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Measuring and Predicting Transient DieselEngine Emissions
TRITA MMK 2009:07ISSN 1400-1179
ISRN/KTH/MMK/R-09/07-SE
Licentiate thesisKTH CICERO
Department of Machine Design
Royal Institute of TechnologySE-100 44 Stockholm
ANDERS WESTLUND
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TRITA MMK 2009:07
ISSN 1400-1179
ISRN/KTH/MMK/R-09/07-SEMeasuring and Predicting Transient Diesel Engine Emissions
Anders Westlund
Licentiate thesis
Academic thesis, which with the approval of Kungliga Tekniska Hgskolan, will be presented
for public review in fulfilment of the requirements for a Licentiate of Engineering in MachineDesign. The public review is held at Kungliga Tekniska Hgskolan, Brinellvgen 23 in room
B3, 24th of March 2009 at 10:00.
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Measuring and Predicting Transient Diesel Engine Emissions 2009Abstract
Dueto
its
impact
on
human
health
and
the
nature
surrounding
us,
diesel
engine
emissions have been significantly reduced over the last two decades. This
reduction has been enforced by the legislating organs around the world that
gradually have made the manufacturers transform their engines to todays
complex hightech products. One of the most challenging areas to meet the
legislationsistransientoperationwheretheinertiaingasexchangesystemmakes
transitionfromoneloadtoanotherproblematic.
Modernengineshavegreatpotential tominimize the problemsassociatedwith
transient operation. However, their complexity also imposes a great challenge
regardingoptimizationandsystematical testingoftransientcontrolstrategies in
anenginetestbedcouldbebothexpensiveandtimeconsuming.
The objective of this project is to facilitate optimization of transient control
strategies.Thisshouldbedonebyidentifyingappropriatemeasurementmethods
forevaluationoftransientsandbyprovidingmodelsthatcanbeusedtooptimize
strategiesoffline.
Measurement methods for evaluation of transients have been tested in several
experiments, mainly focusing on emission but also regarding e.g. EGR flow.
Applicableinstrumentsfortransientemissionmeasurementshavebeenidentified
andused.However,nomethodtomeasuresootemissionscycleresolvedhasyet
been found. Other measurements such as EGR flow and temperatures are
believedtohavesignificantlydecreasedaccuracyduringtransients.
Amodel
for
prediction
of
NOx
emissions
have
been
used
and
complemented
with
a new approach for soot emission predictions that has been developed in this
project. The emission models have been shown to be applicable over a wide
rangeofoperatingconditionswithexception forhighlypremixedcombustion. It
has also been shown that models developed for steadystate conditions can be
usedfortransientsoperation.
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Contents
1 Introduction............................................................................................................................. 5
2 Projectobjectives.................................................................................................................. 13
3 Emissionformationindieselengines.................................................................................... 16
3.1 NOxformation................................................................................................................ 17
3.2 Sootformationandoxidation........................................................................................ 18
3.3 Emissionreduction......................................................................................................... 20
4 Dieselcombustionmodeling................................................................................................. 21
4.1 Simplifieddieselemissionmodeling.............................................................................. 23
4.2 Simplifiedcombustionmodeling................................................................................... 23
4.3 Simplifiedchemicalreactionschemes........................................................................... 25
4.4 Simplifiedemissionmodeling........................................................................................ 26
4.4.1 Phenomenologicalmodels...................................................................................... 26
4.4.2 Semiempiricalmodels............................................................................................ 30
4.4.3 Statisticalmodels.................................................................................................... 31
5 AdvantagesanddisadvantagesofdifferentemissionmodelcategoriesMotivationforchosenmodels.............................................................................................................................. 33
6 Measuringandevaluatingtransientoperation..................................................................... 35
6.1 Gasexchangesystemmeasurements............................................................................ 35
6.1.1 TransientEGRmeasurements................................................................................ 36
6.1.2 Transientairflowandair/fuelratiomeasurements..............................................37
6.2 Emissionmeasurements................................................................................................ 38
6.2.1 NOxmeasurements................................................................................................ 38
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6.2.2 Sootmeasurements................................................................................................ 38
6.3 Postprocessingemissionpredictions............................................................................ 42
7 Introductiontopapersandkeyresults................................................................................. 43
7.1 Paper1,EvaluationofTechniquesforTransientPMmeasurements........................43
7.2 Paper2,PredictionsandMeasurementsofTransientNOEmissionsforaTwostageTurbochargedHDDieselEnginewithEGR.............................................................................. 44
7.3 Paper3,FastPhysicalPredictionofNOandSootinDieselEngines..........................45
7.4 Paper4,FastPhysicalEmissionPredictionsforOfflineCalibrationofTransientControlStrategies.................................................................................................................... 46
8 Summary................................................................................................................................ 47
9 Futurework........................................................................................................................... 48
10 Acknowledgements............................................................................................................ 49
11 References......................................................................................................................... 50
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1 Introduction
In 1892, a German engineer named Rudolf Diesel patented a new kind of
combustionengine.
The
diesel
engines
advantage
over
other
internal
combustion
engines was, and still is, its efficiency. The engines spreading vary in different
markets depending on the geographical position, fuel supply and sometimes
tradition.Italsovariesindifferentmarketsegments;itismoredominatinginhigh
mileagesegmentswhere the improved fueleconomysurpasses itshigherprice.
The diesel engine is dominating in heavy application such as busses and trucks
and itsmarketshare in thepassengercarsegment issteadily increasing,mainly
duetoincreasingfuelpriceandimprovedemission.In2007,53%ofthesoldcars
intheEuropeanUnionwerepoweredbyadieselengine[1].
Figure1Anearlydieselengine,[2].
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Inadieselengine,thefuelisinjecteddirectlyintothecylinderduringthelastpart
ofthecompressionstrokeandthefirstpartoftheexpansionstroke.Theinjected
fuelmixeswiththeairinthecylinderandignitesduetothehightemperatureof
thecompressedgasesinthecylinder.Theignitionusuallyoccursduringtheinitial
partoftheinjectionandthefuelthathasbeeninjecteduptothatpointcombusts
rapidly.Thispartofthecombustioniscalledthepremixedcombustionperiodand
can be a significant fraction of the total combustion period, typically when the
injectiondurationisshort.Thefuelthatisinjectedafterthispointburnsasithas
evaporatedandmixedwithair to incombustableproportionsafteronlyashort
reaction time compared to the initial ignition delay. This combustion period is
thereforecalled
diffusion
combustion.
The
combustion
process
is
both
turbulent
and instationary but a time averaged, conceptual model for the diffusion
combustionperiodhasbeenproposedbyDec([3])andthisprovidesasimplified
way to describe the process. Decs model is shown in a schematic in figure 2
togetherwithapictureofarealcombustioninanopticalaccessengine.
Figure2Dieselcombustionprocess,[3],[4]
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The disadvantage with diesel combustion, compared to e.g. Otto combustion
(normally petrol also called Spark Ignited (SI) combustion), is that it is not
homogenous throughout the cylinder, i.e. parts of the combustion takes place
with a shortage of oxygen, so called rich conditions. Where there is rich
combustion, soot is formed and although most of it is oxidized as the fuel is
furthermixedwith thesurroundingair,someof itwillremainas thegasesare
expelledoutof the cylinderand through the tailpipe.The regionsweresoot is
formedaccordingtoDecsmodelisthebrownregionshowninFigure2.Soot is
emitted from the engine in the form of particles on which water and
hydrocarbons are condensated as the exhaust gases are cooled down. These
particlesare
together
with
other
solid
and
liquid
material
in
the
exhaust
gases
calledParticulateMatter(PM).
