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Article Eﬀects of the Bore to Stroke Ratio on Combustion, Gaseous and Particulate Emissions in a Small Port Fuel Injection Engine Fueled with Ethanol Blended Gasoline

Jihwan Jang 1, Jonghui Choi 2, Hoseung Yi 2 and Sungwook Park 1,*

1 School of Mechanical Engineering, Hanyang University, Seoul 133-791, Korea; [email protected] 2 Department of Mechanical Convergence Engineering, Graduate School of Hanyang University, Seoul 04763, Korea; [email protected] (J.C.); [email protected] (H.Y.) * Correspondence: [email protected]; Tel.: +82-2-2220-0430; Fax: +82-2-2220-4588

Received: 5 December 2019; Accepted: 8 January 2020; Published: 9 January 2020

Abstract: The purpose of this study is to analyze the combustion characteristics of the port fuel injection (PFI) engine considering the fuel mixing ratio, bore to stroke (B/S) ratio and gaseous and particle emissions. Experiments were conducted in a small single-cylinder PFI engine with a displacement of 125 cc. The fuel used in the experiment was a mixture of pure gasoline and ethanol. The engine was operated at 5000 rpm at full load and wide-open throttle. In addition, combustion and exhaust characteristics of the engines with a B/S ratio of 0.88 and 1.15 were analyzed. The combustion pressure inside the combustion chamber was measured to analyze the indicated mean effective pressure (IMEP) and the heat release rate, and the combustion rate was calculated. In the results of combustion characteristics by the difference of B/S ratio, the influence of flame propagation velocity and turbulence intensity is the largest. The 0.88 B/S ratio engine, which has a small bore, has a faster combustion rate than the 1.15 B/S ratio engine due to its larger flame surface area and larger turbulence intensity. This represents a higher efficiency combustion result. Finally, the high oxygen content of ethanol has the characteristic of decreasing soot formation and increasing particle oxidation.

Keywords: bore/stroke ratio; particle number; IMEP; heat release rate

1. Introduction As the depletion of petroleum resources continues and environmental pollution becomes more serious, fuel economy and exhaust regulations of internal combustion engines are rapidly strengthening around the world. Various techniques are being developed to increase the efficiency of gasoline engines to satisfy the emission regulations, such as fuel and knock characteristics [1], engine turbulence [2], exhaust gas recirculation strategies [3] and valve timing strategies [4]. The bore to stroke (B/S) ratio is a factor that should be considered early in the design stage of the engine. As the bore/stroke ratio decreases under the same displacement conditions, the cylinder surface/volume ratio at top dead center (TDC) also decreases, which increases engine efficiency due to the cooling reduction effect [5]. Like this, the B/S ratio is an important factor that directly affects engine efficiency. For this reason, various studies based on the B/S ratio have been previously conducted [6,7]. Most of the studies have been interpreted to compare the turbulence intensity, flame surface and thermal efficiency in the cylinder according to the B/S ratio. Experimental studies based on B/S ratios were often conducted. [8,9]. However, in the case of engine research based on most B/S ratios, the torque and engine power comparison is a study. On the other hand, in this study, the combustion pressure was measured to calculate the combustion rate in the combustion chamber and the engine

Energies 2020, 13, 321; doi:10.3390/en13020321 www.mdpi.com/journal/energies Energies 2020, 13, 321 2 of 15 loss compared to the abnormal cycle. In addition, gas and particle emission were also analyzed. In addition, gasoline engines utilizing increased compression ratio and output along with increasing the fuel octane number to overcome the disadvantages of fuel economy have been applied. In this respect, many studies are being conducted using ethanol, an oxygen fuel, which is harmless to the human body and is less burdened by environmental pollution. By lowering the fuel temperature, the volumetric efficiency can be improved, and as result, a mixture of gasoline and ethanol improves the engine performance [10–12]. Recently, gasoline engines have been studied while focusing on the development of a gasoline direct injection (GDI) engine in which fuel is injected directly into a cylinder. The combustion chamber cooling effect upon using a high compression ratio and latent heat of evaporation of the fuel is shown. Due to this cooling effect, knocking inhibition and volumetric efficiency are increased, thereby improving engine efficiency [13–15]. However, due to the lack of a homogenous air and fuel mixture, locally rich combustion occurs, and particle emission due to fuel wetting of the cylinder wall and the piston becomes a problem. As a result, the regulation of particle emission in gasoline vehicles has gradually strengthened to the same level as that of diesel vehicles [16–19]. Recently, due to the difficulty of satisfying stringent particle emission regulations, interest in port fuel injection (PFI) engines has been increasing since fuel is injected into existing ports to form relatively homogeneous mixtures. Catapano et al. [20] compared the combustion process and emissions from ethanol mixing in GDI and PFI engines. Particular emissions, in particular, also showed dramatic emission reductions from the PFI engine. B. Wahono et al. [21] showed the results of swirl and tumble ratio in the cylinder of PFI engine and studied the internal flow with increasing RPM. As such, research on the PFI engine is steadily progressing. In addition to restrictions on light duty vehicles, enhanced restrictions have recently been levied on motorcycles. In particular, low-emission motorcycles, such as scooters, are used in a high proportion in urban areas, so the emission restrictions have been tightened from EURO 4, which is a European motorcycle emission regulation, to EURO 5. Therefore, it is necessary to study engine efficiency and exhaust gas reduction methods for motorcycles, including particle emission under high rpm and wide open throttle (WOT) conditions. In this study, combustion and exhaust characteristics were analyzed for the bore/stroke ratio in a small single-cylinder PFI engine with 125 cc displacement. Furthermore, combustion and exhaust gas analysis studies were performed using a mixture of gasoline and ethanol. In addition, the particle emission under each experimental condition was measured to analyze the number and size distribution of particles emitted from the PFI engine.

2. Experimental Apparatus and Test Conditions

2.1. Experimental Apparatus The experimental equipment setup and engine information are given in Figure1. In this study, a PFI motorcycle engine was used and two engines with B/S ratios of 0.88 and 1.1, respectively, were analyzed to compare combustion and exhaust characteristics. Table1 shows the information for the two engines. The intake flow rate is the natural flow rate under WOT conditions, and the lambda value was feedback to 1 using a lambda sensor and the NI labview system. Therefore, the AFR was the theoretical air-fuel ratio (14.7). Control factors such as the engine ignition timing and injection timing were controlled in real time using the Labview program, and the combustion characteristics were also analyzed using the Labview program. In the case of pressure data of the engine, the final combustion pressure data was used by performing three repeated experiments for 100 cycles of total combustion, and the heat release rate, combustion duration and engine efficiency were calculated based on the pressure data. Energies 2020, 13, 321 3 of 15

3 of 18

FigureFigure 1. 1.Schematic Schematic diagramdiagram of of the the engine engine test testsystem. system.

