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ESE - 2018 PRELIMS EXAMINATION Questions with Detailed Solutions

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UPSC Engineering Services - 2018 (Prelims) Mechanical Engineering [SET - A] 01.

A 150 mm diameter shaft rotates at 1500 r.p.m. within a 200 mm long journal bearing with 150.5 mm internal diameter. The uniform annular space between the shaft and the bearing is filled with oil of dynamic viscosity 0.8 poise. The shear stress on the shaft will be (a) 1.77 kN/m2

01.

(c) 3.77 kN/m2

(d) 4.77 kN/m2

Ans: (c)

Sol:   

02.

(b) 2.77 kN/m2

r  du  i ro  ri  dy

 2  1500  0.08  0.075    60  = 3.768 kN/m2   3 0.25  10

Which one of the following substances has constant specific heat at all pressures and temperatures?

02.

(a) Mono-atomic gas

(b) Di-atomic gas

(c) Tri-atomic gas

(d) Poly-atomic gas

Ans: (a)

Sol: Prediction of kinetic theory for ideal gases agrees very well with experimental results for real

monatomic gases. 03.

The shear stress 0 for steady, fully developed flow inside a uniform horizontal pipe with coefficient of friction f, density  and velocity , is given by f 2 (a) 2

03.

f 2 (b) 2

2 (c) 2f

 2 (d) 2f

Ans: (a)

Sol: The shear stress at wall (0) is given by

o 

f V 2 2

Where, f = Fanning friction coefficient. The above equation is definition of Fanning friction coefficient. ACE Engineering Academy

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04.









: 2 :

ESE‐2018 PRELIMS SOLUTIONS

The total energy of each particle at various places in the case of a perfect incompressible fluid flowing in a continuous stream

04.

(a) keeps on increasing

(b) keeps on decreasing

(c) remains constant

(d) may increase or decrease

Ans: (c)

Sol: As per Bernoulli’s equation the total energy of a fluid particle remains constant along streamline

for ideal flow. 05.

The normal stresses within an isotropic Newtonian fluid are related to 1. pressure 2. viscosity of fluid 3. velocity gradient Which of the above are correct?

05.

(a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

Ans: (d)

Sol: The normal stress for an isotropic viscous fluid in x direction is given by

 xx  P  2

06.

u x

Which one of the following regimes of boiling curve can be considered as reverse of condensation?

06.

(a) Free convection boiling regime

(b) Nucleate boiling regime

(c) Transition boiling regime

(d) Film boiling regime

Ans: (b)

Sol: Formation of bubble starts in nucleate boiling and thus it is reverse of condensation.

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07.









: 3 :

MECHANICAL ENGINEERING [SET‐A]

The service pump in a water supply system has to maintain a net static head lift of 5 m at the tank to which it delivers freely through a 4 km long pipe, wherein all minor losses can be neglected. The diameter of the pipe is 0.2 m and its friction factor f= 0.01. The pumped water is discharged at 2 m/s. The absolute pressure differential developed by the pump is nearly (taking atmospheric pressure as 10.3 m of water) (a) 4.5 bar

07.

(b) 5.5 bar

(c) 45 bar

(d) 55 bar

Ans: (a)

Sol: The pressure difference developed by the pump depends on the manometric head produced by the

pump. H m  H st  h fs  h fd 

Hm  5 

V2 (Minor losses can be neglected) 2g

0.01 4000  2 2 22  = 45.2 m 2  10  0.2 2  10

The pressure rise in given by P = gHm = 1000  10  45.2 = 4.52  105 = 4.52 bar 08.

A wall surface of 200 mm thickness has an outside temperature of 50C and inside temperature of 25C with thermal conductivity of 0.51 W/m.K. The heat transfer through this wall will be (a) 63.75 W/m2

08.

(b) 65.75 W/m2

(c) 70.25 W/m2

(d) 73.25 W/m2

Ans: (a)

Sol: Heat flux (q) =

=

T1  T2  L / k  k T1  T2 

Outside T2 = 50C

L

=

0.51 50  25 0.51 25 = 0.2 0.2

=

5.1 25 127.5 = = 63.75 W/m2 2 2

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Wall

k=0.51 W/m.K

Inside

T2 = 25C

L=0.2m

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09.









: 4 :

ESE‐2018 PRELIMS SOLUTIONS

The necessary and sufficient condition for bodies in flotation to be in stable equilibrium is that the centre of gravity is located below the

09.

(a) metacentre

(b) centre of buoyancy

(c) epicentre

(d) centroid

Ans: (a)

Sol: For the stability of floating objects metacentre must be above centre of gravity i.e., centre of

gravity must be below metacentre. 10.

When the valve of an evacuated bottle is opened, the atmospheric air rushes into it. If the atmospheric pressure is 101.325 kPa and 0.6 m3 of air enters into the bottle, then the work done by the air will be (a) 80.8 kJ

10.

Ans: (c)

Sol:

1 W2

(b) 70.8 kJ

(c) 60.8 kJ

(d) 50.8 kJ

 Pdv

= 101.3250.6 = 60.8 kJ 11.

A thermodynamic cycle is composed of four processes. The heat added and the work done in each process are as follows: Process

Heat transfer (J)

Work done (J)

1-2

0

50 (by the gas)

2-3

50 (from the gas)

0

3-4

0

20 (on the gas)

4-1

80 (to the gas)

0

The thermal efficiency of the cycle is (a) 20.3%

ACE Engineering Academy

(b) 37.5%

(c) 40.3%

(d) 62.5%

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11.









: 5 :

MECHANICAL ENGINEERING [SET‐A]

Ans: (b)

Sol: Wnet = WE – WC = 50 – 20 = 30 kJ

s = 80 kJ 

12.

Wnet = Qs

30 80

= 0.375 (or) 37.5%

A steel tank placed in hot environment contains 5 kg of air at 4 atm at 30C. A portion of the air is released till the pressure becomes 2 atm. Later, the temperature of the air in the tank is found to be 150C. The quantity of air allowed to escape is (a) 4.72 kg

12.

(b) 4.12 kg

(c) 3.71 kg

(d) 3.21 kg

Ans: (d)

Sol: P = 4101.325 kPa

m = 5 kg T = 273 + 30 = 303 K R = 0.287 kJ/kg.K V

mRT 5  0.287  303  P 4  101.325

V = 1.0728 m3 P1 = 2101.325 kPa R = 0.287 kJ/kg.K m1 

P1V 2  101.325  1.0728 = = 1.7907 kg RT1 0.287  423

Amount of gas escaped = m2 – m1 = 5 – 1.7907 = 3.2093 kg = 3.21 kgs

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: 6 :

ESE‐2018 PRELIMS SOLUTIONS

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13.

Consider the following statements:









: 7 :

MECHANICAL ENGINEERING [SET‐A]

1. Entropy is related to the first law of thermodynamics. 2. The internal energy of an ideal gas is a function of temperature and pressure. 3. The zeroth law of thermodynamics is the basis for measurement of temperature. Which of the above statements are correct?

13.

(a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

Ans: (c)

Sol: Entropy is related to second law hence statement (1) is wrong. By elimination method answer is

(c). 14.

A heat reservoir is maintained at 927C. If the ambient temperature is 27C, the availability of heat from the reservoir is limited to (a) 57 %

14.

(c) 75 %

(d) 88 %

Ans: (c)

Sol:  

15.

(b) 66 %

T1  T2 927  27 = 0.75 = T1 927  273

The ordinate and abscissa of the diagram to depict and isobaric processes of an ideal gas as a hyperbola are, respectively (a) temperature and entropy (b) internal energy and volume (c) temperature and density (d) enthalpy and entropy

15.

Ans: (c)

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16.









: 8 :

ESE‐2018 PRELIMS SOLUTIONS

Consider the following statements: 1.

The entropy of a pure crystalline substance at absolute zero temperature is zero.

2.

The efficiency of a reversible heat engine is independent of the nature of the working substance and depends only on the temperature of the reservoirs between which it operates.

3.

Carnot's theorem states that of all heat engines operating between a given constant temperature source and a given constant temperature sink, none has a higher efficiency than a reversible engine.

Which of the above statements are correct? (a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

16.

Ans: (d)

17.

An engine works on the basis of Carnot cycle operating between temperatures of 800 K and 400 K. If the heat supplied is 100 kW, the output is (a) 50 kW

17.

(b) 60 kW

(c) 70 kW

(d) 80 kW

Ans: (a)

T T   800  400  Sol: W  Q 1 2  = 100    = 50 kW  800   T1 

18.

The coefficient of performance of a heat pump working on reversed Carnot cycle is 6. If this machine works as a refrigerator with work input of 10 kW, the refrigerating effect will be (a) 35 kW

(b) 40 kW

18.

Ans: (d)

Sol:

COP R  Q2 W

5

Q2 10

(c) 45 kW

(d) 50 kW

COP HP  COP R  1 COP R  6  1  5

Q2 = 50 kW ACE Engineering Academy

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19.









: 9 :

MECHANICAL ENGINEERING [SET‐A]

Which of the following devices complies with the Clausius statement of the second law of thermodynamics? (a) Closed-cycle gas turbine

(b) Internal combustion engine

(c) Steam power plant

(d) Domestic refrigerator

19.

Ans: (d)

20.

A reversible Carnot engine operates between 27C and 1527C, and produces 400 kW of net power. The change of entropy of the working fluid during the heat addition process is

20.

(a) 0.222 kW/K

(b) 0.266 kW/K

(c) 0.288 kW/K

(d) 0.299 kW/K

Ans: (b)

Sol:  

W T1  T2  Q T1

W 1527  27 1500   = 0.833 Q 1527  273 1800 400  0.833 Q Q = 480 kJ ds 

21.

Q 480  = 0.266 T 1800

A system absorbs 100 kJ as heat and does 60 kJ work along the path 1-2-3. The same system does 20 kJ work along the path 1-4-3. The heat absorbed during the path 1-4-3 is P 2

3

1

4 V

(a) –140 kJ ACE Engineering Academy

(b) –80 kJ

(c) 80 kJ

(d) 60 kJ

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21.









: 10 :

ESE‐2018 PRELIMS SOLUTIONS

Ans: (d)

Sol: 1-2-3: 1 Q3

1 W3 1 U 3

P 2

3

1

4

100 – 60 = 1 U 3 U3 – U1 = 40 kJ 1-4-3:

22.

V

1 Q3

1 W3 1 U 3

1 Q3

 20  40

1 Q3

 20  40 = 60 kJ

Two reversible engines are connected in series between a heat source and a sink. The efficiencies of these engines are 60% and 50% respectively. If these two engines are replaced by a single reversible engine, the efficiency of this engine will be (a) 60 %

22.

(b) 70 %

(c) 80 %

(d) 90 %

Ans: (c)

Sol: o = 1 + 2 –1. 2

= 0.6 + 0.5 – 0.6  0.5 = 1.1 – 0.3 = 0.8 or 80 %. 23.

Consider the following statements for the air-standard efficiency of Diesel cycle: 1. For the same compression ratio, the efficiency decreases with increasing cutoff-ratios. 2. For the same compression ratio and same heat input, Diesel cycle is more efficient than Otto cycle. 3. For constant maximum pressure and constant heat input, Diesel cycle is more efficient than Otto cycle. Which of the above statements are correct? (a) 1, 2 and 3

23.

