Effects of Ethanol–Diesel on the Combustion and Emissions from A

applied sciences

Article Eﬀects of Ethanol–Diesel on the Combustion and Emissions from a Diesel Engine at a Low Idle Speed

Ho Young Kim, Jun Cong Ge and Nag Jung Choi * Division of Mechanical Design Engineering, Jeonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Korea; [email protected] (H.Y.K.); [email protected] (J.C.G.) * Correspondence: [email protected]; Tel.: +82-63-270-4765

Received: 18 May 2020; Accepted: 14 June 2020; Published: 17 June 2020

Abstract: In this study, detailed experiments were conducted on the combustion and exhaust characteristics of ethanol–diesel blended fuels. The four-stroke four-cylinder common-rail direct injection diesel engine was used. The experiment was carried out at 750 rpm at a low speed idle, and a 40 Nm engine load was applied to simulate the operation of the accessories during the low idle operation of the actual vehicles. The test fuels were four types of ethanol-blended fuel. The ethanol blending ratios were 0% (DE_0) for pure diesel, and 3% (DE_3), 5% (DE_5) and 10% (DE_10) for 3%, 5% and 10% ethanol mixtures (by vol.%). Blending ethanol with diesel fuel increased the maximum combustion pressure by up to 4.1% compared with that of pure diesel fuel, and the maximum heat release rate increased by 13.5%. The brake specific fuel consumption (BSFC) increased, up to 5.9%, as the ethanol blending ratio increased, while the brake thermal efficiency (BTE) for diesel-ethanol blended fuels remained low, and was maintained at 23.8%. The coefficient of variation (COV) of the indicated mean effective pressure (IMEP) was consistently lower than 1% when ethanol was blended. The blending of ethanol increased the ignition delay from a 12.0 degree crank angle (◦CA) at DE_0 to 13.7 ◦CA at DE_10, and the combustion duration was reduced from 21.5 ◦CA at DE_0 to 20.8 ◦CA at DE_10. When ethanol blending was applied, nitrogen oxides (NOx) reduced to 93.5% of the level of pure diesel fuel, the soot opacity decreased from 5.3% to 3% at DE_0, and carbon monoxide increased (CO) by 27.4% at DE_10 compared with DE_0. The presence of hydrocarbon (HC) decreased to 50% of the level of pure diesel fuel, but increased with a further increase in the ethanol blending ratio. The mean size of the soot particulates was reduced by 26.7%, from 33.9 nm for pure diesel fuel, DE_0, to 24.8 nm for DE_10.

Keywords: ethanol–diesel; combustion; emission; low idle speed

1. Introduction The development of next-generation internal combustion engines and new technologies to prevent air pollution and the depletion of resources is underway around the world. Many researchers [1–3] are studying the application of alternative fuels to improve the mechanical functioning and eﬃciency of internal combustion engines. Bajpai et al. [1] and Mahumdul et al. [2] summed up the features of bio-diesel produced with various kinds of raw materials, and reported that biodiesels were suﬃcient as a fuel for internal combustion engines and were likely to reduce emissions and prevent the depletion of fossil fuels. Lim et al. [3] studied GTL (Gas-To-Liquid), a synthetic fuel in the form of a clean diesel fuel generated by the Fischer–Tropsch process, and reported its potential as a fuel for the diesel engine. The representative alternative fuel is biofuel, such as biodiesel and bioethanol, which is produced from vegetable or animal raw materials. Biodiesel and bioethanol have the advantage of containing oxygen in the fuel itself. Biodiesel contains oxygen, ranging from 10 to 12 percent [4–7], and ethanol contains about 35 percent [8,9]. Many studies have been conducted on the application of biodiesel to

Appl. Sci. 2020, 10, 4153; doi:10.3390/app10124153 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4153 2 of 15 diesel engines that have confirmed biodiesel’s positive effect on the reduction of pollutant emissions emitted from diesel engines [2,10–12]. Ethanol is known to be an acceptable fuel for gasoline engines because it has a high octane number [13]. Pure ethanol cannot be used in diesel engines, but it can be used by in blends with diesel fuel. For use as a fuel in an internal combustion or diesel engine, ethanol has many favorable properties [14], such as low viscosity, high oxygen content, high H/C ratio, low sulfur content and high evaporative cooling, which improve its volumetric efficiency. Ethanol has a lower viscosity compared to diesel, which results in the superior atomization of fuel injected into cylinders, and improves the mixing with air when it is blended with diesel. Ethanol also has a high latent heat of evaporation, so using ethanol in a diesel engine by blending it with diesel or biodiesel fuel can increase the volume efficiency by the evaporative cooling of ethanol in the intake and compression strokes. There are three ways to apply ethanol to diesel engines [13,15]. The first method is to supply ethanol fumigation to the intake air using a carburetor or an injector on the manifold. The second method is to build a dual injection system on the cylinder head by modifying the configuration of the system and mechanically changing the engine cylinder head. Lastly, ethanol can effectively be used in diesel engines by blending alcohol and diesel, while preventing phase separation, without modifying the engine system. Many studies have been conducted on the third approach of the above methods. He et al. [16] compared the results of the experiment obtained by applying ethanol 10%-diesel and ethanol 30%-diesel fuels to a four-cylinder direct injection diesel engine. NOx, CO and smoke decreased when using the ethanol blends, but HC increased. Rakopoulos et al. [13] experimented by blending 10% and 15% ethanol (by vol.%) with conventional diesel in a six-cylinder heavy-duty direct injection diesel engine. In that study, CO and NOx tended to decrease slightly, while HC increased. Furthermore, BSFC increased but BTE decreased with increasing alcohol concentration. Sayin [14] applied ethanol–diesel blends, which had 10% ethanol and 15% ethanol by volume, to a single-cylinder four-stroke engine to conduct an experiment with an engine load of 30 Nm at 1000 rpm and 1800 rpm. The results of the experiment showed that CO, HC and smoke opacity decreased, but NOx and BSFC increased. Alptekin [15] tested the effect of a 15% ethanol–diesel blend on three engine speeds (1500, 2000, 2500 rpm) and several engine loads (BMEP 3.3, 5.0, 6.6, 8.3 bar), and reported an increase of pollutant emissions and BSFC. Li et al. [17] applied diesel blended with 5%, 10%, 15% and 20% ethanol by volume to a single-cylinder direct injection diesel engine, and conducted experiments on five engine load conditions (25%, 50%, 75%, 90% and 100%) and two engine speeds (the rated and maximum speed). The addition of ethanol reduced CO and NOx, and increased HC and BSFC. In the study of Author links open overlay panel Lü et al. [18], 15% ethanol–diesel blended fuel was applied to a four-cylinder 3.2-L diesel engine, which demonstrated that CO and HC increased and NOx decreased. Di et al. [19] applied ethanol to diesel and biodiesel to confirm its impact. The concentrations of ethanol in the test fuels were 2, 4, 6 and 8% by volume. It was reported that the BTE improved as ethanol increased, while HC and CO decreased, but NOx increased. Huang et al. [20] conducted a study using five types of ethanol–diesel blended fuels (0, 10, 20, 25 and 30% ethanol by volume). The BTE was reduced because the lower heating value (LHV) of ethanol is low, and the smoke was reduced. The CO decreased above the central load, but increased at low loads and low speeds. Furthermore, the HC increased. However, NOx had different emission trends depending on each fuel condition, engine load and engine speed condition. In addition to the studies of the application of ethanol with diesel, many studies are underway on the application of methanol, an alcohol-based fuel that is less-frequently mixed with diesel than ethanol, but is fully applicable. Jamrozik [21] and Tutak et al. [22] used various proportions of methanol and ethanol–diesel fuel to compare combustion and emission characteristics. In this study, bioethanol was blended with diesel fuel and applied to a four-cylinder common-rail direct injection diesel engine applied to a commercial passenger vehicle. Notably, this experiment was conducted in a low-speed idle condition. Low idling operation is one of the worst operating conditions of the engine, with poor combustion and high exhaust pollutants. Rahman et al. [23] explained that, during idling, the mixture of air and fuel is rich, and combustion is unstable due to the low operating temperature of the engine, resulting in the increased generation of exhaust pollutants, and more Appl. Sci. 2020, 10, 4153 3 of 15 exhaust pollutants being emitted when devices such as air conditioners are operated. Brodrick et al. [24] explained that the engine has a thermal efficiency of 30% under high-speed driving conditions, but only 3–11% at idle. Many researchers reported increased emissions of exhaust pollutants under idle conditions using heavy-duty engines. Khan et al. [25] reported an increase in NOx under idle conditions, Brodrick et al. [24] and McCormick et al. [26] showed an increase in HC and Storey et al. [27] showed an increase in CO. The previous studies were conducted and researched with engines from heavy-duty trucks at high idle, over 1000 rpm. In the above mentioned high idle, the engine runs at a low rated speed with a low load. The experimental conditions of this study were carried out at low-idle speed, rather than a high-idle speed, with an engine for passenger cars. In addition, the fuel injection pressure was 280 bar, the lowest of the engine operating conditions, resulting in relatively poor fuel atomization. In these poor combustion conditions, at a low idle speed, the following objectives were intended to be carried out using bioethanol–diesel fuel, which is an oxygen fuel: (1) study of the effects of combustion due to high oxygen content, low density and low LHV, which are characteristics of bioethanol; (2) check the change in emission of bioethanol; (3) check the validity of bioethanol application.

