4-54 Electrical Charge on Diesel Particules

SOLS: Formation and Reactivity 2nd Int. Aerosol Conf. Berlin i Pergamon Journals Ltd. Printed in Great Britain

ELECTRItrAL CHARGE AN DIESEL PARTICLES

D.B. Kittelson and H.E. llsan Departrnent of Hechanical. Engineering University of Hinnesotâ Hinneapolis, l"lN 55453 u.5. A. N. Cslling:

Carnbridge University Engineering Laboratory trarnbridge CBZ lPZ Engl and

fiRDs:

CHARGE! DIESEL

rducti on

rl exhaust particles carry a significant ele'ctrical charge, recently thig charge was viewed prirnarily as a nuisance t made accurate sampling and characterization a'f the paro{ this charge may l= rnore dif f icult. Hsr'rever, the : i.inportance fer sther reascrns: a)Fresenc€! it gives new insights ints temperature particle charging, b) it may help us ts underI the particle fsrrnatisn procÊss itsel{" c} it may make it rble tr: build sirnple sensors fsr feedback csntrol of diesel te=, and d) it rnay make it pes=ible to build corcnaless This :rcstatic traps {or diegel particle emigsisn csntrsl. ' examines a)rb)n and c! briefly. -i mental

:ypes of experiments have been per{ormed! determinations s{ 1e levels and di=tributinn= cn particleg in diluted exhaust leg and ingtantaneous ,neasurements af the charge present in a Lng exhaust streâm. 1e levels were 'fcund rasuring particle :ntrstirrns upstrean lswnstream of a rdrical electrostatic erisrF. Ccncentra3 Here measured using lectrical aeroscl lzer! a ctlndensation ei ccunter, and 3rs. Ëharge dis,rtisn measurements made using the lity analysis system n in Figure 1. Intaneoug charge conration in the exhaust reasured using an inion senssr like that FIBURE I - l.lobility Anelyeis system 939

940

in Figure 2. The induction senscr ig deeigned so that the charge induced {fn the ring equals the the tstal charge contained within the ring" shcrwn

IlING SENSON

ReslrltE

Several different diesel engines were tested and INSIJLATOR al 1 sho+red =i gni { i cant particle= charge Ên the einitted- The charged {ractions were highegt at high engin€ ltrâds: under these conditiBns trom 7O FIEURE ? - Induction Seneor to ?O 7. of the particles by mass were charged. Charge distributiong were measurÊd tor r rang€ af particle sizes and were feund ttl be biperlar and =ymA typical charge di rnetrical with respect tc charge polaritytribution for {t.133 urn diam. particles is ghtrwn in Figure 3. average charge per particle increased with particle size; ran *rsm abaut l for û.O4 r-rrn diarn- particle= to abeut 4 {or Û.5 ur dism. particlee. The varia,tisn ef average charge r+ith particle size and the shape o{ the charge distribution is sinilar ta trf a high te,mperature Ësltrmann equilibrium charge digtributim *or three dif{er {Lr-ri and Pui, 1?74}. Baltemann digtributisns ternperatures are shswn in Figure 3- The experirnental results fl rea=onably well ts the thesretical distribution far 15ûO K althaugh the experirnental distribution gives çomewhat higher concentratisng ef highly charEed particles. Thig rnismatch becolæs

-8

-6

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FIGURE

HAR GES

o

2

4

6

I

PER PARTICLE

3 - Experimental and Theoretical trharge Dlstrlbutistr

941

for smarler particle= (o.04 o.1o urn diarn. ) and in;:"gses for larger particles to.2 - û.4 - urn diarn. ). srnatr diegel ntr-::.cles tend ts be raughly spherical clusters, while larger Thu= depar=.:= tend to more s{ a chain agglomerate structurell-;-e"s f rtrm the Bsltzmann distrihution, which is calculated asw!r;n:.rg particle=" rnight be expected to be rnast sErvere =pherical rrn larger particles. Sirnilar charge distributions clere sbserved rv- à11 engines tested. "nnteq::.qible