Particulatematterisahazardforhumanshealthandtotheenvironmentandis
togetherwithNOxthemainemissionfromthedieselengineandtherebyoneof
its greatest challenge. Figure 2 also show a green region where most NO is
formed; in the flame front where the air/fuel mixture is approximately
stoichiometricandtheflame isashottest.NOx isalsoformed inSIenginesbut
since
these
are
operated
without
excess
air,
they
can
take
care
of
the
emissions
relatively easy with a threeway catalytic converter. This is not possible in the
diesel engine where overall air excess is necessary to achieve acceptable
combustionefficiencyandsootemissions.
PMemissions have been shown to increase respiratorical symptoms, decrease
lungfunctionand increaseuseofasthmaticmedicine.Increasedhealthcareand
mortalityhasalsobeenprovedregardingcardiovascularandlungdiseases[5].
NOxhas
been
shown
to
cause
increased
sensitivity
to
the
respiratory
passages
[5]
and is also the cause of smog. By the influence of sunlight, NOx forms smog
togetherwithhydrocarbonemissions.
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The maximum allowed quantities of these emissions are limited by the local
emission legislations. In Europe, these have been defined by European Union
directivesin1992andtightenedinseveralstepsafterwards(Euro1,Euro2etc.).
Asof2008,theEuro5legislationiscomingintoeffectandthenextplannedstep
isin2013,howeverthelevelsforEuro6arenotfixedyet.Thelegislatedlevelsare
showninFigure3whereeachmarkerrepresentsaEurolevel(Euro2wasupdated
onetimeandthereforehastwomarkers). Itshouldbenotedthatsincethefirst
Eurolevel,themaximumallowedPMemissionshavedecreaseby95%andNOx
with 75%. When the suggested levels for Euro 6 come into effect, the
correspondingdecreasewillbe97%and95%respectively.
Figure3EmissionlegislationevolutioninEurope,[1]
To control that the legislation is met, the engine manufacturers have to certify
theirenginesaccordingtoproceduresalsodefinedbyEuropeanUniondirectives.
Beforeyear2000,thecertificationprocedureonly includedstationaryoperation
points, i.e.theenginewasoperatedatthirteendefinedstationaryconditions (%
of maximum speed and load) and these were weighted to represent typical
engineusage.After the year2000, sinceEuro 3, transientengineoperationhas
beenincludedinthecertificationproceduretobetterrepresentrealworldusage.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0
2
4
6
8
10
1990 1995 2000 2005 2010 2015
PM[g/kWh]N
Ox[g/kWh]
Year
NOx
PM
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TheEuropeanTransientCycle(ETC)hasbeenusedforEuro3to5andisshownin
Figure 4. This cycle will be replaced by a World Harmonized Transient Cycle
(WHTC)forEuro6.TheWHTChasbeencreatedtocovertypicaldrivingconditions
intheEU,USA,JapanandAustraliawherelocalcertificationcyclesareusedtoday.
ThestationarypartofEuro6willalsobeharmonized.
Figure4EuropeanTransientCycle,ETC
AscanbeseeninFigure4,theETCcycleishighlyvaryingregardingbothloadand
speed and there are significant consequences when adding this part of the
certificationtothestationary.
Insteady
state
conditions,
different
combinations
of
load
and
speed
correspond
to different conditions in the engine regarding the gas flow in the engines gas
exchange circuits and regarding the temperature of all gases, liquids and
components.Whenatransitionfromoneloadandspeedtoanotherisdemanded
from the engine, the steady state flows and temperature will not be obtained
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directlyduetherotational inertiaoftheturbochargerandthethermal inertiaof
the components in the engine. The period between when the new load and
speed were issued until when the corresponding steady state flows and
temperaturearereached iswhat isusuallyreferredtoastransientconditionsor
just transients. In transients, thebiggestproblem is the rotational inertiaof the
turbochargerwhich isthepropertyresponsible for thesocalled turbo lagwhen
thepowerdemandisincreased.Whenmorepowerisdemandedfromtheengine,
the turbine and compressor needs time accelerate its rotational speed and air
flowtomatchthedemand.
InanOttoengine,theinjectedamountoffuelismoreorlessproportionaltothe
airflowsincetheenginehastorunstoichiometrictomakethecatalyticconverter
workproperlyandthereforethetransientpowerdeliveryfromaturbochargedSI
engine is governed by the turbo (after the initial power increase gained by
openingthethrottle).
Atransient inadieselengine isabitdifferent; theenginealwaysoperateswith
excessairandwhenmorepowerisdemanded,theexcessamountisminimizedto
allowtheinjectedamountoffueltoincreasefaster.Theengineisthenranatthis
minimumexcess
amount
of
air
until
the
demanded
amount
of
fuel
is
reached
and
thereaftertheair/fuelratiowill increaseagainuntilitreachesthecorresponding
steadystatevalue.
Thisperiodduringapowerouttakeincrease(e.g.acceleration),whentheair/fuel
ratio is lower than the corresponding steady state value, causes increased soot
emissions due to the impaired conditions for oxidation. The peak values of the
transient soot emission can be more than ten times higher than the
correspondingsteady
state
values
[6].
An
(a
perhaps
abit
extreme)
example
of
suchoccurrencecanbeseeninFigure5.
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Figure5Sootcanbeanissueduringacceleration[7]
There are several different measures to decrease the transient soot peaks but
transient engine control strategies also has to consider other aspects, mainly
engine response (power increase rate) and NOx emissions. Unfortunately the
requirements of the different aspect are often contradictory. An effective and
widespreadwaytodecreaseNOxemissionistheuseofExhaustGasRecirculation
(EGR). However, the engine response is dramatically slower when EGR is used
undertransientsandthesootemissionsaresignificantlyincreasedasEGRsteals
energy from the turbine and thereby slows down the boost build up. Another
contradictionistheminimumallowedair/fuelratioallowedduringthetransient;
a
lower
value
means
that
the
fuel
injection
rate
can
be
increased
faster
andtherebyincreasestheengineresponsebutatthesametime,thismeansahigher
sootemissionpeak.
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TheEGRrateandtheminimumallowedair/fuelratioarecontrolparametersthat
should be optimized as well as VGT strategy (Variable Geometry Turbine) and
injectionstrategy(injectionangle,injectionpressureandmultipleinjections).
Eventhoughaftertreatmentsystemsarebecomingbetter,furtherdecreaseofthe
engineoutemissionsisnecessaryforfutureemission legislationlevels.Insteady
state operation, these parameterscanbe systematically optimized in anengine
testbed toobtain thebestcompromisebetweensoot,NOxand fuelefficiency.
Fortransientoperationontheotherhand,whennotonly loadandspeedhasto
beconsideredbutalsothetransientstate,systematicallytestingcanbeextremely
timeconsumingduetotheveryhighnumberofcombinationsofconditionsand
controlsettings.Transientcontroloptimizationisthereforeagreatchallenge.
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2 Projectobjectives
The aim of this PhD project is to improve the understanding of transient
phenomena
and
to
develop
tools
for
transient
control
optimization.
In
more
concreteterms,theobjectivesaretobeableto:
measureNOxandsootduringtransients
measuretransientflowandgastemperatures
simulatetransientturboperformance
simulatetransient
EGR
flow,
temperature
and
mixing
ratios
simulateNOxandparticleformation
Eventhoughthetransientcyclehasbeen included inthecertificationprocedure
sinceyear2000,mostoftheemissionmeasurementtechniquesthatarestillused
stem from instruments developed for steady state testing. As an effect of this,
mostinstrumenthavegivenprioritytopropertiessuchasmeasurementstability
andreproducibility and long service intervals. Very few instruments therefore
have the response time necessary to study the cycle to cycle phenomena that
occurduringtransients.
Thefirstpartoftheprojecthasbeenmostlyaboutfindingemission instruments
thataresuitable for transientmeasurementsand thishasbeendoneby testing
both instruments developed for steady state measurements and also some
instrumentwithnewertechnologydevelopedfortransientmeasurements.
