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AME436-S08-FinalStudyGuide - AME 436 Prof P Ronney Final...

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AME 436, Prof. P. Ronney Final Exam Study Guide May 7, 2008 Format of the exam The final exam will be open book. You may use any reference materials you want, but no laptop computers or other devices capable of running the aircycles4whatever.xls spreadsheets. The format will be the same as the midterm but will be 2 hours long. The exam will have graphical, numerical and short-answer questions. Short summary of the most important facts Hydrocarbon fuels are the most convenient, high-density way of storing energy; compression, combustion, expansion is the most convenient (high power/weight) way of converting this stored energy into useful work The simplest estimate of adiabatic flame temperature is T ad = T + fQ R /C P (constant pressure), but at high temperatures, C P increases and dissociation of CO 2 and H 2 O causes T ad to fall below this estimate, even if no heat losses are present Practically all chemical reactions of interest in this course have high activation energy, meaning that their rates increase rapidly with increasing temperature. This includes the chemical reactions causing heat release (thus affecting burning velocity of premixed flames), knock and most emissions. If you want to determine how a change in engine operating conditions affects performance, the first thing to check is how temperature is affected Flames come in two flavors – premixed and nonpremixed o Premixed (e.g. Bunsen burner) Fuel and air are completely mixed before combustion is initiated (e.g. via a spark) Most important characteristic is the burning velocity S L ~ ( αΩ ) 1/2 If the mixture is lean, T ad and thus S L will be low (bad) but NO emissions will be low (good) If the mixture is too lean, the flame will extinguish completely (very bad) o Nonpremixed (e.g. Bic lighter) Fuel and air are un-mixed until combustion occurs There are always stoichiometric surfaces (thus stoichiometric-like flame temperatures) somewhere between the regions of pure fuel and pure air As a result, there are always high reaction rates even when the mixture is lean overall (good) but also high NO and soot formation rates (bad) In most cases the burning rate is limited by mixing rates, not chemical reaction rates Engines are air processors – the air takes up most of the spacel, so if you can process more air, you can get more power Thermodynamically, the best way to burn is at the minimum volume or maximum pressure (which is really another way of saying, maximum temperature) because this gives you the most efficient Carnot cycle strips Reciprocating engines o Premixed-charge
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Performance (power, efficiency) is limited by compression ratio, which in turn is limited because of knock Knock is an explosive, homogeneous reaction of the gas ahead of the flame front (“end gas”) before the flame gets to it Knock depends on the temperature of the reactants (T ) (whereas flame propagation depends on product temperature T ad )
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AME436-S08-FinalStudyGuide - AME 436 Prof P Ronney Final...

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