Effects of Injector Spray Angle on Performance of an Opposed

energies

Article Eﬀects of Injector Spray Angle on Performance of an Opposed-Piston Free-Piston Engine

Qinglin Zhang, Zhaoping Xu * , Shuangshuang Liu and Liang Liu School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China; [email protected] (Q.Z.); [email protected] (S.L.); [email protected] (L.L.) * Correspondence: [email protected]; Tel.: +86-25-84303903

Received: 12 June 2020; Accepted: 16 July 2020; Published: 20 July 2020

Abstract: A free-piston engine is a novel internal combustion engine which has the advantages of a variable compression ratio and multi-fuel adaptability. This paper focuses on numerical simulation for combustion process and spray angle optimization of an opposed-piston free-piston engine. The working principle and spray-guided central combustor structure of the engine are discussed. A three-dimensional computational fluid dynamic model with moving mesh is presented based on the tested piston motion of the prototype. Calculation conditions, spray models, and combustion models were set-up according to the same prototype. The effects of spray angle on fuel evaporation rate, mixture distribution, heat release rate, in-cylinder pressure, in-cylinder temperature, and emissions were simulated and analyzed in detail. The research results indicate that the performance of the engine was very sensitive to the spray angle. The combustion efficiency and the indicated thermal efficiencies of 97.5% and 39.7% were obtained as the spray angle reached 40◦.

Keywords: free-piston engine; performance; CFD; spray angle; combustion

1. Introduction The internal combustion engine (ICE) has been widely used in the automotive and aviation fields since its birth in 19th century and has been widely used in people’s daily lives. Its energy source is fossil fuel which is a non-renewable energy. As the automotive market continues to grow, the demand for ICE increases year-on-year, resulting in a rapid increase in demand for oil [1]. Meanwhile, the emissions from the ICE gradually increase the air pollution and greenhouse gas effect. The application of the ICE aggravates the energy crisis and environmental pollution, and that forces mankind to seek out solutions. The current development of the ICE is centered on a new energy engine, taking into account emissions requirements and comprehensively improving engine performance. On the one hand, researchers are trying to find alternative fuels to replace fossil fuel and reduce emissions [2]. On the other hand, they take into account that a new type of power plant can improve the fuel efficiency of conventional engines [3]. In recent years, more and more worldwide scholars pay attention to the free-piston engine which is a novel ICE. Compared with the conventional engine, the free-piston engine eliminates the crankshaft and connecting rod mechanism, and the piston is free to move among its endpoints, only determined by the forces acting on it. Therefore, the free-piston engine can operate with various compression ratios, and it brings potential advantage such as multi-fuel adaptability which means that the free-piston engine can adopt clean fuels, not only solving the energy crisis, but also reducing emissions [4,5]. Due to the fact of its particular structure, the free-piston engine also has the merits of simple structure, low manufacturing cost, high efficiency, multi-combustion mode flexibility as well as high power density [6,7]. In this study, an opposed piston-type free-piston engine with direct injection was applied in terms of its various merits. It is well-known that the fuel direct injection system leads to higher fuel

