Energy & Fuels 2008, 22, 1297–1305
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Effects of Biodiesel from Used Frying Palm Oil on the Performance, Injection, and Combustion Characteristics of an Indirect Injection Diesel Engine Ahmet Necati Ozsezen,† Mustafa Canakci,*,† and Cenk Sayin‡ Department of Mechanical Education, Kocaeli UniVersity, 41380 Izmit, Turkey, AlternatiVe Fuels R&D Center, Kocaeli UniVersity, 41040 Izmit, Turkey, and Department of Mechanical Education, Marmara UniVersity, 34722 Istanbul, Turkey ReceiVed July 27, 2007. ReVised Manuscript ReceiVed December 3, 2007
In this study, biodiesel from used frying palm oil and its blends with diesel fuel were used in a fourcylinder, naturally aspirated indirect injection (IDI) diesel engine. Using petroleum-based diesel fuel (PBDF), biodiesel, and its blends, the engine performance, injection, and combustion characteristics were investigated over a range of engine speeds at full load. When the test engine was fueled with biodiesel and its blends, the brake specific fuel consumption increased slightly relative to PBDF due to its fuel properties and combustion characteristics. Biodiesel and its blends also showed a slight drop in the engine power with increased peak cylinder pressure and reduced ignition delay when compared to PBDF. In the all test conditions, the premixed combustion phase and the start of injection timing of biodiesel and its blends took place earlier than with PBDF.
Introduction Biodiesel as an alternative fuel for diesel engines is an important issue for countries; especially if their energy depends on foreign sources. At the same time, increasing global concern due to environmental pollution from internal combustion engines has generated much attention on clean diesel fuels. In the last two decades, these issues have triggered various research studies to replace petroleum-based diesel fuel (PBDF) with biodiesel in many countries. Some diesel engine manufacturers allow the use of neat biodiesel and biodiesel blends instead of PBDF. The guarantees only apply to biodiesel that fulfills the ASTM D6751 or EN 14214 standards. Some engine manufacturers announced that diesel vehicles can be fueled with up to a 20% blend of biodiesel fuel, and they encouraged the fuel users to require that biodiesel be obtained from sources known to produce quality fuels that meet the specifications.1 The fuel properties of biodiesel can vary with production technologies and feedstock, but they generally have higher cetane number, near-zero sulfur content, and free aromatic content when compared with PBDF. Biodiesel is attractive not only because of its fuel properties but also it can be compatible with the PBDF. Fuel properties of biodiesel are affected by its fatty acids content, which may cause difference in the characteristics of injection, combustion, and emissions. The biodiesels that meet the standards have been used by various academic researchers2–4 who have reported that biodiesel exhibits very close engine performance characteristics to PBDF and reduces the exhaust emissions from diesel engines. But, biodiesel is generally more expensive than PBDF which prevents its wide * Corresponding author: Tel: +90 262 3032285. Fax: +90 262 3032203. E-mail:
[email protected]. † Kocaeli University. ‡ Marmara University. (1) Test specifications for biodiesel fuel, EMA (Engine Manufacturers Association): Chicago, IL, May 31, 2006.
use. The cost of biodiesel is competitive with that of PBDF when using waste cooking or frying oils as a feedstock. Also, using waste cooking or used frying oils in biodiesel production has additional benefits which treats a waste product and provides the efficient use of a resource. However, biodiesel production from used frying oils is not easy because they contains large amounts of free fatty acids that cannot be converted to biodiesel using an alkali catalyst due to the formation of soaps. These soaps can prevent separation of the biodiesel from the glycerin fraction. Alternative processes are available that use an acid catalyst.5 Canakci and Van Gerpen6 have developed a process to produce fuel quality biodiesel from yellow and brown grease using an acid catalyst. Waste vegetable or used frying oils and their methyl esters have been investigated in diesel engines by many researchers. Dorado et al.7 tested a blend of 10% waste vegetable oil-90% diesel fuel in a 3-cylinder direct injection (DI) diesel engine, without any modifications. The results showed an approximately 12% power loss, slight increase in fuel consumption, and drop in combustion efficiency during the testing period. Yu et al.8 compared combustion characteristics of waste cooking oil with diesel fuel in a DI diesel engine. As a result of that study, the ignition delay of waste cooking oil was found to be shorter than (2) Canakci, M. Proc. Inst. Mech. Eng. Part D: J. Auto. Eng. 2005, 219 (7), 915–922. (3) Zhang, Y.; Van Gerpen, J. H. Combustion analysis of esters of soybean oil in a diesel engine; SAE Technical Paper 960765, Society of Automotive Engineers: Warrendale, PA, 1996. (4) Haas, J. M.; Scott, K. M.; Alleman, T. L.; McCormick, R. L. Energy Fuels 2001, 15 (5), 1207–1212. (5) Canakci, M.; Van Gerpen, J. H. Trans. ASAE 1999, 42 (5), 1203– 1210. (6) Canakci, M.; Van Gerpen, J. H. Trans. ASAE 2003, 46 (4), 945– 954. (7) Dorado, M. P.; Arnal, J. M.; Gomez, J.; Gil, A.; Lopez, F. J. Trans. ASAE 2002, 45 (3), 519–523. (8) Yu, C. W.; Bari, S.; Ameen, A. Proc. Inst. Mech. Eng. Part D: J. Auto. Eng. 2002, 216 (3), 237–243.
