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Combustion and Flame 140 (2005) 24–33www.elsevier.com/locate/jnlabr/cn

Kinetic measurements in homogeneous charge compreof dimethyl ether: role of intermediate formaldehydecontrolling chain branching in the low-temperature

oxidation mechanism

Hiroyuki Yamada, Kotaro Suzaki, Hideki Sakanashi, Namil Choi,Atsumu Tezaki∗

Department of Mechanical Engineering, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku,Tokyo 113-8656, Japan

Received 4 September 2003; received in revised form 13 August 2004; accepted 13 September 2004

Available online 23 November 2004

Abstract

Oxidation of dimethyl ether (DME) in compression cycles has been investigated using an externally msingle-cylinder piston engine. With the principal aim being to elucidate the chemical mechanism takingduring two-stage ignition, in situ laser-induced fluorescence (LIF) monitoring of formaldehyde (HCHO) andposition analysis of the exhaust gas were performed. Measurements of formaldehyde fluorescence wereexciting theA1A2–X1A1 41

0 vibronic band near 352.5 nm. From the raw signal intensity profiles, the cangle-resolved concentration profiles were determined from the fluorescence lifetimes, spectral linewidBoltzmann population factors. It was shown that formaldehyde forms rapidly during the first-stage processis the cool ignition, and it disappears during the second-stage process, which is the thermal ignition. Thiwith the predictions of the chemical kinetics simulations, which use the detailed model for DME oxidatCurran et al. It also was confirmed that the composition of the exhaust gas is that which is expected foignition process, when the conditions are such that the onset of thermal ignition is suppressed. Mass specanalysis was applied to monitor emission of CO, CO2, H2O, and HCHO as well as consumption of DME aO2. The engine apparatus was operated mainly without thermal ignition in order to assess the cool ignicomposition. It was found that the dependence of all measured components on equivalence ratio and intemperature was consistent with the detailed model calculation. A notable feature is that the percentagesthat is consumed and the percentages of formaldehyde formed during cool ignition are nearly constant asamount of DME was varied, despite the differences in heat release rates and consequent final temperatbehavior can be explained by the fact that the accumulated formaldehyde can act as a terminator for treaction in the low-temperature oxidation mechanism. A simple chemical mechanism is proposed whiclimited number of rate parameters but adequately represents the observed results. 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

* Corresponding author. Fax: +81-3-5841-6293.E-mail address:tezaki@comb.t.u-tokyo.ac.jp(A. Tezaki).

0010-2180/$ – see front matter 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.doi:10.1016/j.combustflame.2004.09.009

H. Yamada et al. / Combustion and Flame 140 (2005) 24–33 25

Keywords:Compression ignition; HCCI; Dimethyl ether; Formaldehyde; LIF; Low-temperature oxidation; Chain reaction

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1. Introduction

Autoignition or compression ignition is a combution phenomenon in which chemical kinetics plathe dominant role[1]. The chemical mechanism oautoignition has been drawing attention becauseunderstanding of this mechanism is expected tocrucial in controlling HCCI (homogeneous charcompression ignition) engines[2–7]. In addition, itis relevant to conventional diesel ignition and egine knock research. Currently a typical autoignitiprocess is known to undergo two distinct stagesheat release, in which low- and high-temperature odation mechanisms govern the first and second starespectively. Whereas the chemical mechanism ahigh-temperature stage is common to most hydrobon fuels, the low-temperature oxidation (LTO) israther more complicated, fuel-dependent mechanIn LTO, key species such as RO2 (alkyleperoxy radi-cal) and QOOH (hydroperoxy alkyl radical) are dficult to detect. Identification of the elementary ractions frequently is impractical in systems suchan apparatus that uses a flash photolysis pump prTherefore, a variety of techniques are being appliesolve the ignition and LTO mechanism. The elaborprocess of extracting elementary rate constants fhydrocarbon–H2–O2 oxidation systems typically involves an indirect method[8].