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:hargÊ digtributiong obgerved could be produced by bipolar ;:r.r-Eing o* the particles {ormed by combustion early in the com* 1;:: j.Bn protress. Hydr(]carbf]n sornbustiEn Frcrduces high con:gl---ratisns of bipclar isns. Peak isn csncentration= in excegg Î'b "ii1? ml*a have been sbgerved in engines =park ignition l-*nrents and Ërny, I9-74r - Bipclar charging theary indicates that irr:-:zmann equilibrium shsuld be achieved i{ the ian concentratisn :... me product exceeds abcut lO' ml -r s (Liu and pui ! l?74) - Thus" *r: :he peak ion csncentraticn, equilibrium be attained in =hsuld ul:ut ltt uË! or about (t.1 crankangle degree at lEIOO rprn- This :;,nËares ta e csmbu=tion duratisn of absut éû degrees- As the ::dldr:Jstion products expand and cool a.fter csmbuçticn the isn con:s-:ration {alls rapidly by ien-icn reccmbination and icrn:*,r'-i.cIe attachment. This reduces the charging rate until it can At thig tirne, the charge dis't ionger maintain equitibriun. :.-::ution ".freeae=" in a {srm cerre=Fonding to the current :*Illlperature. Assurning that this mechanisn is a csrrect, our ex:,e-:.rnental results indicate a "freezing" ternperature tr+ àbout :,,,1 K. If the mechanissr is csrrectn the bipolar charging rr]:ess would neutralize any chàrgr that might have been as*:,::èted with particle ferrrnation. Con=equently, it wsuld be di{--:-i1t ta learn anything abaut the rele Ef charged species in :.È-r-:-iclÉ {srmation frern rneasurernents Éf charge in the exhauçt. frEi-=urements rnade

t-j{erent

witn È-tre induction Eensor reveal feature= quite {rom what rnight have been expected frorn the charge di=-

:": butian .neasl-lrements alC3ne. ;-:ure 4 ghor.cg typical charge rlIE:Ë{Jre{nents nade with the 5'8,- ssr. General I yo a sharp I - r -=e of pcsi t i ve charge i *:llswed by a much gmaller =but

irràder pulse of negative :-argr. The height o{ the r,:sitive pulEe correlates wel1 u:--h the csncentration of par:-:leg in the exhaust. *'e po=itive pulse of charge :::.ncides with the exhaust r-:wdourn procslss, i.e., the *-Cden drop of cylinder press*-.'e which sccurs when the exra:gt valve first open:i. Peak :*arge levels indicated by :-ll*

8OO RPM

4.O

B

FIBURE

98 Nm Eosch 1.4

4 - €iengor Rrsponre

942

the senssr csrresFond to a charge imbalance o{ about I eler charge per particleThis appeirs inconsistent r+ith the s .'f the observed charge distributisns. rt be that the charal detected by the sens.'r is agsocisted .*ith may particres sutside tfi rang* of our measurernentsn i-e.n particle= rarger than abeut um diarn.. Typically about 1O 7. of the masg o{ particles emit by a diesel engine is in thi= range. such particres FroÈa fsrmed by depositisn sf smaller cnmbustion generated are particres the wallg crf the ccmbustion chamber" {allowed by surface gloneration and subsequent reentrainment. Thermoerectricag_ or might lead ts charge separation and cfrnsequent :lh:r e{fect= charging during reentrainrnËnt- Alternatively, these pàr!i.1" ticles might be charged by irnpacting e,xhaugt valve surfaces during the blswd.'*n proce=s- bre are currentry attempting to mr time resolved charge distributisn rneasurefi*nts f.r particre= r+ ts about l-o um diam- in crder ta help t'understand the s'ur.Ë of charge irnbalance during blowdownCencl usi sns

Farticle charge distribution neasurernents made sn cliluted exhn frcm geveral different type diegel engines reveal a eymmetrical bipclar charge di=tribution which can be clesely appraximated a high temperature Boltzrnann equilibriurn charge distributisn. believe that this distributisn resurts frorn charging by biperr flarne ions earry in the cgmbustion Frt:ceEE. on {he other hand, instantaneous charge measurements rnade in engine exhaugt with n inductian senscrr show a significant excegs oi positively chargd particles during exhaust hlowdat*n. Thig {eatuFe be asgsciated reent*rained particles charged by gurface may rerated pFBce5sesparticle= would be tc]o large to be included in our charge v''"àucn di=tribution rn*asurernents which were con{ined to particles smaller the about O.4 um aianyr. lJhatever the origin the signar produced by the sens.,r, its Fesponse correlates 'f rather well with the particle concentration in the exhaust- lgnsequentry, it might be used in a crosed-ro*n engine control system in r,rhich gmoke emissisns are ngt allor,led r exceed some predetermined value. References

clements, R-H- and P.R. srny (1976) "The Variation gf ronizatim r+ith Air./Fuel Ratio for a spark rgnitian Engine,., J. Asolied Physics 47, sOs Liur B-Y-H- and D-Y-H. pui (tgi4, "Erectrical Aerosols", J. Aerosol Science Sr 465

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