Mechanical Engineering Interview Q&A

[pandikumar]

ULTRA COLLEGE OF ENGINEERING AND TECHNOLOGY FOR WOMEN MADURAI
MECHANICAL ENGINEERING INTERVIEW QUESTIONS & ANSWERS
With Best wishes from Department of Mechanical Engineering
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1. What is the importance of the Thermodynamics in the field of Mechanical
Engineering?
All the mechanical engineering systems are studied with the help of thermodynamics. Hence
it is very important for the mechanical engineers.
2. How many Laws of Thermodynamics are there?
There are three laws of the thermodynamics.
First Law: Energy can be neither created nor destroyed. It can only change forms. In any
process in an isolated system, the total energy remains the same.
Second Law: When two isolated systems in separate but nearby regions of space, each in
thermodynamic equilibrium in itself, but not in equilibrium with each other at first, are at
some time allowed to interact, breaking the isolation that separates the two systems, and they
exchange matter or energy, they will eventually reach a mutual thermodynamic equilibrium.
The sum of the entropies of the initial, isolated systems is less than or equal to the entropy of
the final exchanging systems. In the process of reaching a new thermodynamic equilibrium,
entropy has increased, or at least has not decreased.
Third Law: As temperature approaches absolute zero, the entropy of a system approaches a
minimum.
3. State Laws of conservation of energy?
According to the laws of conservation of energy, “energy can neither be created nor be
destroyed. It can only be transformed from one form to another.”
4. Is the boiler a closed system?
Yes definitely the boiler is a closed system.
5. What is Carnot engine?
It was being designed by Carnot and let me tell you that Carnot engine is an imaginary engine
which follows the Carnot cycle and provides 100% efficiency.
6. Which formula forms a link between the Thermodynamics and Electro chemistry?
Gibbs Helmholtz formula is the formula which forms the link between the thermodynamics
and electromagnetism.
ΔHs/R = [∂ lnp /∂ (1/T)] x
Where: x – mole fraction of CO2 in the liquid phase
p – CO2 partial pressure (kPa)
T – Temperature (K)
R – Universal gas constant
α – mole ratio in the liquid phase (mole CO2 per mole of amine)
7. Which is the hardest compound known?
Diamond
8. What is Hess Law?
According to the Hess law the energy transfer is simply independent of the path being
followed. If the reactant and the product of the whole process are the same then same amount
of energy will be dissipated or absorbed.
9. Which has more efficiency: Diesel engine or Petrol engines?
Off course Diesel engine has the better efficiency out of two.
10.What is the difference between Critical Speed and Whirling Speed?
Ans. In Solid mechanics, in the field of rotor dynamics, the critical speed is the theoretical
angular velocity which excites the natural frequency of a rotating object, such as a shaft,
propeller or gear. As the speed of rotation approaches the objects natural frequency, the
object begins to resonate which dramatically increases system vibration. The resulting
resonance occurs regardless of orientation. Whirling Speed is due to the unbalanced forces
acting on a rotating shaft.
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11. How a Diesel Engine Works as Generator?
Ans. Diesel engine is a prime mover, for a generator, pump,and for vehicles etc. generator
is connected to engine by shaft. mostly in thermal power plat ,there is an engine is used to
drive generator to generate power.
12. Explain Second Law of Thermodynamics?
Ans. The entropy of the universe increases over time and moves towards a maximum value.
13. Compare Brayton Cycle and Otto Cycle?
Ans. The heat addition and rejection processes in Otto cycle are of constant volume, whereas
in Brayton cycle, they are of constant pressure. Otto cycle is the ideal cycle for spark ignition
engines. Brayton cycle is the ideal cycle for gas power turbines.
14. What is the purpose of Scrapper Ring?
Ans. Scrap the excess lube oil from the cylinder walls. There by preventing oil from entering
combustion zone.
15. What is DTSI Technology?
Ans. DTSI stands for Digital Twin Spark Plug Ignition. The vehicles with DTSI Technology
use 2 spark plugs which are controlled by digital circuit. It results in efficient combustion of
air fuel mixture.
 Digital - Since the spark generation will be initiated by a microchip.
 Twin - Since two spark plugs will be used.
 Spark ignition - Since the ignition will be done via a spark.