Dimethyl ether (DME: CH3OCH3) has been considered as a potential alternative fuel for dieselgines because of its high cetane number, which mait suitable for compression ignition. It can leadreduced emissions of soot and NOx , compared toconventional diesel fuels[9,10]. The soot reductionis a direct result of the chemical structure of DMwhich has no C–C bond to act as a seed for polymization, which leads to polyaromatic hydrocarboWhile low NOx is not a feature that is intrinsic tDME combustion, DME provides the capabilityburning at a wide range of equivalence ratios withforming soot, and this leads to less NOx formationand improved application of postcombustion deNxprocesses.

Detailed chemical kinetics mechanisms for DMoxidation have been proposed by Dagaut et al.[11]and by Curran and co-workers[12–14]. Experimen-tal results of a jet-stirred reactor[11], a shock tube[15], and a flow reactor[13,14] were used to vali-date these models. DME exhibits typical two-staignition and negative temperature-coefficient (NTbehavior of the ignition delay time similar to heavifuels like n-heptane. It was demonstrated that inc

sion of the LTO mechanism effective at around 800is essential to account for these non-Arrhenius behiors. A published version of the Curran et al. mod[12] was applied to simulate HCCI operations[6,7]and concentration profiles of 11 species including ricals in a low-pressure DME flame[16]. The generaagreement between the model and a variety ofperiments suggests that the model is fundamencorrect. It is also noted that the detailed mechanof DME consists of less than 400 elementary retions, which is not excessive for a detailed chemmechanism. Given the above points, we regard Das a prototypical fuel that can be used to scrutinizeLTO mechanism taking place during ignition.

In this paper, we present results of DME oxidtion observed in an externally motored single-cylindexperimental engine. For mechanistic informationthe compression ignition process, LIF measuremof formaldehyde were conducted in homogenecharge compression cycles. Corrections for quening and other effects influencing the LIF signal usiexisting procedures[17–19] are described, as thecorrections are essential for obtaining accurate ccentration profiles in the pressure-varying enviroment. In addition, mass spectrometric analysisapplied to quantify several species emitted at thehaust. The chain reaction mechanism governing Lwas clarified by the observations and a simple formlation of the mechanism extracted from the detaimodel is discussed.

2. Experiment

A single-cylinder, side-valve type four-stroke egine of 150 cm3 displacement (Fuji Heavy Industries, EY15-B2) was modified for HCCI operatioand spectroscopic measurement in the cylinder.whole system is schematically shown inFig. 1. Theoriginal engine head equipped with a spark plug wreplaced by a metal plate having ports for three quwindows and a pressure transducer. The engine cis externally driven by a 750 W induction mot(Nippon Densan, RXMR-750) in order to rotate afixed rate regardless of the occurrence of autoigniand consequent internal power generation. The enspeed was usually set at 600–1000 RPM. The land the fluorescence detection system are syncnously triggered by a circuit counting pulses fromrotary encoder connected to the crank axis. Thegine head and the intake are electrically heatedestablish the higher temperature that is necessar

26 H. Yamada et al. / Combustion and Flame 140 (2005) 24–33

Fig. 1. Schematic of the modified engine experimental setup for LIF and mass spectrometric measurements.

atior,ndh-

omup-theinen-the

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en-lyesat

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aintedofs

entersidfordergy

c-the

spontaneous ignition at the current compression rof 6.8. DME is supplied from a commercial cylinderegulated through a 2 SLPM mass-flow controller apremixed with air at the intake. The lubrication mecanism was modified at the nozzle splashing oil frthe crank case so that a minimal quantity of oil is splied to the cylinder to reduce the contamination atwindow surface. It is possible to operate the engcontinuously over 1 h without decreasing the intesity of the probe laser beam transmitted throughwindows and fluorescence signals.