2. Methodology

2.1. Test Fuels In this study, pure diesel fuel was blended with bioethanol with 99.9% purity. Bioethanol was blended with pure diesel fuel at 3%, 5% and 10% by volume. The test fuels were labeled DE_0 (Diesel 100% + Ethanol 0%), DE_3 (Diesel 97% + Ethanol 3%), DE_5 (Diesel 95% + Ethanol 5%) and DE_10 (Diesel 90% + Ethanol 10%). The viscosity of ethanol is about 40% lower than that of diesel fuel, and the LHV is also about 64% lower than diesel fuel. The low viscosity of ethanol is about 40% that of diesel. The cetane number of ethanol is 14.3% that of diesel. Ethanol is further known to have a higher latent heat of vaporization than diesel, by about 3–5 times [8,21,28]. The properties of the diesel and ethanol used in the tests are as shown in Table1.

Table 1. Diesel and Bioethanol properties.

Properties Units Diesel Bioethanol 3 Density at 15 ◦C kg/m 836.8 799.4 2 Viscosity at 40 ◦C mm /s 2.719 1.10 Lower heating value MJ/kg 43.96 28.18 Cetane number - 55.8 8.0 Flash point ◦C 55 12.0 Oxygen content wt.% 0 34.3 Hydrogen content wt.% 13.0 13.1 Carbon content wt.% 85.7 52.2

When blending ethanol and diesel fuel, preventing phase separation is critically important. As shown in Figure1, the experimental fuels of ethanol blended in diesel fuel were observed, and no phase separation occurred. A study by Hansen et al. [29] also reported that the phase separation of a diesel-ethanol blend does not occur when the ambient air temperature is above 10 ◦C. Lapuerta et al. [30] also reported that phase separation began to occur when the temperature drops below 10 ◦C. In this experiment, the test fuels were blended at the ambient temperature of 20 ◦C, and were kept at about 25 ◦C during tests. Appl. Sci. 2020, 10, 4153 4 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 15

Figure 1. Experimental fuels blended with ethanol in diesel diesel..

2.2. Experimental Setup and Measurements 2.2. Experimental Setup and Measurements 2.2.1. Engine and Experimental Setups 2.2.1. Engine and Experimental Setups A four-cylinder 2.0 L common-rail direct injection diesel engine was used for this test. The fuel A four-cylinder 2.0 L common-rail direct injection diesel engine was used for this test. The fuel injection systems (solenoid injectors, fuel pump, common rail) of Bosch and an ECU (engine control injection systems (solenoid injectors, fuel pump, common rail) of Bosch and an ECU (engine control unit) were applied. The detailed specifications are shown in Table2. unit) were applied. The detailed specifications are shown in Table 2. The test engine was installed onTable an 2. eddyEngine current specifications. dynamometer (DY-230 kW, Hwanwoong Mechatronics, Gyeongsangnam-do, Korea). A piezoelectric pressure sensor (Kistler, 6056a, Winterthur,Engine Switzerland) Parameters, located at the position Unit of the glow plug, Specificationwas used to measure the combustion pressureEngine Type, and the data was recorded - and analyzedIn-line 4 Cylinder,by a data WGT acquisition Turbocharged, board EGR (PCI 6040e, NationalMaximum Instruments, Power/Torque Austin, TX, USA). kW/Nm The angular position 84.6(@4000 of the rpm) crankshaft/260(@2000 for rpm)the analysis Bore Stroke mm mm 83 92 was measured by× a rotary encoder (E6B2-CWZ3DE,× Omron, Japan). The emission× levels of NOx and Displacement cc 1991 CO were measured with an MK2 multi-gas analyzer (Euroton, Italy), and HC was measured with an Compression Ratio - 17.7:1 HPCNumber-501 analyzer of Injector (Pantong nozzle holes Huapeng Electronics, - China). The smoke opacity 5 level was measured using a partialInjector flow type collecting soot analyzer -OPA-102 (Qurotech, Korea). Solenoid The fuel flow rate used during theInjector tests holewas diametercalculated by measuring mm the fuel weight change over 10 0.17min on a high-precision digital electronic weighing balance (GP-31K, A&D, Japan). The exhaust gas temperature was measuredThe test just engineafter the was turbocharger. installed on The an soot eddy emitted current from dynamometer the engine (DY-230was collected kW, Hwanwoong by a copper Mechatronics,grid (FCF400-CU, Gyeongsangnam-do, Electron Microscopy Korea). Sciences, A piezoelectric PA, USA), pressure and was sensor used to (Kistler, analyze 6056a, the soot Winterthur, particle Switzerland),sizes by transmission located at the electron position microscopy of the glow (TEM, plug, was H-7650, used to HITACHI measure, the Fukuoka, combustion Japan). pressure, The andexperimental the data was equipment recorded diagram and analyzed is shown by ain data Figure acquisition 2. board (PCI 6040e, National Instruments, Austin, TX, USA). The angular position of the crankshaft for the analysis was measured by a rotary encoder (E6B2-CWZ3DE, Omron, Japan).Table The 2. Engine emission specification levels ofs. NOx and CO were measured with an MK2 multi-gasEngine analyzer Parameters (Euroton, Italy), andUnit HC was measured withSpecification an HPC-501 analyzer (Pantong Huapeng Electronics,Engine Type China). The smoke opacity- levelIn-line was 4 measuredCylinder, WGT using Turbocharged, a partial flow collecting EGR soot analyzerMaximum OPA-102 Power/Torque (Qurotech, Korea).kW/Nm The fuel flow84.6(@400 rate used0 duringrpm)/260(@200 the tests0 wasrpm) calculated by measuringBore the x fuel Stroke weight changemm over × 10mm min on a high-precision83 digital× 92 electronic weighing balance (GP-31K,Displacement A&D, Japan). The exhaustcc gas temperature was measured1991 just after the turbocharger. The soot emittedCompression from the Ratio engine was collected- by a copper grid (FCF400-CU,17.7:1 Electron Microscopy Sciences,Number PA, USA),of Injector and wasnozzle used holes to analyze - the soot particle sizes by transmission5 electron microscopy (TEM, H-7650,Injector HITACHI, type Fukuoka, Japan). The- experimental equipmentSolenoid diagram is shown in Figure2. Injector hole diameter mm 0.17 Appl. Sci. 2020, 10, 4153 5 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 15

FigureFigure 2. 2. SchematicSchematic diagram diagram of of the the experimental experimental setups. setups. 2.2.2. Test Procedure 2.2.2. Test Procedure In this experiment, the engine speed was set to 750 rpm, which was defined as the low idle speed. In this experiment, the engine speed was set to 750 rpm, which was defined as the low idle speed. The engine load was 40 Nm, in order to produce conditions similar to those of real vehicles with The engine load was 40 Nm, in order to produce conditions similar to those of real vehicles with accessory systems such as an alternator and an air conditioner at a low idle speed. The main injection accessory systems such as an alternator and an air conditioner at a low idle speed. The main injection timing was fixed at before top dead center (BTDC) 2 degree crank angle ( CA), the pilot injection timing was fixed at before top dead center (BTDC) 2 degree crank angle (°CA)◦ , the pilot injection timing was fixed at BTDC 20 CA (the separation is 18 CA) and the injection pressure was fixed at timing was fixed at BTDC 20 °CA◦ (the separation is 18 °CA)◦ and the injection pressure was fixed at 280 bar. The main injection duration was 3.4 CA, and the pilot injection’s duration was 1.5 CA. 280 bar. The main injection duration was 3.4 °CA◦ , and the pilot injection’s duration was 1.5 °CA. ◦The The combustion pressure and exhaust levels were measured when the engine speed was stabilized combustion pressure and exhaust levels were measured when the engine speed was stabilized within within 750 10 rpm for each experimental condition. The coolant temperature was maintained at 750 ± 10 rpm± for each experimental condition. The coolant temperature was maintained at 85 ± 5 °C . 85 5 C. During the experiments, the ambient temperature was kept at about 25 C. The combustion During± ◦ the experiments, the ambient temperature was kept at about 25 °C . The combustion◦ pressure pressure was calculated as the average of 200 cycles. The soot emitted from the test engine was directly was calculated as the average of 200 cycles. The soot emitted from the test engine was directly collected from the exhaust pipe onto the copper grid. The grids were evaporated in a 60 °C-vacuum collected from the exhaust pipe onto the copper grid. The grids were evaporated in a 60 ℃-vacuum chamber for 12 h. Soot images were filmed 100,000 times magnified using TEM. The combustion chamber for 12 h. Soot images were filmed 100,000 times magnified using TEM. The combustion pressure and exhaust measurements were initialized when the engine speed was stabilized within pressure and exhaust measurements were initialized when the engine speed was stabilized within 750 10 rpm for each experimental condition. The coolant temperature was maintained at 85 5 C. 750 ±± 10 rpm for each experimental condition. The coolant temperature was maintained at 85 ±± 5 °C◦ . The combustion pressure was calculated as the average of 200 cycles. The experimental conditions are The combustion pressure was calculated as the average of 200 cycles. The experimental conditions summarized in Table3. are summarized in Table 3. Table 3. Test conditions. Table 3. Test conditions. Table Unit Condition Table Unit Condition Engine Speed rpm 750 10 (Low idle speed) ± Engine LoadEngine Speed Nmrpm 750 ± 10 (Low idle 40speed) Cooling Water TemperatureEngine Load CNm 40 85 5 ◦ ± Intake AirCooling Temperature Water Temperature C°C 85 ± 5 25 5 ◦ ± Fuel InjectionIntake Pressure Air Temperature bar°C 25 ± 5 280 InjectionFuel Timing Injection Pressure ◦CAbar Main280 BTDC 2/Pilot BTDC 20 Injection Timing °CA Main BTDC 2/Pilot BTDC 20 3. Results and Discussion 3. Results and Discussion 3.1. Combustion Characteristics 3.1. Combustion Characteristics 3.1.1. Combustion Pressure and Heat Release Rate

