UNIT IV INTERNAL COMBUSTION ENGINES

Transcription

1 UNIT IV INTERNAL COMBUSTION ENGINES Objectives After the completion of this chapter, Students 1. To know the different parts of IC engines and their functions. 2. To understand the working principle of 2 -Stroke and 4-Stroke engines and their applications. 2.1 INTRODUCTION Heat Engines are otherwise called Thermal Engines. It is a machine which converts heat energy into useful mechanical work. Heat engines develop more than 80% of energy generated in the world. They are broadly classified into two types: 1. Internal Combustion Engines 2. External Combustion Engines 2.2 TYPES OF HEAT ENGINES Heat Engines are broadly classified as Internal combustion Engines and External Combustion Engines INTERNAL COMBUSTION ENGINES: In the Internal Combustion Engine, the chemical energy of the fuel is released as a heat by the way of combustion inside the engine cylinder where power is produced. The heat produced is nothing but the products of combustion. By expansion of this hot medium inside the cylinder, heat energy is converted into useful work. The name Internal Combustion Engine is a misnomer since the fuel is burnt internally EXTERNAL COMBUSTION ENGINES: They are steam engines and steam turbines. In these, heat energy is produced during the combustion of fuel in a boiler furnace. This energy is used to produce the steam under the pressure in boiler. The steam expands in turbine and thereby does work.

2 The name External combustion engine is a misnomer since the fuel is burnt externally. 2.3 CLASSIFICATION CLASSIFICATION BASED ON IGNITION: (i) Spark Ignition Engines (S.I engines) In this type of engine, combustible mixture is sucked into the engine cylinder. This mixture is compressed. The compression ratio is about 5:1 to 7:1. At the end of compression, the mixture exists in the cylinder as high pressure and temperature. The Electric spark ignites this mixture. The burning of mixture produces greater pressure and temperature. The product of combustion expands and produced power. Then the products are expelled out. (ii) Combustion Ignition Engines (C.I Engines) In this type, air alone is sucked into the engine cylinder the air is compressed. The compression ratio is about 14:1 to 17:1. The heat of compression in the air is much greater due to high compression. At the end of compression the fuel is injected in the form of fine spray into the engine cylinder. The compression heat ignites the fuel and causes in to burn. Combustion of fuel produces high pressure and temperature. The product of combustion expands and thereby produces power. The combustion products are then exhausted. (iii) Precombustion chamber Engines The mixture is ignited by a spark in a special small anti-chamber, while the takes in the main chamber CLASSIFICATION BASED ON NUMBER OF STROKES: (i) Four stroke Engine In this engine, four strokes of the piston is required to complete a working cycle. In this engine, two revolution of the crankshaft is used to complete the cycle of operation. (ii) Two Stroke Engine

3 In this engine, two strokes of the piston is required to complete a working cycle. In this engine, one revolution of the crankshaft is used to complete the cycle of operation CLASSIFICATION BASED ON CYCLE OF OPERATION: (i) Otto cycle (ii) Diesel cycle (iii) Dual cycle CLASSIFICATION BASED ON THE TYPE OF FUEL USED: (i) Engines using Light Liquid Fuels ---- Petrol Engines (ii) Engines using Heavy Liquid Fuels ---- Diesel Engines (iii) Engines using Gaseous Fuels ---- Gas Engines (iv) Mixed Fuel Engines (v) Multi Fuel Engines CLASSIFICATION BASED ON THE MODE OF CONVERSION OF HEAT ENERGY INTO MECHANICAL WORK: (i) Reciprocating Engines: In this, heat energy gets converted into mechanical energy in the inside of engine cylinder while the piston reciprocates. (ii) Rotary Engines: In this heat energy is converted in to mechanical energy in the inside of engine cylinder while the specially shaped casing and a rotor rotates inside the casing. (iii) Gas Turbine: The heat energy in the hot gases is converted into mechanical work on the rotating blades of the gas turbine. (iv) Combination Engine: In this the heat energy is converted into mechanical work partly in the cylinder of reciprocating engine and partly in the blades of turbine.

