Thermal Engineering laboratory Manual


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Thermal Engineering laboratory Manual

  1. 1. EXPERIMENT NO: Date: STUDY OF TWO STROKE I.C.ENGINES OBJECTIVES: After studying this practical, students are able to know (1) Principles construction & working of 2-S Petrol & Diesel engine. INTRODUCTION: In this engine, the working cycle is completed in two strokes of the piston or one revolution of the crank shaft. In case of two strokes engine, the valves are replaced by ports. Two rows of ports at different levels are cut in the cylinder walls as shown in fig. These are known as exhaust ports and transfer ports. In the case of single cylinder engines, a third row of ports is provided below the first two and these are known as inlet ports. A specific shape is given to the piston crown as shown in fig Which helps to prevent loss of incoming fresh charge entering into the engine cylinder through the transfer port and helps in exhausting only burnt gases. • The charging of cylinder with air fuel mixture in case of petrol engine or with air in case of diesel engine, compression of the mixture or air, expansion of gases and exhausting of the burnt gases from the cylinder are carried out in two strokes. This can be done by using the following two methods. 1. By using closed crank case compression. In this method crank case works as an air pump as the piston moves up and down. The charge or air to be admitted in the cylinder is compressed in crank case, by the pumping action of underside of piston. This method is known as three channel system & used for single cylinder small power engines like scooters & motorcycles. 2. A separate pump outside the cylinder is provided to compress the charge or air before forcing it into the cylinder. This pump is an integral part of an engine & driven by engine it self. This method of charging is used for large capacity multi-cylinder engines. WORKING OF TWO STROKE PETROL ENGINE: It will be easier to describe the cycle beginning at the point when the piston has reached to TDC completing the compression stroke. The position of the piston at the end of compression as shown in fig.( ). The spark is produced by spark plug as the piston reaches the TDC (Top Dead Centre). The pressure and temperature of the gases increases and the gases push the piston downwards producing power stroke, when the piston downwards producing power stroke. When the piston uncovers (opens) the exhaust port as shown in fig ( ) during downward stroke, the expanded burnt gases leave the cylinder through the exhaust port. A little later, the piston uncovers (opens) the transfer port also as shown in (a). In this condition the crank case is directly connected to cylinder through port. During the downward stroke of piston, the charge in crank case is compressed by the underside of the piston to a pressure of 1-4bar. At this position, as shown in fig.( ), the compressed charge (fuel & air) is transferred through the transfer port to the upper port of the cylinder. The exhaust gases are swept out with the help of fresh
  2. 2. charge (scavenging). The piston crown shape helps in this sweeping action as well as it prevents the loss of fresh charge carried with the exhaust gases. This is continued until the piston reaches BDC position. During this stroke of piston (downward stroke) the following processes are completed. 1. Power is developed by the downward movement of piston. 2. The exhaust gases are removed completely from the cylinder by scavenging. 3. The charge is compressed in the crank case with the help of underside of the piston. As the piston moves upward, it covers transfer ports stopping flow of fresh charge into the cylinder. A little later, the piston covers exhaust ports and actual compression of charge begins. This position of piston is shown in fig. The upward motion of the piston during this stroke lowers the pressure in the crank case below atmosphere, therefore, a fresh charge is admitted/induced in the crank case through the inlet port as they are uncovered by the piston. The compression of charge is continued until the piston reaches its original position (TDC) and the cycle is completed. In this stroke of the piston, the following processes are completed. 1. Partly scavenging takes place as the piston moves from BDC to position shown in fig. 2. The fresh charge is sucked in the crank case through the Carburettor. 3. Compression of charge is completed as the piston moves from the position shown in fig.( ) to TDC as shown in fig.( ). The cycle of engine is completed within two strokes of the piston. WORKING OF TWO STROKE DIESEL ENGINE: As the piston moves down on the power stroke, it first uncovers the exhaust port, and the cylinder pressure drops to atmospheric pressure as the products of combustion come out from the cylinder. Further downward movement of the piston uncovers the transfer port (TP) and slightly compressed air enters the engine cylinder from the crank case. Due to deflector on the top of the piston, the air will move up to the top of the cylinder and expels out the remaining exhaust gases through the exhaust port (EP). During the upward movement of the piston, first the transfer port and then the exhaust port closes. As soon as the exhaust port closes the compression of the air starts. As the piston moves up, the pressure in the crank case decreases so that the fresh air is drawn into the crank case through the open inlet port as shown in fig. Just before the end of compression stroke the fuel is forced under pressure in the form of fine spray into the engine cylinder through the nozzle into this hot air. At this moment the temperature of the compressed air is high enough to ignite the fuel. It suddenly increases the pressure and temperature of the products of combustion. The rate of fuel injection is such as to maintain the gas pressure constant during the combustion period. Due to increased pressure the piston is pushed down with a great force. Then the hot products of combustion expand. During expansion some of the heat energy produced is transformed into mechanical work. When the piston is near the bottom of the stroke it uncovers exhaust port which permits the gases to flow out of the cylinder. This completes the cycle and the engine cylinder is ready to suck the air once again.
  3. 3. Assignment: Answer the following questions: (1) What is crankcase compression? Explain? (2) Write function of deflector? (3) When piston moves from TDC to BDC which strokes are performed? When piston moves from BDC to TDC which strokes are performed? (4) Explain working of any two stroke with diagram? (5) Compare 4-S cycle engine & 2-S cycle engine? (6) Why 4-S cycle engine is having more efficiency than two stroke cycle engine? (7) Why 2-S cycle engine develops approximately double power than 4-S cycle engine? (8) Why 4-S cycle engine requires more maintenance?
  4. 4. EXPERIMENT NO: Date: STUDY OF FOUR STROKE I.C.ENGINES Objectives: After Studying this practical students are able to know (1) Principles construction & working of 4-S Petrol & Diesel engine. INTRODUCTION: Any I.C.Engine works on particular cycle and in any I.C.Engines a cycle comprises of four basic operations viz. Suction, compression, expansion and exhaust. If all these four basic operations are performed / completed in four strokes of piston or two revolutions of crank shaft, the engine is called four stroke engines. It may be petrol engine or diesel engine, depending upon type of fuel used. In this there will be one power stroke in two revolution of crank shaft / four strokes of piston. • Valves are used to induct the charges into the cylinder and to push out the exhaust gases from the cylinder. • Valves are actuated by cam & cam shaft mechanism. To open valve, the moment is transferred in this sequence: cam shaft-cam-follower-push rod-rocker arm. • Overall efficiency of 4-Stroke engines is more than 2-Stroke engines. • The speed of cam shaft is half than crank shaft speed. (e.g. if engine is running at 1000 RPM, camshaft speed will be 500 RPM.) Now we will study the basic four operations executed in cylinder to complete the cycle.
  5. 5. 4-STROKE PETROL ENGINE The working cycle of the engine is completed in four strokes or in two revolutions of crank and petrol is used as fuel. Suction stroke: The piston is at the top most position (TDC) and is ready to move down wards the mixture of fuel (petrol) and air. The inlet valve is open and exhaust valve is closed. As the piston moves downwards, a fresh charge of fuel and air mixture enters the cylinder through inlet valve due to suction created. This continues until piston reaches BDC. At this position inlet valve closes. This downward movement of piston is known as suction stroke & crank rotates by 80 during this period. Compression stroke: During this stroke, both valves (inlet and exhaust) are closed and piston moves upward and compresses the charge enclosed in the cylinder. The pressure and temperature of mixture increases continuously, during this process. As the piston reaches, the top dead centre (TDC). Position, the mixture is ignited by an electric spark. The burning of mixture is more or less instantaneous and pressure & temperature of gases increases while the volume remains constant. Power stroke or Expansion stroke: During the expansion stroke, both the valves remain closed. The high pressure and temperature gases push the piston downwards and the gas pressure gradually decreases. During this stroke, piston moves from TDC to BDC. This stroke is known as power stroke, as work is done during this stroke. The exhaust valve opens as the piston reaches BDC position and pressure falls suddenly to atmospheric pressure at constant volume. Exhaust stroke: During upward motion of the piston the exhaust valve is open and inlet valve is closed. The piston moves up in cylinder pushing out the burnt gases through the exhaust valve. As the piston reaches the TDC, again inlet valve opens and fresh charge is
  6. 6. taken in during next downward movement of the piston and cycle is repeated. Working of 4-Stroke Diesel Engine All working operations such as suction, compression, expansion and exhaust are completed in 4-strokes of piston or two revolutions of crankshaft and diesel is used as fuel. Therefore it is known as 4-stroke diesel engine. The liquid fuel like diesel, which cannot be vaporized, is injected into the engine cylinder in the form of fine spray with the help of fuel pump and injector. The working of the 4-stroke diesel engine is described as given below. (1) Suction Stroke : The inlet valve remains open during this operation. The air is taken in through the inlet valve as the piston moves from Top Dead Center (TDC) to Bottom Dead Center (BDC). (2) Compression Stroke : During this stroke, the inlet and exhaust valves remain closed. The piston moves from BDC to TDC and compresses the air, taken in during suction stroke, with an increase in pressure and temperature and decrease in volume. (3) Expansion Stroke : Just before completing the compression stroke, the fuel injector opens and injects the diesel fuel in the form of fine spray inside the cylinder. The injected fuel starts burning due to the high temperature inside the cylinder, as compression ratio of diesel engine is high. The supply of fuel is cut-off after some part of the expansion stroke, and the hot gases expand pushing the piston towards BDC. During this stroke, the piston moves from TDC to BDC. At the end of the stroke exhaust valve opens and the pressure inside the cylinder falls to atmospheric pressure. (4) Exhaust Stroke : The hot gases in the cylinder are driven out through exhaust valve from
  7. 7. the cylinder as the piston move from BDC to TDC. The exhaust valve is closed at the end of the stroke. Again the inlet valve opens and the same operations are repeated. Assignment: Answer the following questions. (1) Why four stroke engines is called four stroke? (2) Write position of valve during each stroke of 4-S cycle engine? (3) Write ratio of rotation of cam shaft to crank shaft? Why? (4) Explain any two stroke of 4-S IC Engine?
