Article pubs.acs.org/EF
Evaluation of the Influence of Diethyl Ether (DEE) Addition on Selected Physicochemical Properties of Diesel Oil and Ignition Delay Period Krzysztof Górski*,† and Marcin Przedlacki‡ †
Faculty of Mechanical Engineering, University of Technology and Humanities in Radom, 45 Chrobrego Street, 26-600 Radom, Poland ‡ Faculty of Civil Engineering, Mechanics and Petrochemistry in Płock, Warsaw University of Technology, 17 Łukasiewicza Street, 09-400 Płock, Poland ABSTRACT: In this paper, selected physicochemical properties (such as lower heating value (LHV), kinematic viscosity, density, lubricity, cold filter plugging point (CFPP), and cetane number (CN), as well as miscibility of diethyl ether (DEE)/ diesel oil blends) were experimentally determined. DEE was added to diesel oil, meeting the requirements of European standard PN-EN 590, in volumetric proportions of 5, 10, 15, and 20 vol %. The LHV, kinematic viscosity, and density of the blends were lower than the values obtained for the tested diesel oil. Especially, it was found that DEE has significant influence on diesel oil viscosity. The addition of merely 5% DEE to diesel oil decreased its viscosity by 26%. It was shown that the lubricity of all tested blends is reduced, but not as significantly as viscosity. It was found that the temperature of the CFPP decreased when DEE was added to diesel oil. The results show that miscibility of all tested fuel blends is excellent in a wide range of temperature changes. The engine experiments were carried out on two different diesel engines. CN values of the tested blends were determined using a Waukesha Co. research engine, according to the procedure presented in European standard EN ISO 5165:2003. It was observed that the addition of DEE slightly increased the CN of the diesel oil. However, it did not have any influence on shortening of the ignition delay period in the tested AD3.152 diesel engine with direct fuel injection.
1. INTRODUCTION Oxygenates added to diesel fuels exhibit significant influence on the decrease of toxic gases emission into the atmosphere. In Europe, the most popular oxygenates used in blend with diesel oil are fatty acid methyl esters (FAME). There have been many different studies investigating the effect of FAME addition to diesel oil on engine performance, emissions, and efficiency. In general, the results of these studies showed that FAME usually decreases emissions of some harmful gases such as carbon monoxide (CO), total hydrocarbons (THC), carbon dioxide (CO2) soot, and particulate matter (PM). On the other hand, emission of nitrogen oxides is often higher, when compared to diesel oil.1−9 It is known that vegetable oils are also unstable and their oxidation can promote the corrosion processes in fuel injection equipment. These problems, as well as some other socio-economic factors associated with the use of food sources in the fuel industry, cause the fact that other oxygenatese.g., alcohols (ethanol, butanol) and ethers (dimethyl ether (DME), diethyl ether (DEE), ethyl tert-butyl ether (ETBE), and others)are also tested as possible components of diesel fuel. Ethanol, which is an oxygenate that is commonly added to gasoline, may also be blended with diesel oil. The addition of ethanol reduces emissions of exhaust gases and PM from diesel oil combustion, but the miscibility of ethyl alcohol and diesel oil and thermal stability of such mixtures are limited.10−13 It should be noted that the poor miscibility of diesel−ethanol blends may be improved with the use of FAME as a third component of a mixture. Nevertheless, such mixtures are still unstable at temperatures below 0 °C, especially when water is present in the fuel. Another possible oxygenate for use in a blend with © 2014 American Chemical Society
