Energy Fuels 2009, 23, 4343–4354 Published on Web 07/28/2009
: DOI:10.1021/ef900448m
Effect of Ethanol on Blending Stability and Diesel Engine Emissions Magı´ n Lapuerta,* Octavio Armas, and Reyes Garcı´ a-Contreras Escuela T ecnica Superior de Ingenieros Industriales, University of Castilla;La Mancha, Edificio Polit ecnico, Avda. Camilo Jos e Cela, s/n 13071, Ciudad Real, Spain Received May 12, 2009. Revised Manuscript Received July 7, 2009
The stability diagrams of ethanol (e)-biodiesel (b)-diesel blends were studied at different temperatures. It was found that biodiesel acts as a stabilizer component in e-diesel blends, except at low temperatures, where it favors the formation of a gelatinous phase. Three blends (two e-diesel and an e-b-diesel) were selected to be tested in a diesel engine, and their performance and emissions were compared to those of a reference diesel fuel. The results show that, with increasing ethanol content in the blends, hydrocarbon emissions increase significantly because of the high heat of vaporization of alcohol, thus promoting the appearance of a nuclei mode. With all of the blends tested, reductions in smoke opacity and particulate matter emissions with respect to diesel fuel are obtained, but these decreases were not lineal with the oxygen content. The oxygen provided by ethanol resulted in more efficiency in the opacity reduction than the oxygen provided by methyl ester (e-b-diesel blend), but in the case of particulate matter emissions, the opposite trend was observed.
experiences, reductions in smoke opacity or particulate matter (PM) emissions were already reported, this currently being an additional benefit of their use as a consequence of the increasingly stringent emission regulations (Euro IV and Euro V), which will soon be entry into force. However, the blending proportion of ethanol is limited because of its low ignitability, its low lubricity, and its reduced heating value.7,8 Another limit for blending is imposed by the limited miscibility with petroleum diesel. Although at ambient temperature, the miscibility is quite good, at temperatures below 10 °C, the miscibility is sharply reduced, which often makes the incorporation of a stabilizing additive necessary.9,10 Additionally, the presence of moisture reduces the miscibility. In fact, the ethanol used for blending in Europe must be anhydrous as specified in the norm EN-15376. The mentioned blending limitations are significantly reduced as a consequence of the progressive incorporation of biodiesel in the commercial diesel fuel, because biodiesel fuels improve the stability of the blends, and compensate for the reduction in cetane number and lubricity derived from the addition of ethanol. Therefore, in this work, a study about the blending stability and the engine emissions with ethanol (e)-biodiesel (b)-diesel blends is presented. This study is an extension to a previous study about ethanol (e)-diesel blends already presented separately in refs 11-13, devoted to blending stability, steady-condition engine emissions, and transientcondition engine emissions, respectively. Other authors have
1. Introduction The recently approved European directive “on the promotion of the use of energy from renewable sources” 2009/28/CE establishes a new objective of 10%, in energy basis, for biofuel consumption in the transportation sector before 2020.1 Different alternatives must be offered by the biofuel producers to fulfill such a target, with them being compatible with the stringent requirements in life-cycle greenhouse emissions, which have also been included in the same directive. In the first period (2010-2017), savings higher than 35% will be required with respect to the greenhouse emissions typically obtained from diesel or gasoline fuels. Ethanol is one of the automotive fuels that will provide the highest life-cycle greenhouse emission savings as soon as second-generation production processes, such as cellulosic fermentation from wheat straw, waste wood, or farmed wood, are consolidated. However, to guarantee the fulfilment of this target, the increasing dieselization process occurring in Europe must also be considered (i.e., in Spain, diesel fuels constitute more than two-thirds of the automotive fuel consumption). Ethanol can participate as a renewable alternative for the consumption of biofuels not only in sparkignition engines but also in diesel ones, in two ways: as biodiesel produced by transesterification with ethanol and as a fuel for direct blending with either petroleum diesel, biodiesel, or both of them. Ethanol was first used in diesel engines in the 1970s in South Africa2 and further on in the 1980s in Japan,3 Germany,4 and United States,5 as reviewed in ref 6. In most of these
(7) Satge de Caro, P.; Mouloungui, Z.; Vaitilingom, G.; Berge, J. Ch. Fuel 2001, 80, 565–574. (8) Xing-cai, L.; Jiang-guang, Y.; Wu-gao, Z.; Zhen, H. Fuel 2004, 83, 2013–2020. (9) Hansen, A. C.; Zhang, Q.; Lyne, P. W. L. Bioresour. Technol. 2006, 96, 277–285. (10) Waterland, L. R.; Venkatesh, S.; Unnash, S. Safety and performance assessment of ethanol/diesel blends (e-diesel), 2003; NREL/SR540-34817. (11) Lapuerta, M.; Armas, O.; Garcı´ a-Contreras, R. Fuel 2007, 86, 1351–1357. (12) Lapuerta, M.; Armas, O.; Herreros, J. M. Fuel 2008, 87, 25–31. (13) Armas, O.; Cardenas, M. D.; Mata, C. SAE Tech. Pap. 200724-0131, 2007.
