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An experimental study on using diethyl ether in a diesel engine operated with diesel-biodiesel fuel blend
Engineering Science and Technology, an International Journal xxx (2018) xxx–xxx
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Full Length Article
An experimental study on using diethyl ether in a diesel engine operated with diesel-biodiesel fuel blend Amr Ibrahim Mechanical Engineering Department, Beirut Arab University, 11 5020 Beirut, Lebanon
a r t i c l e
i n f o
Article history: Received 20 December 2017 Revised 25 June 2018 Accepted 8 July 2018 Available online xxxx Keywords: Diesel Biodiesel Diethyl ether Engine Combustion
a b s t r a c t Although biodiesel has a promising potential to be used as an alternative fuel for compression-ignition engines, its use may deteriorate engine performance. The objective of the current study was to enhance the performance of a compression-ignition engine operated with a diesel-biodiesel blend using diethyl ether (DEE). Four fuels were examined in a diesel engine to assess its performance and analyze the combustion process. These fuels were diesel, biodiesel-diesel mixture, and two mixtures of biodiesel-dieselDEE with DEE proportions of 5% and 10% by volume. It was found that using diesel-biodiesel blend increased the minimum brake specific fuel consumption (bsfc) and reduced the maximum thermal efficiency by 8.1% and 6.8%, respectively, compared to diesel fuel. However, employing 5% DEE in the dieselbiodiesel mixture improved engine performance considerably for most engine loads in comparison with all fuels. Altering the fuel type had no significant impact on combustion start instant. However, the heat release rate was lower and combustion duration was longer for diesel compared to other fuels at higher engine loads. Using DEE did not significantly affect engine stability. Ó 2018 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction The use of alternative renewable fuels for diesel engines has been recommended worldwide due to fossil fuel depletion and the harmful impact of petroleum fuel combustion on the environment [1–8]. Also, using renewable fuels can give the chance for many countries to reduce their dependence on imported oil [9]. Biodiesel is one of the most promising renewable fuels which can be used for diesel engines without engine modification [10,11]. Biodiesel has the potential to improve the combustion efficiency and decrease engine emissions because it is an oxygenated fuel. However, most studies [12] showed that using biodiesel as an alternative to diesel fuel in diesel engines without engine modification can result in some deterioration in engine performance such as a reduction in power and thermal efficiency because biodiesel properties such as viscosity, density, calorific value, etc differ from the corresponding diesel properties. Therefore, biodiesel is usually mixed with diesel in different ratios so that the fuel blend properties are more comparable to diesel fuel properties. Also, fuel additives can be added to biodiesel to make its properties more comparable to diesel properties. For example, biodiesel viscosity is slightly higher compared to the viscosity of diesel. Increasing
E-mail address: [email protected]. Peer review under responsibility of Karabuk University.
the fuel viscosity can increase the atomized fuel droplet diameter during fuel injection leading to a decrease in combustion efficiency. On the other hand, alcohols have low viscosity. Therefore, alcohols are added to diesel-biodiesel blends to reduce the fuel blend viscosity to become more comparable to diesel viscosity. Alcohols are oxygenated renewable fuels, which can be produced from biomass. Alcohol based fuels include methanol, ethanol, butanol, diethyl ether (DEE), etc. Table 1 compares the properties of different alcohol based fuels with the relevant properties of diesel and biodiesel fuels. Although most of the previous investigations found in the literature studied using either butanol or ethanol as alcohol additives for diesel engines, Table 1 indicates that the DEE has the potential to be the most suitable fuel supplement for compression ignition engines because its cetane number and heating value are higher compared to ethanol and butanol. Also, DEE is miscible with diesel and biodiesel fuels [13]. The composition of DEE is C4H10O, making its oxygen content 21.6% by mass. Using DEE as a fuel additive was investigated in only limited number of studies. Kaimal and Vijayabalan [14] found that blending the DEE with waste plastic oil with different proportions up to 15% increased the thermal efficiency. Also, soot and nitrogen oxides (NOx) emissions were significantly reduced. Venu and Madhavan [15] studied adding DEE (up to 10%) to diesel-biodiesel-ethanol and diesel-biodieselmethanol blends. The results showed that the addition of DEE to diesel-biodiesel-ethanol blend increased the combustion duration, cylinder pressure, and brake specific fuel consumption (bsfc) and
https://doi.org/10.1016/j.jestch.2018.07.004 2215-0986/Ó 2018 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: A. Ibrahim, An experimental study on using diethyl ether in a diesel engine operated with diesel-biodiesel fuel blend, Eng. Sci. Tech., Int. J. (2018), https://doi.org/10.1016/j.jestch.2018.07.004
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Table 1 Fuel properties [16,17]. Fuel
Lower heating value (MJ/kg)
Density @20 °C (kg/m3)
Viscosity @40 °C (mPa s)
Flash point (oC)
Cetane number
Diesel Vegetable oil Biodiesel DEE Butanol Ethanol Methanol
44.8 40.4 40.5 33.9 33.1 28.6 19.8
815 916 855 714 808 790 792
2.95 34.2 4.57 0.22 2.63 1.1 0.59
70 274 126 -45 35 13 11
52 37 52 125 25 6 <5
decreased NOx and smoke emissions. On the other hand, adding the DEE to diesel-biodiesel-methanol blend decreased the bsfc, cylinder pressure, and combustion duration. However, smoke emissions increased. Lee and Kim [16] demonstrated that adding the DEE to diesel in different ratios (up to 50% by mass) did not significantly change the engine thermal efficiency. It was also shown that both hydrocarbon (HC) and carbon monoxide (CO) emissions decreased while NOx emissions increased. Patil and Thipse [13] investigated the addition of DEE to diesel in different ratios ranging from 2% to 25% by volume. It was found that the optimum proportion was 15% as it resulted in optimum engine performance. It was also shown that using the DEE reduced the trade-off between particulate matter (PM) and NOx emissions. The DEE was also used as a supplement (up to 4%) to a mixture of tire derived fuel (40%) and diesel (60%) by Tudu and coworkers [17]. It was shown that the bsfc decreased by 6% and NO emission decreased by 25% compared to diesel operation at engine full load condition. Barik and Murugan [18] investigated improving the performance of a diesel engine fuelled by biodiesel-biogas by utilizing the DEE as a supplement (up to 6%). The authors showed that using the DEE increased the thermal efficiency by 2.3% and reduced bsfc by 5.8% in comparison with dual fuel operation at full load condition. It was also shown that CO, HC, and smoke emissions decreased by 12.2%, 10.6%, and 5.7%, respectively, while NO emission increased by 12.7%. Devaraj and coworkers [19] showed that utilizing the DEE (up to 10%) as a supplement for a diesel engine fuelled by waste plastic oil increased the thermal efficiency from 28% to 29% at engine high load condition and decreased NOx emissions. Although most of previous studies showed that using DEE as a supplement for diesel engines had a great potential to enhance engine performance and decrease emissions, there is only limited number of studies that investigated using DEE in diesel engines fuelled with diesel-biodiesel blends. These studies are not sufficient to build a solid conclusion regarding the effect of using DEE on diesel engine performance, emissions, and combustion characteristics. Further studies need to be conducted to investigate wide range of engine design parameters and operating conditions. This research paper aimed to compare engine performance and combustion parameters for four different fuels as summarized in Table 2. Engine speed was fixed to 1500 rpm while engine load changed from small to full-load operating mode.