Energies 2020, 13, 3735; doi:10.3390/en13143735 www.mdpi.com/journal/energies Energies 2020, 13, 3735 2 of 16 efficiency [8]. Due to the special geometric structure of the central combustor of the engine, it has higher requirements on injection. With the narrow structure, the atomization of the fuel spray is limited, which affects the mixture formation significantly. In addition, the small central combustor restricts the spray penetration distance, and it will intensify the impingement phenomenon with long penetration distance. Overall, the injection system of a free-piston engine needs further study to achieve better spray performance. Researchers have done a lot of work on the spray process of the engine. Prabhakara et al. [9] focused on researching the effect of injection pressure on spray process. The results indicated that higher injection pressure is favorable for better fuel atomization and better combustion. However, this finding lacks tested results which should be compared with simulation results. Yuan et al. [10] investigated the spray characteristics of a free-piston engine by comparing with a conventional engine. They found that the spray fuel in a free-piston engine shows smaller sauter mean diameter (SMD) and higher concentration around the spark plug, which is good for mixture formation than a conventional engine. Guo et al. [11] indicated that the trends of the peak in-cylinder pressure versus the injection timing were identical with the convex function curve, and the trapezium fuel injection rate-profile is more appropriate and can keep high efficiency of a free-piston engine than other injection rate-profiles; nevertheless, they did not provide the parameters of the combustion process in detail. Most of the articles, including the above, did not provide the combustion characteristics clearly, and some of those did not compare the calculation results with the tested results which leads to difficulties of validating the models [12–16]. Furthermore, numerous documents mainly studied how the injection timing, injection pressure, as well as injection rate in the spray process affect the performance of the engine. However, studies on the effects of the spray angle on the spray process is still scarce. In fact, the spray angle is one of the key parameters which affect mixture formation and combustion efficiency of the engine, and it has influence on fuel economy and emission properties indirectly [17]. Hence, further study is still necessary regarding the effects of spray angle on the performance of the engine. This paper focuses on numerical simulation for spray angle optimization of an opposed-piston free-piston engine. First, the working principle and spray-guided central combustor structure of the engine are discussed. Then, the computational fluid dynamics (CFD) model of the central combustor was established based on the tested piston motion of the prototype. Finally, the effects of spray angle on performance of the free-piston engine were simulated and analyzed in detail. The results obtained in this paper provide a useful guidance for the improvement of the opposed-piston free-piston engine.

2. System Description

2.1. Working Principle of the Engine The free-piston engine coupled with linear generator (FPEG), as a novel energy conversion device, can convert the chemical energy of fuels to electric energy [18]. The elementary structure of the opposed-piston FPEG prototype is shown in Figure1. It consists of one central combustor, two air-springs, and two linear generators, and the overall structure is centrally symmetrical. Furthermore, the opposed-piston central combustor is equipped with a spark plug and fuel injector. The piston on each side is connected directly to the mover of the linear generator. Due to the elimination of the crankshaft and connecting rod mechanism, the piston assembly moves between inner dead center (IDC) and outer dead center (ODC) without restriction, and its motion is determined by the resultant force acting on the mover. Energies 2020, 13, x FOR PEER REVIEW 3 of 17 Energies 2020, 13, 3735 3 of 16 Energies 2020, 13, x FOR PEER REVIEW 3 of 17