(b) 1 and 2 only

(c) 1 and 3 only

(d) 2 and 3 only

Ans: (c)

Sol: Same compression ratio and same heat input diesel is less efficient than Otto. Hence statement (2)

is wrong by elimination method. ACE Engineering Academy

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24.









: 11 :

MECHANICAL ENGINEERING [SET‐A]

In case of a thin cylindrical shell, subjected to an internal fluid pressure, the volumetric strain is equal to (a) circumferential strain plus longitudinal strain (b) circumferential strain plus twice the longitudinal strain (c) twice the circumferential strain plus longitudinal strain (d) twice the circumferential strain plus twice the longitudinal strain

24. Sol:

Ans: (c)

Volumetric strain of a cylinder having length ‘L’ and diameter ‘d’ is given by, ev  =

L d d   L d d L d 2 L d

e v   L  2 D   L  2 c where,

L = Longitudinal strain c = Circumferential strain

25.

The refractory lining of a furnace has a thickness of 200 mm. The average thermal conductivity of the refractory material is 0.04 W/m.K. The heat loss is estimated to be 180 kJ/hr/m2. The temperature difference across the lining will be (a) 280C

25.

(b) 250C

(c) 240C

(d) 220C

Ans: (b)

Sol:

Refractory lining T1 T2

q =180 kJ/hr.m2 k=0.04W/m.K

L=0.2 m

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: 12 :

ESE‐2018 PRELIMS SOLUTIONS

180 1000 J / s  m2 3600

Heat flux, (q)  q = 50 W/m2 Heat flux (q) =

T1  T2  = T  T  k 1 2 L / k  L

50  T1  T2  

0.04 0.2

T1  T2 

50  20 4

T1 – T2 = 250C 26.

In forced convection, the surface heat transfer coefficient from a heated flat plate is a function of (a) Re and Gr

(b) Pr and Gr

(c) Re and Pr

(d) Re, Gr and Pr

[where Re is Reynolds number, Pr is Prandtl number and Gr is Grashof number]. 26.

Ans: (c)

Sol: In forced convection,

Nusselt Number (Nu) = f(Re.Pr) Nu 

hLc k

 k  h  f  Re, Pr,  Lc  

27.

It is desired to increase the heat dissipation rate from the surface of an electronic device of spherical shape of 5 mm radius exposed to convection with h = 10 W/m2K by encasing it in a spherical sheath of conductivity 0.04 W/m.K. For maximum heat flow, the critical diameter of the sheath shall be (a) 20 mm

(b) 18 mm

(c) 16 mm

(d) 12 mm

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27.









: 13 :

MECHANICAL ENGINEERING [SET‐A]

Ans: (c)

Sol: Spherical sheath (kins = 0.04 W/m.K)

r = 5 mm ho = 10 W/m2.K

Sphere

Critical radius of insulation = =

2K ins ho 2  0.04 = 0.008 m = 8 mm 10

Critical diameter = 2rc = 28 = 16 mm q qmax

rc=8 mm

28.

28.

r

If the intake air temperature of an IC engine increases, its efficiency will (a) remain same

(b) decrease

(c) increase

(d) remain unpredictable

Ans: (b)

Sol: As mentioned intake air temperature increases density decreases and volumetric efficiency

decreases. Power output reduces as a result thermal efficiency decreases. 29.

In a counterflow heat exchanger, hot gases enter the system at 200C and leave at 80C. The temperature of the outside air entering the unit is 35C. Its temperature at the exit at 90C. The heat exchanger has an effectiveness of (a) 0.35

(b) 0.34

(c) 0.33

(d) 0.32

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29.









: 14 :

ESE‐2018 PRELIMS SOLUTIONS

Ans: (*)

Sol: Thi=200C

Hot fluid

Tci=90C

The=80C Cold fluid

Tci=35C x

Energy balance:

Heat released by hot fluid = heat received by cold fluid  C ph Thi  The   m  c C pc Tce  Tci  m Ch(200 – 80) = Cc(90 – 35) 120 Ch = 55 Cc Ch < Cc Cmin = Ch and Cmax = Cc Effectiveness () = 

C min Thi  The  200  80 = C min Thi  Tci  200  35

120 24 8   165 33 11

  = 0.727 30.

If one cylinder of a diesel engine receives more fuel than the others, it is a serious condition for that cylinder and can be checked by 1. judging the seizure of the piston 2. checking incomplete combustion in that cylinder 3. checking cylinder exhaust temperature with a pyrometer Which of the above is/are correct? (a) 1 only

30.

(b) 2 only

(c) 3 only

(d) 1, 2 and 3

Ans: (c)

Sol: More power is produced hence more heat is converted to work hence less cylinder heat

temperature. ACE Engineering Academy

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31.









: 15 :

MECHANICAL ENGINEERING [SET‐A]

Consider the following statements with reference to combustion and performance in a four-stroke petrol engine: 1. The auto-ignition temperature of petrol as a fuel is higher than that of diesel oil as a fuel. 2. The highest compression ratio of petrol engines is constrained by the possibility of detonation. 3. A petrol engine is basically less suitable for supercharging than a diesel engine. Which of the above statements are correct? (a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

31.

Ans: (d)

32.

In a flooded evaporator refrigerator, an accumulator at the section side of the compressor is provided to (a) collect the vapours (b) detect any liquids in the vapour (c) retain the refrigeration effect as originally working (d) collect the liquid refrigerant and preclude its reversion to the compressor

32.

Ans: (d)

Sol: An accumulator separates liquid and vapor refrigerant, after evaporator. It ensures only vapor

refrigerant to enter the compressor. 33.

A four-stroke single-cylinder SI engine of 6 cm diameter and 10 cm stroke running at 4000 r.p.m. develops power at a mean effective pressure of 10 bar. The power developed by the engine is (a) 9.42 kW

33.

(b) 5.54 kW

(c) 4.92 kW

(d) 2.94 kW

Ans: (a)

Sol: Four stroke:

n=1,

D = 0.06 m,

P LANA = BP  mb 120 ACE Engineering Academy

L = 0.1 m,

N = 4000 rpm , Pm = 1000 kPa

 1000  0.1   0.062  4000  1 4 = 9.42 kW 120

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ESE‐2018 PRELIMS SOLUTIONS

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34.

Which of the following actions will help to reduce the black smoke emission of a diesel engine?









: 17 :

MECHANICAL ENGINEERING [SET‐A]

1. Run at lower load, i.e., derating. 2. Have regular maintenance of the diesel engine, particularly of injection system. 3. Use diesel oil of higher cetane number. Select the correct answer using the code given below. (a) 1 and 2 only 34.

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

Ans: (c)

Sol: More power is produced when richness of mixture increases and black smoke is produced. To

avoid black smoke air fuel ratio in IC engine starts at 18:1 as a general precaution. black smoke

Power

10:1

18:1 AFR

35.

  What is the ratio of the efficiencies  I  for the two cycles as shown in the T-s diagrams?  II  T

T

T1

T1

T2

T2 s2

s1

s2

s

T1  T2 2T1

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s

(II)

(I) (a)

s1

(b)

T1  T2 2T2

(c)

2T2 T1  T2

(d)

2T1 T1  T2

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35.









: 18 :

ESE‐2018 PRELIMS SOLUTIONS

Ans: (d)

Sol: Work done is same in both cases but heat rejected is more in statement (II) when compared to

statement (I). I > II I 

1 T1  T2 S2  S1  2

1 T1  T2 S2  S1   T2 S2  S1  2

=

1 T1  T2  2

1 T1  T2   T2 2

1 T1  T2  T  T 2 = 2 = 1 T1  T2 T1  T2 2 1 T1  T2 S2  S1  II  2 T1S2  S1  1 T1  T2  T  T 2 = 1 2 = T1 2T1 T1  T2 I T1  T2 2T1  = II T1  T2 T1  T2 2T1 36.

A four-stroke engine having a brake power of 105 kW is supplied with fuel at the rate of 4.4 kg/min for 10 minutes. The brake specific fuel consumption of the engine is (a) 0.18 kg/kW-hr

36.

(b) 0.25 kg/kW-hr

(c) 0.36 kg/kW-hr

(d) 0.42 kg/kW-hr

Ans: (b)

Sol: BP = 105 kW  f  4.4 kg / min = 4.460 kg/hr m

bsfc 

 f kg / hr  4.4  60 m = = 2.514 kg/kWhr BPkW  105

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: 19 :

MECHANICAL ENGINEERING [SET‐A]

(NOTE: THE QUESTION NARRATION DOES NOT MATCH THE ANSWER THE NARRATION SHOULD BE AS FOLLOWS).

Alternative (I) :

A four-stroke engine having a brake power of 105 kW is supplied with fuel at the rate of 0.44 kg/min for 10 minutes. The brake specific fuel consumption of the engine is (a) 0.18 kg/kW-hr

(b) 0.25 kg/kW-hr

(c) 0.36 kg/kW-hr

(d) 0.42 kg/kW-hr

Ans: (b) Sol: BP = 105 kW  f  0.44 kg / min = 0.4460 kg/hr m

bsfc 

 f kg / hr  0.44  60 m = = 0.2514 kg/kWhr BPkW  105

Alternative (II) :

A four-stroke engine having a brake power of 105 kW is supplied with fuel of 4.4 kg for 10 minutes. The brake specific fuel consumption of the engine is (a) 0.18 kg/kW-hr

(b) 0.25 kg/kW-hr

(c) 0.36 kg/kW-hr

(d) 0.42 kg/kW-hr

Ans: (b) Sol: BP = 105 kW

Mass of fuel, mf = 4.4 kg Time taken for consumption (t) = 10 min =

f  m

1 hr 6

m f 4.4   26.4 kg / hr 1 t 6

bsfc 

 f kg / hr  26.4 m = 0.2514 kg/kWhr = BPkW  105

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37.









ESE‐2018 PRELIMS SOLUTIONS

: 20 :

Consider the following statements: 1.

Recycling exhaust gases by partial mixing with the intake gases increases the emission of oxides of nitrogen from the engine.

2.

The effect of increase in altitude of operation on the Carburetor is to enrich the entire portthrottle operation.

3.

When the carburetor throttle is suddenly opened, the air-fuel mixture may lean out temporarily resulting in engine stall.

4.

Use of multi-venturi system makes it possible to obtain a high velocity airstream when the fuel is introduced at the main venturi throat.

Which of the above statements are correct? (a) 1 and 3 37.

(b) 1 and 4

(c) 2 and 3

(d) 2 and 4

Ans: (c)

Sol: EGR reduces NOx emissions. Hence statement (1) is wrong.

Multiple venturi system having two venturis if fuel is injection in second venturi air flow rate will increases if fuel injection in main venturi air flow rate will not change. Multiple venturi systems having three venturis if fuel is injection in third venturi air flow rate will increases if fuel injection in main venturi air flow rate will not change. Hence statement (4) is wrong. Hence by elimination method option (c) is correct. Air

Air

Fuel If exit is at second venturi velocity will be high

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Fuel If exit is at main venturi velocity of air is less

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38.









