Comparative Study of the Effect of Biodiesel and Diesel Fuel on a

Energy & Fuels 2007, 21, 3627–3636

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Comparative Study of the Effect of Biodiesel and Diesel Fuel on a Compression Ignition Engine’s Performance, Emissions, and Its Cycle by Cycle Variations Muammer Özkan* Department of Mechanical Engineering, IC Engines Laboratory, Yıldız Technical UniVersity, 34349 Istanbul, Turkey ReceiVed January 10, 2007. ReVised Manuscript ReceiVed July 17, 2007

In this study, a compression ignition (CI) engine designed for diesel fuel was operated using biodiesel. No alteration was made to the fuel system or settings of the CI engine during tests. The changes in engine performance, emissions, and cycle by cycle (CBC) variations were observed, and their causes were studied. When biodiesel was used as the fuel, acceptable changes occurred in the performance values. The maximum brake mean effective pressure (BMEP) obtained with the biodiesel was 16% lower than that obtained with the diesel fuel, with the difference being 7.5% under maximum power. While biodiesel reduced the maximum engine power by 8.6%, it increased the brake specific fuel consumption by 9.6%. A comparison of exhaust emissions showed that CO emissions of biodiesel are lower than those of diesel fuel. The difference between the obtained minimum values was around 70%. In terms of hydrocarbon (HC) emissions, diesel fuel has produced better results than the biodiesel fuel. Biodiesel resulted in higher NOx emissions than diesel fuel when the engine operation range was considered. The difference was about 13–15% in the maximum power region. The coefficient of variation of ignition delay of both fuels had similar characteristics. However, due to the difference in the cetane numbers, the ignition delay period of biodiesel was longer. Changes of maximum cylinder pressure have occurred at the same magnitude for both fuels for the same engine speeds. The coefficient of variation of maximum cylinder pressure for both fuels had similar characteristics and considerably increased under maximum power conditions.

1. Introduction Beside the advantages, the widespread use of internal combustion engines has also certain problems. Decrease of fuel consumption, emissions, noise, and vibration has produced important research fields. The emission limits increasingly lowered to reduce air pollution caused by internal combustion engines and fluctuations in oil prices have lead researchers to conduct studies on alternative fuels which have an important effect on engine development.1,2 Since petroleum prices have increased rapidly, researchers have been working on alternative fuel sources. They produced different kinds of vegetable oil based fuels as alternative fuels for compression ignition (CI) engines. However, the idea of using vegetable oils as fuel for CI engines was not new. One hundred years ago, Rudolf Diesel tested vegetable oils as a fuel for his engine.3 Sunflower oil revealed similar fuel properties to diesel fuel, and promising results were obtained through the long term engine tests.4 Biodiesel as a vegetable oil, biodegradable and nontoxic, has low emission profiles, and hence, it is environmentally benefi* Corresponding author. Phone: +90 212 259 70 70 /2541. Fax: +90 212 261 66 59. E-mail: [email protected]. (1) Heywood, J. B. Internal combustion engine fundamentals; McGrawHill: New York, 1988. (2) Ferguson, C. R.; Kirkpatrick, A. T. Internal combustion engines, second ed.; John Wiley & Sons: New York, 2001. (3) Shay, E. G. Diesel fuel from vegetable oils: status and opportunities. Biomass Bioenergy 1993, 4, 227–242. (4) Karaosmanoglu, F.; Kurt, G.; Özaktas¸, T. Long term CI engine test of sunflower oil. Renewable Energy 2000, 19, 219–221.

