In-cylinder soot precursor growth in a low-temperaturecombustion diesel engine : laser-induced fluorescence ofpolycyclic aromatic hydrocarbonsCitation for published version (APA):Leermakers, C. A. J., & Musculus, M. P. B. (2015). In-cylinder soot precursor growth in a low-temperaturecombustion diesel engine : laser-induced fluorescence of polycyclic aromatic hydrocarbons. Proceedings of theCombustion Institute, 35(3), 3079-3086. https://doi.org/10.1016/j.proci.2014.06.101

DOI:10.1016/j.proci.2014.06.101

Document status and date:Published: 01/01/2015

Document Version:Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)

Please check the document version of this publication:

• A submitted manuscript is the version of the article upon submission and before peer-review. There can beimportant differences between the submitted version and the official published version of record. Peopleinterested in the research are advised to contact the author for the final version of the publication, or visit theDOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and pagenumbers.Link to publication

General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.

• Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.

If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, pleasefollow below link for the End User Agreement:www.tue.nl/taverne

Take down policyIf you believe that this document breaches copyright please contact us at:openaccess@tue.nlproviding details and we will investigate your claim.

Download date: 25. Jan. 2022

Available online at www.sciencedirect.comProceedings

ScienceDirect

Proceedings of the Combustion Institute 35 (2015) 3079–3086

www.elsevier.com/locate/proci

of the

CombustionInstitute

In-cylinder soot precursor growth in alow-temperature combustion diesel engine:

Laser-induced fluorescence of polycyclicaromatic hydrocarbons

C.A.J. Leermakers a, M.P.B. Musculus b,⇑

a Department of Mechanical Engineering, Eindhoven University of Technology, P.O. Box 513, GEM-N 1.21, 5600

MB Eindhoven, The Netherlandsb Combustion Research Facility, Sandia National Laboratories, P.O. Box 969, MS 9053, Livermore, CA 94551, USA

Available online 19 July 2014

Abstract

The growth of poly-cyclic aromatic hydrocarbon (PAH) soot precursors are observed using a two-lasertechnique combining laser-induced fluorescence (LIF) of PAH with laser-induced incandescence (LII) ofsoot in a diesel engine under low-temperature combustion (LTC) conditions. The broad mixture distribu-tions and slowed chemical kinetics of LTC “stretch out” soot-formation processes in both space and time,thereby facilitating their study. Imaging PAH–LIF from pulsed-laser excitation at three discrete wave-lengths (266, 532, and 633 nm) reveals the temporal growth of PAH molecules, while soot-LII from a1064-nm pulsed laser indicates inception to soot. The distribution of PAH–LIF also grows spatially withinthe combustion chamber before soot-LII is first detected. The PAH–LIF signals have broad spectra, muchlike LII, but typically with spectral profile that is inconsistent with laser-heated soot. Quantitative natural-emission spectroscopy also shows a broad emission spectrum, presumably from PAH chemiluminescence,temporally coinciding with of the PAH–LIF.� 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords: Polycyclic aromatic hydrocarbons; Particulate matter; Laser-induced fluorescence; Laser-induced incandes-cence; Diesel low-temperature combustion

1. Introduction

Strategies to reduce pollutant emissions fromdiesel engines while maintaining fuel efficiency

http://dx.doi.org/10.1016/j.proci.2014.06.1011540-7489/� 2014 The Combustion Institute. Published by El

⇑ Corresponding author. Fax: +1 (925) 294 1004.E-mail address: mpmuscu@sandia.gov

(M.P.B. Musculus).

are of interest for meeting environmental regula-tions and market demands. Low-temperaturecombustion (LTC) is a class of in-cylinder strate-gies that uses dilute mixtures and enhanced pre-mixing to maintain or even improve fuelefficiency while reducing pollutant emissions,including soot [1]. In addition to its practical util-ity, the broad mixture distributions and slowedchemical-kinetics of LTC conditions “stretch out”

sevier Inc. All rights reserved.

Table 1Engine specifications and operating conditions.