The engine cylinder head was fitted by a pressure sensor. The inlet air was supplied to engine cylinder at ambient conditions. Engine specifications are indicated in Table 3. The test bed contained a hydraulic dynamometer as shown in Fig. 1. The DVF1 volumetric fuel sensor was employed to detect the fuel flow rate and an orifice plate was used to measure the inlet air flow rate. Temperature and pressure of air flowing through the orifice were measured using a thermocouple and a pressure sensor, respectively. A load cell was used to detect the engine torque. An optical rpm transducer was supplied to detect the crankshaft revolutions. Instrument modules were provided to digitally display measured parameters such as flow rates, speed, torque, etc. Also, all the measured data were accurately monitored and recorded on a computer by the TecQuipment Versatile Data Acquisition System (VDAS). Both the engine load and speed were controlled via mechanical governors. The cylinder pressure and engine crankangle were measured simultaneously using a piezoelectric pressure sensor (ECA 101) and a shaft encoder (ECA 102), respectively as indicated in Fig. 1. The cycle analyzer (ECA 100) was supplied by TecQuipment in order to display and record the cylinder pressure data. The ECA 100 was a two-part product, which were interface and software. The ECA 100 interface was connected to pressure sensor, TDC position sensor and shaft encoder. The TDC position sensor produced a signal each time the piston reached the TDC; the crankangle was assigned with zero value at this instant. The interface, which contained charge amplifier and signal conditioning circuits, converted the sensor signals to a format that suited the ECA 100 software. This software did several jobs which included displaying pressure crankangle and pressure volume diagrams, and calculating the indicated power and indicated mean effective pressure. The ECA 100 software was also capable of using the test results and a user controlled animation to visually simulate the engine thermodynamic cycle and the relative position of engine crank, piston, and valves. The software calculated the cylinder volume as a function of measured crankangle using the following equation:
V ¼ Vc þ
p 4
B2 y
ð1Þ
where B is cylinder bore, Vc is clearance volume, and y was calculated as follows:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 y ¼ a þ l a cos h þ l a2 sin2 h
ð2Þ
2. Test bed A single-cylinder, direct- injection, four-stroke, TecQuipment TD212 compression-ignition engine was used to conduct all tests. Table 2 Examined fuel types. Fuel
Acronym
Diesel 70% diesel + 30% biodiesel (% by volume) 70% diesel + 25% biodiesel + 5% DEE (% by volume) 70% diesel + 20% biodiesel + 10% DEE (% by volume)
D100 D70B30 D70B25DEE5 D70B20DEE10
Table 3 Engine specifications. Item
Value
No. of cylinders Maximum power, kW Compression ratio Bore, mm Stroke, mm Connecting rod length, mm Engine capacity, cm3 Injection timing, degrees bTDC
1 3.5 at 3600 rpm 22 69 62 104 232 10
Please cite this article in press as: A. Ibrahim, An experimental study on using diethyl ether in a diesel engine operated with diesel-biodiesel fuel blend, Eng. Sci. Tech., Int. J. (2018), https://doi.org/10.1016/j.jestch.2018.07.004
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Fig. 1. Experimental setup.
where a is crank radius, l is connecting rod length, and h is crank angle. The pressure in the engine intake manifold was measured using a regular pressure sensor, which was then used as a reference pressure for the piezoelectric pressure sensor to modify the pressure data. Table 4 summarizes the uncertainty in the measured variables. The combustion characteristics described in this study were estimated based on calculating the average cylinder pressure data measured for five consecutive thermodynamic cycles. The waste cooking oil biodiesel and DEE, which were utilized in the experiments as blended fuels, were supplied from commercial suppliers. Four different fuels were used in the tests as indicated in Table 2. The density and gross calorific value of these fuels were measured by the chemical lab of an oil company using the ASTM standard procedure. Such properties are indicated in Table 5. The rate of heat release, dQ , varies with in-cylinder pressure p, dh volume V, crank angle h, and specific heat ratio c, via this equation [20]:
dQ r dV 1 dp ¼ p þ V dh r 1 dh r 1 dh
ð3Þ
The coefficient of variation (COV), which was estimated from the cylinder pressure data to assess cyclic variability, was calculated as follows [20]:
COV ¼
rimep
ð4Þ
imep
Table 4 Uncertainty in measurements. Item
Uncertainty
Maximum uncertainty, %
Speed Torque Fuel volume flow rate Air flow rate Cylinder pressure Crank angle
±40 rpm ±0.02 Nm ±0.04 ml/min (max) ±0.05 103 kg/s (max) ±0.01 bar ±0.01 degree
±2.7 ±2 ±1.6 ±1.8 ±1 ±1
Table 5 Examined fuel properties. Fuel type
Gross calorific value, MJ/kg
Density @ 15 °C, kg/m3
Diesel (D100) D70B30 D70B25DEE5 D70B20DEE10
45.5 45.2 44.9 44.6
849 861 852 843
where imep is the mean indicated mean effective pressure estimated for a certain number of thermodynamic cycles, n, while rimep is the standard deviation in indicated mean effective pressure. 3. Discussion of results This section compares the efficiency, fuel consumption, cylinder specific heat ratio, heat release rates, and stability of a compression-ignition engine operated with four fuels at 1500 rpm and different engine loads. 3.1. bsfc and efficiency Figs. 2 and 3 show the change of bsfc and brake thermal efficiency with engine brake power, respectively, for four examined fuels. Figs. 2 and 3 indicate that the bsfc increased while the thermal efficiency decreased when the diesel-biodiesel fuel blend (D70B30) was used as an alternative to diesel for most of engine loads. The minimum fuel consumption increased from 0.246 kg/ kWh for diesel to 0.266 kg/kWh for diesel-biodiesel blend with a percentage increase of 8.1%. Also, the maximum brake thermal efficiency decreased from 32.2% for diesel to 30% for diesel-biodiesel fuel blend with a percentage decrease of 6.8%. This was because the diesel-biodiesel blend had a lower calorific value compared to diesel. Therefore, more fuel needed to burn to produce the same power. In addition, biodiesel had higher viscosity and density compared to diesel fuel. The higher viscosity and density of diesel-biodiesel blend could affect the fuel atomization quality by
Please cite this article in press as: A. Ibrahim, An experimental study on using diethyl ether in a diesel engine operated with diesel-biodiesel fuel blend, Eng. Sci. Tech., Int. J. (2018), https://doi.org/10.1016/j.jestch.2018.07.004
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Fig. 2. bsfc variation with engine brake power.
