BASIC CIVIL AND MECHANICAL ENGINEERING Unit IV Internal Combustion Engines NOtes




DHANALAKSHMI COLLEGE OF ENGINEERING
Dr. V.P.R. Nagar, Manimangalam, Chennai 601 301

Unit IV

Internal Combustion Engines

Introduction:
Heat Engine:
         Heat Engine is a machine which converts heat energy supplied to it into mechanical work.
         Heat energy is supplied to the engine by burning the fuel.

Classification of Heat Engines:
         Internal Combustion Engines (IC Engines)
In IC engines, combustion of fuel takes place inside the engine cylinder.Examples: Diesel Engines, Petrol Engines, Gas engines.
         External Combustion Engines (EC Engines)
In EC engines, combustion of fuel takes place outside the working cylinder.Examples: Steam Engines and Steam turbines
IC Engines are classified into,

(1)   Cycle of operation (No of Strokes per cycle)
         Two Stroke cycle Engines
         Four Stroke Cycle Engines
2) Thermodynamic Cycle or Method of Heat addition:
         Otto Cycle Engines (Combustion at constant volume)
         Diesel Cycle Engines (Combustion at constant Pressure)
         Semi Diesel Engines (Dual Combustion Engines)
 (3) Types of Fuel Used :
         Petrol Engines
         Diesel Engines
         Gas Engines
(4) Ignition Method :
         Spark Ignition (SI)
Compression Ignition (CI)
(5) Cooling System:
         Air cooled Engines
         Water Cooled Engines
(6) Valves Location :
         L head (Side valve) engine
         T Head (Side valve) engine
          I head (over head valve) engine
         F head (over head inlet and side exhaust) engine





Main Components of IC Engines:
Cylinder Block:
         It is the main block of the engine.
         It contains cylinders  accurately finished to accommodate pistons
         The cylinder block houses crank, camshaft, piston and other engine parts.
         In water cooled engines, the cylinder block is provided with water jackets for the circulating cooling water.
         The materials used for cylinder are grey cast iron, aluminium alloys etc., 
         It is usually made of a single casting
                      
Cylinder block of motor cycle                          Cylinder block of car

                                                                                                                                                        
Cylinder Head:
         The cylinder head is bolted to the cylinder
Block by means of studs.
         The water jackets are provided for cooling
water circulation.
         The materials used for cylinder head are cast iron, aluminium alloy etc.,
         This is also generally made of single cast iron.
Cylinder Liners:  
          The liner is a sleeve which is fitted into the cylinder bore.
         It provides wear resisting surface for the cylinder bores.
Liners are classified into:
         (a) Wet liner  (b) Dry liner




Cylinder Liners
Wet Liner : These liners are surrounded or wetted by cooling water. It provides wear resisting surface for the piston to reciprocate.  Also it acts as a seal for the water jacket

Dry Liner :Dry liners have metal to metal contact with the cylinder block. They are not directly in touch with the cooling water.

Liner Materials:
         Liner material should withstand abrasive wear and corrosive.Chromium plated mild teel
         tubes are used as liners.


Crankcase : It may be cast integral with the cylinder
block.Some times, it is cast separately and then attached to the block. These materials are used for crank case are cast iron, aluminium alloys or alloy steels.


Oil pan or oil sump: Oil sump is the bottom part of the engine.It contains lubricating oil.A drain plug is provided the oil sump to drain out the oil.It is made of the pressed sheet.

Piston :
The piston serves the following purposes
          It acts as a movable gas tight seal to keep the gases inside the cylinder
         It transmits the force of explosionin the cylinder to the crankshaft
 through the connecting rod.
         Some of the materials used for piston are cast iron, aluminium alloy,
chrome nickel alloy, nickel iron alloy and cast steel.














Piston rings :

Piston rings are inserted in the grooves provided in the piston. Two types of piston rings are used in the piston.
  1. Compression rings
  2. Oil rings or oil control rings





Main Components of IC Engines
Piston Rings (Compression and Oil rings)






Compression rings :

         Compression rings provide an effective seal for the high pressure gases inside the cylinder.
         They prevent the leakage of high pressure gases from the combustion chamber into the crank case.
         Each piston is provided with at least two compression rings.
          







