Laser Plasma Initiated Ignition of Engines.doc

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LASER PLASMA INITIATED IGNITION OF ENGINES

1. INTRODUCTION

An essential fraction of the surge-current pro uction is !ase on co"!ine heat an

po#er plants #here the "echanical energ$ of internal co"!ustion engines is e"plo$e to po#er electric generators% an the e&haust gas is use for the e&ploitation of ther"al

energ$' (onse)uentl$% the efficienc$ of such po#er plants an hence of the engines "ust

!e as high as pos-si!le #hile the pollutant e"issions shoul !e *ept lo#' Such

re)uire"ents cannot !e reali+e satisfactoril$ an$ "ore #hen using con,entional engine

ignition techni)ues' Spar* plugs reach their li"its at the necessar$ high ignition pressures

e"an ing for e&cessi,el$ high ,oltages' o#e,er% there are se,eral alternati,e concepts

li*e plas"a ignition% high-fre)uenc$ ignition% Diesel "icro-pilot ignition an LASER ignition #hich "ight contri!ute to an i"pro,e-"ent of the o,erall efficienc$' To our

*no#le ge% LASER ignition represents the "ost pro"ising future ignition concept out of

a nu"!er of reasons' The "ain a ,antages of LASER ignition% a"ong "an$ others% are

perfor"ance enhancing high effecti,e "ean pressures in the co"!ustion cha"!er as #ell

as the feasi!ilit$ of ,er$ lean "i&tures lo#ering the fla"e te"perature an conse)uentl$

the NO . e"issions'

In general% the "echanis" of LASER ignition is !ase on non-resonant gas !rea* o#n of

the tightl$ focuse pulse /ns0 LASER !ea"' Initial electrons a!sor! photons to gain

energ$ ,ia the in,erse !re"sstrahlung process' These energetic electrons can ioni+e gas

"olecules lea ing to the !rea* o#n in the focal region ,ia the electron casca e gro#th' It

is i"portant to note that this process re)uires initial see electrons' These electrons "ight

!e pro uce fro" ther"-all$ heate or linearl$ ioni+e i"purities li*e soot or ust in the

gas "i&ture' The plas"a for-"e !$ the "entione "echanis" can ignite the co"!usti!le

"i&ture' Plas"a iagnostics re,ealing shoc* #a,e% ignition *ernels an fla"e propagation #as o!ser,e !$ high-spee Schlieren photograph$' The ifferent phases of

LASER ignition can !e efine in chronological or er1

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FIG3RE 2'21 /A0 M3LTI-E.POS3RE S( LIEREN IMAGE OF S O(45A6E

EMISSION ARO3ND A PLASMA SPAR4 IN AIR AT 27 8AR IN 977-NS STEPS

AFTER IGNITION' /80 S( LIEREN IMAGE OF A STOI( IOMETRI( MET ANE-AIR MI.T3RE AT 27 8AR% 977 :S AFTER IGNITION' /(0 PLANAR LASER-

IND3(ED -FL3ORES(EN(E /PLIF0 IMAGE OF O -RADI(ALS; IN ALL (ASES%

T E LASER 8EAM ENTERS FROM T E RIG T AND SIDE'

i' Electric !rea* o#n an energ$ transfer fro" LASER to plas"a

ii' Shoc*-#a,e generation an propagation

iii' Gas $na"ic effectsi,' (he"ical in uction of !ranching chain reactions of ra icals lea ing to ignition

,' Tur!ulent fla"e initiation

In or er to illustrate plas"a for"ation an the onset of co"!ustion% the co"!ine

Figure is epicte ' The left i"age /a0 threrein sho#s a "ulti-e&posure i"age ena!ling the

o!ser,ation of a shoc*#a,e in 977-ns steps' The te"poral e&pansion of the fla"e *ernel is

also illustrate using Schieren photograph$ /!0 an PLIF i"aging /c0' The LASER pulse

entering fro" the right ha an energ$ E p < 2=7 ">% the initial pressure of the "i&ture

a"ounte p in ? ='@ !ar an the relati,e air fuel ratio B ? 2'@'

Non-resonant !rea* o#n in gases re)uires intensities of se,eral 27 22 5 c"C in the focal

regi-on to !e pro,i e !$ a focuse LASER pulse #ith "ini"u" energ$ E p ? 7'2 "> an

pulse urations p 27 ns' For pulses o#n to se,eral ps the !rea* o#n threshol

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increases accor ing p-2 ' E&peri"ents re,ealing the "ini"u" pulse energies MPE for

ignition of ifferent fuel-air "i&tures /"ethane-air% h$ rogen-air% "ethane-h$ rogen-air0

un er engine li*e con itions ha,e !een reporte in for"er #or*% partl$ !$ our group'

