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Energy 112 (2016) 549e561

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Energy

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Experimental investigation on spray and atomization characteristics ofdiesel/gasoline/ethanol blends in high pressure common rail injectionsystem

Zehao Feng, Cheng Zhan, Chenglong Tang*, Ke Yang, Zuohua HuangState Key Laboratory of Multiphase Flow and Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China

a r t i c l e i n f o

Article history:Received 21 March 2016Received in revised form25 May 2016Accepted 26 June 2016

Keywords:Diesel/gasoline blendsSpray and atomizationEthanolDroplet size and number densitydistribution

* Corresponding author. Room 8405, North 2nd Busity, No. 28 West Xianning Road, Xi'an, 710049, China

E-mail address: chenglongtang@mail.xjtu.edu.cn (

http://dx.doi.org/10.1016/j.energy.2016.06.1310360-5442/© 2016 Published by Elsevier Ltd.

a b s t r a c t

The effects of gasoline and ethanol addition on the spray and atomization characteristics of diesel sprayfrom a common rail injection system, was investigated in a constant volume chamber at differentambient and injection pressures. The fuel spray evolution process was recorded with high spatial andtime resolution and the corresponding spray tip penetration (STP), the cone angle were extracted. Theresults show that increased gasoline proportion in the test blends causes decreased STP and increasedspray cone angle. Additionally, the microscopic spray characteristics such as the local average dropletdiameter and statistical size distributions were measured by particle/droplet image analysis (PDIA)technique. As gasoline blending ratio increases, significantly smaller droplet size was observed whichindicates that the spray breakup and atomization processes were promoted. Further adding ethanolslightly increased the droplet size, but it is still much smaller than the droplet size of neat diesel spray.Moreover, the local droplet size increases along the radial direction but the local droplet volume fractionand normalized droplet number density decreases, indicating a reduced fuel concentration along theradial direction. Along the axial direction, the droplet size, the local droplet volume fraction and thenormalized droplet number density were almost constant.

© 2016 Published by Elsevier Ltd.

1. Introduction

Thanks to its very high compression ratio, the diesel engine asthe so-called compression ignition (CI) engine has higher torqueoutput and higher thermal efficiency compared to petrol engine,which makes it become one of the major power suppliers for ve-hicles nowadays. In addition, the lack of ignition system greatlyimproves its reliability, compared to regular petrol engine. As aconsequence, diesel engine has been widely used for a varietytransport applications such as heavy duty trucks merchant shipsand boats, and passenger buses and cars, especially in Europe andIndia. Diesel engine has also been used in non-transport applica-tions such as power permanent, portable and backup generators.However, in the last few decades, increasing public concerns havebeen raised over the environmental impact of the NOx and par-ticulate emissions from operating these engines. Efforts have been

ilding, Xi'an Jiaotong Unvier-.C. Tang).

made to developing techniques for simultaneously reducing theemission of NOx and particulate matter (PM), which is challengedby the nature of NOx-PM trade-off [1].

These concepts of strategies have been recognized and labeledwith LTC (low temperature combustion). In LTC, two targets areneeded to be achieved, one is that the locally fuel-rich mixtureregions should be avoided and the other is that the in-cylindercombustion temperature should be monitored to be a relativelylow level. Consequently the particulate emission can be reducedbecause of the locally over-rich regions were minimized and ther-mal NOx formation can be correspondingly prohibited as the flametemperature decreased [2]. Representative LTC strategies such aspremixed charged compression ignition (PCCI) combustion, ho-mogeneous charge compression ignition (HCCI) combustion andMK (modulated kinetic) combustion essentially all require theformation of a more homogeneous mixture. One way of increasingthe uniformity of the mixture and reducing the in-cylinder com-bustion temperature is to extend the ignition delay because alonger time before ignition favors the fuel evaporation and mixturetransport [3e7].