*To whom correspondence should be addressed. Telephone: þ34926295431. Fax: þ34-926295361. E-mail:
[email protected]. (1) Directive 28/2009/CE of the European Parliament and the Council on the Promotion of the Use of Energy from Renewable Sources, 2009. (2) Letcher, T. M. S. Afr. J. Sci. 1983, 79, 4–7. (3) Murayama, T.; Miyamoto, N.; Chikahisa, T.; Ogawa, H. SAE Tech. Pap. 830373, 1983. (4) Weidmann, K.; Menrad, H. SAE Tech. Pap. 841331, 1985. (5) Likos, B.; Callahan, T. J.; Moses, C. A. SAE Tech. Pap. 821039, 1983. (6) Rakopoulos, C. D.; Antonopoulos, K. A.; Rakopoulos, D. C. Energy 2007, 32, 1791–1808. r 2009 American Chemical Society
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: DOI:10.1021/ef900448m
Lapuerta et al. Table 1. Fuel Properties
properties
diesel
density at 15 °C (kg/m ) viscosity at 40 °C (cSt) gross heating value (MJ/kg) lower heating value (MJ/kg) flash point (°C) sulfur (ppm w) water (ppm w) CFPP (°C) molecular formula C (% w/w) H (% w/w) O (% w/w) molecular weight (g/mol) stoichiometric fuel/air ratio 3
834.9 2.72 45.54 42.58 58.5 33.9 57 -19 C15.18H29.13 86.13 13.87 0 211.7 1:14.67
soyate biodiesel
bioethanol
885 4.09 39.42 36.87 174 2.5 272 -3 C18.81H34.69O2 77.13 11.94 10.93 292.9 1:12.50
792 1.13 28.05 25.22 12
831 2.41 43.82 40.86
844 2.63 42.37 39.53
827 2.12 41.93 38.99
2024
243 -19 C11.66H22.96O0.27 83.63 13.82 2.55 167.5 1:14.25
343 -14 C12.12H23.61O0.64 81.07 13.26 5.67 179.53 1:13.63
618 -18 C8.96H18.22O0.47 80.60 13.75 5.65 133.56 1:13.74
C2H6O 52.14 13.13 34.73 46.06 1:9
ED7.7
EBD
ED17
Table 2. Engine Specifications
also presented previous studies about the stability of e-b-diesel blends14,15 and their effect on engine emissions,16-18 but only in ref 19 e-diesel and e-b-diesel blends were compared. Finally, an analysis about the effect of the oxygen content on the gaseous emissions, smoke opacity, and PM emissions is presented, trying to discern if the observed reductions can be correlated with the oxygen content of the fuel or if they depend upon the functional group containing oxygen.
fuel injection maximum power maximum torque cylinder arrangement bore stroke displacement compression ratio
common rail, with pilot injection 82 kW at 4000 rpm 248 N m at 2000 rpm 4 cylinders, in line 86.5 mm 94 mm 2.2 L 18:1
The composition and most important properties of pure fuels and tested blends with regard to the engine operation are shown in Table 1. Although the diesel composition has been provided by Repsol, all other properties shown have been measured in our laboratory.
2. Fuels The stability of e-diesel and e-b-diesel blends mainly depends upon temperature, humidity, and fuel composition. A previous study concluded that, with the use of hydrated ethanol, the miscibility of ethanol in diesel decreases.11 In this work, anhydrous ethanol (99.7%) provided by Abengoa Bioenergy has been used. Reference fuel is a typical low-sulfur diesel fuel similar to those available in Spanish petrol stations supplied by Repsol. The biodiesel selected to make stability studies and prepare the e-b-diesel blend for engine testing was soybean methyl ester, also provided by Repsol. The selection of soyate biodiesel is justified because this fuel presents typical properties of biodiesel fuels and has been previously used in other studies with e-b-d diesel blends.17,18,20 From the stability study, three blends have been selected to carry out the engine tests: (1) e-diesel blend (ED7.7), composed of 7.7% ethanol, 0.62% additive O2Diesel (acting as a stabilizer and as a cetane improver), and 91.68% diesel, with all percentages in volume. This ethanol content ensures the stability of the blend in a wide temperature range. This blend is denoted as ED7.7. (2) e-b-diesel blend (EBD), composed of 7.7% ethanol and a blend of 30% of soybean biodiesel-diesel. This biodiesel content has received major commercial interest up to now in both delivery models: captive fleets and petrol stations. (3) e-diesel blend (ED17), composed of 17% ethanol, with this percentage being close to the limit of e-diesel blends stability at ambient temperature. This ethanol content has been selected because this blend has the same oxygen content as EBD fuel, while belonging to a different functional group.
