IES Mechanical Engineering topics Thermodynamics, I. C.Engines .. objective type questions and answers. UPSC IES Mechanical Engineering Part -I Objective type questions. UPSC Indian Engineering services (IES) 2012-13. UPSC (IES) model question papers.
Part I -Conventional Paper (Objective questions)
Thermodynamics : Thermodynamics deals with relations between heat, work and other forms of energy. It predicts many relationships between properties of matter and efficiencies of various processes.
Macroscopic & Microscopic approach of Thermodynamics : While macroscopic is concerned with certain quantity of matter and its behavior, microscopic approach deals with molecular and atomic behaviour of matter and energy.
Thermodynamic Systems : System is a definite quantity of matter bounded by some closed surface (boundary) which is impervious to the flow of matter. Thermodynamic system is defined as any prescribed and identifiable collection of matter upon which attention is focused for investigation. Everything else other than system i.e., the space and matter external to a system is known as surroundings. In the closed system there is no interchange of matter between system and surroundings but energy can cross the boundaries and the boundary itself may move or change its position. However in the open system interchange of both matter and energy takes place. Isolated system is one in which neither mass nor energy cross the boundaries and it is not influenced by the surroundings.
Thermodynamic properties : Properties of system identify the state of system. Property of the system depends solely upon the state of the system and not upon how that state has been reached. A quantity will be the property of the system if it has exact differential. Extensive properties are those whose value , for the entire system, equals the sum of their values for the parts of the system, like total volume, total energy etc.; and the ratio of extensive property to the mass is called the average specific value of that property. The properties whose value is not equal to sum of their values for parts of system are known as intensive properties, like temperature, pressure, density, etc. An intensive property of the system does not depend on the mass of the system whereas extensive property does. Point function is known as property of the system. Path function depends on the process, ie. path followed by the process, like work. However work done in an adiabatic process between a given pair of end states depends only on the end states and not on the particular adiabatic process.
1. A quantity of matter of fixed mass and Identity which is bounded by a closed surface
Ans : System
2. An enclosure which permits thermal interaction
Ans : diathermic
3. An enclosure which prevents thermal interaction
Ans : adiabatic
4. A system in which all measurable variables have the same value as they had inside an enclosure after a long time, irrespective of the interaction that may have taken place.
Ans : equilibrium state
5. A region of interest, that is involved in an analysis.
Ans : System
6. Any Change of state occurring in a system
Ans : process
7. A region in space or control volume or quantity of matter upon which attention is focused for study
Ans : thermodynamic system
8. A change of state which occurs while the system is adiabatically enclosed.
Ans : adiabatic process
9. Any observable characteristic of the system
Ans : Property
10. A type of reversible process, characterized by the fact that the system is at each instant arbitrarily close to equilibrium.
Ans : quasistatic
11. A study of the transfer and conversion of energy
Ans : thermodynamics
12. If a thermodynamic system undergoes an adiabatic process, the net amount of work performed by it depends only on its initial and final states, and not on the sequence of intermediate state or path
Ans :first law of thermodynamics
13. Depends solely upon the state of the system not upon how that state was reached.
Ans : property
14. A change in the state of a system which occurs without any work being done.
Ans :free motion
15. It is impossible to construct a device which, working in a complete cycle, will produce no other effect than the transfer of a quantity to heat from a cooler to a hotter body.
Ans :clausius statement
16. Two states of two systems characterized by an absence of heat flow even when there is no adiabatic wall between them.
Ans : thermal equilibrium
17. The loci of points corresponding to states of the same temperature
Ans : isotherm
18. A system going through some process whose initial and final states are the same.
Ans : Cycle
19. A system which exchanges heat and work with its surroundings while operating in a cyclic process
Ans : thermodynamic machine
20. A hypothetical machine whose operation would violate the laws of thermodynamics
Ans : perpetual -motion machine
21. A property of the system dependent on the mass of the system
Ans :extensive property
22. A cycle consisting of two pieces of isotherms joined by two pieces of reversible adiabatics
Ans :Carnot cycle
23. A property of a system whose value does not depend upon the mass of the system
Ans : intensive property
24. A system in which energy may cross its boundaries, but not the mass.
Ans : closed
25. A system envelope which permits both mass and energy to cross its boundaries
26. A thermodynamic system which is not influenced by the surrounding.
Ans : isolated system
27. An open system in which the rate of passage of mass and energy does not change with time.
Ans : steady flow
28. A process in which no heat transfer takes place between the surroundings and the system but work transfer takes place.
Ans : adiabatic process
29. A process in which fluid expands without doing any work.
Ans : throttling
30. A system in mechanical, thermal and chemical equilibrium
Ans : thermodynamic equilibrium
31. A series of states through which a thermodynamic system passes.
Ans : path
32. A process in which all thermodynamic properties are the same at the start and at the end of the process.
Ans : cyclic process
33. A process carried out such that at every instant the system departs only infinitesimally from the thermodynamic equilibrium state.
Ans : quasi - static process
34. If two systems are both in thermal equilibrium with a third system, they are in thermal equilibrium with each other.
Ans : zeroth law of thermodynamics
35. In thermodynamics, it is defined as the sole effect external to the system that can be reduced to the rise of a weight.