TableTable 1. 1.Speciﬁcations Specifications of of the the test test engines. engines.

SpecificationsSpeciﬁcations Over Square Over Square Under Under Square Square Engine displacement (cc) 125 Engine displacement (cc) 125 B/S ratio 1.15 0.88 BoreB /(mm)S ratio 56 1.15 52 0.88 StrokeBore (mm) (mm) 50.7 56 58.8 52 ConnectingStroke rod length (mm) (mm) 90 50.7 100 58.8 Compression ratio 11.4 11 Connecting rod length (mm) 90 100 Intake valve opens BTDC 362 BTDC 367 Intake valveCompression closes ratio BTDC 143 11.4 BTDC 148 11 CAD IntakeExhaust valve valve opens opens ATDC 148BTDC 362 ATDC BTDC 142 367 IntakeExhaust valve valve closes closes ATDC 354 BTDC 143 ATDC BTDC 351 148 CAD TheExhaust main physical valve opensproperties of gasoline and ethanol used ATDC in this 148 study are ATDCshown 142in Table 2. Ethanol has the advantages of a higher octane number, latent heat of evaporation, and oxygen-rich fuel comparedExhaust to valvegasoline, closes but it requires more fuel for similar ATDC power 354 production ATDCthan conventional 351 gasoline because of lower heating value. The main physical properties of gasoline and ethanol used in this study are shown in Table2. Table 2. Test fuel properties. Ethanol has the advantages of a higher octane number, latent heat of evaporation, and oxygen-rich fuel compared to gasoline, but itMain requires Properties more of fuel Gasoline for similar and Ethanol power production than conventional gasoline because of lower heatingProperties value. Gasoline Ethanol Chemical Formula C5–C11 C2H5OH Density @ 15 °CTable (kg/m 2. 3Test) fuel properties. 46.3 131.3 Research Octane Number 95 108 Lower HeatingMain PropertiesValue (MJ/kg) of Gasoline and Ethanol 42.5 26.9 Latent Heat ofProperties Vaporization (kJ/kg) Gasoline 305 Ethanol 840 Stoichiometric Air to Fuel Ratio 14.7 8.95 Chemical Formula C5–C11 C2H5OH O2 Content (%) 3 - 34 Density @ 15 ◦C (kg/m ) 46.3 131.3 Research Octane Number 95 108 Lower Heating Value (MJ/kg) 42.5 26.9 Latent Heat of Vaporization (kJ/kg) 305 840 Stoichiometric Air to Fuel Ratio 14.7 8.95

O2 Content (%) - 34 Energies 2020, 13, 321 4 of 15

Exhaust gas and particle measurement equipment are shown in Table3. The measurement of NOx and THC, which are gaseous substances, was carried out using a MEXA-9100 exhaust gas measuring instrument. In the case of particle number (PN), which is particulate matter, PN was measured by Pegasor Particle Sensor-Monitoring (PPS-M) and the particle size distribution of PN was measured through the engine exhaust particle sizer (EEPS) instrument. The measuring range of the PPS-M was 2.5 micrometers, and EEPS instruments can measure PN distributions with a size from 5.6 to 560 nm. Detailed equipment information is summarized in Table3.

Table 3. Speciﬁcations of the gas and particle emissions measurement devices.

Emission Measurement Device Item Description Measurement principle Flame Ionization Detector (FID)

MEXA-9100 (THC) Measuring range 10–50,000 ppm Linearity 1.0% of full scale ± Response 1.0% of full scale ± Measurement principle Chemiluminescence method Measuring range 0.00–10,000 ppm MEXA-9100 (NOx) Linearity 1.0% of full scale ± Response 1.0% of full scale ± Measurement principle Measurement of electrical charge Measuring range 1 µg/m3–250 mg/m3 PPS-M (PN) Min: 23 nm Detectable particle size Max: 2.5 µm Response time 0.2s (10Hz data acquisition) Measurement principle Measurement of electrical charge Measuring range 5.6–560 nm EEPS (PN distribution) Concentration range 104–109 #/cm3 Inlet cyclone 50% cut point 1 µm Response 0.1–60 s

2.2. Experimental Conditions The engine was operated at 5000 rpm and the ignition timing was fixed at BTDC 33.5 degrees, at an MBT of 5000 rpm. Fuel injection was carried out at an injection pressure of 4 bar. The fuel used in the test was a mixture of gasoline and ethanol, and the experiment was conducted at 20% v/v intervals from 100% gasoline (E0) to 60% ethanol (E60). For 80% ethanol, combustion stability decreased and misfire occurred. Since ethanol produces less heat than gasoline, more fuel must be injected to maintain a lambda 1 condition. Therefore, the experiment was carried out while advancing start of injection (SOI) based on the fixed EOI (aTDC 330), and the H/C ratio was changed by changing the ethanol blend ratio (Table4). Energies 2020, 13, 321 5 of 15

Table 4. Fuel properties and start of injection (SOI) for ethanol blending ratio.

Fuel Properties E0 (G100) E20 (G80) E40 (G60) E60 (G40) Ethanol (%) 0 20 40 60 H/C ratio 1.99 2.13 2.29 2.48

LHV_mixture (MJ/kg) 43.4 39.9 36.6 33.35 of 18 Heat of vaporization (kJ/kg) 305 414 523 630 HeatSOI of (bTDC vaporization deg.) (kJ/kg) 60 305 414 78 523 93 630 117 SOI (bTDC deg.) 60 78 93 117 3. Results In3. Results this session, the experimental results using two engines with different bore to stroke values are shown. TheIn this bore session, and stroke the experimental values were results 56 mm using and tw 50.7o engines mm inwith the different over-square bore to engine, stroke values yielding a B/S ratioare shown. of 1.1. TheThe under-squarebore and stroke engine values had were a bore 56 mm and and stroke 50.7 of mm 52 mmin the and over-square 58.8 mm, respectively,engine, yieldingyielding a B/S a ratio B/S ratio of 0.88. of 1.1. The The fuel under-square injection timing engine was had bTDC a bore 60 and degrees stroke andof 52 the mm ignition and 58.8 timing mm, was fixedrespectively, at bTDC 33 yielding degrees. aIn B/S addition, ratio of the0.88. engine The fu loadel injection was maintained timing was inbTDC the high60 degrees load section and the of the WOTignition curveand timing the was experiment fixed at bTDC was performed 33 degrees. inIn Lambdaaddition, 1.the engine load was maintained in the high load section of the WOT curve and the experiment was performed in Lambda 1. The motoring pressure results of two engines with different compression ratios are shown in The motoring pressure results of two engines with different compression ratios are shown in FigureFigure2 as follows.2 as follows. The The over over square square engine engine withwith aa highhigh compression ratio ratio of of11.4 11.4 shows shows higher higher pressurepressure than than the underthe under square square engine engine with with aa compressioncompression ratio ratio of of 11.0, 11.0, and and the the maximum maximum cylinder cylinder pressurepressure was alsowas aboutalso about 1.7 bar 1.7 higher. bar higher. It is believedIt is believed that thisthat isthis a general is a general result result where where high pressureshigh at highpressures compression at high ratioscompression were measured.ratios were However,measured. theHowever, combustion the combustion results show results the show opposite the of motoringopposite pressure. of motoring pressure.

