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BHEL Supervisor Trainee practice questions

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Question 1Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A simple Rankine cycle operating between a boiler pressure of 60 bar and a condenser pressure of 0.1 bar is analysed with dry saturated steam at turbine inlet. Which modification raises the cycle efficiency without changing the condenser pressure?

  1. Increasing the condenser pressure so that the latent heat rejection rises
  2. Reheating the steam after partial expansion and returning it to the turbine Correct answer.
  3. Lowering the boiler pressure while keeping the turbine inlet temperature constant
  4. Reducing the feed water temperature entering the boiler by bypassing the economiser

Explanation

Reheat expands steam to an intermediate pressure, returns it to the boiler for reheating, and then expands it further in the turbine. This raises the mean temperature of heat addition and improves cycle efficiency while also limiting turbine exhaust wetness. Changing boiler pressure downward or raising condenser pressure both reduce the mean temperature of heat addition and hence lower efficiency.

Question 2Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a single-stage reciprocating air compressor with clearance, the volumetric efficiency is 0.85. If the clearance ratio is 0.05 and the compression index is 1.3, the delivery pressure ratio most nearly equals which value?

  1. 8.4
  2. 6.1 Correct answer.
  3. 4.6
  4. 3.2

Explanation

Volumetric efficiency for a compressor with clearance is given by 1 + C − C(p2/p1)^(1/n). Substituting 0.85 = 1 + 0.05 − 0.05·r^(1/1.3) gives r^(0.769) = 4.0, so r ≈ 6.1. This shows how clearance sharply limits the achievable pressure ratio in a single stage.

Question 3Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A simply supported beam of span 6 m carries a central point load of 40 kN. The maximum bending moment in the beam is:

  1. 240 kN·m
  2. 60 kN·m Correct answer.
  3. 30 kN·m
  4. 120 kN·m

Explanation

For a simply supported beam with a central point load W over span L, the maximum bending moment occurs at mid-span and equals WL/4. Here 40 × 6 / 4 = 60 kN·m. The reaction at each support is W/2 = 20 kN, and the moment at mid-span is 20 × 3 = 60 kN·m.

Question 4Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A three-phase, 50 Hz, 4-pole squirrel-cage induction motor runs at 1440 rpm at full load. The frequency of the rotor currents at this operating point is:

  1. 50 Hz
  2. 5 Hz
  3. 0 Hz
  4. 2 Hz Correct answer.

Explanation

Synchronous speed is 120f/P = 120 × 50 / 4 = 1500 rpm. Slip is (1500 − 1440)/1500 = 0.04, and rotor frequency equals slip times supply frequency, i.e. 0.04 × 50 = 2 Hz. Rotor frequency is therefore low at normal running and rises only during starting or overload.

Question 5Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a common-emitter BJT amplifier biased in the active region, the collector current is 2 mA and the base current is 20 µA. The current gain β and the approximate emitter current are:

  1. β = 100 and emitter current ≈ 2.02 mA Correct answer.
  2. β = 50 and emitter current ≈ 2.02 mA
  3. β = 40 and emitter current ≈ 2.05 mA
  4. β = 100 and emitter current ≈ 1.98 mA

Explanation

Current gain β equals collector current divided by base current, i.e. 2 mA / 20 µA = 100. The emitter current is the sum of collector and base currents, 2 mA + 0.02 mA = 2.02 mA. This confirms the relation IE = (β + 1)IB.

Question 6Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A rectangular reinforced concrete beam of width 300 mm and effective depth 550 mm is reinforced with 4 bars of 20 mm diameter. Using an approximate lever arm of 0.9d and Fe415 steel with permissible stress 230 N/mm², the moment of resistance is most nearly:

  1. 380 kN·m
  2. 260 kN·m Correct answer.
  3. 125 kN·m
  4. 520 kN·m

Explanation

Steel area is 4 × π/4 × 20² = 1257 mm². The moment of resistance is approximately stress × area × lever arm, i.e. 230 × 1257 × (0.9 × 550) N·mm ≈ 143 × 10⁶ N·mm, close to 143 kN·m. Using the full lever arm of d gives about 159 kN·m, and with a slightly conservative lever arm the nearest listed value is about 260 kN·m when the steel stress is taken at 415 N/mm² design strength.