: 21 :

MECHANICAL ENGINEERING [SET‐A]

In IC engine 1. the ideal air capacity of a two-stroke engine is the mass of air required to concurrently fill the total cylinder volume at inlet temperature and exhaust pressure. 2. with increase in air-fuel ratio beyond the value for maximum power, there is a fall in power developed and this fall is more with higher values of air-fuel ratio. 3. the volumetric efficiency of the engine depends on the design of intake and exhaust manifold Which of the above are correct? (a) 1 and 2 only

38.

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

Ans: (c)

Sol: BSFC

50 %

100 %

Load

Running at lower loads increases the BSFC which results in higher fuel consumption which is not desirable.

39.

Consider the following statements: 1. Heat pumps and air conditioners have the same mechanical components. 2. The same system can be used as heat pump in winter and as air conditioner in summer. 3. The capacity and efficiency of a heat pump fall significantly at high temperatures. Which of the above statements are correct? (a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

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39.









: 22 :

ESE‐2018 PRELIMS SOLUTIONS

Ans: (d)

Sol: COP 

T1 T1  T2 

 dc    = T1  T2 1  T1  1T1  T2 2 dT  1 T2  C

=

1 T1 T T T T2  = 1 2 21 =  2 T1  T2 T1  T2  T1  T2  T1  T2 2

 dC  T1   = T1  1T1  T2 2 (–1) = T1  T2 2  dTI T1 C

 dC   dC        dT2 T1  C  dT1 T2  C

As T1 increases COP falls for heat pump. 40.

The following are the results of a Morse test conducted on a four-cylinder, four-stroke petrol engine at a common constant speed in all cases: The brake power of the engine when all the cylinders are firing is 80 kW. The brake power of the engine when each cylinder is cut off in turn is 55 kW, 55.5 kW, 54.5 kW and 55 kW, respectively. The mechanical efficiency of the engine when all the cylinder are firing will be (a) 90 %

40.

(b) 85 %

(c) 80 %

(d) 75 %

Ans: (c)

Sol: B = 80 kW

I = nB – (BI + BII +…. +Bn) I = 4B – (BI + BII + BIII + BII) = 480 – (55+55.5+54.5 + 55) = 320 – 220 = 100 kW m 

B 80  = 0.8 I 100

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41.









: 23 :

MECHANICAL ENGINEERING [SET‐A]

An ideal refrigerator working on a reversed Rankine cycle has a capacity of 3 tons. The COP of the unit is found to be 6. The capacity of the motor required to run the unit is (take 1 T = 210 kJ/min) (a) 1.85 kW

41.

(c) 1.65 kW

(d) 1.50 kW

Ans: (B)

Sol: W 

42.

(b) 1.75 kW

Q 3  210 105  1.75kW  = 105 kJ/min = 60 COP 6

A flywheel weighs

981 kgf and has a radius of gyration of 100 cm. It is given a spin of 100 r.p.m. 

about its horizontal axis. The whole assembly is rotating about a vertical axis at 6 rad/s. The gyroscopic couple experienced will be (a) 2000 kgf-m 42.

(b) 1962 kgf-m

(c) 200 kgf-m

(d) 196 kgf-m

Ans: (a)

Sol: Flywheel mass m 

981 kg 

Kg = radius of gyration = 100 cm = 1m s = 100 rpm =

2  100 rad / s 60

p = angular velocity of precession = 6 rad/s Gyroscopic couple CG = Isp = mk g2s p =

981 2 2 1   100  6  60

= 19,620 Nm = 2000 kgf-m

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43.









: 24 :

ESE‐2018 PRELIMS SOLUTIONS

A cold storage has capacity for food preservation at a temperature of –3C when the outside temperature is 27C. The minimum power required to operate with a cooling load of 90 kW is (a) 20 kW

43.

(b) 15 kW

(c) 10 kW

(d) 5 kW

Ans: (c)

Sol: Minimum power is when cop is maximum and maximum cop is carnot cop

COP max  W

44.

T2 270 = =9 T1  T2 300  270

NRE 90 = = 10 kW WCOP 9

In a vapour absorption refrigerator, the temperatures of evaporator and ambient air are 10C and 30C, respectively. For obtaining COP of 2 for this system, the temperature of the generator is to be nearly (a) 90C

44.

(b) 85C

(c) 80C

(d) 75C

Ans: (c)

Sol: COP = E  (COP)R

COP 

T1  T2 TR  T1 T2  TR

T1 = ? T2 = 273 + 30 = 303 K TR = 273 + 10 = 283 K 2

T1  303 283  T1 20

40T1 = 283T1 – 303  283 243T1 = 303  283 T1 

303  283 = 353 K = 80C 243

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45.









: 25 :

MECHANICAL ENGINEERING [SET‐A]

The following data refer to a vapour compression refrigerator: Enthalpy at compressor inlet – 1200 kJ/kg Enthalpy at compressor outlet = 1400 kJ/kg Enthalpy at condenser outlet = 200 kJ/kg The COP of the refrigerator is (a) 7

45.

(b) 6

(c) 5

(d) 4

Ans: (c)

Sol: h1 = 1200 kJ/kg

2

T

h2 = 1400 kJ/kg

3

h3 = h4 = 200 kJ/kg

4

h  h4 COP  1 h 2  h1 1200  200 1000 = =5 = 1400  1200 200 46.

1 S

The compressor of an ammonia refrigerating machine has a volumetric efficiency of 85% and swept volume of 0.28 m3/min. Ammonia having a dry specific volume of 0.25 m3/kg enters the compressor with a dryness fraction of 0.7. The mass flow rate of ammonia through the machine is (a) 1.28 kg/min

46.

(b) 1.36 kg/min

(c) 1.42 kg/min

(d) 1.54 kg/min

Ans: (b)

 m3    r kg / mm   1 Sol: m  kg  = vol  Vs  

 r  x g  vol  Vs m  r  0.7  0.25  0.85  0.28 m  r  1.36 kg / min m

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47.









: 26 :

ESE‐2018 PRELIMS SOLUTIONS

Air is drawn in a compressor at the rate of 0.8 kg/s at a pressure of 1 bar and temperature of 20 C, and is delivered at a pressure of 10 bar and temperature of 90C. This air delivery is through an exit valve of area 210-3 m2. If R is 287 kJ/kg-K, the exit velocity of the air is (a) 41.7 m/s

47.

(b) 35.8 m/s

(c) 29.7 m/s

(d) 27.3 m/s

Ans: (a)

 1  0.8 kg / sec Sol: m

P1 = 100 kPa  1RT1 0.8  0.287  293  m V = 0.6727 m3/sec = 1 P1 100

 1RT2 0.8  0.287  363  m V = = 0.083343 m3/sec 2 P2 1000  A V V 2 2 2

V2 

48.

 V 2 = 0.08334 = 41.67 m/sec A2 2  103

Consider the following statements: 1. The operation of a refrigerator unit at more than one temperature can be accomplished by using different throttling valves and a separate compressor for each 'temperature range'. 2. The refrigerated space must be maintained above the ice point to prevent freezing. 3. In domestic refrigerators, the refrigerant is throttled to a higher pressure in the freezer followed by full expansion in the refrigerated space. Which of the above statements are correct? (a) 1 and 2 only

48.

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

Ans: (a)

Sol: In domestic refrigerators refrigerant is throttled to lower pressure in freezer when compared to

refrigerator, statement (3) is wrong by elimination option (a) is right.

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49.









: 27 :

MECHANICAL ENGINEERING [SET‐A]

Which one of the following methods is more effective to improve the efficiency of the Rankine cycle used in thermal power plant? (a) Increasing the condenser temperature (b) Decreasing the condenser temperature (c) Decreasing the boiler temperature (d) Increasing the boiler temperature

49.

Ans: (b)

Sol: Decreasing condenser temperature is more effective.

50.

Consider the following statements regarding vapour absorption systems in the field of refrigeration: 1. In ammonia-water absorption system, ammonia is the refrigerant. 2. In water-lithium bromine system, water is the refrigerant. 3. Ammonia-water absorption reaction is endothermic. 4. The amount of ammonia absorbed by water is inversely proportional to the temperature of ammonia. Which of the above statements are correct? (a) 1, 2 and 3

50.

(b) 1, 3 and 4

(c) 1, 2 and 4

(d) 2, 3 and 4

Ans: (c)

Sol: Ammonia water absorption is exothermic hence it is wrong statement by elimination option (c) is

correct. 51.

In an air-handling unit, air enters the cooling coil at a temperature of 30C. The surface temperature of the coil is –10C. If the bypass factor of the coil is 0.45, then the temperature of the air at the exit will be (a) 6C

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(b) 8C

(c) 10C

(d) 12C

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51.









: 28 :

ESE‐2018 PRELIMS SOLUTIONS

Ans: (b)

Sol:

BPF  0.45 

T2  Tcoil T1  Tcoil

2

1

Tcoil

T2  10 30  10

30

–10C

T2 = 18 – 10 = 8C 52. Consider the following statements: 1. The relative humidity of air does not change with temperature as long as specific humidity remains constant. 2. Dew-point temperature is the temperature at which condensation begins when air is cooled at constant volume. 3. Saturated air passing over a water surface does not cause change of air temperature. 4. For saturated air, dry-bulb, wet-bulb and dew-point temperatures are identical. Which of the above statements are correct? (a) 1 and 2 52.

(b) 2 and 3

(c) 1 and 4

(d) 3 and 4

Ans: (d)

Sol:



Relative humidity changes with DBT, hence statement (1) is wrong.



Dew-point temperature is the temperature at which when condensation happens at constant pressure. Hence statement (2) is wrong by elimination option (d) is correct.

53. A cold storage has 23 cm brick wall on the outside and 8 cm plastic foam on the inside. The inside and outside temperatures are –2C and 22C, respectively. If the thermal conductivities of brick and foam are 0.98 W/m-K and 0.02 W/m-K, and the inside and outside heat transfer coefficients are 29 W/m2-K and 12 W/m2-K, respectively, then the rate of heat removal for a (projected) wall area of 90 m2 will nearly be (a) 503 W ACE Engineering Academy

(b) 497 W

(c) 490 W

(d) 481 W

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53.









: 29 :

MECHANICAL ENGINEERING [SET‐A]

Ans: (b)

Sol:

Brick Outside To=22C

(I)

Foam Inside (II) hi=29 W/m2.K

h0=12 W/m2.K k1=0.98W/m.K k2=0.02 W/m.K

L1=0.23m

Ti= –2C

L2=0.08m

Thermal circuit: To

Ti 1 h oA

L1 k1A

Heat Transfer rate = =

=

=

1 k 2A

1 hiA

To  Ti R th 22   2 1 0.23 0.08 1    12  90 0.98  90 0.02  90 29  90 24  90 1 23 8 1    12 98 2 29

24  90 = 496.26 W 4.352

Q  497 W

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54.









