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Effects of ethyl-tert-butyl ether (ETBE) addition on the physicochemical properties of diesel oil and particulate matter and smoke emissions from diesel engines
Fuel 103 (2013) 1138–1143
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Fuel journal homepage: www.elsevier.com/locate/fuel
Short communication
Effects of ethyl-tert-butyl ether (ETBE) addition on the physicochemical properties of diesel oil and particulate matter and smoke emissions from diesel engines Krzysztof Górski a, Asok K. Sen b,⇑, Wincenty Lotko a, Marek Swat c a b c
Mechanical Faculty, Technical University of Radom, Chrobrego 45 Street, 26-600 Radom, Poland Richard G. Lugar Center for Renewable Energy, Indiana University, 402 N. Blackford Street, Indianapolis, IN 46202, USA Institute for Sustainable Technologies – National Research Institute, K. Pułaskiego 6/10 Street, 26-600 Radom, Poland
a r t i c l e
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Article history: Received 6 March 2012 Received in revised form 2 September 2012 Accepted 3 September 2012 Available online 16 September 2012 Keywords: Diesel fuel ETBE Physicochemical properties Particulate matter and smoke emissions
a b s t r a c t Oxygenates may be added to diesel oil to help reduce exhaust emissions from diesel engines. This study evaluates the effect of adding ethyl-tert-butyl ether (ETBE) to diesel oil on the physicochemical properties of the mixture. ETBE is added in volumetric proportions of 5%, 10%, 20%, 30%, and 40%, and its effects on density, viscosity, lubricity, surface tension, miscibility, and cetane number are investigated. While the effects of ETBE addition on density, viscosity, and cetane number have been examined earlier, this is the first study to investigate the effects of ETBE addition on surface tension, lubricity, and miscibility. The impact of ETBE addition on particulate matter (PM) and smoke emissions is also investigated. The results reveal that ETBE addition can significantly reduce PM emission. In particular, it was found that only 10% ETBE can decrease PM emission by 36%. ETBE addition was also found to reduce the smoke opacity, resulting in 70% reduction with 40% ETBE fraction at the engine speed of 1400 rpm and a load of 80 N m. This is the first study that reports the effect of ETBE addition on PM emission from an engine fueled by diesel–ETBE blends. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Diesel engines are more efficient than gasoline engines and are therefore widely used in heavy-duty applications such as trucks and buses as well as agricultural and mining equipment. However, a major concern with diesel engines is that they often emit undesirable amounts of harmful gases such as carbon monoxide (CO), nitrogen oxides (NOX), total hydrocarbons (THCs), and particulate matter (PM) and smoke into the atmosphere. Different measures have been proposed by researchers to minimize these exhaust emissions and thereby control air pollution. One such measure is to use oxygenates as additives to diesel oil [1–8]. Among the various oxygenates, ethanol has been shown to reduce the emission of some of the exhaust gases and PM [9–13]. Another oxygenate proposed for use with diesel oil is ethyl-tert-butyl ether (ETBE), which is synthesized from ethanol and isobutane. There have been a few studies on investigating the effect of ETBE addition to diesel oil. In a recent work, De Menezes et al. [14] examined the effect of adding ETBE to diesel oil on the physicochemical properties of the mixture. They added ETBE in volumetric proportions of 5%, 10%, 15%, and 20% to Brazilian diesel oil and examined their effect on density, viscosity, volatility, charac⇑ Corresponding author. E-mail address: [email protected] (A.K. Sen). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.09.004
teristics at low temperatures, and cetane number of the various formulations. It was found that ETBE can significantly alter the viscosity, volatility (flashpoint and distillation curve) and reduce the cetane number, impairing the fuel’s performance [14]. In contrast, the effect of ETBE addition on other properties, namely, density, and low temperature characteristics were not found to be very significant. Li et al. [15] investigated the effect of adding ETBE to diesel oil on the combustion characteristics and exhaust emissions of a common rail direct injection diesel engine with high rates of cooled exhaust gas recirculation (EGR). Test fuels were prepared by blending 10, 20, 30 and 40 vol% ETBE to diesel oil. They observed that increasing ETBE fraction in the fuel helps to suppress smoke emission increasing with EGR, but a too high fraction of ETBE leads to misfires at higher EGR rates. While the combustion noise and NOx emissions increase with an increase in ETBE fraction at relatively low EGR rates, they can be suppressed to low levels by increasing the rate of EGR. They also found that there was no significant increase in THC and CO emissions due to ETBE, but ETBE addition resulted in increased aldehyde emissions, compared to diesel oil, especially at low EGR levels. Several researchers have investigated the effect of adding oxygenates other than ETBE to diesel oil on smoke emissions. Among them, Kozak [16] reported a significant decrease in smoke emissions in a diesel engine fueled by a mixture of diesel oil and n-butanol. Lei et al. [17] found that ethanol added to diesel oil can
K. Górski et al. / Fuel 103 (2013) 1138–1143
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Table 1 Selected physicochemical properties of the diesel oil and ETBE. Property 3
Density at 25 °C (kg/m ) Kinematic viscosity at 40 °C (mm2/s) Surface tension (mN/m) Cetane number H/C ratio (%) Ignition temperature (°C) Auto-ignition temperature (°C)
Diesel-B
ETBE
834 2.81 25.8 52.2 16.3 72 240
739 0.45 20.5 8 19.6 19 310
