Energy Fuels 2010, 24, 4490–4496 Published on Web 07/15/2010
: DOI:10.1021/ef100456a
Performance Evaluation of Fuel Blends Containing Croton Oil, Butanol, and Diesel in a Compression Ignition Engine Frank Lujaji,*,† Akos Bereczky,‡ and Makame Mbarawa† †
Department of Mechanical Engineering, Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa, and ‡Department of Energy Engineering, Budapest University of Technology and Economics, H-1111 Budapest, Bertalan Lajos u. 4-6, D208 Hungary Received April 12, 2010. Revised Manuscript Received June 16, 2010
Emission problems associated with the use of fossil fuels have led to numerous research works on the use of renewable fuels. The aim of this study is to evaluate the effects of blends containing croton oil (CRO), 1-butanol (BU), and diesel (D2) on the engine performance, combustion, and emission characteristics. Samples investigated were 20% CRO-80% D2, 15% CRO-5% BU-80% D2, and 10% CRO-10% BU-80% D2, with D2 as the baseline. The density, viscosity, cetane number, and contents of carbon, hydrogen, and oxygen were measured by ASTM standards. A four-cylinder turbocharged direct-injection diesel engine was used for the tests. It was observed that the brake specific energy consumption of blends was found to be high compared to that of the D2 fuel. The addition of BU in the blend reduces the brake thermal efficiency values. BU-containing blends show peak cylinder pressures and heat release rates comparable to those of D2 on higher engine loads. Carbon dioxide and smoke emissions of the BU blends were lower in comparison to those of the D2 fuel.
inherently viscous. High viscosity and poor volatility are major challenges in running modern diesel engines on vegetable oils. It is reported from the literature2,4,7-12 that CI engines run on vegetable oils have lower peak power and torque and low engine speeds and this causes injector cocking, filter clogging, ring sticking, and thickening of the lubrication oil. Two methods have currently been used to solve high-oilviscosity and -volatility problems of vegetable oils. They are oil heating13-15 and the mixing of viscous oil with a lighter diesel fuel.10,16-19 In addition to those two techniques, the mixing of vegetable oils with alcohols can also be used to solve
Introduction The search for sustainable alternative fuels for diesel engines has become important recently because of growing concerns over the future availability of oil reserves and environmental problems. Diesel engines are widely used in agriculture, transportation, and industries. Urbanization in developing counties is growing fast;1 this results in more demand for energy in activities such as agriculture and transportation. Known petroleum reserves are predicted to deplete in the near future. The drive toward a clean energy economy is therefore indispensable. Vegetable oils in this regard provide the opportunity to replace a fraction of petroleum diesel usage in compression ignition (CI) engines in order to achieve significant emission reduction. The use of vegetable oils as fuels reduces carbon dioxide (CO2) emissions; the underlying fact is that they absorb the same amount of CO2 when they grow as when they are released during combustion.2 The use of vegetable oils in diesel engines is as old as the diesel engine itself. German scientist Rudolf Diesel, inventor of the CI engine, tested vegetable oil in one of his engines about 100 years ago.3-6 Vegetable oils consist of mostly triglycerides. Triglycerides are
(7) Murugesan, A.; Umarani, C.; Subramanian, R.; Nedunchezhian, N. Bio-diesel as an alternative fuel for diesel engines;A review. Renewable Sustainable Energy Rev. 2009, 13 (3), 653–662. (8) Ma, F.; Hanna, M. A. Biodiesel production: a review. Bioresour. Technol. 1999, 70 (1), 1–15. (9) Srivastava, A.; Prasad, R. Triglycerides-based diesel fuels. Renewable Sustainable Energy Rev. 2000, 4 (2), 111–133. (10) AltIn, R.; C-etinkaya, S.; Y€ ucesu, H. S. The potential of using vegetable oil fuels as fuel for diesel engines. Energy Convers. Manage. 2001, 42 (5), 529–538. (11) Purushothaman, K.; Nagarajan, G. Experimental investigation on a C.I. engine using orange oil and orange oil with DEE. Fuel 2009, 88 (9), 1732–1740. (12) Purushothaman, K.; Nagarajan, G. Performance, emission and combustion characteristics of a compression ignition engine operating on neat orange oil. Renewable Energy 2009, 34 (1), 242–245. (13) Bialkowski, M. T.; Pekdemir, T.; Towers, D. P.; Reuben, R.; Brautsch, M.; Elsbett, G. Effect of fuel temperature and ambient pressure on a common rail rapeseed oil spray. J. KONES Internal Combust. Engines 2004, 11 (1-2), 53–65. (14) Labeckas, G.; Slavinskas, S. Performance of direct-injection off-road diesel engine on rapeseed oil. Renewable Energy 2006, 31 (6), 849–863. (15) Bernardo, A.; Howard-Hildige, R.; O’Connell, A.; Nichol, R.; Ryan, J.; Rice, B.; Roche, E.; Leahy, J. J. Camelina oil as a fuel for diesel transport engines. Ind. Crops Prod. 2003, 17 (3), 191–197. (16) Saveljev, G. Alternative fuels in agricultural sector. Altern. Fuels 2006, 1 (25), 64–70. (17) Labeckas, G.; Slavinskas, S. Performance and exhaust emissions of direct injection diesel engine operating on rapeseed oil and its blends with diesel fuel. Transport 2005, 20 (5), 186–194.