Motoring pressure 4 Peak pressure[MPa] Over square Over square : 2.6697 Under square Under square : 2.4949 3

1 Cylinder Pressure (MPa) Pressure Cylinder

0 -80 -60 -40 -20 0 20 40 60 Crank Angle (deg)

FigureFigure 2. Motoring2. Motoring pressure pressure forfor over- and and under-square under-square engines engines.. 3.1. Eﬀects of Bore to Stroke Ratio on Combustion Characteristics 3.1. Effects of Bore to Stroke Ratio on Combustion Characteristics FigureFigure3 shows 3 shows the pressure the pressure and heatand generationheat generation rate insiderate inside the cylinder the cylinder according according to the to engine’sthe boreengine's to stroke bore ratio. to stroke In general, ratio. In highgeneral, combustion high combustion pressures pressures are measuredare measured in in engines engines with with high compressionhigh compression ratios, but ratios, this but study this shows study shows higher higher combustion combustion pressures pressures at lower at lower compression compression ratios. This resultedratios. This in highresulted cylinder in high pressure cylinder in pressure high compression in high compression ratio engines ratio beforeengines the before start the of combustion,start of but highcombustion, pressure but in under high squarepressure engines in under with square low compression engines with ratio low due compression to diﬀerences ratio in combustiondue to processdifferences after ignition. in combustion The maximum process after pressure igniti inon. the The combustion maximum chamberpressure wasin the also combustion higher in the under-squarechamber enginewas also than higher in thein the over-square under-square engine, engi withne than 73 barin the and over-square 60 bar, respectively. engine, with In 73 the bar case of and 60 bar, respectively. In the case of the maximum pressure position, the under-square engine the maximum pressure position, the under-square engine had a TDC maximum pressure at 12 degrees had a TDC maximum pressure at 12 degrees and the over-square engine had a TDC maximum and the over-square engine had a TDC maximum pressure at 15 degrees. In the heat generation results, pressure at 15 degrees. In the heat generation results, even under the same ignition timing, faster heat generation results could be seen in the under-square engine. As a result, it was judged that combustion occurs faster in the under-square engine than in the over-square engine. Further, the combustion speed was confirmed by the cumulative heat generation as shown in Figure 4. In this Energies 2020, 13, 321 6 of 15

6 of 18 even under the same ignition timing, faster heat generation results could be seen in the under-square study,engine. the As combustion a result, it was speed judged of each that combustionengine was occursdefined faster by the in thecombustion under-square duration. engine The than shorter6 inof the18 theover-square combustion engine. duration Further, calculated the combustion based on speed the cu wasmulative conﬁrmed heat byrelease, the cumulative the faster the heat combustion generation study,speed,as shown theand incombustion the Figure longer4. In thespeed this combustion study, of each the combustionengine duration, was speedthdefinede slower of eachby thethe engine combustioncombustion was deﬁned speed.duration. by As the Thecan combustion beshorter seen thefromduration. combustion the results The shorter duration in Figure the calculated combustion4, the combustion based duration on durathe calculated cutionmulative in the based under-squareheat on release, the cumulative the engine faster was heatthe short,combustion release, which the speed,wasfaster determined theand combustion the longer that combustion speed, the combustion and the speed longer duration, was the faster combustion th thane slower the duration, over-square the combustion the slowerengine. speed. the combustion As can be speed.seen fromAs can the be results seen from in Figure the results 4, the in combustion Figure4, the dura combustiontion in the duration under-square in the under-squareengine was short, engine which was wasshort, determined which was that determined combustion that speed combustion was faster speed than was the faster over-square than the engine. over-square engine. 8 80 Over square 5000RPM 7 Under square 70 8 Throttle open 100% 80 Over square 6 5000RPMLambda 1 60 7 Under square 70 Throttle open 100% 5 50 6 Lambda 1 60 4 40 5 50 3 30 4 40 2 20 3 30 1 10 2 20

Cylinder Pressure (MPa) Pressure Cylinder 0

0 (J/deg) Rate Heat Release 1 10 -1 -10

Cylinder Pressure (MPa) Pressure Cylinder 0 -60 -40 -20 0 20 40 60 800 (J/deg) Rate Heat Release -1 Crank Angle (deg) -10 -60 -40 -20 0 20 40 60 80 Figure 3. In-cylinder pressure andCrank heat release Angle ra tes(deg) for over- and under-square engines.

FigureFigure 3. 3. In-cylinderIn-cylinder pressure pressure and and heat heat release release ra ratestes for for over- over- and and under-square under-square engines engines..

350 Over square 350300 Under square Over square 250 300 5000RPM Under square Throttle open 100% 250200 5000RPMLambda 1 Throttle open 100% 200150 Lambda 1

150100

10050

500 Integrated heat release (J) -500 Integrated heat release (J) Under-50

Under CA10 CA30 CA50 CA70 CA90

CA10 CA30 CA50 CA70 CA90 Over

Over Combuston duration -20 0 20 40

Combuston duration Crank Angle (deg) -20 0 20 40 FigureFigure 4. 4. IntegratedIntegrated heat heat release release and and combustion combustionCrank Angledu durationration (deg) for for over- over- and and under-square under-square engines engines..

In Figureparticular, 4. Integrated the combustion heat release speed and combustion of initial codumbustionration for over-was andfaster under-square because the engines difference. in CA10 was apparent. The cause of the difference in combustion speed could be largely divided into In particular, the combustion speed of initial combustion was faster because the difference in CA10 was apparent. The cause of the difference in combustion speed could be largely divided into Energies 2020, 13, 321 7 of 15