Compression ratio 11.22:1Common-rail injector Cummins XPINozzle hole arrangement 8 � 140 lm � 152�Fuel rail pressure 1000 barCommand start of injection 352 CADCommand injection duration 2.5 msTDC motored density 16.6 kg/m3

TDC motored temperature 975 KIntake pressure 177 kPa (abs)Intake temperature 155 �C(Simulated 16:1 intake pressure) 116 kPa (abs)(Simulated EGR rates) 30–59%Air excess ratio 2.39 at 21% O2

Fig. 1. Schematic of the single-cylinder engine, laserconfiguration, and optical detector system. The cameraand spectrometer field-of-view are shown in the upperright corner.

3080 C.A.J. Leermakers, M.P.B. Musculus / Proceedings of the Combustion Institute 35 (2015) 3079–3086

soot-formation processes in both space and time,thereby facilitating their study.

Poly-cyclic aromatic hydrocarbons (PAH) arean important class of soot precursors [2]. SmallerPAH with one to five rings may be either fuel-borne [3] or combustion-generated. Larger PAHwith more than five rings are almost entirely dueto combustion-induced formation and/or growth.In addition to toxicity/carcinogenicity [4], PAHformation is undesirable because PAH contributeto particulate matter (soot) [5].

PAH growth during combustion can beobserved to some degree through spectroscopictechniques. The absorption/emission spectra ofPAH molecules shift to longer wavelengths withincreasing number of rings [6,7]. Individual PAHspectra are broadband and overlap with eachother, however, so specific PAH species generallycannot be isolated spectrally.

One technique to probe the absorption redshiftdue to PAH growth is laser-induced fluorescence(LIF). If soot is present, the laser-induced emis-sion can also include significant laser-inducedincandescence (LII) of soot, which is also spec-trally broad. One way to discriminate soot-LIIfrom PAH–LIF is to use a second laser pulse ata wavelength that yields only soot-LII. TheNd:YAG laser fundamental at 1064 nm shouldyield only soot-LII, because even very largePAH do not display single-photon absorption atnear-infrared wavelengths [8,9]. Comparisons ofthe two signals, acquired virtually simultaneously,thus allow discrimination of PAH-only regionsfrom regions with both PAH and soot.

This two-laser LIF-LII approach for dieselcombustion follows previous work [10], using only532-nm PAH–LIF. Here, the PAH–LIF isextended over a broader excitation wavelengthrange, adding 266 and 633 nm, to study the spa-tio-temporal evolution of both small and verylarge PAH. Excitation at 266 nm probes smallPAH that do not absorb at 532 nm, while verylarge PAH could conceivably absorb at 633 nmand thus fluoresce upon 633-nm excitation. Suchabsorption, if present, would confound sootextinction techniques that use 633-nm (i.e. HeNe)laser light [11]. Both the soot-LII and the PAH–LIF signals are also spectrally resolved to furtherdiscriminate LIF from LII, and quantitativechemiluminescence spectra provide further insightinto PAH growth.

2. Experiment set-up

2.1. Optical engine and operating conditions

The optical engine is a single-cylinderBowditch-piston version of a Cummins N14heavy-duty diesel (bore 139.7 mm, stroke152.4 mm, 2.34 L). Table 1 shows operating

conditions and some engine specifications (see[12,13] for more engine details).

In the experiment schematic of Fig. 1, a flatUV-grade fused-silica piston-crown window pro-vides imaging access from below to the open,right-cylindrical bowl (diameter 97.8 mm). A 30-mm-wide curved window matching the pistonbowl-wall contour and a flat rectangular cylin-der-wall window provide laser access into the pis-ton bowl, even near top-dead center.

A target load of 6 bar gross indicated mean-effective pressure (gIMEP) is investigated at1200 rpm with a single fuel injection per cycle.The fuel is n-heptane, selected for its low fluores-cence compared to pump diesel fuel to avoid inter-ference with the PAH–LIF and soot-LII signals. Italso has essentially zero fuel-borne PAH, meaning

C.A.J. Leermakers, M.P.B. Musculus / Proceedings of the Combustion Institute 35 (2015) 3079–3086 3081

all PAH in this study is combustion-generated.The ignition properties of n-heptane are similarto European diesel fuel, with a cetane numberbetween 52.5 and 56 [14]. Different intake oxygenmole fractions (not including fuel) were achievedby diluting the intake stream with nitrogen. Anoxygen mole-fraction of 18% mimics typical cur-rent diesel combustion using exhaust aftertreat-ment [15], while 10.5% intake oxygen is lowenough that exhaust aftertreatment for soot andnitrogen oxides (NOx) may not be required [15].Additionally, the low combustion temperaturesat 10.5% oxygen further slow PAH/soot kineticsso that their formation pathways can be studiedin more detail.

