Experimental Studies of a Naturally Aspirated, DI Diesel Engine

Energy Fuels 2010, 24, 652–663 Published on Web 12/07/2009

: DOI:10.1021/ef900814r

Experimental Studies of a Naturally Aspirated, DI Diesel Engine Fuelled with Ethanol-Biodiesel-Water Microemulsions Dong Hui Qi,*,† Hao Chen,† Chia Fon Lee,‡ Li Min Geng,† and Yao Zhang Bian† †

School of Automobile, Chang’an University, Xi’an, China and ‡Department of Mechanical Science and Engineering, the University of Illinois at Urbana-Champaign, Urbana, Illinois Received July 30, 2009. Revised Manuscript Received November 15, 2009

The main objective of this paper is to study the performance, emissions, and combustion characteristics of the diesel engine using ethanol-biodiesel-water microemulsions. The results indicated that although the microemulsions exhibited similar combustion stages to that of biodiesel, some differences were observed between biodiesel and microemulsions. Biodiesel showed an earlier start of combustion at all engine operating conditions, while the ignition delays of the microemulsions were longer. The peak cylinder pressures of the microemulsions were lower at low engine loads, and almost similar to that of biodiesel at medium and high engine loads. The peaks of heat release rate of the microemulsions were almost similar to that of biodiesel at low engine loads, but slightly higher at medium and high engine loads. The combustion durations of the microemulsions were longer at low engine loads and speed, but shorter at medium and high engine loads. For the microemulsions, there were slightly higher brake specific fuel consumptions (BSFC), while lower brake specific energy consumptions (BSEC). Drastic reduction in smoke was observed with the microemulsions at high engine loads. Nitrogen oxides (NOx) emissions were found slightly lower under almost all engine operating conditions for the microemulsions.

without engine modification.7-9 The increase in NOx emission is the major impediment to widespread use of biodiesel. To reduce this adverse effect, investigations have been carried out on different approaches for reducing NOx emission when biodiesel is used. Fernando et al.10 reviewed the NOx reduction methods for biodiesel fuels. They concluded that the thermal NOx mechanism is the major contributor to NOx emission, thus NOx can be reduced through the application of water injection, water emulsified biodiesel, ignition timing retardation, or exhaust gas recirculation, which can lead to reduction in flame temperature. Kumar et al.11,12 applied methanol in jatropha oil and ethanol in animal fat to a diesel engine. The results showed that drastic reduction in smoke, NOx, HC and CO emissions were observed with the emulsion as compared to neat fat at high power outputs. Prommes et al.13 studied the phase diagram of dieselbiodiesel-ethanol blends at different purities of ethanol and different temperatures, examined the fuel properties of the selected blends and their emissions performance in a diesel

Introduction Oxygenated fuels are known to reduce particulate matter for motor vehicles and have been evaluated as potential sources of renewable fuels. Among the alternative fuels, biodiesel and ethanol have been proposed as alternatives for internal combustion engines.1-5 In particular, biodiesel has received wide attention as a replacement for diesel fuel because it is biodegradable, is nontoxic, and can reduce overall life cycle emission of carbon dioxide (CO2) from the engine.6 The high oxygen content in biodiesel results in the improvement of its burning efficiency, as well as the reduction of particulate matter (PM), carbon monoxide (CO), and hydrocarbon (HC), but at the same time produces larger nitrogen oxides (NOx). It is estimated that the burning of neat biodiesel would produce higher NOx than that of petroleum-based diesel *To whom correspondence should be addressed. Telephone: 86-2982334784. Fax: 86-29-82334476. E-mail: [email protected]. (1) Agarwal, A. K. Biofuels (alcohols and biodiesel) applications as fuels for internal combustion engines. Prog. Energy Combust. Sci. 2007, 33 (3), 233–271. (2) Demirbas, A. Progress and recent trends in biofuels. Prog. Energy Combust. Sci. 2007, 33 (1), 1–18. (3) Korres D. M.; Painesaki Arg.; Karonis D.; Lois E.; Kalligeros S. Use of Ethanol along with Biodiesel in Diesel and Jet Fuels on a Stationary Diesel Engine; SAE 2005-01-3676, Society of Automotive Engineers: Warrendale, PA, 2005. (4) Masuda, Y.; Chen, Z. Combustion and Emission Characteristics of a PCI Engine Fueled with Ethanol-Diesel Blends; SAE 2009-01-1854, Society of Automotive Engineers: Warrendale, PA, 2009. (5) Kim, H.; Choi, B. The effect of biodiesel and bioethanol blended diesel fuel on nanoparticles and exhaust emissions from CRDI diesel engine. Renewable Energy 2010, 35 (1), 157–163. (6) Cvengros, J.; Povazanec, F. Production and treatment of rapeseed oil methyl esters as alternative fuels for diesel engines. Bioresour. Technol. 1996, 55 (2), 145–152. (7) Hess, M. A.; Haas, M. J.; Foglia, T. A.; Marmer, W. N. Effect of antioxidant addition on NOx emissions from biodiesel. Energy Fuels 2005, 19 (4), 1749–1754. r 2009 American Chemical Society

