BHARATHIYAR COLLEGE OF ENGINEERING AND TECHNOLOGYKARAIKAL

B.Tech ODD SEM Question Bank

T 105 THERMODYNAMICS(Common to all branches)TWO MARKS QUESTION AND ANSWER

UNIT- IV GAS POWER CYCLES

1. Define: Thermodynamic cycles.

Thermodynamic cycle is defined as the series of processes performed on the system, so that the system attains to its original state.

2. Classification of thermodynamics cycles Thermodynamics cycles can be classified into different categories depending on fluid used or the different processes: Gas and vapor cycles 1- Gas cycle: the working fluid remains in gaseous phase throughout the entire cycle. 2- Vapor cycle: the working fluid exists in the vapor phase during one part of the cycle and in the liquid phase during another part.

Open and closed cycles 1- Open cycle: the working fluid is returned to the initial state at the end of the cycle and is re circulated. 2- Closed cycle: the working fluid is renewed at the end of each cycle instead of re circulated. 3. How to increase the efficiency of a cycle?a) By increasing the net work.b) By decreasing the amount of heat rejection.c) By increasing the mean temperature of heat addition or by decreasing the mean temperature of heat rejection.4. Define the term compression ratio.Compression ratio is the ratio between total cylinder volumes to clearance volume. It is denoted by the letter r

5. Write the expression for efficiency of the Otto cycle? 1Efficiency otto = 1 - --------- (r)-16. What is meant by cut-off ratio?

Cut-off ratio is defined as the ratio of volume after the heat addition to before the heat addition. It is denoted by the letter

7. What are the assumptions made for an air standard Cycle?

Air is the working substance. Throughout the cycle, air behaves as a perfect gas and obeys all the gas laws. No chemical reaction takes place in the cylinder Both expansion and compression are strictly isentropic The values of specific heats of the air remain constant throughout the cycle.

8. What is the difference between Otto and Diesel Cycle.

Otto CycleDiesel Cycle

1. Otto cycle consist of two adiabatic and two constant volume process.1. It consists of two adiabatic, one constant volume and one constant pressure processes.

2. Compression ratio is equal to expansion ratio2. Compression ratio is greater than expansion ratio.

3. Heat addition takes place at constant volume.3. Heat addition takes place at constant pressure

4. Compression ratio is less. It is varies from 6 to 8.4. Compression ratio is more. It varies from 12 to 18.

9. What is meant by air standard efficiency of the cycle?

It is defined as the ratio of work done by the cycle to the heat supplied to the cycle.

Work doneEfficiency = --------------Heat supplied

10. Define: Mean effective pressure of an I.C. engine.

Mean effective pressure is defined as the constant pressure acting on the piston during the working stroke. It is also defined as the ratio of work done to the stroke volume or piston displacement volume. It is denoted by Pm.

11. Define: Specific fuel consumption.

SFC is defined as the amount of fuel consumed per brake power hour of work.

12. What is meant by calorific value of a fuel?Calorific value of a fuel is defined as the amount of heat liberated by the compete combustion of unit quantity of a fuel.

13. Give the expression for efficiency of the Dual cycle.

1k - 1Efficiency = 1 - ------- X ----------------- (r)-1 (k-1) + k (-1)where,

r Compression ratiok Pressure or Expansion ratio Cut off ratio and Adiabatic index

14. What are the factors influencing of the Dual cycle?

1. Compression ratio 2.cut off ratio 3. Pressure ratio and 4. Heat supplied at constant volume and constant pressure.

15. Give the expression for efficiency of the Brayton cycle.

1Efficiency = 1 - ---------- where K Pressure ratio. (K)/

16. What is the fuel injector?

Fuel injector is used in diesel engine to inject and atomize the diesel at the end of the compression stroke.

17. What is meant by SI engine? Why it is called so?

SI engine means spark ignition engine. In SI engine air fuel mixture is ignited by spark plug hence it is called spark ignition engine. It is also called as petrol engine.

18. What is meant by CI Engine? Why it is called so?

CI engine means compression ignition engine. In CI engine the fuel is injected by a fuel injector in atomized form because of high compressed air it gets ignited automatically. Hence it is called as compression ignition engine.

19. What is a two stroke engine?

A two stroke engine is an engine in which one cycle of operation is completed in two stroke of the piston or one revolution of the crank shaft.

20. What is a four stroke engine?

A four stroke engine is an engine in which one cycle of operation is completed in four stroke of the piston or two revolution of the crank shaft.

