An Experimental Study on the Atomization and Combustion

Energy & Fuels 2005, 19, 2201-2208

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An Experimental Study on the Atomization and Combustion Characteristics of Biodiesel-Blended Fuels Chang Sik Lee,*,† Sung Wook Park,‡ and Sang Il Kwon§ Department of Mechanical Engineering and Graduate School, Hanyang University, 17 Haengdang-dong, Sungdong-gu, Seoul 133-791, and National Institute of Environmental Research, Environmental Research Complex, Kyungseo-dong, Seo-gu, Incheon 404-170, Korea Received January 24, 2005. Revised Manuscript Received April 28, 2005

In this paper we describe the atomization and combustion characteristics of biodiesel fuels in a common-rail diesel engine. To investigate the effect of the mixing ratio of biodiesels on the emission characteristics and engine performance, the experiments were conducted at various mixing ratios of the biodiesel and engine operation conditions. In addition, the physical properties such as kinematic viscosity and cetane number of the biodiesel-blended fuel were analyzed to study relations with the fuel atomization and combustion characteristics. The atomization characteristics of biodiesel-blended fuels were investigated in terms of spray tip penetration, SMD, and mean velocity distributions by using a spray visualization system and phase Doppler particle analyzer. The effect of the mixing ratio on the combustion characteristics was studied on the basis of the results of the combustion pressure obtained from the single-cylinder engine at various experimental conditions. The emission characteristics of HC, NOx, and CO were also measured to reveal the effect of the mixing ratio of the biodiesel fuel on the pollutant emissions. The results indicate that the mean size of the droplets increases in accordance with the mixing ratio of the biodiesel because the viscosity and surface tension of the biodiesel are higher than those of the conventional diesel fuel. As the ratio of the biodiesel becomes higher, HC and CO emissions are decreased, whereas the NOx emission increases because of oxygen in the biodiesel and a shorter ignition delay.

1. Introduction The attention on biodiesel fuels derived from vegetable oils or animal fats has increased as they are alternative and clean fuels for compression ignition engines. It is well-known that biodiesels can be used for diesel engines as blended forms with the conventional diesel without modifications of the engine. In addition, the biodiesel can be used as a proper method to reduce the pollutant emissions from the engine because the oxygen in biodiesel fuel promotes combustion. Therefore, it is important to analyze the relationship between the mixing ratio of biodiesels and engine emissions. To analyze the emission characteristics of diesel engine fueled with biodiesels, a lot of research on the effect of biodiesel fuel has been conducted. Chang and Gerpen1 compared the emission characteristics between conventional diesel and biodiesel blends in terms of total particulate, SOF, and HC both experimentally and numerically. They showed that pure biodiesel and its blends with conventional diesel can significantly reduce total particulate and HC emissions in comparison with conventional diesel at full load engine conditions. The * To whom correspondence should be addressed. Phone: +82-2-22200427. Fax: +82-2-2281-5286. E-mail: [email protected]. † Department of Mechanical Engineering, Hanyang University. ‡ Graduate School, Hanyang University. § National Institute of Environmental Research. (1) Chang, D. Y.; Gerpen, J. H. V. SAE Tech. Pap. Ser. 1998, 982527.

regulated and unregulated emissions from diesel engines fueled with biodiesel-blended fuels were measured by Sharp et al.2,3 In their research, it was shown that measurable HC emissions were generally eliminated and CO was reduced roughly 40% by using biodiesels whereas NOx emissions increased by 12% because oxygen in the fuel increases the combustion temperature. To overcome the increase of NOx emission by using biodiesels, Yoshimoto and Takami4 suggested that exhaust gas recirculation (EGR) and water emulsion can be a proper method to reduce the combustion temperature and NOx emission. Also they showed that NOx emission decreases significantly without an increase in smoke emissions in the case of a single-cylinder engine fueled with biodiesels by combining 21% EGR and 30% water emulsion by volume. Ramadhas et al.5 found that the compression ignition engine fueled with biodiesel derived from rubber seed oil has higher carbon deposits inside the combustion chamber than engines fueled with conventional diesel; therefore, more frequent cleaning of the fuel filter, the pump, and the combustion chamber is required for the use of biodiesel-blended fuels. Also, (2) Sharp, C. A.; Howell, S. A.; Jobe, J. SAE Tech. Pap. Ser. 2000, 2000-01-1967. (3) Sharp, C. A.; Howell, S. A.; Jobe, J. SAE Tech. Pap. Ser. 2000, 2000-01-1968. (4) Yoshimoto, Y.; Tamaki, H. SAE Tech. Pap. Ser. 2001, 2001-010649. (5) Ramadhas, A. S.; Jayaraj, S.; Muraleedharan, C. Renewable Energy 2005, 30, 795-803.