Oil rings :

         Oil rings wipe off the excess oil from the cylinder walls.
         It also returns excess oil to the oil sump, through the slots provided in the rings.
The materials used for piston rings should be wear resistant.
Normally piston rings are made of alloy steel iron containing silicon, manganese alloy steels etc.
Connecting Rod:

         It connects the piston and crank shaft.
         It transmits the force of explosion during power stroke to the crankshaft.
         The connecting rod has bearings at both ends.
         The small end of the connecting has a solid or split eye and contains a bush.
         This end is connected to the piston by means of a gudgeon pin.
         The other end is called as big end of the connecting rod.
         The connecting rods must withstand heavy thrusts.
         Hence it must have strength and rigidity.
         They are usually drop forged I sections.
         The materials used are plain carbon steel, aluminium alloys, nickel alloy steels etc,

Crank Shaft :

         It is the main rotating shaft of the engine.
         Power is obtained from the crank shaft.
         The crank shaft is combination with connecting rod converts reciprocating motion of the piston into rotary motion.
         The crank shaft is held in position by the main bearings.
         There are two main bearings to support the crank shaft.
         The materials used for crank shaft are billet steel, carbon steel, nickel chrome and other heat treated alloy steels.

Camshaft:
         Camshaft contains number of cams.
         It is used to convert rotary motion into linear or straight line motion.
         It has so many cams as the number of valves in an engine.
         An additional cam is also provided to drive the fuel pump.
         A gear is provided in the cam shaft to drive the distributor or oil pump.
         The opening and closing of the engine valves are controlled by the cams provided on the cam shaft.












Petrol Engines

Classification of Petrol Engines

         Two Stroke cycle Petrol Engines

         Four Stroke cycle petrol Engines


Two Stroke cycle Petrol Engines:


Two Stroke Cycle Petrol Engine – Construction

Construction :
         A piston reciprocates inside the cylinder
         It is connected to the crankshaft by means of connecting rod and crank
         There are no valves in two stroke engines, instead of valves ports are cut on the cylinder walls.
         There are three ports, namely inlet, exhaust and transfer ports.
         The closing and opening of the ports are obtained by the movement of piston. The crown of piston is made in to a shape to perform this.
         A spark plug is also provided.

First Stroke : (Compression, ignition and inductance) (Upward stroke of piston)
(a)    compression:
         The piston moves up from Bottom Dead Centre (BDC) to Top Dead Centre (TDC)
         Both transfer and exhaust ports are covered  by the piston.
         Air fuel mixture which is transferred  already into the engine cylinder is compressed       by moving piston.
         The pressure and temperature increases at the end of compression.


First Stroke : (Compression, ignition and inductance) (Upward stroke of piston)
(b) Ignition and Inductance:
          Piston almost reaches the top dead centre
         The air fuel mixture inside the cylinder is ignited by means of an electric spark produced by a spark plug
         At the same time, the inlet port is uncovered by the plane.
         Fresh air fuel mixture enters the crankcase through the inlet port.

Second Stroke: (Downward Stroke of the engine) :
(c)Expansion and Crankcase compression
         The burning gases expand in the cylinder
         The burning gases force the piston to move down. Thus useful work is obtained.
         When the piston moves down, the air fuel mixture in the crankcase is partially compressed. This compression is known as Crank case compression.

Second Stroke: (Downward Stroke of the engine) :
(d) Exhaust and transfer:
         At the end of expansion, exhaust port is uncovered.
         Burnt gases escape to the atmosphere.
         Transfer port is also opened. The partially compressed air fuel mixture enters the cylinder through the transfer port.
         The crown of the piston is made of a deflected  shape. So the fresh charge entering  the cylinder is deflected upwards in the cylinder.
         Thus the escape of fresh charge along with the exhaust gases is reduced.

Two stroke cycle Diesel Engines- Construction

Construction :
          Two stroke cycle diesel engines require air supply
         This air is used to blow out the exhaust gases and to fill the cylinder with clean air
         This air is supplied by a blower or air compressor which is driven by engine itself.
         These engines may be valve or port type.
         A plate is provided in the crank case to admit air into the crank case.
         Transfer and exhaust ports are provided in the cylinder.
         These ports are covered and uncovered by the moving piston.

