Fuel 102 (2012) 264–273

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Fuel

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Combustion process of JP-8 and fossil Diesel fuel in a heavy duty diesel engineusing two-color thermometry

Jinwoo Lee, Heechang Oh, Choongsik Bae ⇑Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon, Republic of Korea

h i g h l i g h t s

" Emission of JP-8 and fossil Diesel fuel in terms of direct imaging and 2-color thermometry." Stronger and longer flame luminosity with fossil Diesel fuel." Higher decreasing rate of flame luminosity with JP-8." Locally high temperature region and lower level of KL factor with JP-8." Combustion analysis provides the reason for high NOx and low smoke of JP-8.

a r t i c l e i n f o

Article history:Received 3 March 2011Received in revised form 19 June 2012Accepted 12 July 2012Available online 26 July 2012

Keywords:JP-8Fossil Diesel fuelDirect imagingTwo-color thermometry

0016-2361/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.fuel.2012.07.029

⇑ Corresponding author. Tel.: +82 42 350 3044; faxE-mail address: csbae@kaist.ac.kr (C. Bae).

a b s t r a c t

An experimental study was performed to analyze the combustion processes of JP-8 and fossil Diesel fuelin an optically-accessible single-cylinder heavy-duty diesel engine equipped with a high pressure com-mon-rail injection system. In terms of emission, JP-8 emitted less smoke with more HC and NOx. Directimaging and two-color thermometry were applied to verify the emission trend for both fuels. The com-bustion process was characterized by means of image analysis focusing on the luminosity intensity andits spatial distribution (flame spatial fluctuation (FSF) and flame non-homogeneity (FNH)). The resultsfrom the two-color thermometry were analyzed by the flame temperature and KL factor distribution.From the combustion process analysis of the direct imaging, it was verified that JP-8 had a longer ignitiondelay compared to fossil Diesel fuel regardless of injection pressure. However, flame luminosity of JP-8was vanished more rapidly. The flame luminosity intensity analysis showed that fossil Diesel fuel hadstronger flame luminosity overall and duration of visible flame luminosity was longer than JP-8. Thisimplies that fossil Diesel fuel had more diffusion dominant combustion. From the flame luminosity var-iation rate analysis, decreasing rate of flame luminosity for JP-8 was higher compared with fossil Dieselfuel, showing that oxidation rate of JP-8 was much higher than fossil Diesel fuel. From FSF and FNH anal-ysis, JP-8 showed lower value for both FSF and FNH in the later stage of combustion, because the laterstage of combustion with JP-8 has less jet structure in comparison with fossil Diesel fuel. The flametemperature field from two-color thermometry showed that locally high temperature region existed withJP-8. KL factor distribution of JP-8 was distributed more uniformly with a relatively lower level of KLintensity in comparison with fossil Diesel fuel in the late stage of combustion.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Jet Propellant type 8 (JP-8) is kerosene jet fuel equivalent to civilJET A-1 with the inclusion of three additives; static dissipater, cor-rosion inhibitor, and icing inhibitor. For logistical reasons, theNATO nations have initiated using single fuel for all land based mil-itary aircraft, vehicles and equipment [1]. This idea has become theSingle Fuel Concept (SFC), which JP-8 was selected for the SFC.

ll rights reserved.

: +82 42 350 5023.

Therefore, it is necessary that diesel engines operate with JP-8 suc-cessfully, when fully substituted by JP-8 [2–5]. Most enginesequipped with the conventional fuel injection system, such as amechanical rotary pump and unit injector, operated successfullywith JP-8 [3,4], while penalties in torque and fuel economy dueto the lower density of JP-8 were reported [6]. With regard to ex-haust emissions, using JP-8 was reported to have the potential toreduce smoke emission compared to fossil Diesel fuel [6], while re-search with opposite result was also published [7]. Smoke emissionis a crucial issue for military training and combat missions, as wellas for civil application. However, an explanation for not only smoke

Nomenclature

a spectral range constantk wave length (m)e light emissivityK absorption coefficient (1/cm)i,j pixel position�I normalized flame luminosity for an image at a certain

crank angleIi,j captured flame radiation intensity at pixel position (i,j)@I/@x partial differentiatrion in x direction@I/@y partial differentiatrion in y directionN total pixel numberL line of sight path length through flame (cm)Pinj injection pressure (MPa)Qinj injection quantity (mg/stroke)Ta apparent temperature at k