Fig. 3. Brake thermal efficiency variation with engine brake power.
producing larger fuel droplets causing a poorer fuel-air mixing and a reduction in combustion efficiency [12]. Figs. 2 and 3 indicate that there was a significant enhancement in engine bsfc and efficiency for most of engine loads when DEE with a percentage of 5% was added to the diesel-biodiesel mixture (D70B25DEE5). Adding 5% DEE to diesel-biodiesel mixture decreased the bsfc and increased the thermal efficiency in comparison
with diesel for most of engine loads. Although the DEE had lower calorific value in comparison with diesel and biodiesel, its lower viscosity and density improved the fuel atomization quality leading to higher combustion efficiency. Lee and kim [16] found that the higher volatility of DEE facilitated air-fuel mixing and helped form leaner and more homogeneous fuel-air mixtures, which consequently increased the fuel conversion efficiency. The
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improvement in combustion efficiency required less fuel to be burned to produce the same power as indicated in Fig. 4, which shows the variation of engine operating air to fuel ratio against brake power for all tested fuels. Fig. 4 indicates that the highest air to fuel ratio was obtained for most of engine loads when D70B25DEE5 was used as an alternative to all tested fuels. However, the bsfc increased and thermal efficiency decreased for most of engine loads when the DEE proportion in the dieselbiodiesel mixture increased to 10% (D70B20DEE10). Although DEE is an oxygenated fuel which can result to more complete combustion with a consequent increase in engine thermal efficiency and a decrease in fuel consumption, DEE has a lower calorific value and higher latent heat of vaporization compared to diesel fuel, which can lead to a decrease in engine efficiency and an increase in fuel consumption [9,16]. Therefore, the proportion of DEE in the fuel blend needs to be optimized for best engine performance because increasing the proportion of DEE in fuel blend up to a certain limit can decrease fuel blend calorific value more significantly leading to deterioration in engine performance. Barik and Murugan [18] investigated improving a diesel engine performance operated with biodiesel-biogas dual fuel mode using DEE as an additive fuel with three proportions of 2%, 4%, and 6%. The authors concluded that the optimum DEE proportion was 4% as it led to higher efficiency and lower bsfc. Furthermore, previous studies [9] showed that increasing the proportion of DEE to a certain limit resulted in engine instability due to the high volatility of DEE which caused vapor lock in the fuel lines connected to fuel pump. When the DEE percentage was raised to 10%, the blend calorific value decreased more significantly; this caused a reduction in thermal efficiency and a rise in fuel consumption. It can be concluded that the optimum fuel among examined fuels was D70B25DEE5 because this blend caused the engine to produce the lowest bsfc and highest thermal efficiency for most of engine loads in comparison with all tested fuels including the diesel fuel. On the other hand, using the diesel-biodiesel blend (D70B30) resulted in higher bsfc and lower thermal efficiency for most of engine loads in comparison with all tested fuels.
5
3.2. Specific heat ratio It is important to estimate an accurate value for the specific heat ratio (c) of cylinder combustion content because this specific heat ratio is used to estimate the heat release rate according to Eq. (3). A fixed value of c ranging from 1.3 to 1.35 [20,21] was usually assumed by most of previous studies in order to estimate the heat release rate. However, c is a function of in-cylinder composition and temperature [20]. Significant errors could be induced in the estimation of rate of heat release if inaccurate values of c were assumed [21]. Also, inaccurate combustion characteristics results could be produced if c was assigned with a fixed value for all engine operating modes and examined fuels [21]. Although c varies instantaneously with the change of engine crankangle during combustion, it was shown that satisfactory heat release rate results can be obtained by assigning c with an acceptable average value [21]. However, this value of c should be a function of engine operating characteristics. A methodology described by Abbaszadehmosayebi and coworkers [21] proposed to use the measured cylinder pressure data to estimate the average specific heat ratio during combustion. The isentropic relationships of pV cu = constant & pV cb = constant are employed during engine compression and expansion processes, respectively [20]. cu and cb represent the specific heat ratio of cylinder gas through compression and expansion processes, respectively. Hence, cu and cb can be estimated by calculating the slopes of the two approximate straight lines that are created by plotting the log p-log V relationships during compression and expansion processes, respectively [20]. Abbaszadehmosayebi [21] demonstrated that an appropriate value of c can be estimated as the average of both cu and cb. This described strategy was utilized in the current study to estimate c as a function of engine indicated power and type of tested fuel using the measured cylinder pressure as shown in Fig. 5. Fig. 5 demonstrates that the specific heat ratio, c, decreased as indicated power increased for all examined fuels. The increase of mass of burned fuel with increasing engine power, as indicated
Fig. 4. Change of engine air to fuel ratio with brake power.
Please cite this article in press as: A. Ibrahim, An experimental study on using diethyl ether in a diesel engine operated with diesel-biodiesel fuel blend, Eng. Sci. Tech., Int. J. (2018), https://doi.org/10.1016/j.jestch.2018.07.004
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Fig. 5. Variation of c against power.
Fig. 6. Increase of maximum cylinder pressure for different fuels.
in Fig. 4, resulted in an increase in in-cylinder pressure, as indicated in Fig. 6, and temperature, which led to a reduction in c. 3.3. Heat release rate Figures from 7 to 9 show the rate of heat release variations with engine crank-angle for all tested fuels at small, medium, and full
load, respectively. The small, medium, and full loads corresponded to indicated powers of 0.8, 1.4, and 2.1 kW, respectively. The corresponding brake powers were 0.3, 0.8, and 1.6 kW, respectively. Although all fuels were injected about 10 degrees before the piston reached the top dead center (350 degrees), the heat release rate started to increase rapidly a few degrees later due to the fuel ignition delay period. This rapid increase in heat release rate
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Fig. 7. Rate of heat release change with crank angle at small load.