3.1.1. TheCombustion blending Pressure of ethanol and has Heat a great Release effect Rate on combustion in the cylinder. By weight, the ethanol molecule is approximately 33% oxygen, which promotes combustion. On the other hand, the low The blending of ethanol has a great effect on combustion in the cylinder. By weight, the ethanol LHV of ethanol is low, at 64% of that of diesel fuel, but it has a high latent heat of vaporization, which molecule is approximately 33% oxygen, which promotes combustion. On the other hand, the low LHV of ethanol is low, at 64% of that of diesel fuel, but it has a high latent heat of vaporization, which Appl.Appl. Sci. Sci. 20202020, ,1010, ,x 4153 FOR PEER REVIEW 6 6of of 15 15 provides an ethanol evaporation cooling effect during combustion. Also, the low density and viscosityprovides of an ethanol ethanol promote evaporation the atomization cooling effect of during the injected combustion. fuel. Also, the low density and viscosity of ethanolFigure promote 3a,b and the Table atomization 4 shows of the the combustion injected fuel. pressure and heat release rates for each fuel condition.Figure The3a,b maximum and Table combustion4 shows the pressure combustion of DE_0 pressure is 5828 and kPa heat, and release the ethanol rates-blended for each DE_3, fuel DE_5condition. and DE_10 The maximum are 4.1%, combustion3.1% and 2.9% pressure higher, of resp DE_0ectively, is 5828 at kPa, 6070 and kPa, the 6013 ethanol-blended kPa and 5998 DE_3, kPa, respectivelyDE_5 and DE_10. The position are 4.1%, of 3.1%maximum and 2.9% cylinder higher, pressure respectively, was at 11 at °CA 6070 after kPa, top 6013 dead kPa center and 5998 (ATDC) kPa, underrespectively. all fuel The conditions. position of The maximum maximum cylinder heat pressurerelease rate was (HRR) at 11 ◦ CA is also after higher top dead when center ethanol (ATDC) is blended.under all The fuel maxi conditions.mum HRR The for maximumDE_3 is increased heat release by 9.3%, rate from (HRR) 28.64 is J/°CA also higher to 31.30 when J/°CA ethanol at DE_0, is DE_3blended. is increased The maximum by 11.2% HRR to 31.86 for DE_3 J/°CA, is increasedand DE_10 by is 9.3%, increased from by 28.64 13.5% J/◦CA to 32.53to 31.30 J/°CA. J/◦CA The at maximumDE_0, DE_3 heat is release increased rate by occurred 11.2% to at 31.86ATDC J/ ◦7CA,–8 °CA and under DE_10 all is fuel increased conditions. by 13.5% The maximum to 32.53 J HRR/◦CA. forThe DE_3 maximum is increased heat release by 9.3%, rate from occurred 28.64 atJ/°CA ATDC at 7–8DE_0◦CA to 31.30 under J/°CA, all fuel DE_5 conditions. is increased Themaximum by 11.2% toHRR 31.86 for J/°CA, DE_3 is and increased DE_10 isby increased 9.3%, from by 28.64 13.5% J/◦ CA to 32.53 at DE_0 J/°CA. to 31.30 The maximum J/◦CA, DE_5 heat is increased release rate by occurred11.2% to 31.86 at ATDC J/◦CA, 7- and8 °CA DE_10 under is increased all fuel conditions by 13.5% to. Jamrozik 32.53 J/◦CA. [21] The reported maximum that heat the mixturerelease rate of alcoholoccurred increased at ATDC the 7–8 HRR◦CA due under to allthe fuel increase conditions. in the ignition Jamrozik delay [21] reportedbecause thatof the the low mixture cetane of number alcohol andincreased high latent the HRR heat due of to evaporation, the increase thereby in the ignition increasing delay the because maximum of the combustion low cetane pressure. number The and studyhigh latentby Gnanamor heat of evaporation,iti et al. [31] also thereby reported increasing that, with the maximuman increased combustion ethanol blending pressure. rate, The the study heat releaseby Gnanamoriti rate increased et al. [dramatically31] also reported due to that, the with increase an increased in the adiabatic ethanol blendingflame template. rate, the On heat the release other hand,rate increased combustion dramatically after the main due injection to the increase tends to in be the different adiabatic from flame the template.combustion On of the pilot other injection hand, fuel.combustion When the after main the main injection injection is sprayed, tends to combustion be different fromis generated the combustion partially of in pilot the injection combustion fuel. chamber,When the so main the injectiondiffuse combustion is sprayed, stage combustion is initiated. is generated The greater partially the increase in the combustionin the blend chamber, ratio of ethanol,so the di theffuse stronger combustion the improvement stage is initiated. of combustion The greater by the increase in theoxygen blend content. ratio of Also, ethanol, the additionthe stronger of ethanol the improvement improves the of atomization combustion of by the the injected increase fuel. in oxygen Sayin [14] content. said that Also, the the advantage addition ofof using ethanol alcohol improves as fuel the is atomization improving mixing of the injected with the fuel. air by Sayin improving [14] said the that saturat the advantageion of the injected of using fuelalcohol through as fuel its is low improving viscosity. mixing Li et withal. [17] the reported air by improving that the theincrease saturation in ethanol of the blending injected fuel resulted through in theits lowlower viscosity. density and Li et reduced al. [17] reportedsurface tension that the of increase the blended in ethanol fuel, which blending improved resulted the in air the mixing lower duedensity to the and evaporation reduced surface of the tensionethanol of spray, the blended which improved fuel, which the improved macroscopic the air spray mixing behavior due to and the atomizationevaporation performance. of the ethanol Garai spray, et which al. [32] improved also confirmed the macroscopic the injection spray characteristics behavior and of atomizationpure diesel fuelperformance., pure ethanol,Garai and et al.ethanol [32] also 10 and confirmed 20% blended the injection fuel, and characteristics reported that of the pure addition diesel of fuel, ethanol pure facilitatesethanol, and the ethanolimprovement 10% and of 20%the spray blended and fuel, atomization. and reported In addition, that the with addition the addition of ethanol of facilitates ethanol, fuelthe improvementin parts of the of pilot the spray injection and atomization.that have not In been addition, burned with due the to addition the slow of burning ethanol, fuelof the in pilot parts injectionof the pilot in injectionDE_10 will that be have burned not been with burned the main due injection to the slow, and burning the heat of therelease pilot rate injection will inincrease DE_10 rapidly.will be burned with the main injection, and the heat release rate will increase rapidly.

Figure 3. (a) Combustion pressure and (b) heat release rate of test fuels. Figure 3. (a) Combustion pressure and (b) heat release rate of test fuels.

Appl. Sci. 2020, 10, 4153 7 of 15

Table 4. Combustion characteristics under test conditions.