The modified head plate has two horizontaaligned 6-mm aperture quartz windows that passprobe laser beam parallel to the piston top surfaand a 9-mm aperture window to detect the fluorcence at a normal angle to the probe beam. The plight source is an excimer laser-pumped dye la(Lambda Phisik, LPD3002). The probe radiationaround 350 nm is directly produced using a UV dsolution (p-terphenyl and DMQ mixture). The fluorescence is detected by a miniature photomultipmodule (Hamamatsu H5783-56) through a combition of glass filters arranged to cut the probe and althe passage of longer wavelengths. The intensitthe probe pulse passed through the cylinder is mitored with a photodiode. These signals are resptively averaged by gated boxcar integrators (StanfResearch Systems, SR250) and stored in a PC.photomultiplier is supported separately from thegine to avoid mechanical vibration while opticalcoupled to the observation volume with two lenshaving a focal length of 30 mm. It was confirmed ththe vibration of the engine does not influence theserved fluorescence signal after being averaged.

A quadrupole mass analyzer (Anelva M-40QA-M) was used to measure gas composition atexhaust. The exhaust gas pressure is reducedsampling pipe and introduced into the analyzer chaber via a leak valve. At the standard electron impionization energy of 70 eV, DME generates highfragment signals than the parent peak ofm/e = 46, sothat signals of smaller products are obscured. Hewe set the EI energy at 12 eV (nominal), whereparent DME signal is far higher than any fragmesignals and sensitivity remains at a sufficient levelthe current measurements. Only a signal atm/e = 45was observed at 20% intensity of the parent sigwhich has no influence on the detection of othspecies observed in this study. Since this energalready below the ionization limit of N2, CO can beselectively detected atm/e = 28. DME, CO2, O2,HCHO, CO, and H2O were measured atm/e = 46,44, 32, 30, 28, and 18, respectively. In order to obtcorrect mole fractions, calibrations were conducfor DME, O2, and HCHO using standard samplesknown mole fractions. Formic acid (HCOOH) waadditionally included as an intermediate in the recversion of the mechanism of Curran and co-work[13,14]; however, the mass number of formic acis the same as that of DME and the sensitivityHCOOH was found to be two orders of magnituless than that of DME at the current ionization eneof 12 eV. Therefore, the signalm/e = 46 is essentiallyfrom DME. Typical uncertainties estimated from flutuations of the mass signals and repeatability ofcalibrations are 10% for DME, O2, and HCHO, 5%for CO2 and H2O, and 20% for CO.

H. Yamada et al. / Combustion and Flame 140 (2005) 24–33 27

as-on-Nasfit

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-ho

es-ula-etion

atureatic

andni-relts.is

uelsig-

htm-

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t aer-ak

ureit

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dinin

ec-dthy-ther-

cog-

owsnal

esof

3. Numerical simulation

Numerical integration of the homogeneous gphase reaction scheme for DME oxidation was cducted by using the SENKIN code in the CHEMKIII package[20]. The specific volume was treateda function of time given by a user-made routine tothe experimental compression cycle with the folloing formula[2–4,21],

V/Vc =[1+ 1

2(εc − 1)

{R + 1− cosθ

(1)− (R2 − sin2 θ

)1/2}],

whereVc is the volume at TDC,εc is the compression ratio, R is the ratio of connecting rod lengtto crank radius, andθ is the crank angle relative tTDC. In the present case,εc = 6.8 andR = 83/23.The intake valve closing time, at which the comprsion from atmospheric pressure begins in the calction, was adjusted to 125◦ BTDC so as to reproducexperimental pressure profiles. The detailed reacmechanism published in 1998 by Curran et al.[12]was used as the input scheme. Pressure, temperand species concentration profiles under the adiabassumption were calculated as a function of timethe time was converted to crank angle. Thermal igtion limits and final composition in the exhaust wealso derived and compared with experimental resuIt is noted that the occurrence of thermal ignitionvery clear and the selection of criteria, e.g., total fconsumption or temperature over 1500 K, has nonificance for the determination of limits.

