Energy Fuels 2009, 23, 5228–5235 Published on Web 08/27/2009
: DOI:10.1021/ef9004847
Mixing Stability and Spray Behavior Characteristics of Diesel-Ethanol-Methyl Ester Blended Fuels in a Common-Rail Diesel Injection System Su Han Park,† Se Hun Kim,† and Chang Sik Lee*,‡ † Graduate School of Hanyang University, Department of Mechanical Engineering, Hanyang University, 17 Haengdang-dong, Sungdong-gu, Seoul, 133-791, Korea, and ‡Department of Mechanical Engineering, Hanyang University, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, Korea
Received May 19, 2009. Revised Manuscript Received August 3, 2009
The purpose of this study is to investigate the mixing stability, fuel properties, and spray-atomization characteristics of diesel-ethanol blended fuels in a common rail diesel injection system. In the diesel blended with ethanol fuel, the most important characteristics are the phase separation and low cetane number of blended fuel. To solve the phase separation, some amount of biodiesel fuel as an additive was added to diesel-ethanol blended fuels. The physical properties of diesel and ethanol blended fuels;such as density, kinematic viscosity, and surface tension;were measured. Based on the fuel properties, the overall spray characteristics were investigated. Spray tip penetration and spray cone angle were obtained from a visualization system. Droplet sizes were measured with a droplet measuring system. The results showed that the addition of biodiesel into the blended fuel prevented the phase separation. With the addition of biodiesel fuel, the diesel-ethanol blended fuels showed no occurrence of phase separation under the closed condition. On the other hand, the increased ethanol blending ratio and the increased fuel temperature resulted in a slight decrease in the spray tip penetration, because of a reduction in spray momentum. However, an increased ethanol blending ratio induced a decrease in the droplet size distribution of diesel-ethanol blended fuels.
ratio increased. Xu et al.5 reported that the brake thermal efficiency was also increased, and the soot emission was lower than a conventional diesel fuel with the increase of the ethanol ratio in the blended fuels, although the increased ethanol blending ratio induced the increase in the specific fuel consumption. In addition, they reported that the addition of ethanol fuel to the conventional diesel fuel improved the diesel combustion performance in the combustion chamber by investigating the ignition delay and combustion duration. Sahin et al.6 numerically analyzed the effect of ethanol-diesel blended fuels on diesel engine performance, combustion, and exhaust emissions. They reported that the brake specific fuel consumption decreased, brake effective efficiency improved significantly, and brake effective power increased slightly as the percentage of ethanol in the mixture increased. In addition to these studies, research about the fuel properties and combustion characteristics of ethanol blended fuels are consistently investigated. Also, studies on the effect of temperature on biodiesel and biodiesel-ethanol blended fuel and the physical and chemical properties of ethanol-diesel blend on the performance and emissions of diesel engines have been completed.7,8 The most convenient method to use diesel-ethanol blended fuels is by directly blending diesel and ethanol fuels. However, if even a slight amount of water is blended, it will result in phase separation, because of hydrogen bonding with the hydroxyl of ethanol and water molecules. Another problem is the low cetane number of the blended fuel,