4 2.3.6 CLASSIFICATION BASED ON THE METHOD OF MIXTURE FORMATION: (i) External Mixture Formation Engines: Used in spark ignition engines, and gas engines in which the fuel is injected into the intake pipe or intake manifold and mixes with air externally to the cylinder, i.e., in the carburetor. (ii) Internal Mixture Formation Engines: Used in diesel engines, with injection of fuel into the cylinders and in gas engines in which the gas is fed into the cylinder at the beginning of compression CLASSIFICATION BASED ON THE METHOD OF COOLING (i) Air cooled engines (ii) Water cooled engines CLASSIFICATION BASED ON THE METHOD OF GOVERNING (i) Quantity governing (ii) Quality governing (iii) Hit and miss governing CLASSIFICATION BASED ON THE ARRANGEMENT OF CYLINDERS (i) Inline Engine All cylinders are arranged in a line and the power is taken from a single crankshaft. This arrangement is used in automobiles. (ii) V-Type It is a combination of two inline engines set at an angle of V varies from 30 to 75.

5 (iii) Opposed piston engines The piston reciprocates in a common cylinder having common combustion chamber at the centre. (iv) Radial Engine All the cylinders are set along the radius of a circle. The connecting rods point towards the centre of the circle. The connecting rods of all pistons work on a single crank pin which rotates around the centre of the circle. This occupies little floor space and simplifies the balancing problems. This is popular in aircrafts. (v) Rotary Engine The engine consists of three sided converse type of piston rotating in a cylinder. This engine is known as Wankel engine. It is of high speed, lighter weight and works on spark ignition system CLASSIFICATION BASED ON THE METHOD OF CONTROL OF CHARGE UNDER VARIABLE LOAD (i) Quality control engines In which the composition of the mixture which undergoes combustion is changed by admitting more or less quantity of fuel in accordance with the variation of load. Air quantity remains almost constant. All diesel engines are quality control engines. (ii) Quantity control engines In which the composition of the mixture which remains almost constant when the load varies and the quantity of the mixture admitted is changed. All Petrol engines are quantity control engines. (iii) Combination control engines In which both quality and quantity of the mixture are varied depending on the load CLASSIFICATION BASED ON THE PURPOSE (OR) APPLICATION

6 (i) Stationary Engines (ii) Mobile Engines (iii) Aero Engines (iv) Marine Engines 2.4 COMPONENTS For effective functioning of the internal combustion engine every components of the engine has to work properly. The following components of the engine are, Components of I.C Engine Cylinder It is a cylindrical space (or) container in which piston reciprocates. The working substance contained with in the cylinder is subjected to different thermodynamics processes. The cylinder is supported in cylinder block. Piston It is a reciprocating cylinder component which is fitted in to the cylinder. The power generated by the working substance during the expansion stroke is

7 transmitted into the piston, hence it forms the first link in transmitting the gas force to crankshaft Piston Ring These piston rings are fitted into the slots around the piston, provide a tight seal between piston and cylinder wall, thus preventing leakage of combustion gases Combustion chamber It is the space enclosed in the upper part of the cylinder, below the cylinder head and above the top of the piston surface during the combustion process. The combustion of the fuel takes place with in this space. Connecting Rod The connecting rod inter connects the piston and the crank-shaft and transmits the gas forces from the piston to the crankshaft. It has two ends called small end and big end. The small end of the connecting rod is connected with piston by using a pin called gudgeon pin. The big end of the connecting rod is connected with crank pin by using a pin called crank pin. Crankshaft It converts the reciprocating motion of the piston in to useful rotary motion of the output shaft. The crankshaft is enclosed within crankcase. The crankshaft is attached with big end of the connecting rod. Spark plug It is usually mounted on the cylinder head. It is a component which initiates the combustion process in spark ignition engines. Fuel injector This component is present in the case of combustion ignition (CI) engines. This component atomizes the fuel into fine droplets, thus injecting it at correct timing, in correct proportion during the working cycle. Inlet Manifold It is a piping system which connects the intake system to the inlet opening. Air, as in the case of CI engine (or) air fuel mixture, as in the case of SI engine, will follow through the inlet manifold.

