6 Multiple-Choice Questions Lead compounds were earlier used to be added to gasoline in S.1, engines so as to (a) reduce hydrocarbon emissions (b) reduce knocking (©) reduce exhaust gas temperature (2) increase power output Decrease of air-fuel ratio in SI engines results in (a) an increase NO, (b) a decrease of CO and unburnt hydrocarbon (©) an increase of CO and unburnt hydrocarbon (d) none of the above If. is the rpm, the number of power strokes per minute in a four-stroke engine is (a) 2N () 2 wn (@) an Volumetric eff ney is a measure of (a) speed of the engine (b) power of the engine (©) breathing capacity ofthe engine (d) pressure rise in the eylinder If. is the rpm, the number of power strokes per minute in a two-stroke wine is @N ) 2 (©) 2N @ 4y ‘The volumetric efficiency of a well-desig (a) 30 10.40 (b) 40 t0 60% (©) 60 10 70% (4) 75 10.90% is inthe range of If Lis the stroke and WV isthe rpm, the mean piston speed of a two-stroke engine is (@ LN () Ly2 (©) 2LN (4) none of the above Equivalence ratio is (a) Seta feta ratio (py Soichiometric fuel~air ratio stoichiometric fuel—air ratio actual fuel air ratio (o) Michiometse fuel-air ratio. gy Soichiomette a ‘actual air—fuel ratio actual air-fuel ratio ‘The compression ratio in diesel engines is of the order of (57 () 7-10 (©) 10-12 (a) 14-20 ‘The compression ratio ofan SI engine is in the range of (a) 406 (b) 6108 (©) 6010 (@) 10w 14 ‘The octane number of iso-octane is @o (b) 30 () 60 (4) 100ae ed a ee 12 13 4 16, fe 18 19, 20. 21 24, Applied Thermodynamics The ignition quality of diesel fuel is indicated by its (a) octane number (b) cetane number (©) flash point (4) fire point A simple carburetor supplies rich mixture during (a) starting () idling (e) enuisin (4) accelerating Modern carburetors provide the correct quality of air-fuel mixture during (@) starting (b) idling (©) cruising (4) all conditions Fuel is injected in four-stroke CI engine (a) atthe end of the suction stroke (b) at the end of the expansion stroke (©) atthe end of the compression stoke (@) at the end of the exhaust stroke ‘The advantage of fuel injection in SI engine is (a) low initial cost (b) Low maintenance requirement (6) increased volumetric efficiency (d) none of the above For engines operating with rich mixtures, the optimum spark timing (@) must be advanced (by must be retarded —_(c) must be at TDC (d) none of the above By decreasing the cooling water temperature in SI engines, the knocking tendency (a) increases (b) decreases (©) is not affected (4) none of the above In Cl engines knocking tendency increases with (a) inerease in compression ratio (b) nerease in inlet temperature of air (©) decrease in compression ratio (d) increasing coolant water temperature In turbocharging, the supercharger is driven by (a) a gas turbine using the exhaust gases (b) the engine itself (©) a separate electtic motor (@) none of the above For a multi-stage compressor, the polytropic efficiency is (a) the efficiency ofall stages combined together (b) the isentropic efficiency of one stage (©) constant throughout forall stages (@) adirect consequence of pressure ratio ‘When the flow of air through the compressor is parallel to the axis of the compressor, it is a (2) rotary compressor (b) reciprocating compressor (©) centrifugal compressor (4) axial flow compressor ‘The stip in a centrifugal compressor will (a) increase with inereasing number of vanes (b) decrease with increasing number of vanes (©) remain the same with increasing number of vanes (d) have none of the above Assertion A. The polytropic efficiency of a compressor decreases with increasing pressure ratio, Reasoning R_ Because more energy is required to compress air at higher pressures and temperatures. (a) Both A and R are true, but R is not the correct answer of A (b) both A and R are true and R is the correct answer of A (©) Aistrue, R is false (@) Ais false, Ris true ‘The maximum pressure rato in an actual sing-stage axial flow compressor will be approximately (@) Lis (b) 110 (Lt (@) 1:12ae ed 26. 28 29. 30. 34. 36. MatipeChoie Questions Faas The degree of reaction in a turbo compressor is defined as (a) enthalpy drop in rotor/enthalpy drop in stage _(b) enthalpy increase in the stage/enthalpy drop in rotor (©) enthalpy drop in stage’enthalpy drop in rotor (4) enthalpy tise in rotorienthalpy inerease in stage ‘The volumetric efficiency of a reciprocating compressor having a given pressure ratio decreases if the index compression (2) increases (b) decreases (©) remains constant (2) isequal to 1 For the same compression ratio, the efficiency of the Brayton eycle is (a) equal tothe Diesel eycle (b) equal to the Otto cycle (©) equal to the dual eyele (@) greater than the Diesel cycle Ifthe temperature at the turbine inlet is kept constant, the net output of a simple gas turbine plant would (a) increase with increasing pressure ratio (b) decrease