10.1021/ef700447z CCC: $40.75 2008 American Chemical Society Published on Web 01/12/2008
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that of diesel fuel. However, the researchers7–12 unanimously agree that long-term tests have resulted some engine problems such as piston rings being stuck, carbon buildup on injectors, fuel system failure, and lubricating oil contamination. These problems have related with the high viscosity and low volatility of waste vegetable oils. When diesel engines are fueled with neat vegetable or used frying oil, decreased spray angle, higher spray penetration, and reduced atomization may lead to poor combustion. This may also result in an increase in combustion chamber deposits. Therefore, vegetable or used frying oils are converted to biodiesel to improve fuel properties. Comparisons of engine performance between biodiesel-diesel fuel blends and PBDF in unmodified diesel engines generally show some reductions in engine power that are approximately equivalent to the reductions in energy content of the blends relative to PBDF. Canakci and Van Gerpen13 prepared two different biodiesels from soybean oil and yellow grease with 9% free fatty acids to investigate the effect of the biodiesel on a DI diesel engine. Similar performance characteristics were found by using biodiesels produced from different sources. The start of injection for two biodiesel fuels occurred earlier than that for No. 2 diesel fuel. They also found that an approximately 14% increase in brake specific fuel consumption (BSFC) for two biodiesels when compared with No. 2 diesel fuel. Reed et al.14 converted waste cooking oil to its methyl and ethyl esters and tested neat biodiesel and a 30% blend of biodiesel with diesel fuel in a diesel engine. They reported that no significant difference occurred in the power and performance. Ulusoy et al.15 investigated the effects of biodiesel from used frying oil on the performance and emissions of a Fiat Doblo 1.9 DS diesel engine by using a chassis dynamometer. Biodiesel showed a 3.35% reduction in wheel force, and wheel power was reduced by 2.03%. When the fuel consumptions were compared, they saw that biodiesel consumption was 2.43% less than that of No. 2 diesel fuel. Yoshimoto et al.16 has reported a similar result for BSFC. In that study, the BSFC of neat biodiesel was lower than that of diesel fuel at high loads. Dorado et al.17 worked on waste olive oil methyl ester as a fuel for a DI diesel engine. After running the engine for 50 h, biodiesel was used instead of No. 2 diesel fuel; a minor loss (2%) in power and a larger increase (26%) in BSFC took place. In an indirect injection (IDI) diesel engine, the fuel is injected into a small precombustion chamber attached to the main chamber. The IDI combustion system is used almost exclusively for the size of engines required for passenger cars and light (9) Schlick, M. L.; Hanna, M. A.; Schinstock, J. L. Trans. ASAE 1988, 31 (5), 1345–1349. (10) Hemmerlein, N.; Korte, V.; Richter, H. Performance, exhaust emissions and durability of modern diesel engines running on rapeseed oil; SAE Technical Paper 910848, Society of Automotive Engineers: Warrendale, PA, 1991. (11) Ziejewski, M.; Goettler, H. J.; Haines, H.; Huang, C. EMA durability tests on high oleic sunflower and safflower oils in diesel engines; SAE Technical Paper 961846, Society of Automotive Engineers: Warrendale, PA, 1996. (12) Masjuki, H. H.; Kalam, M. A.; Maleque, M. A.; Kubo, A.; Nonaka, T. Proc. Inst. Mech. Eng. Part D: J. Auto. Eng. 2001, 215 (3), 393–404. (13) Canakci, M.; Van Gerpen, J. H. Trans. ASAE 2003, 46 (4), 937– 944. (14) Reed, T. B.; Graboski, M. S.; Gaur, S. Biomass and Bioenergy 1992, 3 (2), 111–115. (15) Ulusoy, Y.; Tekin, Y.; Cetinkaya, M.; Karaosmanoglu, F. Energy Sourc. 2004, 26 (10), 927–932. (16) Yoshimoto, Y.; Onodera, M.; Tamaki, H. Reduction of NOx, smoke, and bsfc in a diesel engine fueled by biodiesel emulsion with used frying oil; SAE Tecnical Paper 1999-01-3598, Society of Automotive Engineers: Warrendale, PA, 1999. (17) Dorado, M. P.; Ballesteros, E.; Arnal, J. M.; Gomez, J.; Lopez Gimenez, F. J. Energy Fuels 2003, 17 (6), 1560–1565.
