Comparison of Ethanol and Butanol as Additives in Soybean Biodiesel

Published: March 28, 2011

r 2011 American Chemical Society 1837 dx.doi.org/10.1021/ef200111g | Energy Fuels 2011, 25, 18371846

ARTICLE

pubs.acs.org/EF

Comparison of Ethanol and Butanol as Additives in Soybean BiodieselUsing a Constant Volume Combustion ChamberHaifeng Liu,, Chia-fon Lee,*, Ming Huo, and Mingfa Yao

State Key Laboratory of Engines, Tianjin University, Tianjin 300072, Peoples Republic of ChinaDepartment ofMechanical Science and Engineering, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, United States

ABSTRACT: To investigate the eects of dierent alcohol additives in biodiesel fuel on the spray, combustion characteristics, andsoot formation and oxidation, a detailed comparative study between the butanolbiodiesel blend and the ethanolbiodiesel blendwas carried out in an optical constant volume combustion chamber. Two dierent volumetric blend fuels were tested in this studywith dierent ambient temperatures at the start of injection (from 800 to 1200 K). The volumetric ratios were the 20% butanol/80%soybean biodiesel referred to as B20S80 and 20% ethanol/80% soybean biodiesel referred to as E20S80. Results demonstrated thatthe microexplosion occurred for B20S80 and E20S80 fuels at 800 and 900 K ambient temperature because of the volatility dierencebetween the additives (butanol or ethanol) and the base fuel (biodiesel). The E20S80 fuel presented higher peak pressure andshorter combustion duration compared to the B20S80 fuel. The autoignition was earlier for the B20S80 fuel at 1000 and 1200 Kambient temperature, while the autoignition of the B20S80 and E20S80 fuels was nearly the same at 800 K ambient temperature.The E20S80 fuel had a lower ame luminosity compared to the B20S80 fuel. The soot distribution was increased downstream of thespray jet with a higher ambient temperature for both tested fuels, and E20S80 had a lower value of normalized time-integrated sootmass (NTISM). Therefore, E20S80 has more advantages to reduce the soot emission compared to the B20S80 fuel. Also, increasingthe ambient temperature from 800 to 1200 K led to a rapid increase in the value of NTISM for both tested fuels. Therefore, a lowerambient temperature with the piston at top dead center (TDC) should have more advantages to combustion and soot control in areal diesel engine.

1. INTRODUCTION

Diesel fuel is a specic fractional distillate of petroleum fuel oiland is largely consumed by the transportation sector. Alternativesthat are not derived from petroleum, such as biodiesel, areincreasingly being developed and adopted. Biodiesel is denedas the monoalkyl esters of long-chain fatty acids derived fromvarious feedstocks, such as vegetable oil, animal fat, algae, etc.Biodiesel has many similar properties to diesel fuel, and it canblend with conventional diesel fuel in any proportion. Studieswith various biodiesel blend ratios or the neat biodiesel havedemonstrated that biodiesel-fueled diesel engines could reduceemissions of carbon monoxide (CO), total hydrocarbons(THCs), and particulate matter but slightly increase brake-specic fuel consumption because of the reduction in heatingvalue, while the power output for biodiesel was almost the sameas that for diesel fuel.19 Most of the studies reported a slightincrease in nitrogen oxides (NOx) emissions using biodieselfuels. However, this NOx emissions problem could be eliminatedby advanced injection strategies and exhaust gas recirculation(EGR).2,4,9 In addition, studies have also demonstrated thatbiodiesel had low or no greenhouse gas emissions (in net) by thewell-to-wheels analysis.10,11 Thus, it can be seen that biodiesel isable to be used as a substitute for diesel fuel in a diesel engine withlower harmful gas emissions.