Gasflows
such
as
EGR
flow
and
the
air
flow
into
the
engine
are
also
difficult
to
measurewithhightimeresolutionandsoaremosttemperatures.Thereasonfor
this is also that the established techniques for these measurements are
developed forsteadystatemeasurements.EGR isusuallymeasuredbyemission
instrumentswhichalreadyhavebeendiscussed.Airflow isoftenmeasuredabit
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upstream from the engine, meaning that e.g. pressure build up after this point
mustbeaccountedforseparately.
Temperatures
are
often
measured
with
thermocouples
which
function
by
assumingthegastemperatureandtheresponsetimehere isdeterminedbythe
thermal inertia of the thermocouple. The transient responseofa thermocouple
can be improved by making it smaller but it cannot be made to small since it
wouldbecometoosensitivefortheroughenvironmentintheengine.
While the two first objectives are all about increasing the understanding and
quantifyingtransientphenomena,thethree lastobjectivesareaboutdeveloping
toolsfor
transient
control
optimization.
One
way
to
get
around
the
problem
with
highlytimeconsumingtesting inenginetestbeds istoodeveloppredictivetools
sothatallcontrolsettingscanbeoptimizedbyofflinesimulations.Oneversionof
this solution would be to use somewhat simplified models for shorter
computationaltimesandthendothefinetuninginatestbed.
Suchpredictivetoolscanbedividedintothreecategories:
Firstlyengineperformancemodels.Theseshouldbeabletopredictthebehavior
ofthecompletegasexchangesystemincludingturbochargers,EGRcircuitetcand
howitrespondstodifferentcontrolsettingsduringtransients.Themodelsinthis
categoryareoften1Dmodelsormeanvaluemodels.
The secondpart is prediction of the incylinderconditions. The simplest way to
modelsthisistoassumethattheincylindermixtureishomogenous.Thisstrongly
simplifiedapproachcannotpredictanydetailsaboutthecombustionbutcanbe
enough to use in the first category; engine performance models. The most
realisticway
to
model
the
in
cylinder
conditions
is
with
CFD
(Computational
Fluid
Dynamics), where many details in the combustion process can be resolved by
dividingthecylinderintoafinemeshofsubvolumes.ThedisadvantagewithCFD
isthat it isverytimeconsumingandthat itsometimescanbehardtoverifythe
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detailsofthecombustionsincethesearedifficulttomeasure.Therealsoexista
number of approaches that lie somewhere between these two approaches
regardingthelevelofdetailsthatcanberesolved,e.g.2zonecombustionmodels
andmultizonecombustionmodels.
Thethirdandfinalpartistheemissionformationcalculations.Emissionformation
rate is calculated with chemical reaction schemes. These schemes occur in
literaturewithawiderangeofcomplexityandtheydescribehow fuelandair is
turned intowater,carbondioxideandbyproductsviaanumberof intermediate
reactions and species. The most detail schemes include thousands of reactions
butsometimesonlyusingafewofthemcanbeuseful.
Thedifferentmodelsandmodelcategorieswillbedescribedmorethoroughly in
thenextchapters.
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3 Emissionformationindieselengines
Figure6Emissionformationinconventionaldieselcombustion,[8].
Figure6showsasocalledphiTmap.Inthismap,thetypicalregions, interms
of temperature and equivalence ratios (phi), for NO formation and soot (PM)
formation/oxidationare
shown.
The
figure
also
contains
amixing
line
which
represents the typical path of a fuel package. There are also lines that show
whereinthephiTmapdifferentsectionsoftheflametypicallyare.
A phiT map is commonly used to describe how different combustion concepts
affecttheemissionformation.Oneimportantandwidelyacceptedtheoryshown
insuchmap is thatsoot is formed in the rich regionsof the flameand that its
onlyformedwithinacertaintemperatureinterval.HCCIcombustionforexample
avoidssoot
formation
by
not
having
rich
regions
and
the
newer
concept
Low
Temperature Combustion (LTC) avoids soot formation by moving the mixing
linetothecoldsideofthesootformationpeninsula.
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The figurealsoshowsthatNOx formationandsootoxidationoccur inthesame
region,i.eunderthesameconditions.Thisistheexplanationtowhymeasuresto
decreaseoneoftheemissionsoftenleadtoanincreaseintheother.Thisresults
inthewellknownsootNOxtradeoffcurveshowninfigure7.
Figure7ExampleofasootNOxtradeoffcurve.[9]
3.1
NOxformation
ThemostimportantNOxformationmechanismistheZeldovichmechanism,also
calledthethermalmechanismsincethisisresponsibleforthestrongtemperature
depedenceofNOxemissions.ThemechanismwasproposedbyZeldovichin1946
[9,10]:
NNONO ++2
(1)
ONOON ++2
(2)
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Themechanismwaslaterextendedtoincludeathirdreaction:
HNOOHN ++
(3)
As longasthemaximumcombustiontemperature isfairlyhigh(above~2000K),
theZeldovichmechanismisdominatingtheNOproduction.Otherwise,e.g.when
high EGR rates are used or with HCCI combustion, other mechanisms can have
significantcontribution.BesidestheZeldovichmechanism,therearethreeother
suggestedmechanisms[9]:
The
promt
(Fenimore)
mechanism
in
which
NO
is
formed
from
an
intermediatespecieintherichzonesoftheflame
Thenitrousoxide(N2O)mechanism
NOformationfromfuelnitrogen
Anautomotivedieselenginecantypicallynotavoidhightemperatureregions in
its entire operational range (of loadand speedand in transients)and therefore
theZeldovichmechanismisoftenconsideredtobedominating.
3.2
Sootformationandoxidation
Sootformationandsootformationmodelingis,inseveralaspects,morecomplex
than NOx formation. One reason is that NOx is formed only via gasphase
reactions while soot formation also includes solid phase reactions. Another
reason is that the exact mechanisms are not known and no simple dominating
soot formation mechanism have been identified (comparable to the Zeldovich
mechanism
for
NOx
formation).
However,
several
reaction
schemes
and
mechanisms have been proposed and some of the most widely accepted are
brieflydescribedhere:
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Theprecursors forsoot formationare formedvia thegasphasereactionduring
fuel pyrolysis. One important precursor is believed to be PAHs (PolyAromatic
Hydrocarbons) and the growth process of these precursors appears to involve
additionofC2,C3orothersmallunitsas,e.g.,C2H2,acetylene[11].
The reaction step when these gas phase species becomes particles is called
nucleation. Several models for this has been proposed and one of the most
popularmodels isthePAHmodel inwhichthePAHsstarttosticktoeachother
after they have reached a certain size. These PAH clusters then evolve to solid
particles[11].
Whenthese
particles
have
been
formed
they
increase
in
size
in
what
is
called
the
mass growth process. This process can be divided into three subprocesses;
coagulation, surface growth and dehydrogenation. Coagulation starts
immediately after the particle formation and can be described as a sticking
collision between particles which significantly increases particle mass and
decreases particle number without affecting the total particle mass [12]. The
surfacegrowthprocess iswhengasphasespeciessuchasacetyleneandPAHs is
adsorbed to the particles surface [11]. In the final stage of soot formation, the
particlesare
dehydrogenated
to
progressively
more
graphitic
carbon
material.
At
the same time, the number of active sites on the particle for surface growth is
reducedandthereforethedehydrogenation leadstothe formationofchain like
structures[11].
Simultaneouslyasthesootformationprocessesandinmanycasesuntilthegases
areemittedfromthecylinder,thesootoxidationprocessistakingplace.Themain
oxidating reactants are OH under fuel rich conditions and O2 under fuel lean
conditions[11].