2.2. Excitation setup

Two independent 10-Hz Spectra PhysicsQuanta Ray Nd:YAG laser systems provide thetwo excitation beams. One (model GCR-250) isused at its fundamental (1064 nm) for soot-LII.A second (PRO-200) excites PAH–LIF usingeither its second (532 nm) or fourth (266 nm) har-monics. Additionally, a Spectra-Physics opticalparametric oscillator provides pulsed laser lightat 633 nm.

One at a time, the 266/532/633-nm laser beamsare combined with the 1064-nm beam and co-aligned with the approximate symmetry plane ofone of the eight fuel jets, angled at 14� relativeto the fire deck (152� included angle). The beamsare formed into 30-mm wide sheets that are lessthan one mm thick over the region of interest withpulse energies entering the engine of 50–60 mJ at266 and 532 nm, 40 mJ at 633 nm, and 250 mJat 1064 nm. The effect of divergence through thecurved window on laser fluence is minimal withinthe LII plateau. The 1064-nm fluence exceeds0.8 J/cm2, which was found to be sufficient toreach the plateau in the LII signal strength, suchthat a further increase in fluence does not signifi-cantly affect the LII signal. In lower-pressureflames, at the beginning of the observed plateausoot vaporization is occurring but not excessive.The laser fluence in UV and visible is significantlylower than for IR (as can be determined from theratio of total laser energies, since the beam foot-prints are similar).

2.3. Detector setup

As shown in Fig. 1, the soot-LII signal is direc-ted by the 45-degree mirror and a 50-50 beamsplitter to a Xybion gated, intensified CCD cam-era (model ISQ-350-W-3). The camera is equippedwith SWP450 (450-nm short-wave-pass) andGG385 (385-nm long-pass) filters and a 105-mmfocal-length glass lens at f/4. As described in[16], laser-heated (LII) soot particles should fol-low Planck’s law and therefore emit more strongly

at shorter wavelengths than combustion-heatedsoot, so using a shorter spectral band from 385to 450 nm improves the signal-to-noise ratio rela-tive to combustion-heated soot. The laser pulse isbracketed with the minimum intensifier gate widthof 60 ns.

The PAH–LIF signal is imaged by a PI-MAX3ICCD camera with an HQf intensifier and another105-mm glass lens at f/4. The intensifier gate over-laps the laser pulse with the minimum achievable2.47-ns width to maximize the signal-to-noiseratio. A short-wave-pass filter with a 50% cut-offnear 850 nm (CVI/Melles-Griot FSWP-850)removes scattered 1064-nm light. Narrow-bandnotch filters and a WG285 color filter reject elas-tically scattered light at 532/633 and 266 nm,respectively.

In each engine run, 20 images are acquired at afixed crank angle, with every fourth image captur-ing background luminosity without any laserlight. The LII laser pulse trails the LIF pulse by1 ls, which is sufficient to separate the two signalswhile being virtually simultaneous relative toengine time scales. The order of the two pulsesdid not affect the acquired images, as observedpreviously [10]. From each image set, a single rep-resentative image pair is objectively identifiedusing a statistical image-correlation technique[17].

For spectral measurements, an Oriel 77250monochromator is used as a spectrograph bymounting a Xybion intensified CCD camera(model ISG-250-UX-3) at the exit plane. A 300lines/mm grating (Oriel #77939) resolves the400–700 nm range with a spectral resolution of8 nm at 500 nm. The entrance slit is lens-coupledto the probe volume using a 50-mm glass lens atf/4 (Fig. 1). Spectrograph images are acquired insets of 20 at each crank angle. For laser-inducedspectra, the minimum intensifier gate of 110 nsbrackets the desired laser pulse, whereas forchemiluminescence spectra the gate is increasedto 140 microseconds, or about 1 �CA. The laser-induced spectra are background-luminosity-sub-tracted (removing very weak natural emission),median-filtered, spatially binned over the head ofthe jet (35–47 mm from the injector along thenominal jet axis), and ensemble-averaged. Thechemiluminescence spectra are only spatially-and ensemble-averaged. All spectra are madequantitative in spectral radiance by correctingfor the filter transmission and camera spectral sen-sitivity using a Hoffman calibrated integratingsphere, model LR-6Z.

