Judah Levine, 3070L13 1http://physics.colorado.edu/phys3070http://physics.colorado.edu/phys3070Judah LevineJudah LevineJILA SJILA [email protected]@colorado.edu303 492303 492--7785 (27785 (2--7785)7785)M,F 11M,F 11--Noon, W 1Noon, W 1--3 pm3 pmalso by appointmentalso by appointmentJudah Levine, 3070L13 23070, Exam 102468101230405060708090100BinFrequencyAverage=74Median=77Judah Levine, 3070L13 3The Otto cycle engineThe Otto cycle engineAirPistonConnecting rodFlywheel/crankshaftGasExhaust gasesEngine size istotal volume Of cylindersCompressionRatioJudah Levine, 3070L13 4Summary of the Otto CycleSummary of the Otto CycleQQStroke 1Stroke 1––Piston moves downward, draws in air/fuel Piston moves downward, draws in air/fuel mixture mixture (Fuel must vaporize rapidly)(Fuel must vaporize rapidly)QQStroke 2Stroke 2––Piston moves upward, compresses and heats Piston moves upward, compresses and heats air/fuel mixture air/fuel mixture (Fuel must not ignite!)(Fuel must not ignite!)QQStroke 3Stroke 3––Fuel/air mixture is ignited driving piston Fuel/air mixture is ignited driving piston downward (downward (Only stroke that produces powerOnly stroke that produces power))QQStroke 4Stroke 4––Piston moves upward, driving out exhaust gasesPiston moves upward, driving out exhaust gasesJudah Levine, 3070L13 5WhatWhat’’s wrong with the Otto cycle?s wrong with the Otto cycle?QQPower stroke is 1 stroke out of 4Power stroke is 1 stroke out of 4––1 power stroke for 2 revolutions1 power stroke for 2 revolutionsQQFlywheel drives piston for the other 3Flywheel drives piston for the other 3QQStored flywheel energy ~ (rpm)Stored flywheel energy ~ (rpm)22QQEngine cannot run at low rpm (even Engine cannot run at low rpm (even with multiple cylinders)with multiple cylinders)––Minimum about 600 rpmMinimum about 600 rpmJudah Levine, 3070L13 6WhatWhat’’s wrong with the Otto cycle?s wrong with the Otto cycle?QQEfficiency is about 25%Efficiency is about 25%--30%30%––Exhaust gases are HOT and corrosiveExhaust gases are HOT and corrosive––Engine cooled with water (or air)Engine cooled with water (or air)••Some cogeneration recoverySome cogeneration recoveryJudah Levine, 3070L13 7WhatWhat’’s wrong with the Otto cycle?s wrong with the Otto cycle?QQIncreasing efficiency pushes towards Increasing efficiency pushes towards higher compression ratios and higher compression ratios and combustion temperaturescombustion temperatures––Increases NIncreases N22O, NO, NOO, NO, NO22, HNO, HNO33……––Makes preMakes pre--ignition problem worseignition problem worseJudah Levine, 3070L13 8TwoTwo--stroke variantstroke variantFlywheel/crankshaftStroke 1a:Piston moves belowIntake, drawingIn fuelStroke 2:Piston moves upwardBlocking input andOutput and Compressing mixStroke 1b:Ignition, piston pusheddownward, unblockingportsExhaustValveFuel/air mixJudah Levine, 3070L13 9The Diesel cycle engine The Diesel cycle engine --11AirPistonConnecting rodFlywheel/crankshaftIntake strokeAir Onlydrawn inFlywheelDrives pistonJudah Levine, 3070L13 10The Diesel cycle engine The Diesel cycle engine --22AirPistonConnecting rodFlywheel/crankshaftCompressionstrokeAir heatedFlywheelDrives pistonVery high compressionratio:Pre-ignition not a ProblemJudah Levine, 3070L13 11The Diesel cycle engine The Diesel cycle engine --33AirPistonConnecting rodFlywheel/crankshaftPowerstrokeFuel injectedFlywheelDrives pistonHot air ignites fuel.Fuel characteristicsnot very importantJudah Levine, 3070L13 12The Diesel cycle engine The Diesel cycle engine --44AirPistonConnecting rodFlywheel/crankshaftExhaust gases +unburned fuelExhauststrokeFlywheelDrives pistonJudah Levine, 3070L13 13Comparing Diesel/Otto cyclesComparing Diesel/Otto cyclesQQDiesel engine compresses only air, injects fuel Diesel engine compresses only air, injects fuel into cylinder at start of power stroke.into cylinder at start of power stroke.––Allows higher compression and higher Allows higher compression and higher temperature since pretemperature since pre--ignition is not a problemignition is not a problem••implies higher efficiencyimplies higher efficiencyQQFuel injection makes engine less sensitive to Fuel injection makes engine less sensitive to fuel characteristicsfuel characteristicsQQHigher combustion temperature makes Higher combustion temperature makes NONOxxworseworseQQEngine cannot run at very low speedEngine cannot run at very low speedJudah Levine, 3070L13 14What does What does ““cleanclean--burningburning””mean?mean?Judah Levine, 3070L13 15Model heat engine (encore)Model heat engine (encore)ForcegasPressure on piston comes from gas molecules striking itPressure proportional to temperature (kinetic energy)Judah Levine, 3070L13 16Model heat engine (encore)Model heat engine (encore)ForcegasSuppose it could be made to workJudah Levine, 3070L13 17Model heat engine (encore)Model heat engine (encore)ForcegasKinetic energy of gas converted to external workKinetic energy (and temperature) decreaseResidual gas would be colderJudah Levine, 3070L13 1822ndndLaw of ThermodynamicsLaw of ThermodynamicsIt is impossible in principle to construct a refrigerator that requires no energy inputeven if it obeys the principle of the conservation of energyJudah Levine, 3070L13 19Heat Engine Energy FlowHeat Engine Energy FlowTHTLQHWQLW= QH–QLJudah Levine, 3070L13 20Refrigerator Energy FlowRefrigerator Energy FlowTHRefrigerator transfers heatFrom the inside to the room.Room warms up by QHRefrigerator cools off by QLQH=QL+W > QLTLQHWQLWattshrBTUWQeerL/==Judah Levine, 3070L13 21Energy Efficiency RatioEnergy Efficiency RatioWattshrBTUWQeerL/==Suppose eer=10 (a typical value).What is the real fractional efficiency obtainedBy expressing both quantities in the same units?3/41.31/10≈×≈ratioenergyhrBTUwattWattshrBTUeerJudah Levine, 3070L13 22Heat Pump Energy FlowHeat Pump Energy FlowTH1>=>+=WQCOPWQWQHLHRoom warms upBy QHTLQHWQLGreat outdoorsJudah Levine, 3070L13 23RefrigeratorRefrigeratorEasy to liquefyEvaporates at cold tempCool (inside) Warm (outside)Gas flowLiquidGasLiquid flowCompressorGas to liquidQLQHExpansionvalveJudah Levine, 3070L13 24RefrigerantsRefrigerantsFFHCClClFFreon 12FFC C FHHFC-134Chlorine primarily responsible for ozone depletionJudah Levine, 3070L13 25Electric power generationElectric power generationNSCoil ACoil BCoil C3-phase“Delta”O3O2 O1Judah
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