lead to about 60% reduction in smoke emission compared to diesel oil. Choi and Oh [18] showed that smoke emission can be reduced approximately 26% when the diesel oil is added with 20% (v/v) of butyl ether. Ogawa et al. [19] used dimethoxy methane (DMM) as the oxygenate, and observed lower smoke emissions. Using 2-methoxyethyl ether (diglyme) blends with diesel oil, both Nabi and Hustad [20], and Di et al. [21] found significant reductions in smoke emissions. Kannan and Marappan [22] used diethyl ether (DEE) as an oxygenated additive to diesel oil and observed major improvements in engine performance and exhaust emissions. In particular, they found that DEE addition lowers the viscosity improving fuel atomization and thereby increasing brake thermal efficiency. The higher oxygen content of DEE also reduced the smoke opacity. The objective of this study is to reexamine the effect of ETBE addition to diesel oil on the physicochemical properties of the mixture, and to estimate its effect on PM and smoke emissions from a diesel engine. Through this investigation we confirm some of the findings of De Menezes et al. [14], and also extend their work to investigating the effect of ETBE on other physicochemical properties such as surface tension, lubricity, and miscibility. In addition, we present, for the first time, the effect of ETBE addition to diesel oil on PM emission from a diesel engine. 2. Materials and methods In our experiments we used grade B European diesel oil. ETBE was blended with the diesel oil in volumetric proportions of 5%, 10%, 20%, 30% and 40%. Selected physicochemical properties of the tested diesel oil and ETBE are listed in Table 1. Our engine tests were performed as follows. The cetane numbers of diesel oil and diesel–ETBE blends were determined on a Waukesha engine. This engine meets the requirements of the European standard EN 5165:2003 [23], which is equivalent to the US norm ASTM D613-10a [24]. The PM emission was measured on an ADCR engine using the PIERBURG PTP 2000 test bench. This is a 4-stroke, 4-cylinder, water-cooled, turbocharged direct-injection unit equipped with a BOSCH common rail fuel injection system. The PTP 2000 system measures the soot particles in the exhaust stream. This is done by sampling a portion of the exhaust gases which are mixed with air and then the particles are collected on the filter surface. The measurement procedure uses a 13-mode test cycle introduced by ECE regulation No. 49 and adopted by the European Economic Community [25]. Smoke emission tests were conducted at engine speeds of 1000, 1400 and 1800 rpm under loads of 80 and 120 N m. An AVL 465 diGas analyzer was used. It contains a smoke meter that measures the smoke opacity by measuring the amount of light transmitted from a source which is prevented from reaching a light detector.
Fig. 1. Density variations of diesel oil and diesel–ETBE blends.
[26,27]. Diesel fuel injection systems are designed to precisely meter the required volume of fuel into the combustion chamber. All diesel injection systems meter the fuel on a volume basis; so the fuel density affects the mass of fuel injected. Accordingly, a fuel with higher density will produce more power; however, a denser fuel may also increase smoke emission. In their work with Brazilian diesel oil and diesel–ETBE blends, De Menezes et al. [14] found that an increase in the content of ETBE in the blend reduced the density of the blend. This is attributed to the fact that ETBE has lower density than diesel oil (see Table 1). They used ETBE fractions of 5%, 10% and 20% by volume. Their results indicate that addition of 20% ETBE decreases the density by about 2%. Exactly the same density reduction was observed in our experiments when ETBE was mixed with European diesel oil (see Fig. 1). As we increased the proportion of ETBE, the density continued to decrease almost linearly with the amount of ETBE added. Overall, the changes in density due to ETBE addition were not very significant. It should be noted that the European standard EN 590:2006 [28] specifies a narrow band of 820–845 kg/m3 for fuel density measured at 15 °C. Accordingly, only two of the tested diesel–ETBE blends which contain 5% and 10% of ETBE meet the requirements of the European standard. The other ETBE blends have densities lower than the specified low value of 820 kg/m3. 3.2. Viscosity
3. Results and discussion
Viscosity is a measure of the fuel’s resistance to flow shear. It impacts the fuel spray characteristics through flow resistance inside the injection system and in the nozzle holes [26]. The viscosity should not be too high or too low. Higher viscosity generally results in reduced flow rates for equal injection pressure and degrades atomization. On the other hand, low viscosity increases the flow rates leading to higher efficiency, but may also result in excessive leakage in the fuel pump and fuel injector. According to the European standard EN 590:2006 [28], the kinematic viscosity of the fuel measured at 40 °C should be kept in the range of 2–4.5 mm2/s. We measured the viscosity of the diesel oil and its blends with ETBE. The results are illustrated in Fig. 2. It is seen from this figure that ETBE addition can significantly reduce the viscosity. These findings are in agreement with those reported by De Menezes et al. [14]. It should be pointed that some of diesel–ETBE blends containing higher than 20% ETBE have smaller viscosity values than those required by EN 590:2006.
3.1. Density
3.3. Surface tension
The fuel properties that have a significant impact on the injection system include density, viscosity, and surface tension
Surface tension of a fuel is the property that describes its tendency to form droplets. As a consequence, it can have a major im-
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Fig. 2. Variations of kinematic viscosity of diesel oil and diesel–ETBE blends.