16. How to Find, Ductile-Brittle Transition Temperature in Metals?
Ans. The point at which the fracture energy passes below a pre-determined point for a
standard Impact tests, DBTT is important since, once a material is cooled below the DBTT, it
has a much greater tendency to shatter on impact instead of bending or deforming.
17. What is the importance of Thermodynamics?
Ans. All the mechanical engineering systems are studied with the help of thermodynamics.
Hence it is very important for the mechanical engineers.
18. What is the difference between P11 and P12 Pipes?
Ans. P11 the chromium molybdenum composition that is 1% of chromium and 1/4% of
molybdenum P12 the chromium molybdenum composition that is 1% of chromium and 2%
of molybdenum
19. State difference between Anti-Friction Bearing and Journal Bearing?
Ans. Generally, journal bearings have higher friction force, consume higher energy and
release more heat, but they have larger contact surface, so normally used in low speed high
load applications. In anti-friction bearings friction is less. One object just rolls over each
other.
20. What is the difference between Technology and Engineering?
Ans. Engineering is application of science. Technology shows various methods of
Engineering. A bridge can be made by using beams to bear the load,by an arc or by hanging
in a cable; all shows different technology but comes under civil engineering and science
applied is laws of force/load distribution.
21. How to Measure Temperature in Wet Bulb Thermometer?
Ans. Wet bulb temperature is measured in a wet bulb thermometer by covering the bulb with
a wick and wetting it with water. It corresponds to the dew point temperature and relative
humidity.
22. What are the Advantages and Disadvantages of using LPG in Car?
Ans. Advantages: 1. complete combustion 2. Fuel saving 3. Homogenous combustion
Disadvantages: 1. as complete combustion is occurring, more heat liberated, not advised for
long journey, engine will be over heated 2. Installation is difficult 3. Reduce engine life
efficiency
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MECHANICAL ENGINEERING INTERVIEW QUESTIONS & ANSWERS
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23. What is the difference between Speed and Economic Speed?
Ans. The rated speed tells us about the maximum speed which can be achieved by a vehicle
or some other machine but the economical speed means the speed limit at which the machine
works efficiently with least consumption of fuel.eg-in normal bikes (not racing),the max.
Speed limit shown on speedometer is upto 120 kmph but companies always advice their
customers to drive such bikes at around 60 kmph to have maximum mileage.
24. What is Powder Technology?
Ans. Powder technology is one of the ways of making bearing material. In this method metals
like bronze, Al, Fe are mixed and compressed to make an alloy.
25. State all the laws of Thermodynamics?
Ans. There are three laws of the thermodynamics.
First Law: Energy can be neither created nor destroyed. It can only change forms. In any
process in an isolated system, the total energy remains the same.
Second Law: When two isolated systems in separate but nearby regions of space, each in
thermodynamic equilibrium in itself, but not in equilibrium with each other at first, are at
some time allowed to interact, breaking the isolation that separates the two systems, and they
exchange matter or energy, they will eventually reach a mutual thermodynamic equilibrium.
The sum of the entropies of the initial, isolated systems is less than or equal to the entropy of
the final exchanging systems. In the process of reaching a new thermodynamic equilibrium,
entropy has increased, or at least has not decreased.
Third Law: As temperature approaches absolute zero, the entropy of a system approaches a
minimum.
26. State the difference between Unilateral and Bilateral Tolerance?
Ans. A unilateral tolerance is tolerance in which variation is permitted only in one direction
from the specified direction. e.g. 1800 +0.000/-0.060
Bilateral tolerance is tolerance in which variation is permitted in both direction from the
specified direction. e.g. 1800 +0.060/-0.060
27. What is the abbreviation of welding rod 7018?
Ans. 7018 =
70=tensile strength 70000psi 1= welding position 8=current flux
28. What is difference between Welding and Brazing?
Ans. In Welding concentrated heat (high temperature) is applied at the joint of metal and fuse
together.
In Brazing involves significantly lower temperatures and does not entail the melting of base
metals. Instead, a filler metal is melted and forced to flow into the joint through capillary
action.
29. Which has more Efficiency Diesel Engine or Petrol Engine?
Ans. Diesel engine has the better efficiency out of two.