In particular, the combustion speed of initial combustion was faster because the difference in CA10 was apparent. The cause of the difference in combustion speed could be largely divided into flame propagation and turbulence intensity according to the combustion chamber shape. In the case of flame propagation, differences could be seen in the result of the area occupied by the flame surface at the instantaneous volume of the combustion chamber. In fact, in a previous study, the flame surface was compared according to the B/S ratio. In short stroke engines, the flame surface area vs. momentary volume results were high only up to about 20% of the total volume of the combustion chamber, whereas in long stroke engines, the area occupied by the flame surface in the instantaneous volume of the combustion chamber approaches 60%; this reduces the combustion duration. As a result, the flame propagation speed was faster because the flame surface area was larger in the long stroke engine. Therefore, the results of this study also show that the long stroke, under-square engine produced a shorter combustion time than an over-square engine [5]. Next, when the turbulent intensity in the combustion chamber increased, flame propagation proceeded rapidly, resulting in a shorter combustion duration. Turbulence intensity in gasoline engines had a great influence on combustion. As the turbulence intensity increased, the flame propagation speed increased. The turbulence intensity inside the combustion chamber changed according to the B/S ratio, and as the B/S ratio decreased, the turbulence intensity increased near the initial stage of the intake planet and TDC. This results in shorter combustion duration for under-square engines with a B/S ratio of 0.88. Figure5 shows the pressure and volume (P–V) diagram for the B /S ratio and three different losses and indicated mean effective pressure (IMEP) compared to the ideal cycle. In general, it is judged that the loss of the engine is consumed in the range (energy conversation efficiency). In the case of IMEP, the result was about 50% including the pump loss. In addition, other studies have shown that the IMEP results are around 35–40% [7]. The IMEP results of this study show a result between 60% and 70%, which was considered to have a high result because it did not include exhaust heat loss. In addition, the result of heat loss might be somewhat inaccurate because it was measured based on the pressure data, but the tendency of the loss result was reasonable and judged. From the minimal heat loss to combustion time loss and exhaust blowdown loss, the relative values for each loss are shown in Figure6. Heat losses via the cylinder wall during the second half of the compression stroke (during the combustion duration) and during the expansion stroke were 26.6% and 23.2%, followed by combustion losses of 11.6% and 8.7%, and finally, blowdown losses of 2.5% and 2.3%, respectively. The most significant factors in the heat losses observed were the flame propagation and combustion time mentioned above. Differences in flame propagation due to engine shape also affect heat transfer. In particular, when the bore and stroke size were changed, the timing and the contact area of the flame and the contact between the piston and the cylinder wall change—both of which affect heat transfer. In the case of long stroke engines, they have a relatively narrow bore and the piston is far from the spark plug, so that the flame surface first contacts the cylinder wall before contacting the piston. In short stroke engines, on the other hand, the flame first contacts the piston before the cylinder wall. This causes greater heat transfer to the piston tower. In the case of the cylinder wall, constant temperature control is possible through the coolant, but in the case of the piston tower, since no additional temperature control is performed, more heat transfer occurs and a large heat loss occurs. Next, in the case of the loss of combustion time, the burning time due to the flame propagation and turbulence intensity in the long stroke, under-square engine was reduced and the loss corresponding to this duration was reduced. Finally, the 2.5% blowdown losses were similar between models. 8 of 18

Energies 2020, 13, 321 88 of 1518

16 5000RPM 16 Ideal cycle 14 Throttle open 100% Ideal cycle(exhaust) 5000RPM Ideal cycle Lambda 1 Under square 14 Throttle open 100% Ideal cycle(exhaust) 12 Over square Lambda 1 Under square 1012 Over square

108

Cylinder pressure (Mpa) Cylinder pressure 24

Cylinder pressure (Mpa) Cylinder pressure 02 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0 0.00 0.02 0.04In-cylinder 0.06 volume 0.080.10 (m3) 0.12 0.14

In-cylinder volume (m3) Figure 5. The P–V diagram for over- and under-square engines compared to the ideal cycle. Figure 5. The P–V diagram for over- and under-square enginesengines comparedcompared toto thethe idealideal cycle.cycle.

2.5 2.3 100 Blowdown loss 2.5 2.3 11.6 8.7 Time loss 10090 Blowdown loss 8.7 Heat loss 8090 11.6 Time loss 23.2 Indicated work 26.6 Heat loss 80 70 23.2 Indicated work 26.6 6070 5060 4050 3040 59.3 65.8 Engine loss (%) Engine 2030 59.3 65.8 Engine loss (%) Engine 1020 100 Over Under 0 Figure 6. Three difference engineOver losses and indicated Under work for over- and under-square engines. Figure 6. Three difference engine losses and indicated work for over and under square engines. 3.2. EffectsFigure of Ethanol 6. Three Blend difference on Combustion engine losses Characteristics and indicated work for over and under square engines. 3.2. Effects of Ethanol Blend on Combustion Characteristics Figure7 shows the results of cylinder pressure and heat release according to ethanol content. 3.2. Effects of Ethanol Blend on Combustion Characteristics AccordingFigure to 7 theshows B/S ratio,the results combustion of cylinder characteristics pressure ofand 20% heat intervals release from accordin 0% tog 60%to ethanol of ethanol content. were Accordingcompared.Figure to As7 theshows can B/S be the seenratio, results from combustion theof cylinder results, characteristics aspressure the ethanol and of heat20% blending intervalsrelease ratio accordin from increased, 0%g toto ethanol the60% combustion of content.ethanol pressurewereAccording compared. increased to the AsB/S and canratio, the be combustion combustion seen from duration characteristicsthe results, shortened. as of the 20% This ethanol intervals is because blending from ethanol 0% ratio hadto 60%increased, higher of ethanol oxygen the combustioncontentwere compared. than pressure gasoline, As increasedcan which be resultsseen and fromthe in morecombustion the activeresults, combustion.duration as the shortened.ethanol Therefore, blending This the is combustionbecauseratio increased, ethanol pressure hadthe washighercombustion gradually oxygen pressure increasedcontent increased than and thegasoline, and initial the combustion combustionwhich resu ltsduration speed in more was shortened. fast.active Figure combustion. This8 andis because Table Therefore,5 ethanol show the hadthe combustionhigher oxygen durationpressure content resultswas than gradually from gasoline, CA10 increased towhich CA90 andresu for over-squarethelts ininitial more combustion and active under-square combustion. speed engines.was Therefore, fast. As Figure can the be 8 andseencombustion Table in Table 5 showpressure5, the the di ff wascombustionerence gradually between duration increased the combustion result ands fromthe speeds initial CA10 of combustionto the CA90 over for and speedover-square under was square fast. and Figure engines under- 8 wassquareand largelyTable engines. 5 aboutshow As 4 thecan degrees combustion be seen in CAin Table duration 50, and 5, the the result di differenceffserence from between CA10 in combustion to the CA90 combustion for speed over-square was speeds about andof 2 the degrees under- over andonsquare average. under engines. square This As result engines can seemsbe seenwas to inlargely be Table due about to5, thethe 4 didi degreefffferenceerences in inbetween CA the flame50, theand surface combustion the difference mentioned speeds in above.combustion of the Bothover speedenginesand under was also aboutsquare have shorter2 engines degrees combustion was on average.largely duration about This 4result as degree ethanol seemss in blending CAto be 50, due and rate to increases.the the difference difference This in is in combustion because the flame the surfacelaminarspeed was mentioned flame about propagation 2 above. degrees Both speedon enginesaverage. of ethanol also This have isresult faster shorter seems than combustion thatto be of due gasoline. toduration the Noteworthydifference as ethanol in is theblending that flame the rateflamesurface increases. propagation mentioned This speed above.is because in Both the the combustionengines laminar also flame chamberhave propagation shorter was notcombustion significantlyspeed of durationethanol affected is as faster ethanol by the than turbulentblending that of gasoline.flamerate increases. propagation Noteworthy This speed, is becauseis that but more the laminarflame so by thepropag flame laminar ationpropagation flame speed propagation inspeed the ofcombustion ethanol speed. is chamberfaster than was that not of gasoline. Noteworthy is that the flame propagation speed in the combustion chamber was not 9 of 18