*To whom correspondence should be addressed. Tel: þ27 12 382 5177. Fax: þ27 12 382 5602. E-mail:
[email protected] or lujajifc@ tut.ac.za. (1) Cohen, B. Urbanization in developing countries: Current trends, future projections, and key challenges for sustainability. Technol. Soc. 2005, 28 (1-2), 63–80. (2) Demirbas, A. Biodiesel: a realistic fuel alternative for diesel engines; Springer Verlag: Berlin, 2008. (3) Shay, E. G. Diesel fuel from vegetable oil: status and opportunities. Biomass Bioenergy 1993, 4 (4), 227–242. (4) Pinto, A. C.; Guarieiro, L. L. N.; Rezende, M. J. C.; Ribeiro, N. M.; Torres, E. A.; Lopes, W. A.; Pereira, P. A. d. P.; Andrade, J. B. d. Biodiesel: an overview. J. Braz. Chem. Soc. 2005, 16, 1313–1330. (5) Meher, L. C.; Vidya Sagar, D.; Naik, S. N. Technical aspects of biodiesel production by transesterification;a review. Renewable Sustainable Energy Rev. 2006, 10 (3), 248–268. (6) Demirbas, A. Progress and recent trends in biodiesel fuels. Energy Convers. Manage. 2009, 50 (1), 14–34. r 2010 American Chemical Society
4490
pubs.acs.org/EF
Energy Fuels 2010, 24, 4490–4496
: DOI:10.1021/ef100456a
Lujaji et al. 20
the viscosity problem. It can be seen from the literature that ethanol can mix properly with rapeseed vegetable oil up to 9%. Above this value, a phase separation may occur because of the incompatibility of vegetable oils and the ethanol-water content. The mixing of ethanol with vegetable oils actually has two contradictory behaviors on the blend properties. First, ethanol has a viscosity (1.2-1.4 mm2/s at 40 °C) that is around 23-25 times lower than that of vegetable oils (28-34.9 mm2/s). This may reduce the blend oil viscosity and improve cold-flow properties, fuel-spray penetration, and atomization quality.21-25 On the other hand, the low cetane number (CN) of ethanol, which varies between 5 and 7, and its high autoignition temperature of approximately 420 °C along with high volatility and absorbed water content reduce the autoignition character of the fuel blend injected during idling conditions. Blending higher alcohols such as 1-butanol (BU) with vegetable oils can improve the fuel properties. The phase stability of the vegetable oil, alcohol, and diesel blends can also be improved by using higher alcohols such as BU, propanol, etc. In contrast to ethanol, the miscibility of BU with vegetable oils at a wide range of operating conditions is excellent. The blends of BU with vegetable oils maintain their stability for a long period without any phase separation. Furthermore, BU has heat of vaporization of about 570 kJ/kg, while that of ethanol is about 900 kJ/kg. Therefore, the earlier start of BU evaporation intensifies the preparation of the combustible mixture and improves autoignition of heavy, less volatile vegetable oil molecules in the blend. In addition, BU has a higher CN (17) than ethanol, which is below 8. It should be emphasized that the high CN causes the fuel to flare up easily and therefore improves the combustion of the fuel blend. Additionally, the net heating value of BU is higher than that of ethanol, which is another advantage for the engine performance and fuel consumption. Because of these limitations of ethanol as a blend component, BU could be used in the vegetable oil blends instead of ethanol. The main obstacle of using BU as a blending component with vegetable oil is its low CN, which may deteriorate the autoignition capability of the blend during the combustion process. On the other hand, the higher CN of vegetable oils may compensate for the lower CN of BU in the blend. It is reported from the study by Chotwichien et al.26 that three-component fuel;85% diesel, 10% palm oil ethyl ester, and 5% BU;blends have shown better fuel properties. In another study, hydrocarbon (HC), smoke, and particulate
Figure 1. Engine test experimental setup.
