energies
Article Experimental Comparative Study on Performance and Emissions of E85 Adopting Different Injection Approaches in a Turbocharged PFI SI Engine
Cinzia Tornatore *, Luca Marchitto , Maria Antonietta Costagliola and Gerardo Valentino Istituto Motori-CNR-via Marconi, 4-80125 Napoli, Italy; [email protected] (L.M.); [email protected] (M.A.C.); [email protected] (G.V.) * Correspondence: [email protected]
Received: 29 March 2019; Accepted: 17 April 2019; Published: 24 April 2019
Abstract: This study examines the effects of ethanol and gasoline injection mode on the combustion performance and exhaust emissions of a twin cylinder port fuel injection (PFI) spark ignition (SI) engine. Generally, when using gasoline–ethanol blends, alcohol and gasoline are externally mixed with a specified blending ratio. In this activity, ethanol and gasoline were supplied into the intake manifold into two different ways: through two separated low pressure fuel injection systems (Dual-Fuel, DF) and in a blend (mix). The ratio between ethanol and gasoline was fixed at 0.85 by volume (E85). The initial reference conditions were set running the engine with full gasoline at the knock limited spark advance boundary, according to the standard engine calibration. Then E85 was injected and a spark timing sweep was carried out at rich, stoichiometric, and lean conditions. Engine performance and gaseous and particle exhaust emissions were measured. Adding ethanol could remove over-fueling with an increase in thermal efficiency without engine load penalties. Both ethanol and charge leaning resulted in a lowering of CO, HC, and PN emissions. DF injection promoted a faster evaporation of gasoline than in blend, shortening the combustion duration with a slight increase in THC and PN emissions compared to the mix mode.
Keywords: E85; ethanol; port fuel injection; dual fuel; spark ignition engine; particles; particulate
1. Introduction Ethanol can be considered a proper alternative fuel for spark ignition (SI) engines because it shows several physical and combustion properties similar to gasoline and it can be produced from renewable energy sources. While most ethanol today is produced from grain starch (in US), sugar cane (in Brazil), and rapeseed oil (Europe) [1], industry is adopting new technologies to process different types of waste biomass materials, corn, and barley to produce new low-carbon bio fuels that can achieve renewable energy targets for the transport sector. In the European Union, the market petrol may contain up to 10% ethanol by volume (E10). So far, E10 petrol has only been introduced in Finland, France, Germany, and Belgium, whereas the other countries continue to use E5. One of the EU objectives is to have 10% of transport fuel provided from renewable sources by 2020. New generation biofuels are required to reduce the greenhouse gas intensity deriving from fuels; for this reason, the EU sets rigorous sustainability criteria in order to avoid the process of indirect land use change (Directive 2018/2001). Ethanol in SI engines increases the combustion performance, mainly due to higher flame speed and oxygen content. The latent heat of evaporation of ethanol is 2–3 times higher than that of gasoline; this cools the air entering into the engine and increases volumetric efficiency and power density. Moreover, the higher auto-ignition temperature, latent heat of vaporization, and research octane number of ethanol could reduce engine knock tendency [2].
Energies 2019, 12, 1555; doi:10.3390/en12081555 www.mdpi.com/journal/energies Energies 2019, 12, 1555 2 of 15
Despite these advantageous characteristics, neat ethanol is not easily used as transport fuel mainly because its low heating value and low volatility make cold starting very challenging, especially in cold climates. The most common way to overcome this problem is to blend ethanol with a much more volatile fuel such as gasoline. Ethanol-gasoline blends were extensively studied in engine research [3,4], the results showed that ethanol-gasoline blended fuels provide higher engine efficiency, compared to neat gasoline. However, low levels of ethanol can induce gasoline to evaporate more easily, thus low-level ethanol blends can increase evaporative emissions in vehicles. For this reason, car manufacturers have developed flexible fuel vehicles able to run with ethanol levels ranging from 0% to 85%. These vehicles are currently very popular in Brazil and in Sweden and their future popularity will mainly depend on the strategy adopted to promote the market share of biofuels. Greater ethanol percentages are commonly used in Brazil and the United States aiming to reduce the dependence on fossil fuels and improve the engine performance [5]. E85 (15% gasoline plus 85% ethanol by volume) is generally the mixture with the greatest amount of bioethanol that can be found in the United States with more than 3000 fueling stations [6]. E85 has an octane number of 106, quite lower than pure ethanol but still higher than gasoline (95). Such a high-level of gasoline–ethanol blend is less volatile than gasoline and low-level ethanol blends and results in lower evaporative emissions. On the other hand, in comparison to gasoline, ethanol requires a higher fuel/air ratio for stoichiometric combustion and it shows a lower LHV. This means that ethanol fuel needs higher fuel mass flow and thus a longer injection pulse width, leading to greater cylinder wall and piston wetting tendencies. Usually, gasoline and ethanol are externally mixed in a definite blending ratio before the use as fuel. However, a novel port dual-injection strategy can offer greater flexibility: different quantities of two different fuels can be separately and simultaneously injected into the intake manifold allowing for different mixing ratios. The blending proportion of alternative and fossil fuels can be instantly changed according to the engine request and in-vehicle fuel availability. Therefore, the port dual-injection strategy offers an alternative approach to meet stringent emissions targets and future biofuel legislation [7,8]. At the present time, there is no information about the impact that the mixture formation can have on performance, combustion, and emissions. For this reason, in this paper the pre-blended PFI strategy and the equivalent port dual-injection were compared for E85. Performance and emissions were studied in different SI engines fueled with E85 in terms of thermal efficiency, carbon monoxide (CO), oxides of nitrogen (NOx), carbon dioxide (CO2), and unburned hydrocarbons (HC) [9,10]. Many researchers reported that blended ethanol fuel in SI engine decreased HC and CO, remarkably. However, the changes in CO2 and NOx were very dependent on the kind of engine and on operating condition. It is generally accepted that ethanol blended fuel exhibits higher particulate mass (PM) and particles number (PN) emissions than that of gasoline in direct injection (DI) SI engines, mainly due to the lower evaporation characteristics of ethanol at ambient temperature below 300 K. Anyway, some researchers showed that PM emissions from DI engines can either increase or decrease with ethanol content depending on fuel injection timing [11]. This result indicates that engine-out particles emissions from ethanol-blended gasoline are still not nearly as well understood as those for diesel or gasoline [12]. It was consistently reported [13] that in SI-PFI vehicles the addition of ethanol caused a decrease in the number of particles and a significant reduction in PM mass emissions. Even if the particle formation could be influenced by mixture formation, at the present time, no studies have been carried out on the influence of the injection strategy (port dual fuel and single injection) on particle emission. The purpose of this investigation is to assess the effects of ethanol-gasoline injection mode on performance and exhaust emissions of a downsized PFI twin-cylinder turbocharged engine. Ethanol was supplied into the intake manifold through a fuel injection system separated from that for the gasoline (dual-fuel, DF) and in blend with gasoline (mixed). The ratio between ethanol and EnergiesEnergies 20192019,, 1212,, 1555x FOR PEER REVIEW 33 of 1515
mass was fixed at 0.85. The initial reference conditions were set running the engine, fueled with full gasoline mass was fixed at 0.85. The initial reference conditions were set running the engine, fueled gasoline at the knock limited spark advance (KLSA) boundary, a spark timing sweep was carried out with full gasoline at the knock limited spark advance (KLSA) boundary, a spark timing sweep was at lean, stoichiometric, and reach conditions. carried out at lean, stoichiometric, and reach conditions. 2. Experimental Setup 2. Experimental Setup The experiments were carried out on a PFI twin-cylinder SI engine whose main characteristics The experiments were carried out on a PFI twin-cylinder SI engine whose main characteristics are reported in Table 1. Further information can be found in [14]. The engine was equipped with an are reported in Table1. Further information can be found in [ 14]. The engine was equipped with an electro-hydraulic intake variable valve actuation system (VVA) that allows managing the electro-hydraulic intake variable valve actuation system (VVA) that allows managing the combustion combustion air with a limited throttling of the intake duct at part load. A waste-gated Mitsubishi air with a limited throttling of the intake duct at part load. A waste-gated Mitsubishi turbocharger, with turbocharger, with a dump-valve for surge control on the compressor, could attain boost pressures a dump-valve for surge control on the compressor, could attain boost pressures up to 2.4 bar. The air up to 2.4 bar. The air was constantly supplied to the engine at 293 ± 1 K by an air conditioning unit. was constantly supplied to the engine at 293 1 K by an air conditioning unit. Piezo-quartz pressure Piezo-quartz pressure transducers (accuracy± of ±0.1%) were installed within both cylinders to detect transducers (accuracy of 0.1%) were installed within both cylinders to detect instantaneous in-cylinder instantaneous in-cylinder± pressure signals that were acquired through AVL INDICOM. Afterwards, pressure signals that were acquired through AVL INDICOM. Afterwards, ensemble averages were ensemble averages were performed on 270 consecutive cycles. Furthermore, the boost pressure at performed on 270 consecutive cycles. Furthermore, the boost pressure at the compressor outlet and the compressor outlet and the turbine inlet pressure were acquired through piezo-resistive the turbine inlet pressure were acquired through piezo-resistive low-pressure indicating sensors. low-pressure indicating sensors. Thermocouples were installed to monitor intake and exhaust Thermocouples were installed to monitor intake and exhaust temperatures, primarily to check the temperatures, primarily to check the maximum gas temperature allowed by the turbine blades. maximum gas temperature allowed by the turbine blades.