3. Experimental Equipment and Test Conditions The stability and rate of separation of e-b-diesel blends were obtained by means of optical equipment (Turbiscan), specifically designed for the characterization of liquid emulsion suspensions and solutions. The device has an infrared light source of 850 nm wavelength and two detectors operating simultaneously. The first one detects the light transmitted with no refraction and is located in the prolongation of the light beam (0°), while the second one detects the backscattered light in the direction 135° with respect to the incident light beam. Other characteristics of Turbiscan are described in ref 11. Different tests were carried out to study the influence of ethanol and biodiesel content on the stability of blends at different temperatures. As the temperature decreases, the region of unstable blends becomes larger and, therefore, a higher number of blends must be tested to determine the boundary between the stable/unstable region and the limit between the different unstable subregions. To determine the composition of the separated phase in unstable blends (Section 6), a Shimadzu GC-14 gas chromatograph has been used. The detector was a flame ionization detector (FID) with helium as a carrier gas and a capillary column Supelwax adequate for alcohol detection. The column oven temperature started at 40 °C was held for 4 min and increased at 250 °C at a rate of 25 °C/min. Engine tests were carried out in a 4-cylinder, 4-stroke, turbocharged, intercooled, common-rail, 2.2 L Nissan diesel engine (which can be considered typical of those used in European diesel vehicles) joined to an asynchronous electric brake Schenck Dynas III LI 250. The main specifications of the engine are given in Table 2, and the experimental layout is shown in Figure 1. The brake control system allowed for the measurement and control of the engine speed, throttle position, and torque. Because emissions are strongly dependent upon the exhaust gas recirculation (EGR) ratio, it is important to keep this parameter fixed when comparing different fuels. For this reason, the EGR valve was driven by a stepper motor, which allows for acheiving the
(14) Fernando, S. Energy Fuels 2004, 18, 1695–1703. (15) Makareviciene, V.; Sendzikiene, E.; Janulis, P. Bioresour. Technol. 2005, 96, 611–616. (16) Chen, H.; Shuai, S.-J.; Wang, J.-X. Proc. Combust. Inst. 2006, 31, 2981–2989. (17) Shi, X.; Yu, Y.; He, H.; Shuai, S.; Wang, J.; Li, R. Fuel 2005, 84, 1543–1549. (18) Xiaoyan, S.; Xiaobing, P.; Yujing, M. Atmos. Environ. 2006, 40, 2567–2574. (19) Kwanchareon, P.; Luengnaruemitchai, A.; Jai-In, S. Fuel 2007, 86, 1053–1061. (20) Fernando, S.; Hanna, M. Trans. ASAE 2005, 48 (3), 903–908.
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Figure 1. Experimental installation. Table 3. Set Values of Tested Engine Modes speed (rpm)
effective torque (N m)
bmep (bar)
EGR ratio (%)
mode
value
RSD (%)
value
RSD (%)
value
RSD (%)
value
RSD (%)
U10 C0 F G0 H
2126 1410 1526 1743 1853
0.017 0.016 0.007 0.017 0.026
8 26 46 77 110
8.807 3.987 0.684 0.811 1.007
0.46 1.50 2.65 4.44 6.34
8.807 3.987 0.684 0.811 1.007
25 14 10 0 0
5.766 6.469 8.028 0 0
(DMA), where a voltage is applied and particles are classified according to their electrostatic mobility ratio. Finally, they are counted in a condensation particle counter (CPC). A correction for particles with multiple charges was automatically applied by the control software. The total concentration and the mean diameter of the distribution were calculated by integrating over the particle diameter variation range of the distributions. The concentration was appropriately scaled to express it as in the raw exhaust, because particles were sampled from the dilution minitunnel and not from the exhaust pipe. Nitrogen oxide (NOx) emissions were recorded with a chemioluminiscence analyzer Topaze 3020, and total hydrocarbon emissions (THC) were measured with a flame ionization detector Graphite 52M-D (errors below 0.5%), both integrated in a sampling and conditioning system Environnement. A smokemeter AVL 439 was used to measure the smoke opacity (in percentages of the light absorbed or dispersed with respect to the intensity of the emitted beam, with less than 0.1% errors). Five operating modes were selected among the collection of steady stages which reproduce the transient cycle that vehicles with this type of engine must follow for certification according to the European Emission Directive 70/220, amendment 2001/C 240 E/01. The selection of modes C0 , F, G0 , and H was made based on the highest contribution to the total PM emissions,25 while mode U10 was chosen because it was the mode with the highest total hydrocarbon (THC) emissions. Table 3 shows the set values established for engine speed, effective torque, brake mean effective pressure (bmep), and EGR ratio at each operation mode, together with the relative standard deviation (RSD) of the values attained during the tests. It can be observed that the repeatability was lower in mode U10 than in the other modes, as a consequence of the typically low engine stability at such a low load.
