Effects of Biodiesel from Used Frying Palm Oil on the Exhaust

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Energy & Fuels 2008, 22, 2796–2804

Effects of Biodiesel from Used Frying Palm Oil on the Exhaust Emissions of an Indirect Injection (IDI) 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 March 11, 2008. ReVised Manuscript ReceiVed May 14, 2008

In our previous paper, the influences of biodiesel and its blends on the performance, combustion, and injection characteristics of an indirect injection (IDI) diesel engine have been discussed. The results have indicated that, when the test engine was fueled with biodiesel and its blends, the maximum brake torque, brake thermal efficiency, and brake power dropped, while the brake-specific fuel consumption increased compared to the petroleum-based diesel fuel (PBDF). The main differences in the combustion and injection characteristics of biodiesel and its blends are earlier premixed combustion, shorter ignition delay, higher cylinder gas pressure, and earlier start of injection in terms of the PBDF. This paper discusses the exhaust emission results obtained in the same study. The emission results showed that carbon monoxide (CO), unburned hydrocarbon (HC) emissions, and smoke opacity decreased with the increase of biodiesel percentage in the fuel blend for all engine speeds under the full-load condition. However, NOx and CO2 emissions showed different behaviors in terms of the engine speed.

Introduction Exhaust emissions from diesel engines fueled with conventional diesel fuel have caused environmental pollution and global problems. The European Commission has published the some directives (2005/55/EC for Euro 4/5, etc.) to reduce exhaust emissions from light- and heavy-duty diesel engines. Hence, the vehicle manufacturers and academic researchers have directed searching the commercial diesel engines with high performance and low emissions. At this time, the vehicle manufacturers can meet the diesel engine emissions within the accepted level by using the highest injection pressure, multipoint injection, different catalyst type (oxidation catalyst, NOx absorber, etc.), exhaust gas recirculation, particulate traps, and controlling the timing of the start of injection. Nevertheless, the exhaust emissions from diesel engines are difficult to reduce simultaneously. One approach to solving this problem is to use oxygenated fuels. The development of alternative fuel sources has become increasingly important for diesel engines. Biodiesel has attracted increasing attention within the alternative fuels, owing to its fuel properties in reducing emissions.1 Biodiesel can be produced from various feedstocks. The properties of biodiesel can vary depending upon processing technology and feedstock, but they generally have high cetane number, oxygen content, near-zero sulfur content, and very low * To whom correspondence should be addressed. Telephone: +90-2623032285. Fax: +90-262-3032203. E-mail: [email protected]. † Department of Mechanical Education, Kocaeli University. ‡ Alternative Fuels R&D Center, Kocaeli University. § Marmara University. (1) Ozsezen, A. N. Investigation of the effects of biodiesel produced from waste palm oil on the engine performance and emission characteristics. Ph.D. Dissertation, Kocaeli University, Turkey, 2007; pp 4-7.

aromatic content when compared to conventional diesel fuel.2,3 The molecular structure of biodiesel is similar to diesel fuel, and it contains additional oxygen, which is useful to reduce unburned HC, CO, and smoke opacity in the exhaust. However, a diesel engine fueled with biodiesel or its blend generally releases higher NOx emission than that of petroleum-based diesel fuel. Some researchers4–8 have shown the effect of fuel properties on NOx emissions. Signer et al.9 expressed an increase in the NOx emissions for a 3.5% increase in the fuel density. Peterson et al.10 found that lower iodine numbers correlated with reduced NOx. As the iodine number increased from 7.88 to 129.5, the NOx amount in the exhaust increased to 29.3%. McCormick et al.11 reported that increasing the number of double bonds, quantified as iodine number, correlated with increasing emissions of NOx. A number of studies12–26 have shown reductions in the emissions of CO, HC, and PM (2) Knothe, G.; Dunn, R.; Bagby, M. Biodiesel: The use of vegetable oils and their derivatives as alternative diesel fuels. In ACS Symp. Ser. 666: Fuels and Chemicals from Biomass; American Chemical Society: Washington, D.C., 1997; pp 172-208. (3) Last, R. J.; Kru¨ger, M.; Du¨rnholz, M. SAE Paper 950054, 1995. (4) Environmental Protection Agency (EPA). A comprehensive analysis of biodiesel impacts on exhaust emissions. Report EPA420-P-02-001, 2002. (5) Ullman, T. L.; Spreen, K. B.; Mason, R. L. SAE Paper 941020, 1994. (6) Yamane, K.; Ueta, A.; Shimamoto, Y. Int. J. Eng. Res. 2001, 2 (4), 249–261. (7) Schmidt, K.; Van Gerpen, J. H. SAE Paper 961086, 1996. (8) Yuan, W.; Gratten, M. R.; Hansen, A. C. Parametric investigation of NOx emissions from biofuels for compression-ignition engines. ASAE Annual Meeting, 2005; 056115. (9) Signer, M.; Heinze, P.; Mercogliano, R.; Stein, H. J. SAE Paper 961074, 1996. (10) Peterson, C. L.; Taberski, J. S.; Thompson, J. C.; Chase, C. L. Trans. ASAE 2000, 43 (6), 1371–1381. (11) McCormick, R. L.; Alvarez, J. R.; Graboski, M. S. NOx solutions for biodiesel. Report NREL/SR-510-31465; National Renewable Energy Laboratory (NREL), Golden, CO, 2003. (12) Canakci, M.; Van Gerpen, J. H. Trans. ASAE 2003, 46 (4), 945– 954.