Fig. 3. Impact of ETBE addition to diesel oil on surface tension.
pact on fuel atomization, which is the first stage of the combustion process [29]. A correct atomization allows proper mixing of fuel with air and facilitates complete combustion in a diesel engine, reducing emissions and increasing engine efficiency [30,31]. A low value of surface tension helps to form smaller fuel droplets, whereas higher surface tensions make droplet formation difficult. In this study, the surface tensions of the diesel oil and diesel– ETBE blends were measured at 20 °C using a LAUDA ring tensiometer, according to the European standard EN 14370:2005 [32]. The ring was initially dipped into the fuel and then slowly withdrawn from the fuel. The force at the surface of contact between the ring and the fuel was measured. The maximum value of this force divided by the ring’s length gives a measure of the surface tension of the fuel. Fig. 3 depicts the values of the surface tension for the various formulations. It can be seen from this figure that increasing the fraction of ETBE decreases the surface tension. Although these reductions are not large, they demonstrate that addition of ETBE to diesel oil can improve the fuel atomization process, thereby improving the thermal efficiency of the engine and decreasing exhaust emissions. Surface tension of a liquid depends on the magnitude of the cohesive forces between the molecules of liquid. In the case of ETBE, these forces are less in comparison with those which are typical for intermolecular interactions in diesel oil. For homogenous diesel–ETBE blends, all molecules interact with each other, but the intermolecular forces are still less than those for diesel oil, but higher than those for ETBE.
lubricity is the property that characterizes the extent to which the fuel can provide boundary lubrication at a metal-to-metal contact surface. A fuel with poor lubricity can increase the fuel injection pump and injection nozzle wear. For this reason, diesel fuel lubricity has to be kept at an adequately high level [33]. We measured the lubricity of diesel oil and various diesel–ETBE blends using the high-frequency reciprocating rig (HFRR) method. The HFFR method is approved by the European standard EN 121561:2006 [34]. This test uses a steel ball which is held against a stationary disk dipped in the fuel. The ball is moved back and forth across the disk at a frequency of 50 Hz, under an applied load of 200 g. The test duration is 75 min. The wear scar produced on the disk is viewed under a microscope, and its diameter is measured in micrometers. The wear scar diameter (WSD) is used as a measure of fuel lubricity. A higher value of WSD represents a lower lubricity, i.e., the fuel has poorer lubricating properties. The HFRR test can be carried out at different temperatures. Due to the fact that ETBE tends to vaporize easily, our experiments were performed in a closed cup at 25 °C. It is recommended that at this temperature the WSD should not be larger than 380 lm [35]. Fig. 4 shows the variation of WSD for diesel oil and diesel–ETBE blends. We see that increasing the ETBE fraction increases the WSD indicating that lubricity of the tested diesel–ETBE blends is lower than that of diesel oil. Nevertheless, the lubricity of all the tested diesel–ETBE blends is at an acceptable level, i.e. the WSD is significantly below the limit of 380 lm. In the case of diesel oil mixed with 10% ETBE, the WSD is 253 lm, which is 14% higher than diesel oil. The lubricity of the diesel–ETBE blends can be enhanced by adding suitable lubricity additives [36]. These additives contain a polar or hydrophilic group at one end of the molecule and a hydrophobic group such as a long-chain alkyl group at the other end. The polar group attaches to the metal surface as a monomolecular layer, whereas the non-polar tail is miscible with the hydrocarbon fuel phase. Fatty acid methyl ester (FAME) is one of the commonly used oxygenated additives which is found to significantly improve the lubricity properties of diesel oil. 3.5. Miscibility All oxygenates are not easily miscible with diesel oil. It is well known that plant oils as well as FAME can be easily blended with diesel oil. Furthermore, water which may be present in such fuel blends does not promote phase separation that is typical for diesel–ethanol blends [37,38]. One of the main barriers to the use of ethanol in diesel oil is its limited miscibility, especially at lower temperatures [39]. A practical way to improve diesel–ethanol blend stability is to use FAME as a third component of mixture.
3.4. Lubricity The fuel should prevent elements of the injection system such as the fuel pump and fuel injector against excessive wear. Fuel
Fig. 4. Impact of ETBE addition to diesel oil on wear scar diameter.
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In this case, FAME acts as an emulsifier which helps blending of ethanol with diesel oil [37,38]. However, FAME has a tendency to oxidize and it also has poor low-temperature properties. For this reason, FAME–ethanol–diesel blends are not stable at temperatures typically below 0 °C [37]. Diesel–ETBE blends do not have these disadvantages; accordingly, it may be more attractive as an ecofriendly fuel for diesel engines. The ETBE molecule, in contrast to ethanol, does not contain a polarized OH group. By virtue of this, ETBE can be easily mixed with diesel oil over a wide range of temperatures. We have checked the stability of such mixtures using two fuel samples, which were stored in a dark place over several weeks at temperatures ranging from 10 to 30 °C. One of the samples contained neat ethanol mixed with diesel oil in the proportion 50:50% by volume and the other sample contained a diesel–ETBE blend in the same proportion. It was found that both samples stored at 30 °C were homogeneous without visible phase separation. We also found that the diesel–ethanol blends begin to separate when the temperature went below 20 °C. At temperatures of 10 °C and 0 °C, the diesel– ethanol blend was separated into two clear phases. At 10 °C, the phase separation was even more visible, with a clear phase separated by a gel-like phase. In contrast, the diesel–ETBE sample was still in one clear phase over the entire period of observation. These tests reveal that the diesel–ETBE blends are more easily mixed and are more stable in a wide range of temperatures than the diesel–ethanol blends. We also found that water does not promote phase separation of diesel–ETBE blends. These observations collectively suggest that, from the standpoint of miscibility and stability, ETBE, compared to ethanol, is a more effective oxygenate for use an additive with diesel oil. 3.6. Cetane number The cetane number measures the readiness of the fuel to autoignite when injected into the combustion chamber and is one of the most significant properties to specify the ignition quality of any fuel for compression ignition engines. A higher cetane number decreases the time delay between fuel injection and ignition. A shorter ignition delay implies a lower rate of pressure rise and lower peak temperature, resulting in reduced NOX, CO, THC and PM emissions. One of the more obvious effects of running on a low cetane number fuel is an increase in engine noise [36]. The cetane numbers of the diesel oil and diesel–ETBE blends considered here were determined in a Waukesha engine by comparison with two reference hydrocarbon fuels, using the procedure described in EN 5165:2003 [23] or ASTM D613-10a [24]. The results are depicted in Fig. 5. These results indicate that the CN of all tested blends were under the lower limit (CN = 51) specified
Fig. 5. Impact of ETBE addition to diesel oil on cetane number.