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commercial vehicles. Compared with the direct injection (DI) diesel engine, it is relatively insensitive to fuel quality. Furthermore, the injection pressure of IDI diesel engines is low through a pintle nozzle as a single spray. IDI diesel engines also produce lower carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbons (HC) than DI diesel engines. IDI engines have the advantages of less noise, lower ignition delay, and faster combustion. This permits small engines to run at higher engine speed and thus with larger output over a broad speed range. Additionally, its structural requirements are not particularly demanding, which means that it is more compact and less expensive to manufacture. However, IDI engines have somewhat poorer fuel economy than DI diesel engines. Most of this loss is due to greater heat transfer losses in the swirl chamber and connecting passageway.18–22 As mentioned above, an IDI diesel engine exhibits two important advantages; it does not depend on fuel quality and produces low exhaust emissions depending on the combustion chamber design. For this reason, in this study, an IDI diesel engine was used. The objectives of this study are to investigate the effects of biodiesel and its blends on the performance, injection timing, and combustion characteristics of an unmodified IDI diesel engine and to compare them with those of PBDF.
Fuel Characteristics In this study, biodiesel was produced from used frying palm oil in the Fuel Laboratory of the Department of Mechanical Education at Kocaeli University. The feedstock was supplied by Kocaeli Uzay Gıda (Frito-Lay Chips Factory). Because of the low acid value of the oil (0.578 mg KOH/g), direct transesterification reaction was selected. To produce biodiesel from the used oil, first, the small-scale transesterification reaction had been carried out in laboratory conditions, and the reaction inputs such as catalyst amount, molar ratio, reaction temperature, and time were determined. And then, a large scale production process was applied using a stainless steel reactor tank and other equipment. Biodiesel was prepared using a methanol to oil ratio of 6:1 with potassium hydroxide (KOH) as catalyst (1% of oil by weight). After dissolving KOH catalyst in methanol at room temperature, the moisture-free used frying oil was added to the reaction tank to start the transesterification reaction. To increase the reaction completion, the mixture was agitated throughout 4 h at 55–60 °C. After glycerol separation, the biodiesel was washed with warm distilled water. The washing was repeated four times. PBDF was purchased from a commercial supplier. Fuel specifications of the PBDF and biodiesel were determined by TUBITAK (The Scientific and Technological Research Council of Turkey) using the standard test methods. The fuel properties of the biodiesel and PBDF are shown in Table 1. PBDF was selected as the base fuel. The biodiesel were blended with PBDF in four volumetric proportions. They include (18) Rakopoulos, C. D.; Antonopoulos, K. A.; Rakopoulos, D. C.; Giakoumis, E. G. Appl. Therm. Eng. 2006, 26 (14–15), 1611–1620. (19) Owen, K.; Coley, T. AutomotiVe Fuels Reference Book, second ed.; SAE: Warrendale, PA, 1995; p 375. (20) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill Int. Editions: New York, 1988; pp 491–497. (21) Abdel-Rahman, A. A. Int. J. Energy Res. 1998, 22 (6), 483–513. (22) Challen, B.; Baranescu, R. Diesel Engine Reference Book, second ed.; Butterworth Heinemann, Woburn, MA, 1999; p 97.