1. Introduction To meet the strengthened exhaust emission regulations, various new engine technologies, such as a high-pressure and multiple-injection system, as well as the use of the alternative fuels, are being developed. These methodologies are being applied to the direct injection diesel engine with a high thermal efficiency, compared to that of the gasoline engine. Biodiesel and dimethyl ether (DME) fuels are in the spotlight as alternatives to diesel fuel, and they are actively studied in various research fields. Recently, research to improve the combustion performance and exhaust emission characteristics is also progressing, using ethanol fuel and diesel blended fuels, as well as biodiesel and DME fuels.1-3 Lu et al.4 performed a study on the combustion and emission characteristics of diesel-ethanol blended fuels in a singlecylinder diesel engine, and they reported the superior characteristics of ethanol blended diesel fuel. These characteristics include low emissions of carbon monoxide (CO), particulate matter (PM), and nitrogen oxides (NOx). After the combustion visualization experiments were performed and analyzed, they confirmed that the ignition delay was increased, and the combustion duration was decreased, as the ethanol blending *Author to whom correspondence should be addressed. Tel.: þ82-22220-0427. Fax: þ82-2-2281-5286. E-mail:
[email protected]. (1) Park, S. W. Energy Fuels 2009, 23, 3909-3918. (2) Junjun, Z.; Xinqi, Q.; Zhen, W.; Bin, G.; Zhen, H. Energy Fuels 2009, 23 (1), 170–174. (3) Fang, T.; Coverdill, R. E.; Lee, C.-F. F.; White, R. A. Int. J. Automot. Technol. 2009, 10 (3), 285–295. (4) Lu, X.; Huang, Z.; Zhang, W.; Li, D. Int. J. Automot. Technol. 2005, 6 (1), 15–21. (5) Xu, B. Y.; Qi, Y. L.; Zhang, W. B.; Cai, S. L. Int. J. Automot. Technol. 2007, 8, 9–18. r 2009 American Chemical Society
(6) Sahin, Z.; Durgun, O. Energy Fuels 2009, 23, 1707–1717. (7) Park, S. H.; Yoon, S. H.; Suh, H. K.; Lee, C. S. Oil Gas Sci. Technol. 2008, 63 (6), 737–745. (8) Li, D.; Zhen, H.; Lu, X.; Zhang, W.; Yang, J. Renewable Energy 2005, 30, 967–976.
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Energy Fuels 2009, 23, 5228–5235
: DOI:10.1021/ef9004847
Park et al.
Figure 3. Definition of spray characteristics in the auto measuring program. Table 1. Specifications of the Droplet Analysis System and HighSpeed Camera parameter
frame rate shutter speed resolution light source wavelength focal length collection angle
Figure 1. Test injector with single hole nozzle and schematic of the injector nozzle, visualization system, and droplet measuring system.
value Visualization System 10 000 fps 1/20 000 s 512 512 PDPA System Ar-ion laser 514.5 nm, 488 nm 500 mm for transmitter and receiver 30°
Table 2. Experimental Conditions for the Spray Visualization and Droplet Measuring parameter
value Test Fuels
D100 DE10 DE20 DE30
diesel (100%) diesel (85%) þ ethanol (10%) þ biodiesel (5%) diesel (75%) þ ethanol (20%) þ biodiesel (5%) diesel (65%) þ ethanol (30%) þ biodiesel (5%)
Visualization Experiment 60 MPa, 120 MPa injection pressure, Pinj 2 MPa, 3 MPa ambient pressure, Pamb 0.7 ms energizing duration, teng 290 K ambient temperature, Tamb 290 K, 330 K, 370 K fuel temperature, Tfuel
Figure 2. Schematic of the heat exchange device between hightemperature steams and fuels.
which decreases rapidly as the ethanol blending ratio increases. The main factors affecting the phase separation of diesel-ethanol blended fuels are the water content, temperature, and the ethanol blending ratio in the blended fuel. The phase separation occurs easily in blending conditions with high water content and low temperature.9 To solve the problem of phase separation in ethanol-diesel blended fuels, various additives are used. Recent research reported that biodiesel fuel is a good additive to prevent the phase separation.10 In addition, the use of biodiesel fuel results in an increase in the low cetane number. In this study, biodiesel fuel was used as an additive to prevent phase separation. The mixing stability and fuel properties (fuel density, viscosity, and surface tension) of diesel-ethanol blending fuels were studied. Based on the
Droplet Measuring Experiment 60 MPa injection pressure, Pinj 0.1 MPa ambient pressure, Pamb 1.3 ms energizing duration, teng 290 K ambient temperature, Tamb 290 K fuel temperature, Tfuel
results of the mixing stability for the diesel-ethanol blended fuels with an additive of small amounts of biodiesel, the sprayatomization characteristics of blending fuels were investigated under various injection conditions to clarify the effect of the ethanol fuel and additive (biodiesel) to the blended fuels. 2. Experimental Apparatus and Procedure
(9) Lapuerta, M.; Armas, O.; Garcia-Contrearas, R. Fuel 2007, 86, 1351–1357. (10) Kwanchareon, P.; Luengnar uemitchai, A.; Jai-In, S. Fuel 2007, 86, 1053–1061.
