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 02

Question No. 51
Difference between Boiling Point and Melting Point.
(A) The melting point is a defined for solids when it transfers from solid state to liquid state.
(B) The boiling point is defined for liquids for a state change from liquid to gas.
(C) Boiling point is highly dependent on the external pressure whereas the melting point is independent of the external pressure.

Question No. 52
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.”

Question No. 53
Is the boiler a closed system?
Yes definitely the boiler is a closed system.

Question No. 54
What is Carnot engine?
It was being designed by Carnot and Carnot engine is an imaginary engine which follows the Carnot cycle and provides 100% efficiency.

Question No. 55
What is the use of flash chamber in a vapour compression refrigeration cycle to improve the COP of refrigeration cycle?
When liquid refrigerant as obtained from condenser is throttled, there are some vapours. These vapours if carried through the evaporator will not contribute to refrigerating effect. Using a flash chamber at some intermediate pressure, the flash vapour at this pressure can be bled off and fed back to the compression process. The throttling process is then carried out in stages. Similarly compression process is also done in two separate compressor stages.

Question No. 56
Why pistons are usually dished at top?
Pistons are usually hollowed at top to
(A) Provide greater space for combustion
(B) Increase surface for flue gases to act upon, and
(C) Better distribution of stresses.

Question No. 57
What is the function of thermostat in cooling system of an engine?
Thermostat ensures optimum cooling because excessive cooling decreases the overall efficiency. It allows cooling water to go to radiator beyond a predetermined temperature.

Question No. 58
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.
           ∆Hs/R = [∂ ln.p /∂(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 amines)

Question No. 59
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.

Question No. 60
Which has more efficiency: Diesel engine or Petrol engines?
Off course Diesel engine has the better efficiency out of two.

Question No. 61
Explain Otto cycle.
Otto cycle can be explained by a pressure volume relationship diagram. It shows the functioning cycle of a four stroke engine. The cycle starts with an intake stroke, closing the intake and moving to the compression stroke, starting of combustion, power stroke, heat exchange stroke where heat is rejected and the exhaust stroke. It was designed by Nicolas Otto, a German engineer.

Question No. 62
Why iso-octane is chosen as reference fuel for S.I. engines and allotted 100 values for its octane number?
Iso-octane permits highest compression without causing knocking.

Question No. 63
Why thermal efficiency of I.C. engines is more than that of gas turbine plant?
In I.C. engine maximum temperature attained is higher than in gas turbine.

Question No. 64
Which are the reference fuels for knock rating of S.I. engines?
n-heptane and ISO-octane.

Question No. 65
What will be the position of Piston Ring?
In 180 degree angle the Top ring, Second ring and Oil ring are fixed. Position the ring approximately 1 inch gap below the neck.

Question No. 66
What is Heat rate of a Power plant?
Heat rate is a measure of the turbine efficiency. It is determined from the total energy input supplied to the Turbine divided by the electrical energy output.

Question No. 67
When effect of variations in specific heats is considered then how do maximum temperature and pressure vary compared to air standard cycle?
Temperature increases and pressure decreases.

Question No. 68
Quantities like pressure, temperature, density, viscosity, etc. are independent of mass. What are these called?
Intensive properties.

Question No. 69
Why we do not use same technology to start both SI /CI engine?
The S.I. or spark ignition engine uses petrol as a fuel and the C.I. or compression ignition engine uses diesel as a fuel. Both the fuels has different compression ratio. In SI engine the compression ratio is 8-12:1. In CI engine the compression ratio is 16-22:1. So in case of SI engine, the compression ratio is not sufficient for fuel to burn so a spark plug is used, whereas in CI engine, the compression ratio is so high that due to its internal heat the fuel is combusted so there is no need for a spark plug. So the technology used in SI engine is different from CI engine.

Question No. 70
What VVTi written on new cars of Toyota stands for?
VVTi: Variable Valve Timing with Intelligence. It is the advanced version of the VVT engine.
It changes the cam shaft position by using oil pressure. It is similar to CVVT in Hyundai.

Question No. 71
In convection heat transfer, if heat flux intensity is doubled then temperature difference between solid surface and fluid will?
Get doubled.

Question No. 72
How you can define coal?
Coal is a naturally occurring hydrocarbon that consists of the fossilised remains of buried plant debris that have undergone progressive physical and chemical alteration, called coalification, in the course of geologic time.