Appl. Sci. 20Max20, 10 Combustion, x FOR PEER REVIEWLocation of Max Heat Release Location of Exhaust Gas 7 of 15 Test BSFC BTE Pressure (P ) P Rate (HRR ) HRR Temperature Fuel max max max max (bar) ( CA) (J/ CA) ( CA) (K) (g/kWh) (%) Table◦ 4. Combustion characteristic◦ s under◦ test conditions. DE_0 58.3 ATDC 11 28.64 ATDC 7 501 336.9 24.3 DE_3Max Combustion 60.7 ATDCLocation 11 of Max Heat 31.30 Release Location ATDC 7of Exhaust 498 Gas 348.3 23.8 Test BSFC BTE DE_5Pressure 60.1 (Pmax) ATDCPmax 11 Rate 31.86 (HRRmax) HRR ATDCmax 8 Temperature 495 350.3 23.8 DE_10Fuel 60.0 ATDC 11 32.53 ATDC 8 494 356.7 23.8 (bar) (°CA) (J/°CA) (°CA) (K) (g/kWh) (%) DE_0 58.3 ATDC 11 28.64 ATDC 7 501 336.9 24.3 DE_3 60.7 ATDC 11 31.30 ATDC 7 498 348.3 23.8 FigureDE_5 4 shows60.1 (a) the BSFCATDC and 11 BTE and31.86 (b) the exhaustATDC gas 8 temperature495 for each350.3 fuel condition.23.8 The BSFCDE_10 increased 60.0 from 336.9ATDC g/kWh 11 (DE_0)32.53 to348.3 g/kWhATDC (DE_3) 8 as 3.3%,494 350.3 g356.7/kWh (DE_5)23.8 as 4.0% and 356.7 g/kWh (DE_10) as 5.9% with an increasing blend ratio of ethanol. This is because more fuel was consumedFigure 4 shows due (a) to the the BSFC lower and LHV BTE and of ethanol. (b) the exhaust The BTE, gas temperature on the other for hand, each fuel is lower condition. than that of dieselThe BSFC fuel whenincreased ethanol from 336.9 is blended, g/kWh (DE_0) and remains to 348.3 at g/kWh a similar (DE_3) level as 3 of.3%, 23.8% 350.3 asg/kWh the ethanol(DE_5) as blend 4.0% and 356.7 g/kWh (DE_10) as 5.9% with an increasing blend ratio of ethanol. This is because more ratio increases. In all studies using ethanol-blended or alcohol-blended fuel, the BSFC was seen to fuel was consumed due to the lower LHV of ethanol. The BTE, on the other hand, is lower than that increase.of diesel In afuel study when by ethanol Lü et al.is blend [18],ed ethanol, and remains had a lowerat a similar BTE level than of diesel 23.8% fuel, as the and ethanol in the blend study of Huangratio et increases. al. [20] under In all lowstudies engine using load ethanol conditions,-blended or the alcohol BTE- tendedblended tofuel, be the lower BSFC than was that seen for to pure dieselincrease. fuel. Sayin In a study [14] reported by Lü et thatal. [18] the, ethanol BTE of had alcohol a lower (ethanol BTE than and diesel methanol) fuel, and blended in the study fuel is of lower thanHuang that of et diesel al. [20] fuel under because low engine of the load higher conditions, BSFC. Gnanamoritithe BTE tended et to al. be [31 lower] reported than that that for the pure reason for thediesel reduced fuel. Sayin BTE of[14] blended reported ethanol that the wasBTE theof alcohol low LHV, (ethanol resulting and methanol) in more blended fuel being fuelconsumed is lower to producethanthe that same of diesel power, fuel anbecause increased of the ignitionhigher BSFC. delay Gn dueanamoriti to the lowet al. cetane[31] reported number that and the areason decreased combustionfor the reduced temperature BTE of dueblend toed the ethanol quenching was the elowffect LHV of, ethanol. resulting Rakopoulosin more fuel being [13] andconsumed Li et al.to [17], on theproduce other hand,the same reported power, that an increased the BTE increasesignition delay with due the to blending the low cetane of ethanol. number Di and et al. a decreas [19] explaineded combustion temperature due to the quenching effect of ethanol. Rakopoulos [13] and Li et al. [17], on that there are several factors that affect the BTE. (i) The combustion is improved by the increase of the other hand, reported that the BTE increases with the blending of ethanol. Di et al. [19] explained oxygenthat due there to are the several inclusion factors of that ethanol. affect (ii)the TheBTE. temperature (i) The combustion of the is flame improved decreases by the dueincrease to the of high evaporativeoxygen due latent to the heat, inclusion reducing of ethanol. the heat (ii) loss The of temperature the cylinder. of the (iii) flame The increasedecreases in due the to ignition the high delay due toevaporative the low cetane latent heat, number reducing increases the heat the loss fuel of burned the cylinder. during (iii) the The premixed increase in period. the ignition (iv) Indelay order to producedue theto the same low outputcetane number with a lowerincreases LHV, the morefuel burned fuel is during injected the and premixed burned period. by the (iv) expansion In order to stroke, and theproduce diffusion the same combustion output with is alsoa low increased.er LHV, more These fuel is factors injected have and burned complex bye theffects expansion depending stroke, on the engineand load, the diffusion speed and combustion fuel conditions. is also increased. Additionally, These thefactors exhaust have complex gas temperature effects depending decreases on withthe the increasingengine ethanolload, speed blending and fuel ratio. conditions. A study Additionally, by Tutak the et e al.xhaust [22] gas with temperature ethanol and decreases methanol with blendedthe fuel,increasing an alcohol ethanol fuel, alsoblending reported ratio. thatA study the exhaustby Tutak temperatureet al. [22] with was ethanol lowered and methanol as the blend blended ratio of fuel, an alcohol fuel, also reported that the exhaust temperature was lowered as the blend ratio of alcohol increased. alcohol increased.

FigureFigure 4. (a ) 4. The (a BSFC) The and BSFC BTE, and and BTE, (b) exhaust and (b) gas exhaust temperature gas temperature with changing with ethanol changing concentrations. ethanol concentrations. 3.1.2. Combustion Phase The3.1.2. combustion Combustion Phase phase is known to directly affect the thermal efficiency of internal combustion engines.The The combustion mass fraction phase burned is known (MFB) to directly provides affect a the convenient thermal efficiency way to of analyze internal thecombustion phase of the combustionengines. inThe the mass cylinder fraction by burned the duration (MFB) ofprovides the crank a convenient angle. The way MFB to isanalyze calculated the phase by dividing of the the amountcombustion of accumulated in the cylinder heat during by the duration the combustion of the crank period angle. by The the MFB total is heat calculated released by [dividing12,33]. From the the startamount of fuel injection,of accumulated the crank heat positionduring the where combustion the MFB period is 10% by isthe noted total heat as CA10, release andd [12,33] the point. From where the MFBthe start is 90% of fuel is noted injection, as CA90.the crank The position difference where between the MFBthe is 10% fuel is injection noted as CA10 start, point and the and point CA10 is where the MFB is 90% is noted as CA90. The difference between the fuel injection start point and called the ignition delay, or the flame-development angle, and the difference between CA10 and CA90 Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 15

CA10Appl. is Sci. called2020, 10the, 4153 ignition delay, or the flame-development angle, and the difference between CA108 of 15 and CA90 is called the combustion duration, or the rapid-burning angle [33–35]. CA50 denotes where Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 15 the MFB is 50%, which means that 50% of the injected fuel is converted to energy [36]. is called the combustion duration, or the rapid-burning angle [33–35]. CA50 denotes where the MFB is FigureCA10 5 is shows called thethe MFBignition for delay, each fuelor the condition. flame-development Figure 5a angle, is the andinitial the area difference of combustion, between CA10 which 50%, which means that 50% of the injected fuel is converted to energy [36]. shows theand CA10CA90 islocation called the of combustion each fuel condition,duration, or and the rapid (b) is-burning the end angle of the [33 MFB,–35]. CA50 which denotes shows where CA90. Figure5 shows the MFB for each fuel condition. Figure5a is the initial area of combustion, which With anthe increasing MFB is 50%, ethanol which means blend that ratio, 50% CA10 of the isinjected retarded fuel is from converted BTDC to 8.0 energy °CA [36] at. DE_0 to BTDC shows the CA10 location of each fuel condition, and (b) is the end of the MFB, which shows CA90. With 6.3 °CA at DE_10.Figure 5 Withshows increasingthe MFB for ethanol,each fuel condition.CA90 was Figure retarded 5a is thefrom initial ATDC area of13.5 combustion, °CA, at DE_0,which to an increasingshows the ethanolCA10 location blend of ratio, each CA10 fuel condition, is retarded and from (b) is BTDC the end 8.0 of theCA MFB, at DE_0 which to shows BTDC CA90. 6.3 CA at ATDC 14.5 °CA, at DE_10. Figure 6 presents the ignition delay and◦ combustion duration. As◦ the DE_10.With With an increasing ethanol ethanol, blend CA90 ratio, was CA10 retarded is retarded from ATDC from 13.5 BTDCCA, 8.0 at °CA DE_0, at DE_0 to ATDC to BTDC 14.5 CA, ethanol in the blend is increased, the ignition delay is increased from ◦12.0 °CA to 13.7 °CA, and the◦ at DE_10.6.3 °CA Figure at DE_10.6 presents With theincreasing ignition ethanol, delay andCA90 combustion was retarded duration. from ATDC As the 13.5 ethanol °CA, at in DE_0, the blend to is combinationATDC duration14.5 °CA, decreasedat DE_10. Figurfrome 21.56 presents °CA to the 20.8 ignition °CA. delayThis meansand combustion that the combustionduration. As othef the increased, the ignition delay is increased from 12.0 ◦CA to 13.7 ◦CA, and the combination duration pilot injectionethanol inbecomes the blend slower is increased, with an the increased ignition delay ethanol is increased blend ratio from, 12.0but °CAthe diffusionto 13.7 °CA combustion, and the decreased from 21.5 CA to 20.8 CA. This means that the combustion of the pilot injection becomes after thecombination main injection duration◦ becomes decreased faster.◦ from When 21.5 ethanol°CA to 20.8 is blended, °CA. This the means ignition that thedelay combustion increases of due the to slower with an increased ethanol blend ratio, but the diffusion combustion after the main injection the highpilot evaporation injection becomes latent heat slower and with low an cetane increased number ethanol of blendethanol. ratio As, but the the diffusion diffusion combustion combustion of thebecomes mainafter injection faster.the main Whenis injection initiated ethanol becomes, rapid isblended, faster.combustion When the ethanoloccurs ignition isdue delayblended, to the increases the effect ignition of due oxygen delay to the increases added high evaporation fromdue to the blendedlatentthe heatethanol. high and evaporation Studies low cetane by latent Xingcai number heat andet of al. low ethanol. [18] cetane, Jamrozik As number the [21] di offfusion ethanol.and Tutak combustion As theet al. diffusion [22] of theal socombustion main reported injection of that is blendinginitiated,the ethanol main rapid injection combustionor alcohol is initiated fuels occurs increases, rapid due combustion to thethe eignitionffect occurs of oxygendelay due dueto added the to effect the from low of the oxygen cetane blended added number ethanol. from and the Studies the blended ethanol. Studies by Xingcai et al. [18], Jamrozik [21] and Tutak et al. [22] also reported that coolingby Xingcai effect etcaused al. [18 by], Jamrozik the high [latent21] and heat Tutak of evaporation, et al. [22] also and reported decreases that the blending combination ethanol duration. or alcohol blending ethanol or alcohol fuels increases the ignition delay due to the low cetane number and the CA50fuels is increasesthe point thewhere ignition the maximum delay due heat to the release low cetane rate occurs number at the and center the cooling of the eMFBffect profile caused, byand the cooling effect caused by the high latent heat of evaporation, and decreases the combination duration. is highused latentto set heatthe maximum of evaporation, brake and torque decreases of the the engine. combination CA50 is duration. retarded CA50 fromis ATDC the point 6.8 where°CA to the maximumCA50 is heat the poin releaset where rate the occurs maximum at the heat center release of the rate MFB occurs profile, at the center and is of used the MFB to set profile the maximum, and ATDC is7.6 used °CA to as set the the ethanol maximum blend brake ratio torque increases. of the engine. It is noted CA50 thatis retarded these pofromints ATDC are similar 6.8 °CA to to the brake torque of the engine. CA50 is retarded from ATDC 6.8 ◦CA to ATDC 7.6 ◦CA as the ethanol locationsATDC of HRR 7.6 max°CA. as the ethanol blend ratio increases. It is noted that these points are similar to the blend ratio increases. It is noted that these points are similar to the locations of HRRmax. locations of HRRmax.