30. What is Difference between stamina and strength?
Ans. Strength is capability over a short length of time and Stamina is the ability to keep
going continuously.
31. What is Hydrostatic System?
Ans. Hydrostatics is the study of fluid bodies that are
 At rest
 Moving sufficiently slowly so there is no relative motion between adjacent parts of
the body
For hydrostatic situations
 There are no shear stresses
 There are only pressure forces that act perpendicular to any surface.
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It’s a closed loop hydraulic systems. It comprises of motor and pump. Here pump supplies
energy to motor and motor gives return energy to pump supply.
32. What is Cotter joint?
Ans. A cotter joint is used to connect rigidly two co-axial rods or bars which are subjected
to axial tensile or compressive forces. Here shaft is locked in place by a smaller pin that
passes through the side of the lug and partly or completely through the shaft itself. This
locking pin is named as cotter.
33. How is the excess discharge pressure prevented?
Ans. Discharge pressure prevented by a pressurized spike cushion. Here the system
employs a pressurized cushion of air and a two o-ring piston, which permanently separates
this air cushion from the water system. When the valve closes and the water flow is suddenly
stopped, the pressure spike pushes the piston up the arrester chamber against the pressurized
cushion of air. The air cushion in the arrester reacts instantly, absorbing the pressure spike
that causes water hammer.
34. What is the difference between Strainer and Fitler?
Ans. Strainer for coarse size, Filter is more accurate than Strainer.
35. What is the position of Piston Ring?
Ans. In 180 degree angle the Top ring, Second ring and Oil ring are fixed. Position the ring
approximately 1 inch gap below the neck.
36. Why Deareator are placed at Height, In Thermal Power Plant?
Ans. To build a Very high pressure and the temperature for a boiler feed water pump and it
discharge high pressure water to the boiler and to provide the required Net Positive Suction
Head (NPSH) for the BFW pump and to serve as a storage tank to ensure a continuous supply
of feed water during rapid changes in BFP.
37. What is meanst by One Tonn Air-Conditioner?
Ans. 1 ton refrigeration means 210 kJ/min extracts heat from the system.
38. State 1st Law of Thermodynamics?
Ans. Heat and mechanical work are mutually convertable. Energy can be neither created
nor be destroyed but it can be transferred from one form to another form.
39. If you heat a steel pipe with the hole at center, does heat affects the hole diameter?
Ans. It gets bigger.
1. Why the Centrifugal Pump is called High Discharge pump?
Ans. Centrifugal pump is a kinetic device. The centrifugal pump uses the centrifugal force
to push out the fluid. So the liquid entering the pump receives kinetic energy from the
rotating impeller. The centrifugal action of the impeller accelerates the liquid to a high
velocity, transferring mechanical (rotational) energy to the liquid. So it discharges the liquid
in high rate. It is given in the following formulae:
Centrifugal force F= (M*V2)/R.
Where,
M-Mass
V-Velocity
R-Radius
2. How Cavitation can be eliminated by Pump?
Ans. Cavitation means bubbles are forming in the liquid.
To avoid Cavitation, we have to increase the Pump size to one or Two Inch;
To increase the pressure of the Suction Head, or Decrease the Pump Speed.
3. Why Cavitation will occur in Centrifugal Pump and not in Displacement Pump?
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Ans. The formation of cavities (or bubbles) is induced by flow separation, or non-uniform
flow velocities, inside a pump casing. In centrifugal pumps the eye of the pump impeller is
smaller than the flow area of pipe. This decrease in flow area of pump results in increase in
flow rate. So pressure drop happened between pump suction and the vanes of the impeller.
Here air bubbles or cavities are formed because of liquid vapour due to increase in
temperature in impeller. This air bubbles are transmitted to pump which forms cavitation.
4. Which Pump is more Efficient Centrifugal Pump or Reciprocating Pump?
Ans. Centrifugal pump because flow rate is higher compared to reciprocating pump. Flow
is smooth and it requires less space to install. Lower initial cost and lower maintenance cost.
5. Why Centrifugal Pump is not called as a Positive Displacement Type of Pump?
Ans. The centrifugal has varying flow depending on pressure or head, whereas the Positive
Displacement pump has more or less constant flow regardless of pressure.