Table 1. Engine specification Table 1. Engine specification. PFI Twin-Cylinder Engine PFI Twin-Cylinder Engine Displacement (cm3) 875.4 3 DisplacementLayout (cm ) 4 valve/cyl875.4 Layout 4 valve/cyl BoreBore (mm) (mm) 80.5 80.5 StrokeStroke (mm) (mm) 86 86 Connecting Rod Rod Length Length (mm) (mm) 136.85 136.85 CompressionCompression Ratio Ratio 10:1 10:1 Rated Power (kW) 63.7@5500 rpm Rated Power (kW) 63.7@5500 rpm Max Torque (Nm) 146.7@2000 rpm Max Torque (Nm) 146.7@2000 rpm Cooled water temperature (◦C) 85 Cooled water temperature (°C) 85
In this paper, we investigated two different fuel injection approaches: mixed fuel port injection In this paper, we investigated two different fuel injection approaches: mixed fuel port injection and dual-fuel injection. The schematic of experiment setup is shown in Figure1. and dual-fuel injection. The schematic of experiment setup is shown in Figure 1.
. Figure 1. Schematic of intake manifold and dual fuel injection system. Figure 1. Schematic of intake manifold and dual fuel injection system. In the first case (mix mode) ethanol and gasoline were externally mixed and then injected in the intakeIn manifold the first case through (mix themode) standard ethanol gasoline and gasoline injectors. were Compared externally to mixed the second and then case injected (dual mode), in the ethanolintake manifold was injected through by twothe standard solenoid gasoline injectors injectors. installed Compared in the runners, to the upstream second case of the(dual standard mode), gasolineethanol was ones. injected The ethanol by two injection solenoid system injectors included installed an externalin the runners, pump andupstream a rail toof accumulatethe standard a propergasoline amount ones. The of alcohol, ethanol the injection system wassystem controlled included by an an external external pump driver and that alsoa rail allowed to accumulate handling a theproper injection amount timing of andalcohol, interval. the Thesystem injection was pressurecontrolled was by kept an constantexternal atdriver 4 bar. that also allowed handling the injection timing and interval. The injection pressure was kept constant at 4 bar.
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For each injection mode, the injection interval was adjusted to keep the same energy content at λ value set by ECU standard map. Then, the fuel amount was modified in order to reach the chosen λ value taking into account the different ethanol properties compared to gasoline. Table2 shows a number of selected properties which are relevant to the analysis performed in this paper, they are given for 1.0 bar ambient pressure and are obtained from scientific literature [15,16].
Table 2. Test fuel properties.
Properties Gasoline (95 RON EN228) Ethanol E85
Chemical formula C5–C11 C2H5OH C2H5OH + gasoline 3 Density@15 ◦C (kg/m ) 735 790 780 Research Octane Number 95 108 106 Lower heating value (MJ/kg) 42.5 26.9 29.2 Carbon intensity (gCO2/MJ) 74.4 71 71.5 Latent Heat of Vaporization (kJ/kg) 370 840 770 Stoichiometric Air to Fuel Ratio 14.7 8.95 10 Boiling Point (◦C) 30–190 78 75–84.8
The engine was controlled by a prototype driver managed by a software tool developed within LabView environment able to switch from the commercial ECU to external control. In this way, the main engine parameters—such as spark advance (SA), fuel injection timing, and interval—of both ethanol and gasoline could be handled and monitored. The other engine parameters, based on manufacturer-developed logics, such as valve and turbocharger strategies were controlled by the standard ECU map. The in-cylinder pressure data were acquired selecting a high sampling resolution of 0.1 CAD within the angular window between 90 and 90 CAD ATDC, for combustion analysis − assuming a polytropic thermodynamic process. Outside of the former angular interval, the sampling resolution was set at 1 CAD. Exhaust gas emissions were sampled by a ultra-violet gas analyzer (UV Limas 11, ABB, Zürich, Switzerland) for NOx and cold extractive IR gas analyzer (URAS, ABB, Zürich, Switzerland) for CO, CO2 and O2 while a FID analyzer (Siemens, Munich, Germany) was used for THC. An electrical low-pressure impactor (–ELPI, Dekati Ltd., Kangasala, Finland) was used to measure total particle number (PN) and size distribution in the range 7 nm–10 µm. PN sampling was carried out upstream of the three-way catalyst by using a double stage diluter (FPS, Dekati Ltd., Kangasala, Finland) which controlled the dilution temperature at 150 ◦C and the dilution ratio at 30. PN size distribution acquired by ELPI was also used to estimate the particulate mass, assuming the soot density as 1 g/cm3. In order to minimize the artifacts associated with ELPI currents, only particles with aerodynamic diameters lower than 1 µm were considered for PM estimation (PM1) [17].
Engine Test Conditions Tests were carried out at 3000 rpm considering as reference 17.5 bar of net IMEP, corresponding to high load condition. The net IMEP was estimated by the in-cylinder pressure over the complete engine cycle, computed as the difference between the gross IMEP (in-cylinder pressure over compression and expansion strokes) and pumping mean effective pressure (PMEP), evaluated over intake and exhaust strokes. This condition was chosen to keep the maximum allowable turbine inlet temperature (TIT) below 950 C and the average maximum in-cylinder pressure below 85 bar ( 5 bar) to preserve the ◦ ± engine components from mechanical failure. To this aim, the maximum boost level was automatically controlled in the range 1.85–2.0 bar, acting on the waste-gate valve opening. Tests were carried out at ethanol to gasoline fraction of 0.85 and the relative air-fuel ratio (λ) was swept from rich (0.9) to stoichiometric (1) and lean condition (1.1) to this aim, the fuel injection amount was fine monitored to realize the correct air/fuel ratio, estimated by the exhaust lambda sensor meter, based on the changing oxygen mass, located at the exhaust upstream of the three-way catalyst. The initial spark timing was set according to the standard ECU map corresponding to the KLSA under full gasoline Energies 2019, 12, 1555 5 of 15 operation. Then, E85 was used and the spark timing was swept out up to the new KLSA. These conditions were repeated for both injection modes. The overall test matrix is illustrated in Table3. Energies 2019, 12, x FOR PEER REVIEW 5 of 15
the new KLSA. These conditions were Tablerepeated 3. Test for bo matrix.th injection modes. The overall test matrix is FUELillustrated Injection in Table Mode 3. SA (CAD ATDC) SOI = SOI (CAD ATDC) Intake P [bar] λ [ 0.01] G E ± Gasoline Single fuel 11 165 1.81 0.90 − Table 3. Test matrix− E85 Dual fuel 11 to 15 165 1.74 0.90 − − − E85FUEL Dual Injection fuel Mode SA11 (CAD to 17ATDC) SOIG = SOIE165 (CAD ATDC) Intake 1.75P [bar] λ [±0.01] 1.00 − − − E85 Dual fuel 11 to 17 165 1.77 1.10 Gasoline Single fuel − −−11 −−165 1.81 0.90 E85 Mix 11 to 15 230 1.72 0.90 E85 Dual fuel − −11 to− −15 −−165 1.74 0.90 E85 Mix 11 to 17 230 1.80 1.00 E85 Dual fuel − −11 to− −17 −−165 1.75 1.00 E85 Mix 11 to 19 230 1.83 1.10 E85 Dual fuel − −11 to− −17 −−165 1.77 1.10 E85 Mix −11 to −15 −230 1.72 0.90 3. ResultsE85 and Discussion Mix −11 to −17 −230 1.80 1.00 E85 Mix −11 to −19 −230 1.83 1.10 3.1. Effects of Ethanol–Gasoline Injection Mode on Engine Combustion and Performance 3. Results and Discussion As already mentioned in the previous section, the selected reference condition corresponded to full3.1. gasoline, Effects of richEthanol–Gasoline case (λ = 0.90, Injection SA =Mode11 on CAD Engine ATDC, Combus IMEPtion and= Performance17.5 bar),according to the ECU − standard engineAs already map, mentioned related to in the the most previous advanced section, SA the without selected knock. reference To condition illustrate corresponded the effect of ethanol to injectionfull modegasoline, on rich combustion case (λ = and0.90, performance,SA = −11 CAD the ATDC, main IMEP engine = 17.5 parameters bar), according were acquiredto the ECU at rich conditionsstandard as shownengine inmap, Table related4. to the most advanced SA without knock. To illustrate the effect of Withethanol E85, injection the IMEP mode slightly on combustion diminishes and performa if comparednce, the with main gasoline, engine parameters as well as were the turbineacquired inlet temperatureat rich conditions even if the as thermal shown in e ffiTableciency 4. remains almost unvaried (~0.31). Looking at the combustion parameter,With E85 E85, leads the to IMEP a faster slightly combustion diminishes than if compared gasoline with with gasoline, higher as and well earlier as the in-cylinder turbine inlet peak pressuretemperature closer to theeven TDC. if the These thermal results efficiency are well-illustrated remains almost in Figureunvaried2 which (~0.31). reports Looking the in-cylinderat the combustion parameter, E85 leads to a faster combustion than gasoline with higher and earlier pressure and rate of heat release profiles. in-cylinder peak pressure closer to the TDC. These results are well-illustrated in Figure 2 which reports the in-cylinder pressure and rate of heat release profiles. Table 4. Rich condition main engine parameters.