First Stroke (Upward Stroke of the piston)
(a)    Compression and inductance:
         The piston moves upwards from Bottom Dead Centre (BDC) to Top Dead Centre (TDC).
         Both transfer and exhaust ports are covered.
         Air which is transferred already into the engine cylinder is compressed by moving piston.
         The pressure and temperature of the air increases.
         At the same time, fresh air is admitted into the crankcase through the plate valve (reed valve)







First Stroke (Upward Stroke of the piston)
(b) Ignition and inductance.
         Piston almost reaches the top dead centre.
         The fuel is injected into the hot compressed air inside the cylinder. The fuel mixed with hot air and burns.
         The admission of fresh air into the crankcase continues till the piston reaches the top centre.

Second Stroke  (Downward Stroke of the piston)
(c) Expansion and crank case compression:
         The burning gases expand in the cylinder.
         Burning gases force the piston to move down. Thus useful work is obtained.
         At the same time, the air in the crank case is compressed by the movement of piston.
         All the ports and the plate valve are in closed position

Second Stroke  (Downward Stroke of the piston)
(d) Exhaust and Transfer:
          At the end of expansion, the exhaust port is uncovered.
         The burnt escape to the atmosphere through the exhaust port.
         Transfer port is also uncovered shortly after the exhaust port is opened.
         The partially compressed air from crank case enters the cylinder the transfer port.
         This air is deflected upwards by the deflected shape of the piston.
         Thus the entering air helps in forcing out the combustion products from the cylinder
         The plate valve remains during this period.


Four stroke cycle Petrol Engines

Construction :
         A piston reciprocates inside the cylinder
         The piston is connected to the crank shaftby means of a connecting rod and crank.
          The inlet and exhaust valves are Mounted on the cylinder head.
         A spark is provided on the cylinderHead.
          The fuel used is petrol
 





Four Stroke Petrol Engine- Working


(a)    Suction Stroke (First Stroke of the Engine)
         Piston moves down from TDC to BDC
         Inlet valve is opened and the exhaust valve is closed.
         Pressure inside the cylinder is reduced below the atmospheric pressure.
         The mixture of air fuel is sucked into the cylinder through the inlet valve.

      (b) Compression Stroke : (Second Stroke of the piston)
         Piston moves up from BDC to TDC
         Both inlet and exhaust valves are closed.
         The air fuel mixture in the cylinder is compressed.

      (c) Working or Power or Expansion Stroke: (Third Stroke of the Engine)

         The burning gases expand rapidly. They exert an impulse (thrust or force) on the piston.
             The piston is pushed from TDC to BDC
         This movement of the piston is converted into rotary motion of the crankshaft through connecting rod.
         Both inlet and exhaust valves are closed.

      (d) Exhaust Stroke (Fourth stroke of the piston)

         Piston moves upward from BDC
         Exhaust valve is opened and the inlet valve is closed.
         The burnt gases are forced out to the atmosphere through the exhaust valve (Some of the burnt gases stay in the clearance volume of the cylinder)
         The exhaust valve closes shortly after TDC
         The inlet valve opens slightly before TDC and the cylinder is ready to receive fresh charge to start a new cycle.






Summary :
         Compression ratio varies from 5 to 8
         The pressure at the end of compression is about 6 to 12 bar.
         The temperature at the end of the compression reaches 250o C to 350o C




Four Stroke Diesel Engine



Construction:

         A piston reciprocates inside the cylinder
         The piston is connected to the crankshaft by means of a connecting rod and crank.
          The inlet and exhaust valves are mounted on the cylinder head.
         A fuel injector is provided on the cylinder head
         The fuel used is diesel.

(a)    Suction Stroke (First Stroke of the piston)

         Piston moves  from TDC to BDC
         Inlet valve is opened and the exhaust valve is closed.
         The pressure inside the cylinder is reduced below the atmospheric pressure.
         Fresh air from the atmosphere is sucked into the engine cylinder through air cleaner and inlet valve.


      (b) Compression stroke (Second stroke of the piston)

         Piston moves from BDC to TDC
         Both inlet and exhaust valves are closed.
         The air is drawn during suction stroke is compressed to a high pressure and temperature




 (c) Working or power or expansion stroke (Third stroke of the piston)

         The burning gases (products of combustion) expand rapidly.
         The burning gases push the piston move downward from TDC to BDC
         This movement of piston is converted into rotary motion of the crank shaft through connecting rod.
         Both inlet and exhaust valves are closed.