10.1021/ef050026h CCC: $30.25 © 2005 American Chemical Society Published on Web 07/27/2005

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Usta6 showed that tobacco seed oil methyl ester can be partially substituted for the diesel fuel at most engine operating conditions without any engine modifications or preheating of the biodiesel-blended fuel. The atomization characteristics of biodiesel fuel such as spray tip penetration and mean droplet size play an important role in the engine performance and the emission characteristics. Moreover, the biodiesels or their bends have different physical properties compared to the conventional diesel. Therefore, the relationship between the fuel physical properties and the atomization characteristics of biodiesels needs to be investigated. Ramadhas et al.7 studied correlations between the physical properties of biodiesels and the combustion characteristics. They concluded that biodiesel-blended fuels can be used as alternative fuels for diesel engines without modification and even the pure biodiesel can be used for diesel engines with some minor modifications. Lang et al.8 showed that the biodiesels have properties compatible with those of conventional diesels on the basis of GC analysis. To study the effect of the high viscosity of biodiesels on the spray characteristics, Grimaldi and Postrioti9 compared the process of spray development between the conventional diesel and biodiesels using a common-rail injection system. Their results indicated that the spray tip penetrations increase in accordance with the increase in mixing ratio of the biodiesels because the biodiesels are hardly atomized in comparison to conventional diesel due to the high surface tension. To overcome the effect of the high surface tension of biodiesels on the fuel injection, Postrioti et al.10 suggested the tuning in injection method for the use of bioderived fuels in a common-rail HSDI diesel engine. Despite these efforts, the fuel atomization and combustion characteristics of biodiesels are not fully investigated. In particular, research on the correlations of fuel atomization and combustion performance of the biodiesel and its blends is needed. In this study, the spray characteristics of biodiesels, spray tip penetration, mean droplet size, velocity distributions, and injection profiles were measured using the spray visualization system and phase Doppler particle analyzer (PDPA) system. The effects of different mixing ratios of biodiesel fuel on the combustion and emission characteristics were also analyzed using a single-cylinder diesel engine equipped with a common-rail system. 2. Experimental Apparatus and Procedure In this experiment, the effects of the mixing ratio of the biodiesel on the atomization and combustion characteristics were studied by using an injection rate meter and spray visualization and PDPA systems. The experimental apparatus for the combustion analysis consists of a single-cylinder engine, dc dynamometer, and emission measurement system. 2.1. Test Fuels. Biodiesel derived from unpolished rice and soybeans is used in this experiment. The biodiesel fuel is (6) Usta, N. Biomass Bioenergy 2005, 28, 77-86. (7) Ramadhas, A. S.; Jayaraj, S.; Muraleedharan, C. Renewable Energy 2004, 29, 727-742. (8) Lang, X.; Dalai, A. K.; Bakhshi, N. N.; Reaney, M. J.; Hertz, P. B. Bioresour. Technol. 2001, 80, 53-62. (9) Grimaldi, C.; Postrioti, L. SAE Tech. Pap. Ser. 2000, 2000-011252. (10) Postrioti, L.; Battistoni, M.; Grimaldi, C. N.; Millo, F. SAE Tech. Pap. Ser. 2003, 2003-01-0786.

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Figure 1. Spray measurement systems. Table 1. Test Fuels fuel composition

ref

conventional diesel unpolished rice oil (10%) + diesel (90%) unpolished rice oil (20%) + diesel (80%) unpolished rice oil (40%) + diesel (60%) soybean oil (10%) + diesel (90%) soybean oil (20%) + diesel (80%) soybean oil (40%) + diesel (60%)