(8) Sharp, C. A.; Howell, S. A.; Jobe, J. The Effect of Biodiesel Fuels on Transient Emissions from Modern Diesel Engines-Part I: Regulated Emissions and Performance; SAE 2000-01-1967; Society of Automotive Engineers: Warrendale, PA, 2000. (9) Schumacher, L. G.; Borgelt, S. C.; Fosseen, D.; Goeta, W. Heavyduty engine exhaust emissions tests using methyl ester soybean oil/diesel fuel blends. Bioresour. Technol. 1996, 57 (1), 31–36. (10) Fernando, S.; Hall, C.; Jha, S. NOx reduction from biodiesel fuels. Energ Fuels 2006, 20 (1), 376–382. (11) Kumar, M. S.; Ramesh, A.; Nagalingam, B. An experimental comparison of methods to use methanol and jatropha oil in a compression ignition engine. Biomass Bioenergy 2003, 25 (3), 309–318. (12) Kumar, M. S.; Kerihuel, A.; Bellttre, J.; Tazerout, M. Ethanol animal fat emulsions as a diesel engine fuel;Part 2: Engine test analysis. Fuel 2006, 85 (17-18), 2646–2652. (13) Prommes, K.; Apanee, L.; Samai, J. I. Solubility of a dieselbiodiesel-ethanol blend, its fuel properties, and its emission characteristics from diesel engine. Fuel 2007, 86 (7-8), 1053–1061.

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engine and compared to those of base diesel. They concluded that a blend of 80% diesel, 15% biodiesel, and 5% ethanol was the most suitable ratio because of the acceptable fuel properties and the reduction of emissions. Jha et al.14 studied the emission characteristics of diesel-biodiesel-ethanol (DBE) fuel blends on one used engine and two new engines. The results with DBE showed a significant reduction in NOx emissions in new engines with increased ethanol concentration, whereas with the used engine under similar conditions, an increased NOx emissions profile was observed. Park et al.15 studied the atomization of ethanol biodiesel blends and concluded that the ethanol addition improved the atomization performance of biodiesel because the ethanol blended fuel has a low kinematic viscosity and surface tension, then that has more active interaction with the ambient gas, compared with biodiesel. Shi et al.16 carried on experimental test to reduce NOx and particulate matter (PM) emissions from a diesel engine using both ethanol-selective catalytic reduction (SCR) of NOx over an Ag/Al2O3 catalyst and a biodiesel-ethanol-diesel fuel blend (BE-diesel). They concluded that the use of BE-diesel increased PM emissions by 14% because of the increase in the soluble organic fraction (SOF) of PM, but it greatly reduced the Bosch smoke number by 60%-80% according to the results from 13-mode test of European Stationary Cycle (ESC) test compared with diesel. The SCR catalyst was effective in NOx reduction by ethanol, and the NOx conversion was approximately 73%. The emulsification technique is being applied to reduce NOx emission and to promote the combustion efficiency for fossil fuels 17. Hence, emulsions of biodiesel are considered to curtail emissions of NOx from burning biodiesel. Masjuki et al.18 studied the performance of palm oil methyl esters and diesel emulsions containing 5 and 10% of water by volume in an Isuzu 4FB1 horizontally arranged four-cylinder indirect injection engine. They concluded that the emulsified fuels showed a lower tendency to NOx formation because the increment of water content in diesel and palm oil methyl esters fuels vaporizes and absorbs more energy from the surrounding air because of its relatively high specific heat value, and therefore, resulting in lower combustion temperatures. Samec et al.19 concluded that blending water into diesel fuel lowers the flame temperature and thereby decreases the emissions of the NOx. Manuel et al.20 studied the impact of water in diesel fuel microemulsions on CIDI engine performance and exhaust emissions.