Further"ore% LASER ignition for !iogas applications has also !een stu ie successfull$

2H ' 5hile plas"a can !e for"e !$ pulse energies of onl$ a fe# 277 :> % the

e,elop"ent of a fla"e *ernel re)uires the "ore energ$ in or er to ignite the "i&ture the

higher B an the lo#er the te"perature T'This circu"stance is clearl$ illustrate in Figure'

Moreo,er% for stoichio"etric "i&tures MPE is in epen ent of T% as it can !e seen in the

figure'

The higher p in an B are% the higher is the engine efficienc$ an the lo#er are the NO .

e"issions% B J is esira!le' In or er to ensure relia!le ignition for ,arious operation

con itions of the gas engine% a ns-LASER pulse #ith E p ? 9 K 27 "> is necessar$'

Further"ore% #e coul pro,e that shorter ns-pulses /


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Q A separate ignition LASER is "ounte on e,er$ c$lin er hea !eing supplie !$ an

e&ternal pu"p source /LASER io e0% ter"e LASER spar* plug'

Q LASER unit an engine are separate an the ignition pulses are transporte ,ia

optical fi!ers to the c$lin ers'

The conceptual i eas an e&peri"ents ealing #ith the e,elop"ent of a co"pact ignition

LASER can !e foun in our recent pu!lication' Such a LASER s$ste" "ust !e s"all in

si+e an has to !e "anufacture in a ro!ust for" in or er to #ithstan the a ,erse

influences on the c$lin er hea ' The regi"e of short ns-pulses can !e #ell co,ere !$

passi,el$ -s#itche LASERs #hich are% ue to their nature% ro!ust% si"ple in esign an

cheap' The pu"p pulse generating the in,ersion in the LASER acti,e "e iu" can !e propagate through a con,entional otical step-in e& fi!er fro" an e&ternal LASER io e

to the soli -state LASER'

The transportation of ignition pulses ,ia optical fi!ers i"plicates so"e plain a ,antages%

!ut as a "atter of fact% the restricti,e factor is the a"age threshol of the fi!er "aterial'

Such pulses "a$ in uce intensities in a s"all core far !e$on 2'9 27 5 c"C #hich is

the !rea*- o#n threshol of fuse silica for ns-LASER pulses' An enlarge"ent of the solifi!er core area% #here the !ea" is gui e % in or er to re uce the intensit$ is not a apti,e

since for fi&e #a,elengths higher core ia"eters i"pl$ "ulti-"o e !ea" profiles' The

focusa!ilit$ of the ignition pulse epen s strongl$ on the !ea" profile% hence

isa ,antageous "ulti-"o e ra iation #oul enhance the MPE'

o#e,er% there e&ist so"e concepts for the transportation of ignition LASER pulses ,ia

hollo#-core optical fi!ers 'The e"plo$"ent of Photonic 8an Gap /P8G0 fi!ers #oul !ring so"e a ,antages li*e single-"o e propagation' As reporte in a recent paper !$ our

group %the highest trans"itte single pulse energ$ of appro&i"atel$ 7'H "> is not a e)uate

for LASER ignition' The further i"pro,e"ent of hollo# fi!ers has to !e successfull$

achie,e !efore an increase of the eli,ere pulse energ$ up to a le,el sufficient for

LASER ignition can !e reali+e ' In contrast to the ignition pulse% the pu"p !ea" can

!e gui e ,ia con,entional optical fi!ers since the intensit$ is lo#er !$ a!out si& or ers of

"agnitu e' In a ition% the pu"p !ea" ge-nerate fro" io e !ars generall$ is not of

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single "o e nature an therefore onl$ "ulti-"o e fi!ers are appropriate'

A further cornerstone of LASER ignition is the #in o# representing the gate#a$ !et#een

the hot co"!ustion cha"!er an the LASER oscillator' Pre,ious in,estigations on the#in o# inclu e co"!ustion-si e eposits an their for"ation as #ell as LASER-in uce

eposit for"ation' E,en a fe# eposits on the #in o# lea to su!stantial a!sorption of

LASER ra iation' Deposits on the co"!ustion #in o# are not #ell efine su!stances

ranging fro" nutt$% car!onaceous "atters to s"all inorganic particles'

FIG3RE 2'@1 S( EMATI( ILL3STRATION OF T E P3RS3ED LASER IGNITION

(ON(EPT' EA( LASER ON A ( LINDER EAD IS S3PPLIED 8 T E E.TERNAL

P3MP SO3R(E 6IA A SEPARATE OPTI(AL FI8ER' T E M3LTIPLE.ER

DISTRI83TES T E P3MP RADIATION FROM T E (OMMON P3MP SO3R(E TO

EA( FI8ER .