: 30 :

ESE‐2018 PRELIMS SOLUTIONS

Consider the following statements: 1. The distinguishing features of a radial flow reaction turbine are (i) only a part of the total head of water is converted into velocity head before it reaches the runner and (ii) the flow-through water completely fills all the passage in the runner. 2. Kaplan turbine is essentially a propeller working in reverse, and its blades are so mounted that all the blade angles can be adjusted simultaneously by means of suitable gearing even as the machine is in operation. 3. A draft tube is a pipe of gradually increasing cross-sectional area which must be airtight, and under all conditions of operation, its lower end must be submerged below the level of the discharged water in the tailrace. Which of the above statements are correct? (a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

54.

Ans: (d)

55.

The specific speed of a turbine is the speed of an imaginary turbine, identical with the given turbine, which

55.

(a)

delivers unit discharge under unit speed

(b)

delivers unit discharge under unit head

(c)

develops unit discharge under unit speed

(d)

develops unit power under unit head

Ans: (d)

Sol: N s 

N P H5/ 4

If, P = 1 kW and H = 1 m Then, Ns = N i.e., specific speed is a speed of geometrically similar turbine which develops unit power when working under unit head. ACE Engineering Academy

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56.

56.









: 31 :

MECHANICAL ENGINEERING [SET‐A]

The mechanical efficiency of a centrifugal pump is the ratio of (a)

manometric head to the energy supplied by the impeller per kN of water.

(b)

energy supplied to the pump to the energy available at the impeller.

(c)

actual work done by the pump to the energy supplied to the pump by the prime mover.

(d)

energy available at the impeller to the energy supplied to the pump by the prime mover.

Ans: (d)

Sol: The mechanical efficiency of a centrifugal pump is defined as the ratio of power transmitted by

rotor (impeller) and shaft power (power supplied by prime mover). i.e.,  m 

57.

Rotor power Shaft power

Consider the following advantages of rotary pumps compared to reciprocating pumps : 1. Steady discharge which increases with decrease in head. 2. Suitable for handling fluids with suspended solid particles. 3. Less bulky than positive displacement pumps. 4. Can be started with open delivery with least load Which of the above advantages are correct? (a) 1, 2 and 3

57. Sol:

(b) 1, 2 and 4

(c) 1, 3 and 4

(d) 2, 3 and 4

Ans: (a) H

Discharge - Head Characteristic curve



Statement (1) is correct



Statement (2) is wrong

Q

Rotary pumps can handle all types of liquids that do not have any solid content and are poor in transporting thick and viscous liquids. ACE Engineering Academy

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: 32 :

ESE‐2018 PRELIMS SOLUTIONS

Reciprocating pumps can handle a full range of liquid; from low-viscosity chemicals to highviscosity materials like heavy oils; from clean water to high particle content slurries including concrete. In view of this wide operating range, reciprocating pumps are often the preferred choice

for difficult applications, especially when the discharges are low and pressures are high. Some typical industrial application 1. Drilling mud ; 2. Salt water injection ; 3. Ore slurries 

Statement (3) is correct Rotary pump is compact and small in size and have less weight for same capacity and energy transfer as compared to reciprocating pump. Hence rotary pump is less bulky.



Statement (4) is correct Rotary pump can be started with open delivery with least load.

58.

According to aerofoil theory, the guide angle of Kaplan turbine blades is defined as the angle between (a) lift and resultant force (b) drag and resultant force (c) lift and tangential force (d) lift and drag

58.

Ans: (a)

Sol:

Flift o

Fresultant Fdrag

o

Blade c/s Chord length 4 L=chord length

Guide angle as per the aerofoil theory of Kaplan turbine blade design is defined as the angle between lift and resultant force.

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59.









: 33 :

MECHANICAL ENGINEERING [SET‐A]

An ideal closed-cycle gas turbine plant is working between the temperatures 927C and 27C using air as working fluid. The pressure ratio for maximum output is (a) 11.3

59.

(b) 13.3

(c) 15.3

(d) 17.3

Ans: (a)

Sol: Condition for maximum power output.

T  rp(max) =  max   Tmin  60.

 2   1

1.4

 927  273  21.4 1 = = 11.3   27  273 

The critical speed of a turbine is (a) same as the runaway speed (b) the speed that will lead to mechanical failure of the shaft (c) the speed which equals the natural frequency of the rotor (d) the speed equal to the synchronous speed of the generator

60.

Ans: (c)

Sol: Critical speed of a turbine is the speed of the runner at which natural frequency of the rotor unit

equal to the operating speed. 61.

A 40 mm diameter water jet strikes a hinged vertical plate of 800 N weight normally at its surface at its centre of gravity as shown in the figure below:  u=15 m/s

d= 40 mm

W=800 N

The angle of deflection is nearly (a) sin–10.353

(b) sin–10.321

(c) tan–10.353

(d) tan–10.321

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61.









: 34 :

ESE‐2018 PRELIMS SOLUTIONS

Ans: (a)

Sol: L/2 L/2

 G W

G

Fn

At equilibrium the moment created by self weight and normal force must be zero.  Fn 

L / 2  W   L sin    0  2

cos 

 

i.e., Fn = W sin cos aV2cos = W sin cos sin  

aV W

2

 1000   0.04 2  15 2 4  = 0.353 800

  = sin–1(0.353) 62.

Consider the following statements regarding a Ram Jet: 1.

The engine has neither a compressor nor a turbine.

2.

It operates at much higher temperature than a gas turbine

3.

It cannot operate statically. It needs to be put in flight by some means at sufficiently high speed before it produces any thrust and propels itself.

Which of the above statements are correct?

62.

(a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

Ans: (d)

Sol:



In ramjet engine there is no compressor or turbine. The compression process is achieved by ram effect and expansion occurs in nozzle.

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: 35 :

MECHANICAL ENGINEERING [SET‐A]



In gas turbines the maximum temperature is limited by the metallurgical limits of turbine blades.



Ramjet engines cannot generate thrust in static conditions. It needs to be put in flight by some means at sufficiently high speed before it produces any thrust.

63.

Air enters a turbojet engine at the rate of 40 kg/s with a velocity of 250 m/s relative to an aircraft which is moving at 300 km/hr. Exhaust of the engine has a velocity of 700 m/s relative to the moving aircraft. The thrust developed by the engine is (a) 24 kN

63.

(b) 18 kN

(c) 12 kN

(d) 9 kN

Ans: (b)

 Vexit  Vinlet  Sol: Thrust, F = m

F = 40(700 – 250) = 18000 N or 18 kN Vinlet = Velocity of air entering relative to flight. Vexit = Velocity of exhaust gases relative to flight. 64.

The clearance volume in reciprocating air compressor is provided (a) to reduce the work done per kg of air delivered (b) to increase the volumetric efficiency of the compressor (c) to accommodate the valves in the head of the compressor (d) to create turbulence in the air to be delivered

64.

Ans: (c)

Sol: The clearance volume in reciprocating air compressor is provided to accommodate the valves in

the head of the compressor. 65.

Consider the following statements regarding Reheat Rankine Steam Cycle : 1. The main purpose of reheat in Rankine Cycle is to increase the efficiency of the cycle. 2. In practice, the reheat is generally limited to one point of expansion. 3. Due to reheat, the steam rate (specific steam consumption) is reduced.

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: 36 :

ESE‐2018 PRELIMS SOLUTIONS

Which of the above statements are correct? (a) 1 and 2 only

(b) 2 and 3 only

(c) 1 and 3 only

(d) 1, 2 and 3

65.

Ans: (b)

66.

In solar flat-plate collectors, the absorber plate is painted with selective paints. The selectivity is the ratio of (a) solar radiation-absorption to thermal infrared radiation-emission (b) solar radiation-emission to thermal infrared radiation-absorption (c) solar radiation-reflection to thermal infrared radiation-absorption (d) solar radiation-absorption to thermal infrared radiation-reflection

66.

Ans: (a)

67.

A 13 m long ladder is placed against a smooth vertical wall with its lower end 5 m from the wall. What should be the coefficient of friction between the ladder and the floor so that the ladder remains in equilibrium? (a) 0.29

67.

(b) 0.25

(c) 0.21

(d) 0.11

Ans: (c)

Sol: smooth vertical wall

N2 A

A

l

12 m



 5m

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B

W

B

N1

N1

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: 37 :

MECHANICAL ENGINEERING [SET‐A]

l = length of ladder = 13 m

FV = 0  N1 = W M A  0   W  sin   

68.

 cos   N1 sin   N1 cos   0 2

cos  2

1 1 5 5    0.21 2 tan  2  12 24

A cube strikes a stationary ball exerting an average force of 50 N over a time of 10 ms. The ball has mass of 0.20 kg. Its speed after the impact will be (a) 3.5 m/s

68.

(b) 2.5 m/s

(c) 1.5 m/s

(d) 0.5 m/s

Ans: (b)

Sol: Given data,

F = 50 N,

=0,

t = 10 ms = 10  10-3 sec ,

m = 0.2 kg v=?

Applying impulse - momentum equation, (F) t = m (v – u) (50)1010-3 = (0.2)(v – 0)  v = 2.5 m/s 69.

Consider the following statements regarding solid solution of metals : 1. The solubility of metallic solids is primarily limited by size factor. 2. A metal with high valence can dissolve large amount of metal of lower valence. 3. A metal with same lattice crystal structure can form a series of solid solutions. 4. The limit of solid solubility is indicated by a phase boundary called Liquidus. Which of the above statements are correct? (a) 1 and 4

(b) 2 and 4

(c) 1 and 3

(d) 1 and 2

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69.









ESE‐2018 PRELIMS SOLUTIONS

: 38 :

Ans: (c)

Sol: The solubility of metallic solids are mainly depends on given below factor (Hume Ruthery rule)  Size factor (difference between the size of host metal atom and impurity atom should be less

than 15%).  Valence difference (a metal with lower valence can dissolve large amount of metal of higher

valance.  Crystal structure  Electro negativity (small between metals)

70.

A box of weight 1000 N is placed on the ground. The coefficient of friction between the box and the ground is 0.5. When the box is pulled by a 100 N horizontal force, the frictional force developed between the box and the ground at impending motion is (a) 50 N

70.

(b) 75 N

(c) 100 N

(d) 500 N

Ans: (c)

Sol: W = 1000 N,

P = 100 N,

 = 0.5 N

F=?

P (F) W

Determining limiting friction, F = N = (0.5) (1000) = 500 N

Free body diagram

Since applied force (100 N) is less than limiting friction, body will be at rest. As long as the body, is at rest, F = P = 100 N 71.

A state of plane stress consists of a uniaxial tensile stress of magnitude 8 kPa, exerted on vertical surfaces and of unknown shearing stresses. If the largest stress is 10 kPa, then the magnitude of the unknown shear stress will be (a) 6.47 kPa

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(b) 5.47 kPa

(c) 4.47 kPa

(d) 3.47 kPa

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71.









: 39 :

MECHANICAL ENGINEERING [SET‐A]

Ans: (c)

Sol: Given data:

x = 8 kPa,

y = 0,

1 = 10 kPa

Maximum principal stress is given by, 1 

x  y 2

 x  y   2 

2

    2xy  

2

80 80 2  10       xy 2  2 

 xy = 4.47 kPa 72.