Results indicated that NOx, PM, and CO can be significantly reduced because of water in diesel fuel microemulsions. Fernando et al. 21 determined the relative compatibilities of ethanol, biodiesel, and diesel fuel. They revealed that ethanol-biodiesel-diesel (EB-diesel) fuel blend microemulsions are stable well below subzero temperatures and have shown equal or superior fuel properties to regular diesel fuel. Despite ethanol having a considerably lower energy value, cetane number, and lubricity value than biodiesel or diesel fuel alone, the heat of combustion and cetane numbers of the EBdiesel blends remained steady, without significant reduction. The present study aims to investigate the performance and combustion behaviors of the ethanol-biodiesel-water microemulsions at different engine operating conditions and to compare them with the baseline data of biodiesel in an unmodified diesel engine. Equipment and Experiments Microemulsion Preparation. The microemulsion process consisted of introducing the chosen quantity of surfactant, cosurfactant, and water into the biodiesel. It has been reported that microemulsions are instantaneously formed when all the components are put together in required proportions.22 Biodiesel is defined as mono alkyl esters of long-chain fatty acids derived from renewable feed stocks, such as vegetable oils and animal fats, or other triglyceride-bearing biomass, such as microalgae. Biodiesel’s production includes the transesterification stage that is followed by separation and evaporation stages. Any material that contains triglycerides can be used as raw material for this production. In this study, the biodiesel produced from soybean crude oil was prepared by a method of alkaline-catalyzed transesterification. The aim of surfactant addition was to reduce oil and water superficial tension, activated their surfaces and maximized the superficial contact area to make microemulsions.23 The surfactant used in this study was Span 80. Span 80 has a HLB number equal to 4.3. This surfactant was more lipophilic than hydrophilic and hence appropriated for making water-in-oil emulsions. The cosurfactant permitted to improve the migration of surfactant and enhanced the stability of the microemulsion.24,25 Ethanol is produced through distillation of a liquid product coming from fermentation of sugars or lignocelluloses containing biomass. The type of biomass that is used as raw material affects the yield ratio of each production process. The fermentation stage is followed by distillation and separation stages. In this study, ethanol was chosen as cosurfactant because it was entirely miscible with biodiesel at ambient temperature.26 Since ethanol has lower cetane number and heating value, the amount of ethanol added with the biodiesel was limited to a maximum of 20% by volume to prevent combustion troubles.

(14) Jha, S. K.; Fernando, S.; Columbus, E.; Willcutt, H. A comparative study of exhaust emissions using diesel-biodiesel-ethanol blends in new and used engines. Trans. ASABE 2009, 52 (2), 375–381. (15) Park, S. H.; Suh, H. K.; Lee, C. S. Nozzle flow and atomization characteristics of ethanol blended biodiesel fuel. Renewable Energy 2010, 35 (1), 144–150. (16) Shi, X. Y.; Yu, Y.; He, H.; Shuai, S. J.; Dong, H. Y.; Li, R. L. 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. (17) Defries, T. H.; Kishan, S.; Smith, M. V.; Anthony, J.; Ullman. The Texas Diesel Fuels Project, Part 1: Development of Txdot-Specific Test Cycles with Emphasis on a “Route” Technique for Comparing Fuel/ Water Emulsions and Conventional Diesel Fuels; SAE 2004-01-0090; Society of Automotive Engineers: Warrendale, PA, 2004. (18) Masjuki, H.; Abdulmuin, M. Z.; Sii, H. S. Indirect injection diesel engine operation on palm oil methyl esters and its emulsions. Proc. Inst. Mech. Eng. 1997, 211, 291–299. (19) Samec, N.; Kegl, B.; Dibble, R. W. Numerical and experimental study of water/oil emulsified fuel combustion in a diesel engine. Fuel 2002, 81 (16), 2035–2044. (20) Manuel, A.; Gonzalez, D.; Hercilio, R.; Xiomara, G.; Aymara L. Performance and Emissions Using Water in Diesel Fuel Microemulsion; SAE 2001-01-3525; Society of Automotive Engineers: Warrendale, PA, 2001