matter emissions were reported to decrease when a three-component fuel (ethanol-biodiesel-diesel) was used to run the diesel engine.27 Emission reduction was also reported by Shi et al. in their experimental works on three-component fuels (alcoholbiodiesel-diesel).28-30 In this work, combustion characteristics, the engine performance, and emissions of three-component fuel [croton oil (CRO), BU, and diesel (D2)] blends will be investigated. It is expected that the use of BU blended with vegetable oil and D2 may provide a solution for the phase stability problem of the vegetable oil-simple alcohol blends, improve combustion characteristics, and minimize emission problems related to the burning of petroleum diesel fuel in a diesel engine. Experimental Methods Test Fuels. Croton megalocarpus oil (CRO) was supplied by Diligent Tanzania Ltd. 1-Butanol (BU; CAS no. 71-36-3) purchased from a chemical shop and diesel fuel (D2) from a local petrol station (Budapest, Hungary) were used to make blends and as a base reference for engine tests. Blends were prepared in the laboratory by mixing the components in 5000 mL beakers at room temperature; a transesterification process was not involved. On a volumetric basis, D2 was fixed at 80% for all blends; the remaining 20% consisted of vegetable oil, while BU varied from 0% to 10% in steps of 5%. The blends were made in batches, where the measured amounts of components were poured into the beaker and stirred until they were mixed. The uniform mixtures of different samples of 20% CRO-80% D2, 15% CRO-5% BU-80% D2, and 10% CRO-10% BU-80% D2 were then taken for further analysis. Equipment Setup. Figure 1 shows the equipment connections for the engine test. The engine (1) was coupled with a dynamometer (2) to provide variable brake load, while the engine throttling
(18) Dorado, M. P.; Arnal, J. M.; G omez, J.; Gil, A.; Lopez, F. J. The effect of a waste vegetable oil blend with diesel fuel on engine performance. Trans. ASAE 2002, 45 (3), 519–523. (19) Nwafor, O. M. I.; Rice, G. Performance of rapeseed oil blends in a diesel engine. Appl. Energy 1996, 54 (4), 345–354. (20) Yoshimoto, Y.; Onodera, M. Performance of a Diesel Engine Fueled By Rapeseed Oil Blended with Oxygenated Organic Compounds. SAE Tech. Pap. Ser. 2002, document no. 2002-01-2854. (21) Akih-Kumgeh, B.; Bergthorson, J. M. Shock Tube Study of Methyl Formate Ignition. Energy Fuels 2009, 24 (1), 396–403. (22) Huber, M. L.; Lemmon, E. W.; Kazakov, A.; Ott, L. S.; Bruno, T. J. Model for the Thermodynamic Properties of a Biodiesel Fuel. Energy Fuels 2009, 23 (7), 3790–3797. (23) Kegl, B.; Hribernik, A. Experimental Analysis of Injection Characteristics Using Biodiesel Fuel. Energy Fuels 2006, 20 (5), 2239– 2248. (24) Tat, M. E.; Van Gerpen, J. Measurement of biodiesel speed of sound and its impact on injection timing. National Renewable Energy Laboratory, NREL/SR-510-31462, 2003. (25) Boehman, A. L.; Morris, D.; Szybist, J.; Esen, E. The Impact of the Bulk Modulus of Diesel Fuels on Fuel Injection Timing. Energy Fuels 2004, 18 (6), 1877–1882. (26) Chotwichien, A.; Luengnaruemitchai, A.; Jai-In, S. Utilization of palm oil alkyl esters as an additive in ethanol-diesel and butanol-diesel blends. Fuel 2009, 88 (9), 1618–1624.