cial.5 Due to its structural nature and carbon cycle, biodiesel is a fuel that does not contribute to the greenhouse effect.6 It has no aromatics and contains 10–11% oxygen by weight.7 The viscosity of vegetable oils is several times higher than that of diesel fuels, and the higher viscosity affects the flow properties of the fuel, such as spray atomization, consequent vaporization, and air–fuel mixing in the combustion chamber.8–10 Therefore, it is necessary to reduce the viscosity of vegetable oils in several ways, such as preheating, blending with diesel fuel, thermal cracking, and transesterification.8,9 With the decreasing viscosity of biodiesel, the injectors were observed to be cleaner. Additionally, no carbonization was observed on the surface of cylinder and piston heads.11 Biodiesel (5) Krawezyk, T. Biodiesel Alternative fuels makes inroads but hurdles remain. INFORM 1996, 7, 801–829. (6) Ahouissoussi, N. B. C.; Wetzstein, M. E. A comparative cost anaysis of biodiesel, compressed natural gas, methanol, and Diesel for transit bus systems. Resour. En. Economics 1997, 20, 1–15. (7) Canakci, M.; Erdil, A.; Arcakliog˘lu, E. Performance and exhaust emissions of a Biodiesel engine. Applied Energy 2005,Article In Press. (8) Ramadhas, A. S.; Muraleedharan, C.; Jayaraj, S. Performance and emission evaluation of a diesel engine fueled with methyl esters of rubber seed oil. Renewable Energy 2005, 30, 1789–1800. (9) Pramanik, K. Properties and use of jatropha curcas oil and diesel fuel blends in compression ignition engine. Renewable Energy 2003, 28, 239–248. (10) Sukumur, P.; Vedaraman, N.; Sankaranarayanan, G.; Boppana Bharat Ram, V. Performance and emission study of mahua oil (madhuca indica oil) ethy ester in a 4-stroke natural aspirated direct injection Diesel engine. Renewable Energy 2005, 30, 1269–1278. (11) Cetinkaya, M.; Ulusoy, Y.; Tekin, Y.; Karaosmanoglu, F. Engine and winter road test performances of used cooking oil originated Biodiesel. Energy ConVers. Manage. 2005, 46, 1279–1291.

10.1021/ef070013z CCC: $37.00  2007 American Chemical Society Published on Web 09/19/2007

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could be used directly in CI engines with no substantial modifications to the engine.12 Also, biodiesel contains no sulphur, and this is greatly advantageous when future environmental regulations are considered. Compared to petroleum-based diesel fuels, the high cost of biodiesel is a major barrier for its commercialization. It costs approximately 1.5 times the cost of petroleum-based diesel depending on feedstock oils.13 Therefore, the use of waste cooking oil should reduce the cost of biodiesel as a result of the recycling of raw materials, but waste cooking oil alone is not enough to reduce the costs. Thus, the farmers who raise oil crops (sunflower and rapeseed) should increase their crop capacity. In addition, biodiesel obtained from energy crops produces favorable effects on the environment, such as a decrease in acid rain and the greenhouse effect caused by combustion.13 Combustion of fossil fuels produces CO2, which accumulates in the atmosphere and causes many environmental problems. However, CO2 produced by the combustion of biofuels is absorbed by crops, and hence, CO2 levels are kept in balance.8,14 Therefore, the use of biodiesel in automobiles has been continuously increasing in recent years all over the world.15 Biodiesel has an approximately 12% lower heat value compared to fossil diesel fuels, and this has a significant effect on engine performance.16,17 Some studies showed that biodiesel causes a slightly higher exhaust temperature than diesel fuel.18–20 However, some other studies proved that the temperature of the biodiesel exhaust gas was lower than that of diesel fuel.21 The differences in the rate of flame development are particularly important in internal combustion engines, and they may cause cycle by cycle (CBC) variations, which reduce the total power as well as cause a reduction in fuel economy and engine reliability. The CBC variation also causes an increase in the exhaust emissions, noise, and engine roughness in addition to uneven torque from the engine. A substantial saving in fuels and reduction of engine vibrations would be obtained if these CBC variations could be controlled or eliminated, such that all (12) Laforgia, D.; Ardito, V. Biodiesel fueled IDI engines, performances emissions and heat release investigation. Bioresour. Technol. 1995, 51, 53– 59. (13) Zhang, Y.; Dube, M. A.; Mclean, D. D.; Kates, M. Biodiesel production from waste cooking oil: 2.Economic assessment and sensitivity analysis. Bioresour. Technol. 2003, 90, 229–240. (14) Al-Widyan, M.; Tashtoush, G. Abu-Qudais Moh’d, Utilization of ethyl ester of waste vegetable oils as fuel in Diesel engines. Fuel Process. Technol. 2002, 76, 91–103. (15) Carraretto, C.; Macor, A.; Mirandola, A.; Stoppato, A.; Tonon, S. Biodiesel as alternative fuel: Experimental analysis and energetic evaluations. Energy 2004, 29, 2195–2211. (16) Antolin, G.; Tinaut, F. V.; Briceno, Y.; Castano, V.; Perez, C.; Ramirez, A. I. Optimization of biodiesel production by sunflower oil transesterificition. Biores. Technol. 2002, 83, 111–114. (17) Özkan, M.; Ergenç, A. T.; Deniz, O. Experimental performance analysis of Biodiesel Diesel fuel and Biodiesel with glycerine. Turkish J. Eng. EnViron. Sci. 2005, 29, 89–94. (18) Tashtous, G.; Al-Widyan, M. I.; Al-Shyoukh, A. O. Combustion performance and emissions of ethyl ester of a waste vegetable oil in a watercooled furnace. Appl. Therm. Eng. 2003, 23, 285–293. (19) Usta, N.; Öztürk, E.; Can, Ö.; Conkur, E. S.; Nas, S.; Çon, A. H.; Can, A. Ç.; Topçu, M. Combustion of Biodiesel fuel produced from hazelnut soapstock/waste sunflower oil mixture in a Diesel engine. Energy ConVers. Manage. 2005, 46, 741–755. (20) Nwafor, O. M. I.; Rice, G.; Ogbonna, A. I. Effect of advanced injection timing on the performance of rap seed oil in diesel engines. Renewable Energy 2000, 21, 433–444. (21) Rakopoulos, C. D.; Antonopoulos, K. A.; Rakopoulos, D. C. Development and application of multi-zone model for combustion and pollutants formation in direct injection diesel engine running with vegetable oil or its bio-diesel. Energy ConVers. Manage. 2007, 48, 1881–1901.