2.1. Experimental Setup. In this work, the single hole injector with a nozzle orifice geometry of 0.3 mm (diameter)0.8 mm (orifice depth) (L/D=2.67), as illustrated in Figure 1a, was used 5229
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: DOI:10.1021/ef9004847
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Figure 5. Separation ratio of the ethanol blended diesel fuels according to the blending ratio, additive ratio, and time after the start of blending (Pamb =0.1 MPa, Tamb =290-293 K, humidity= 45%-50%).
Figure 4. Pictures of the phase separation phenomena of dieselethanol blended fuels (DE30) (Pamb=0.1 MPa, Tamb=290-293 K, humidity=45%-50%).
were obtained using a high-speed camera (Photron, FastcamAPX RS) with a metal-halide lamp (Photron, HVC-SL) as a light source, as illustrated in Figure 1b. To synchronize the fuel injection and camera shutter signal, a multichannel digital delay/pulse generator (Berkeley Nucleonics Corp, Model 555) and an image grabber installed in a computer were utilized. The droplet measuring system consisted of an argon-ion laser (INNOVA 70C, Coherent) with a 0.7 W laser output, a transmitter,
to investigate the overall spray characteristics of diesel-ethanol blended fuels. This injector was controlled by the current signal of the injector driver (TEMS, TDA-3200H). In addition, the test injector was controlled by a peak current of 21.0 A and a hold current of 11.0 A. Spray images for analyzing the spray characteristics, such as spray tip penetration and spray cone angle, 5230
Energy Fuels 2009, 23, 5228–5235
: DOI:10.1021/ef9004847
Park et al.
Figure 6. Fuel properties of diesel-ethanol blended fuels for the blending ratio.
a receiver, and a droplet size and velocity analyzer, as shown in Figure 1b. To obtain time-resolved data, the signal analyzer was synchronized with the injector driver using a digital delay/pulse generator. Figure 2 shows a schematic of the fuel heating device. Diesel and ethanol blended fuels were heated by the high-pressure and high-temperature steam generated from a steam boiler. While the fuel went through the fuel supply line wrapped by a duplication tube, the fuel temperature varied among temperatures of 290, 330, and 370 K. Fuel temperature and steam temperature were measured by two K-type thermocouples (Omega, KMTSS-020G-12): one in the fuel supply line, and one in the duplication tube. The fuel temperature was controlled by the relief valve. In addition, the ambient gas in a highpressure chamber was pressurized to 4 MPa (maximum) using nitrogen gas. 2.2. Experimental Procedure. By investigating the mixing stability of the diesel and ethanol blended fuels, the phase separation phenomenon was determined to rely on the ethanol blending ratio, the contents of the additive (biodiesel fuel), and the condition of the test bottle cap (“open” or “closed”). The ethanol blending ratio was changed from 10% to 50% (by increments of 10%), and the additives were changed from 0% to 10% (by increments of 5%). In addition, test materials were observed under the “closed cap” and “open cap” conditions to study the effect of the contact between the ambient air and blended fuel. After finding a stable blending ratio of diesel, ethanol, and biodiesel fuel, the fuel properties of blended fuels, such as density, viscosity, and surface tension were measured using a hydrometer, viscometer, and surface tension measuring meter (Itoh Seisakusho Ltd., No. 514-B2). Macroscopic spray characteristics, including spray tip penetration and spray cone angle, were analyzed using spray images obtained from the visualization system, as illustrated in Figure 1b. The spray tip penetration is defined as the maximum distance from a nozzle tip that an injected spray can reach. The spray cone angle was measured from the angle between two lines
Figure 7. Comparison of the spray images for the ethanol blending ratio and fuel temperature.
formed from the nozzle tip and the outer contour of the injected spray. The spray tip penetration and spray cone angle were measured the image process software, which was created by the FORTRAN program. In this program, the spray tip penetration and spray cone angle were measured according to the definition given in Figure 3. To investigate atomization performance, the diameter subrange for the droplet measurement was set from 2 μm to 80 μm, and ∼15 000 spray droplets were accumulated at each measuring point. The measuring points were set every 10 mm, from 20 mm to 60 mm from the nozzle tip along the axial direction, and every 2 mm, to a maximum of 10 mm along the radial direction. The detailed specification and criteria for the droplet measuring system and the high-speed camera for the visualization system are listed in Table 1. The test fuels and detailed experimental conditions are listed in Table 2.