D100 BD10a BD20a BD40a BD10b BD20b BD40b

blended with conventional diesel with mixing ratios of 10%, 20%, and 40% by volume. For the experiments of combustion analysis, biodiesel-blended fuels in mixing ratios of 10%, 20%, and 40% were used, whereas biodiesel-blended fuels in 20% and 40% ratios were utilized in the cases of experiments for spray measurements. Table 1 lists the test fuels and their compositions used in this experiment. 2.2. Spray Measurement Systems. For the analysis of the spray characteristics of the biodiesel and its blends, the injection profile, images of spray development, mean droplet size, and velocity distributions were obtained. The injection rate meter based on Bosch’s suggestion11 was used to analyze the effect of the mixing ratio of the biodiesel on the injection profile. This experimental apparatus calculates the injection rate using the pressure variation in a tube filled with fuel. When the fuel is injected into the tube, a pressure wave is generated by the fuel injection. Then a pressure sensor installed in the tube detects the pressure variation as a function of time after the start of injection. By analyzing the pressure variation in a tube, an injection profile can be obtained. In this experiment, the pressure within the tube was set constant at 2 MPa. In this experiment, 300 continuous injections were averaged for each test case. Figure 1 shows the schematic diagrams of spray measurement systems such as spray visualization and phase Doppler particle analyzer systems. As shown in Figure 1a, the processes of spray development were visualized by using the spray visualization system composed of an ICCD camera (DiCam(11) Bosch, W. SAE Tech. Pap. Ser. 1966, 660749.

Atomization and Combustion of Biodiesel-Blended Fuels

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Figure 3. Schematic diagram of the engine test bench.

Figure 2. Measurement points for the PDPA experiment. Table 2. Experimental Conditions for Spray Measurement injection system injector nozzle nozzle diameter injection pressure (Pinj) ambient conditions energizing duration

common-rail single hole 0.3 mm 40, 60, and 80 MPa atmosphere 1.0 ms

Pro, Cooke), a Nd:YAG laser (SL2-10, Continnum), and a synchronization system. The Nd:YAG laser emits a beam approximately 10 mm in diameter and 532 nm in wavelength, with a 5 ns pulse duration that is short enough to ignore the movement of droplets during the exposure duration of the ICCD camera. The guided beam by the mirrors passes through a cylindrical lens with 10 mm of focal length and expands the beam height vertically. The beam then passes through a positive cylindrical lens with a 300 mm focal length to reduce the thickness of the beam horizontally. The line beam generated by two cylindrical lenses illuminates the spray, and the spray axis is located in the horizontal focus of the cylindrical lenses. Two high-pressure pumps compress the fuel up to 200 MPa, and a digital delay generator controls the fires of the Nd:YAG laser, the ICCD camera, and the injector driver. The microscopic characteristics such as the Sauter mean diameter (SMD) and mean velocity distributions were obtained from the phase Doppler particle analyzer illustrated in Figure 1b. In this system, an Ar-ion laser was used as a light source, and the signal analyzer was synchronized with the injection signal to obtain the transient characteristics of the fuel atomization process. By using a beam expander for the transmitter, a measurement range for velocity was expanded from -292 to +292 m/s. The subrange of diameter measurement was from 2 to 100 µm, and about 20000 droplets were collected and averaged for each measurement point. To analyze the atomization characteristics of biodiesel sprays, the droplet size and velocity distributions were measured in the range from 10 to 70 mm in the axial direction and from 0 to 12 mm in the radial direction with constant intervals. Figure 2 shows the measurement points and the coordination systems of the PDPA experiment. For each measurement point, approximately 20000 droplets are collected and averaged to obtain the SMD and mean velocity of the point. The overall SMD at a specific time is determined by averaging the captured droplets at all of the measurement points of Figure 2. The experimental conditions for spray measurement are listed in Table 2. 2.3. Combustion and Emission Analysis System. The influences of different mixing ratios of biodiesel fuel on the combustion and emission characteristics are investigated using a single-cylinder engine installed on a dc dynamometer as illustrated in Figure 3. The rail pressure was controlled by a

Table 3. Engine Specifications and Operating Conditions injection system injector nozzle nozzle diameter spray angle rail pressure bore × stroke swept volume valve fuel mass injected injection timing (BTDC) compression ratio

common-rail six holes 0.128 mm 156° 100 MPa 75.5 mm × 83.5 mm 373.6 cm3 DOHC 4 8 mg 2°, 4°, 6°, 8°, 10° 20.85

programmable ECU, and an encoder for the detection of the crank angle was installed. The combustion pressure was obtained using a piezoelectric pressure sensor and data acquisition board. The exhaust gases such as soot, NOx, HC, and CO are measured by using an exhaust gas analyzer. The injected fuel quantity is set constant at 8 mg, and the rail pressure was 100 MPa. The test engine is operated at 1000 rpm, and the detailed engine specifications are listed in Table 3.