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3.3
Emissionreduction
The trade off relationship between NOx and soot (Figure 6) is fortunately only
fixedfor
aspecific
engine
and
ways
to
reduce
emissions
has
continuously
moved
this tradeoff curve towards origo to meet new legislations (Figure 3). New
combustionconceptsuchasHCCI,PCCIetc.canbreakthetradeoffbyalteringthe
mixing and ignition parts of the combustion. However, conventional diesel
combustion is still dominating and the greatest contribution to the emission
reductioncomes fromcomponentdevelopment.Oneclear trendover theyears
hasbeento increasethe injectionpressureandthereby improvethemixingand
reduce the rich regions that cause soot formation. Exhaust Gas Recirculation
(EGR)had itsbreakthrough forEuro4.TheuseofEGRdecreasesthemaximum
combustion temperature and thereby the NOx formation. Lower combustion
temperature means worse soot oxidation but this can be compensated for by
increasingtheboostpressureandtherebytheair/fuelratioorby increasingthe
injectionpressurefurther.
SincetheEuro4emissionlevel,manyenginemanufacturershavebeenforcedto
equiptheengineswithaftertreatmentsystemssincetheywerenolongerableto
sufficientlyreduce
the
engine
out
emissions.
The
most
common
aftertreatment
systemsareParticulateTrapsandSelectiveCatalyticReduction(SCR)whereNOx
isreducedwithareductante.g.ammonia.However,theengineoutemissionsstill
need tobereducedandcontrolled.AnSCRaftertreatmentsystemonly reduces
7095%sotheengineoutemissionscanstillnotbetoohigh.Anotherfactoristhe
costof the reductant thathas tobeadded.Forallaftertreatmentsystems,size
and service interval are also strong arguments to reduce the engine out
emissions.
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4 Dieselcombustionmodeling
Figure7CFDModelingofthedieselcombustionprocess[12]
Since the combustion process in a diesel engine is very complex, the only
possibilitytomodel itrealistically isbyusingCFDcoupledtoachemicalreaction
model.Arealisticmodelhastobeabletodescribeincylinderflowinducedduring
the intakestrokewith properties such as swirl and tumble. It has to be able to
predict how the fuel spray breaks up, evaporates and mixes with the air in the
cylinder,howthefirstreactionsstartandthemixtureautoignitesandthereafter
the diffusion combustion. It also has to be able to predict the incylinder flow
after combustion and heat transfer to the cylinder walls and roof and to the
piston. Many of these processes are difficult to predict separately and are
extremelychallengingcombined.
In CFD combustion modelings, the combustion chamber is divided into sub
volumes.Thenumberofsubvolumesthatareusedrangesfromthousandsupto
threeordersofmagnitudemore.Thegreatestadvantageoverothermodelsisthe
highlevelofdetailthatcanberesolved.Ifamodelhasbeenthoroughlyvalidated,
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itcan increase understanding of thecombustionprocessand the model can be
usedtopredicttheinfluenceofe.g.injectorpropertiesandpistonbowldesign.It
canalsoshowwherethegreatestcontributiontotheemissionoriginandhowitis
avoided.
The formation of soot and NO emission can be predicted via chemical kinetics
reactionschemesthatoftenareextensive.Theseschemesdescribeshowfueland
air are converted to carbon dioxide and water via thousands of reactions and
hundredsof intermediatespeciesandarealsousedtopredictautoignitionand
heat release rate. NOx formation can be included in the schemes while soot
formation can only be partly included since soot formation also consist of solid
matterreactionsandnotonlygaseous.
Someexamplesofsootmodelswherethesecomplexgasphasechemicalreaction
schemesareused togetherwithparticlegrowthmodels inCFDmodelsare [13
16].
The greatest disadvantage of using these complex models is the computational
time.Dependingonthenumberofsubvolumes inthemodel,thecomplexityof
themodel,
computer
capacity,
etc.,
computing
one
engine
cycle
require
from
a
couple of hours up to days. If one considers engine transients; one large load
transient in a heavy duty engine has a duration of over 50 cycles and to find
optimumcontrolsetting forthistransient,hundredsofcombinationsofsettings
would have to be simulated. In practice, this means that it isnt feasible to use
thesemodelsfortransientcontroloptimization,i.e.simplifiedmodelsareneeded
forthatapplication.
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4.1
Simplifieddieselemissionmodeling
Todecreasethecomputationaltimeneededforemissionpredictions,themodels
canbe
simplified
in
three
aspects:
how
the
combustion
process
is
modeled,
the
chemicalreactionschemesandthemodelsthatdescribestheemissionformation
processes.
4.2
Simplifiedcombustionmodeling
Simplificationsofthecombustionprocesscanbefoundinmanydifferentdegrees
inliterature.Onecommonsimplificationthatsignificantlyreducesthecomplexity
ofthemodelistodisregardthespatialresolution, i.e.use0Dmodels.Themost
common0Dmodelsare:
Homogenousincylinderconditions[17]
Twozone combustion models [1820]: One zone with reactants and one
zonewithproducts.
Multizone combustion models [2124]: One zone with reactants and one
zonewithproductsforeachtimestepafterthestartofcombustion.
Thetwo
zone
approach
is
considered
to
be
afairly
accurate
way
to
describe
SI
combustion [10] since the fuel/air mixture is homogenous throughout the
cylinder and the flamefront propagates outwards from the sparkplug in the
centerofthecylinderwithproductsbehinditandreactantsinfrontofit.Thetwo
zone approach is however less straight forward when it comes to diesel
combustiondueto inhomogenousincylinderconditionsandamuchlessregular
combustion process. The common way to solve this is to assume that the
combustion takes place at stoichiometric conditions and that the combustion
productsarethengraduallymixedwiththeexcessairinthecylinder.Thus,theincylindermixturebecomeshomogenousbeforetheexhaustvalvesopen[18,20].
Themaindrawbackofusingthetwozonemodelfordieselcombustionmodeling
isthatthemaximumtemperatureisunderestimatedsincethetemperatureinthe
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burnedzoneatevery instant isanaverageofallburningandpreviouslyburned
elements.Thisunderestimationhassignificant impactonthehighlytemperature
dependentNOxformationprocess.
To better imitate the real combustion process, multizone models have been
developed.Inthesemodels,thecombustionprocessisdividedintodiscretetime
steps and a new zone with combustion products is formed for each one. The
temperature and the mixing processes are still averaged but to a lower extent.
TheincylindertemperatureswiththedifferentmodelsareshowninFigure8.
Figure
8 Incylinder temperatures. Tmean is the homogenous temperature. Tu is the
temperatureintheunburnedzone,i.e.thezonecontainingthereactantsinatwozone
model.Tbistheburnedzone,i.e.thezonecontainingtheproductsinatwozonemodel.
Tzone is the temperature in the product zones in a multizone model. In this case four
zonesareusedbutnormallyonezoneperdegreecombustiondurationisused,i.e.the
numberofzonesattheendofthecombustionistypicallyaround50.
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4.3
Simplifiedchemicalreactionschemes
As previously mentioned, the combustion chemistry in a diesel flame is very
complexwith
hundreds
of
species
and
thousands
of
intermediate
reactions.
On
the lean side of the flame, where the combustion is almost completed, the
chemistry is dominated by only a few species and is thereby much simpler to
model.Thetenspecieswithhighestconcentrationare;CO2,CO,H2O,H2,O2,N2,H,
O,NO,andOH [10,21].As the localchanges in temperatureandairsupplyare
slowon the leansideof the flameunlike in the richandstoichiometricsidesof
the flame, the concentration of these species can be assumed to equal the
equilibriumconcentrations.Thefractionsofthedifferentspeciesarefoundwith
carbon,hydrogen
,oxygen
and
nitrogen
balance
and
by
finding
the
equilibrium
balanceforthefollowingreactions:
(4)
(5)
(6)
HH 22
1 (7)
(8)
(9)
Thiscombustionchemistrycanbeusedtopredictprocessesontheleansideof
theflamesuchasNOxformationandsootsurfaceoxidation.
