[su_heading]Top 100 Mechanical Engineering Interview Questions & Answer[/su_heading]
1. What is the difference between scavenging and supercharging ?
Ans: Scavenging is process of flushing out burnt gases from engine cylinder by introducing fresh air in the cylinder before exhaust stroke ends. Supercharging is the process of supplying higher mass of air by compressing the atmospheric air.
2. What are the names given to constant temperature, constant pressure, constant volume, constant internal energy, constant enthalpy, and constant entropy processes.Ans: Isothermal, isochroic, isobaric, free expression, throttling and adiabatic processes respectively.
3. In a Rankine cycle if maximum steam pressure is increased keeping steam temperature and condenser pressure same, what will happen to dryness fraction of steam after expansion ?Ans: It will decrease.
4. Why entropy change for a reversible adiabatic process is zero ?
Ans: Because there is no heat transfer in this process.
5. What are two essential conditions of perfect gas ?
Ans: It satisfies equation of state and its specific heats are constant.
6. Enthalpy and entropy are functions of one single parameter. Which is that ?
7. Why rate of condensation is higher on a polished surface compared to rusty surface ?
Ans: Polished surface promotes drop wise condensation and does not wet the surface.
8. How much resistance is offered to heat flow by drop wise condensation ?
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9. What is the relationship between COP of heating and cooling ?
Ans: COP of heating is one(unity) more than COP of cooling.
10. How much is the work done in isochoric process ?
11. When maximum discharge is obtained in nozzle ?
Ans: At the critical pressure ratio.
1. Under what condition the work done in reciprocating compressor will be least ?
Ans: It is least when compression process approaches isothermal. For this purpose, attempts are made to cool the air during compression.
13. What is the difference between stalling and surging in rotary compressions ?
Ans: Stalling is a local phenomenon and it occurs when How breaks away from the blades. Surging causes complete breakdown of flow and as such it affects the whole machine.
14. Why the electric motor of a fan with backward curved blades is never got overloaded under any condition ?
Ans: The maximum power is consumed at about 70% of maximum flow in case’of fan with backward blades. For higher flow, power consumption gets lower.
15. Why the work per kg of air flow in axial flow compressor is less compared to centrifugal compressor for same pressure ratio ?
Ans: Isentropic efficiency of axial flow compressor is higher.
16. What is the name given to portion of thermal energy to be necessarily rejected to environment ?
17. What is pitting ? How it is caused ?
Ans: Non uniform corrosion over the entire metal surface, but occuring only in small pits is called pitting. It is caused by lack of uniformity in metal.
18. What is caustic embrittlement ?
Ans: It is the actual physical change in metal that makes it extremely brittle and filled with minute cracks. It occurs particularly in the seams of rivetted joints and around the rivet holes.
19. Which impurities form hard scale and which impurities soft scale ?
Ans: Sulphates and chlorides of lime and magnesium form hard scale, and carbonates of lime and magnesium form soft scale.
20. What is the difference between hard water and soft water ?
Ans: Hard water contains excess of scale forming impurities and soft water contains very little or no scale forming substances.
21. Which two elements in feed water can cause corrosion of tubes and plates in boiler ? ‘
Ans: Acid and oxygen in feed water lead to corrosion.
22. What should be done to prevent a safety valve to stick to its seat ?
Ans: Safety valve should be blown off periodically so that no corrosion can take place on valve and valve seat.
23. Why large boilers are water tube type ?
Ans: Water tube boilers raise steam fast because of large heat transfer area and positive water circulation. Thus they respond faster to fluctuations in demand. Further single tube failure does not lead to catastrophy.
24. What type of boiler does not need a steam drum ?
Ans: Super-critical pressure boiler.
25. Why manholes in vessels are usually elliptical in shape ?
Ans: Elliptical shape has minimum area of opening and thus plate is weakened the least. Further it is very convenient to insert and take out the cover plate from elliptical opening.
26. Low water in boiler drum is unsafe because it may result in overheating of water tubes in furnace. Why it is unsafe to have high water condition in boiler drum ?
Ans: High drum level does not allow steam separation to be effective and some water can be carried over with steam which is not desirable for steam turbine.
27. Why boiler is purged everytime before starting firing of fuel ?
Ans: Purging ensures that any unburnt fuel in furnace is removed, otherwise it may lead to explosion.