(21) Fernando, S; Hanna, M. Development of a Novel Biofuel Blend Using Ethanol-Biodiesel-Diesel Microemulsions: EB-Diesel. Energy Fuels 2004, 18 (6), 1695–1703. (22) Neuma de Castro Dantas, T.; da Silva, A. C.; Neto, A. A. D. New micro-emulsion systems using diesel and vegetable oils. Fuel 2001, 80 (1), 75–80. (23) Lin, C. Y.; Wang, K. H. Effects of an oxygenated additive on the emulsification characteristics of two and three-phase diesel emulsions. Fuel 2003, 83 (4), 507–515. (24) Lin, C. Y.; Lin, S. A. Effects of emulsification variables on fuel properties of two- and three-phase biodiesel emulsions. Fuel 2007, 86 (1), 210–217. (25) Wang, F.; Fang, B.; Zhang, Z. Q.; Zhang, S. Y.; Chen, Y. D. The effect of alkanol chain on the interfacial composition and thermodynamic properties of diesel oil micro-emulsion. Fuel 2008, 87 (12), 2517– 2522. (26) Prommes, K.; Apanee, L.; Samai, J. I. Solubility of a dieselbiodiesel-ethanol blend, its fuel properties, and its emission characteristics from diesel engine. Fuel 2007, 86 (7), 1053–1061.

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and spray pattern. Ethanol has lower heating value which is approximately 30.9% less than that of biodiesel. Therefore, it is necessary to increase the fuel amount to be injected into the combustion chamber to produce same power output. Ethanol contains almost 34.8% of oxygen by weight, has lower cetane number, and correspondingly, has prolonged ignition delay. Experimental Setup and Procedure. The engine used was a single cylinder, naturally aspirated, four stroke, water cooled, 16.5:1 compression ratio, direct injection diesel engine with a bowl in piston combustion chamber. The maximum torque was 52.3 N m at 1800 rev/min, and the maximum engine power was 11.03 kW at 2000 rev/min. The injector nozzle was located in the center of the combustion chamber and had an opening pressure of 18 MPa. A high precision flow meter was used to measure the fuel flow per 30 s. A Kistler piezoelectric transducer was installed in the engine cylinder head to acquire the combustion pressure. Signals from the pressure transducer were amplified using charge amplifier. The cylinder pressure data were recorded for 100 consecutive cycles and then averaged to eliminate the effect of cycle-to-cycle variations. A high-precision photoelectric sensor was used for delivering signals for TDC and crank angle with a precision of 0.1° to crank angle generator. The signals from the charge amplifier and crank angle generator were recorded by CB566 combustion analyzer. Gaseous emissions were measured by AVL Digas 4000, a chemiluminescent detector (CLD) for NOx, a flame ionization detector (FID) analyzer for HC, and a nondispersive infrared (NDIR) analyzer for CO. Light absorption coefficient of smoke was measured by a part-flow smoke opacimeter (AVL Dismoke 4000). Fuel including biodiesel and the microemulsions were tested at six different engine loads at engine speeds of 1500 rev/min and 1800 rev/min. All tests were carried out under steady state engine conditions. A schematic diagram of the experimental setup is shown in Figure 1. Combustion Methods. The details about combustion stages can often be determined by analyzing the heat release rate. The trend of heat release can be obtained by the experimental cylinder pressure data. The model assumes thermodynamic equilibrium during combustion in the cylinder but ignores temperature gradients, pressure waves, nonequilibrium conditions, fuel vaporization, mixing, and so on.32 From the first law of thermodynamics dQB dQn dQw ¼ þ ð1Þ dj dj dj

Table 1. Maximum Dissolved Water with Different Addition of Span80 biodiesel (mL) ethanol (mL) SPAN80 (g) water (mL) 80

20

1

1.4

80

20

4

1.6

80

20

8

1.8

80

20

10

appearance Clear Transparent Well-distributed

2

Table 2. Composition of the Test Microemulsions microemulsion biodiesel (mL) ethanol (mL) SPAN80 (g) water (mL) ME1 ME2