Recently, significant attention has focused on the fuel options

Z. Feng et al. / Energy 112 (2016) 549e561550

for LTC. Gasoline has low cetane number and is an ideal option to beblended with diesel for longer ignition delays [7]. In addition,gasoline is easy to be vaporized and potentially favors the reducingof the locally rich region, if blended in diesel. Both diesel and gas-oline are easily available and have been used as practical fuels,respectively in compression ignition and spark ignition engines formany years. Premixed charge compression ignition (PCCI) is a formof LTC and it basically monitors the level of pre-combustion mixingby tuning the fuel injection. Hanson et al. [8] investigated thecombustion and emission of an engine with diesel direct injectionin combination with gasoline port fuel injection and found thatgasoline injection reduces the combustion phasing, which in turnreduces heat transfer, fuel consumption, and emission of NOx andPM. Kalghatgi [9] investigated the performance of running 84 RON,95 RON and diesel in a single cylinder diesel engine and found thatgasoline fuel, because of its high ignition delay, is very beneficial foradjusting the premixed combustion phasing compared with aconventional diesel fuel. Subsequently, they extended their study[10] to an Euro VI multi cylinder diesel engine and the comparisonsshow that evenwith lower injection pressure and lower EGR levels,gasoline yields similar level of NOx emission, much lower smoke,lower maximum pressure rise and BSFC. In addition, by usingfuels with lower cetane number also extends the ignition delaytime because of reduced chemical reactivity of the premixed charge[11].

In addition, studies have been conducted on the LTC conceptengines by using gasoline-diesel fuel blends to simultaneouslyreduce NOx and soot emissions. Zhong et al. [12] experimentallyinvestigated the HCCI engine combustion and emissions using neatgasoline and diesel/gasoline blends and found that the blended fuelHCCI combustion in fact produced much less harmful emissionsthan pure gasoline HCCI combustion, especially HC and NOx. Ker-schgens et al. [13] numerically estimated the CO and NOx emissioncharacteristics of the mixture of gasoline and diesel fuels in PCCImode. They reported that an increase in mixing time and areduction in squish volume resulted in a decrease in CO andNO� emissions, respectively. Li et al. [14] used KIVA4eCHEMKIN toexam the effect of gasoline-diesel blend fuel on combustion andemission characteristics in a conventional diesel engine and foundthat the blends showa better performance on emissions at mediumand high load condition.

A very important aspect toward the goal of using blended fuelsin direct injection diesel engines is to characterize the fuel sprayand atomization. Due to the changed physical properties, the fuelblends may yield a modified spray and atomization characteristics,which subsequently impact the in-cylinder mixture formation, andsubsequent engine combustion and emissions [15]. Park et al. [16]studied droplet atomization, combustion, and exhaust emissioncharacteristics of gasoline and diesel fuels in a compression ignitionengine, they found that the blended fuel caused a decrease indroplet size and the ignition delay time was extended, and the NOxand soot emissions was reduced. A well spray and atomizationcharacteristic can improve the combustion and emission charac-teristics [11,17,18]. There have been some studies to find out thechanged spray characteristics when use the gasoline and dieselblending fuels but still exist some disagreements. Payri et al. [19]observed similar spray momentum and macroscopic behaviorwhen injecting diesel and gasoline, respectively. Kim et al. [18] andHan et al. [20] indicated that gasoline could produce shorter liquidpenetration distance and larger spray cone angle. It is necessary tomore accurately study the spray and atomization characteristics ofdiesel/gasoline fuel blends for better LTC strategy.

In addition, recently, ethanol has been emerging as a promisingbiofuel for petroleum replacement because ethanol is a renewablealternative fuel that has drawn much attention in recent years.

Using ethanol fuel on the internal combustion engine has profoundeffect to relieve the pressure of energy shortage and greenhousegas emissions. However, due to the poor solubility of ethanol indiesel, the ethanol volume fraction in the blends can't be higherthan 5% at normal pressure and temperature [21]. Fortunately,ethanol has no solubility problem if therewere gasoline presence indiesel because using gasoline as a co-solvent can improve the sol-ubility of ethanol in diesel such that uniform and stable mixturescan be prepared for laboratory experiments. Unfortunately, there islittle research on the spray and atomization characteristics whenadd ethanol to the diesel and gasoline blends.

To identify the influences of diesel gasoline and ethanol blendson spray and atomization processes, spray penetration distance,spray cone angle, droplet characteristics of the blends are investi-gated on a common rail injection system in this study.

2. Experimental setup and procedure

2.1. Experimental apparatus

The experimental system is sketched in Fig. 1. Fuel blends wereprepared and stored in a tank, and pumped into the common railafter a filter. The injector was purchased from Bosch (8Z1F8E4) andwas adapted with a single hole nozzle with an orifice of 0.18 mm.The spray was produced by triggering the magnetic valve of theinjector and it was ejected into the constant volume vessel withcontrolled ambient pressure by filling compressed nitrogen.Leakage test has been carried out at 50 bar before the experiments,and the leakage's rate was negligibly small (0.5 bar/h). Two quartzwindows with the diameter of 100 mm were installed on theopposite sides of the vessel to provide an optical access for thespray visualization.