significantly affected by the turbulent flame propagation speed, but more so by the laminar9 offlame 18 propagation speed. significantly affected by the turbulent flame propagation speed, but more so by the laminar flame Energiespropagation2020, 13, 321 speed. 9 of 15 Over square Under square 10 80 E0 5000RPM 9 E20 70 Throttle open 100% Over square Under square 10 8 E40 80 Lambda 1 E0 5000RPM E60 60 9 7 E20 70 Throttle open 100% E40 50 8 6 Lambda 1 E60 60 7 5 40 50 6 4 30 40 5 3 20 4 2 30 3 10 1 20 Cylinder Pressure (MPa) 2 0 0 Heat Release Rate (J/deg) 10 1-1 -10 Cylinder Pressure (MPa) 0 -60 -40 -20 0 20 40 600 80 Heat Release Rate (J/deg) -1 Crank Angle (deg) -10 -60 -40 -20 0 20 40 60 80 Figure 7. In-cylinder pressure and heatCrank release Angle rate for (deg) over- and under-square engine vs. ethanol blending ratio. Figure 7. In-cylinder pressure and heat release rate for over- and under-square engine vs. ethanol Figure 7. In-cylinder pressure and heat release rate for over- and under-square engine vs. ethanol blendingblending ratio. ratio.

CA10 CA30 CA50 CA70 CA90 CA10 CA30 CA50 CA70 CA90

E60 E60 CA10 CA30 CA50 CA70 CA90 CA10 CA30 CA50 CA70 CA90

E60E40 E60E40

E40E20 E40E20

E20Combuston duration E0 E20Combuston duration E0

-30 -20 -10 0 10 20 -30 -20 -10 0 10 20

Combuston duration E0 Combuston duration E0 Crank Angle (deg) Crank Angle (deg) -30 -20 -10 0 10 20 -30 -20 -10 0 10 20 (a) Over square engine (b) Under square engine Crank Angle (deg) Crank Angle (deg) Figure 8. Combustion duration for over- and under-square engine as ethanol blending ratio. Figure 8.(aCombustion) Over square duration engine for over- and under-square ( engineb) Under as square ethanol engine blending ratio.

Table5.TableFigureCrank 5. Crank8. angle Combustion angle of combustion of combustionduration duration for duration over- for and over- for under-sq over- and and under-squareuare under-square engine as engine ethanol engine as blending ethanolas ethanol ratio. blending blending ratio. ratio. Table 5. CrankOver angle Square of combustion duration CA10 for over- CA30 and under-square CA50 engine as ethanol CA90 blending Over Square CA10 CA30 CA50 CA90 ratio. E0 (G100) 4.58 3.45 8.61 23.78 E0 (G100) −−4.58 3.45 8.61 23.78 Over SquareE20 (G80) CA104.59 3.39CA30 8.54CA50 23.16CA90 E20 (G80) −−4.59 3.39 8.54 23.16 E0 (G100)E40 (G60) −4.585.22 2.893.45 8.12 8.61 23.0923.78 E40 (G60) −−5.22 2.89 8.12 23.09 E20 (G80)E60 (G40) −4.595.45 2.563.39 7.69 8.54 22.5923.16 E60 (G40) −−5.45 2.56 7.69 22.59 E40Under (G60) Square − CA105.22 CA302.89 CA50 8.12 CA9023.09 Under Square CA10 CA30 CA50 CA90 E60 (G40)E0 (G100) −5.457.19 0.072.56 4.43 7.69 21.7822.59 E0 (G100) −−7.19 − −0.07 4.43 21.78 Under E20Square (G80) CA107.83 CA300.49 4.07CA50 21.06CA90 E20 (G80) −−7.83 − −0.49 4.07 21.06 E40 (G60) 8.29 1.18 3.26 21.24 E0 (G100) −−7.19 −−0.07 4.43 21.78 E60 (G40) 8.25 1.06 3.35 20.35 E20 (G80) −−7.83 −−0.49 4.07 21.06

E40 (G60) −8.29 −1.18 3.26 21.24 E60 (G40) −8.25 −1.06 3.35 20.35

Figure 9 shows the P–V diagram according to ethanol content and shows the calculated losses under each condition. As the content of ethanol increased, the net loss of time indicated by the Energieschange2020, 13 of, 321 combustion time increased, and the net indicated work could be relatively increased. 10 of 15 However, the results upon varying the ethanol content did not show much difference. As can be seen from these results, when ethanol was mixed with gasoline, the combustion characteristics of not significantlythe engine were affected. not significantly However, affected. ethanol Howeve fuel hadr, ethanol a lower fuel low had calorific a lower value low calorific than gasoline, value so the differencethan gasoline, in fuel consumption so the difference was in large.fuel consumpt Figure ion10 showswas large. the Figure results 10 of shows fuel the consumption results of fuel compared consumption compared to the same output according to B/S ratio and ethanol content. Figure 11 to the same output according to B/S ratio and ethanol content. Figure 11 shows the results of IMEP shows the results of IMEP under each condition. First, the fuel consumption and IMEP results underaccording each condition. to the B/S First, ratio thecould fuel beconsumption confirmed by the and high IMEP IMEP results resulting according in the relatively to the B /low-lossS ratio could be confirmedin under-square by the high engines. IMEP As resulting a result inof theincreasing relatively ethanol low-loss content, in the under-square IMEP also increased engines. in Asboth a result of increasingover-square ethanol and content, under-square the IMEP engines. also As increased shown in inthe both combustion over-square chamber and internal under-square pressure engines. As shownresults in and the loss combustion results, it was chamber determined internal that pressurethe combustion results speed and was loss also results, increased. it was In determinedthe case that the combustionof fuel consumption, speed was the alsoamount increased. of fuel injected In the was case increased of fuel consumption,due to the low heat the value amount of ethanol. of fuel injected was increased due to the low heat value of ethanol.