Question No. 73
Why is the Suction pipe of Vapour Compression Refrigeration system insulated?
(A) It prevents the suction line from sweating and dripping water inside the house.
(B) The insulation also prevents the suction line attracting heat from the outdoors on its way to the condenser coil.

Question No. 74
How to determine the capacity of Refrigeration system? How we use Condenser coils, Compressor, Capillarity?
To determine the Refrigeration system by test of C.O.P and use the Condenser coils, Compressor, Capillarity, based on the properties

Question No. 75
What do you mean by super critical above 500 MW in Thermal power plant?
In super critical boiler means all the steam to convert in to superheated steam at outlet of boiler no need to sent to super heater once again.

Question No. 76
Which pollutant is major greenhouse gas and what is its effect?
CO is major greenhouse gas and it traps the radiation of heat from the sun within earth's atmosphere.

Question No. 77
In order to increase efficiency and reduce CO emissions and other emissions, clear coal technologies are receiving major attention. What are these?
(A) Advanced pulverized and pressurized pulverized fuel combustion.
(B) Atmospheric fluidized bed combustion and pressurized fluidized bed combustion.
(C) Supercritical boilers.
(D) Integrated gasification combined cycle systems.
(E) Advanced integrated gasification, including fuel cell systems.
(F) Magneto hydrodynamic electricity generation.

Question No. 78
What does CC Stand for?
CC is the abbreviated form of cubic centimeter. It is the unit by which the capacity of an engine is designated. It is the volume between TDC and BDC. It represents the quantity of fuel-air mix or exhaust gas that is pumped out in a single piston stroke. Alternatively it can represent the volume of the cylinder itself.

Question No. 79
State the difference between ultimate and proximate analysis of coal?
In ultimate analysis, chemical determination of following elements is made by weight: Fixed and combined carbon, H, O, N, S, water and ash. Heating value is due to C, H and S.
In proximate analysis following constituents are mechanically determined by weight. Moisture, volatile matter, fixed carbon and ash heating value is due to fixed carbon and volatile matter.

Question No. 80
What is fuel ratio?
Fuel ratio is the ratio of its percentage (%) of fixed carbon to volatile matter.

Question No. 81
How the analyses and calorific values of fuels can be reported?
It may be reported as:
(A) As received or fired (wet) basis
(B) Dry or moisture free basis
(C) Combustible or ash and moisture free basis

Question No. 82
We have read that when the piston goes up and down then the engine works i.e. the suction, compression etc. then what happens in the case of big vehicles, which start at stable condition, i.e. how does their piston moves when they are at rest. How suction, compression etc?
Smaller vehicles like bikes, cars are started with the help of motors. Initially, motors turn the crank shaft till sufficient suction pressure is reached. When sufficient suction pressure is reached, the engine starts to suck the fuel in and then the cycle begins when the fuel is taken in and ignited. Similarly, for huge engines, instead of motors, we use starting air. air at a pressure of 10-30 bar is fed to the engine which is at rest. This air rotates the engine till it attains sufficient suction pressure. Once the pressure is reached, the cycle starts and it starts firing.

Question No. 83
The Compression ratio of Petrol engine is always less than Compression Ratio of Diesel engine why?
Petrol is not self igniting; it needs spark to flame up in chamber. Whereas diesel is self igniting in diesel engine, to attain that state it requires high temp &pressure. This temperature & pressure is more than what’s required in Petrol Engines by property of that fluid.

Question No. 84
What are the important operational performance parameters in design of fuel firing equipment?
Fuel flexibility, electrical load following capability, reliability, availability, and maintenance ease.

Question No. 85
What is the difference between total moisture and inherent moisture in coal?
The moisture content of the bulk as sampled is referred to as total moisture, and that of the air dried sample is called inherent moisture.

Question No. 86
What is the temperature of space?
The short answer is that the temperature in space is approximately 2.725 Kelvin. That means the universe is generally just shy of three degrees above absolute zero, the temperature at which molecules themselves stop moving. That’s almost -270 degrees Celsius, or -455 Fahrenheit.

Question No. 87
Proximity analysis of coal provides data for a first, general assessment of a coal's quality and type. What elements it reports?
Moisture, volatile matter, ash and fixed carbon.