Finally, a large-area (1 cm2) silicon, photo-conductive mode photodiode (UDT PIN-10D)viewing through the piston-crown windowrecords the natural combustion-luminosity, withsystem sensitivity set to resolve the strongsoot luminosity, but not the relatively weakchemiluminescence.

3082 C.A.J. Leermakers, M.P.B. Musculus / Proceedings of the Combustion Institute 35 (2015) 3079–3086

3. Results and discussion

Figure 2 shows image sets for 10.5% and 18%intake-oxygen, with 532-nm LIF and 1064-nmLII acquired virtually simultaneously in eachimage, but each image-pair at a different crankangle is from a different engine run. LIF and LIIimages are false-colored green and red, respec-tively, such that overlap regions appear yellow.To improve the yellow visibility relative to green(which appears very bright to the eye), the gamma[18] of the red image is set to 0.8. Because of thepossibility of soot-LII in the signal from 532-nm(as well as 266- and 632-nm) excitation, overlapareas may or may not have PAH, but certainlyhave soot. The injector position and bowl-rimposition are indicated in the images, with one ofthe eight fuel jets propagating horizontally leftto right across the image.

Because of the different dynamic ranges of thecameras and to make initial soot-formation visi-ble, both the LII and the LIF signal images areartificially amplified such that 0.1 percent of the

36820

250

36920

250

37020

250

37120

100

3722070

3747

40

3764

40

3786

20

Fig. 2. Top: ROHR, injection rate and natural soot luminositfractions. Bottom: composite single-shot images, false colorgamma = 0.8) with image crank angle and color-coded signal greferences to color in this figure, the reader is referred to the e

pixels in the area of interest saturates. Thesepost-processing gains, color-coded according tothe false-color scheme, are displayed in eachimage. The left side of each image in Fig. 2 showsvarious degrees of scattering/interference from the1064-nm beam (red). The scattering appears worsein the early images because of higher gains (asindicated on the images).

3.1. 532-nm LIF and 1064-nm soot LII imaging at10.5% and 18% intake oxygen

The top of Fig. 2 shows the measured injectionrate, the apparent rate of heat release (ROHR,from an air-standard first-law analysis [19]) andthe soot-luminosity photodiode signal. Verticaldashed-lines indicate the crank angles of theimages. The ROHR and soot luminosity showthat ignition and soot-formation processes areslower at 10.5% intake oxygen than at 18%oxygen.

With 10.5% intake oxygen, 532-nm PAH–LIF(green) is first detected immediately after the

36120

250

361.520

250

361.7520

200

36210

100

3633

30

3642

30

3652

40

3662

30

y signals for 10.5% (left) and 18% (right) intake oxygened for 532-nm LIF (green, gamma = 1) and LII (red,ains are denoted in each image. (For interpretation of thelectronic version of this paper.)

C.A.J. Leermakers, M.P.B. Musculus / Proceedings of the Combustion Institute 35 (2015) 3079–3086 3083

premixed heat-release peak, near 370 CAD.Thereafter, the PAH–LIF area grows withoutany detectable soot-LII until 374 CAD, at whichtime the photodiode also detects soot naturalluminosity (top). After this 4 �CA inception delay,soot-LII signal quickly fills the regions wherePAH were present, yielding increasing signal onboth cameras (note decreasing gains in laterimages).

Also for 10.5% intake oxygen, a conventionalcombusting diesel jet structure [16] is not apparentbecause of the long mixing time. Instead, the sootdistribution is a consequence of jet/wall/swirldynamics preceding soot formation. Prior to igni-tion and combustion, the jet impinges and spreadsalong the bowl wall. The counterclockwise swirl-flow sweeps the originally horizontal jet partiallyout of the laser sheet, while parts of the adjacentclockwise jet are transported into the bottom ofthe laser sheet. Hence, the large structure in thebottom of the images at the end of the sequence(378 CAD) is a remnant of the interactionbetween two adjacent jets.