89 89 90 Figure 1. Configuration of the engine. Figure 1. Configuration of the engine. 90 Figure 1. Configuration of the engine. 91 In the the FPEG FPEG system, system, a complete a complete working working cycle cycle consists consists of two of strokes two strokes including including expansion expansion stroke 91 In the FPEG system, a complete working cycle consists of two strokes including expansion stroke 92 strokeand compression and compression stroke. stroke. During During the expansion the expansion stroke, stroke, the fuel the mixture fuel mixture is ignited is ignited by the by spark the sparkplug, 92 and compression stroke. During the expansion stroke, the fuel mixture is ignited by the spark plug, 93 plug,and the and gas the generated gas generated in combustion in combustion process process pushes pushes the piston the piston assembly assembly to move to movefrom IDC from to IDC ODC, to 93 and the gas generated in combustion process pushes the piston assembly to move from IDC to ODC, 94 ODC,and the and air theis sucked air is sucked into the into cylinder the cylinder to accomp to accomplishlish the gas theexchange gas exchange process. process. Meanwhile, Meanwhile, the air- 94 and the air is sucked into the cylinder to accomplish the gas exchange process. Meanwhile, the air- 95 thespring air-spring is compressed is compressed to store to energy store energy for the for ne thext nextstroke. stroke. During During the compression the compression stroke, stroke, the 95 spring is compressed to store energy for the next stroke. During the compression stroke, the 96 theexpansion expansion of the of air-spring the air-spring releases releases the stored the stored energy, energy, and andthe piston the piston assembly assembly moves moves towards towards IDC 96 expansion of the air-spring releases the stored energy, and the piston assembly moves towards IDC 97 IDCcompressing compressing the in-cylinder the in-cylinder mixture mixture [19]. [19 Most]. Most of ofthe the time, time, the the linear linear generator generator works works in in the 97 compressing the in-cylinder mixture [19]. Most of the time, the linear generator works in the 98 generating mode because the engineengine worksworks inin thethe combustioncombustion cycle.cycle. The linear generatorgenerator works in 98 generating mode because the engine works in the combustion cycle. The linear generator works in 99 motoring mode only inin thethe startingstarting timetime oror no-fireno-fire cyclecycle ofof thethe engine.engine. 99 motoring mode only in the starting time or no-fire cycle of the engine. 100 2.2. Central Combustor of the Engine 100 2.2. Central Combustor of the Engine 101 Figure2 2 shows shows thethe structurestructure of of the the central central combustor combustor discussed discussed inin this this paper. paper. ItIt consistsconsists ofof twotwo 101102 opposedFigure pistons, 2 shows twotwo the intake structure ports,ports, of twotwo the exhaust central ports,port combustos, oneone sparksparkr discussed plug,plug, andandin this one paper. fuel injector. It consists The of shape two 102103 ofopposed piston pistons, top is one two of intake thethe keykey ports, factorsfactors two aaffecting exhaustffecting port spraysprays, one andand spark combustioncombustion plug, and processes, one fuel andinjector. it has The an shape eeffectffect 103104 onof piston the gas top flow,flow, is one fuelfuel of atomization,atomization, the key factors andand affectingflameflame propagationpropagat spray andion in incombustion thethe cylindercylinder processes, [[20].20]. TheThe and pistonpiston it has isis designeddesignedan effect 104105 ason athe flat-toppedflat-topped gas flow, fuel piston atomization, due to the and fact flame of itsits advantagespropagatadvantagesion ofin largethe cylinder combustorcombusto [20].r volume, The piston uniform is designed force, 105106 smallas a flat-topped heatheat absorptionabsorption piston area,area,due to simplesimple the fact structure,structure, of its ad andvantagesand easyeasy of processing.processing. large combusto In order r volume, to obtain uniform higher force, gas 106107 exchangesmall heat eefficiency, ffiabsorptionciency, thethe area, centralcentral simple combustorcombustor structure, isis equippedequipped and easy withwith processing. twotwo intakeintake In andand order exhaust to obtain ports, higher which cangas 107108 increaseexchangeincrease intakeintake efficiency, flowflow andtheand central reducereduce combustor backflowbackflow phenomenon.phenomenon. is equipped with two intake and exhaust ports, which can 108 increase intake flow and reduce backflow phenomenon.

109 109 Figure 2. Central combustor of the engine. 110 Figure 2. Central combustor of the engine. 110 Figure 2. Central combustor of the engine. In Figure2, xin and xex represent the distance from intake port and exhaust port to the center of the 111 In Figure 2, xin and xex represent the distance from intake port and exhaust port to the center of combustor, respectively. In order to improve the gas exchange process, the position of the exhaust port 111112 the combustor,In Figure 2, respectively. xin and xex represent In order tothe improve distance the from gas intakeexchange port process, and exhaust the position port to of the the center exhaust of and the width of the intake port are adjusted. By gradually increasing xex, it was found that the change 112113 theport combustor, and the width respectively. of the intake In order port to are improve adjusted. the By gas gradually exchange increasing process, the xex position, it was found of the exhaustthat the in the exhaust port position had a slight influence on the gas exchange efficiency. With the width of 113114 portchange and in the the width exhaust of theport intake position port had are aadjusted. slight influence By gradually on the increasing gas exchange xex, it efficiency. was found With that the the intake port increasing, the gas exchange efficiency increases, but the gas exchange duration also 114115 changewidth of in thethe intakeexhaust port port increasing, position had the a gasslight exch influenceange efficiency on the gas increases, exchange but efficiency. the gas exchangeWith the increases. From the perspective of the entire working cycle, longer gas exchange duration results in 115116 widthduration of alsothe intakeincreases. port Fromincreasing, the perspective the gas exch of angethe entireefficiency working increases, cycle, butlonger the gas exchange 116 duration also increases. From the perspective of the entire working cycle, longer gas exchange

Energies 2020, 13, 3735 4 of 16 shorter combustion duration, which causes the combustion eﬃciency decrease. In the end, the main structure parameters of the central combustor are listed in Table1.

Table 1. Main parameters of the engine.