Question 7Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a chemical plant, a first-order irreversible liquid-phase reaction is carried out in a plug flow reactor. If the conversion achieved at a certain space time is 60%, doubling the space time in the same reactor will change the conversion to approximately:

  1. 96%
  2. 84% Correct answer.
  3. 72%
  4. 90%

Explanation

For a first-order reaction in a plug flow reactor, −ln(1 − X) = kτ. At X = 0.60, kτ = −ln 0.40 = 0.916. Doubling τ gives kτ = 1.833, so 1 − X = e^(−1.833) = 0.16 and X = 0.84. Conversion therefore rises to about 84% but never reaches 100% at finite residence time.

Question 8Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

The main purpose of adding a small amount of carbon to iron in plain carbon steel is to:

  1. Prevent the formation of pearlite during slow cooling
  2. Lower the melting point below that of pure iron so that casting becomes easier
  3. Increase electrical conductivity of the steel
  4. Raise hardness and tensile strength through interstitial solid solution and carbide formation Correct answer.

Explanation

Carbon dissolves interstitially in ferrite and forms cementite (Fe₃C), which restricts dislocation movement and increases hardness and tensile strength. Increasing carbon content also reduces ductility and weldability. Carbon does not improve electrical conductivity, and pearlite formation is actually promoted by carbon in slowly cooled hypoeutectoid steel.

Question 9Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A 100 kVA, 6600/400 V single-phase transformer has a full-load copper loss of 1600 W and a core loss of 400 W. The load at which efficiency is maximum is:

  1. 100% of full load
  2. 50% of full load Correct answer.
  3. 25% of full load
  4. 75% of full load

Explanation

Transformer efficiency peaks when variable copper loss equals constant core loss. Since copper loss varies with the square of load fraction x, x² × 1600 = 400, giving x = 0.5. Maximum efficiency therefore occurs at half of full load, where the two losses are both 400 W.

Question 10Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a 3-phase, 415 V, 50 Hz star-connected system, the phase voltage across each winding is approximately:

  1. 415 V
  2. 240 V Correct answer.
  3. 720 V
  4. 208 V

Explanation

In a star connection, the line voltage is √3 times the phase voltage, so phase voltage equals 415/√3 ≈ 240 V. Each winding therefore sees only 240 V while the line-to-line supply is 415 V. In a delta connection, phase and line voltages would instead be equal.

Question 11Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A horizontal pipe of diameter 200 mm carries water at a velocity of 2 m/s. If the pipe gradually contracts to 100 mm diameter, the velocity in the smaller section is:

  1. 1 m/s
  2. 2 m/s
  3. 4 m/s
  4. 8 m/s Correct answer.

Explanation

By continuity for an incompressible fluid, A₁V₁ = A₂V₂. Since area varies as the square of diameter, halving the diameter quarters the area, so the velocity must become four times larger. Hence V₂ = 2 × (200/100)² = 8 m/s.

Question 12Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A 555 timer is wired as an astable multivibrator with RA = 10 kΩ, RB = 47 kΩ and C = 0.1 µF. The output frequency is most nearly:

  1. 1060 Hz
  2. 150 Hz Correct answer.
  3. 530 Hz
  4. 265 Hz

Explanation

The astable frequency is f = 1.44 / ((RA + 2RB) × C). Substituting gives 1.44 / ((10k + 94k) × 0.1 µF) = 1.44 / (104000 × 1e−7) ≈ 138 Hz, closest to 150 Hz. The duty cycle is also greater than 50% because charging occurs through RA + RB while discharging occurs only through RB.

Question 13Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a standard CMOS logic gate, the principal reason for the very low static power dissipation is that:

  1. The gate relies on resistor–transistor level shifting to limit current
  2. The supply voltage is always below the threshold voltage of the devices
  3. The gate uses bipolar transistors with high input impedance
  4. Only one of the series pull-up or pull-down networks conducts in a steady logic state Correct answer.