with increasing pressure ratio (©) first inerease and then decrease with increasing pressure ratio (@) remain unaffected with changes in pressure ratio, ‘The specific output of a gas turbine plant ean be increased by water injection. The water should be injected between the (2) regenerator and the combustion chamber (b) combustion chamber and the turbine (c) compressor and the regenerator (@) turbine and compressor ‘The thermal efficiency ofa gas turbine cycle can be increased by (2) reheating (b) regeneration (©) intercooling (2) all the above ‘The maximum temperature in a gas turbine is approximately (a) 300°C (b) 600°C fe) 900°C (a) 120°C Assertion A.A regenerator always increases the efficiency of a gas turbine unit, Reasoning R : Because it recovers the energy which was going as a waste and saves fuel consumption, (@) Both A and R are false (b) A is false, Ris true (©) Ais true, R is false (@) Both A and R are true For an axial-flow gas turbine, the polytrapic efficiency is equal to the (a) isentropic efficiency (b) overall efficiency (©) isentropic efficiency of an infinitely small stage (@) stage efficiency Ina gas turbine unit with a regenerator, perfect regeneration means @) TT, (T=T, (@) none of the above where T, is the temperature of air coming out of the regenerator and T, is the temperature of gases leaving the turbine. Assertion A_ The output and thermal efficiency of a closed-« ceyele gas turbine, Reasoning R : Because a closed cycle gas turbine can use helium as a working fluid (2) Both A and R are true and R is the correct explanation of A (b) Both A and R are false (c) Ais false and R is true (d) Both A and R are true and R is not the correct explanation of A le gas turbine ean be greater than that of an openae ed 856A Basic and Appl Thrmadynanics 3. 38, 39. 40. 41 2 44 45, 46. 4. 48, 49. 50. For a gas turbine unit, the pressure ratio for maximum outpat is given by 0 4 @ GTO) (IT) ©) (GIT, w@) G/T) where 7, is the temperature of air at the compressor inlet and T, is the temperature atthe turbine inlet. Ina gas turbine, eeheating is done mainly to (a) increase the outlet temperature (b) reduce the size of the turbine (©) increase the power output (@) reduce the peak temperature Higher air-fuel ratio in a gas turbine would (@) increase thermal efficiency (b) inerease power output (6) decrease the outlet temperature (4) make all ofthe above ‘The maximum thrust power is obtained in a turbojet engine when (a) aireraft velocity is equal tothe jet velocity (b) aircraft velocity is twice the jet velocity (©) aircraft velocity is half the jet velocity (@) aireraft velocity isthe square root of the jet velocity Restricted burning in a solid propellant rocket is adopted when (a) a large thrust is required fora small period of time (b) a small thrust is required for a small period of time (©) a small thrust is required for a large period of time (A) a large thrust is required for a large period of time Assertion A. tis hot possible to attain 100% propulsive efficiency of a turbojet. Reasoning R Because that would require the thrust and thrust power equal to zero. (a) Both A and R are false (b) Both A and R are true (©) Ais true, but Ris false (@) Ais false and R is true Propulsive efficiency is defined as (a) thrust power/propulsive power (b) propulsive powerlthrust power (©) propulsive power x thrust power (@) none of the above ‘The maximum propulsive efficiency for maximum thrust power of a turbojet engine is (a) 50% (b) 66.7% (©) 18% (2) 100% ‘The thrust of a jet-propulsion power input ean be increased by’ (a) burning fuel after gas turbine (b) injecting water into the compressor (©) injecting ammonia into the combustion chamber (@) none of the above ‘The propeller of a turboprop engine is driven by (2) compressor (b) turbine (©) both a’ and b’ (d) none of the above Assertion A :A turbojet employs a smaller gas turbine unit Reasoning R Because aircratts fly at comparatively lower speeds. (2) Both A and R are false (b) Both A and R are true (©) Ais false, Ris true (@) Ais true, Ris false ‘The efficiency of a jet engine is higher at (a) high altitudes (b) tow altitudes (©) low speeds (@) high speeds Ina refrigeration plant, ifthe condenser temperature inereases, the power input to the compressor will (a) decrease (b) increase (©) remain the same (d) be unpredictable Ata place where there is no electricity, we can use the following system to obtain refrigeration. (2) Vapour compression (b) Vapour absorption (ec) Steam jet refrigeration (d) Air eycle refrigerationae ed sl 38, 59. 