FigureFigureFigure 5. Mass 5. 5.Mass Mass Fraction Fraction Fraction Burned. Burned. (a )(a (CA10)a )CA10 CA10 and and and ( (bb)) ( CA90bCA90) CA90...

ATDC 20°CA ATDC 20°CA ■ : Ignition Delay ■ : Combustion Duration ■ : Ignition Delay ■ : Combustion Duration

ATDC 10°CA ATDC 10°CA 20.8 21.5 21.3 21.1 20.8 TDC 21.5 21.3 21.1 TDC

BTDC 10°CA 12.5 12.9 13.7 BTDC 10°CA 12.0

BTDC 20°CA 12.0 12.5 12.9 13.7 (SOI) DE_0 DE_3 DE_5 DE_10 BTDC 20°CA (SOI) DE_0Figure 6. The ignitionDE_3 delay and combustionDE_5 duration. DE_10

Figure 6. The ignition delay and combustion duration. 3.1.3. Combustion StabilityFigure 6. The ignition delay and combustion duration. 3.1.3. CombustionThe coefficient Stability of variation (COV) of the indicated mean effective pressure (IMEP) represents 3.1.3. Combustion Stability theThe robustness coeﬃcient of ofthe variation engine to cyclical (COV) variability of the indicated per cycle mean, and is eﬀ aective way ofpressure verifying (IMEP)the stability represents of Thecombustion. coefficient If ofthe variation COVIMEP exceeds (COV) 10% of the in the indicated internal combustionmean effective engines pressure used in (IMEP)automobiles, represents it is the robustness of the engine to cyclical variability per cycle, and is a way of verifying the stability of the robustnessjudged that of thethere engine is a problem to cyclical with variabilitythe operation per of cycle the engine,, and isand a way if it is of less verifying than 5%, the it isstability judged of combustion. If the COVIMEP exceeds 10% in the internal combustion engines used in automobiles, it is combustion. If the COVIMEP exceeds 10% in the internal combustion engines used in automobiles, it is judged that there is a problem with the operation of the engine, and if it is less than 5%, it is judged to judged that there is a problem with the operation of the engine, and if it is less than 5%, it is judged have a stable combustion state [21,22]. Figure7a shows the IMEP for 200 cycles under each combustion Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 15 Appl. Sci. Sci. 20202020,, 1010,, x 4153 FOR PEER REVIEW 99 of of 15 to have a stable combustion state [21,22]. Figure 7a shows the IMEP for 200 cycles under each to have a stable combustion state [21,22]. Figure 7a shows the IMEP for 200 cycles under each combustion condition, and (b) shows the COVIMEP. Under pure diesel fuel conditions, the COVIMEP of combustioncondition, and condition (b) shows, and the (b) COV shows the. UnderCOVIMEP pure. Under diesel pure fuel diesel conditions, fuel conditions, the COV the COVof DE_0IMEP of is DE_0 is less than 1.5%, and thoseIMEP of the ethanol-blended fuels were lower thanIMEP 0.8%, which DE_0less than is less 1.5%, than and 1.5%, those and of the those ethanol-blended of the ethanol fuels-blended were lower fuels than were 0.8%, lower which than demonstrates 0.8%, which demonstrates very stable combustion. For ethanol additions above 3%, further increases in ethanol demonstratesvery stable combustion. very stable Forcombustion. ethanol additions For ethanol above addition 3%, furthers above increases 3%, further in ethanolincreases very in ethanol slightly very slightly increase the COVIMEP. Due to the low LHV of bioethanol and the high latent heat of veryincrease slightly the COVincrease .the Due COV toIMEP the. lowDue LHV to the of low bioethanol LHV of and bioethanol the high and latent the heat high of latent evaporation, heat of evaporation, the combustionIMEP temperature and pressure in the cylinder were reduced when injected evaporatthe combustionion, the temperature combustion andtemperature pressure and in the pressure cylinder in were the cylinder reduced were when reduced injected when with fuel, injected and with fuel, and rapid combustion occured subsequently. It was believed that COVIMEP increased withrapid fuel, combustion and rapid occured combustion subsequently. occured It was subsequently. believed that It COV was believedincreased that slightly COVIMEP as increased the blend slightly as the blend ratio of bioethanol increased. However, it variesIMEP less than 0.2%. Similar to the slightlyratio of bioethanolas the blend increased. ratio of bioethanol However, increased it varies less. However, than 0.2%. it varies Similar less to than the results0.2%. Similar of this to study, the results of this study, Tutak et al. [22] reported that the COVIMEP was 2.5% at low ethanol blending resultsTutak et of al. this[22 ]study, reported Tutak that et the al. COV [22] reportedwas 2.5% that atthe low COV ethanolIMEP was blending 2.5% ratios,at low and ethanol did not blending exceed ratios, and did not exceed 10% until theIMEP ethanol fraction was 35%. The study of Jamrozik [21] also ratios10% until, and the did ethanol not exceed fraction 10% was until 35%. the Theethanol study fraction of Jamrozik was 35%. [21 ]The also study showed of Jamrozik that the COV[21] IMEPalso showed that the COVIMEP increased with an increasing mixture of alcohol fuels, and that the COVIMEP showedincreased that with the an COV increasingIMEP increased mixture with of alcoholan increasing fuels, andmixture that of the alcohol COV fuels,increased and that by the more COV thanIMEP increased by more than 10% when the ethanol blend ratio was more thanIMEP 30%. increased10% when by the more ethanol than blend 10% when ratio was the moreethanol than blend 30%. ratio was more than 30%.

FigureFigure 7. 7. (a(a) )Distributions Distributions of of the the IMEP IMEP and and ( (bb)) the the COV COV of of the the IMEP IMEP.. Figure 7. (a) Distributions of the IMEP and (b) the COV of the IMEP. 3.2.3.2. Emissions Emissions Characteristics Characteristics 3.2. Emissions Characteristics 3.2.1.3.2.1. Gaseous Gaseous Emissions Emissions 3.2.1. Gaseous Emissions TheThe mixture mixture of of ethanol ethanol has has a a very very substantial substantial effect eﬀect on on combustion. combustion. The The properties properties of of ethanol, ethanol, The mixture of ethanol has a very substantial effect on combustion. The properties of ethanol, suchsuch as as its its low cetane number,number, lowerlower LHV, LHV, high high latent latent heat heat of of evaporation evaporation and and low low viscosity, viscosity aﬀ, ectaffect the such as its low cetane number, lower LHV, high latent heat of evaporation and low viscosity, affect thegeneration generation of exhaust of exhaust pollutants. pollutants. Figure Figure8 shows 8 theshow emissionss the emissions characteristics characteristics of exhaust of pollutants exhaust the generation of exhaust pollutants. Figure 8 shows the emissions characteristics of exhaust pollutantson the gaseous on the emissions gaseous emissions of the test of fuel the conditions. test fuel conditions. Figure8a showsFigureNOx 8a shows andPM, NOx (b) and shows PM, the(b) pollutants on the gaseous emissions of the test fuel conditions. Figure 8a shows NOx and PM, (b) showsemission the rates emission of NO rates2 and of NO NO22/ NOx,and NO and2/NOx (c) shows, and the(c) shows emission the characteristics emission characteristics of HC and of CO HC by and test shows the emission rates of NO2 and NO2/NOx, and (c) shows the emission characteristics of HC and COfuel by conditions. test fuel conditions. CO by test fuel conditions.