The common rail injection system can withstand a maximuminjection pressure of 1400 bar. The ECU (optical diagnosticcontroller OD2301) is used to control the rail pressure, start of in-jection and energizing pulsewidth. The camera trigger signal is alsocontrolled by the ECU, providing variable synchronization signal fortriggering the spray visualization.

In order to record the development of the macroscopic fuelspray, a high-speed camera (Phantom V611, equipped with two100 mm focal length ZEISS lens) was used. A Xenon lamp, twomagnifying lens, and slit devices were aligned to provide schlierenaccess for high speed photography. The high speed camera wasoperated at a sampling rate of 15,000 fps with a resolution of352 � 640 pixel. The window view was calibrated with a Calibra-tion scale and the dimension/pixel ratio is 0.121 mm/pixel. Themaximum spray penetration length that can be recorded was100 mm due to the window size limit. The high speed cameraexposure time was 90 ms.

With regard to the particle/droplet image analysis (PDIA) sys-tem for droplet size measurement, as shown in Fig. 2, we replaceWCL schlieren optics system. A CCD camera (ImagerProSX 5M)linked with a telephotomicroscope to capture the droplets near theedge of the spray, and get the local microscopic images. Illumina-tion for the images was provided by a pulsed Nd:YAG laser with thewavelength of 532 nm utilized and intensity of 400 mJ/pulse inconjunction with a diffuser with the diameter of 120 mm con-taining fluorescent plates for uniform backlighting. Kashdan et al.[22,23] compared the PDIA results with those obtained from PhaseDoppler Anemometry (PDA). Their experiments having most of thedroplets size in the range of 5 mme30 mm showed that PDIA hasvery similar results with PDA. That compares very well with ourexperiments, and we are confident that the present PDIA systemcan provide reliable statistics for droplet distribution.

Fig. 1. Experimental setup for STP and cone angle measurements.

Fig. 2. The sketch of the PDIA system.

Fig. 3. The tested fuel blends after freshly prepared for three days. It is seen that theblends were uniformed mixed and there is no fuel stratification.

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2.2. Tested fuels

The test fuels in this study are neat diesel and gasoline-dieselblends named G0, G20, G40, which indicates that the gasolinevolumetric fraction in the gasoline-diesel blends are, respectively,0%, 20% and 40%. Treating these fuels as a basis, we use certainproportion of ethanol to replace gasoline to examine the effect ofethanol substitution on the spray and atomization characteristics.The new blend fuels were named G10E10, G10E30, G30E10, G10E10represents the fuel blends with 10% gasoline, 10% ethanol, and theremainder 80% diesel. Before experiments, we have to make surethat the mixtures are stable and do not have the issue of stratifi-cation. Fig. 3 shows the image of the tested fuel blends after theywere freshly prepared for three days. It is seen that all the fuelblends are well uniformly mixed and very stable. The physicalproperties of all test fuels are shown in Table 1. The density, vis-cosity, surface tension were measured according to Chinese na-tional standard GB/T 1884-1992, GB/T 265-1988 and GB/T 6541-1986, respectively.

2.3. Test conditions and procedure

First of all, in the whole experiment process, we keep the fueland the environmental temperature at 293 K and the injectionduration was 1.5 ms. In the macroscopic spray experiments, threeinjection pressures Pinj were tested (600, 900 and 1200 bar) and attwo ambient pressures Pamb (20 and 40 bar). The experiments wererepeated at least three times under each condition and the averagequantifying parameters and standard deviation of the spray evo-lution were used for experimental results demonstration.

The original image obtained from a typical spray evolutioninstant is shown in Fig. 4(a). In the image processing procedure, the

Table 1Properties of the fuels tested in this work.

Mixtures and their abbreviations in this work (diesel/gasoline/ethanol, vol%) Density @ 293 K (g/mL) Viscosity @ 293 K (mPa s) Surface tension @ 293 K (mN/m)

Gasoline (0%/100%/0%) 0.730 0.60 18.99Ethanol (0%/0%/100%) 0.789 1.07 21.60G0 (100%/20%/0%) 0.812 3.28 24.86G20 (80%/20%/0%) 0.807 2.08 23.56G40 (60%/40%/0%) 0.782 1.50 22.33G10E10 (80%/10%/10%) 0.802 2.26 23.95G10E30 (60%/10%/30%) 0.799 1.96 23.30G30E10 (60%/30%/10%) 0.785 1.56 22.78

Fig. 4. The macroscopic image and definition of the spray tip penetration and spray cone angle.