same EGR ratio for all tested fuels. Herein, the EGR ratio was defined as the mass flow of the recirculating exhaust normalized by the total mass flow entering the cylinder. Fuel consumption was measured with a gravimetric fuel balance AVL 733-S (measurement errors below 0.12%), and air consumption was measured with a hot-wire sensor Siemens 5WK9628. The engine was also equipped with the necessary instrumentation for the measurement and monitoring of operating temperatures and pressures (intake air, fuel, exhaust gases, lube oil, etc.). PM was collected using a partial flow mini-tunnel (Nova Microtrol) equipped with mass flow controllers to achieve the set value of the dilution ratio. A portion of the exhaust gas was sampled in the center of the exhaust pipe to avoid capturing wall deposits. This portion of gas was routed into the mini-tunnel, where it was mixed with filtered ambient air. This diluted flow was then driven across a Whatman glass microfiber 70 mm filter, which was weighed before and after sampling using a Sartorius M5P analytical balance to determine the total mass of PM collected on the filter. The filters were conditioned before every weighing in a climatic chamber (Minitest CCM-0/81), to obtain the same temperature and humidity conditions. The collection and conditioning procedure was optimized as described in ref 21. Finally, the filters were subjected to a thermal extraction method (providing the volatile organic fraction of the PM, VOF) by means of a thermogravimetric analyzer, with this method being described in ref 22. Particle size distributions were determined using the partial dilution mini-tunnel and a scanning mobility particle sizer (SMPS) TSI 3936L10. This method was previously optimized in ref 23. Particles were sampled from the dilution mini-tunnel, and the large ones were removed by a 610 nm impactor. These particles are transported to a differential mobility analyzer (21) Lapuerta, M.; Armas, O.; Ballesteros, R.; Duran, A. SAE Tech. Pap. 99-01-3531, 1999. (22) Lapuerta, M.; Ballesteros, R.; Rodrı´ guez-Fernandez, J. Measurement Science and Technology 2007, 18, 650–658. (23) Armas, O.; G omez, A.; Herreros, J. M. Meas. Sci. Technol. 2007, 18, 2121–2130.
(24) Lapuerta, M.; Armas, O.; Hernandez, J. J. Appl. Therm. Eng. 1999, 19, 513–529. (25) Lapuerta, M.; Armas, O.; Ballesteros, R.; Fernandez, J. Fuel 2005, 84, 773–780.
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crystals are mixed in the bottom of the cell, presenting a gelatinous aspect, whereas a liquid phase is distinguished in the upper part of the cell glass. The phase laying above the gelatinous one is not always a homogeneous liquid phase. There are some conditions under which two liquid phases appear separated by an interphase.
After each change of fuel, all lines were drained and then filled with the next fuel. Before beginning a new test, the sample lines were cleaned to remove deposits and hydrocarbons of the previous tests. After that, the engine was warmed with the new fuel for at least 1 h to purge any remains of the previously tested fuel from the engine fuel system. Additionally, the reference fuel (REF) was tested prior to every biodiesel blend. The duration of each operation mode was chosen, so that three different particulate filters could accumulate a particulate mass of at least 1.3 mg for each one and six particle size distributions could be taken with the SMPS.
5. Parameters of Stability From the transmission profiles of unstable blends, some information about the composition of the phases can be obtained. A non-dimensional parameter, which is defined as the separation ratio (SR), indicates the final/maximum volume of separated ethanol (when the phase separation process has finished, Vsep) with respect to the initial ethanol content in the blend (Ve)
4. Interpretation of Stability Profiles The transmission profiles provided by Turbiscan (instrument described in Section 3) must be interpreted to obtain the stability results. e-diesel or e-b-diesel blends are considered stable when components that constitute these blends are mixed completely, leading to a single homogeneous, clear, and transparent phase. In this case, the obtained Turbiscan profiles along the test time are superposed. On the contrary, when separation of phases occurs (unstable blend), the profiles do not superpose each other with time.11 The phase separation process could generate different types of unstable blends, such as two liquid phases, a gelatinous interphase, or the formation of a gelatinous phase in the bottom of the glass cell. Each one of these unstable patterns is described in the following paragraphs: (A) Two liquid phases. At positive temperatures and when blends contain between 15 and 75% of ethanol, two different liquids phases are distinguished after the separation. The separation process begins when the sample turns quickly into a turbid phase, with no distinction between the two phases. Later, a more transparent phase in the upper part of the mixture is distinguished, corresponding to ethanol, although the phase below the interphase maintains the turbidity. Finally, the width of the bioethanol phase increases, moving the interphase toward a lower position until equilibrium is reached, whereas the phase located below becomes transparent. (B) Gel formation in the interphase. At temperatures below 0 °C and when blends contain between 10 and 60% ethanol, a gelatinous phase is generated and located in the interphase between the two liquid phases. The main difference between a liquid and gel interphase is its thickness, with the latter being thicker (>1 mm). From its very early formation, the appearance of this interphase is already gelatinous. The stages of this phase separation process are similar to that described in A. (C) Gel formation in the inferior part of the glass cell. When the ethanol content is high and tests are carried out at low temperature, a gelatinous phase appears in the inferior part of the cell glass. First, it was thought that biodiesel prompted this gel formation because this temperature is similar to the coldfilter plugging point (CFPP) value of methyl ester. However, the gelatinous phase is generated in ediesel blends too. Although the e-diesel interaction is the main cause for this gel formation, the biodiesel presence favors this phase. After an initial turbidity is observed in these unstable blends, small crystals appear along the glass cell, which are suspended in the liquid phase during the separation process. These