diesel oil is ETBE. It is manufactured from ethanol and commonly used in Europe as an additive to gasoline. The ETBE molecule, in contrast to ethanol, does not contain a polarized −OH group. For this reason, the miscibility of ETBE with diesel oil is unlimited over a wide range of temperatures. Tests of ETBE−diesel oil mixtures were carried out by Menezes et al.,14 and Li et al.,15 as well as by Górski et al.16,17 They examined the effect of adding ETBE (up to 40% (by volume)) to diesel oil on the physicochemical properties of the mixture, engine performance, and emissions. The results of these tests suggest that a small amount of ETBE may be blended with diesel oil and used as a fuel in diesel engines. It decreases the emission of smoke and PM into the atmosphere, but increasing fraction of ETBE added to diesel oil decreases the cetane number (CN) value. As a consequence, the ignition delay period is adequately longer. It leads to misfires, especially at higher EGR rates, and also promotes a higher emission of toxic gases. DEE is another oxygenate that shows promise as a possible component of diesel fuels. DEE has been selected for its several favorable features in application to diesel engines. It is known that, in contrast to ethanol, as well as ETBE, the CN of DEE is very high. It is estimated to be 125, whereas the CN of ETBE and ethanol is lower than 8.18−21 DEE has also higher energy density, when compared with ethanol. Finally, it should be noted that ethanol tends to promote corrosion processes, whereas DEE is not corrosive to common metals.22 In most Received: December 19, 2013 Revised: March 4, 2014 Published: March 5, 2014 2608
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emission of HC. DEE−diesel oil blends were also tested by Cinar et al.36 in an HCCI engine. They observed unfavorable knock phenomenon in the tested engine when 30% and 40% DEE was added to diesel oil. Furthermore, Clothier et al.37 reported that DEE ignition was inhibited by diesel fuel and that adding DEE to diesel fuel actually decreased the cetane number (CN) of the diesel fuel. Their experiments suggest that DEE can interact with aromatics in diesel fuel and, for this reason, the fuel ignition delay process can be delayed. The influence of DEE on ignition delay in a diesel engine fuelled with diesel oil does not seem to be fully intelligible. As it was mentioned above, many authors have reported an extremely high CN of DEE. On this basis, they usually conclude that the addition of DEE should decrease the ignition delay period. However, these opinions are often unconfirmed in engine tests. The purpose of this article is to show the effect of DEE addition to diesel oil on the physicochemical properties such as lower heating value, kinematic viscosity, density, cold filter plugging point, miscibility, and CN. In addition, it shows, for the first time, the effect of DEE addition on the lubricity of diesel oil. DEE, as a low viscosity liquid, could have a negative influence on diesel fuel lubricity. However, it was observed that the lubricity of a commercially available diesel fuel is not deteriorated by the addition of DEE up to 20% (by volume). Moreover, it showed an impact of DEE−diesel oil blends on ignition delay period. These results extend the knowledge of the DEE−diesel oil blend as a potential fuel for CI engines and confirm some of the findings of other authors.
cases, DEE is produced as a byproduct of ethylene hydration, which is a process used to make synthetic ethanol.23 DEE is also produced on a large scale by dehydration of ethanol carried out in the presence of sulfuric acid or in vapor phase over solid acid catalysts with yield higher than 95%.24 If DEE is to be added to a fuel as a biocomponent, bioethanol can be used for its production. In order to evaluate the cost of DEE production from biomass via bioethanol, the National Renewable Energy Laboratory (NREL) conducted a process simulation that showed that hydrous ethanol could be converted to DEE and resulting liquid/liquid phases of water−ethanol/ethanol−DEE could be easily separated in a simple decanter.20 At the end of biomass fermentation process and subsequent rectification, hydrous ethanol would be generated. The conversion of ethanol to ether could take place before final drying of the alcohol. The analysis shows that the net conversion cost for producing DEE in this manner is similar to the cost of the final drying process step, which means that the cost of fuel-grade DEE would be only slightly higher than that of anhydrous bioethanol.20 The physicochemical properties of DEE are well-described in chemical databases, but the knowledge about DEE as a component of diesel oil still seems to be incomplete. Probably, the first results of the literature search on DEE as a renewable diesel fuel were presented by Bailey et al.20 They found 16 papers related to