Fluorescence-Based Method Designed for Quantitative Measurement of Fuel Film Thickness during Cold-Start of Engines

Volume 54, Number 4, 2000 APPLIED SPECTROSCOPY 5650003-7028 / 00 / 5404-0565$2.00 / 0

q 2000 Society for Applied Spectroscopy

Fluorescence-Based Method Designed for QuantitativeMeasurement of Fuel Film Thickness during Cold-Startof Engines

JIEMIN YANG and LYNN A. MELTON *Department of Chemistry, University of Texas at Dallas, Richardson, Texas 75083-0688

The liquid fuel that accumulates in the engine manifold and cylin-

ders during cold start conditions is thought to contribute signi® -cantly to excess hydrocarbon emissions. Two ¯ uorescent dopants,

cyclohexanone and 2-methyl-cyclopentanone, have been tested for

use as ¯ uorescent markers for quantitative two-dimensional (2D)imaging of the thickness of automotive fuel ® lms in the range 0± 1

mm. These dopants are co-evaporative with synthetic automotive

fuel and have ¯ uorescence that is virtually independent of oxygenconcentration equivalent to saturation under 5 atm air at temper-

atures of 20± 200 8 C. Selection, calibration procedures, and (nonen-

gine) demonstration laser-induced-¯ uorescence (LIF) imaging ex-periments are discussed.

Index Headings: Fluorescence; Diagnostics; Engines; Liquid ® lms;

Cold start.

INTRODUCTION

Further reduction of hydrocarbon (HC) emissions is akey factor in meeting the emission regulations institutedby the 1990 U.S. Clean Air Act Amendments,1,2 and thus,a thorough understanding of fuel behavior inside an en-gine is essential. The heaviest release of unburned hy-drocarbon emissions, as measured in the Federal TestProcedure, occurs during the engine cold-start period,when the intake port and the cylinder wall are not yet hotenough to vaporize the liquid fuel ef® ciently. A signi® -cant fraction of the injected liquid fuel remains on theintake port and the cylinder wall and is carried over tolater cycles, which results in fuel-rich combustion andhigh hydrocarbon emissions. In addition, the catalyticconverter, which would oxidize the hydrocarbons fromthe engine, has not reached its minimum operating tem-perature.3±6 Quader et al.7 showed that different ways offuel delivery affect the emission levels during cold startquite differently: when the ambient temperature de-creased from 24 to 2 7 8 C, engines with port fuel injectionshowed a 54% increase in the mean hydrocarbon emis-sions, while those with premixed charge showed only a19% increase. Effective engineering solutions of this liq-uid retention problem will require the ability to determinethe temporal and spatial behavior of the liquid fuel duringthe cold-start period. This paper describes the develop-ment of a ¯ uorescence-based method that should make itpossible to image quantitatively the liquid fuel ® lm thick-ness during this critical period.

Many studies have been conducted in order to under-stand the fuel behavior in the engine chamber. Earlymethods include direct photography and shadowgraph or

Received 2 September 1999; accepted 23 November 1999.* Author to whom correspondence should be sent.

schlieren imaging with either a transparent engine/intakeair tube or a glass cylinder.8±10 These methods, while pro-viding the most straightforward observation of the fuel,are necessarily line-of-sight and nonquantitative tech-niques. They require the use of glass engine parts whosethermal behavior may not mimic that of the metal partsof a production engine. Gas chromatographic (GC) anal-ysis has been used in order to measure quantitatively theamount of fuel at each stage during cold start.3,10 GCprovides the most accurate measurement of the amountof fuel left in the engine chamber; however, since theengine has to be stopped at each measurement point, thetest is time consuming, and it provides neither spatialresolution nor real-time observation of the fuel behaviorin the engine.

In recent years, laser-induced ¯ uorescence (LIF) mea-surements have become the method of choice for enginediagnostics. Usually the engine is modi® ed so that laserlight can be introduced into the engine directly via ® beroptics or as a laser sheet. A charge-coupled device (CCD)camera then captures the laser-induced ¯ uorescence fromeither the gasoline itself or from an added dopant. TheLIF measurement is nonintrusive, provides direct obser-vation of the fuel behavior in the engine, is instantaneousand in situ, provides high spatial resolution, and intro-duces minimal alteration of the ¯ ow conditions.

The ¯ uorescent molecule is a vital element in LIF mea-surements. It should be minimally perturbing to the fuelbehavior and should provide an accurate determination ofthe fuel ® lm thickness. The most convenient and leastperturbing molecules are the ¯ uorescent components thatare already in the gasoline. Witze and Green 11 used theexisting heavy components in commercial gasoline as thesource of ¯ uorescence, and Felton et al.12 used a purpledye, which is an added marker in a standard test fuel, asthe source of ¯ uorescence. Since no extra chemicals areadded into the commercial gasoline, these ¯ uorescentmolecules are perceived as not perturbing the fuel be-havior. However, since multiple components, with un-known thermal and ¯ uorescence quenching behavior,contribute to the overall ¯ uorescence, it is dif® cult todetermine quantitatively the amount of fuel on the wall.In a different approach, the strongly ¯ uorescent dye ¯ uo-rescein was added to the fuel, but quantitative measure-ments of the amount of fuel on the wall could not bemade since the boiling point of ¯ uorescein is much high-er than that of gasoline.10

An ideal dopant must possess the following qualities:(1) signi® cant absorption at an available laser wave-length; (2) satisfactory quantum yield; (3) good solubility

566 Volume 54, Number 4, 2000

FIG. 1. Schematic diagram of laser-induced ¯ uorescence imaging experimental setup.