10.1021/ef800174p CCC: $40.75  2008 American Chemical Society Published on Web 06/28/2008

Used Frying Palm Oil for an IDI Diesel Engine

significantly, but NOx emissions increased about 5-20% for biodiesel fuels. However, some studies reported no changes or reduction in NOx emissions. McDonald27 studied a 50% blend of yellow grease methyl ester with No. 2 diesel fuel (B50) used in a General Motors L65 GMT 600 turbo-charged, IDI diesel engine. As a result of that study, the use of B50 reduced brakespecific polyaromatic hydrocarbon (PAH) emissions by 35% and PM emissions by 23% relative to the diesel fuel. NOx emissions were not increased by the use of B50. Similar results were obtained by Purcell et al.,28 who tested emissions of an IDI diesel engine (Caterpillar 3304) using neat soybean oilbased biodiesel and B30. The blend (B30) had 4% less power and 4% higher fuel consumption than the No. 2 diesel fuel; while the neat biodiesel had 9% less power and 13% higher fuel consumption than No. 2 diesel fuel. Emissions of CO and HC reduced, but NOx emissions did not changed. McDonald29 studied an IDI diesel engine (Caterpillar 3304 PCNA) using oxygenated fuels (including 100% soybean oil-based biodiesel and B30). Emission test results showed that NOx emissions from B30 did not change significantly but they decreased with the neat biodiesel fuel. Peterson30 conducted some road tests using 100% biodiesel on a Cummins B5.9 L engine in a Dodge pickup. The emission test results showed an 11.8% NOx reduction and a 10.3% PM increase. As explained above, different fuel properties and engine types exhibit different emission ratios. At the same time, the shape of the combustion chamber is one of the decisive factors in determining the quality of combustion. IDI diesel engines held an advantage over direct-injection (DI) diesel engines in terms of noise and exhaust emissions.31 In an IDI diesel engine, the fuel is injected into a small precombustion chamber attached to the main combustion chamber. The IDI combustion system is used almost exclusively for the size of engines required for passenger cars and light commercial vehicles. In comparison to the DI diesel engine, it is relatively insensitive to fuel quality. (13) Haas, J. M.; Scott, K. M.; Alleman, T. L.; McCormick, R. L. Energy Fuels 2001, 15 (5), 1207–1212. (14) Graboski, M. S.; Ross, J. D.; McCormick, R. L. SAE Paper 961166, 1996. (15) Ziejewski, M.; Goettler, H. J.; Haines, H.; Huang, C. SAE Paper 961846, 1996. (16) Scholl, K. W.; Sorenson, S. C. SAE Paper 930934, 1993. (17) Ulusoy, Y.; Tekin, Y.; Cetinkaya, M.; Karaosmanogjlu, F. Energy Sources 2004, 26, 927–932. (18) Schumacher, L. G.; Marshall, W.; Krahl, J.; Wetherell, W. B.; Grabowski, M. S. Trans. ASAE 2001, 44 (6), 1465–1468. (19) Gomez Gonzales, M. E.; Howard-Hildige, R.; Leahy, J. J.; O’Reilly, T. O.; Supple, B.; Malone, M. EnViron. Monit. Assess. 2000, 65, 13–20. (20) Dorado, M. P.; Ballesteros, E.; Arnal, J. M.; Gomez, J.; Lopez, F. J. Fuel 2003, 82 (11), 1311–1315. (21) Clark, N. N.; Lyons, D. W. Trans. ASAE 1999, 42 (5), 1211–1220. (22) Senatore, A.; Cardone, M.; Rocco, V.; Prati, M. V. SAE Paper 2000-01-0691, 2000. (23) Schumacher, L.; Borgelt, S. C.; Hires, W. G.; Wetherell, W.; Nevils, A. SAE Paper 962233, 1996. (24) Chang, Y. Z.; Van Gerpen, J. H.; Lee, I.; Johnson, L. A.; Hammond, E. G.; Marley, S. J. J. Am. Oil Chem. Soc. 1996, 73 (11), 1549–1555. (25) Sharp, C. A. Characterization of biodiesel exhaust emissions for EPA 211(b). Report 08-1039A; National Biodiesel Board (NBB), Jefferson City, MO, 1998. (26) Sharp, C. A.; Howell, S.; Jobe, J. SAE Paper 2000-01-1967, 2000. (27) McDonald, J. F. Evaluation of a yellow grease methyl ester and petroleum diesel fuel blend. Final Report to the Agricultural Utilization Research Institute, 1997. (28) Purcell, D. L.; McClure, B. T.; McDonald, J.; Basu, H. N. J. Am. Oil Chem. Soc. 1996, 73 (3), 381–388. (29) McDonald, J. F.; Purcell, D. L.; McClure, B. T.; Kittelson, D. B. SAE Paper 950400, 1995. (30) Peterson, C. L.; Reece, D. L.; Thompson, J. C.; Beck, S. M.; Chase, C. Biomass Bioenergy 1996, 10 (5/6), 331–336. (31) Bosch. AutomotiVe Handbook; Robert Bosch GmbH: Stuttgart, Germany, 2000, pp 380-382.