Most of the properties of biodiesel fuels are comparable tothose of diesel fuel, except for cloud point and pour point, whichindicate that the cold ow behavior of a fuel is poor. In addition,the viscosity of biodiesel is higher than that of the diesel fuel, whichwill aect the spray characteristics and subsequent combustion

processes. Studies have demonstrated that the low-temperatureow properties of biodiesel could be improved by blendingethanol with various volumetric ratios (020%).1214 Further,a 20 vol % ethanol blending into biodiesel has been reported toachieve improved combustion with a reduction in CO, THC,NOx, and smoke emissions without aecting the break thermaleciency.12,15 The addition of ethanol with a 1030% blendratio decreased the droplet size and improved the atomizationperformance of biodiesel fuel because of the lowered kinematicviscosity.1618

On the other hand, butanol is a very competitive alcohol to beapplied in diesel engines and is becoming popular recently.Similar to ethanol, butanol is a biomass-based fuel that can beproduced by alcoholic fermentation of the biomass feedstocks. Astudy has shown that the solubility of dieselbiodieselbutanolwas higher than the solubility of dieselbiodieselethanol attemperatures of 1030 C,19 and another study has alsodemonstrated that ethanoldiesel depicted poor blending sta-bility compared to butanoldiesel blends.20 The miscibility ofbutanol with biodiesel was excellent compared to ethanol at awide range of operating conditions.21,22 Furthermore, fuel prop-erties illustrated in Table 1 indicate that butanol has the potentialto overcome the drawbacks brought by ethanol in diesoholfuels,1926 i.e., higher heating value, less vapor lock tendencybecause of lower volatility, less ignition problems because of

Received: January 20, 2011Revised: March 28, 2011

1838 dx.doi.org/10.1021/ef200111g |Energy Fuels 2011, 25, 18371846

Energy & Fuels ARTICLE

lower latent heat, and higher cetane number. A study hasdemonstrated that the viscosity of the biodiesel was decreasedby the butanol additives and the soot and NOx emissionsreduction with 20 vol % butanol blend in biodiesel.27 Further,a detailed chemical kinetic reaction mechanism was proposed forthis biodieselbutanol surrogate (methyl octanoatebutanol)in a jet-stirred reactor, and the study has also indicated that themethyl octanoatebutanol mixtures are less prone to emittingacetaldehyde than the corresponding methyl octanoateethanolmixtures oxidized in a jet-stirred reactor.28

On the basis of the literature survey, it can be found that thebenet of alcoholbiodiesel blends is its capability of osetting thedisadvantage of higher viscosity and worse cold ow behavior forbiodiesel and the lower cetane number for alcohols; thus, propertiesof the blend fuels are similar to conventional diesel fuel. Meantime,the spray and emission characteristics for neat biodiesel may beimproved by adding ethanol or butanol. However, there was fewreports on the detailed comparative study between the buta-nolbiodiesel blend and the ethanolbiodiesel blend in terms ofspray, combustion, and emissions. In this paper, two dierent blendfuels, butanol/soybean biodiesel and ethanolsoybean biodiesel,were tested in an optical constant volume combustion chamber.Miescattering, natural ame luminosity, and forward illumination lightextinction (FILE) were used to explore the spray, combustioncharacteristics, and soot distribution, respectively, at various ambienttemperatures, ranging from 800 to 1200 K. The study is signicantto reveal the eects of dierent alcohol additives in biodiesel fuel onthe spray, combustion characteristics, and soot formation andoxidation.

2. EXPERIMENTAL SECTION

2.1. Experimental Setup. A constant volume chamber withoptical access was used in this study. The chamber has a bore of 110 mmand a height of 65 mm. Figure 1a shows the schematic of the chamberand soot measurement method. A fused silica (Dynasil 1100) end windowwith a high UV transmittance down to 190 nm was installed opposite tothe injector. A Caterpillar hydraulic-actuated electronic-controlled unitinjector (HEUI) was mounted at the center of the chamber bottom, andthe configurations of the injector are listed in Table 2. The cylinder wallwas heated to 380 K by eight heaters before the test to mimic the walltemperature of a diesel engine as well as to prevent water condensationon the optical windows. The oil line and fuel line inside the chamberhead were kept at 350 K to simulate the situation in an actual engine. Aquartz pressure transducer (Kistler 6121) was embedded in the chamberwall in conjunction with a dual mode differential charge amplifier forrecoding the in-chamber pressure during the experiments.2.2. Laser Diagnostic Methods. The working principle of liquid

spray light scattering is the different reflective rates from the liquiddroplets and its surrounding background. The camera could observestronger reflection of the laser beam from spray, resulting in differentpixel values on the images; therefore, the spray behavior could be taken.For the natural flame luminosity measurement, images were obtained bya high-speed camera (Phantom V7.1) with a 105 mm focal length lens(Nikkor), located above the chamber. A neutral-density filter wasadopted to fit the light intensity within the camera measurement range.The forward illumination light extinction (FILE) method was used forthe soot distribution and two-dimensional (2D) time-resolved quanti-tative measurements, and it has been used tomeasure the total soot massin previous studies.23,2933 Sootmeasurements using the light extinction