Parameters Content Engine speed 3500 r/min Cylinder bore 50 mm Exhaust port position 22 mm Intake port position 26 mm Intake port width 12 mm Exhaust port width 12 mm Inner dead center (IDC) 2.3 mm Outer dead center (ODC) 38 mm Area of intake port 562 mm2 Area of exhaust port 562 mm2

The central combustor is also equipped with one spark plug and one fuel injector which can provide accurate injection duration and fuel supply. This engine adopts the gasoline direct injection (GDI) system to achieve lean combustion and conserve fuel accordingly. During lean combustion, direct injection system reduces pumping loss. Because the directly injected fuel is more likely to impinge on the piston and cylinder wall, the air-guided direct injection system and the wall-guided direct injection system have disadvantages of low fuel efficiency and unfavorable emissions. Therefore, the engine in this paper adopted a spray-guided direct injection system to avoid the disadvantages mentioned above. For the spray-guided stratified direct injection system, the fuel injector and the spark plug were close-spaced, and the fuel injector confined the spray fuel reducing contact with the piston, so it was easier to form a proper ignitable mixture near the spark plug to improve combustion and emissions. Clearly, the combustion process depends more on the spray characteristics of the injection system than on the in-cylinder air flow [21].

3. CFD Model

3.1. Mesh Model of the Engine At present, most researchers apply a zero-dimensional or one-dimensional procedure to simulate the free-piston engine. These models neglect the air flow and the fuel distribution in the cylinder, so it is difficult to define the initial and boundary conditions causing the calculation results to be inaccurate. In this paper, a three-dimensional CFD model was established in AVL-Fire which is a commercial software to calculate the entire working cycle of the free-piston engine. Piston motion curve is one of the basic conditions for calculation. As can be seen, the working process of one cycle is divided into two stages including compression stroke and expansion stroke. Figure3 shows the piston motion curve with the specific open and close times of intake and exhaust ports. The equivalent crank angle (ECA) instead of the time variable was employed to describe the motion time. To obtain this curve, first, a computation model of the free-piston engine is built in MATLAB/Simulink. Then, the force output from the sub-models, such as the cylinder and linear generator, was used as input. Under the control and conversion of the sub-models, the force situation and motion trajectory of the engine piston were analyzed. The main ECA of the engine is listed in Table2. Energies 2020, 13, 3735 5 of 16 Energies 2020, 13, x FOR PEER REVIEW 5 of 17

149 Figure 3. Piston motion of the engine. 150 Figure 3. Piston motion of the engine. Table 2. Main equivalent crank angle of the engine. 151 Table 2. Main equivalent crank angle of the engine. Parameters ECA (◦CA) ExhaustParameters port open ECA (°CA) 80 IntakeExhaust port open port open 80 95 OuterIntake dead centerport open 95 154 IntakeOuter port dead close center 154 223 ExhaustIntake port port close close 223 241 InnerExhaust dead center port close 241 360 Inner dead center 360 Based on the CFD software AVL-Fire, a mesh model of central combustor was established in 152 this paper.Based As on shown the CFD in software Figure4, AVL-Fire, the mesh a model mesh model consisted of central of two combustor intake ports, was established two exhaust in ports,this 153 andpaper. a cylinder, As shown and in the Figure whole 4, the computational mesh model consisted mesh was of centrallytwo intake symmetrical. ports, two exhaust The computational ports, and a 154 meshescylinder, of theand connectionthe whole computational sections between mesh thewas intakecentrally and symmetrical. exhaust ports The computational and the cylinder meshes were 155 encryptedof the connection to obtain sections sufficient between fine meshes, the intake eliminating and exhaust the gridports e andffects the and cylinder increasing were theencrypted accuracy to of 156 obtain sufficient fine meshes, eliminating the grid effects and increasing the accuracy of the the calculation results. The mesh size was the primary factor affecting calculation results. The smaller 157 calculation results. The mesh size was the primary factor affecting calculation results. The smaller mesh size makes higher accuracy but takes a longer calculating time. In the process of the mesh 158 mesh size makes higher accuracy but takes a longer calculating time. In the process of the mesh established, the meshes were of hexahedral structure with a maximal cell size of 1 mm which can 159 established, the meshes were of hexahedral structure with a maximal cell size of 1 mm which can ensureEnergies the 2020 stability, 13, x FOR of PEER thecomputation REVIEW and save calculation time. 6 of 17 160 ensure the stability of the computation and save calculation time. 161 In the moving mesh tool Fame Engine, there are two options to define the free-piston movement. 162 One is to set the piston displacement function (connecting rod, stroke, and piston offset). The other, 163 which is adopted in the paper, was to import a readable data file. The data file had two columns of 164 numbers: one column was the ECA and the other was the piston displacement. After importing the 165 data file and defining the directions of the opposed pistons, the computation software creates the 166 moving mesh model. As shown in Figure 4a,b, there are two moving surfaces on each piston moving 167 between IDC and ODC.