3. Results and Discussion 3.1. Physical Properties of Biodiesel-Blended Fuels. The physical properties of biodiesel fuel are different compared with conventional diesel. Therefore, it is important to investigate the physical properties of biodiesels, which can affect the atomization and combustion characteristics of the fuel. Figure 4 shows the effect of the mixing ratio of unpolished rice oil and soybean oil in fuel on the physical properties such as kinematic viscosity, surface tension, and cetane number. It can be guessed that the kinematic viscosity and surface tension are closely related to the spray characteristics such as spray tip penetrations and distributions of mean droplet sizes as shown in the research of Grimaldi and Postrioti.9 The kinematic viscosity and surface tension increase in accordance with the increase of the mixing ratio of the biodiesel as can be seen in this figure. The cetane number of the blended fuel, which influences the ignition delay and ignition ability, also increases in accordance with the increase of the mixing ratio. This shows that the ignition delay can get shorter as the mixing ratio of the biodiesel increases. Another characteristic of biodiesel fuel is that it contains an oxygen content of 10% by weight. The oxygen in the biodiesel fuel can promote the combustion process and make the peak combustion pressure higher. The promoted combustion of the biodiesel can reduce the HC emission but increase the NOx emissions.

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Figure 6. Effect of the mixing ratio on spray tip penetrations (Pinj ) 60 MPa).

Figure 4. Effect of the mixing ratio of biodiesels on the physical properties.

Figure 5. Process of spray development (Pinj ) 60 MPa).

On the basis of the physical properties of the biodiesel, it can be summarized that the advantages of the biodiesel are shorter ignition delay due to a higher cetane number and an enhanced combustion process caused by oxygen in the biodiesel. On the other hand, the disadvantages of the biodiesel are higher kinematic viscosity and surface tension, thereby making the atomization of the biodiesel difficult. 3.2. Spray Characteristics of Biodiesel-Blended Fuels. The spray characteristics of biodiesel-blended fuels are analyzed to study the influence of higher viscosity and surface tension of the biodiesel on the atomization performance. Figure 5 shows the effect of different mixing ratios of the biodiesel on the process of spray development at a 60 MPa injection pressure as a function of time after the start of injection (Tasoi). As illustrated in this figure,

it can be observed that the mixing ratio of the biodiesel has a minimal effect on the spray development. It can be seen that the injection velocity of the biodiesel is lower than that of conventional diesel because the higher viscosity of the biodiesel increases the friction between the biodiesel and nozzle surface. At the same time, the SMD of the biodiesel spray is higher than that of conventional diesel because the higher surface tension and viscosity of the biodiesel influence the spray atomization. Therefore, it can be seen that the lower injection velocity causes shorter spray tip penetration and a higher SMD brings about longer spray tip penetration of the biodiesel. It can be summarized that the biodieselblended fuels have similar spray tip penetration compared with conventional diesel by these two effects. The effects of the mixing ratio of the biodiesel on the spray tip penetrations are analyzed quantitatively in Figure 6. The error bars for D100 are shown together to indicate the reliability of the results of spray visualization. As shown in this figure, the biodiesel-blended fuels show spray tip penetration similar to that of conventional diesel. On the basis of this result, it can be said that the mixing ratio of the biodiesel has little effect on the spray tip penetration. In addition, it can be suggested that the research on the vaporizing sprays of biodiesel-blended fuels would help the optimization of mixture formation of biodiesel-blended fuels. Figure 7 shows the effect of the injection pressure on the spray tip penetration of conventional diesel and biodiesel-blended fuels with a 20% mixing ratio. The results indicate that the spray tip penetrations become longer at higher injection pressure because the injection velocity becomes higher. The distributions of SMD and spray tip penetration are key parameters in analyzing spray characteristics because they are closely related to the combustion and emission characteristics in the engine. To study the atomization characteristics of biodiesel fuels, it is necessary to analyze the SMD distributions of biodiesel fuels because the physical properties such as the surface tension and viscosity of the biodiesel are different from those of conventional diesel. The effect of the mixing



Figure 9. Distributions of the axial mean velocity as a function of time after the start of injection (Pinj ) 60 MPa).

Figure 7. Effect of the injection pressure on spray tip penetrations.