Table 1. Properties of Ethanol, Butanol, Soybean Biodiesel, and the Blend Fuels

properties ethanola butanolasoybean biodiesel

(ASTM method)

B20S80

(ASTM method)

E20S80

(ASTM method)

molecular formula C2H5OH C4H9OH CH3OOCR

cetane number 8 25 51 (D 613) 49.2 (D 976) 47.8 (D 976)

octane number 108 96

oxygen content (wt %) 34.8 21.6 10 12.32 14.96

density at 15 C (g/mL) 0.795 0.813 0.887 (D 1298) 0.871 (D 1298) 0.869 (D 1298)autoignition temperature (C) 434 385 363b

ash point at closed cup (C) 8 35 173.9 (D 93)lower heating value (MJ/kg) 26.8 33.1 37.53 (D 240) 36.64 35.38

boiling pointc (C) 78.4 117.7 329.7 (D 1160) 318.6 (D 86) 303.5 (D 86)stoichiometric ratio 9.02 11.21 12.5 12.24 11.80

latent heating at 25 C (kJ/kg) 904 582 200b 276.4 340.8ammability limits (vol %) 4.319 1.411.2saturation pressure at 38 C (kPa) 13.8 2.27viscosity at 40 C (mm2/s) 1.08 2.63 4.0 (D 445) 3.68 (D 445) 3.08 (D 445)total glycerin (wt %) 0.142 (D 6584)

free glycerin (wt %)



method have been conducted in a number of studies,3437 and the FILEmethod shares a similar principle but requires only one window foroptical access, as explained in the following.

As seen in Figure 1b, without soot in the light path, only a part of thelight is reected back to the camera after the diuse reection. Thisreected light is labeled as I0 and represents the light intensity reachingthe camera without the soot cloud. With the incident light labeled as Ii, itis obtained by I0 =RIi, whereR represents the ratio between the incidentlight and the light collected by the camera after the diuse reection. It isdetermined by surface roughness and optical properties. The reectedlight should be much smaller than the incident light because of thediuse reection character. With the soot cloud, the incident light will beattenuated before it reaches the light diuser, as shown in Figure 1b. Thelight intensity after the soot cloud is labeled as I1, which obeysLambertBeers law and is given by I = I1 exp(

R0LKextdx), where Kext

is the extinction coecient for a cloud of soot (m1) and L is the pathlength of the light beam or the thickness of the soot cloud (m). Afterreaching the light diuser, I1 will be further attenuated by the light diusereection. It will obey the same rule as without soot: I2 = RI1. Thereected light will then go through the soot cloud again and once morebe attenuated. The light recorded by the camera after the soot cloud isnow labeled as I; I = I2 exp(

R0LKextdx). Combining the above relations

together, it obtains that the light intensity changes resulting from thepresence of soot are similar to LambertBeers law.

I I0 exp Z 2L0

Kextdx 1

.The only dierence is that the light extinction is related to 2L instead ofL because of the two passes through the soot cloud.

The extinction coecient for the soot cloud is related to the particlenumber density, size, and optical properties. With the Rayleigh approx-imation, a simple expression can be obtained. The soot volume fractioncan be shown as

Cv 2LKa1 RsalnI0I

2LKeln

I0I

2

,where Rsa is the scattering/absorption ratio. With the Rayleigh approx-imation, the extinction is mainly dominated by the absorption of sootrather than the scattering; therefore,Rsa is equal to 0, is the wavelengthof monocolor light, Ka is the dimensionless absorption constant, whichcan be calculated using the following formula with the Rayleighscattering approximation:

