Energy & Fuels 2008, 22, 4229–4234
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Effects of Biodiesel from Palm Kernel Oil on the Engine Performance, Exhaust Emissions, and Combustion Characteristics of a Direct Injection Diesel Engine Bai-Fu Lin, Jyun-Han Huang,* and Dao-Yi Huang Department of Vehicle Engineering, National Taipei UniVersity of Technology, No. 1, Sec. 3, Jhongsiao E. Rd., Taipei City, 106, Taiwan, Republic of China ReceiVed May 13, 2008. ReVised Manuscript ReceiVed August 5, 2008
Palm kernel oil is extracted from palm fruit as well as palm oil and is considered to be a potential feedstock for biodiesel production. The objective of this paper is to evaluate the feasibility of using biodiesel from palm kernel oil on a direct injection (DI) diesel engine under three different engine speeds and at various gradational engine load conditions. Experimental results demonstrate that the brake specific fuel consumption (BSFC) increased as the percentage of palm kernel oil methyl ester (PKOME) fuel in blends increased, producing the same level of engine power as petroleum diesel (PD), because of the decreased lower heating value (LHV). In addition, increasing the percentage of PKOME fuel in blends reduces the exhaust gas temperature (EGT), the amount of smoke and total hydrocarbon (THC) emissions, and the formation of nitrogen oxides (NOx) emissions, because of the shorter carbon-chain lengths, more saturated carbon bonds, and higher oxygen content of PKOME fuel when compared with the same percentage of palm oil methyl ester (POME) fuel in blends.
Introduction Biodiesel, which is produced from vegetable oil and animal fat through transesterification, is widely used as a substitute fuel for petroleum diesel (PD) in diesel engines, because it is renewable and has environmentally friendly characteristics.1,2 Biodiesel that has fuel properties similar to those of PD can be used directly or mixed with other substitute fuels in diesel engines without or with few engine modifications.3,4 Several researchers have demonstrated that using biodiesel in diesel engines reduces the production of carbon dioxide (CO2) based on the well-to-wheel perspective.5,6 Also, some studies have reported that biodiesel has a high cetane number and intrinsic oxygen content available for combustion, resulting in increased combustion efficiency. The improved combustion efficiency reduces the production of smoke and total hydrocarbon (THC) emissions. In addition, a slight increase in nitrogen oxides (NOx) * Author to whom correspondence should be addressed. Tel.: 886-227712171 ext. 3673. Fax: 886-2-27314990. E-mail address: lukas613@ yahoo.com.tw. (1) Altı´n, R.; C¸etinkaya, S.; Yu¨cesu, H. S. The potential of using vegetable oil fuels as fuel for diesel engines. Energy ConVers. Manage. 2001, 42, 529–538. (2) Graboski, M. S.; McCormick, R. L. Combustion of fat and vegetable oil derived fuels in diesel engines. Prog. Energy Combust. Sci. 1998, 24, 125–164. (3) Rakopoulos, C. D.; Antonopopulos, K. A.; Rakopoulos, D. C.; Hountalas, D. T.; Giakoumis, E. G. Use of vegetable oils as I.C. engine fuelssA review. Energy ConVers. Manage. 2006, 47, 3272–3287. (4) Shudo, T., Fujib, A.; Kazahaya, M.; Aoyagi, Y.; Ishii, H.; Goto, Y.; Noda, A. The Cold Flow Performance and the Combustion Characteristics with Ethanol Blended Biodiesel Fuel, SAE Paper 2005-01-3707, Society of Automotive Engineers, Warrendale, PA, 2005. (5) Sheehan, J., Camobreco, V.; Duffield, J., Craboski, M., Shapouri, H. An OVerView of Biodiesel and Petroleum Diesel Life Cycles, A joint study sponsored by U.S. Department of Agriculture and Energy, Washington, DC, 1998. (6) Carraretto, C.; Macor, A.; Mirandola, A.; Stoppato, A.; Tonon, S. Biodiesel as alternative fuel: Experimental analysis and energetic evaluations. Energy 2004, 29, 2195–2211.