2.5 2.4 2.5 8 100 2.5 Blowdown loss E0 90 11.5 11.4 11.3 11.2 Time loss E20 Heat loss E40 80 6 Indicated work E60 70 26.2 24.6 25.5 24.4 60 4 50 40 30 2 59.8 61.5 60.8 Engine loss (%) loss Engine 20 61.9 Cylinder pressure (Mpa) 10 0 0 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 E0 E20 E40 E60 3 In-cylinder volume (m ) (a) Over square engine

2.3 2.2 2.3 2.2 8 100 Blowdown loss 8.2 8.0 E0 90 8.7 8.2 Time loss E20 Heat loss E40 80 6 23.2 22.7 22.8 22.4 Indicated work E60 70 60 4 50 40 30 2 65.8 66.9 66.7 67.4 Engine loss (%) Engine loss 20 Cylinder pressure (Mpa) 10 0 0 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 E0 E20 E40 E60 3 In-cylinder volume (m ) (b) Under square engine

FigureFigure 9. The 9. The P–V P–V diagram diagram and and three three engine losses losses for for over over and and under under square square engines engines and ethanol and ethanol blending ratio. 11 of 18 blending ratio.

Over square Under square ISFC 250

200

150

100

50 Fuel consumption (g/kWh) consumption Fuel 0 E0 E20 E40 E60 FUEL

Figure 10.FigureFuel 10. consumption Fuel consumption for for over- over- and and under-squareunder-square engine engine and andethanol ethanol blending blending ratio. ratio.

Over square Under square 3

IMEPnet COV [%] 6 IMEPnet [MPa]

2 4

2 1 IMEP (MPa) IMEP COV (%)

0 E0 E20 E40 E60 FUEL

Figure 11. Indicated mean effective pressure (IMEP) and coefficient of variation (COV) for over- and under-square engine and ethanol blending ratio.

3.3. Emissions of B/S Ratio and Ethanol Blend In this session, we measured and compared the exhaust gas NOx and THC emissions from the two engines used in the experiment, and then analyzed the emission trends and concentrations by particle size via additionally measuring the particle number. For particulate matter, PPS-M and EEPS equipment were measured at the same time to compare the emission trends. In addition, analysis of gaseous and particulate matters released with increasing ethanol content was also conducted.

3.3.1. Gas Emissions Figure 12 shows the results for NOx and THC emissions for each experimental condition. In the case of NOx, when only gasoline fuel was used, the emission from the over-square and under- square engines was similar. As the ethanol content was increased, NOx was reduced. Since the 11 of 18

Over square Under square ISFC 250

200

150

100

50 Fuel consumption (g/kWh) consumption Fuel 0 E0 E20 E40 E60 FUEL

Energies 2020, 13, 321 11 of 15 Figure 10. Fuel consumption for over- and under-square engine and ethanol blending ratio.

Over square Under square 3

IMEPnet COV [%] 6 IMEPnet [MPa]

2 4

2 1 IMEP (MPa) IMEP COV (%)

0 E0 E20 E40 E60 FUEL

Figure 11.FigureIndicated 11. Indicated mean mean eﬀective effective pressure pressure (IMEP) (IMEP) and coefficient coeﬃcient of ofvariation variation (COV) (COV) for over- for and over- and under-squareunder-square engine engine and ethanoland ethanol blending blending ratio. ratio.

3.3. Emissions3.3. Emissions of B/S of Ratio B/S Ratio and and Ethanol Ethanol Blend Blend In thisIn session, this session, we measured we measured and and compared compared thethe exhaust gas gas NOx NOx and and THC THC emissions emissions from the from the two enginestwo engines used inused the in experiment,the experiment, and and then then analyzed analyzed the the emission emission trends trends and andconcentrations concentrations by by particle size via additionally measuring the particle number. For particulate matter, PPS-M and particle size via additionally measuring the particle number. For particulate matter, PPS-M and EEPS EEPS equipment were measured at the same time to compare the emission trends. In addition, equipmentanalysis were of measuredgaseous and at theparticulate same timematters to comparereleased with the emissionincreasing trends. ethanol Incontent addition, was analysisalso of gaseousconducted. and particulate matters released with increasing ethanol content was also conducted.

3.3.1. Gas3.3.1. Emissions Gas Emissions Figure Figure12 shows 12 shows the results the results for NOxfor NOx and and THC THC emissions emissions for for each each experimentalexperimental condition. condition. In In the case of NOx,the case when of NOx, only when gasoline only gasoline fuel was fuel used, was theused, emission the emission from from the the over-square over-square and and under-square under- square engines was similar. As the ethanol content was increased, NOx was reduced. Since the engines was similar. As the ethanol content was increased, NOx was reduced. Since the latent heat of evaporation of ethanol was higher than that of gasoline, this results in a cooling effect by lowering the ambient temperature at the time of combustion. It was found that the NOx emission results decreased as the ethanol content increased in both the over-square and under-square engines. In the case of under-square engine, as the intake valve seat was smaller than that of the over-square, the fuel injected into the port decreased and the amount of droplets flowing into the combustion chamber increased, thus increasing the latent heat of vaporization in the combustion chamber. This result was due to the effect of reducing the internal temperature, however, the difference was not significant. In the case of THC, another gaseous substance, its emission was reduced with increasing ethanol content. In the case of gasoline, which was basically a hydrocarbon paraffinic, and since ethanol contains oxygen, HC emission was estimated to be lower than 100% gasoline condition. On the other hand, THC emission results according to the B/S ratio did not show a big difference and appear to show similar levels of emission. 12 of 18