Ka 6 Im m2 1

m2 2

! 36nkn2 k2 22 4n2k2 3

Here, m is the refractive index of soot m = n ik and changes with thecomposition of soot and the wavelength of light. The Rayleigh approx-imation is valid for soot measurements with primary soot sizes below50 nm and laser wavelengths of 511 nm. The scattering/absorption ratiocan be assumed to be 0. The dimensionless extinction coecient Ke willbe the same as the dimensionless absorption coecientKa. In this paper,a value of 5.47 is adopted with m = 1.62 i0.66. The images recordedwith and without soot clouds are processed pixel by pixel to calculate thesoot volume fraction using eq 2. However, for a non-axisymmetric dieselame, the thickness of the soot cloud is unknown and what is obtained isonly a line-of-sight measurement as computed by CvL = (/2Ke)ln(I0/I). If the area represented by each pixel is known as s and the sootdensity is chosen as Fs = 2.0 g/cm3, the soot mass at each pixel will beknown asmi = FsCvLs. When all pixel values are summed together, thetotal soot mass in the ame can be calculated. For the soot density, somestudies have demonstrated that the value was about 1.82.0 g/cm3.3841 Inthis paper, the soot density was chosen as 2.0 g/cm3 based on the recentultra-small-angle X-ray scattering work by Braun et al.40 and heliumpycnometry measurements by di Stasio.41

The current optical setup of FILE is shown in Figure 1a. The incidentlight was supplied by a copper vapor laser (Oxford Lasers LS20-50). Thecamera and laser were synchronized up to 15 037 frames/s to producetime-resolved measurements at a resolution of 256 256 pixels. Thecopper vapor laser has two-color output at 511 and 578 nm, with a powerratio of 2:1. To lter out the light at 578 nm for this monochromatic lightextinction and suppress the ame emission, two interference lters at510 and 515 nm with 10 nm full width at half-maximum (fwhm) were

Table 2. Conguration of the HEUI Injector and the Con-dition of Fuel Injections

parameter value

nozzle style valve-covered orice

number of nozzle holes 6

spray angle (deg) 140

orice diameter (mm) 0.145

injection pressure (MPa) 134

injection duration (ms) 3.5

fuel volume (mm3) 120

fuel temperature (K) 350

Figure 1. Schematic of the constant volume chamber and light extinction by the soot cloud.



adopted. The two lters will achieve a 5 nm fwhmwhen aligned together.The light emitted from the ber was condensed by an asphericcondenser lens. The laser beam entered the constant volume chambervia a reecting mirror of 6 mm in diameter placed in front of thecondenser lens.2.3. Experimental Procedure. The test procedure was started by

filling the chamber to the premixed, combustible-gas mixture with aspecified density. The mixture, including acetylene (C2H2), oxygen, andnitrogen, whose concentration can be precisely controlled through theirsource pressure, was then ignited with a spark plug and burned to createa high-temperature, high-pressure environment in the chamber. Acet-ylene was used as the combustible gas because of its flammability and lowwindow contamination. Equation 4 shows the chemical reaction of themixture

4C2H2 10 O2 65N2 f 8CO2 4H2O O2 65N24

where denotes the amount of excess oxygen. As the products ofcombustion cool over a relatively long time (2 s) because of heattransfer to the chamber walls, the pressure slowly decreased. The sprayinjection begins when the pressure of the cooling burned mixturereaches the preset conditions, as illustrated in Figure 2. The temperature,density, and oxygen concentrations of the gas in the chamber at the timeof injection can be varied widely with this simulation procedure. Thecamera was triggered to start the recording by the injection signal, andthe record duration was long enough to cover the whole spray,

combustion, and soot measurements. The exposure time was 3 s,and the image interval was 66.5 s, for all test conditions.

For the experiments presented in this paper, dierent ambienttemperatures were considered for the spray, combustion, and sootmeasurement ranging from 800 to 1200 K that covered both low-temperature combustion and conventional combustion in a dieselengine. The ambient temperature means the in-chamber temperatureat the start of injection, which was calculated from the premixedcombustion pressure at the time of injection. The ambient density waskept constant at 14.8 kg/m3. Diesel combustion with 1000 K ambienttemperature, 21% oxygen concentration, and 14.8 kg/m3 ambientdensity is a typical diesel in-cylinder environment with the piston attop dead center (TDC). Two dierent volumetric blend fuels were usedin this study: one was the 20% butanol/80% soybean biodiesel blendreferred to as B20S80, and the other was the 20% ethanol/80% soybeanbiodiesel blend referred to as E20S80. The physical and chemicalproperties of soybean biodiesel, ethanol, butanol, and the blend fuelsare listed in Table 1. It can be found that the B20S80 fuel has a highercetane number and lower oxygen content compared to the E20S80 fuel,while the density and heating values of B20S80 and E20S80 fuels onlyhave a slight dierence.