DEE as a transportation fuel (for spark as well as compression ignition (CI) engines). In 2008, Kapilan et al. presented results of their experiments on “Performance and Emission Studies of Diesel Engine Using Diethyl Ether as Oxygenated Fuel Additive”.25 They found that the addition of DEE to diesel fuel improves the performance and thermal efficiency of a diesel engine, and it reduces the emission of smoke, carbon monoxide, and hydrocarbons. Also, Subramanian and Rajesh26 demonstrated that DEE added to a water− diesel oil emulsion helped to decrease smoke, NOx, HC, and CO emissions without adverse effect on brake thermal efficiency. Similar results were presented by Anand et al.27 They achieved a simultaneous reduction of NOx and smoke emissions using 10%−30% DEE by volume in blend with diesel oil. Many papers have been focused on the blending of DEE with biodiesel to reduce emissions.28−33 Generally, the results of these works showed that DEE blended with biodiesel resulted in better engine performance and lower emissions. Kannan et al.28,29 reported decrease in NOx and smoke emissions (by ∼15%) for a diesel engine fuelled with a blend of biodiesel with DEE. Iranmanesh et al.30 also reported lower NOx and smoke emission when DEE was added to biodiesel. They found that the addition of 15% and 20% DEE reduced NOx by 40% and 51%, respectively. Similar results were presented by Zhang et al.31 They demonstrated that DEE− biodiesel blends can effectively reduce NOx emission and simultaneously PM. It is also known that CI engines fuelled with biodiesel are prone to cold-starting problems, especially in winter seasons. For this reason, Ramadhas et al.32 suggest that DEE blended with biodiesel can be used effectively to overcome these problems. Mohanan et al.34 reported that 5% DEE blend provides better performance and lower emissions compared with other blends of DEE and diesel fuel. Hence, 5% DEE can be blended with diesel fuel to improve the performance and to reduce emissions of a diesel engine. Rakopoulos et al.35 found that DEE in blend with diesel fuel decreases emissions of smoke, CO, and NOx. However, they also reported the increased
2. EXPERIMENTAL SECTION 2.1. Engine Test Stand and Testing Procedure. The impact of DEE added to diesel oil on the ignition delay period was tested on an AD3.152 engine (Table 1), which was produced by URSUS Co. (Warsaw, Poland).
Table 1. Technical Specification of the AD3.152 Diesel Engine parameter
value
number of cylinders and arrangement engine capacity compression ratio maximum power maximum torque crankshaft speed at idle run fuel injection system cylinder bore injection pump timing fuel injector opening pressure type of fuel injector number of injector nozzle holes
3, in line 2502 cm3 16.5 34.6 kW at 2150 rpm 165 N m at 1200 rpm 750 rpm Lucas - CAV type DPA 91.4 mm 17 °CA before TDC (set at idle run) 17.5 MPa multihole (made by WZM Warsaw Co.) 4
AD3.152 is a four-stroke, three-cylinder, naturally aspirated, directinjection engine equipped with a mechanically controlled fuel injection system. The engine (denoted as “7” in Figure 1) was loaded by a hydraulic dynamometer (“8” in Figure 1) that was controlled by a steering module (“5” in Figure 1). The in-cylinder pressure was measured with an AVL Model QC34D water-cooled piezoelectric pressure transducer (“6” in Figure 1) with a sensitivity of 190 pC/bar and a measuring range of 0−25 MPa. The piezoelectric sensor was mounted directly in the combustion chamber. The engine was also equipped with a CL80 needle lift sensor 2609
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fuel injection. It should be noted that the same rules as those used to determine the SOI and SOC were used for all of the tested fuels. 2.2. Testing Methods of the Fuel Properties. Tests were carried out using grade B European diesel oil (DO), meeting standard EN 590:2006.39 Selected physicochemical properties of the tested diesel oil and DEE are listed in Table 2.
Table 2. Physicochemical Characteristics of the Tested Diesel Oil and DEE Figure 1. Test stand diagram. (Legend: 1, PC station equipped with KPCI 3110 measurement board; 2, amplifier equipped with a crank angle encoder; 3, crank angle transmitter; 4, needle lift sensor; 5, steering module; 6, in-cylinder pressure sensor; 7, tested engine; and 8, dynamometer.)