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Table 1. Fuel Properties of the Biodiesel and PBDF property
units
typical formula average molecular weight heating value density kinematic viscosity flash point sulfated ash content cold filter plugging point carbon residue cetane number total contamination copper strip corrosion oxidation stability acid value iodine value free glycerol total glycerol ester content phosphorus content distillation initial boiling point (IBP) 90% recovered
g/mol kJ/kg kg/m3, 15 °C mm2/s, 40 °C °C % mass °C % mass mg/kg 3 h, 50 °C h, 110 °C mg KOH/g % mass % mass % mg/kg
EU EN 14214 limits
860–900 3.5–5.0 120 min 0.02 max 0.30 max 51 min 24 max No. 1 max 6.0 min 0.50 max 120 max 0.02 max 0.25 max 96.5 min 10 max
°C °C
Table 2. Engine Specification model of engine combustion chamber engine type number of cylinder bore × stroke compression ratio injection pump injector opening pressure nozzle hole diameter maximum power
1.8 VD diesel BMC indirect injection water-cooled, four strokes and naturally aspirated 4 80.26 × 88.9 mm 21.47:1 mechanically controlled distributor type 130 bar 0.2 mm 38.8 kW at 4250 rpm
B5 (5% biodiesel + 95% PBDF), B20 (20% biodiesel + 80% PBDF), B50 (50% biodiesel + 50% PBDF), and B100 (pure biodiesel).
USA ASTM D6751 limits
biodiesel
diesel fuel C14.16H25.21 195.50 42930 840.3 3.177 61.5 0.0015 -14 0.067 56.5 4.14 No. 1A
10 max
C18.08H34.86O2 284.17 38730 875 4.401 70.6 0.0004 +10 0.0004 60.4 9.03 No. 1A 10.1 0.15 62 0.01 0.06 96.5 2.9
360 max
331 348
164.7 351.1
1.9–6.0 130 min 0.02 max 47 min No. 3 max 0.80 max 0.02 max 0.24 max
of cylinder gas pressure data. The injector opening pressure specified by the manufacturer is 130 bar which was used in ignition delay calculation. All fuel tests were completed without any modifications on the test engine. The tests were carried out under steady-state conditions. A variable speed, from 1000 to 3000 rpm with an increment of 500 rpm at full load, was selected for the performance tests. The engine was sufficiently warmed up at each test and the engine oil temperature was maintained at 65–70 °C. The engine was allowed to run for a few minutes until it reached steady-state conditions, and then, the data were collected subsequently. To reduce the experimental uncertainties, the tests were repeated there times and average values were presented. During the tests, the engine did not show any starting difficulties when fueled with biodiesel and its blends, and the engine ran satisfactorily throughout the entire tests.
Result and Discussions Experimental Setup Engine tests were carried out on a water-cooled, naturally aspirated IDI diesel engine. Engine specifications are shown in Table 2. Figure 1 shows the schematic diagram of the experimental setup. To provide brake load, the engine was coupled to a MOTOSAN brand hydraulic dynamometer which has an ESIT model SP200 digital load cell with 1 g uncertainty. A magnetic pickup was fixed over the engine flywheel gear to determine the crankshaft position. Fuel consumption was determined by weighing fuel used for a period of time on an electronic scale with a precision of 0.1 g. Air consumption was measured using a sharp edged orifice plate (ISO 5167 (1980)) and inclined manometer. Six different digital thermocouples monitored the temperatures of the intake air, fuel, engine oil, exhaust gas, and coolant inlet and outlet. The fuel line and cylinder gas pressures were measured to obtain useful information about the combustion process. To measure the cylinder gas pressure, a Kistler model 6061B pressure transducer was mounted on the first cylinder head. An AVL model 8QP500c pressure transducer was installed in the fuel line of the first cylinder to obtain the change in the fuel line pressure. A Kistler 5051A model charge amplifier was used to produce an output voltage proportional to the charge which was then converted to digital signals. The cylinder gas and fuel line pressure signals were recorded by a computer using a digital device, Advantech PCI 1716 multifunctional data acquisition board. The pressure data of 50 engine cycles were collected with a resolution of 0.25° crank angle (CA). A plot of log P versus log V was created to check the quality
Brake Torque and Specific Fuel Consumption. When the engine was fueled with the biodiesel and its blends, the brake torque was slightly dropped compared with PBDF. The maximum brake torque (95.23 N m) at 2000 rpm was obtained with PBDF, followed by B5 (94.90 N m), B20 (93.57 N m), B50 (91.79 N m), and B100 (89.91 N m). On average over the speed range at full load condition, the brake torques of B100, B50, B20, and B5 decreased by 7.16, 5.16, 4.28, and 2.01%, respectively, compared with those of the PBDF. Figure 2 shows a comparison of the brake torque and BSFC values obtained for PBDF, B100, B50, B20, and B5. As illustrated in Figure 2, the BSFC slightly increased with the increase of biodiesel percentage in the fuel blend. By increasing the engine speed from 1000 to 2000 rpm, the fuel consumption for all tested fuels decreased due to the increase in atomization ratio. From 2000 to 3000 rpm, the fuel consumption for the fuels increased due to the decreasing of volumetric efficiency. On average, BSFCs of B100, B50, B20, and B5 were 16.76, 9.42, 5.78, and 2.17% higher than that of PBDF, respectively. Because of the lower energy content of the biodiesel and its blends, the BSFCs increased when compared to PBDF. Since the heating value of biodiesel and its blends per unit mass was lower than PBDF, the fuel consumption had to be higher to maintain maximum brake torque at the full load condition.