FIG. 2. Emission ( l ex 5 308 nm) and excitation ( l em 5 419 nm) spectra of 0.1 M cyclohexanone/cyclohexane solution.

in the fuel; (4) co-evaporation with the fuel; (5) insen-sitivity to oxygen quenching; (6) insensitivity to temper-ature changes; and (7) ready availability.

Quantitative LIF measurement of the thickness of theliquid fuel in the cylinder of an operating engine has beenattempted.12±14 However, each of these attempts is subjectto criticism because the dopant system is not ideal. Thevapor pressure and the temperature dependence of the¯ uorescence intensity of the purple dye used by Feltonet al. are not known.12 Meyers et al.13 used ¯ uoranthene,which has negligible temperature quenching and readilycorrectable oxygen quenching, but its boiling point (3848 C) is far higher than that of gasoline. The temperatureand oxygen quenching of the ¯ uorescence signal fromthe heavy components in fuel, used as a dopant by Parkset al.,14 were not evaluated.

The dynamic environment inside the engine chambersmakes quantitative LIF measurement dif® cult. Conditionssuch as temperature, pressure, and oxygen concentrationsare changing throughout the engine cycle. Most common¯ uorescent materials are sensitive to these changes, es-pecially to oxygen quenching. However, the ¯ uorescenceof acetone and 3-pentanone has been shown to be virtu-ally independent of the above mentioned environmental

changes,15,16 and they can be used as dopants for quan-titative analysis. These dopants are, however, more vol-atile than conventional automotive gasoline, and there-fore dopants are needed that are also co-evaporative withthe fuel.

This paper describes the selection and evaluation ofpotential dopants that will be applicable to quantitativemeasurement of the thin liquid fuel ® lm inside an enginemanifold or cylinder during cold start. Two ¯ uorescentdopants, cyclohexanone and 2-methyl-cyclopentanone,which have ¯ uorescence that is virtually independent ofoxygen concentration (0 ±5 atm air) and temperature (20±200 8 C), appear to be appropriate for use as ¯ uorescentmarkers for two dimensional (2D) imaging of automotivefuel thickness in the range 0±1 mm. Alkane-based syn-thetic fuel systems have been developed so that thesedopants are co-evaporative with the synthetic fuel. Selec-tion, calibration procedures, and demonstration imagingexperiments will be discussed.

EXPERIMENTAL

Experimental Strategy. A list of 40±50 chemicalswas compiled on the basis of (1) the approximate match

APPLIED SPECTROSCOPY 567

FIG. 3. UV absorption spectra of 2-methyl cyclopentanone red-shiftsat 25 8 C (± ± ±) and 100 8 C ( ).

FIG. 4. UNIFAC calculation of vapor±liquid equilibrium for 1.4%(w/w) cyclohexanone in nonane/iso-octane solution.

of the boiling point to that of the automotive gasoline(100±170 8 C), and (2) the likelihood of their ¯ uorescenceto be inert to oxygen and temperature changes (see Re-sults and Discussion for details). UV absorption spectraof these chemicals were obtained in order to select theexcitation wavelength for ¯ uorescence measurement, andmeasurement of the ¯ uorescence spectra was attempted.Those chemicals having measurable ¯ uorescence spectrain the visible region (400±800 nm) were then further test-ed for their sensitivity to oxygen quenching and temper-ature changes. Those molecules whose ¯ uorescence wasinsensitive to the oxygen quenching and temperaturechanges were then designated as potential dopants. Oncea candidate dopant was identi® ed, a synthetic fuel systemwas designed in which this speci® c dopant was co-evap-orative with the fuel system. These dopant/fuel systemswere then used in calibration tests of ¯ uorescence inten-sity vs. liquid ® lm thickness at both room temperatureand under heated conditions.

Chemicals and Sample Preparation. All chemicalsused in the experiments, 2,2,4-trimethyl pentane (iso-oc-tane, 99.7% HPLC grade), nonane (anhydrous 99 1 %), 2-methyl cyclopentanone (98% ), and cyclohexanone(99.8%), were purchased from Aldrich Chemical Co.(Milwaukee, WI) and used as received without furtherpuri ® cation. Hydrogen chloride/helium mixture(99.995 1 % ), neon gas (99 .999% ), and xenon gas(99.999%) for XeCl excimer laser operation were pur-chased from Spectra Gas (Branchburg, NJ) and used asreceived.

Solutions used for laser-induced ¯ uorescence calibra-tion experiments were 1.4 wt % 2-methyl cyclopentanonein 55 wt % iso-octane and 43.6 wt % nonane mixture,and 1.4 wt % cyclohexanone in 25 wt % iso-octane and73.6 wt % nonane mixture.