Figure 8. Emissions of (a) NOx and soot opacity, (b) NO and NO /NOx, and (c) HC and CO. Figure 8. Emissions of (a) NOx and soot opacity, (b) NO2 and NO22/NOx, and (c) HC and CO. Figure 8. Emissions of (a) NOx and soot opacity, (b) NO2 and NO2/NOx, and (c) HC and CO. Appl. Sci. 2020, 10, 4153 10 of 15

NOx was emitted at 872 ppm under the conditions of DE_0. With the ethanol-mixed fuel DE_3, NOx was reduced by 3.5% to 841 ppm; with DE_5, it decreased by 6.5% to 815 ppm; and with DE_10, NOx decreased 6.9% to 812 ppm. As the ethanol blend ratio increases, the emissions of NOx tend to decrease. It was judged that NOx decreases in this manner because the low LHV and high latent heat of evaporation of ethanol reduce the combustion temperature, even though the pre-mixed combustion is increased due to the longer ignition delay caused by the low cetane number of ethanol. Many studies [13,16,19,20] have also reported that the blend of ethanol or alcohol fuels reduces the production of NOx due to their lower LHV and high latent heat of evaporation, which makes the splitting of nitrogen molecules less likely. In the studies of He et al. [16], the generation of NOx was reduced by blending ethanol in fuels, due to the high latent heat of vaporization and low LHV, and the suppression of thermal NO generation by the trace amounts of H2O in ethanol-blended fuel. Also, Huang et al. [20] reported NOx reduction by ethanol blending for a low engine load condition. Huang explained that the reason for NOx reduction is that the evaporation and lower LHV of ethanol in the blends after injection reduced the gas temperature in the cylinder. Di et al. [19] showed that NOx decreased by 14.1% and 6.1% at low engine loads (0.08 MPa and 0.2 MPa) when using ethanol–diesel blend fuels. In the study of Li et al., using a single-cylinder direct-injection engine, NOx concentrations decreased by 2.2% and 4.2%, respectively, when using 10% and 15% ethanol–diesel fuels. Rakopoulos [13] reported that the emission of NOx was the same or very slightly lower, when an ethanol-blended fuel was used, than that of pure diesel fuel. The reasons were that the oxygen content and the longer ignition delay from the low cetane number of ethanol, and the low combustion temperature due to the lower caloric value of ethanol, counterbalanced each other for NOx generation. On the other hand, there are other studies [14,22] that show that NOx increases with the blend of ethanol. Sayin [14] showed that NOx emissions increased with the use of ethanol and methanol blends, and explained that the NOx increase was due to the fact that the low cetane number and oxygen content were more effective than the latent heat vaporization. Based on the studies above, several factors affect the generation of NOx. First, the high combustion temperature caused by the high oxygen content of the alcohol generates thermal NOx. Second, the reduced combustion temperature caused by the cooling effect of the high evaporation cooling of alcohol reduces NOx. More NOx is produced with the rapid increase of the combustion temperature caused by the increase of premixed combustion, which is related to the low cetane number of ethanol. Tutak et al. [22] reported that the generation of NOx increased with the increase in the ethanol blending ratio, and that it was discharged to a maximum of 5.5 g/kWh from 30% ethanol–diesel fuel. Another reason for the decrease in NOx is the hydrous ethanol that can occur during fuel production or management. Due to its molecular structure, ethanol can be dissolved in water [37]. Kun-Balog et al. [38] reported that aqueous ethanol could reduce NOx. In the study, the combustion of aqueous ethanol and natural gas was compared using a Swirl burner, and the NOx generated by the combustion of aqueous ethanol was found to be reduced by 25%. Tutak explained that this was because the low cetane number of the blended ethanol increased the ignition delay. It is noted that NOx includes both NO and NO2. NO2 reacts with volatile organic compounds (VOCs) in the atmosphere to generate optical smog, known as fine dust [39,40]. As shown in Figure8b, NOx decreases but NO 2 increases as the ethanol mix increases. This is due to the increase in NO2 generation due to the added oxygen from the ethanol in the mixture. The smoke opacity is reduced by ethanol blending, as shown in Figure8a. The levels of smoke opacity were reduced from 5.3% for DE_0 to 3% for DE_10 by ethanol blending, and slightly decreased as the ethanol blending ratio increased. This is because of the effect of oxygen from ethanol blending resulting in more complete combustion. Appl. Sci. 2020, 10, 4153 11 of 15

The HC emissions, addressed by Figure8c, initially decrease as 3% ethanol is added to pure diesel fuel, and then increase as the ethanol blending ratio increases. The HC at DE_0 was 34 ppm, and these levels were reduced by 50% to 17 ppm for DE_3, by 44% to 19 ppm for DE_5 and by 15% (vs. diesel) to 29 ppm for DE_10. By blending ethanol, the HC emissions are lower than the diesel fuel condition. Under idle conditions with a low rotational speed and low fuel injection pressure, slightly rich combustion conditions are created for the stability of combustion. Blending ethanol with diesel reduces HC emissions because of the reduction of partial fuel-rich areas due to the eﬀect of the oxygen and the improved atomization caused by the reduced viscosity of the injection fuel. Also, according to the report of Sayin [14], the addition of alcohol increases the laminar ﬂame speed, which reduces the temperature of the combustion but promotes the oxidation of the HC. However, in this study, the level of HC increases with the further increase in the ethanol blending ratio. This is believed to be caused by the partial over-lean area being increased by the blending of ethanol and the bulk quenching caused by the high latent heat of the evaporation of ethanol. Most studies [13,15–22] also report an increase in HC as the blending ratio of ethanol increases. CO emissions increased as the blending ratio of ethanol increased. The CO concentration at DE_0 was 252 ppm, that of DE_3 increased by 5%to 266 ppm, DE5 increased by 13.5% to 286 ppm, DE10 increased by 27.4% to 321 ppm. Figure8c shows that CO emissions for the blended fuels are higher than for unmixed diesel fuel. Several studies have reported that CO increases with the blending of ethanol. Lü et al. [18] explained that the increase in CO due to the ethanol mixture is caused by the incomplete combustion caused by the higher latent heat of the evaporation of ethanol. On the other hand, several other studies have reported that CO decreases with ethanol blending. Sayin [14] reported that the addition of alcohol adds oxygen to the combustion gas, resulting in a partially leaner fuel, which converts more CO to CO2 than under the diesel fuel conditions.

3.2.2. Soot Morphology The size and morphology of the soot particles are changed by the operating conditions of the engine, for example, the engine load, the speed of rotation, the injection timing and the effect of the fuel used. The sizes of the soot particles emitted under each fuel condition can be identified by TEM images, as shown in Figure9. As the blend ratio of ethanol increases, the size of the particles decreases. As shown in Figure 10, the mean size of the particles was 33.9 nm for DE_0, down 12.8% to 29.6 nm for DE_3, down 20.0% to 27.1 nm for DE_5, and down 26.7% to 24.9 nm for DE_10. The sizes of particles is between 10 nm and 60 nm, and the distribution of small particles increases with the increase in the ethanol blending ratio. This is believed to be due to the promotion of soot oxidation due to the introduction of more oxygen from the blending of ethanol. Lapuerta et al. [41] applied ethanol in the same way as this test, and reported that the oxidation of the soot nuclei produced during combustion decreases with the increase in the ethanol blending ratio. Likewise, Wei et al. [42] experimented by applying methanol 5%, 10% and 15% (by vol.%) to diesel fuel. The study reported that, due to the increase in the methanol blending ratio, the fractal dimension of soot particles decreased, because the methanol promoted branches of soot aggregates, and the soot activation occurred in the outer area, thus inhibiting soot maturity. Wei et al. [43] applied oxygenated fuel in a methanol blend, a dimethyl carbonate blend and a dimethyl methane blend to confirm the distribution of the primary particle diameter for each fuel condition. The distribution for diesel fuel was 9.5 nm to 50 nm. For the methanol blend, the size range was 7.5 nm to 42.5 nm; for the dimethyl carbonate blend, it was 12.5 nm to 43.5 nm; and for the dimethyl methane blend, the range was 6.5 nm to 41.0 nm. When using an oxygenated fuel, the soot has a smaller particle size distribution. Also, the mean primary particle meter of the methanol blend was the smallest, at 21.66 nm. Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 15

and the improved atomization caused by the reduced viscosity of the injection fuel. Also, according to the report of Sayin [14], the addition of alcohol increases the laminar flame speed, which reduces the temperature of the combustion but promotes the oxidation of the HC. However, in this study, the level of HC increases with the further increase in the ethanol blending ratio. This is believed to be caused by the partial over-lean area being increased by the blending of ethanol and the bulk quenching caused by the high latent heat of the evaporation of ethanol. Most studies [13,15–22] also report an increase in HC as the blending ratio of ethanol increases. CO emissions increased as the blending ratio of ethanol increased. The CO concentration at DE_0 was 252 ppm, that of DE_3 increased by 5%to 266 ppm, DE5 increased by 13.5% to 286 ppm, DE10 increased by 27.4% to 321 ppm. Figure 8c shows that CO emissions for the blended fuels are higher than for unmixed diesel fuel. Several studies have reported that CO increases with the blending of ethanol. Lü et al. [18] explained that the increase in CO due to the ethanol mixture is caused by the incomplete combustion caused by the higher latent heat of the evaporation of ethanol. On the other hand, several other studies have reported that CO decreases with ethanol blending. Sayin [14] reported that the addition of alcohol adds oxygen to the combustion gas, resulting in a partially leaner fuel, which converts more CO to CO2 than under the diesel fuel conditions.