Figure 8. Mean size distributions of biodiesel-blended fuels (Pinj ) 60 MPa).

ratio of the biodiesel on the SMD distribution is shown in Figure 8. In both cases of fuels mixed with unpolished rice oil and soybean oil, the SMD of biodiesel-blended fuel is higher than that of the conventional diesel. The breakup characteristics of a droplet are mainly influenced by the Weber number,12,13 and the surface tension of the biodiesel is higher than that of commercial diesel. Therefore, it can be seen that the lower Weber number of biodiesels caused by higher surface tension is the reason for higher SMD distributions. As shown in Figure 4, the surface tension of the biodiesel is higher than that of conventional diesel fuel, which makes the Weber number lower. With the lower Weber number, the fuel spray is minimal because the viscosity and surface tension are higher than those of diesel fuel. The (12) Lee, C. S.; Reitz, R. D. Atomization Sprays. 2001, 11, 1-19. (13) Liu, A. B.; Reitz, R. D. Atomization Sprays 1993, 3, 55-75.

lower injection velocity can be another reason for the higher SMD of biodiesel-blended fuels. The higher kinematic viscosity increases the friction between the nozzle surface and fuel; as a result, the injection velocity of the biodiesel-blended fuel is reduced. This assumption can be supported by the results of axial mean velocity distributions measured by the PDPA system. Figure 9 illustrates the distributions of the axial mean velocity as a function of time after the start of injection. In this figure, the velocity of the conventional diesel is higher than that of the biodiesel-blended fuel before 1.0 ms after the start of injection. Then the velocity of the conventional diesel decreases rapidly and becomes lower than that of the biodiesel-blended fuel. The droplets of the conventional diesel spray are smaller than those of biodiesel-blended fuels as illustrated in Figure 8. Therefore, the momentum of the conventional diesel droplet is lower than that of the biodiesel-blended fuel. The droplets with lower momentum decelerate more rapidly due to the drag from ambient gas. Analyzing the distributions of SMD and axial mean velocity, it can be said that the biodiesel-blended fuels are inferior to the conventional diesel in fuel injection and atomization performance. 3.3. Combustion and Emission Characteristics. The combustion pressure, rate of heat release, and emission characteristics were obtained using a singlecylinder common-rail engine. In these experiments, the injected fuel quantity per cycle (Qinj) is set constant as 8 mg to study the effect of the mixing ratio of the biodiesel. Figure 10 shows the injection profiles of the injector installed in the single-cylinder engine when the injected fuel quantity is fixed at 8 mg. In the figure, normal and bold lines indicate 50 and 100 MPa injection pressures, respectively. In all cases, the injection delay, which is defined as the time interval between the start of energizing of the injector solenoid and the start of the rapid increase in the injection rate, is shorter at a 100 MPa injection pressure than at a 50 MPa injection pressure. It can be said that the injection delay is reduced with an increase of the injection pressure because the nozzle is opened using the back-pressure of the sac volume. Furthermore, as shown in Figure 9,

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Figure 10. Effect of the mixing ratio on the injection profiles at 8 mg of injected fuel quantity. Bold and normal lines indicate 100 and 50 MPa injection pressures, respectively.

the variation in the injection delay according to the mixing ratio of the biodiesel is negligible. Generally, the peak injection rate of the fuel is determined by the nozzle geometry and injection pressure. However, the peak injection rate is decreased with an increase of the mixing ratio of the biodiesel as indicated in Figure 10. It can be conjectured that the increase in the friction between the nozzle surface and fuel has been shown to reduce the injection velocity and reduce the peak injection rate. On the basis of this assumption, it can be said that it is necessary to increase the injection pressure in the case of a diesel engine fueled with a biodiesel-blended fuel to achieve higher combustion performance. Figure 11 shows the influence of injection timing on the combustion pressure and rate of heat release at a 100 MPa injection pressure in a diesel engine fueled with D100, BD20a, and BD20b. In the test range of this experiment, all fuels show similar patterns in the combustion pressure and rate of heat release as shown in this figure. For the analysis of the ignition delay and combustion peak combustion pressure, the combustion pressure and rate of heat release are illustrated at the same injection timing and injection pressure in Figure 12. In this figure, it can be observed that the ignition delay decreases as the mixing ratio of the biodiesel becomes higher at both injection pressures. The cetane number plays an important role in the ignition delay in the combustion chamber. Therefore, it can be said that the ignition delay becomes shorter because the cetane number increases with an increase of the mixing ratio in soybean and unpolished rice oils as illustrated in

Figure 11. Effect of the injection timing on the combustion pressure and rate of heat release (Pinj ) 100 MPa).

Figure 4. On the basis of the results of ignition delay, it is suggested that the fuel injection timing be modified in the diesel engine fueled with biodiesel for higher engine performance. The peak combustion pressure is a main index that shows the state of combustion. In the case of a 100 MPa injection pressure as illustrated in Figure 12, the peak combustion pressure increases with an increase of the mixing ratio of the biodiesel due to the promotion of combustion by oxygen in the biodiesel and shorter ignition delay. The main properties of the biodiesel that are related to the combustion performance are the oxygen in the biodiesel and the higher cetane number.