λ = 0.90Table 4. Rich condition IMEP main Pmax engine parameters MFB50 TIT 0.90 SA = 11 CADλ = ATDC (bar)IMEP (bar)Pmax (CADMFB50 ATDC) TIT (K) SA− = −11 CAD ATDC (bar) (bar) (CAD ATDC) (K) GasolineGasoline 17.517.5 75.475.4 17.66 17.66 833 833 E85 dualE85 dual fuel fuel 17.3 17.3 79.2 79.2 15.42 15.42 799 799 E85 mixE85 mix 17.2 17.2 77.2 77.2 15.27 15.27 813 813
100 250 3000 rpm Gasoline 90 SA =-11 E85 DF E85 mix λ=0.9 80 200
70
60 150
50
40 100
30 ROHR [J/CAD] In-cylinder Pressure [bar] Pressure In-cylinder 20 50
10
0 0 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Crank Angle [deg] .
FigureFigure 2. In-cylinder 2. In-cylinder pressure pressure and and rate rate of heatof heat release release in in Cyl. Cyl. #2 #2 for for di differentfferent injectioninjection conditions conditions at at SA = 11SA CAD = −11 ATDC CAD ATDC and λ =and0.9. λ = 0.9. −
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In the compression stroke, the pressure for E85 is lower than gasoline. This is due to E85 having a higherIn the latent compression heat of stroke,vaporiza thetion. pressure In addition, for E85 isthe lower slight than difference gasoline. between This is due the to two E85 injection having a highermodes latentcan be heat explained of vaporization. by fuel evaporation In addition, rate. the slightMore diinff erencedetails, between unblended the twogasoline injection evaporates modes canfaster be than explained the gasoline by fuel evaporationin the blend. rate. Thus, More when in details, dual unblendedfuel injection gasoline is used, evaporates less liquid faster fuel than is theintroduced gasoline in in the the combustion blend. Thus, chamber when dual if compared fuel injection to ismix used, mode less and liquid a lower fuel is amount introduced of heat in the is combustionsubtracted to chamber the air (temperature if compared an tod mix pressure mode are and higher a lower for amount E85 DF). of heat is subtracted to the air (temperatureE85 speeds and up pressure the rate are of higher heat forrelease E85 DF).triggering higher in-cylinder peak temperature and pressureE85 speedswith an up advanced the rate of combustion. heat release A triggering slight IMEP higher reduction in-cylinder for peakE85 can temperature be explained and pressurethrough withthe ROHR an advanced analysis. combustion.Even if gasoline A slightshows IMEPa lower reduction peak, a larger for E85 amount can be of explainedheat is released through during the ROHRthe expansion analysis. stroke. Even if gasoline shows a lower peak, a larger amount of heat is released during the expansionThe following stroke. sections illustrate results of engine performance and exhaust emissions obtained at rich,The stoichiometric, following sections and illustrate lean conditions results of for engine the two performance injection andmodes, exhaust as a emissions result of obtained the spark at rich,timing stoichiometric, sweep up to andthe new lean knock conditions limited for condition. the two injection The effect modes, of injection as a result mode of the on sparkIMEP timing at the sweepdifferent up spark to the timings new knock is displayed limited condition.in Figure 3. The The e plotffect reports of injection the reference mode on condition IMEP at the (black diff erentsolid sparkstar) and timings the isthree displayed relative in Figureair-to-fuel3. The ratios plot reports(λ = 0.9, the 1, reference and 1.1). condition The effect (black of charge solid star) cooling and thecombined three relative with the air-to-fuel inherent ratios chemical (λ = anti-knock0.9, 1, and 1.1).capability The eff ofect ethanol of charge allows cooling to set combined advanced with spark the inherenttimings compared chemical anti-knockto the full gasoline capability case of and ethanol even allows stoichiometric to set advanced and lean spark conditions. timings The compared IMEP is toreduced the full when gasoline switching case andfrom even rich stoichiometricto stoichiometric and and lean lean conditions. conditions The at a IMEP fixed SA. is reduced Anyway, when the switchingload can be from recovered rich to stoichiometricby advancing the and spark lean conditionstiming considering at a fixed that SA. E85 Anyway, allows the extending load can the be recoveredknock limit by up advancing to −19. On the the spark other timing side, considering the increase that in E85 the allows in-cylinder extending peak the pressure knock limitabove up the to threshold19. On the value other (~90 side, bar) the inhibits increase more in the advanced in-cylinder spark peak timings pressure to above protect the engine threshold components value (~90 from bar) − inhibitsmechanical more breakdown. advanced For spark all timingsthe conditions, to protect the engine standard components deviation fromof IMEP mechanical value is breakdown.lower than For3%. all the conditions, the standard deviation of IMEP value is lower than 3%.
20 Gasoline E85mix E85 DF λ = 0.90 19 λ = 1.00 λ = 1.10
18
17
IMEP [bar] 16
15
14 -19 -17 -15 -13 -11 Spark Advance [CAD ATDC] FigureFigure 3. IndicatedIndicated mean mean effective effective pressu pressurere against SA at different different λ..
Very lowlow IMEPIMEP levelslevels werewere measuredmeasured forfor λλ == 1.1. At fixed fixed spark advance, the IMEP was higher for the mixed blend compared with the dual injection except for the rich case where the two values were almost overlapped. overlapped. The The best best engine engine pe performancerformance was was provided provided by by the the mixed mixed blend blend @ λ @ = λ1,= SA1, SA 17 that gave the highest IMEP value. −17 −that gave the highest IMEP value. In-cylinder peak pressure profile profile (Figure 44a),a), clearlyclearly showsshows thatthat thethe E85E85 richrich casecase resultsresults inin significantsignificant higher peak values if compared toto stoichiometricstoichiometric andand leanlean casecase forfor allall SA,SA, asas expected.expected. Dual fuelfuel injection injection modes modes always always provide provide higher higher Pmax Pmax than mix than case, mix this case, behavior this isbehavior in agreement is in withagreement combustion with combustion phasing (MFB50) phasing trends, (MFB50) shown trends, in Figure shown4b. In in fact, Figure E85 4b. DF shortensIn fact, E85 the DF combustion shortens phasingthe combustion at each SAphasing and air at to each fuel SA ratio and except air to for fuel rich ratio conditions except wherefor rich the conditions two behaviors where are the almost two overlapped.behaviors are This almost makes overlapped. the pressure This peak makes in combustion the pressure closer peak to thein combustion TDC, resulting closer in higher to the values. TDC, resultingAs regards in higher the values. effect of air to fuel ratio, as expected a more advanced combustion phasing is given by richAs caseregards while the the effect lean of condition air to fuel reduces ratio, as the expected in-cylinder a more peak advanced pressure andcombustion produces phasing a slower is combustiongiven by rich with case higher while MFB50 the lean values. condition reduces the in-cylinder peak pressure and produces a slower combustion with higher MFB50 values.
Energies 2019, 12, x FOR PEER REVIEW 7 of 15 Energies 2019, 12, 1555 7 of 15 100 30 EnergiesEnergies 20192019,, 1212,, xx FORFOR PEERPEER REVIEWREVIEW Gasoline E85mix E85 DF 77 ofof 1515 λ = 0.90 27 90 λ = 1.00 ] 100100 3030 λ ar = 1.10 24 GasolineGasoline E85mixE85mix E85E85 DFDF [b λλ == 0.900.90 80 2727 9090 λλ == 1.001.00 ] ] 21 λλ ar ar == 1.101.10
ressure ressure 2424 [b [b P 70 18 k 8080 2121 ea P
ressure ressure ressure ressure 15 2 P P
60 7070 1818 k k er d ea ea
n 12 P P li
Gasoline E85mix E85 DF 1515 2 2 50 6060λ = 0.90 ATDC][CAD 2Cyl. MFB50 er er n-cy
I 9 d d λ = 1.00 n n 1212 li li λ = 1.10 GasolineGasoline E85mixE85mix E85 E85 DFDF MFB50 Cyl. 2 [CAD ATDC][CAD 2Cyl. MFB50 40 5050 λλ == 0.90 0.90 ATDC][CAD 2Cyl. MFB50 6 n-cy n-cy I I 99 λλ-19 == 1.00 1.00 -17 -15 -13 -11 -19 -17 -15 -13 -11 λλ == 1.10 1.10 Spark Advance [CAD ATDC] 4040 66 Spark Advance [CAD ATDC] -19-19 -17 -17 -15 -15 -13 -13 -11 -11 -19-19 -17 -17 -15 -15 -13 -13 -11 -11 SparkSpark Advance(Advancea) [CAD[CAD ATDC]ATDC] SparkSpark AdvanceAdvance(b) [CAD[CAD ATDC]ATDC]
Figure 4. In-cylinder peak(( aapressure)) profile (a) and combustion phasing-MFB50(b)(b) (b) profile against SA Figureat FiguredifferentFigure 4. 4.In-cylinder4. In-cylinderλIn-cylinder. peak peakpeak pressure pressurepressure profile profileprofile (a (()aa and)) andand combustion combustioncombustion phasing-MFB50 phasing-MFB50phasing-MFB50 ( (b(bb))) profile profileprofile againstagainstagainst SASA at diffataterent differentdifferentλ. λλ.. Figure 5 shows the in-cylinder pressure and rate of heat release profiles of E85 at λ = 1.0 and 1.1. FigureIn FigureFigurethe 5mixed shows 55 showsshows fuel the thethe injection in-cylinder in-cylinderin-cylinder condition, pressure pressurepressure for and andand every rate raterate ofλ ofof value, heat heatheat releaserelease the use profiles profilesprofiles of ethanol of of E85E85 E85 blend atat at λλ λ== gives 1.0=1.01.0 andand anda 1.1.lower1.1. 1.1. peakIn pressureIn theIn thethe mixed mixedmixed and fuel a fuelfuel longer injection injectioninjection combustion condition, condition,condition, interval. forforfor everyeveryevery In spite λλ value,value, of the thethe lower useuse ofof of pressure ethanolethanol ethanol blendblend peak, blend givesgives the gives increase aa lower alower lower in peakcombustionpeakpeak pressure pressurepressure duration and andand a longeraa results longerlonger combustion combustionincombustion a constantly interval. interval.interval. higher-pressure InIn spite spite of of thethe value lowerlower during pressurepressure pressure the peak,peak,expansion peak, thethe the increaseincrease increasestroke ininthat in combustioninducescombustioncombustion an increase duration durationduration in results thermal resultsresults in inin efficiency, a aa constantly constantlyconstantly as higher-pressure cohigher-pressurehigher-pressurenfirmed by the valuevalue results duringduring during reported thethe the expansionexpansion expansionin Figure stroke stroke6. stroke thatthat that inducesinducesinduces an anan increase increaseincrease in inin thermal thermalthermal e ffiefficiency,efficiency,ciency, as asas confirmed coconfirmednfirmed by byby the thethe resultsresults reported reported inin FigureFigure Figure 6.66.. 100 240 E85 DF λ=1.0 100100 3000rpm 240240 90 E85 mix λ=1.0 SA=-11 CAD ATDC E85 E85 DF DF λλ=1.0=1.0 3000rpm3000rpm E85 DF λ=1.1 9090 E85 E85 mix mix λλ=1.0=1.0 200 SA=-11SA=-11 CAD CAD ATDC ATDC λ 80 E85 mixλλ =1.1 E85 E85 DF DF =1.1=1.1 200200 8080 E85 E85 mix mix λλ=1.1=1.1 70 160 7070 60 160160 6060 50 120 5050 120120 40 4040 80
30 ROHR [J/CAD] 8080 In-cylinder Pressure[bar] ROHR [J/CAD] 3030 ROHR [J/CAD] In-cylinder Pressure[bar] In-cylinder Pressure20[bar] 2020 40 4040 10 1010 0 0 00 00 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 -30-30 -20 -20 -10 -10 0 0 10 10 20 20 30 30 40 40 50 50 60 60 70 70 80 80 90 90 Crank Angle [deg] CrankCrank AngleAngle [deg][deg] FigureFigureFigureFigure 5.5. In-cylinder5.5.In-cylinder In-cylinderIn-cylinder pressurepressure pressurepressure andand andand rate rate raterate of of ofof heat heheheatat release releaserelease ininin Cyl.Cyl. #2 #2#2 forfor for E85E85 E85 atat atat λλλ λ == =11 1 1andand andand 1.11.1 1.11.1 atatat at SASASA SA == =− −=1111 − 11 11 − CADCADCADCAD ATDC.ATDC. ATDC.ATDC.