SR ¼
Vsep hsep ¼ Ve he
ð1Þ
Because the section along the glass cell is constant, the separation ratio can be expressed as a ratio of heights (eq 1), where hsep is the final/maximum height of the separated phase and he is the height of the initial content of ethanol. These values can be obtained from the Turbiscan software. The SR increases as the ethanol content becomes higher in both e-diesel and e-b-diesel blends. In some conditions, values of SR higher than unity are obtained. This result would indicate that the separated phase is composed of more ethanol than the initial volume in the blend, which is not possible. In fact, in the migration process of ethanol toward the superior part of the cell, either diesel or biodiesel is dragged, indicating that the separated phase is not only composed of ethanol. Therefore, another non-dimensional parameter has been defined, dragging ratio (DR), indicating the content of diesel, biodiesel, or both that is present in the separated phase. This parameter is defined as the ratio between the height of the separated phase (diesel and biodiesel, hd,sep and hb,sep) and that corresponding to the initial diesel-biodiesel blend. To calculate this parameter, all of the blended ethanol is considered to belong to the separated phase. DR ¼
hd, sep þ hb, sep hsep - he ¼ hblend - he hd þ hb
ð2Þ
6. Composition of the Separated Phase The composition of the separated phase of an unstable e-b-diesel blend (only in the case of two liquid phases) was analyzed using a Shimadzu GC-14 chromatograph (described in Section 3). A previous study concluded that the separated phase from the e-diesel blend with 20% ethanol was majority composed of ethanol,11 with this conclusion allowing for the use of the parameter SR. Chromatographic analysis of pure components that form e-b-diesel blends (ethanol, biodiesel, and diesel) have been carried out, and they are shown in Figure 2. While in the figure of ethanol analysis a single peak is shown for low retention time, in the diesel profile, many peaks appear because it is composed by many different hydrocarbons. The detection of biodiesel fuel occurs from the 20th minute, indicating that it is the heaviest component of the three tested fuels. Figure 3 shows the chromatography analysis of the separated phase of the E45B5 blend at 0 °C, which presents a SR 4346
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Figure 2. Chromatographic analysis of pure components.
diagram is widely used in the study of ternary blends15,19,20 because it allows us to study simultaneously the influence of the three components of the blends at a single temperature. At each temperature tested, the limit of the stability region is depicted. The shape of the unstable zone is similar to that obtained by other authors.15 The limit between the stable and unstable regions corresponding to 10 °C is shown in Figure 4a. This was chosen as the first temperature to be tested because most of the literature reports engine problems derived from unstability below this temperature, with these problems becoming even greater at lower temperatures.9,10 Two zones can be distinguished in the unstable region corresponding to two different types of unstable blends: blends that present a certain separation ratio and blends defined by their dragging ratio. Iso-lines indicate different values of SR or DR at the final equilibrium state. In the SR zone, it can be observed that, the higher the ethanol content, the higher the separated phase, while biodiesel generates the opposite effect (it improves the stability of the blends) because it acts as a stabilizer. In the DR region, the separated phase is not only composed of ethanol (see Section 6). Ethanol presents a good miscibility with biodiesel because of its polar character,19 but there are e-diesel blends (containing no biodiesel) that also present DR, indicating that biodiesel is not the only component that can be dragged by ethanol. Biodiesel improves stability, but in the DR region, the higher the biodiesel content in the blends, the higher the DR values. Figure 4b shows the stable region obtained at 5 °C, which is smaller than the zone corresponding to 10 °C. Again, the increase of ethanol content promotes the phase separation, while biodiesel stabilizes the blends. Although this is a positive temperature and it is far from the CFPP values of fuels, a gelatinous phase appears in the bottom of the cell glass when the ethanol content is very high (75%) and the content of
Figure 3. Chromatographic analysis of the separated phase of the E45B5 blend at 0 °C.
value higher than 1 (dragging ratio zone). In this figure, a large peak appears in the first moments, indicating the presence of ethanol. From minutes 10 to 20, several peaks are shown corresponding to diesel fuel, and in last minutes, some peaks corresponding to biodiesel fuel are observed. Therefore, when ethanol content in the blend is higher than 25% (blends situated in the DR zone), the alcohol, in its migration process, drags diesel and biodiesel toward the upper phase, as described above. This result justifies the use of the DR instead of the SR. Another analysis made from a sample taken from the lower phase of this blend (although not presented here) showed a very minor ethanol content, justifying the definition given in eq 2. 7. Effect of the Composition and Temperature on Stability The effect of ethanol and biodiesel content at different temperatures has been studied. Ternary diagrams are used to show the stability results of e-b-diesel blends. This kind of 4347
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Figure 4. Stability results at different temperatures.
biodiesel is small (less than 10%). Above this gelatinous layer, a homogeneous and clear phase is formed. The region corresponding to these blends is defined as G-1L. The gel formation could be caused by the presence of biodiesel because this is the component with the highest CFPP value. However, in e-diesel blends this phase appears too.
Therefore, it must be concluded that, although the gel formation is derived from the combination of e-diesel, the presence of biodiesel highly influences the formation of this gel. Ethanol has a very low freezing point. Therefore, it could be expected that e-diesel and e-b-diesel improve cold-flow properties with respect to diesel and biodiesel fuels. However, while 4348
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Figure 5. Brake-specific fuel consumption and thermal efficiency.