8 Inlet Valve It is mounted on the cylinder head. It is used to regulate the charge (either air or air fuel mixture) coming into the cylinder. Exhaust Manifold It is a piping system which connects the exhaust system with the exhaust (or) outlet opening. Products of combustion from the cylinder will escape into the atmosphere through this system. Exhaust Valve It is also mounted on the cylinder head. It is used to control and regulate the discharge of combustion products from the cylinder into the atmosphere. In general the exhaust valve is subjected to higher temperature and corrosive atmosphere than the intake or inlet valve. Cam Shaft The cam shaft is driven by crank shaft through timing gears having gear ratio of 2. The cam shaft is used to control the opening and closing of inlet and exhaust valves. Cam These are integral parts of the cam shaft. They are designed in such a way to open the valve at the correct timing and keep them in the same position for necessary duration and to close it. Flywheel It is mounted on the crank shaft and its function is to maintain the speed of the engine as a constant. It is done by storing excess energy during the power stroke and is utilized during remaining strokes of operation.

9 FOUR STROKE PETROL ENGINE DEFINITION: In 4 stroke there is one power stroke in every 4 strokes (or) two rows of cranks. It is also known as S.I. Engine because spark plug ignites the air fuel mixture. SUCTION: During suction stroke inlet value opens and air fuel mixture is sucked into cylinder - The piston moves from TDC to BDC - During suction stroke the exhaust value is closed COMPRESSION STROKE: Both inlet and exhaust valve are closed. The piston moves from BDC to TDC. The air fuel mixture is compressed.

10 Shortly below piston reaches TDC, the charge is ignited by means of spark plug and pressure. The combustion occurs. These two strokes complete one revolution. POWER STROKE: During this Stroke both valves remain closed. Due to rise in pressure the piston is pushed down with great force and the piston reaches BDC. his is also working stroke as the work is down by expression of hot gases. EXHAUST: During this stroke the exhaust valve opens. The movement of piston from BDC to TDC pushes out the hot exhaust gases through exhaust valve. This computes the cycle again the inlet value opens and operations are repeated. FOUR STROKE DIESEL ENGINE: Diesel engine is also knows as IC since ignitions takes place due to high temperature produced during the compression as air. The fuel is injected in the form as fine spray with help of fuel injector. Suction: During suction stroke the inlet value opens and air is down into cylinder. The piston moves from TDC to BDC The exhaust value is closed in this stroke.

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12 Compression Stroke: o Both inlet and exhaust value are closed. o The piston moves from BDC to TDC. o The air is compressed to high pressure & temperature. o Just below piston reaches TDC, the fuel ignited in the form of spray through fuel injector. o At this movement the fuel ignited and started burning and high pressure produced. Power Stroke: Exhaust: o This high pressure focuses the piston down from exhausted TDC to BDC. It is also known as working stroke because the work is done by the gases. During this strokes both values remains closed. Here the inlet valve is closed and the exhaust valve is kept opened and the waste gases are sent out through this valve only. The piston moves from BDC to TDC The crank shaft will make 2 complete revolutions. TWO STROKE PETROL - In 2 strokes engine there is one power stroke for every 2 strokes. - It will be easier to describe the cycle beginning at end as compression stroke. FIRST STROKE (Power and Exhaust): o Shows the piston at the end of compression. o Spark plug is ignited.

13 o The pressure and temperature increased and hence the gases push the piston downwards producing power stroke. o When the piston moves down the exhaust port opens and the products of combustion are exhausted to the atmosphere causing exhaust stroke. o A little later the compressed mixture of air and fuel is transferred to upper part of cylinder, and the exhaust gases are pushed out with help of compressed charge. This is known as scavenging.