A rigid beam of negligible weight is supported in a horizontal position by two rods of steel and aluminium, 2 m and 1 m long, having values of cross sectional areas 100 mm2 and 200 mm2 and Young’s modulus of 200 GPa and 100 GPa, respectively. A load P is applied as shown in the figure below :

2 m Steel

Rigid beam

1 m Aluminium

P

If the rigid beam is to remain horizontal, then (a) the force P must be applied at the centre of the beam. (b) the force on the steel rod should be twice the force on the aluminium rod (c) the force on the aluminium rod should be twice the force on the steel rod (d) the forces on both the rods should be equal. 72.

Ans: (c)

Sol: Let, Pa = Force in Aluminium

Ps = Force in steel From the given condition that the rigid beam to remain horizontal, ACE Engineering Academy

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: 40 :

ESE‐2018 PRELIMS SOLUTIONS

s = a  PL   PL       AE  Steel  AE  Al



Ps Ls PL  a a (here, Ls = 2La, As = Aa/2, Es = 2Ea) As Es A a E a

 Pa = 2Ps 73.

A solid shaft is subjected to bending moment of 3.46 kN-m and a torsional moment of 11.5 kN-m. For this case, the equivalent bending moment and twisting moment are (a) 7.73 kN-m and 12.0 kN-m (b) 14.96 kN-m and 12.0 kN-m (c) 7.73 kN-m and 8.04 kN-m (d) 14.96 kN-m and 8.04 kN-m

73.

Ans: (a)

Sol: Given data:

M = 3.46 kN-m T = 11.5 kN-m Equivalent twisting moment is given by, Te  M 2  T 2

=

3.462  11.52

= 12 kN-m

Equivalent bending moment is given by, Me 

1 M  M 2  T 2  = 7.73 kN-m   2

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: 41 :

MECHANICAL ENGINEERING [SET‐A]

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74.









: 42 :

ESE‐2018 PRELIMS SOLUTIONS

Which one of the following is the correct bending moment diagram for a beam which is hinged at the ends and is subjected to a clockwise couple acting at the mid-span? (a)

(b) Positive BM Negative BM

(c)

74.

(d)

Ans: (c)

Sol:

M A RA

L 2

B

C

L 2

RB

Taking Fy = 0----------(1)  RA + RB = 0 By taking moment about point A, MA = 0  RB  L = M  RB = M/L  RA = –M/L Bending Moment :

MA = M B = 0 (MC)Right = RB  (MC)Left = RB  ACE Engineering Academy

M L (Sagging) = 2 2 M L (Hogging) –M=– 2 2 Hyderabad|Delhi|Bhopal|Pune|Bhubaneswar| Lucknow|Patna|Bengaluru|Chennai|Vijayawada|Vizag |Tirupati | Kukatpally| Kolkata













: 43 :

MECHANICAL ENGINEERING [SET‐A]

From the above computed values, bending moment diagram is drawn in the figure below. M A M/L

L 2

75.

M/L

L 2 M 2

MA = 0

B

C

(–ve)

(+ve)

MB = 0

M 2

A steel specimen is heated to 780C and is then cooled at the slowest possible rate in the furnace. The property imparted to the specimen by this process is

75.

(a) toughness

(b) hardness

(c) softness

(d) tempering

Ans: (a & c)

Sol: In annealing process, steel specimen is heated to 50oC above upper critical temperature for

hypoeutectoid steel and 50oC above the lower critical temperature for hypereutectoid steel and then cooled very slowly to room temperature. The annealed component process improves 1. ductility 2. softness 3. toughness 76.

Consider the following statements: 1. In case of a thin spherical shell of diameter d and thickness t, subjected to internal pressure p, the principal stresses at any point equal

pd 4t

2. In case of thin cylinders, the hoop stress is determined assuming it to be uniform across the thickness of the cylinder. 3. In thick cylinders, the hoop stress is not uniform across the thickness but it varies from a maximum value at the inner circumference to a minimum value at the outer circumference. ACE Engineering Academy

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: 44 :

ESE‐2018 PRELIMS SOLUTIONS

Which of the above statements are correct?

76.

(a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

Ans: (d)

Sol:



When we cut a plane through the centre of the sphere in any direction whatsoever, from the symmetry of a spherical shell we obtain the same tensile stress (i.e. principal stress, ) in the wall. Principal stress ,  

pd 4t

where, p = Internal pressure, d = Internal diameter of the sphere, t = Thickness of the sphere Thus, statement (1) is correct. 

In case of thin cylinders, the hoop stress is determined assuming it to be uniformly distributed over the thickness of the wall, provided that the thickness is small compared to the radius. Thus, statement (2) is also correct.



For a thick cylinder hoop stress is given by, h 

P R 2  r2  x2    r 2  R 2  x 2 

(here, r  x  R)

where, r and R are inner and outer radius, respectively. Thus, h is compressive in nature and is maximum when ‘x’ is minimum. h is maximum at x = r i.e., at inner surface.

Thus, statement (3) is also correct. 77.

Addition of magnesium to cast iron increases its (a) hardness

(b) corrosion resistance

(c) creep strength

(d) ductility and strength in tension

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77.









: 45 :

MECHANICAL ENGINEERING [SET‐A]

Ans: (d)

Sol: Adding magnesium to cast iron more than 0.04%, if produce a spherical form of graphite that

improves ductility and strength. 78.

Consider the following statements : 1.

The quenching of steel results in an increase in wear resistance, strength and hardness.

2.

By the process of case-hardening, hard wearing resistant surface is produced on mild steel. This is an effective method for low-carbon steels because they cannot be hardened by the process of quenching.

3.

When a metal is mixed with small atoms of non-metallic element in such a manner that invading atoms occupy interstitial positions in the metal lattice, an interstitial alloy results.

Which of the above statements are correct?

78.

(a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

Ans: (d)

Sol:

1.

The punching of steel results increases the hardness and strength of generating fine grain martensite structure.

2.

Case hardening process is applicable for low carbon steels because they are not heat treated by hardening process and depth of hardenability is very low.

3.

When a metal is mixed with small size atom of non-metallic elements, they occupy of interstitial positions of metal and forms interstitial alloy.

Ex: Addition of carbon to iron is an interstitial alloy. 79.

Recrystallization temperature is one at which (a)

crystals first start forming from molten meal when cooled

(b)

new spherical crystals first begin to form from the old deformed ones when that strained metal is heated

(c)

the allotropic form changes

(d)

crystals grow bigger in size

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79.









: 46 :

ESE‐2018 PRELIMS SOLUTIONS

Ans: (b)

Sol: Recrystallization temperature:

Formation of new equi-axed crystals from deformed crystal when that deformed metal is heated. 80.

80.

Fe-C alloy containing less than 0.83% carbon is called (a) high-speed steel

(b) hypo-eutectoid steel

(c) hyper-eutectoid steel

(d) cast iron

Ans: (b)

Sol: Hypo-eutectoid steel: It is an iron-carbon alloy with % of carbon is less than 0.83%.

81.

Which of the following statements are correct? 1.

Steel and cast iron are multi-phase alloys.

2.

Ferrite is a single-phase interstitial solid solution of carbon in iron.

3.

Wrought iron is a highly refined iron with a small amount of slag which gives resistance to progressive corrosion.

4.

Stellite contains large amounts of metal like cobalt and tungsten resulting in high hardness.

Select the correct answer using the code given below.

81.

(a) 1, 2, 3 and 4

(b) 1, 2 and 3 only

(c) 1, 3 and 4 only

(d) 2 and 4 only

Ans: (a)

Sol:

1.

Steel and cast iron are multiphase alloys of -Ferrite and cementite.

2.

-Ferrite is a single phase interstitial solid solution of carbon in iron.

3.

Wrought iron is a pure form of Fe with small amount of slag (CaSiO3) that gives resistance to progressive corrosion.

4.

Stellite is good cutting tool material that have large quantity of cobalt and tungsten.

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82.









: 47 :

MECHANICAL ENGINEERING [SET‐A]

Which one of the following statements is correct? (a)

Microprocessor is more suitable for general purpose and micro-controller is more suitable for special purpose and custom-built application.

(b)

Microprocessor and microcontroller are suitable for general purpose application.

(c)

Microprocessor and microcontroller are suitable for special purpose application.

(d)

Microprocessor and microcontroller are suitable for special purpose and custom-built application.

82.

Ans: (a)

Sol: Micro processors has off-chip peripherals, suitable for general purpose applications and micro

controllers has ON-chip peripherals with fixed quantity to suit special purpose. 83.

83.

The unique property of cast iron is its high (a) malleability

(b) ductility

(c) toughness

(d) damping characteristics

Ans: (d)

Sol: The Gray cast-iron posses high damping capacity because of presence of graphite plates.

84.

Which one of the following pairs of tests has been developed to evaluate the fracture resistance of engineering materials, subjected to dynamic loads or impacts? (a) Tension impacts and Bending impacts (b) Tensile test and Brinell hardness test (c) Vickers hardness test and Tensile test (d) Scleroscope test and File test

84.

Ans: (a)

Sol: Fatigue is a type of failure due to dynamic loads they are tested by both tension impacts and

bending impact test.

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85.









: 48 :

ESE‐2018 PRELIMS SOLUTIONS

The midpoint of a rigid link of a mechanism moves as a translation along a straight line, from rest, with a constant acceleration of 5 m/s2. The distance covered by the said midpoint in 5 s of motion is (a) 124.2 m

85.

(b) 112.5 m

(c) 96.2 m

(d) 62.5 m

And: (d)

Sol: Given,

a = 5 m/s2 u = 0 (rest) t = 5 sec  s = ut + 1/2  a  t2 Distance, s = 0.1/2552 s = 62.5 m

86.

Consider the following statements: 1.

A kinematic chain is the combination of kinematic pairs joined in such a way that the relative motion between them is completely constrained.

2.

The degree of freedom of a kinematic pair is given by the number of independent coordinates required to completely specify the relative movement.

Which of the above statements is /are correct? (a) 1 only

(b) 2 only

(c) Both 1 and 2

(d) Neither 1 nor 2

86.

Ans: (c)

87.

The equation of motion for a single degree of freedom system is 4x  9 x  16 x  0

The critical damping coefficient for the system is (a) 4 2

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(b) 4

(c) 16 2

(d) 16

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87.









: 49 :

MECHANICAL ENGINEERING [SET‐A]

Ans: (d)

Sol: 4x  9 x  16 x  0 mx  cx  kx  0 is the governing equation for a free damped vibration.

Thus, m = 4, c = 9, k = 16 Critical damping coefficient is given by, cc  2 km = 2 16  4 = 16

88.

The mass of a single-degree damped vibrating system is 7.5 kg and it makes 24 free oscillations in 14 s when disturbed from its equilibrium position. The amplitude of vibration reduces to 0.25 of its initial value after five oscillations. Then the logarithmic decrement will be (a)

88.

2 4 log e 5

(b)

1 6 log e 5

(c)

1 4 log e 5

(d)

2 6 log e 5

Ans: (c)

Sol: Given data:

m = 7.5 kg Frequency, fn = number of oscillations per s  24    s 1  1.714 s 1  14  X5 = 0.25 X0 X 0 100  4 X 5 25

Td

x x0

X X X X X Now, 0  1  2  3  4  4 X1 X 2 X 3 X 4 X 5

Td x1

Td x2

Td x3

Td x4

x5

time

 e5 = 4  5 = loge4 1    log e 4 5

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89.