(27) Chen, H.; Wang, J.; Shuai, S.; Chen, W. Study of oxygenated biomass fuel blends on a diesel engine. Fuel 2008, 87 (15-16), 3462–3468. (28) Shi, X.; Yu, Y.; He, H.; Shuai, S.; Wang, J.; Li, R. Emission characteristics using methyl soyate-ethanol-diesel fuel blends on a diesel engine. Fuel 2005, 84 (12-13), 1543–1549. (29) Shi, X.; Pang, X.; Mu, Y.; He, H.; Shuai, S.; Wang, J.; Chen, H.; Li, R. Emission reduction potential of using ethanol-biodiesel-diesel fuel blend on a heavy-duty diesel engine. Atmos. Environ. 2006, 40 (14), 2567–2574. (30) Shi, X.; Yu, Y.; He, H.; Shuai, S.; Dong, H.; Li, R. Combination of biodiesel-ethanol-diesel fuel blend and SCR catalyst assembly to reduce emissions from a heavy-duty diesel engine. J. Environ. Sci. 2008, 20 (2), 177–182.
4491
Energy Fuels 2010, 24, 4490–4496
: DOI:10.1021/ef100456a
Lujaji et al.
Table 1. Description of the Engine Test Experimental Setup S/N 1 2 3 4 5 6 7 8 9 10 11 12 13
equipment
Table 2. Engine Details engine model capacity bore stroke compression ratio maximum power maximum torque fuel system
description
engine dynamometer fuel tank and flowmeter exhaust pipe gas analyzer smoke meter computer computer computer
Audi, 1.9 L, TDI type: FE350S-BORGBI and SAUERI type: AVL 7030 connected to the laboratory exhaust system type: Horiba system (Mexa-812) type: AVL 415 (variable sampling) PC recording emission data PC installed with an indication system PC installed with an engine loading controls program type: Kistler KIAG 6005
Audi, 1.9 L, TDI 1896 cm3 79.5 mm 95.5 mm 19.5:1 66 kW, at 4000 rpm 202 N m, at 1900 rpm direct injection with an electronic distributer pump
BU-80% D2 blend varies from 9.44 to 55.67 MJ/kWh. Under full load conditions, BSEC for the D2 fuel sample was 9.24 MJ/kWh. The maximum value was observed on the 10% CRO-10% BU-80% D2 fuel sample because of its high oxygen content and low lower heating value (LHV; Table 3). Similar results were observed in the literature.32,33 Brake Thermal Efficiency (BTE). Table 5 shows the BTEs for different fuel samples at different loads. It can be observed that BTE increases as the load increases for all test fuels. At 25% load, the 20% CRO-80% D2 records a maximum BTE of 29.17%, while the 10% CRO-10% BU-80% D2 blend exhibits a minimum BTE of 27.63%. It can also be observed that, at 25% load, BTE decreases as the percentage of alcohol increases, with the 20% CRO-80% D2 blend having a BTE close to that of D2. A similar trend is observed at 100% load, that is, with D2 exhibiting the highest BTE of 38.98% and the 10% CRO-10% BU-80% D2 blend the lowest at 38.150%. This could be attributed to the percentage increase of alcohol in the fuel. The high BTE value of 20% CRO-80% D2 is influenced by the presence of intrinsic oxygen in vegetable oils (Table 3), which supports combustion. Combustion. Cylinder Pressure. At no load condition, the maximum peak cylinder pressure was found to be 62.20 bar at -0.35 CAD with the 10% CRO-10% BU-80% D2 blend sample and 62.10 bar at -035 CAD with the D2 fuel sample. The minimum peak cylinder pressure was found to be 54.20 bar at -035 CAD with the 20% CRO-80% D2 sample (Figure 2). For 25% load condition, the maximum peak cylinder pressure was found to be 69.83 bar at -1.10 CAD with the 10% CRO-10% BU-80% D2 blend sample, and with the D2 sample, the peak pressure was 68.89 bar at -0.70 CAD, while the minimum peak cylinder pressure was found to be 65.59 bar at -0.70 CAD with the 15% CRO-5% BU-80% D2 sample (Figure 2). At 50% load condition, the maximum peak cylinder pressure was found to be 83.33 bar at 6.68 CAD with the 10% CRO-10% BU-80% D2 blend sample and 79.67 bar at -4.57 CAD with the D2 sample. D2 was established to record a minimum peak cylinder pressure at this load condition among all of the test samples (Figure 2). For 75% load condition, the maximum peak cylinder pressure was found to be 98.98 bar at 7.38 CAD with the 10% CRO-10% BU-80% D2 blend sample and 96.54 bar at -7.38 CAD with the D2 sample. The minimum peak cylinder pressure was determined to be 93.44 bar at 6.33 CAD with the 20% CRO-80% D2 sample (Figure 2). At full load condition, the maximum peak cylinder pressure was established as 126.25 bar at 7.73 CAD with the 15% CRO-5% BU-80% D 2 blend sample and 125.73 bar at 5.98 CAD with