Energy & Fuels, Vol. 22, No. 4, 2008 2797

Furthermore, the injection pressure of IDI engines is low through a pintle nozzle as a single spray. IDI engines also produce lower CO, NOx, and unburned HC exhaust emissions than DI 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.32–36 As mentioned above, an IDI diesel engine exhibits two important advantages: not depend upon fuel quality and produce low exhaust emissions depending upon combustion chamber design. For these reasons, in this study, an IDI diesel engine was used. In our previous study,37 it was seen that biodiesel and its blends have a slight drop in the engine power, increasing peak cylinder pressure, earlier premixed combustion phase, shorter ignition delay, and advancing start of fuel injection timing when compared to petroleum-based diesel fuel (PBDF). Especially, these differences in the premixed combustion phase, the cylinder gas pressures, and the fuel line pressures of the fuels were seen more explicit with increasing engine speed. The objectives of this study are to investigate the effects of biodiesel and its blends on the exhaust emissions characteristics of an unmodified IDI diesel engine and to compare them with those of PBDF. Fuel Characteristics For this study, biodiesel was produced from used frying palm oil in the Fuel Laboratory of Department of Automotive Technologies in Kocaeli University. The feedstock was supplied by Kocaeli Uzay Gida (Frito-Lay Chips Factory). To produce biodiesel from the used vegetable oil, the small-scale transesterification reaction had been carried out in laboratory conditions; thus, the reaction inputs, such as catalyst amount, molar ratio, and reaction temperature and time, have been determined. Then, a big-scale production process was applied using a stainless-steel reactor tank and other equipment. Finally, biodiesel was prepared using a methanol/oil ratio of 6:1 with potassium hydroxide (KOH) as a catalyst (1% of oil by weight). After solving the KOH catalyst in methanol at room temperature, the moisture-free used frying oil was added to the reaction tank to start the transesterification reaction. 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 4 times. PBDF was purchased from a commercial supplier. Fuel specifications of the PBDF and biodiesel were determined by The Scientific and Technological Research Council of Turkey (TUBITAK) 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 three volumetric proportions. They (32) Rakopoulos, C. D.; Antonopoulos, K. A.; Rakopoulos, D. C.; Giakoumis, E. G. Appl. Therm. Eng. 2006, 26 (14-15), 1611–1620. (33) Owen, K.; Coley, T. AutomotiVe Fuels Reference Book; Society of Automotive Engineers (SAE): Warrendale, PA, 1995; p 375. (34) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill: New York, 1988; pp 491-497, 209, 571. (35) Abdel-Rahman, A. A. Int. J. Energy Res. 1998, 22 (6), 483–513. (36) Challen, B.; Baranescu, R. Diesel Engine Reference Book; Butterworth Heinemann: Woburn, MA, 1999; pp 97, 479. (37) Ozsezen, A. N.; Canakci, M.; Sayin, C. Energy Fuels 2008, 22 (2), 1297–1305.

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Ozsezen et al.

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 1 h, 110 °C mg of KOH/g % mass % mass % mg/kg

EU EN 14214

USA ASTM D 6751

limits

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

1.8 L diesel BMC

combustion chamber engine type number of cylinder bore × stroke compression ratio injection pump injector opening pressure nozzle hole diameter maximum power

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

include B5 (5% biodiesel plus 95% PBDF), B20 (20% biodiesel plus 80% PBDF), and B50 (50% biodiesel plus 50% PBDF). Experimental Section 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 experimental setup. The engine was coupled to a hydraulic dynamometer to provide brake load. A magnetic pickup was fixed over the engine flywheel gear to determine the crankshaft position. A water-cooled cylinder pressure transducer (Kistler model 6061B) was mounted on the first cylinder head to measure in the cylinder gas pressure. A pressure transducer (AVL model 8QP500c) was installed in the fuel line of the first cylinder to obtain the fuel line pressure. A charge amplifier (Kistler model 5051A) was used to produce an output voltage proportional to the charge and 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 cylinder gas pressure data of 50 engine cycles were collected with a resolution of 0.25° crank angle (CA). The injector opening pressure specified by manufacturer is 130 bar, which was used in the ignition delay calculation. Fuel consumption was determined by weighing fuel used for a period of time on an electronic scale (error (1 g). Air consumption was measured using a sharp-edged orifice plate [ISO 5167 (1980)] and inclined manometer (error (3%). The relative humidity and ambient temperature were monitored by a hygrometer (error (3% Rh+1). During the tests, six different digital thermometers monitored the temperatures of intake air, fuel, engine oil, exhaust gas, and coolant inlet and outlet. The fuel, engine oil, coolant inlet, and outlet temperature measurements were carried out by digital thermometers