(d) Exhaust Stroke  (Fourth stroke of the piston)

         Piston moves from BDC to TDC
         Exhaust valve is opened the inlet valve is closed.
         The burnt gases are forced out to the atmosphere through the exhaust valve. (some of the burnt gases stay in the clearance volume of the cylinder)
         The exhaust valve closes shortly after TDC
         The inlet valve opens slightly before TDC and the cylinder is ready to receive fresh air to start a new cycle.

Scavenging

Scavenging :

         It is the process of forcing out the burnt exhaust gases from the cylinder for admitting the fresh charge into the cylinder.
         This action takes place in the two stroke cylinder.

Scavenging Process

         The charge (air fuel mixture or air) enters the engine cylinder from the crank case at a pressure higher than the exhaust gases.
          This fresh charge forces the exhaust gases to the atmosphere through the exhaust port.
          During the period both the transfer and exhaust ports are kept open for a short period.
          Hence there is a possibility of the fresh charge escaping out with the burnt gases.
          This is over come by designing the piston to have a deflected shape.
          This shape of piston deflects the fresh charge upward in the engine cylinder.
         It also helps out in forcing out the exhaust gases to atmosphere.
         This process is known as Scavenging.













Comparison between SI and CI Engines
(General Comparison):

S.No.
Spark Ignition Engines (SI)
Compression Ignition Engines (CI)
1
It draws air fuel mixture into the cylinder during suction stroke
It draws only air into the cylinder during suction stroke.
2
Petrol engines operate with low pressure and temperature
Diesel engines operate with high pressure and temperature
3.
Pressure ranges from 6 to 12 bar
Temperature ranges from 250o to 300o C
Pressure ranges from 35 to 40 bar
Temperature ranges from 600o to 700o C
4
It is fitted with carburettor and spark plugs
It is fitted with fuel injection pump and injectors
5
The burning  of fuel takes place at constant volume
The burning of fuel takes place at constant pressure
6.
Ignition of air fuel mixture takes place by an electric spark produced by spark plug
Ignition of air fuel takes placed by a injection of fuel into the hot compressed air.
7
Petrol engines are quality governed engines. The speed of petrol engines are controlled by varying the quantity of air fuel mixture.
Diesel engines are quantity governed engines. The speed of diesel engines are controlled by varying quality of air fuel mixture. (rich or weak mixture)
8
Petrol engines are widely used in automobiles and aeroplanes etc.,
Diesel engines are widely used in heavy vehicles, such as buses, lorries, trucks etc.,















Comparison between SI and CI Engines
(Merits and Demerits):

S.No.
Spark Ignition Engines (SI)
Compression Ignition Engines (CI)
1
Merits: Otto cycle is employed in petrol engine. Otto cycle is more efficient for a given compression ratio.
Demerits: Diesel engines works on diesel cycle. Diesel cycle is less efficient than Otto cycle for a given compression ratio.
2
Operating speed is more. Speed range is 3000 to 6000 rpm
Operating speed is less. Speed range is 400 to 3500 rpm.
3.
Starting is easy, since cranking effort required is less
Starting is difficult since more cranking effort is required.
4
Merits: Initial cost and maintenance cost are less
Demerits: More initial and maintenance costs since the construction is heavy and sturdy.
5
Produces less noise.
Produces more noise.
6
Weight per unit power is less
Weight per unit power is more.
4
Demerits: Thermal efficiency is less, since compression ratio is limited. 5 – 8
Merits: Thermal efficiency is high since compression ratio is high. 12 to 18.
5
Specific fuel consumption is more.
Specific fuel consumption is less
6
The fuel used is petrol. It is costlier than diesel. It is volatile and fire hazard is more
The fuel used is diesel. It is cheaper than petrol. It is less volatile and fire hazard is less.















Comparison between Four stroke cycle and two stroke cycle engine
(Merits and Demerits):

S.No.
Two Stroke Cycle Engine
Four Stroke Cycle Engine
1
Merits: One power stroke in one revolution of the crankshaft
Demerits: One power stroke in two revolutions of the crank shaft
2
Power developed for the same engine speed theoretically twice that of a four stroke engine
Power developed for the same engine speed is theoretically half that of two stroke engine.
3
Simple design and lighter in construction for the same power
For the same power complicated design and heavier in construction
4
Merits:
Uniform torque is obtained. Hence a lighter fly wheel can be used
Demerits:
Non uniform torque on the crankshaft. Hence a heavier flywheel is required for balancing.
5
Design of ports is simpler. Hence initial cost is less
Design valve mechanism is difficult. Hence initial cost is more.
6
Mechanical efficiency is high. No moving parts like cam, follower, rocker arm valves etc.,
Mechanical efficiency is less. Power is lost due to friction caused by valve mechanism
7
Merits: Starting is easy
Demerits: Starting is not so easy
8
These engines are generally air cooled
These engines are generally water cooled.
