pressure transducer crank angle type: optical encoder-HENGSTLER RI 32 transducer charge amplifier type: Kistler KIAG 5001 temperature sensor type K thermocouple
and dynamometer settings were controlled by a computer (9). A pressure transducer (10) was installed in one of the piston cylinders. Cylinder pressure signals from a pressure transducer were amplified by a charge amplifier (12) and connected to the SMETech COMBI-PC indication system (8) for data acquisition. The data acquisition system was externally triggered 1024 times in one revolution by an incremental crank angle transduceroptical encoder (11). Fuel was introduced from a fuel tank (3) equipped with fuel-flow measurement system. During fuel switching, the fuel tank was drained from the engine fuel filter, a new fuel was introduced into the tank until the fuel filter was full, and the engine was then started and allowed to run for a few minutes to clear the fuel lines and stabilize. The emission was measured by a Horiba emission gas analyzer system (5) equipped with analyzer modules NDIR (AIA-23), H.FID (FIA-22), and H.CLDC (CLA-53M) for measuring (CO, CO2, and HC), total HC (THC), and nitrogen oxides (NOx), respectively, and a smoke meter (6) connected before the oxidative converter at the engine exhaust pipe (4). The system was connected to the computer (7), and emission gas data were recorded. Table 1 shows equipment descriptions. Engine Test Cycle. During the engine test, the engine was run at a constant speed of 3000 rpm at different loads, from no load condition to 100% load in steps of 25% full load. The engine was run for 2 min at each test condition. Table 2 shows engine details. Combustion Analysis. The heat release rate was calculated from cylinder pressure data and crank angle readings. The analysis was derived from the first law of thermodynamics for an open system that is quasi-static.31
Results and Discussion Engine Performance. Brake Specific Energy Consumption (BSEC). BSEC represents a more reliable factor for comparing different fuels with different heating values and different values of density. Table 3 shows measured fuel properties for different samples. Table 4 shows the BSECs of tested fuels at different engine loads and at constant engine speed. It can be seen that BSEC deceases as the load increases. The reason for the sharp decrease is that the amount of fuel required for operating the engine per unit energy output at higher loads decreases. The BSEC values vary from 9.21 MJ/kWh, observed on 20% CRO-80% D2 at full load, to the maximum value of 58.66 MJ/kWh, observed in D2 under no load. BSEC for 20% CRO-80% D2 blend was observed to be 49.62 MJ/kWh at no load, while the 15% CRO-5% BU-80% D2 blend varies from 9.43 to 50.93 MJ/kWh and the 10% CRO-10%
(32) Murugan, S.; Ramaswamy, M. C.; Nagarajan, G. Performance, emission and combustion studies of a DI diesel engine using Distilled Tyre pyrolysis oil-diesel blends. Fuel Process. Technol. 2008, 89 (2), 152–159. (33) Agarwal, D.; Kumar, L.; Agarwal, A. K. Performance evaluation of a vegetable oil fuelled compression ignition engine. Renewable Energy 2008, 33 (6), 1147–1156.
(31) Heywood, J. B. Internal combustion engine fundamentals; McGraw-Hill: New York, 1998.