Z. Feng et al. / Energy 112 (2016) 549e561552

color images were first converted to the grayscale style; then apreset threshold value was chosen to extract the spray contour asshown in Fig. 4(b). We can measure the macroscopic spray char-acteristics STP which is defined as vertical distance from the nozzletip to the bottom of the spray edge. The spray cone angle is definedas the angle covered by the two tangent lines, which connects thenozzle tip and the periphery points at the position of 1/2 pene-tration distance from the nozzle tip.

For investigation of the droplet size statistics, we fixed the in-jection pressure and ambient pressure respectively at Pinj¼ 900 barand Pamb ¼ 20 bar. It is noted that due to the limit of the presentPDIA system, a full field we choose the location at L ¼ Lo ¼ 50 mmbelow the nozzle tip and R ¼ R0 ¼ 6 mm from the spray axis as themeasurement window as demonstrated in Fig. 5 and the windowsize is 1.8 � 1.5 mm for all the six fuels tested in this study. Due tothe high droplets density at the central of the spray that attenuatethe background illumination light, clear images showing themicroscopic liquid droplets can be captured by using the laser and

CCD camera and one typical image is shown in Fig. 5. Measure-ments are conducted at 3 ms after the start of injection energizingwhen the spray shows quasi stationary period. In order to eliminatethe influence of the contingency we adopt the method of statistics,so every fuel we manage the location and size information of thedroplets from 60 distinct pictures after filter. The recorded imageswere then processed by the software DaVis 8.0.0 of LaVision. Atypical processed image of the local window with droplet sizedistribution is present in Fig. 6 and the data information of thisimage is provided as supplemental material. In the calibration, 1pixel represents 1.5 mm, hence the droplets with a diameter under8 mm are removed because the minimum droplet diameter that canbe accurately resolved is 5 pixel equivalent. Then the software canpresent the information of location and size of the droplets in everyimages. Unlike the well-established PDA technique which onlydetects the spherical droplets and is a point-wise method, PDIA candirectly measure even non-spherical droplets on the focal plane.

Fig. 5. Typical image illustrating the local droplets.

Fig. 6. Typical image showing droplet size distribution in the target window.

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3. Results and discussion

3.1. Spray macroscopic characteristics

The spray macroscopic features were acquired based on the

images captured by the high speed photography system. The Fig. 7shows the result of the three repeated experiments of pure diesel ata typical condition (Pinj ¼ 900 bar Pamb ¼ 20,40 bar). It is seen thatthe three data set almost overlap, thus the standard deviation isnegligibly small, as shown by the error bars in Fig. 8. Thus we don't

Fig. 7. The STP of three repeated experiments on a typical condition.

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add error bars in the rest of figure. Fig. 8(aec) illustrates the effectof gasoline blending ratio on the STP at injection pressures of 600,900 and 1200 bar and ambient pressures of 20 and 40 bar. For allthe three tested fuels and pressure conditions, the STP evolves in avery similar pattern: initially after the start-of-injection (aSOI), theSTP increases linearly with time, and no significant difference inSTP evolution among the three fuels at neither ambient pressureswas observed. After certain critical time tbreak (about 0.2 ms aSOI),the rate of STP increase slows down. This is because for t < tbreak, theSTP is mainly affected by the fuel density [24]. Because all the testfuels show such small difference in the liquid density that themethod we adopted can't capture it. However, after tbreak, the dif-ference in STP evolution among the three fuels was observed. Therewas a significant decrease of STP as the ambient pressure increasedfrom 20 bar to 40 bar, and this is because aerodynamic drag forceincreases with increasing environmental gas density. Comparingthe three kinds of fuel, we could see a slight decrease on STP withthe increase of gasoline blending, and G40 presents the lowest STPfor all the injection and ambient pressures studied in this work.This is because the blends have lower viscosity and surface tensionand the main spray body is easier to deform and breakup, resultingin a decrease of spray droplet size and increase the aerodynamicdrag force, loss in spray momentum which eventually slows downthe penetration. To what extent the jet atomization and dropletbreakup impact the spray tip penetration of the blended fuel sprayswill be amain point of discussion in sec. 3.2. Furthermore, we foundafter 1.6 ms the difference on STP between the three fuels becomemore obvious at Pinj ¼ 600 bar while the time could be broughtforward to 1.0 ms at Pinj ¼ 1200 bar, this means that high injectionpressure could prompt droplets deform and breakup. This isconsistent with the study by Agarawal et al. [25] for both diesel andbiodiesel.

