Mechanical Engineering Interview Questions with Answers - ObjectiveBooks

Mechanical Engineering Interview Questions with Answers

Mechanical Engineering Interview Questions with Answers on Thermodynamics, Power Plant Engg., Heat & Mass Transfer, I.C Engine, Automobile, Refrigeration & Air-Conditioning: Set 01

Question No. 01
State all the laws of Thermodynamics?
Generally thermodynamics contains four laws;
(A) Zeroth law: deals with thermal equilibrium and establishes a concept of temperature.
(B) First law: throws light on concept of internal energy.
(C) Second law: indicates the limit of converting heat into work and introduces the principle of increase of entropy.
(D) Third law: defines the absolute zero of entropy.

Question No. 02
What is the difference between scavenging and supercharging?
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.

Question No. 03
What are the names given to constant temperature, constant pressure, constant volume, constant internal energy, constant enthalpy, and constant entropy processes?
Isothermal, isobaric, isochoric, free expression, throttling and adiabatic processes respectively.

Question No. 04
Explain Second Law of Thermodynamics?
The entropy of the universe increases over time and moves towards a maximum value.

Question No. 05
Compare Brayton Cycle and Otto Cycle?
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.

Question No. 06
What is the importance of Thermodynamics?
All the mechanical engineering systems are studied with the help of thermodynamics. Hence it is very important for the mechanical engineers.

Question No. 07
How to Measure Temperature in Wet Bulb Thermometer?
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.

Question No. 08
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?
It will decrease.

Question No. 09
Why entropy change for a reversible adiabatic process is zero?
Because there is no heat transfer in this process.

Question No. 10
What is the purpose of Scrapper Ring?
Scraps the excess lube oil from the cylinder walls. There by preventing oil from entering combustion zone.

Question No. 11
What are two essential conditions of perfect gas?
It satisfies equation of state and its specific heats are constant.

Question No. 12
Enthalpy and entropy are functions of one single parameter. Which is that?

Question No. 13
What is DTSI Technology?
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.

Question No. 14
Why rate of condensation is higher on a polished surface compared to rusty surface?
Polished surface promotes drop wise condensation and does not wet the surface.

Question No. 15
How much resistance is offered to heat flow by drop wise condensation?
No resistance offered.

Question No. 16
What are the Advantages and Disadvantages of using LPG in Car?
1. Complete combustion
2. Fuel saving
3. Homogeneous combustion

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

Question No. 17
Which has more Efficiency Diesel Engine or Petrol Engine?
Diesel engine has the better efficiency out of two.

Question No. 18
What is the relationship between COP of heating and cooling?
COP of heating is one (unity) more than COP of cooling.

Question No. 19
How much is the work done in isochoric process?

Question No. 20
Why Entropy decreases with increase in temperature?
ds = dQ/T, i.e.  Entropy is inversely proportional to the temperature. So, as temperature Increases, entropy decreases.

Question No. 21
Why different types of sound are produced in different bikes, though they run on SI Engines?
Engine specifications are different in different manufactures like as Bore Diameter (CC), Ignition timing. Also the exhaust passage takes more responsible for sound.

Question No. 22
What should be done to prevent a safety valve to stick to its seat?
Safety valve should be blown off periodically so that no corrosion can take place on valve and valve seat.

Question No. 23
Define Octane Number and 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.

Question No. 24
Why efficiency of gas turbines is lower compared to I.C. engines?
In gas turbines, 70% of the output of gas turbine is consumed by compressor. I.C. engines have much lower auxiliary consumption. Further combustion temperature of I.C. engines is much higher compared to gas turbine.

Question No. 25
What do you understand by timed cylinder lubrication?
For effective lubrication, lube oil needs to be injected between two piston rings when piston is at bottom of stroke so that piston rides in oil during upward movement. This way lot of lube oil can be saved and used properly.

Question No. 26
What is HUCR in relation to petrol engine?
HUCR is highest useful compression ratio at which the fuel can be used in a specific test engine, under specified operating conditions, without knocking.

Question No. 27
Which Mechanism is used in Automobile gearing System?
Differential mechanism

Question No. 28
In some engines glycerin is used in place of water for cooling of engine. Why?
Glycerin has boiling point of 90°C which increases its heat carrying capacity. Thus weight of coolant gets reduced and smaller radiator can be used.

Question No. 29
Why consumption of lubricating oil is more in two-stroke cycle petrol engine than four-stroke cycle petrol engine?
In two-stroke engine lube oil is mixed with petrol and thus some lube oil is blown out through the exhaust valves by scavenging and charging air. There is no such wastage in four stroke petrol engine.