4492
Energy Fuels 2010, 24, 4490–4496
: DOI:10.1021/ef100456a
Lujaji et al. Table 3. Fuel Properties
test
viscosity at 40 °C [mm /s]
LHV [MJ/kg]
density at 28 °C [kg/m3]
CN
% C, % H, % O
test method BU 20% CRO-80% D2 15% CRO-5% BU-80% D2 10% CRO-10% BU-80% D2 100% CRO D2
ASTM D 445 2.63 8.50 4.33 3.82 33.38 2.30
ASTM D 240 33.94 41.73 41.58 41.43 36.98 42.92
ASTM D 1298 811.95 842.56 837.16 831.76 920.00 823.20
ASTM D 613 17.0 51.8 50.6 49.4 40.7 54.6
65.0, 14.0, 22.0 85.2, 12.0, 2.8 84.6, 12.1, 3.3 83.9, 12.2, 3.9 78.0, 12.0, 10.0 87.0, 12.0, 0.0
2
Table 4. BSECs at Different Loads BSEC [MJ/kWh] fuel
0%
25%
50%
75%
100%
20% CRO-80% D2 15% CRO-5% BU-80% D2 10% CRO-10% BU-80% D2 D2
49.62 50.93 55.67 58.66
12.34 12.45 13.03 12.39
9.98 10.45 10.88 10.30
9.86 9.82 9.58 9.82
9.21 9.43 9.44 9.24
Table 5. BTEs at Different Engine Loads BTE [%] fuel
25%
50%
75%
100%
20% CRO-80% D2 15% CRO-5% BU-80% D2 10% CRO-10% BU-80% D2 D2
29.171 28.926 27.631 29.057
36.077 34.436 33.098 34.949
36.506 36.644 37.597 36.649
39.085 38.177 38.150 38.981
Figure 3. Cylinder pressure at full load (100%) condition.
extent on fuel-air mixing. D2 indicates a slightly lower peak cylinder pressure than 10% CRO-10% BU-80% D2, which also was caused by its density, which is proportional to its bulk modulus. Another reason is oxygen-fuel mixing, which it is more efficient in fuel that contains intrinsic oxygen (Table 3). More complete combustion leads to high cylinder pressure.11,12 Heat Release. At no load condition, the maximum peak heat release rate was found to be 11.43 J/CAD at 9.84 CAD with the 10% CRO-10% BU-80% D 2 blend sample and 7.19 J/CAD at 8.79 CAD with the D2 sample. The minimum heat release rate was established as 6.43 J/CAD at 8.44 CAD with the 15% CRO-5% BU-80% D2 sample (Figure 4). For a 25% load condition, the maximum peak heat release rate was determined to be 25.36 J/CAD at 7.03 CAD with the 10% CRO-10% BU-80% D2 blend sample and 23.32 J/CAD at 7.03 CAD with the D2 sample. The minimum heat release rate was found to be 17.76 J/CAD at 6.68° after the top dead center with the 15% CRO-5% BU-80% D2 sample (Figure 4). At 50% load condition, the maximum peak heat release rate was found to be 26.55 J/CAD at 9.50 CAD with the 20% CRO-80% D2 blend sample and 25.63 J/CAD at 9.84 CAD with the D2 sample. The minimum heat release rate under this load condition was identified as 24.34 J/CAD at 9.50° after the top dead center with the 10% CRO-10% BU-80% D2 sample (Figure 4). For a 75% load condition, the maximum peak heat release rate was found to be 34.05 J/CAD at 10.55 CAD with the D2 sample. Slightly lower than the D2 sample, the 15% CRO-5% BU-80% D2 blend was observed to exhibit about a 2% less peak heat release rate than D2; this was 33.38 J/CAD at 10.55 CAD for the D2 sample. The minimum heat release rate was established as 32.92 J/CAD at 10.55° after the top dead center with the 15% CRO-5% BU80% D2 sample (Figure 4). At full load condition, the maximum peak heat release rate was found to be 35.70 J/CAD at 6.33 CAD using the 15% CRO-5% BU-80% D2 blend
Figure 2. Peak cylinder pressure at different load conditions.
