You must answer ALL the questions, and upload your answers the Moodle space before the deadline.

Additional notes for candidates:

This is an ‘open-book’ assessment, and you are entitled to consult module-relevant resources, such as textbooks, your own notes, and other reference materials when answering the questions. However, answers should be written in your own words.  You are not expected to undertake additional research in order to answer the questions.

This is an individual assessment; you must not confer or collude with other students/ individuals. Plagiarism is an academic offence.

You may use an electronic calculator, a spreadsheet or other computational tools. However, you must show all relevant workings.

Each answer page should include the question number being answered and the page number. You will not receive marks for the style/formatting of your answers; therefore, hand-written or word-processed submissions are equally acceptable, provided all of your answers are legible and the language is coherent. If you choose a hand-written submission, you must ensure the work is clearly recognisable and understandable for marking.

You should upload your answers to the module Moodle site as a single pdf file, which should include your entire submission (i.e. all the scanned/photographed images of each of your answer pages). If you have decided to hand-write your answers, then Microsoft Lens is the   recommended application for combining scanned images/photographs into a single pdf file. If you have decided to write your answers using a word-processed document, you should produce a pdf file directly from the word-processed document and check that the transcription has been completed accurately before you upload the document; half an hour has been added to your assessment time to enable you to complete this step.

Only one submission will be accepted into Moodle. It is your responsibility to check your work is legible and the correct pdf has been uploaded.  This check should be done within the submission window.  Once the deadline has passed, no alterations can be made.

This assessment will be marked anonymously.  Your uploaded file will be assigned a unique, anonymous ID when uploaded into the Moodle submission area. You must not put your name or your student ID on any part of your uploaded answer, and nor should your name be included in the pdf file name.

A1        Answer ALL parts (a) – (d)

A 700 kg sports car is powered with a 1.6 litre, 4-stroke, 6-cylinder engine. At 5,000 rpm, the engine produces 100 kW of power. The engine has an overall efficiency of 46% and the breathing and mechanical efficiencies are 90% and 82% respectively. We will model this as an Otto cycle operating on an ideal gas and you may assume that Rg  = 0.287 kJ/kg/K and γ = 1.4, where γ is the adiabatic index.

(a)        Determine  the   compression   ratio,  brake  work  and  net  work   output  per cylinder, per cycle. (10 marks)

(b)        At the intake, the air pressure and temperature are p1  = 101 kPa  and T1  = 288 K. Determine the temperature at each stage in the process, the work done during compression  and  expansion  and the  heat  rejection  per  cylinder  per cycle. (8 marks)

(c)        Suggest  two  ways  in  which  the  thermal  efficiency  of  the  engine  could  be improved and briefly explain how each method works. (2 marks)

(d)        The air-fuel  mix consists of 1.6% petrol by mass and petrol has a density of 0.8 kg/litre. Assuming the car travels at 70 mph (112 km/hour) in 6th  gear at 5000 rpm, determine the fuel economy of the engine in km/litre. (5 marks)

A2        Answer ALL parts (a) – (d)

A gas power plant operates a gas turbine, which is modelled as an ideal Brayton cycle. At the compressor inlet, a mixture of natural gas and air is drawn in at a pressure of 5 bar at 32oC. After combustion, the temperature of the gas reaches 2800oC. You may assume that the working fluid is an ideal gas, with Rg   = 0.301 kJ/kg/K and γ =  1.42.

(a)        Determine   the   required   pressure   ratio   if   the   specific   heat   addition   is 1800 kJ/kg. What is the resulting thermal efficiency and work ratio? (7 marks)

(b)       The gas inlet duct has a cross-sectional area of 3.5 m2, determine the mass flow and flow velocity of the gas mixture in the inlet duct, given that the turbine provides a net power output of 10 MW. (4 marks)

(c)        A  regenerative heat exchanger is added to increase the thermal efficiency of the turbine. 10% of the excess heat from the exhaust is lost to the surroundings before the heat exchanger and after the heat exchanger, the compressed gas and exhaust are at thermal equilibrium (T2(′) = T4′ ). Determine this equilibrium temperature,  T2(′),  the  specific  heat  exchanged  as  well  as  the  new  thermal efficiency and work ratio. (10 marks)

(d)        Explain under what conditions a regenerative heat exchanger wouldn’t be able to increase the thermal efficiency of the turbine. At what is the pressure ratio and thermal efficiency when a regenerative heat exchanger stops improving efficiency? You may assume that the specific heat addition does not change and neglect any heat loss for the exhaust gases prior to the heat exchanger. (4 marks)