Özkan Table 1. Specifications of Test Fuels ester content density viscosity flash point sulphur cetane number water content total contamination LHV

% (m/m) @15 °C, kg/m3 @40 °C, m2/s °C mg/kg mg/kg mg/kg MJ/kg

biodiesel

diesel fuel

96.5 883.9 4.678 178 3.3 51.3 250 3.0 37.2

831.8 3.48 76 38.9 65.5 200 2.89 42.1

Table 2. Technical Specifications of the Test Engine 1753 cm3 4 82 mm 82,5 mm 21.5:1 140 Nm 44 kW

total displacement number of cylinders stroke bore compression ratio max torque (at 2000 rpm) max power (at 3500 rpm)

cycles were the same and optimum.22 A coefficient has been introduced to numerically express the CBC variation. This coefficient named the coefficient of variation (COV) has been defined as the ratio of the multiplication of values such as ignition delay period and maximum cylinder pressure measured in consecutive cycles with standard deviation determined by the number of cycles to the total sum of the values measured. The purpose of this study is to observe the effect of the use of biodiesel as a fuel on the performance and CBC variations of an engine provided that all hardware and settings of the engine designed for use with diesel fuel remain the same. For maximum cylinder pressure and ignition delay, the coefficient of variation was calculated per the following formula (1).22 COV )

σnn Σ x

(1)

n

The ideal operation condition is when the same value is obtained for parameters used in determining the CBC variation in consecutive cycles, in which circumstance the standard deviation is equal to zero. In this ideal circumstance without cyclic variation, COV will also be equal to zero. Therefore, in terms of the cyclic variation, it is desired that COV be as close to zero as possible. 2. Materials and Experimental Setup In the experimental study, biodiesel per EN14214 and diesel fuel per EN590 were used. The specifications for these test fuels are given in Table 1. A four-cylinder, four-stroke, turbocharged IDI diesel engine whose characteristics are listed in Table 2 was used in the experiments. The test bed included a water-cooled hydraulic dynamometer, measurement instruments, and a control panel. The dynamometer load was measured by using a load cell. An inductive speed sensor was employed to measure the engine speed. The air consumption was measured by a mass air flow sensor. The fuel consumption was determined by fuel flow meter. Air inlet, fuel, engine coolant, and exhaust temperatures were measured by K type thermocouples. An AVL 8QP500c quartz pressure transducer and a charge amplifier were utilized to measure the cylinder pressure. A Krom Shröder RGA 33 has been used to measure the exhaust (22) Beshai, S.; Deniz, O.; Chomiak, J.; Gupta, A. An experimental study of the variations in cyclic energy release rate in a spsrk ignition engine. AIAA/ASME/SAE/ASEE 25th Joint Propulsion Conference, Monterey, CA, July 10–12, 1989.

