AME 436 Energy and Propulsion

AME 436AME 436

Energy and PropulsionEnergy and Propulsion

Lecture 5Lecture 5Unsteady-flow (reciprocating) engines 1:Unsteady-flow (reciprocating) engines 1:

Basic operating principles, Basic operating principles, design & performance parametersdesign & performance parameters

AME 436 - February 8, 2005AME 436 - February 8, 2005 22

OutlineOutline Classification of unsteady-flow enginesClassification of unsteady-flow engines Basic operating principlesBasic operating principles

Premixed-charge (gasoline) 4-strokePremixed-charge (gasoline) 4-stroke Premixed-charge (gasoline) 2-strokePremixed-charge (gasoline) 2-stroke Premixed-charge (gasoline) rotary or WankelPremixed-charge (gasoline) rotary or Wankel Nonpremixed-charge (Diesel) 4-strokeNonpremixed-charge (Diesel) 4-stroke Nonpremixed-charge (Diesel) 2-strokeNonpremixed-charge (Diesel) 2-stroke

Design and performance parametersDesign and performance parameters Compression ratio, displacement, bore, stroke, etc.Compression ratio, displacement, bore, stroke, etc. Power, torque, work, Mean Effective PressurePower, torque, work, Mean Effective Pressure Thermal efficiencyThermal efficiency Volumetric efficiencyVolumetric efficiency EmissionsEmissions


Classification of unsteady-flow enginesClassification of unsteady-flow engines

Most important distinction: Most important distinction: premixed-charge vs. nonpremixed-chargepremixed-charge vs. nonpremixed-charge Premixed-chargePremixed-charge: frequently called “Otto cycle”, “gasoline” or “spark : frequently called “Otto cycle”, “gasoline” or “spark

ignition” engine but most important distinction is that the fuel and air ignition” engine but most important distinction is that the fuel and air are mixed before or during the compression process and a premixed 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. flame is ignited (usually by a spark, occasionally by a glow plug (e.g. model airplane engines), occasionally homogeneous ignition model airplane engines), occasionally homogeneous ignition (Homogenous Charge Compression Ignition (HCCI), under development)(Homogenous Charge Compression Ignition (HCCI), under development)

Nonpremixed-chargeNonpremixed-charge: frequently called “Diesel” or “compression : frequently called “Diesel” or “compression ignition” but key point is that only air is compressed (not fuel-air ignition” but key point is that only air is compressed (not fuel-air mixture) and fuel is injected into combustion chamber after air is mixture) and fuel is injected into combustion chamber after air is compressedcompressed

Either premixed or nonpremixed-charge can be 2-stroke or 4-stroke, and Either premixed or nonpremixed-charge can be 2-stroke or 4-stroke, and can be piston/cylinder type or rotary (Wankel) typecan be piston/cylinder type or rotary (Wankel) type

Why is Why is premixed-charge vs. nonpremixed-chargepremixed-charge vs. nonpremixed-charge the most important the most important distinction? Because it affectsdistinction? Because it affects Choice of fuels and ignition systemChoice of fuels and ignition system Choice of compression ratio (gasoline - lower, diesel - higher)Choice of compression ratio (gasoline - lower, diesel - higher) Tradeoff between maximum power (gasoline) and efficiency (diesel)Tradeoff between maximum power (gasoline) and efficiency (diesel) Relative amounts of pollutant formation (gasoline engines have lower NORelative amounts of pollutant formation (gasoline engines have lower NOxx & &

particulates; diesels have lower CO & UHC)particulates; diesels have lower CO & UHC)



Flame front Fuel spray flame

Premixed charge (gasoline)

Non-premixed charge (Diesel)

Spark plug Fuel injector

Fuel + air mixture Air only


4-stroke premixed-charge engine4-stroke premixed-charge engine Animation: Animation: http://auto.howstuffworks.com/engine3.http://auto.howstuffworks.com/engine3.htmhtm

Note: ideally combustion occurs in zero time when piston is at the Note: ideally combustion occurs in zero time when piston is at the top of its travel between the compression and expansion strokestop of its travel between the compression and expansion strokes

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.