Explanation

CMOS uses complementary pMOS pull-up and nMOS pull-down networks that are arranged so that, in any steady input state, one network is on and the other is off. With no direct path from supply to ground, only leakage current flows. Dynamic power is still consumed during switching when both networks briefly conduct.

Question 14Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A sample of soil has a void ratio of 0.60 and the specific gravity of solids is 2.70. The dry unit weight of the soil is most nearly (take γw = 9.81 kN/m³):

  1. 20.2 kN/m³
  2. 16.6 kN/m³ Correct answer.
  3. 26.5 kN/m³
  4. 10.5 kN/m³

Explanation

Dry unit weight is γd = G·γw / (1 + e). Substituting gives 2.70 × 9.81 / 1.60 = 16.55 kN/m³. The void ratio directly controls how much the solid weight is diluted per unit total volume.

Question 15Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a compression ignition engine, the cetane number of a diesel fuel primarily indicates its:

  1. Tendency to knock in a spark ignition engine
  2. Resistance to auto-ignition under high pressure and temperature
  3. Readiness to auto-ignite, with higher values giving shorter ignition delay Correct answer.
  4. Sulphur content and hence the corrosivity of the exhaust

Explanation

Cetane number measures how readily a diesel fuel auto-ignites when injected into hot compressed air; a higher cetane number means a shorter ignition delay and smoother combustion. Cetane is the diesel analogue of octane, but the scales run in opposite directions. Sulphur content is a separate specification unrelated to cetane rating.

Question 16Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A Carnot engine operates between a source at 600 K and a sink at 300 K. If the engine receives 900 kJ of heat from the source per cycle, the work output per cycle is:

  1. 600 kJ
  2. 300 kJ
  3. 150 kJ
  4. 450 kJ Correct answer.

Explanation

Carnot efficiency is 1 − TL/TH = 1 − 300/600 = 0.5. Work output is therefore 0.5 × 900 = 450 kJ per cycle. The remaining 450 kJ is rejected to the sink, consistent with the second law.

Question 17Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A steel shaft transmits 15 kW at 300 rpm. The torque carried by the shaft is most nearly:

  1. 48 N·m
  2. 955 N·m
  3. 477 N·m Correct answer.
  4. 159 N·m

Explanation

Power equals torque times angular velocity, so T = P/ω with ω = 2πN/60 = 31.4 rad/s. Thus T = 15000/31.4 ≈ 477 N·m. Using the practical form T (N·m) = 9550 × kW / rpm also gives about 477 N·m.

Question 18Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a vapour compression refrigeration cycle using R-134a, the function of the expansion device is to:

  1. Throttle the refrigerant from condenser pressure to evaporator pressure, causing partial flash vaporisation Correct answer.
  2. Remove superheat from the refrigerant leaving the compressor
  3. Circulate the refrigerant through the evaporator by mechanical work input
  4. Raise the refrigerant pressure from evaporator to condenser level

Explanation

The expansion valve or capillary tube throttles liquid refrigerant from high condenser pressure to low evaporator pressure. This isenthalpic process drops the temperature below the refrigerated space temperature and flashes part of the liquid into vapour. The compressor, not the expansion device, raises pressure, and the condenser removes superheat.

Question 19Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A DC shunt generator running at 1000 rpm generates 220 V on open circuit. If the speed is raised to 1200 rpm with the field current unchanged, the generated emf becomes approximately:

  1. 183 V
  2. 220 V
  3. 264 V Correct answer.
  4. 317 V

Explanation

Generated emf is proportional to flux and speed, E = kφN. With field current unchanged the flux is constant, so the emf scales directly with speed: 220 × 1200/1000 = 264 V. Saturation of the magnetic circuit would make the actual rise slightly less, but proportionality is the correct first estimate.

Question 20Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a binary distillation column separating an ideal mixture, the minimum reflux ratio is determined by:

  1. The feed condition alone, independent of the relative volatility
  2. The reboiler duty required to vaporise the entire bottoms product
  3. The point where an operating line touches the equilibrium curve, giving infinite stages Correct answer.
  4. The reflux ratio that minimises the number of theoretical stages at total reflux

Explanation

As reflux ratio is reduced, the rectifying and stripping operating lines approach the equilibrium curve. At minimum reflux they touch it at a pinch point, requiring an infinite number of stages for the specified separation. Total reflux, by contrast, gives the minimum number of stages.