80 80

20 20

4 4

0.5 1.0

Table 3. Main Properties of Biodiesel, Ethanol, and Microemulsions properties density (g mL-1) latent heat of evaporation (kJ kg-1) lower heating value (kJ kg-1) kinematic viscosity at 20 °C (mm2 s-1) stoichiotric air-fuel ratio (kg kg-1) flash point (°C) boiling point (°C) oxygen content (wt %)

biodiesel ethanol 0.8700 200 38812 7.8 12.5 166 330 10.0

0.7880 840 26800 1.2 9.0 13.5 78 34.8

ME1

ME2

0.8545 0.8552 333 360 31294 31283 11.252 11.249

15.0

15.0

The water quantity dispersed in the biodiesel was very important because it allowed microexplosion during combustion.27 It was noticed in past studies that the increase in water fraction in the emulsified fuel resulted in significant reduction in pollutant emissions in case of vegetable oil.28 In this study, the maximum water quantities in the microemulsions are shown in Table 1. It can be seen that, with Span 80 addition increasing, the maximum water quantity for maintaining the steady state of the microemulsion is increased. But the water addition decreases the lower heating value (LHV) of the fuel and increases ignition delay in a compression ignition engine.29 For the preparation of the microemulsified fuel, initially, the known quantity of 20% by volume of ethanol was added to the 80% volume based biodiesel at ambient temperature (25 °C). Then, 4 mg Span80 was mixed along with the mixture. Last, the water was added slowly to the mixture. It was necessary to stir the mixture when Span80 and water were being mixed. This paper mainly studied two ethanol-biodiesel-water microemulsions, which can keep steady for one month at ambient temperature, and its compositions are shown in Table 2. The microemulsions are denoted as ME1 and ME2, respectively. Table 3 shows the main properties of biodiesel, ethanol, and the microemulsions.30,31 Without engine modification, the fuel properties of the microemulsion will affect the engine performance and emissions, since ethanol has different physical and chemical properties from biodiesel. Ethanol has lower values of viscosity and density that affect fuel quantity, injection timing,

dQB/dj is the gross heat release rate, dQn/dj is net heat release rate, and dQw/dj is the heat transfer rate to the walls. dQn/dj equals to the rate at which work is done on the piston plus the rate of change of sensible internal energy of the cylinder contents, assuming that the contents of the cylinder can be modeled as an ideal gas dQn dV dUs dV dT þ þ mcv ð2Þ ¼p ¼p dj dj dj dj dj For the ideal gas law, pv = mRT, with R assumed constant, it follows that dp dV dT þ ¼ ð3Þ p V T

(27) de Caro Satge, P.; Mouloungui, Z.; Vaitilingom, G.; Berge, J. C. Interest of combining an additive with diesel-ethanol blends for use in diesel engines. Fuel 2001, 80 (4), 565–574. (28) Ali, Y.; Hanna, M. A.; Borg, J. E. Optimization of diesel, methyl tallowate and ethanol blend for reducing emissions from diesel engine. Bioresour. Technol. 1995, 52 (3), 237–243. (29) Ali, Y.; Eskridge, K. M.; Hanna, M. A. Testing of alternative diesel fuel from tallow and soybean oil in Cummins N14-410 diesel engine. Bioresour. Technol. 1995, 53 (3), 243–254. (30) Qi, D. H.; Geng, L. M.; Chen, H.; Bian, Y. Z.; Ren, X. C.; Liu, J. Combustion and performance evaluation of a diesel engine fueled with biodiesel produced from soybean crude oil. Renewable Energy 2009, 34 (12), 2706–2713. (31) Chen, H.; Shuai, S. J. Study on combustion characteristics and PM emission of diesel engines using ester-ethanol-diesel blended fuels. Proc. Combust. Inst. 2007, 31 (2), 2981–2989.

Equation 3 can be used to eliminate T from eq 2 to give dQn cv dV cv dp þ V ¼ 1þ p dj R dj R dj

ð4Þ

Then dQn γ dV 1 dp p þ V ¼ γ -1 dj γ -1 dj dj

ð5Þ

(32) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw Hill, New York, 1988.

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Figure 1. Schematic layout of the test installation.

Figure 2. Ignition delay at (a) 1500 and (b) 1800 rev/min.