biodiesel

diesel fuel C14.16H25.21 195.50 42 930 840.3 3.177 61.5 0.0015 -14 0.067 56.5 4.14 No. 1A

10 max

C18.08H34.86O2 284.17 38 730 875 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

with the thermocouple probes of K type. The effective range of the measurement was from -40 to 350 °C, with an error (1 °C. The intake air temperature was measured by a digital thermometer (error (1 °C). The exhaust gas temperature was monitored using a standard probe thermocouple K type (NiCr-Ni) placed in the flow of the exhaust fumes, as close as possible to the engine. The effective range of the measurement is from 0 to 1200 °C, with an error (1 °C. In this study, three different gas analyzers were used to measure exhaust gas concentrations. Table 3 gives information about the exhaust gas analyzers and their accuracies. Full-load characteristics of the engine were determined at constant engine speed mode (1000-3000 rpm, with an increment of 500 rpm), which controlled within the (25 rpm range. In this mode, the objective was always to keep the engine speed at the maximum engine torque output levels for each fuel; hence, the dynamometer load was increased manually to match the greater engine torque output. The test procedure is repeated 3 times to verify the maximum engine torque level at a constant engine speed for each fuel. All fuel tests were completed without any modifications on the test engine. The tests were carried out under steady-state conditions. The engine was sufficiently warmed up for each test, and the engine oil temperature was maintained around 65-70 °C. 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 test.

Results and Discussion Brake Torque and Brake-Specific Fuel Consumption (BSFC). A comparison of the brake torque and BSFC values obtained for the fuels tested over the speed range at the fullload condition is shown in Figure 2. The maximum brake torque (95.23 N m) was obtained for PBDF, followed by B5 (94.90 N m), B20 (93.57 N m), B50 (91.79 N m), and B100 (89.91 N m) at 2000 rpm. On average, the brake torques of B100, B50, B20, and B5 compared to those of the PBDF over the speed range at full-load condition decreased by 7.16, 5.16, 4.28, and 2.01%, respectively. As seen in Figure 2, the BSFC slightly increased with the increase of biodiesel percentage in the fuel blend. On average, BSFCs for B100, B50, B20, and B5 were 16.76, 9.42, 5.78, and 2.17% higher than that of PBDF, respectively. As seen in Table 1, the heating value of the biodiesel is 9.78% lower than that of PBDF. Therefore, when the test engine was fueled with biodiesel and its blends, the



Figure 1. Schematic diagram of experimental setup. Table 3. Exhaust Gas Analyzers and Their Accuracies emission devices

measuring values

technology

Kane-May Quintox KM9106

NOx

electrochemical

Bilsa MOD 500

CO, CO2, total unburned HC

infrared

Bosch RTM 430 smoke opacity tester

smoke opacity

Bosch technology

BSFC increased, while the brake torque was dropped because of the lower energy content of the biodiesel. At the same time, for the same volume, more biodiesel fuel based on the mass was injected into the combustion chamber than that of PBDF because of the higher density of the biodiesel than that of PBDF. Exhaust Gas Temperature. A comparison of the exhaust temperature values obtained for PBDF, B100, B50, B20, and B5 is shown in Figure 3. The average exhaust gas temperature of all engine speeds for each fuel B100, B50, B20, B5, and PBDF was calculated as 535.62, 561.64, 562.88, 558.26, and 574.36 °C, respectively. The maximum exhaust temperature for B100, B50, B20, B5, and PBDF was measured as 646.6, 635.8, 642.7, 638.7, and 655.9 °C at 3000 rpm, respectively. The oxygen content of biodiesel provides better combustion, which causes the exhaust temperature close to those of PBDF. Indeed,

Figure 2. Brake torque and BSFC versus engine speed for the fuels.

accuracy (5 ppm < 100 ppm (5% > 100 ppm 0.001 vol %, 0.01 vol %, 1 ppm, respectively 0.1% degree of opacity

because the heating value of B5, B20, B50, and B100 is relatively lower than that of PBDF, these small differences in the exhaust temperatures are normal. Emission Characteristics Carbon Monoxide (CO) Emissions. The CO emissions in the exhaust gases represent lost chemical energy that is not fully used in the engine. Generally, CO emission is affected by air-fuel equivalence ratio, fuel type, combustion chamber design, atomization rate, start of injection timing, engine load, and speed. The most important among these parameters is the air-fuel equivalence ratio.34,35,38 In this study, the calculated equivalence ratio for PBDF and B100 is 1.09 < Φ(A/F,PBDF) < 1.14 and 0.72 < Φ(A/F,B100) < 0.90, respectively. Table 4 shows


Ozsezen et al.