TheTheTheThe indicatedindicated indicatedindicated thermal thermal thermalthermal effi efficiencies cienciesefficienciesefficiencies of E85, ofofof E85,E85, reported reporeported inrted Figure ininin FigureFigureFigure6 are always6 6 6 are areare alwaysalways higheralways thanhigherhigher higher the thanthan referencethan thethe the gasolinereferencereferencereference knock gasoline gasolinegasoline limited knock knockknock case limited limitedlimited except case casecase for except except E85-DFexcept forfor rich E85-DFE85-DF at SA richrich11. at atat TheSA SASA −− highest−11.11.11. TheThe The highesthighest e highestfficiencies efficienciesefficiencies efficiencies were reached werewere were − byreachedreachedreached E85-mix, by byby E85-mix, about E85-mix,E85-mix, 0.39 about aboutabout compared 0.39 0.390.39 compared comparedcompared to 0.31 reached tototo 0.310.31 reachedreached by the gasoline bybyby the thethe gasoline gasolinegasoline reference referencereference reference case. Thiscase.case. case. result ThisThis This resultresult can result be attributedcancancan be be beattributed attributedattributed to the advancedto toto the thethe advanced advancedadvanced spark timingsspark sparkspark timings timingstimings and leaner anandd leanerleaner mixtures mixtures mixturesmixtures allowed allowedallowed allowed by ethanol byby by ethanolethanol ethanol anti-knock anti-knockanti-knock anti-knock effect inducingeffecteffecteffect inducing inducinginducing better better combustion betterbetter combustion combustioncombustion phasing phasing phasingphasing and faster and and burningfasterfaster burningburning rate. rate. rate.rate.
0.450.45 0.45 Gasoline Gasoline E85mix E85mix E85 E85 DF DF 0.430.43 λ λ = = 0.90 0.90 Gasoline E85mix E85 DF 0.43 λ λλ = = 1.00 0.901.00 0.410.41 λ = 1.00 0.41 λλ = = 1.10 1.10 0.390.39 λ = 1.10 0.39 0.370.37 0.37 0.350.35 0.35 0.330.33 0.33 0.310.31 Thermal Efficiency Thermal Efficiency 0.31
Thermal Efficiency 0.290.29 0.29 0.270.27
0.270.250.25 -19-19 -17 -17 -15 -15 -13 -13 -11 -11 0.25 -19Spark -17Spark Advance Advance -15 [CAD [CAD ATDC] ATDC] -13 -11 Spark Advance [CAD ATDC] FigureFigure 6.6. IndicatedIndicated thermalthermal efficienciesefficiencies profilprofilee againstagainst sparkspark advanceadvance atat differentdifferent λλ.. Figure 6. Indicated thermal efficiencies efficiencies profileprofile against spark advanceadvance atat didifferentfferent λλ..
Energies 2019, 12, 1555 8 of 15 Energies 2019, 12, x FOR PEER REVIEW 8 of 15
3.2.3.2. EEffectsffects of Ethanol–Gasoline Injection Mode onon ExhaustExhaust EmissionsEmissions Figures7 –7–99 show show the the variation variation of specific of specific exhaust exhaust emissions emissions versus versus the spark the advance spark foradvance di fferent for air-to-fueldifferent air-to-fuel ratios and ratios injection and modes. injection Figure modes.7a,b Figure shows the7a,b e showsffect of the injection effect modeof injection and air-fuel mode ratio and onair-fuel CO and ratio CO on2. CO and CO2. Switching fromfrom gasolinegasoline toto E85,E85, a more completecomplete combustioncombustion duedue toto the ethanol oxygen content resultsresults inin aa reductionreduction inin CO.CO. For E85,E85, thethe richrich casecase showsshows highhigh COCO andand lowerlower COCO22 compared to the other lambda values, indicatingindicating aa lowlow combustioncombustion eefficiency.fficiency. At stoichiometricstoichiometric and leanlean conditions,conditions, thethe higherhigher oxygenoxygen contentcontent allowsallows aa veryvery strongstrong reductionreduction inin COCO emissionsemissions upup toto valuesvalues closeclose toto zero.zero. ForFor eacheach injectioninjection mode,mode, whenwhen switchingswitching fromfrom richrich toto stoichiometricstoichiometric andand leanlean condition,condition, anan increaseincrease inin COCO22 isis foundfound inin spitespite ofof thethe lowerlower fuelfuel amountamount injected.injected. This is due to a better combustion for stoichiometric andand leanlean mixture,mixture, confirmedconfirmed byby thethe simultaneoussimultaneous reductionreduction inin CO.CO. Negligible didifferencesfferences inin specificspecific COCO emissionsemissions versusversus injectioninjection modemode areare obtained.obtained. SpecificSpecific COCO22 isis higherhigher inin dualdual fuelfuel injectioninjection conditioncondition forfor leanlean andand stoichiometricstoichiometric mixture.mixture.
800 140 Gasoline E85mix E85 DF Gasoline E85mix E85 DF λ = 0.90 λ 120 = 0.90 λ = 1.00 λ = 1.00 λ = 1.10 100 λ = 1.10 700 80
60 [g/kWh] [g/kWh]
2 CO 20 CO 600
10
0 500 -21-19-17-15-13-11 -19-17-15-13-11 Spark Advance [CAD ATDC] Spark Advance [CAD ATDC] (a) (b)
Figure 7.FigureSpecific 7. COSpecific and CO2 andemissions CO2 emissions against theagainst spark the advance spark advance at different at differentλ.(a) CO; λ. ( b) CO2.
Figure8 8shows shows the the specific specific HC HC emissions emissions versus vers theus sparkthe spark advance advance for di forfferent different air-to-fuel air-to-fuel ratios andratios fuel and injection fuel injection modes. modes. Since theSince FID the used FID for us THCed for measurements THC measurements is a carbon is a ioncarbon counter ion counter and its responseand its response is directly is directly proportional proportional to the carbon to the countcarbon of count the analyzed of the analyzed gas, in the gas, case in the of HC case coming of HC fromcoming pure from ethanol pure theethanol sensitivity the sensitivity is reduced is ofreduced 0.46 [18 of]. 0.46 As a [18]. result, As the a result, following the HCfollowing emissions HC wereemissions adjusted, were taking adjusted, into taking account into the account ethanol massthe ethanol fraction, mass according fraction, to according the equation to the proposed equation by Karproposed et al. [ 19by]. Kar et al. [19]. The switchswitch from gasolinegasoline to E85 inin richrich conditionsconditions does not result inin lowerlower HC.HC. AA reductionreduction inin HCHC could be expected since ethanol allows a faster and more complete combustion alsoalso thanksthanks toto the oxygenoxygen inin thethe molecule. molecule. Anyway, Anyway, for for the the selected selected operating operating conditions, conditions, when when E85 isE85 injected, is injected, the total the masstotal mass of fuel of is fuel higher is higher andmore and more HC would HC would be emitted. be emitted. These These two factorstwo factors compensate compensate each each other other and HCand concentrationHC concentration remains remains unvaried. unvaried. A decrease A decrease in HC in is HC obtained is obtained by increasing by increasing the air-to-fuel the air-to-fuel ratio. Theratio. dual The fuel dual injection fuel injection mode producesmode produces higher higher HC levels HC thanlevels the than mix the case mix for case every for condition every condition except forexcept the richfor case,the rich where case, the worstwhere combustion the worst ecombfficiencyustion reduces efficiency the di ffreduceserence untilthe overlappingdifference until the trends.overlapping The di thefference trends. between The difference the two injectionbetween modesthe two can injection be explained modes with can thebe explained fact that mix with mode the inducesfact that an mix increase mode in induces combustion an increase duration in resulting combustion in a constantlyduration resulting higher pressure in a constantly value during higher the expansionpressure value stroke. during The higherthe expansion pressure stroke. (and temperature) The higher pressure of E85 mix (and in thetemperature) expansion of stroke E85 mix promotes in the theexpansion oxidation stroke of unburned promotes fuel the trapped oxidation in the of crevices unburned during fuel the trapped compression in the stroke. crevices during the compression stroke.