3. RESULTS AND DISCUSSION

3.1. Spray Images and the Potential Microexplosion.Figure 3 demonstrates the spray image at 800 and 900 K ambienttemperatures. It can be observed that the tip of the spray jeterupted into a plume and expanded the spray tip. Some dropletsmoved away from the spray jet, and these droplets had a higherevaporation rate, showing lower reflective light intensity. Thesephenomena indicated that the microexplosion should be oc-curred at these experimental conditions. The microexplosionphenomenon is caused by the volatility difference between theadditives (butanol or ethanol) and the base fuel (biodiesel). Theinterior additives and base fuel become superheated as thedroplet is heated by convective and radiative heat transfer fromthe surrounding ambient temperature. The superheat of liquid,which is in the thermodynamically metastable condition, ismaintained as long as no phase transformation occurs withinthe droplet. As the droplet temperature approaches the superheatlimit, the occurrence of bubble nucleation dominates, leading tothe internal formation of vapor bubbles, rapid evaporation, andconsequently, disintegration of the superheated liquid. The

Figure 2. Chamber pressure versus time, illustrating the method usedto generate experimental conditions simulating engine conditions.

Figure 3. Spray jet images for microexplosion at 800 and 900 K ambient temperature.



violent disintegration produces the momentum to disperse thefine secondary droplets into a large physical volume and, conse-quently, enhances the fuel air mixing in the combustion field.This has the potential to suppress the formation of soot andunburned hydrocarbons.The microexplosion in water and oil emulsied fuel has been

studied widely,42,43 but these studies just focused on a singledroplet with a diameter of about 1 mm. Few studies was found inthe literature about the microexplosion in a diesel spray from aninjector. In recent studies by Lee et al.44 and Shen et al.,45

microexplosions in mixtures of ethanolbiodiesel and buta-nolbiodiesel were studied using the numerical model proposedby Zeng and Lee;46 however, there was no experimental valida-tion in these papers. In this study, for the blend of B20S80 andE20S80, the butanol or ethanol has a much lower boiling pointcompared to biodiesel. Therefore, it was quite possible that abubble formed in the fuel droplet because of this drastic fuelproperty dierence and nally incurred the microexplosion.Furthermore, the fuel downstream of the jet atomized bettercompared to upstream, as illustrated by Figure 3, indicatingdownstream of the jet would be easier to reach the superheatlimit and to form bubble nucleation. However, when the ambient

temperature increased (>900 K), all of the blended fuel vapor-ized very fast, resulting in no time to form bubble nucleation inthe blended fuels.3.2. Combustion Pressure andHeat Release Rate Analysis.

Figure 4 presents the combustion pressure (averaged over 10injections) and heat release rate with B20S80 and E20S80 atdifferent ambient temperatures. The combustion pressure is thetotal pressure derived from the transducer minus the initialambient pressure at the time of injection. It should be notedthat the energy input was 3.83 and 3.69 kJ for B20S80 andE20S80 fuels, respectively, and the energy difference was 3.65%for these two blend fuels. Figure 4 shows that the combustionpressure of the B20S80 fuel was lower than the E20S80 fuel,although the B20S80 fuel had a little higher energy input becauseof its higher heating value and density. The pressure rise isdirectly proportional to combustion efficiency in constant-vo-lume adiabatic combustion.47 However, the heat loss is inevitablein a real constant-volume combustion chamber, and the magni-tude of the heat loss is dependent upon a number of aspects, suchas flame area interacting with the chamber wall, flame radiation,and the ratio of premixed and diffused combustion. A largerflamewall interaction area and higher flame temperature willenhance both convection and radiation heat transfer through thewall, causing higher heat loss. Meanwhile, a premixed dominantcombustion reduces the combustion duration and, correspond-ingly, reduces the time of heat transfer compared to mixingcontrolled dominant combustion, resulting in a further increaseof the maximum combustion pressure. Therefore, the peakpressure in this experiment can be regarded as an indication ofthe heat release ability of the fuel under certain experimentalconditions rather than ideal constant-volume adiabatic condi-tions. It can be observed from Figure 4 that the E20S80 fuel had ahigher peak pressure compared to the B20S80 fuel, indicating apotentially higher thermal efficiency of the E20S80 fuel. Thepossible explanation of this observation is that less combustionoccurred near the wall region with E20S80, leading to lower heatloss. Also, the higher latent heat for ethanol could result in alower flame temperature, leading to a lower flame radiation loss,and a lower soot formation for E20S80 induced a lower flameradiation loss. In addition, the laminar burning velocity of ethanolis higher than that of butanol,48,49 thereby the E20S80 fuel had ashorter combustion duration and less heat loss.For the autoignition timing, it can be seen from Figure 4 that

