AME 436 Energy and Propulsion

AME 436Energy and Propulsion Lecture 5 Unsteady-flow (reciprocating) engines 1: Basic operating principles, design & performance parameters

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 - February 8, 2005

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 (Homogenous 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) and 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 • 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 - February 8, 2005

Classification of unsteady-flow engines AME 436 - February 8, 2005

4-stroke premixed-charge 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 - February 8, 2005

4-stroke premixed-charge engine • Source: http://auto.howstuffworks.com/engine3.htm AME 436 - February 8, 2005

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 - February 8, 2005

2-stroke premixed-charge engine • Source: http://science.howstuffworks.com/two-stroke2.htm AME 436 - February 8, 2005

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-strokes AME 436 - February 8, 2005

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 in fuel) • 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) - same design can use gasoline, Diesel, methanol.. • 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 AME 436 - February 8, 2005

Rotary or Wankel engine • Source: http://auto.howstuffworks.com/rotary-engine4.htm AME 436 - February 8, 2005

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 • Source: http://auto.howstuffworks.com/diesel.htm AME 436 - February 8, 2005

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 is not a problem • Because of the previous 3 points, 2-stroke diesels have far fewer environmental problems than 2-stroke gasoline engines AME 436 - February 8, 2005

2-stroke Diesel engine • Why can’t gasoline engines use this concept? They can in principle but fuel must be injected & fuel+air fully mixed after the intake ports are covered but before spark is fired • Also, difficult to control ratio of fuel/air/exhaust residual precisely since intake & exhaust paths are open at same time - ratio of fuel to (air + exhaust) critical to premixed-charge engine performance (combustion in non-premixed charge engines always occurs at stoichiometric surfaces anyway, so not an issue) • 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 - February 8, 2005

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. 5.2 liter engine means 5.2 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), 5.2 liter, 8-cylinder engine, B = 9.4 cm • Power = Angular speed (N) x Torque () = 2πN AME 436 - February 8, 2005

Classification of unsteady-flow engines AME 436 - February 8, 2005

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 much weight you can pull up a hill, but power to torque ratio ~ N tells you what gear ratios you’ll need to do the job AME 436 - February 8, 2005

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 = ∫ PdV over 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 and n = 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 - February 8, 2005

Net indicated work (+) (-) Gross indicated work Pumping work Engine design & performance parameters • Animation: gross & net indicated work, pumping work AME 436 - February 8, 2005

Engine design & performance parameters • Brake work (Wb) or brake power (Pb) = work power that appears at the shaft at the back 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 - February 8, 2005

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 - February 8, 2005

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) & 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) • See figure on p. 217 of Heywood for good summary of all these effects AME 436 - February 8, 2005

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 Brake torque = BMEP*Vd/2πn • I like to think of MEP as the first moment of pressure with respect to cylinder volume, or average pressure, with volume as the weighting function for the averaging process • 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 ≈ 8 atm - how to get more? Turbocharge - increase Pintake above 1 atm, more fuel & air stuffed into cylinder, more heat release, more power AME 436 - February 8, 2005

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 th,i,g ≈ 30%, v ≈ 85%, f = 0.068 (at stoichiometric), QR = 4.5 x 107 J/kg, R = 287 J/kg-K, Tintake = 300K  IMEPg / Pintake ≈ 9.1 • In reality, we have to be more careful about accounting for the exhaust residual and the fact that its properties are very different from the fresh gas, but this doesn’t change the results much AME 436 - February 8, 2005

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. Brake Specific NOx (BSNOx) = mdotNOx / (Brake power) • 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 • Larger vehicles (and stationary engines for power generation) are regulated based on brake specific emissions directly AME 436 - February 8, 2005

AME 436 Energy and Propulsion