Energies 2019, 12, 1555 9 of 15 EnergiesEnergies 20192019,, 1212,, xx FORFOR PEERPEER REVIEWREVIEW 99 ofof 1515
1010
88
66
44 HC [g/kWh] HC HC [g/kWh] HC
GasolineGasoline E85mixE85mix E85E85 DFDF 22 λλ == 0.900.90 λλ == 1.001.00 λλ == 1.101.10 00 -19-19 -17 -17 -15 -15 -13 -13 -11 -11 SparkSpark AdvanceAdvance [CAD[CAD ATDC]ATDC] FigureFigure 8.8. HCHC emissions emissions against against thethe sparkspark advanceadvance atat didifferentdifferentfferent λ λ...
TheThe eeffecteffectffect of ofof injection injectioninjection mode modemode and andandλ λλon onon specific specificspecific NO NONO emissions emissionsemissions is shownisis shshownown in Figureinin FigureFigure9. As 9.9. suggestedAsAs suggestedsuggested by manybyby manymany researchers researchersresearchers [16], [16],[16], thermal thermalthermal NOx NOx formationNOx formatioformatio is thenn is dominantis thethe dominantdominant mechanism mechmech foranismanism NOx forfor emission NOxNOx emissionemission from SI enginesfromfrom SISI engines forengines gasoline forfor gasolinegasoline as well as asasethanol wellwell asas fuel.ethanolethanol According fuel.fuel. AccordingAccording to this mechanism, toto thisthis mechanism,mechanism, NO formation NONO formationformation is directly isis proportionaldirectlydirectly proportionalproportional to the temperature toto thethe temperaturetemperature reached in reache thereache combustiondd inin thethe combustion chambercombustion and chamberchamber to the oxygen andand toto concentration. thethe oxygenoxygen Inconcentration.concentration. the rich condition, InIn thethe switchingrichrich condition,condition, from gasolineswitchingswitching to fromfrom E85 there gasolinegasoline is a slighttoto E85E85 reduction therethere isis aa in slightslight NOx. reductionreduction This can beinin dueNOx.NOx. to ThisThis the lowercancan bebe adiabatic duedue toto thethe flame lowerlower temperature adiabaticadiabatic flfl ofameame ethanol temperaturetemperature which slows ofof ethanolethanol down whichwhich the procedure slowsslows downdown of NOx thethe formationprocedureprocedure for ofof NOx E85.NOx formationformation forfor E85.E85. LeaningLeaning the thethe mixture mixturemixture induces inducesinduces an increaseanan increaseincrease in NOx inin NOx emissionsNOx emissionsemissions probably probablyprobably due to moreduedue to oxygento moremore available oxygenoxygen foravailableavailable N2 oxidation. forfor N2N2 Atoxidation.oxidation. lean and AtAt stoichiometric leanlean andand stoichiometricstoichiometric conditions, the conditions,conditions, dual fuel injection thethe dualdual mode fuelfuel produces injectioninjection highermodemode NOproducesproduces levels higher thanhigher the NONO mix levelslevels case. thanthan This thethe is in mixmix agreement case.case. ThisThis with isis inin the agreementagreement higher peak withwith reached thethe higherhigher by the peakpeak pressure reachedreached when byby dualthethe pressurepressure fuel injection whenwhen is dualdual used. fuelfuel injectioninjection isis used.used.
4040 GasolineGasoline E85mixE85mix E85E85 DFDF λλ 3535 == 0.900.90 λλ == 1.001.00 λλ 3030 == 1.101.10
2525
2020
1515 NO [g/kWh] NO [g/kWh]
1010
55
00 -19-19 -17 -17 -15 -15 -13 -13 -11 -11 SparkSpark AdvanceAdvance [CAD[CAD ATDC]ATDC] FigureFigure 9.9. NONO emissions emissions against against the the sparkspark advanceadvance atat didifferentdifferentfferent λ λ...
AccordingAccording toto thethe equationequation byby SuSu etet al.al. [[20],[20],20], thethethe combustioncombustioncombustion eefficiencyefficiencyfficiency ((ηηc)c)c) cancancan bebebe estimatedestimatedestimated asas functionfunctionfunction ofofof the thethe measured measuredmeasured values valuesvalues of ofof emission emissionemission concentration concentrationconcentration (HC, (HC,(HC, CO, CO,CO, CO COCO2).22). The). TheThe results resultsresults are areare reported reportedreported in Figureinin FigureFigure 10 against1010 againstagainst the sparkthethe sparkspark advance advanceadvance for the forfor di thethefferent diffdiff testerenterent cases. testtest cases. Thecases. combustion TheThe combustioncombustion efficiency efficiencyefficiency is almost isis independentalmostalmost independentindependent on the spark onon thethe advance. sparkspark advance.advance. As a general AsAs trend,aa generalgeneral the maintrend,trend, parameter thethe mainmain influencingparameterparameter influencing theinfluencing combustion thethe ecombustioncombustionfficiency is λ efficiencyefficiency, with a significant isis λλ,, withwith increase aa significantsignificant for the increaseincrease lean case, forfor for thethe both leanlean injection case,case, forfor modes. bothboth injectioninjection The E85 modes. emodes.fficiency TheThe is alwaysE85E85 efficiencyefficiency higher thanisis alwaysalways gasoline higherhigher reference thanthan gasolinegasoline case for bothreferencereference injection casecase modes. forfor bothboth Dual injectioninjection fuel mode modes.modes. shows DualDual combustion fuelfuel modemode eshowsshowsfficiency combustioncombustion values slightly efficiencyefficiency higher valuesvalues than mix slightlyslightly mode inhigherhigher agreement thanthan withmixmix amodemode more in completein agreementagreement evaporation withwith aa ofmoremore the fuelcompletecomplete before evaporationevaporation entering in theofof combustionthethe fuelfuel beforebefore chamber. enteringentering Nevertheless, inin thethe combustioncombustion DF injection chamber.chamber. induces Nevertheless,Nevertheless, a worse phasing DFDF withinjectioninjection engine inducesinduces strokes aa worseworse reducing phasingphasing the thermal withwith engineengine efficiency. strokesstrokes reducingreducing thethe thermalthermal efficiency.efficiency.
Energies 2019, 12, 1555 10 of 15 Energies 2019, 12, x FOR PEER REVIEW 10 of 15 Energies 2019, 12, x FOR PEER REVIEW 10 of 15
100 100
95 95
] 90 %
] 90 [ %
C [
C η η
85 Gas oli ne E 85 mix E 85 DF 85 Gas oli ne E 85 mix E 85 DF = 0. 90 λ = 0. 90 λ = 1. 00 λ = 1. 00 λ = 1. 10 λ = 1. 10 80 λ 80 -19 -17 -15 -13 -11 -19 -17 -15 -13 -11 S park Advance [ CAD AT DC] S park Advance [ CAD AT DC]
Figure 10. Combustion efficiency against the spark advance at different λ. FigureFigure 10.10. CombustionCombustion eefficiencyfficiency againstagainst thethe sparkspark advanceadvance atat didifferentfferent λ λ. . 3.3. Effects of Ethanol–Gasoline Injection Mode on PN and PM1 Emissions 3.3.3.3. EffectsEffects ofof Ethanol–GasolineEthanol–Gasoline InjeInjectionction ModeMode onon PNPN andand PM1PM1 EmissionsEmissions This section illustrates the influence of fuel injection mode on engine-out PN emissions. ThisThis sectionsection illustratesillustrates thethe influenceinfluence ofof fuelfuel injeinjectionction modemode onon engine-outengine-out PNPN emissions.emissions. FigureFigure Figure 11 summarizes PN emissions (particles/kWh), measured for gasoline (reference λ = 0.9, 1111 summarizessummarizes PNPN emissionsemissions (particles/k(particles/kWh),Wh), measuredmeasured forfor gasolinegasoline (reference(reference λλ == 0.9,0.9, SASA == −−1111 SACAD= ATDC),11 CAD E85 ATDC), dual fuel E85 and dual mix fuel mode and injection mix mode for injectioneach tested for λ each and testedSA. Theλ boxand plot SA. indicates The box CAD −ATDC), E85 dual fuel and mix mode injection for each tested λ and SA. The box plot indicates plot indicates the data distribution of each condition (minimum, first quartile, median, third quartile, thethe datadata distributiondistribution ofof eacheach conditioncondition (minimum,(minimum, firstfirst quartile,quartile, median,median, thirdthird quartile,quartile, andand and maximum). The addition of ethanol in gasoline causes a strong PN reduction compared to the maximum).maximum). TheThe additionaddition ofof ethanoethanoll inin gasolinegasoline causescauses aa strongstrong PNPN reductionreduction comparedcompared toto thethe referencereference gasolinegasoline casecase thatthat exhibitsexhibits aa meanmean valuevalue ofof about about 8 8 ×10 101313 particlesparticles/kWh./kWh. ComparedCompared toto reference gasoline case that exhibits a mean value of about 8× × 1013 particles/kWh. Compared to gasoline, E85 DF and E85 mix achieve a PN average reduction of 87% and 94%, respectively, with a gasoline,gasoline, E85E85 DFDF andand E85E85 mixmix achieveachieve aa PNPN averageaverage reductionreduction ofof 87%87% andand 94%,94%, respectively,respectively, withwith aa statistically significant result (F test: 95% confidence interval). The main reason for PN reduction with statisticallystatistically significantsignificant resultresult (F(F test:test: 95%95% confidconfidenceence interval).interval). TheThe mainmain reasonreason forfor PNPN reductionreduction alcohol blend fuels is the increased fuel oxygen content within ethanol molecule, which leads to an withwith alcoholalcohol blendblend fuelsfuels isis thethe increasedincreased fuelfuel oxygenoxygen contentcontent withinwithin ethanolethanol molecule,molecule, whichwhich leadsleads toto increased yield in the combustion reaction [21,22]. anan increasedincreased yieldyield inin thethe combustioncombustion reactionreaction [21,22].[21,22]. The analysis of the ethanol/gasoline injection modes pointed out that, although most of the data TheThe analysisanalysis ofof thethe ethanol/gasoliethanol/gasolinene injectioninjection modesmodes pointedpointed outout that,that, althoughalthough mostmost ofof thethe datadata overlap in the two tested configurations, the average PN emissions are slightly lower for the mixed overlapoverlap inin thethe twotwo testedtested configurations,configurations, thethe averageaverage PNPN emissionsemissions areare slightlyslightly lowerlower forfor thethe mixedmixed fuel (E85 mix). fuelfuel (E85(E85 mix).mix).