Tflame flame temperature on the two-color thermometry im-age [K]

T true soot particle temperature

AbbreviationsATDC after top dead centerCAD crank angle degreeFNH flame non-homogeneityFSF flame spatial fluctuationFSN filtered smoke numberHC hydrocarbonHFR hydraulic flow rateJP-8 Jet Propellant type 8NOx nitrogen oxideSFC Single Fuel Concept

J. Lee et al. / Fuel 102 (2012) 264–273 265

but also other emissions has not yet been adequately provided.Moreover, deep observation for combustion process using JP-8 indiesel engine has not been fully investigated. Generally, it is knownthat low cetane number fuel delays the start of combustion, whilelow distillation temperature fuel shows the shortened ignition de-lay. Therefore, it would be meaningful to figure out the effect oftwo conflicting features on combustion process.

In the current work, the whole cycle combustion was visualizedby using a high-speed digital video camera, which provided a com-prehensive comparison of combustion processes to correlate withemission results. Tests were made at the fixed injection quantityby varying the injection pressure and the injection timing. Thecombustion process was characterized by means of image analysisfocusing on the luminosity intensity and its spatial distribution(flame spatial fluctuation (FSF) and flame non-homogeneity(FNH)). Finally, two-color optical thermometry was applied to pro-vide the flame temperature and KL factor distribution for the morequantitative insight about emission formation.

2. Experimental setup

2.1. Engine performance and optical diagnostics

Fig. 1a shows a schematic diagram of the experimental appara-tus used in this study. The detailed specifications of the test engineare presented in Table 1. The engine was a four-stroke, water-cooled, single-cylinder, naturally-aspirated, direct injection dieselengine. The engine was a typical heavy-duty diesel engine with abore of 128 mm and a stroke of 142 mm, yielding a displacementof 1.818 � 10�3 m3. It was equipped with a high pressure com-mon-rail injection system, operated by a programmable injectordriver (Zenobali Co., IDU 5000B) capable of controlling injectionpressure (up to 160 MPa), injection quantity, and injection timing.An eight-hole, SAC type nozzle injector was employed withhydraulic flow rate (HFR) of 860 cc/30 s and injection angle of146�. The fuel supply system was set up for JP-8 and fossil Dieselfuel, respectively. Engine speed was controlled constantly by aDC dynamometer (90 kW). A rotary encoder (Autonics,3600 pulses/revolution) mounted on the camshaft was used tocontrol the fuel injection timing. A smoke meter (AVL 415s) wasused to measure the engine-out smoke emission. Nitrogen oxide(NOx), hydrocarbon (HC) emissions were measured using exhaustgas analyzer (HORIBA, MEXA 1500D).

An elongated piston was installed to enable the mounting of a45� mirror beneath the piston quartz window, which allowed a

76 mm diameter visual field in 128 mm bore. A high speed digitalvideo camera (Vision Research Inc., Phantom V.7.0) was used totake images of the natural luminosity produced by combustion.This high speed imaging system can record up to 10,000 framesper second with resolution of 512 � 384 pixels, thus images couldbe taken every 0.72 CAD at an engine speed of 1200 rpm. The expo-sure duration was 10 ls to obtain clear images.

The optics layout for two-color thermometry is shown inFig. 1b. Two-color thermometry is based on continuous radiationof soot particles during the combustion of fuel–air mixture, whichare measured at two different colors within the broadband sootemission spectrum. This method is regarded as one of the mostmature diagnostics for soot measurement [8,9]. The light emittedby glowing soot is reflected by a 45� mirror directly to a bi-prism.A bi-prism is used to split a primary light beam into two interferingbeams, which is made up of two equal prisms with a very smallrefraction angle [10]. In refracting a beam by the bi-prism, twobeams occur: one after refraction by one half of the bi-prism, theother by the other half of the bi-prism. These two beams passthrough two band-pass filters (550 nm and 750 nm), which are in-serted behind the bi-prism and in front of CCD camera to obtainimages at selected wavelengths. The band-pass filter of 550 nmwas selected for high accuracy at high temperature condition,while 755 nm filter was chosen for the measurement for widerange of temperature. These selected wavelengths are thought tohave high slope of change in the monochromatic emission in therange of 1500–2500 K and to be influenced little by chemilumines-cence [8]. The mathematics of the two-color thermometry, as wellas the determination and analysis of flame temperature and the KLfactor has been described in previous studies [8,9,11–13]. Theamount of light intensity that is produced is dependent upon thenumber of soot particles per unit volume, as shown in equation be-low [14].

ek ¼ 1� e�KLkað Þ ð1Þ

where e is light emissivity, k is wave length (m), a is spectral rangeconstant (1.39 for visible band), K is soot absorption coefficient (1/cm), directly proportional to number density of soot particles, and Lis line of sight path length through flame (cm).