Likewise viscosity is constant for positive displacement pump where centrifugal pump have
up and down value because the higher viscosity liquids fill the clearances of the pump
causing a higher volumetric efficiency. When there is a viscosity change in supply there is
also greater loss in the system. This means change in pump flow affected by the pressure
change. One more example is, positive displacement pump has more or less constant
efficiency, where centrifugal pump has varying efficiency rate.
1. Why Entropy decreases with increase in temperature?
Ans. ds=dQ/T Entropy is inversely proportional to the temperature so, as temp. Increases,
entropy decreases.
2. Why different types of sound are produced in different bikes, though they run on SI
Engines?
Ans. Engine specifications are different in different manufactures like as Bore Diameter
(CC), Ignition timing. Also the exhaust passage takes more responsible for sound.
3. How much Watt means 1Hp?
Ans. 746.2 Watt
4. Explain Bicycle Rear Wheel Sprocket working?
Ans. Rear wheel sprocket works under the principle of ratchet and pawl.
5. Definition of Octane Number and Cetane Number?
Ans. Octane No.- Octane number is defined as the percentage, by volume, of iso octane in
the mixture of iso octane and h-heptane. It is the measure of rating of SI engine.
Cetane No.- Cetane number is defined as the percentage, by volume, of n-cetane in the
mixture of n-cetane and alpha methyl naphthalene. It is the measure of rating of CI engine.
6. Which Mechanism is used in Automobile gearing System?
Ans. Differential mechanism
7. When Crude Oil is Heated, Which Hydro Carbon comes first?
Ans. Natural gas (Gasoline)… at 20 Celsius
8. How to calculate Bearing number ti Diameter of the inner and outer?
Ans. Divide the shaft diameter size by 5, it will give last two digit of the bearing no. and
according to type of load we have to choose the type of bearing and that will give prior
number of the bearing.
9. The Fatigue life of a part can be improved by?
Ans. Improving the surface finish by Polishing & providing residual stress by Shot peening
10. What happens if gasoline is used in a Diesel Engine, Siesel Engine will work?
Ans. No, it will not work, as the Compression ratio of Petrol engine is 6 to 10 & that of
Diesel engine is 15 to 22. Thus on such high compression, gasoline gets highly compressed &
it may blast.
11. Poisson Ratio is higher in, Rubber/Steel/Wood?
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Ans. When a material is compressed in one direction, it usually tends to expand in the other
two directions perpendicular to the direction of compression. This phenomenon is called the
Poisson effect. Poisson’s ratio is a measure of the Poisson effect.
For rubber = 0.5
For steel = 0.288
For wood < 0.2
Thus Poisson’s ratio is higher in RUBBER.
12. What are the different types of fits? Explain?
On the basis of Indian standards fits can mainly be categorized into three groups:
Clearance Fit: These types of fits are characterized by the occurrence of a clearance
between the two mating parts. The difference between the minimum size of the hole
and the maximum size of the shaft is called the minimum clearance, the difference
between the maximum size of the hole and the minimum size of the shaft is known as
maximum clearance.
Interference Fit: In these types of fits the size of the mating parts are predefined so
that interference between them always occurs. The tolerance zone of the hole is
completely below the tolerance zone of the shaft.
Transition Fit: As the name suggests these type of fit has its mating parts sized limited
to allow either clearance or interference. The tolerance zone of the hole and the shaft
overlaps in case of such fits.
For a shaft designated as 40 H8/f7, calculate the tolerances.
Given: Shaft designation = 40 H8/f7
The shaft designation 40 H8/f 7 means that the basic size is 40 mm and the tolerance grade
For the hole is 8 ( i. e. I T 8) and for the shaft is 7 ( i. e. I T 7).
Since 40 mm lies in the diameter steps of 30 to 50 mm, therefore the geometric mean
diameter, D = Square root of (30 x 50) = 38.73 mm
We know that standard tolerance unit,
i = 0.45 x Cube root of (D) + 0.001 D
i = 0.45 × 3.38 + 0.03873 = 1.559 73 or 1.56 microns
i = 1.56 × 0.001 = 0.001 56 mm ...(1 micron = 0.001 mm)
The standard tolerance for the hole of grade 8 (IT8) = 25 i = 25 × 0.001 56 = 0.039 mm
The standard tolerance for the shaft of grade 7 (IT7) = 16 i = 16 × 0.001 56 = 0.025 mm
13. What are the factors that can affect the Factor of safety selection?
The factor of safety is used in designing a machine component. Prior to selecting the
correct factor of safety certain points must be taken into consideration such as:
The properties of the material used for the machine and the changes in its intrinsic
properties over the time period of service.