14 SECOND STROKE (Suction and Compression): As the piston moves upwards it covers the transfer port. Therefore the flow of charge is stopped. The upward motion of piston lowers the pressure in the crank case below the atmosphere and fresh air fuel mixture is introduced into crank case through inlet port producing suction stroke. A little later, the piston covers the exhaust port and actual compression of charge starts causing compression stroke and the operations are repeated. Thus the cycle is completed within two strokes. TWO-STROKE DIESEL ENGINE Definition: In 2 strokes engine there is one power stroke for every 2 strokes. It will be easier to describe the cycle beginning at end as compression stroke. Introduction: The air enclosed in the cylinder and it is compressed and its temperature rises. Diesel which can t be vaporized and it is injected with the help of fuel injector. The working is similar to that of petrol engine except fuel injector is used instead 0f spark plug and air is used as inlet instead of air fuel mixture in petrol engine. FIRST STROKE (Power and Exhaust): o Shows the piston at the end of compression. o Fuel is injected from fuel injector. o The pressure and temperature increased and hence the gases push the piston downwards producing power stroke. o When the piston moves down the exhaust port opens and the products of combustion are exhausted to the atmosphere causing exhaust stroke.

15 o A little later the air is transferred to upper part of cylinder, and the exhaust gases are pushed out with help of compressed charge. This is known as scavenging. SECOND STROKE (Suction and Compression): As the piston moves upwards it covers the transfer port. Therefore the flow of charge is stopped. The upward motion of piston lowers the pressure in the crank case below the atmosphere and fresh air is introduced into crank case through inlet port producing suction stroke. A little later, the piston covers the exhaust port and actual compression of charge starts causing compression stroke and the operations are repeated. Thus the cycle is completed with in two strokes.

16 2.5 COMPARISON BETWEEN FOUR STROKE AND TWO STROKE CYLINDER ENGINES: Four Stroke Cylinder Engine 1. For every two revolution of the crank shaft, there is one power stroke. 2. Because of the above, turning moment is not so uniform and hence heavier flywheel is needed. 3. For the same power more space is required. 4. Because of one power stroke in two revolutions, lesser cooling and lubrication requires. Lower rate of wear and tear. 5. Valves are required inlet and Two Stroke Cylinder Engine 1. For every one revolution of the crank shaft, there is one power stroke. 2. Because of the above, turning moment is more uniform and hence a lighter flywheel is used. 3. For the same power less space is required. 4. Because of one power stroke for every revolution, greater cooling and lubrication requirements. Higher rate of wear and tear.

17 exhaust valves. 6. Because of heavy weight, complicated valve mechanism and water cooled, making it complicated design and difficult to maintain. 7. The air-fuel mixture is completely utilized thus efficiency is higher. 8. Volumetric efficiency is high due to more time for induction. 9. Lower fuel consumption per horse power. 10. Used in heavy vehicles, e.g. Buses, lorries, trucks etc. 11. The engine cost is more. 12. The exhaust is less noisy. 5. Ports are made in the cylinder walls inlet, exhaust, and transfer port. 6. Simple in design, light weight and air cooled and easy to maintain. 7. As inlet and exhaust port open simultaneously, some times fresh charges may escape with exhaust gases. The exhaust gases are not always completely removed. This cause lower efficiency. 8. Volumetric efficiency is low due to lesser time for induction. 9. The fuel consumption per horse power is more because of fuel dilution by the exhaust gas. 10. Used in light vehicles, e.g. Motor cycle, scooter, etc. 11. The engine cost is less. 12. The exhaust is noisy due to short time available for exhaust. 2.6 COMPARISON OF S.I. AND C.I. ENGINES: S.I. Engines 1. The fuel used is gasoline (Petrol). 2. Air + Fuel mixture is taken during suction. 3. For mixing air and fuel a separate device called carburetor is required. 4. Since homogeneous mixture is produced in carburetor, no need of injector. 5. Pressure at the end of compression is about 10 bar. 6. A spark plug is used to ignite the air fuel mixture. 7. Self ignition temperature of fuel is not attained. In other words, the fuel is not self ignited. 8. S.I. Engines works on Otto cycle (i.e.) C.I. Engines 1. Fuel used is Diesel. 2. Only air taken during suction. 3. No need of carburetor. 4. For atomizing and spraying the fuel inside the cylinder, fuel injector is necessary. 5. Pressure at the end of compression is about 35 bar. 6. Spark plug is not necessary. 7. The fuel get ignited due to the high temperature of compressed air. 8. C.I. Engines works on diesel cycle (i.e.) combustion takes place at constant