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Figure 1. Experimental setup.
Figure 2. Brake torque and BSFC versus engine speed for the fuels.
The maximum BSFC (442.24 g/(kW h)) was obtained while the test engine was fueled with B100 at a 1000 rpm engine speed. The reason for this may be explained by the fact that the test engine has a mechanically controlled distributor type injection pump; the fuel delivery quantity was decreased at low engine speeds. This affects the atomization ratio that causes a decrease in fuel-air mixing rates. The effect of increased injection pressure reflects that on BSFC and brake torque and is much more significant at higher speeds. Thus, while at 2000 rpm, an increase in injection pressure has led to a maximum level in brake torque and a minimum level in BSFC. Another possible reason is the in-cylinder temperature which is low when the engine is running under 2000 rpm at full load. When the neat biodiesel is injected into the combustion chamber, it can adhere to the cylinder wall due to higher spray penetration. The B100 with a high boiling points is not adequately evaporated and is expelled without combustion; oxidation may also occur in the tailpipe. Brake Power and Thermal Efficiency. The brake power decreased with increasing biodiesel concentration in the fuel blend at all speeds compared to PBDF. Figure 3 shows a comparison of the brake power values obtained for PBDF, B100, B50, B20, and B5. The maximum brake power value for each fuel occurred at the maximum speed. The maximum brake power (26.88 kW) at 3000 rpm was obtained with PBDF, followed by B5 (26.58 kW), B20 (26.05 kW), B50 (25.84 kW), and B100 (25.21 kW). The primary reason for the decreasing brake power with an increasing biodiesel percentage in the blend is the effect of the energy content of biodiesel.
Figure 3. Brake power versus engine speed for the fuels.
Figure 4. Brake thermal efficiency versus engine speed for the fuels.
The brake thermal efficiency of a diesel engine is the efficiency in which the chemical energy of a fuel is turned into useful work. It can be determined by dividing the useful work by the lower heating value of the fuel.20–23 Figure 4 shows the brake thermal efficiency for PBDF, B100, B50, B20, and B5 over the speed range at the full load condition. At 2000 rpm, the maximum brake thermal efficiencies for B100, B50, B20, B5, and PBDF were calculated as 24.44, 24.48, 24.52, 25.18, and 25.76%, respectively. Exhaust Gas Temperature. The exhaust gas temperature indicates the effective use of the heat energy of a fuel. The heat (23) Pulkrabek, W. W. Engineering Fundamentals of the Internal Combustion Engine, Prentice-Hall: New York, 1997; pp 68–69.