Figure 3. Exhaust temperatures versus engine speed for the fuels. Table 4. Air-Fuel Equivalence Ratios for Each Engine Speed in the Test Conditions of the B100 and PBDF B100 PBDF

1000 rpm

1500 rpm

2000 rpm

2500 rpm

3000 rpm

0.90 1.14

0.76 1.06

0.79 1.14

0.77 1.13

0.72 1.09

the air-fuel equivalence ratio for each engine speed in the test conditions of the B100 and PBDF. Figure 4 shows a comparison of the CO emission values obtained for PBDF, B100, B50, B20, and B5. This figure clearly showed that the biodiesel and its blends produced the lowest CO emissions than those of PBDF, although the biodiesel has a lower air-fuel equivalence ratio for all engine speeds. This case indicates that the oxygen content of biodiesel influences the amount of hydrocarbon oxidation. From Table 4 and Figure 4, it is seen that the CO emissions changed as a function of the air-fuel equivalence ratio, resulting from the fact that reducing the air-fuel ratio increased the CO formation. The maximum CO emission for both fuels was measured at 3000 rpm. This is typical for diesel engines because the air-fuel equivalence ratio slightly decreases with an increasing engine speed. At the same time, as illustrated in the figure, biodiesel and PBDF have similar CO emission slopes in terms of the engine speed; these similar behaviors in CO emission slopes between biodiesel with PBDF are based on their combustion characteristics (premixed phase, cylinder gas pressure, etc.), which are close to each other. Also, the figure shows that the CO emission gradually decreased with the increase of the biodiesel percentage in the fuel blend. On average, CO emissions of all engine speeds for each fuel B100, B50, B20, and B5 compared to those of the PBDF decreased by 56.85, 32.72, 20.14, and 15.39%, respectively. The maximum CO emission for five fuels was measured at 3000 rpm. This is typical with all diesel engines because the air-fuel equivalence ratio slightly decreases with an increasing engine speed. In this study, the ignition delay was calculated in terms of the crank angle between the start of fuel injection timing and the start of combustion timing. Table 5 shows the ignition delays of the tested fuels. The B100 exhibited, on average, 1.02 °CA shorter ignition delay, owing to its higher cetane number, when compared to PBDF (see Table 1). These behaviors caused the extension of the combustion or oxidation timing for biodiesel when compared to PBDF. Hence, CO emissions are reduced by the combustion of biodiesel or its blends. Carbon Dioxide (CO2) Emissions. CO2 emission is produced by complete combustion of fuel. CO2 is an important component

in global warming. Biodiesel is a renewable fuel. The same researcher claimed that all of the carbon released by the combustion of biodiesel has been fixed by the plant through the process of photosynthesis.39–42 Figure 5 shows a comparison of the CO2 emission values obtained for the fuels tested at the full load over the speed range. The figure shows that most of the carbon goes to carbon dioxide under the full-load condition for all test fuels, except 1000 rpm. As mentioned previously, while the test engine was running with biodiesel, the air-fuel equivalence ratio was lower than that of PBDF at all engine speeds. Probably, at 1000 rpm, higher BSFC amount, viscosity, and density of biodiesel, which leads to poor injection characteristics relative to PBDF, affected the complete combustion reaction of the biodiesel. CO2 emissions of the biodiesel and its blends were measured lower for 1000, 1500, and 2000 rpm but higher for 2500 and 3000 rpm than those of PBDF. The shorter ignition delays and higher boiling point of biodiesel increase the combustion duration. This behavior becomes more obvious at the high engine speeds. Also, the BSFC of the biodiesel or its blends improved at the high engine speeds because of the increasing fuel line pressure, which influences the fuel droplet size in the mechanically controlled fuel injection pump. Thus, at the high engine speeds, biodiesel and its blends emitted more CO2 emissions compared to that of PBDF. Nonetheless, the average CO2 emission of all engine speeds for each fuel B100, B50, B20, and B5 decreased by 3, 2.10, 2.25, and 1.95%, respectively, compared to those of the PBDF. In addition, CO2 formation connects to the carbon-hydrogen ratio in the fuel. In the literature, this case has been explained by Peterson and Hustrulid39 in detail. Stoichiometrically, combustion of a hydrocarbon fuel should produce only CO2 and water (H2O). The relative proportion of these two depends upon the carbon-hydrogen ratio in the fuel. In this study, these ratios are about 1:1.78 for PBDF and 1:1.93 for biodiesel (see Table 1). As seen in the eqs 1 and 2, if the complete combustion reactions are established, PBDF releases 3.18 kg of CO2/kg of fuel and biodiesel releases 2.79 kg of CO2/kg of fuel. Thus, CO2 emissions from an engine can be reduced by reducing the carbon content of the fuel per unit energy. The complete combustion reaction for biodiesel is C18.08H34.86O2 + 25.79(O2 + 3.79N2) f 18.08CO2 + 17.43H2O + 96.97N2 (1)

(38) Pulkrabek, W. W. Engineering Fundamentals of the Internal Combustion Engine; Prentice-Hall: Upper Saddle River, NJ, 1997; p 338.