ComparatiVe Study on Biodiesel and Diesel Fuels

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Figure 1. Experimental setup: (1) Diesel engine, (2) turbocharger, (3) cooler, (4) hydrokinetic dynamometer, (5) incremental encoder, (6) load cell, (7) cylinder pressure sensor, (8) charge amplifier, (9) fuel tank, (10) fuel flow meter, (11) mass air flow sensor, (12) gas analyzer, (13) thermocouple (air temperature), (14) thermocouple (air inlet temperature), (15) thermocouple (engine temperature), (16) thermocouple (coolant inlet temperature), (17) thermocouple (coolant output temperature), (18) data logger, (19) computer, (20) air inlet, (21) exhaust outlet.

emissions. A data logger collected the results. The experimental setup is shown in Figure 1.

3. Test Procedure Before the data collection, the engine was started with the reference certified desel fuel and warmed up. The warm-up period ended when the coolant temperature was stabilized. During all tests, the injection timing was not altered and the rack position was maintained at full open throttle. The data was collected at seven load conditions. A 160 kW hydrokinetic dynamometer was used to load the engine. The measured performance results were the engine speed, brake torque, and fuel consumption. The results were converted to the standard conditions in accordance with the DIN 70020 power correction standard. The temperatures were measured at six different points: exhaust, coolant, fuel, air inlet (before and after the turbocharger), and ambient temperature. In addition, the air mass flow rate and exhaust emissions were also measured. An incremental encoder and a pressure transducer were used for the cylinder pressure analysis. A data logger collected the cylinder pressure variations. About 200 cycles of cylinder pressure variations were measured per one-half crank angles for each loading condition. Cylinder pressure variations resulting from crank angles for each cycle were analyzed. Then, the maximum cylinder pressure and ignition delay probability graphs and variation coefficient graphs were plotted against engine speed. 4. Results and Discussion 4.1. Engine Performance. 4.1.1. Brake Mean EffectiVe Pressure and Brake Power. The tests and data collection have been performed at seven different engine speeds for each fuel. Brake mean effective pressure and brake power versus engine speed graphs with biodiesel and the diesel fuel are given in Figures 2 and 3, respectively. Figure 2 shows the variation of brake mean effective pressure with engine speed in the wide open throttle (WOT) condition. The maximum brake mean effective pressure values are about 9.47 bar at 2000 rpm for the DF and 8 bar at 2500 rpm for the biodiesel. The experimental results with biodiesel showed significant reduction in brake mean effective pressure values between 1500 and 3500 rpm, but above 3500 rpm, the brake mean effective pressure values were nearly the same. While the highest mean effective pressure was obtained for diesel fuel

Figure 2. Variation of brake mean effective pressure versus engine speed.

and biodiesel at engine speeds of 2000 and 2500, respectively, the brake mean effective pressure was at the lowest at 1000 and 4000 rpm, respectively, for the two types of fuel. This happened because at high engine speed the time left for combustion gets shorter while the mixture composition is violated at low speeds due to insufficient air motion in the cylinder. Both effects decrease the combustion efficiency. In general, the brake mean effective pressure values of the diesel fuel are greater than biodiesel. The maximum brake power values of biodiesel and the diesel fuel were obtained at 3500 rpm. Figure 3a indicates that the diesel fuel produced higher power in general. The limited brake mean effective pressure during biodiesel use puts a limitation on the brake power as well (Figure 3b). The heat value of biodiesel is lower than that of diesel fuel. This explains the lower power performance of biodiesel compared to diesel fuel. Approximately an 8.6% power loss occurred with biodiesel at the maximum brake power output condition. 4.1.2. Fuel Consumption and Thermal Efficiency. The variations of brake specific fuel consumption (BSFC) and thermal efficiency of these fuels are presented in Figures 4 and 5, respectively. The BSFC graphs show similar behavior. The BSFC at the maximum power was 316.7 g/(kW h) with the diesel fuel and 352.1 g/(kW h) with biodiesel. BSFC increase rates have been calculated as 11.2% and 10.6% for maximum and minimum power, respectively. Figure 4a showed that, for both fuels, BSFC assumes a smaller value at low engine speeds compared to high ones. The difference between the BSFC values of two fuels at the highest


Özkan

Figure 3. (a) Variation of brake power values versus engine speed. (b) Variation of brake power values versus brake mean effective pressure.