  8. 8. Experiment No: Date: STUDY OF AIR COMPRESSOR Objectives: After studying this practical, students will be able to know (1) Use of air compressor (2) Classifications of compressor (3) Basic terminology of compressor. (4) Principle construction & working of following air compressor. • Reciprocating compressor • Rotary compressor  Centrifugal Compressor  Axial Flow compressor  Roots Blower  Vane type blower A compressor is a device, which receives gas or vapour, increases its pressure and delivers it at a high pressure. Use of compressed air: Compressed air is used as motive power in the operation of drills, hammers, hoist sand blasters, controls, air brakes, sprays, chucks, lift gates etc. Air compression is a major factor in performance of I.C.engines and gas turbines. Compression of vapour is met with in many refrigeration plants. Classification of compressors: The compressors are broadly classified as positive displacement compressors or rotary compressors. Positive displacement compressors includes, reciprocating compressors and compression blowers while later includes radial flow or centrifugal compressors. Axial flow compressors and Mixed flow compressors. Any one of these compressors may be compounded in series to increase pressure range or may be compounded in parallel to increase the capacity. Air compressor terminology: (1) Single acting compressors are those compressors in which suction compression and delivery of air take place on one side of the piston. (2) Double acting compressors are those compressors in which suction, compression and delivery take place on both side of the piston. (3) Single stage compressors are those compressors in which the compression of air from the initial pressure to final pressure is carried out in one cylinder only. (4) Multi-stage compressors are those in which the compression of air from initial pressure to the final pressure is carried out in more than one cylinder. (5) Ratio of Compression is defined as the ratio of absolute discharge pressure to absolute intake pressure.
  9. 9. (6) Volumetric efficiency is the ratio of actual capacity of compression to its displacement. The reciprocating air compressors: It consists of a piston that operates in a cylinder driven through a connecting rod and crank mounted on crank mounted on crankcase. There are inlet and delivery valves mounted on the head of cylinder. These valves are usually of the pressure differential type, meaning that they will operate as a result of the difference of pressure across the valve. The operation of this type of compressor is as follows. When the piston is moving down the cylinder, any residual compressed air left in the cylinder after the previous compression will expand and will eventually reach a pressure slightly below intake pressure early on in the stroke. This means that the pressure outside the inlet valve is now higher than on the inside and hence the inlet valve will lift off its seat. Thus a fresh charge of air will be aspirated into the cylinder for the remainder of the induction stroke as it is called. During this stroke the delivery valve will remain closed, since the compressed air on the outside of this valve is at a much higher pressure. The piston is now moving upwards. At the beginning of this upward stroke, slight increase in cylinder pressure will have closed the inlet valve since both the inlet and outlet valves are now closed the pressure of the air will rapidly rise because it is now locked up in the cylinder. Eventually a pressure will be reached which is slightly in excess of the compressed air on the outside of the delivery valve and hence the delivery valve will lift. The compressed air is now delivered from the cylinder for the stroke. At the end of compression stroke the piston once again begins to move down the cylinder, the delivery valve closes, the inlet valve eventually opens and the cycle is repeated. ROTARY AIR COMPRESSORS: There are three basic types of rotary air compressors, namely, the radial or centrifugal compressors, the axial flow compressors, and blower. NON POSITIVE DISPLACEMENT COMPRESSOR CENTRIFUGAL COMPRESSOR:- Radial compressor consists of impeller rotating usually at high speed (something like, 20,000 to 30,000 rpm in some cases) in a casing. The impeller consists of disc onto which radial blades are attached. The blades break up the air into cell. If the impeller is rotated the cells of air will also be rotated with the impeller. Centrifugal force will then come in to action and thus the air in the cells will move out from the outside edge of the impeller and more will move into centre of the impeller to take its place. The centre is called the eye of impeller. The air as it moves away from the outside edge of impeller passes into a diffuser ring, which helps to direct the air into the volute. Also, in the diffuser ring the air is decelerated, the deceleration producing a pressure rise in air.
  10. 10. The volute is a collecting device for this compressor and it will be noticed that its cross-section increases round the compressor. The reason for this is that as the air is collected round the volute, so a greater section will be required to pass the increasing quantity of air. This type of compressor is a continuous flow device and will deal with large quantities of air through a moderate pressure range. Pressure compression ratios of 4:1 to 6:1 are common. Axial Flow Compressor:- In axial type of compressor there are alternate rows of fixed and moving blades. The fixed blades are fixed in an outer casing while the moving blade are fixed to a central drum which can be rotated by means of a drive shaft. The moving blade can be locked at in a simple way as a set of fans in series. These blades progress the air through the compressor, the fan before, boosting the fan after, as it were. The fixed blades act both as guide vanes and diffusers. The angles of all blade rows are set such that there is a smooth progression of air from blade row to blade row. The air passes axially along the compressor, hence its name. Air is removed by suitable duct at the end of the compressor. Once again, this type of compressor is high speed and in general, deals with large quantities of air. Pressure compression ratio of 10:1 or more can be obtained. This compressor design is commonly used in aircraft gas turbines. (b) Working: The compression is performed in a similar manner to that of the centrifugal type. The work input to the rotor shaft to the air is by the moving blades. This will accelerate the air. The velocity of the air relative to the blades is decreased as the air passes through them because of the space provided between blades. This reduction in velocity increases the arranged to form diffuser passages. The fixed stator blades guide the air by changing its angle in order to enter the second row of moving rotor blades. The numbers of stages are usually large. These compressors are capable of developing pressure ratios of 1.2 to 1.3 per stage. These compressors can develop pressure up to 8bar and deliver 1 to 5 m3 /s of air. Thus, in order to achieve above pressure ratio of 8 about 6 stages are required. POSITIVE DISPLACEMENT COMPRESSOR: ROOTS BLOWERS:
  11. 11. (a) Construction: The two-lobe type roots blower is shown in fig. For higher- pressure ratios, three and four lobe versions are in use. One of the rotors is connected to the drive. The second rotor is gear driven from the first. Thus both the rotors rotate in phase. In order to seal delivery side from the inlet side, the profile of the lobs is of cycloidal or involute. This scaling continues until delivery commences. To reduce wear, there must be some clearance between the lobes and between the casing and the lobes. Although this clearance will form a leakage path for compressed air and will have an adverse effect on efficiency when pressure ratio increases. In order to achieve the acceptable efficiency of the blower, a very small clearance of about 0.2 to 0.5 mm is provided. The pressure at the delivery is not constant. Single acting blower can develop the pressure up to twice the inlet pressure. (b) Principle of Operation: The operation can be considered as talking place in two distinct phases suction and discharge as described below Suction phase: The rotation of the rotors produces a space, which increases the volume as rotation continues. The gas therefore flows into the machine to fill the space
  12. 12. The flow of gas into the blower continues involving both the rotors. A quantity of gas is trapped between one rotor and the casing for a very brief interval. This part of the blower is not open to the suction. No gas flows into it. The gas continues to flow into the space produced by the rotation of the other rotor. This rotor is carrying out the same cycle as the first but 90o behind it. Discharge Phase: The trapped volume is at suction pressure as it has simply been drawn in by the movement of the rotors. There is no internal compression of gas as there is no meshing of rotors prior to the release to discharge. Continued rotation of rotors opens the trapped space to the discharge part. As the pressure in the discharge port is higher than suction pressure, the movement that the trapped volume opens to the discharge port, a back flow of gas from discharge takes place. This will increase its pressure up to discharge level. The continued rotation then
  13. 13. pushes out the gas in the chamber (Receiver). Summery of Principles of operation: 1. A root blower has only two moving parts, the two rotors, which are normally identical in shape and size. 2. Operation is entirely rotary 3. As the rotors are symmetric about their centre of rotation, the operation is dynamically in balance. 4. Suction takes place at one side of the rotors and discharge at the other. 5. Discharge of the compressed gas is complete and there is no clearance volume. 6. The operation is positive displacement as the gas is drawn in, trapped and discharged by the movement of the components. VANE TYPE COMPRESSOR OR BLOWER: (a) Construction: Fig shows this type of compressor. It consists of a rotor mounted eccentrically in the body. This rotor is supported by ball and roller bearings in the end cover of the body. The rotor is slotted to take the blades. These blades are made from non-metallic material usually fiber. The casing of the compressor is circular in which the drum rotates during rotation. The vanes remain in contact with the casing. The slots in which the blades are fixed are cut radially into the rotor. The blades or vanes can slide in and out due to the centrifugal forces setup by the rotary motion of the rotor. The circular casing is provided with one inlet and one outlet port. The space between the rotor and its casing is subdivided into a number of compartments by these vanes. Two consecutive vanes form one compartment. Due to eccentric motion of the rotor the volume of each compartment keeps on changing. It can develop the pressure up to 9bar and delivered 15 m3 /min of air. (b) Principle of Operation: The operation can be considered to take place in three phases. Suction internal compression and discharge. (1) Suction phase: The relative position of the rotor and the casing is clearly shown in fig. The start of the suction phase is indicated for one space between
  14. 14. two of the vanes. The rotation of the rotor causes space to be formatted between the vanes, the rotor and casing. This space is connected to the suction port so that gas from the suction passes into the space to fill it. The continued rotation of the rotor increases the space open to the suction. Hence more gas flows in, to fill it. This process continues until the space into which the gas is flowing stops increasing. This will depends on the number of vanes in the compressor. After this point the enclosed volume starts to reduce if it is still connected to the compressor suction gas will be dispelled. The suction port is positioned to be cut off as soon as the vanes move beyond this point. Internal Compression Phase: The reduction in volume which occurs due to continued rotation thus causes internal compression to take place. This will continue until the leading vane moves over the discharge port. This will allow the trapped gas to be released. The positioning of the discharge port determines the amount of internal compression. Discharge Phase:
  15. 15. It is the final phase of operation. When the leading vane passes the discharge port, the gas is open to discharge and is expelled. Assignment: Write answers for following questions: 1. Write use of air compressor. 2. Classify air compressor. 3. Differentiate single stage compressor & multistage compressor. 4. Valves of reciprocating compressor works on which principle? How? 5. Why cross section of volute increases round the compressor? 6. Why axial flow compressor is called axial flow? 7. Due to which process pressure rise takes place in roots blower? Briefly explain. 8. What you understand by positive displacement type compressor? 9. Why rotor of vane type blower is eccentrically mounted? 10.Due to which two processes pressure rise takes place in vane type blower? Briefly explain?
