measure the engine performance. The cylinder pressure varies palpably while the gas expands during the power stroke in an engine. Peak pressure is available just after TDC, but pressure drops readily as the piston moves towards BDC. While signifying the cylinder pressure, it is helpful to refer to the average or mean effective pressure throughout the whole power stroke.





Where

T = torque in newton-metre
nc = number of revolutions per power stroke (for a 4-stroke engine nc =2)
Vd = displacement volume in cubic metre
The units used for mean effective pressure may be either kilo Newton / square meter (kN/m2) or bars (where: 1 bar ¼ 100 kN/m2).

Power-to-weight ratio is a type of measurement of the actual performance of any engine. It is also used as a measurement of performance of a vehicle, wherein the produced external power of the engine is divided by the mass of the vehicle.

Power-to-weight is often expressed at the maximum value, but the actual value may vary in use and variations will affect performance. The opposite of power-to-weight, the weight-to-power ratio or power loading is a ratio commonly applied to aircraft, cars, and vehicles in general to compare them.

Power-to-weight ratio is equal to thrust applied per unit mass multiplied by the velocity of a vehicle. The power to weight ratio is mathematically expressed as






Where
a(t) is acceleration of the center of mass of the body, changing with time.
V(t) is linear speed of the center of mass of the body.
g is the gravitational acceleration.

Spark burning engines (SI engines) use gasoline as fuel, while in compression ignition (CI) engines, the fuel used is usually diesel. Petrol engines have less weight and therefore they can achieve higher speed. Diesel engines are heavy and hence less efficient.

In the case of SI engines, the Otto cycle is used where addition of heat or fuel combustion occurs at a constant volume. In CI engines, in the Diesel cycle, the fuel combustion occurs at a constant pressure.
In the case of SI engines, a mixture of air and fuel is automatically injected from cylinder head portion. In CI engines, the quantity of air to be injected is not controlled.

To generate this spark in SI engines, the spark plug is placed in the cylinder head of the engine. in the case of CI engines, there is no need for spark plugs.

In the case of CI engines the value of compression ratio is higher; they have the potential to achieve higher thermal efficiency.

Clearance Volume
The Clearance Volume is the empty space remaining in the cylinder when the piston is at the Top Dead Centre (TDC).  It is denoted by Vc

Bore
The internal diameter of the engine cylinder is known as the bore.  The cylinder is made using a boring process, hence then name.

Top Dead Centre
The upper most point in the cylinder which the piston reaches is called the Top Dead Centre or TDC.  Once this position is reached, the piston reverses its direction and moves downwards.

Bottom Dead Centre
The bottom most point in the cylinder which the piston reaches is called the Bottom Dead Centre or BDC.  The piston reverses direction and moves upwards once this point is reached.

Stroke Length

The distance travelled by the piston in one stroke is called the stroke length.  It is the distance between the Top Dead Centre and the Bottom Dead Centre.  It is represented by L.

Swept Volume

The volume of the total space covered by the piston when moving between the Bottom Dead Centre and the Top Dead Centre is called the Swept Volume.

            Four Stroke Engine                Two Stroke Engine
One power stroke is obtained for every four strokes
One power stroke is available for every two strokes
Heavier flywheel is required as there is only one power stroke for two revolutions
Lighter flywheel is sufficient.
Power produced for the same size is lesser when compared with a Two stroke engine.
Power produced is more for the same size.
High initial cost 
Lower initial cost
Higher efficiency due to turbocharging and positive scavenging
Lower efficiency 
Contains Valves and their operational mechanisms
 Ports are used instead of valves
Suitable for applications where efficiency should be high.
Suitable for applications where smaller size is required such as in ships and in automobiles.
All the fuel is burnt in the four stroke engine.  This results in lesser pollution. 
Some of the unburnt fuel is ejected along with the exhaust gases.
Hence, the two stroke engine will cause relatively more pollution

The Specific Fuel Consumption is an important parameter from the economical point of view.  Precise measurements of this value are made during testing and throughout the life of the engine.  Particular  calculations of this parameter are made before and after overhauling the engine.