222 COHOHCO ++
2221 COOCO +
OO 22
1
HOOOH ++2
NONOON ++22
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4.4
Simplifiedemissionmodeling
Simplifiedemissionmodelingcanbedividedintothreecategories:
Phenomenologicalmodels
Semiempiricalmodels
Statistical(cycleresolved)models
4.4.1
Phenomenologicalmodels
A
phenomenological
model
is
one
which
relates
empirical
observations
of
phenomenatoeachother,inawaywhichisconsistentwithfundamentaltheory,
butisnotdirectlyderivedfromtheory. Thiscategoryofmodelingisawaytodeal
with the complexity of soot formation where important and representative
species and mechanisms for the soot formation process are modeled. One
exampleofaphenomenologicalmodelwasproposedbyFuscoetalin1994[17].
Each part of the soot formation and oxidation process was represented by one
equationreactionasillustratedinFigure9.
Figure9IllustrationofFuscoetalsSootmodel[17].Schematicfigurefrom[25]
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Thereactionscanbesummarizedinfourequations:
(4)
(5) (6) 5 (7)andare the concentrations of soot precursor species and growthspecie (acetylene).andare the stoichiometric coefficient and reaction ratecoefficients. and are soot volume fractions and particle numberconcentration. , , , and are the soot mass, molarmass,density,averagediameterandsurfaceareaconcentration(cm2/cm3).isAvogradosnumber. isthesootoxidationratecalculatedwiththeNagleandStricklandConstable(NSC)model:
1 (8)WhereRNSCistheoxidationrate[molCatoms/(cm2*s)],kAkBandkZ,aswellaskT
whichisusedinthenextequation,aretemperaturedependentratefunctions,
pO2isthepartialpressureofoxygen.ThemolarfractionofO2wasapproximated
bytheequilibriummolarfraction,whichisalreadycalculatedfortheNO
formationrate.xAisthefractionofsootwithamorereactivesurfacesite.
xAis
calculated
with
the
following
equation:
1
(9)
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WhenFuscoetalpresentedthismodeltheyusedaverage incylinderconditions
[18],butithasalsobeenusedtogetherwithCFD(ComputationalFluidDynamics)
modeled incylinderconditions [three].Extendedversionsof themodelhasalso
beenused,e.g.[26,27]andtherearealsoalternativemodelsproposed,e.g.[28,
29].
Anotherexampleofphenomenologicalmodelsismodelswheredifferentreaction
ratesarecurvefittedtootherparameterssuchastemperature.Forexample,the
complexcalculationsforsootformationratethatareshowninFigure6onaphiT
maphavebeencurvefittedasafunctionofphi(fuel/airratio)andtemperatures
andusedinCFD[8].
A common method to predict NO formation is to assume that the thermal NO
formationisdominatingandthatthethermal(Zeldovich)mechanismcanbeused
topredictthetotalNOformation.FromtheextendedZeldovichmechanism,eq.1
3,therateofNOformationcanthenbewrittenas:
(10)Where [ ] denotes concentration of the different species and ki denotes the
forward and reversed rate coefficients. The rate constants are strongly
temperaturedependentandcanbefoundine.g.[10]
The concentration of the species included in the equation, and thereby the
formationrate,canbeobtainedifcomplexgasphasereactionschemesareused.
Examples
can
be
found
in
in
cylinder
CFD
combustion
and
emission
models,
e.g.
[13,14].Ifreactionschemesarenotused,theequilibriumassumption,[10],can
beusedsincemostNO isformedonthe leansideoftheflame.Thisapproach is
usedbye.g.[21]andleadstothefollowingsimplifiedexpression:
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(11)
Where[]edenotesequilibriumconcentrationand (12)
(13)
(14)
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4.4.2 Semi-empiricalmodels
Semiempirical models can be described as a further simplification of the
phenomenologicalmodels.
The
most
popular
example
is
the
two
step
Hiroyasu
modelwhichhaswaspresentedin1983[30]withonereactionforsootformation
andoneforoxidation:
.exp (15) .exp (16)Where
and are the mass of soot and fuel vapor.andare thepreexponentialconstantsforsootformationandsootoxidationrespectivelyandtheyareadjustedempirically. andaretheactivationenergiesandarealsoadjusted empirically. is the oxygen molar fraction.,andare incylinderpressureandtemperatureandthegasconstant.
Thismodelhasbeenwidelyused,thankstoitssimplicity.Adisadvantageisthatit
only is valid in a narrow range of operating conditions. The model can be used
withone
,two
,and
multizone
models
as
well
as
with
CFD
models
[8,
18,
24
and
28].
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4.4.3
Statisticalmodels
Statisticalmodels([22and3133])areusuallycycleresolvedblackboxmodels
wherethe
emissions
are
predicted
by
curve
fitting
cycle
averaged
and/or
cycle
characteristicdatatoemissions.Thistypeofmodelingisthefastestanditdoesnt
require any deeper understanding of the underlying mechanisms. The
disadvantagesarethathighaccuracyusuallycantbeobtainedoverwideranges
ofoperatingconditionsandthattheyareunreliableforextrapolation.
Statistical modeling could be a useful tool for rough offline optimization of
transientcontrolstrategies thanksto thesimplicityof themodelsandtheshort
computational
time.
The
problem
is
that
more
or
less
all
measurements
are
somewhatdelayedandfilteredorsmoothened.Puredelaycanoftenbeidentified
and corrected for rather easy but the filtering caused by the instruments rise
timesishardertoidentifyandcorrectfor.Thismakesitmoredifficulttoidentify
whatparametersthatarethemostimportantandtheircauseeffectrelationship.
To illustrate this, an artificial signal and a filtered version of the same signal is
shownintheupperpartofFigure10.Thefloatingaveragefilterthatisusedhere
representsaninstrumentrisetimeof~0.7second.Areasonableexamplefor,e.g.,
thesoot
measurements.
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Figure10Effectsofinstrumentrisetimeonstatisticalmodels.
The lower part of Figure 10 shows the correlation between the signal and its
filtered version and shows why it is difficult to create statistical models for
transientoperation,especiallyiftheinstrumentsrisetimeisunknown.Thecaseis
often that both cause and effect parameters have significant (and different)
filtering.
Itwouldbeanoptiontodevelopthemodelsduringstationaryoperationwhere
theinstrumentsrisetimesisntaproblem.However,theextremeconditionsthat
occurduringtransients(extremelylowair/fuelratio)aredifficulttoachievein
stationaryoperation.
-0.5 0 0.5 1 1.5 2 2.5 3 3.50
2
4
6
Time
signal
filtered signal
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
2
4
6
signal
filtered
signal
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5 Advantagesanddisadvantagesofdifferentemissionmodel
categories-Motivationforchosenmodels
The typeof modelsused for emission predictions in thisproject was chosen by
evaluating the advantages anddisadvantages of the differentmodelcategories.
The CFD models with complex chemistry and particle growth models were
eliminatedatanearlystageduetocomputationalcost.Thecycleresolvedmodels
on the other hand have a very low computational cost but do not give any
physicalunderstandingoftheunderlyingprocessesandthereisalsothepractical
problem with establishing causeeffect relationships due to the rise times of
different
measurement
methods.
The
model
category
that
was
chosen
for
this
projectwasthereforethesimplifiedphysicalmodelcategory.
One specific work that influenced the model development in this project was
EgnellsNOxmodels[21].Inthismodelamultizoneapproachisusedtomodelthe
combustion process, the equilibrium assumption is used to predict the
composition of different species in the cylinder and the NO formation rate is
calculatedwiththeextendedZeldovichmechanism.