For 18% intake oxygen (Fig. 2, right), a moreconventional combusting diesel jet is apparent.Over the narrower crank-angle range of images(to 366 CAD) than at 10.5% oxygen (to 378CAD), the jet is transported counterclockwiseonly slightly by swirl. PAH–LIF is again firstdetected immediately after the premixed burn(361 CAD). Within 1 �CA, the PAH–LIF is typi-cally surrounded by soot-LII. While some por-tions display pure PAH (green), the rapidincrease in soot (note the decreasing ‘red’ gains)yields LII signal on both cameras. Soot luminosityis also much higher than at 10.5% intake oxygen(Fig. 2, top).

3.2. Multi-wavelength imaging comparison

LIF originating from 266, 532, and 633-nmexcitation is compared at 10.5% intake oxygenfraction, where the evolution of PAH is slowestand thus most easily studied. Figure 3 shows threetime-sequences of representative compositeimages from LIF excitation at these three wave-lengths (false-colored green) with simultaneous1064-nm LII (false-colored red).

Comparing the left and middle sets of imagesin Fig. 3 shows that combustion-generated spe-cies, very likely PAH, can be detected earlier with266-nm than with 532-nm excitation. The onset offluorescence resulting from excitation at 266 nmoccurs between 368 and 369 CAD. In the 370and 371 CAD images, the signal for 266-nm exci-tation is stronger than for 532-nm excitation.Recall that the images use different gains as indi-cated on each image.

Excitation at 633 nm shows signal only at latercrank angles, starting from 371 CAD. Further-more, signal from 633-nm excitation, presumably

fluorescence of very large PAH, arises beforeany LII. This implies that (very large) PAH canabsorb light at the red HeNe-laser wavelength(�633 nm). Similar long-wavelength PAH absorp-tion/fluorescence has also been observed in atmo-spheric-pressure flames, even to 660 nm [21].While this to some extent may open possibilitiesfor fluorescence detection of such species, absorp-tion by very large PAH may also interfere withsoot-extinction measurements that use HeNelasers; the extinction will be increased beyond thatcaused by soot alone, especially near soot incep-tion. At later crank angles when soot is moredense, absorption by PAH would likely becomeless significant, or even insignificant, relative toextinction by soot.

All three image sequences show a consistentPAH signal development. A temporal redshift inabsorption (excitation) wavelength, and hencegrowth in PAH molecular size (i.e., number ofrings), occurs over a short 3–4 �CA range. In theensemble, no spatial differences are apparentamong the different excitation-wavelength/sizeclasses; in regions where long-wavelength LIF isobserved, similar regions also show short-wave-length LIF (though from different cycles).

Figure 3 shows that to detect the earliest (andsmallest) PAH, 266-nm laser light is appropriate.However, laser-light as well as the correspondingemission are more readily attenuated at shorterwavelengths. While most of the window-depositedsoot in the path of the laser is blasted away bylaser pulses between skip-fired cycles, attenuationby the soot cloud itself [20] and soot deposits onthe piston window can be problematic, especiallyat high-shooting conditions. Furthermore, com-ponents of commercially available fuels, as wellas (intermediate) combustion products, may beexcited by 266-nm laser light, thereby confound-ing the fluorescence signal. Hence, while 266-nmPAH–LIF can be useful for fuels/conditions suchas those employed here, excitation of PAH–LIF at532 nm has a broader utility.

3.3. Spectral analysis of LIF and LII at 10.5%intake oxygen

Figure 4 shows quantitative LIF and LII spec-tra for the 10.5% oxygen condition of Fig. 3. Spec-tral ranges where the filters have high absorbanceare artificially masked for clarity. At 374 CAD, atwhich time the soot-LII (not shown) is relativelyweak, the 532-nm LIF spectrum is broad, peakingnear 550 nm, and without any narrow peaksattributable to small combustion intermediatessuch as CH or C2. Shortly thereafter, at 376CAD, the broad 532-nm LIF emission is moreredshifted, peaking near 600 nm. The 1064-nmsoot-LII spectrum at 376 CAD is similarly broad,but exhibiting an even more redshifted peakbetween 600 and 650 nm. As explained earlier,

36850

250

36950

250

37050

200

37150

200

37250

200

3742040

3767

70

3784

80

36820

250

36920

250

37020

250

37120

100

3722070

3747

40

3764

40

3786

20

36850

250

36950

250

37050

250

37130

250

37240

100

37420

100

37620

100

633 nm532 nm266 nm

Fig. 3. Composite single-shot images, false-colored for LII excitation at 1064 nm (red, gamma = 0.8) and LIF excitation(green, gamma = 1) at 266, 532 or 633 nm, as indicated by the column titles. Operating conditions correspond to Fig. 2(10.5% O2). Image crank angle and color-coded signal gains are denoted in each image. (For interpretation of thereferences to color in this figure, the reader is referred to the electronic version of this paper.)