168 Figure 4. Three-dimensional mesh model of the engine. 169 Figure 4. Three-dimensional mesh model of the engine. In the moving mesh tool Fame Engine, there are two options to define the free-piston movement. 170 One3.2. is Calculation to set thepiston Conditions displacement of the Engine function (connecting rod, stroke, and piston offset). The other, 171 which isBefore adopted calculation in the paper, starts, wasit is necessary to import to a set-up readable the datainitial file. conditions The data and file boundary had two conditions. columns of

172 The calculation starts at the exhaust ports’ open time (80 °CA) and ends at exhaust ports’ open time 173 of next cycle (440 °CA). Table 3 lists the initial conditions and boundary conditions tested from the 174 prototype such as pressure and temperature at 80 °CA.

175 Table 3. The initial conditions and boundary conditions.

Parameters Content Intake pressure 1.3 bar Intake temperature 320 K Exhaust pressure 1.1 bar Exhaust temperature 700 K Cylinder pressure 5.7 bar Cylinder temperature 1680 K Exhaust gas recycle (EGR) 1

176 A series of sub-models in the CFD software was employed to describe the spray and combustion 177 process clearly in the free-piston engine. Turbulent flow was the disordered airflow in the cylinder, 178 and it affected the mixture formation, flame propagation, and heat transfer in the cylinder. The k-ε 179 model was chosen as turbulence model to calculate the in-cylinder airflow. k represents turbulent 180 kinetic energy, and ε represents diffusion rate of turbulent kinetic energy. The k-ε model is widely 181 used in engineering applications due to the fact of its good adaptability and calculation stability [22]. 182 The spray process is one of the indispensable stages in the cylinder. It has a significant effect on 183 the fuel atomization, evaporation and mixing process, so it directly determines the quality of the 184 combustion process. This paper adopts an outer open-loop injector whose needle valve opens 185 outward, and the fuel is injected from the gap between the needle valve forming an umbrella spray. 186 The outer diameter and inner diameter of the nozzle were 3.80 mm and 3.76 mm, respectively. As 187 shown in Figure 5, the whole spray process consisted of four stages including spray, breakup, 188 evaporation, and wall interaction. The Taylor analogy breakup (TAB) model was used as the breakup 189 model due to the fact of its suitability for hollow cone spray, and it is based on the theory of elastic 190 mechanics which means the droplets will break up when vibrating and twisting to a certain extent. 191 Equation (1) shows the governing equation of droplets oscillation deformation: 𝑑 𝑦 𝐶 𝜌 𝑢 𝐶𝜎 𝐶𝜇 𝑑𝑦 = − 𝑦− (1) 𝑑𝑡 𝐶 𝜌 𝑟 𝜌𝑟 𝜌𝑟 𝑑𝑡

Energies 2020, 13, 3735 6 of 16

numbers: one column was the ECA and the other was the piston displacement. After importing the data ﬁle and deﬁning the directions of the opposed pistons, the computation software creates the moving mesh model. As shown in Figure4a,b, there are two moving surfaces on each piston moving between IDC and ODC.