General Apparatus. UV Absorption Spectroscopy.Solutions used in UV measurement were prepared in sucha way that the total absorption at the absorption maxi-mum had an optical density between 0.1 and 1.0. A dual-beam absorption spectrometer (Hitachi, San Jose, CA,Model U2000) was used for acquiring UV absorption

spectra of the compounds. All spectra were acquired witha scan rate of 200 nm/min and 1 nm resolution.

Fluorescence Emission Spectroscopy. A Spex Fluorol-og 2 series spectro¯ uorometer equipped with a Peltier-cooled photomultiplier tube was used to obtain all ¯ uo-rescence spectra. All ¯ uorescence spectra were obtainedby using the spectral bandwidths of 2 nm for excitationand 4 nm for emission, with a scan increment of 1.0 nmand a 0.1 integration time. Datamaxt software (Jobin-Yvon Spex Instruments SA Group, Edison, NJ) was usedfor the steady-state ¯ uorescence data acquisition and postdata processing.

Each compound was excited at its UV absorption max-imum, and its ¯ uorescence spectrum was then collectedat 90 8 relative to the incident light. Once a ¯ uorescenceemission spectrum was obtained, it was compared to theresult published in the literature. In the event no literaturespectrum could be located, the excitation spectrum wasacquired in order to con® rm that the emission spectrumwas indeed that of the intended compound.

High-Temperature UV Absorption Fluorescence Spec-tra. For experiments at high temperature, pure vapor ofthe chemical under investigation was introduced to aquartz cuvette, ® tted with a Te¯ on stopcock, by using afreeze/pump/thaw method. An aluminum heating blockheld the cuvette. The aluminum block was then heatedby a set of four cartridge heaters (15 W) connected to avariac. The temperature of the heating block was moni-tored by a thermocouple (Type J, Omega Model 199,Omega Engineering, Inc., Stamford, CT). Details of thisapparatus, speci® cally designed for such high-tempera-ture experiments, and its operation have been describedpreviously.17 The cuvette was maintained at a temperaturefor at least 2 min before the spectrum was collected. Aweak interference peak from ¯ uorescent room lights waseliminated by covering the instrument with a dark cloth.

Oxygen Quenching of Fluorescence Emission. In orderto investigate the quenching of the ¯ uorescence by oxy-gen, solutions in the sealable cuvette were purged withnitrogen, air, or oxygen, respectively, for 15 min. Thesethree gases provided different levels of oxygen dissolvedin the solution. The cuvette was then sealed, and the ¯ uo-rescence spectrum was acquired.

Laser-Induced-Fluorescence (LIF) Imaging. A pulsedtunable XeCl excimer laser (Model EMG 150 MSC,


FIG . 5. Calibration curve for cyclohexanone LIF image vs. sample weight.

FIG. 6. Cyclohexanone LIF intensity vs. corrected liquid thickness.

Lambda Physik, Santa Clara, CA), was used as the ex-citation source for the laser-induced-¯ uorescence imagingexperiments. The typical average laser power was 30 mJ/pulse at a 10 Hz repetition rate, measured with the py-roelectric Joulemeter (Model ED-500, Gen-Tec Inc., Can-ada). A cooled CCD camera (Star-1, Photometrics Ltd,Tucson, AZ) equipped with a Nikon 50 mm lens and aTiffen 1 1 close-up lens was used as the detection systemto capture the LIF images ( f 5 1/8, exposure time 5 0.8s, XeCl excimer laser operated at 8 Hz). The heart of thecamera head is a scienti ® c-grade 384 3 576 pixel CCDchip with 23 m m square pixels, a 160 000 electron full-well capacity, and a typical readout noise of 25 electrons.The camera head was thermoelectrically cooled and wasregulated to operate at a temperature of 2 45 8 C. Coolingreduced the dark current to approximately 15 electrons/second/pixel. For room-temperature calibration experi-ments, the average intensity of three images at each thick-ness was used. For heated experiments, sequential imageswere captured every 12 s by use of ZStar-1 software.Image acquisition was controlled by a PC through anIEEE-488 interface. The original 12-bit `ìmg’ ’ formatimages could then be either stored on a PC or convertedto standard 8-bit TIFF or BMP formats for further dataanalysis.

A block diagram for the laser-induced-¯ uorescence im-

aging experiments is shown in Fig. 1. Light from the laserpassed through a quartz plate, used as a beamsplitter at45 8 , onto an Omega 337DRLP dichroic beamsplitter(Omega Optical, Brattleboro, VT) that was also set at a45 8 angle relative to the laser beam. The quartz beam-splitter re¯ ected a ® xed percentage of the total laser in-tensity to the reference cell. The quartz reference cell was® lled with 1.00E-7M (1,4-D i[2-(5-phenyloxazo-lyl)]benzene (POPOP) solution in hexadecane. The PO-POP solution was chosen since it has a high quantumyield and its ¯ uorescence was in the same wavelengthrange as the best dopant candidates, 2-methyl cyclopen-tanone and cyclohexanone (broad peak at 400±500 nm,centered at 420 nm). When set at a 45 8 angle, the dichroic® lter re¯ ected any light with a wavelength lower than 337nm with 95% ef® ciency; i.e., the XeCl excimer laser lightat 308 nm was re¯ ected into the sample cell. The ® lteralso transmitted any light with wavelength longer than337 nm, and the ¯ uorescence signal from the sample wastransmitted into the CCD camera located directly abovethe sample cell. The CCD camera was set to include bothimages from the sample cell and the reference cell in thesame frame. The sample image could then be ratioed tothat of the reference cell so deviations due to the shot-to-shot variation of the laser were eliminated.