3. Results and Discussions 3.1. Mixing Stability and Fuel Properties of Diesel-Ethanol Blended Fuels. In this study, the 5% and 10% biodiesel fuel volumetric ratios were added to the diesel-ethanol blended fuel to prevent phase separation. Moreover, the effect of “closed” and “open” cap conditions on the phase separation was also investigated. Figure 4 shows the phase separation phenomena of 30% ethanol blending fuels with 0%, 5%, and 10% biodiesel fuel. As shown in Figure 4a, in the case of “open cap”, phase separation occurred immediately after blending, and the separation ratio increased with as the elapsed time increased. In this experiment, the phase separation ratio was calculated from the ratio of the separated upper layer volume to the initial ethanol volume. On the other hand, when 10% of the biodiesel fuel was added to the diesel-ethanol blended fuel, there was no occurrence 5231
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: DOI:10.1021/ef9004847
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Figure 8. Effect of the ethanol blending ratio on the spray characteristics.
Figure 9. Effect of the fuel temperature on the spray characteristics of diesel fuel (D100).
of phase separation. In addition, in the case of the “closed” condition with some biodiesel fuel as an additive, there is also no occurrence the phase separation between the diesel and ethanol fuels. This homogeneity was due to the fact that biodiesel can act as an amphiphile (a surface-active agent) and form micelles that have nonpolar tails and polar heads. These molecules are attracted to liquid/liquid interfacial films and to each other. These micelles acted as polar or nonpolar solutes, depending on the orientation of the biodiesel molecules.10 From these results, it can be said that the contact between the ambient air and the blend of diesel and ethanol are very important factors regarding the formation of the phase separation. In addition, it can be confirmed that biodiesel fuel, as an additive, is able to contribute to the prevention of phase separation. Figure 5 shows the results for various blending ratios and additive ratios in the case of an “open cap”. As illustrated in Figure 5, the addition of 10% ethanol fuel to diesel fuel shows the most stable blending characteristics, regardless of the additive ratio. In addition, the separation ratio increased as the ethanol blending rate increased. Therefore, it can be said that the separation ratio was determined by the ethanol blending ratio in both cases of 0% and 5% additive. In the case of 10% BD additive (Figure 5c), the separation phenomenon occurred between
24 h and 48 h. From these results, it can be confirmed that the addition of biodiesel fuel affected the relaxation of the occurrence of the phase separation between diesel and ethanol fuels. Through the mixing stability experiment, it can be concluded that the diesel-ethanol blended fuels are sufficient stable fuels to use in a diesel engine if those fuels include some biodiesel fuel with storage under the closed condition. Of course, they have the proper cetane number, because of the addition of biodiesel fuel. From the experimental results about the mixing stability of diesel-ethanol blended fuels, DE10, DE20, and DE30 with 5% BD were selected as test fuels, considering the mixing stability and cetane number. During the performance of the experiment, test fuels were stored in the closed fuel tank. Figure 6 shows the fuel properties, such as fuel density, kinematic viscosity, and surface tension. Fuel properties were measured at an ambient temperature of ∼20 °C, and the fuel temperature was ∼20 °C. As shown in Figure 6, an increase in the ethanol blending ratio caused a decrease in the fuel properties, because the ethanol fuel has low fuel density, kinematic viscosity, and surface tension. 3.2. Spray Tip Penetration and Spray Cone Angle Characteristics. Two important spray parameters include the spray tip penetration and spray cone angle in the direct 5232
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: DOI:10.1021/ef9004847
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Figure 10. Effect of the fuel temperature on the spray characteristics of diesel-ethanol blended fuel (DE30).