the ignition delay became longer at a lower ambient temperature.A longer autoignition delay allowed more time for ambient air tobe entrained into the jet to form a premixed zone with fuel vapor,thereby more fuel was burnt by premixed combustion at a lowerambient temperature. The heat release rate plots in Figure 4demonstrate this transition from amixing controlled combustionat 1200 K to a premixed combustion dominant one at 800 K.With regard to the fuel eects, Figure 4 demonstrates that theautoignition delay for B20S80 was shorter than that of E20S80 inthe 1000 and 1200 K ambient temperature cases because of thehigher cetane number for B20S80. Meanwhile, the autoignitiondelay for B20S80 and E20S80 was nearly the same at 800 Kambient temperature, which can be explained by the fact that, atthe lower ambient temperature, the chemical reaction ratereduced and weakened the eect of the chemical autoignitiondelay because of the dierent cetane numbers, whereas thephysical delay because of higher volatility and lower viscosityfor the E20S80 fuel played a more important role on theautoignition.

Figure 4. Combustion pressure and heat release rate at dierentambient temperatures.



3.3. Natural Flame Images and Soot Formation Process.Figures 57 demonstrate the natural flame luminosity and sootmeasurement of B20S80 and E20S80 at various ambient tempera-tures. The time on the upper left corner represented the time atwhich the image was captured, and the labels F and S on theupper right corner represented flame emission and soot measure-ment, respectively. For natural flame and soot measurement, theaperture was set to f/32 and f/4.5, respectively. The aperture is theopening that determines the cone angle of a bundle of rays thatcome to focus in the image plane. Some images that representeddifferent combustion and soot development phases were demon-strated in this section. It can be seen from Figure 5 that, for theB20S80 fuel at 800 K ambient temperature, the flame was firstobserved near the wall region at around 3.66 ms. Less air wasentrained to reach this region because of the ongoing combustion,and as a consequence, therewas a greater chance for soot generationin this region, which was confirmed by the soot image at the sametime. Subsequently, the natural flame luminosity became brighter(larger pixel intensity value), and the flame distribution became

larger at 4.85 ms. More soot was generated near the wall region atthe same time. In the following image, the diffusion flame propa-gated toward the injector but the intensive combustion was stillvisible near the wall region because of a slow chemical reaction rateat lower ambient temperature. Therefore, the soot distributioncould be observed near the wall region and downstream of thespray jet. By 6.45 ms, the fading flame luminosity indicated that thecombustion had nearly completed in the chamber and the soot hadbeen oxidizedmostly. In comparison to the B20S80 fuel, the naturalflame luminosity decreased for the E20S80 fuel, which is reasonablebecause the flame temperature was reduced as a result of the higherlatent heat for ethanol. The soot was first observable downstream ofthe spray jet at around 3.99 ms. Next, the flame propagated towardthe injector; less combustion occurred near the wall region; and thesoot distribution also moved toward the injector at 4.52 ms. After4.52 ms, the main soot distribution region was in the reacting sprayjet. Finally, by 6.18ms, the fading flame luminosity indicated that thecombustion had nearly completed in the chamber and the soot hadbeen oxidized mostly.

Figure 5. Natural ame luminosity and soot distribution for E20S80and B20S80 fuels at 800 K ambient temperature.




Figure 6 demonstrates the natural ame luminosity and sootdistribution of the B20S80 and E20S80 fuels at 1000 K ambienttemperature. The ame luminosity was brighter than that of800 K ambient temperature because of the more intensivecombustion reaction. Only a little premixed combustion oc-curred, as shown in Figure 4; therefore, the diusion ame as wellas the soot emission could be seen downstream of the spray at2.39 ms for B20S80 and E20S80 fuels. Subsequently, the amepropagated to the chamber wall, and more regions began tocombust. Meanwhile, a higher soot concentration was perceiva-ble near the wall region. For the B20S80 fuel, the ameluminosity was observed near the wall region and downstreamof the spray jet after 3.46 ms; therefore, the soot was alsoperceivable at these two regions. For the E20S80 fuel, however,more combustion was detected downstream of the spray jet after3.59 ms and there was little combustion ame near the wallregion. Therefore, the main soot distribution was also down-stream of the spray jet.

