2.61 Internal Combustion Engines Spring 2008

Size: px

Start display at page:

Download "2.61 Internal Combustion Engines Spring 2008"

Kevin Miles
6 years ago
Views:

1 MIT OpenCourseWare Internal Combustion Engines Spring 2008 For information about iting these materials or our Terms of Use, visit:

2 2.615 Internal Combustion Engine Final Examination - Spring, 2008 Open book, 3 hour Exam Problem 1 (20 points) (i) (ii) At engine operating temperature, automotive lubriants omprise long hain hydroarbon moleules whih are normally tangled up, but align themselves at high shear rates. Thus the visosity dereases as the shear rate inreases (shear- thinning). Explain why would this property help or hurt engine lubriation. (2 points) Experimentally, the fritional mean effetive pressure (fmep) needed to overome the valve train frition dereases with engine speed; e.g., see Fig of text. Explain this trend. (2 points) (iii) Syntheti diesel fuel (e.g. made by the Fisher Tropsh proess whih was invented in war time Germany to derive liquid fuel from oal and wood) has a very high etane number (CN~80 ompared to the normal diesel fuel with CN in the high 40 s). Give two omments on the impat of this high CN value on diesel engine design and operation. (2 points) (iv) An inventor proposed that a turbo-ompound diesel engine be whih uses an additional power turbine to extrat mehanial energy out of the exhaust gas to the drive shaft. The novelty of the proposal was that muh of the shaft power was to be derived from the power turbine instead of the engine. Give two omments on whether the onept is good or bad (eah omment ould either be positive or negative respetively). (2 points) (v) Give two reasons why retarding the spark timing at onstant idle speed at old start would make the atalyst to light-off faster. (2 points) (vi) Give two onsequenes from operating a SI engine on Miller yle (i.e. with a onfiguration suh that the ompression ratio is less than the expansion ratio. (2 points) (vii) Give two reasons why for a given maximum power output of a SI engine, turbo-harging would lead to better fuel eonomy. (2 points) (viii) The US Army is interested in developing a diesel engine that does not use liquid oolant. The primary reason for suh an engine is the elimination of the radiator whih is vulnerable at ombat. Give two detrimental effets to the engine operation if suh a onfiguration is used. (2 points) (ix) Why would one pursue a ylinder-deativation strategy, i.e. deativate some of the ylinders in ertain part of the engine map? (1 point). Do you want to keep the valves open or losed for the deativated ylinders and why (1 point). (x) One way to lower NOx emissions in diesel engines is to add water to the fuel. Sine water is not mixable with diesel fuel, it has to been emulsified by a surfatant. Explain how it works and why this arrangement is more effetive than injeting water in the intake manifold (2 points).

3 Problem 2 (20 points) There has been substantial interest in using fuel ells as power soures in automobiles. The only pratial reatants for urrent fuel ell systems are hydrogen and oxygen. It is diffiult to use hydrogen diretly in a vehile beause of the problems of storage and refueling. A proposed onept is to use gasoline as the fuel on-board and make hydrogen out of it. See press release below. Image removed due to opyright restritions. Please see Akre, Brian S. "Chrysler Developing Eletri Car Using Gasoline to Power Fuel Cell." Los Angeles Daily News, January 7, The method is to partially oxidize the fuel to CO and H 2 first. Then the CO is reated with the H 2 O (whih is one of the end produts in the exhaust) by the Water-Gas shift reation to form H 2 and CO 2. The H 2 is used in the fuel ell. You are to analyze the energetis of the proess (i.e., energy balane analysis only; ignoring all the irreversibility losses). You may represent the gasoline as iso-otane and assume ideal reations, i.e. the partial oxidation only forms CO and H 2., and that all CO is onverted to CO 2 and H 2 in the Water-Gas shift reation. (a) On the basis of the energetis of the reations, what is the maximum energy onversion effiieny of suh a system? (b) How would your answer hange if the fuel is not iso-otane but ethanol? (What is the major differene?)

4 Problem 3 (20 points) To examine the behavior of turboharging, we shall onsider the following example of a four stroke 2.0 L displaement passenger ar diesel engine turboharged with a single stage entrifugal ompressor, and a single stage turbine. For simpliity, there is no waste-gate and no inter-ooler. You may assume the following: The ambient onditions are T 1 =300K and P 1 =1 bar. The working fluid may be assumed to have onstant properties of γ = 1.35 and moleular weight of 29. The turbine and ompressor have onstant effiienies: η t = 0.85 and η = You may also assume that the Mah no. is small so that the kineti energy of the fluid is negligible. The ompressor and turbine maps are shown in the aompanied figures. (For this simplified example, ignore the atual values of the η ontours on the ompressor map and assume η to be onstant.) The referene onditions of these maps are the same as the ambient onditions: T ref = T 1 ; P ref = P 1. The engine volumetri effiieny based on the intake manifold ondition is onstant: η v = 0.8. The heating value of the diesel fuel is 43 MJ/kg. Assume that 30% of the fuel energy is lost as heat transfer in the engine The engine indiated net fuel onversion effiieny is η f = 0.33 We shall look at the engine operating at 3000 rpm with A/F ratio = 22, and ompressor pressure ratio (π ) of 1.5. (a) What is the ompressor outlet temperatures (T 2 ) at ompressor pressure ratios π = 1.5? (b) What is the engine intake air mass flow rate at this value of π? () What is the ompressor speed at this value of π? (d) Draw on the ompressor map the operating point orresponding to your answers of (b). (e) What is the orresponding turbine inlet temperature? (f) What is the turbine pressure ratio (π t ) -1 that is required to power the ompressor? (Assume that all mehanial losses are already inorporated into the definition of the ompressor/turbine effiienies.) (g) The turbine and ompressor run at the same speed. From the turbine map, using the result of (f), determine the mass flow rate required by the turbine to power the ompressor. (h) Put this point also on the ompressor map; i.e. plot the mass flow rate required by the turbine versus the ompressor pressure ratio π. Does this point math up to the point plotted in part (d)?

5 Simple turbo-harged diesel engine Pressure ratio π Pressure ratio % 70% % % % 60% Ignore η6070 ontours. 1.8 Assume 5550η = 0.65 everywhere Compressor or 4025 N/ ssor T 1 Map Map "Correted" Flow rate m T 1 /P 1 Surge limit Compressor (π (π t ) -1 t ) =P3/P4-1 =P3/P4 Correted orreted mass mass flowrate flowrate m m orreted, kg/s, kg/s Figure by MIT OpenCourseWare. Adapted from Haddad, Sam David, and Watson, N. Priniples and Performane in Diesel Engineering... N. Priniples and Performane in Diesel Engineering. Chihester, England: Ellis Horwood, P ref T ref P ref =1 bar T ref = 300K Turbine Map p N/ (T 3 /T ref )

SCHOOL OF COMPUTING, ENGINEERING AND MATHEMATICS SEMESTER 2 EXAMINATIONS 2014/2015 ME340. Power Train Engineering

s SCHOOL OF COMPUTING, ENGINEERING AND MATHEMATICS SEMESTER 2 EXAMINATIONS 2014/2015 ME340 Power Train Engineering Time allowed: TWO hours Answer: Any FOUR questions only Items permitted: This is a losed-book