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Figure 5. Exhaust temperatures versus engine speed for the fuels.
loss in the exhaust pipe or an increase in the exhaust temperature reduces the conversion of heat energy of the fuel to work.20–22 Figure 5 shows a comparison of the exhaust temperature values obtained for the fuels tested over the speed range at the full load condition. The average exhaust gas temperatures for B100, B50, B20, B5, and PBDF were measured as 535.62, 561.64, 562.88, 558.26, and 574.36 °C, respectively. As seen in Figure 5, except the B100 value at 1000 rpm, no remarkable difference has been observed in the exhaust temperature of B5, B20, B50, and B100 compared to PBDF over the speed range. The maximum exhaust temperatures for B100, B50, B20, B5, and PBDF were measured as 646.6, 635.8, 642.7, 638.7, and 655.9 °C at 3000 rpm, respectively. Since the heating values of B5, B20, B50, and B100 are relatively lower than that of PBDF, these small differences in the exhaust temperatures are normal. The oxygen content of biodiesel provides better combustion which causes the exhaust temperatures to be close to those of PBDF. Simultaneously, except B100 at 1000 rpm, obtained exhaust temperature values for B5, B20, B50, and B100 can be an indication of complete combustion. For the B100 case, much more fuel was injected into the combustion chamber at 1000 rpm as shown in Figure 2. For this reason, the cylinder gas temperature is probably reduced by evaporation of the unburned fuel until the exhaust valves open. At the same time, due to the fact that the density and kinematic viscosity of biodiesel is higher than PBDF, the spray penetration for biodiesel at low engine speed may be long and this causes a poor atomization rate. Consequently, these factors reduced the reaction temperature. Combustion Characteristics. The combustion characteristics of the biodiesel and its blends can be compared by means of injection timing, heat release, and cylinder gas pressure. The heat release analysis was based on the cylinder gas pressure data collected during the tests. Analysis of Cylinder Gas Pressure. To analyze the cylinder gas pressure, the pressure data of 50 cycles with a resolution of 0.25° CA was averaged and then used. The pressure variation in the cylinder during the combustion indicates engine knock or noise. Figure 6 shows the changes in cylinder gas pressures with respect to crank angle at various engine speeds. As seen in the figure, all fuels had no trace of knock and the cylinder gas pressure smoothly varied over the engine speed range. During the experimental study, it was detected that the engine noise was qualitatively less than that of PBDF when the engine was running with B100. The maximum gas pressure did not show any significant difference among the fuels over the speed range. This result indicates that the engine converts the fuel energy to mechanical energy equally as well for the B100, B50, B20, and B5 as for PBDF.
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The maximum cylinder gas pressure occurred within the range of 2.5-6° CA after top dead center (ATDC) for all tested fuels. When biodiesel or its blends was used, the peak of cylinder gas pressure slightly closed to top dead center (TDC). In the maximum torque condition, 2000 rpm, the peak cylinder gas pressure for B100 was 8.79 MPa occurring at 2.75° CA ATDC, while the peak cylinder gas pressure in the case of PBDF was 8.73 MPa occurring at 3.5° CA ATDC. This shows that the peak cylinder gas pressure for B100 was 0.06 MPa higher and occurred 0.75° CA earlier than for PBDF. Analysis of Heat Release Rate. Heat release calculations are an attempt to learn something about the combustion process in an engine. A number of approaches to heat release analysis have been present in the literature, but the most widely used one is that developed by Krieger and Borman.22–24 Therefore, in this study, the heat release combustion model of Krieger and Borman was used. In this model, the heat release rate was calculated according to the first law of thermodynamics applied to a control volume consisting of the engine cylinder. The cylinder volume was calculated from the geometry as a function of the crank position. Li et al.25 expressed that the main combustion chamber and the precombustion chamber can be combined into a single zone thermodynamic model. It is assumed that no passage throttling losses exist between both the chambers. Temperature gradients, pressure waves, nonequilibrium conditions, fuel vaporization, and mixing can be ignored. By using these ideas, calculated heat release rates as functions of the crank position are showed in Figure 7. All test fuels indicated rapid premixed burning followed by a diffusion (start of mixing-controlled) combustion period as is typical of natural aspiration. In all test speeds, biodiesel and its blends completed the premixed combustion phase earlier than PBDF due to their earlier start of combustion and having a less premixed combustible mixture. Also, because of the shorter ignition delay, the time for occurrence of the maximum heat release is earlier for biodiesel or its blends in comparison with PBDF. Another reason for the higher rate of premixed burning for PBDF may be the higher volatility of PBDF compared to biodiesel. Biodiesel vaporizes more slowly than PBDF and contributes less to premixed combustion. It was found that when the engine was fueled with biodiesel or its blends, the combustion process started in an advanced operating condition, a feature which became more evident with increasing engine speed. Also, as seen in the figure, the duration of combustion of biodiesel and its blends is almost same as that of PBDF in the maximum torque condition (2000 rpm). This result shows that an increase in the oxygen fraction of the injected fuel provides an increase in the maximum heat release rate and in the fraction of fuel burned in the premixed combustion phase; this case is more obvious at a high engine speed. Note that the increased fuel amount when using B100, B50, and B20 caused a higher diffusion burning rate at 1500, 2000, 2500, 3000 rpm, but the duration of burning was not changed much for the three fueling rates. Analysis of Injection Line Pressure and Ignition Delay. One of the important parameters in combustion phenomenon is the ignition delay. The ignition delay is influenced by the fuel’s ignition quality (defined by the cetane number), the compression ratio, engine speed, cylinder gas pressure, the intake-air tem(24) Borman, G. L.; Ragland, K. W. Combustion Engineering; McGrawHill: New York, 1998; pp 234–240. (25) Li, J.; Zhou, L.; Pan, K.; Jiang, D.; Chae, J. EValuation of the thermodynamic process of indirect injection diesel engines by the first and second law; SAE Technical Paper 952055, Society of Automotive Engineers: Warrendale, PA, 1995.