Comparison between Four stroke cycle and two stroke cycle engine
(Merits and Demerits):

S.No.
Two Stroke Cycle Engine
Four Stroke Cycle Engine
1
DeMerits:
Consumption of lubricating oil is more, because less time is available to remove the heat
Merits:
Consumption of lubricating oil is less, because more time is allowed for removing heat from the cylinder.
2
More wear and tear of moving parts.
Less wear and tear of parts is less
3
Some of the fresh air fuel mixture may escape with exhaust gases. Hence fuel consumption is more
Fuel cannot escape with exhaust gases. Hence fuel consumption is less.
4
DeMerits:
Thermal efficiency is less.
Merits:
Thermal efficiency is more.
5
It produces more noise due to sudden release of exhaust gases
Noise is less is less. Exhaust gases are released in separate stroke.
6
Scavenging is poor, since exhaust port is open only for a short time
Scavenging is better, since there is a separate exhaust stroke for the removal of exhaust gases
7
Merits:
Poor scavenging leads to mixing of fresh charge with exhaust gases. This results in poor performance, slow running
Demerits:
Better performance and efficiency is more
8
Used in light vehicles, like bikes, scooters, mopeds, etc.,
Used in heavy vehicles, like buses, lorries, trucks etc.,












I.C ENGINE TERMINOLGOGY

The standard terms used in I.C Engines are

1. Bore: Inside diameter of the cylinder is termed as Bore.

2. Top Dead Center (TDC): The extreme position reached by the piston at the top of the
cylinder in the vertical engine is called Top Dead center.

3. Bottom Dead Center (BDC): The extreme position reached by the piston at the Bottom of the cylinder in the vertical engine is called Bottom Dead center.

4. Stroke: The nominal distance travelled by the piston in the cylinder between the extreme upper and lower positions of the piston (TDC &BDC) is termed as stroke.

5. Compression ratio (r): It is the ratio of Maximum cylinder volume to the Clearance volume.

6. Cylinder volume (v): It is the sum of swept volume and the Clearance volume.
              V = Vs + Vc
7. Swept volume (Vs): It is the volume of space generated by the movement of piston from one dead center to another dead center.

8. Clearance Volume( Vc): It is the space in the cylinder, when the piston is at Top Dead Center



Major parts of an IC engine

1. Cylinder
         It is a round cylindrical casting in which a piston slides in and out to make strokes.
         Combustion take place inside the cylinder. The cylinder is closed by a cylinder head.
Material: Grey cast iron, Aluminium

2. Cylinder head
       It is fitted to the top of the cylinder. It has inlet and outlet values, spark plug, Fuel injector, Water jackets.
       Material: C.I, Aluminium
   3. Piston
      It is a device which transmits the energy (or) force of the expanding gas to the connecting rod. It slides up and down inside the cylinder.
       Material: C.I, Aluminium alloy,  Cast steel
4. Piston rings:  Piston rings are inserted in the grooves of piston. There are two types of rings.
         1) Oil ring ( One ring is used)
         2) Compression ring( Two ring is used)
5. Connecting rod: It converts the reciprocating motion of the piston into rotary motion of crankshaft. The small end of the connecting rod is connected to piston and the big end is connected to the crankshaft.
Material: Plain carbon steel, Aluminium alloys 
6. Crank shaft: It is the device used for getting power from the motion of the piston and connecting rod and this power is applied to the flywheel.
   Material: Alloys steel.
7. Camshaft: It operates the opening and closing of the engine values. It has number of cams which are driven by crank shaft through timing gears. The function of the cam is to convert the rotary motion into the linear reciprocating motion
    Material: Alloys steel
8. Crank case: It is the bottom portion of the I.C engine and holds the cylinder and the crank case. It also serves as a pump for the lubricating oil.
      Material: Aluminium alloy,  Cast iron
 9. Flywheel: It is a big wheel attached with crankshaft. It maintains the speed of the engine.
10. Valves: The function of the value is to admit the fresh charge in the cylinder and to send the exhaust gases out. There are two values namely inlet value and outlet value.
Material: Inlet value: Nickel chrome.
                 Outlet value: Nickel chrome, Stainless steel etc
11. Water Jackets: Water jackets are provided in the cylinder head. The purpose of water jackets is to keep the walls of the engine cool.