Specific fuel Consumption = Mass Flow Rate in kg per hour / Power Generated in kilowatt per hour


The Volumetric Efficiency of an IC engine is the ratio of the total air drawn  inside the engine cylinder during the suction process to the amount of air which would normally be inside the cylinder of the given swept volume

Volumetric Efficiency = Total Air Present at the end of the suction process/ Air Present under ideal conditions.

The Volumetric efficiency is affected by the resistance offered to the air by the components in the air intake system such as filters and coolers.

Indicated Thermal Efficiency is the ratio of Indicated Power to the Fuel Power.


Where Ip is the indicated Power
mf is the mass of fuel
CV is the calorific value

The Brake Power of the engine is the power that is available at the crankshaft.  The Brake Power of the engine is lesser than the indicated power of the engine.

Some of the power developed by the cylinders is lost in overcoming the friction of the engine components.  Some power is also used to drive the engine accessories such as the pumps.  The Brake Power of the engine is the useful power available.

Thus,

Brake Power = Indicated Power - Frictional Power

Friction Power

The power used to overcome the frictional resistance and make the engine rotate is called the Friction Power.

An engine which runs without load is developing only Friction Power.


The piston is continually moving inside the cylinder.  The piston of the cylinder is not constant throughout the cycle.  The piston speed is zero at TDC and BDC where it reverses direction.

The speed is zero when the crank angle is 0 and 180 degrees.  The piston speed is maximum when the crank angle is 90 and 270 degrees.

The piston traverses a distance equal to the stroke length in half the rotation of the crank.  In one complete rotation of the crank, the piston would have covered a distance equal to twice the stroke length, 2L

Therefore if N is the speed of the engine, the mean speed of the piston will be




In the IC Engine, as the piston moves up and down the cylinder, many other things have to take place simultaneously.

The fuel has to be injected, the injected fuel has to be ignited and air has to be drawn in for combustion. After combustion, the products of combustion have to be expelled.

When each of these processes occurs is known as the Timing of the engine.  The Timing of the engine is denoted in degrees with reference to the TDC or BDC of the engine.

A general timing of an IC Engine is as follows.

Inlet valve opening           30 degrees before TDC
Inlet valve closure            50 degrees after BDC
Exhaust valve opening     45 degrees before BDC
Exhaust valve closure      30 degrees after TDC

Injection of the Fuel        15 degrees before TDC

Knocking in IC engines refers to undesirable combustion of fuel outside the optimized time zone.

The combustion in an IC engine occurs when the piston is at the Top Dead Center (TDC) at the beginning of the power stroke.

The combustion causes the expansion of the gases which pushes the piston down.

Knocking occurs when the combustion occurs at during the compression stroke when the piston is still moving towards the top dead center.  The power developed in such a situation is not useful. The power developed during knocking can damage the engine components.  For instance, knocking may remove material from the walls of the cylinder.  If knocking occurs for many cycles, the valves may get damaged.

Knocking

Knocking is usually accompanied with a pinging sound.

Inline Engines

In Inline Engines, the cylinders are placed in a single bank.  This type of engine design is used when the number of cylinder is less, such as 8 or 10.

Advantages

  • Vibrations are lesser 
  • No balancing shafts are required
  • Only one set of cams are required
  • Cheaper to manufacture
  • Can be naturally balanced. No balancing weights are required


Disadvantages

  • Difficult to design engines with more number of cylinders.
  • Larger in size.
V Engines

In V engines, the cylinders are placed in two banks, the banks are usually at an angle of 60 degrees. The V engine design is used for engines of large capacity.  

Advantages
  • Design is complex. 
  • Large engines can be designed using the V design

Disadvantages
  • Not naturally balanced
  • Two sets of cam shafts are required
  • Higher vibration than inline engines






The requirements for a good fuel are


  1. The Fuel should have high calorific value
  2. It should have low ignition temperature
  3. There should be less residue after combustion
  4. It should be cheap
  5. It should be easy to transport and to store
  6. It should cause less pollution

Advantages

  1. Liquids are easily tranportable through pipes.
  2. They burn without any solid residue like ash, clinkers etc.
  3. They are easy to ignite.
  4. They can be stored for long periods of time.


Disadvantages

  1. The cost of liquid fuels is more than that of solid fuels
  2. Specially designed expensive tanks are required to store liquid fuel
  3. They have an unpleasant odour.
  4. There is a greater risk of fire hazards with liquid fuels.

