Multiple choice question for engineering
Set 1
1. A flat wall of fire clay, 50 cm thick and initially at 25 degree Celsius, has one of its faces suddenly exposed to a hot gas at 950 degree Celsius. If the heat transfer coefficient on the hot side is 7.5 W/m2 K and the other face of the wall is insulated so that no heat passes out of that face, determine the time necessary to raise the center of the wall to 350 degree Celsius. For fire clay brick
Thermal conductivity = 1.12 W/m K
Thermal diffusivity = 5.16 * 10 -7 m2/s
a) 43.07 hours
b) 53.07 hours
c) 63.07 hours
d) 73.07 hours
Answer
Answer: a [Reason:] t – t a/t 0 – t a = 0.86. Also, α T/l 2 = 0.32.
2. Glass spheres of 2 mm radius and at 500 degree Celsius are to be cooled by exposing them to an air stream at 25 degree Celsius. Find maximum value of convective coefficient that is permissible. Assume the following property values
Density = 2250 kg/m3
Specific heat = 850 J/kg K
Conductivity = 1.5 W/m K
a) 245 W/m2K
b) 235 W/m2K
c) 225 W/m2K
d) 215 W/m2K
Answer
Answer: c [Reason:] l = volume/surface area = r/3. So, h = (0.1) (k) (3)/r.
3. The transient response of a solid can be determined by the equation
a) – 4 p V c = h A (t – t0)
b) – 3 p V c = h A (t – t0)
c) – 2 p V c = h A (t – t0)
d) – p V c = h A (t – t0)
Where,
P is density, V is volume, c is specific heat and A is area.
Answer
Answer: d [Reason:] It can be determined by relating rate of change of internal energy with conductive heat exchange at the surface.
4. A 2 cm thick steel slab heated to 525 degree Celsius is held in air stream having a mean temperature of 25 degree Celsius. Estimate the time interval when the slab temperature would not depart from the mean value of 25 degree Celsius by more than 0.5 degree Celsius at any point in the slab. The steel plate has the following thermal physical properties
Density = 7950 kg/m3
C P = 455 J/kg K
K = 46 W/m K
a) 6548 s
b) 6941 s
c) 4876 s
d) 8760 s
Answer
Answer: b [Reason:] t – t a/t I – t a = exponential (- h A T/p V c). Now A/V = 100 per meter.
5. An average convective heat transfer coefficient for flow of air over a sphere has been measured by observing the temperature-time history of a 12 mm diameter copper sphere (density = 9000 kg/m3 and c = 0.4 k J/kg K) exposed to air at 30 degree Celsius. The temperature of the sphere was measured by two thermocouples one located at the center and the other near the surface. The initial temperature of the ball was 75 degree Celsius and it decreased by 10 degree Celsius in 1.2 minutes. Find the heat transfer coefficient
a) 27.46 W/m2 K
b) 21.76 W/m2 K
c) 29.37 W/m2 K
d) 25.13 W/m2 K
Answer
Answer: d [Reason:] t – t a/t I – t a = exponential (- h A T/p V c). So, h = 25.13 W/m2 K.
6. Transient condition means
a) Conduction when temperature at a point varies with time
b) Very little heat transfer
c) Heat transfer with a very little temperature difference
d) Heat transfer for a short time
Answer
Answer: a [Reason:] The term transient or unsteady state designates a phenomenon which is time dependent.
7. Which of the following is not correct?
In a transient flow process
a) The state of matter inside the control volume varies with time
b) There can be work and heat interactions across the control volume
c) There is no accumulation of energy inside the control volume
d) The rate of inflow and outflow of mass are different
Answer
Answer: c [Reason:] In transient heat conduction there is accumulation of energy inside the control volume.
8. A cylindrical stainless steel (k = 25 W/m K) ingot, 10 cm in diameter and 25 cm long, passes through a heat treatment furnace which is 5 meter in length. The initial ingot temperature is 90 degree Celsius, the furnace gas is at 1260 degree Celsius and the combined radiant and convective surface coefficient is 100 W/m2 K. Determine the maximum speed with which the ingot moves through the furnace if it must attain 830 degree Celsius temperature. Take thermal diffusivity as 0.45 * 10 -5 m2/s
a) . 000116 m/s
b) .000216 m/s
c) . 000316 m/s
d) . 000416 m/s
Answer
Answer: b [Reason:] t – t a/t I – t a = exponential (- h A T/p V c). Now, A/V = 2(r + L)/r L = 0.48 per cm. Also, T = 1158.53 second so required velocity is 0.25/1158.53.
9. The curve for unsteady state cooling or heating of bodies is
a) Hyperbolic curve asymptotic both to time and temperature axis
b) Exponential curve asymptotic both to time and temperature axis
c) Parabolic curve asymptotic to time axis
d) Exponential curve asymptotic to time axis
Answer
Answer: d [Reason:] α/α 0 = exponential [- h A T/p c V], which represents an exponential curve.