16 1x10 16 1x10 Gasoline Gasoline E85 mix E85 mix 1x1015 E85 DF 1x1015 E85 DF
1x1014 1x1014
13 PN [1/kWh] 1x10 13 PN [1/kWh] 1x10
1x1012 1x1012
1x1011 1x1011 Gasoline E85 DF E85 mix Gasoline E85 DF E85 mix FigureFigure 11.11. PN emissions displayed by box and whiskers plot.plot. Figure 11. PN emissions displayed by box and whiskers plot. Figures 12 and 13 report PN emissions for E85 DF and E85 mix respectively, displayed in semi-log FiguresFigures 1212 andand 1313 reportreport PNPN emissionsemissions forfor E8E855 DFDF andand E85E85 mixmix respectively,respectively, displayeddisplayed inin graphs,semi-log as graphs, a function as a offunction the spark of the advance spark atadvance different at different air-to-fuel air-to-fuel ratios. Each ratios. plot Each also plot reports also semi-log graphs, as a function of the spark advance13 at different air-to-fuel ratios. Each plot also thereports gasoline the gasoline reference reference value (PN(E0) value (PN(E0)8 10 ≅ particles8 × 1013/ kWh)particles/kWh) which gives which the highestgives the measured highest reports the gasoline reference value (PN(E0)× ≅ 8 × 1013 particles/kWh) which gives the highest PN emission. measuredmeasured PNPN emission.emission. ForFor E85E85 DFDF (Figure(Figure 12),12), atat richrich condition,condition, PNPN emissionsemissions followfollow aa slightslight decreasingdecreasing trendtrend inin thethe range between −11 and −15 CAD, with a minimum of 1.6 × 1013 particles/kWh at the most advanced range between −11 and −15 CAD, with a minimum of 1.6 × 1013 particles/kWh at the most advanced
Energies 2019, 12, 1555 11 of 15
Energies 2019, 12, x FOR PEER REVIEW 11 of 15 Energies 2019, 12, x FOR PEER REVIEW 11 of 15 For E85 DF (Figure 12), at rich condition, PN emissions follow a slight decreasing trend in the spark timing. A different trend against the spark advance is found at the stoichiometric and the lean rangespark betweentiming. A different11 and 15trend CAD, against with the a minimum spark advance of 1.6 is10 found13 particles at the /stoichiometrickWh at the most and advanced the lean conditions. In this− case, −a minimum is evident at SA = −13× CAD. The lowest PN emission values are sparkconditions. timing. In Athis di ffcase,erent a trendminimum against is evident the spark at advanceSA = −13 isCAD. found The at thelowest stoichiometric PN emission and values the lean are attained for λ = 1.1: they range from 1 × 1012 to 3 × 1012 particles/kWh, for spark advance between −11 conditions.attained for Inλ = this 1.1: case, they a range minimum from is1 × evident 1012 to 3 at × SA 1012= particles/kWh,13 CAD. The for lowest spark PN advance emission between values − are11 and −17 CAD ATDC. − attainedand −17 CAD for λ =ATDC.1.1: they range from 1 1012 to 3 1012 particles/kWh, for spark advance between 11 These results confirm that E85 mix× injection× mode further reduces PN compared to E85 −DF, and These17 CAD results ATDC. confirm that E85 mix injection mode further reduces PN compared to E85 DF, with− a maximum reduction at advanced spark timing at both stoichiometric and lean conditions (i.e., with Thesea maximum results reduction confirm that at advanced E85 mix injection spark timing mode at further both stoichiometric reduces PN compared and lean to conditions E85 DF, with (i.e., a −15 and −17 CAD), where PN emissions, unlike E85 DF, are almost independent on the spark maximum−15 and −17 reduction CAD), atwhere advanced PN sparkemissions, timing unlike at both E85 stoichiometric DF, are almost and leanindependent conditions on (i.e., the 15spark and advance. − advance.17 CAD), where PN emissions, unlike E85 DF, are almost independent on the spark advance. − 1x1015 1x1015 Gasoline E85 DF Gasoline E85 DF λ = 0.90 λ = 0.90 λ = 1.00 λ 1x1014 = 1.00 14 λ = 1.10 1x10 λ = 1.10
1x1013 1x1013 PN [1/kWh] PN [1/kWh] 1x1012 1x1012
1x1011 11 1x10 -19 -17 -15 -13 -11 -19 -17 -15 -13 -11 Spark Advance [CAD ATDC] Spark Advance [CAD ATDC]
Figure 12. PN as a function of the spark advance for E85 DF. Figure 12. PN as a function of the spark advance for E85 DF.
1x1015 1x1015 Gasoline E85 mix Gasoline E85 mix λ = 0.90 λ = 0.90 λ = 1.00 λ 1x1014 = 1.00 14 λ = 1.10 1x10 λ = 1.10
1x1013 1x1013 PN [1/kWh] PN [1/kWh] 1x1012 1x1012
1x1011 11 1x10 -19 -17 -15 -13 -11 -19 -17 -15 -13 -11 Spark Advance [CAD ATDC] Spark Advance [CAD ATDC]
Figure 13. PN as a function of the spark advance for E85 mix. Figure 13. PN as a function of the spark advance for E85 mix. λ λ λ Figures 1414–16–16 show show the the particle particle size size distribution distribution at at= λ0.9, = 0.9,= λ1, = and1, and= λ1.1, = 1.1, respectively. respectively. Each Each log Figures 14–16 show the particle size distribution at λ = 0.9, λ = 1, and λ = 1.1, respectively. Eachλ diagramlog diagram summarizes summarizes particle particle size size distribution distribution of fuels of fuels at di atfferent different spark spark advances advances except except thecase the case at log diagram summarizes particle size distribution of fuels at different spark advances except the case =at 0.9λ =where 0.9 where only only the trendthe trend at SA at= SA11 = − CAD11 CAD ATDC ATDC is shown. is shown. at λ = 0.9 where only the trend at SA− = −11 CAD ATDC is shown. At richrich conditioncondition (Figure (Figure 14 14),), gasoline gasoline provides provides the the highest highest PN PN emissions, emissions, while while E85 E85 DF andDF and E85 At rich condition (Figure 14), gasoline provides the highest PN emissions, while E85 DF and mixE85 amix significantly a significantly lower lower value value over theover entire the entire dimensional dimensional range. range. The highest The highest accumulation accumulation peak E85 mix a significantly lower value over the entire dimensional range. The highest accumulation (particlespeak (particles with diameterwith diameter higher higher than 100than nm) 100 isnm) reached is reached by E85 by DF, E85 suggesting DF, suggesting the presence the presence of local of peak (particles with diameter higher than 100 nm) is reached by E85 DF, suggesting the presence of richlocal regions rich regions which which promote promote solid particle solid particle formation formation [23]. For [23]. all theFor fuels, all the more fuels, than more 95% than of particles 95% of local rich regions which promote solid particle formation [23]. For all the fuels, more than 95% of haveparticles the diameterhave the lowerdiameter than lower 70 nm, than and 70 more nm, thanand 75%more are than smaller 75% thanare smaller 40 nm. than The variation40 nm. The of particles have the diameter lower than 70 nm, and more than 75% are smaller than 40 nm. The variation of spark advance carried out with E85 DF and E85 mix does not introduce any variation in variation of spark advance carried out with E85 DF and E85 mix does not introduce any variation in particle size distribution compared with those reported in Figure 14. particle size distribution compared with those reported in Figure 14.