Figure 4. Carbon monoxide emissions versus engine speed for the fuels. Table 5. Ignition Delay 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

The complete combustion reaction for PBDF is C14.16H25.21 + 20.46(O2 + 3.76N2) f 14.16CO2 + 12.60H2O + 76.92N2 (2) Hydrocarbon (HC) Emissions. Unburned hydrocarbon emissions consist of fuel droplets that are completely unburned or only partially burned. HC emissions result from problems of fuel and air mixing and are largely unaffected by the overall air-fuel equivalence ratio.36 A comparison of the HC emission quantities obtained for PBDF, B100, B50, B20, and B5 was showed in Figure 6. On average, unburned HC emission of all engine speeds for each fuel B100, B50, B20, and B5 compared to those of the PBDF decreased by 40.26, 23.38, 22.08, and 16.88%, respectively. As seen in the figure, the unburned HC emission was gradually increased when the amount of PBDF increased in the blend. The similarities in CO emission slopes of the biodiesel and PBDF are seen for unburned HC emission slopes of the biodiesel and PBDF. This case supported the above prescience, which explained that the oxygen content of biodiesel influenced the amount of hydrocarbon oxidation. It is known that the higher oxygen in the combustion region provides more complete combustion. 35,36,43 Thus, when the test engine was fueled with biodiesel or its blends, the unburned CO and HC amount reduced compared to PBDF because of the high oxygen content of biodiesel. The maximum unburned HC emissions appeared to be 11 ppm for PBDF at 1500 rpm because of the air-fuel equivalence ratio reduced at this speed. Unburned HC emissions were significantly lower for all fuels in the IDI diesel engine, (39) Peterson, C. L.; Hustrulid, T. Biomass Bioenergy 1998, 14 (2), 91– 101. (40) Agarwal, A. K.; Das, L. M. J. Eng. Gas Turbines Power 2001, 123 (2), 440–447. (41) Ko¨rbitz, W. Renewable Energy 1999, 16 (1-4), 1078–1083. (42) Wu, Y. G.; Lin, Y.; Chang, C. T. Fuel 2007, 86 (17-18), 2810– 2816. (43) Borman, G. L.; Ragland, K. W. Combustion Engineering; McGrawHill: New York, 1998; pp 216, 415.

which has a homogeneous charge condition. An IDI diesel engine uses the heat of the piston recess wall to vaporize the fuel. If the air flow inside of the combustion chamber is properly adapted, an extremely homogeneous air-fuel mixture with a long combustion period, low pressure increase, and therefore, quite combustion and higher oxidation can be achieved.44 The cetane number of biodiesel shortens the ignition delay (see Table 5); this situation extents oxidation duration of biodiesel and its blends and reduces the level of unburned HC emission. At the same time, the shortness in ignition delay affects the premixed combustion phase and gives higher cylinder gas pressure for the biodiesel and its blends. Likely, the shorter ignition delay and higher boiling point of biodiesel increase the cylinder gas temperatures. These behaviors could be seen more clearly at the high engine speeds. Consequently, when the test engine was accelerated, unburned HC dropped to nearly 3-5 ppm for all test fuels. Oxides of Nitrogen (NOx) Emissions. In the diesel engine, the fuel distribution is non-uniform. The pollution formation process is strongly dependent upon the fuel distribution and how that distribution changes with time because of mixing. Nitrogen oxides (NOx) form in the high-temperature burned gas region, which is non-uniform, and formation rates are highest in the close to stoichiometric regions.34 Humidity has a large influence on NOx emissions. For that reason, in this study, the relative humidity of air was measured for the NOx correction factor, which was calculated as assured in ref45 A comparison of the NOx emission level obtained for PBDF, B100, B50, B20, and B5 was shown in Figure 7. The average NOx emissions of all engine speeds for each fuel B100, B50, B20, and B5 were calculated to be 14.72, 13.20, 8.87, and 3.83% higher than that of PBDF, respectively. As can be seen in the figure, NOx emissions are more complex than CO and HC emissions. The NOx emissions gradually increased with the increasing fraction of biodiesel in the blend at 1500, 2000, and 2500 rpm. In these engine speeds, it is obvious that the increased NOx with the use of biodiesel and its blends is a result of increasing oxidation. The oxygen content of biodiesel is an important factor in the high NOx formation levels, because oxygen content of biodiesel provides high local peak temperatures and a corresponding excess of air. In the (44) Bosch. Diesel-Engine Management: An OVerView; Robert Bosch GmbH: Stuttgart, Germany, 2003; p 27. (45) Society of Automotive Engineers, Inc. SAE Handbook; SAE: Warrendale, PA, 2001; Vol. 1, pp 1304-1306.