Intake (piston Intake (piston moving down, moving down, intake valve intake valve open, exhaust open, exhaust valve closed)valve closed)

Compression Compression (piston moving (piston moving up, both valves up, both valves closed)closed)

Expansion Expansion (piston moving (piston moving down, both down, both valves closed)valves closed)

Exhaust (piston Exhaust (piston moving up, intake moving up, intake valve closed, valve closed, exhaust valve open)exhaust valve open)


4-stroke premixed-charge engine4-stroke premixed-charge engine Source: Source: http://auto.howstuffworks.com/engine3.http://auto.howstuffworks.com/engine3.htmhtm


2-stroke premixed-charge engine2-stroke premixed-charge engineMost designs have fuel-air mixture Most designs have fuel-air mixture

flowing first INTO CRANKCASE (?)flowing first INTO CRANKCASE (?)Fuel-air mixture must contain Fuel-air mixture must contain

lubricating oil lubricating oil On down-stroke of pistonOn down-stroke of piston

Exhaust ports are exposed & Exhaust ports are exposed & exhaust gas flows out, crankcase is exhaust gas flows out, crankcase is pressurizedpressurized

Reed valve prevents fuel-air mixture Reed valve prevents fuel-air mixture from flowing back out intake from flowing back out intake manifoldmanifold

Intake ports are exposed, fresh fuel-Intake ports are exposed, fresh fuel-air mixture flows into intake portsair mixture flows into intake ports

On up-stroke of pistonOn up-stroke of piston Intake & exhaust ports are coveredIntake & exhaust ports are covered Fuel-air mixture is compressed in Fuel-air mixture is compressed in

cylindercylinder Spark & combustion occurs near top Spark & combustion occurs near top

of piston travelof piston travel Work output occurs during 1st half Work output occurs during 1st half

of down-strokeof down-stroke

QuickTime™ and aTIFF (Uncompressed) decompressor



2-stroke premixed-charge engine2-stroke premixed-charge engine Source: Source: http://science.howstuffworks.com/two-stroke2.htmhttp://science.howstuffworks.com/two-stroke2.htm


2-stroke premixed-charge engine2-stroke premixed-charge engine

2-strokes gives ≈ 2x as much power2-strokes gives ≈ 2x as much power since only 1 crankshaft since only 1 crankshaft revolution needed for 1 complete cycle (vs. 2 revolutions for 4-revolution needed for 1 complete cycle (vs. 2 revolutions for 4-strokes)strokes)

Since intake & exhaust ports are open at same time, some fuel-air Since intake & exhaust ports are open at same time, some fuel-air mixture flows directly out exhaust & some exhaust gas gets mixture flows directly out exhaust & some exhaust gas gets mixed with fresh gasmixed with fresh gas

Since oil must be mixed with fuel, oil gets burnedSince oil must be mixed with fuel, oil gets burned As a result of these factors, As a result of these factors, thermal efficiency is lower, thermal efficiency is lower,

emissions are higher, and performance is near-optimal for a emissions are higher, and performance is near-optimal for a narrower range of engine speedsnarrower range of engine speeds compared to 4-strokes compared to 4-strokes


Rotary or Wankel engineRotary or Wankel engine Uses non-cylindrical combustion chamberUses non-cylindrical combustion chamber Provides one complete cycle per engine revolution without “short circuit” flow Provides one complete cycle per engine revolution without “short circuit” flow

of 2-strokes (but still need some oil in fuel)of 2-strokes (but still need some oil in fuel) Simpler, fewer moving parts, higher RPM possibleSimpler, fewer moving parts, higher RPM possible Very fuel-flexible - can incorporate catalyst in combustion chamber since fresh 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 gas is moved into chamber rather than being continually exposed to it (as in piston engine) - same design can use gasoline, Diesel, methanol.. piston engine) - same design can use gasoline, Diesel, methanol..