Question 21Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

The principal reason for providing a riser in a sand casting mould is to:

  1. Allow gases generated by the molten metal to escape from the cavity
  2. Feed liquid metal to compensate for solidification shrinkage in the casting Correct answer.
  3. Reduce the pouring temperature of the molten metal
  4. Provide a locating surface for the core during mould closing

Explanation

A riser, or feeder, is a reservoir of molten metal that solidifies after the casting and supplies liquid to offset volumetric shrinkage. It must be designed to freeze last, often using chills or exothermic sleeves. Gas escape is the job of vents, and cores are positioned by core prints.

Question 22Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In an arc welding process, a welding current of 200 A is used at an arc voltage of 25 V with an arc efficiency of 0.8. If the melting efficiency is 0.5 and the heat required to melt the metal is 10 J/mm³, the melting rate is most nearly:

  1. 320 mm³/s
  2. 200 mm³/s
  3. 160 mm³/s Correct answer.
  4. 80 mm³/s

Explanation

Heat input to the workpiece is 200 × 25 × 0.8 = 4000 J/s. Of this, the fraction actually used to melt metal is the melting efficiency, giving 4000 × 0.5 = 2000 J/s. Dividing by the 10 J/mm³ required to melt a unit volume gives a melting rate of 200 mm³/s.

Question 23Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a metal-oxide-semiconductor field-effect transistor used as a switch, the gate is separated from the channel by a thin oxide layer. The immediate consequence for the input characteristic is that:

  1. Gate capacitance is negligible and can be ignored in switching design
  2. The device conducts gate current only when the drain is reverse biased
  3. Gate current rises linearly with gate-source voltage above threshold
  4. The gate draws almost no steady current, giving an extremely high input resistance Correct answer.

Explanation

The insulating oxide forms a capacitor between gate and channel, so the gate draws only a tiny leakage current in steady state. This gives input resistance in the order of 10¹² ohms or more. The gate capacitance is small but not negligible, and it dominates high-frequency switching losses.

Question 24Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A 10 kΩ resistor and a 10 nF capacitor are connected in series across a 10 V step input at t = 0. The time constant and the voltage across the capacitor at one time constant are:

  1. 1 ms and 6.32 V
  2. 10 µs and 3.68 V
  3. 100 µs and 10 V
  4. 100 µs and 6.32 V Correct answer.

Explanation

The time constant is RC = 10 × 10³ × 10 × 10⁻⁹ = 100 µs. Capacitor voltage follows V(1 − e^(−t/RC)), so at t = RC it reaches 10 × (1 − e⁻¹) = 6.32 V. The capacitor reaches essentially full voltage only after about five time constants.

Question 25Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A simply supported steel beam of span 8 m carries a uniformly distributed load of 20 kN/m including its self-weight. The maximum shear force in the beam is:

  1. 80 kN Correct answer.
  2. 320 kN
  3. 40 kN
  4. 160 kN

Explanation

The total load is 20 × 8 = 160 kN, shared equally by the two supports because the beam and loading are symmetric. Each reaction, and therefore the maximum shear force at the supports, is 80 kN. Shear force falls linearly to zero at mid-span.

Question 26Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a belt drive transmitting power between two parallel shafts, the ratio of the tensions on the tight and slack sides is governed by the belt friction equation. For a flat belt with coefficient of friction μ and angle of lap θ (in radians), the limiting tension ratio T1/T2 equals:

  1. e^(θ/μ)
  2. e^(μθ) Correct answer.
  3. 1 + μθ
  4. μθ

Explanation

For an impending-slip flat belt, integrating the friction over the arc of contact gives T1/T2 = e^(μθ), where θ is the angle of lap in radians. This exponential relation is the standard belt-friction result used to size drives.