Question No. 30
When Crude Oil is Heated, Which Hydro Carbon comes first?
Natural gas (Gasoline), at 20 Celsius.

Question No. 31
What happens if gasoline is used in a Diesel engine? Will diesel engine 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.

Question No. 32
Why boiler is purged every time before starting firing of fuel?
Purging ensures that any unburnt fuel in furnace is removed; otherwise it may lead to explosion.

Question No. 33
What is the principle of mechanical refrigeration?
A volatile liquid will boil under the proper conditions and in so doing will absorb heat from surrounding objects.

Question No. 34
Why high latent heat of vaporisation is desirable in a refrigerant?
A high latent heat of vaporisation of refrigerant results in small amount of refrigerant and thus lesser circulation system of refrigerant for same tonnage.

Question No. 35
How a diesel engine works in generator?
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 plant, there is an engine is used to drive generator to generate power.

Question No. 36
What is flash-point?
Flash point: the lowest temperature at which the vapour of a combustible liquid can be ignited in air.

Question No. 37
What is the critical temperature of a refrigerant?
Critical temperature is the maximum temperature of a refrigerant at which it can be condensed into liquid and beyond this it remains gas irrespective of pressure applied.

Question No. 38
What is refrigerant?
Any substance that transfers heat from one place to another, creating a cooling effect. Water is the refrigerant in absorption machines.

Question No. 39
What is the effect of reheat on Rankine cycle?
This prevents the vapour from condensing during its expansion which can seriously damage the turbine blades, and improves the efficiency of the cycle, as more of the heat flow into the cycle occurs at higher temperature.

Question No. 40
As compression ratio increases, thermal n increases. How is thermal n affected by weak and rich mixture strength?
Thermal n is high for weak mixture and it decreases as mixture strength becomes rich.

Question No. 41
How engine design needs to be changed to burn lean mixture?
Engine to burn lean mixture uses high compression ratio and the highly turbulent movement of the charge is produced by the geometry of the combustion chamber.

Question No. 42
Horse power of I.C. engines can be expressed as RAC rating, SAE rating, or DIN rating. To which countries these standards belong?
U.K., USA and Germany respectively.

Question No. 43
Why are Head Gaskets blown?
Normally head gasket blows, when the engine overheats and they can also blow from incorrect installation or poor design. Head gaskets expand and contract according to engine temperature, these cycles may happen after a long period of time, causes the gasket to fail. If you’re replacing the gasket, check the engine block, and head for warping. Follow proper cleaning and torque specifications during assembly.

Question No. 44
What is the Difference between a Humidifier and Vaporizer?
(A) The basic difference between humidifiers and vaporizers is that humidifiers disperse cool mist into the air, and vaporizers heat the water to disperse hot steam. Humidifiers are normally used in cooler climates, when due to the usage of heater, the air in the house becomes too dry for comfort and also, it becomes very difficult to breathe. Humidifiers release cool moisture droplets into the air.
(B) Vaporizers also help in moistening the dry air in the house, but vaporizers release hot vapour into the air. There is a heating element in the vaporizers, which help in releasing steam.
(C) Vaporizers heat the water and then release vapours.
(D) The basic difference between them is that one emits cold vapours, while the other one emits hot vapours.

Question No. 45
What do you understand by fuel cycle in nuclear plants?
Fuel cycle a series of sequential steps involved in supplying fuel to a nuclear power reactor. The steps include : Mining, refining uranium, fabrication of fuel elements, their use in nuclear reactor, chemical processing to recover remaining fissionable material, re-enrichment of fuel from recovered material, re-fabrication of new fuel elements, waste storage etc.

Question No. 46
What is heavy water and what is its use in nuclear plants?
Water containing heavy isotopes of hydrogen (Deuterium) is known as heavy water. Heavy water is used as a moderator. Heavy water has low cross section for absorption of neutrons than ordinary water. Heavy water slows down the fast neutrons and thus moderates the chain reaction.

Question No. 47
What is a converter reactor?
A reactor plant which is designed to produce more fuel than it consumes. The breeding is obtained by converting fertile material to fissile material.

Question No. 48
Difference between Absorption and Adsorption.
In absorption, one substance (matter or energy) is taken into another substance. But in adsorption only the surface level interactions are taking place.

Question No. 49
Difference between Liquid and Aqueous.
Liquid is a state of matter, while aqueous is a special type of liquid formed by dissolving a compound in water
All aqueous solutions are liquids, but not all liquids are aqueous solutions.

Question No. 50
Difference between Gas and Vapour.
(A) Vapour can turn back and forth into liquid and solid states but a gas cannot.
(B) Gases cannot be seeing while vapours are visible.
(C) Vapours settle down on ground while gases do not.

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