  16. 16. EXPERIMENT NO: Date: STUDY OF SIMPLE CARBURETTOR OF PETROL ENGINE OBJECTIVES: After studying this practical, students are able to know…… (1) Carburetion process (2) Function of Carburettor. (3) Construction & working of simple Carburettor. (4) Limitations of simple carburetor. THEORY: In petrol engines, the air and fuel is mixed outside the engine and partly evaporated mixture is supplied to the engine. The fuels such as petrol, benzol and alcohol used in S.I. engine vaporizes easily if injected in the flow of air, therefore, the engine suction is sufficient to create the air flow and fuel injected easily evaporates. The oil fuels which are used in C.I.engines do not vaporize easily. Therefore, a separate injection system is used. The process of preparing air-fuel mixture in S.I.engine outside the engine cylinder is known as Carburetion. The device used for this purpose is known as Carburettor. During the suction, the air is sucked as vacuum is created inside the engine cylinder. The fuel is injected in the air from the Carburettor and a mixture is supplied to the engine cylinder. It is desirable to have a complete vaporized mixture in the engine cylinder but some of the larger droplets may reach the cylinder in the form of liquid and they are mixed and vaporized during the compression stroke. The time available for atomization, mixing and vaporisation is so small, (0.02 second when engine is running at 3000 RPM) the design of the system becomes more difficult. Temperature is one of the factors which accelerates vaporization but this would reduce the power output due to reduction is mass flow. Carburettor: Carburettor is a device, which is used for atomizing and vaporizing the fuel (petrol) and mixing it with the air in varying proportions, to suit the changing operating conditions of the engine. Atomization is the breaking up the liquid fuel (petrol) into very small particles so that it is properly mixed with the air. But vaporization is the change of state of the fuel from liquid to vapor. Carburettor performs both the process i.e., atomization of the fuel and vaporization of the fuel. SIMPLE CARBURETTOR: Fig. (A)shows a simple Carburettor, which consists of, (a) float and float chamber, (b) venturi and throttle valves and (c) choke valve. Float and float chamber: The petrol is supplied to the float chamber from the fuel tank through the filter and fuel pump. The arrangement in float chamber is such that when the petrol reaches a particular level, the needle valve blocks the inlet passage and thus cuts off the petrol supply. On the fall of the petrol level in the float chamber, the float
  17. 17. descends down and inlet passage opens. The petrol is supplied to the chamber again. Thus a constant fuel (petrol) level is maintained in the float chamber. The float chamber is vented to atmosphere. The level of fuel float chamber is kept slightly below the top of the jet to prevent the leakage when not operating. The difference of level between the top of the jet and the source of the fuel in the float chamber is usually kept as 1.5 mm. Venturi and throttle valve: The main body of the Carburettor consists of a narrower passage at its centre. This narrower passage is known as venturi. One end of the Carburettor is mounted on the intake manifold of the engine. During the suction stroke of the piston, vacuum is created in the cylinder of the engine. Due to vacuum, the air is drawn through the Carburettor. The velocity of the air increases as it passes through the venturi where the area of cross section is minimum. Due to increased velocity of air at the venturi, the pressure at the venturi decreases. Therefore a low-pressure zone is created in the ventruri. The outlet of the discharged jet which is located at the venturi is in the zone of low pressure. The fuel issues out from the nozzle in the form of fine spray. This fuel spray is mixed with air in the mixture chamber and then the mixture is supplied to the intake manifold of the engine. The throttle valve is placed between the mixing chamber and the intake manifold of the engine. The supply of the mixture is controlled by means of throttle valve. Choke valve: During starting or warm up in cold weather the engine requires extra rich mixture. This is done by introducing a choke valve in the air passage before the venturi. For this purpose the choke valve is closed to allow only a limited supply of air and creates high vacuum near the fuel jet. The fuel flow increases as the vacuum near the jet increases. This enriches the mixture as desired. WORKING: During suction stroke air is drawn through the venturi. Venturi is a tube of decreasing cross-section, which reaches a minimum at throat. (Venturi tube is also known as choke tube and is so shaped that it gives minimum resistance to air flow.) The air passing through the venturi increases in velocity and the pressure in the venturi throat decreases. From the float chamber, the fuel is fed to discharge jet, the tip of which is located in the throat of the venturi. Now because the pressure in the float chamber is atmospheric and that at the discharge jet is below atmospheric, a pressure differential, called Carburettor depression, exits which causes the discharge of fuel into the air stream and the rate of the flow is controlled or metered by the size of smallest section in the fuel passage. (This is provided by the discharged jet and the size of this jet is chosen empirically to give the required engine performance). The pressure at the throat at the fully open throttled condition lies between 4 and 5 cm Hg below atmospheric, and seldom exceeds 8 cm Hg below atmospheric. To avoid wastage of fuel, the level of the liquid in the jet is adjusted by the float chamber needle valve to maintain the level a short distance below the tip of the discharge jet.
  18. 18. The petrol engine is quantity governed which means that when less power is required at a particular speed the amount of charge delivered to the cylinders is reduced. This is achieved by means of throttle valve of the butterfly type which is situated after venturi tube. As the throttle is closed less air flows through the venturi tube and less quantity of air-fuel mixture delivered to the cylinders and hence less is the power developed. As the throttle is opened, more air flow through the choke tube, and the power of the engine increases. A simple Carburettor of the type described above suffers from a fundamental fault in that it provides increasing richness as the engine speed and air flow increases with full throttle because the density of the air tends to decrease as the rate of air flow increases. Also it provides too lean mixture at low speed and part open throttle. Limitations of a Simple Carburettor: (a) A simple Carburettor is suitable only for engines running at constant speeds and at constant load condition as it gives proper mixture at only one engine speed and load. (b) The working of the simple Carburettor is, effected by changes of atmospheric temperature and pressure. (c) Simple Carburettor does not have arrangements for providing rich mixture during starting and warm up. (d)It cannot provide very rich mixture required for sudden acceleration of the engine. ASSIGNMENT: (1) Why a separate ignition system is used for diesel engine? (2) Define carburetion process? (3) Write function of Carburettor? Explain atomization & vaporization? (4) Write function of needle valve, throttle valve & choke valve? (5) Why the level of petrol in float chamber is kept slightly lower than the top of jet? (6) What is ventury? Which pressure act at throat of ventury? How? FILL IN THE BLANK: 1. Theoretical air-fuel ratio required for complete combustion is __________. (20:1, 15:1, 8:1) 2. The throttle valve is used to control __________ of mixture going in to cylinder. (quality, quantity ). 3. When the choke valve is operated the flow of air into venturi ____________ (increases, decreases, remain constant) 4. The function of venturi shape in Carburettor is to decrease ____________ (velocity, pressure, temperature). 5. The breaking up and mixing the petrol with air is known as _____________ (Mixing, carburetion, scavenging). 6. Idling engine requires ____________ air fuel mixture. (lean, rich, no) 7. Carburettor is not used in ____________ engine (petrol, diesel, gas).
  19. 19. 8. In a simple Carburettor as the speed of the engine increases the mixture will become ____________. (rich, lean ).
  20. 20. EXPERIMENT NO. Date: STUDY OF COOLING SYSTEMS OF I.C.ENGINE Objective: After studying this practical, students are able to know. (1) Need of cooling system. (2) Applications of air cooling system & water cooling system. (3) Construction, working, advantages & disadvantages of air cooling system. (4) Construction working of mentioned water cooling system. 1. Direct or non-return system. 2. Thermosyphon system 3. Forced circulation system. Theory: In internal combustion engines large amount of heat is produced during the combustion of fuel in the engine cylinder. It is found that only 25-30% of the total heat produced is converted into useful work. About 40% of the total heat is carried away by the exhaust gases and the remaining about 30% of the total heat is absorbed by the material of the cylinder and other components of the engine. The large amount heat production is due to the fact that the temperature of the explosion is about 2000 to 2600 o C. The temperature of the gases produced at the time of burning of the fuel is above the melting point of aluminum which is 657o C and that of iron which is 1400o C. No metal can withstand such a high temperature continuously. If this heat absorbed by the engine components is not dissipated by some method of cooling, the temperature of the engine will soon reach a dangerous temperature and will seriously harm the functioning of the engine. The piston will seize in the cylinder due to expansion. Lubricating oil will break down, which may result scoring of cylinders, the valves, will burn and wrap. Moreover the higher temperature may also cause reduced. It is because of the fact that the mass of the charge taken in during suction stroke is considerably reduced, due to its excessive heating in the intake manifold. Consequently the power output is reduced. It is therefore, very important that the unwanted heat which causes overheating of the engine, should be dissipated by some methods of ~ooling. ApplicaZions of air cooling: Air cooling is usually used for small engines and for engines whose application gives extreme importance to weight such as aircraft engines. Other areas for air-cooled engines are industrial and agricultural engines wh~re there can be~a strong objection to use of water as coolant. 20or air cooling Zhe cylinder head transfer area is increased by finning and air is passed over these fins to affect cooling. Application of water-cooling: In case of water-cooled engines the cylinder and the cylinder head are enclosed in a water jacket. This water jacket is connected to a radiator (heat exchanger). Water is caused to flow in the jacket where it cools the engine, then it gives up this heat to air 20n the radiator Znd is again circulated in the water jacket.
  21. 21. TY21ES OF COOLING SÛSTEM: In order to cool the engine a cooling medium is required. This can either be air or liquid: Accordingly there are two types of systems in general use for cooling of I.C.engines. They are- • Air or Direcç cooling systemà • Water or Indir<ct cooling systÛm. AIR COOLING SYSTEM: In this system, air is used as a cooling medium and it is used for small capacity engines. Earlier it is used for big capacity air-craft engines as the water cooling was not practically possible as the weight of water cooling system is very high compared with air-cooling system. The heat transfer coefficient for air-cooling is very low as mentioned earlier. This heat transfer coefficient can be increased further by using the forced flow of air over the engine surface. The heat transfer coefficient with air-cooling with forced circulation is also considerably lower (50 W / m2 -K) compared with water cooling system. The other method of increasing the heat transfer rate from the cylinder surface is to increase the surface area by increasing the heat transfer rate from the cylinder surface is to increase the surface area by providing the tins. The use of fins increases the heat transfer surface by 5 to 10 times of its original value. In the air- cooled systems, the forced circulation of air with increased surface area by using the fins is commonly used in practice for aero-engines and motor cycle engines. The sectional view of an engine-cylinder with fins is shown in Fig.( ). More fins are used near the exhaust valve and cylinder heat where the possibility of occurrence of maximum temperature exists. The cooling fins are either cast integral with the cylinder and cylinder head or they are fixed to the cylinder block separately. The number of the fins, used are 2 to 3 in case of cast fins and 4 to 5 in case of machined fins per centimeter. The height of fin depends on the type of material and manufacturing process used. Generally the height of fin used lies between 2 cm to 5cm and fin spacing is limited to 2.5 mm. Advantages and Disadvantages of air-cooled system: 1. The design of air-cooled system is simple and less costly. 2. Each cylinder of the multi-cylinder engine can be removed separately as no common cooling system is used for the engine. It is easy to renew in case of accident. 3. There is no danger of leakage of the coolant. 4. The freezing in the cooling system is not at all a danger which is very common in water-cooled systems. 5. The installation of air-cooled system is easier as it does not require radiator, headers and piping connections. 6. The weight of the cooling system per B.P. of the engine is far less than water cooled system. Air cooling results in higher engine temperature. This necessitates the provision of bigger clearances between the various parts of the engine.