Question No. 88
Ultimate analysis of coal is elementary analysis. What it is concerned with?
Carbon, hydrogen, nitrogen, and sulfur in coal on a weight percentage basis.

Question No. 89
What are the causes of main engine black smoke?
There is many cause of black smoke.
(A) Is improper mixture of fuel supply by carburettor like very rich mixture so the fuel improper burn.
(B)  It is when piston or piston ring is fail so back side cooling oil release in combustion chamber it cause black smoke.
(C) Improper ignition system like not sufficient time of pressure rise delay period.

Question No. 90
What is the significance of torque (in Nm) given in the engine specification?
It give the moment about any point or simple rotation.

Question No. 91
Explain the difference between AFBC, BFBC, PFBC and CFBC in regard to fluidized bed technologies.
AFBC (Atmospheric fluidized bed combustion) process consists of forming a bed of inert materials like finely sized ash or ash mixed with sand, limestone (for sulfur removal), and solid fuel particles in a combustor and fluidising it by forcing combustion air up through the bed mixture. The gas flows through bed without disturbing particles significantly but gas velocity is high enough to support the total weight of bed (fluidisation). At slightly higher velocity excess gas passes through the bed as bubbles (fluidized bed) and gives the bed the appearance of a boiling liquid.

BFBC (Bubbling fluidized bed combustion) has a defined height of bed material and operates at or near atmospheric pressure in the furnace.

PFBC (Pressurized fluidized bed combustion) system operates the bed at elevated pressure. Exhaust gases have sufficient energy to power a gas turbine, of course, gases need to be cleaned.
In fluidized combustion, as ash is removed some unburned carbon is also removed resulting in lower efficiency.

CFBC (circulating fluidized bed combustion) system, bed is operated at higher pressure leading to high heat transfer, higher combustion efficiency, and better fuel feed. Circulating fluidized beds operate with relatively high gas velocities and fine particle sizes. The maintenance of steady state conditions in a fast fluidized bed requires the continuous recycle of particles removed by the gas stream (circulating bed). The term circulating bed is often used to include fluidized bed systems containing multiple conventional bubbling beds between which bed material is exchanged.

Question No. 92
What is the difference between nuclear fission and fission chain reaction?
The process of splitting of nucleus into two almost equal fragments accompanied by release of heat is nuclear fission. Self sustained, continuing, sequence of fission reactions in a controlled manner is fission chain reaction.

Question No. 93
What is BHP?
Brake horsepower is the amount of work generated by a motor under ideal conditions. This work is calculated without the consideration of effects of any auxiliary component that may slow down the actual speed of the motor. Brake horsepower is measured within the engines output shaft and was originally designed to calculate and compare the output of steam engines.
As per the conventions, 1 BHP equals to:
(1) 745.5 watts
(2) 1.01389 ps
(3) 33,000 ft lbf/min
(4) 42.2 BTU/min

Question No. 94
Which parameter remains constant in a throttling process?

Question No. 95
What is the difference between isentropic process and throttling process?
In isentropic process, heat transfer takes place and in throttling process, enthalpy before and after the process is same.

Question No.96
What is D-4D Technology used in Toyota Vigo?
D-4D is widely recognized as one of the most advanced diesel technologies on the market today. Diesel engines relied on relatively simple technology with a low-pressure mechanical injector delivering fuel to a pre-combustion chamber in the cylinder head where a single ignition fired by the intense heat of high compression takes place.

Question No. 97
What for Schmidt plot for is used in heat transfer problems?
Schmidt plot is a graphical method for determining the temperature at any point in a body at a specified time during the transient heating or cooling period.

Question No. 98
What is ATFT Technology used in Honda Hunk?
ATFT means Advance Tumble Flow Induction Technology, Tumble flow means swirling. In this technology, fuel air mixture from the carburettor into the engine cylinder with a swirl action. The advantage being one gets a more efficient burning of fuel hence more power and better fuel economy with lesser emissions.

Question No. 99
What is big advantage of fast breeder reactor?
It has rapid self breeding of fissile fuel during the operation of the reactor, and thus, it offers about sixty times the output with same natural uranium resources through ordinary non breeder nuclear reactor.

Question No. 100
What is the purpose of biological shield in nuclear plants?
Biological shield of heavy concrete prevents exposure to neutrons, beta rays and gamma rays which kill living things.
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