Value property
DO
CAS no. Chemspider ID cetane number, CN lubricity at 25 °C friction coefficient cold filter plugging point, CFPP kinematic viscosity at 40 °C density at 25 °C lower heating value molecular weight hydrogen contenta carbon contenta oxygen contenta H/C ratio ignition temperature autoignition temperature boiling point vaporization latent heat
(“4” in Figure 1), with a sensitivity of 0.5 V/mm and measuring range of 0−2 mm. The needle lift sensor was made by the Polish company ZEPWN. The sensors, as well as the crank angle transmitter (“3” in Figure 1) were connected to an amplifier (“2” in Figure 1) equipped with a crank angle (CA) encoder. Necessary signals were recorded on a personal computer (“1” in Figure 1) equipped with KPCI 3110 measurement board produced by Keithley Co. The computer code necessary to control this high-speed data acquisition system was prepared at our university, using the TestPoint v7 software. Engine measurements were carried out at a constant speed of 1000, 1400, and 1800 rpm under partial load condition of 80 and 120 N m. It is ∼60% and ∼85% of the maximum load of the tested engine fuelled with diesel oil. Sensor signals were sampled with an accuracy of 1024 samples per cycle of engine work (0.7 °CA). In this way, we had the possibility to determine the influence of DEE on the beginning of the fuel injection process. However, our measurements were also carried out in the time domain with a sampling frequency of 100 kHz. It helps to increase measurement accuracy and also creates a possibility to process the recorded data using advanced mathematical techniques designed for time-data analysis. For example, the noise was separated from the recorded signals using a discrete wavelet transform (DWT). In particular, a multiresolution analysis (MRA) was performed using a four-level DWT, according to a theory presented by G. Mallat.38 The analysis of recorded data was carried out using MathCad Wavelet Extension Pack for 100 consecutive cycles of engine work. In each cycle, according to the example shown in Figure 2, the ignition delay period was found. In our investigation, the ignition delay period was defined as the time between the start of injection (SOI) and combustion (SOC).
a
51.1 222 μm 0.164 −10 °C 2.81 mm2/s 0.834 g/cm3 42.7 MJ/kg ∼200 g/mol 14% 86% 0% 16.3% 72 °C 240 °C 180−360 °C 250 kJ/kg
DEE 60−29−7 3168 >125
0.23 mm2/s 0.710 g/cm3 33.9 MJ/kg 74.1 g/mol 13.5% 64.9% 21.6% 20.8% −40 °C 165 °C 34.6 356 kJ/kg
Mass basis (w/w).
DEE was blended with the DO in ratios of 5%, 10%, 15%, and 20% (by volume). These blends were coded in this paper as DEE5, DEE10, DEE15, and DEE20, respectively. Physicochemical properties of the tested diesel oil and DEE−diesel oil blends were determined using the methods listed in Table 3. Blends containing more than 20% (v/v) of
Table 3. Methods of Physicochemical Properties Measurements of Tested Fuel Blends property
EU standard
ref
density viscosity lubricity lower heating value cetane number, CN cold filter plugging point, CFPP
EN ISO 3838:2005 EN ISO 3104 EN ISO 12156-1:2006 ASTM D240-02:2007 EN ISO 5165:2003 EN 116:2003
40 41 42 43 44 45
DEE were not tested, because they caused hard starting and overall poor performance of the test engine. Moreover, because of the very low viscosity of these compositions, leakages in the fuel system were observed. The CN value of DEE reported in the literature is 125. It should be pointed out that such a high CN value of DEE was determined using a constant-volume combustion bomb. In our tests, the CN of the diesel oil and DEE−diesel oil blends were determined in a Waukesha Co. research engine by comparison with two reference hydrocarbon fuels. Technical specification of the Waukesha engine, as well as testing procedures, are described in EN ISO 5165:200344 or equivalent ASTM standards (D 613-10a).46 It should be emphasized that, in this paper, we present, for the first time, the effect of DEE on the lubricity of diesel oil. The European standard for measuring the lubricity of diesel oil is EN ISO 121561:2006; however, in this case, it was necessary to modify the test procedure, because of the high volatility of DEE, compared to typical
Figure 2. Graphical interpretation of the self-ignition delay period. (Legend of terms: SOI, start of fuel injection; SOC, start of fuel combustion; and dpc/dt, the first derivative of in-cylinder pressure.)