the D2 sample. The minimum peak cylinder pressure was found to be 124.07 bar at 5.98 CAD with the 20% CRO-80% D2 sample. This may be due to the presence of BU in the blends, which contains intrinsic oxygen. It may also be due its high density, 837.16 kg/m3, and CN, 50.6, which were higher than those of the 10% BU blend, 831.76 kg/m3 and 49.4, respectively (Table 3). The latter results in a strong premixed combustion phase and gives rise to higher gas pressure in the cylinder at full load with the 15% CRO-5% BU-80% D2 sample, followed by 10% CRO-10% BU-80% blend samples. The increase in the ignition delay with respect to the 10% CRO-10% BU-80% D2 and 15% CRO-5% BU-80% D2 samples also increases the amount of fuel burned within the premixed burning phase, causing high values of the peak pressure and rate of pressure rise (Figures 2 and 3). The high viscosity and presence of oxygen from CRO of 20% CRO-80% D2 (Table 3) are the reasons for its lower peak cylinder pressure because the fuel spraying is affected by the viscosity. Complete combustion depends to a large 4493
Energy Fuels 2010, 24, 4490–4496
: DOI:10.1021/ef100456a
Lujaji et al.
Figure 4. Peak heat release rates at different load conditions.
Figure 6. CO2 emissions at different load conditions.
Figure 5. Heat release rates at full load (100%) condition. Figure 7. THC emissions at different load conditions.
sample and 35.65 J/CAD at 6.68 CAD with the D2 sample. The minimum peak cylinder pressure was found to be 34.45 bar at 5.98 CAD with the 10% CRO-10% BU-80% D2 sample (Figure 4 and Figure 5). Blend fuels follow a trend similar to that of diesel fuel: the peak heat release value increases as the load increases, which is illustrated in Figure 4. It can be seen that the blends record an improvement in the heat release rate at the premixed combustion period. The presence of BU and oxygen in the blends decreases the CN of the blends and increases the ignition delay period. This is caused by high temperature and high cylinder pressure, better fuel-air mixing, and higher flame velocity associated with higher loads. A larger percentage of alcohol in the blend is the reason for slightly lower heat release rates at higher loads than that of the D2 sample because the samples containing alcohol recorded a low LHV value. D2 exhibits the highest LHV value and lower viscosity (Table 3): these two properties influence the heat release characteristics of the fuel.11,12 This could also be influenced by the fuel injection characteristics of the D2 sample because the engine fuel injection system is optimized for D2. A slight injection advance may occur for blends because of their properties and the speed of sound, which may alter the fuel injection system settings.23-25,34
Emissions. CO2. Fuel derived from vegetable oils reduces the overall CO2 emissions when it is used to run diesel engines because plants absorb CO2 during growth.2,35 Figure 6 shows CO2 emission results. It can be observed that the CO2 emission increases as the load increases at the same engine speed. At 0% load, 15% CRO-5% BU-80% D2 indicates the highest percentage of CO2 emissions of 2.84 vol % of the exhaust gas, whereas D2 and the rest of the blends record almost similar percentages of CO2 emission. At 100% load, it was also observed that 20% CRO-80% D2 indicates the maximum percentage of CO2 emission of 9.36 vol %, while the 10% CRO-10% BU-80% D2 blend evidences a minimum percentage of CO2 emission of 8.63 vol %. It can also be observed that, at maximum load, the CO2 percentage decreases as the amount of BU is increased in the blend. Oxygen present in vegetable oils and alcohol supports combustion and hence will lead to more CO2 emissions; more complete combustion leads to more CO2 emissions. This fact has been observed from the literature36 because diesel exhibits lower CO2 emissions. (35) Balat, M.; Balat, H. A critical review of bio-diesel as a vehicular fuel. Energy Convers. Manage. 2008, 49, 2727–2741. (36) Mbarawa, M. Performance, emission and economic assessment of clove stem oil-diesel blended fuels as alternative fuels for diesel engines. Renewable Energy 2008, 33 (5), 871–882.
(34) Tat, M.; Van Gerpen, J.; Soylu, S.; Canakci, M.; Monyem, A.; Wormley, S. The speed of sound and isentropic bulk modulus of biodiesel at 21 °C from atmospheric pressure to 35 MPa. J. Am. Oil Chem. Soc. 2000, 77 (3), 285–289.