Fig. 8. Rate of heat release change with crank angle at medium load.
indicated the instant of combustion beginning. The rate of heat release was rapidly increased after combustion started due to the burning of fuel that accumulated within the combustion chamber during the delay period. At low load condition, the instant of combustion start remained unaffected when fuel type was varied as the combustion started almost at 355 degrees for all examined fuels. Although the cetane
number (CN) of DEE was much higher compared to diesel and biodiesel fuels, the cetane number of diesel-biodiesel-DEE blends might remain close to diesel CN. A previous study [22] showed that mixing the DEE with diesel reduced the mixture cetane number and became less than the diesel CN because the DEE interacted with diesel aromatics, which delayed the start of ignition. However, the maximum heat release rate obtained during the rapid
Please cite this article in press as: A. Ibrahim, An experimental study on using diethyl ether in a diesel engine operated with diesel-biodiesel fuel blend, Eng. Sci. Tech., Int. J. (2018), https://doi.org/10.1016/j.jestch.2018.07.004
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20 D100 D70B30 D70B25DEE5 D70B20DEE10 15
HRR,J/deg
10
5
0
−5 340
350
360
370
380 Crank angle,deg
390
400
410
420
Fig. 9. Rate of heat release change with crank angle at full load.
combustion phase was significantly higher for diesel in comparison with other tested fuels, as shown in Fig. 7. Although the delay periods were comparable for all fuels, the engine operated at significantly higher air to fuel ratio during the small load engine operation as demonstrated in Fig. 4. The existence of oxygenated fuels in the very lean fuel-air mixture could have the potential to reduce the maximum heat release rate [20]. The rapid combustion stage ended almost 1.5 degrees after the piston reached the TDC (361.5 degrees) for all fuels at low load condition. However, the combustion continued with lower heat release rates as the fuel issuing from the injector was mixed and burned with in-cylinder air in the mixing controlled combustion phase. When engine load increased, the combustion beginning instant was more advanced for all examined fuels. The combustion started almost one degree earlier for all fuels when the load was changed from small to full load. The increase of burned fuel mass with increasing engine load increased cylinder pressure and temperature, which caused a reduction in the ignition delay duration. Similar trend was also indicated by Rakopoulos and coworkers [23] who investigated the variations of ignition delay with diesel engine load for different fuels of neat cotton seed vegetable oil, neat cotton seed biodiesel, and their blends with either 20% DEE or 20% n-butanol. The authors showed that the ignition delay period decreased from a range of almost 4.5–6 degrees at engine brake mean effective pressure (bmep) of 1 bar to a range of 4–5 degrees at bmep of about 5.5 bar. The crankangle at which the rapid combustion phase ended was more advanced when load increased due to the earlier start of combustion. However, the heat release rate was significantly lower for diesel compared to other tested fuels at higher engine load operation as shown in Fig. 9. That led to a slight delay in the end of combustion timing for diesel in comparison with other examined fuels. The engine operated at richer air-fuel mixture when the load increased. Therefore, the presence of oxygenated fuels,
and higher volatility of DEE that enhanced the air-fuel mixing led to a slight increase in the combustion rate. Raising up the mass of burned fuel with increasing the load led to a significant increase in the rate of heat release during the mixing controlled combustion phase at higher engine load operation as can be noticed by comparing Fig. 7 with Fig. 9. In addition, the increase of injected fuel mass at higher engine loads extended the duration of combustion process. The combustion duration increased by an average value of 11 crank angle degrees as the load changed from small to full-load for all examined fuels. 3.4. Engine stability Cycle to cycle fluctuation in indicated mean effective pressure (IMEP) was estimated via the calculation of the coefficient of variation (COV) in order to assess the engine stability for all tested fuels. Cylinder pressure-crank angle data differs from one thermodynamic cycle to another mainly due to the associated variations in combustion process [20]. Heywood [20] illustrated that engine stability deteriorated when the COV increased above 10%. However, different studies demonstrated that increasing the COV above 5% could badly affect engine stability [12]. The change of COV with load for all examined fuels is indicated in Fig. 10. This figure demonstrates that the COV was lower than 5% for most of engine operating conditions. Therefore, it can be concluded that mixing the DEE (up to 10%) with diesel-biodiesel mixture did not negatively affect engine stability. Fig. 10 shows also that the COV generally reduced with the increase of load indicating better engine stability. That can be explained as the engine consumed a richer air-fuel mixture at higher load conditions. Decreasing the operating air to fuel ratio can decrease engine cyclic variations [20]. Similar results were also obtained by Rakopoulos and coworkers [23] who compared the COV of a diesel engine
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Fig. 10. Change of engine COV with indicated power.
fuelled by different fuels of neat cotton seed vegetable oil, neat cotton seed biodiesel, and their blends with either 20% DEE or 20% n-butanol. The authors found that the COV ranged from 8% to 9% for all fuels at low load condition while it significantly reduced to a range of 1.5%–2% at engine high load condition. Fig. 10 shows that adding DEE to diesel-biodiesel blend with a proportion up to 10% generally decreased the COV compared to diesel at higher engine load conditions. Similar results were also obtained by Lee and Kim [16] who found that adding DEE to diesel with a proportion of 25% decreased the COV at higher engine load conditions. Also, Uyumaz [24] demonstrated that adding mustard oil biodiesel to diesel with proportions of 10%, 20%, and 30% resulted in a significant decrease in COV at all engine load conditions. The author explained that as the addition of oxygenated fuel to diesel resulted in more complete combustion throughout engine cycles which decreased the cycle to cycle variation. The D100 COV calculated for this study was around 3% for most of engine loads as shown in Fig. 10. That could be considered slightly higher compared to other diesel engines because the air flow rate was measured using an orifice meter which is a flow restriction device which can induce relatively higher fluctuations in inlet pressure and air flow rate. However, the COV can vary significantly from engine to engine according to engine specifications, operating conditions, and fuel properties [25]. Uyumaz [24] found that the COV in imep for D100 ranged from 11.62% at light load (brake torque of 3.75 Nm) to about 6.5% at high load (brake torque of 18.75 Nm). 4. Conclusions Four fuels were examined in a diesel engine to assess its performance and stability and analyze the combustion process at various loads and a constant speed of 1500 rpm. These fuels were diesel, biodiesel-diesel mixture, and two mixtures of biodiesel-dieselDEE with DEE proportions of 5% and 10% by volume. The following results were found:
The higher viscosity and lower calorific value of biodiesel compared to diesel resulted in some engine performance deterioration when diesel-biodiesel blend was used as an alternative to diesel as the minimum bsfc increased by 8.1% and the maximum thermal efficiency decreased by 6.8%. Although the high volatility and low viscosity of DEE in addition to its oxygen content can enhance the combustion process, its lower calorific value and higher latent heat of vaporization can deteriorate engine performance. It was found that the optimum blending proportion was 5% because blending the dieselbiodiesel mixture with 5% DEE led to a significant improvement in engine performance as the bsfc decreased and thermal efficiency increased at most engine loads compared to diesel. However, increasing the proportion of DEE to 10% decreased thermal efficiency. The oxygen content of oxygenated fuels reduced the maximum heat release rate of blended fuels compared to diesel at light load condition where the air to fuel ratio was the highest. However, the heat release rate was lower and combustion duration was longer for diesel compared to other tested fuels at higher engine load conditions. The high cetane number of DEE did not significantly affect the start of combustion timing for the blended fuels. The COV decreased with increasing engine load for all fuels due to the richer fuel-air mixture operation at higher engine load conditions. The use of oxygenated fuels generally decreased the COV compared to diesel at higher engine load conditions. The COV was lower than 5% for all examined fuels at most loads indicating a stable engine operation.
