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Thermodynamics Question Paper

Thermodynamics

Course:Diploma In Mechanical Engineering

Institution: Technical University Of Mombasa question papers

Exam Year:2017

1 a. Define the following terms
(a) Enthalpy
(b) Entropy (4 marks)

b. 3 kg of air at 1 bar and 20 degree Celsius undergoes a change of state to 5 bars and 227 degree
Celsius. Determine ;
(a) The initial and final volumes
(b) The change in entropy. Take Cp and Cv as 1.005 KJ/Kg K and 0.718 KJ/Kg K, respectively. (8 marks)

c.(a) Distinguish between the ideal and actual cycles.
(b) Derive the expression for the Carnot efficiency in terms of temperatures. (8 marks)

d. A Carnot engine uses 0.05 kg of air as the working substance. The temperature limits of the cycle
are 27 degree Celsius and 567 degree Celsius, the maximum pressure is 84 bar, and the heat added per
cycle is 4.2 kJ. Determine the temperature, pressure and volume at each state of the cycle. (10 mrks)

2. a. Draw a heat engine in a steady operation state and derive its thermal efficiency (10 marks)

b. Explain why the efficiency of heat must be less than 100% (3 marks)

c. A heat engine produces 10 KW of power at the output shaft. If the engine receives heat at a rate
of 2 MJ/min, determine;

(a) The efficiency of the engine.

(b) The rate of heat rejection. (7 marks)

3 a. Show that for steam being generated between temperatures t1 to t2, the change in entropy is given
by S2- S1= Cp ln(T2/T1)

b. Determine the entropy steam at a pressure of 7 bar and temperature of 425 degree Celsius.

c. Steam at 40 bar and 425 degree Celsius kg is expanded isentropically to a pressure of 0.07 bar.
Draw the T-s diagram and determine the final state of the steam. (8 marks)

4. An engine operating on the ideal diesel cycle has a maximum pressure of 41.4 bar and a maximum
temperature of 1093 degree celsius. The air at the beginning of compression is at 21.1 degree Celsius
and 1.013 bar.

a. Draw the P-v diagrams for the cycle

b. Determine the air standard efficiency of the cycle (take y= 1.4) (20 marks)

5. An engine operates on ideal Otto cycle. the bore, stroke and compression ratio are 100 mm, 127 mm,
and 7:1 respectively. The air at the beginning of compression stroke is at 15.6 degree celsius and 1
bar and the maximum temperature is 1650 degree celsius. Draw the P-v diagram and determine for the
cycle;

a. The temperature, volume and pressure at the process points.

b. The air standard efficiency.

c. The mean effective pressure.
Take for air, Cv= 0.718 KJKg/K and Cp= 1.005 KJKg/K (20 marks)

Thermodynamics Question Paper

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