Using Eq. (1), the two-color equation is obtained as followed.

KL ¼ �kaln 1� exp �C2

k1Ta� 1

T

� �� �� �ð2Þ

where C2 is 14,388 lm K, Ta is apparent temperature at k, and T istrue soot particle temperature.

Fig. 1. The schematic diagram for (a) engine performance and natural luminosity imaging and (b) two-color thermometry.

266 J. Lee et al. / Fuel 102 (2012) 264–273

Since KL is constant, if one chooses two wavelengths the righthand side of Eq. (2) can be solved for each wavelength, as shownin Eq. (3).

1� exp �C2

k1

1Ta1� 1

T

� �� ��ka11

¼ 1� exp �C2

k2

1Ta2� 1

T

� �� �� �ka22

"

ð3Þ

If k, k2, a1 and a2 are known, and the apparent temperatures aremeasured, Eq. (3) may then be solved for the only unknown, T.

2.2. Test fuel and experimental condition

The properties of JP-8 and fossil Diesel fuel evaluated in thisstudy are listed in Table 2. Cetane number, liquid density and ki-netic viscosity are lower with JP-8, while lower heating value isslightly higher for JP-8. Through the whole range, JP-8 presentslower distillation temperature than fossil Diesel fuel. Experimentalconditions for engine operating are summarized in Table 3. The en-gine was operated at 1,200 rpm under both motored and fired con-ditions. The coolant temperature was set to 353 K. Two differentinjection pressures (30 and 140 MPa) were applied to test theeffect of injection pressure. The injection timings were sweptfrom �24 to 4 CAD after top dead center (ATDC). The same static

Table 2Properties of JP-8 and fossil Diesel fuel.

Properties ASTM Method JP-8 Fossil Diesel fuel

Cetane number D 976 39 47Liquid density (kg/m3) D 4052 800 840Low heating value (MJ/kg) D 3338 43.3 42.5Kinetic viscosity @ 313 K D 445 1.2 2.6Sulfur content (ppm) D 4294 100 �300Aromatic content (vol.%) D 1319 19.3 27.5Distillation (�C) D 8610% 162 21020% 170 23050% 189 28290% 234 330

Table 3Experimental conditions for engine operating.

Engine speed (rpm) 1200Fuel JP-8 and fossil Diesel fuelInjection quantity [mg/stroke] 60 for fossil Diesel fuel, 58.8 for JP-8Injection timing (CAD ATDC) �24, 20, 16, �12, �8, �4, 0, 4Injection pressure (MPa) 30, 140

Table 1Engine specification.

Engine Single-cylinder, direct injection, four-valves,compression-ignition engine

Bore � Stroke 128 � 142 mmDisplacement 1.818 � 10�3 m3

Compression ratio 17:1Fuel injection type Common rail injection system (up to 160 MPa)Injector 8 holes, HFR 860 cc/30s, injection angle 146�

J. Lee et al. / Fuel 102 (2012) 264–273 267

injection timing applied for each condition with JP-8 and diesel.However, keeping the constant combustion phasing with differentinjection timing for JP-8 and diesel is also meaningful in engineperformance aspect, which will be performed as a next researchstep. Injection quantity of each fuel was set to represent a mid-loadcondition, which were 60 mg/stroke for fossil Diesel fuel and58.8 mg/stroke for JP-8 considering its higher heating value asshown in Table 2. The difference in fuel mass between JP-8 andfossil Diesel fuel can cause some errors in interpreting two-colorthermometry result due to its line-of-sight-integrated measurement.However, the injection quantity difference is small (1.2 mg/stroke)comparing with difference in fuel characteristics (cetane numberand distillation temperature as shown in Table 2), which will beneglected in analyzing two-color thermometry images.

Fig. 2. Comparison of in-cylinder pressure and heat release rate with differentinjection pressure of (a) 30 MPa, (b) 140 MPa.