References [1] K.V. Krishna, G.R.K. Sastry, M.M. Krishna, J.D. Barma, Investigation on performance and emission characteristics of EGR coupled semi adiabatic diesel engine fuelled by DEE blended rubber seed biodiesel, Eng. Sci. Technol. An Int. J. 21 (2018) 122–129.
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Full Length Article
An experimental study on using diethyl ether in a diesel engine operated with diesel-biodiesel fuel blend Amr Ibrahim Mechanical Engineering Department, Beirut Arab University, 11 5020 Beirut, Lebanon
a r t i c l e
i n f o
Article history: Received 20 December 2017 Revised 25 June 2018 Accepted 8 July 2018 Available online xxxx Keywords: Diesel Biodiesel Diethyl ether Engine Combustion
a b s t r a c t Although biodiesel has a promising potential to be used as an alternative fuel for compression-ignition engines, its use may deteriorate engine performance. The objective of the current study was to enhance the performance of a compression-ignition engine operated with a diesel-biodiesel blend using diethyl ether (DEE). Four fuels were examined in a diesel engine to assess its performance and analyze the combustion process. These fuels were diesel, biodiesel-diesel mixture, and two mixtures of biodiesel-dieselDEE with DEE proportions of 5% and 10% by volume. It was found that using diesel-biodiesel blend increased the minimum brake specific fuel consumption (bsfc) and reduced the maximum thermal efficiency by 8.1% and 6.8%, respectively, compared to diesel fuel. However, employing 5% DEE in the dieselbiodiesel mixture improved engine performance considerably for most engine loads in comparison with all fuels. Altering the fuel type had no significant impact on combustion start instant. However, the heat release rate was lower and combustion duration was longer for diesel compared to other fuels at higher engine loads. Using DEE did not significantly affect engine stability. Ó 2018 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction The use of alternative renewable fuels for diesel engines has been recommended worldwide due to fossil fuel depletion and the harmful impact of petroleum fuel combustion on the environment [1–8]. Also, using renewable fuels can give the chance for many countries to reduce their dependence on imported oil [9]. Biodiesel is one of the most promising renewable fuels which can be used for diesel engines without engine modification [10,11]. Biodiesel has the potential to improve the combustion efficiency and decrease engine emissions because it is an oxygenated fuel. However, most studies [12] showed that using biodiesel as an alternative to diesel fuel in diesel engines without engine modification can result in some deterioration in engine performance such as a reduction in power and thermal efficiency because biodiesel properties such as viscosity, density, calorific value, etc differ from the corresponding diesel properties. Therefore, biodiesel is usually mixed with diesel in different ratios so that the fuel blend properties are more comparable to diesel fuel properties. Also, fuel additives can be added to biodiesel to make its properties more comparable to diesel properties. For example, biodiesel viscosity is slightly higher compared to the viscosity of diesel. Increasing
E-mail address: [email protected]. Peer review under responsibility of Karabuk University.
the fuel viscosity can increase the atomized fuel droplet diameter during fuel injection leading to a decrease in combustion efficiency. On the other hand, alcohols have low viscosity. Therefore, alcohols are added to diesel-biodiesel blends to reduce the fuel blend viscosity to become more comparable to diesel viscosity. Alcohols are oxygenated renewable fuels, which can be produced from biomass. Alcohol based fuels include methanol, ethanol, butanol, diethyl ether (DEE), etc. Table 1 compares the properties of different alcohol based fuels with the relevant properties of diesel and biodiesel fuels. Although most of the previous investigations found in the literature studied using either butanol or ethanol as alcohol additives for diesel engines, Table 1 indicates that the DEE has the potential to be the most suitable fuel supplement for compression ignition engines because its cetane number and heating value are higher compared to ethanol and butanol. Also, DEE is miscible with diesel and biodiesel fuels [13]. The composition of DEE is C4H10O, making its oxygen content 21.6% by mass. Using DEE as a fuel additive was investigated in only limited number of studies. Kaimal and Vijayabalan [14] found that blending the DEE with waste plastic oil with different proportions up to 15% increased the thermal efficiency. Also, soot and nitrogen oxides (NOx) emissions were significantly reduced. Venu and Madhavan [15] studied adding DEE (up to 10%) to diesel-biodiesel-ethanol and diesel-biodieselmethanol blends. The results showed that the addition of DEE to diesel-biodiesel-ethanol blend increased the combustion duration, cylinder pressure, and brake specific fuel consumption (bsfc) and
https://doi.org/10.1016/j.jestch.2018.07.004 2215-0986/Ó 2018 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Table 1 Fuel properties [16,17]. Fuel
Lower heating value (MJ/kg)
Density @20 °C (kg/m3)
Viscosity @40 °C (mPa s)
Flash point (oC)
Cetane number
Diesel Vegetable oil Biodiesel DEE Butanol Ethanol Methanol
44.8 40.4 40.5 33.9 33.1 28.6 19.8
815 916 855 714 808 790 792
2.95 34.2 4.57 0.22 2.63 1.1 0.59
70 274 126 -45 35 13 11
52 37 52 125 25 6 <5
decreased NOx and smoke emissions. On the other hand, adding the DEE to diesel-biodiesel-methanol blend decreased the bsfc, cylinder pressure, and combustion duration. However, smoke emissions increased. Lee and Kim [16] demonstrated that adding the DEE to diesel in different ratios (up to 50% by mass) did not significantly change the engine thermal efficiency. It was also shown that both hydrocarbon (HC) and carbon monoxide (CO) emissions decreased while NOx emissions increased. Patil and Thipse [13] investigated the addition of DEE to diesel in different ratios ranging from 2% to 25% by volume. It was found that the optimum proportion was 15% as it resulted in optimum engine performance. It was also shown that using the DEE reduced the trade-off between particulate matter (PM) and NOx emissions. The DEE was also used as a supplement (up to 4%) to a mixture of tire derived fuel (40%) and diesel (60%) by Tudu and coworkers [17]. It was shown that the bsfc decreased by 6% and NO emission decreased by 25% compared to diesel operation at engine full load condition. Barik and Murugan [18] investigated improving the performance of a diesel engine fuelled by biodiesel-biogas by utilizing the DEE as a supplement (up to 6%). The authors showed that using the DEE increased the thermal efficiency by 2.3% and reduced bsfc by 5.8% in comparison with dual fuel operation at full load condition. It was also shown that CO, HC, and smoke emissions decreased by 12.2%, 10.6%, and 5.7%, respectively, while NO emission increased by 12.7%. Devaraj and coworkers [19] showed that utilizing the DEE (up to 10%) as a supplement for a diesel engine fuelled by waste plastic oil increased the thermal efficiency from 28% to 29% at engine high load condition and decreased NOx emissions. Although most of previous studies showed that using DEE as a supplement for diesel engines had a great potential to enhance engine performance and decrease emissions, there is only limited number of studies that investigated using DEE in diesel engines fuelled with diesel-biodiesel blends. These studies are not sufficient to build a solid conclusion regarding the effect of using DEE on diesel engine performance, emissions, and combustion characteristics. Further studies need to be conducted to investigate wide range of engine design parameters and operating conditions. This research paper aimed to compare engine performance and combustion parameters for four different fuels as summarized in Table 2. Engine speed was fixed to 1500 rpm while engine load changed from small to full-load operating mode.