Ans : work
36. The product of the Characteristic gas constant and the molecular weight of an ideal gas.
Ans : universal gas constant
37. the molecular weights of all the perfect gases occupy the same volume under the same conditions of pressure and temperature.
Ans : Avagradro's hypothesis
38. A property of a system, the change in whose value is the algebraic sum of the heat supply and the work done during any change in state.
Ans : internal energy
39. Change of state taking place without any heat transfer.
Ans : adiabatically
40. In an isolated system, the energy of the system remains constant.
Ans : law of conservation of energy
41. A machine that will give continuous work without receiving any energy from other system or surroundings.
Ans : perpetual motion machine
42. A process in which the heat transfer is equal to change in enthalpy.
Ans : isobaric process
43. The pressure value product is a constant for a perfect gas.
Ans : isothermal process
44. In a process following pvn = C, if n = 0, 1 and µ means the process is - - - - -- - -
Ans : constant pressure, constant volume, and constant temperature
45. The work done in constant volume process is - - - - - - - - - - -
46. As per ----------------, heat transfer in a process is equal to work transfer.
Ans : Joule
47. Internal energy of an ideal gas is a function of - - - - - - - - - -
Ans : temperature
48. Internal energy change of an ideal gas is proportional to ---------------
Ans : Cv
49. --------------------cycle provides concept of maximizing work output between two temperature limits.
Ans : Carnot
50. Carnot cycle is independent of ----------------------
Ans : working substance
51. All reversible engines operating between two fixed temperatures have -------- efficiency.
Ans : same
52. A Carnot cycle operating between temperature limits of 800oK and 400oK will have efficiency of 50 % when the working substance is -----------
Ans : any
53. If a system in a given state undergoes a reversible process till its state is in thermodynamic equilibrium with the surrounding, then work done by the system is maximum.
Ans : Availability of system
54. Because of the diverging nature of constant pressure lines, the value of reheat factor is -----------
Ans : greater than 1
55. The quantity H-TS is termed as -----------------
Ans :Gibbs function
56. The quantity U-TS is termed as ------------------
Ans : Helmholtz function
57. Each unique condition of the system.
58. Homogeneous composition of the matter in a particular state.
Ans : phase
59. A series of processes that begin and end at the same state of working fluid.
Ans :thermodynamic cycle
60. On which scales absolute zero is - 273 and -460.
Ans : Celsius and Fahrenheit
61. On which scale the boiling point of water is 373.16 and 672.
Ans : Kelvin and Rankine
62. A device that transfers heat from a body at a lower temperature to a body at higher temperature.
Ans : heat pump
63. Friction, throttling, heat flow, mixing of different fluids are examples of ---------------
Ans : irreversibility
64. In -------- process, system tends to approach the state of equilibrium with its surroundings.
65. All spontaneous processes are ------------
I. C. Engines
In internal combustion engines, the combustion takes place inside the engine whereas in stem engines the combustion takes place outside the engine.
The internal combustion engine possesses certain advantages over steam engine.
(i) I.C. engine is more compact unit as it does not involve several cumbersome auxiliaries required in steam engine.
(ii) The efficiency of I.C. engine is of the order of 35% and the steam engine is about 15%. This is because of the fact that very high temperature can be achieved in I.C. engines.
(iii) I.C. engines are mostly single acting, thereby avoiding the necessity of leak proof stuffing box, piston rod, D-slide valve etc.
(iv) As combustion takes place inside the cylinder, therefore overall size is reduced and these become portable. In stem engines, most of the area is occupied by boiler for producing steam.
Efficiency of Air Standard Otto Cycle
From the above expression it is obvious that greater the compression ratio, higher will be the air standard cycle efficiency.
In actual practice, petrol engine cycle is based on otto cycle. In petrol engine, the fuel is sent along with air in suction stroke. So if we try to achieve very high compression ratio, the fuel mixture will automatically ignite without getting spark. This auto ignition produces very high pressure which may burst piston. This phenomenon is called detonation. Also the combustion of the fuel will take place during compression stroke in case the high compression ratio is used. This is because the ignition temperature of petrol is low. But if combustion takes place during compression stoke, there will be loss of power, rather than increase in efficiency. Therefore high compression ratios can't be used in petrol engines. In case of otto engines the compression ratio is of the order of 4 to 8.
Efficient of Ideal Diesel Cycle
Here for all values of σ, expression
therefore for same compression ratio, n otto > n diesel. But in case of diesel engine high compression ratio can be achieved, as fuel is injected at the end of compression stoke.
Engines working on ottto cycle use petrol as fuel and are known as spark ignition (S.I.) engines, and those working on diesel cycle use diesel oil and are known as compression ignition (C.I.) engines.
Dual combustion cycle . It is a combination of Otto and Diesel cycle and hence also known as mixed or composite cycle. Engines working on this cycle are called compression ignition engines.
Injection and ignition of fuel. Fue! in I.C. engines may be injected by (a) air injection or (b) airless or solid injection. Fuel may be ignited by (a) electric spark, (b) compression ignition, (c) hot tube ignition, or (d) hot combustion chamber.
1. The constant volume process in I.C. engines is characteristic of the ---------
Ans : Spark ignition