Figure 6. Absolute and relative fuel/air ratio.
the pour point of e-diesel blends decreases significantly with respect to that of diesel fuel, the cloud point increases.26 At 0 °C (Figure 4c), the influence of ethanol and biodiesel content is similar to that at the previous studied temperatures, but the region in which the gelatinous phase (G-1L) appears becomes larger as the biodiesel content becomes higher (until 30%). At this temperature, a new region appears located between DR and G-1L, where unstable blends present a gelatinous phase in the bottom of glass cell and two liquid phases appear in the upper part (G-2L). Two negative temperature values have been chosen (-2 and -5 °C) above and below the biodiesel CFPP value. At -2 °C (Figure 4d), an important increase of the unstable region is observed with respect to that obtained at 0 °C. This could be explained by the enlargement of the G-1L region with respect to positive temperatures. While biodiesel benefits stability when ethanol content is low, it increases the gel formation at high ethanol percentages. Above this temperature, all biodieseldiesel and ethanol-biodiesel blends remain stable, but at this value, there are some ethanol-biodiesel blends in which the gelatinous phase appears. Most of the blends situated in the SR or DR zone at positive temperatures show a gelatinous interphase (new phase, GI) between two liquid phases. The lowest temperature tested was -5 °C, being below the CFPP value of biodiesel. In this case, only pure ethanol, pure diesel, and the e-diesel blends up to 5% of alcohol remain stable (Figure 4e), indicating that, at this temperature, biodiesel promotes gel formation (biodiesel does not act as a stabilizer any more).
8. Engine Emissions 8.1. Engine Operating Conditions and Performance. At each operation mode, the EGR ratio, effective torque, and engine speed were established, as listed in Table 3, regardless of fuel consumption. All of the variables are plotted in Figures 5-11 against the bmep, with the latter being derived from the effective torque. Mode U10 is plotted separately from the rest of the modes, because no increasing bmep accompanies the increased engine speed in this case. Small variations are obtained in the values of the EGR ratio with different fuels (Table 3), especially at low load. To obtain the same torque and power output for every tested fuel, the break-specific fuel consumption (bsfc) was higher for ethanol blends because these fuels present a lower heating value than diesel (left panel of Figure 5), but ED7.7 shows similar values to diesel. The brake thermal efficiency (right panel of Figure 5) of different fuels is similar in low-load conditions, although in mode H, e-diesel blends (especially ED7.7) present slightly higher values, with this trend being in agreement with that observed by other authors.27,28 The measured differences in absolute fuel/air ratios of ethanol blends with respect to diesel (left panel of Figure 6) were compensated by the different combustion stoichiometric values, leading to similar relative fuel/air ratios (right panel of Figure 6), except the ED7.7 blend that presents relative fuel/air ratios lower than other fuels at high load because of its lower (27) Li, D.-G.; Zhen, H.; Xingcai, L.; Wu-gao, Z.; Jian-guang, Y. Renewable Energy 2005, 30, 967–976. (28) Abu-Qudais, M.; Haddad, O.; Qudaisat, M. Energy Convers. Manage. 2000, 41, 389–399.
(26) McCormick, R. L.; Parish, R. Advanced petroleum based fuels program and renewable diesel program, 2001; NREL/MP-540-32674.
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Figure 7. THC and NOx emissions.
Figure 8. Smoke opacity and PM emissions.
fuel distribution processes are less favorable.29 The higher heat of vaporization of ethanol favors an increment in the hydrocarbon emission with respect than that with diesel. The results obtained in this work are in agreement with that observed by other authors.6,27,30 Therefore, little ethanol content and biodiesel presence have no influence on THC emissions, but a higher alcohol content in the blends affects these gaseous emissions. NOx emissions of fuels tested are shown in the right panel of Figure 7. It is observed that emissions from ethanol blends are slightly lower than those from reference fuel in C0 , F, and G0 modes. No difference can be observed between ED17 and EBD, thus showing that neither further ethanol content nor the addition of biodiesel provides further reductions in NOx emissions. 8.3. Smoke Opacity and PM. Smoke opacity is indicative of dry soot emissions, which are one of the main components of PM.16,31 The smoke opacity decreases as the ethanol content in the blend becomes higher (right panel of Figure 8), in agreement with all of the literature reviewed.6,8,16,31-33 This reduction is more important at high-load modes.34,35 The lower opacity can be explained because the presence of fuel oxygen
Figure 9. Volatile organic fraction of the PM.
bsfc. The relative fuel/air ratio is a parameter with an important influence in emissions; therefore, the similar values obtained with different fuels tested indicate that the differences in emissions could be attributed to the composition and properties of these fuels and not the relative fuel/air ratio. 8.2. Gaseous Emissions. Total hydrocarbon emissions for different fuels are shown in the left panel of Figure 7. While blends with 7.7% of ethanol present THC emissions similar to diesel, the gaseous emissions from the blend with the highest ethanol content are higher, especially in mode U10. In low-load conditions, the atomization, evaporation, and
(30) Kass, M. D.; Thomas, J. F.; Storey, J. M.; Domingo, N.; Wade, J.; Kenreck, G. SAE Tech. Pap. 2001-01-2018, 2001. (31) Chen, H.; Wang, J.; Shuai, S.; Chen, W. Fuel 2008, 87, 3462– 3468. (32) Shih, L. K. SAE Tech. Pap. 982573, 1998. (33) He, B. Q.; Shuai, S. J.; Wang, J. X.; He, H. Atmos. Environ. 2003, 37, 4965–4971. (34) Di, Y.; Cheung, C. S.; Huang, Z. J. Aerosol Sci. 2009, 40, 101– 112. (35) Choi, C. Y.; Reitz, R. D. Fuel 1999, 78, 1313–1317.