4. Results and discussion

4.1. Pressure profile and ignition mode

Fig. 2 shows typical profiles of pressure and ligemission as a function of crank angle at different cobinations of equivalence ratio and preheat tempture. When no DME fuel is injected, the pressure sply varies with the inverse of the volume during tpiston motion. In the presence of DME, the pressrises above the simple compression profile, whiccaused by heat release during the fuel oxidation. Afuel-air equivalence ratio of 0.5 and an intake tempature of 480 K, two-stage ignition takes place; a wepressure rise is followed by a final strong pressrise. The first stage is called a cool ignition andcauses a partial consumption of fuel with some reual oxygen remaining. The second stage is a therignition, and it completes the combustion and thetal consumption of fuel or oxygen. Weak emissionlight is seen during the cool ignition stage, while

,

Fig. 2. Pressure and light emission profiles in piston copression of DME/air mixture at 1000 RPM. Fuel–oxygequivalence ratio and intake gas temperature are indicfor each curve. The emission sensitivity for the two contions is different.

strong emission of light occurs during the thermalnition stage.

The observed ignition behavior was reproducedthe chemical kinetics simulation.Fig. 3shows the cal-culated profiles of pressure, temperature, and spemole fractions under the same conditions as theperiments shown inFig. 2. The pressure rise and pepressure are considerably overestimated in thetially nondimensional calculation; nevertheless,two-stage ignition at higher load and the single cignition event at lower load are satisfactorily repsented. It was reported in a multizone model stuof HCCI combustion[22] that the ignition timing atthe center core well correlates with that of adiabasingle-zone results, although heat loss effect dethe ignition near the wall and moderates peak cyder pressure.

4.2. LIF detection of formaldehyde

LIF excitation spectra of HCHO traced at fixecrank angles in the engine cylinder are shownFig. 4, together with a reference spectrum takena static cell and a simulation. The simulated sptrum was obtained by giving an assumed linewito the line-by-line intensities calculated with Asrot [23]. It is clear that the spectra observed incylinder are predominantly from HCHO and interfeences from other fluorescent species are not renized. While the spectrum at 90◦ ATDC is similar tothe low-pressure reference, the TDC spectrum shsignificant pressure broadening so that the rotatiostructure is almost dissolved.

The crank angle-resolved LIF intensity profilwere taken at the fixed excitation wavelength41

0RR3 bandhead (28,370 cm−1). In order to de-

28 H. Yamada et al. / Combustion and Flame 140 (2005) 24–33

s,/airtio

ro-ing,olli-thebyure-

sedd

ne

ies,

ningec-

entctionith

l

ofTheas

er-inglu-wn

Fig. 3. Calculated profiles of DME and HCHO fractionpressure and temperature in compression cycles of DMEmixture. (a) Single cool ignition case at equivalence raφ = 0.47 and intake temperature= 400 K, (b) two-stage ig-nition atφ = 0.50 and intake temperature= 480 K.

rive an accurate concentration or mole fraction pfile, integrated corrections for pressure broadentemperature-dependent state population, and csional quenching are conducted according tofollowing procedure, which was previously usedShin et al. for an atmospheric burner flame measment[19].

Fig. 4. LIF excitation spectra obtained in the compresDME/air mixture (a), from stored HCHO in a cell (b) anfrom a simulation with an assumed linewidth of 0.3 cm−1.Vertical ticks with connecting bars indicate rotational lipositions for each subband.

The observed LIF intensityS is expressed as

(2)S = NBEΓ

�νLfBΦ

g

4π,

whereN is the concentration of the detecting specB the Einstein absorption coefficient,E the laserenergy, Γ the lineshape overlap,�νL the laserlinewidth,fB the Boltzmann population factor,Φ thefluorescence quantum yield, andg the transmissionefficiency of the detecting system. Here,Γ andΦ de-pend mainly on pressure through pressure broadeand physical quenching of the excited state, resptively, andfB depends on temperature. In the prestreatment, these variables are evaluated as a funof crank angle at the same operating condition wthe fluorescence intensity measurement.