3. Results and discussion

3.1. Heat release rate and emission of JP-8 and fossil Diesel fuel

The authors previously performed the evaluation of JP-8 andfossil Diesel fuel in terms of basic spray characteristics and com-bustion characteristics by measuring macroscopic spray images,in-cylinder pressure and exhaust emissions [15]. The previousstudy concluded that JP-8 had a shorter spray tip penetrationand wider spray angle than fossil Diesel fuel mainly due to the fas-ter vaporization characteristics of JP-8. Fig. 2 shows the in-cylinderpressure and the heat release rate curve for JP-8 and Diesel fuelwith the injection pressures of (a) 30 MPa and (b) 140 MPa at injec-tion timing of �20 CAD ATDC and �8 CAD ATDC, respectively,which showed the maximum IMEP at each injection pressure.

Combustion commenced earlier with fossil Diesel fuel due to itsshorter ignition delay even though the injection timing was thesame for each fuel, as can be seen in Fig. 2. It was expected thatJP-8 would have shorter ignition delay due to superior vaporizationand consequently faster mixing. At this point, the main reason forthis result is attributed to different cetane number (JP-8: 39, fossilDiesel fuel: 47) as listed in Table 2. The peak heat release rate of JP-8 was higher than that of fossil Diesel fuel regardless of injectionpressure. The premixed burn portion of JP-8 was greater than thatof fossil Diesel fuel. This phenomenon clearly resulted from thesuperior evaporation rate and longer ignition delay of JP-8, becausethese characteristics can provide enhanced fuel–air mixing withmore fuel during the premixed burn phase. This confirms that low-er cetane number of JP-8 has benefit to improve the mixing qualityby prolonging the ignition delay. Although Exhaust Gas Recircula-tion (EGR) was not applied in this study, this result implied that thecombination of EGR with JP-8 has the possibility of simultaneousreduction of NOx and smoke. These combustion phenomena withJP-8 corresponds with recent research on the PPC combustionmode using various fuel designs, which means that JP-8 has the po-tential to be applied for the PPC combustion mode [16]. In terms ofemission, JP-8 showed a benefit in smoke reduction with greateramount of HC and NOx as can be seen in Fig. 3. The higher evapo-ration rate and longer ignition delay of JP-8 provided greaterchance for the formation of a premixed mixture close to the stoi-chiometric condition. This larger portion of premixed burn resultedin more NOx. The potential to form a locally over-lean mixture dueto a higher vaporization rate and longer ignition delay was thoughtto result in incomplete combustion in some parts of the cylinder,leading to higher HC with JP-8. This resulted is supported by the

Fig. 3. Comparison of emission characteristics (HC, NOx, smoke) with different injection pressure ((a): Pinj: 30 MPa, (b) 140 MPa).

268 J. Lee et al. / Fuel 102 (2012) 264–273

calculation of combustion efficiency, which showed that the com-bustion efficiency of JP-8 combustion was lower than that of dieselcombustion [15]. Furthermore, less smoke was emitted with JP-8because fuel-rich zone was reduced due to the enhanced fuel–airmixing. From the following section, the result of combustionvisualization by directing imaging and two-color thermometry ispresented to verify the combustion and emission features of JP-8and fossil Diesel fuel shown in previous study.

3.2. Combustion visualization of JP-8 and fossil Diesel fuel

The combustion images obtained using a high-speed digital vi-deo camera are shown in the top (fossil Diesel fuel) and bottom (JP-8) of Fig. 4 for the different injection pressures. The color-map onthe right side of Fig. 4 indicates the intensity of the natural lumi-nosity. In the case of the injection pressure of 30 MPa, the flameluminosity was detected at an earlier crank angle with fossil Diesel

J. Lee et al. / Fuel 102 (2012) 264–273 269

fuel, indicating that fossil Diesel fuel has a shorter ignition delaythan JP-8. At 4.5 CAD ATDC, the diffusion flame already spreadout over the entire combustion chamber, showing very strongflame luminosity. After this crank angle, the flame luminosity de-creased, while the remaining diffusion flames were carried down-stream to the bowl wall. Especially, after 23.9 CAD ATDC, the flameluminosity near the injector became weak and nearly disappearedat 30.4 CAD ATDC. This phenomenon was recently verified by somestudies with the conclusion that stagnant mixtures remaining nearthe injector that are not carried by residual jet momentum to thebowl wall, but simply become very lean due to an ‘‘entrainmentwave’’ [17–19]. This observation suggests that the flame luminos-ity is removed by its oxidation rather than the momentum im-parted by the diesel jet. On the other hand, in the case of JP-8,flame luminosity maintained a lower level and was mostly con-fined in the bowl with little in the squish region compared to fossilDiesel fuel combustion through the combustion process. This im-plies that the diffusion burn phase was not dominant as much as

