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AME 436 Energy and Propulsion

AME 436 Energy and Propulsion. Lecture 6 Unsteady-flow (reciprocating) engines 1: Basic operating principles, design & performance parameters. Announcements. Pick up graded HW #1 and solutions if you didn’t last week Midterm “party” #1 next week F irst 90 minutes of class

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AME 436 Energy and Propulsion

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  1. AME 436Energy and Propulsion Lecture 6 Unsteady-flow (reciprocating) engines 1: Basic operating principles, design & performance parameters

  2. Announcements • Pick up graded HW #1 and solutions if you didn’t last week • Midterm “party” #1 next week • First 90 minutes of class • Party covers lectures 1 – 5 and HW 1 – 2 only (not lecture 6) • Review material posted online as per email yesterday • Open to lecture notes, HWs and solutions and optional textbooks ONLY • “Humongous file” of first 6 lectures posted online, useful for searching • During the exam, you may use laptop computers ONLY to view pdf versions of lecture notes (either individual or humongous file); laptops may NOT be used to surf websites, open Excel files, IM-a-friend, etc. • 10 minute break after exam, then 60 minutes lecture (#7) • HW #2 due Friday, 4:30 pm • At that time solutions will be available where you drop off HW • Graded HW available Monday where you dropped it off • Did I forget anything? AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  3. Outline • Classification of unsteady-flow engines • Basic operating principles • Premixed-charge (gasoline) 4-stroke • Premixed-charge (gasoline) 2-stroke • Premixed-charge (gasoline) rotary or Wankel • Nonpremixed-charge (Diesel) 4-stroke • Nonpremixed-charge (Diesel) 2-stroke • Design and performance parameters • Compression ratio, displacement, bore, stroke, etc. • Power, torque, work, Mean Effective Pressure • Thermal efficiency • Volumetric efficiency • Emissions AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  4. Classification of unsteady-flow engines • Most important distinction: premixed-charge vs. nonpremixed-charge • Premixed-charge: frequently called “Otto cycle,”“gasoline” or “spark ignition” engine but most important distinction is that the fuel and air are mixed before or during the compression process and a premixed flame is ignited (usually by a spark, occasionally by a glow plug (e.g. model airplane engines), occasionally homogeneous ignition (Homogeneous Charge Compression Ignition (HCCI), under development) • Nonpremixed-charge: frequently called “Diesel” or “compression ignition” but key point is that only air is compressed (not fuel-air mixture) & fuel is injected into combustion chamber after air is compressed • Either premixed or nonpremixed-charge can be 2-stroke or 4-stroke, and can be piston/cylinder type or rotary (Wankel) type AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  5. Classification of unsteady-flow engines • Why is premixed-charge vs. nonpremixed-charge the most important distinction? Because it affects • Choice of fuels and ignition system • Choice of compression ratio (gasoline - lower, diesel - higher) • Tradeoff between maximum power (gasoline) and efficiency (diesel) • Relative amounts of pollutant formation (gasoline engines have lower NOx & particulates; diesels have lower CO & UHC) AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  6. Classification of unsteady-flow engines AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  7. 4-stroke premixed-charge piston engine Animation: http://auto.howstuffworks.com/engine3.htm Note: ideally combustion occurs in zero time when piston is at the top of its travel between the compression and expansion strokes Intake (piston moving down, intake valve open, exhaust valve closed) Exhaust (piston moving up, intake valve closed, exhaust valve open) Compression (piston moving up, both valves closed) Expansion (piston moving down, both valves closed) AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  8. 2-stroke premixed-charge engine • Most designs have fuel-air mixture flowing first INTO CRANKCASE (?) • Fuel-air mixture must contain lubricating oil • On down-stroke of piston • Exhaust ports are exposed & exhaust gas flows out, crankcase is pressurized • Reed valve prevents fuel-air mixture from flowing back out intake manifold • Intake ports are exposed, fresh fuel-air mixture flows into intake ports • On up-stroke of piston • Intake & exhaust ports are covered • Fuel-air mixture is compressed in cylinder • Spark & combustion occurs near top of piston travel • Work output occurs during 1st half of down-stroke AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  9. 2-stroke premixed-charge engine http://science.howstuffworks.com/two-stroke2.htm AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  10. 