28. What is the principle of mechanical refrigeration ?
Axis. A volatile liquid will boil under the proper conditions and in so doing will absorb heat from surrounding objects.
29. Why high latent heat of vaporisation is desirable in a refrigerant ?
Ans: A high latent heat of vaporisation of refrigerant results in small amount of refrigerant and thus lesser circulation system of refrigerant for same tonnage.
30. What is the critical temperature of a refrigerant ?
Ans: Critical temperature is the maximum temperature of a refrigerantrat which it can be condensed into liquid and beyond this it remains gas irrespective of pressure applied.
31. Maximum combustion temperature in gas turbines is of the order of 1100 to 10°C whereas same is around 00°C in I.C. engine ? Why ?
Ans: High temperature in I.C. engine can be tolerated because it lasts for a fraction of second but gas turbines have to face it continuously which metals can’t withstand.
32. Why efficiency of gas turbines is lower compared to I.C. engines ?
Ans: In gas turbines, 70% of the output of gas turbine is consumed by compressor. I.C. engines have much lower auxiliary consumption. Further combustion temperature of I.C. engines is much higher compared to gas turbine.
33. What do you understand by timed cylinder lubrication ?
Ans: For effective lubrication, lub oil needs to be injected between two piston rings when piston is at bottom of stroke so that piston rides in oi during upward movement. This way lot of lub oil can be saved and used properly.
34. What is IIUCR in relation to petrol engine ?
Ans: HUCR is highest useful compression ratio at which the fuel can be used in a specific test engine, under specified operating conditions, without knocking.
35. In some engines glycerine is used in place of water for cooling of engine. Why ?
Ans: Glycerine has boiling point of 90°C which increases its heat carrying capacity. Thus weight of coolant gets reduced and smaller riadiator can be used.
36. Why consumption of lubricating oil is more in two-stroke cycle petrol engine than four-stroke cycle petrol engine ?
Ans: In two-stroke engine lub oil is mixed with petrol and thus some lub oil is blown out through the exhaust valves by scavenging and charging air. There is no such wastage in four stroke petrol engine.
37. As compression ratio increases, thermal n increases. How is thermal n affected by weak and rich mixture strength ?
Ans: Thermal n is high for weak mixture and it decreases as mixture strength becomes rich.
38. How engine design needs to be changed to burn lean mixture ?
Ans: Engine to burn lean mixture uses high compression ratio and the highly turbulent move¬ment of the charge is produced by the geometry of the combustion chamber.
39. Horse power of I.C. engines can be expressed as RAC rating, SAE rating, or DIN rating. To which countries these standards belong ?
Ans: U.K., USA and Germany respectively.
40. What is the use of flash chamber in a vapour compression refrigeration cycle to improve the COP of refrigeration cycle ?
Ans: When liquid refrigerant as obtained from condenser is throttled, there are some vapours. These vapours if carried through the evaporator will not contribute to refrigerating effect. Using a flash chamber at some intermediate pressure, the flash vapour at this pressure can be bled off and fed back to the compression process. The throttling process is then carried out in stages. Similarly compression process is also done in two separate compressor stages.
41. Why pistons are usually dished at top ?
Ans: Pistons are usually hollowed at top to (i) provide greater spa’e for combustion, (ii) increase surface for flue gases to act upon, and (iii) better distribution of stresses.
42. What is the function of thermostat in cooling system of an engine ?
Ans: Thermostat ensures optimum cooling because excessive cooling decreases the overall efficiency. It allows cooling water to go to radiator beyond a predetermined temperature.
43. What are the causes of failure of boiler tubes ?
Ans: Boiler tubes, usually are made from carbon steel and are subject to (a) high rates of heat transfer,( b ). bending stresses due to uneven heating, especially at expanded or welded joints into headers or drums, © external erosion from burners and flue gas, (d) possible corrosion on the boiler side, and (e) occasional manufacturing defects.
Failure may occur due to following reasons :
(a) High thermal ratings may lead to rapid failure if the internal fluid flow is reduced for any reason. The resultant overheating leads to a failure by creep, characterised by the bulging of the tube with the eventual development of a longitudinal split.
(b ) Fatigue cracking due to bending stresses occur. These are associated with change of section and/or weld undercut, where tubes are expanded or welded into headers.