In an effort to minimize background emission and re-


FIG. 7. Calibration curves for 1.4% cyclohexanone solution at roomtemperature ( ) and under heated conditions ( m m m ).

FIG. 8. Relative mole% of cyclohexanone in liquid as a function ofmole% of total liquid evaporated.

FIG. 9. Calibration curves for 1.4% 2-methyl cyclopentanone solutionat room temperature ( ) and under heated conditions ( m m m ).

¯ ection of the incident light, a black-anodized aluminumsample cup (1.5 in. o.d., 1.0 in. i.d., 2 in. overall height,1.5 in. deep) was used. The inside bottom of the cup waspolished to a roughness , 0.3 mil.

For heated experiments, the sample cell was heated bya ¯ exible heating tape (8.6 W/in 2, Omega Engineering)wrapped around it. The heating rate was controlled by avariac. The temperature of the sample cell was monitoredby a thermocouple (Type J, Omega Engineering, Model199) inserted in the cup wall and was measured by adigital thermometer (Omega Engineering, HH82). Liquidsamples were delivered into the sample cell by volumet-ric pipettes (Rainin, Woburn, MA, Model #P-1000, 1 mLcapacity 6 0.0013 mL). For the calibration, the thicknessof the liquid was calculated on the basis of either thevolume of the sample added or the weight at the time theLIF image was acquired (the difference in results basedon these two calculations will be discussed in detail inResults and Discussion). Sample weight changes duringimage acquisition were recorded automatically every 2 sby a top-loading balance (Sartorius, Model #BP310S, 3106 0.001 g, Edgewood, NY) logged by a PC. A compositedensity of the solution was used for thickness calculationfrom sample weight. The sensitivity of the balance wasveri® ed with a set of certi ® ed weights (1±10 mg) underthe same experimental conditions used for the LIF imageacquisition. Adequate ventilation was provided to allowevaporation of the heated solution and to prevent con-densation of the vapor onto the optics.

Demonstration Experiment. A demonstration experi-ment was performed by depositing the dopant solutionsonto the bottom of the heating cup. The initial tempera-ture of the cup was either room temperature or 100 8 C,and the cup was heated further to evaporate the solution.LIF images of the liquid on the metal surface were ac-quired every 12 s throughout the evaporation processwith the same optic and electronic parameters as in thethickness calibration experiments. Images from the dem-onstration experiments were then processed, and thethickness of the liquid at each pixel was calculated fromthe calibration curves.

RESULTS AND DISCUSSION

An ideal dopant must possess the following qualities:(1) signi® cant absorption at an available laser wave-length; (2) satisfactory quantum yield; (3) good solubility

in the fuel; (4) co-evaporation with the fuel; (5) insen-sitivity to oxygen quenching; (6) insensitivity to temper-ature changes; and (7) ready availability. This section de-scribes the selection and use of such dopants.

Molecular Properties. Selection of Proper Dopants.Previous work has shown that it is possible to ® nd oxy-gen-insensitive ¯ uorescence dopants, in which high in-ternal decay rates suppress the effect of external quench-ing. When no photochemical reaction is involved, the¯ uorescence quantum yield, F F, is given by Eq. 1.

kFF 5 (1)F k 1 k 1 k 1 k [Q ]F isc d q

where kF 5 rate constant for ¯ uorescence emission (s 2 1);kisc 5 rate constant for intersystem crossing (s 2 1); kd 5rate constant for internal conversion (s 2 1); kq 5 quenchingrate constant (s 2 1 M 2 1); and [Q ] 5 quencher concentra-tion (M).

Since the internal conversion rate is unlikely to dom-inate the ¯ uorescence or intersystem crossing rates,18 ahigh k isc (kisc . kq[Q ]) is essential for F F to be relativelyinert to changes in quencher concentration. Two groupsof compounds are usually considered as having enhancedinter-system crossing: (1) compounds in which the mo-


FIG. 10. Demonstration experiment of the evaporation of the solution containing 1.4% cyclohexanone dopant. The initial temperature of the metalsurface was at room temperature.

lecular excitation is an n ® p * process instead of a p ®p * process, as in the case of acetone, and (2) compoundspossessing ``heavy atom’ ’ substituents.18

A list of potential dopants that were expected to haveenhanced intersystem crossing rates was compiled. Sincea potential dopant has to be co-evaporative with gasoline,for which the average 50% and 90% distillation temper-atures (T50 and T90) are 103 8 C and 166 8 C, respectively,7

compounds on the list were restricted to those with nor-mal boiling points between 100 and 170 8 C. Compoundsfor which the ¯ uorescence maximum was in the UV re-gion ( l em , 400) were also excluded since most com-mercial CCD cameras have signi® cantly lower sensitivityin the UV region than in the visible region. The list ofpotential dopants contained about 40 compounds in twomajor categories: (1) compounds with atoms having lone-pair electrons in their molecular orbitals (various mono-and di-carbonyl compounds including straight-chain, cy-clic, aromatic and a , b -unsaturated, aromatic ethers andsul® des, and amines), and (2) compounds with at leastone halogen atom (mono-halogenated aromatics and di-halogenated aromatics).