The ignition LASER s$ste" "ust fulfil the afore "entione re)uire"ents in or er to

ensure relia!le operation un er engine-li*e con itions' 3nfortunatel$% such s"all ns-pulse

LASER s$ste"s are co""erciall$ not a,aila!le' Moreo,er% custo"ar$ LASER s$ste"s

fulfilling the nee s of LASER ignition are large in si+e% hea,$% e&pensi,e an consu"e

se,eral *ilo#atts of electrical po#er' Thus the nee to e,elop an ignition LASER of our

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o#n e"erge

An en -pu"pe fi!er-couple passi,el$ -s#itche soli -state LASER see"s to !e the

"ost pro"ising can i ate' Such a LASER s$ste" satisfies the re)uire"ents li*eco"pactness% ro!ustness% lo# cost an the pu"pa!ilit$ ,ia optical fi!ers' Figure sho#s the

sche"atic concept of LASER ignition appl$ing a LASER spar* plug #ith an e&ternal

pu"p source' 8esi es this a ,antageous concept% other research groups fa,our the i ea of a

trans,ersall$-pu"pe soli -state LASER for ignition' To our *no#le ge% trans,ersall$-

pu"pe LASER s$ste"s are #orse in !ea" )ualit$ an pulse uration' Moreo,er% such

si e-pu"pe s$ste"s are "ore co"ple& an e&-pensi,e than en -pu"pe ones' The

follo#ing section intro uces the sche"atic setup of the LASER s$ste" as #ell as thee&peri"ental setup'

2. EXPERIMENTAL SETUP

A suita!le LASER ignition s$ste" co,ering the re)uire"ents li*e ro!ustness% si"ple

esign an co"pactness is a fi!er couple % en -pu"pe passi,el$ -s#itche soli -state

LASER' Figure = epicts the sche"atic setup of such a LASER unit' 8esi e the financial

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aspect the a,aila!ilit$ of the co"ponents is a "a or point #ithin the selection criteria'

Therefore N @U1 AG as LASER ac-ti,e "e iu" an (r =U1 AG as passi,e a!sor!er has

!een chosen'

FIG3RE '21 S( EMATI( 6IE5 OF T E LASER S STEM' T E P3LSE LENGT

5AS MEAS3RED 8 A P OTODIODE 5 ILE FOR T E ENERG

MEAS3REMENT A P ROELE(TRI( SENSOR 5AS 3SED'

The N - oping of the LASER cr$stal #as aroun 2'7 at' V an the fluorescence lifeti"e

of the upper LASER le,el =F@ a"ounts to fl < 99 :s' The a!sorption line of N @U1 AG

is centere aroun B io e ? H7H n" !eing suita!le for a GaAs high po#er LASER io e as

pu"p source' Pu"p pulses up to a po#er P pu"p ? @77 5 at 977 :s can !e generate !$ this

LASER io e' Since the LASER io e sho#s a te"perature gra ient of 7'@ n" 4%

te"perature sta!ili+ation #as necessar$' Further"ore% as sho#n in Figure 9% te"perature

sta!ili+ation ena!les the e&act a - ust"ent of the io e e"ission line an the cr$stal

a!sorption #a,elength #ith the ai" of highest con,ersion efficienc$' A step-in e& fi!er for

transportation of the pu"p pulse #ith a core ia"eter of 77 :" an a nu"erical aperture

NA ? 7' #ere e"plo$e '

Different aspheric colli"ating lenses #ith effecti,e focal lengths l f%eff ? 2'9 W 2H ""

#ere teste % ho#e,er% the lens #ith l f%eff ? H "" an NA ? 7'9 turne out to !e the "ost

suita!le for in-coupling the pu"p !ea" into the LASER cr$stal'

The in coupling "irror of the resonator #as irectl$ coate onto the cr$stal in or er

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to re uce the length of the s$ste" an % as a result% the pulse uration' Moreo,er% the en

face of the cr$-stal #as coate #ith an anti-reflection /AR0 la$er for the H7H n" pu"p

!ea" re ucing the re-flection losses' The output-coupler #as separate fro" the cr$stals

since the a ust"ent of the "irror allo#s fle&i!ilit$ in pulse energ$ an !ea" profile'

Passi,e a!sor!ers #ith initial trans"issions T 7 ? 9V W X7V an output-couplers

#ith reflecti,ities R ? 9V W 7V co,er the fiel of LASER operation' All passi,e

a!sor!er cr$stals #ere coate #ith an AR la$er for the LASER #a,elength B e" ? 27 = n"'