© Failure may arise due to overstressing of a reduced section of metal.
(d) Sudden failure of the boiler tube due to corrosion arises from embrittlement of the carbon steel due to interaction between atomic hydrogen from the corrosion process and the iron carbide present in the steel.
(e) Defects in tube manufacture, although far from being a regular occurrence, can be a cause of serious trouble. Lamination in boiler tubes or score marks arising from the cold drawing of tubes, give rise to premature failure and may promote corrosion at these regions.
44. What are the causes of failure of superheater tubes ?
Ans: Superheater tubes are subjected to the most severe combination of stress, temperature and corrosive environment. In addition to high-temperature strength, resistance to corrosion is also important. For example, low-alloy ferritic steel such as -1/% Cr, 1% Mo would not be used at metal temperatures above 580°C because of inadequate resistance to corrosion and oxidation over a full service life of 100,000/150,000 hr.
Failures in superheater tubes may arise from :
(a) Prior fabrication history (b ) Faulty heat treatment
© Consequences of welding (d) Overheating of the tube metal
(e) Gas-side corrosion (f) Stress corrosion (austenitic steels).
45. Why supercritical boilers use less amount of steel compared to non-supercritical boilers ?
Ans: Supercritical boilers do not head heavy drum for separation of steam from mixture of water and steam.
46. Out of electric heater and heat pump, which is economical in operation ?
Ans: Heat pump.
47. Which furnace burns low-ash fusion coal and retains most of the coal ash in the slag?
Ans: Cyclone furnace.
48. How the thickness of thermal boundary layer and thickness of hydrodynamic boundary layer related ?
Ans: Ratio of their thickness = (Prandtl number)-1/3.
49. What is the effect of friction on flow of steam through a nozzle ?
Ans: To decrease both mass flow rate and wetness of steam.
50. Why gas turbine power plant needs efficient compressor ?
Ans: Because a large portion of turbine work is eaten away by compressor and its inefficiency will affect net power output and cost of generation.
51. Why rockets using liquid hydrogen have higher specific impulse compared to liquid hydrocarbon ?
Ans: Liquid hydrogen has higher burning velocity.
52. Why axial flow compressor is preferred for gas turbines for aeroplanes ?
Ans: Because it has low frontal area.
53. What is the effect of inter cooling in gas turbines ?
Ans: It decreases thermal efficiency but increases net output.
54. Why iso-octane is chosen as reference fuel for S.I. engines and allotted 100 value for its octane number ?
Ans: Iso-octane permits highest compression without causing knocking.
55. Why thermal efficiency of I.C. engines is more than that of gas turbine plant ?
Ans: In I.C. engine maximum temperature attained is higher than in gas turbine.
56. Which are the reference fuels for knock rating of S.I. engines ?
Ans: n-heptane and ISO-octane.
57. When effect of variations in specific heats is considered then how do maximum temperature and pressure vary compared to air standard cycle ?
Ans: Temperature increases and pressure decreases.
58. Quantities like pressure, temperature, density, viscosity, etc. are independent of mass. What are these called ?
Ans: Intensive properties.
59. The amount of radiation emitted per scm per sec is called …. ?
Ans: Emissive power.
60. In convection heat transfer, if heat flux intensity is doubled then temperature
difference between solid surface and fluid will ?
Ans: Get doubled.
61. How you can define coal ?
Ans: Coal is a naturally occurring hydrocarbon that consists of the fossilised remains of buried plant debris that have undergone progressive physical and chemical alteration, called coalification, in the course of geologic time.
62. Which pollutant is major greenhouse gas and what is its effect ?
Ans: CO is major greenhouse gas and it traps the radiation of heat from the sun within earth’s atmosphere.
63. In order to increase efficiency and reduce CO emissions and other emissions, clear coal technologies are receiving major attention. What are these ?
Ans: (i) Advanced pulverised and pressurised pulverised fuel combustion.
(ii) Atmospheric fluidised bed combustion and pressurised fluidised bed combustion.
(iii) Supercritical boilers.
(iv) Integrated gasification combined cycle systems.
(v) Advanced integrated gasification, including fuel cell systems.
(vi) Magneto hydrodynamic electricity generation.