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in EN 5165:2003 standard [23]. Both ethanol and ETBE have very low cetane numbers compared to diesel oil [40,41]. The CN of ethanol is estimated to be 8, whereas that of ETBE is estimated to be 2.5. Thus, increasing the fraction of ETBE in diesel oil leads to a decrease in the CN-value. As can be seen in Fig. 5, only 5% (v/v) of ETBE blended with diesel oil reduces its CN by about 7%, whereas addition of 40% ETBE reduces the CN by 38%. Lowered cetane number due to ETBE addition may cause a high rate of in-cylinder pressure rise and thus deteriorate thermal efficiency or fuel economy [15]. The ignition quality of the diesel–ETBE blends can be improved by adding a cetane number improver which decreases the ignition delay. One of the cetane number improvers that have been used effectively is 2-ethyl-hexyl nitrate (EHN). The improvement in ignition quality is achieved through an increase in cetane number [36]. 3.7. PM emission As mentioned in the Introduction, the PM emission from an engine fueled by diesel oil and various diesel–ETBE blends was estimated by measuring the concentration of the soot particles in the exhaust stream. A 13-mode test cycle was used for measurement [25]. Fig. 6 depicts the variation of particulate matter concentration with the volume fraction of ETBE addition. It is apparent from this figure that only 10% ETBE can decrease PM emission by about 36%. With the addition of 20%, 30% and 40% ETBE, these reductions are 45%, 48%, and 61%, respectively. Clearly, these results reveal that ETBE has a major impact on reducing PM emission. It should be pointed out that the H/C ratio of an ETBE molecule is higher than that of diesel oil. In other words, an ETBE molecule contains more hydrogen atoms than a molecule of diesel oil. Since the combustion of hydrogen is known to be one of the safest modes of combustion from an environmental viewpoint, it is not surprising that addition of ETBE to diesel oil can significantly reduce the PM emission. 3.8. Smoke emission Smoke emission measurements were carried out at engine speeds of 1000, 1400, and 1800 rpm, and at loads 80 and 120 N m. The results are illustrated in Figs. 7–9. It is apparent from these figures that, with the addition of 10% ETBE, smoke opacity was reduced by 15–22%, 16–18%, and 14–19%, respectively, at speeds of 1000, 1400 and 1800 rpm, compared to diesel oil. Higher volume fractions of ETBE further reduced the smoke opacity, and in particular, 70% reduction in smoke opacity was achieved with the addition of 40% ETBE at the speed of 1400 rpm and at the load of 80 N m.
Fig. 6. Effect of ETBE addition to diesel oil on PM emission.
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(1) Adding ETBE to diesel oil does not significantly alter the density and viscosity of the formulation. These observations are in agreement with those of De Menezes et al. [14]. (2) ETBE addition can lower surface tension and improve fuel atomization thereby increasing thermal efficiency and reducing exhaust emissions. (3) Compared to ethanol, ETBE is more readily miscible with diesel oil, and the mixture remains stable at a wider range of temperatures (from 10 to 30 °C). The miscibility of ETBE with diesel oil was found to be significantly better than the diesel–ethanol blends, especially at low temperatures. (4) Blending diesel oil with ETBE can significantly reduce PM and smoke emissions from a diesel engine. Fig. 7. Effect of ETBE addition to diesel oil on smoke emission for the diesel engine operating at 1000 rpm.
In summary, our results indicate that small amounts of ETBE can be used effectively as an oxygenated additive to diesel oil in a diesel engine. Such mixtures can significantly reduce PM and smoke emissions while altering the physicochemical properties within acceptable limits. References
Fig. 8. Effect of ETBE addition to diesel oil on smoke emission for the diesel engine operating at 1400 rpm.
Fig. 9. Effect of ETBE addition to diesel oil on smoke emission for the diesel engine operating at 1800 rpm.
Smoke is produced when the combustible mixture is deficient in oxygen, especially if fuel atomization is poor. Due to its higher oxygen content, ETBE can reduce smoke emission when added to diesel oil. In addition, ETBE decreases the surface tension thereby improving the fuel atomization process and reducing smoke emission. 4. Summary and conclusions We have investigated the effect of adding ETBE to European diesel oil on the physicochemical properties of the mixture, and PM and smoke emissions. Based on the present study, the following conclusions can be made.
