Transcript AME 436 Energy and Propulsion

AME 436
Energy and Propulsion
Lecture 5
Unsteady-flow (reciprocating) engines 1:
Basic operating principles,
design & performance parameters
Outline
 Classification of unsteady-flow engines
 Basic operating principles

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
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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


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
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Compression ratio, displacement, bore, stroke, etc.
Power, torque, work, Mean Effective Pressure
Thermal efficiency
Volumetric efficiency
Emissions
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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

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
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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)
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Classification of unsteady-flow engines
Spark plug
Flame front
Fuel + air mixture
Premixed charge
(gasoline)
Fuel injector
Fuel spray flame
Air only
Non-premixed charge
(Diesel)
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4-stroke premixed-charge engine
 Animation: http://auto.howstuffworks.com/engine3.htm
QuickTime™ and a
QuickTime™ and a
QuickTime™ and a
QuickTime™ and a
TIFF (LZW) decompressorTIFF (LZW) decompressor TIFF (LZW) decompressor TIFF (LZW) decompressor
are needed to see this picture.
are needed to see this picture.
are needed to see this picture.
are needed to see this picture.
Intake (piston
moving down,
intake valve
open, exhaust
valve closed)
Compression
(piston moving
up, both valves
closed)
Expansion
(piston moving
down, both
valves closed)
Exhaust (piston
moving up, intake
valve closed,
exhaust valve open)
Note: ideally combustion occurs in zero time when piston is at the
top of its travel between the compression and expansion strokes
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4-stroke premixed-charge engine
 Source: http://auto.howstuffworks.com/engine3.htm
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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 fuelair mixture flows into intake ports
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
 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
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2-stroke premixed-charge engine
 Source: http://science.howstuffworks.com/two-stroke2.htm
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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 4strokes)
 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
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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
QuickT ime™ and a
T IFF (Uncompressed) decompressor
are needed to see thi s pi cture.
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Rotary or Wankel engine
 Source: http://auto.howstuffworks.com/rotary-engine4.htm
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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Rotary or Wankel engine
 Source: http://auto.howstuffworks.com/rotary-engine4.htm
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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
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2-stroke Diesel engine
 Used in large engines, e.g. locomotives
 More differences between 2-stroke
gasoline vs. diesel engines than 4stroke 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, 2stroke diesels have far fewer
environmental problems than 2-stroke
gasoline engines
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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
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Engine design & performance parameters
 See Heywood Chapter 2 for more details
 Compression ratio (rc)
rc 

maximum cylinder volume Vc  Vd

minimum cylinder volume Vc
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
N (revolutions per minute,
RPM) x (in foot pounds)
P (in horsepower)
5252
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Classification of unsteady-flow engines
Clearance
volume
Bore
Displacement
volume
Stroke
Piston at bottom
of travel
Piston at top
of travel
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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
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Engine design & performance parameters
 Indicated work - work done for one cycle as 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 PV 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)
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Engine design & performance parameters
 Animation: gross & net indicated work, pumping work
(-)Pumping work
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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
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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)
Power output (brake or indicat ed)
th 
ÝfuelQR
m
 Specific fuel consumption (sfc) = (mdotfuel)/(Power)
isfc 

Ýfuel
Ýfuel
m
m
;bsfc 
indicated power
brake power
units usually pounds of fuel per horsepower-hour (yuk!)
 Note also

th,i 
1
1
;th,b 
(isfc)QR
(bsfc)QR

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Engine design & performance parameters
 Volumetric efficiency (v) = (mass of air actually drawn into
cylinder) / (mass of air that ideally could be drawn into cylinder)
v 
Ýair (measured)
m
airVd N /n
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
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Engine design & performance parameters
 Mean effective pressure (MEP)
MEP 
Work per cycle

Displacement volume
 PdV
cycle
Vd

(Power)n /N (Power)n

Vd
Vd N
 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
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
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
(Gross indicated power) n (th,i,g mÝfuelQR )n
IMEPg 

Vd N
Vd N
(th,i,g mÝair [ f /(1 f )]QR )n th,i,g (v  air,ambientVd N /n)QR n f


Vd N
Vd N
1 f
 Pambient
 f
IMEPg th,i,gv fQR Pambient
 th,i,gv QR 
(1 f )


Pintake
RTambient Pintake
RTambient
1 f
 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
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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
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