emissions occurs when biodiesel is used, because of an advanced combustion process and a rapid combustion rate.7-9 Furthermore, a reduced exhaust gas temperature (EGT) was observed when biodiesel was used, because of the reduced energy content and superior combustion process.10,11 The current use of biodiesel is related to specific climates, agricultural policies, and the environmental laws of countries. In the United States, the use of biodiesel made from soybean oil is common. In Europe, rapeseed oil is the most readily available oil for producing biodiesel. In tropical climates, such as those in Malaysia and Indonesia, palm oil is the most common base oil for biodiesel production.12 Table 1 shows the world major vegetable oil production in the period of 2003-2007. Roughly 30.06 million metric tons of palm oil, which is the second-most widely produced vegetable oil, after soybean oil, were produced worldwide in 2003. In addition, palm oil surpassed soybean oil as the most widely produced vegetable oil in 2004. Palm fruit can produce not only palm oil (extracted from palm fruit), but also palm kernel oil (extracted from fruit seeds). A bunch of palm fruit can produce ∼20% palm oil and (7) Lapuerta, M.; Armas, O.; Rodrı´guez-Ferna´ndez, J. Effect of biodiesel fuels on diesel engine emissions. Prog. Energy Combust. Sci. 2008, 34, 198–223. (8) Senatore, A.; Cardone, M.; Buono, D.; Rocco, V.; Allocca, L.; Vitolo, S. Performance and Emissions Optimization of a CR Diesel Engine Fuelled with Biodiesel, SAE Paper 2006-01-0235, Society of Automotive Engineers, Warrendale, PA, 2006. (9) Kawano, D.; Ishii, H.; Goto, Y.; Noda, A.; Aoyagi, Y. Application of Biodiesel Fuel to Modern Diesel Engine, SAE Paper 2006-01-0233, Society of Automotive Engineers, Warrendale, PA, 2006. (10) Subramanian, M.; Malhotra, R. K., Kanal, P. C. Performance EValuation of Biodiesel-Diesel Blends in Passenger Car, SAE Paper 200428-0088, Society of Automotive Engineers, Warrendale, PA, 2004. (11) Sureshkumar, K.; Velraj, R.; Ganesan, R. Performance and exhaust emission characteristics of a CI engine fueled with Pongamia pinnata methyl ester (PPME) and its blends with diesel. Renew. Energy 2008, 33, 2294– 2302. (12) Demirbas, A. Importance of biodiesel as transportation fuel. Energy Policy 2007, 35, 4661–4670.
10.1021/ef800338j CCC: $40.75 2008 American Chemical Society Published on Web 09/10/2008
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Table 1. World Vegetable Oil Productiona Production (million metric tons) oil
2003
2004
2005
2006
2007
coconut cottonseed olive palm palm kernel peanut rapeseed soybean sunflower seed
3.29 3.85 3.06 30.06 3.67 5.07 14.17 30.16 9.19
3.44 4.78 2.97 33.44 4.13 5.09 15.77 32.53 9.17
3.40 4.65 2.59 35.97 4.38 4.97 17.14 34.50 10.50
3.25 4.89 2.99 37.02 4.48 4.81 17.71 36.29 10.97
3.32 4.86 3.02 40.20 4.79 4.85 18.28 37.97 10.05
a
Source: USDA (www.usda.gov).
Table 2. Specifications of the Prototypical Engine description
specification
engine model engine type bore × stroke displacement compression ratio intake system exhaust system injection system number of nozzle holes diameter of plunger injection pressure injection timing injection angle rated output
Yanmar industrial engine (TF110-F) horizontal single-cylinder, four-stroke, water-cooled, direct-injection diesel engine 88 mm × 96 mm 0.583 L 17.9 naturally aspirated, single valve, swirl port single valve pump-line-nozzle system 4 8 mm 19.6 MPa 17° CA BTDC 150° 8.1 kW/2400 rpm
2% palm kernel oil at the same time.13 In 2007, palm oil and palm kernel oil ranked first and seventh by amount, respectively, of all vegetable oils produced worldwide (see Table 1). Biodiesel production from palm kernel oil is called palm kernel oil methyl ester (PKOME). The fatty acids of PKOME fuel are high in saturated carbon bonds. Therefore, the cloud point of PKOME fuel is relatively higher than that of other biodiesel fuels.14 Some researchers demonstrated that fuel with a high cloud point causes problems in diesel engines under low temperature conditions.4,15 To investigate the effects of using PKOME fuel on diesel engines in practical aspects, this study blended 20% and 50% v/v PKOME fuel with PD as test fuels and compared the experimental results with the same percentage of palm oil methyl ester (POME) fuel in blends. Therefore, POME20 (20% v/v POME fuel and 80% v/v PD), PKOME20, POME50, and PKOME50 fuels were used in an unmodified, single-cylinder direct injection (DI) diesel engine to investigate the effects of biodiesel from palm kernel oil on DI diesel engine performance, exhaust emissions, and combustion characteristics. Materials and Methods Experimental Apparatus. This study used a single-cylinder, four-stroke, water-cooled, DI diesel engine (YANMAR TF110-F) in a series of engine tests. Table 2 lists the engine specifications. Figure 1 shows a schematic diagram of the engine setup and its instrumentation. The hydraulic dynamometer (SCHENCK, Model (13) Jekayinfa, S. O.; Bamgboye, A. I. Development of equations for estimating energy requirements in palm-kernel oil processing operations. J. Food Eng. 2007, 79, 322–329. (14) Shiotani, H.; Goto, S. Studies of Fuel Properties and Oxidation Stability of Biodiesel Fuel, SAE Paper 2007-01-0073, Society of Automotive Engineers, Warrendale, PA, 2007. (15) Kono, N.; Fukumoto, J.; Lizuka, M.; Takeda, H. Influence of FAME Blends in Diesel Fuel on driVeability performance of diesel Vehicles at low Temperatures, SAE Paper 2006-01-3306, Society of Automotive Engineers, Warrendale, PA, 2006.