Very difficult to seal BOTH vertices and flat sides of rotor!Very difficult to seal BOTH vertices and flat sides of rotor! Seal longevity a problem alsoSeal longevity a problem also Large surface area to volume ratio means more heat lossesLarge surface area to volume ratio means more heat losses

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Rotary or Wankel engineRotary or Wankel engine Source: Source: http://auto.howstuffworks.com/rotary-engine4.http://auto.howstuffworks.com/rotary-engine4.htmhtm












Rotary or Wankel engineRotary or Wankel engine Source: Source: http://auto.howstuffworks.com/rotary-engine4.http://auto.howstuffworks.com/rotary-engine4.htmhtm


4-stroke Diesel engine4-stroke Diesel engine Conceptually similar to 4-stroke gasoline, but only air is Conceptually similar to 4-stroke gasoline, but only air is

compressed (not fuel-air mixture) and fuel is injected into compressed (not fuel-air mixture) and fuel is injected into combustion chamber after air is compressedcombustion chamber after air is compressed

Source: Source: http://auto.howstuffworks.com/diesel.htmhttp://auto.howstuffworks.com/diesel.htm


2-stroke Diesel engine2-stroke Diesel engine Used in large engines, e.g. locomotivesUsed in large engines, e.g. locomotives More differences between 2-stroke More differences between 2-stroke

gasoline vs. diesel engines than 4-gasoline vs. diesel engines than 4-stroke gasoline vs. dieselstroke gasoline vs. diesel Air comes in directly through intake Air comes in directly through intake

ports, not via crankcaseports, not via crankcase MustMust be turbocharged or supercharged be turbocharged or supercharged

to provide pressure to force air into to provide pressure to force air into cylinder cylinder

No oil mixed with air - crankcase has No oil mixed with air - crankcase has lubrication like 4-strokelubrication like 4-stroke

Exhaust valves rather than ports - not Exhaust valves rather than ports - not necessary to have intake & exhaust necessary to have intake & exhaust paths open at same timepaths open at same time

Because only air, not fuel/air mixture Because only air, not fuel/air mixture enters through intake ports, “short enters through intake ports, “short circuit” of intake gas out to exhaust is circuit” of intake gas out to exhaust is not a problemnot a problem

Because of the previous 3 points, 2-Because of the previous 3 points, 2-stroke diesels have far fewer stroke diesels have far fewer environmental problems than 2-stroke environmental problems than 2-stroke gasoline enginesgasoline engines


2-stroke Diesel engine2-stroke Diesel engine

Why can’t gasoline engines use this concept? They can in Why can’t gasoline engines use this concept? They can in principle but fuel must be injected & fuel+air principle but fuel must be injected & fuel+air fully mixedfully mixed after the after the intake ports are covered but before spark is firedintake ports are covered but before spark is fired

Also, difficult to control ratio of fuel/air/exhaust residual precisely Also, difficult to control ratio of fuel/air/exhaust residual precisely since intake & exhaust paths are open at same time - ratio of fuel since intake & exhaust paths are open at same time - ratio of fuel to (air + exhaust) critical to premixed-charge engine performance to (air + exhaust) critical to premixed-charge engine performance (combustion in non-premixed charge engines always occurs at (combustion in non-premixed charge engines always occurs at stoichiometric surfaces anyway, so not an issue)stoichiometric surfaces anyway, so not an issue)

Some companies have tried to make 2-stroke premixed-charge Some companies have tried to make 2-stroke premixed-charge engines operating this way, e.g. engines operating this way, e.g. http://www.orbeng.com.au/http://www.orbeng.com.au/, but , but these engines have found only limited applicationthese engines have found only limited application


Engine design & performance parametersEngine design & performance parameters See Heywood Chapter 2 for more detailsSee Heywood Chapter 2 for more details Compression ratio (rCompression ratio (rcc))

VVdd = displacement volume = volume of cylinder swept by piston (this is = 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 what auto manufacturers report, e.g. 5.2 liter engine means 5.2 liters is combined displacement volume of ALL cylinderscombined displacement volume of ALL cylinders

VVcc = clearance volume = volume of cylinder NOT swept by piston = clearance volume = volume of cylinder NOT swept by piston Bore (B) = cylinder diameterBore (B) = cylinder diameter Stroke (L) = distance between maximum excursions of pistonStroke (L) = distance between maximum excursions of piston Displacment volume of 1 cylinder = πBDisplacment volume of 1 cylinder = πB22L/4; if B = L (typical), 5.2 L/4; if B = L (typical), 5.2

liter, 8-cylinder engine, B = 9.4 cmliter, 8-cylinder engine, B = 9.4 cm Power = Angular speed (N) x Torque (Power = Angular speed (N) x Torque () = 2πN) = 2πN

rc maximum cylinder volume

minimum cylinder volume

Vc Vd

Vc

P (in horsepower) N (revolutions per minute, RPM) x (in foot pounds)