The accuracy and authenticity of test results to the actual machine parts
The applied load reliability
The limit of stresses (localized)
The loss of property and life in case of failures
The limit of initial stresses at the time period of manufacture
The extent to which the assumptions can be simplified
The factor of safety also depends on numerous other considerations such as the
material, the method of manufacturing, the various types of stress, the part shapes etc.
14. What is heat treatment and why is it done?
Heat treatment can be defined as a combination of processes or operations in which the
heating and cooling of a metal or alloy is done in order to obtain desirable characteristics
without changing the compositions. Some of the motives or purpose of heat treatment are as
follows: In order to improve the hardness of metals
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For the softening of the metal
In order to improve the machinability of the metal
To change the grain size
To provide better resistance to heat, corrosion, wear etc.
Heat treatment is generally performed in the following ways:
Normalizing
Annealing
Spheroidising
Hardening
Tempering
Surface or case hardening
What are the rules that must be kept in mind while designing castings?
Some of the points that must be kept in mind during the process of cast designing are as
follows:
To avoid the concentration of stresses sharp corners and frequent use of fillets should be
avoided.
Section thicknesses should be uniform as much as possible. For variations it must be done
gradually.
Abrupt changes in the thickness should be avoided at all costs.
Simplicity is the key; the casting should be designed as simple as possible.
It is difficult to create true large spaces and henceforth large flat surfaces must be avoided.
Webs and ribs used for stiffening in castings should as minimal as possible.
Curved shapes can be used in order to improve the stress handling of the cast.
What are the points that should be kept in mind during forging design?
Some of the points that should be followed while forging design are:
A radial flow of grains or fibers must be achieved in the forged components.
The forged items such as drop and press forgings should have a parting line that should
divide the forging into two equal halves.
The ribs in a forging should not be high or thin.
In order to avoid increased die wear the pockets and recesses in forgings should be minimum
In forgings the parting line of it should lie as far as possible in a single plane.
For ease of forging and easy removal of forgings the surfaces of the metal should contain
sufficient drafts.
Describe briefly the different cold drawing processes.
Some of the important cold drawing processes are as follows:
Bar and Rod Drawing: In the case of bar drawing the hot drawn bars are at first pickled,
washed and coated to prevent oxidation. Once this is done a draw bench is used for the
process of cold drawing. In order to make an end possible to enter a drawing die the diameter
of the rod is reduced by the swaging operation. This end is fastened by chains to the draw
bench and the end is gripped by the jaws of the carriage. In this method a high surface finish
and accuracy dimensionally is obtained. The products of this process can be used directly
without any further machining.
Wire Drawing: Similar to the above process the bars are first pickled, washed and coated to
prevent any oxidation. After this the rods are passed through several dies of decreasing
diameter to provide a desired reduction in the size (diameter). The dies used for the reduction
process is generally made up of carbide materials.
Tube Drawing: This type of drawing is very similar to the bar drawing process and in
majority of cases it is accomplished by the use of a draw bench.
What are the different theories of failure under static load, explain briefly?
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The main theories of failure of a member subjected to bi-axial stress are as follows:
Maximum principal stress theory (Rankin’s theory): This theory states that failure occurs at a
point in member where the maximum principal or normal stress in a bi-axial system reaches
the maximum strength in a simple tension test.
Maximum shear stress theory ( Guest’s or Tresca’s theory): This theory states that failure
occurs when the biaxial stress reaches a value equal to the shear stress at yield point in a
simple tension test.
Maximum principal strain theory (Saint Venant theory): This theory states that failure occurs
when bi-axial stress reaches the limiting value of strain.
Maximum strain energy theory ( Haigh’s theory): This theory states that failure occurs when
strain energy per unit volume of the stress system reaches the limiting strain energy point.
Maximum distortion energy theory ( Hencky and Von Mises theory): This theory states that
failure occurs when strain energy per unit volume reaches the limiting distortion energy.