(29) Sayin, C.; Uslu, K.; Canakci, M. Renewable Energy 2008, 33, 1314–1323.
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Figure 10. Particle size distributions for C0 and H engine modes.
Figure 11. Mean diameter (dm) and total particle number concentration (PNC).
reduces the probability of rich-zone formation (high local fuel/ air ratio) and promotes the oxidation of soot nuclei generated in fuel combustion. Among the two fuels with the same ethanol content, the smoke opacity of EBD is lower, corresponding to the additional biodiesel content. The left panel of Figure 8 shows the PM emissions. At low load, there is no important PM reduction of ethanol blends with respect to diesel, but at high load, a significant decrease with biofuel blends is observed. Different from the opacity results, EBD presents the highest reduction in PM emission. This can be explained because THC emissions from ED17 are much higher than with the other fuels (the probability of adsorption/condensation onto the surface of soot particles is higher). Such an increase partly compensates the significant reduction in opacity obtained with this blend. The volatile organic fraction (VOF) with the blends tested increases as the ethanol content in the blends becomes higher (Figure 9). This result confirms that the increase in adsorbed/ condensated HC is higher than the soot reduction (the latter derived from the opacity measurement). Particle size distributions of tested fuels corresponding to C0 and H engine modes are shown, as examples, in Figure 10. No clear tendency can be found in the ethanol blends because, while the ED7.7 blend presents distributions similar to reference fuel in both modes, the number concentration of particles of EBD is lower than that of diesel fuel. The size distribution obtained from the blend with the highest ethanol content is composed of two modes: the accumulation mode, which is the characteristics of diesel particles (composed of soot and adsorbed THC), and the nuclei mode, which indicates the presence of condensed hydrocarbon drops.
The parameters derived from particle size distribution that are generally used for quantification are the mean diameter and total concentration (Figure 11). Although ED17 presents a nuclei mode, in these figures, only the results corresponding to the accumulation mode have been used to compare with the rest of the fuels. The mean diameter (dm) decreases as the ethanol content in the blends increases. ED7.7 and EBD show similar values of mean diameter, with the effect of biodiesel on this parameter being negligible. The reduction of the mean diameter obtained with EBD and ED7.7 is due to the sharp decrease of large particles. Although the nuclei mode has not been taken into account, the mean size of the particles emitted with ED17 is even much lower than that corresponding to the other fuels. The trends obtained with the particle number concentration (PNC) are different from those obtained with the mean diameter. While the PNC obtained with ED7.7 is similar to that with reference fuel, the results corresponding to EBD and ED17 are lower than that from diesel fuel. No important differences in accumulation PNC can be found between these two fuels. 8.4. Effect of Fuel Oxygen on Engine Emissions. It is wellknown that the oxygen presence in the fuel composition favors the reduction of opacity and PM. However, there is no clear tendency in THC, NOx, and CO emissions with oxygen fuel (especially with ethanol blends), and there is almost no literature about the particle size from blends with alcohol. In this paragraph, the influence of fuel oxygen in emissions is studied. While in the previous figures, the x axis represented the bmep values, in the following figures, the oxygen 4351
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Figure 12. THC and NOx variation rates.
Figure 13. Smoke opacity and PM variation rates.
fuel content is represented in this axis. In all cases, the y axis represents variation rates, defined as variations in percentage with respect to the emissions from diesel fuel. In each studied emission, each fuel is represented by two points, corresponding to low and high loads. Low-load conditions indicate the mean variation results of U10 and C, and high load corresponds to the average of F, G0 , and H. This type of representation permits us to distinguish whether the engine load has any significant effect or not. Two line types are drawn to distinguish between the oxygen provided by biodiesel and ethanol, with the continuous line corresponding to the ethanol effect and the dashed line indicating the biodiesel effect. The fuels tested are identified, for clarity, just in the left panel of Figure 12. The variation of gaseous emissions with the oxygen content is shown in Figure 12. The oxygen presence in the fuels favors higher oxidation of the hydrocarbons remaining from the combustion process. The results shown in Figure 12 with ED7.7 and EBD are similar to those obtained with diesel fuel, while ED17 presents much higher THC emissions. Therefore, oxygen is not the only parameter affecting THC emissions. The higher heat of vaporization of ethanol reduces the local combustion temperature, thus leading to increasing incomplete combustion.16,31 These effects could compensate each other for low ethanol and biodiesel presence not affecting THC emissions, while a significant increase of the emitted hydrocarbons is obtained for a higher ethanol content, especially at high load. In the case of NOx emissions (left panel of Figure 12), the results obtained with the blends are slightly lower than those
Figure 14. VOF variation rate.
with diesel fuel. Although the oxygen presence and the lower cetane number (derived from the addition of ethanol) favor NOx formation, the results show a slight decrease in this emission. This can be explained because the adiabatic flame temperature is much lower for ethanol and because the heat of vaporization of ethanol is much higher, with both effects counteracting the previous ones and leading to slight reductions of NOx emissions with respect to those from diesel fuel.36 Figure 13 shows the smoke opacity and PM emissions, respectively. The higher the ethanol content, the higher the (36) Can, O.; C-elikten, I.; Usta, N. Energy Convers. Manage. 2004, 45, 2429–2440.