5252



Piston at bottom of travel

Piston at top of travel

Bore

Stroke Displacement volume

Clearance volume


Engine design & performance parametersEngine design & performance parameters

Engine performance is specified in both in terms of Engine performance is specified in both in terms of power and power and engineengine torque - which is more important? torque - which is more important? Wheel torque = engine torque x gear ratioWheel torque = engine torque x gear ratio tells you whether you tells you whether you

can climb the hillcan climb the hill Gear ratio in transmission typically 3:1 or 4:1 in 1st gear, 1:1 in Gear ratio in transmission typically 3:1 or 4:1 in 1st gear, 1:1 in

highest gear; gear ratio in differential typically 3:1highest gear; gear ratio in differential typically 3:1» Ratio of engine revolutions to wheel revolutions varies from 12:1 in Ratio of engine revolutions to wheel revolutions varies from 12:1 in

lowest gear to 3:1 in highest gearlowest gear to 3:1 in highest gear Power Power tells you how fast you can climb the hilltells you how fast you can climb the hill Torque can be increased by transmission (e.g. 2:1 gear ratio Torque can be increased by transmission (e.g. 2:1 gear ratio

ideally multiplies torque by 2)ideally multiplies torque by 2) Power can’t be increased by transmission; in fact because of Power can’t be increased by transmission; in fact because of

friction and other losses, power will decrease in transmissionfriction and other losses, power will decrease in transmission Power really tells how fast you can accelerate or how much 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 weight you can pull up a hill, but power to torque ratio ~ N tells you what gear ratios you’ll need to do the jobyou what gear ratios you’ll need to do the job


Engine design & performance parametersEngine design & performance parameters Indicated workIndicated work - work done - work done for one cyclefor one cycle as determined by the cylinder as determined by the cylinder

P-V diagramP-V diagram = work acting on piston face = work acting on piston faceNote: it’s called “indicated” power because historically (before oscilloscopes) 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 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 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 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!)by cutting out the P-V and weighting that part of the card!)

Net indicated work = WNet indicated work = Wi,neti,net = ∫ PdV = ∫ PdV over whole cycleover whole cycle = net area inside P-V = net area inside P-V

diagramdiagram Indicated work consists of 2 partsIndicated work consists of 2 parts

Gross indicated work WGross indicated work Wi,grossi,gross - work done during power cycle - work done during power cycle Pumping work WPumping work Wi,pi,p - work done during intake/exhaust pumping cycle - work done during intake/exhaust pumping cycle

WWi.neti.net = W = Wi,grossi,gross - W - Wi,pumpi,pump

Indicated power = WIndicated power = Wi,xi,xN/n, where x could be net, gross, pumping andN/n, where x could be net, gross, pumping and n n

= 2 for 4-stroke engine, n = 1 for 2 stroke engine (since 4-stroke needs 2 = 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 complete revolutions of engine for one complete thermodynamic cycle as seen on P-V diagram whereas 2-stroke needs only 1 revolution)as seen on P-V diagram whereas 2-stroke needs only 1 revolution)



AnimationAnimation: gross & net indicated work, pumping work: gross & net indicated work, pumping work

Gross indicated work

Gross indicated workPumping workPumping work

Net indicated work

Net indicated work(+)(+)

(-)(-)



Brake work (WBrake work (Wbb) or brake power (P) or brake power (Pbb) = work power that appears at ) = work power that appears at

the shaft at the back of the enginethe shaft at the back of the engine Historically called “brake” because a mechanical brake [like that Historically called “brake” because a mechanical brake [like that

on your car wheels] was used in laboratory to simulate the “road on your car wheels] was used in laboratory to simulate the “road load” that would be placed on an engine in a vehicle)load” that would be placed on an engine in a vehicle)