Figure 2 shows that the start of injection was determined using the needle lift signal. In our mathematical algorithms, the time-point of the SOI was found when the needle lift reached 0.03 mm. It is ∼10% of the maximum needle lift in the tested engine. The SOC was determined taking the point at which the calculated trace of in-cylinder pressure rise rate (first derivative of pressure) starts to increase after 2610
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Table 4. Physicochemical Properties of Tested Blends property
DEE5
DEE10
DEE15
DEE20
lower heating value kinematic viscosity at 40 °C density at 25 °C lubricity at 25 °C friction coefficient cold filter plugging point, CFPP cetane number, CN
42.3 MJ/kg 2.31 mm2/s 0.826 g/cm3 223 μm 0.170 −11 °C 51.2
41.8 MJ/kg 1.86 mm2/s 0.819 g/cm3 226 μm 0.175 −12 °C 52.1
41.4 MJ/kg 1.64 mm2/s 0.815 g/cm3 234 μm 0.181 −13 °C 53.7
40.9 MJ/kg 1.50 mm2/s 0.812 g/cm3 233 μm 0.190 −14 °C 53.8
diesel oil components. The deviation from EN ISO 12156-1:2006 standard was that the lubricity tests were conducted at 25 °C instead of 60 °C. According to ASTM D6079-02 standard, lubricity of a fuel can be tested using a high-frequency reciprocating rig (HFRR) apparatus at 25 °C if it is too volatile to be tested at 60 °C.47 The lubricity was determined using a HFRR tribological tester manufactured by PCS Instruments Co. A special setup, designed for lubricity measurements of gasoline, was used, with an increased fluid container volume (to 10 cm3) and equipped with a PTFE seal, reducing the evaporation of volatile components. It should be noted that one of the main barriers for the use of ethanol in diesel fuel is its limited miscibility at lower temperatures.10,12 For this reason, we would also like to present the results of miscibility tests of DEE−diesel blends, which were stored in a dark place over one week in the temperature range from −20 °C to 10 °C.
v) of DEE added to diesel oil increases its CN by 0.6 and 4.9%, respectively. The further increase of the fraction of DEE in diesel oil from 15% to 20% (v/v) does not significantly change the CN value. In the case of conventional diesel oil, the variations of CN have a well-known impact on the self-ignition delay period. Also, the high CN value of DEE should improve the quality of ignition, i.e., the fuel becomes easier to self-ignite in a shorter period of time. However, it was not confirmed in our engine tests. First of all, the impact of DEE on CA position at the beginning of fuel injection was checked. As it can be seen in Figure 4, the addition of DEE to diesel oil does not have a significant impact on the beginning of the fuel injection process. This observation means that physical conditions such as temperature and pressure in the combustion chamber at the beginning of injection process were the same for all of the tested fuels. For this reason, it was concluded that the variations of ignition delay period, for all the tested fuels, were not determined by these beginning parameters. Probably, in this case, other factors and their impact on the quality of fuel atomization, vaporization, and mixing with air were the most dominant. As it can be seen in Figure 5, the addition of DEE to diesel oil does not have a pronounced impact on the variation of the ignition delay period under the testing conditions used for the AD3.152 engine. It is evident that the ignition delay period was dependent on the rotational speed of the crankshaft but not on the composition of the mixtures. The increase of the rotational speed of the crankshaft causes the growth of fuel injection pressure and, in consequence, results in better fuel atomization and mixing with the air. Probably, in this case, these physical factors had a dominant impact on the shortening of the ignition delay period. Moreover, we expected that the increasing fraction of DEE in diesel oil should decrease the ignition delay period. According to Table 2, the autoignition temperature of DEE is ∼75 °C lower than that of diesel oil. For this reason, a smaller amount of energy supplied as heat is necessary to begin the chemical reactions of combustion. In consequence, the chemical ignition delay period should be shortened when DEE is added to diesel oil. Also, the high CN value of DEE should improve the quality of ignition. On the other hand, the latent heat of vaporization of DEE is higher, compared to the case of diesel oil alone (see Table 2). It should decrease the temperature in the combustion chamber, because of the cooling effect of the DEE vaporization process, and increase the physical delay period in the first step of combustion. It seems that, in our tests, the final impact of all these different factors on ignition delay period was neutral. As previously mentioned, Clothier et al.37 reported that DEE ignition is inhibited by diesel fuel and that adding DEE to diesel fuel will actually decrease the CN of the diesel fuel. Our experience shows that
3. RESULTS AND DISCUSSION The results of our tests on selected physicochemical properties of DEE−diesel blends are listed in Table 4. It does not contain data of the tested diesel oil and DEE. These properties were presented earlier in Table 2. In particular, the results of all the performed tests are discussed in the following (sections 3.1−3.6. Each figure presented in these sections contains the results of the measurements as well as a regression line, which was shown to statistically describe the trend of the points in the scatter plots. The linearity assumption was tested by the value of the R2 coefficient. 3.1. Cetane Number and Ignition Delay Period. Cetane numbers of the diesel oil and tested DEE−diesel blends were determined on a Waukesha research engine by comparison with two reference hydrocarbon fuels, according to the procedure described in EN ISO 5165:2003.44 The results of CN measurements are depicted in Figure 3. The Waukesha engine tests showed that the CN of all tested blends were above the lower limit (CN = 51) specified in the EN ISO 5165:2003 standard. As it can be seen in Figure 3, the increase of the fraction of DEE added to diesel oil leads to the increase in the CN value. It was observed that 5% and 15% (v/
Figure 3. Impact of the addition of DEE to diesel oil on cetane number (CN) value. 2611
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Figure 4. Variations of needle lift recorded for the AD3.152 engine powered with tested fuels.