latent heat of evaporation of ethanol was higher than that of gasoline, this results in a cooling effect by lowering the ambient temperature at the time of combustion. It was found that the NOx emission results decreased as the ethanol content increased in both the over-square and under- square engines. In the case of under-square engine, as the intake valve seat was smaller than that of the over-square, the fuel injected into the port decreased and the amount of droplets flowing into the combustion chamber increased, thus increasing the latent heat of vaporization in the combustion chamber. This result was due to the effect of reducing the internal temperature, however, the difference was not significant. In the case of THC, another gaseous substance, its emission was reduced with increasing ethanol content. In the case of gasoline, which was basically a hydrocarbon paraffinic, and since ethanol contains oxygen, HC emission was estimated to be lower than 100% gasoline condition. On the other hand, THC emission results according to the B/S Energies 2020, 13, 321 12 of 15 ratio did not show a big difference and appear to show similar levels of emission. 13 of 18 13 of 18 the particle sizeOver ofsquare the PN appeared Under square to be larger than that of theOver PN square emitted from Under the square over-square engine.the particle100 size of the PN appeared to be larger than that20 of the PN emitted from the over-square engine.The PN results with the ethanol blending ratio indicate that the PN emission was reduced as 80 the ethanolThe PN blending results with ratio the increased. ethanol Overblending square ratio engine indicate15 shows that low the PNPN resultsemission except was forreduced the E20 as condition.the ethanol Ethanol blending is ratio a fuel increased. having Overa high square latent engine heat showsof vaporization low PN results and exceptoxygen for content, the E20 60 andcondition. the latent Ethanol heat is ofa fuelevaporation having a adverselyhigh latent affected heat of PNvaporization emission becauseand oxygen it relatively content, 10 cooledand the the latent ambient heat temperature. of evaporation On adverselythe contra ry,affected the oxygen PN emission content hadbecause a great it relatively effect on 40 reducingcooled the the ambient PN emission temperature. since theOn thecombus contrationry, reactionthe oxygen was content actively had progressed. a great effect Under on 5 NOx (g/kg fuel) thereducing E2020 condition, the PN theemission effect of since high latentthe combus heat oftion evaporation THC (g/kg fuel) reaction of was ethanol actively was expressedprogressed. more, Under and the E20atomization condition, was the loweredeffect of highdue latentto the heatcooling of evaporation effect in the of ethanolport, resulting was expressed in increased more, andPN emission.the atomization0 However, was after lowered the E20 due condition, to the cooling the oxygen effect 0content in the was port, greatly resulting increased, in increased which wasPN consideredemission. However, toE0 decrease after E20 the the PN E20 again. E40 condition, Under E60 squarethe oxygen engines, content onE0 the was other greatly E20 hand, increased, tended E40 towhich decrease E60 was FUEL FUEL withconsidered increasing to decrease ethanol the content. PN again. This Underwas because square the engines, oxygen on content the other of hand,the fuel tended increased to decrease as the ethanolwith increasing content increased,ethanol content. so that This combustion was because inside the the oxygen cylinder content was moreof the active fuel increasedand incomplete as the (a) (b) combustionethanol content decreased increased, [22,23]. so that The combustion particle also inside had the a closecylinder relation was moreto the active polycyclic and incomplete aromatic hydrocarbonscombustionFigureFigure 12.12.decreased (PAHs), ((aa)) NOx NOx and and[22,23]. (b ()bthe THC) THC Theaddition emissions emissions particle of for alcoholfor over-also over- andhad to and under-square agasoline closeunder-square relation decreases engine engine to vs. the ethanol vs. aromatic polycyclicethanol blending blending content ratio.aromatic in blendhydrocarbons ratio.fuels [24,25]. (PAHs), Finally, and inthe the addition case of of the alcohol under to squa gasolinere engine, decreases PN increasedthe aromatic by contentthe small in 3.3.2. Particle Emissions heatblend capacity fuels [24,25]. of the Finally, intake in valve the case and of the the fuunderel wetting square on engine, the wall PN ofincreased the cylinder by the due small to 3.3.2. Particle Emissions andheat theFinally,capacity bore Figures sizeof the was 13 intake and reduced. 14 valve compare Since and thethese the particle fuhaveel wetting numbersa direct oneffect (PN) the withon wall PN increasing ofemissions, the cylinder ethanol it is believed blending due to ratio in over- and under-square engines. In the case of PN measured in PPS-M, the average value of thatand PNtheFinally, emissions bore Figures size are was 13the andreduced. main 14 causecompare Since of decrea thethese particlese have as the numbersa directethanol effect(PN) blending withon PN increasingratio emissions, increases. ethanol it is believedblending PN measured for certain duration of time was shown, and in the case of emission concentration by thatratio PN in emissionsover- and areunder-square the main cause engines. of decrea In these case as the of PNethanol measured blending in PPS-M, ratio increases. the average value of particlePN measured size, the for cumulative certain duration result wasof time shown was at shown, the same and time. in the case of emission concentration by 7.0x106 9.0x108 particle size, the cumulative result was shown PN at the same time. E0 6 8

6 )

7.0x10 3 9.0x108 6.0x10 7.5x10 E20 First of all, the difference in results PNbetween the over-square E0 and under-square engines was

66 ) E40 6.0x10 3 8 evident.5.0x10 Compared with the over-square engine, the 7.5x10PN 8emission E20 was higher in the under-square 6.0x10 E60 66 E40 engine,4.0x105.0x10 and the particle size of the emitted PN was larger 8in the under-square engine. In the case of 6.0x108 E60 4.5x10 6 the under-square,3.0x104.0x106 100% gasoline yielded 80.58 nm and then decreased by about 60.43 nm upon 8 4.5x108 increasing66 ethanol ratio. On the other hand, in the 3.0x10case of PN discharged from an over-square Particle Number 2.0x103.0x10 8 engine, 52.33 nm at 100% gasoline injection was observed3.0x108 and then decreased by about 33.98 nm 66 Particle Number 1.0x102.0x10 1.5x10 PN concentration PN concentration (#/cm upon increasing ethanol ratio. As such, there is a difference8 in PN emission results depending on 1.0x100.06 1.5x100.0 the B/S ratio. ThisE0 might E20 be dueE40 to the fuel E60 injection PN concentration (#/cm method1 of the PFI engine. 10 Fuel 100 was injected 1000 into 0.0 0.0 the port due to the characteristicsFuel of the PFI engine. However, if the intakeParticle sizeport (nm) and valve seat do E0 E20 E40 E60 1 10 100 1000 not have enough heat to vaporizeFuel all the fuel, or if there is not enough timeParticle for size the (nm) fuel to evaporate, dropletsFigure of 13.fuel Particle are numberintroduced and sizeinto distribution the combus for tionethanol chamber. blending Theratio ininflow over-square of droplets engine. causes incompleteFigureFigure 13.combustion, 13. ParticleParticle number number which and and causes size size distribution distribution PN emission for for et ethanolshanol [20]. blending blendingParticularly, ratio ratio in in theover-square over-square under-square engine. engine. engine

emits7.00x10 more6 PN than the over-square engine, because 9.0x10the 8size of the intake valve must decrease as PN E0 the bore size6 decreases. It is judged that the heat for vaporizing8 fuel decreases in the interior of the

6 )

7.00x10 3 9.0x108 6.00x10 7.5x10 E20 combustion chamber. In addition, as the droplet PN size is larger than E0 that of the conventional engine,

66 ) E40 6.00x10 3 8 5.00x10 7.5x108 E20 6.0x10 E60 66 E40 4.00x105.00x10 8 6.0x108 E60 4.5x10

6 3.00x104.00x106 8 4.5x108

6 3.0x10 2.00x103.00x106 8 Particle Number 6 3.0x108 1.00x102.00x106 1.5x10 PN concentration PN concentration (#/cm