Dopant Fluorescence. Potential dopants were evaluat-ed by measuring their ¯ uorescence spectra at a concen-tration for which the o.d.(l 5 1 cm) ù 0.1. Most of thecompounds on the candidate list did not have suf® cientlyintense ¯ uorescence spectra, and several of them did not

have a detectable ¯ uorescence signal at all. Reasonableassumptions were made to exclude some compounds aspotential dopants. For instance, the ¯ uorescence of monoiodo-benzene was not investigated since experimentswith chloro-benzene and bromo-benzene did not result indetectable ¯ uorescence. Only carbonyl compounds dis-played suf® cient ¯ uorescence signal in the visible region( l em . 400 nm). The rejected compounds included ha-logenated aromatics (mono-substituted and di-substitut-ed), a , b -unsaturated carbonyl compounds, ethers, sul-® des, and amines.

The emission spectra of all the carbonyl compoundstested are similar to that of cyclohexanone (Fig. 2, l ex 5308 nm). They usually have a broad emission maximumnear 420 nm and a second broad emission shoulder be-tween 450 and 470 nm, in agreement with the literature.19

Quantum yields were not measured in this work, but theyare expected to be in the range of 1±2 3 10 2 3, as foundfor other carbonyls.18

As a peripheral point, the potential risk of biacetyl for-mation from photodissociation of carbonyl compounds isnot a particular concern in this case. 20 The presence ofoxygen effectively suppresses the formation of biacetyland its ¯ uorescence interference (two peaks near 512 and561 nm). Since the biacetyl ¯ uorescence is usually muchstronger than the carbonyl ¯ uorescence, its presencewould be a signi® cant concern for carbonyl ¯ uorescence


FIG. 11. Demonstration experiment of the evaporation of the solution containing 1.4% cyclohexanone dopant. The initial temperature of the metalsurface was 100 8 C.

measurements. 21,22 In a sealed system with no oxygen, theinitially absent biacetyl signal at 512 and 561 nm ap-peared after a few minutes when an acetone solution wasirradiated at 308 nm. However, once the oxygen was pre-sent, there were no signs of biacetyl ¯ uorescence evenafter 1 h of continuous irradiation.

Straight-chain carbonyl compounds, including biace-tyl, have been used in qualitative studies for fuel distil-lation imaging during engine cold start.23±25 However,concerns about the thermal stability of straight-chain al-dehydes and ketones 26 made cyclic-carbonyl compoundspreferable. Among the two types of thermal decomposi-tion reactions, the a -cleavage processes are likely to beunimportant since the C±C bond that is broken in theprimary cleavage process has a bond energy ( ; 325 kJ/mol) that is very close to the (n, p *) excited-state ener-gies of simple aliphatic ketones. 27 For acyclic carbonylcompounds, the Norrish type II reaction, which involvesan intramolecular hydrogen abstraction by the excitedcarbonyl group from the g -position, could be more im-portant. For the cyclic carbonyl compounds, the ring sys-tem prevents the g -H from moving into the position fora Norrish type II reaction. Therefore, the cyclic ketoneswere chosen as dopants.

The two most promising cyclic-carbonyl compoundsin our study are 2-methyl cyclopentanone (b.p. 139 8 C)and cyclohexanone (b.p. 155 8 C). They are in the rightboiling point range to be considered to be approximately

co-evaporative with automobile fuel, and the cyclic ke-tones are unlikely to participate in Norrish type II reac-tions.

Oxygen Sensitivity. Oxygen is usually a highly effec-tive ¯ uorescence quencher. Since the air/fuel ratio is nothomogeneous during the precombustion process, theamount of oxygen dissolved in the fuel ® lm is presum-ably not constant. Even under constant air/fuel ratio con-ditions, the equilibrium oxygen concentration will de-crease as the liquid phase temperature rises. To achievenear quantitative thickness imaging for thin fuel ® lmmeasurement, the dopant ¯ uorescence intensity has to bevirtually independent of the oxygen and temperaturechanges in the engine. The two dopants that were testedfor oxygen and temperature sensitivity are 2-methyl cy-clopentanone and cyclohexanone.

For 2-methyl cyclopentanone, at room temperature, the¯ uorescence intensity from air-saturated and oxygen-sat-urated solutions was 4 and 20%, respectively, less thanthat of a nitrogen purged solution. The O2 quenching re-sults for cyclohexanone were similar.