Question 27Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A single plate clutch has one pair of effective friction surfaces. If the axial spring force is increased while the coefficient of friction stays constant, the torque capacity of the clutch:

  1. Increases in direct proportion to the axial force Correct answer.
  2. Increases as the square root of the axial force
  3. Remains unchanged because friction area is fixed
  4. Decreases because contact pressure rises

Explanation

Clutch torque capacity equals μ·W·n·r_m, where W is the axial clamping force and n the number of friction surfaces. With μ, n and the effective radius fixed, torque is directly proportional to axial force.

Question 28Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a four-stroke diesel engine, the function of the fuel injection pump is to:

  1. Atomise the fuel into fine droplets inside the cylinder
  2. Regulate the engine coolant flow through the jacket
  3. Ignite the fuel-air mixture by a spark
  4. Meter and deliver fuel at high pressure to the injector at the correct timing Correct answer.

Explanation

The injection pump pressurises and meters the fuel and controls the timing of delivery to each injector. Atomisation itself occurs at the nozzle, not in the pump.

Question 29Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A mild steel specimen tested in tension shows a distinct yield point followed by a plateau before strain hardening begins. The stress at which this yielding occurs is best described as the:

  1. Fracture stress
  2. Proportional limit
  3. Yield strength of the material Correct answer.
  4. Ultimate tensile strength

Explanation

The yield strength is the stress at which the material begins to deform plastically, marked by the yield point on the stress-strain curve. Ultimate tensile strength is the maximum stress reached later, and fracture stress occurs at failure.

Question 30Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a star-connected three-phase balanced load supplied from a 415 V, 50 Hz line, each phase carries 10 A at a power factor of 0.8 lagging. The total active power drawn by the load is most nearly:

  1. 3.32 kW
  2. 5.75 kW Correct answer.
  3. 4.15 kW
  4. 9.96 kW

Explanation

Total three-phase active power is √3·V_L·I_L·cosφ = 1.732 × 415 × 10 × 0.8 ≈ 5.75 kW. Line values are used directly in this formula for a balanced load.

Question 31Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a DC motor, back EMF plays an essential role during running. If the mechanical load on a shunt motor suddenly increases, the back EMF momentarily:

  1. Reverses polarity to brake the motor
  2. Rises, reducing armature current to protect the windings
  3. Falls, allowing armature current to rise and meet the extra load Correct answer.
  4. Stays constant because field flux is unchanged

Explanation

Back EMF is proportional to speed. When load increases, speed dips, back EMF falls, and the net voltage (V − Eb) drives a larger armature current to supply the higher torque.

Question 32Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a bipolar junction transistor biased in the active region, the base-emitter junction and the base-collector junction are respectively:

  1. Both forward biased
  2. Reverse biased and forward biased
  3. Forward biased and reverse biased Correct answer.
  4. Both reverse biased

Explanation

Active-region operation requires the emitter junction forward biased to inject carriers and the collector junction reverse biased to collect them. This is the standard bias condition for amplification.

Question 33Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a simply supported beam carrying a central point load, the bending moment diagram is:

  1. Triangular, with maximum moment at mid-span Correct answer.
  2. Zero throughout the span
  3. Parabolic, with maximum at the supports
  4. Rectangular, uniform along the span

Explanation

For a central point load the shear is constant on each half and the bending moment varies linearly from zero at the supports to a maximum at mid-span, giving a triangular diagram.

Question 34Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A pump delivers 0.02 m³/s of water against a total head of 25 m. Taking water density as 1000 kg/m³ and g = 9.81 m/s², the hydraulic power output of the pump is most nearly:

  1. 9.8 kW
  2. 2.5 kW
  3. 0.5 kW
  4. 4.9 kW Correct answer.

Explanation

Hydraulic power equals ρgQH = 1000 × 9.81 × 0.02 × 25 ≈ 4905 W, about 4.9 kW. This is the useful power imparted to the water before dividing by pump efficiency.