Advantages

  1. They can be transported easily through pipelines.
  2. They can be ignited easily.
  3. They have higher heat content
  4. They burn without any solid residue.
  5. They can be preheated using exhaust gases.


Disadvantages

  1. They require special storage tanks
  2. They are highly inflammation and hence there is a greater risk of fire accidents.

The octane number of a fuel describes the tendency of a fuel to detonate.  Detonation is the undesirable combustion of fuel inside and engine.  The combustion occurs when the piston is not in the power stroke.  The detonation produces a pinging sound and can damage the engine components.

Detonation is chiefly caused by octane.

The octane number is the percentage of octane in a mixture of iso-octane and iso-heptane.

To calculate the octane number of a fuel, an engine is run with the fuel.  Trials are conducted in which detonation occurs in the engine.

The engine is then run with different mixtures of iso-octane and iso-heptane in the same conditions.  The trial is repeated with different mixtures.  The conditions at which knocking occurs are noted.

The combination of iso-octane and iso-heptane which causes knocking at the same conditions as that of the fuel is identified.

For instance, let us say that a combination of 65% iso octane and 35% iso heptane causes knocking in the same conditions as that of the fuel.

Then, the fuel is said to have an octane number of 65.

The octane number of a fuel describes the tendency of a fuel to detonate.  Detonation is the undesirable combustion of fuel inside and engine.  The combustion occurs when the piston is not in the power stroke.  The detonation produces a pinging sound and can damage the engine components.

Detonation is chiefly caused by octane.

The octane number is the percentage of octane in a mixture of iso-octane and iso-heptane.

To calculate the octane number of a fuel, an engine is run with the fuel.  Trials are conducted in which detonation occurs in the engine.

The engine is then run with different mixtures of iso-octane and iso-heptane in the same conditions.  The trial is repeated with different mixtures.  The conditions at which knocking occurs are noted.

The combination of iso-octane and iso-heptane which causes knocking at the same conditions as that of the fuel is identified.

For instance, let us say that a combination of 65% iso octane and 35% iso heptane causes knocking in the same conditions as that of the fuel.

Then, the fuel is said to have an octane number of 65.

Enthalpy  of a system is the sum of the internal Energy and the product of pressure and volume.

Enthalpy represents the total energy stored in the system.

H = U + pV

where
H is the enthalpy
U is the internal energy
p is the pressure and
V is the volume

Enthalpy and Temperature
The change in enthalpy is directly proportional to the change in temperature.

dH = m Cp(T2-T1) Where Cp is the specific heat at constant pressure

The internal energy of the system is the sum of the kinetic and potential energies of the system.

The kinetic energy of the system is the energy of molecules which are moving inside the system.  The kinetic energy of the system increases with the increase in temperature.

The potential energy of the molecules in the system is the result of the interatomic forces acting on the molecules.  For example, electrostatic forces can cause the attraction and repulsion of the atoms.

The sum of the kinetic and potential energies of the sytem is called the internal energy.

Temperature and Internal Energy

The Internal Energy is directly proportion al to the temperature of the system.  This is because, the a rise in temperature increases the kinetic energy of the system.

Consequently, this causes a change in Internal Energy as well.

At absolute zero (0 K or -270 deg. C), the internal energy of a system is zero.

Change in Internal Energy

The change in internal  energy depends on the change in temperature of the system.

dU = mCv (T1 - T2)

where
m is the mass of gas in kg
Cv is the specific heat at constant volume in kJ/kg.k

A system in Thermodynamics is defined as an area where a thermodynamic process is taking place. A system has boundaries.

Anything outside the system which may affect the system is called the surroundings.

The system and the surroundings are separated by a boundary.  The boundary may be real or imaginary.  Energy or mass may pass through the boundary from the system to the surroundings or vice versa.

Consider a cylinder in an internal combustion engine.  The gas enclosed in the cylinder is the system, the rest of the engine and the atmosphere is  the system.




In Thermodynamics, energy is classified into two types viz. stored energy and transit energy

Stored Energy

Stored energy is the energy possessed by a system within its boundaries.  Examples are kinetic energy, potential energy, internal energy.  These types of energy are contained within the boundary of the system itself.

Transit Energy

Transit Energy is the energy which can cross the boundary of a system. Examples of Transit energy are electricity, heat, etc.