The engine cylinder head was fitted by a pressure sensor. The inlet air was supplied to engine cylinder at ambient conditions. Engine specifications are indicated in Table 3. The test bed contained a hydraulic dynamometer as shown in Fig. 1. The DVF1 volumetric fuel sensor was employed to detect the fuel flow rate and an orifice plate was used to measure the inlet air flow rate. Temperature and pressure of air flowing through the orifice were measured using a thermocouple and a pressure sensor, respectively. A load cell was used to detect the engine torque. An optical rpm transducer was supplied to detect the crankshaft revolutions. Instrument modules were provided to digitally display measured parameters such as flow rates, speed, torque, etc. Also, all the measured data were accurately monitored and recorded on a computer by the TecQuipment Versatile Data Acquisition System (VDAS). Both the engine load and speed were controlled via mechanical governors. The cylinder pressure and engine crankangle were measured simultaneously using a piezoelectric pressure sensor (ECA 101) and a shaft encoder (ECA 102), respectively as indicated in Fig. 1. The cycle analyzer (ECA 100) was supplied by TecQuipment in order to display and record the cylinder pressure data. The ECA 100 was a two-part product, which were interface and software. The ECA 100 interface was connected to pressure sensor, TDC position sensor and shaft encoder. The TDC position sensor produced a signal each time the piston reached the TDC; the crankangle was assigned with zero value at this instant. The interface, which contained charge amplifier and signal conditioning circuits, converted the sensor signals to a format that suited the ECA 100 software. This software did several jobs which included displaying pressure crankangle and pressure volume diagrams, and calculating the indicated power and indicated mean effective pressure. The ECA 100 software was also capable of using the test results and a user controlled animation to visually simulate the engine thermodynamic cycle and the relative position of engine crank, piston, and valves. The software calculated the cylinder volume as a function of measured crankangle using the following equation:
V ¼ Vc þ
p 4
B2 y
ð1Þ
where B is cylinder bore, Vc is clearance volume, and y was calculated as follows:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 y ¼ a þ l a cos h þ l a2 sin2 h
ð2Þ
2. Test bed A single-cylinder, direct- injection, four-stroke, TecQuipment TD212 compression-ignition engine was used to conduct all tests. Table 2 Examined fuel types. Fuel
Acronym
Diesel 70% diesel + 30% biodiesel (% by volume) 70% diesel + 25% biodiesel + 5% DEE (% by volume) 70% diesel + 20% biodiesel + 10% DEE (% by volume)
D100 D70B30 D70B25DEE5 D70B20DEE10
Table 3 Engine specifications. Item
Value
No. of cylinders Maximum power, kW Compression ratio Bore, mm Stroke, mm Connecting rod length, mm Engine capacity, cm3 Injection timing, degrees bTDC
1 3.5 at 3600 rpm 22 69 62 104 232 10
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Fig. 1. Experimental setup.
where a is crank radius, l is connecting rod length, and h is crank angle. The pressure in the engine intake manifold was measured using a regular pressure sensor, which was then used as a reference pressure for the piezoelectric pressure sensor to modify the pressure data. Table 4 summarizes the uncertainty in the measured variables. The combustion characteristics described in this study were estimated based on calculating the average cylinder pressure data measured for five consecutive thermodynamic cycles. The waste cooking oil biodiesel and DEE, which were utilized in the experiments as blended fuels, were supplied from commercial suppliers. Four different fuels were used in the tests as indicated in Table 2. The density and gross calorific value of these fuels were measured by the chemical lab of an oil company using the ASTM standard procedure. Such properties are indicated in Table 5. The rate of heat release, dQ , varies with in-cylinder pressure p, dh volume V, crank angle h, and specific heat ratio c, via this equation [20]:
dQ r dV 1 dp ¼ p þ V dh r 1 dh r 1 dh
ð3Þ
The coefficient of variation (COV), which was estimated from the cylinder pressure data to assess cyclic variability, was calculated as follows [20]:
COV ¼
rimep
ð4Þ
imep
Table 4 Uncertainty in measurements. Item
Uncertainty
Maximum uncertainty, %
Speed Torque Fuel volume flow rate Air flow rate Cylinder pressure Crank angle
±40 rpm ±0.02 Nm ±0.04 ml/min (max) ±0.05 103 kg/s (max) ±0.01 bar ±0.01 degree
±2.7 ±2 ±1.6 ±1.8 ±1 ±1
Table 5 Examined fuel properties. Fuel type
Gross calorific value, MJ/kg
Density @ 15 °C, kg/m3
Diesel (D100) D70B30 D70B25DEE5 D70B20DEE10
45.5 45.2 44.9 44.6
849 861 852 843
where imep is the mean indicated mean effective pressure estimated for a certain number of thermodynamic cycles, n, while rimep is the standard deviation in indicated mean effective pressure. 3. Discussion of results This section compares the efficiency, fuel consumption, cylinder specific heat ratio, heat release rates, and stability of a compression-ignition engine operated with four fuels at 1500 rpm and different engine loads. 3.1. bsfc and efficiency Figs. 2 and 3 show the change of bsfc and brake thermal efficiency with engine brake power, respectively, for four examined fuels. Figs. 2 and 3 indicate that the bsfc increased while the thermal efficiency decreased when the diesel-biodiesel fuel blend (D70B30) was used as an alternative to diesel for most of engine loads. The minimum fuel consumption increased from 0.246 kg/ kWh for diesel to 0.266 kg/kWh for diesel-biodiesel blend with a percentage increase of 8.1%. Also, the maximum brake thermal efficiency decreased from 32.2% for diesel to 30% for diesel-biodiesel fuel blend with a percentage decrease of 6.8%. This was because the diesel-biodiesel blend had a lower calorific value compared to diesel. Therefore, more fuel needed to burn to produce the same power. In addition, biodiesel had higher viscosity and density compared to diesel fuel. The higher viscosity and density of diesel-biodiesel blend could affect the fuel atomization quality by
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Fig. 2. bsfc variation with engine brake power.