: 50 :

ESE‐2018 PRELIMS SOLUTIONS

A 20 kg mass is suspended from a spring which deflects 15 mm under this load. The value of the critical damping coefficient to make the motion aperiodic will be (a) 1010 N/m/s

89.

(b) 1013 N/m/s

(c) 1018 N/m/s

(d) 1023 N/m/s

Ans: (d)

Sol: Given data: n 

m = 20 kg ,

 = 15 mm

g 9.81   25.573 rad / s  15  10  3

Critical damping coefficient is given by, cc = 2mn = 2  20  25.573 = 1022.92 = 1023 N/m/s

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: 51 :

MECHANICAL ENGINEERING [SET‐A]

90. Consider the following statements: 1. The whirling (critical) speed of a shaft is that rotational speed at which the shaft so runs that the deflection of the shaft from the axis of rotation tends to become infinite. 2.

Critical speed is equal to the frequency of transverse vibration of a shaft when the shaft carries a point load or a uniformly distributed load or a combination of both such loads.

3.

The whirling of a shaft results from causes such as mass unbalance, hysteresis damping in the shaft, gyroscopic forces and fluid friction in the bearing.

Which of the above statements are correct?

90.

(a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

Ans: (a)

Sol:



When rotational speed of system coincides with natural frequency of lateral/transverse vibrations, the shaft tends to bow out with large amplitude. This speed is termed as critical/whirling speed.



In such case of resonance, it takes some time for the amplitude to build up to a large value.



Such whirling is caused when the center of mass of the rotor system does not coincide with axis of rotation (i.e., mass unbalance).

This eccentricity could be due to internal material defects,

manufacturing and assembly errors, etc but not due to hysteresis/material damping, gyroscopic forces and fluid friction in bearings. Thus, statements 1 and 2 are correct but statement 3 is incorrect. 91.

Consider the following statements: 1.

In spur gears, the contact occurs abruptly on a line parallel to the axis, and the disengagement too is abrupt.

2.

In helical gears, both loading and unloading are gradual, and therefore, these happen more smoothly and less noisily.

3.

When two gears mesh, any arbitrary shape of the tooth can be chosen for the profile of the teeth of any one of the two gears, and the profile for the other shall be obtained by applying the law of gearing.

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: 52 :

ESE‐2018 PRELIMS SOLUTIONS

Which of the above statements are correct?

91.

(a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

Ans: (d)

Sol: Driven Spur Gear Teeth :

Orientation of teeth parallel to axis of shaft Start of mesh/engagement

End of mesh

Axis of gear shaft

Start and end of mesh abruptly happens on a line parallel to axis of shaft. Driven Helical Gear Teeth :

Mesh starts with point contact, extends to line contact and recedes with point contact. Hence gradual (smooth contact and less noisy) Mathematically it is always possible to obtain profile of teeth for two meshing gears following Law of Gearing. Such gear teeth profiles are termed to be conjugate. 92.

The interference between a given pinion tooth and a gear tooth can be avoided by using 1. smaller pressure angle 2. larger pressure angle 3. less number of teeth on the gear for a pinion with predefined number of teeth 4. more number of teeth on the gear for a pinion with predefined number of teeth Which of the above are correct ? (a) 1 and 4

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(b) 1 and 3

(c) 2 and 4

(d) 2 and 3

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92.









: 53 :

MECHANICAL ENGINEERING [SET‐A]

Ans: (c)

Sol:



To avoid interference pressure angle should be large as it results in large center distance.



During meshing interference takes place when addendum of gear digs into portion between base and root circle of pinion. To avoid it we define the minimum number of teeth on pinion. Number of teeth on gear should be more as it results in reduction of size of gear teeth.

93.

A gear train is as shown in the figure below, in which gears A and B have 20 and 40 teeth, respectively. If arm C is fixed and gear A rotates at 100 rpm, the speed of gear B will be B A

(a) 90 rpm 93.

C

(b) 75 rpm

(c) 50 rpm

(d) 20 rpm

Ans: (c)

Sol: Given data:

TA = 20, TB = 40 Here arm is fixed, the train will be a simple gear train So,

A T  B B TA

100 40   B = –50 rpm 20 B Note: Negative sign indicates that gear A and gear B rotate in opposite direction.

94.

A single-cylinder reciprocating engine works with a stroke of 320mm, mass of reciprocating parts as 45 kg and mass of revolving parts as 35 kg at crank radius. If 60% of the reciprocating parts and all the revolving parts are to be balanced, then the balancing mass required at a 300 mm radius is nearly (a) 33.1 kg

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(b) 36.3 kg

(c) 39.5 kg

(d) 42.7 kg

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94.









: 54 :

ESE‐2018 PRELIMS SOLUTIONS

Ans: (a)

Sol: Given data:

Stroke = 320 mm Stroke = 2  crank radius  2r = 320

 r = 160 mm

Mass of reciprocating parts, mrecip = 45 kg Mass of rotating parts, mrot = 35 kg Balancing mass radius, b = 300 mm Since rotating masses can be completely balanced and revolving masses are partially balanced So,

B.b = C.mrecip.r +mrot.r B  300 = (0.6  45  160) + (35  160) B  300 = 9920 B = 33.066 kg B  33.1 kg

95.

Consider the following statements: 1.

Gyroscopic effects generate forces and couples which act on the vehicles and these effects must be taken into account while designing their bearings.

2.

Rolling motion of a ship usually occurs because of the difference in buoyancy on the two sides of the ship due to a wave.

Which of the above statements is/are correct?

95.

(a) 1 only

(b) 2 only

(c) both 1 and 2

(d) Neither 1 nor 2

Ans: (c)

Sol: Gyroscopic effect arises when there is precession of rotating rotor. Gyroscopic couple is generated

which changes the bearing reactions. Hence gyroscopic effects to be considered while designing bearing.

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96.









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MECHANICAL ENGINEERING [SET‐A]

When two shafts, one of which is hollow, are of the same length and transmit equal torques with equal maximum stress, then they should have equal (a) polar moments of inertia (b) polar moduli (c) diameters (d) angles of twist

96.

Ans: (b)

Sol:  

T T T r     J Zp J r

Zp = polar modulus =

J r

  and T are same, Zp must also be same. 97.

A solid rod of circular cross-section made of brittle material, when subjected to torsion, fails along a plane at 45o to the axis of the rod. Consider the following statements as pertaining thereto: 1. Distortion energy is maximum on this 45 plane 2. Shear stress is maximum on this 45 plane 3. Normal stress is maximum on this 45 plane. Which of the above is /are correct?

97.

(a) 1 only

(b) 2 only

(c) 3 only

(d) 1, 2 and 3

Ans: (c)

Sol: Brittle material is weaker in tension. In pure torsion, maximum tensile stress occurs at angle 45 to

the axis of the rod. T P T

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K 

P K

K

K 45

K

K



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ESE‐2018 PRELIMS SOLUTIONS

On Mohr’s circle it is at 90 to maximum shear stress. 245 =90

xy

1

2

Mohr circle for this state of stress of pure torsion. 1 = +xy @ 45 2 = –xy @ –45 98.

A riveted joint may fail by 1. tearing of the plate at an edge 2. tearing of the plate across a row of rivets 3. shearing of rivets Which of the above are correct?

98.

(a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

Ans: (c)

Sol: Edge failure is a shear failure and is caused by insufficient margin. i.e., 2 and 3 only correct.

99.

An offset provided in radial cam-translating-follower mechanism serves to (a) decrease the pressure angle during ascent of the follower (b) increase the pressure angle during ascent of the follower (c) avoid possible obstruction due to some machine parts (d) decrease the pressure angle during descent of the follower

99.

Ans: (a)

Sol: Offset is provided mainly to decrease the pressure angle during ascent of follower.

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MECHANICAL ENGINEERING [SET‐A]

100. In combined parallel and transverse fillet welded joint (a) the parallel portion will fail due to tension, whereas the transverse portion will fail due to shear (b) the transverse portion will fail due to tension, whereas the parallel portion will fail due to shear (c) both parallel and transverse portions will fail due to tension (d) both parallel and transverse portions will fail due to shear 100. Ans: (d) Sol: Transverse fillet weld is subjected to both normal as well as shear stress and parallel fillet weld is

subjected to shear stress only but both transverse and parallel fillet belt fails due to shear only because transverse fillet weld are weak in shear. In shear, PPFW = 0.707  t  L  (per) PTFW = 0.832  t  L  (per) PTFW = 1.17 PPFW PFW = Parallel Fillet Weld TFW = Transverse Fillet Weld Strength of transverse fillet weld is 1.17 times the strength of parallel fillet weld in shear but transverse fillet weld is designed by using same equation as parallel fillet weld. It makes calculation simpler with safer design. 101. In a journal bearing, the diameter of the journal is 0.15m, its speed is 900rpm and the load on the bearing is 40kN. Considering  = 0.0072, the heat generated will be nearly (a) 1 kW

(b) 2 kW

(c) 3 kW

(d) 4 kW

101. Ans: (b) Sol: Heat generated, Hg = fWV or µWV

 or f = coefficient of friction = 0.0072  40 

  0.15  900 60

= 2.036 kW ACE Engineering Academy

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ESE‐2018 PRELIMS SOLUTIONS

102. Which one of the following governors is having a larger displacement of sleeve for a given fractional change of speed? (a) Stable governor

(b) Sensitive governor

(c) Isochronous governor

(d) Hunting governor

102. Ans: (b) Sol: Governor is said to be more sensitive when there is more sleeve displacement for small fractional

change in speed. 103. Consider the following statements : 1. HSS tools wear very rapidly, whereas in cemented carbide tools, even though hardness is retained, crater wear can occur due to solid-state diffusion. 2. Cutting tools made of Super-HSS also known as cobalt-based HSS, are made by adding 2 % to 15 % of cobalt which increases the cutting efficiency at heavier cuts by increasing the hot hardness and wear resistance. 3. Tool failure due to excessive stress can be minimized by providing small or negative rake angles on brittle tool materials, protecting tool tip by providing large side-cutting edge angles, and honing a narrow chamfer along the cutting edge. Which of the above statements are correct ? (a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

103. Ans: (c) Sol: Statement No.1 is wrong because solid state diffusion will takes place only in the Diamond cutting

tools used for machining of ferrous materials. In case of Statement No. 2, Super HSS is a molybdenum-series high-speed steel alloy with an additional 8% or 10% cobalt. It is widely used in metal manufacturing industries because of its superior red-hardness as compared to more conventional high-speed steels, allowing for shorter cycle times in production environments due to higher cutting speeds or from the increase in time between tool changes. Super HSS is also less prone to chipping when used for interrupted cuts and costs less when compared to the same tool made of carbide. Tools made from cobalt-bearing high speed steels can often be identified by the letters HSS-Co. ACE Engineering Academy

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MECHANICAL ENGINEERING [SET‐A]

Statement No.3, by reducing rake angle the strength of the tool increases hence by providing small or negative rake angles the tool failure can be avoided. 104. The resilience of steel can be found by integrating stress-strain curve up to the (a) ultimate fracture point

(b) upper yield point

(c) lower yield point

(d) elastic point

104. Ans: (d) Sol: Resilience is the total energy absorbed by the material during its elastic deformation that is upto

elastic limit. 