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Figure 6. Cylinder gas pressure versus crank angle at various engine speeds.
perature, and the quality of fuel atomization. The definition of the ignition delay is the time from the start of fuel injection to the start of combustion.3,19–22 In this study, the ignition delay was calculated in terms of the crank angle between the start of fuel injection and the start of combustion. The start of fuel injection was determined from fuel injection line pressure data. Figure 8 shows the comparison of the injection line pressures versus crank angle degree at various engine speeds. As seen in the figure, when the test engine was fueled with biodiesel, the start of the nozzle needle is carried out at earlier crank angles than that of PBDF over the speed range. The start of injection timing for B100 is 1.12°, 1.21°, 0.66°, 0.81°, and 2.14° CA earlier than that of PBDF at 1000, 1500, 2000, 2500, and 3000 rpm, respectively. This behavior concerns the different density, viscosity, and compressibility of the fuels. As mentioned earlier, the biodiesel has a slightly higher density (see Table 1), which affects the fuel compression process in the volumetric injection pump. The higher density of biodiesel causes advance in the injection timing. Also, this case causes a different quantity of fuel to be injected per stroke for the same volume of crank
angle. Kegl26 reported that the higher viscosity of biodiesel leads to reduced fuel losses during the injection process, to faster evolution of pressure, and thus to advanced injection timing. Furthermore, a lower vapor content in a high pressure injection system could also be the reason for the advanced injection timing. By decreasing the vapor volume, the injection delay decreases which results in advanced injection timing. Another possible reason for changing the injection timing is the fuel’s compressibility. Isothermal compressibility and kinematic viscosity of the biodiesel affect the injection process. The compressibility of biodiesel and its blends is lower than that of PBDF. Thus, for all fuels, using the same fuel pump at the same speed, the injection characteristics of biodiesel and its blends are not the same as each other. Some researchers27–29 have shown that the injection timing of biodiesel is effectively advanced relative to that PBDF. A critical difference between (26) Kegl, B. Fuel 2006, 85 (17–18), 2377–2387. (27) Yamane, K.; Ueta, A.; Shimamoto, Y. Int. J. Engine Res. 2001, 2 (4), 249–261. (28) Tat, M. E.; Van Gerpen, J. H.; Soylu, S.; Canakci, M.; Monyem, A.; Wormley, S. J. Am. Oil Chem. Soc. 2000, 77 (3), 286–289.
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Figure 7. Heat release rate versus crank angle at various engine speeds.
the PBDF and biodiesel fuel properties was the bulk modulus. The bulk modulus of biodiesel was higher than that of diesel fuel; then, the rate of liquid pressure rise has gone up and the injection timing has advanced. Table 3 shows the ignition delays of the tested fuels. Ignition delays of the fuels showed almost similar trends at 1000 rpm, but the start of injection timing for B100, B50, B20, and B5 is 1.12°, 1.05°, 0.70°, and 0.53° CA earlier than that of PBDF. The maximum difference between the ignition delays of B100 and PBDF at 1500 and 3000 rpm has occurred at 2.04° and 1.36° CA, respectively. It can be also observed that generally the ignition delay slightly decreased with the increase of biodiesel in the blend. It is known that the fuels with high cetane number make autoignition easily and give short ignition delay. So, primary reason for the decrease in ignition delay is the cetane number of the biodiesel which is higher than that of PBDF, as (29) Rodriguez-Anton, L. M.; Casanova-Kindelan, J.; Tardajos, G. High pressure physical properties of fluids used in diesel injection systems; SAE Technical Paper 2000-01-2046, Society of Automotive Engineers: Warrendale, PA, 2000.