Fig. 9(aef) illustrate the effect of ethanol blending ratio on theSTP at injection pressures of 600, 900 and 1200 bar and ambientpressures of 20 and 40 bar. For mixtures with 80% diesel and 20%gasoline (G20), its STP evolution is very similar to that of themixture G10E10 (80% diesel/10% gasoline, and 10% ethanol), asshown in Fig. 9(aec). This is because ethanol and gasoline haveclose physical properties, as shown in Table 1. In addition, since themajority of the mixture is diesel, the fraction of ethanol and gaso-line is not large enough to result in noticeable difference in vis-cosity and surface tension. This is consistent with previous studieswhich state that the main influencing factors that affect the STP

between different fuels is the viscosity and surface tension. Formixtures with a higher gasoline/ethanol fraction, the effect ofethanol addition on the STP evolution becomes discernible.Fig. 9(def) shows the comparison of the STP for neat diesel (G0),diesel/gasoline/ethanol blends (G10E30, G30E10) and diesel/gaso-line blends (G40). The STP for neat diesel (G0) develops fastest,

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Fig. 9. The effect of blending ethanol on STP evolution.

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followed by the mixture of diesel/gasoline/ethanol blends (G10E30,G30E10) and diesel/gasoline (G40) mixture shows the slowest STPevolution. When adding ethanol into diesel/gasoline blends whilekeeping the diesel fraction constant, the STP increases slightly withthe increase of ethanol fraction.

Due to the transient nature of the spray, the spray cone anglevaries with time. Fig. 10(a) shows a typical measured spray coneangle variation. It is seen that after a certain period of time, thespray cone angle becomes a constant. This is because initially theneedle opening of the injector increases, and after the needle isfully opened, the instantaneous momentum flux becomes a con-stant. Thus the spray cone angle discussed below is the timeaverage of their values during the period of needle full opening. It isseem that for given fuel mixture, the averaged cone angle isinsensitive to the variation of the injection pressure. It is noted that

previously Kang et al. [26] found that as the injection pressureincreased, the spray cone angle decreased, and they stated that theincrease of injection pressure makes lower gas entrainment degreedue to the increased ratio of Pinj/Pamb. It was then conjectured thatincreasing the ambient pressure weakens the influence of injectionpressure because high environment gas density will increase theresistance of axial movement, making the spray evolution processtowards radial direction, and this was evidenced by the comparisonof the spray cone angle shown in Fig. 10(b) and (c) for the samemixture. Furthermore, with the increase of gasoline blending ratio,the spray cone angle is increased. This is because the lower vis-cosity of gasoline decreases the overall viscosity of the blends, thespray body is more easily to be broken. This is consistent withprevious study of Valentino et al. [27] and Anand et al. [28], inwhich they showed that the higher fuel viscosity leads a smaller

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spray cone angle and lower level of air entrainment.Fig. 11(aec) illustrate the effect of ethanol blending ratio on the

spray cone angle of diesel/gasoline blends at injection pressures of600, 900 and 1200 bar and ambient pressures of 20 and 40 bar.Fig. 9(a) shows that for high diesel fraction blends (80% diesel),there was no significant difference between the two fuels for allconditions, which was similar to the dependence of the STP for this80% diesel fuel blends. For relatively lower diesel fraction fuel

blends (60% diesel) as shown in Fig. 9(b) and (c), the addition ofethanol results in a noticeable increase in the spray cone angle.When adding ethanol to G40, similarly we compared the threeblend fuels with neat diesel. We found that the fuels blend withethanol have smaller spray cone angle than G40 and with the

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increase of ethanol fraction in the mixture, the spray cone angledecreases. Neat diesel presents significantly smaller spray coneangle, compared to these three fuels of diesel/gasoline/ethanolmixtures. That's because for these 60% diesel/gasoline blends,substitution of gasoline by ethanol do not change much of theoverall viscosity but these blends show significantly smaller vis-cosity compared to neat diesel.

3.2. Spray microscopic characteristics

In most practical combustion systems, fine droplet atomizationleads to higher volumetric heat release rates, a wider range ofstabilized operating condition and lower pollutant emissions [16].Therefore, the study on the fuel atomization level is important andshould be conducted for the optimization of the combustion sys-tem. From this point of view, the different fuels' droplet atomiza-tion performancewere investigated and compared in the following.