2-stroke premixed-charge engine • 2-strokes gives ≈ 2x as much power since only 1 crankshaft revolution needed for 1 complete cycle (vs. 2 revolutions for 4-strokes) • Since intake & exhaust ports are open at same time, some fuel-air mixture flows directly out exhaust & some exhaust gas gets mixed with fresh gas • Since oil must be mixed with fuel, oil gets burned • As a result of these factors, thermal efficiency is lower, emissions are higher, and performance is near-optimal for a narrower range of engine speeds compared to 4-stroke engines AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  11. Rotary or Wankel engine • Uses non-cylindrical combustion chamber • Provides one complete cycle per engine revolution without “short circuit” flow of 2-strokes (but still need some oil injected at the rotor apexes) • Simpler, fewer moving parts, higher RPM possible • Very fuel-flexible - can incorporate catalyst in combustion chamber since fresh gas is moved into chamber rather than being continually exposed to it (as in piston engine) – one engine could possibly use gasoline, Diesel, methanol, etc. • Very difficult to seal BOTH vertices and flat sides of rotor! • Seal longevity a problem also • Large surface area to volume ratio means more heat losses http://static.howstuffworks.com/flash/rotary-engine-exploded.swf AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  12. Rotary or Wankel engine • Source: http://auto.howstuffworks.com/rotary-engine4.htm AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  13. Rotary or Wankel engine http://auto.howstuffworks.com/rotary-engine4.htm AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  14. 4-stroke Diesel engine • Conceptually similar to 4-stroke gasoline, but only air is compressed (not fuel-air mixture) and fuel is injected into combustion chamber after air is compressed http://auto.howstuffworks.com/diesel2.htm AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  15. 2-stroke Diesel engine • Used in large engines, e.g. locomotives • More differences between 2-stroke gasoline vs. diesel engines than 4-stroke gasoline vs. diesel • Air comes in directly through intake ports, not via crankcase • Must be turbocharged or supercharged to provide pressure to force air into cylinder • No oil mixed with air - crankcase has lubrication like 4-stroke • Exhaust valves rather than ports - not necessary to have intake & exhaust paths open at same time • Because only air, not fuel/air mixture enters through intake ports, “short circuit” of intake gas out to exhaust not a problem • Because of the previous 3 points, 2-stroke diesels have far fewer environmental problems than 2-stroke gasoline engines AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  16. 2-stroke Diesel engine • Why can’t gasoline engines use concept similar to 2-stroke Diesel? They can in principle but fuel must be injected & fuel+airfully mixed after the intake ports are covered but before spark is fired • Also, difficult to control ratio of fuel/air/exhaust residual precisely since relative amounts of exhaust & air leaving exhaust ports varies from cycle to cycle (due to turbulence) - ratio of fuel to (air + exhaust) critical to premixed-charge engine performance (combustion in non-premixed charge engines always occurs at stoichiometric surfaces in overall lean mixtures anyway, so not an issue for non-premixed charge engines) • Some companies have tried to make 2-stroke premixed-charge engines operating this way, e.g. http://www.orbeng.com.au/, but these engines have found only limited application AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  17. Engine design & performance parameters • See Heywood Chapter 2 for more details • Compression ratio (rc) Vd = displacement volume = volume of cylinder swept by piston (this is what auto manufacturers report, e.g. 5.7 liter engine means 5.7 liters is combined displacement volume of ALL cylinders Vc = clearance volume = volume of cylinder NOT swept by piston • Bore (B) = cylinder diameter • Stroke (L) = distance between maximum excursions of piston • Displacment volume of 1 cylinder = πB2L/4; if B = L (typical), 5.7 liter, 8-cylinder engine, B = 9.7 cm • Power = Angular speed (N) x Torque () = 2πN AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  18. Classification of unsteady-flow engines AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  19. Engine design & performance parameters • Engine performance is specified in both in terms of power and engine torque - which is more important? • Wheel torque = engine torque x gear ratio tells you whether you can climb the hill • Gear ratio in transmission typically 3:1 or 4:1 in 1st gear, 1:1 in highest gear; gear ratio in differential typically 3:1 • Ratio of engine