Fromthis
NOx
model,
anew
approach
for
soot
predictions
was
developed.
Soot
oxidationontheleansideoftheflameisbelievedtobedominatedbyO2surface
oxidation [11] and can thus be predicted by using the Nagle and Strickland
ConstablemodelintheframeworkoftheNOxmodel,i.e.amultizonecombustion
modelandequilibriumcombustionchemistry.
Sincethemultizonemodelonlydescribestheproductsofcombustion,i.e.starting
inthestoichiometricandleanregionsofthecombustion,thesootformationand
OHoxidation
processes
cannot
be
included
since
these
occurs
at
rich
conditions.
To get around that problem, a new approach was attempted based on two
observations; firstly in a phiT map, e.g. Figure 6, it can be seen that NOx
formation and soot oxidation occur under the same conditions, i.e. high
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temperatureandwithavailableoxygen.Thesecondobservation istheNOxsoot
tradeoffcurve,whichclearlyshowsthatoperatingconditionsthatresult inhigh
NOxemissionsalsoresultinlowsootemissionsandviceversa. Theconclusionof
these observations is thatsince it ispossible to predict NOxemissionswith the
multizonemodelitshouldbepossibletopredictsootemissionsaswell.
Asecondaryproductofthatconclusionisthatthevariationsinthesootformation
processontherichsideoftheflamearelesssignificantfortheemissionsthanthe
variationsintheoxidationprocessontheleansideoftheflame.Thereby,agood
estimation of the emitted soot should be found if a typical soot formation
process for the specific engine can be found and if the oxidation process is
describedcorrectly.
In thisapproach it wasassumed that the typicalsoot formationprocess for the
specific engine could be represented as a typical fraction of fuel, with a typical
particlediameter,survivingthroughtheflameandtothestoichiometricandlean
sideswherethemultizonemodelandsootoxidationmodelarevalid.
The typical fraction and diameter for the soot model was found together with
typicalmixing
rate
and
typical
combustion
air/fuel
ratio
for
the
multizone
model
by running an error minimization script for the models over a wide range of
steadystateconditions.
One of the fundamental assumptions in the multizone approach is that the
combustiontakesplaceinadevelopedflameandasaconsequence,theaccuracy
ofthemodeldecreaseswhensignificantpartofthecombustionispremixed.
Moredetailsofthemodelsarefoundinthearticlesintheappendix.
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6 Measuringandevaluatingtransientoperation
As described in the introduction section, measuring and evaluating transient
operationis
in
most
aspect
more
difficult
compared
to
stationary
operation
as
it
demands information with high time resolution from instruments and
measurement methods that were developed for stability, repeatability and
durability. The extra precautions and data processing that is necessary to
evaluatetransientoperationwillbedescribedinthissection.
6.1
Gasexchange
system
measurements
Theperformanceofthegasexchangesystem, includingthe intake andexhaust
systemsand theEGRcircuit, isa very important factor duringengine transients
and is therefore very important to be able to measure. The performance
parametersthataremostinterestingtoevaluateareintakeairflowandEGRrate.
Theyaretwoofthemostimportantparametersfortheincylinderconditionsand
arethereforeessentialasinputsforemissionpredictions.
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6.1.1
TransientEGRmeasurements
Insteadystateconditions,EGRrate is typicallymeasuredbycomparing theCO2
fractionin
the
intake
with
the
exhaust
concentration
but
since
CO2
is
measured
with a conventional instrument, this cannot be used during the transients.
Instead, the EGR rate was estimated from air based volumetric efficiency (only
fresh air flow is included) via a steady state correlation. This correlation origin
fromawiderangeofoperatingconditionsandisshowninFigure11.
Figure11SteadystatecorrelationbetweenairbasedvolumetricefficiencyandEGRrate
forawiderangeofoperatingconditions
AscanbeseeninFigure11,themaximummeasuredvolumetricefficiencyisonly
0.85.Sincethevolumetricefficiency iscalculatedfromthesweptvolume,100%
efficiency will not be achieved as the intake valves are closed somewhat later
thanthe
bottom
dead
position
of
the
piston.
A
few
percent
is
also
lost
due
to
residualgasesinthecylinder,i.e.internalEGR.VariationsininternalEGRrateare
whatcausetheerrorsintheexternalEGRestimationsinthefigureandwhenthe
estimationisusedduringtransients.
y=114.01x+92.809R=0.9574
10
0
10
20
30
40
50
0.4 0.5 0.6 0.7 0.8 0.9
EGRrate[%]
Airbasedvolumetricefficiency
Measured Linear(Measured)
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6.1.2
Transientairflowandair/fuelratiomeasurements
Sincetheairflowtotheengineismeasuredbeforeitscompressors,therewillbea
measurement
error
when
the
engine
is
operating
transient.
This
is
due
to
accumulation effects in the intake system during pressure build up. This can
correctedforwiththefollowingequation:
(17)Thiscorrectionshouldbedoneforallsubvolumes intheengines intakesystem.
P,V,RandTarepressure,volume,gasconstantandtemperature.
Toget thecorrectglobalair/fuel ratio in thecylinder, theexcessair in theEGR
alsohastobeaccountedfor.Thesetwocorrectionshavesignificantimpactonthe
estimatedair/fuel ratio.Air/fuel ratiowasalsomeasuredwithawidebandunit.
However,thisalsohastobecorrectedfortheairintheEGR.
Figure12Correctionoftheestimatedair/fuelratio
-10 0 10 20 30 40 50 600
0.5
1
1.5
2
2.5
Lambda
Cycle
-10 0 10 20 30 40 50 60
0
15
30
EGR[%]
Air & Fuel flow based
A & F flow + Accumulation corr.
A & F flow Accumulation + EGR corr.
Wideband unit
Wideband +EGR corr
lambda min. setpoint
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6.2
Emissionmeasurements
6.2.1
NOxmeasurements
Instationaryoperationconditions,NOx ismeasuredwithaECOPhysicsCLD700
chemiluminescencedetector.This instrumentmeasuresthetotalofNOandNO2
measurements but since it has a rise time in the order of 20 seconds it is not
useful for transientevaluation.Forthatapplication,aCambustion fNOx400Fast
CLD chemiluminescence detector was used instead. This instrument has a rise
time
of
4
milliseconds
but
only
measures
NO.
That
is
however
an
acceptable
compromise since significant NO2/NOx ratios only occurs at high global air/fuel
ratiosandthustypicallynotundertransientconditions[10].
6.2.2
Sootmeasurements
ParticulateMatter (PM)andsoothasbeenmeasured inanumberofaspects in
thisproject;
PM
mass
concentration
[mg/m3],
opacity
(%),
Filter
Smoke
Number
(FSN),particlenumberconcentration[N/cm3]andsizedistribution.Thedifferent
measurement methods has been evaluated and compared [34] and it was
concluded that a good characterization of the PM emissions can be achieved if
some kind of mass related measurements are performed as well as some size
distribution related measurement. Mass related measurements are besides PM
mass concentration also opacity and FSN. The most repeatable measurements
have been found to be FSN. However, FSN has to be averaged for about 30
secondsand
are
thereby
not
useful
during
transients.
Therefore
opacity
was
used
for transient measurements instead. Opacity is a bit less stable since it can be
affectedbyliquidPM,humidityandlightabsorbingspeciesintheexhaust,but it
hasarisetimeoflessthanonesecond.
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Size distribution measurements and particle number concentration
measurementscanprovideverydetailedinformationabouttheemittedPM.One
ofthereasonswhythesetypesof instrumenthavegainedpopularityrecently is
that researchers have found correlations between human health and particle
number concentrations [5]. It is believed that the reductions in PMmass
emissionshavebeenaccomplishedbydecreasing theaverageparticlemassand
notthenumberofparticles.Thenextemission legislation level,Euro6,willalso
includeparticlenumbermeasurements.
