100 Latest Thermodynamics & Power plant Engineering Interview Questions
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.
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.
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.
Explain Second Law of Thermodynamics?
The entropy of the universe increases over time and moves towards a maximum value.
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.
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.
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.
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.
Why entropy change for a reversible adiabatic process is zero?
Because there is no heat transfer in this process.
What is the purpose of Scrapper Ring?
Scraps the excess lube oil from the cylinder walls. There by preventing oil from entering combustion zone.
What are two essential conditions of perfect gas?
It satisfies equation of state and its specific heats are constant.
Enthalpy and entropy are functions of one single parameter. Which is that?
Temperature.
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.
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.
How much resistance is offered to heat flow by drop wise condensation?
No resistance offered.
What are the Advantages and Disadvantages of using LPG in Car?
Advantages
1. Complete combustion
2. Fuel saving
3. Homogeneous 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
Which has more Efficiency Diesel Engine or Petrol Engine?
Diesel engine has the better efficiency out of two.
What is the relationship between COP of heating and cooling?
COP of heating is one (unity) more than COP of cooling.
How much is the work done in isochoric process?
Zero.
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.
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.
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.
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.
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.
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.
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.
Which Mechanism is used in Automobile gearing System?
Differential mechanism
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.
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.
When Crude Oil is Heated, Which Hydro Carbon comes first?
Natural gas (Gasoline), at 20 Celsius.
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.
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.
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.
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.
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.
What is flash-point?
Flash point: the lowest temperature at which the vapour of a combustible liquid can be ignited in air.
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.
What is refrigerant?
Any substance that transfers heat from one place to another, creating a cooling effect. Water is the refrigerant in absorption machines.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.”
Is the boiler a closed system?
Yes definitely the boiler is a closed system.
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.
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.
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.
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.
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)
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.
Which has more efficiency: Diesel engine or Petrol engines?
Off course Diesel engine has the better efficiency out of two.
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.
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.
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.
Which are the reference fuels for knock rating of S.I. engines?
n-heptane and ISO-octane.
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.
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.
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.
Quantities like pressure, temperature, density, viscosity, etc. are independent of mass. What are these called?
Intensive properties.
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.
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.
In convection heat transfer, if heat flux intensity is doubled then temperature difference between solid surface and fluid will?
Get doubled.
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.
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.
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
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.
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.
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.
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.
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.
What is fuel ratio?
Fuel ratio is the ratio of its percentage (%) of fixed carbon to volatile matter.
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
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.
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.
What are the important operational performance parameters in design of fuel firing equipment?
Fuel flexibility, electrical load following capability, reliability, availability, and maintenance ease.
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.
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.
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.
Ultimate analysis of coal is elementary analysis. What it is concerned with?
Carbon, hydrogen, nitrogen, and sulfur in coal on a weight percentage basis.
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.
What is the significance of torque (in Nm) given in the engine specification?
It give the moment about any point or simple rotation.
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.
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.
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
Which parameter remains constant in a throttling process?
Enthalpy.
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.
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.
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.
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.
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.
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.