Fig. 4. Normalized flame luminosity and flame luminosity variation rate and natural flam

fossil Diesel fuel combustion. Moreover, at 8.8 CAD ATDC, abouthalf of the visible domain was covered with weak flame luminosityonly at the perimeter of the combustion chamber. A more vigorousoxidation process was confirmed over 17.4 CAD ATDC, showingthat the flame luminosity near the injector dissipated more rapidlycompared with fossil Diesel fuel combustion. This indicates thatthe combustion might be accelerated by the fast vaporization char-acteristics of JP-8. The series of flame luminosity values obtained athigher injection pressure are totally different, as can be seen inFig. 4b. With fossil Diesel fuel, the flame luminosity appeared ini-tially near the wall region in the bowl. Moreover, the site nearthe injector was not covered by the flame luminosity during entirecombustion process, showing the weaker flame luminosity inten-sity compared with the lower injection pressure at 30 MPa. Theseresults suggest that the flame lift-off length was elongated withhigher injection pressure, which implies that more ambient air isentrained into the fuel spray, resulting in less smoke formation[20]. Using JP-8, the flame luminosity was almost invisible

e luminosity of diesel and JP-8 at injection pressure of (a) 30 MPa and (b) 140 MPa.

Fig. 5. Flame spatial fluctuation and flame non-homogeneity for JP-8 and dieselwith different injection pressure (Pinj: (a): 30 MPa, (b): 140 MPa).

270 J. Lee et al. / Fuel 102 (2012) 264–273

throughout the combustion process contrary to fossil Diesel fuelcase which showed flame luminosity a little. At 7.8 CAD ATDC,the flame luminosity with very low intensity was detected onlyaround the perimeter of the combustion chamber, which lastedonly about 5 CAD. Later in the cycle at approximately 12.2 CADATDC, some flame luminosity appeared near the injector, whichwas formed by fuel dribbled from the injector after the end ofinjection. This kind of flame luminosity was also detectable withother experimental results, which is known as a source of HCemissions.

The time resolved flame luminosity varying with crank anglewas obtained for JP-8 and fossil Diesel fuel by normalizing pixelvalues of the flame luminosity, as shown in the middle of Fig. 4with different injection pressures. Luminosity intensity was aver-aged over the flame field, while the maximum value of intensityfor each pixel was set to ‘1’, as the normalized flame luminositycan be expressed in equation be low.

�I ¼P

i

PjIi;j

Nð4Þ

where Ii,j is the flame radiation intensity at pixel position (i,j) and Nis the total pixel number.

The flame luminosity variation rate was depicted in the bottomof the normalized flame luminosity. The overall flame luminositywas weaker and the duration of visible flame luminosity wasshorter for JP-8, as discussed in the combustion image analysisregardless of injection pressure. The flame luminosity of fossil Die-sel fuel appeared in the earlier crank angle than that of JP-8. Thepeak value of the flame luminosity and flame luminosity variationrate were higher for fossil Diesel fuel, indicating the diffusion burnphase of fossil Diesel fuel was more vigorous than JP-8. However,the location of peak flame luminosity was shifted to an earliercrank angle than fossil Diesel fuel and the decreasing rate of flameluminosity for JP-8 was higher compared with fossil Diesel fuel atthe low injection pressure. The former confirms that the combus-tion of JP-8 progressed faster, even though combustion com-menced later. And the latter suggests that the late stage flamewas burnt out faster for JP-8 than fossil Diesel fuel. In other words,the oxidation rate of JP-8 was much higher than fossil Diesel fuel,which resulted in less smoke emission through the tailpipe. Thisassumption is supported by a previous study [21]. In the case ofhigher injection pressure, even the decreasing rate of the flameluminosity of fossil Diesel fuel was higher compared with JP-8.However, it is just because the absolute value of the peak flameluminosity with JP-8 is much lower than that with fossil Dieselfuel. In other words, the ratio of the decreasing rate to the increas-ing rate is larger with JP-8.