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Figure 5. Carbon dioxide emissions versus engine speed for the fuels.

Figure 6. Unburned hydrocarbon emissions versus engine speed for the fuels.

Figure 7. Nitrogen oxide emissions versus engine speed for the fuels.

literature, some researchers4,46,47 showed that increased oxygen levels increased the maximum temperature during the combustion, which increased NOx formation. At 1000 rpm, B100 and B50 exhibit the minimum NOx emissions (64 and 62 ppm, respectively) that are lower than (46) Beatrice, C.; Bertoli, C.; D’Alessio, J.; Del Giacomo, N.; Lazzaro, M.; Massoli, P. Combust. Sci. Technol. 1996, (120), 335–355. (47) Song, J.; Cheenkachorn, K.; Wang, J.; Perez, J.; Boehman, A. L. Energy Fuels 2002, 294–301.

those of PBDF, B20, and B5. When the test engine was fueled with B100 and B50, more fuel was injected into the combustion chamber. Because of this, the cylinder gas temperature of the B100 reduced by the evaporation of unburned fuel continued until the exhaust valves open. However, the exhaust gas temperature of the B50 did not reduce because it contains 50% (volumetric) PBDF. Oxidation timing of B50 may extend to the exhaust stroke because of the high boiling points and low volatility of biodiesel. Thus, thermal NOx formation reduced



Figure 8. Smoke opacity versus engine speed for the fuels.

especially for B100. NOx formation strongly depends upon the combustion temperature, which determines the exhaust temperature. Figure 3 shows the exhaust temperatures. Higher exhaust temperatures lead to significantly increases in the NOx formation. Therefore, the NOx emission curve increased from 1000 to 3000 rpm. The 3000 rpm condition has the highest peak pressure, and the rate of pressure rise at a given load and timing also tends to have the highest concentration of NOx. The maximum NOx emission for PBDF was obtained to be 125 ppm at the speed of 3000 rpm. At this engine speed, minimum NOx was measured for B100. In the literature, this contrary tendency was explained with the NOx emissions formed as a function of the peak cylinder pressure and peak rate of pressure rise.16,43 Ignition delay can have a substantial effect on the exhaust emissions, because it controls the proportion of fuel burned. Increasing the fraction of the fuel burned in the premixed phase increases the NOx emission level. The shorter ignition delay advances the combustion timing, increases peak pressure and temperature, and enhances NOx formation. Thus, many researchers have demonstrated that retarding the fuel injection timing and lengthen the ignition time is an effective method for controlling NOx emissions of biodiesel.48 This is quite effective for some reduction in fuel economy, because it reduces the amount of premixed burning.43 In this study, when the test engine fueled with biodiesel, the start of the nozzle needle carried out an average 1.12 °CA earlier than that of PBDF. All of these factors have the tendency to increase the combustion temperature of biodiesel fuels while decreasing the exhaust gas temperatures. At 1500, 2000, and 2500 rpm for B100 and B50, the higher NOx emissions were mainly caused by the biodiesel having a shorter ignition delay. As seen in Figure 3, the exhaust gas temperatures for biodiesel dropped below compared to the PBDF in all test speeds. Low exhaust gas temperatures are one indicator of earlier combustion. At the same time, the reason of changing the NOx emission level can be explained by the fact that the test engine has a mechanically controlled distributor type injection pump; the injection characteristics, atomization ratio, and fuel-air mixing rates changed with the engine speed. Smoke Opacities. Soot is produced by oxygen-deficient thermal cracking of long-chain molecules.35 Biodiesel consists of long-chain hydrocarbons. Schmidt and Van Gerpen6 reported that, by adding the long-chain esters, the aromatic content of (48) Wang, W. G.; Lyons, D. W.; Clark, N. N.; Gautam., M.; Norton, P. M. EnViron. Sci. Technol. 2000, 34, 933–939.