What’s the difference between brake and indicated work or What’s the difference between brake and indicated work or power? power? FRICTIONFRICTION Gross Indicated work = brake work + friction work (WGross Indicated work = brake work + friction work (W ff))

WWi,gi,g = W = Wbb + W + Wff

Note that this definition of friction work includes not only the Note that this definition of friction work includes not only the “rubbing friction” but also the pumping work; I prefer“rubbing friction” but also the pumping work; I prefer

WWi,gi,g = W = Wbb + W + Wff + W + Wpp

which separates rubbing friction (which cannot be seen on which separates rubbing friction (which cannot be seen on a P-V diagram) from pumping friction (which IS seen on the P-V) a P-V diagram) from pumping friction (which IS seen on the P-V)

The latter definition makes friction the difference between your actual The latter definition makes friction the difference between your actual (brake) work/power output and the work seen on the P-V(brake) work/power output and the work seen on the P-V

Note the friction work also includes work/power needed to drive the Note the friction work also includes work/power needed to drive the cooling fan, water pump, oil pump, generator, air conditioner, …cooling fan, water pump, oil pump, generator, air conditioner, …

Moral - know which definition you’re usingMoral - know which definition you’re using



Mechanical efficiency = (brake power) / (indicated power) - Mechanical efficiency = (brake power) / (indicated power) - measure of importance of friction lossmeasure of importance of friction loss

Thermal efficiency (Thermal efficiency (thth) = (what you get / what you pay for) = ) = (what you get / what you pay for) =

(power ouput) / (fuel heating value input)(power ouput) / (fuel heating value input)

Specific fuel consumption (sfc) = (mdotSpecific fuel consumption (sfc) = (mdotfuelfuel)/(Power))/(Power)

units usually pounds of fuel per horsepower-hour (yuk!)units usually pounds of fuel per horsepower-hour (yuk!) Note alsoNote also

th Power output (brake or indicated)

Ý m fuelQR

isfc Ý m fuel

indicated power;bsfc

Ý m fuel

brake power

th,i 1

(isfc)QR

;th,b 1

(bsfc)QR



Volumetric efficiency (Volumetric efficiency (vv) = (mass of air actually drawn into ) = (mass of air actually drawn into

cylinder) / (mass of air that ideally could be drawn into cylinder) cylinder) / (mass of air that ideally could be drawn into cylinder)

where where airair is at ambient conditions = P is at ambient conditions = Pambientambient/RT/RTambientambient

Volumetric efficiency indicates how well the engine “breathes” - Volumetric efficiency indicates how well the engine “breathes” - what lowers what lowers vv below 100%? below 100%? Pressure drops in intake system (e.g. throttling) & intake valvesPressure drops in intake system (e.g. throttling) & intake valves Temperature rise due to heating of air as it flows through intake Temperature rise due to heating of air as it flows through intake

systemsystem Volume occupied by fuelVolume occupied by fuel Non-ideal valve timingNon-ideal valve timing ““Choking” (air flow reaching speed of sound) in part of intake system Choking” (air flow reaching speed of sound) in part of intake system

having smallest area (passing intake valves)having smallest area (passing intake valves) See figure on p. 217 of Heywood for good summary of all these See figure on p. 217 of Heywood for good summary of all these

effectseffects

v Ý m air (measured)

airVd N /n



Mean effective pressure (MEP)Mean effective pressure (MEP)

Power could be brake, indicated, friction or pumping power, leading to Power could be brake, indicated, friction or pumping power, leading to BMEP, IMEP, FMEP, PMEPBMEP, IMEP, FMEP, PMEP

Note Power = Torque x 2πN, thus Note Power = Torque x 2πN, thus Brake torque = BMEP*VBrake torque = BMEP*Vdd/2πn/2πn I like to think of MEP as the I like to think of MEP as the first moment of pressure with respect to first moment of pressure with respect to

cylinder volumecylinder volume, or , or average pressure, with volume as the weighting average pressure, with volume as the weighting function for the averaging processfunction for the averaging process