Temperature Sensitivity and Thermal Stability. The ef-fect of temperature on ¯ uorescence intensity can be com-plex. Usually, k0

f is temperature independent, but someradiationless processes from S 1 are temperature depen-dent. As the temperature increases, enhancement of ra-diationless processes from S 1, i.e., intersystem crossingand/or internal conversion, with increasing temperature


results in decreased ¯ uorescence quantum yield (as islikely in these carbonyl compounds).19 However, the ab-sorption spectra of these same carbonyl compounds showa red shift as the temperature increases (Fig. 3). Thus, ifthe excitation wavelength used were shorter than that ofthe absorption maximum, the effect of lower absorptionextinction coef® cient coupled with the decreased ¯ uores-cence quantum yield at higher temperature would resultin a much-decreased total ¯ uorescence intensity. For ex-ample, with 2-methyl cyclopentanone, as the temperatureincreased from room temperature to 190 8 C, excitation at266 nm resulted in an 80% decrease of the ¯ uorescenceintensity. By contrast, when 2-methylcyclohexanone wasexcited at 308 nm, under the same conditions, there wasless than 4% change in its ¯ uorescence intensity. There-fore, the 308 nm excitation wavelength was chosen toensure that the dopant ¯ uorescence intensity is as insen-sitive as possible to temperature changes. The results forcyclohexanone were similar. All experiments were per-formed in the gas phase and in the presence of air. Theseresults are similar to the results obtained with 3-pentan-one.28±30

Both 2-methyl cyclopentanone and cyclohexanone arethermally very stable. When tested at 170 8 C for up to10 h, neither demonstrated a measurable decrease in ¯ uo-rescence intensity. The high thermal stability of the dop-ants minimizes the potential interference of thermal deg-radation on signal measurement.

Design of Co-evaporative System. For quantitativemeasurement of fuel ® lm thickness, if (1) the measure-ment remains in the optical thin region, and (2) the dop-ant concentration remains constant throughout the evap-oration process, then the ¯ uorescence intensity of the so-lution will be directly proportional to the ® lm thickness.Under the optically thin conditions, the ¯ uorescence sig-nal, If, can be calculated as follows:

If 5 (4.6kÎF F )clI0 (2)

where k 5 instrument constant (collection ef® ciency,CCD sensitivity, etc.); Î 5 molar absorptivity (103 cm 3 /mol cm); F F 5 quantum yield of ¯ uorescence; I0 5 in-cident light intensity; If 5 ¯ uorescence intensity; c 5concentration (mol/cm3); and l 5 pathlength/thickness(cm).

The results described in ``Temperature Sensitivity andThermal Stability’ ’ show that ÎF F is virtually independentof temperature and oxygen concentration. Thus, the ¯ uo-rescence intensity of a sample can be expressed by Eq. 3:

(If)T /I0 5 Klc (3)

where K 5 ( F FkÎ) is constant.If the dopant concentration in the solution remains con-

stant throughout the evaporation process, then the ¯ uo-rescence intensity of the sample will be proportional tothe thickness of the liquid.

To have a solution in optical thin region (o.d. , 0.1 at1 mm thickness), for cyclohexanone (Î 5 15 M 2 1 cm 2 1

at 308 nm), a dopant concentration , 0.067 M is required.However, a ; 0.1 M solution was used to provide ade-quate ¯ uorescence intensity. For a solution of cyclohex-anone in mixed alkanes, a 0.1 M solution is approxi-mately equal to 1.4% (w/w). For 2-methyl cyclopentan-

one, Î 5 16 M 2 1 cm 2 1 at 308 nm, and an approximately0.1 M [1.4 wt % (w/w)] solution was also used.

The co-evaporative system design is based on the Uni-versal Quasi-Chemical Functional-group Activity Coef-® cient (UNIFAC) approach,31 which uses group contri-butions as the basis for nonideal vapor±liquid equilibriumcalculations. UNIFAC can predict vapor±liquid equilibriain systems for which speci® c data are not available.

In order to design a co-evaporative system that (1)closely represents the fuel system used in engine testsand (2) does not introduce any extraneous ¯ uorescenceor energy transfer, only alkanes are used as solvents forthe ketones. For cyclohexanone (NBP 5 155 8 C), nonane(NBP 5 151 8 C) was identi® ed as the potential solventin the synthetic fuel system, which would contain onlycyclohexanone (1.4 wt %) and nonane. A computer pro-gram modi® ed from Fredenslund et al. was used to cal-culate the UNIFAC vapor±liquid equilibria of the two-component system.31 Figure 4 shows the fraction of cy-clohexanone evaporated as a function of the fraction ofthe total solution evaporated for two different systems.The diagonal line is ideal behavior. Note that even thoughthe boiling point of cyclohexanone is higher than that ofnonane, after the nonidealities caused by polarity differ-ence between the ketone dopant and straight-chain hy-drocarbon solvent are accounted for, the cyclohexanonedopant escaped to the vapor phase much faster than theoverall solution. Thus cyclohexanone dopant concentra-tion in the liquid would decrease as the temperature in-creases during a LIF imaging experiment. Addition of alighter component, iso-octane (NBP 5 99 8 C), resultedin faster evaporation of the solution and more nearly idealbehavior of the dopant evaporation. An optimized con-centration of 1.4% cyclohexanone dopant, 20% iso-oc-tane, and 78.6% nonane [all (w/w)] was subsequentlyidenti® ed as the best co-evaporative system for cyclohex-anone ® lm thickness imaging. Similar calculations werealso performed for 2-methyl cyclopentanone, and an op-timized concentration of 1.4% 2-methyl cyclopentanonein 55% iso-octane and 43.6% nonane [all (w/w)] wasidenti® ed. This concentration of the dopant [1.4 wt % (w/w)] is much lower than the previously used 10% (w/w)dopant,24 and is unlikely to cause deviations from engineperformance with undoped fuels.