the fuel reduced. Some investigators49–51 showed that the presence of aromatic compounds in the fuel facilitates the particulate formation process in the diesel engine. Therefore, by reducing aromatics, smoke opacity should be reduced. Smoke opacity curves of the five fuels are shown in Figure 8. In comparison to PBDF, the average reduction for the smoke opacity for all engine speeds for each fuel was calculated as 22.52% for B100, 10.94% for B50, 2.13% for B20, and 0.89% for B5. As presented in Figure 8, the B100 and B50 exhibits the greater reductions in smoke opacity compared to those of the PBDF, especially at 2500 and 3000 rpm. As seen Table 3, the biodiesel and its blends exhibit shorter ignition delay compared to PBDF. This case causes early combustion. This advance in combustion timing allows for more time for soot oxidation, therefore reducing smoke opacity, especially at 2500 and 3000 rpm. Furthermore, the soot formation took place during the initial premixed combustion phase. This phase occurs at near stoichiometric fuel-air equivalence ratios, where oxygen content of biodiesel provided oxygen to the fuel-rich zone. Thus, the reduction of smoke opacity was obtained in the high engine speeds. At 1000 rpm, the maximum BSFC was obtained while the test engine was fueled with B100 because of the lower energy content of the biodiesel. At the same time, when the B100 or B50 fuel was injected into the combustion chamber, higher spray penetration occurred because of the high viscosity of biodiesel. These factors increased incomplete fuel oxidation or partial oxidation in the center of fuel jets in low engine speeds. Partially burned fuel in localized high-temperature regions of the combustion system formed soot emissions. Conclusion In this study, the biodiesel produced from used frying palm oil was blended with PBDF and tested in an IDI diesel engine to investigate the effects of the fuels on the performance and exhaust emissions over the speed range at full-load condition. The following conclusions can be drawn from the present paper: (1) CO emissions gradually decreased with the increase of biodiesel percentage in the fuel blend for all tests. In comparison to those of the PBDF, on average CO emission of all engine speeds for each fuel B100, B50, B20, and B5 decreased by (49) Asaumi, Y.; Shintani, M.; Watanabe, Y. SAE Paper 922214, 1992. (50) Betts, W. E.; Floysand, S. A.; Kvinge, F. SAE Paper 922190, 1992. (51) Rosenthal, M. L.; Bendinsky, T. SAE Paper 932800, 1993.


56.85, 32.72, 20.14, and 15.39%, respectively. Although biodiesel has a lower air-fuel equivalence ratio, it produced the lowest CO emissions than those of PBDF for all engine speeds. This case shows that the oxygen content and combustion characteristics (shorter ignition delay, higher cylinder gas pressure, and advanced premixed combustion phase) of biodiesel influenced the amount of hydrocarbon oxidation. (2) On average, CO2 emission of all engine speeds for each fuel B100, B50, B20, and B5 decreased by 3, 2.10, 2.25, and 1.95%, respectively, compared to that of PBDF. CO2 formation is related to the fuel injection characteristics, oxygen content, and carbon-hydrogen ratio in the fuel. In this study, the carbon-hydrogen ratio is about 1:1.78 for PBDF and 1:1.93 for biodiesel. CO2 emissions of biodiesel reduced by reducing its carbon content per unit energy. (3) When the test engine fueled with biodiesel or its blends, the amount of unburned HC emissions reduced. On average, unburned HC emission of all engine speeds for each fuel B100, B50, B20, and B5 decreased by 40.26, 23.38, 22.08, and 16.88%, respectively, compared to that of the PBDF. The oxygen content of biodiesel may cause this result because the higher oxygen in the combustion region provides more complete combustion. (4) The oxygen content, higher boiling points, density, and kinematic viscosity of biodiesel provided the high local combustion temperatures. When the test engine was fueled with biodiesel, the injector nozzle needle carries out at earlier than that of PBDF. These complex structures affected NOx formation for biodiesel. In this study, on average, NOx emissions of all engine speeds for each fuel B100, B50, B20 and B5 were calculated as 14.72, 13.20, 8.87, and 3.83% higher than that of PBDF, respectively. (5) Biodiesel has oxygen, near-zero sulfur, and low aromatic content when compared to conventional diesel fuel. These properties of biodiesel are beneficial from the

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standpoint of reducing smoke opacities. Particularly, the oxygen content of biodiesel provided oxygen to the fuel-rich zone. In comparison to PBDF, the average reduction for the smoke opacity was calculated as 22.52% for B100, 10.94% for B50, 2.13% for B20, and 0.89% for B5. Acknowledgment. This study was supported by the grants from TUBITAK (Project 104M372) and the Scientific Research Foundation of Kocaeli University (Projects 2003/79 and 2004/24). The authors thank the individuals at the engine test laboratory who were involved in making this work possible.

Nomenclature ASTM ) American Society for Testing and Materials B5 ) 5% biodiesel plus 95% PBDF (volumetric) B20 ) 20% biodiesel plus 80% PBDF (volumetric) B50 ) 50% biodiesel plus 50% PBDF (volumetric) B100 ) 100% biodiesel BSFC ) brake-specific fuel consumption CO ) carbon monoxide CO2 ) carbon dioxide DI ) direct injection EPA ) Environmental Protection Agency HC ) hydrocarbons IDI ) indirect injection KOH ) potassium hydroxide NOx ) nitrogen oxides PAH ) polyaromatic hydrocarbon PBDF ) petroleum-based diesel fuel PM ) particulate matter TUBITAK ) The Scientific and Technological Research Council of Turkey EF800174P

Effects of Biodiesel from Used Frying Palm Oil on the Exhaust

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