revolutions to wheel revolutions varies from 12:1 in lowest gear to 3:1 in highest gear • Power tells you how fast you can climb the hill • Torque can be increased by transmission (e.g. 2:1 gear ratio ideally multiplies torque by 2) • Power can’t be increased by transmission; in fact because of friction and other losses, power will decrease in transmission • Power really tells how fast you can accelerate or how fast you can climb a hill, but power to torque ratio ~ N tells you what gear ratios you’ll need to do the job AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  20. Engine design & performance parameters • Indicated work - work done for one cycleas determined by the cylinder P-V diagram = work acting on piston face Note: it’s called “indicated” power because historically (before oscilloscopes) the P and V were recorded by a pen moving in the x direction as V changed and moving in the y direction as P changed. The P-V plot was recorded on a card and the area inside the P-V was the “indicated” work (usually measured by cutting out the P-V and weighting that part of the card!) • Net indicated work = Wi,net = ∫ PdVover whole cycle = net area inside P-V diagram • Indicated work consists of 2 parts • Gross indicated work Wi,gross - work done during power cycle • Pumping work Wi,p - work done during intake/exhaust pumping cycle • Wi.net = Wi,gross - Wi,pump • Indicated power = Wi,xN/n, where x could be net, gross, pumping andn = 2 for 4-stroke engine, n = 1 for 2 stroke engine (since 4-stroke needs 2 complete revolutions of engine for one complete thermodynamic cycle as seen on P-V diagram whereas 2-stroke needs only 1 revolution) AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  21. Net indicated work (+) (-) Gross indicated work Pumping work Engine design & performance parameters Animation: gross & net indicated work, pumping work AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  22. Real engine P-V diagram Clearance volume Displacement volume Expansion stroke Compression stroke Exhaust stroke Exhaust pressure ≈ 1 atm Intake pressure ≈ 0.3 atm Intake stroke AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  23. Engine design & performance parameters • Brake work (Wb) or brake power (Pb) = work or power that appears at the shaft coming out of the engine • Historically called “brake” because a mechanical brake [like that on your car wheels] was used in laboratory to simulate the “road load” that would be placed on an engine in a vehicle) • What’s the difference between brake and indicated work or power? FRICTION • Gross Indicated work = brake work + friction work (Wf) Wi,g = Wb + Wf • Note that this definition of friction work includes not only the “rubbing friction” but also the pumping work; I prefer Wi,g = Wb + Wf + Wp which separates rubbing friction (which cannot be seen on a P-V diagram) from pumping friction (which IS seen on the P-V) • The latter definition makes friction the difference between your actual (brake) work/power output and the work seen on the P-V • Note the friction work also includes work/power needed to drive the cooling fan, water pump, oil pump, generator, air conditioner, … • Moral - know which definition you’re using AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  24. Indicated vs. brake work or power • Indicated • Measure cylinder P vs. time • Measure crank angle vs. time • Translate crank angle to volume (V) with engine geometry (piston / crankshaft / connecting rod) • Plot P vs. V • ∫PdV = indicated work (Wind) • Power = WindN/n • Brake • Connect engine to dynamometer (typically electrical generator) to simulate load that vehicle places on engine • Measure torque required to spin generator at a particular speed (N) • Horsepower = Torque x N / 5252 AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  25. Engine design & performance parameters • Mechanical efficiency = (brake power) / (indicated power) - measure of importance of friction loss • Thermal efficiency (th) = (what you get / what you pay for) = (power ouput) / (fuel heating value input) • Specific fuel consumption (sfc) = (mdotfuel)/(Power) units usually pounds of fuel per horsepower-hour (yuk!) • Note also AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  26. Engine design & performance parameters • Volumetric efficiency (v) = (mass of air actually drawn into cylinder) / (mass of air that ideally could be drawn into cylinder) where air is at ambient conditions = Pambient/RTambient • Volumetric efficiency indicates how well the engine “breathes” - what lowers v below 100%? • Pressure drops in intake system (e.g. throttling to reduce power) & intake valves • Temperature rise due to heating of air as it flows through intake system • Volume occupied by fuel • Non-ideal valve timing • “Choking” (air flow reaching speed of sound) in part