Here γ is the ratio of specific heats, cp/cv. An appropriate range for γ for diesel heat release analysis is 1.3-1.35. dQw ¼ hc AðT -Tw Þ dj

Heat transfer coefficient, hc, uses the Woschni’s heat transfer coefficient hc ¼ 3:26B -0:2 p0:8 T -0:55 w0:8

ð6Þ

Here w is the average cylinder gas velocity. 655

ð7Þ

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Figure 3. Cylinder pressure for 0.177 MPa at (a) 1500 and (b) 1800 rev/min.


period of ignition delay. It can be further explained by the low cetane number of ethanol, which results in longer ignition delay. Cylinder Pressure and Pressure Rise Rate. The cylinder pressures variation of the test fuels under different engine operating conditions are shown in Figures 3-5. From these figures, it is clear that the peak cylinder pressure is increased with the rising of engine load and does not vary significantly with the increase of engine speed. The peak cylinder pressure is lower for the microemulsions at low engine loads (Figure 3) and is almost similar to that of biodiesel at medium and high engine loads (Figures 4 and 5). The peak cylinder pressure hardly varies with the different water quantity in the microemulsions, especially at high engine speed (1800 rev/min). At low engine loads, the peak cylinder pressure attains a lower value for the microemulsions since the ignition delay is longer and combustion starts near the TDC for the microemulsions and premixed burning phase extends to the expansion stroke. However, at high engine loads the ignition delay of microemulsions is longer than that of biodiesel and more fuel is burned in the premixed burning phase. Further, the water drops microexplosions can improve the spray atomization, which results in the faster combustion of the microemulsions 18. It also can be seen that the peak cylinder pressure occurs earlier in the crank angle for biodiesel than for the microemulsions at low engine loads. At medium and high engine loads, the peak cylinder pressure occurs at nearly the same crank angle for all test fuels.

Results and Discussion Combustion Characteristics. Ignition Delay. Ignition delay is a very important parameter, which greatly influences the combustion duration. The ignition delay is the time interval from the beginning time of nozzle valve lift to the beginning time of rapid pressure rising. The ignition delay of the test fuels at different engine operating conditions is shown in Figure 2. From the figure, it can be seen that the ignition delay decreases as the engine load increases, and slightly increases with the increasing of engine speed. Ignition delay represents the time taken in physical and chemical reactions and does not change much on a time scale of milliseconds. However, it will increase in terms of crank angle degrees with increasing engine speed since higher engine speed corresponds to a larger crank angle for the same time duration. As the engine load decreases, the residual gas temperature and wall temperature decrease, which leads to lower charge temperature at injection timing, and lengthens the ignition delay. At high engine load, the gas temperature inside the cylinder is higher, which reduces the physical ignition delay period. The ignition delay of the microemulsions is longer than that of biodiesel under all engine operating conditions, and slightly increases with the growth of water quantity in the microemulsions. It can be explained that the latent heat of evaporation of ethanol and water is larger than that of biodiesel, which cause the injected fuel spray into a relatively low temperature environment and increases the 656

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Figure 6. Pressure rise rate for 0.177 MPa at (a) 1500 and (b) 1800 rev/min.


Figures 6-8 show the pressure rise rates of biodiesel and the microemulsions under different engine operating conditions. It is clear that the peak pressure rise rate increases with the growth of engine load, and does not vary significantly with the increase of engine speed. The peak of pressure rise rate is lower for the microemulsions at low and medium engine loads (Figures 6 and 7) but is higher at high engine loads (Figure 8). There is no evident variation of the peak of

pressure rise rate with the quantity of water in the microemulsions. The main reason is the same as that mentioned above for cylinder pressure. Heat Release. The variation of heat release rates of the test fuels under different engine operating conditions are shown in Figures 9-11. Because of the heat loss from the cylinder and the cooling effect of the fuel vaporization when it is injected into the cylinder, the heat release rate is slightly 657

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Figure 9. Heat release rate for 0.177 MPa at (a) 1500 and (b) 1800 rev/min.


negative during the ignition delay. The initial phase of combustion, called the premixed burning phase, is very rapid because of the combustion of the fuel that has mixed with air during the ignition delay. After this phase, the combustion continues slowly until most of fuel is burned. This phase of combustion is called mixing-controlled combustion. The final combustion phase is the late combustion, which continues until the end of the expansion stroke. It can be seen that the peak of heat release rate increases with the growth of engine load, and hardly varies with the engine speed increasing. The peak of heat release rate is almost similar at low

engine loads (Figure 9) but is slightly higher for the microemulsions at medium and high engine loads (Figures 10 and 11). The reason is that the presence of ethanol and water fraction in the microemulsions decreases the cetane number and increases the ignition delay, which results in increased amount of combustible fuel to be prepared within the period of ignition delay and increases the heat release rate at medium and high engine loads. At low engine loads, the combustion becomes inferior with the microemulsions because the premixed burning phase shifts afterward to expansion stroke. Hence the peak heat release rate is lower for the 658