Steam Boilers

         Generates steam by transferring heat by burning of fuel to water.
         Energy released by burning fuel (solid, liquid or gaseous) is transferred to the water in the boiler.

Classification of boilers:
The steam boilers are classified as
         According to flow of water and hot gases.
  1. Fire Tube Boilers
  2. Water Tube Boilers
         According to the method of firing.
  1.  Internally fired boilers
  2. Externally fired boilers
         According to the Pressure developed
  1. Low pressure boilers
  2. High pressure boilers

In fire tube boilers,
          The hot gases pass through the tubes surrounded by water.
         The water is get heated up and converted into steam
         The exhaust gases are sent to atmosphere through chimney.
E.g Locomotive boiler, Lancashire boiler.












Fire Tube & Water Tube Boilers


According to flow of water and hot gases:

In water tube boilers,
          Water is circulated through number of tubes and the hot flue gases flow over these tubes.
          A number of tubes are connected with boiler drum through headers.
          The hot gases flow over these tubes many times before escaping through the stack.
          The water is converted into steam and steam occupies steam space.
E.g. Babcock & Wilcox, stirling, BHEL boiler, Velox, Lamont, Lo-effler boilers.

According to the method of firing:

In internally fired boilers,
          The furnace grate is provided inside the boiler shell.(E.g Lancashire, Locomotive boilers) In externally fired boilers,
          The furnace grate is provided outside or built under the boiler shell.













Internally Fired Boiler:



Externally Fired Boiler:


According to the Pressure Developed:


In Low Pressure Boilers,Steam is produced at a pressure lower than 80 bar.
(E.g. Cochran, Lancashire, Locomotive)
In High Pressure Boilers,Steam is produced at a pressure more than 80 bar.
(E.g. Lamont, Velox, Benson, Lo-effler boiler)













Cochran Boiler:


         Coal is fed into the grate through the fire hole and burnt.
          Ash formed during the burning is collected in the ash pit provided just below the grate.
          Ash is then removed manually.
          The hot gases from the grate pass through the combustion chamber to the horizontal fire tubes and transfer the heat by convection.
          The flue gases coming out of fire tubes pass through the smoke box and escape to the atmosphere through the chimney.
          Smoke box is provided with a door for cleaning the fire tubes and smoke box.
          The working pressure and steam capacity of cochran boiler are 6.5 bar and 3500 kg /hr respectively.
















Lamont Boiler:


           It is a water tube, forced circulation and externally fired high pressure boiler.
          The capacity of the plant is 50 tonnes/hr
          Pressure of the steam generated is 170 bar.
          Temperature of the steam produced is 500o C

Working:

          Feed water is pumped to the boiler by the feed pump through the economiser.
          Economiser preheats the feed water by using hot gases leaving the boiler.
          The circulating pump circulates the water from the drum under high pressure to prevent the tubes from being overheated
         Water is evaporated into steam when passing through these tubes.
          The water and steam from the tube enters the boiler drum where the steam is separator.
          This steam is passed through a convection superheater and the steam is superheated by the flue gases.
          This super heated steam is supplied to the prime mover through steam outlet.
         The water level in the drum is kept constant by pumping the feed water into the boiler drum.
          The air is preheated by the flue gases before entering the combustion chamber to aid the combustion of the fuel.
          This type of boiler has a working pressure of 170 bar.
          They can produce the steam at the rate of 45000 kg per hour.







Boiler Mountings & Accessories:

Boiler Mountings:
          Boiler mountings are primarily intended for the safety of the boiler and for complete control of steam generation process.
Boiler  Accessories :
          Boiler accessories are installed to increase the efficiency of the boiler plants to help in proper working of boiler unit.

Boiler Mountings:

Boiler Mountings:
          Dead weight safety valve.
          Spring loaded safety valve
          Fusible plug
          Pressure gauge
Dead Weight Safety Valve


Dead Weight Safety Valve:
         Weights are placed sufficiently in the weight carrier.
          The total load on the valve includes the weight of the carrier, the weight of the cover, the weight of the discs and the weight of the valve itself.
          When the steam pressure exceeds the normal limit, the valve along with the weight carrier is lifted off its seat.
          Thus the steam escapes through the discharge pipe.