Flame spatial fluctuation (FSF) and Flame Non-Homogeneity(FNH) concepts were computed in this paper to provide moreinformation for the flame geometric characteristics compared toflame luminosity [22]. FSF is defined as follows:

FSF ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiX

i

Xj

Ii;j ��I 2

sð5Þ

where Ii,j is the flame radiation intensity at pixel position (i,j) and �I isthe mean flame radiation intensity for an image at a certain crankangle. FNH is defined as the sum of the lengths of spatial gradientsfor the images over all pixels:

FNH ¼X

i

Xj

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi@I=@xð Þ2i;j þ @I=@yð Þ2i;j

qð6Þ

where oI/ox and oI/oy are the partial differentiation in x and y direc-tions, respectively. The time evolutions of both FSF and FNH areplotted in Fig. 5 for different injection pressures. The meaning ofFSF is statistically similar to the definition of standard deviation,

which shows the data deviation from the mean value with no infor-mation about the data variation around the mean value. However,FNH compensates the information of the data variation, which pro-vides spatial distribution of non-homogeneity characteristics [23].In the case of an injection pressure of 30 MPa, at an early stage,especially during the most vigorous diffusion burn phase, the FSFof JP-8 was higher than that of fossil Diesel fuel, which indicates dif-fusion flames were well distributed with strong and similar level offlame luminosity throughout the combustion chamber with fossilDiesel fuel, while the diffusion flames with JP-8 were more variable.One hypothesis can be drawn here that a locally lean or locally richmixture, due to the superior vaporization property and unsymmet-rical spray development of JP-8, resulted in the relatively non-uni-form flame field. This hypothesis can be supported further by theobservation that the crank angle with the lowest FNH value of fossilDiesel fuel is coincident with the crank angle which showed themost intense luminosity, e.g. 5.9 CAD ATDC. Specifically, a lowerFNH value does not always correspond to the more homogeneouscombustion. However, both the FSF and FNH of JP-8 showed lowervalues than that of fossil Diesel fuel for the later stage of combus-tion, because the later stage of combustion with JP-8 has less jetstructure in comparison with fossil Diesel fuel due to faster oxida-tion process as confirmed from the combustion images in Fig. 4.At the injection pressure of 140 MPa, it is obvious that JP-8 hadmuch lower FSF and FNH values for all crank angles. This result sug-gests that JP-8 had a much more premixed and homogeneous dom-inant combustion, while fossil Diesel fuel still showed relativelydiffusion dominant combustion. From the analysis of combustionimages above, the emission trends discussed earlier can be ex-plained, especially for smoke emission.

Fig. 6. Flame temperature and KL factor distribution at injection pressure of (a) 30 MPa and (b) 140 MPa.

J. Lee et al. / Fuel 102 (2012) 264–273 271

For more quantitative analysis, the results of two-color opticalthermometry are depicted in terms of the flame temperature andKL factor distribution in Fig. 6 for different injection pressures.The color of each pixel represents a temperature and KL factor va-lue for each pixel matched with the color-bar. To some extent, theflame temperature and KL factor obtained by two-color opticalthermometry can be interpreted as an indication of NOx formationand soot concentration inside the fuel spray [24]. The signal fromcombustion could hardly be detected during the premixed burnphase for both injection pressure cases, which shows that smokewas rarely generated during the premixed burn phase, especiallyfor JP-8. In the case of a 30 MPa injection pressure, the flame tem-perature field of fossil Diesel fuel was distributed more uniformlythan that of JP-8, while the right-side region of the combustionchamber showed higher temperature regions with JP-8. This lessuniform flame temperature field with JP-8 combustion seems tobe resulted from asymmetric spray pattern with JP-8 due to lowerkinetic viscosity characteristic, which led to different degree offuel–air mixing for each spray [15]. Therefore, it was found thatnot only the premixed burn dominant combustion, but also theexistence of the locally high temperature in diffusion flame con-tributed to a higher level of NOx for JP-8 combustion. The flametemperature distribution within the combustion chamber at a gi-ven instant (10 CAD ATDC) is shown in Fig. 7a. For Fig. 7a, the num-ber of pixel was evaluated corresponding to each temperatureprofile, which pixel number can be interpreted to flame are for thatflame temperature [25]. For both fuels, most flame area was at thetemperature of around 2000 K. However, high temperature areaover 2000 K appeared with JP-8 combustion, while fossil Dieselfuel combustion showed larger area with relatively lower temper-ature of around 1500 K. This observation also supports the higherNOx level of JP-8 combustion to some extent. KL factor distributionin Fig. 6 showed that fossil Diesel fuel combustion resulted in amore uniformly distributed soot concentration with higher valuethan JP-8 combustion at 2 CAD ATDC. In general, the soot is knownto be formed more in the jet periphery, especially toward the lead-ing edge (head vortex region) [26]. In that point of view, the KL dis-tribution shown in this study seems not to be in agreement with