Droplet size statistics represent the spray atomization perfor-mance and in this study, we apply the PDIA technique for thedroplet size statistics. It is difficult to configure the droplet detec-tion algorithm to correctly identify droplets over thewhole range ofsizes without either missing high intensity droplets or identifyingimage noise as very low intensity droplets. As a result, we choosedifferent threshold values (A pixel will belong to a certain segmentof the particle when its intensity is above the global threshold [29])to make sure every droplet in the target view is counted and sub-sequently processed through statistics. Then the parameters Lowintensity level (35%) and High intensity level (65%) are chosen forparticle size validation, which indicates that the droplet has a sizesmaller than the maximum and larger than the minimum, and weuse the averaged value as the real diameter of each droplet.

Fig. 12(a) and (c) respectively presents the droplet size distri-bution along the radial and axial directions of the diesel/gasolineblends. D10 represents the arithmetic mean diameter which is amore intuitive parameter to quantify the droplet size. Becausethose droplets were located at discrete points within the observa-tion window (a square with a dimension of 1.8 mm 1.5 mm), wedivided the target window into infinitesimals with a width of50 mm and the droplets within each infinitesimal was averaged andused as the representative of the droplet size at the local coordi-nate. The droplet size along the radial direction for diesel anddiesel/gasoline blends were shown in Fig. 12(a). It is found that thedroplet size slightly increases along the radial direction. Twopossible mechanisms for the size distribution are collisional effectsand vortex effects.Wu et al. [30] suggested that the droplets growthwith radial distance from the center can be attributed to the dropcollisions and coalescence. They pointed out that because of theshear forces the outer layer of the jet has a lower velocity than thecentral region and the droplets' movement is more disorderly at theouter layer, so collisions between droplets become more frequent.This increase in collisions lead to the increase of the average dropletsize. The computational results obtained by Chung et al. [31] can beanother explanation which demonstrated that large-size dropletsentrained in the large-scale vortices of a turbulent shear flow tendto be more easily centrifuged away from the vortex core which candisperse larger size droplets to the outer edge of the spray. Inaddition, the normalized droplet number density and droplet vol-ume fraction, were calculated. It was found that the droplets

Fig. 12. Droplets size, number density and droplet volume fraction distribution alongthe radial and axial directions. Note that normalized droplet number density is theratio of number of droplets within the local infinitesimal to the total number ofdroplets. The droplet volume fraction is the summation of volume of each dropletwithin the local infinitesimal to the total volume of droplets.

Fig. 13. Droplets size distribution along the axial and radial direction for diesel/gas-oline/ethanol blends.

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number slightly decreases along the radial direction as shown inFig. 12(b). The droplet volume fraction also decreases along withthe radial direction which is consistent with the study by Jennifer[32], indicating that the droplet volume fraction relies more on thedroplet number density. So, it is reasonably expected that themixtures in the regions away from the spray center line is leaner,compared with themixtures in the regions close to the spray centerline. It is noted that previously we reported the droplet size dis-tribution along the radial direction of biodiesel/di-n-butyl etherblends [33], in which we found that the diameter decreases slightlyas it shifts from the central to the edge of the spray. This differentdroplet size distribution may be caused by the different test loca-tions we choose (40 mm below the nozzle tip and 6 mm from thespray axis in the previous study). Thus we believe that the dropletsize distribution varies with the location.

The droplet size and the number and volume probabilities isalmost constant along the axial direction. This means that the axialdistance has little influence on the distribution of the droplets. Inaddition, neat diesel produces the largest droplets, compared to themixture of diesel/gasoline and the blended fuels give the smallerdroplets with the blending ratio of gasoline increases. This isbecause the addition of gasoline leads to a more active breakupprocess due to reduced viscosity and surface tension thus theblends have a better atomization characteristic.

Fig. 13 shows the effect of ethanol addition on the droplets sizedistribution for diesel/gasoline/ethanol mixture. The droplet sizealong the axial and the radial direction were similar to thatobserved in Fig. 12. For these diesel/gasoline/ethanol mixtures,with the increase of ethanol fraction, the droplet size increases,though this effect is not significant. In addition, this is in accordancewith the macroscopic spray behavior such as the STP and spraycone angle shown previously in Figs. 9 and 11. Because from the STPdependence we see that the ethanol substitution results in a longerSTP and smaller spray cone angle which reduces the intensity of airentrainment. Thus it is conjectured that adding ethanol to thediesel/gasoline blends will lead to a worse atomization result,which was evidenced by the droplet size distribution as a functionof ethanol fraction shown in Fig. 13. However, we note that thoughethanol addition slightly moderates the atomization performanceof diesel/gasoline blends, it still results in significantly finer dropletsize distribution, compared with the neat diesel.