Figure 7 demonstrates the natural ame luminosity and sootdistribution of the B20S80 and E20S80 fuels at 1200 K ambienttemperature. The ame luminosity was the brightest amongthree ambient temperatures. No premixed combustion occurred,as shown in Figure 4. The diusion ame can be observed muchearlier, and this initial diusion ame kernel was closer to theinjector. Correspondingly, the soot was also observed earlier thanthose of 800 and 1000 K ambient temperatures, and the sootdistribution was also closer to the injector at 2.00 ms for B20S80and 2.13 ms for E20S80. The ame then propagated down-stream, and the main combustion occurred downstream of thespray jet for B20S80 and E20S80 because of the fast combustionreaction at 1200 K ambient temperature. Little combustion amewas observed near the wall region for B20S80 and E20S80 fuels.Therefore, themain soot distribution was also downstream of thespray jet for B20S80 and E20S80 fuels.From Figures 57, it can be found that the soot distribution

was altered with the increase of the ambient temperature. For theB20S80 fuel, the main soot distribution was near the wall regionat 800 K ambient temperature, the soot distribution was reducednear the wall region with the increase of the ambient tempera-ture, and more soot was formed downstream of the spray jet at1000 and 1200 K ambient temperatures. For E20S80 fuel, thesoot formation was also increased downstream of the spray jetwith the increase of the ambient temperature. As reported inprevious studies,32 the ame lift-o length decreases as theambient temperature increases. The shorter lift-o length re-sulted in less air entrained downstream of the spray jet, and thus,more soot was observed at this region.


Figure 8. Total soot mass for B20S80 and E20S80 fuels at dierentambient temperatures.



3.4. Total Soot Mass Analysis. Figure 8 shows the total sootmass and the normalized time-integrated soot mass (NTISM) ofB20S80 and E20S80 fuels at three different ambient temperatures.Time-integrated total soot mass was introduced because it con-sidered not only the soot amount but also the soot existing timewithin the flame. For a better comparison, all of the time-integratedtotal soot masses were normalized by that of combustion at 800 Kambient temperature with the B20S80 fuel. A single value ofNTISM was thus introduced to demonstrate the soot tendency toget away from oxidation and appear in the exhaust pipe in a realdiesel engine, as shown in Figure 8b. As aforementioned, the energyinput for B20S80 and E20S80 fuels was 3.83 and 3.69 kJ, respec-tively. To eliminate the effect of different energy inputs, the value oftime-integrated total soot mass was divided by the energy input fordifferent fuel blends, which has also been shown in Figure 8b. It wasalso demonstrated that the different energy inputs have little effecton the value of NTISM.As shown in Figure 8a, soot formation and oxidation demon-

strated dierent behavior at dierent ambient temperatures forboth tested fuels. At a low ambient temperature of 800 K, a peakvalue of total soot mass was observed, which clearly demarcatedthe soot formation and oxidation process, while at high ambienttemperature, the soot formation dominated at the beginning ofthe combustion and the total soot mass reached a quasi-steadystate during the combustion before it dropped to zero approach-ing the end of the combustion. The soot oxidation rate has beenillustrated with a gray dash line, which was dened as the slopeconnecting between the peak value of the total soot mass and thelocation where it rst dropped to zero. It can be found that thesoot oxidation rate was increased with the increase of the ambienttemperature. The net soot release is the competition results ofsoot formation and oxidation. Although the lower ambienttemperature reduced the soot oxidation rate, the soot formationrate had also been reduced because of the lower combustiontemperature. Because the soot formation was inhibited eectivelyat lower ambient temperature, the total soot mass was reducedwith the decrease of ambient temperatures. Therefore, inhibitionof the soot formation was more important than acceleration ofsoot oxidation at present experimental conditions.As shown in Figure 8b, the value of NTISM was increased for