What happens if gasoline is used in a Diesel engine?Diesel engine will work ?
No, It will not work,as the Compression ratio of Petrol engine is 6 to 10 & that of Diesel
engine is 15 to 22. Thus on such high compression, gasoline gets highly compressed & it may
blast.
Which Mechanism is used in Automobile gearing system
Differential mechanism
Why different types of sound are produced in different bikes though they say run on SI
engine
Engine specifications are different in different manufactures like as Bore Diameter(CC),
Ignition timing. Also the exhaust passage take more responsible for sound.
Why entropy decreases with the increase in temperature?
ds=dQ/TEntropy is inversely proportional to the temperature so.as temp. increases, entropy
decreases.
1 hp how much watt?
746.2Watt
How to calculate bearing number to diameter of the inner and outer
Divide the shaft diameter size by 5, it will give last two digitof the bearing no. and according
to type of load we have tochose the type of bearing and that will give prior no. ofthe bearing.
Explain Bicycle rear wheel Sprocket working?
Rear wheel sprocket works under the principle of ratchet and pawl.
Definition of Octane number & Cetane number
Octane No.- Octane number is defined as the percentage, by volume, of iso octane in the
mixture of iso octane and h-heptane. It is the measure of rating of SI engine.
Cetane No.- Cetane number is defined as the percentage, by volume, of n-cetane in the
mixture of n-cetane and alpha methyl naphthalene. It is the measure of rating of CI engine.
Poisons ratio is higher in (rubber/steel/wood)
What is important to remember about radial bearings?
A turbine rotor is supported by two radial bearings, one on each end of the steam cylinder.
These bearings must be accurately aligned to maintain the close clearance between the shaft
and the shaft seals, and between the rotor and the casing. If excessive bearing wear lowers the
he rotor, great harm can be done to the turbine.
How many governors are needed for safe turbine operation? Why?
Two independent governors are needed for safe turbine operation. One is an over speed or
emergency trip that shuts off the steam at 10 percent above running speed (maximum speed).
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The second, or main governor, usually controls speed at a constant rate; however, many
applications have variable speed control.
How is a fly ball governor used with a hydraulic control?
As the turbine speeds up, the weights are moved outward by centrifugal force, causing
linkage to open a pilot valve that admits and releases oil on either side of a piston or on one
side of a spring-loaded piston. The movement of the piston controls the steam valves.
What is meant by critical speed?
It is the speed at which the machine vibrates most violently. It is due to many causes, such as
imbalance or harmonic vibrations set up by the entire machine. To minimize damage, the
turbine should be hurried through the known critical speed as rapidly as possible. (Caution,
be sure the vibration is caused by critical speed and not by some other trouble).
How is oil pressure maintained when starting or stopping a medium-sized turbine?
An auxiliary pump is provided to maintain oil pressure. Some auxiliary pumps are turned by
a hand crank; others are motor-driven. This pump is used when the integral pump is running
too slowly to provide pressure, as when starting or securing a medium-sized turbine.
Besides lubrication, what are two functions of lubricating oil in some turbines?
In larger units, lube oil cools the bearings by carrying off heat to the oil coolers. Lube oil in
some turbines also acts as a hydraulic fluid to operate the governor speed-control system.
Do you stop cooling-water flow through a steam condenser as soon as the turbine is
slopped?
You should keep the cooling water circulating for about 15 mill or more so that the condenser
has a chance to cool down gradually and evenly. Be sure to have cooling water flowing
through the condenser before starting up in order to prevent live steam from entering the
condenser unless it is cooled. Overheating can cause severe leaks and other headaches.
How can the deposits be removed?
o Water soluble deposits may be washed off with condensate or wet steam.
o Water insoluble deposits are removed mechanically after dismantling the
turbine.
o Experience shows that water soluble deposits are embedded in layers of waterinsoluble
deposits. And when the washing process is carried out, water soluble
parts of the deposit dissolve away leaving a loose, friable skeleton of waterinsoluble
deposits which then break loose and wash away.
How can the fatigue damage on high-pressure blades be corrected?
Fatigue-damage on high-pressure blades arises due to vibration induced by partial-arc
admission. This can be corrected by switching over to full arc admission technique.

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