3.2.2. Soot Morphology The size and morphology of the soot particles are changed by the operating conditions of the engine, for example, the engine load, the speed of rotation, the injection timing and the effect of the fuel used. The sizes of the soot particles emitted under each fuel condition can be identified by TEM images, as shown in Figure 9. As the blend ratio of ethanol increases, the size of the particles decreases. As shown in Figure 10, the mean size of the particles was 33.9 nm for DE_0, down 12.8% to 29.6 nm for DE_3, down 20.0% to 27.1 nm for DE_5, and down 26.7% to 24.9 nm for DE_10. The sizes of particles is between 10 nm and 60 nm, and the distribution of small particles increases with the increase in the ethanol blending ratio. This is believed to be due to the promotion of soot oxidation due to the introduction of more oxygen from the blending of ethanol. Lapuerta et al. [41] applied ethanol in the same way as this test, and reported that the oxidation of the soot nuclei produced during combustion decreases with the increase in the ethanol blending ratio. Likewise, Wei et al. [42] experimented by applying methanol 5%, 10% and 15% (by vol.%) to diesel fuel. The study reported that, due to the increase in the methanol blending ratio, the fractal dimension of soot particles decreased, because the methanol promoted branches of soot aggregates, and the soot activation occurred in the outer area, thus inhibiting soot maturity. Wei et al. [43] applied oxygenated fuel in a methanol blend, a dimethyl carbonate blend and a dimethyl methane blend to confirm the distribution of the primary particle diameter for each fuel condition. The distribution for diesel fuel was 9.5 nm to 50 nm. For the methanol blend, the size range was 7.5 nm to 42.5 nm; for the dimethyl carbonate blend, it was 12.5 nm to 43.5 nm; and for the dimethyl methane blend, the range was 6.5 Appl.nm to Sci. 41.02020 ,nm.10, 4153 When using an oxygenated fuel, the soot has a smaller particle size distribution.12 Also, of 15 the mean primary particle meter of the methanol blend was the smallest, at 21.66 nm.

Appl. Sci. 2020, 10, x FOR PEERFigure REVIEW 9. Images of TEM (a) DE_0, (b) DE_3, (c) DE_5, (d) DE_10. 12 of 15 Figure 9. Images of TEM (a) DE_0, (b) DE_3, (c) DE_5, (d) DE_10.

FigureFigure 10. 10. ((aa)) Mean Mean soot soot particle particle sizes, sizes, and and ( (bb)) distributions distributions of of particles particles.. 4. Conclusions 4. Conclusions Different ethanol–diesel fuel blends were applied to a diesel engine operating at a low idle speed Different ethanol–diesel fuel blends were applied to a diesel engine operating at a low idle speed to investigate their combustion characteristics, exhaust pollutants and soot morphology. Based on the to investigate their combustion characteristics, exhaust pollutants and soot morphology. Based on experimental results, it is possible to draw the following conclusions: the experimental results, it is possible to draw the following conclusions: The maximum combination pressure and maximum heat release rate of ethanol-blended fuels •• The maximum combination pressure and maximum heat release rate of ethanol-blended fuels were higher than those of pure diesel fuel. By increasing the ethanol blend ratio, the maximum were higher than those of pure diesel fuel. By increasing the ethanol blend ratio, the maximum combustion pressure decreased and the maximum heat release increased. combustion pressure decreased and the maximum heat release increased. The BSFC increased when ethanol was blended, and increased with the blend ratio; however, the •• The BSFC increased when ethanol was blended, and increased with the blend ratio; however, theBTEs BTEs of ethanol-blended of ethanol-blended fuels fuels were were lower lower than than when when pure pure diesel diesel fuel fuel was was used. used. As ethanol was blended and the blending ratio increased, the ignition delay increased, and the •• As ethanol was blended and the blending ratio increased, the ignition delay increased, and the combustioncombustion duration duration decreased. decreased. • The COVIMEPIMEP values of the ethanol-blended fuels were lower than those of the pure diesel fuel, • The COV values of the ethanol-blended fuels were lower than those of the pure diesel fuel, andand tended tended to to increase increase as the ethanol blending ratio increased above 3%. • WhenWhen ethanol ethanol was was blended blended and and the blendingthe blending ratio increased,ratio increased, the NOx the and NOx soot and opacity soot decreased, opacity • decreased,but CO emissions but CO increased. emissions The increased. emission The ratio emission of NO2 ratioin NOx of NO also2 increasedin NOx also with increased more ethanol. with moreThe levels ethanol. of HC The showed levels aof tendency HC showed to increase a tend asency the to ethanol increase blending as the ratio ethanol increased, blending although ratio increased,the HC emissions although of the ethanol–diesel HC emissions blended of ethanol fuels– arediesel lower blended than those fuels ofare pure lower diesel than fuel. those of pureAs the diesel ethanol fuel. blending ratio increased, the mean size of the soot particles decreased, and the • • Asdistribution the ethanol of smallblending particles ratio increased.increased, the mean size of the soot particles decreased, and the distribution of small particles increased. In low idling conditions with poor combustion conditions, the mixture of bioethanol, a high-oxidant fuel,In improves low idling combustion conditions and with improves poor combustion idle stability. conditions, Increasing the NOx mixture generation of bioethanol, due to aa longerhigh- oxidantignition fuel, delay improves due to the combustion low cetane and number improves of the idle bioethanol stability. was Increasing suppressed NOx by generation the cooling due effect to ofa longerthe high ignition latent delay heat of due evaporation, to the low andcetane as thenumber blend of ratio the ofbioethanol bioethanol was increased, suppressed NOx by was the reducedcooling effectstill further. of the high latent heat of evaporation, and as the blend ratio of bioethanol increased, NOx was reducedThis still experiment further. confirmed its effect by blending pure diesel with bioethanol at a low rate. The low viscosityThis ofexperiment bioethanol confirmed can adversely its effect affect by the blending lubrication pure of di dieselesel with engines. bioethanol In the future, at a low it israte. thought The lowthat viscosity we will need of bioethanol to study not can only adversely the limits affect of the the blending lubrication ratio of of diesel bioethanol, engines. but In also the thefuture, methods it is thoughtfor higher that blending we will ratiosneed to (e.g., study the not addition only the of biodiesel).limits of the blending ratio of bioethanol, but also the methods for higher blending ratios (e.g., the addition of biodiesel).

Acknowledgments: This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (No. 2019R1I1A1A01057727) and the government of Korea (MSIT) (No. 2019R1F1A1063154). The authors also thank the teachers in the Center for University-Wide Research Facilities (CURF) at Jeonbuk National University for their help in collecting some experimental data.

Conflicts of Interest: The authors declare no conflicts of interest. Appl. Sci. 2020, 10, 4153 13 of 15

Author Contributions: H.Y.K. suggested this research, performed the experiments, analyzed all experimental data, and wrote this paper. J.C.G. performed the experiments and contributed to combustion analysis. N.J.C. performed the data analysis and contributed to the discussion, and supervised the work and the manuscript. All authors participated in the evaluation of the data. All authors have read and agreed to the published version of the manuscript. Acknowledgments: This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (No. 2019R1I1A1A01057727) and the government of Korea (MSIT) (No. 2019R1F1A1063154). The authors also thank the teachers in the Center for University-Wide Research Facilities (CURF) at Jeonbuk National University for their help in collecting some experimental data. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

Abbreviations The following abbreviations are used in this manuscript.

◦CA Crank Angle ATDC After Top Dead Center BMEP Brake Mean Effective Pressure BSFC Brake Specific Fuel Consumption BTDC Before Top Dead Center BTE Brake Thermal Efficiency CA10 The crank angle of 10% Mass Fraction Burned CA50 The crank angle of 50% Mass Fraction Burned CA90 The crank angle of 90% Mass Fraction Burned CO Carbon Monoxide COV Coefficient of Variation DE_0 100% Diesel + 0% Ethanol, Pure petroleum diesel DE_3 97% Diesel + 3% Ethanol DE_5 95% Diesel + 5% Ethanol DE_10 90% Diesel + 10% Ethanol HC Hydrocarbon IMEP Indicated Mean Effective Pressure LHV Lower Heating Value MFB Mass Fraction Burned NOx Nitric Oxide TEM Transmission Electron Microscopy

References

1. Bajpai, D.; Tyagi, V.K. Biodiesel: Source, production, composition, properties and its beneﬁts. J. Oleo Sci. 2006, 55, 487–502. [CrossRef] 2. Mahmudul, H.; Hagos, F.; Mamat, R.; Adam, A.A.; Ishak, W.; Alenezi, R.J.R.; Reviews, S.E. Production, characterization and performance of biodiesel as an alternative fuel in diesel engines—A review. Renew. Sustain. Energy Rev. 2017, 72, 497–509. [CrossRef] 3. Lim, Y.-K.; Shin, S.-C.; Kim, J.-R.; Yim, E.-S.; Song, H.-O.; Kim, D. Characteristic analysis of GTL fuel as an automobile diesel. Appl. Chem. Eng. 2008, 19, 617–623. 4. Suh, H.K.; Lee, C.S. A review on atomization and exhaust emissions of a biodiesel-fueled compression ignition engine. Renew. Sustain. Energy Rev. 2016, 58, 1601–1620. [CrossRef] 5. Rajasekar, E.; Selvi, S. Review of combustion characteristics of CI engines fueled with biodiesel. Renew. Sustain. Energy Rev. 2014, 35, 390–399. [CrossRef] 6. Hoekman, S.K.; Broch, A.; Robbins, C.; Ceniceros, E.; Natarajan, M. Review of biodiesel composition, properties, and speciﬁcations. Renew. Sustain. Energy Rev. 2012, 16, 143–169. [CrossRef] 7. Alptekin, E.; Canakci, M. Determination of the density and the viscosities of biodiesel–diesel fuel blends. Renew. Energy 2008, 33, 2623–2630. [CrossRef] 8. Çelebi, Y.; Aydın, H.J.F. An overview on the light alcohol fuels in diesel engines. Fuel 2019, 236, 890–911. [CrossRef] Appl. Sci. 2020, 10, 4153 14 of 15