of motor fuels. It is defined as the amount of heat released by combusting a specified quantity of fuel (initially at 25 °C) and returning the temperature of the combustion products to 150 °C, which assumes that the latent heat of vaporization of water in the reaction products is not recovered. For this reason, LHV seems to be the best measure of the energy content of motor fuels. The lower heating value (LHV) of the tested diesel fuel is 42.7 MJ/kg (36 MJ/dm3) and DEE has a LHV of 33.9 MJ/kg (24 MJ/dm3). Therefore, blending DEE with diesel oil should reduce the LHV of the diesel oil mixture. In fact, as can be seen in Figure 6, these reductions are not significant. For example: the addition of 10% DEE to diesel oil reduces the LHV by only ∼3.4%. The lower energy content of DEE−diesel blends should
Figure 5. Impact of DEE addition to diesel oil on the ignition delay period in AD3.152 engine operated at partial load of (a) 80 Nm and (b) 120 Nm.
DEE increases the CN value of diesel oil; however, it does not have any impact on variations of the ignition delay period in the tested diesel engine. 3.2. Lower Heating Value. The measure of the energy content of a fuel is its heating value (also called combustion heat). It is usually tested using a bomb calorimeter. The lower heating value (LHV) is the best measure of the energy content
Figure 6. Lower heating value (LHV) of tested diesel oil and its blends with DEE in ratios of 5%, 10%, 15%, and 20% by volume (v/v). 2612
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3.4. Lubricity. Lubricity is often defined as the ability of the lubricant (in this case, diesel fuel) to reduce friction and damage to surfaces in relative motion under load. Lubricity cannot be directly measured, and friction and wear tests must be carried out to quantify a lubricant’s performance. In the case of diesel fuel, the HFRR system is most commonly used equipment to evaluate its lubricity. The HFRR lubricity test uses a hardened steel ball vibrating in loaded contact with a hardened steel plate immersed in the test fuel. The result from the HFRR lubricity test is based on the measurement of the diameter of the wear scar produced on the ball. A smaller wear scar diameter means better fuel lubricity. In a modern diesel engine, the fuel plays a significant part in the engine lubrication system. High-pressure fuel pumps and injectors are lubricated by the fuel, and if its lubricity is inadequate, these mechanisms are subject to excessive wear and reduced durability. Modern, ultralow sulfur diesel fuels have low lubricity, because of the removal of oxygen-containing compounds, along with sulfur, during the hydrodesulfurization (HDS) of diesel fuel components. This is compensated by the addition of highly effective lubricity improvers to the base diesel fuels. The HFRR test rig also measures the coefficient of friction (COF) between sliding surfaces submerged in the tested fuel. The COF is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together. The addition of DEE to diesel oil reduces its lubricity only by ca. 5% when the content of DEE reaches 20% (v/v). As it can be seen in Figure 9, the wear scar diameter (WSD) measured at
increase fuel consumption. It means that the engine powered with DEE−diesel blends may consume slightly more fuel (by volume), when compared with pure hydrocarbon diesel fuel. 3.3. Viscosity and Density. The viscosity of fuel is a measure of its resistance to flow. It also has an impact on the atomization process and combustion quality. According to European standard EN ISO 590:2006, the fuel kinematic viscosity measured at a temperature of 40 °C should remain in the range of 2−4.5 mm/s2. As it can be seen in Figure 7,
Figure 7. Kinematic viscosity of diesel oil and DEE−diesel blends.
blending just 10% (v/v) DEE with the tested diesel oil can decrease its viscosity to