18 combustion takes place at constant volume. 9. Compression ratio is around 6 to Cold starting of engine is easy. 11. These are very lighter. 12. Cost is comparatively low. 13. Running cost is high. 14. Less maintenance. 15. η thernal is about 25%. 16. Over heating trouble is more. 17. Spark plug needs frequent maintenance. 18. These are high speed engines. 19. Noiseless operation due to less compression ratio. 20. Engine weight / kw is less. 21. Vibration is less. 22. Generally employed for light duty vehicles e.g. two wheeler, Otto etc. pressure. 9. Compression ratio is around 15 to Cold starting of engine is difficult. 11. Heavier engine. 12. Cost is high. 13. Running cost is not high. 14. High maintenance is needed. 15. η thernal is about 35 to 45%. 16. Over heating trouble is less. 17. Fuel injector needs less maintenance. 18. These are low speed engines. 19. Very noisy operation due to high compression ratio. 20. Engine weight / kw is more. 21. More vibration is there. 22. Generally employed for heavy duty vehicles e.g. trucks, buses, etc. Two Marks Questions 1. What is an engine? An engine is a device used for converting heat energy into mechanical energy by combustion of a fuel. 2. What are the types of heat engine? Internal combustion engine (I.C. Engine) External Combustion engine (E.C. Engine) 3. What is mean by S.I. Engine? Why is it called so? Petrol engine is called is as spark ignition, because the combustion of fuel takes place by means of a spark produced by the spark plug. 4. What is mean by C.I. Engine? Why it is called so?

19 Diesel is called as compression ignition engine, because the combustion takes place due to the heat produced by the compression of air-fuel mixture. 5. Spark plug is necessary to run a engine. Ans: Petrol 6. Give the main components of a petrol engine. Cylinder, cylinder head, piston, connecting rod, valves, spark plug, crank shaft, cam shaft and fly wheel. 7. Number of working strokes per minute for a four stroke cycle engines are the speed of the engine. Ans: Half. 8. A petrol engine works on cycle. Ans: Otto is used to mix fuel and air in a petrol engine. Ans: Carburetor. 10. What is a four stroke engine? In a four stroke engine, one power stroke is completed for every stokes of the piston or during two revolutions of the crank shaft. 11. Diesel engine works on the principle of cycle. Ans: Diesel cycle. 12. What is the function of a carburetor? To mix the fuel with air in correct proportion and to evaporate the fuel with fast moving air.

20 To regulate the supply of air -fuel mixture entering into the engine cylinders. 13. What is the fundamental difference between two-stroke and four-stroke engine? In four- stroke engine one power stroke is obtained in two revolutions of crank shaft where as in two- stroke engine, one power stroke is obtained in each revolution of crank shaft. 14. Why fuel is injected in a C.I. engine? The fuel used in C.I. engine cannot be vaporized and hence injected into the cylinder in the form of fine spray. 15. Define carburetor. Carburetor is a device used for mixing the air with petrol in correct proportion. 16. Mention the types of ignition systems used in petrol engine. Battery (or) coil ignition system. Magneto ignition system 17. State the function of choke in a petrol engine. Choke is provided for easy starting of the engine. It allows rich mixture into the cylinder by reducing the amount of air present in the mixture. 18. What is the function of a spark plug? Spark plug is a device used to ignite the compressed air fuel mixture by producing an electric spark. 19. The gap between the central electrode and earth electrode of a spark plug is Ans: 0.4mm