Fig. 4. Spectra of the laser-induced signal for excitationlaser wavelengths of 266, 532 and 633 nm (labeled“LIF”) and 1064 nm (labeled “LII”) at 374 and 376CAD. 10.5% intake oxygen condition (Fig. 3).

3084 C.A.J. Leermakers, M.P.B. Musculus / Proceedings of the Combustion Institute 35 (2015) 3079–3086

the emission from 532-nm excitation likelyincludes soot LII when soot is present, so the sim-ilarity to the soot-LII spectrum from 1064-nmexcitation is not unexpected. At even later timings(not shown), the LIF spectra become even moresimilar to the soot-LII spectra, suggesting thatthe signal from 532-nm excitation becomes domi-nated by soot-LII.

The 266 and 633-nm LIF emissions are alsospectrally resolved at the same two crank anglesas for 532 nm. Considering 266-nm excitationfirst, at the early crank angle of 374 CAD, theLIF spectrum is much different than the LIIsignal, peaking at <450 nm, which is inconsistentwith blackbody incandescence at the soot

vaporization temperature (peaking at640–720 nm at 4000–4500 K). (The peak mayactually lie in the ultraviolet, which is not resolvedin these measurements.) Hence, emission from266-nm excitation at this and earlier crank anglescan be interpreted as being almost solely LIF, nothaving significant LII, consistent with the imagesin Fig. 3. At later crank angles, the signal likelycontains more and more LII, resulting in a red-shift of the spectral peak.

The 633-nm induced LIF spectra are some-what weaker than for 266 and 532-nm excitation(see Fig. 3), especially at 374 CAD. Nonetheless,the spectrum at 374 CAD displays proportionallymore light red-shifted from the 633-nm excitationwavelength than those of the shorter wavelengthexcitations. At the later crank angle (376 CAD)the shape of the LII and 633-nm LIF spectra areso similar that no LIF signal can be distinguishedfrom LII.

A significant portion of the early LIF emission(at 374 CAD) for both 532-nm and 633-nm excita-tion is blue-shifted from the laser wavelength. (Asimilar effect may occur for 266-nm, but is notresolved here). A similar blueshift has beenobserved in both atmospheric pressure premixedmethane and diesel-spray diffusion flames [21], aswell as a previous study with this same engineunder conditions with no detectable 1064-nm sootLII signal [10]. The source of the blue-shiftedemission is likely laser-excitation of PAH [21].We speculate that combustion chemistry yieldsPAH in excited ro-vibrational states, which canrelax to lower ground states after electronic exci-tation by the laser light, thereby yielding blue-shifted emission. For large PAH molecules with

C.A.J. Leermakers, M.P.B. Musculus / Proceedings of the Combustion Institute 35 (2015) 3079–3086 3085

so many ro-vibrational modes, much of theground electronic state can be in higher ro-vibra-tional states even for thermal distributions atcombustion-heated temperatures. Non-thermalro-vibrational distributions resulting from com-bustion chemistry can increase the amount ofblue-shifted emission. Chemiluminescence spec-troscopy described in the next section providessome support for the possibility of non-thermalro-vibrational distributions.

3.4. Chemiluminescence spectroscopy

Figure 5 shows natural chemiluminescenceemission spectra for the 10.5% oxygen condition,using a much longer (140 microsecond) cameragate than the PAH–LIF experiments (110 ns).The spectra span from first heat release to the lastcrank angle before soot-LII and soot luminosityappear. From the earliest crank angle, the emis-sion spectrum is broadband, peaking in the blue.This early broadband emission spectrum is consis-tent with the CO2

* emission [22] spectrum, rangingfrom 340 to about 650 nm with its peak around400 nm. Over the next few �CA, the broadbandsignal strength increases, especially at longerwavelengths. A slight hump is visible near431 nm, corresponding to the excited CH* radical[23], and a distinct chemiluminescence peak near590 nm may be from sodium impurities in the fuel,as sodium-doped flames have a strong isolatedline near 589 nm [22].