Spring Loaded Safety Valve:


         The steam pressure acts below the valves.
          When the steam pressure is normal the valves are held in their seats tightly by the spring force.
          When the steam pressure in the boiler exceeds the working pressure, both valves are lifted off their seats.
          Thus the steam from the boiler escapes the boiler and steam pressure is reduced.
          The blow off pressure is adjusted by loosening or screwing the nut.

Fusible Plug:





         Under normal working conditions, the fusible plug is completely covered with water.
          Hence the temperature of the plug is not increased appreciably during combustion process.
          When the water level falls below the safe limit the fusible plug is uncovered from water and exposed to steam.
          The furnace heat over heats the plug and it melts the fusible metal and copper plug falls down.
          Due to this water steam mixture rushes into the furnace and the fire is extinguished.

Bourdan Tube Pressure Gauge


         The steam pressure is applied to the Bourdon’s tube.
          The elliptical cross section of the tube to straighten out slightly.
          The closed end of the Bourdon tube moves.
          This movement actuates the toothed sector and pinion rotates.
          The pointer is mounted on the pinion. Hence the pointer moves on the graduated dial in clockwise, to indicate the steam pressure.
















Water Level Indicator


         To know the water level in the boiler the handles of the steam cock and water cock are kept in vertical positions.
          Water rushes through the bottom casting and steam rushes through the upper casting to the gauge glass tube.
          The level of water corresponds to the water level in the boiler.


Boiler Accessories
         Economiser
          Air Preheater
          Super Heater
          Steam Separator
          Steam Trap Feed Pump

Economiser:


         The feed water is pumped to the bottom header and this water is carried to the top header number of vertical tubes.
          Hot flue gases are allowed to pass over the external surface of the tubes.
          The feed water which flows upward in the tubes is heated by the flue gases.
          This preheated water is supplied to the water.
          Scrappers are moved slowly moved up and down to clean the surface of the tubes.


Air Preheater:


         Hot flue gases pass through the tubes of air preheater after leaving the boiler or economiser.
          Air and flue gases flow in opposite directions.
          Baffles are provided in the air preheater and the air passes number of times over the tubes.
         Heat is absorbed by the air from the flue gases.
          This preheated is supplied to the furnace to aid combustion.
          
Super heater


         Steam stop valve is opened.
          The steam from the evaporator drum is passed through the super heater tubes.
          First the steam passed through the radiant super heater and then to the convective super heater.
          The steam is heated when it passes through these super heaters and converted into the super heated steam.
          This superheated steam is supplied to the turbine through the valve.

Steam Separator


         The steam is allowed into the separator.
          The steam strikes the baffle plates and the direction of flow is changed.
          As a result, heavier particles in steam falls down to the bottom of the separator.
          The separated steam is free from water particles.
          It is passed to the turbine or engine through the outlet pipe.
Stream Trap

         The condensed water enters the steam trap by gravity.
          When the water level in the trap rises high enough, the ball float is lifted.
          This causes the valve to open and the water is discharged through the outlet.
          After the discharge of water, the float moves down.
          This causes the valve to close again.


Differences between Boiler mountings and Accessories:

Sl. No.
Boiler Mountings
Boiler Accessories
1
Mountings are fitted for the safety of the boiler.
Accessories are fitted to increase the efficiency
2
They form integral parts of the boiler
They are not integral part of the boiler.
3
They are usually mounted on the boiler shell.
They are usually installed outside the boiler shell.
4
A boiler should not be operated without mountings
A boiler can be operated without accessories.

References:
         Ganesan.V, “Internal Combustion Engines”,
         Ballaney.P.L, “ Thermal Engineering”, Dhanpatrai & sons.
          R.S. Khurmi, J.K.Gupta, “ Thermal Engineering”, S.Chand & Co.,
         P.K. Nag, “Basic and Applied  Thermodynamics”, Tata McGraw Hill Publishing Co.,


Prepared by

A.R. PRADEEP KUMAR, M.E., (Ph.D).,
Assistant Professor/Mechanical – S& H

Email : dearpradeepkumar@gmail.com
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