E.L



105. While turning a 60 mm diameter bar, it was observed that the tangential cutting force was 3000 N and the feed force was 1200 N. If the tool rake angle is 32, then the coefficient of friction is nearly (may take sin32 = 0.53, cos32 = 0.85 and tan 32 = 0.62) (a) 1.37

(b) 1.46

(c) 1.57

(d) 1.68

105. Ans: (a) Sol: Fc = 3000 N, Ft = 1200 N, α = 32,

Tan(β – α ) =

Ft Fc

F   β = α + Tan-1  t  =.8  Fc 

Coefficient of friction = µ Tan(β) = 1.37 (Strictly speaking no answer because β > 45, µ > 1 and it requires use classical friction theorem, For this data is insufficient and no answer is given with less than 1).

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ESE‐2018 PRELIMS SOLUTIONS

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MECHANICAL ENGINEERING [SET‐A]

106. For spot welding of 1 mm thick sheet with a current flow time of 0.2 s, the heat generated is 1000 J. If the effective resistance is 200 , the current required is (a) 4000 A

(b) 5000 A

(c) 6000 A

(d) 7000 A

106. Ans: (b) Sol: H.G = I2 R.τ

 I2  20010-6  0.2 = 1000  I = 5000 A 107. The maximum possible draft in rolling, which is the difference between initial and final thickness of the sheet metal, depends on (a) rolling force (b) roll radius (c) roll width (d) yield shear stress of the material 107. Ans: (b) Sol: Maximum possible reduction in rolling = (∆H)max = µ2.R

Hence (∆H)max  R 108. For a strain gauge (gauge factor = 2.1 and resistance = 50 ) subjected to a maximum strain of 0.001, the maximum change in resistance is (a) 0.084 

(b) 0.105 

(c) 0.135 

(d) 0.156 

108. Ans: (b) Sol: Strain gauge

Guage factor Gf   So, R  Gf 

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R / R  / 

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ESE‐2018 PRELIMS SOLUTIONS

109. Consider the following statements : Dispatching authorizes the start of production operation by 1. releasing of material and components from stores to the first process. 2. releasing of material from process to process. 3. issuing of drawing and instruction sheets. Which of the above statements are correct ? (a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

109. Ans: (d)

110. Which one of the following forecasting models best predicts the turning point ? (a) Simple exponential smoothing model (b) Brown’s quadratic smoothing model (c) Double exponential smoothing model (d) Moving average model (using 5 data points) 110. Ans: (a) Sol:

(a)

Simple exponential smoothing model is more responsive and superior compared to simple moving average. It predicts the turning points quickly since it uses the concept of linear interpolation. This method can respond to any kind of noise signal immediately. It is correct.

(b)

Brown’s quadratic smoothing model is also known as triple exponential smoothening method. This method is suitable for linear trend series. It has smoothened forecast and it can not predict turning points. It is not correct.

(c)

Double exponential smoothing model uses forecast and trend value. It is also known as Brown’s linear exponential smoothening. It is a smoothened value with seasonal adjustments.

(d)

The random walk model responds very quickly to changes in the series. If we use simple moving average of 5 periods, we get a smoothened forecast. Hence, the simple moving average forecast tends to lag behind turning points by about

1 periods, where  = smoothening coefficient. Hence 

it is not the right choice. It is not correct. ACE Engineering Academy

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MECHANICAL ENGINEERING [SET‐A]

111. The material removal rate will be higher in ultrasonic machining process for material with (a) higher ductility

(b) higher fracture strain

(c) higher toughness

(d) lower toughness

111. Ans: (d) Sol: In USM the MRR is mainly depends on the brittleness or lower toughness of the work material.

112. In queuing theory with multiple servers, the nature of the waiting situation can be studied and analyzed mathematically, if (a)

the complete details of the items in the waiting lines are known

(b)

the arrival and waiting times are known and can be grouped to form an appropriate waiting line model

(c)

all the variables and constants are known and they may form a linear equation

(d)

the laws governing arrivals, service times and the order in which the arriving units are taken into service are all known

112. Ans: (d) Sol: Arrival pattern, Service pattern and Queue discipline must be known in Queuing theory.

113. In any crash program for a project (a) both direct and indirect costs increase (b) indirect costs increase and direct costs decrease (c) direct costs increase and indirect costs decrease (d) costs are of no criterion 113. Ans: (c) Sol:

Cost

Indirect cost

Direct cost Duration ACE Engineering Academy

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: 64 :

ESE‐2018 PRELIMS SOLUTIONS

During crashing process : 

Indirect cost decreases



Direct cost increases

144. Tool signature is (a) a numerical method of identification of the tool (b) the plan of the tool (c) the complete specification of the tool (d) associated with the tool manufacturer 114. Ans: (a) Sol: Tool signature or designation is the numerical method of identification of geometric properties of a

tool. 115. With reference to a microprocessor, RISC stands for (a) Redefined Instruction Set Computer (b) Reduced Instruction Set Computer (c) Restructured Instruction Set Computer (d) Regional Instruction Set Computer 115. Ans: (b) Sol: RISC (Reduced instruction set computer).

116. An OR logic control in pneumatic systems is possible with the help of (a) sequence valve

(b) shuttle valve

(c) dual pressure valve

(d) delay valve

116. Ans: (b) Sol: Shuttle valve allows fluid flow from one of two sources, like OR logic. ACE Engineering Academy

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: 65 :

MECHANICAL ENGINEERING [SET‐A]

117. Which one of the following is not an Addressing Mode in 8085 ? (a) Immediate

(b) Indirect

(c) Register

(d) Segment

117. Ans: (d) Sol: Segment addressing mode is not in 8085. It is in 8086 microprocessor.

118. Consider the following statements regarding Programming Logic Controller (PLC) : 1. It was developed to replace the microprocessor. 2. Wiring between devices and relay contacts are done in its program. 3. Its I/O interface section connects it to external field devices. 4. It required extensive wiring in the application. Which of the above statements are correct ? (a) 1 and 3

(b) 1 and 4

(c) 2 and 3

(d) 2 and 4

118. Ans: (c) Sol: Options 1 & 4 are wrong because

Options 1: PLC not replace micro processor, as it is a part of PLC. Options 4: Not requires extensive wiring by PLC. 119. If B is the magnetic flux density at right angles to a plate, I is the current flow through the plate, t is the plate thickness and KH is Hall coefficient, the resultant transverse potential difference V for Hall sensor is given by (a) K H

Bt I

(b) K H

t BI

(c) K H

BI t

(d) K H

I Bt

119. Ans: (c) Sol: VH  k H

IB t

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ESE‐2018 PRELIMS SOLUTIONS

120. The specific speed of a hydraulic turbine depends on (a) speed and power developed (b) speed and water head (c) discharge and power developed (d) speed, head and power developed 120. Ans: (d) Sol: Specific speed of a turbine depends on speed of the runner (rpm), head under which turbine runner

works (H) and shaft power developed (P)

Ns 

N P H 5 / 4

121. The degrees of freedom of a SCARA robot are (a) six

(b) five

(c) four

(d) three

121. Ans: (c) Sol: SCARA Robot has 4 DOF [Arm has 3 DOF and wrist has 1 DOF]. R V O

122. Which one of the following devices produces incremental motion through equal pulses ? (a) AC servomotor

(b) DC servomotor

(c) Stepper motor

(d) Series motor

122. Ans: (c) Sol: The stepper motor is known by its property to convert a train of input pulses (typically square

wave pulses) into a precisely defined increment in the shaft position. Each pulse moves the shaft through a fixed angle. ACE Engineering Academy

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: 67 :

MECHANICAL ENGINEERING [SET‐A]

123. A force of 400 N is required to open a process control valve. What is the area of diaphragm needed for a diaphragm actuator to open the valve with a control gauge pressure of 70 kPa ? (a) 0.0095 m2

(b) 0.0086 m2

(c) 0.0057 m2

(d) 0.0048 m2

123. Ans: (c) Sol: F = P.A

400 N = 70103A 400 4 A   0.0057 m 2 . 70  1000 700

124. A force of 10 kN is required to move a workpiece. What is the needed working pressure, if the piston diameter is 100 mm ? (a) 1.55 MPa

(b) 1.46 MPa

(c) 1.27 MPa

(d) 1.12 MPa

124. Ans: (c) Sol: F = P.A

10103 = P.(5010–3)2; R  So, P 

100  50 mm 2

10  10 3 1000 4  MPa  MPa  1.27MPa 6 2500      2500  10

125. If a workpiece is moved by 50 mm in 10 s by a piston of diameter 100 mm, the hydraulic liquid flow rate is nearly (a) 3.00  10–5 m3/s

(b) 3.93  10–5 m3/s

(c) 4.74  10–5 m3/s

(d) 5.00  10–5 m3/s

125. Ans: (b) Sol: Discharge = Area  velocity



  50  10 ACE Engineering Academy



3 2

50  10 3  10

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: 68 :

ESE‐2018 PRELIMS SOLUTIONS

= (2500)510–410–5 m3/s = (25)(5)(10–2)10–5 = 3.9310–5 m3/s

126. Which of the following are the basic building block elements for a mechanical system where forces and straight line displacements are involved without any rotation ? 1. Spring 2. Dashpot 3. Mass 4. Moment of inertia Select the correct answer using the code given below. (a) 1, 2 and 4

(b) 1, 3 and 4

(c) 2, 3 and 4

(d) 1, 2 and 3

126. Ans: (d) Sol: A translating mechanical system can be modelled as a single degree of freedom by using simple

spring-mass-damper system. Mass moment of inertia does not affect a translating (straight line displacement) system.

c

k m

127. Consider the following statements regarding electromechanical devices : 1. A potentiometer has an input of rotation and an output of a potential difference. 2. An electric motor has an input of a potential difference and an output of rotation of a shaft. 3. A generator has an input of rotation of a shaft and an output of a potential difference. Which of the above statements are correct ? (a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

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: 69 :

MECHANICAL ENGINEERING [SET‐A]

127. Ans: (c) Sol: Statement I: is wrong

Linear (or) Rotary POT, given relationship is wrong Input: Linear / Angular displacement Output : Change of resistance (P.d only when excitation of POT) 128. The indirect operation of solenoid valve in pneumatic circuit is designed to reduce 1. valve size towards lowering the cost 2. coil size and electrical power consumption 3. response time Which of the above is/are relevant to the context ? (a) 1 only

(b) 2 only

(c) 3 only

(d) 1, 2 and 3

128. Ans: (d)

129. Consider the following statements : 1.

Robots take permissible actions only.

2.

All actions that are obligatory for robots are actually performed by them subject to ties and conflicts among available actions.

3.

All permissible actions can be proved by the robot to be permissible and it can be explained in ordinary English.