21 20. The device used to supply correct quantity of fuel into the cylinder of a diesel engine is Ans: Fuel pump 21. Define fuel injector Fuel injector is a device is device used to atomize the fuel and to deliver the fuel to the cylinder of a diesel engine in the form of fine spray. 22. What are the types of cooling systems used in I.C. engine? Air cooling. Water cooling. 23. Mention the types of water cooling on I.C. engines. Natural circulation system Forced circulation system 24. Define lubrication. Lubrication is the process of applying lubricant between the surfaces of contact of two moving parts. 25. Mention some engine parts that require frequent lubrication. Cylinder, piston and piston rings. Main bearings Crank shaft Valves etc. 26. What are the types of lubrications for I. C. engines? Mist lubrication. Wet lubrication. 27. Compression ratio of petrol engine is in the range of (a) 2 to 3 (b) 3 to 5 (c) 7 to 10 (d) 16 to 20 Ans: (c) 7 to 10

22 28. Compression ratio of diesel engine is in the rang e of (a) 8 to 10 (b) 10 to 15 (c) 16 to 20 (d) 20 and above Ans: (c) 16 t Fuel injector is used in (a) S.I. engines (b) C.I. engines (c) Gas engines. Ans: (b) C.I. engines. 30. What is mean by internal combustion engine? Internal combustion engine (I.C. engine) is a heat engine where combustion of fuel (petrol/diesel) with air takes place inside the engine cylinder. 31. What are the main parts of the internal combustion engine? The main parts of an internal combustion engine are cylinder, piston, piston rings, piston pin, crank shaft, connecting rod, crank case, valves and flywheel. 32. How I.C. engines are classified? I.C. engines are classified as spark ignition engines (S.I. engines) and compression ignition engines (C.I. engines). Another classification of I.C. engines are (1) Four stroke engine (2) two stroke engine. 33. What are the four stroke of an I.C. engine? Suction stroke, compression stroke, expansion or power stroke, working stroke and exhaust stroke. 34. What is mean by stroke in an I.C. engine? A stroke refers to the linear distance between the two extreme positions of the piston. This distance is measured parallel along the axis of the cylinder. 35. Name the two extreme positions occupied by the piston?

23 The two extreme positions occupied by the occupied by the piston in a cylinder are Top dead centre (TDC) and Bottom dead centre (BDC). 36 Name the important systems in an I.C. engine. Fuel system, ignition system, cooling system, lubricating system and air system. 37. What is the main purpose of fuel system in an I.C. engine? Storage and supply of fuel for combustion of engines is the main purpose of fuel system. 38. What are the important components of fuel system? Fuel filters, fuel tank, carburetor (for S.I.engine), fuel pump and fuel injector (for C.I. engine). 39. What is the basic function of ignition system? Ignition system is used in a spark ignition engine to ignite the air fuel mixture in the cylinder 40. What are the two types of ignition system? (1) Battery ignition system (2 ) Magneto ignition system 41. Why cooling system is necessary in an I.C. engine? Combustion of air-fuel mixture in the cylinder produces very high temperature in the range of C. This heat may cause expansion of cylinder wall, cylinder head, piston and other parts resulting in abnormal ignition. Cooling system is used to maintain the temperature within limits so that normal ignition occurs. 42. What is the purpose of lubrication system used in I.C. engines? Lubricating system is used to reduce the friction between the moving parts and thus reduces its wear and tear. It also cools and cleans the moving parts.

24 43. What is the compression ratio of an I.C. engine? The compression ratio of an I.C. engine is defined as the ratio of maximum cylinder volume to minimum cylinder volume. 44. What is meant by scavenging in I.C. engine? Scavenging refers to the process of removing burnt gases during exhaust stroke with the help of incoming charge and deflector. 45. State the purpose of flywheel. Flywheel is used to sustain the movement of the piston even during nonpower strokes.(suction, compression and exhaust strokes) 46. What is meant by TDC? TDC means top dead centre, it is the extreme position of the piston at the top end of the cylinder in a vertical engine. 47. What is meant by BDC? BDC means Bottom dead centre, it is the extreme position of the piston at the bottom of the cylinder in vertical engine. 48. What is function of I.C. engine? I.C. engine is used to produce mechanical energy by burning fuel such as petrol or diesel inside the cylinder. 49. What is the cycle used in SI engine? The cycle used in SI engine is OTTO CYCLE. 50. What is the fuel used in SI & CI engine? The fuel used in SI & CI engines are PETROL & DIESEL respectively.