D230-1e) was connected to the engine to control engine torque. For combustion analysis, a quartz pressure transducer (AVL, Model QC43D) was mounted on the cylinder head and a rotary encoder (BEI, Model H25) was attached directly to the crankshaft to measure in-cylinder combustion pressure and corresponding crank angles during tests. These two data were delivered to the combustion analyzer (AVL, Model 620 Indiset), which was linked to a personal computer that was running IndiWin 2.0 software to calculate the rate of heat release. The diesel smoke meter (IYASAKA, Model GSM-2) and two exhaust emissions analyzers (HORIBA, Model MEXA-1160CLT-HA, and CAI, Model 300M-HFID) were used to record smoke emissions (as a percentage), NOx emissions (in units of parts per million (ppm)), and THC emissions (expressed in terms of ppm) during tests, respectively. Test Fuels. The biodiesel used in this study was supplied by Chant Oil Co., Ltd., Taiwan. Palm oil and palm kernel oil were used as raw materials to conduct the transesterification reaction. Sodium methoxide (CH3NaO) was added to the transesterification process as a catalyst, to accelerate the reaction rate. Finally, the POME and PKOME fuels were produced through the water washing and purification process, and then blended with PD (i.e., 20% v/v POME and 80% v/v PD (POME20); 20% v/v PKOME and 80% v/v PD (PKOME20); 50% v/v POME and 50% v/v PD (POME50); and 50% v/v PKOME and 50% v/v PD (PKOME50)) as test fuels. The PD was purchased from the Chinese Petroleum Co., Taiwan. In addition, the biodiesel and PD were examined by the Formosa Petrochemical Co., Taiwan, using the standards of the American Society for Testing and Materials (ASTM). Table 3 presents the main physical and chemical fuel properties of the biodiesel and PD. Table 4 shows the fatty acid compositions of POME and PKOME fuels. Test Procedures. The PD and four biodiesel blends (POME20, PKOME20, POME50, and PKOME50) were used as test fuels fed into an unmodified DI diesel engine. The fuel injection pressure and injection timing for each test fuel were set at 19.6 MPa and 17 °CA BTDC during the tests, respectively (see Table 2). [Note: The term °CA BTDC represents the degrees of crank angle before top dead center.] Pure PD was first used to obtain baseline data. Engine speed was progressively set at 1200, 1800, and 2400 rpm. At each constant engine speed, the engine torque was divided into various gradations, to simulate different load conditions. Table 5 shows all engine testing conditions at the three different engine speeds and different load conditions. For each engine testing condition (engine speed and load), the brake specific fuel consumption (BSFC), engine power, exhaust emissions, and combustion characteristics were measured. The combustion characteristics (i.e., the rate of heat release and the in-cylinder combustion pressure) were averaged over 100 combustion cycles that were acquired in sequence. Furthermore, all experimental data were replicated at least three times and averaged for improved accuracy and reliability.