Figure 11. Spray atomization characteristics of diesel-ethanol blended fuels (Pinj =60 MPa, Pamb =0.1 MPa, teng =1.3 ms, Tfuel = 293 K, Tamb=293 K).
injection diesel engine system, and studies on spray tip penetration are currently ongoing.11,12 Spray tip penetration is affected by the relative magnitude of two opposing forces: the kinetic energy of the initial liquid jet and the aerodynamic resistance of the surrounding gas.13 Comparisons of the spray images according to the ethanol blending ratio and the fuel temperature of diesel-ethanol blended fuels are illustrated in Figure 7. As shown in Figure 7a, the spray images roughly seem to be similar for the ethanol blending ratio, except for the increase of the spray cone angle. In addition, Figure 7b also showed the similar spray behavior in the variation of the fuel temperature. Detailed and quantitative spray characteristics were analyzed from these sprays images and are represented by Figures 8-12. Figure 8 shows the effect of the ethanol blending ratio on the spray tip penetration and spray cone angle. The injection pressures are 60 and 120 MPa, and the ambient pressure and energizing duration were fixed at 2 MPa and 0.7 ms, respec-
tively. At the initial stage after injection, the spray tip penetration of D100 is slightly higher than other fuels, because of a high injection momentum due to the higher fuel density. However, the minimal difference among test fuels is observed at later times after the start of energizing. In addition, this observation can be explained by the following reason: the spray tip penetration was affected by the fuel density before the breakup of the injected liquid column, but it was mainly affected by the ambient gas density and the pressure difference after the breakup of the liquid column.8 On the other hand, as shown in Figure 8b, D100 showed the highest spray cone angle among the test fuels immediately after the injection. However, the spray cone angles of test fuels are similar, for the duration of the test after the start of energizing. Therefore, it can be said that there is no effect of the ethanol fuel on the macroscopic spray characteristics of blended fuels. The effects of the fuel temperature on the macroscopic spray characteristics of D100 and DE30 are illustrated in Figures 9 and 10, respectively. As shown in Figure 9a, an increase in the fuel temperature caused a small decrease in the spray tip penetration, because the fuel density is reduced by an increase of the fuel temperature. In addition, the reduction
(11) Hiroyasu, H.; Arai, M. SAE Tech. Pap., 1990, SAE900475. (12) Desantes, J. M.; Payri, R.; Salvador, F. J.; Gil, A. Fuel 2006, 85, 910–917. (13) Lefebvre, A. H. Atomization and Sprays; Taylor & Francis: New York, 1989; pp 274-275.
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Figure 12. SMD distribution of diesel and diesel-ethanol-methyl ester blended fuels at the specific region of fuel spray (Pinj=60 MPa, Pamb= 0.1 MPa, teng=1.3 ms).
fuel decreased from 825 kg/m3 to 773 kg/m3 (an ∼6% reduction) and that of DE30 fuel decreased from 814 kg/m3 to 770 kg/m3 (an ∼5.5% reduction) as the fuel temperature increased to 370 K. This is related to a small variation of spray momentum. 3.3. Atomization Characteristics and Contour of Droplet Size Distribution. The atomization characteristics of fuel directly affect the combustion and emissions characteristics in a diesel engine system. The improvement of the injected droplet size can increase the combustion performance and reduce the particulate matter (PM) emissions. Therefore, an investigation into atomization performance is essential for any study involving the diesel injection system. Figure 11a shows the spray atomization characteristics of diesel-ethanol blended fuels. The definition of the local Sauter mean diameter (SMD) is the transient droplet size at each measuring point over the entire injection period, while the overall SMD is the mean time-dependent droplet size at a specific time at a measuring point. As illustrated in Figure 11a, the local droplet size (SMD) decreased as the ethanol blending ratio increased, because the low kinematic viscosity and low surface tension induced the active droplet
of the axial droplet velocity by the increase of the fuel temperature induced the decrease of the spray tip penetration.14 The evaporation of small droplets by the increase of the fuel temperature makes the mixing between the droplets and ambient air active. It is conjectured that this phenomenon affected the decrease of the spray tip penetration. After having the peak spray cone angle immediately after injection, it suddenly decreased 0.3-0.6 ms after the start of energizing. Since then, the spray cone angles of test fuels have values in the range of 18°-28°, regardless of the fuel temperatures. Based on these observations, it can be concluded that an increase in the fuel temperature decreased the spray tip penetration, and the fuel temperature has little effect on the spray cone angle. Alternatively, in the case of DE30, the increased fuel temperature has little effect on the spray tip penetration when compared to D100. This may result from the lower fuel density of DE30, compared to D100; therefore, the density reduction by the increased fuel temperature is small. The density of D100 (14) Park, S. H.; Kim, H. J.; Suh, H. K.; Lee, C. S. Int. J. Heat Fluid Flow, 2009 DOI:10.1016/j.ijheatfluidflow.2009.04.003.