Particle Number 8 1.00x100.006 1.5x100.0 E0 E20 E40 E60 E80 PN concentration (#/cm 1.0 10.0 100.0 1000.0 0.00 0.0 Fuel Particle size (nm) E0 E20 E40 E60 E80 1.0 10.0 100.0 1000.0 Fuel Particle size (nm) FigureFigure 14. 14. ParticleParticle number number and and size size distribution distribution for for et ethanolhanol blending blending ratio ratio in in under-square under-square engine engine.. Figure 14. Particle number and size distribution for ethanol blending ratio in under-square engine. 4. ConclusionsFirst of all, the difference in results between the over-square and under-square engines was evident. Compared with the over-square engine, the PN emission was higher in the under-square 4. ConclusionsIn this study, the main focus was placed on improving the combustion characteristics and particulateIn this matterstudy, theemission main focuscharacteristics was placed of onPF I improvingsystem small the combustiongasoline engines. characteristics To analyze and particulate matter emission characteristics of PFI system small gasoline engines. To analyze Energies 2020, 13, 321 13 of 15 engine, and the particle size of the emitted PN was larger in the under-square engine. In the case of the under-square, 100% gasoline yielded 80.58 nm and then decreased by about 60.43 nm upon increasing ethanol ratio. On the other hand, in the case of PN discharged from an over-square engine, 52.33 nm at 100% gasoline injection was observed and then decreased by about 33.98 nm upon increasing ethanol ratio. As such, there is a difference in PN emission results depending on the B/S ratio. This might be due to the fuel injection method of the PFI engine. Fuel was injected into the port due to the characteristics of the PFI engine. However, if the intake port and valve seat do not have enough heat to vaporize all the fuel, or if there is not enough time for the fuel to evaporate, droplets of fuel are introduced into the combustion chamber. The inflow of droplets causes incomplete combustion, which causes PN emissions [20]. Particularly, the under-square engine emits more PN than the over-square engine, because the size of the intake valve must decrease as the bore size decreases. It is judged that the heat for vaporizing fuel decreases in the interior of the combustion chamber. In addition, as the droplet size is larger than that of the conventional engine, the particle size of the PN appeared to be larger than that of the PN emitted from the over-square engine. The PN results with the ethanol blending ratio indicate that the PN emission was reduced as the ethanol blending ratio increased. Over square engine shows low PN results except for the E20 condition. Ethanol is a fuel having a high latent heat of vaporization and oxygen content, and the latent heat of evaporation adversely affected PN emission because it relatively cooled the ambient temperature. On the contrary, the oxygen content had a great effect on reducing the PN emission since the combustion reaction was actively progressed. Under the E20 condition, the effect of high latent heat of evaporation of ethanol was expressed more, and the atomization was lowered due to the cooling effect in the port, resulting in increased PN emission. However, after the E20 condition, the oxygen content was greatly increased, which was considered to decrease the PN again. Under square engines, on the other hand, tended to decrease with increasing ethanol content. This was because the oxygen content of the fuel increased as the ethanol content increased, so that combustion inside the cylinder was more active and incomplete combustion decreased [22,23]. The particle also had a close relation to the polycyclic aromatic hydrocarbons (PAHs), and the addition of alcohol to gasoline decreases the aromatic content in blend fuels [24,25]. Finally, in the case of the under square engine, PN increased by the small heat capacity of the intake valve and the fuel wetting on the wall of the cylinder due to and the bore size was reduced. Since these have a direct effect on PN emissions, it is believed that PN emissions are the main cause of decrease as the ethanol blending ratio increases.

4. Conclusions In this study, the main focus was placed on improving the combustion characteristics and particulate matter emission characteristics of PFI system small gasoline engines. To analyze combustion and exhaust characteristics for engine combustion chamber shape, two engines with the same volume but different bore and stroke ratios (1.1 and 0.88) were compared and analyzed. Additionally, we analyzed the efficiency on engine performance by mixing with ethanol and gasoline, which are alternative fuels for reducing engine exhaust gas. In addition, emission analyses were conducted for particle emissions not regulated in the PFI engine. The following are the summary and conclusions of this study: The combustion pressure in the combustion chamber was measured as higher in the long stroke (0.88 B/S ratio) engine compared to the short stoke (1.15 B/S ratio) engine. This was influenced by flame propagation and turbulent flow in the combustion chamber. Due to this combustion speed difference, the time loss in the long stroke engine was reduced and the engine indicated a greater mean effective pressure. Increasing the mixing ratio of ethanol to gasoline fuel increased the combustion pressure and combustion speed of the engine, which was considered to be the result of the laminar flame propagation speed of ethanol being faster than that of gasoline. As a result, engine performance was improved, but fuel consumption was increased due to the low heating value of ethanol. Energies 2020, 13, 321 14 of 15

In the case of NOx emission, the effect of B/S ratio was not large, but when ethanol was present, NOx was reduced. This is because ethanol, which had a higher latent heat of vaporization than gasoline, increased the cooling effect in the combustion chamber. This results in lower combustion temperatures and lower NOx emissions. In the case of THC, since ethanol contained less hydrocarbons and more oxygen than gasoline, the THC decreased as the ethanol mixing ratio increases. Finally, PN emissions were greatly affected by engine geometry. For engines with a B/S ratio of 1.1, the heat from the intake port was greater than that of the 0.88 B/S ratio engine, because the engine head and intake valve were larger in the former. This caused an evaporation difference of the injected fuel due to the characteristics of the PFI engine injecting the fuel into the intake port, causing a difference in the liquid film generated in the port and the valve. Subsequently, more PN emissions were observed from the 0.88 B/S ratio engine. In addition, depending on the ethanol mixing ratio, the PN content and sizes were reduced.

Author Contributions: Conceptualization, J.J. and S.P.; Data measurement J.J. and H.Y.; Experimental setup, H.Y.; Analyze data, J.J. and J.C.; Investigation, J.C. and H.Y.; Writing—original draft prepration, J.J.; Writing—review and editing, J.J. and H.Y.; Data post-processing programming, J.C. and J.J.; Supervision, S.P.; Project administration, S.P.; Funding acquisition, S.P.; All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the CEFV (Center for Environmentally Friendly Vehicle) as Global-Top Project of KMOE (Ministry of Environment, Korea) (No. 2016002070010). Acknowledgments: This research was supported by the CEFV (Center for Environmentally Friendly Vehicle) as Global-Top Project of KMOE (Ministry of Environment, Korea) (No. 2016002070010) and the research fund of Hanyang University (HY-2019). Conﬂicts of Interest: The authors declare no conﬂict of interest.

Abbreviations

PFI Port Fuel Injection GDI Gasoline Direct Injection HRR Heat Release Rate SOI Start of Injection EOI End of Injection IMEP Indicated Mean Eﬃciency Pressure PN Particle Number WOT Wide Open Throttle EEPS Engine Exhaust Particle Sizer TDC Top Dead Center

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