The chemiluminescence spectrum departs fromthe initial CO2

* emission spectrum starting at 368CAD, which is the same crank angle as the firstdetectable PAH–LIF in the images of Fig. 3 for266-nm excitation (small PAH). After this initialrise in natural emission (potentially PAH chemilu-minescence), its red-peaked broadband emission

Fig. 5. Quantitative chemiluminescence emission spec-tra for an early crank angle range of the 10.5% intakeoxygen condition (Fig. 2, left). Legend indicates thestarting crank angle of the 1 �CA camera gate.

rises through the same crank-angle range inFig. 3 where PAH–LIF signals arise with 532-nm (370 CAD) and then 633-nm (371 CAD) exci-tation. At later crank angles, the natural emissiongrows rapidly, especially after 374 CAD whensoot luminosity and soot-LII become significantin Figs. 2 and 3, respectively. For clarity, the verybright spectra from these later crank angles arenot shown here. As the soot luminosity signaleventually becomes orders of magnitude strongerthan the chemiluminescence, any narrow spectralfeatures disappear into a broad continuum whosespectral peak shifts further towards the infrared.However, the detection of significant PAH chemi-luminescence (molecules in excited electronicstates), make it reasonable to expect that alsoPAH molecules are in excited vibrational states,which could cause the PAH–LIF blue-shift inFig. 4. Furthermore, the increased short-wave-length emission from PAH chemiluminescencewould also explain the unreasonably high appar-ent soot temperatures observed near the onset ofsoot formation in two-color pyrometry measure-ments, especially for LTC conditions [24].

4. Summary and conclusions

In an optically accessible diesel engine, imag-ing PAH–LIF from pulsed-laser excitation at266, 532, and 633 nm reveals the temporalgrowth of PAH molecules, while soot-LII froma 1064-nm pulsed laser indicates inception tosoot. At a LTC engine operating condition with10.5% intake-oxygen, PAH is first detected by266-nm excitation just after the premixed heatrelease peak, followed by 532-nm and then 633-nm excitation, indicating a likely temporalgrowth of PAH molecular size. The PAH–LIFregion also grows spatially during the approxi-mately 5–6 �CA before soot-LII is first detected.For 18% intake oxygen, first PAH are alsodetected just after the premixed burn, but within1 �CA, the PAH region is surrounded by soot,though some images display regions of purePAH.

The LIF signals have broadband spectra,much like LII, but typically with a blue-shiftedpeak, such that the LIF spectrum is spectrallyinconsistent with laser-heated soot. A substantialportion of the LIF emission is at wavelengthsshorter than the excitation laser, suggesting thatmany of the combustion-generated PAH are inexcited ro-vibrational states. The LIF signal with633-nm excitation is weaker than for 266 or 532-nm excitation, and more difficult to distinguishspectrally from LII because the broadband peaksare at similar wavelengths. Imaging comparisonswith 1064-nm soot-LII show that 633-nm LIFdoes precede LII, however. At later crank angles,the contribution of LII grows to dominate the

3086 C.A.J. Leermakers, M.P.B. Musculus / Proceedings of the Combustion Institute 35 (2015) 3079–3086

LIF signals, so that the emission is spectrallyindistinguishable from LII.

Quantitative chemiluminescence spectroscopyalso shows CH (430 nm) and sodium (590 nm)features on a broad emission spectrum, probablyfrom PAH chemiluminescence, whose initialemergence temporally coincides with first detec-tion of the 266-nm PAH–LIF. The existence ofelectronically excited PAH yielding chemilumi-nescence is consistent with the hypothesis thatthe strong short-wavelength portion of the LIFemission spectrum is from ro-vibrationallyexcited PAH, and may also explain anomalouslyhigh apparent temperatures at soot inception insoot optical thermometry measurements. At latercrank angles, a soot-luminosity signal appears,coinciding with the first soot-LII, and the nar-row spectral features disappear as the LIIquickly strengthens by orders of magnitude andthe spectral peak shifts further toward theinfrared.