  22. 22. WATER COOLING SYSTEM: In this system mainly water is used and made to circulate through the jackets provided around the cylinder, cylinder-head, valve ports and seat sand other hot spots where it extracts most of the heat. The diagrammatic sketch of water circulating passage, viz, water jacket is shown in Fig.( ). It consists of a long flat, thin walled tube with an opening, facing, the water pump outlet and a number of small opening along its length that direct the water against the exhaust valves. The tubes, fits in the water jacket and can be removed from the front end of the block. The heat is transferred from the cylinder walls and other parts by convection and condition. The water becomes heated in its passage through the jackets and is in turn cooled by means of an air cooled radiator system. The heat from water in turn is transferred to air. Hence it is called the indirect cooling system. Water cooling can be done by any one of the following five methods.  Direct or non-return system  Thermosyphon system  Forced circulation cooling system  Evaporative cooling system  Pressure cooling system Direct or Non-Return System: This system is useful for large installation where plenty of water is available. The water from a storage tank is directly supplied through an inlet valve to the engine cooling water jacket. The hot water is not cooled for reuse but simply discharged. Thermo-siphon: The arrangement of this system is shown in fig.( ). When the circulation of water is achieved by virtue of its density difference, it is known as thermosyphon circulation. Thermo-siphon circulation is based upon the fact that when water is heated up its density decreases. Due to decrease in its density it tends to rise up. The cold water take its place, as its density is relatively more. The rate of circulation is less as the force causing the flow of water is limited. The water as passed through the radiator is cooled by the flow of air passed over the radiator tubes by the cooling fans. The cooled water rises to the cylinder jacket, takes the heat from the cylinder wall and then it enters into the radiator from the top header and comes down. It is cooled as it is passed through the radiator tubes. The limitations of this system are listed below: 1. The engine should be placed as low as possible in relation to the radiator as the force causing the flow is limited by the temperature difference of hot and cold water. 2. The water level in the system should not fall below the level of the delivery pipe, otherwise the circulation of water in the system will stop. This causes the boiling of water and formation of steam resulting in further loss of water which may damage the engine in a short period. 3. The use of this system is recommended for small capacity engines.
  23. 23. 4. The main drawback of this system is that, the cooling depends only on the temperature and is independent of engine speed. The rate of circulation is slow. Because of many limitations this system is rarely used presently. The main advantage of this system is its simplicity and automatic circulation of cooling water. Forced Circulation Cooling System: This system is used in large number of automobiles like cars, buses and even heavy trucks. Here, flow of water from radiators to water jackets is by convection assisted by pump. The main principle of the system is explained with the help of block diagram shown in fig ( ). The water is circulated through jackets around the parts of engine to be cooled, and is kept in motion by a centrifugal pump which is driven from the engine. The water is passed through the radiator where it is cooled by air drawn through the radiator by a fan and by the air flow developed by the forward motion of the vehicle. A thermostat is used to control the water temperature required for cooling. The system mainly consists of four component, viz., a radiator, fan, water pump and a thermostat. ADVANTAGES OF WATER COOLING SYSTEM: 1. Compact design of engines with appreciably smaller frontal area is possible. 2. The fuel consumption of high compression water cooled engines are rather lower than for air cooled engines. 3. Because of even cooling of cylinder barrel and head due to jacketing makes it possible to reduce the cylinder head and valve seat temperatures. 4. The size of engine does not involve serious problems as far as the design of cooling system is concerned. In case of air cooled engines particularly in high horse power range difficulty is encountered in the circulation of requisite quantity of air for cooling purposes. LIMITATIONS OF WATER COOLING SYSTEM: This is dependent system in which supply of water for circulation in the jackets is required. Power absorbed by the pump for water circulation is considerable and this affects the power output of the engine. Cost of the system is considerably high. In the event of failure of the cooling system serious damage may be caused to the engine. ASSIGNMENT: Answer the following questions: (1) Why cooling of engine is necessary? If we don't provide effective cooling system than write down adverse effects to the engine? (2) How heat transfer coefficient of air can be increased?
  24. 24. (3) Water cooling system is called indirect cooling system and air cooling system is called direct cooling system? (4) In thermosyphon system circulation of water takes place on the base of which principle? How? (5) Which is thermostat? Explain its function?
  25. 25. EXPERIMENT NO: DATE: OBJECTIVES: After studying this practical, students are able to know (1) About common faults & remedies in IC engine. AIM: Common Faults and Remedies in I.C.Engines. Symptoms Causes Tests or Checks Remedies (1) Engine fails to start Lack of Ignition current. (2) For battery Ignition – weak battery. (3) Objectionable Ignition spark. (4) No fuel in the tank. (5) No fuel in the carburettor. (6) Too much fuel in the engine or flooded engine. (1) See if Ignition switch is on. (2) Check the battery strength by removing one of the spark plug wire and holding it 10 mm away from the plug when cranking the engine. For no spark or weak spark check the broken or loose connecting wires. (3) Check spark plug gap. (4) Check fuel level and the fuel shut off cock. (5) Check whether the fuel pump supplies sufficient fuel, after loosening fuel supply line to carburettor ? If it supplies sufficient fuel check the carburettor. Check float, float needle & starter device. (6) Check whether the spark plugs are wet or sooted. (1) Switch on Ignition-c light. (2) Recharge the batte Join the broken or l (3) Set the plug gap be Tighten seat of all c (4) Refuel or change ov the fuel shut off coc (5) Clean the fuel pump air pump. Wash all Clean the carburett float, float needle & insertion of gasket w (6) Dry & clean the spa throttle wide open. (2) Engine stalls at low speed (1) Air-fuel mixture very rich. (1) Check position of choke. (2) Check fuel level in float chamber. (3) Check exhaust pipe for clogging. (4) Check valve tappet clearance. (5) Check Ignition timing. (1) Set it to proper clos (2) Refill after cleaning Set the float for pro (3) Clean the exhaust p wire. (4) Set its right amount (5) Advance the timing (2) Air fuel mixture very lean. (1) Check Ignition timing. Check carburettor for clogging. (2) Check fuel supply lines & (1) Clean the carburett speed adjustment. S level. (2) Repair the joints. If
  26. 26. intake pipe for leakage. pipeline. (3) Spark plug fouled. (1) Check for soot or wetness. Check the electrode gap. (2) Contact breaker point misadjusted, solied or pitted. (3) Check if water in the fuel. (1) Clean the spark plu (2) Clean the contact p the contact breaker (3) Drain fuel & filter it. (3) Engine is not turned by the starter. (1) Defective starter switch or lead. (2) Battery discharged, connections or lead plates defective. (3) Wrong grade of engine oil in used or frozen water pump or defective starter. (1) If lights are bright check the starter switch. (2) Check brilliance of head lights. (3) Test starter & compare grade of oil according specification of vehicle manufacturer. (1) Replace starter swit (2) Charge battery & re plates. (3) Use specified grade replace the defectiv (4) Engine does not start although starter turns at proper speed. (1) Inadequate petrol supply. (2) Defective pump. (3) Ignition system unsatisfactory. (4) Carburetion faults. (1) Check petrol supply. (2) Check operation of pump. (3) Check Ignition system. (4) Check carburettor for faults. (1) Clean choked filter (2) Correct pump faults pump connections. (3) Tight the connection contact breaker stic system faults. (4) Correctly set the mi carburettor faults lik piston, needle valve carburettor. (5) Engine has poor starting in warm condition. (1) Rich Air : Fuel mixture. (1) Check choke & See whether the engine is flooded? (1) Close the choke & O while starting in case th (6) Engine does not start in cold condition. (1) Cranking motor speed very low. (2) Storage battery discharged. (3) Defective starting Motor. (4) Petrol level in carburettor very low. (5) Weak or no spark at the spark plug gap. (1) Check temp. of motor oil. (2) Check the battery for proper discharge. (3) Check the motor. (4) Check petrol level in the carburettor after priming. (5) Check spark at plug points. (1) Heat it if it is very co (2) Replace battery or W while starting depre (3) Start the engine by (4) Repair the petrol su filter & air vent hole (5) Clean & remove ca the plug point.
  27. 27. (7) Engine stops at high speed. (1) Insufficient fuel supply to the carburettor. (2) Fuel supply line broken or closed. (3) Fuel pump filter defective. (4) Fuel pump leaking. (5) Sticking valve or broken valve spring. (1) Check the fuel supply pipe. (2) Check fuel supply line for broken pipe. (3) Check it. (4) Check fuel pump leakage. (5) Check the valve. (1) Clean the fuel supp necessary. (2) Clean fuel supply lin line by means of ins (3) Clean the filter or re (4) Seal it with new gas (5) Clean the valve and Repair or replace if (8) Thick smoke emitted by engine. (1) Very rich Air Fuel mixture. (2) Sticking float and not closing properly. (3) Main jet very large or wrongly adjusted. (4) Wet air filter. (1) Check the choke and throttle valve. (2) Check the adjustment of float needle. (3) Check the main jet. (4) Check air filter. (1) Close the choke an properly. (2) Replace the float ne (3) Make standard adju replace it, if require (4) Clean and dry air fil (9) Engine Stops suddenly. (1) Seized piston due to over heating. (2) Failure of Ignition. (3) Interrupted fuel supply. (1) Check oil level. (2) Check the Ignition system. (3) Check fuel supply line & carburettor. (1) (a) Allow it to cool. (b) Top up with oil, (c) Start with care. (2) Set the Ignition syst defective parts. (3) Clean the fuel supp Carburettor. (10) Engine Stalls temporarily. Defective Ignition system. (1) Check Ignition system, contact breaker points & the spark plugs. (1) Replace defective p adjust breaker poin gap. (11) Engine produces noise when (a) idling. (b) running light. (1) Worn out gudgeon pin & small end bearing. (2) Piston slaps due to cylinder & ring wear, broken ring. (3) Big end bearing worn out. (1) Short circuit each spark plug in turn. (2) Noise will disappear when each plug is short circuited in turn. (3) Short circuit plug in each cylinder. (1) Replace the worn o (2) Replace the piston (3) Repair big end bear (12) Poor Engine performance (1) Ignition fails to operate properly. (2) Choke opening. (3) Fuel supply line clogged. (4) Insufficient fuel supplied by the fuel (1) Check the Ignition system (2) Check choke valve. (3) Check the fuel supply line. (4) Check the fuel pump. (1) Replace the defecti and do necessary s Close the choke & ope Clean & repair. Repair the pump.