Energies 2019, 12, 1555 12 of 15 spark advance carried out with E85 DF and E85 mix does not introduce any variation in particle size Energies 2019, 12, x FOR PEER REVIEW 12 of 15 distributionEnergies 2019, 12 compared, x FOR PEER with REVIEW those reported in Figure 14. 12 of 15
15 1x10 15 1x10 λ Gasoline SA= -11 λ= 0.90 SA= -11 ATDC Gasoline SA= -11 E85E85 mixmix SA=SA= -11-11 1x1014 E85 DF SA= -11 1x1014 E85 DF SA= -11
1x1013 1x1013
1x1012 1x1012 dN/dlogDp [kWh] dN/dlogDp [kWh] 1x1011 1x1011
1x1010 1x1010 0,01 0,1 1 0,01 0,1 1 μ Aerodynamic diameter [μm]
FigureFigure 14.14. ParticleParticle sizesize distributiondistribution forfor λλ= = 0.90.9 andand SASA == −1111 CAD. CAD. Figure 14. Particle size distribution for λ = 0.9 and SA = −11 CAD. The higher PN emissions in the range between 7 nm up to 1 µm of E85 DF compared with E85 The higher PN emissions in the range between 7 nm up to 1 µm of E85 DF compared with E85 mix is confirmed at stoichiometric conditions (Figure 15). Although the two injection modes provide mix is confirmed at stoichiometric conditions (Figure 15). Although the two injection modes provide similar particle distributions, ultrafine particles (diameter lower than 35 nm) cover a greater percentage similar particle distributions, ultrafine particles (diameter lower than 35 nm) cover a greater with E85 mix than E85 DF (almost 55% vs. 45%) for all tested spark advances with the exception of SA percentage with E85 mix than E85 DF (almost 55% vs. 45%) for all tested spark advances with the = 17 CAD ATDC. In this case, contribution of ultrafine particles is close to that measured with E85 exception− of SA = −17 CAD ATDC. In this case, contribution of ultrafine particles is close to that DF (almost 45% of total PN). measured with E85 DF (almost 45% of total PN). DF mode particle size distribution is just slightly influenced by the spark timing. An increase in DF mode particle size distribution is just slightlyly influencedinfluenced byby thethe sparkspark timing.timing. AnAn increaseincrease inin particle emissions by increasing the spark advance is instead evident with E85 mix, especially in the particle emissions by increasing the spark advance is instead evident with E85 mix, especially in the diameter range between 28 and 150 nm. This behavior is strictly related to the THC increasing with diameter range between 28 and 150 nm. This behavior isis strictlystrictly relatedrelated toto thethe THCTHC increasingincreasing withwith SA, discussed above. Hydrocarbons contribute to particle emissions throughout nucleation process at SA, discussed above. Hydrocarbons contribute to particle emissions throughout nucleation process the engine out. at the engine out. The same influence of SA on particle size distribution is found in lean conditions (Figure 16). The same influence of SA on particle size distribution is found in lean conditions (Figure 16). The increase in fine particles by advancing the spark timing is evident for both fuel injection modes. The increase in fine particles by advancing the spark timing is evident for both fuel injection modes. A reduction in PN larger thanthan 200 nm is found comparedcompared toto stoichiometricstoichiometric conditions.conditions. Their value is A reduction in10 PN larger than 200 nm is found compared to stoichiometric conditions. Their value is lower than 1010 pt/kWh which is close to detection limit of measuring system. lowerlower thanthan 101010 pt/kWhpt/kWh whichwhich isis closeclose toto detectiondetection limitlimit ofof measuringmeasuring system.system. 15 1x10 15 1x10 E85 mix SA=-11 CAD ATDC E85 mix SA=-11 CAD ATDC λ E85 mix SA=-13 CAD ATDC λ= 1.00 E85 mix SA=-13 CAD ATDC = 1.00 E85 mix SA=-15 CAD ATDC E85 mix SA=-15 CAD ATDC 1x1014 E85 mix SA=-17 CAD ATDC 1x1014 E85 mix SA=-17 CAD ATDC E85 DF SA=-11 CAD ATDC E85 DF SA=-11 CAD ATDC E85 DF SA=-13 CAD ATDC E85 DF SA=-13 CAD ATDC E85 DF SA=-15 CAD ATDC 13 E85 DF SA=-15 CAD ATDC 1x10 13 E85 DF SA=-17 CAD ATDC 1x10 E85 DF SA=-17 CAD ATDC
1x1012 1x1012 dN/dlogDp [kWh] dN/dlogDp dN/dlogDp [kWh] dN/dlogDp 1x1011 1x1011
1x1010 1x1010 0,01 0,1 1 0,01 0,1 1 μ Aerodynamic diameter [μm]
Figure 15. Particle size distribution for λ = 1. Figure 15. Particle size distribution for λ == 1.1.
EnergiesEnergies 20192019,, 1212,, 1555x FOR PEER REVIEW 1313 of 1515 Energies 2019, 12, x FOR PEER REVIEW 13 of 15
1x1015 1x1015 E85 mix SA=-11 CAD ATDC λ= 1.10 E85E85 mixmix SA=-11SA=-13 CADCAD ATDCATDC λ= 1.10 E85E85 mixmix SA=-13SA=-15 CADCAD ATDCATDC 1x1014 E85E85 mixmix SA=-15SA=-17 CADCAD ATDCATDC 14 1x10 E85E85 mixmix SA=-17SA=-19 CADCAD ATDCATDC E85E85 mixDF SA=-19SA=-11 CAD ATDC E85 DF SA=-11 CAD ATDC 13 E85 DF SA=-13 CAD ATDC 1x10 E85E85 DFDF SA=-13SA=-15 CADCAD ATDCATDC 1x1013 E85E85 DFDF SA=-15SA=-17 CADCAD ATDCATDC E85 DF SA=-17 CAD ATDC 1x1012 1x1012 dN/dlogDp [kWh] dN/dlogDp [kWh] 1x1011 1x1011
1x1010 1x10100,01 0,1 1 0,01 0,1 1 Aerodynamic diameter [μm] μ Aerodynamic diameter [ m] FigureFigure 16.16. ParticleParticle sizesize distributiondistribution forfor λλ == 1.1. Figure 16. Particle size distribution for λ = 1.1.
FigureFigure 1717 reportsreports PMPM11 emissions (particles less than 1.0 µµmm in aerodynamic diameter) estimatedestimated Figure 17 reports PM1 emissions (particles less than 1.0 µm in aerodynamic diameter) estimated asas integralintegral of of PN PN size size distribution distribution for for gasoline, gasoline, E85 E85 DF, andDF, E85and mix E85 at mix the diatff theerent differentλ. Adding λ. Adding ethanol as integral of PN size distribution for gasoline, E85 DF, and E85 mix at the different λ. Adding stronglyethanol strongly lowers the lowers particulate the particulate mass. With mass. rich With conditions, rich conditions, PM1 goes PM1 from goes 18 mgfrom/kWh 18 mg/kWh with gasoline with ethanol strongly lowers the particulate mass. With rich conditions, PM1 goes from 18 mg/kWh with togasoline less than to less 4 mg than/kWh 4 mg/kWh with E85. with E85. gasoline to less than 4 mg/kWh with E85. SuchSuch asas forfor PN,PN, E85E85 DFDF providesprovides higherhigher PMPM emissionsemissions thanthan E85E85 mix,mix, with a didifferencefference ofof aboutabout Such as for PN, E85 DF provides higher PM emissions than E85 mix, with a difference of about 60%60% withwith richrich andand stoichiometricstoichiometric conditionsconditions andand 30%30% withwith leanlean conditions.conditions. AnAn evidentevident reductionreduction isis 60% with rich and stoichiometric conditions and 30% with lean conditions. An evident reduction is obtainedobtained whenwhen leaningleaning thethe mixturemixture forfor bothboth injectioninjection modes.modes. obtained when leaning the mixture for both injection modes. Generally,Generally, thethe E85E85 DFDF (15%v(15%v gasolinegasoline andand 85%v85%v ethanol)ethanol) resultsresults inin aa greatgreat reductionreduction ofof PNPN andand Generally, the E85 DF (15%v gasoline and 85%v ethanol) results in a great reduction of PN and PM1PM1 engine-outengine-out emissions emissions compared compared to to gasoline, gasoline, but bu furthert further reduction reduction is obtained is obtained with with the mixedthe mixed fuel PM1 engine-out emissions compared to gasoline, but further reduction is obtained with the mixed gasolinefuel gasoline/ethanol./ethanol. fuel gasoline/ethanol. 20 20 Gas oli ne GEa8s5o lDi nFe E 85 DF 16 E 85 mix 16 E 85 mix
] 12 h
] 12 W h k W / g k / m g [
m 1 [
M 1 P M P
048 0480. 9 1. 0 1. 1 0. 9 1. [0-] 1. 1 λ [-] λ FigureFigure 17.17. PMPM11 estimated by PN size distribution. Figure 17. PM1 estimated by PN size distribution. 4.4. Conclusions 4. Conclusions InIn thisthis study, study, the the eff ecteffect of E85 of injectionE85 injection mode mode on performance on performance and gaseous and andgaseous particle and emissions particle In this study, the effect of E85 injection mode on performance and gaseous and particle wasemissions investigated was investigated on a turbocharged on a turbocharged port fuel SI port engine fuel atSI 3000engine rpm, at 3000 high rpm, engine high load, engine and load, different and emissions was investigated on a turbocharged port fuel SI engine at 3000 rpm, high engine load, and air-fueldifferent ratios. air-fuel ratios. different air-fuel ratios. TwoTwo injectioninjection modes modes of E85of E85 were were compared: compared: dual fuel—ethanoldual fuel—ethanol and gasoline and gasoline separately separately injected Two injection modes of E85 were compared: dual fuel—ethanol and gasoline separately withininjected the within intake the manifold; intake manifold; and mixed—ethanol and mixed—ethanol and gasoline and mixed gasoline to formmixed E85 to andform then E85 injected. and then injected within the intake manifold; and mixed—ethanol and gasoline mixed to form E85 and then injected.The main advantage in using dual fuel injection is the flexibility of the system that can feed the injected. engineThe with main diff advantageerent percentages in using of dual ethanol fuel ininjection an easy is way. the flexibility Conversely, of the injectionsystem that system can feed is more the The main advantage in using dual fuel injection is the flexibility of the system that can feed the complexengine with because different it requires percentages two separate of ethanol circuits in andan easy tanks. way. Conversely, the injection system is engine with different percentages of ethanol in an easy way. Conversely, the injection system is more complex because it requires two separate circuits and tanks. more complex because it requires two separate circuits and tanks.
