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Cork Institute of Technology
Higher Certificate in Engineering in Mechanical Engineering – Award
(NFQ Level 6)
Summer 2007
Mechanical Technology-Thermofluids
(Time: 3Hours)
INSTRUCTIONS Answer 5 questions, at least two questions from Section A and two from Section B and one other.
Examiners: Ms. M. Kavanagh Mr. J. Connolly Dr. P. Delassus
Section A
Q1. (a) A cycle consists of the following processes in order:
(i) adiabatic compression from a pressure of 1 bar and volume of 2m^3 to a volume of 0.2m^3 (ii) constant pressure expansion to 0.4m 3 (iii) adiabatic expansion back to original volume (iv) constant volume cooling to its initial state Sketch the cycle on a P-V diagram (5 marks)
(b) 0.113 m^3 of air at 825kN/m^2 is expanded in a cylinder until the volume is 0.331 m^3. The temperature before expansion is 500 o^ C. Calculate the final pressure and the work done if the expansion is polytropic, n=1.4. Find the heat energy transferred during the process. Take R = 287J/kgK and c (^) V =718J/kgK. (15 marks)
Q2. Sketch a line diagram for the water-steam circuit in a steam power plant, indicating the relative positions of the main components. Describe the function of the main components in the diagram. (5 marks) In a steam plant, steam enters a turbine at 44bar dry saturated and exhausts into the condenser at 0.04 bar. Sketch the T-s diagram. Determine the dryness fraction of the steam entering the condenser and the Rankine efficiency of the plant. (15 marks)
Q3. Draw an indicator diagram for both a petrol engine and a diesel engine and discuss the essential differences between them. (5 marks)
A six cylinder, four stroke internal combustion engine has a bore diameter of 90mm and a piston stroke of 110mm. The indicated mean effective pressure is 782kN/m^2 , the engine runs at a speed of 3700revs/min and the fuel consumption 26kg/hour. The mechanical efficiency is 80%. The calorific value of the fuel is 41.86MJ/kg. Calculate the indicated power, the brake power and the brake thermal efficiency. Determine the brake specific fuel consumption. ( marks)
Q4. What would be the effect of burning a hydrocarbon fuel with an excessively weak mixture. (2 marks)
Write a balanced chemical equation for the combustion of heptane (C 7 H 16 ) and determine the stoichiometric mass of air required to completely burn 1kg of the gas. If the actual air to fuel ratio is 19:1, determine the percentage excess air supplied. Determine the percentage analysis of the products of combustion by mass. (18 marks)
Composition of air by mass: 77% Nitrogen 23% Oxygen Atomic mass: Carbon(C) 12 Hydrogen(H) 1 Oxygen(O) 16