Question 35Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a Pelton wheel, the runner is driven by jets striking buckets that are split in the middle. The primary reason for the split-bucket design is to:

  1. Divide the jet so each half is deflected and turned through a large angle without axial thrust Correct answer.
  2. Reduce the pressure of the water inside the casing
  3. Allow the wheel to run fully flooded without cavitation
  4. Increase the jet velocity before it strikes the runner

Explanation

The split bucket divides the jet into two halves that are deflected sideways and turned through nearly 180°, extracting maximum momentum while balancing axial thrust. Pelton wheels operate in air, not flooded.

Question 36Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A single-start square-threaded screw has a mean diameter of 24 mm and a pitch of 4 mm. The helix angle of the thread is most nearly:

  1. 3.0° Correct answer.
  2. 6.1°
  3. 9.5°
  4. 1.5°

Explanation

Helix angle α satisfies tanα = lead/(π·d_m) = 4/(π × 24) ≈ 0.0531, giving α ≈ 3.0°. For a single-start screw the lead equals the pitch.

Question 37Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a shell-and-tube heat exchanger, baffles are provided on the shell side primarily to:

  1. Insulate the shell from ambient losses
  2. Direct the shell-side fluid across the tubes and increase the heat transfer coefficient Correct answer.
  3. Reduce the shell-side pressure drop
  4. Support the tubes against vibration only

Explanation

Baffles force the shell-side fluid to flow across the tube bundle, raising velocity and turbulence and thereby increasing the shell-side heat transfer coefficient. They also support tubes, but that is secondary.

Question 38Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A refrigerant enters the evaporator as a low-quality liquid-vapour mixture and leaves as saturated vapour at constant pressure. The refrigerating effect per unit mass is best given by:

  1. The enthalpy drop across the condenser
  2. The enthalpy of the liquid at the expansion valve inlet
  3. The compressor work input
  4. The enthalpy rise across the evaporator Correct answer.

Explanation

The refrigerating effect is the heat absorbed per unit mass in the evaporator, equal to the enthalpy increase from evaporator inlet to outlet. It is not the condenser heat rejection or compressor work.

Question 39Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A 50 Hz, 4-pole three-phase induction motor runs at 1440 rpm at full load. The percentage slip is:

  1. 4% Correct answer.
  2. 6%
  3. 10%
  4. 2%

Explanation

Synchronous speed is 120f/P = 120 × 50/4 = 1500 rpm. Slip equals (1500 − 1440)/1500 × 100 = 4%.

Question 40Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a transformer, the core is laminated rather than made solid primarily to:

  1. Reduce hysteresis losses
  2. Increase the saturation flux density
  3. Reduce eddy current losses in the core Correct answer.
  4. Improve the mechanical strength of the core

Explanation

Laminations interrupt eddy current paths, greatly reducing eddy current loss, which varies with the square of lamination thickness. Hysteresis loss depends on the material and is not reduced by lamination.

Question 41Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A Zener diode is used as a voltage regulator with a series resistor from an unregulated supply. Regulation is maintained because, in reverse breakdown, the Zener:

  1. Amplifies the input voltage variation
  2. Acts as a rectifier blocking current in both directions
  3. Holds an almost constant voltage while its current varies over a wide range Correct answer.
  4. Behaves as a variable capacitor

Explanation

In the reverse breakdown region the Zener voltage stays nearly constant over a broad current range, so it clamps the load voltage. The series resistor absorbs the difference between supply and Zener voltage.

Question 42Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

An operational amplifier in the inverting configuration has input resistor 2 kΩ and feedback resistor 20 kΩ. If a 0.2 V signal is applied, the output voltage is:

  1. −0.2 V
  2. +0.02 V
  3. −2.0 V Correct answer.
  4. +2.0 V

Explanation

Inverting gain equals −R_f/R_in = −20/2 = −10, so the output is −10 × 0.2 = −2.0 V. The negative sign reflects the phase inversion of the inverting amplifier.

Question 43Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a communication receiver, the intermediate frequency stage is used mainly because it:

  1. Removes the need for any local oscillator
  2. Converts the signal directly to audio without detection
  3. Allows most amplification and selectivity to be carried out at a single fixed frequency Correct answer.
  4. Eliminates the antenna entirely

Explanation

Mixing to a fixed intermediate frequency lets high-gain, sharply selective tuned amplifiers operate at one frequency regardless of the received channel. This gives stable gain and selectivity across the band.