The lineshape overlapΓ is defined by the integraover wavenumber,

(3)Γ =+∞∫

−∞Y (ν)L(ν) dν,

where Y (ν) and L(ν) are the spectral shapesthe absorbing media and the laser, respectively.linewidth (FWHM) of the laser source at 352 nm westimated to be 0.10 cm−1, which is comparable tothat of HCHO at atmospheric pressure. The ovlap integral decreases with increasing pressure owto the further broadening of absorption. The evaated HCHO linewidth under compression is sho

H. Yamada et al. / Combustion and Flame 140 (2005) 24–33 29

insional-were

line-d

intithThen

k

-

e,nO

eely

x-n-

inedolid

eef-

ndn-deras

ecay

n ine5e-t isl tore,di-tial

b-or-

rti-ro-

res-n in

ve

Fig. 5. Spectral linewidth of formaldehyde 410

RR3 rota-tional lines determined from observed excitation spectrumthe cylinder. The superimposed curve is the scaled collifrequency= PT −1/2, in which measured pressure and cculated temperature from the ideal gas state equationused.

Fig. 6. Crank angle-resolved pressure, temperature, andshape overlap integralΓ . Γ is obtained from observelinewidth.

in Fig. 5 as a function of crank angle. Each powas obtained by fitting the simulated spectrum wthe observation assuming Lorentzian lineshape.result is approximately proportional to the collisiofrequency given byP/T 1/2. The calculated cranangle-dependent overlap integral is shown inFig. 6with pressure and temperature curves.

The quantum yieldΦ is determined by the competition of the rate of fluorescenceA with the rate ofquenchingQ as

(4)Φ = A

A + Q.

A is defined as the reciprocal of radiation lifetimτrad, which includes contribution of predissociatioand is known to be 85 ns in the case of HCHA–X 41

0 transition [24]. In high pressures wherQ � A is valid; it can be represented approximat

Fig. 7. Effective fluorescence lifetime of formaldehyde ecited at 410

RR3 transition in the engine compression evironment. Square symbols are experimentally determfrom fluorescence decay observation, and the thin scurve is a scaled reciprocal collision frequency.

as

(5)Φ ≈ A

Q= τ2

τrad,

where τ2 is the lifetime of the excited state at thobserving condition. Since the quenching rate coficient varies considerably with collision partner asufficient HCHO data for the combustion enviroment are not available, fluorescence lifetime unpractical compression conditions in the cylinder wmeasured separately by tracing fluorescence dat a photomultiplier load resistance of 50. Themeasured crank angle-dependent lifetime is showFig. 7. The range over which the lifetime could bdirectly measured is limited to crank angles after 4◦ATDC, where plenty of HCHO is present and the liftime is not shorter than the laser pulse duration. Iassumed that the lifetime is inversely proportionathe collision frequency as superimposed in the figuand this curve was used for the correction. The incated errors are statistical in the fitting to exponendecay (2σ ).

The thermally distributed population of the oserved state was calculated with Boltzmann’s fmula:

(6)

fB = exp(−Ev(v′′)

kT)(2J ′′ + 1)exp(−ER(J ′′,K ′′)

kT)

QvibQrot.

The vibrational and rotational energies, and the pation functions were calculated using Asyrot. Thetational quantum number of the observedRR3 band-head was represented as (6′′, 3′′). The temperature inthe cylinder was estimated from the measured psure profile using the equation of state as showFig. 6.