  28. 28. pump. (5) Loose air intake manifold. (6) Insufficient throttle valve opening. (7) Leaky pistons & valves. (8) Wrong Ignition timing. (9) Brakes blocking or dragging. (5) Check manifold for air leakage. (6) Check throttle linkage. (7) Check for the leakage. (8) Check for Ignition timing. (9) Check brake mechanism & locate the trouble. Tighten the bolts and m with new gasket, if requ Adjust throttle opening. Repair or replace pisto Adjust the Ignition timin specification. Repair brake mechanis clearance & adjustmen (13) Engine continues to run even after switching off the Ignition. (1) Defective Ignition lock. (1) Check Ignition lock. (1) Loose the cable of t required, short circu (14) Engine shows poor compression (1) Cylinder or ring wear, broken or stuck rings. (2) Burnt valve and seating. (3) Sticking valves. (1) Check for blue smoke from exhaust. (2) Remove air filter and listen for hiss from carburettor on compression stroke. (3) Check gasket for leakage. (1) Provide proper seal cylinder. (2) Replace the burnt v components. (3) Provide proper tapp gasket. (15) Engine overheats. (1) Insufficient water in cooling system. (2) Radiator furred or clogged. (3) Fan belt slipping. (4) Oil very low. (1) Check water level in the cooling water system. (2) Check cooling system. (3) Check fan and fan belt. (4) Check oil level. (1) Fill with water, if lev (2) Clean radiator. (3) Tighten fan belt. (4) Top up the oil level, EXPERIMENT NO. Date:
  29. 29. STUDY OF INTERNAL COMBUSTION ENGINES. OBJECTIVES: After studying this practical students are able to know (1) About heat engine (2) Advantages of IC Engine over EC Engine (3) Classification of IC Engine. (4) Basic nomenclature of IC engine (5) Various components of IC engine & its functions (6) Application of IC engine. HEAT ENGINE: In general, an engine is defined as a device which converts one form of energy into mechanical work. Heat engine is a device which transforms heat energy into mechanical energy. Transformation of one form of energy into another required form is always associated with losses. Therefore efficiency of conversion plays an important role. In every heat engine, some form of fuel (solid, liquid, gas or nuclear) is used. The chemical energy of fuel is converted into thermal or heat energy & that is further used to perform useful work. Heat engines are mainly classified as • External Combustion Engines. (E.C.engine) • Internal Combustion Engines. (I.C.engine) In E.C. engine working fluid is not mixed with fuel, therefore, the same working fluid is used again & again. Steam engine / Steam turbine falls under this category. • In I.C. engine, fuel is mixed with the air & burned, the combustion of fuel takes place inside the engine cylinder and power is produced. This type of engines are used in trucks & buses, scooters & cars, ships & locomotives, agricaltural & earth moving machinery, many industrial applications & for power generation. ADVANTAGES OF I.C.ENGINE: Internal combustion engines have some special advantages over external combustion engines. 1. The thermal efficiengy of I.C.engine ( 35-40% ) is much higher than E.C.engine (15-25% ). 2. Greater mechanical simplicity 3. Power developed by the I.C.engine per Kg weight of engine is higher, therefore it is lighter & occupies less space. 4. The I.C.engine can be started quickly whereas an E.C.engine (Steam engine) requires much more time as the steam has to be generated in the boiler.
  30. 30. 5. The I.C.engine being more compact, practically it has no competitor for small & portable power range. 6. The temperature in I.C.engine are very high (about 2000 C) as the combustion takes place inside the engine cylinder, therefore, a cooling arrangement is necessary to prevent overheating of engine cylinder. 7. In case of steam engines, fresh steam is circulated in the jackets to prevent condensation & power loss. 8. I.C.engines are commonly single acting whereas steam engines are commonly double acting CLASSIFICATION OF I.C.ENGINE The I.C.engines are classified as follows: 1. According to type of fuel used. On this basis, I.C.engines are classified as petrol engine, diesel engine and gas engine. 2. According to number of strokes required to complete the cycle. On this basis they are classified as two stroke and four strokes engines. In four stroke engines, the cycle is completed in four strokes of piston or two revolutions of crank. And there will be one power stroke in two revolution of crank shaft. In two stroke engines, the cycle is completed in two strokes of piston or one revolution of crank shaft. And there will be one power stroke in each revolution of crank shaft. 3. According to methods of ignition: They are classified as S.I.( Spark Ignition) engines and C.I.( Combustion Ignition) engines. S.I.engines: The compressed mixture of petrol and air ( i.e. charge) is ignited with the help of introducing an electric spark with the help of spark plug, used in petrol engines. (pressure: 70 bar) C.I.engines: The fuel is ignited as it comes in contact with high temperature and high pressure air (600 C & 3500 bar) i.e. compressed air in the cylinder. Diesel engines works on this system.
  31. 31. 4. According to cycle of operation: They are mainly classified as Otto cycle engines and Diesel cycle engines. Otto cycle : Combustion of fuel takes place at constant volume. Petrol engine works on this cycle. Diesel cycle: Combustion of fuel takes place at constant pressure. Diesel engine works on this cycle. 5. According to method of cooling: They are classified as air cooled or water cooled engines. Air cooled engines: These engines are cooled with the help of passing air flow over the surface of engines. Generally applicable for small capacity engines. Water cooled engines: These engines are cooled by circulating cooling water through the engine jackets. Generally used for high capacity engines. (bus,trucks ,etc.) 6. According to method of governing used: They are classified as quantity governing & quality governing: Quantity governing: In this method the quantity of charge (mixture of petrol & air) supplied to engine cylinder is controlled by throttle valve as per the requirement. Petrol engines are governed by this method. Quality governing: In this method the quantity of diesel injected into the cylinder is controlled by control rack provided in fuel pump as per the requirement. Air fuel ratio will very continuously.
  32. 32. 7. According to speed of engine: They are classified as low speed, medium speed and high speed engines. Petrol engines are high speed engines whereas diesel engines fall under low or medium speed engines. 8. According to arrangement of cylinder. They are classified as horizontal, vertical, incline, V-type & radial engines. Incline engine: All cylinders are arranged linearly and transmit power to a single crank shaft. This type is very popular with automobiles, where 4 & 6 cylinder in line engines are commonly used. V-Type: In this arrangement, two cylinders are inclined at an angle (30-75) to each other & with one crank shaft. Radial engines: In this arrangement the cylinders are arranged along the periphery of a circle. Previously this type engine was used in air craft engine. Opposed piston engine: In this arrangement a single cylinder is used with pistons moving in opposite direction. Therefore it is known as opposed piston engines. The combustion takes place at the center for both pistons and no cylinder head is required. Better balanced. Opposed cylinder engine: In this arrangement, an engine with two cylinder banks is located in same plane on opposite sides of the crankshaft. BASIC ENGINE NOMENCLATURE: 1. Bore: Inside diameter of cylinder is known as bore 2. Stroke: The maximum linear distance travelled by the piston in cylinder in one direction is known as stroke and it is equal to twice the crank radius. 3. Top dead centre (TDC): The extreme position of the piston at the top of the cylinder (head end side) is clalled Top dead center position. In case of horizontal engines this is known as Inner dead center (IDC) position. 4. Bottom dead centre: The extreme position of piston at the bottom of cylinder is called Bottom dead center (BDC) position. In case of horizontal engine this is known as (ODC) Outer dead centre position. The distance between these two extreme positions is known as stroke length. 5. Clearance Volume : The volume contained in cylinder above top of piston when the piston is at TDC is called the clearance volume. It is denoted by Vc.
  33. 33. 6. Piston displacement or swept volume: The volume swept through by the piston in moving between TDC & BDC is defined as piston displacement or swept volume and it is denoted by Vs. Vs = ×D×L Where, L =Length of stroke in m N =Speed of the engine in RPM. The cylinder volume = Swept volume(Vs) + Clearance volume(Vc). 7. Compression Ratio: The ratio of volume when the piston is at BDC to volume when piston is at TDC is called compression ratio & It is denoted by r = V1/V2 = (Vs+Vc) / Vc. Or It is the ratio of volume, of working fluid, before compression to volume of working fluid after compression. For petrol engine the compression ratio varies from 5:1 to 9:1. For diesel engine the compression ratio varies from 12:1 to 22:1 8. Piston speed: The distance travelled by the piston in one minute is known as piston speed. Piston speed = 2LN m/min Where, L =Length of stroke in m N =Speed of the engine in rpm. VARIOUS COMPONENTS OF I.C.ENGINE: Depending upon the design & application, one I.C.engine may be different from the other I.C.engine, as regards its size, shape & dimensions. But the main essential components becomes necessary to understand the purpose of various components for complete understanding of I.C.engine. So now we shall discuss the purpose of main components of I.C.engines. • Frame: It generally consists of base plate, crankcase, and it also supports different moving parts. The base plates are rigidly fixed to the foundation of the floor. The lower part of crankcase contains oil for lubrication purpose. • Cylinder and Cylinder Block: Single cylinder engine has a single cylinder. Multi cylinder engine has a cylinder block which contains cylinder bores and openings for the valves. To avoid wear and tear of cylinder block, cylinder liners are provided. It may contain passage for flow of cooling water. The cylinder of I.C.engine is considered as main body of the engine in which piston reciprocates to develop power. It has to withstand very high pressure and temperature (about 2200 C) because there is direct combustion inside the cylinder therefore its material should be such that it can retain strength at high temperature, should be good conductor of heat and should resist to rapid wear and tear due to reciprocating parts. Generally ordinary cast iron is used, but in case of heavy duty engines, alloy steels are used. Sometimes, when engine blocks are heavy & for easy maintenance sleeves or liners are inserted into cylinder which can be replaced when worn out Liners are generally made of nickel chrome iron.
  34. 34. • Cylinder head: The cylinder head closes one end of cylinder. It houses inlet and exhaust valves through which charge is taken inside the cylinder and burned gases are exhausted to atmosphere from the cylinder. Cylinder head is usually cast as one piece and bolted to the top of the cylinder. A copper and asbestos gaskets are provided between the cylinder & cylinder head to obtain a gas tight joint. The material used for cylinder head is also cast iron. • Piston & Piston Rings: The functions of piston are to compress the charge during compression stroke and to transmit the gas force to the connecting rod and then to the crank during power stroke. The piston of I.C.engines is made of cast iron, cast steel and aluminum alloy. The aluminum alloy has the advantage of higher thermal conductivity and lower specific gravity. Piston is the heart of the engine. The piston rings are housed in the circumferential grooves provided on the outer surface of the piston. It gives gas tight fitting between piston and the prevents the leakage of high pressure gases. These are made of special grade cast iron. This material retains its elastic property at very high temperature. The upper piston rings are called the compression rings and the lower piston rings are called the oiling or oil control ring ( oil scrapper rings). • Connecting rod: It is usually a steel forging of circular, rectangular, I, T or H section and is highly polished for increased endurance strength. Its small end forms a hinge and pin joint with piston and its big end is connected to crank by crank pin. It has a passage for the transfer of lubricating oil from the big end bearing to small end bearing (gudgeon pin). • Crank & Crank shaft: Both crank and crank shafts are steel forged and machined to a smooth finish. These are held together by means of a key. Crank shaft is supported in main bearings and has a heavy wheels called flywheel, to even out the fluctuation of torque. The power required for any useful purpose is taken from crank shaft only. The crank shaft is the back bone of the engine. • Cam shaft: The function of the cam shaft is to operate the intake and exhaust valves through the cams, cam followers, push rods and rocker arms. The cam shaft is driven positively from the crankshaft at half speed of the crankshaft. • Piston pin or wrist pin: Piston pin connects the piston and small end of connecting rod. It provides the bearing for the oscillating small end of connecting rod. Inlet valve: It controls the admission of charge into the petrol engine or air into diesel engine during suction stroke of the engine. • Exhaust valve: The removal of exhaust gases after doing work on the piston is controlled by this valve.