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Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Short communication
Effects of ethyl-tert-butyl ether (ETBE) addition on the physicochemical properties of diesel oil and particulate matter and smoke emissions from diesel engines Krzysztof Górski a, Asok K. Sen b,⇑, Wincenty Lotko a, Marek Swat c a b c
Mechanical Faculty, Technical University of Radom, Chrobrego 45 Street, 26-600 Radom, Poland Richard G. Lugar Center for Renewable Energy, Indiana University, 402 N. Blackford Street, Indianapolis, IN 46202, USA Institute for Sustainable Technologies – National Research Institute, K. Pułaskiego 6/10 Street, 26-600 Radom, Poland
a r t i c l e
i n f o
Article history: Received 6 March 2012 Received in revised form 2 September 2012 Accepted 3 September 2012 Available online 16 September 2012 Keywords: Diesel fuel ETBE Physicochemical properties Particulate matter and smoke emissions
a b s t r a c t Oxygenates may be added to diesel oil to help reduce exhaust emissions from diesel engines. This study evaluates the effect of adding ethyl-tert-butyl ether (ETBE) to diesel oil on the physicochemical properties of the mixture. ETBE is added in volumetric proportions of 5%, 10%, 20%, 30%, and 40%, and its effects on density, viscosity, lubricity, surface tension, miscibility, and cetane number are investigated. While the effects of ETBE addition on density, viscosity, and cetane number have been examined earlier, this is the first study to investigate the effects of ETBE addition on surface tension, lubricity, and miscibility. The impact of ETBE addition on particulate matter (PM) and smoke emissions is also investigated. The results reveal that ETBE addition can significantly reduce PM emission. In particular, it was found that only 10% ETBE can decrease PM emission by 36%. ETBE addition was also found to reduce the smoke opacity, resulting in 70% reduction with 40% ETBE fraction at the engine speed of 1400 rpm and a load of 80 N m. This is the first study that reports the effect of ETBE addition on PM emission from an engine fueled by diesel–ETBE blends. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Diesel engines are more efficient than gasoline engines and are therefore widely used in heavy-duty applications such as trucks and buses as well as agricultural and mining equipment. However, a major concern with diesel engines is that they often emit undesirable amounts of harmful gases such as carbon monoxide (CO), nitrogen oxides (NOX), total hydrocarbons (THCs), and particulate matter (PM) and smoke into the atmosphere. Different measures have been proposed by researchers to minimize these exhaust emissions and thereby control air pollution. One such measure is to use oxygenates as additives to diesel oil [1–8]. Among the various oxygenates, ethanol has been shown to reduce the emission of some of the exhaust gases and PM [9–13]. Another oxygenate proposed for use with diesel oil is ethyl-tert-butyl ether (ETBE), which is synthesized from ethanol and isobutane. There have been a few studies on investigating the effect of ETBE addition to diesel oil. In a recent work, De Menezes et al. [14] examined the effect of adding ETBE to diesel oil on the physicochemical properties of the mixture. They added ETBE in volumetric proportions of 5%, 10%, 15%, and 20% to Brazilian diesel oil and examined their effect on density, viscosity, volatility, charac⇑ Corresponding author. E-mail address: [email protected] (A.K. Sen). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.09.004
teristics at low temperatures, and cetane number of the various formulations. It was found that ETBE can significantly alter the viscosity, volatility (flashpoint and distillation curve) and reduce the cetane number, impairing the fuel’s performance [14]. In contrast, the effect of ETBE addition on other properties, namely, density, and low temperature characteristics were not found to be very significant. Li et al. [15] investigated the effect of adding ETBE to diesel oil on the combustion characteristics and exhaust emissions of a common rail direct injection diesel engine with high rates of cooled exhaust gas recirculation (EGR). Test fuels were prepared by blending 10, 20, 30 and 40 vol% ETBE to diesel oil. They observed that increasing ETBE fraction in the fuel helps to suppress smoke emission increasing with EGR, but a too high fraction of ETBE leads to misfires at higher EGR rates. While the combustion noise and NOx emissions increase with an increase in ETBE fraction at relatively low EGR rates, they can be suppressed to low levels by increasing the rate of EGR. They also found that there was no significant increase in THC and CO emissions due to ETBE, but ETBE addition resulted in increased aldehyde emissions, compared to diesel oil, especially at low EGR levels. Several researchers have investigated the effect of adding oxygenates other than ETBE to diesel oil on smoke emissions. Among them, Kozak [16] reported a significant decrease in smoke emissions in a diesel engine fueled by a mixture of diesel oil and n-butanol. Lei et al. [17] found that ethanol added to diesel oil can
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Table 1 Selected physicochemical properties of the diesel oil and ETBE. Property 3
Density at 25 °C (kg/m ) Kinematic viscosity at 40 °C (mm2/s) Surface tension (mN/m) Cetane number H/C ratio (%) Ignition temperature (°C) Auto-ignition temperature (°C)
Diesel-B
ETBE
834 2.81 25.8 52.2 16.3 72 240
739 0.45 20.5 8 19.6 19 310