21. Name the four strokes of an IC engine?

Suction, Compression, Power and Exhaust stroke.

22. Differentiate petrol and Diesel engines.

Petrol or SI enginesDiesel or CI engine

1. Combustion of air fuel mixture takes place by spark produced by sparkplug.1. Combustion takes place by high compressed air.

2. Carburetor is used to mix the air fuel mixture.2. Fuel injector is used to inject the fuel in atomized form.

3. Compression ratio varies from 6 to 8.3. Compression ratio varies from 12 to 18.

4.It works on Otto cycle.4. It works on Diesel or Dual cycle.

23. What is compression ratio?Compression ratio is defined as the ratio between total cylinder volume and clearance volume.

Total volume Vc+VsCompression ratio = ------------------- -----------Clearance Volume Vc

24. What are the processes involved in Otto cycle?

Otto cycle consists of four processes, namely, adiabatic compression, constant volume heat addition, adiabatic expansion and constant volume heat rejection.

25. What are the processes involved in Diesel cycle?

Diesel cycle consists of four operations, namely, adiabatic compression, constant pressure heat addition, adiabatic expansion and constant volume heat rejection.

26. What are the processes involved in Duel cycle?

Duel cycle consists of five operations, namely, adiabatic compression, constant volume heat addition, constant pressure heat addition, adiabatic expansion and constant volume heat rejection.

27. Compare Otto, Diesel and Duel cycles with respect to their performance?

Otto cycle has the highest work output and efficiency. Diesel cycle has the lowest and Duel cycle has the efficiency between two

28. Define Top Dead Center and Bottom Dead Center?

The top most position of the piston inside the cylinder. i.e. when it is at the maximum distance from the crankshaft axle is known as Top Dead Center (TDC).The bottom most position of the piston inside the cylinder is called Bottom Dead Center (BDC)

29. Define Bore and Stroke of an Engine?

The diameter of the cylinder is called Bore. The distance between TDC and BDC is known as stroke or piston travel.

30. Define Clearance volume, Swept or Stroke volume, and Cylinder volume?

The space above the piston, when the piston is at TDC is called Clearance volume The volume swept through by the piston, as it moves from one dead center to the other is called swept volume or stroke volume. The volume above the piston, when the piston is at BDC is called cylinder volume. The cylinder volume is the sum of the clearance volume and stroke volume

31. What is meant by the Cubic Capacity of an engine?

The cubic capacity of engine or engine displacement or engine size is the product of the stroke volume in one cylinder and the number of the cylinders in the engine.

32. Sketch P-V and T-S diagram for Otto cycle?

Process 1-2: Reversible adiabatic compression of air. Process 2-3: Heat addition at constant volume. Process 3-4: Reversible adiabatic expansion of air. Process 4-1: Heat rejection at constant volume.

33. Sketch P-V and T-S diagram for Diesel cycle?

34. Sketch P-V and T-S diagram for Brayton or Joule cycle?

Additional Questions

1. State Clausius inequality?Clausius inequality states that when the system undergoes a cyclic process, the summation of dQ/T around a closed cycles is less than or equal to ZERO

(dQ/T)rev 0 This equation is called inequalityIf (dQ/T)rev = 0 cycle is reversible, (dQ/T)rev< 0 cycle is irreversible,

(dQ/T)rev> 0 cycle is impossible

2. State the types of irreversible process?

An irreversible process is one in which heat is transferred through a finite temperature.Irreversibilitys are of two types

External irreversibilitys: these are associated with dissipating effects outside the working fluid Internal irreversibilitys: These are associated with dissipating effects within the working fluid3. Mention the various processes of the Brayton cycle? Isentropic compression Constant pressure heat supplied Isentropic expansion Constant pressure heat rejection4. Conversion factors

Pressure 1 Bar = 105 N/m21 pa = 1 N/m21 atm = 760 mm of Hg , 1.01325 bar1 joule = 1 n-m , 1 watt-sec1 watt = 1 joule/sec1 KW = 1000 watts

Ratio of specific heats = 1.4 ( for air)Sp.Heat at constant pressure Cp = 1.005 KJ/Kg KSp.Heat at constant volume Cv = 0.718 KJ/Kg KGas constant R = 0.287 KJ/Kg KUniversal gas constant M= 8.314 KJ/Kg.mol K = Cp/CvR = Cp-CvCv/Cp = 1/Cv = R/-1

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