The HCHO concentration,N , or mole fraction,N/Ntotal, could be finally obtained after the abo

30 H. Yamada et al. / Combustion and Flame 140 (2005) 24–33

edni-inla-

mnals,

leo-

isd af-theonsisme-ptedni-

sthehisthe

t gas

ug-the

ig-

en-O

al

p-ies

nceset

fordO,eer-e-er

en-arlytios,

Fig. 8. Profiles of raw LIF signal intensity and correctformaldehyde mole fraction obtained at single cool igtion (upper) and two-stage ignition (lower) conditions. Thdashed lines are mole fraction of chemical kinetics simutions. Revolution= 1000 RPM. Error bars are derived frothe scattering of measured lifetime and fluorescence sigin which the former is a larger factor.

procedure. The corrected profiles of HCHO in mofraction for a single cool ignition event and a twstage ignition event are shown inFig. 8with raw LIFintensity profiles. The validity of this correctiondemonstrated by the fact that the curves before anter the correction are totally different in shape, andcorrected profiles are consistent with the predictimade according to the detailed chemical mechanin both cases. The model prediction of HCHO bhavior, i.e., formation in the cool ignition and promconsumption in the thermal ignition, has been provby this measurement. In the case of a single cool igtion (upper trace inFig. 8), the corrected profile showa gradual decrease in the ATDC region, whereasmodel prediction shows an almost constant level. Tvariation, however, seems to be within the error incorrection factors and is subject to further study.

4.3. Mass spectroscopic measurement of exhauscomposition

The results presented in the previous section sgest that even exhaust gas analysis is useful inexamination of the transient composition of cool

Fig. 9. Species mole fractions in the exhaust of DME/airgine compression as a function of equivalence ratio, (a)2,(b) CO, (c) H2O, (d) CO2. Revolution= 600 RPM and in-take gas temperature= 414 K. Dashed lines are chemickinetics simulations.

nitions when the thermal ignition occurrence is supressed.Fig. 9shows measured and simulated specmole fractions at exhaust as a function of equivaleratio. Engine speed and intake temperature areat 600 RPM and 414 K, respectively. FractionsDME, HCHO, and O2 in the experiment are scaleaccording to separate calibration, while those for CCO2, and H2O are given to fit the calculation. Thpoint above which fuel is totally consumed is the thmal ignition limit. The match is excellent overall blow the thermal limit, although the limit occurs earliin the calculation at this RPM.

In Fig. 10, the quantities of DME and HCHO arshown relative to the initial amount of DME at the itake. A notable feature is that these ratios are neconstant across a wide range of equivalence ra

H. Yamada et al. / Combustion and Flame 140 (2005) 24–33 31

eldf

arere-

rac-e

uente ism-

this

ate-

nsof

av-ingys-

inas

rxi-hylf

i-iti-

-at

f the

veingofn-

oferi-ed-w,wby

Fig. 10. Fuel consumption and HCHO generation yirelative to initial amount of DME as a function oequivalence ratio. Revolution= 600 RPM and intaketemperature= 414 K. Dashed and dot-dashed lineschemical kinetics simulations for 600 and 1200 RPM,spectively.

i.e., about 30% of fuel is consumed and an equal ftion of HCHO is formed at the cool ignition stagregardless of the load. In other words, the conseqheat release per unit mass at the cool ignition stagproportional to the fuel load; hence the first stage teperature rise differs. The mechanism governingbehavior is discussed in the following section.

4.4. Mechanism controlling LTO chain reaction

The detailed chemical simulation predicts thDME consumption occurs only during the short priod of cool ignition and the rest of the fuel remaiin the interval before thermal ignition regardlesswhether sufficient oxygen is still present. This behior should be discussed in terms of chain branchand the termination mechanism of this reaction stem.

The DME oxidation at LTO is described as a chareaction cycle whose chain carrier is OH radicalshown below:

CH3OCH3 + OH → CH3OCH2 + H2O, (R1)

CH3OCH2 → HCHO+ CH3, (R2)

CH3OCH2 + O2 ↔ CH3OCH2O2, (R3)

Fig. 11. Chain branching indexα, defined by the numbeof OH reproduction per cycle in DME low-temperature odation, and decomposition efficiency of hydroperoxymetformyl ether,r(HFE), atφ = 0.29, defined by the ratio oHFE generation rate to HFE decomposition rate.