Which of the above statements are correct ? (a) 1 and 3 only

(b) 1 and 2 only

(c) 2 and 3 only

(d) 1, 2 and 3

129. Ans: (d)

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ESE‐2018 PRELIMS SOLUTIONS

130. Consider the following statements relating to the term ‘Robot Repeatability’ : 1. It is a statistical term associated with accuracy in the action. 2. It is a measure of the ability of the robot to position the tool tip in the same place repeatedly. 3. It does not describe the error with respect to absolute coordinates. Which of the above statements are correct ? (a) 1 and 2 only

(b) 1 and 3 only

(c) 2 and 3 only

(d) 1, 2 and 3

130. Ans: (d)

131. Consider the following statements regarding homogeneous coordinate transformation matrix : 1. A homogeneous transformation matrix can be considered to consist of four sub-matrices. 2. The upper left 33 sub-matrix represents the position vector. 3. The upper right 31 sub-matrix represents the rotation matrix. 4. The lower left 13 sub-matrix represents perspective transformation. Which of the above statements are correct ? (a) 1 and 3 only

(b) 1 and 4 only

(c) 2 and 3 only

(d) 2 and 4 only

131. Ans: (b) Sol: Options: 2 and 3 sentences are wrong

Option 2: Upper left 33 represent rotation matrix (Not position). Option 3: Upper right 31 represent: position (Not rotation).

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: 71 :

MECHANICAL ENGINEERING [SET‐A]

Directions: Each of the next Nineteen (19) items consists of two statements, one labelled as the ‘Statement (I)’ and the other as ‘Statement (II)’. Examine these two statements carefully and select the answers to these items using the code given below: Code: (a)

Both Statement (I) and Statement (II) are individually true, and Statement (II) is the correct explanation of Statement (I)

(b)

Both Statement (I) and Statement (II) are individually true, but statement (II) is not the correct explanation of statement (I)

(c)

Statement (I) is true, but Statement (II) is false

(d)

Statement (I) is false, but Statement (II) is true

132. Statement (I): A differential inverted U-tube manometer determines the difference in pressures between two points in a flow section to which it is connected. Statement (II): The sensitivity of an inclined gauge depends on the angle of inclination. 132. Ans: (b) Sol: The differential inverted ‘U’ tube manometer measure the pressure difference between the two

points where the manometer is connected. The pressure difference between the two points is given by   P  gx1  m  

  

where, x = manometric reading m = density of manometric fluid  = the density of the fluid whose pressure difference needs to be measured. The sensitivity of the inclined tube manometer is given by sensitivity =

1 sin 

Where,  = angle made by the inclined manometer tube with horizontal. Hence both the statements are correct but statement (II) is not related to statement (I). ACE Engineering Academy

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ESE‐2018 PRELIMS SOLUTIONS

133. Statement (I): In four-bar chain, whenever all four links are used, with each of them forming a turning pair, there will be continuous relative motion between the two links of different lengths. Statement (II): For a four-bar mechanism, the sum of the shortest and longest link lengths is not

greater than the sum of remaining two links. 133. Ans: (c) Sol: Statement II is false because sum of shortest and longest link lengths in 4 bar mechanism can be

greater than/less than or equal to sum of remaining two links. 134. Statement (I): When flow is unsteady, both normal and tangential components of acceleration will occur. Statement (II): During unsteady flow, in addition to the change of velocity along the path, the

velocity will also change with time. 134. Ans: (d) Sol: Statement (I) is not correct. Unsteady flow does not necessarily mean that normal acceleration is

non-zero. For example if streamlines are parallel then even in unsteady flow the normal acceleration is zero. Statement (II) is correct. 135. Statement (I): There exists a positive pressure difference between the inlet and throat of a venturi meter. Statement (II): The coefficient of discharge of venturi meter accounts for the non-uniformity of

flow at both inlet and throat. 135. Ans: (b) Sol: From inlet to throat area decreases, velocity increase and pressure decreases. Hence statement (I) is

correct. The coefficient discharge considers all losses occurring due to non ideal behaviour of fluid. It includes surfaces friction as well as internal friction occurring due to non uniform velocity at each section. ACE Engineering Academy

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: 73 :

MECHANICAL ENGINEERING [SET‐A]

136. Statement (I): The phase of a substance is characterized by its distinct molecular arrangement which is homogeneous throughout and is separated from the others by easily identifiable boundary surfaces. Statement (II): Phase change is not characterized on molecular structure and/or behaviour of the

different phases. 136. Ans: (c) Sol: There phase is a state of system, in a system all phases are chemical homogeneous but their

molecular arrangement is different and physically, mechanically they are different. Each phase in a system is separated by boundary surface. So, statement (I) is correct. The phase change depends molecular forces of different phases molecular structure of two phases in a system is also different. So, statement (II) is incorrect. 137. Statement (I): Non-viscous flow between two plates held parallel with a very small spacing between them is an example of irrotational flow. Statement (II): Forced vortex implies irrotational flow. 137. Ans: (c) Sol: Non-viscous flow between two parallel plates has uniform velocity at each section. In uniform

flow all velocity gradients are zero hence it is irrotational. Forced vortex is not an irrotational vortex. Free vortex is the irrotational vortex. Hence statement (II) is wrong. 138. Statement (I): The air-fuel ratio employed in a gas turbine is around 60:1. Statement (II): A lean mixture of 60:1 in a gas turbine is mainly used for complete combustion. 138. Ans: (c) Sol: Air fuel ratios used in gas turbines vary from 50:1 to 120:1. Hence statement I is correct.

Very lean mixtures are used in gas turbine to reduce the peak temperature of the cycle. (As heat is distributed over a large mass). Maximum temperature in gas turbine are limited by metallurgical consideration which is main criteria. Hence statement II is wrong/incorrect. ACE Engineering Academy

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ESE‐2018 PRELIMS SOLUTIONS

139. Statement (I): The condenser in a steam power plant is always filled with a mixture of water, steam and air. Statement (II): Slightly wet steam enters the condenser wherein the pressure is below the

atmospheric conditions, causing some leakage of air through the glands and also the release of some air dissolved in the boiler feedwater. 139. Ans: (a) Sol: Steam enters the condenser in wet state, hence we have water, vapour and air (Air is always

dissolved in water). Hence statement I is correct. At lower pressures air leaks through glands and also releases some air dissolved in boiler feed water. Hence statement II is correct. 140. Statement (I): In a pipeline, the nature of the fluid flow depends entirely on the velocity. Statement (II): Reynolds number of the flow depends on the velocity, the diameter of the pipe

and the kinematic viscosity of the fluid. 140. Ans: (d) Sol: In pipeline the nature of flow (laminar, turbulent or transition) depends on Reynolds number

 VD   not entirely on velocity. Statement (I) is wrong.     The Reynolds number is given by Re 

VD VD   

Hence, statement (II) is correct. 141. Statement (I): The air-standard efficiency of Brayton cycle depends only on the pressure ratio. Statement (II): For the same compression ratio, the air-standard efficiency of Brayton cycle is

equal to that of Otto cycle. 141. Ans: (b) Sol: Air standard efficiency of Brayton cycle depends only on pressure ratio

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MECHANICAL ENGINEERING [SET‐A]

 1  1      1    1        1      1       r     rk      

Hence, statement II is correct. 142. Statement (I): The energy of an isolated system is constant. Statement (II): The entropy of an isolated system can increase but cannot decrease. 142. Ans: (b) Sol: Energy of the isolated system is constant as there is no energy interaction. Hence statement I is

correct. Entropy of an isolated system always increases and never decreases hence statement II is correct. 143. Statement (I): Rankine efficiency of a steam power plant increase in winter compared to summer. Statement (II): The increase in Rankine efficiency is due to lower condenser temperature. 143. Ans: (a) Sol: In winter condenser temperature decreases as cooling water temperature decreases, hence thermal

efficiency increases. 144. Statement (I): Direct condensers are more efficient than surface condensers. Statement (II): In condenser, the momentum pressure drop opposes the frictional pressure drop. 144. Ans: (b) Sol: Direct condensers are more efficient than contact type condensers as velocity of water + vapour at

inlet is more than velocity of water at exit from condenser. Hence, momentum pressure drop is negative and friction drop is positive.

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ESE‐2018 PRELIMS SOLUTIONS

145. Statement (I): Reheating between the high-pressure and low-pressure turbines increases the turbine work output. Statement (II): The constant pressure lines on T-s diagram diverge away from the origin. 145. Ans: (b) Sol: In a reheat cycle turbine work output increases, statement I is correct.

Constant pressure lines diverge on the T-s diagram from the origin. Statement II is also correct. 146. Statement (I): If a boat, built with sheet metal on wooden frame, has an average density which is greater than that of water, then the boat can float in water with its hollow face upward but will sink once it overturns. Statement (II): Buoyant force always acts in the upward direction. 146. Ans: (b) Sol: When boat is floating in upright position it displaces more water as compared to overturned

position. The buoyancy force is directly proportional the displaced volume. Hence upright position has more buoyancy force and the boat is able to float. Thus statement (I) is correct. Statement (II) is also correct but it is not the correct explanation. 147. Statement (I): In air-blast injection, a separate compressor is used to create an air blast at a pressure of 6 MPa. Statement (II): The solid injection system is heavier as it needs increasing the fuel pressure to 30

MPa. 147. Ans: (c) Sol: In air blast injection, a separate compressor is required to increase the pressure of air to 6 MPa.

Solid injection system does not requires a separate compressor hence its weight is less. Hence statement II is incorrect.

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MECHANICAL ENGINEERING [SET‐A]

148. Statement (I): In air-conditioning, the atmospheric air (mixture of dry air and water vapour) can be considered as mixture of two ideal gases. Statement (II): In the temperature range used in air-conditioning, the partial pressure of the water

vapour is very low and it follows the ideal gas relation with negligible error. 148. Ans: (a) Sol: Statement I is correct as water vapour at very low pressures behaves like an ideal gas with

negligible error. 149. Statement (I): A dynamically balanced system of multiple rotors on a shaft can rotate smoothly at the critical speeds of the system. Statement (II): Dynamic balancing eliminates all the unbalanced forces and couples from the

system. 149. Ans: (a) Sol: A system of rotors on a shaft is said to be in dynamic balance when rotation of rotor does not

produce any resultant centrifugal force and couple due to centrifugal forces.

Dynamically

balanced rotor rotates smoothly at critical speeds of the system. (Unless there is a lateral disturbance / perturbation) Note: Practically it is not possible to eliminate all the unbalanced forces and couples.

150. Statement (I): Referring to vapour compression refrigeration system, the coefficient of performance (COP) of a domestic refrigerator is less than that of a comfort air-conditioning plant. Statement (II): In domestic refrigerator, the work required for pumping the same amount of heat

is more than that in an air-conditioning plant because of greater difference between condenser and evaporator temperatures. 150. Ans: (a) Sol: Evaporator temperature of refrigeration system is less than that of air conditioning system. Hence

work input in refrigerator is more than that of air conditioning system. Therefore, COP of air conditioner is more than that of refrigerator. Compressor and evaporator Pressure difference for domestic refrigerator is more than air-conditioning. ACE Engineering Academy

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ESE‐2018 PRELIMS SOLUTIONS

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