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breakup. Moreover, as shown in the decreasing trend, it can be concluded that the fuel atomization can be improved by the addition of ethanol fuel. Figure 11b shows the mean axial velocity of droplets of diesel-ethanol blended fuels at 60 MPa of injection pressure and 1.3 ms of energizing duration. In this case, the mean axial velocity of droplets smoothly decreased according to the axial distance, and it decreased with the increase of the ethanol ratio. This is the reason why the small droplets have a large momentum loss in the process of the atomization. The reduced axial velocity could affect the decrease of the spray tip penetration when the ethanol blending ratio increased in the diesel-ethanol blended fuels. Figure 12 shows the SMD distribution of diesel and diesel-ethanol blended fuels under the following experimental conditions: injection pressure, 60 MPa; ambient gas pressure, 0.1 MPa; and the spray droplet characteristics illustrated at the specific spray area are located 20-60 mm in the axial direction and 0-10 mm in the radial direction. Generally, the droplet size shows the decreasing tendency as increasing the radial distance, except the results of 10 mm in a radial distance. It can be explained that the spray outer region was actively mixed with the ambient gas; therefore, the contact area became wider than the inner region of the injected spray. In addition, looking through the SMD distribution at the entire radial direction, D100 has a range of 29 μm, while DE30 has a droplet size range of 23-24 μm. From these results, it can be concluded that the increase of the ethanol blending ratio positively influences the improvement of the diesel atomization performance.
fuel density, kinematic viscosity, and surface tension. As increasing the blending ratio of ethanol fuel, the blended fuels with the lowered surface tension and density were wellmixed with the ambient air, because of the evaporation of ethanol droplets. From this result, it can be expected that the macroscopic spray behavior and atomization performance are improved by the variation of the ambient air flow. (3) The added ethanol fuel slightly affected spray tip penetration and spray cone angle, whereas the droplet size of diesel-ethanol blended fuels decreased with an increase in the ethanol blending ratio. On the other hand, the spray tip penetration was slightly decreased by an increased fuel temperature, because of a reduction in the spray momentum. (4) The mean droplet size decreased as the ethanol blending ratio increased because of the low kinematic viscosity and low surface tension of ethanol blended fuels. From the analysis of the SMD distribution at the specific spray region, the range of the droplet size distribution shrank and was reduced when the ethanol blending ratio increased. From these analyses, the fuel atomization of diesel fuel can be improved by the addition of ethanol fuel. Acknowledgment. This study was supported in part by the CEFV (Center for Environmentally Friendly Vehicle) of the EcoSTAR Project of the MOE (Ministry of the Environment) in Seoul, Republic of Korea, and the Second Brain Korea 21 Project in 2009. This work was also financially supported by a manpower development program for Energy & Resources supported by the Ministry of Knowledge and Economy (MKE).
Nomenclature 4. Conclusions
L = distance from the nozzle tip P=pressure (MPa) SMD=Sauter mean diameter (μm) t=time (ms) T=temperature (K)
In this study, the mixing stability, fuel properties, and spray-atomization characteristics of diesel-ethanol blended fuels were investigated at various ethanol blending ratios and ambient conditions. The conclusions from the experimental results and analysis are summarized as follows. (1) To prevent the phase separation between diesel and ethanol fuels, biodiesel fuel as an additive must be added to the blended fuels. In addition, the addition of biodiesel fuel with a high cetane number improves the low cetane number of diesel-ethanol blended fuels. (2) The increase in the percentage of ethanol fuel in the diesel-ethanol blended fuels induced the reduction of the
Subscripts amb=ambient asoe = after the start of energizing inj=injection eng=energizing fuel=fuel z = axial direction
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