seen in Table 1. However, the ignition delay for all fuels increased with the increase of engine speed. Conclusion In this study, the biodiesel from used frying palm oil was blended with PBDF and tested in an IDI diesel engine to investigate the effects on the engine performance, injection, and combustion characteristics. The following conclusions can be drawn from the present paper. 1. All test fuels burned easily in the IDI diesel engine. When the engine was fueled with B100, the engine noise was qualitatively lower compared to PBDF. 2. Compared with the PBDF, the brake torques and powers produced by the blends decreased with the increasing amount of biodiesel in the fuel blend. The primary reason for the decreasing brake torque and power with an increasing amount of biodiesel in the fuel blend is the lower energy content of the biodiesel. This also caused a higher BSFC for B100 and the blends.
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Figure 8. Fuel line pressure versus crank angle at various engine speeds. Table 3. Ignition Delays for the Test Fuels over the Speed Range ignition delay (°CA) 1000 1500 2000 2500 3000
rpm rpm rpm rpm rpm
PBDF
B5
B20
B50
B100
4.81 6.53 7.59 11.43 13.16
4.59 6.45 7.91 11.39 13.46
4.51 6.50 8.33 11.26 12.98
4.36 5.63 8.34 10.67 13.51
4.18 4.49 7.25 10.74 11.80
3. The maximum brake thermal efficiencies were obtained at the maximum torque condition for all test fuels. However, on average, the thermal efficiency decreased with the increasing amount of biodiesel in the fuel blend. 4. No remarkable difference was observed in the exhaust temperatures of the fuels tested at the same conditions. However, small differences in the exhaust temperatures are normal, since the heating values of B5, B20, B50, and B100 are relatively lower than that of PBDF. 5. No trace of knock appeared in the engine for all tested fuels. It was observed that the cylinder gas pressure smoothly varied over the range of speeds for the fuels. The maximum
cylinder gas pressure occurred within the range of 2.5-6° CA ATDC for all tested fuels. When the biodiesel or blends were used, the peak of cylinder gas pressure closed to TDC compared with PBDF. 6. Biodiesel and blends completed the premixed combustion phase earlier than PBDF due to their earlier start of combustion and having less premixed combustible mixture. When the engine was fuelled with the biodiesel or blends, the combustion process started earlier with increasing engine speed. 7. When the test engine was fueled with biodiesel, the start of the nozzle needle was carried out earlier than that of PBDF over the speed range. Therefore, the start of injection timing for B100 was earlier than that of PBDF at all test conditions. Similar behaviors were observed for the blends. This behavior concerns the different density, viscosity, and compressibility of the fuel. 8. Generally, ignition delay decreased with the increasing amount of biodiesel in the fuel blend. The primary reason for the decrease in ignition delay is the cetane number of the biodiesel which is higher than that of PBDF. In addition, the ignition delay for all fuels increased with the increase of engine speed.
Used Frying Palm Oil in an IDI Diesel Engine Acknowledgment. This study was supported by the grants from TUBITAK (Project No. 104M372) and Scientific Research Foundation of Kocaeli University (Project Nos. 2003/79 and 2004/24). The authors would like to thank the institutes and the individuals at the engine test laboratory who were involved in making this work possible. Abbreviations
ASTM ) American Society for Testing and Materials ATDC ) after top dead center BSFC ) brake specific fuel consumption B5 ) 5% biodiesel + 95% PBDF, volumetric B20 ) 20% biodiesel + 80% PBDF, volumetric B50 ) 50% biodiesel + 50% PBDF, volumetric B100 ) 100% biodiesel
Energy & Fuels, Vol. 22, No. 2, 2008 1305
CA ) crank angle CO ) carbon monoxide DI ) direct injection HC ) hydrocarbons IDI ) indirect injection KOH) potassium hydroxide NOx ) nitrogen oxides PBDF ) petroleum-based diesel fuel rpm ) revolutions per minute TDC ) top dead center TUBITAK ) The Scientific and Technological Research Council of Turkey EF700447Z