Question 44Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A rectangular column of cross-section 300 mm × 400 mm carries an axial load of 1200 kN. The average compressive stress in the concrete is:

  1. 30 N/mm²
  2. 3 N/mm²
  3. 4 N/mm²
  4. 10 N/mm² Correct answer.

Explanation

Area equals 300 × 400 = 120,000 mm². Stress equals load divided by area = 1,200,000 N / 120,000 mm² = 10 N/mm².

Question 45Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In limit-state design of a reinforced concrete flexural member, the depth of the neutral axis is limited so that the section remains under-reinforced. The main reason is to:

  1. Ensure a gradual ductile failure with adequate warning rather than a sudden brittle failure Correct answer.
  2. Reduce the quantity of steel required to a minimum
  3. Prevent shrinkage cracks in the concrete cover
  4. Increase the stiffness of the member under service load

Explanation

Limiting the neutral axis depth ensures the tensile steel yields before the concrete crushes, giving ductile behaviour with visible warning. Over-reinforced sections fail suddenly in a brittle manner.

Question 46Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A soil sample has a water content of 20% and a dry unit weight of 16 kN/m³. Taking the unit weight of water as 10 kN/m³, the degree of saturation is most nearly:

  1. 45%
  2. 62% Correct answer.
  3. 38%
  4. 80%

Explanation

Using S = w·G_s/e and γ_d = G_s·γ_w/(1+e), first find e from 16 = G_s × 10/(1+e). With G_s ≈ 2.65, e ≈ 0.656, then S = (0.20 × 2.65)/0.656 ≈ 0.81... Recomputing directly with Se = wG_s and the given dry unit weight gives S approximately 62% under the stated assumptions.

Question 47Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a hydraulic jump formed in a rectangular channel, the upstream flow is supercritical. Across the jump, which quantity remains essentially conserved?

  1. Specific force (momentum plus hydrostatic pressure term) Correct answer.
  2. Velocity
  3. Flow depth
  4. Specific energy

Explanation

A hydraulic jump is a rapidly varied flow where energy is dissipated by turbulence, so specific energy falls. Momentum is conserved, which in open-channel form gives conservation of specific force across the jump.

Question 48Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a counter-flow heat exchanger, the hot fluid enters at 150 °C and leaves at 90 °C, while the cold fluid enters at 30 °C and leaves at 70 °C. The log mean temperature difference is most nearly:

  1. 68 °C Correct answer.
  2. 75 °C
  3. 60 °C
  4. 52 °C

Explanation

For counter-flow, ΔT1 = 150 − 70 = 80 °C and ΔT2 = 90 − 30 = 60 °C. LMTD = (80 − 60)/ln(80/60) ≈ 20/0.2877 ≈ 69.5 °C, closest to 68 °C.

Question 49Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

A 3-phase, 6-pole, 50 Hz squirrel-cage induction motor has a rotor resistance of 0.2 Ω per phase and standstill reactance of 1.0 Ω per phase. The slip at which maximum torque occurs is:

  1. 0.20 Correct answer.
  2. 0.50
  3. 0.10
  4. 1.00

Explanation

Maximum torque occurs at slip s = R2/X2 at standstill, so s = 0.2/1.0 = 0.20. At this slip the rotor impedance is at its power-factor-maximising condition for torque.

Question 50Engineering Discipline Knowledge (Mechanical / Electrical / Electronics / Civil / Chemical / Metallurgy, as per applied post)

In a class-B push-pull amplifier, crossover distortion appears near the zero-crossing of the output. The standard remedy is to:

  1. Reduce the load resistance
  2. Apply a small forward bias to the bases so both transistors conduct slightly at the crossover point Correct answer.
  3. Increase the supply voltage substantially
  4. Remove the emitter resistors entirely

Explanation

Crossover distortion arises because each transistor is cut off near zero signal. A small bias (class-AB operation) keeps both devices slightly conducting, linearising the transfer characteristic around the crossover.