LIF Image Calibration. Film Thickness Calculationfrom Sample Weight /Total Volume. Thin fuel ® lm thick-ness was calculated on the basis of either the volume ofthe sample added (Eq. 4a) or the weight and density ofthe sample (Eq. 4b).

21 5 V /( p r ) (4a)

21 5 W /(r p d ) (4b)

where l 5 ® lm thickness (cm); V 5 volume of sampleadded (mL); W 5 sample weight (g); d 5 solution com-posite density (g/cm 3); and r 5 radius of sample cell(cm).

The average ¯ uorescence intensity (background cor-rected) in the area of interest (center 40% of the cellwhere laser intensity was uniform) was normalized to thatof the reference sample to eliminate the effects of shot-to-shot variation of the laser. A minimum of three ex-periments were averaged to establish the calibration


curve for each compound at each thickness. The average¯ uorescence intensity was plotted against the calculated® lm thickness to obtain the LIF intensity vs. ® lm thick-ness calibration curve.

Since the ® lm thickness can be calculated either fromthe weight of the sample, as measured by the balance atthe time the ¯ uorescence image was taken, or from thetotal volume of the sample added, the results of the twomethods were compared. The thickness calculated fromthe sample weight was consistently about 4% less thanthat calculated from the sample volume. Since wetting ofthe metal sample cup wall was observed in the sampleaddition process, it was assumed that some thinning oc-curred due to evaporation from the wall. The ® lm thick-ness was calculated from the sample weight, taken at thesame time the image was taken ( 6 0.2 s), since error dueto evaporation between the time of the addition of thesample and the taking of the images was eliminated. Thedensity used to calculate liquid thickness for each samplewas the average of three experimental measurementsfrom the sample weight and volume at room temperature.For 1.4% (w/w) cyclohexanone solution, d 5 0.712 60.001 g/mL, and for 1.4% (w/w) 2-methyl cyclopentan-one, d 5 0.702 6 0.001 g/mL. The sensitivity of thebalance was checked with 1, 3, and 10 mg standardweights at both room temperature and under heated con-ditions; the maximum deviation was 6 1 mg at room tem-perature and 6 3 mg under heated conditions. The thick-ness calculated from the sample weight was used in bothroom-temperature and heated calibration experiments.

LIF Image Calibration at Room Temperature. Figure5 shows the typical raw results for a calibration experi-ment. The background-corrected ¯ uorescence intensitydoes not rise above zero until a small amount of solutionis added. The intensity then increases linearly with theamount of the sample and eventually rises more slowly,due to the effect of increasing optical thickness. The ini-tial amount of sample added does not go to the center ofthe sample cup, where the ¯ uorescence intensity is mea-sured. Instead, it partially wets the walls before liquidaccumulates in the center of the cell.

The method used for (corrected) ® lm thickness calcu-lation is similar to the standard addition method. Thesample weight due to the initial amount of the solution,which wets the walls but does not accumulate in the cen-ter of the cup, is determined from the x-axis intercept ofthe linear ® t. The true sample thickness at the center ofthe sample cell was calculated on the basis of a ``cor-rected weight’ ’ , obtained by subtracting this initial weightfrom the total sample weight. The ® lm thickness was cal-culated on the basis of the corrected sample weight, andthe resulting ® lm thickness is the ``corrected thickness’ ’ .The LIF intensity increases linearly as the corrected ® lmthickness increases up to about 1.3 mm. For cyclohexa-none solution (Î 5 15 M 2 1 cm 2 1,33 l 5 1.3 mm, and c ù01 M), A ù 0.2, which is already above the ideal opticalthin region (A 5 0.1). Similar results are obtained for the2-methyl cyclopentanone solution. However, if a moresensitive CCD camera were used, the concentration ofthe dopant could be further lowered, and then the appli-cable ® lm thickness could be expanded beyond 1.3 mm.Within the thickness limit ( , 1.3 mm), LIF imaging in-tensity increases linearly with the ® lm thickness (Fig. 6).