Dec.’s previous research, because the middle region of the jet flameshows higher KL factor values than the region with cylinder wallside, which are appeared in the same pattern for both fuels. Thisphenomenon is thought to be resulted due to the fact that the re-gion near the cylinder wall side already reached to the conditionwhich the soot can be oxidized by OH radical attack, which meansthat the combustion process is under the late soot burnout periodfor both fuels [27]. However, this assumption requires further re-search for the verification with another method like OH-PLIF. At10 CAD ATDC, soot concentration of JP-8 decreased drastically nearthe injector, while the soot concentration remained almost con-stant with fossil Diesel fuel. This can be confirmed more quantita-tively by Fig. 7b. KL factor distribution of JP-8 was found moreuniformly with a relatively lower level of KL intensity comparedto fossil Diesel fuel. Moreover, the fossil Diesel fuel case showedmore pixels with greater KL intensity. The averaged KL factor valuealso showed that JP-8 had lower value than fossil Diesel fuel as canbe seen in Fig. 8. Particularly, the difference in KL factor becameslightly larger when the timing was retarded to 10 CAD ATDC. KLfactor analysis supports the discussion that the oxidation rate ofJP-8 was more dominant than that of fossil Diesel fuel. Conse-quently, it can be concluded that this observation is one of themost influencing factors explaining why JP-8 combustion resultedin less smoke emission. The flame temperature of fossil Diesel fuelseemed much higher than that of JP-8 at first glance, when theinjection pressure increased to 140 MPa, as can be seen in Fig. 6.However, this is because the flame temperature field of JP-8 hadnull pixel value over the majority of the combustion chamberdue to superior premixed combustion. KL factor distributionshowed that fossil Diesel fuel still showed some parts with soot,while the existence of soot was hardly detected with JP-8.

4. Conclusions

Optical measurements were carried out in a transparent heavy-duty direct injection diesel engine with a single-cylinder engineconfiguration and a common rail injection system. The combustion

Fig. 7. Flame temperature distribution (a) and histogram of KL intensity (b) at 10 CAD ATDC with injection pressure of 30 MPa.

Fig. 8. Comparison of averaged KL factor at 2 CAD ATDC and 10 CAD ATDC withinjection pressure of 30 MPa.

272 J. Lee et al. / Fuel 102 (2012) 264–273

process was characterized by means of image analysis, luminosityintensity analysis and spatial distribution analysis (flame spatialfluctuation (FSF) and flame non-homogeneity (FNH)). The result

from the two-color thermometry was analyzed by the flame tem-perature and KL factor distribution. The main findings from thisstudy are summarized as follows:

(1) From the flame images, flame luminosity of JP-8 was van-ished more rapidly and earlier than that of fossil Diesel fuel.This suggests that late stage combustion of JP-8 was acceler-ated by superior vaporization characteristics of JP-8.

(2) The analysis on flame luminosity intensity showed that fos-sil Diesel fuel had stronger flame luminosity overall andduration of visible flame luminosity was longer than JP-8.This implies that fossil Diesel fuel had more diffusion dom-inant combustion, while relatively premixed combustionwas predominated in JP-8 combustion. From the flame lumi-nosity variation rate analysis, decreasing rate of flame lumi-nosity for JP-8 was found higher compared with fossil Dieselfuel.

(3) From FSF and FNH analysis, applied to provide flame geo-metric characteristics, JP-8 showed lower value for bothFSF and FNH in the later stage of combustion, because thelater stage of combustion with JP-8 has less jet structurecompared to fossil Diesel fuel. This analysis is another prooffor faster oxidation process of JP-8 combustion.

J. Lee et al. / Fuel 102 (2012) 264–273 273

(4) The flame temperature field from two-color thermometryshowed that locally high temperature region was existedwith JP-8, which contributed to higher level of NOx. KL fac-tor distribution of JP-8 was distributed more uniformly witha relatively lower level of KL intensity in comparison withfossil Diesel fuel in the late stage of combustion.

Acknowledgments

The authors would like to appreciate Doosan Infracore and theAgency for Defense Development for their financial support. Wewould also like to thank Dr. Jungseo Park in Zenobalti Co. for thetechnical support.

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