ME 2301 – THERMAL ENGINEERING

1. Define air standard efficiency of Diesel cycle.

Air standard efficiency is defined as the ratio of work done by the cycle to the heat supplied

to the cycle.

2. What is the significance of mean effective pressure?

Mean effective pressure is the quantity related to operation of an IC engine and it is valuable

measure of an engine capacity to do work that is independent of engine displacement.

3. Show that the efficiency of the Brayton cycle depends only on the pressure ratio.

The efficiency of the Brayton cycle is given by

ηBRAYTON=1−T 4−T 1

T 3−T 2

Process 1-2 Isentropic compression process

T 3=T 4 .R pΥ−1Υ

Process 3-4 Isentropic expansion process

T 2=T1 .R pΥ−1Υ

ηBRAYTON=1−T 4−T 1

T 4 .Rp

Υ−1Υ −T 1 . Rp

Υ−1Υ

ηBRAYTON=1− 1

RpΥ−1Υ

4. What does the area enclosed by the cycle represent on P – V diagram?

The area enclosed on P-V diagram represents the work done by the cycle.

5. A Diesel engine has a compression ratio of 14 and cut off takes place at 6% of the

stroke. Find the air standard efficiency.

r=14

V1 = 14 V2

V2 + VS = 14 V2

VS = 13 V2

V3 = 0.06 VS

V3 =0.06 x 13 V2

(1)

ρ=V 3

V 2

=0.06 x13V 2

V 2

=0.78

ηDiesel=1− 1Υ (r )Υ−1 [ ρΥ−1

ρ−1 ] =54%

6. Draw actual P – V Diagram for four stroke SI Engine.

7. Mention the various processes of Dual cycle.

Process 1-2 Isentropic compression process

Process 2-3 Constant volume heat addition process

Process 3-4 Constant pressure heat addition process

Process 4-5 Isentropic expansion process

Process 5-1 Constant volume heat rejection process

8. How does the change in compression ratio effect air standard efficiency of an ideal

OTTO cycle?

When the compression ratio increases, the air standard efficiency of the OTTO cycle

increases and vice versa.

9. Define mean effective pressure.

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.

10. Give the expression for the mean effective pressure of Dual cycle.

Pm=P1 r

Υ (k Υ ( ρ−1 )+(k−1)−r1−Υ (k ρΥ−1 ))(Υ−1)(r−1)

11. Define “Degree of reaction” in a steam turbine.

It is defined as the ratio of isentropic heat drop in the moving blades to isentropic

heat drop in the entire stage of the reaction turbine.

12. What is the effect of friction on the flow through a steam nozzle?

The expansion is no more isentropic and enthalpy drop is reduced. There by resulting

in the lower exit velocity. The final fraction of the steam is increased as a part of kinetic

energy gets converted into heat due to friction and absorbed by steam with in increases in

enthalpy.

The specific volume of steam is increased as the steam becomes drier due to frictional

reheating.

13. Give the working principle of an impulse turbine.

The high velocity jet of steam which is obtained form the nozzle impinged on the blades

fixed on a rotor. The blades change direction of steam flow without changing its pressure.

This causes change in momentum and the force developed driver the turbine rotor.

14. Define nozzle efficiency.

It is defined as the ratio of actual enthalpy drop to the isentropic enthalpy drop.

15. What is blading efficiency?

It is defined as the ratio between work done on the blades and energy supplied to the

blade.

16. What is the effect of friction on the flow through a steam turbines?

The losses due to friction between fluid layers causing change in relative velocity at entry

and exit.

17. What is critical pressure ratio of a steam nozzle?

On the critical pressure ratio, it produces the maximum discharge.

18. What is the need for compounding in steam turbines?

The steam is expanded from the boiler pressure to condenser pressure in one stage the

speed of the rotor becomes tremendously high which crops up practical complicacies. The

multiple system of rotor in series, keyed on common shaft and the steam pressure of jet

velocity is absorbed in stages as the steam flows over the blades. This is known as

compounding.

19. What do you mean by supersaturated flow?

When the supersaturated steam expands in the nozzle, the condensation will occur in the

nozzle. Since the steam has more velocity the condensation will not take place at the

expected rate. So the equilibrium between the liquid and vapor place is delayed and steam

continues to expand in dry state. The steam is such set of condition is said to be

supersaturated of metastable flow.

20. What is the function of governors in steam turbine?

Maintaining the speed of the turbine is constant irrespective of variation of the load on

the turbine known as governing of turbines.

21. What is the function of Camshaft and Crankshaft?

Camshaft is used to convert rotary motion of the camshaft into linear motion of the

follower or lifter.

Crankshaft is used to convert reciprocating motion of the piston into rotary motion.

22. List out the effects of Detonation?

The impact of the engine components and structures may cause failure and creates

undesirable noise

The lack of control of combustion process leads to pre ignition and local overheating.

Therefore, piston may be damaged by overheating

23. What are the various methods to determine the FHP of the engine?

William’s method

Morse test

Motoring test

Deceleration method

24. What re the basic requirement of the fuel injection system of a diesel engine?

a) The beginning as well as end of injection should take place sharply

b) Atomize the fuel to the required degree

c) Distribute the fuel throughout the combustion chamber for better mixing.