  35. 35. • Valve spring: Valves are kept closed by the valve spring (compression spring). • Inlet manifold: It is the passage which carries the charge from carburettor to the petrol engine. It connects all the inlet valves in case of multi-cylinder engine. • Exhaust manifold: It is the passage which carries the exhaust gases from the exhaust valve to the atmosphere. • Cam shaft: The function of cam shaft is to actuate/operate the intake and exhaust valves through the cams, cam followers, push rods and rocker arms. It is driven positively from crank shaft at half the speed of the crank shaft. • Cam & Cam follower: It gives the desired motion to the valves through the follower. • Push rod and Rocker arm: The motion of cam is transmitted to the valve through push rod and rocker arm. These links together are also known as valve gear. • Crank case: It is the base which holds the cylinder and crank shaft. It also serves as sump for lubricating oil. • Water jacket: The jackets are the integral opening/passage provided around the cylinder & other hot spots through which water is passed for cooling the engine. • Flywheel: It is a wheel mounted on the crank shaft which stores excess energy during power stroke and returns that energy during other strokes and maintains a fairly constant output torque on the crankshaft. • Governor: It is run by a drive from the crankshaft. The function of governor is to regulate charge in case of petrol engine and amount of fuel in case of diesel engine to maintain the speed of the engine constant, when the load requirement varies. • Carburettor: The function of carburetor is to supply the uniform air fuel to the cylinder of a petrol engine through the intake manifold. The mass of mixture entering the cylinder is controlled by a throttle valve. • Spark plug: To ignite the compressed charge in petrol engine. • Fuel pump: It forces fuel oil at high pressure through fuel nozzle into the cylinder at the end of compression stroke in diesel engine.
  36. 36. • Fuel nozzle: The function of fuel nozzle is to break up the oil into a fine spray as it enters the cylinder of diesel engine. Applications of I.C.engines: Application Power(kW) SI or CI 2-stroke or 4-stroke Cooling method 1 Road Vehicles 1.Mopeds 2.Scooters and Motor cycles 3.Small car 4.Large car 5.Lawn movers 0.75-1.5 3-7 15-75 75-200 0.75-3 SI SI SI SI SI 2 2,4 4 4 2,4 A A W W A 2 Locomotives 400-4000 CI 2,4 w 3 Small Air crafts 1.Helicopters 2.Airplanes 40-1500 40-3000 SI SI 4 4 A A 4 Marine Applications 1.Motor boats 2.Ships 0.5-75 3000-20000 SI,CI CI 4 2,4 W W 5 Industrial Applications 1.Electric power 2.Gas Pipe lines 40-25,000 700-5000 CI SI 2,4 2,4 W W 6 Off-Road Vehicles 1.Light vehicles (Factory,Airport) 2.Agriculture 3.Earthmoving machinery 4.Military purpose 1.5-15 3-150 40-800 40-2000 SI SI,CI CI CI 2,4 2,4 2,4 2,4 A A,W W A,W
  37. 37. Assignment: Answer the following questions. (1) Define heat engine? How energy is liberated? (2) Compare EC Engine & IC Engine? (3) Define compression ratio. Write range of compression ratio for Diesel Engine & Petrol Engine? Why compression ratio of Diesel Engine is more than compression ratio of Petrol engine? (4) Write down function & material of following components? 1. Frame 2. Cylinder block 3. Cylinder head 4. Piston 5. Piston rings 6. Connecting rod 7. Crank & Crankshaft. (5) Write function of following ? 1. Camshaft 2. Piston pin 3. Cam & follower 4. Push rod & rocker arm 5. Crank case 6. Flywheel 7. Governor.
  38. 38. EXPERIMENT NO: Date: STUDY OF THE IGNITION SYSTME OF PETROL ENGINE OBJECTIVES: After studying this practical, students are able to know (1) Requirements of ignition system. (2) Construction & working of battery ignition system. (3) Construction & working of magneto ignition system (4) Firing order of IC Engine. (5) Construction & working of spark plug. THEORY: The petrol engines are spark ignition engines and in S.I.engines, the compressed (air and petrol vapour mixture) charge must be ignited at the correct instant so that resulting rise in pressure of hot gases acts on the piston when the piston is near top dead centre. The expanding gases force the piston out in power stroke. A high voltage is required to jump the gap of spark plug and give a spark of sufficient energy to ignite the mixture & this is produced by ignition system. REQUIREMENT OF IGNITION SYSTEM: The important requirements of the spark ignition systems are listed below: 1. The voltage across the spark plug electrodes should be sufficiently large to produce an arc required to initiate the combustion. The voltage necessary to overcome the resistance of the spark gap and to release enough energy to initiate the self-propagating flame front in the combustible mixture is about 10,000 to 20,000 volts. 2. The intensity of spark should lie in a specified limit because too high intensity may burn the electrodes and too low intensity may not ignite the mixture properly. 3. The volume of the mixture (clearance volume) at the end of compression should not be too large, otherwise the spark produced may not be sufficient to ignite the whole charge. There is definite relation between the size of the spark and clearance volume. 4. There should be no missing cycle due to failure of spark. 5. In a multi-cylinder engine, there must be arrangement (distributor) to carry this voltage to the right cylinder at the right time. Now we will study the Battery ignition system & magneto ignition system used in petrol engine. BATTERY OR COIL IGNITION SYSTEM: Construction: This system is used in cars and other vehicles using petrol engines. Fig(a) Shows the circuit diagram of a battery or coil ignition system. The main components
  39. 39. of this system are: (a) a battery of 6 to 12 volts,(b) ignition switch, (c) induction coil, (d) circuit or contact breaker, (e) condenser, (f) distributor, and (g) spark plugs. There are two circuits in this system- One is primary circuit and the other is the secondary circuit. The primary circuit consists of a battery, ignition switch, ammeter, primary winding in the induction coil, contact breaker and a condenser. The secondary circuit consists of secondary winding which has large number of turns of fine wire in the induction coil, distributor, rotor and spark plugs. The primary winding and secondary winding are wound on a laminated soft iron core and are insulated from each other. One end of the secondary winding is earthed and other end is connected to the distributor cap. The contact breaker is driven by a cam which rotates at half the engine speed (for four stroke engines). There is a condenser in the primary circuit. The condenser prevents the sparking at the contact breaker points. Working of the battery or Coil ignition system: When the ignition switch is switched on and the contact breaker point touches a current flows the battery through the switch to the primary winding of the coil to the circuit breaker points and the circuit is completed through the ground. The current which flows through the primary winding of the coil produces a magnetic field in the coil. When the primary circuit is opened by the contact breaker points, the magnetic field collapses. Electromotive force is induced in the secondary winding of the coil. A condenser is connected across the contact breaker in the primary circuit which helps to collapse the field very quickly and produces a very high voltage in the secondary coil as there are more turns of fine wire than in the primary coil. The voltage is increased up to 20,000 volts. One end of the secondary coil is connected to the ground and the other end is connected to the external terminal of the distributor. The distributor connects the secondary coil to the different spark plugs. The distributor directs this high voltage to the proper spark plug where it jumps the air gap of the spark plug electrodes and the charge in that cylinder is ignited. In a single cylinder engine the distributor is not required as in the case of motor cycle engine, scooter engine and a single cam is sufficient for giving the spark. MAGNETO IGNITION SYSTEM: Magneto ignition system is generally used in small spark ignition engines, such as in motor cycle and scooters. Fig.(b) Shows the circuit diagram of a magneto ignition system. This system consists of a magneto instead of battery, which produces and supplies current in the primary winding. The magneto consists of a fixed armature having primary and secondary windings and a rotating magnetic assembly which is driven by the engine. It also consists of contact breaker, condenser, distributor rotor, distributor and spark plugs. As the magnet turns, a magnetic field is produced from a positive from a positive maximum to a negative maximum and back again. As this valve falls from a positive maximum valve, as alternating current is induced in the primary winding. This current flows in the primary circuit till the contact points are closed. When the contacts open, a very high voltage is induced in the secondary winding. This high voltage is then directed to the proper spark plug by the distributor.