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Figure 12. Crank angle for 5% mass burned at (a) 1500 and (b) 1800 rev/min.

microemulsions. The crank angle corresponding to the peak heat release rate is later for microemulsions under all operating conditions. Combustion Process. Figure 12 shows the crank angle for 5% mass fraction burned. This figure shows that 5% fuel burns earlier for biodiesel than for the microemulsions, which is the result of the earlier start in combustion for

biodiesel, as suggested above. Figure 13 shows the crank angle for 95% mass fraction burned, which increases with increasing engine load since a large quantity of fuel needs to be injected for higher engine loads. This figure suggests that 95% mass fraction burns earlier for the microemulsions under almost all operating conditions, except at low engine loads and speed. The reason is that the addition of ethanol 659

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Figure 13. Crank angle for 95% mass burned at (a) 1500 and (b) 1800 rev/min.

causes lower viscosity of the microemulsions and improves vaporization and atomization in better mixing with air. In addition, the presence of water in the microemulsions leads to secondary atomization (microexplosion) of the fuel and results in more complete combustion and rapid heat release. Figure 14 shows the variation of combustion duration under different engine operating conditions. Crank angle duration from 5% mass burned to 95% mass burned has been taken as the combustion duration. Combustion duration increases with higher engine speed and engine load owing to the increase of the fuel quantity. At low engine loads and speed, the combustion duration of the microemulsions is slightly longer than that of biodiesel, but at medium and high engine loads, it becomes shorter. As explained in the above section, the faster combustion rate in the premixed burning phase and shorter mixing-controlled combustion phase decrease the total combustion duration of the microemulsions at medium and high engine loads. At low engine loads and speed, the longer ignition delay of the microemulsions will postpone the combustion to a late expansion stage, which results in inferior combustion because of the low temperature of the cylinder. The crank angle of 95% mass

burned is later for microemulsions than for biodiesel (seen in Figure 13), so the combustion duration is longer for microemulsions. Performance and Emissions Characteristics. Figure 15 shows the BSFC variation of the biodiesel and the microemulsions with respect to BMEP at engine speed of 1500 and 1800 rev/min. In general, the BSFC values of the microemulsions are slightly higher than those of biodiesel under all range of engine loads. The BSFC of diesel engine depends on the relationship among volumetric fuel injection system, fuel density, viscosity, and lower heating value. More microemulsions are needed to produce the same amount of power due to its lower heating value in comparison with biodiesel. The improvement in BSEC (seen in Figure 16) with the microemulsions is attributed to the changes occurring in the combustion process. The addition of ethanol causes lower viscosity of the microemulsions compared to biodiesel, which improved vaporization and atomization in better mixing with air and leads to complete combustion. In addition, the internal droplet microexplosions of water, induced by the volatility difference between the water and the fuel, produce a secondary atomization and improve the mixing process, which result in more complete combustion and 660

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Figure 14. Variation in the combustion duration at (a) 1500 and (b) 1800 rev/min.

rapid heat release. 33,34 All these factors lead to lower BSEC for the microemulsions. Figure 17 shows the variations of NOx emissions with respected to engine loads at engine speed of 1500 and 1800 rev/min. The NOx emission is increased with the rising of engine load and slightly reduced with the increase of engine speed. The microemulsions show slightly lower NOx emissions as compared with biodiesel except at high engine loads and engine speeds. The main reason is that the heat absorption by ethanol and water vaporization causes a decrease of local adiabatic flame temperature and therefore reduces the chemical reaction in gas phase to produce thermal NO.35 Figure 18 shows the variation of the light absorption coefficient of smoke (K value) with respect to engine loads at engine speed of 1500 and 1800 rev/min. With the increase of engine loads, smoke emission is increased. Smoke emission is slightly decreased at high engine speed (1800 rev/min). The microemulsions exhibit evident reduction of smoke emissions at high engine loads in comparison with biodiesel. It can be explained by the reasons that the oxygen enriched Figure 15. Variation of BSFC with respect to engine loads at (a) 1500 and (b) 1800 rev/min.