60. Multiple Choice Questions Pear In an aircraft-reftigeration system, the pressure atthe cooling turbine outlet is equal to (a) ambient pressure (b) cabin pressure (c) pressure at inlet to compressor (a) none of the above ‘Match list I and list IT and select the answer from the code given below: List List It Equipment in a reftigeration system Variation of properties A. Compressor 1, Enthalpy remains constant B. Evaporator 2. Enthalpy increases . Throttle valve 3. Enthalpy increases but pressure remains constant D. Condenser 4, Enthalpy decreases but pressure remains constant Code: 4 8B CD M3 2 1 4 m2 3 4 4 @2 3 1 4 @4 2 3 4 Ina 2-stage vapour compression system, a flash intercooler is installed between the (a) HP compressor and condenser (b) LP and HP compressor (©) Evaporator and LP compressors (@) none of the above In a water-vapour refrigeration system, a steam ejector maintains a very low pressure in the (a) condenser (b) absorber (©) flash chamber (@) none of the above Ina vapour compression system, the working fluid isa superheated vapour at inlet to (2) evaporator (b) condenser (©) compressor (2) throttle valve A good refrigerant should have a (2) large latent heat at condensing pressure (b) large latent heat at evaporator pressure (©) condensing pressure close to critical pressure (d) high eritical pressure ‘The effects of superheating the vapour in the evaporator and subcooling in the condenser (a) decrease of COP (b) inerease the COP (©) superheating increases COP, but undercooling decreases COP (d) supetheating decreases COP, but subcooling increases COP A humidification process means (@) a decrease in relative humidity (b) a decrease in specific humidity (©) am increase in specific humility (@) an inerease in temperature In an adiabatic saturation process (a) the enthalpy remains constant (b) the temperature remains constant (©) the absolute humidity remains constant (@) the relative humidity remains constant ‘When air is adiabatically saturated, the temperature attained is, (a) dew point temperature (b) dey bulb temperature (©) wet bulb temperature (@) tiple point temperatureae ed Ps ee 61 2 64. 66. 67. 68, 69. 10. Applied Thermodynamics ‘The saturation temperature at the partial pressure of water vapour in an air-water mixture is the (a) dow point temperature (b) wet bulb temperature (o) dry bulb temperature (@) adiabatic saturation temperature ‘The use of rcheating of steam in a steam-power plant (a) permits the use of higher boiler steam pressure fo increase the net power output (b) reduces the steam rate and the heat rate (©) limits the permissible quality of steam at turbine exhaust (@) accomplishes all the above The ideal regenerative steam cycle which yields an efficiency equal to that of Carnot cycle is not practicable because (2) reversible heat transfer cannot be obtained in finite time (b) heat exchanger in the turbine is mechanically not practicable (6) the moisture content of the steam in the turbine will be high (@) ofall the above reasons Ifthe number of feedwater heaters used in a steam power plant are seven, the maximum gain in cycle efficiency ‘occurs when the overall temperature rise of feedwater is about x times the difference between the condenser and boiler saturation temperature (with dry saturated steam at turbine inlet), Where 1 1 8 @ x55 ) x= (@ x55 (@) x=5 2 4 8 9 Ina steam power plant while the energy loss in the condenser is about 60%, the energy loss is nearly. (a) zero (b) 4% fe) 60% (2) 100% The overall efficiency of a steam power plant is expressed as the product of efficiencies of its components. If the boiler efficiency takes care of the energy loss in the boiler, the cycle efficiency takes care of the energy Toss in the (a) turbine (b) condenser (@) electric generator (4) the auxiliaries ‘Two Thermodynamic cycles are coupled in series where heat lost By one is absorbed by the other. Ifthe topping cycle has an efficiency of 30% and the bottoming cycle has an efficiency of 20%, the overall efficiency of the combined eyele is (a) 50% (b) 60% fe) 44% (a) 34% This cycle is applicable to a pulse jet engine: (@) Brayton cycle (b) Lenoir eyele (©) Atkinson eyele (@) Ericsson cycle A gas turbine plant operates on an ideal Brayton cycle. If the minimum temperature is 300 K and the maximum temperature is 1200 K, the eyele efficiency corresponding to maximum net work is (@) 030 () 0.40 (©) 050 (6) 075 In the expansion of steam is a turbine with friction and heat loss, the entropy decrease of steam due to heat loss happens to be equal to the entropy increase of steam due to friction. Thea, the entropy of steam at the turbine exit ‘minus that at turbine inlet is, (a) zero (b) positive (©) negative (4) more or less than zeroThe McGraw-Hill Companies Mutipe Choice Quetons Pras ANSWERS 1.) @ 32.0) 55. (b) 20) 1 (a) 38. (©) 56. (b) 3b) @ 39. (d) 57. (b) 40 @ 40. (@) 58. (¢) 5 @) @ 41. ©) 59. (a) 6) io) 42. (b) 60. (c) 7) @ 8. (@) 61. (a) 8.) @ 44. (b) 2. (a) 9. (a) (b) 45. (2) 63. (a) 10. (©) io 46, (b) 64. (0) Ww 29. (e) 4.) 65. (b) 12.) 30. (e) 48. (@) 66. (b) 1B. @) 31d) 49. (b) 67. (¢) 14. @) 32. (d) 50. (b) 68. (b) 15. (©) 33d) 51. (b) 69. (0) 16. (©) 34. (e) 52. ©) 70. (a) 17. @) 35. (@) 53. (b) 18. (b) 36. (a) 54. (0)