The overall droplet size statistics of the six test fuels are shownin Table 2. It was seen that the total number of the droplets forstatistics were higher than 5000. The widely used Sauter MeanDiameter (SMD) are also presented in this work. SMD is also namedas D32 as defined by the ratio of the sum of the volume of thedroplets and the surface area of the droplets. Because D32 is anaveraged statistic parameter that represents the potential ofmixture formation performance. The number of droplets for thisaveraging should be large enough. Fig. 14 shows the SMD as afunction of droplet number for statistics. It is seen that when thetotal number is larger than 3000, the SMD is independent of thenumber of droplets for statistics. Table 2 showed that our statisticsof SMD is reliable and meaningful because droplet number is largerthan 5000. We found the D32 of neat diesel, G20 and G40 isrespectively 29.64, 28.24 and 24.95 mm, and the corresponding D32reduction by gasoline addition into diesel is 5% and 15%. In addition,for given diesel fraction mixture, it is found that D32 of G10E10 isslightly higher than that of G20 (3% larger), indicating that higherethanol fraction leads to larger D32. For lower diesel fractionmixture (G30E10, G10E30, and G40), we found similar behavior:blending gasoline in diesel show significant reduced D32. D32 ofG40 is 15.8% smaller than that of diesel, while substitution of gas-oline in G40 by ethanol increases D32, however, the D32 is stillsmaller (26.68 and 26.75 mm) than that of neat diesel (29.64 mm).

Table 2Parameters that quantify the droplet size statistics for the fuels studied in this work.

Fuel tested G0 G20 G10E10 G40 G30E10 G10E30

Total number of droplets 5893 6517 5743 6665 5436 6360D10/mm 23.20 22.08 22.77 20.25 21.00 21.18D32/mm 29.64 28.24 29.02 24.95 26.68 26.75Dv10/mm 20.19 19.19 20.09 17.28 18.12 18.26Dv50/mm 31.54 30.34 31.09 26.54 28.61 28.57Dv90/mm 47.3 45.57 46.72 39.01 42.83 43.45

Fig. 14. Typical SMD statistics as a function of droplet number (Diesel, Pinj ¼ 900 bar,Pamb ¼ 20 bar). The droplet number has to reach a minimum value such that thestatistics is meaningful.

Fig. 15. Probability density of droplet size versus droplet diameter for different diesel/gasoline blends.

Fig. 16. Probability density of droplet size versus droplet diameter for different diesel/gasoline/ethanol blends.

10 20 30 40 50 600

20

40

60

80

100

Cum

ulat

ive

Volu

me

(%)

Diameter (μm)

G0 G10E10 G20 G30E10 G40 G10E30

Dv10 Dv50 Dv90

Fig. 17. The cumulative volume percentage as a function of droplet diameter for thetested fuels.

Z. Feng et al. / Energy 112 (2016) 549e561 559

This indicates that the atomization performance and rate of evap-oration of the diesel is enhanced by addition of gasoline. In addi-tion, for given diesel/gasoline blends, further addition of ethanolslightly reduces the evaporation rate of the mixture, yet it is stillexpected to result in better evaporation process that the neat diesel.

In order to clarify the quantitative effect of gasoline addition onthe droplets size of diesel spray, the droplet numbers were profiledas a function of the droplet size for neat diesel and diesel/gasoline