lead to about 60% reduction in smoke emission compared to diesel oil. Choi and Oh [18] showed that smoke emission can be reduced approximately 26% when the diesel oil is added with 20% (v/v) of butyl ether. Ogawa et al. [19] used dimethoxy methane (DMM) as the oxygenate, and observed lower smoke emissions. Using 2-methoxyethyl ether (diglyme) blends with diesel oil, both Nabi and Hustad [20], and Di et al. [21] found significant reductions in smoke emissions. Kannan and Marappan [22] used diethyl ether (DEE) as an oxygenated additive to diesel oil and observed major improvements in engine performance and exhaust emissions. In particular, they found that DEE addition lowers the viscosity improving fuel atomization and thereby increasing brake thermal efficiency. The higher oxygen content of DEE also reduced the smoke opacity. The objective of this study is to reexamine the effect of ETBE addition to diesel oil on the physicochemical properties of the mixture, and to estimate its effect on PM and smoke emissions from a diesel engine. Through this investigation we confirm some of the findings of De Menezes et al. [14], and also extend their work to investigating the effect of ETBE on other physicochemical properties such as surface tension, lubricity, and miscibility. In addition, we present, for the first time, the effect of ETBE addition to diesel oil on PM emission from a diesel engine. 2. Materials and methods In our experiments we used grade B European diesel oil. ETBE was blended with the diesel oil in volumetric proportions of 5%, 10%, 20%, 30% and 40%. Selected physicochemical properties of the tested diesel oil and ETBE are listed in Table 1. Our engine tests were performed as follows. The cetane numbers of diesel oil and diesel–ETBE blends were determined on a Waukesha engine. This engine meets the requirements of the European standard EN 5165:2003 [23], which is equivalent to the US norm ASTM D613-10a [24]. The PM emission was measured on an ADCR engine using the PIERBURG PTP 2000 test bench. This is a 4-stroke, 4-cylinder, water-cooled, turbocharged direct-injection unit equipped with a BOSCH common rail fuel injection system. The PTP 2000 system measures the soot particles in the exhaust stream. This is done by sampling a portion of the exhaust gases which are mixed with air and then the particles are collected on the filter surface. The measurement procedure uses a 13-mode test cycle introduced by ECE regulation No. 49 and adopted by the European Economic Community [25]. Smoke emission tests were conducted at engine speeds of 1000, 1400 and 1800 rpm under loads of 80 and 120 N m. An AVL 465 diGas analyzer was used. It contains a smoke meter that measures the smoke opacity by measuring the amount of light transmitted from a source which is prevented from reaching a light detector.
Fig. 1. Density variations of diesel oil and diesel–ETBE blends.
[26,27]. Diesel fuel injection systems are designed to precisely meter the required volume of fuel into the combustion chamber. All diesel injection systems meter the fuel on a volume basis; so the fuel density affects the mass of fuel injected. Accordingly, a fuel with higher density will produce more power; however, a denser fuel may also increase smoke emission. In their work with Brazilian diesel oil and diesel–ETBE blends, De Menezes et al. [14] found that an increase in the content of ETBE in the blend reduced the density of the blend. This is attributed to the fact that ETBE has lower density than diesel oil (see Table 1). They used ETBE fractions of 5%, 10% and 20% by volume. Their results indicate that addition of 20% ETBE decreases the density by about 2%. Exactly the same density reduction was observed in our experiments when ETBE was mixed with European diesel oil (see Fig. 1). As we increased the proportion of ETBE, the density continued to decrease almost linearly with the amount of ETBE added. Overall, the changes in density due to ETBE addition were not very significant. It should be noted that the European standard EN 590:2006 [28] specifies a narrow band of 820–845 kg/m3 for fuel density measured at 15 °C. Accordingly, only two of the tested diesel–ETBE blends which contain 5% and 10% of ETBE meet the requirements of the European standard. The other ETBE blends have densities lower than the specified low value of 820 kg/m3. 3.2. Viscosity
3. Results and discussion
Viscosity is a measure of the fuel’s resistance to flow shear. It impacts the fuel spray characteristics through flow resistance inside the injection system and in the nozzle holes [26]. The viscosity should not be too high or too low. Higher viscosity generally results in reduced flow rates for equal injection pressure and degrades atomization. On the other hand, low viscosity increases the flow rates leading to higher efficiency, but may also result in excessive leakage in the fuel pump and fuel injector. According to the European standard EN 590:2006 [28], the kinematic viscosity of the fuel measured at 40 °C should be kept in the range of 2–4.5 mm2/s. We measured the viscosity of the diesel oil and its blends with ETBE. The results are illustrated in Fig. 2. It is seen from this figure that ETBE addition can significantly reduce the viscosity. These findings are in agreement with those reported by De Menezes et al. [14]. It should be pointed that some of diesel–ETBE blends containing higher than 20% ETBE have smaller viscosity values than those required by EN 590:2006.
3.1. Density
3.3. Surface tension
The fuel properties that have a significant impact on the injection system include density, viscosity, and surface tension
Surface tension of a fuel is the property that describes its tendency to form droplets. As a consequence, it can have a major im-
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Fig. 2. Variations of kinematic viscosity of diesel oil and diesel–ETBE blends.