64. What are the important operational performance parameters in design of fuel firing equipment ?
Ans: Fuel flexibility, electrical load following capability, reliability, availability, and maintenance ease.
65. What is the differenc between total moisture and inherent moisture in coal ?
Ans: The moisture content of the bulk as sampled is referred to as total moisture, and that of the air dried sample is called inherent moisture.
66. Proximity analysis of coal provides data for a first, general assessment of a coal’s quality and type. What elements it reports ?
Ans: Moisture, volatile matter, ash and fixed carbon.
67. Ultimate analysis of coal is elementary analysis. What it is concerned with ?
Ans: Carbon, hydrogen, nitrogen, and sulphur in coal on a weight percentage basis.
68. Explain the difference between AFBC, BFBC, PFBC and PCFB in regard to fluidised bed technologies.
Ans: AFBC (Atmospheric fluidised bed combustion) process consists of forming a bed of inert materials like finely sized ash or ash mixed with sand, limestone (for sulphur removal), and solid fuel particles in a combustor and fluidising it by forcing combustion air up through the bed mixture. The gas flows thorugh bed without disturbing particles significantly but gas velocity is high enough to support the total weight of bed (fluidisation). At slightly higher velocity excess gas passes through the bed as bubbles (fluidised bed) and gives the bed the appearance of a boiling liquid.
Bubbling fluidised bed combustion (BFBC) has a defined height of bed material and operates at or near atmospheric pressure in the furnace.
Pressurised fluidised bed combustion (PFBC) system operates the bed at elevated pressure. Exhaust gases have sufficient energy to power a gas turbine, of course, gases need to be cleaned.
In fluidised combustion, as ash is removed some unburned carbon is also removed resulting in lower efficiency. In circulating fluidised bed combustion (CFBC) system, bed is operated at higher pressure leading to high heat transfer, higher combustion efficiency, and better fuel feed. Circulating fluidised beds operate with relatively high gas velocities and fine particle sizes. The maintenance of steady state conditions in a fast fluidised bed requires the continuous recycle of particles removed by the gas stream (circulating bed). The term circulating bed is often used to include fluidised bed sys¬tems containing multiple conventional bubbling beds between which bed material is exchanged.
69. What for Schmidt plot for is used in heat transfer problems ?
Ans: Schmidt plot is a graphical method for determining the temperature at any point in a body at a specified time during the transient heating or cooling period.
70. In which reactor the coolant and moderator are the same ?
Ans: Pressurised water reactor.
71. Which reactor has no moderator ?
Ans: Fast breeder reactor.
72. What are thermal neutrons ?
Ans: Thermal neutrons are slow neutrons (having energy below 1 eV) which are in thermal equilibrium with their surroundings.
73. What is big advantage of fast breeder reactor ?
Ans: It has rapid self breeding of fissile fuel during the operation of the reactor, and thus, it offers about sixty times the output with same natural uranium resources through ordinary non-breeder nuclear reactor.
74. What is the purpose of biological shield in nuclear plants ?
Ans: Biological shield of heavy concrete prevents exposure to neutrons, beta rays and gamma rays which kill living things.
75. Which two elements have same percentage in proximate and ultimate analysis of coal?
Ans: Moisture and ash.
76. On which analysis is based the Dulong’s formula for the heating value of fuel ?
Ans: On ultimate analysis.
77. Which element causes difference in higher and lower heating values of fuel ?
78. Which heating value is indicated by a calorimeter and why ?
Ans: Gross heating value because steam is condensed and heat of vapour formed is recovered.
79. State the difference between ultimate and proximate analysis of coal ?
Ans: In ultimate analysis, chemical determination of following elements is made by weight: Fixed and combined carbon, H, O, N, S, water and ash. Heating value is due to C, H and S.
In proximate analysis following constituents are mechanically determined by weight. Moisture, volatile matter, fixed carbon and ash. Heating value is due to fixed carbon and volatile matter.
80. What is fuel ratio ?
Ans: Fuel ratio is the ratio of its % age of fixed carbon to volatile matter.
81. How the analyses and calorific values of fuels can be reported ?
Ans: It may be reported as
(a) as received or fired (wet) basis
(b ) dry or moisture free basis
© combustible or ash and moisture free basis
82. What is the difference between nuclear fission and fission chain reaction.
Ans: The process of splitting of nucleus into two almost equal fragments accompanied by re¬lease of heat is nuclear fission. Self sustained, continuing, sequence of fission reactions in a con¬trolled manner is fission chain reaction.