Figure 12. Effect of the mixing ratio of biodiesels on the combustion characteristics (tinj ) -8°, ATDC).

Therefore, it can be said the peak combustion pressure of biodiesel-blended fuels becomes higher because the oxygen in biodiesels can promote the combustion process and the higher cetane number reduces the ignition delay. Figure 13 shows the emission characteristics of biodiesel-blended fuels according to the injection timing at a 100 MPa injection pressure. In the test range of this experiment, the soot emission was too low to measure by using a soot analyzer in both cases for the conventional diesel and biodiesel. In the figure, HC and CO emissions are decreased with an increase of the mixing ratio of the biodiesel because of the oxygen in the biodiesel. Also it can be seen that there is little difference in emission characteristics between unpolished rice oil and soybean oil. On the other hand, NOx emissions become higher as the mixing ratio increases. This may be the reason that the oxygen in the biodiesel promotes the combustion process and increases the combustion temperature as indicated by the previous studies of other researchers.2,14 It is also shown that the NOx emission is lower in the case of late injection (tinj ) -2°) than in the case of early injection (tinj ) -8°) because the peak combustion pressure becomes lower as the injection timing is retarded. To overcome the increase of NOx emissions by using the biodiesel-blended fuels, homogeneous charge compression ignition (HCCI) can be a proper method.15 Yoshimoto and Tamaki4 also showed that the water (14) Schmidt, K.; Gerpen J. V. SAE Tech. Pap. Ser. 1996, 961086.

Figure 13. Emission characteristics of biodiesel-blended fuels.

emulsion and EGR can be used for the reduction of NOx emission in the compression ignition engine fueled with biodiesel-blended fuels. 4. Conclusions The effect of different mixing ratios of biodieselblended fuels on the atomization and combustion characteristics were performed using a common-rail engine system. To study the spray characteristics of biodieselblended fuels, the spray tip penetration, SMD, and axial mean velocity distributions were obtained using the spray measurement systems. The combustion characteristics were also investigated using a single-cylinder engine. On the basis of the results, it can be said that the performance of diesel engines fueled with biodieselblended fuels can be promoted by optimizing the injection timing and injection pressure. The conclusions of this study can be summarized as follows. (1) The kinematic viscosity, surface tension, and cetane number of biodiesels such as unpolished rice oil (15) Kim, D. S.; Kim, M. Y.; Lee, C. S. Energy Fuels 2004, 18, 12131219.

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and soybean oil become higher as the mixing ratio of the biodiesel increases. Because of the different physical properties of biodiesels, the atomization and combustion characteristics of biodiesel-blended fuels are different from those of commercial diesel fuel. (2) Despite the different physical properties, there is little difference in the spray tip penetrations according to the mixing ratio of the biodiesel. On the basis of this result, it can be said that the biodiesel can be used for a compression ignition engine with no or little modification. However, research on the spray characteristics of evaporating biodiesel sprays is required for optimizing the mixture formation of biodiesel-blended fuels. (3) The surface tension of the biodiesel is higher than that of commercial diesel, which gives a lower Weber number. In addition, the higher viscosity of the biodiesel decreases the injection velocity of biodiesel-blended fuels. Because of the lower Weber number and lower injection velocity of the biodiesel, biodiesel-blended fuels show inferior performance in atomization compared to the commercial diesel. (4) The peak injection rate becomes lower as the mixing ratio of the biodiesel increases. It can be conjectured that the high viscosity of the biodiesel

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increases the friction between the fuel and nozzle wall, resulting in a reduction of the injection velocity. Therefore, it is necessary to increase the injection pressure in the case of a diesel engine fueled with biodieselblended fuels. (5) The ignition delay becomes shorter with an increase of the mixing ratio due to the higher cetane number of the biodiesel. The peak combustion pressure is increased with an increase of the mixing ratio because of the oxygen in the fuel and shorter ignition delay of the biodiesel. (6) HC emissions can be reduced by using biodieselblended fuels up to 55%, whereas NOx emissions are increased. It can be seen that oxygen in the biodiesel promotes the combustion process and increases the combustion temperature. Acknowledgment. This work was supported by the CEFV (Center for Environmentally Friendly Vehicle) of the Eco-STAR project from the MOE (Ministry of Environment, Republic of Korea). EF050026H

An Experimental Study on the Atomization and Combustion

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