Figure13RisetimeevaluationforPMinstruments
The rise times for the different instruments are showed in Figure 13. This was
testedbydecreasingtheair/fuelratiosignificantlyfromonecycletotheother.In
this figure, normalized opacity and particle number measurements in three
different
size
ranges
are
shown.
The
particle
counters
CPC
3025
and
CPC
3010
measuredthetotalnumberofparticleslargerthan3and50nmrespectively.The
size distribution measured with the EEPS was integrated and shows the total
numberofparticleslargerthan6nminthefigure.Itcanbeseeninthefigurethat
-1 0 1 2 3 4 50
0.1
0.5
0.9
1.1
Time [s]
Normalizedsignal
CPC 3025
Opacimeter
EEPS total
CPC 3010
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theconcentrationof largerparticlescorrelatesbetterwithopacity (and thereby
PMmass)thanwhensmallerparticlesalsoareincluded.
The
CPC
3025
was
chosen
to
measure
particle
number
concentrations
during
transientssince it is the fastest instrument. ItwasequippedwithaParticleSize
Selectorsothatitonlymeasuredparticleslargerthan20nm,i.e. tocomplywith
theproposedEuro6legislation.
Figure14CorrelationbetweenPMmeasurementmethods
Figure14showsopacityandparticlenumberconcentration,vs.FSN.Itshowsthat
OpacityandFSNcorrelatesverywellwhiletherearesomespreadwhenparticle
number concentration and FSN is compared, probably due to particle size
variations. This data comes from a wide range of steady state operating
conditionsandcurvefittedlineartrendlineswaslaterusedtocorrelatetransient
measurementstosteadystateandtoestimatesizechangesduringthetransients.
AnexampleisshowninFigure15.
0
5000
10000
15000
20000
25000
02468
10121416
1820
0 0.5 1 1.5 2 2.5
ParticleNumberConcentration
[N/cm3]
Opacity
FSN
Opacity CPC3025
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Figure15Variationsinmassaveragedparticlediameterduringatransientcanbe
estimatedbycomparingopacityandparticlenumbermeasurements.
Figure 15 shows how measured opacity and particle number concentration
correlatesduringatransient.Fromthiscorrelation,variationsintheaveragedsize
ofthe
emitted
particles
can
be
estimated.
The
ratio
between
opacity
and
particle
numberconcentrationaboutthreetimeslargeraftercycle20thanbefore. Ifthe
mass per particle is increased three times, that corresponds to a diameter
increaseof~50%.
Thismethodforestimationofhowtheaverageparticlesizevariescanbeusedas
astepinthevalidationofsootpredictionmodels.
-10 0 10 20 30 40 50 600
20
40
60
Opacity[%]
Cycle
-10 0 10 20 30 40 50 60
2
3x 10
4
Particlenumberconcentration[N
/cm3]
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6.3 Postprocessingemissionpredictions
Asdescribed,mostinstruments,andespeciallysootinstruments(figure13),have
significantrise
times.
This
rise
time
is
often
caused
by
internal
volumes
in
the
instruments and the result is an averaging/filtering effect. Some averaging also
occursbeforetheexhaustgasesreachestheinstruments,asgasesfromdifferent
cylindersandcyclesaremixedwitheachother.Sincethemeasurementsarenot
cycleresolved,somepostprocessingisnecessarytomakethepredictedemission
comparable.
Figure15Postprocessingofpredictedsootemissionstoimitateexhaustsystemand
instrumentaveragingandtodecreaseeffectsofnoisyinputdata.
Figure15showsthepredictedsootemissionswithandwithoutanaveragingfilter
togetherwiththemeasurements.Thestrengthofthenecessaryfilteringdepends
onthe instrumenttowhichthepredictionsarecompared. Noisy inputdatacan
alsoincreasetheneedforfiltering.
-10 0 10 20 30 40 50 600
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Cycle
Soot[%o
ffuel]
Measured
Predicted
Predicted + filtered
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7 Introductiontopapersandkeyresults
This section contains very brief summaries of the papers and some of the key
results
presented
in
them.
The
lessons
learned
from
each
paper
have
been
carriedontothenextoneandthelongtermsgoalhasbeentofulfilltheproject
objectives:
measureNOxandsootduringtransients
measuretransientflowandgastemperatures
simulatetransientturboperformance
simulatetransientEGRflow,temperatureandmixingratios
simulateNOxandparticleformation
7.1
Paper1,EvaluationofTechniquesforTransientPM-measurements
Asthe
title
reveals,
paper
1presents
an
evaluation
of
different
PM
instruments
regarding their suitability for transient measurements. The tested instruments
were a TEOM and an opacimeter for PMmass and two CPCs and an EEPS for
particlenumberconcentrationandsizedistribution.Thetestswereconductedon
asinglecylinderresearchengine.Themostimportantconclusionfromthispaper
was that the measurements with the highest time resolution were achieved by
one of the CPC and the opacimeter, with rise timesjust below 1 second (see
Figure 13). It was also found that the proportionality between PM mass and
particle
number
concentrations
vary,
mainly
due
to
variations
in
particle
size
distribution and that these variations can be decreased if the very smallest
particlesaredisregarded.
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7.2
Paper2,PredictionsandMeasurementsofTransientNOEmissions
foraTwo-stageTurbochargedHDDieselEnginewithEGR
In
paper
2,
an
engine
load
transient
was
evaluated
with
different
control
strategies, or more specifically, with different EGR rates during the first part of
the load increase.Thetestedheavydutyenginewasequippedwithatwostage
turbochargingsystemandahighpressureEGRroute.Theaimwastopredictthe
enginebehaviorwiththedifferentstrategieswitha1Dmodelandtocouplethat
modeltoamultizoneNOpredictionmodel. Theobjectivesthatwerefocusedon
inthispaperweretransientNOxmeasurementsandsimulationoftransientturbo
performance,EGRflowandNOxformation.
ThemainconclusionwerethatthemultizoneNOxmodelpredictedtheemissions
wellandthatthismodeleasilycouldbecoupledtothe1Dmodelwithoutslowing
itdownsignificantly.However,duetothehighpressurelevelsinthisengine,itis
difficult predict the EGR flow in the high pressure routeand thishassignificant
influenceontheNOxpredictions.InthisexperimenttheEGRratewasmeasured
by measuring the NO in the intake and in the exhaust. Examples of results are
showninFigure16.
Figure16PredictedandmeasuredIMEPandEGRratewiththreedifferentEGRvalvesettingsduringtheinitialpartofatransient.
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7.4
Paper4,FastPhysicalEmissionPredictionsforOff-lineCalibrationof
TransientControlStrategies
Inpaper
4,
the
models
developed
in
paper
3were
used
to
predict
the
emission
duringtransientoperation.Similarlytopaper2,a fullengine loadtransientwas
evaluatedwithdifferentcontrolstrategiesbutthistimewithmorestrategiesand
also includingsoot. Inthisexperiment,onlymeasurementswereusedas inputs.
Withoutmakinganychangestothemodelsfrompaper3,thepredictedemissions
agreedwellwithmeasurements,asshowninfigure18.Inmostcaseswherethere
are discrepancies in one of the predicted emissions, there is a corresponding
discrepancyintheotherandthisprobablymeansthattheseshortcomingsorigin
frominput
data
or
modeling
the
in
cylinder
conditions.