10. What is the wavelength band for TV rays?
a) 1 * 10 3 to 34 * 10 10 micron meter
b) 1 * 10 3 to 2 * 10 10 micron meter
c) 1 * 10 3 to 3 * 10 10 micron meter
d) 1 * 10 3 to 56 * 10 10 micron meter
Answer
Answer: b [Reason:] This is the maximum and minimum wavelength for TV rays.
Set 2
1. Time loss factor in Actual Cycle is due to
a) progressive combustion
b) heat loss through cylinder walls
c) gas leakage
d) friction
Answer
Answer: a
2. If the spark timing is at TDC, the work is
a) more
b) less
c) equal
d) none of the mentioned
Answer
Answer: b [Reason:] If the spark timing is at TDC, the work is less as the peak pressure is low.
3. When the spark is advanced, the work output is less as
a) the peak pressure is low
b) the peak temperature is low
c) additional work is required to compress the burning gas
d) frictional losses increase
Answer
Answer: c [Reason:] When the spark is advanced, the work output is less as additional work is required to compress the burning gas.
4. Optimum spark timing gives
a) higher mean effective pressure
b) higher efficiency
c) higher mean effective pressure and higher efficiency
d) none of the mentioned
Answer
Answer: c
5. The major loss in a S.I. engine is due to
a) exhaust blow down
b) pumping
c) incomplete combustion
d) variation in specific heat and chemical equilibrium
Answer
Answer: d [Reason:] The major loss in a S.I. engine is due to variation in specific heat and chemical equilibrium while the major loss in a C.I. engine is loss due to incomplete combustion.
6. The major loss in a C.I. engine is
a) direct heat loss
b) loss due to incomplete combustion
c) rubbing friction loss
d) pumping loss
Answer
Answer: b
7. In an actual S.I engine the pumping loss with respect to speed
a) increases
b) decreases
c) remains same
d) none of the mentioned
Answer
Answer: b
8. The volumetric efficiency is affected by
a) the exhaust gas in the clearance volume
b) the design of intake and exhaust valve
c) valve timing
d) all of the mentioned
Answer
Answer: d
9. In petrol engine the actual pressure developed compared to the predicted maximum pressure is
a) 90%
b) 70%
c) 50%
d) 25%
Answer
Answer: c [Reason:] In petrol engine the actual pressure developed compared to the predicted maximum pressure is 50% while energy released in actual cycle is about 90% of the fuel input.
10. Energy released in actual cycle is about _____________ of the fuel input.
a) 90%
b) 70%
c) 50%
d) 25%
Answer
Answer: a
Set 3
1. Air injected in IC engines refers to injection of
a) air only
b) liquid fuel only
c) liquid fuel and air
d) none of the mentioned
Answer
Answer: c [Reason:] Liquid fuel and air are injected in IC engines.
2. The m.e.p. of a petrol engine first increases as the fuel air ratio is increased and then decreases on further increase in fuel air ratio. The m.e.p. is maximum in the zone of
a) lean mixture
b) chemically correct mixture
c) rich mixture
d) none of the mentioned
Answer
Answer: c [Reason:] The m.e.p. is maximum in the zone of rich mixture.
3. The specific fuel consumption for a petrol engine first decreases with increase in fuel air ratio and then increases with further increase in air fuel ratio. The maximum value occurs in the range of
a) lean mixture
b) chemically correct mixture
c) rich mixture
d) none of the mentioned
Answer
Answer: a [Reason:] The maximum value occurs in the range of lean mixture.
4. Auto-ignition reaction time for petrol engine first decreases with increase in fuel air ratio, reaches a minimum value and then increases with subsequent increase in fuel air ratio. The maximum value occurs in the region of
a) lean mixture
b) chemically correct mixture
c) rich mixture
d) none of the mentioned
Answer
Answer: b [Reason:] The maximum value occurs in the region of chemically correct mixture.
5. The most effective air cleaner in case of diesel engines is
a) dry type
b) wet type
c) whirl type
d) oil bath type
Answer
Answer: d [Reason:] Oil bath type is the most effective air cleaner in case of diesel engines.
6. A high flame speed is obtained in diesel engine when air fuel ratio is
a) uniform throughout the mixture
b) chemically correct mixture
c) about 3-5% rich mixture
d) about 10% rich mixture
Answer
Answer: d [Reason:] When air fuel ratio is about 10% rich mixture, then a high flame speed is obtained in diesel engine.
7. During idling, a petrol engine requires ______________ mixture.
a) lean
b) rich
c) chemically correct
d) none of the mentioned
Answer
Answer: b [Reason:] Rich mixture is required by a petrol engine when it is in idling condition.
8. The ___________ engines can work on very lean mixture of fuel.
a) spark ignition
b) compression ignition
c) all of the mentioned
d) none of the mentioned
Answer
Answer: b [Reason:] Only the compression ignition engines can work on very lean mixture of fuel while spark ignition requires richer mixture.