(33) Cheng, C. H.; Cheung, C. S.; Chan, T. L.; Lee, S. C.; Yao, C. D.; Tsang, K. S. Comparison of emissions of a direct injection diesel engine operating on biodiesel with emulsified and fumigated methanol. Fuel 2008, 87 (10), 1870–1879. (34) Kadota, T.; Yamasaki, H. Recent advances in the combustion of water fuel emulsion. Prog. Energy Combust. Sci. 2002, 28 (5), 385–404. (35) Mark, P. B.; Musculus; John, E. D. Effects of Water-Fuel Emulsions on Spray and Combustion Processes in a Heavy-Duty DI Diesel Engine; SAE 2002-01-2892; Society of Automotive Engineers: Warrendale, PA, 2002.

fuels with high OH radical concentration contributes to the reduction of smoke.36 In addition, the charge cooling in(36) Ashok, M. P.; Saravanan, C. G. The Effect of Preheating the Inlet Air to Study the Performance and Combustion Characteristics of Diesel Engine using Ethanol Emulsion; SAE 2007-01-0628, Society of Automotive Engineers: Warrendale, PA, 2007.

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: DOI:10.1021/ef900814r

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Figure 18. Variation of smoke with respect to engine loads at (a) 1500 and (b) 1800 rev/min.

Figure 16. Variation of BSEC with respect to engine loads at (a) 1500 and (b) 1800 rev/min.

under all engine operating conditions because of the different physical properties of the test fuel. Lower peak cylinder pressure and pressure rise rate were found at low and medium engine loads, and higher were found at high engine loads for microemulsions. The crank angle corresponding to the peak values were later for microemulsions under all engine operating conditions. Higher premixed combustion and lower combustion duration were found for the microemulsions under almost all engine operating conditions. (3) The microemulsions showed slightly higher BSFC because of the lower heating value. The reduction of BSEC under all engine operating conditions was found for the microemulsions compared with biodiesel. NOx emissions exhibited slight improvement under almost all engine operating conditions, and smoke showed drastically reduction at high engine loads for the microemulsions. Acknowledgment. The authors thank the West Transport Construction Foundation from Ministry of Transport of People’s Republic of China (No. 200631826253).

Nomenclature 2

A = wall area (m ) B = cylinder bore (m) cp = constant pressure specific heat (kJ/kgK) cv = constant volume specific heat (kJ/kgK) dQB/dj = heat release rate with crank angle dQn/dj = net heat release rate with crank angle dQW/dj = heat transfer rate with crank angle hc = heat transfer correlation (J/m2 s K) m = mass of cylinder gases (kg) p = cylinder pressure (MPa) R = specific gas constant (J/kgK) T = absolute temperature (K) Tw = wall temperature (K) V = cylinder volume (m3) j = crank angle (degree)

Figure 17. Variation of NOx with respect to engine loads at (a) 1500 and (b) 1800 rev/min.

creases ignition delay and thus, enhances the mixing of the microemulsions with air, which in turn makes better air utilization at high engine loads. Conclusions The objective of this study was to characterize the ethanol-biodiesel-water microemulsion on the combustion characteristics, performance, and exhaust emissions of a diesel engine. On the basis of the experimental results, the following conclusions can be drawn: (1) Emulsification of biodiesel with ethanol and water can be a promising technique for using biodiesel efficiently in diesel engines without any modifications in the engine. (2) Ethanol-biodiesel-water microemulsion showed longer ignition delay as compared with biodiesel

Abbreviations BMEP = brake mean effective pressure (MPa) 662

Energy Fuels 2010, 24, 652–663

: DOI:10.1021/ef900814r

Qi et al.

BSEC = brake-specific energy consumption (MJ/kWh) BSFC = brake specific fuel consumption (g/kW 3 h) CA = crank angle CI = compression ignition CO = carbon monoxide (%) CO2 = carbon oxides (%) DI = direct injection

HC = hydrocarbon (ppm) k = coefficient of light absorption of the smoke (1/m) LHV = lower heating value (kJ/kg) NOx = nitrogen oxide (ppm) PM = particulate matter ppm = parts per million TDC = top dead center

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Experimental Studies of a Naturally Aspirated, DI Diesel Engine

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