Figure 4. (a) Variation of specific fuel consumption values versus engine speed. (b) Variation of specific fuel consumption values versus brake mean effective pressure.

Figure 5. (a) Variation of thermal efficiency of the fuels versus engine speed. (b) Variation of thermal efficiency of the fuels versus brake mean effective pressure.

brake effective pressure was determined as 15%. On the other hand, the BSFC value for the close brake mean effective pressure was obtained at high and low engine speeds for diesel and biodiesel fuels, respectively. While an increase in BSFC has been observed for both fuels during the engine speed increase at a fixed brake mean effective pressure, a faster BSFC increase was observed for biodiesel. At a brake mean effective pressure of 7 bars, BSFC values were 295 and 320 g/(kW h) for diesel fuel and biodiesel fuels, respectively. At a higher engine speed, this value increased to 325 for diesel and 365 g/(kW h) for biodiesel with the increased rates for diesel fuel and biodiesel fuels being 10% and 14.6%, respectively (Figure 4b). The higher BSFC value of biodiesel was the result of a lower calorific value. On the basis of BSFC and calorific value calculations, the variation of thermal efficiency values of both fuels showed a

similar behavior. The maximum thermal efficiency provided was about 30% (Figure 5a). Figure 5b shows that thermal efficiency values for both fuels are close at the same brake mean effective pressure and tend to decrease as the engine speed increases. 4.1.3. Emissions. The variation of excess air value based on the engine speed and brake mean effective pressure has been given in Figure 6. Under the WOT operating condition, high excess air value has been observed for low and high engine speed varying with the load while low excess air value has been observed for high brake mean effective pressure. For biodiesel, the excess air value has been higher under all operating conditions than that of the diesel fuel. This is caused by the high oxygen content in



Figure 6. (a) Variation of excess air value versus engine speed. (b) Variation of excess air value versus brake mean effective pressure.

Figure 7. (a) Variation of CO concentration versus engine speed. (b) Variation of CO concentration versus brake mean effective pressure. (c) Variation of CO concentration versus excess air value.

biodiesel, which accounts for a lower calorific value of biodiesel compared to diesel fuel. Considering Figures 2 and 6 together, it can be concluded that suitable conditions for combustion are satisfied at middle rotation speeds, during which the excess air value approaches the stoichiometric ratio and the brake mean effective pressure increases as a result of better combustion. The increase in brake mean effective pressure indicates good combustion. When brake mean effective pressure increases and the excess air value decreases, the decrease in the amount of free oxygen in the exhaust gas because the oxygen in the combustion chamber reacts to produce CO2 as a result of good combustion causes the excess air value to decrease. 4.1.4. Carbon Monoxide and Carbon Dioxide Emission. Figure 7 shows the variation of CO emission values depending on engine speed, brake mean effective pressure, and excess air

value for the experimental fuels. The experimental results with biodiesel showed significant reductions in CO emissions. The minimum CO emissions were at about 4000 rpm for each fuel (Figure 7a). The fuels produced low CO emissions at low load levels and more emissions at high load conditions. The CO emission values were measured as 0.075 and 0.023 vol % for diesel fuel and biodiesel, respectively, under the maximum power conditions. This corresponds to a 69% decrease in the CO emissions. The oxygen content of the biodiesel increases the oxygen concentration in the combustion chamber. Therefore, CO emission of biodiesel drops. This is typical with all CI engines, since the air–fuel ratios decrease with an increase in load. It can be observed in Figure 7b and c that, for the same brake mean effective pressure and excess air value, biodiesel produces lower CO emission. This shows that a large amount


Özkan

Figure 8. (a) Variation of CO2 concentration versus engine speed. (b) Variation of CO2 concentration versus brake mean effective pressure. (c) Variation of CO2 concentration versus excess air value.