Z. Feng et al. / Energy 112 (2016) 549e561560

blends, as illustrated in Fig. 15. It calculates the ratio of the numberof droplets with a specific size to the total number of droplets. Ascan be seen from the figure for given diesel/gasoline mixture, theprobability of the intermediate size droplets is the largest whiledroplets with minimized and maximized size were very few and itgets close to a normal distribution though slightly spread out moreon the larger droplet size side. In addition, with the increase of thegasoline fraction in diesel/gasoline blends, the size of dropletswhich have the maximum probability decreases. Specifically, forneat diesel, the peak frequency occurs for droplets with diameter ofaround 23 mm. When gasoline is blended into diesel with volumefraction of 20%, the diameter with maximum frequency decreasesto around 20 mm, and for G40, the droplets with diameter of around18 mm have the maximum probability. Moreover, the percentage ofthe diameter with maximum frequency is 13% for G40, which ishigher than another two fuels G20 and neat diesel (11%). Theseobservations indicate that with the increase of gasoline fraction indiesel/gasoline blends, the number of small droplets increased. As aconsequence, the total mean droplet size decreased as shown byFig. 12. This can be explained by the fact that the higher viscosityand larger surface tension of neat diesel inhibits its atomizationprocess, and as gasoline is added, the viscosity and the surfacetension of the blended fuel is decreased, which favors the break ofdroplets due to reduced resistance to shear stress and thus betteratomization performance is achieved. The practical implication isthat the increase of small droplets gasoline addition in diesel favorsthe mixture formation due to enhanced average surface to volumeratio and if the environmental condition is at high temperature, theoverall rate of evaporation will significantly increases due to thissurface to volume ratio increase.

Fig. 16 shows the probability density of droplet diameter versusdroplet diameter for diesel/gasoline/ethanol blends. From thecomparison between two fuels with 80% diesel shown in Fig. 16(a),it is seen that the peak point in the number of droplets decreased asethanol was added. We could also found this tendency in Fig. 16(b),which presents the comparison among the number density ofdroplet sizes of G40, G30E10 and G10E30. Moreover, the peak pointmoved right when more ethanol was blended. This indicates thatthe number of small droplets decreased slightly when ethanol waspresent in the blends. As the number of small droplets decreased,and the large droplets increased, the total average droplet sizeincreased, as shown in Fig.13. However, it is noted that even thoughthe ethanol addition increases the number of large droplets formixtures with 60% diesel (G40, G10E30 and G30E10), this effect isvery weak, compared to the case of neat diesel because all of thesemixtures show noticeably smaller droplet compared to G0.

Fig. 17 shows the cumulative volume of all the droplets as afunction of droplet size all the tested fuels. The small dropletswithin the spray do not contribute significantly to the total sprayvolume and the cumulative volume is significantly influenced bythe larger droplets because the volume is proportional to the cubicof the average droplet diameter and the droplet number. It isobserved that with the increase of gasoline blending ratio, thecurve shifts to the left, which further indicates that addition ofgasoline increases the number of the small droplets. However, withthe increase of ethanol blending ratio, the curve shifts to the largerdroplets side. The characteristic diameters Dv10, Dv50 and Dv90[34] which are extracted from the cumulated volume of all thedroplets as is shown in Fig.17. They represent that 10%, 50% and 90%of the total volume is made up of droplets with diameters smallerthan Dv10, Dv50 and Dv90 respectively. These values were alsolisted in Table 2.

The results show that neat diesel gives the largest characteristicsdiameter. With the increase of gasoline blending ratio, the char-acteristic diameter is decreased. It is concluded that gasoline

blending decreases the viscosity and surface tension and conse-quently the resistance of droplets to shear stress which favors thebreakup of droplets or liquid ligaments. The atomization perfor-mance of biodiesel is enhanced by the addition of DBE. However,when adding ethanol into the blends, the increase of the diameteris weak, compared to the effect of gasoline addition to diesel.

4. Conclusions

This study experimentally investigated the macroscopic sprayevolution process and the droplet characteristics within the sprayfor 6 kinds of liquids including neat diesel, diesel/gasoline blends,and diesel/gasoline/ethanol blends. The main conclusions aresummarized as follows:

1. The increased gasoline fraction in diesel/gasoline blends leads tosignificantly decreased spray tip penetration and increasedspray cone angle, while adding ethanol into the diesel/gasolineblends would lead to slightly increased spray tip penetrationand subtle decrease in the spray cone angle.

2. For all the fuels studied in this work, in the target window ofview, the local droplet size slightly increases along with theradial direction, but the local droplet number density anddroplet volume fraction decreases along the radial direction,indicating that the region along the radial direction have ahigher fuel concentration if the droplets were evaporated. Alongthe axial direction, the droplet size, local number density,droplet volume fraction are almost constant.

3. With the increase of gasoline fraction in the diesel/gasolineblends, the droplet size decreases significantly. Further addingethanol into the diesel/gasoline blends slightly increases thedroplet size, but it is still smaller than that of neat diesel.

Acknowledgement

This work is supported by the National Natural Science Foun-dation of China (91541107, 51206131, and 91441203), and the Na-tional Basic Research Program (2013CB228406). The support fromState Key Laboratory of Engines, Tianjin University (K2015-01) isalso acknowledged.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.energy.2016.06.131.

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