CH3OCH2O2 ↔ CH2OCH2O2H, (R4)

CH2OCH2O2H → 2HCHO+ OH, (R5)

CH2OCH2O2H + O2 ↔ O2CH2OCH2O2H, (R6)

O2CH2OCH2O2H → HO2CH2OCHO+ OH, (R7)

HO2CH2OCHO→ HCHO+ HOCO+ OH. (R8)

This constitutes the simplest form of the typcal “degenerate chain branching” consisting of ination forming an alkyl radical(R1), the first O2 ad-dition (R3), isomerization(R4), the second O2 addi-tion (R6), and decompositions reproducing OH(R7),(R8). Reactions(R2) and(R5) are competing decomposition steps that lower the OH reproduction ratehigher temperatures; hence they are the cause onegative temperature coefficient.

Fig. 11shows two indices relevant to the effectitemperature range of the LTO. One is the branchindex of OH reproduction, defined as the numberOH reproduced in a cycle starting with one OH cosumption by(R1)and represented by the formula,

α = {k3[O2]/(

k3[O2] + k2)}

(7)× {(2k6[O2] + k5

)/(k6[O2] + k5

)},

where the rate constants cited in the mechanismCurran and co-workers were used for the numcal evaluation of this index. Since an index exceing unity is required for the chain system to grothis limits the effective temperature of LTO to belo850 K. Another index is the kinetic rate governedthe slowest reaction(R8) defined as

r(HFE) = k8[HO2CH2OCHO](8)× (

k6[CH2OCH2O2H][O2])−1,

32 H. Yamada et al. / Combustion and Flame 140 (2005) 24–33

thecesit-

r:ingx-ster

be-K)

ys-, it

nge;n-cti-

ys-

ns

as-

thee

wthea

er-the

toen-a-theot

x

he

c-

.theted

-

min

e-mliero-

eri-ca-tsed.

ioned

ac-

where the actual species concentration obtained insimulation of compression ignition was used. Henthe index depends on the equivalence ratio and theuation whereφ = 0.29 is shown inFig. 11. The stableHO2CH2OCHO (hydroperoxymethyl formyl etheHFE) accumulates, reducing the effective branchwhen the index is below unity. When the index eceeds unity, the accumulated HFE decomposes fathan its generation. Consequently the cool ignitiongins in a narrow range of temperature (720–740under every condition studied in this work.

A significant intermediate in LTO of DME isformaldehyde (HCHO) and the effect to the chain stem should be considered. When HCHO is formedshares in OH consumption with(R1) through the re-action:

HCHO+ OH → HCO+ H2O. (R9)

HCO readily reacts with O2 to form CO + HO2.These are not sources of OH at this temperature rahence,(R9) acts as a chain termination step. Cosequently the chain reaction system may be deavated by the accumulation of HCHO before the stem reaches 850 K.

The chemical mechanism consisting of reactio(R1)–(R9)can be summarized as

CH3OCH3 + OH→ αOH + βHCHO+ other products, (R1′)

HCHO+ OH → inert products, (R9′)where α is the same index of OH reproductionshown inFig. 11, andβ is the number of formaldehyde produced in a cycle.β is a weak function oftemperature and is estimated to be around 1.3 inLTO region. A rate equation for OH growth can brepresented by

d[OH]/dt = {(α − 1)k1[DME] − k9[HCHO]}[OH]

(9)= G[OH],where the chain mechanism is active when the grofactor G is positive. TheG factor tends to decreaswith increasing HCHO and will eventually turn intonegative. This endpoint of cool ignition is

(10)[HCHO][DME] = (α − 1)k1

k9.

Both k1 and k9 have strong dependence on tempature; however, they are mostly canceled out inratio k1/k9, which is close to 0.5.α is also withina limited range when the temperature rise duea cool ignition is small. These factors are indepdent of equivalence ratio. This criterion is qualittively consistent with the observed steadiness ofHCHO/DME ratio; however, the HCHO yield cannbe derived from this alone.