25 51. What is the standard ratio of air-fuel mixture for an SI engine? The standard ratio of air-fuel mixture for SI engine is 15: What is the crank shaft revolution in 2 Stroke and 4 Stroke? 2 Stroke - one revolution. 4 Stroke - two revolutions. 53. What are the sequences in 4 Stroke engine? Suction stroke Compression stroke Power stroke Exhaust stroke 54. What is the use of piston ring? The piston is used to lubricating oil on the cylinder walls 55. What is the use of connecting rod? Connecting rod is used as the linear or reciprocating motion of the piston into the rotary motion of crankshaft. 56. What is the use of flywheel? It is used to minimize the fluctuation of energy in crankshaft 57. What is the ratio of petrol engine and diesel engine? Petrol engine- 7 to 10. Diesel engine- 15 to What is the fundamental difference between 2-stroke and 4-Stroke engine?

26 In four stroke engine one power stroke is obtained in two revolutions of crankshaft where as in two stroke engine, one power stroke is obtained in each revolution of crankshaft. 59. Write Short notes on Fly wheel? Fly wheel is a fairy large wheel mounted on the crank shaft. During the power stroke, fly wheel stores excess energy and releases it during the other strokes. Thus fly wheel ensures minimum variation in speed. 60. What is clearance volume? The volume of cylinder above the top of the piston when the piston is at TDC is called clearance volume. 61. What is gudgeon pin? The pin which is used to connect connecting rod and piston is known as gudgeon pin. 62. Define Otto cycle. Ina four stroke cycle petrol engine (also known as spark ignition engine), four strokes of the piston, namely suction, compression, expansion and exhaust take place in one cycle of operation in two revolutions of the crank shaft. This system of operation is known as Otto cycle. 63. What is Scavenging? The process of cleaning or removing the burnt exhaust gas by the incoming compressed air-fuel (petrol) mixture is known as scavenging. 1. How boilers are classified? STEAM GENERATORS

27 According to flow of water and gases Fire tube boilers Water tube boilers According to pressure Low pressure boilers High pressure boilers 2. Mention the advantages of high pressure boilers? Rate of steam production is high Steam produced at a pressure pf more than 890 bar. Superheated steam can be produced. 3. State the main function of a boiler The main function of a boiler is to evaporate water into steam at a higher pressure. 4. How modern boilers differ from olden day boilers? Give four important points. Steam is produced at high rate. Steam is produced at a higher pressure. Suitable mountings avoid the danger and provide safe operation and control. Boiler accessories fitted increases the efficiency. 5. What is the use of an economizer in a high pressure boiler? Economizer extracts the heat from the hot flue gases going out of the boiler and preheats the water that is fed to the boiler. 6. What is the difference between mountings and accessories in a boiler?

28 Boiler mountings are fitted in the boiler for safe operation and control steam generation where as boiler accessories increases the boiler efficiency of the boiler. 7. Name any two mountings of a boiler? Pressure gauge Water gauge 8. What is a boiler? Boiler is a closed vessel in which steam is generated from water by the application of heat and the pressure being higher than the atmosphere. 9. State the main components of a boiler. Shell, furnace, chimney, manhole, super heater etc. 10. What is the purpose of super heater in a high pressure boiler? Super heater is used to increase the temperature of the steam above its saturation temperature. 11. Name any two steam boiler accessories. Economizer Air preheater. 12. What do you understand by forced circulation boiler? In this type of boiler, water is circulated by a pump driven by a motor. Example: La-Mont Boiler. 13. State the different types of safety valves used in a boiler. Dead weight safety valve. Lever safety valve Spring loaded safety valve. 14. State the function of air preheater.

29 It preheats the air supplied to the combustion chamber by using the heat of the flue gases. 15. Give an example for a water tube boiler. Babcock and Wilcox boiler.