Figure18Predictedemissionsduringaloadtransientwiththreedifferentstrategies.Thestrategiesareidenticalincase1and2besidestheEGRvalvepositionduringthefirst10cycles.Case2and3areidenticalbesidestheVGTpositionduringthefirst20
cyclesofthetransient
-10 0 10 20 30 40 50 60
0
100
200
300
400
500
600
700
C cle
NO
[ppm
]
-10 0 10 20 30 40 50 60
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
C cle
Soot[%of
fuel]
Pred. 1
Meas. 1
Pred. 2
Meas. 2
Pred. 3
Meas. 3
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8 Summary
The long term objective for this project is to develop models that allow
optimizationof
engine
control
strategies
for
transient
operation
via
simulations.
Themotivationforthis istheneedfor improvedunderstandingofthisoperating
conditionandalsoawishtominimizethetimeneededforcalibrationinanengine
testbed.
Thesteppingstonestowardsthatgoalareidentifyingmeasurementmethodsthat
canresolve thetransientbehaviorof theengine in termsofgasexchange flows
and temperatures and in terms of emissions. Also to develop models that can
predictthat
behavior.
Due
to
the
high
number
of
possible
control
settings,
an
additionalrequirementforthemodelsislowcomputationaltime.
At this point, halfway into the project, a range of measurement methods and
simulation tools have been tested. Methods for measuring emissions with
acceptabletimeresolutionhavebeenidentifiedaswellasacceptablemethodsto
estimatetheair/fuelratioandEGRrateduringtransients. However,thereisstill
room for improvements. No effort has yet been made to improve temperature
measurementsand
this
makes
it
impossible
to
resolve
the
dramatic
variations
in
intaketemperaturethatoccurastheEGRvalve isclosedandopenedduringthe
transient.
Regardingthemodelingobjectives,modelsforsootandNOxemissionshavebeen
validatedwithawiderangeofstationaryoperatingconditionsand for transient
conditions and the models appear to be applicable for their purpose. However,
simulatedinputshaveonlybeenusedinlesserextentwhena1Dmodelwasused
topredict
an
engines
transient
behavior
and
provide
inputs
to
amultizone
NOx
modelandmoreworkisneededbeforethelongtermsobjectivescanberegarded
asfulfilled.
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9 Futurework
Examplesofareaswhereadditionalworkisneededand/orwouldbeinteresting:
Toprovethevalidityoftheemissionmodelingapproach,itisnecessary
toverifyitonotherengines
To test themodels in their intended application, i.e. to use them with
engine models, 1D and/or meanvalue models, to optimize transient
controlstrategies.
Since the models predict the incylinder emissions during the engine
cycle, it would be interesting to compare the models with incylinder
measurements
Oneofthefundamentalassumptionsinthesootmodelisthatatypical
soot formation ratecanbe found foraspecificengine.However, ifan
engine with variable injection pressure was used, the typical soot
formationratewouldmostlikelybedifferentatdifferentpressuresand
therebyalsointerestingtotrytomodel.
The largestsourcesoferror for thetransientemissionspredictionsare
believed to be measured inputs such as EGR rate, air/fuel ratio and
temperatures. It would be interesting to see how the emission
predictionsimprovedifthesewereimproved.
Investigatehowthemodelscanbeadjustedorextendedtoimprovethe
accuracyforpredictingemissionsinpremixedcombustion.
Try more time steps/number of zones in the multizone to see how it
affectsthetimerequirementforthemodelanditsaccuracy.
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10Acknowledgements
ThisworkwascarriedoutwithinCICERO(CentreforInternalCombustionEngine
ResearchOpus),
acompetence
centre
at
KTH,
sponsored
by
the
Swedish
Energy
Agency,vehicle industryinSwedenandKTH.CICERO isgreatlyacknowledgedfor
thesupport.
I would also like to thank my supervisor HansErik ngstrm and all my friends
andcolleaguesatthedivisionofinternalcombustionengines.
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11
References
1.
http://www.dieselnet.com/news/2008/02acea.php, 200901202. [peswiki.com/.../DieselStationaryOnecylinder.jpg], 200901203.
Dec J. E., A conceptual model of DI diesel combustion based on lasersheetimaging, SAE 970873. 19974. Courtesy of Mikael Lindstrm, KTH Royal Institute of Technology andSCANIA, 200901205.
Jungnelius S. and Svartengren M., Hlsoeffekter av trafikavgaser, 20006. Hagena J. R., Filipi Z. S. and Assanis D.N., Transient Diesel Emissions:Analysis of Engine Operation During a TipIn, SAE paper 20060111517.
http://www.hankstruckpictures.com/bullnose_rmtf2001.htm, 200901208.
Kaario O., Antila E. and Larmi M. Applying soot phiT maps for engineering
CFD applications in diesel engines, SAE 2005013856, 20059. Heywood J. B. Internal combustion engine fundamentals, McGrawHill,199810.
Rente T., Injection strategies for heavy duty DI diesel engines, doctoralthesis, Chalmers University, 200411.
Xi J. and Zhong BJ. Review: soot in diesel combustion systems. ChemicalEngineering Technology, 29 No. 6, 200612. https://reader009.{domain}/reader009/html5/0330/5abdcecf5fa4e/5ab,20090120
-
7/26/2019 Formacion de Las Emisiones en Diesel
53/55
Measuring and Predicting Transient Diesel Engine Emissions 2009
51
13.
Akihama K., Takatori Y. Inagaki K. Sasaki S. and Dean A. M. Mechanism
of the smokeless rich combustion by reducing temperature, SAE 2001010655, 200114. Kitamura T, Ito T, Senda J and Fujimoto H. Mechanism of smokelessdiesel combustion with oxygenated fuels based on the dependence of theequivalence ratio and temperature on soot particle formation.International Journal for Engine Research, Vol 3. No 4, 200215.
Saric F. A detailed kinetic model for calculation of the soot partile sizedistribution function. Doctoral thesis, Lund University, 200616. Netzell K. Development and application of detailed kinetic models forthe soot particle size distribution function, Doctoral thesis. LundUniversity, 2006
17.
Fusco A., KnoxKelecy A.L. and Foster D.E. Application of aphenomenological soot model to diesel engine combustion, Internationalsymposium Comodia 94, 199418.
C.D. Rakopoulos, D.C. Rakopoulos and D.C. Kyritsis, Development andvalidation of a comprehensive twozone model for combustion andemissions formation in a DI diesel engine, International journal of energy,2003:27, 200319. Ericson C., NOx modelling of a complete engine/SCR system, licentiatethesis, Lund University, 2007
20.
Lapuerta M, Hernandez J J and Armas O. Kinetic modelling of gaseousemissions in a diesel engine, SAE paper 2000012939, 200021. Egnell R. On zerodimensional modelling of combustion and NOxformation in diesel engines. Doctoral thesis, Lund University, 2001
-
7/26/2019 Formacion de Las Emisiones en Diesel
54/55
-
7/26/2019 Formacion de Las Emisiones en Diesel
55/55
Measuring and Predicting Transient Diesel Engine Emissions 200930.
Hiroyasu H., Kadota T. and Arai M., Development and Use of a Spray
Combustion Modeling to Predict Diesel Engine Efficiency and PollutantEmissions, Bulletin of the JSME, Vol. 26, 198331. Desantes J. M., Arregle J. and Molina S. Influence of the EGR rate, oxygenconcentration and equivalent fuel/air ratio on the combustion behaviourand pollutant emissions of a heavy duty diesel engine, SAE 2000011813,200032.
Boulouchos K. and Kirchen P. A phenomenological mean value sootmodel for transient engine operation, MTZ 070812008, Vol. 69, 200833. Wenzel S. P., Modellierung der Russ und NOxEmissionen desDieselmotors, Doctoral thesis, OttovonGuerickeUniversitt Magdeburg,2006
34.
Westlund A and ngstrm HE, Evaluation of techniques for transientPMmeasurements, SAE Paper 2008011680, 2008