of carbon in the structure of biodiesel oxidates to CO2 during the oxidation process. Figure 8 shows the variation of CO2 values with engine speed, brake mean effective pressure, and excess air value for the experimental fuels. The maximum CO2 emissions were at about 3000 rpm with diesel fuel and 3500 rpm with biodiesel. Although the CO2 variations show similar characteristics for both fuels, the values obtained with diesel fuel are higher than the values obtained with biodiesel. The CO2 emission indicates the combustion quality, and it is preferable to have it as high as possible. In the study with biodiesel, the CO2 emissions at low levels are accompanied by an increase in the HC emissions. From Figure 8b, it is possible to conclude that, for the same brake mean effective pressure, diesel fuel produces better combustion under all speed conditions. At excess air value levels close to the stoichiometric ratio, the CO2 concentration in the exhaust gas has increased for both types of fuel. Emission values of diesel fuel are higher than those of biodiesel. The high viscosity and density of biodiesel adversely affects the CO2 emission. This is the reason for the difference between biodiesel and diesel fuels in terms of CO2 emissions (Figure 8c). However, it should be taken into consideration that the same excess air values are obtained at different engine speeds which implies that the air motion conditions inside the cylinder, which considerably affect combustion and emissions, also vary. 4.2. Hydrocarbon Emission. The results of HC emissions are presented in Figure 9. The HC emissions for both fuels were at low level because the diesel engines were operated at higher excess air value. HC emissions increased at low load and high engine speed conditions. The increments of HC emission of biodiesel were more than HC emission of diesel fuel. In different

studies the combustion rate of biodiesel is revealed being lower than that of diesel fuel.19,20 Low combustion rate of biodiesel vis a vis diesel fuel shifts the combustion towards the exhaust phase under high speed and hence short cycle time conditions. This results in incomplete combustion that leads to increased exhaust emissions. On the other hand, with increasing kinematic viscosity unburnt HC emissions increase.23 The kinematic viscosity of biodiesel used in the study is higher than that of diesel fuel. This has been one of the causes of the increase in HC in the exhaust with the engine running on biodiesel. It has been observed that, as the percentage of biodiesel increases in a mixture of diesel fuel and biodiesel, the HC emission considerably decreases.24 4.2.1. Oxides of Nitrogen Emission. Figure 10 shows the variation of NOx with engine speed, brake mean effective pressure, and excess air value for diesel fuel and biodiesel. The maximum NOx observed was 639 ppm at 4000 rpm for biodiesel and 557 ppm at 3500 rpm for diesel fuel (Figure 10a). At all measurement points, biodiesel produced more NOx emissions than diesel fuel. Although the difference between NOx emissions is higher at low speed conditions, it varies between 4 and 50% within the engine operation range. This is due to the high oxygen content of biodiesel. The increase of the oxygen concentration within the combustion chamber increases the NOx emission. (23) Miyamoto, N. Description of diesel emissions by indiVidual fuel properties, SAE Paper No. 922221; Society of Automotice Engineers: Warrendale, PA, 1992. (24) Rakopoulos, C. D.; Rakopoulos, D. C.; Hountalas, D. T.; Giakoumis, E. G.; Andritsakis, E. C. Performance and emissions of bus engine using blends of diesel fuel with bio-diesel of sun flower or cottonseed oils derived fron Greek feedstock. Fuel, May 21, 2007, http://dx.doi.org/10.1016/ j.fuel.2007.04.011.



Figure 9. (a) Variation of HC concentration versus engine speed. (b) Variation of HC concentration versus brake mean effective pressure. (c) Variation of HC concentration versus excess air value.

Figure 10. (a) Variation of NOx concentration versus engine speed. (b) Variation of NOx concentration versus brake mean effective pressure. (c) Variation of NOx concentration versus excess air value.


Özkan

Figure 11. Probability changes of ignition delay at 2000 rpm.

Figure 12. Probability changes of ignition delay at 3500 rpm.