of intake system having smallest area (passing intake valves) • Will be > 100% with turbocharging or supercharging • See figure on p. 217 of Heywood for good summary of all these effects AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  27. Engine design & performance parameters • Mean effective pressure (MEP) • Power could be brake, indicated, friction or pumping power, leading to BMEP, IMEP, FMEP, PMEP • Note Power = Torque x 2πN, thus • MEP can be interpreted as the first moment of pressure with respect to cylinder volume, or average pressure, with volume as the weighting function for the averaging process AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  28. Engine design & performance parameters • MEP is useful for 2 reasons • Since it’s proportional to power or work, we can add and subtract pressures just like we would power or work • (More important) it normalizes out the effects of engine size (Vd), speed (N) and 2-stroke vs. 4-stroke (n), so it provides a way of comparing different engines and operating conditions • Typical 4-stroke engine, IMEP ≈ 120 lb/in2 ≈ 9 atm - how to get more? Turbocharge - increase Pintake above 1 atm, more fuel & air stuffed into cylinder, more heat release, more power AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  29. Engine design & performance parameters • Pumping power = (pumping work)(N)/n = (P)(V)(N)/n = (Pexhaust - Pintake)VdN/n but PMEP = (pumping power)n/(VdN), thus PMEP = (Pexhaust - Pintake) (wasn’t that easy?) (this assumes “pumping loop” is a rectangle) • Estimate of IMEP • Typical engine at wide-open throttle (Pintake = Pambient): th,i,g ≈ 35%, v ≈ 90%, f ≈ 0.064 (at stoichiometric), QR = 4.3 x 107 J/kg, R = 287 J/kgK, Tintake = 300K  IMEPg / Pintake ≈ 10.1 AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  30. Engine design & performance parameters • Emissions performance usually reported in grams of pollutant emitted per brake horsepower-hour (yuk!) or grams per kilowatt hour (slightly less yuk), e.g. • One can also think of this as (mass/time) / (energy/time) = mass / energy = grams of pollutant per Joule of work done • …but Environmental Protection Agency standards (for passenger vehicles) are in terms of grams per mile, not brake power hour, thus smaller cars can have larger BSNOx (or BSCO, BSHC, etc.) because (presumably) less horsepower (thus less fuel) is needed to move the car a certain number of miles in a certain time, and still fit into a particular emissions “bin” (see lecture 5) • Larger vehicles (> 8500 lb) and stationary engines (e.g. for power generation) are regulated based on brake specific emissions directly AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  31. Example #1 How much power does a 5.7 liter (= 0.0057 m3) Hemi 4-stroke (n = 2) engine at 6000 RPM with thermal efficiency th = 30% = 0.30 and volumetric efficiency v = 90% = 0.90 generate using a stoichiometric gasoline-air mixture (fstoich = 0.0641, QR = 4.3 x 107 J/kg)? AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  32. Example #2 In a laboratory test of a 4-stroke engine with Vd = 3.05 liters at N = 3000 RPM the following performance data are measured: net IMEP 107.9 lbf/in2, 70.32 brake horsepower, fuel flow rate 16.66 kg/hr, air flow rate 269.6 kg/hr. The fuel is C8H18 (QR = 4.3 x 107 J/kg.) The ambient air temperature is 295K. The intake pressure gauge is broken, so the intake pressure is not known. Determine: a) BMEP b) Friction MEP FMEP = IMEP – BMEP IMEP = (107.9 lb/in2)(4.448N/lb)(in/0.0254m)2 = 7.44 x 105 N/m2 FMEP = 7.44 x 105 N/m2 – 6.88 x 105 N/m2 = 5.6 x 104 N/m2 = 0.55 atm c) Equivalence ratio C8H18 + 12.5(O2 + 3.77N2)  8 CO2 + 9 H2O + 12.5(3.77) N2 Stoichiometric fuel/air: (8(12)+18(1))/[12.5((32)+3.77(28))] = 0.0663 Actual fuel/air: (16.66 kg/hr)/(269.6 kg/hr) = 0.06179 Equivalence ratio = 0.06179/0.0663 = 0.932 AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  33. Example #2 (continued) d) Brake thermal efficiency e) Indicated torque f) Is this engine throttled, turbocharged or neither? Explain. (Hint: compute the volumetric efficiency.) AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

  34. Summary - engine design & performance parameters • Many mechanical implementations of unsteady-flow engines exist, but all are based on a thermodynamic cycle consisting of compression, combustion, expansion • The factor that affects engine design and performance more than any other is whether the engine is premixed-charge or nonpremixed-charge • Many measures of engine performance are employed - be careful! • Work and power - indicated vs. brake • Efficiencies - thermal vs. volumetric • Mean Effective Pressure - brake, indicated, pumping, friction AME 436 - Spring 2013 - Lecture 6 - Unsteady flow engines I: principles

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