Fig. 12. HCHO formation yieldy and chain branching indeG versus normalized fuel consumptionq, as a solution ofEq. (11). The point thatG crosses zero corresponds to tendpoint of LTO chain reaction.

A time-independent expression for the HCHO acumulation versus fuel consumption is

dy/dq = {βk1(1− q) − k9y

}/k1(1− q)

(11)= β − (k9/k1)y/(1− q),

where y = [HCHO]/[DME]0 is the normalizedamount of accumulated HCHO, andq = ([DME]0 −[DME]t)/[DME]0 the normalized fuel consumptionThis is universal for any equivalence ratio whendifference in temperature is neglected. An integraresult using values at 720 K is shown inFig. 12. Asthe original nature ofβ exceeding unity is partly compensated with the HCHO consumption by(R9)so thaty grows almost along the liney = q. The endpoint ofthe chain is also given byG(q) = 0, which in thiscase lies atq = 0.22.

The latter version of the DME kinetic mechanisof Curran and co-workers, which was published2000[13,14], has become available on line very rcently. Our preliminary application of the mechanisto the current problem showed a tendency of earignition than experiments, in contrast to the oppsite relationship of the former version and the expments. Besides the author-describing major modifition of branching to formic acid formation in producof (R8), a number of rate constants were modifiMost significant ones for LTO are(R5) and (R8);i.e., k5 was reduced 5 times andk8 was enhanced40 times at 700 K. Consequently DME consumptand HCHO formation in cool ignitions are enhancthrough the increase of indexα. Formic acid is sup-posed to act as a chain terminator through the retion:

HCOOH+ OH → H2O + CO2 + H. (R10)

H. Yamada et al. / Combustion and Flame 140 (2005) 24–33 33

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Here H atom is not an active chain carrier in the crent low temperature and high pressure, since it tuinto relative inactive species HO2 through

H + O2 + M → HO2 + M. (R11)

Considering that the rate constant of(R10) is an or-der of magnitude less than that of(R9), formic acidshould rather be regarded as inert. Hence the braing to HCOOH, instead of dominant productionHCHO in (R5), leads to a decrease of indexβ and aslight shift to higher fuel consumption. In any casthe effect of HCOOH is insignificant provided ththe yield is within the simulated value of 20%. Thrate constants of(R5)–(R8), as well as the equilibriumconstant in(R4), are crucial for the LTO characteristics. As far as the fuel consumption and HCHO yiein cool ignitions are concerned, the 1998 versionconsistent with the current experiments whereas2000 version tends to overestimate these values.

It has been argued that hydrogen peroxide (H2O2)is an important species that once accumulatester cool ignitions and triggers hot ignition througthe thermal decomposition producing OH[1]. In thisstudy, however, our mass-spectrometric setup wasensitive to H2O2 and we did not pay attention to thspecies further. Optimization of the mass detectand improvement of the sampling to avoid condsation in the tube, or another probing method, mayrequired to extend the methodology to the H2O2 is-sue.

5. Conclusions

Two separate experiments and a simple mohave elucidated the chemical mechanism that demines the characteristics of DME autoignition. In slaser-induced fluorescence measurement of formahyde in an engine cylinder confirmed that the intermdiate is promptly formed in the first-stage cool igntion and totally consumed in the second stage therignition. The time-resolved LIF monitoring also sugested that the cool ignition composition couldquantified by exhaust gas analysis where the therignition is suppressed. Mass-spectroscopic analwas effective in revealing that the percentage of fconsumed and the ratio of formaldehyde formationinitial fuel load in cool ignitions tended to be indpendent of the equivalence ratio. This observationsuccessfully accounted for by the consideration ofLTO chain reaction in which accumulated formaldhyde acts as a chain terminator. A simple set ofmulations with a limited number of relevant reactirate parameters for the critical point could satisf

torily estimate the fuel consumption and subsequheat release of cool ignitions.

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