Specific Heat Capacity

Specific Heat Capacity of a substance is defined as the amount of heat required to increase the temperature of unit mass (1 kg) the substance through one degree Celsius.

Its unit is kJ/kgK

Specific Heat Capacity is denoted by C

Specific Heat Capacity at constant volume

Specific heat capacity at constant volume is the amount of heat required to raise the temperature of a unit mass of a substance by one degree celsius at constant volume


Q = m.Cv.dT

where
Q is the amount of heat transferred in Joules
m is the mass in kg
Cv is the specific heat capacity at constant volume in kJ/kgK
dT is the difference in temperature in K

Specific Heat Capacity at constant pressure

Specific heat capacity at constant pressure is the amount of heat required to raise the temperature of a unit mass of a substance by one degree celsius at constant pressure


Q = m.Cp.dT

where
Q is the amount of heat transferred in Joules
m is the mass in kg
Cp is the specific heat capacity at constant pressure in kJ/kgK
dT is the difference in temperature in K

     Hydraulic Systems    Pneumatic Systems
Low Operating SpeedHigh Operating Speed
Oil is the operating fluidAir is the operating fluid
Operating cost is lowOperating cost is high
Compact. Space requirements are lessSpace requirements are more
Leakages cause dirt accumulation in componentsLeakages do not cause dirt accumulation
They are used in applications where more power is requiredUsed in applications where less power is required.

The Accumulator is an important part of a hydraulic circuit.  It is a cylinder vertically mounted.  The cylinder is filled with air. The cylinder is connected to the hydraulic pipes.   The pressurized oil enters the accumulator and pushes against the air, compressing it.

The function of the accumulator are as follows.

1. It absorbs the fluctuations in pressure which occur when a motor is started and stopped.
2. It provides temporary compensation to losses due to leakages
3. It acts as a short absorber and absorbs pressure changes during operation.


The liquid used in hydraulic systems are usually oils.  Only liquids which have specific qualities can be used in hydraulic systems.

The required properties of the hydraulic fluid.

Viscosity

Viscosity is the property by which a liquid resists motion between the different layers.  Hydraulic liquids should have an optimum value of viscosity.  If the viscosity is too low, there will be leakages from small gaps.  If the viscosity is too high, the liquids may not be able to flow through small passages and pipes.

Oxidation stability
This refers to the property of the oil to resist oxidation.  The oxidation of the oil can be reduced by adding special additives.

Viscosity Index
Viscosity Index refers to the change in viscosity to a change in temperature.  If the viscosity index is high, the oil will have a very small change in viscosity to a change in temperature.  A hydraulic system requires oils with high Viscosity Index

Flash Point
It is the temperature at which the oil flashes momentarily when it is exposed to a fire.  Oils in Hydraulic systems should have a high flash point.

Cloud Point
A Cloud Point is the temperature at which the oil turns to wax.  This is an important criteria for hydraulic systems which are used in cold climates.

Lubricity
The oil used in hydraulic systems should have excellent lubricating properties to minimize wear and tear.

Rust and Corrosion prevention
The oils chosen should be able to prevent rusting of components and prevent corrosion.


Hydraulic systems are used in situations where higher forces and more precise controls are required.

Hydraulic systems have their merits and demerits.

The advantages of hydraulic systems are

  1. They are more compact.  Mechanical linkages are not required.
  2. They require less maintenance
  3. Hydraulic fluids contain oil.  Hence, they are self lubricating.  
  4. Very precise and smooth movements can be obtained.
  5. They have high reliability
Disadvantages of hydraulic systems

 1. Hydraulic systems are prone to leakages.
2. The leakage of oil causes accumulation of dust particles on the surface.
3.  The parts of the hydraulic system such as seals, gaskets, packings, etc need frequent replacement
4.  The oil is expensive.  


The main elements of a hydraulic system are

The Reservoir
The Reservoir contains the hydraulic liquid.  The hydraulic liquid is drawn by the pump into the hydraulic circuit.  The liquid is again returned to the reservoir.

Pump
The Pump is the centre of any hydraulic system.  The pump pressurizes the hydraulic liquid to the rated pressure and makes it available for the other components.  The pump is driven by a prime mover such as an engine or a motor.

