# Most Important 200 Mechanical Engineering Interview Questions and Answers PDF

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1.   What is the difference between scavenging and
supercharging ?
Ans:
Scavenging is process of flushing out burnt gases from engine cylinder by
introducing fresh air in the cylinder before exhaust stroke ends. Supercharging
is the process of supplying higher mass of air by compressing the atmospheric
air.
2.   What are the names given to constant
temperature, constant pressure, constant volume, constant internal energy,
constant enthalpy, and constant entropy processes.
Ans:
Isothermal, isochroic, isobaric, free expression, throttling and adiabatic
processes respectively.
3.   In a Rankine cycle if maximum steam pressure
is increased keeping steam temperature and condenser pressure same, what will happen to dryness
fraction of steam after expansion
?Ans: It will decrease.
4.   Why entropy change for a reversible adiabatic
process is zero ?
Ans:
Because there is no heat transfer in this process.
5.   What are two essential conditions of perfect gas ?
Ans:
It satisfies equation of state and its specific heats are constant.
6.   Enthalpy and entropy are functions of one
single parameter. Which is that ?
Ans:
Temperature.
7.   Why rate of condensation is higher on a polished
surface compared to rusty surface ?
Ans:
Polished surface promotes drop wise condensation and does not wet the surface.
8.   How much resistance is offered to heat flow by drop wise condensation ?
Ans:
Nil
How
9.   What is the relationship between COP of
heating and cooling ?
Ans:
COP of heating is one(unity) more than COP of cooling.
10.    How much is the work done in isochoric
process ?
Ans:
Zero.
11.    When maximum discharge is obtained in nozzle ?
Ans:
At the critical pressure ratio.
12.    Under what condition the work done in
reciprocating compressor will be least ?
Ans:
It is least when compression process approaches isothermal. For this purpose,
attempts are made to cool the air during compression.

13.    What is the difference between stalling and
surging in rotary compressions ?
Ans: Stalling
is a local phenomenon and it occurs
when How breaks
causes complete breakdown of flow and as such it affects
the whole machine.
14.    Why the electric motor of a fan with backward
Ans: The maximum power
is consumed at about 70% of maximum
flow in case’of
fan with backward
blades. For higher flow, power consumption gets lower.
15.    Why the work per kg of air flow in axial flow compressor is less compared
to centrifugal compressor for same pressure ratio ?
Ans:
Isentropic efficiency of axial flow compressor is higher.
16.    What is the name
given to portion
of thermal energy
to be necessarily rejected to environment ?
Ans:
Anergy.
17.    What is pitting
? How it is caused
?
Ans:
Non uniform corrosion over the entire metal surface, but occuring only in small
pits is called pitting. It is caused by lack of uniformity in metal.
18.    What is caustic embrittlement ?
Ans:
It is the actual physical change in metal that makes it extremely brittle and
filled with minute cracks. It occurs particularly in the seams of rivetted
joints and around the rivet holes.
19.    Which impurities form hard scale and which
impurities soft scale ?
Ans:
Sulphates and chlorides of lime and magnesium form hard scale, and carbonates
of lime and magnesium form soft scale.
20.    What is the difference between hard water and
soft water ?
Ans:
Hard water contains excess of scale forming impurities and soft water contains
very little or no scale forming substances.
21.    Which two elements
in feed water
can cause corrosion of tubes and plates in boiler ? ‘
Ans: Acid and oxygen in feed water lead to corrosion.
22.    What should be done to prevent a safety valve to stick to its seat ?
Ans:
Safety valve should be blown off periodically so that no corrosion can take
place on valve and valve seat.
23.    Why large boilers
are water tube type ?
Ans:
Water tube boilers raise steam fast because of large heat transfer area and
positive water circulation. Thus they respond faster to fluctuations in demand.
Further single tube failure does not lead to catastrophy.

24.    What type of boiler does not need a steam
drum ?
Ans:
Super-critical pressure boiler.
25.    Why manholes in vessels are usually elliptical in shape ?
Ans:
Elliptical shape has minimum area of opening and thus plate is weakened the
least. Further it is very convenient to insert and take out the cover plate
from elliptical opening.

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1. asif ali shah