Results and Discussion The effects of POME20, PKOME20, POME50, and PKOME50 fuels on a DI diesel engine performance, exhaust emissions, and combustion characteristics at variable engine speeds and load conditions were investigated. Engine Performance. Figure 2 shows that PKOME50 fuel produced the highest BSFC, followed by POME50, PKOME20, and POME20 fuels. Generally, differing the carbon, hydrogen, oxygen, and sulfur content in a fuel influences its computation of the lower heating value (LHV).16 The PKOME fuel has lower carbon, hydrogen, and sulfur contents and higher oxygen content than those of POME and PD fuels. Therefore, PKOME fuel has the lowest LHV (see Table 3). Although the high density of biodiesel compensates for the low LHV, the lowest LHV per unit volume of PKOME50 fuel still results in the highest (16) Channiwala, S. A.; Parikh, P. P. A unified correlation for estimating HHV of solid, liquid and gaseous fuels. Fuel 2002, 81, 1051–1063.
Biodiesel Effects on DI Diesel Engine Performance
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Figure 1. Experimental setup for engine testing. Table 3. Main Fuel Properties of the Biodiesel and Petroleum Diesel (PD) property
unit
density (@ 15 °C) cetane number kinematic viscosity (@ 40 °C) sulfur content hydrogen content carbon content oxygen content flash point cloud point acid value lower heating value glycerin content monoglyceride content diglyceride content triglyceride content a
kg/m3 cSt wt % wt % wt % wt % °C °C mg KOH/g
method
PD
ASTM-D4052 837.8 ASTM-D613 50.6 ASTM-D445 3.275
POME PKOME 878.4 62 4.698
876.6 62.1 3.248
MJ/kg
ASTM-D2622 0.0019 0.0005 ASTM-D5291 13.25 12.17 ASTM-D5291 86.1 75.66 11.34 ASTM-D93 68 189 ASTM-D2500 -4 15 ASTM-D974 0.28 EN14111 44.63 ASTM-D4809 45.85 39.91
0.0001 11.91 72.66 13.78 131 10 0.21 16.79 38.53
wt % wt %
ASTM-D6584 ASTM-D6584
NDa 0.041
NDa 0.029
wt % wt %
ASTM-D6584 ASTM-D6584
0.056 NDa
0.044 NDa
Figure 2. Percentage change in brake specific fuel consumption (BSFC) for four biodiesel blends, in comparison with that for petroleum diesel (PD).
Not detected.
Table 4. Fatty Acid Compositions of Palm Oil Methyl Ester (POME) and Palm Kernel Oil Methyl Ester (PKOME) Fuels fuel type C8:0 C10:0 C12:0 C14:0 C16:0 C18:0 C18:1 C18:2 POME PKOME
3.60
3.10
0.50 48.00
1.60 14.70
49.80 11.50
2.90 1.40
38.60 15.90
6.60 1.80
Table 5. Engine Testing Conditions engine speed (rpm)
engine load (Nm)
1200 1800 2400
12 15 20 25 30 35 40 maximum 12 15 20 25 30 35 40 maximum 12 15 20 25 30 maximum
increase in BSFC of all test fuels.17 However, the brake specific energy consumption (BSEC) for all biodiesel blends is lower than that of PD (see Figure 3). This could be due to that the POME and PKOME fuels both have higher cetane numbers,
increased density, and extra oxygen content than those of PD (see Table 3). Therefore, blending POME and PKOME fuels with PD have a superior combustion process. In addition, the improved combustion and high BSFC compensate for the low volumetric LHV of biodiesel blends, which produce the same level of engine power as PD in a DI diesel engine.7 Therefore, the maximum difference in engine power under full load conditions at the three engine speeds between PD and the four biodiesel blends was only 0.95% (see Figure 4). Exhaust Emissions. The PKOME50 fuel reduced the smoke emissions the most, followed by POME50, PKOME20, and POME20 fuels (see Figure 5). Biodiesel has extra oxygen (17) De Almeida, S. C. A.; Belchior, C. R.; Nascimento, M. V. G.; Vieira, L. S. R.; Fleury, G. Performance of a diesel generator fuelled with palm oil. Fuel 2002, 81, 2097–2102.
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Figure 3. Percentage change in brake specific energy consumption (BSEC) for four biodiesel blends, in comparison with that for PD.
Figure 4. Percentage change in maximum engine power for four biodiesel blends, in comparison with that for PD.