In or er to achie,e highest output po#er the in-coupling optics #ere al#a$s a apte #ith

respect to the resonator con itions /length% reflecti,it$% initial trans"ission0' The results are

su""ari+e in the ne&t section'

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FIG3RE ' 1 EMISSION AND A8SORPTION SPE(TR3M OF T E LASER DIODE

AND T E LASER (R STAL% RESPE(TI6EL ' AT A EMISSION 5A6ELENGT

DIODE ? H79'X NM% (ORRESPONDING TO A DIODE TEMPERAT3RE /T DIODE 0 OF

APPRO.IMATEL Y(% T E MA.IM3M A8SORPTION /MINIM3M

TRANSMISSION0 5AS NOTED'

3. RESULTS

The "ain para"eters strongl$ influencing the "a&i"u" output po#er are the pu"p

uration pu"p % T7 an R' At a pu"p po#er P pu"p ? X7 5% pu"p ? @77 :s% R ? 97V an T 7 ?


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=7V the "a&i"u" pulse energies of appro&i"atel$ '2 "> #ere $iel e ' This result

correspon s to an optic-to-optic efficienc$ Z optic ? X'2V' The length of the resonator #as

a!out 29 "" #hile the length of the N - ope LASER ro ha,ing [ ? "" thic*ness #as

9 ""' Pulse urations p < 2'7 ns #ere achie,e lea ing to a pea* po#er P p% pea* M5'

An a itional para"eter influencing E P is the cr$stal oping !eing t$picall$ !et#een 7'H W

2'= at'V' o#e,er% so"e "anufacturers offer '9 at'V- ope N 1 AG cr$stals #hich

result in a higher cross-section an % therefore in higher a!sorption efficienc$% !ut the

fluorescence lifeti"e fl ecreases ra"aticall$' For our purposes% a co"pro"ise !et#een

the a!sorption efficienc$ an the storage a!ilit$ of the upper LASER le,el ha to !e foun '

It turne out that N oping aroun 2'= at'V le to the "a&i"u" output po#er'

An increase of P pu"p ? @77 5 an pu"p ? @77 :s lea to E p < "> at p < 2'7 ns using the in-

coupling lens sho#n in Figure =% !ut #ith an effecti,e focal length l f%eff ? =' ""' 5e foun

out% that E p is ,er$ sensiti,e to pu"p ue to the ther"al loa of the LASER cr$stal' Figure

epicts the e&peri"ental ata of E p ,ersus pu"p ' o#e,er% a!o,e pu"p < 77 :s E pre"ains constant at a ,alue of roughl$ ">' Further"ore% the ti"ing itter !eing the

fluctuation of the te"poral position of the pulse increases ra"aticall$ for large pu"p an

the LASER s$ste" !eco"es unsta!le' The afore "entione in-coupling optics pla$s also a

"a or role concerning the LASER output' E"plo$ing the 22 an 29'@ "" aspheric lenses%

the $iel e pulse energ$ #as "uch lo#er than #hen using the =' an H "" lens' The

focal spot of the 22 an 29'@ "" lenses #ere larger than the en -face of the cr$stal an %

therefore% a certain a"ount of pu"p energ$ coul not !e e"plo$e to create in,ersion'


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FIG3RE @'21 T E P3LSE ENERG /E P0 6ERS3S T E P3MP D3RATION /\ P3MP 0' AT

A (ERTAIN \ P3MP < 77 :S E P DOES NOT RISE AN MORE'

3sing a [ ? 9 "" thic* an l ? 27 "" long LASER ro % the pu"p !ea" a!sorption

efficienc$ coul !e increase an % as a conse)uence% E p' On the one han % a lens #ith a

higher effecti,e focal length /an conse)uentl$ larger focal spots0 can !e applie an % on

the other han % lon-ger cr$stals can a!sor! the pu"p !ea" "ore efficientl$ accor ing to

8eer]s la#'

E"plo$ing output couplers #ith R ? =7 an 7V% an a!sor!ers #ith T 7 ? 27 W 7V% as sho#n

in Figure X% le to E p ranging fro" X to 27 ">' These output ,alues correspon to Z optic 22V'

In this case again pu"p ? @77 :s% !ut in contrast to the ""-thic* cr$stal% an increase of E p in

the range of pu"p ? 77 W @77 :s #as o!ser,e ' This e&peri"ental fact "ight !e ue to the

larger in,ersion ,olu"e an the re uce ther"al loa per ,olu"e' The use in-coupling lens

ha an effecti,e focal length of l f%eff ? H "" an a NA ? 7'97 ena!ling goo illu"ination

of the LASER ro ' p is in the or er of 2'9 ns correspon ing to a pea* po#er P p%pea*