both tested fuels with the increment of ambient temperatures. Athigher ambient temperatures, less ambient air was entrained bythe spray jet because of the shorter ame lift-o and less fuel wasburnt by premixed combustion. The weak premixed combustionwith less ambient air entrainment resulted in some higher fuel-rich zones, thus leading to higher soot formation. In comparisonto the B20S80 fuel, the E20S80 fuel had a lower value of NTISMat all tested ambient temperature conditions and the reductionwas as much as 3060% depending upon dierent ambienttemperatures. The reduction can be attributed by a number offactors. First of all, the higher oxygen content of E20S8 com-pensating for the lack of air entrained in the ame resulted inlower soot generation. Second, a higher volatility and a lowerviscosity for E20S80 fuel would improve the mixture formationand reduce the local equivalence ratio favoring the soot exhibi-tion. As a conclusion, the E20S80 fuel presented more advan-tages to reduce the soot emission compared to the B20S80 fuel.Furthermore, increasing the ambient temperature from 800 to1200 K led to a rapid increase in soot formation for B20S80 andE20S80 fuels. Therefore, a lower environment temperature withthe piston at TDC should have more advantages to the combus-tion and soot emissions control for a real diesel engine.

4. CONCLUSION

To investigate the eects of dierent alcohol additives in biodieselon the spray, combustion characteristics, and soot formation andoxidation, a detailed comparative study between the buta-nolbiodiesel blend and the ethanolbiodiesel blend was carriedout in an optical constant volume combustion chamber. TheB20S80 (20 vol % butanol 80 vol % soybean biodiesel) andE20S80 (20 vol % ethanol 80 vol % soybean biodiesel) fuels wereused as the test fuel at dierent ambient temperatures, ranging from800 to 1200 K. The main conclusions are as following: (1) Themicroexplosion occurred for the B20S80 and E20S80 fuels at 800and 900 K ambient temperatures because of the volatility dierencebetween the additives (butanol or ethanol) and the base fuel(biodiesel). (2) The E20S80 fuel had a higher peak pressurecompared to the B20S80 fuel, indicating a higher potential thermaleciency. Meanwhile, the E20S80 fuel had a shorter combustionduration compared to the B20S80 fuel. (3) The autoignition wasearlier for the B20S80 fuel at 1000 and 1200K ambient temperaturebecause of its higher cetane number. However, the autoignition ofthe B20S80 and E20S80 fuels was nearly the same at 800 K ambienttemperature because the eect of chemical autoignition delay as aresult of the dierent cetane numbers was weakened, whereas thephysical delay because of a higher volatility and lower viscosity forE20S80 had a larger eect on the autoignition. (4) The naturalameluminosity was increased with elevated ambient temperatures. TheE20S80 fuel had a lower ame luminosity compared to the B20S80fuel. (5) The soot distribution was increased downstream of thespray jet at higher ambient temperatures. The E20S80 fuel had alower value of NTISM compared to the B20S80 fuel because ofits higher oxygen content, which can compensate for less airentrained in the ame. In addition, a higher volatility, lowerviscosity, and lower cetane number improved themixture formationand reduced the local equivalence ratio. Therefore, the E20S80fuel exhibited more advantages to reduce the soot emissioncompared to the B20S80 fuel. (6) The total soot mass weresensitive to the ambient temperature for both fuels, indicating that alower environment temperature near the TDC should be moreadvantageous to the combustion and emissions control for a realdiesel engine because the soot formation can be inhibited and theNOx emission can also be reduced as a result of the lower environ-ment temperature, leading to lower combustion temperature.

AUTHOR INFORMATION

Corresponding Author*Telephone: 1-217-333-5879. Fax: 1-217-244-6534. E-mail: [email protected].

ACKNOWLEDGMENT

The authors appreciate John Scheider and JonathonMcCradyfrom the University of Illinois at UrbanaChampaign for theirhelp during the research. The help of the American AnalyticalChemistry Laboratories and Incobrasa Industries, Ltd. with thefuel tests is also gratefully acknowledged. This work was sup-ported in part by the U.S. Department of Energy (DOE) underGrant DE-FC26-05NT42634, the DOE-sponsored GraduateAutomotive Technology Education (GATE) Centers of Excel-lence under Grant DE-FG26-05NT42622, and the China Scho-larship Council under Project [2009]3012. This work wasperformed using the equipment at the University of Illinois,and the support of the university sta is greatly appreciated.



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