Fig. 3. Brake thermal efficiency variation with engine brake power.
producing larger fuel droplets causing a poorer fuel-air mixing and a reduction in combustion efficiency [12]. Figs. 2 and 3 indicate that there was a significant enhancement in engine bsfc and efficiency for most of engine loads when DEE with a percentage of 5% was added to the diesel-biodiesel mixture (D70B25DEE5). Adding 5% DEE to diesel-biodiesel mixture decreased the bsfc and increased the thermal efficiency in comparison
with diesel for most of engine loads. Although the DEE had lower calorific value in comparison with diesel and biodiesel, its lower viscosity and density improved the fuel atomization quality leading to higher combustion efficiency. Lee and kim [16] found that the higher volatility of DEE facilitated air-fuel mixing and helped form leaner and more homogeneous fuel-air mixtures, which consequently increased the fuel conversion efficiency. The
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improvement in combustion efficiency required less fuel to be burned to produce the same power as indicated in Fig. 4, which shows the variation of engine operating air to fuel ratio against brake power for all tested fuels. Fig. 4 indicates that the highest air to fuel ratio was obtained for most of engine loads when D70B25DEE5 was used as an alternative to all tested fuels. However, the bsfc increased and thermal efficiency decreased for most of engine loads when the DEE proportion in the dieselbiodiesel mixture increased to 10% (D70B20DEE10). Although DEE is an oxygenated fuel which can result to more complete combustion with a consequent increase in engine thermal efficiency and a decrease in fuel consumption, DEE has a lower calorific value and higher latent heat of vaporization compared to diesel fuel, which can lead to a decrease in engine efficiency and an increase in fuel consumption [9,16]. Therefore, the proportion of DEE in the fuel blend needs to be optimized for best engine performance because increasing the proportion of DEE in fuel blend up to a certain limit can decrease fuel blend calorific value more significantly leading to deterioration in engine performance. Barik and Murugan [18] investigated improving a diesel engine performance operated with biodiesel-biogas dual fuel mode using DEE as an additive fuel with three proportions of 2%, 4%, and 6%. The authors concluded that the optimum DEE proportion was 4% as it led to higher efficiency and lower bsfc. Furthermore, previous studies [9] showed that increasing the proportion of DEE to a certain limit resulted in engine instability due to the high volatility of DEE which caused vapor lock in the fuel lines connected to fuel pump. When the DEE percentage was raised to 10%, the blend calorific value decreased more significantly; this caused a reduction in thermal efficiency and a rise in fuel consumption. It can be concluded that the optimum fuel among examined fuels was D70B25DEE5 because this blend caused the engine to produce the lowest bsfc and highest thermal efficiency for most of engine loads in comparison with all tested fuels including the diesel fuel. On the other hand, using the diesel-biodiesel blend (D70B30) resulted in higher bsfc and lower thermal efficiency for most of engine loads in comparison with all tested fuels.
5
3.2. Specific heat ratio It is important to estimate an accurate value for the specific heat ratio (c) of cylinder combustion content because this specific heat ratio is used to estimate the heat release rate according to Eq. (3). A fixed value of c ranging from 1.3 to 1.35 [20,21] was usually assumed by most of previous studies in order to estimate the heat release rate. However, c is a function of in-cylinder composition and temperature [20]. Significant errors could be induced in the estimation of rate of heat release if inaccurate values of c were assumed [21]. Also, inaccurate combustion characteristics results could be produced if c was assigned with a fixed value for all engine operating modes and examined fuels [21]. Although c varies instantaneously with the change of engine crankangle during combustion, it was shown that satisfactory heat release rate results can be obtained by assigning c with an acceptable average value [21]. However, this value of c should be a function of engine operating characteristics. A methodology described by Abbaszadehmosayebi and coworkers [21] proposed to use the measured cylinder pressure data to estimate the average specific heat ratio during combustion. The isentropic relationships of pV cu = constant & pV cb = constant are employed during engine compression and expansion processes, respectively [20]. cu and cb represent the specific heat ratio of cylinder gas through compression and expansion processes, respectively. Hence, cu and cb can be estimated by calculating the slopes of the two approximate straight lines that are created by plotting the log p-log V relationships during compression and expansion processes, respectively [20]. Abbaszadehmosayebi [21] demonstrated that an appropriate value of c can be estimated as the average of both cu and cb. This described strategy was utilized in the current study to estimate c as a function of engine indicated power and type of tested fuel using the measured cylinder pressure as shown in Fig. 5. Fig. 5 demonstrates that the specific heat ratio, c, decreased as indicated power increased for all examined fuels. The increase of mass of burned fuel with increasing engine power, as indicated
Fig. 4. Change of engine air to fuel ratio with brake power.
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Fig. 5. Variation of c against power.
Fig. 6. Increase of maximum cylinder pressure for different fuels.
in Fig. 4, resulted in an increase in in-cylinder pressure, as indicated in Fig. 6, and temperature, which led to a reduction in c. 3.3. Heat release rate Figures from 7 to 9 show the rate of heat release variations with engine crank-angle for all tested fuels at small, medium, and full
load, respectively. The small, medium, and full loads corresponded to indicated powers of 0.8, 1.4, and 2.1 kW, respectively. The corresponding brake powers were 0.3, 0.8, and 1.6 kW, respectively. Although all fuels were injected about 10 degrees before the piston reached the top dead center (350 degrees), the heat release rate started to increase rapidly a few degrees later due to the fuel ignition delay period. This rapid increase in heat release rate
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Fig. 7. Rate of heat release change with crank angle at small load.
Fig. 8. Rate of heat release change with crank angle at medium load.
indicated the instant of combustion beginning. The rate of heat release was rapidly increased after combustion started due to the burning of fuel that accumulated within the combustion chamber during the delay period. At low load condition, the instant of combustion start remained unaffected when fuel type was varied as the combustion started almost at 355 degrees for all examined fuels. Although the cetane
number (CN) of DEE was much higher compared to diesel and biodiesel fuels, the cetane number of diesel-biodiesel-DEE blends might remain close to diesel CN. A previous study [22] showed that mixing the DEE with diesel reduced the mixture cetane number and became less than the diesel CN because the DEE interacted with diesel aromatics, which delayed the start of ignition. However, the maximum heat release rate obtained during the rapid
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20 D100 D70B30 D70B25DEE5 D70B20DEE10 15
HRR,J/deg
10
5
0
−5 340
350
360
370
380 Crank angle,deg
390
400
410
420
Fig. 9. Rate of heat release change with crank angle at full load.