83. Explain difference between fissile and fertile materials.
Ans: The materials which can give nuclear fission e.g. U 35, Pu 39, U 33 are fissile materi¬als. Fertile material itself is not fissionable, but it can be converted to a fissionable material by irradiation of neutrons in a nuclear reactor.
84. What do you understand by fuel cycle in nuclear plants ?
Ans: Fuel cycle a series of sequential steps involved in supplying fuel to a nuclear power reactor. The steps include : Mining, refining uranium, fabrication of fuel elements, their use in nuclear reactor, chemical processing to recover remaining fissionable material, re-enrichment of fuel from recovered material, refabrication of new fuel elements, waste storage etc.
85. What is heavy water and what is its use in nuclear plants ?
Ans: Water containing heavy isotopes of hydrogen (Deuterium) is known as heavy water. Heavy water is used as a moderator. Heavy water has low cross section for absorption of neutrons than ordinary water. Heavy water slows down the fast neutrons and thus moderates the chain reaction.
86. What is a converter reactor ?
Ans: A reactor plant which is designed to produce more fuel than it consumes. The breeding is obtained by converting fertile material to fissile material.
87. Explain nuclear reactor in brief.
Ans: A plant which initiates, sustains, controls and maintains nuclear fission chain reaction and provides shielding against radioactive radiation is nuclear reactor.
88. What is the difference between conversion and enrichment ?
Ans: The process of converting the non fissile U 38 to fissile U-35 is also called “Conversion”. The material like U 38 which can be converted to a fissile material by the neutron flux is called “fertile material”. The conversion is obtained within the nuclear reactor during the chain reaction.
Enrichment is the process by which the proportion of fissile uranium isotope (U-35) is increased above 0.7% (original % in natural uranium).
The concentration of U-35 in the uranium hexafluoride is increased from the 0.7% in natural uranium to to 4%. This is called enrichment and is accomplished in an enrichment plant.
89. Disposal of radioactive waste materials and spent fuel is a major and important technology. How the waste radioactive material is disposed off ?
Ans: Nonusable fission products are radioactive and take short/medium/long time for radioactive decay to reach safe level of radioactivity. Accordingly three methods of disposal are :
(a) Zero or low radioactivity material is dispersed or stored without elaborate shielding.
(b ) Medium radioactivity material is stored for short duration of about 5 years to allow decay of radioactivity.
© High radioactive material. They are stored in water for several months to permit radioactive decay to an accepetable low level.
90. Which nuclear reactor uses water as a coolant, moderator and reflector ?
Ans: Pressurised water reactor.
91. Which reactor produces more fissionable material than it consumes ?
Ans: Breeder reactor.
92. Which reactor uses natural uranium as fuel ?
Ans: Gas cooled reacator.
93. Which reactor uses heavy water as moderator ?
94. Which reactor requires no moderator ?
Ans: Breeder reactor.
95. Which reactor uses primary coolant as fluoride salts of lithium, beryllium, thorium and uranium ?
Ans: Molten salt breeder reactor.
96. Why an increase in area is required to produce an increase of velocity in case of supersonic flow ?
Ans: Increase in area for increase in velocity for supersonic flow is required because the density decreases faster than velocity increases at supersonic speeds and to maintain continuity of mass, area must increase.
97. Under what circumstances would there be an increase in pressure in a diver¬gent nozzle ?
Ans: For subsonic flow at inlet section of a diffuser a lower velocity and higher pressure will exist at the exit section. For supersonic isentropic flow at the inlet section a higher velocity and lower pressure will exist at the exit but if a shock wave occurs in the diffuser then a higher pressure will exist at the exit.
98. Why water can’t be used as refrigerant for small refrigerating equipment ?
Ans: The refrigerant should be such that vapour volume is low so that pumping work will be low. Water vapour volume is around 4000 times compared to R- for a given mass.
99. Which parameter remains constant in a throttling process ?
100. What is the difference between isentropic process and throttlinglprocess ?
Ans: In isentropic process, heat transfer takes place and in throttling process, enthalpy before and after the process is same.
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