Under operating conditions with excess air values between 1.13 and 1.17 for diesel fuel and between 1.23 and 1.27 for biodiesel NOx emissions of both fuel types were at the highest level (Figure 10b). In these ranges, the value of brake mean effective pressure obtained for the two fuels will be maximum. This is caused by the good combustion of the fuels as a result of the excess air value approaching the stoichiometric ratio (Figure 10c). 4.3. Cycle by Cycle Variation. 4.3.1. Ignition Delay. The data collected at 2000 and 3500 rpm engine speeds were used for comparison of CBCs of fuels. Figures 11 and 12 indicate that, while the ignition delay curve characteristics of both fuels are similar, biodiesel, which has a lower cetane number than diesel, generally operates with a higher ignition delay. Characteristics of the fuels used for the study indicate that biodiesel evaporates less than diesel fuel. The volatility directly affects the combustion rate. This result indicated that the combustion rate had a significant effect on exhaust gas temperatures. The coefficient of variation of ignition delay characteristics of the fuels used in the study based on the engine speed are similar (Figure 13a). Despite assuming the ignition delay to be a constant value in studies relating to engine cycles, the inequality of the coefficient of variation of ignition delay to zero increases the absolute error where the ignition delay is expressed in the form of a certain constant value. Therefore, the standard deviation of the engine cycles analysis should be determined by conducting a sufficient number of cycle analyses or the cycle being analyzed should be identified to represent the majority through the determination of its occurrence percentage. While the coefficient of variation of ignition delay of diesel fuel assumes its highest level at a low engine speed, this coefficient

is also at its lowest at the highest engine speed. At 3500 rpm, where maximum power has been obtained for both types of fuel, the coefficient of variation of ignition delay is at a high level. The coefficient of variation of ignition delay varies between 0.1 and 0.36 for diesel fuel and between 0.17 and 0.24 for biodiesel (Figure 13b). The values calculated as 0.24 at maximum brake mean effective pressure for biodiesel and 0.24 at maximum brake mean effective pressure for diesel are considerably close to each other. The minimum values obtained under maximum power conditions indicate that it is possible to decrease the mean value of cycle by cycle variation. In CI engines, the ignition delay is desired to be as short as possible. This will allow avoiding diesel knock and high concentration of NOx gathering at the combustion chamber during the ignition delay, caused by the sudden pressure and temperature increase during the combustion phase by diffusion. 4.3.2. Cylinder Pressure. The probability changes of maximum cylinder pressure at 2000 and 3500 rpm were given in Figures 14 and 15, respectively. The maximum cylinder pressure of diesel fuel was higher than that of biodiesel. A higher calorific value and higher combustion speed caused higher cylinder pressure. In addition, the density of biodiesel was higher than diesel fuel’s density. Therefore, the rate of energy per cycle was lower than the rate of calorific values and this was the reason why the rate of peak pressure was also lower than the rate of calorific values. When diesel fuel is substituted by biodiesel, the maximum pressure most probable to occur at 2000 rpm decreases by around 5.2% and the maximum pressure most probable to occur at 3500 rpm decreases by 6.4%. From among the data obtained at the engine speed of 2000 rpm, the magnitude of the change in the maximum cylinder pressure has been around 3 bars for both fuel types. This value is around 5 bars for both fuel types at 3500 rpm, where the maximum power is obtained. The coefficient of variation of maximum cylinder pressure was given in Figure 16 for different engine speeds and brake mean effective pressures. The coefficient of variation of maximum cylinder pressure has generally been at low and close values for different engine speeds and brake mean effective pressures. However, it visibly increased under the maximum power condition for both fuel types (Figure 16). Hints of this increase can be found in the slope of the curves appearing in Figures 14 and 15. The decrease in the slope of the curves indicates that the cycles occur at a wide range with different maximum pressures. The variation of the maximum cylinder pressure will lead to variation of the maximum cylinder temperature. During cycles when the pressure and the temperature exceeded nominal values, emissions of NOx increased while emissions of CO and HC decreased. During cycles when



Figure 13. (a) Coefficient of variation of ignition delay versus engine speed. (b) Coefficient of variation of ignition delay versus brake mean effective pressure.

Figure 14. Probability changes of maximum cylinder pressure at 2000 rpm.