Fig. 3. Impact of ETBE addition to diesel oil on surface tension.
pact on fuel atomization, which is the first stage of the combustion process [29]. A correct atomization allows proper mixing of fuel with air and facilitates complete combustion in a diesel engine, reducing emissions and increasing engine efficiency [30,31]. A low value of surface tension helps to form smaller fuel droplets, whereas higher surface tensions make droplet formation difficult. In this study, the surface tensions of the diesel oil and diesel– ETBE blends were measured at 20 °C using a LAUDA ring tensiometer, according to the European standard EN 14370:2005 [32]. The ring was initially dipped into the fuel and then slowly withdrawn from the fuel. The force at the surface of contact between the ring and the fuel was measured. The maximum value of this force divided by the ring’s length gives a measure of the surface tension of the fuel. Fig. 3 depicts the values of the surface tension for the various formulations. It can be seen from this figure that increasing the fraction of ETBE decreases the surface tension. Although these reductions are not large, they demonstrate that addition of ETBE to diesel oil can improve the fuel atomization process, thereby improving the thermal efficiency of the engine and decreasing exhaust emissions. Surface tension of a liquid depends on the magnitude of the cohesive forces between the molecules of liquid. In the case of ETBE, these forces are less in comparison with those which are typical for intermolecular interactions in diesel oil. For homogenous diesel–ETBE blends, all molecules interact with each other, but the intermolecular forces are still less than those for diesel oil, but higher than those for ETBE.
lubricity is the property that characterizes the extent to which the fuel can provide boundary lubrication at a metal-to-metal contact surface. A fuel with poor lubricity can increase the fuel injection pump and injection nozzle wear. For this reason, diesel fuel lubricity has to be kept at an adequately high level [33]. We measured the lubricity of diesel oil and various diesel–ETBE blends using the high-frequency reciprocating rig (HFRR) method. The HFFR method is approved by the European standard EN 121561:2006 [34]. This test uses a steel ball which is held against a stationary disk dipped in the fuel. The ball is moved back and forth across the disk at a frequency of 50 Hz, under an applied load of 200 g. The test duration is 75 min. The wear scar produced on the disk is viewed under a microscope, and its diameter is measured in micrometers. The wear scar diameter (WSD) is used as a measure of fuel lubricity. A higher value of WSD represents a lower lubricity, i.e., the fuel has poorer lubricating properties. The HFRR test can be carried out at different temperatures. Due to the fact that ETBE tends to vaporize easily, our experiments were performed in a closed cup at 25 °C. It is recommended that at this temperature the WSD should not be larger than 380 lm [35]. Fig. 4 shows the variation of WSD for diesel oil and diesel–ETBE blends. We see that increasing the ETBE fraction increases the WSD indicating that lubricity of the tested diesel–ETBE blends is lower than that of diesel oil. Nevertheless, the lubricity of all the tested diesel–ETBE blends is at an acceptable level, i.e. the WSD is significantly below the limit of 380 lm. In the case of diesel oil mixed with 10% ETBE, the WSD is 253 lm, which is 14% higher than diesel oil. The lubricity of the diesel–ETBE blends can be enhanced by adding suitable lubricity additives [36]. These additives contain a polar or hydrophilic group at one end of the molecule and a hydrophobic group such as a long-chain alkyl group at the other end. The polar group attaches to the metal surface as a monomolecular layer, whereas the non-polar tail is miscible with the hydrocarbon fuel phase. Fatty acid methyl ester (FAME) is one of the commonly used oxygenated additives which is found to significantly improve the lubricity properties of diesel oil. 3.5. Miscibility All oxygenates are not easily miscible with diesel oil. It is well known that plant oils as well as FAME can be easily blended with diesel oil. Furthermore, water which may be present in such fuel blends does not promote phase separation that is typical for diesel–ethanol blends [37,38]. One of the main barriers to the use of ethanol in diesel oil is its limited miscibility, especially at lower temperatures [39]. A practical way to improve diesel–ethanol blend stability is to use FAME as a third component of mixture.
3.4. Lubricity The fuel should prevent elements of the injection system such as the fuel pump and fuel injector against excessive wear. Fuel
Fig. 4. Impact of ETBE addition to diesel oil on wear scar diameter.
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In this case, FAME acts as an emulsifier which helps blending of ethanol with diesel oil [37,38]. However, FAME has a tendency to oxidize and it also has poor low-temperature properties. For this reason, FAME–ethanol–diesel blends are not stable at temperatures typically below 0 °C [37]. Diesel–ETBE blends do not have these disadvantages; accordingly, it may be more attractive as an ecofriendly fuel for diesel engines. The ETBE molecule, in contrast to ethanol, does not contain a polarized OH group. By virtue of this, ETBE can be easily mixed with diesel oil over a wide range of temperatures. We have checked the stability of such mixtures using two fuel samples, which were stored in a dark place over several weeks at temperatures ranging from 10 to 30 °C. One of the samples contained neat ethanol mixed with diesel oil in the proportion 50:50% by volume and the other sample contained a diesel–ETBE blend in the same proportion. It was found that both samples stored at 30 °C were homogeneous without visible phase separation. We also found that the diesel–ethanol blends begin to separate when the temperature went below 20 °C. At temperatures of 10 °C and 0 °C, the diesel– ethanol blend was separated into two clear phases. At 10 °C, the phase separation was even more visible, with a clear phase separated by a gel-like phase. In contrast, the diesel–ETBE sample was still in one clear phase over the entire period of observation. These tests reveal that the diesel–ETBE blends are more easily mixed and are more stable in a wide range of temperatures than the diesel–ethanol blends. We also found that water does not promote phase separation of diesel–ETBE blends. These observations collectively suggest that, from the standpoint of miscibility and stability, ETBE, compared to ethanol, is a more effective oxygenate for use an additive with diesel oil. 3.6. Cetane number The cetane number measures the readiness of the fuel to autoignite when injected into the combustion chamber and is one of the most significant properties to specify the ignition quality of any fuel for compression ignition engines. A higher cetane number decreases the time delay between fuel injection and ignition. A shorter ignition delay implies a lower rate of pressure rise and lower peak temperature, resulting in reduced NOX, CO, THC and PM emissions. One of the more obvious effects of running on a low cetane number fuel is an increase in engine noise [36]. The cetane numbers of the diesel oil and diesel–ETBE blends considered here were determined in a Waukesha engine by comparison with two reference hydrocarbon fuels, using the procedure described in EN 5165:2003 [23] or ASTM D613-10a [24]. The results are depicted in Fig. 5. These results indicate that the CN of all tested blends were under the lower limit (CN = 51) specified
Fig. 5. Impact of ETBE addition to diesel oil on cetane number.