A linear ® t of the corrected ® lm thickness vs. LIF inten-sity resulted a correlation coef® cient R 2 5 0.99. Calibra-tion with 2-methyl cyclopentanone as the dopant alsogave linear response of ¯ uorescence intensity to the liq-uid thickness ( , 1.3 mm) with a correlation coef® cient R 2

5 0.99.LIF Image Calibration under Heated Conditions. The

goal was to develop a method which could be used underconditions that simulate engine cold start, including heat-ing. Since the two dopants selected are virtually insen-sitive to oxygen concentration and temperature changes,it was predicted that the linear correlation between the® lm thickness and the LIF intensity observed at roomtemperature should be the same under heated conditions.Calibration experiments of ® lm thickness vs. LIF inten-sity under heated conditions were also conducted by add-ing known amounts of sample solutions at room temper-ature to the sample cell. The cell was then heated to 1708 C over a period of 10 min to evaporate the solution. Theslow heating rate was necessary to minimize the devia-tion in sample weight measurement caused by the con-vective motion of the surrounding heated air. Fluores-cence images were acquired during this process, and thesample weight was recorded at the same time each ¯ uo-rescence image was captured ( 6 0.2 s). Film thicknesswas again calculated from the corrected sample weightand the composite solution density at room temperature,and the ¯ uorescence intensity was plotted vs. correctedthickness.

Results of the heated cup calibration for cyclohexa-none are shown in Fig. 7. The solid line is the ® ttedcalibration curve obtained at room temperature, and thedata points are the ones obtained under heated conditions.The two parts of the graph were processed in the sameway and were not normalized to each other. The resultsindicated that, although the data are somewhat scatteredunder the heated experimental conditions, they clearlymatch the calibration curve obtained at room tempera-ture. Thickness resolution was limited by the deviationfrom weight measurement (1 mg and 3 mg at room tem-perature and under heated conditions, respectively) andCCD sensitivity. An uncertainty of 1 mg (at room tem-perature) and 3 mg (under heated conditions) in weightmeasurement results in uncertainties of 0.01 mm and 0.03mm in the thickness, respectively, at a nominal thicknessof 1.00 mm. Deviations caused by oxygen quenching ofdopant ¯ uorescence would result in uncertainties of nomore than 20%.

Potentially, several factors could cause deviation of theheated cell results from those obtained at room temper-ature. First, although the dopant ¯ uorescence remains vir-tually insensitive (within a few percent) to either tem-perature or oxygen concentration changes in the solution,the total ¯ uorescence could change slightly at high tem-perature as a result of these effects. Second, the calcu-lated ® lm thickness towards the very end of the evapo-ration might deviate slightly from the true value due tochanges in the solution composition and consequentchanges in the solution density. Third, although thedopant concentration in the liquid ® lm remains almostunchanged throughout the evaporation procedure, it even-tually decreases rapidly towards the very end of the evap-oration (Fig. 8). This decrease could cause some under-


representation of the ® lm thickness. Last, the measure-ment of the sample weight throughout the experimentcould be affected by the convective motion of the sur-rounding heated air, no matter how carefully this effectwas minimized. Despite all the concerns, the results ofthe two calibration methods are completely consistent.

Similar results were obtained with 2-methyl-cyclopen-tanone (Fig. 9).

Demonstration Experiment. A demonstration exper-iment was performed to show a potential application ofthe method. The calibration curve established in the pre-vious session was used to calculate the ® lm thickness onthe basis of the ¯ uorescence intensity of an image of anevaporating liquid ® lm. An approximately 0.5 mL testsolution was sprayed on the metal surface, and a se-quence of images was obtained. The initial temperatureof the metal surface was either room temperature or 1008 C, and the experiments were concluded once the cuptemperature reached 170 8 C. The intensity image at a par-ticular point was converted to a thickness image by useof the calibration curve. Figure 10 shows the liquid ® lmthickness of the cyclohexanone solution sprayed onto ametal surface at room temperature, and Fig. 11 shows theliquid ® lm thickness of the same solution sprayed onto apreheated metal surface at 100 8 C, for which the evapo-ration is much more rapid. With these dopant systems,quantitative 2D real-time in situ determination of the liq-uid ® lm thickness should now be possible.

CAUTIONS REGARDING USE IN ENGINES

The systems described in this paper represent a care-fully designed response to many of the diagnostic prob-lems associated with use in test engines, but potentialusers should be aware of, at least, the following potentialproblems: (1) The synthetic fuel described here does notinclude components characteristic of the light and heavyends of distillation range. In addition, the high proportionof straight-chain alkanes may make the octane number ofthe synthetic fuel too low for normal engine operation.Users interested in the effects of the light ends on cold-start performance should include lighter carbonyl com-pounds, such as acetone and 3-pentanone, 23±25 in their di-agnostic formulations. (2) Oil and fuel in a engine com-monly contain ¯ uorescent materials. In order to obtainaccurate results from the systems described in this paper,users must test for the background signals from theseother sources and ® nd ways to minimize the interferenc-es.

ACKNOWLEDGMENTS

This work was supported by Ford Motor Co. through its UniversityResearch Program and by U.S. ARO (Grants DAAH04-94-G-0020 andDAAG 55-97-1-0265). The authors wish to express their gratitude toProf. Richard. A. Caldwell, Prof. John P. Ferraris, Mr. Yadong Zhao,and Mr. Qingzheng Lu for their useful suggestions and discussions. Ourspecial thanks to Advanced Optical Diagnostics Group of the UnitedTechnology Research Center for use of their excimer laser at their site.

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Fluorescence-Based Method Designed for Quantitative Measurement of Fuel Film Thickness during Cold-Start of Engines