4494
Energy Fuels 2010, 24, 4490–4496
: DOI:10.1021/ef100456a
Lujaji et al.
Figure 8. NOx emissions at different load conditions.
Figure 9. CO emissions at different load conditions.
THC. The results of various fuels’ THC emissions are shown in Figure 7. It can be observed that the THC emissions are at maximum levels upon idling, whereas lower THC ppm values are obtained at 50% and 75% brake loads at a steady engine speed. D2 releases low THC emissions at all load levels. The 15% CRO-5% BU-80% D2 blend records the highest value for THC emissions upon idling of 35.7 ppm, whereas D2 shows the lowest, at 75% load at a steady engine speed, 14.8 ppm. The reason for this is the slightly higher percentage of hydrogen and slightly higher viscosity values for the blends (Table 3). From the literature, it has also been observed that the presence of alcohol, in the blends, contributes to an increase in THC emissions. A similar trend was also observed from the literature, whereas high THC emission is observed at lower loads and maximum loads.29 NOx. Figure 8 shows NOx emissions versus loads. The results indicate a general increase in NOx emissions as the load increases at a steady engine speed. There is not much difference with regards to NOx emissions between blends and D2. Upon idling, 10% CRO-10% BU-80% D2 records minimum values of NOx emission of 36 ppm, whereas the rest of the blends possess values very close to that of D2. Upon 100% load, D2 reports a minimum value of NOx emission, 896 ppm, whereas 15% CRO-5% BU-80% D2 indicates a maximum value, 966 ppm. More NOx for blends is caused by increased velocity of sound in blends owing to their properties (Table 3) as opposed to D2; it slightly advances the injection timing, different from the manufacturer settings.24,25 The results conform to those in the literature.11,12 The presence of intrinsic oxygen contributes to the slightly increased NOx emissions for blends. Higher cylinder pressure also contributes to increased NOx emissions because of the increased peak combustion temperature at higher engine loads. Carbon Monoxide (CO). Figure 9 shows CO emissions of fuels at different loads. High CO emissions are observed on lower loads, with minimum emissions recorded at 75% engine load. The 15% CRO-5% BU-80% D2 blend indicates a maximum value of CO emission of 244 ppm upon idling, while D2 records a minimum CO emission, 148 ppm. Under full load (100%) conditions, the 20% CRO-80% D2 blend evidences the highest CO emission, 164 ppm. The results of these CO emissions agree with those reported from the literature;11 that is, for lower loads, CO emissions are increased because of incomplete combustion, whereas at
Figure 10. Smoke emissions at different load conditions.
100% load, CO emissions are slightly increased because of the local presence of a richer mixture in the combustion chamber.27 Smoke. Smoke emission results are shown in Figure 10. The 10% CRO-10% BU-80% D2 blend records a minimum value of 6 mg/m3, with a 20% CRO-80% D2 maximum (21 mg/m3) upon idle running conditions. Maximum smoke emission values are observed at 100% load, whereas D2 indicates a maximum value of 49 mg/m3 while 15% CRO5% BU-80% D2 shows a minimum value of 31 mg/m3 under the same load conditions. The presence of oxygen in the blends contributes to the lower values of smoke emissions for the alcohol blends.12,27 Conclusions The following conclusions can be deduced based on the results and discussion above: Fuel properties of vegetable oils are improved by the blending of vegetable oil, BU, and D2. Another significant improvement is in the phase stability of the resulting fuel blend. BU in the blends results in increased BSEC at higher engine loads; it also reduces the engine BTE. A small amount of BU in CRO-diesel blends results to a high cylinder pressure and 4495
Energy Fuels 2010, 24, 4490–4496
: DOI:10.1021/ef100456a
Lujaji et al.
emissions compared to that of D2 at all load conditions. D2 shows high smoke emissions at higher engine loads.
improved heat release rate as compared to diesel fuel. BU blends also result in reductions in emissions. CO2 levels at high loads are reduced. There is no noticeable improvement in the NOx emissions of CRO blends compared to D2. 20% CRO-80% D2 and BU blends show higher ppm values of CO
Acknowledgment. The authors gratefully acknowledge support from a Hungary/South Africa Research fund (Grant IUD 65429).
4496