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in EN 5165:2003 standard [23]. Both ethanol and ETBE have very low cetane numbers compared to diesel oil [40,41]. The CN of ethanol is estimated to be 8, whereas that of ETBE is estimated to be 2.5. Thus, increasing the fraction of ETBE in diesel oil leads to a decrease in the CN-value. As can be seen in Fig. 5, only 5% (v/v) of ETBE blended with diesel oil reduces its CN by about 7%, whereas addition of 40% ETBE reduces the CN by 38%. Lowered cetane number due to ETBE addition may cause a high rate of in-cylinder pressure rise and thus deteriorate thermal efficiency or fuel economy [15]. The ignition quality of the diesel–ETBE blends can be improved by adding a cetane number improver which decreases the ignition delay. One of the cetane number improvers that have been used effectively is 2-ethyl-hexyl nitrate (EHN). The improvement in ignition quality is achieved through an increase in cetane number [36]. 3.7. PM emission As mentioned in the Introduction, the PM emission from an engine fueled by diesel oil and various diesel–ETBE blends was estimated by measuring the concentration of the soot particles in the exhaust stream. A 13-mode test cycle was used for measurement [25]. Fig. 6 depicts the variation of particulate matter concentration with the volume fraction of ETBE addition. It is apparent from this figure that only 10% ETBE can decrease PM emission by about 36%. With the addition of 20%, 30% and 40% ETBE, these reductions are 45%, 48%, and 61%, respectively. Clearly, these results reveal that ETBE has a major impact on reducing PM emission. It should be pointed out that the H/C ratio of an ETBE molecule is higher than that of diesel oil. In other words, an ETBE molecule contains more hydrogen atoms than a molecule of diesel oil. Since the combustion of hydrogen is known to be one of the safest modes of combustion from an environmental viewpoint, it is not surprising that addition of ETBE to diesel oil can significantly reduce the PM emission. 3.8. Smoke emission Smoke emission measurements were carried out at engine speeds of 1000, 1400, and 1800 rpm, and at loads 80 and 120 N m. The results are illustrated in Figs. 7–9. It is apparent from these figures that, with the addition of 10% ETBE, smoke opacity was reduced by 15–22%, 16–18%, and 14–19%, respectively, at speeds of 1000, 1400 and 1800 rpm, compared to diesel oil. Higher volume fractions of ETBE further reduced the smoke opacity, and in particular, 70% reduction in smoke opacity was achieved with the addition of 40% ETBE at the speed of 1400 rpm and at the load of 80 N m.
Fig. 6. Effect of ETBE addition to diesel oil on PM emission.
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(1) Adding ETBE to diesel oil does not significantly alter the density and viscosity of the formulation. These observations are in agreement with those of De Menezes et al. [14]. (2) ETBE addition can lower surface tension and improve fuel atomization thereby increasing thermal efficiency and reducing exhaust emissions. (3) Compared to ethanol, ETBE is more readily miscible with diesel oil, and the mixture remains stable at a wider range of temperatures (from 10 to 30 °C). The miscibility of ETBE with diesel oil was found to be significantly better than the diesel–ethanol blends, especially at low temperatures. (4) Blending diesel oil with ETBE can significantly reduce PM and smoke emissions from a diesel engine. Fig. 7. Effect of ETBE addition to diesel oil on smoke emission for the diesel engine operating at 1000 rpm.
In summary, our results indicate that small amounts of ETBE can be used effectively as an oxygenated additive to diesel oil in a diesel engine. Such mixtures can significantly reduce PM and smoke emissions while altering the physicochemical properties within acceptable limits. References
Fig. 8. Effect of ETBE addition to diesel oil on smoke emission for the diesel engine operating at 1400 rpm.
Fig. 9. Effect of ETBE addition to diesel oil on smoke emission for the diesel engine operating at 1800 rpm.
Smoke is produced when the combustible mixture is deficient in oxygen, especially if fuel atomization is poor. Due to its higher oxygen content, ETBE can reduce smoke emission when added to diesel oil. In addition, ETBE decreases the surface tension thereby improving the fuel atomization process and reducing smoke emission. 4. Summary and conclusions We have investigated the effect of adding ETBE to European diesel oil on the physicochemical properties of the mixture, and PM and smoke emissions. Based on the present study, the following conclusions can be made.
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