Figure 15. Probability changes of maximum cylinder pressure at 3500 rpm.

the pressure and the temperature stayed below nominal values, the above statement relating to the emissions worked in the opposite way. However, vibration occurs in the engine in both situations, with a negative impact on the performance and comfort. 5. Conclusions In this study, biodiesel was tested and compared to diesel fuel in a four-cylinder, four-stroke, turbocharged CI engine. The engine was operated with same settings for both fuel types

during the experiments, and no alteration has been made in the fuel system elements. The purpose was to determine and evaluate variations in performance, emission, and motor cycles where an engine designed for diesel fuel is run on a biofuel of known characteristics. It was found that with biodiesel the engine operated smoothly without notable problems. Compared to the diesel fuel, an approximately 8.6% power loss occurred with biodiesel at maximum brake power output condition. The observed maximum brake mean effective pressure value of biodiesel was 16% less than that for diesel fuel operations. However, the minimum break specific fuel consumption value of biodiesel was measured at 9.6% more than that of diesel fuel operations. In general, the performance characteristics of biodiesel were closer to those of diesel fuel. Using biodiesel, the CO concentration decreases considerably. In the maximum power region, the difference between CO concentrations of diesel fuel and biodiesel was about 69%. Also, similar values were gathered for the whole engine operation range. Similarly CO2 concentration of biodiesel has been measured to be lower than that of diesel fuel within the engine operation range. This indicates a better combustion of diesel fuel. The reversal of carbon compounds, which are formed by combustion, to their original source determines their atmospheric stay and influence durations. This is called the life cycle. Since the life cycle of the CO2 produced by fossil fuels is quite long, it leads to CO2 accumulations in the atmosphere and results in significant environmental problems. However, the life cycle of


Özkan

Figure 16. (a) Coefficient of variation of maximum pressure versus engine speed. (b) Coefficient of variation of maximum pressure versus brake mean effective pressure.

CO2 caused by biodiesel fuels is significantly shorter than that of diesel fuel. This helps maintain atmospheric balance by eliminating the problems of fossil fuels.8,14 Biodiesel is a renewable energy because it transforms itself into a reusable raw material in a short period of time. Diesel fuel yielded better results in terms of HC emissions. While there were insignificant differences at lower speeds, the differences at higher speeds were around 35%. The increasing HC concentration of biodiesel at higher engine speed is caused by a lower combustion rate. Another factor behind the increase of HC emission in biodiesel is its higher kinematic viscosity compared to that of diesel fuel. The difference between the NOx concentration levels of both fuels ranges from 4% to 50%. At every measurement taken, biodiesel emitted higher NOx than diesel fuel. Nevertheless with increasing engine speed, the difference between the NOx emissions of both fuels went down. The high oxygen content of biodiesel furthers the oxygen concentration of the combustion chamber and the flame temperature. This situation leads to the increases in NOx reaction rate and NOx concentration. With increasing engine speed, biodiesel’s low combustion rate and lowered excess air coefficient reduces the amount of atmospheric N2 and hence NOx formation. Biodiesel has a lower cetane number, which accounts for its operation with a longer ignition delay compared to diesel fuel. The studies conducted have shown that the evaporation percentage of biodiesel based on the crank angle is lower than that of diesel fuel.21 Therefore, the combustion rate of biodiesel is lower

than that of diesel fuel. This is due to the physical characteristics of biodiesel. The difference between the maximum cylinder pressures is an indicator of power loss. When biodiesel is used to replace diesel fuel, the decrease in the maximum cylinder pressure most probable to occur is 5.2% at 2000 rpm and 6.4% at 3500 rpm. The difference between the minimum and maximum values of the maximum cylinder pressure at a constant engine speed did not vary depending on the type of fuel. The difference between the minimum and maximum values of the maximum cylinder pressure was 3 bars at an engine speed of 2000 rpm and 5 bars at 3500 rpm. Shrinkage of this range indicates stable engine operation. Ideally, this range should be equal to zero, in which case the coefficient of variation of maximum pressure will also be equal to zero. The similarity of coefficients of variation means that biodiesel would be an alternative fuel for a diesel engine with respect to cycle-by-cycle variation. Biodiesel represents one of the best alternatives as a renewable fuel for diesel engines from economic, energy, and environmental protection perspectives. In general, the findings of this work clearly indicate that biodiesel is a potential candidate as a fuel for diesel engines. Nomenclature

σn ) standard deviation x ) value of parameter n ) number of cycle analyzed EF070013Z

Comparative Study of the Effect of Biodiesel and Diesel Fuel on a

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