9. In order to mix air and petrol in the required proportion and to supply it to the engine during suction stroke, then ______________ is employed.
a) fuel pump
b) injector
c) carburetor
d) none of the mentioned
Answer
Answer: c
Set 4
1. The air standard efficiency of an I.C. engine depends on
a) fuel used
b) speed of engine
c) compression ratio
d) none of the mentioned
Answer
Answer: c
2. The specific fuel consumption per B.P. hour for a petrol engine is about
a) 0.2 kg
b) 0.25 kg
c) 0.3 kg
d) 0.35 kg
Answer
Answer: b [Reason:] Generally, the specific fuel consumption per B.P. hour for a petrol engine is about 0.25 kg while the specific fuel consumption per B.P. hour for a diesel engine is about 0.2 kg.
3. The specific fuel consumption per B.P. hour for a diesel engine is about
a) 0.2 kg
b) 0.25 kg
c) 0.3 kg
d) 0.35 kg
Answer
Answer: a [Reason:] Generally, the specific fuel consumption per B.P. hour for a diesel engine is about 0.2 kg while the specific fuel consumption per B.P. hour for a petrol engine is about 0.25 kg.
4. If the compression ratio in I.C. engine increases, then its thermal efficiency will
a) increase
b) decrease
c) remain same
d) none of the mentioned
Answer
Answer: a [Reason:] The compression ratio in I.C. engine is directly proportional to the thermal efficiency, so as it increases, then its thermal efficiency will increase.
5. The thermal efficiency of petrol and gas engines is about
a) 15%
b) 30%
c) 50%
d) 70%
Answer
Answer: b [Reason:] The thermal efficiency of petrol and gas engines is about 30% and the thermal efficiency of diesel engines is about 70%.
6)The thermal efficiency of diesel engines is about
a) 15%
b) 30%
c) 50%
d) 70%
Answer
Answer: d [Reason:] The thermal efficiency of diesel engines is about 70% while the thermal efficiency of petrol and gas engines is about 30%.
7. The thermal efficiency of diesel engines on weak mixtures is
a) unaffected
b) lower
c) higher
d) dependent on other factors
Answer
Answer: c
8. The volumetric efficiency of a well designed engine may be
a) 30 to 40%
b) 40 to 60%
c) 60 to 70%
d) 75 to 90%
Answer
Answer: d
9) The working medium of a air standard cycle has constant ___________ throughout the cycle.
a) pressure
b) volume
c) specific heat
d) mass
Answer
Answer: c
10. It is assumed that all the processes of a air standard cycle are
a) reversible
b) irreversible
c) adiabatic
d) isothermal
Answer
Answer: a
Set 5
1. Advantage of gaseous fuel is that
a) it can be stored easily
b) it can mix easily with air
c) it can displace more air from the engine
d) all of the mentioned
Answer
Answer: b
2. Paraffins are generally represented by
a) CnHn
b) CnH2n
c) CnH2n+2
d) CnH2n-6
Answer
Answer: c [Reason:] Paraffins are generally represented by CnH2n+2 while olefins are generally represented by CnHn .
3. Paraffins have molecular structure of
a) chain saturated
b) chain unsaturated
c) ring saturated
d) ring unsaturated
Answer
Answer: a
4. Olefins are generally represented by
a) CnHn
b) CnH2n
c) CnH2n+2
d) CnH2n-6
Answer
Answer: a [Reason:] Olefins are represented by CnHn and paraffins are generally represented by CnH2n+2.
5. Hydrocarbons are decomposed into smaller hydrocarbons by
a) reforming
b) refining
c) cracking
d) polymerization
Answer
Answer: c [Reason:] Hydrocarbons are decomposed into smaller hydrocarbons by cracking while the molecular structure of the straight-run gasoline is changed by reforming.
6. The molecular structure of the straight-run gasoline is changed by
a) reforming
b) refining
c) cracking
d) boiling
Answer
Answer: a [Reason:] The molecular structure of the straight-run gasoline is changed by reforming and hydrocarbons are decomposed into smaller hydrocarbons by cracking.
7. For S.I. engines fuel most preferred are
a) aromatics
b) paraffins
c) olefins
d) napthenes
Answer
Answer: a [Reason:] For S.I. engines fuel most preferred are aromatics while for C.I. engines fuel most preferred are paraffins.
8. For C.I. engines fuel most preferred are
a) aromatics
b) paraffins
c) olefins
d) napthenes
Answer
Answer: b [Reason:] For C.I. engines fuel most preferred are paraffins and for S.I. engines fuel most preferred are aromatics.
9. In spark ignition engines, the knocking tendency can be decreased by
a) decreasing compression ratio
b) controlling intake throttle
c) controlling ignition timing
d) adding dopes like tetraethyl lead and ethylene dibromide
Answer
Answer: d
10. Detonation in petrol engines can be suppressed or reduced by the addition of small quantity of
a) lead nitrate
b) iso-octane
c) n-heptane
d) none of the mentioned
Answer
Answer: a