Figure 5. Percentage change in smoke emissions generated by the four biodiesel blends, in comparison with that generated by PD.
available for fuel-rich zones in the combustion process, which reduces the formation of smoke emissions.18,19 In addition, the fatty acid compositions of biodiesel from vegetable oil generally
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Figure 6. Percentage change in NOx emissions generated by the four biodiesel blends, in comparison with that generated by PD.
have carbon-chain lengths in C18 groups.20 However, POME and PKOME fuels have short carbon-chain lengths. Specifically, the length of carbon-chain in POME fuel is ∼50% of that for C16 and the length of the carbon chain in PKOME fuel is ∼50% of that for C12 (see Table 4). Fuel with short carbon-chain lengths also benefits the combustion process.21 Therefore, a significant reduction in smoke emissions was observed with increasing content of PKOME in the fuel blends, because of the high oxygen content and short carbon-chain lengths of PKOME fuel. Production of smoke emissions decreased by 37.33% and 59.56%, when using PKOME20 and PKOME50 fuels at 2400 rpm, respectively (see Figure 5). Many studies demonstrated that reducing smoke emissions increases NOx emissions, because of the tradeoff between smoke and NOx emissions.3,22 In this study, the PKOME50 fuel reduced smoke emissions the most of all of the tested fuels; however, its NOx emissions were even less than those of POME20 fuel (see Figures 5 and 6). Biodiesel has different fuel properties and specific fatty acid compositions, which influences the formation of NOx emissions during the combustion process. The production of NOx emissions has a tendency to decrease with oxygen content and increase with carbon-chain length.23,24 In addition, highly saturated fuels (those with no double bonds in the fatty acid chains) seem to have low NOx emissions.25 The (18) Lapuerta, M.; Armas, O.; Ballesteros, R.; Ferna´ndez, J. Diesel emissions from biofuels derived from Spanish potential vegetable oils. Fuel 2005, 84, 773–780. (19) Shuepeng, S., Tsolakis, A., Theinnoi, K.; Xu, H. M.; Wyszynski, M. L.; Qiao, J. A Study of QuantitatiVe Impact on Emissions of High Proportion RME-Based Biodiesel Blends, SAE Paper 2007-01-0072, Society of Automotive Engineers, Warrendale, PA, 2007. (20) Agarwal, A. K. Biofuels (alcohols and biodiesel) applications as fuels for internal combustion engines. Prog. Energy Combust. Sci. 2007, 33, 233–271. (21) Ramadhas, A. S.; Jayaraj, S.; Muraleedharan, C. Use of vegetable oils as I.C. engine fuelssA review. Renew. Energy 2004, 29, 727–742. (22) Heywood, J. B. International Combustion Engine Fundamentals; McGraw-Hill: Singapore, 1988. (23) Sharp, C. A.; Ryan, T. W., III.; Knothe, G. HeaVy-Duty Diesel Engine Emissions Tests Using Special Biodiesel Fuels, SAE Paper 200501-3671, Society of Automotive Engineers, Warrendale, PA, 2005. (24) Sendzikiene, E.; Makareviciene, P.; Janulis, P. Influence of fuel oxygen content on diesel engine exhaust emissions. Renew. Energy 2006, 31, 2505–2512. (25) McCormick, R. L.; Graboski, M. S.; Alleman, T. L.; Herring, A. M. Impact of biodiesel source material and chemical structure on emissions of criteria pollutants from a heavy-duty engine. EnViron. Sci. Technol. 2001, 35 (9), 1742–1747.
Biodiesel Effects on DI Diesel Engine Performance
Figure 7. Percentage change in THC emissions generated by the four biodiesel blends, in comparison with that generated by PD.
fatty acid compositions of biodiesel made from vegetable oil generally consist of unsaturated carbon bonds primarily.20 However, POME and PKOME fuels primarily consist of saturated carbon bonds. Specifically, ∼50% of the carbon bonds in POME fuel are saturated, whereas ∼80% of carbon bonds in PKOME fuel are saturated (see Table 4). Therefore, blending PKOME fuel with PD suppresses the increase in NOx emissions, whereas a significant reduction in smoke emissions is observed because of the high oxygen content, short carbon-chain lengths, and numerous saturated carbon bonds of PKOME fuel. The production of NOx emissions slightly increased by 1.46% and 2.33% for PKOME20 and PKOME50 fuels at 2400 rpm, respectively (see Figure 6). Figure 7 shows that PKOME50 fuel produced the highest reduction in THC emissions, followed by POME50, PKOME20, and POME20 fuels. As mentioned previously, the high oxygen content, short carbon-chain lengths, and numerous saturated carbon bonds of PKOME fuel are beneficial to combustion. Therefore, blending PKOME fuel with PD effectively reduces THC emissions. The THC emissions decreased by 15.53% and 22.09% when using PKOME20 and PKOME50 fuels at 1200 rpm, respectively (see Figure 7). In addition, the PKOME50 fuel produced the lowest EGT, followed by POME50, PKOME20, and POME20 fuels (see Figure 8). The PKOME fuel has low energy content, which results in reduced total heat release.26 Therefore, low EGT was observed by blending PKOME fuel. Combustion Characteristics. The rate of heat release, calculated from the in-cylinder combustion pressure data versus crank angle positions, is commonly used to represent the history of the combustion process, to analyze the effects of different fuel properties and specific fatty acid compositions of biodiesel on combustion, which further impact engine performance and exhaust emissions. Figure 9 shows the heat release rate for all test fuels. The rate of heat release passes through the zero before its steep rise is defined as the start of combustion. As can be seen in Figure 9, increasing the percentage of PKOME fuel in blends generates an early start of combustion, a fast increase in the rate of heat release, and a high peak in the rate of heat release. This could be due to the improvements in the combustion process, because of one or more of the following factors: (26) Usta, N. An experimental study on performance and exhaust emissions of a diesel engine fuelled with tabacco seed oil methyl ester. Energy ConVers. Manage. 2005, 46, 2373–2386.