9. Gerdes, K.; Suppes, G.J. Miscibility of ethanol in diesel fuels. Ind. Eng. Chem. Res. 2001, 40, 949–956. [CrossRef] 10. Ge, J.C.; Yoon, S.K.; Kim, M.S.; Choi, N.J.J.A.S. Application of canola oil biodiesel/diesel blends in a common rail diesel engine. Appl. Sci. 2017, 7, 34. [CrossRef] 11. Kim, H.Y.; Ge, J.C.; Choi, N.J.J.A.S. Application of palm oil biodiesel blends under idle operating conditions in a common-rail direct-injection diesel engine. Appl. Sci. 2018, 8, 2665. [CrossRef] 12. Kim, H.Y.; Ge, J.C.; Choi, N.J.J.E. Effects of fuel injection pressure on combustion and emission characteristics under low speed conditions in a diesel engine fueled with palm oil biodiesel. Energies 2019, 12, 3264. [CrossRef] 13. Rakopoulos, D.; Rakopoulos, C.; Kakaras, E.; Giakoumis, E. Effects of ethanol–diesel fuel blends on the performance and exhaust emissions of heavy duty DI diesel engine. Energy Convers. Manag. 2008, 49, 3155–3162. [CrossRef] 14. Sayin, C.J.F. Engine performance and exhaust gas emissions of methanol and ethanol–diesel blends. Fuel 2010, 89, 3410–3415. [CrossRef] 15. Alptekin, E.J.F. Evaluation of ethanol and isopropanol as additives with diesel fuel in a CRDI diesel engine. Fuel 2017, 205, 161–172. [CrossRef] 16. He, B.-Q.; Shuai, S.-J.; Wang, J.-X.; He, H.J.A.E. The effect of ethanol blended diesel fuels on emissions from a diesel engine. Atmos. Environ. 2003, 37, 4965–4971. [CrossRef] 17. Li, D.-G.; Zhen, H.; Xingcai, L.; Wu-Gao, Z.; Jian-Guang, Y.J.R.E. Physico-chemical properties of ethanol–diesel blend fuel and its effect on performance and emissions of diesel engines. Renew. Energy 2005, 30, 967–976. [CrossRef] 18. Xing-Cai, L.; Jian-Guang, Y.; Wu-Gao, Z.; Zhen, H.J.F. Effect of cetane number improver on heat release rate and emissions of high speed diesel engine fueled with ethanol–diesel blend fuel. Fuel 2004, 83, 2013–2020. [CrossRef] 19. Di, Y.; Cheung, C.; Huang, Z.J.A.E. Comparison of the effect of biodiesel-diesel and ethanol-diesel on the gaseous emission of a direct-injection diesel engine. Atmos. Environ. 2009, 43, 2721–2730. [CrossRef] 20. Huang, J.; Wang, Y.; Li, S.; Roskilly, A.P.; Yu, H.; Li, H.J.A.T.E. Experimental investigation on the performance and emissions of a diesel engine fuelled with ethanol–diesel blends. Appl. Therm. Eng. 2009, 29, 2484–2490. [CrossRef] 21. Jamrozik, A.J.E.C. The effect of the alcohol content in the fuel mixture on the performance and emissions of a direct injection diesel engine fueled with diesel-methanol and diesel-ethanol blends. Energy Convers. Manag. 2017, 148, 461–476. [CrossRef] 22. Tutak, W.; Jamrozik, A.; Pyrc, M.; Sobiepański,M.J.A.T.E. A comparative study of co-combustion process of diesel-ethanol and biodiesel-ethanol blends in the direct injection diesel engine. Appl. Therm. Eng. 2017, 117, 155–163. [CrossRef] 23. Rahman, S.A.; Masjuki, H.; Kalam, M.; Abedin, M.; Sanjid, A.; Sajjad, H.J.E.C. Impact of idling on fuel consumption and exhaust emissions and available idle-reduction technologies for diesel vehicles—A review. Energy Convers. Manag. 2013, 74, 171–182. [CrossRef] 24. Brodrick, C.-J.; Dwyer, H.A.; Farshchi, M.; Harris, D.B.; King, F.G., Jr.; Association, W.M. Effects of engine speed and accessory load on idling emissions from heavy-duty diesel truck engines. J. Air Waste Manag. Assoc. 2002, 52, 1026–1031. [CrossRef][PubMed] 25. Khan, A.S.; Clark, N.N.; Thompson, G.J.; Wayne, W.S.; Gautam, M.; Lyon, D.W.; Hawelti, D. Idle emissions from heavy-duty diesel vehicles: Review and recent data. J. Air Waste Manag. Assoc. 2006, 56, 1404–1419. [CrossRef][PubMed] 26. McCormick, R.L.; Graboski, M.S.; Alleman, T.L.; Yanowitz, J. Idle emissions from heavy-duty diesel and natural gas vehicles at high altitude. J. Air Waste Manag. Assoc. 2000, 50, 1992–1998. [CrossRef] 27. Storey, J.M.; Thomas, J.F.; Lewis, S.A.; Dam, T.Q.; Edwards, K.D.; DeVault, G.L.; Retrossa, D.J. Particulate Matter and Aldehyde Emissions from Idling Heavy-Duty Diesel Trucks; SAE Technical Paper 0148-7191; SAE: Warrendale, PA, USA, 2003. 28. Gao, Z.; Lin, S.; Ji, J.; Li, M.J.E. An experimental study on combustion performance and flame spread characteristics over liquid diesel and ethanol-diesel blended fuel. Energy 2019, 170, 349–355. [CrossRef] 29. Hansen, A.C.; Zhang, Q.; Lyne, P.W. Ethanol–diesel fuel blends—A review. Bioresour. Technol. 2005, 96, 277–285. [CrossRef] Appl. Sci. 2020, 10, 4153 15 of 15

30. Lapuerta, M.; Armas, O.; Garcia-Contreras, R.J.F. Stability of diesel–bioethanol blends for use in diesel engines. Fuel 2007, 86, 1351–1357. [CrossRef] 31. Gnanamoorthi, V.; Devaradjane, G. Effect of compression ratio on the performance, combustion and emission of DI diesel engine fueled with ethanol–Diesel blend. J. Energy Inst. 2015, 88, 19–26. [CrossRef] 32. Garai, A.; Mondal, S.; Pal, S.; Chatterjee, S.; Sen, S.; Mukhopadhyay, A.J.E.T.; Science, F. Experimental investigation of spray formation in a hybrid atomizer using diesel, ethanol and ethanol blended diesel. Exp. Therm. Fluid Sci. 2019, 100, 158–170. [CrossRef] 33. Heywood, J. Internal Combustion Engine Fundamentals; McGraw-Hill: New York, NY, USA, 1988. 34. Cooney, C.P.; Worm, J.J.; Naber, J.D. Combustion characterization in an internal combustion engine with ethanol gasoline blended fuels varying compression ratios and ignition timing. Energy Fuels 2009, 23, − 2319–2324. [CrossRef] 35. Li, B.; Li, Y.; Liu, H.; Liu, F.; Wang, Z.; Wang, J.J.A.E. Combustion and emission characteristics of diesel engine fueled with biodiesel/PODE blends. Appl. Energy 2017, 206, 425–431. [CrossRef] 36. Caton, J.A. Combustion phasing for maximum efficiency for conventional and high efficiency engines. Energy Convers. Manag. 2014, 77, 564–576. [CrossRef] 37. Martins, F.P.; Lacava, P.T.; de Andrade, C.R.; Garzuzi, S. Alternative Fuels: A Review about Anhydrous and Hydrous Ethanol Properties; SAE Technical Paper 0148-7191; SAE: Warrendale, PA, USA, 2016. 38. Kun-Balog, A.; Sztankó, K.; Józsa, V. Pollutant emission of gaseous and liquid aqueous bioethanol combustion in swirl burners. Energy Convers. Manag. 2017, 149, 896–903. [CrossRef] 39. He, C.; Li, J.; Ma, Z.; Tan, J.; Zhao, L. High NO2/NOx emissions downstream of the catalytic diesel particulate filter: An influencing factor study. J. Environ. Sci. 2015, 35, 55–61. [CrossRef] 40. Ge, J.C.; Kim, H.Y.; Yoon, S.K.; Choi, N.J. Reducing volatile organic compound emissions from diesel engines using canola oil biodiesel fuel and blends. Fuel 2018, 218, 266–274. [CrossRef] 41. Lapuerta, M.; Armas, O.; Garcia-Contreras, R. Effect of ethanol on blending stability and diesel engine emissions. Energy Fuels 2009, 23, 4343–4354. [CrossRef] 42. Wei, J.; Fan, C.; Qiu, L.; Qian, Y.; Wang, C.; Teng, Q.; Pan, M. Impact of methanol alternative fuel on oxidation reactivity of soot emissions from a modern CI engine. Fuel 2020, 268, 117352. [CrossRef] 43. Wei, J.; Zeng, Y.; Pan, M.; Zhuang, Y.; Qiu, L.; Zhou, T.; Liu, Y. Morphology analysis of soot particles from a modern diesel engine fueled with different types of oxygenated fuels. Fuel 2020, 267, 117248. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).