The detection of apparent PAH–LIF from633-nm excitation suggest that very large PAHabsorb light at HeNe-laser wavelengths in a nar-row crank-angle range prior to the formation ofsoot. While this to some extent may open possibil-ities for fluorescence detection of such species,absorption by large PAH may also interfere withsoot extinction measurements that use HeNelasers by increasing the extinction beyond thatcaused by soot alone, especially near soot incep-tion. At later crank angles when soot is moredense, absorption by PAH would become less sig-nificant, or even insignificant, relative to extinc-tion by soot.

Acknowledgements

Support for this research at the CombustionResearch Facility, Sandia National Laboratories,Livermore, CA, was provided by the U.S.Department of Energy, Office of Vehicle Tech-nologies. Sandia is a multi-program laboratoryoperated by Sandia Corporation, a LockheedMartin Company for the United States Depart-ment of Energy’s National Nuclear SecurityAdministration under contract DE-AC04-94AL85000. Keith Penney and David Ciconeare gratefully acknowledged for their assistancewith experiments.

References

[1] M. Musculus, P. Miles, L. Pickett, Prog. EnergyCombust. Sci. 39 (2–3) (2013) 246–283.

[2] J. Warnatz, U. Maas, R. Dibble, Combustion,Springer, Berlin-Heidelberg, Germany, 2006.

[3] R.N. Westerholm, T.E. Alsberg, A.B. Fromelln,et al., Environ. Sci. Technol. 22 (8) (1988) 925–930.

[4] D.W. Dockery, C.A. Pope, X. Xu, J.D. Spengler,J.H. Ware, M.E. Fay, B.G. Ferris, F.E. Speizer,N. Engl. J. Med. 329 (1993) 1753–1759.

[5] H. Bockhorn (Ed.), Soot Formation in Combustion,Springer, Berlin-Heidelberg, 1994.

[6] T. Aizawa, H. Kosaka, Int. J. Engine Res. 9 (2006)79–96.

[7] K. Hayashida, K. Amagai, K. Satoh, M. Arai, J.Eng. Gas Turbines Power 128 (2006) 241–246.

[8] R. Rieger, K. Mullen, J. Phys. Org. Chem. 23(2010) 315–325.

[9] R.L. Vander Wal, K.A. Jensen, M.Y. Choi, Com-bust. Flame 109 (1997) 399–414.

[10] M.K. Bobba, M.P.B. Musculus, Combust. Flame159 (2012) 832–843.

[11] M. Musculus, L. Pickett, Combust. Flame 141 (4)(2005) 371–391.

[12] J. O’Connor, M. Musculus, SAE Int. J. Engines 6(1) (2013) 379–399 (2013-01-0910).

[13] J. O’Connor, M. Musculus, Int. J. Eng. Res. 15 (4)(2013) 421–443.

[14] M. Murphy, J. Taylor, R. McCormick, Compen-dium of Experimental Cetane Number Data,National Renewable Energy Laboratory, NREL/SR-540-36805, 2004.

[15] P. Brijesh, S. Sreedhara, Int. J. Automot. Technol.14 (2) (2003) 195–206.

[16] J.E. Dec, SAE Trans. 106 (3) (1997) 1319–1348(970873).

[17] C. Genzale, Optimizing Combustion ChamberDesign for Low-Temperature Diesel Combustion,Ph.D. thesis, University of Wisconsin-Madison,USA, 2008.

[18] A. Yadav, P. Yadav, Digital Image Processing,University Science Press, New Delhi, 2009.

[19] J. Heywood, Internal Combustion Engine Funda-mentals, McGraw-Hill, 1988.

[20] M.Y. Choi, K.A. Jensen, Combust. Flame 112 (4)(1998) 485–491.

[21] S. Bejaoui, R. Lemaire, P. Desgroux, E. Therssen,Appl. Phys. B. (2014). http://dx.doi.org/10.1007/s00340-013-5692-y (in press).

[22] A.G. Gaydon, The Spectroscopy of Flames, Chap-man and Hall, London, UK, 1974.

[23] Y. Hardalupas, M. Orain, Combust. Flame 139 (3)(2004) 188–207.

[24] E. Huestis, P.A. Erickson, M.P.B. Musculus, SAETrans. 116 (4) (2007) 860–879 (2007-01-4017).