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Figure 8. Percentage change of EGT for the four biodiesel blends, in comparison with that for PD.
Figure 9. Rate of heat release for all of the tested fuels.
the short carbon-chain lengths, the numerous saturated carbon bonds, and the higher oxygen content of PKOME fuel. The rapid and superior combustion further reduces the production of smoke and THC emissions (see Figures 5 and 7). Conversely, a slight increase in NOx emissions is observed (see Figure 6). However, the lower kinematic viscosity of PKOME fuel may allow more air to enter the fuel spray, which reduces the combustion temperature and NOx emissions simultaneously.27 Therefore, blending PKOME fuel with PD results in lower NOx emissions, compared to the same percentage of POME fuel in blends, although the peak rate of heat release of PKOME fuel is higher. In addition, an increased PKOME fuel in blends has a rapid rate of combustion and excellent combustion, because of its high oxygen content, short carbon-chain lengths, and numerous saturated carbon bonds, resulting in a rapid rate of heat release, and high in-cylinder combustion pressure occurs near the TDC position (see Figures 9 and 10). Therefore, most of the release of fuel energy and the conversion of heat to work occurs in the premixed phase of combustion. This is a possible reason why the PKOME fuel blends, which have a lower energy content (27) Kinoshita, E.; Myo, T.; Hamasaki, K.; Tajima, H.; Kun, R. Z. Diesel Combustion Characteristics of Coconut Oil and Palm Oil Biodiesels, SAE Paper 2006-01-3251, Society of Automotive Engineers, Warrendale, PA, 2006.
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Figure 10. In-cylinder combustion pressure for all of the tested fuels.
than PD, produce the same level of engine power as does PD (see Figure 4). At the same time, the energy released at the late combustion phase was less, and a low EGT was observed when the amount of PKOME fuel in blends was increased (see Figure 8). Conclusion Biodiesel from palm kernel oil was blended with petroleum diesel (PD) and tested in an unmodified direct injection (DI) diesel engine to investigate experimentally the effects of
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biodiesel blends on engine performance, exhaust emissions, and combustion characteristics. In total, 22 engine testing conditions with varying engine speeds and gradational load conditions were used to test each fuel. The different fuel properties and specific fatty acid compositions of palm kernel oil methyl ester (PKOME) fuel significantly affect the combustion timing, combustion rate, and combustion efficiency. These combustion characteristics further improve engine performance and reduce exhaust emissions. Increasing the percentage of PKOME fuel in blends produces an early start of combustion, a rapid increase in the rate of heat release, and an increase in the peaks of the rate of heat release, because of the short carbon-chain lengths, the numerous saturated carbon bonds, and the increased oxygen content of PKOME fuel. These further reduce the production of smoke and total hydrocarbons (THC) emissions and suppress the increase in nitrogen oxides (NOx) emissions. In addition, a rapid combustion rate and an excellent combustion also benefit the conversion of heat energy to work during the early portion of the expansion stroke. Therefore, blending PKOME fuel with PD results in the same level of engine power as that from PD. Furthermore, less energy is released at the late combustion phase and a lower exhaust gas temperature (EGT) is observed (see Figure 8). Acknowledgment. The authors would like to thank the National Science Council of Taiwan, for financially supporting this research, under Contract No. NSC 96-2221-E-027-042. We appreciate Chant Oil Co., Ltd., Taiwan, for supplying the POME and PKOME fuels used in the study. EF800338J