  40. 40. FIRING ORDER: The order in which the firing takes place in the different cylinders of a multi- cylinder engine is known as the firing order. Proper firing order maintains proper engine balancing and reduces engine vibration. Firing orders for various engines are given below: No. of Cylinders Firing order 2 1,2 3 1,3,2 4 1,2,4,3 1,3,4,2 6 1,5,3,6,2,4 1,4,2,6,3,5 1,3,2,6,4,5 1,2,4,6,5,3 8 1,6,2,5,8,3,7,4 1,4,7,3,8,5,2,6 SPARK PLUG: Spark plug is used in S.I. engines (Petrol engines) to produce electric spark to ignite the compressed air fuel mixture inside the engine cylinder. The spark plug consists of three main parts: 1. A central electrode. 2. A threaded metallic body with a ground electrode. 3. An insulator separating the two electrodes. The central electrode in the spark plug is surrounded by a porcelain insulator. The central electrode extends for a short length through the bottom of the insulator. The upper end of the central electrode is connected to the cable from the ignition coil. A metal screw surrounds the bottom part of the insulator. The lower portion of the screw is attached to a short electrode and bent towards the central electrode so that there is a gap between the two electrodes. The air gap is kept between 0.6 mm to 1 mm. The high-tension current is given to the terminal of central electrode. This current jumps the air gap between the central electrode and ground electrode. The electrode metals are, Nickel, alloy of Nickel and Manganese, platinum alloy, alloy of Nickel, manganese and silicon. Too large or too small air gap reduces the efficiency of the entire ignition system. ASSIGNMENT: State the functions of following components of Battery Ignition System: 1. Battery 2. Induction coil
  41. 41. 3. Circuit or contact breaker 4. Condenser 5. Distributor 6. Spark Plug QUIZ: 1. ____________ is required to produce an are in S.I.engine (Injector, Spark plug, Fuel pump). 2. Spark gap in spark plug is about __________. (0.7 mm, 1.6 mm, 0.7 mm). 3. Pre ignition means __________ 4. In ignition system a spark is produced when contact breaker points _____. (closes, opens) 5. The voltage induced in secondary coil depends on __________________. (Thickness of wire, Conductivity of wire, Number of turns of coil) 6. The voltage required to produce spark is about __________. (2300, 2400, 10,000 V) 7. Firing order of four cylinder petrol engine is ___________.(1-2-3-4, 1-2-4-3, 4-2- 3-1, 1-3-4-2)
  42. 42. EXPERIMENT NO: DATE: STUDY OF LUBRICATING SYSTEMS OF I.C.ENGINES Objectives: After studying this practical students are able to know (1) Need of lubricaating system (2) Functions of lubrication system (3) Properties of lubricating system (4) Working of mentioned lubricating system 1. Splash lubricating system 2. Pressure feed lubrication system 3. Wet sump lubrication system 4. Dry sump lubrication system Theory: The process of inserting a film of oil in between the moving parts so as to reduce friction is known as lubrication. Any metallic surface, regardless how highly it has been finished or polished, shows crests or depressions when it is seen with the help of microscope. These crests and depressions on one surface interlock with crests and depressions on the other mating surface. When one surface slides over the other, friction is developed. The friction produces heat which results wear and tear of sliding or rotating surfaces. Moreover, a large amount of power produced by engine is wasted to overcome the force of friction. Almost all machine parts of an I.C.engine have relative motion and rub against each other. The lubrication is required to reduce this rubbing action and increases the life of engine. The purpose of lubrication in I.C.engine are reducing the rubbing action between different machine parts having relative motion with each other and removing the heat generated inside the engine cylinder. The power developed inside the engine is known as Indicated Power (I.P.), but the power available at the crankshaft (Brake Power) is always less than I.P. This is because, part of the power is lost in bearings, cylinder and piston, gears and many other parts due to friction. It can be reduced by using lubrication between the parts which have relative motion with each other, as film of lubricant does not allow metal contact. The frictional resistance between two mating parts having relative motion is mostly dependent on lubricating oil properties, surface condition, material of surfaces, rate of relative motion, nature of relative motion and quantity of lubricating oil. FUNCTIONS OF LUBRICATION: 1. To reduce friction between the mating parts, so as to keep the moving parts sliding freely over each other, and increases the power output. 2. To reduce wear and tear of the moving parts. 3. To reduce noise and to increase engine life. 4. To act as a cooling medium for removing heat from the bearings, cylinders & pistons etc.
  43. 43. 5. To form a good seal between piston rings and cylinder walls, thus avoiding the loss of power which is caused due to leakage of pressure. 6. To absorb and carry away harmful substances resulting from incomplete combustion. 7. To act as a cleaning agent, cleaning the bearing and piston rings from dust, carbon & micro metal chips and thus it keeps the parts clean. 8. To prevent metallic components from corrosive attack due to acid formation during combustion process. PROPERTIES OF THE LUBRICATION OIL: For proper functioning of reciprocating and rotating parts of the engine, the lubricating oil must possess certain properties. Some of the important properties are discussed here: 1. Viscosity: Viscosity of an oil is a measure of its resistance to flow and is usually measured in terms of Saybolt Universal Seconds (SUS), which is the time required, in seconds, for a given quantity of oil to flow through a capillary tube under specified test conditions. Viscosity is measured at two temperatures –18o C (0o F) and 99o C (210o F). It is also expressed in centi-stokes, centi-poise and Redwood seconds, depending upon the type of viscometer used for its determination. 2. Viscosity Index: The viscosity of oil is affected by its temperature. Higher the temperature lower is the viscosity. This variation of viscosity of oil with changes in a temperature is measured by term Viscosity Index (VI). A high viscosity index number indicates relatively smaller changes in viscosity of the oil with temperature. The oil is compared with two reference oils having same viscosity at 99o C. One is paraffinic base oil (considerable change in viscosity with temperature) is arbitrarily assigned an index of zero (0) and the other is a nepthenic base oil (little change in viscosity with temperature) assigned an index of 100. As a thumbrule, oil having VI below 50 are considering to be low viscosity index oil, oils having VI between 50 to 80 are considered medium viscosity index oil & oils having VI more than 80 are considered high VI oils. TYPES OF LUBRICATION SYSTEMS: (1) Splash Lubrication System (2) Pressure Feed Lubrication System (3) Mist/Charge Lubrication System (4) Wet Sump Lubrication System (5) Dry Sump Lubrication System
  44. 44. (1) Splash Lubrication System: The arrangement of the system is shown in Fig.( ). This method generally used for vertical engines with a closed crankcase. The sump is located at the bottom of the crankcase. When the engine crankshaft rotates, the big end of the connecting rod splashes oil by centrifugal action. The connecting rod big eng has a hollow pipe called a scoop which is fitted to the bearing gap and pointed towards the direction of the rotation of the crank shaft. The lubricating oil passing through the scoop, lubricate the big end bearing and gudgeon pin bearing. All other parts are lubricated by the splash. Excess oil is collected in the troughs located as shown in figure and are provided with overflows and collected in the main sump. The level of the oil in trough is maintained constant. The dripping from the cylinders is also collected in the sump. The oil from the sump is recirculated with the help of the pump as shown in figure. The inability to regulate the quantity of oil splashed against the cylinder wall or inability to keep the oil from getting past the piston head into the combustion chamber, burning with the fuel and passing out with exhaust gases are the limitations of this system. (1) Pressure Feed Lubricating system: All modern car and bus engines are lubricated by high pressure feed system as shown in Fig.(B). Such a system supplies oil under pressure ( 2 to 5 bar) directly to the connecting rod bearings, to the camshaft bearings, to the valve gear and to the camshaft drive. Indirect supplies reach the cylinder walls, the gudgeon pin, the distributor and pump drives. Oil is carried in the sump, a deep tray which closes the bottom of the crankcase and is circulated by the by the gear pump which sucks from the sump through a strainer as shown in figure. The pump delivery pressure is controlled by a relief valve and the oil passes through a very fine filter before it reaches the main distributor gallery. From the various bearings, surfaces and gears, the oil simply drains into the sump. After lubricating the big end bearings, the oil is fed to the gudgeon pins through the oil-way in the connecting rod and further squirted into the cylinder wall. (2) Mist OR Charge Lubrication System: This is the simplest method of lubrication and does not require oil-further and oil pump. In this system the lubricating oil is per-mixed with the petrol therefore the fuel carries the lubricating oil in the cylinder which helps for lubricating the petrol therefore the fuel caries the lubricating oil in the cylinder which helps for lubricating the piston and cylinder. Most of the oil burns with the fuel due to high temperature and burnt oil is carried with the exhaust gases. The lubricating oil cannot be recovered in this system. This type of lubrication is generally used for two stroke spark ignition engines of scooter and motorcycle. The quantity of lubricating oil mixed with the petrol is 3 to 6 % of petrol. The advantages of this system are listed below:
  45. 45. (1) It does not require separate lubricating system so it is most economical. 2 (2) There is no risk of failure of lubrication system. (3) The lubricating oil supplied is regulated at various loads and speeds by the increased fuel flow. The carbon deposits due to the burning of the oil on the spark plug and on other parts and non-recovery of the oil used are the main disadvantages of this system. The lubricating systems are also classified as wet-sump lubrication and dry sump lubrication system. (3) Wet sump Lubricating System: The arrangement is shown in Fig.( ). This is called wet sump as sump is always full of oil. The working is just similar to the pressure feed lubricating system. Oil is drawn from the sump by an oil pump through an oil strainer. A pressure relief valve is provided which automatically maintains the delivery pressure constant. If the pressure exceeds than the predetermined pressure, the valve opens and allows some of the oil to return to the sump and relives the oil pressure in the system. The oil from the pump goes to the bearings and part of it passes through a filter which removes solid particles from the oil. As all the oil is not passed through the filter, the system is known as by-pass filtering system. Advantage of this system is that a clogged filter will not restrict the flow of oil to the engine. (4) Dry sump Lubricating System: The dry sump lubricating system is shown in Fig.( ). This is known as dry-sump as the sump does not contain oil and all the oil required for lubrication remains in the circulation only. High-speed racing cars and military jeeps use this type of lubricating system as the oil in the wet sump is subjected to large back and forth acceleration. An auxiliary tank is used to supply the oil to the main bearings with the help of the pump. The oil returns back to tray and then returned back to auxiliary tank by scavenging pump, the capacity of which is always 20 to 30% more than the pressure pump to avoid flooding of the crank-case. If the filter is clogged, the pressure relief valve opens permitting oil to flow bypassing the filter and reaches the supply tank. The oil is then circulated to the bearing form the supply tank. A separate oil cooler is used to cool the oil to remove the heat form the oil, as heating of oil is rapid because of rapid circulation of oil and high speed of the engine. ASSIGNMENT: (1) Define lubrication? Write adverse effects if lubricating system is not proper? (2) Frictional resistance depends up on which factors? (3) Define viscosity index? Write unit of viscosity? (4) Why splash lubricating system is named splash? (5) Write benefits & draw back of mist lubricating system? (6) What is the main difference between wet sump lubricating system & dry sump lubricating system?
  46. 46. EXPERIMENT NO. DATE: PREPARATION OF HEAT BALANCE SHEET OF I.C.ENGINE Objectives: After studying this practical, students are able to know (1) Measurements necessary to draw heat balance sheet. (2) Take necessary readings for following measurement • Brake power • Rate of fuel consumption • Heat carried away by cylinder jacket cooling water • Heat carried away by exhaust gases (3) Prepare observation table (4) Do calculation of • Brake power • Heat supplied by combustion of fuel • Heat carried away by exhaust gases • Heat carried away by cooling water • Unaccounted heat (5) Prepare heat balance sheet (6) Conclude the practical. Thermodynamic tests: They are carried out for the purpose of comparing actual results with the theoretical or ideal performance. For such tests it is necessary to measure losses in addition to the useful part of the energy and also to draw up a heat balance account. The measurements necessary to draw up the heat balance account are 1. Brake power 2. Rate of fuel consumption 3. Heat carried away by exhaust gases 4. Heat carried away by cooling water BRAKE POWER MEASUREMENT: Brake power can be obtained by the use of either machanical, electrical, hydraulic or pneumatic dynamometer etc. Here mechanical brake is used to measure the brake power. A rope brake dynamometer consists of one or more ropes (in our case one rope) wrapped around the fly wheel of an engine whose power is to be measured. The upper end of the rope is attached to a spring balance and the downward end is kept in place by a dead weight. The rotation of flywheel produces frictional force and the rope tightens. Consequently a force is induced in the spring balance. Generation of heat is enormous and that necessitates a cooling arrangement for the brake. The rim is made through and kept in place by the centrifugal force.