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Figure 15. Mean diameter (dm) and particle number concentration (PNC) variation rates.
reduction in opacity, in both load conditions (except ED7.7 in low load). In this case, the effect of oxygen fuel in opacity is very significant. However, the reduction observed is not linear with the oxygen content, with the e-diesel blends presenting higher opacity reduction for the same oxygen content. This result indicates that the oxygen provided by ethanol is more efficient than the oxygen corresponding to biodiesel, trends in agreement with other authors.37 It can be concluded that the reduction in opacity does not only depend upon the fuel oxygen content but also the functional group providing the oxygen atoms. In the case of PM emissions, the trends are not so clear as in opacity. While with ED7.7 and EBD blends lineal reductions are obtained for PM emissions for increasing oxygen content, the results obtained with ED17 do not follow this trend; they show higher PM values than the other blends. The high THC emissions observed with this blend are probably contributing to its higher particle matter emissions. Therefore, although oxygen favors PM reduction, some fuel properties (volatility and heat of evaporation) should be taken in account to explain this emission. This is in agreement with some authors that consider that, from 20% ethanol content onward, PM reductions are not observed any more.31 The VOF values obtained with oxygenated blends are higher than the diesel one, as observed in Figure 14. The VOF values corresponding to ED17 are consistent with its high THC emissions. The VOF increase is not linear with the increasing fuel oxygen content. In this case, the oxygen provided by ethanol favors a higher VOF increase than that provided by biodiesel. The variations corresponding to dm and total PNC are shown in Figure 15. Some dm reduction was obtained with every other fuel tested, with these reductions being higher at high-load conditions. The dm corresponding to the ED17 blend is lower than that from EBD at high load. Therefore, the fuel oxygen provided by ethanol favors higher dm reductions than those provided by biodiesel. This can be explained by the reduced agglomeration derived from the lower soot concentration, corresponding to the lower smoke opacity observed (left panel of Figure 13). Again, the reduction obtained is not linear with the oxygen content. The left panel of Figure 15 shows the total PNC results with respect to the fuel oxygen content. The results indicate
that, while the blend with the lowest oxygen content presents similar values to the reference fuel, the values corresponding to blends with a higher oxygen content are significantly lower. Therefore, although no linear trend was observed for PNC with the oxygen content, it is clear that high oxygen fuel composition favors the reduction of the total PNC emitted and the differences between fuel oxygen derived from ethanol or biodiesel are small. 9. Conclusions The effect of biodiesel on the stability of e-diesel blends and the engine performance and emissions using different ethanol blends has been studied. The main conclusions are the following: (1) The phase separation process of e-b-diesel blends generates different types of unstable blends: two liquid phases, a gelatinous interphase, or the formation of a gelatinous phase in the bottom of the cell glass. The composition of the blend (ethanol and biodiesel content) and the temperature are the factors that have some influence on the type of phase. (2) Biodiesel fuel acts as a stabilizing component in e-diesel blends, but at low temperatures and with high ethanol content, it favors the appearance of a gelatinous phase. (3) The higher the ethanol content in the blend, the higher the THC emissions and VOF, while smoke opacity shows the opposite trend. With regard to NOx emissions, PM, and PNC, no clear trend was found when the ethanol content increases in the blends. (4) The variation of different emissions with respect to fuel oxygen content is not linear. Although ED17 and EBD present the same oxygen content, the results obtained with both fuels are different in most of the emissions. Therefore, it can be concluded that the effect of biodiesel or ethanol addition is different. (a) When the ethanol content is high, its higher heat of vaporization (with respect to diesel fuel) leads to increases in the hydrocarbons remaining from the combustion, thus becoming the main cause for the high THC emissions with ED17. (b) The opacity reduction obtained with ED17 is much higher that that corresponding to EBD, thus indicating that the oxygen provided by ethanol is more efficient that the oxygen provided by biodiesel. This trend is opposite in PM emissions (the emissions obtained with ED17 are higher than those obtained with EBD) because of the high THC emissions of the ED17 blend. (c) The blend with the highest ethanol content presents a nuclei mode because the high THC emissions favor the nucleation of hydrocarbons. The mean particle diameter corresponding to the accumulation mode of ED17 is lower than that obtained with EBD.
(37) Pepiot-Desjardins, P.; Pitsch, H.; Malhotra, R.; Kirby, S. R.; Boehman, A. L. Combust. Flame 2008, 154, 191–205.
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Therefore, the effect of fuel oxygen on the particle size depends upon its functional group.
project CENIT 2007-1031, IþDEA) and by the company Abengoa Bioenergy S.A. The companies Repsol and Nissan are also acknowledged for the supply of the reference and biodiesel fuels and for the donation of the engine tested, respectively.
Acknowledgment. The authors gratefully acknowledge the financial support provided by the Spanish CDTI (research
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