Useful for 2 reasonsUseful for 2 reasons Since it’s proportional to power or work, we can add and subtract pressures Since it’s proportional to power or work, we can add and subtract pressures

just like we would power or workjust like we would power or work (More important) It normalizes out the effects of engine size (V(More important) It normalizes out the effects of engine size (Vdd), speed (N) ), speed (N)

and 2-stroke vs. 4-stroke (n), so it provides a way of comparing different and 2-stroke vs. 4-stroke (n), so it provides a way of comparing different engines and operating conditionsengines and operating conditions

Typical 4-stroke engine, IMEP ≈ 120 lb/inTypical 4-stroke engine, IMEP ≈ 120 lb/in22 ≈ 8 atm - how to get more? ≈ 8 atm - how to get more? TurbochargeTurbocharge - increase P - increase Pintakeintake above 1 atm, more fuel & air stuffed into above 1 atm, more fuel & air stuffed into

cylinder, more heat release, more powercylinder, more heat release, more power

MEP Work per cycle

Displacement volume

PdVcycle

Vd

(Power)n /N

Vd

(Power)n

Vd N



Pumping power = (pumping work)(N)/n = (Pumping power = (pumping work)(N)/n = (P)(P)(V)(N)/nV)(N)/n= (P= (Pexhaustexhaust - P - Pintakeintake)V)VddN/nN/n

but PMEP = (pumping power)n/(Vbut PMEP = (pumping power)n/(VddN), thus N), thus PMEP = (PPMEP = (Pexhaustexhaust - P - Pintakeintake))

(wasn’t that easy?) (wasn’t that easy?) (this assumes “pumping loop” is a rectangle)(this assumes “pumping loop” is a rectangle) Estimate of IMEPEstimate of IMEP

Typical engine Typical engine th,i,gth,i,g ≈ 30%, ≈ 30%, vv ≈ 85%, f = 0.068 (at stoichiometric), Q ≈ 85%, f = 0.068 (at stoichiometric), QRR = 4.5 = 4.5

x 10x 1077 J/kg, R = 287 J/kg-K, T J/kg, R = 287 J/kg-K, Tintakeintake = 300K = 300K

IMEPIMEPgg / P / Pintakeintake ≈ 9.1 ≈ 9.1 In reality, we have to be more careful about accounting for the exhaust 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 residual and the fact that its properties are very different from the fresh gas, but this doesn’t change the results muchgas, but this doesn’t change the results much

IMEPg (Gross indicated power) n

Vd N

(th,i,g Ý m fuelQR )n

Vd N

(th,i,g Ý m air[ f /(1 f )]QR )n

Vd N

th,i,g (vair,ambientVd N /n)QR n

Vd N

f

1 f

th,i,gvQR

Pambient

RTambient

(1 f )

f

1 f

IMEPg

Pintake

th,i,gv fQR

RTambient

Pambient

Pintake



Emissions performance usually reported in grams of pollutant Emissions performance usually reported in grams of pollutant emitted per brake horsepower-hour (yuk!) or grams per kilowatt emitted per brake horsepower-hour (yuk!) or grams per kilowatt hour (slightly less yuk), e.g.hour (slightly less yuk), e.g.

Brake Specific NOBrake Specific NOxx (BSNO (BSNOxx) = mdot) = mdotNOxNOx / (Brake power) / (Brake power) One can also think of this as (mass/time) / (energy/time) = mass / One can also think of this as (mass/time) / (energy/time) = mass /

energy = grams of pollutant per Joule of work doneenergy = grams of pollutant per Joule of work done ……but Environmental Protection Agency standards (for passenger but Environmental Protection Agency standards (for passenger

vehicles) are in terms of grams per vehicles) are in terms of grams per milemile, not brake power hour, , not brake power hour, thus smaller cars can have larger BSNOthus smaller cars can have larger BSNOxx (or BSCO, BSHC, etc.) (or BSCO, BSHC, etc.)

because (presumably) less horsepower (thus less fuel) is needed because (presumably) less horsepower (thus less fuel) is needed to move the car a certain number of miles in a certain timeto move the car a certain number of miles in a certain time

Larger vehicles (and stationary engines for power generation) are Larger vehicles (and stationary engines for power generation) are regulated based on brake specific emissions directlyregulated based on brake specific emissions directly

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AME 436 Energy and Propulsion