Motor or actuator
The motor or actuator converts the hydraulic energy into mechanical energy.  The motion can be either linear or rotary.

For linear motion, a piston and cylinder are used.  For rotary motion, a turbine is used.  The is called a hydraulic motor.

Pipelines and Valves
The hydraulic system contains numerous valves and pipelines.  The liquid passes through the pipelines.  The flow and direction are controlled by the valves.


Discharge of a reciprocating pump

The discharge of a reciprocating pump is given by the following formula

Qt = A L N

where Qt is the theoretical discharge in m3
           A  is the Area of the piston in m2         
           L  is the length of the stroke in m          
           N  is the number of the revolutions of the crank per second (rps)


Coefficient of Discharge

The Coefficient of Discharge of a reciprocating pump is the ratio of the actual discharge to the theoretical discharge.

Cd = Qa / Qt

 Where
             Cd is the Coefficient of Discharge
             Qa is the actual discharge in m3
             Qt is the theoretical discharge in m3

Slip in a Reciprocating pumps

Slip in a Reciprocating pump is the difference between the theoretical discharge and the actual discharge.

Slip = Qt - Qa

A reciprocating pump works by moving a piston inside a cylinder.  The pump has two strokes

The Suction Stroke and
The Delivery Stroke

Suction Stroke
In the suction stroke, the piston moves from the bottom of the cylinder to the top.  This opens the suction valve and closes the delivery valve.  Water enters the pump cylinder from the suction side.

Delivery Stroke
In this stroke, the piston moves the top of the cylinder to the bottom.  In this stroke, the suction valve is closed and the delivery valve is open.  The pressurized water is expelled through the delivery stroke.


The advantages of multistage pumps are

  1. The Head per stage is less.  This results in reduced leakage loss.
  2. The pump can operate at lower speeds.
  3. The efficiency of the pump will increase.  
  4. The impeller size can be small as the head per stage is less.
  5. Multistage pumps can pump water to greater heights than single stage pumps.

The Surge Tank is a reservoir which is located at the end of the penstock.  It is located at the entrance to the turbine.

The function of the surge tank is to absorb the excess water when there is a sudden change in the load of the generator.  In such a situation, the governor will quickly reduce the water flow into the turbine.  As a result of this, very high pressure will be generated which can damage the penstock.

The Surge Tank prevents such a scenario by storing the excess water during a drop in the requirement.  Similarly, when there is a sudden increase in the load, a large amount of water will be needed in a very short period.  The water flow from the penstock may not be sufficient.  The Surge Tank can supply the excess water and ensure that the turbine is able to deliver the required load.  

The braking jet is a special jet of water which is used to bring the impulse turbine , such as a Pelton Wheel, to a stop in a short time.

The impulse turbine works when a jet of water impinges on the buckets in the turbine.  The turbine rotates at a high speed.

When the turbine is to be stopped, the flow of water is stopped.  However, due to the momentum, the turbine will run for a long time.

In order to reduce the time taken for the turbine to come to a stop, a jet of water is made to hit the turbine from the direction opposite to the direction of rotation.

This jet of water is known as the braking jet as it applies a "brake" on the turbine.

Turbines can be classified on different aspects and criteria

Some of the methods of classification of turbines are

According to position and mounting

Vertical Turbine
Horizontal Turbine

According to the head of water

Low Head Turbines
Medium Head Turbines
High Head Turbines

Eponymous classification (According to the name of the originator)

Francis Turbine
Kaplan Turbine
Pelton Wheel
Girard Turbine

According to the Flow Direction

Radially outward flow turbine
Radially inward flow turbine
Axial Flow Turbine
Mixed Flow turbine

According to Discharge

Low discharge
Medium discharge
High Discharge

Reaction Turbines work by converting the pressure as well as the velocity of the water into mechanical energy.  The water enters the high pressure region of the turbine at the inlet and moves to the low pressure region in the outlet.

Examples: Francis Turbine, Kaplan Turbine

An Impulse Turbine is a turbine in which the energy of the water is converted into kinetic energy by passing it through a nozzle.

The water enters the casing of the turbine at atmospheric pressure.  The jet of water which emanates from the nozzle impinges on the buckets of the turbine.  The kinetic energy of the water is used to rotate the turbine.

An example of this type of turbine is the Penton Wheel