25. What is meant by compensation in carburetor?

The process of providing additional air of fuel when it requires and maintaining the

correct air fuel mixture is called compensation in Carburetor.

26. List out the various circuits available in Solex Carburetor?

a) Starting circuits

b) Normal running circuits

c) Acceleration circuits

d) Idling and slow running circuits

27. Compare CI engine and SI engine?

S.

No.

CI ENGINE SI ENGINE

1 It is operated by OTTO cycle It is operated by Diesel cycle

2 Spark plug is required to ignite the fuel

air mixture

Fuel is ignited automatically by high

pressure & temperature air

3 Compression ratio varies from 6 to 8 Compression ratio varies from 12 to 18

28. Which is better efficient two stroke or four stroke engines?

Four stroke engines are always efficient because the two stroke engines give lesser

efficiency due to incomplete combustion and poor scavenging.

29. What do you mean by Scavenging in IC engines?

The process of removing the burnt gases from the combustion chamber of engine

cylinder is known as scavenging.

30. Define Mean effective pressure?

It is defined as the algebraic sum of the mean pressure acting on the piston during one

complete cycle.

31. Why clearance is necessary in reciprocating compressors?

Clearance is necessary in reciprocating compressors to trap the gas after each stroke

32. Differentiate positive and non-positive displacement compressors?

S.NO

.

POSITIVE DISPLACEMENT

COMPRESSORS

NON-POSITIVE DISPLACEMENT

COMPRESSORS

1These compressors are smaller at a

given flow rate.

These compressors are bigger at a given

flow rate.

2The air is compressed adiabatically

but uncooled

The air is not trapped in specific

boundaries but it flows continuously and

steadily though the machine.

3

Pressure rise is either by back flow

of air or both by variation in the

volume and back flow

Pressure rise in primarily due to dynamic

effects

33. Differentiate VCRS and VARS

S.No. VAPOUR COMPRESSION

REFRIGERATION SYSTEM

VAPOUR ABSORPTION

REFRIGERATION SYSTEM

1 Electric power is needed to drive the

system

No need of electric power

2 Wear and tear is more because of moving

components

Wear and tear is less

3 Tonne capacity is low Tonne capacity is high

4 Charging of refrigerant is simple Charging of refrigerant is difficult

5 More chances for leakage of refrigerant There is no leakage of refrigerant

6 Mechanical energy is supplied Heat energy is supplied

7 Performance at part load is poor At part loads, the performance is not

affected

8 Space requirement is more Space requirement is less

9 Energy requirement is low Energy requirement is high

34. Mention the various processes of the Otto cycle

i. Isentropic Compression

ii. Constant volume heat supplied

iii. Isentropic expansion

iv. Constant volume heat rejection

35. Mention the various processes of the Diesel cycle

i. Isentropic Compression

ii. Constant pressure heat supplied

iii. Isentropic expansion

iv. Constant volume heat rejection

36. Mention the various processes of the Dual cycle

a. Isentropic Compression

b. Constant volume heat supplied

c. Constant pressure heat supplied

d. Isentropic expansion

e. Constant volume heat rejection

37. Define Air Standard Efficiency

Air standard efficiency is defined as the ratio of work done by the cycle to the heat

supplied to the cycle.

38. Define: Compression Ratio

Compression ratio is defined as the ratio between total cylinder volumes to

clearance volume.

39. Define: Cut off ratio

Cut off ratio is defined as the ratio of volume after the heat addition to volume

before the heat addition

40. Define: Expansion ratio

Expansion ratio is the ratio of volume after the expansion to the volume before

expansion.

41. What are all the modifications are carried out in Brayton cycle? Why?

In Brayton we incorporate Regenerator, reheater and intercooler, because of

increasing thermal efficiency of the cycle.

42. When will the intercooler is provided between two compressors?

When the pressure ratio is very high, then the intercooler is provided between

compressors.

43. What are the effects of reheat cycle?

Thermal efficiency is less since the heat supplied is more. Turbine output is

increased fir same expansion ratio.

44. What is the function of mixing chamber?

The mixing chamber has two butterfly valves. One is to allow air inti the

mixing chamber and known as choke valve. The other is to allow air fuel mixture ti

the engine and known as throttle valve.

45. What is meant by compensation in carburetor?

The process of providing additional air or fuel when it requires maintaining

the correct air fuel mixture is called compensation in carburetor.

46. What are the basic types of carburetor?

a) Simple carburetor

b) Solex carburettor

47. What is carburetion?

The process of vaporizing the fuel and mixing it with air in a carburetor is

called carburetion.

48. What are the different methods of fuel supply system?

a. Gravity feed system

b. Pressure or Pump feed system

49. What is a carburetor?

A carburetor is a device which vaporizes the fuel and mixed it with the air

50. What are the different types of fuel pumps commonly used?

a. Mechanical fuel pump

b. Electrical fuel pump