M5 representing a respecta!le ,alue for such a co"pact soli -state LASER' The slightl$

longer pulse uration

p ?2'9 ns in co"parison to the a!o,e "entione pulses #ith energies of an "> is ue

to a higher T 7 ? 7V of the satura!le a!sor!er'

Recapitulating% this i"pro,e"ent in perfor"ance is ue to the thic*er LASER

cr$stal% on the one han % an ue to a "ore efficient in-coupling process% on the other han ' A further interesting aspect is the intensit$ profile of the outgoing !ea"' 5hile the

intensit$ istri!ution for pulse #ith E p ? "> is )uasi-Gaussian /Figure Ha0% the profile for


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"a&i"u" o!ser,e pulse energ$ therein #as E p ? 2@2 :> at p ? ns an an Z optic ? 'XV'

Another s"all N - ope LASER has !een pu!lishe in @9 using a 77 5 )uasi-c#

io e' The achie,e output para"eters of the 22 "" long resonator #ere pulse urations

of a!out p ? = ns an pulse energies E p ? ='9 ">' An interesting pu"p arrange"ent% #here

the LASER ro #as pu"pe at !oth en -faces lea ing to !etter a!sorption efficienc$ can

!e foun in @@ ' Ne,ertheless% such a LASER setup is not useful for our purposes since

the ignition LASER shoul !e asse"!le into a s"all% ro -li*e housing'

5. CONCLUSION AND OUTLOOK

This paper re,ie#s the "ilestones of LASER ignition in short% !eing !ase on LASER-

in uce plas"a for"ation% an the correspon ing ignition para"eters' These facts gi,e

infor"ation a!out the rele,ant LASER specifications' The !asic re)uire"ents #hich are

pulse energies aroun 27 "> at pulse urations of a!out 2 ns #ere reali+e ' For a realistic

application to co"!ustion engines% the LASER s$ste" shoul !e asse"!le in s"all

housing #ith i"ensions of a con,entional spar* plug' Moreo,er% the LASER s$ste" has

to #ithstan the etri"ental influences of the engine inherent ,i!rations as #ell as ther"al

an "echanical stresses' One of the pro"ising #a$s to "eet these re)uire"ents #ill !e a

"onolithic LASER esign'


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REFERENCES

2 ' 4OPE(E4% et al'% ̂ LASER ignition of "ethane-air "i&tures at high pressures

an iagnostics^% >ournal of Engineering for Gas Tur!ines an Po#er% 6ol' 2 X/20%

pp' 2@- 2 % / 77=0'

' 4OPE(E4% et al'% ^LASER-in uce ignition of "ethane-air "i&tures at pressures

up to =MPa^% LASER Ph$sics Letters% 6ol' 2@/220% pp'2-9% / 77@0'

@ ' 4OPE(E4% et al'% ^LASER-sti"ulate Ignition in a o"ogeneous

(harge (o"pression Ignition Engine^% SAE International% / 77@0'

= P' D' RONNE % ^LASER ,ersus con,entional ignition of fla"es^% Optical

Engineering% 6ol' @@/ 0% pp'927-9 2% /2 =0'

9 M' LA(4NER% et al'% ̂ In,estigation of the earl$ stages in LASER-in uce ignition

!$ Schlieren photograph$ an LASER-in uce fluorescence spectroscop$^% Optics

E&press% 6ol' 2 % pp'=9= -=99X% / 77=0'

M' LA(4NER% et al' ^Optical Diagnostics of LASER Ignition for Future

A ,ance Engines^% Procee ings of ASME Fall Technical (onference% / 77=0'

X L' >' RAD_IEMS4I% et al'% ^LASER-In uce Plas"as an Applications^% Tho"pson

8rian >'% 2 e '% Ne# or*% Marcel De**er Inc'% pp' ==9% IS8N1 7-H =X-H7XH-X% /2 H 0'

H T'.' P 3O(% ^LASER-in uce spar* ignition fun a"ental an applications^%

Optics an LASERs in Engineering% 6ol' ==/90% pp'@92-@ X% / 77 0'

D' 8RADLE % et al'% ^Fun a"entals of high-energ$ spar* ignition #ith

LASERs^% (o"!ustion an Fla"e% 6ol' 2@H/2- 0% pp'99-XX% / 77=0'

27 ' 4OPE(E4% et al'% ^LASER ignition of "ethane-air "i&tures at high

pressures^% E&peri"ental Ther"al an Flui Science% 6ol' X% pp'= -97@% / 77@0'

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