  47. 47. Let W be the dead weight. S be the spring balance reading. D be the brake drum diameter and d be the rope diameter. Then effective radius of the brake drum would be Reff = (D+d)/2 and braking torque would be (S- W) Reff Brake Power = 2πN (S- W) Reff = 2πN(S-W)(D + d/2) 60 60 MEASUREMENT OF RATE OF FUEL CONSUMPTION: Burette is used to measure the mass of fuel consumed. During normal working, fuel is supplied from the tank. To measure the mass of fuel consumed three way cock is turned in such a way that fuel is supplied from burette and supply of fuel from tank is stopped. Measure the time required to consume certain amount of fuel. Now turn the three-way cock such that fuel is supplied from tank. MEASUREMENT OF HEAT CARRIED BY CYLINDER JACKET COOLING WATER: In ordinary I.C.engines, the circulation of cylinder jacket cooling water is maintained by means of natural gravitation current of water or by force circulation from a pump. (First one is used here). To measure the rate of flow of jacket cooling water, water meter can be fitted in the inlet pipe or one can collect the outflow water in a measuring vessel in a given interval. It is also necessary to measure the inlet and outlet temperature of water. Let Mw = mass of cylinder jacket cooling water supplied in Kg/min T1 = Inlet temp. of jacket cooling water K. T2 = Outlet temp. of jacket cooling water K. Cp = Specific heat of water kJ/Kg K MEASUREMENT OF HEAT CARRIED AWAY BY THE EXHAUST GASES: The actual determination of heat carried away by exhaust gases is concerned with three quantities, namely, the temp. of exhaust gases and room temperature, the mass of exhaust gases, and the mean specific heat of exhaust gases. Temperature of exhaust gases: The temperature of exhaust gases (tg) as they leaves the engine cylinder can be measured by a thermocouple or thermometer. The thermocouple or thermometer is encased in a tube which is screwed into the exhaust connection of the cylinder. Mass of exhaust gases: It may be calculated from the measured air consumption by air-box orifice method or by air flow meter in a given time. Mass of exhaust gases per minute = Air consumption per minute + Fuel consumption per minute = (1 + Air Fuel ratio) mf HEAT BALANCE SHEET: In order to complete the heat balance sheet for an I.C.engine, it should be tested over a period of the time under condition of constant load and speed. All measurements listed earlier should be taken at regular interval of time. At the
  48. 48. completion of the trial the necessary data should be averaged out and a heat account should be drawn up as follows: Heat supplied In kJ % Heat Expenditure In kJ % (1) Heat equivalent to brake power. (2) Heat carried away by cooling water (3) Heat carried away by exhaust gases (4) Observation error and radiation losses. OBSERVATION: 1. Dead weight W = _________N. 2. Spring balance reading S = __________N 3. Engine speed N = ________rpm 4. Pulley diameter D = 300 mm 5. Rope diameter d = 12.7 mm 6. Reff. = _______mm 7. Time required to consume ______ml of fuel = ______sec. 8. Rate of fuel consumption mf = _________kg/sec 9. Calorific value of fuel C.V. = 44,000 kJ/kg 10.Mass of jacket cooling water supplied mw = _________Kg/sec 11.Inlet temperature of cooling water T1 = ________K 12.Outlet temperature of cooling water T2 = _________K 13.Temperature of exhaust gas T3 = _________K 14.Ambient temperature T4 = _______K 15.Air fuel ratio = 20 16.Mass of exhaust gas mg = (1 + air fuel ratio) mf = ________kj/sec
  49. 49. 17.Specific heat of exhaust gas Cpg = 2.1 kJ/kg-K 18. ρp = 0.86 gm/cm3 19.Cpw = 4.2 kJ/kgK 20. ρw = 1000 kg/m3 CALCULATION: (1) Brake power: Brake power = 2πN |S- W| Reff 60 Heat Equivalent to brake power = __________Kj (2) Heat supplied by combustion of fuel: Heat supplied = mf × C.V. (3) Heat carried away by exhaust gases: Heat carried away by exhaust gases = mg Cpg (T3 – T4) (4) Heat carried away by cooling water: Heat carried away by cooling water = mw Cpw (T2 –T1) CONCLUSION:
  50. 50. OBSERVATION TABLE: Sr. No. Time required to pass 10 ml of diesel td sec Load W Kg Spring Scale Reading S Kg Speed N rpm Inlet Temp. of Cooling Water T1° C Outlet Temp. of Cooling Water T2°C Time required to 2 liter fla with water tw sec 1 2 3 4 Heat Suppli ed ∆Hs kW.B rake Powe r bp kW (Heat Utilis ed). ∆Hu kW.H eat Carri ed 5
  51. 51. away by coolin g water ∆Hw kW.H eat carrie d away by exha ust gas ∆Hg kW.U nacc ounte d Heat Loss ∆HL kW. RESU LT TABL E: Sr. No. 1 2 3 4 5 Avg.
  52. 52. EXPERIMENT NO. DATE: THE VALVE TIMING DIAGRAM OF A 4-STROKE AUTOMOBILE ENGINE OBJECTIVES: After Studying this practical students are able to know. (1) Ideal valve timing diagram. (2) Factor which affects ideal valve timing diagram. (3) Actual valve timing diagram. APPARATUS: 4-Stroke I.C.engine, marking pencil, feeler gauge, a device for measuring crank angles. THEORY: In 4-stroke SI engine the opening and closing of the valves, and the ignition of air-fuel mixture do not take place exactly at the dead centre positions. The valves open slightly earlier and close after their respective dead centre positions. The ignition also occurs prior, to the mixture is fully compressed, and the piston, reaches the dead centre position. A typical valve-timing diagram of a SI engine is shown in fig. Similarly, in a CI engine both valve do not open and close exactly at dead centre positions, rather operate some degrees on either side in terms of crank angles from the dead centre positions. The injection of the fuel (diesel) is also timed to occur earlier. A typical valve timing diagram of a diesel engine is shown in fig. There are two factors, on mechanical and other dynamic, for the actual valve to be different from theoretical valve timing. (A) Mechanical factor: The poppet valve of the reciprocating engines are opened and closed by cam mechanisms. The clearance between cam, tappet and valve must be slowly taken up and valve slowly lifted, at first, if the noise and wear to be avoided. For the same reason valve cannot be closed abruptly, else it will bounce on its seat. Thus the valve opening and closing periods are spreaded over a considerable number of crank shaft degrees. As a result, the opening of the valve must commence ahead of the time at which it is fully opened (before dead centre). The same reason applies for the closing time and the valve must be closed after the dead centers. (B) Dynamic factor: Besides the mechanical factor of opening and closing of valves, the actual timing is set taking into consideration the dynamic effect of the gas flow. INTAKE VALVE TIMING: Intake Valve timing has bearing on actual quantity of air sucked during the suction stroke i.e. it affects the volumetric efficiency. For both low and high speed
  53. 53. engine the intake valve opens 10o before the arrival of the piston at TDC on the exhaust stroke. This is to insure that the valve will be fully open and fresh charge starting to flow into the cylinder as soon as possible after TDC. As the piston moves out in the suction stroke, the fresh charge is drawn in through the intake valve. When the piston reaches charge tends to cause it to continue to move into the cylinder. To take advantage of this, the intake is closed after TDC so that maximum air is taken in. This called ram effect. However, if the intake valve is to remain open for too long a time beyond BDC, the up moving piston on the compression stroke would tend to force some of the charge, already in the cylinder, back into the intake manifold. The time the intake valve should remain open after TDC is decided by the speed of the engine. At low engine speed the charge speed is low and so the air inertia is low, and hence the intake valve should be close relatively early after BDC. In high-speed engines, the charge speed is high, and consequently the inertia is high and hence to induct maximum quantity of the charge due to ram effect the intake valve should be close relatively late after BDC (up to 60o after BDC). There is limit to the high speed engine for advantage of ram effect. At very high speed the effect of the fluid friction may be more than offset the advantage of ram effect and the charge for cylinder per cycle falls off. EXHAUST VALVE TIMING: The exhaust valve is set to open before BDC (say about 25o before BDC in low speed engines and 55o before BDC in high speed engines). If the exhaust valve did not start to open until BDC. The pressure in the cylinder would be considerably above the atmospheric pressure during the first portion of the exhaust stroke, increasing the work required to expel the exhaust gases. But opening of the exhaust valve earlier reduces the pressure near end of the power stroke and thus causes some loss of useful work on this stroke. How ever the overall effect of opening the valve prior to the time the position reaches BDC results in overall gain in output. The closing time of the exhaust valve effects the volumetric efficiency. By closing the exhaust valve few degrees after TDC (about 15o in case of the low speed engine and 20o in case of the high speed engines) the inertia of the gases tends to scavenge the cylinder by carrying out a greater mass of the gas left in the clearance volume. This results in increased volumetric efficiency. There may be period when both inlet and exhaust valve are open at the same time. This is called valve over-lap (15o in case of low speed engine and 30o in case of high-speed engine). This overlap should not be excessive otherwise it will allow the burned gases to be sucked into the intake manifold, or the fresh charge to escape through the exhaust valve. PROCEDURE: 1. Fix a plate on the body of the engine touching the flywheel. 2. Mark the positions of both dead center on the flywheel with reference to the fixed plate TDC and BDC in case of vertical engines, and IDC and ODC in case of horizontal engine. 3. Mark on the flywheel when the inlet and exhaust valves open and close as the flywheel is rotated slowly. 4. Measure periphery of flywheel. 5. Measure distances of valve opening closing from BDC or TDC.
  54. 54. 6. After calculations plot actual valve timing diagram. OBSERVATION: Periphery = _______cm. Sr. No. Valve timings Distance from TDC or BDC in cm Angles from TDC of BDC in degree 1. IVO 2. IVC 3. EVO 4. EVC PRECAUTIONS: 1. Marking plaZe should be fixZd properly withBthe flywheel. 2. T54e positions of he dead centers™and locations oç opening and clÿsing of valves hould be marked™carefully. ASSIGNMENT: Answer the following questions: (1) What do you mean by valve timing diagram? (2) What is mechanical factors? (3) Why inlet vafve is opened before TDC & closed after BDC? (4) Why exhaust valve ~s opened before™BDC & closed after TDC? (5) Why ignition takes plaÛe before TDC? (6) Zhat is valve ovZrlap? Write useBulness of valve overlap. What are adverse effecZs if we keep vaZve overlap more<than required? Z