combustion phase was significantly higher for diesel in comparison with other tested fuels, as shown in Fig. 7. Although the delay periods were comparable for all fuels, the engine operated at significantly higher air to fuel ratio during the small load engine operation as demonstrated in Fig. 4. The existence of oxygenated fuels in the very lean fuel-air mixture could have the potential to reduce the maximum heat release rate [20]. The rapid combustion stage ended almost 1.5 degrees after the piston reached the TDC (361.5 degrees) for all fuels at low load condition. However, the combustion continued with lower heat release rates as the fuel issuing from the injector was mixed and burned with in-cylinder air in the mixing controlled combustion phase. When engine load increased, the combustion beginning instant was more advanced for all examined fuels. The combustion started almost one degree earlier for all fuels when the load was changed from small to full load. The increase of burned fuel mass with increasing engine load increased cylinder pressure and temperature, which caused a reduction in the ignition delay duration. Similar trend was also indicated by Rakopoulos and coworkers [23] who investigated the variations of ignition delay with diesel engine load for different fuels of neat cotton seed vegetable oil, neat cotton seed biodiesel, and their blends with either 20% DEE or 20% n-butanol. The authors showed that the ignition delay period decreased from a range of almost 4.5–6 degrees at engine brake mean effective pressure (bmep) of 1 bar to a range of 4–5 degrees at bmep of about 5.5 bar. The crankangle at which the rapid combustion phase ended was more advanced when load increased due to the earlier start of combustion. However, the heat release rate was significantly lower for diesel compared to other tested fuels at higher engine load operation as shown in Fig. 9. That led to a slight delay in the end of combustion timing for diesel in comparison with other examined fuels. The engine operated at richer air-fuel mixture when the load increased. Therefore, the presence of oxygenated fuels,
and higher volatility of DEE that enhanced the air-fuel mixing led to a slight increase in the combustion rate. Raising up the mass of burned fuel with increasing the load led to a significant increase in the rate of heat release during the mixing controlled combustion phase at higher engine load operation as can be noticed by comparing Fig. 7 with Fig. 9. In addition, the increase of injected fuel mass at higher engine loads extended the duration of combustion process. The combustion duration increased by an average value of 11 crank angle degrees as the load changed from small to full-load for all examined fuels. 3.4. Engine stability Cycle to cycle fluctuation in indicated mean effective pressure (IMEP) was estimated via the calculation of the coefficient of variation (COV) in order to assess the engine stability for all tested fuels. Cylinder pressure-crank angle data differs from one thermodynamic cycle to another mainly due to the associated variations in combustion process [20]. Heywood [20] illustrated that engine stability deteriorated when the COV increased above 10%. However, different studies demonstrated that increasing the COV above 5% could badly affect engine stability [12]. The change of COV with load for all examined fuels is indicated in Fig. 10. This figure demonstrates that the COV was lower than 5% for most of engine operating conditions. Therefore, it can be concluded that mixing the DEE (up to 10%) with diesel-biodiesel mixture did not negatively affect engine stability. Fig. 10 shows also that the COV generally reduced with the increase of load indicating better engine stability. That can be explained as the engine consumed a richer air-fuel mixture at higher load conditions. Decreasing the operating air to fuel ratio can decrease engine cyclic variations [20]. Similar results were also obtained by Rakopoulos and coworkers [23] who compared the COV of a diesel engine
Please cite this article in press as: A. Ibrahim, An experimental study on using diethyl ether in a diesel engine operated with diesel-biodiesel fuel blend, Eng. Sci. Tech., Int. J. (2018), https://doi.org/10.1016/j.jestch.2018.07.004
A. Ibrahim / Engineering Science and Technology, an International Journal xxx (2018) xxx–xxx
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Fig. 10. Change of engine COV with indicated power.
fuelled by different fuels of neat cotton seed vegetable oil, neat cotton seed biodiesel, and their blends with either 20% DEE or 20% n-butanol. The authors found that the COV ranged from 8% to 9% for all fuels at low load condition while it significantly reduced to a range of 1.5%–2% at engine high load condition. Fig. 10 shows that adding DEE to diesel-biodiesel blend with a proportion up to 10% generally decreased the COV compared to diesel at higher engine load conditions. Similar results were also obtained by Lee and Kim [16] who found that adding DEE to diesel with a proportion of 25% decreased the COV at higher engine load conditions. Also, Uyumaz [24] demonstrated that adding mustard oil biodiesel to diesel with proportions of 10%, 20%, and 30% resulted in a significant decrease in COV at all engine load conditions. The author explained that as the addition of oxygenated fuel to diesel resulted in more complete combustion throughout engine cycles which decreased the cycle to cycle variation. The D100 COV calculated for this study was around 3% for most of engine loads as shown in Fig. 10. That could be considered slightly higher compared to other diesel engines because the air flow rate was measured using an orifice meter which is a flow restriction device which can induce relatively higher fluctuations in inlet pressure and air flow rate. However, the COV can vary significantly from engine to engine according to engine specifications, operating conditions, and fuel properties [25]. Uyumaz [24] found that the COV in imep for D100 ranged from 11.62% at light load (brake torque of 3.75 Nm) to about 6.5% at high load (brake torque of 18.75 Nm). 4. Conclusions Four fuels were examined in a diesel engine to assess its performance and stability and analyze the combustion process at various loads and a constant speed of 1500 rpm. These fuels were diesel, biodiesel-diesel mixture, and two mixtures of biodiesel-dieselDEE with DEE proportions of 5% and 10% by volume. The following results were found:
The higher viscosity and lower calorific value of biodiesel compared to diesel resulted in some engine performance deterioration when diesel-biodiesel blend was used as an alternative to diesel as the minimum bsfc increased by 8.1% and the maximum thermal efficiency decreased by 6.8%. Although the high volatility and low viscosity of DEE in addition to its oxygen content can enhance the combustion process, its lower calorific value and higher latent heat of vaporization can deteriorate engine performance. It was found that the optimum blending proportion was 5% because blending the dieselbiodiesel mixture with 5% DEE led to a significant improvement in engine performance as the bsfc decreased and thermal efficiency increased at most engine loads compared to diesel. However, increasing the proportion of DEE to 10% decreased thermal efficiency. The oxygen content of oxygenated fuels reduced the maximum heat release rate of blended fuels compared to diesel at light load condition where the air to fuel ratio was the highest. However, the heat release rate was lower and combustion duration was longer for diesel compared to other tested fuels at higher engine load conditions. The high cetane number of DEE did not significantly affect the start of combustion timing for the blended fuels. The COV decreased with increasing engine load for all fuels due to the richer fuel-air mixture operation at higher engine load conditions. The use of oxygenated fuels generally decreased the COV compared to diesel at higher engine load conditions. The COV was lower than 5% for all examined fuels at most loads indicating a stable engine operation.
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Please cite this article in press as: A. Ibrahim, An experimental study on using diethyl ether in a diesel engine operated with diesel-biodiesel fuel blend, Eng. Sci. Tech., Int. J. (2018), https://doi.org/10.1016/j.jestch.2018.07.004