Energies 2019, 12, 1555 14 of 15
In the experiments, the relative air–fuel ratio (λ) was swept from rich (0.9) to stoichiometric (1) and lean condition (1.1). The initial spark timing was set according to the standard ECU map corresponding to the KLSA under full gasoline operation. Then, E85 was used and the spark timing was swept out up to the new knock limit. E85 allowed advancing the spark timing and removing charge over-fueling: the increase in spark advance resulted in a slight increase of thermal efficiency without engine load penalties; leaning mixture provided a further increase in thermal efficiency with a maximum for E85-mix of about 0.39 compared to 0.31 reached by the gasoline reference case. The combined effect of ethanol oxygen content and charge leaning resulted in a more complete oxidation process of burned gases with a lowering of CO and HC emissions. On the contrary, NO emissions increased due to the higher air-to-fuel ratio and the raise in the peak pressure and temperature, due to advanced spark timings. The injection mode had significant effects on the combustion process: dual fuel injection promoted a faster evaporation of the gasoline than mix mode, shortening the combustion duration and reducing the thermal efficiency. The higher in-cylinder pressure and temperature peak provided a slight increase in NO and THC, compared to mix mode. Regarding particulate emissions, the oxygen content of ethanol promoted the particle matter oxidation as demonstrated by the strong reduction in both particle size distribution number and particulate mass (PM1), obtained by E85 in the rich case. Furthermore, the E85 showed a reduced tendency to larger particle agglomeration at each air–fuel ratio, because the oxygen helps the post oxidation processes. Dual fuel injection mode produced a larger amount of particles at each investigated condition compared to E85 mix. This behavior was due to the increase in gasoline evaporation with a corresponding increase of the maximum pressure and combustion velocity, which partially reduces the soot post-oxidation during the expansion stroke. This paper gives insights on the phenomena that take place in the combustion process when fuel is injected in DF mode. The analysis reported can be useful in optimizing this injection mode allowing to take advantage of its high flexibility.
Author Contributions: Conceptualization, C.T.; Data curation, C.T., L.M., and M.A.C.; Formal analysis, C.T., L.M., and M.A.C.; Funding acquisition, G.V.; Investigation, C.T., L.M., and M.A.C.; Project administration, G.V.; Supervision, G.V.; Writing—original draft, C.T., L.M., M.A.C., and G.V.; Writing—review & editing, C.T. Funding: This research received no external funding. Acknowledgments: The authors thank Alfredo Mazzei for his precious technical support. Conflicts of Interest: The authors declare no conflict of interest.
References
1. Solomon, B.D. Biofuels and sustainability. Ann. N. Y. Acad. Sci. 2010, 1185, 119–134. [CrossRef][PubMed] 2. Al-Muhsen, N.F.O.; Huang, Y.; Hong, G. Effects of direct injection timing associated with spark timing on a small spark ignition engine equipped with ethanol dual-injection. Fuel 2019, 239, 852–861. [CrossRef] 3. Turner, D.; Xu, H.; Cracknell, R.F.; Natarajan, V.; Chen, X. Combustion performance of bio-ethanol at various blend ratios in a gasoline direct injection engine. Fuel 2011, 90, 1999–2006. [CrossRef] 4. Chen, K.H.; Chao, Y.C. Characterization of Performance of Short Stroke Engines with Valve Timing for Blended Bioethanol Internal Combustion. Energies 2019, 12, 759. [CrossRef] 5. Costa, R.C.; Sodré, J.R. Hydrous ethanol vs. gasoline-ethanol blend: Engine performance and emissions. Fuel 2010, 89, 287–293. [CrossRef] 6. Alternative Fuels Data Center. Available online: https://afdc.energy.gov/fuels/ethanol_locations.html# /analyze?country=US&fuel=E85 (accessed on 20 March 2019). 7. Wu, X.; Daniel, R.; Tian, G.; Xu, H.; Huang, Z.; Richardson, D. Dual-injection: The flexible, bi-fuel concept for spark-ignition engines fuelled with various gasoline and biofuel blends. Appl. Energy 2011, 88, 2305–2314. [CrossRef] Energies 2019, 12, 1555 15 of 15
8. Trimbake, S.; Malkhede, D. Investigations of Port Dual Injection (PDI) Strategies in Single Cylinder SI Engine Fueled with Ethanol/Gasoline Blends; SAE Technical Paper 2016-01-0573; SAE International: New York, NY, USA, 2016. [CrossRef] 9. Türköz, N.; Erku¸s,B.; Karamangil, M.I.;˙ Sürmen, A.; Arslano˘glu,N. Experimental investigation of the effect of E85 on engine performance and emissions under various ignition timings. Fuel 2014, 115, 826–832. [CrossRef] 10. Qian, Y.; Liu, G.; Guo, J.; Zhang, Y.; Zhu, L.; Lu, X. Engine performance and octane on demand studies of a dual fuel spark ignition engine with ethanol/gasoline surrogates as fuel. Energy Convers. Manag. 2019, 183, 296–306. [CrossRef] 11. Raza, M.; Chen, L.; Leach, F.; Ding, S. A Review of Particulate Number (PN) Emissions from Gasoline Direct Injection (GDI) Engines and Their Control Techniques. Energies 2018, 11, 1417. [CrossRef] 12. Sakai, S.; Rothamer, D. Impact of ethanol blending on particulate emissions from a spark-ignition direct-injection engine. Fuel 2019, 236, 1548–1558. [CrossRef] 13. Karavalakis, G.; Short, D.; Vu, D.; Villela, M.; Asa-Awuku, A.; Durbin, T.D. Evaluating the regulated emissions, air toxics, ultrafine particles, and black carbon from SI-PFI and SI-DI vehicles operating on different ethanol and iso-butanol blends. Fuel 2014, 128, 410–421. [CrossRef] 14. Tornatore, C.; Siano, D.; Marchitto, L.; Iacobacci, A.; Valentino, G. Water Injection: A Technology to Improve Performance and Emissions of Downsized Turbocharged Spark Ignited Engines. SAE Int. J. Engines 2017, 10, 2319–2329. [CrossRef] 15. Aleiferis, P.G.; Serras-Pereira, J.; van Romunde, Z.; Caine, J.; Wirth, M. Mechanisms of spray formation and combustion from a multi-hole injector with E85 and gasoline. Combust. Flame 2010, 157, 735–756. [CrossRef] 16. Masum, M.; Masjuki, H.H.; Kalam, M.A.; Rizwanul Fattah, I.M.; Palash, S.M.; Abedin, M.J. Effect of ethanol–gasoline blend on NOx emission in SI engine. Renew. Sustain. Energy Rev. 2013, 24, 209–222. [CrossRef] 17. Maricq, M.M.; Xu, N.; Chase, R.E. Measuring Particulate Mass Emissions with the Electrical Low Pressure Impactor. Aerosol Sci. Technol. 2006, 40, 68–79. [CrossRef] 18. Scanlon, J.T.; Willis, E.D. Calculation of Flame Ionization Detector Relative Response Factors Using the Effective Carbon Number Concept. J. Chromatogr. Sci. 1985, 23, 333–340. [CrossRef] 19. Kar, K.; Cheng, W.K. Speciated engine-out organic gas emissions from a PFI-SI engine operating on ethanol/gasoline mixtures. SAE Int. J. Fuels Lubr. 2010, 2, 91–101. [CrossRef] 20. Su, J.; Xu, M.; Li, T.; Gao, Y.; Wang, J. Combined effects of cooled EGR and a higher geometric compression ratio on thermal efficiency improvement of a downsized boosted spark-ignition direct-injection engine. Energy Convers. Manag. 2014, 78, 65–73. [CrossRef] 21. Costagliola, M.A.; De Simio, L.; Iannaccone, S.; Prati, M.V. Combustion efficiency and engine out emissions of a S.I. engine fueled with alcohol/gasoline blends. Appl. Energy 2013, 111, 1162–1171. [CrossRef] 22. Qian, Y.; Li, Z.; Yu, L.; Wang, X.; Lu, X. Review of the state-of-the-art of particulate matter emissions from modern gasoline fueled engines. Appl. Energy 2019, 238, 1269–1298. [CrossRef] 23. Liu, H.; Wang, Z.; Long, Y.; Xiang, S.; Wang, J.; Fatouraie, M. Comparative study on alcohol–gasoline and gasoline–alcohol Dual-Fuel Spark Ignition (DFSI) combustion for engine particle number (PN) reduction. Fuel 2015, 159, 250–258. [CrossRef]
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