3.2. Calculation Conditions of the Engine Before calculation starts, it is necessary to set-up the initial conditions and boundary conditions. The calculation starts at the exhaust ports’ open time (80 ◦CA) and ends at exhaust ports’ open time of next cycle (440 ◦CA). Table3 lists the initial conditions and boundary conditions tested from the prototype such as pressure and temperature at 80 ◦CA. Energies 2020, 13, x FOR PEER REVIEW 7 of 17 Table 3. The initial conditions and boundary conditions. 192 where 𝑦 is the displacement of the droplets; 𝜌 and 𝜌 represent the gas and liquid density; 𝑢 is 193 the relative velocity of the droplets;Parameters 𝑟 is the radius of the droplets; Content 𝜎 is the gas–liquid surface 194 tensions; 𝜇 represents the viscosityIntake of pressure the liquid; 𝐶, 𝐶, 𝐶, and 1.3 bar𝐶 are dimensionless constants 195 [23]. Intake temperature 320 K 196 The fuel evaporation processExhaust involves pressure gas–liquid two-phase 1.1 flow bar which is complicated and has 197 a significant effect on the gas Exhaustformation. temperature The Dukowicz model is 700adopted K as evaporation model due 198 to the fact of its advantage ofCylinder low calculation pressure cost to describe 5.7the bar mass and heat transfer of fuel Cylinder temperature 1680 K 199 evaporation. This model is based on the following assumptions: (1) the droplets are spherical Exhaust gas recycle (EGR) 1 200 symmetry; (2) quasi steady gas-film around the droplet; (3) uniform droplet temperature along the 201 droplet diameter; (4) uniform physical properties of the surrounding fluid; (5) liquid–vapor thermal 202 equilibriumA series ofon sub-models the droplet surface. in the CFD This softwaremodel assume was employed that the temperature to describe of the the spray droplets and is combustion uniform, 203 processso the clearlyrate of droplets in the free-piston temperature engine. change Turbulent is determined flow by was the the energy disordered balance airflowequation: in the cylinder, and it affected the mixture formation, flame𝑑𝑇 propagation,𝑑𝑚 and𝑑𝑄 heat transfer in the cylinder. The k-ε 𝑚 𝑐 =𝐿 + (2) model was chosen as turbulence model to calculate𝑑𝑡 the𝑑𝑡 in-cylinder𝑑𝑡 airflow. k represents turbulent ε ε kinetic energy, and represents diffusion𝑑𝑄 rate of turbulent kinetic energy. The k- model is widely =α𝐴 (𝑇 −𝑇) (3) used in engineering applications due to the𝑑𝑡 fact of its good adaptability and calculation stability [22]. The spray process is one of the indispensable stages in the cylinder. It has a significant effect 204 where 𝑚 means the mass of droplets; 𝑐 is the drag coefficient; 𝑇 is the temperature of droplets; on the fuel atomization, evaporation and mixing process, so it directly determines the quality of the 205 α represents the convective heat transfer coefficient through the film around the droplets; 𝐴 is the combustion process. This paper adopts an outer open-loop injector whose needle valve opens outward, 206 area of the droplets surface; 𝑇 means the far-field temperature; 𝑇 is the temperature of droplets and the fuel is injected from the gap between the needle valve forming an umbrella spray. The outer 207 surface [24]. 208 diameterThe and Walljet1 inner model diameter is employed of the nozzle as the wall were intera 3.80ction mm model and 3.76 due mm, to the respectively. fact of its suitability As shown for in 209 Figureheated5, thewalls, whole and it spray ignores process the mass consisted exchange of fourwith stagesoil film includingon the cylinder spray, wall breakup, [25]. The evaporation, Schmidt 210 andmodel wall interaction.is used as the The particle Taylor analogyinteraction breakup model (TAB) which model has wasadvantages used as theof high breakup computational model due to 211 theefficiency fact of its and suitability low grid for sensitivity hollow cone [26]. spray, The select and ited is sub-models based on the in theorythe spray of elastic process mechanics are listed which in 212 meansTable the 4. droplets will break up when vibrating and twisting to a certain extent.

213 214 FigureFigure 5. In-cylinder fuel fuel spray spray process. process.

215 Table 4. Sub-models of spray process.

Sub-Models Content Breakup Taylor analogy breakup (TAB) Evaporation Dukowicz Wall interaction Walljet1 Particle interaction Schmidt

216 For the simulation on spark ignition engines, there are several different ignition models. The 217 coherent flame model was applied as the flame model in this paper. The initial flame core of the 218 coherent flame model was spherical. This model had the merit of good adaptability which can be

Energies 2020, 13, 3735 7 of 16

Equation (1) shows the governing equation of droplets oscillation deformation:

2 2 d y C ρg u C σ Cdµl dy = F k y (1) 2 2 3 2 dt Cb ρ1 r − ρ1r − ρ1r dt where y is the displacement of the droplets; ρg and ρl represent the gas and liquid density; u is the relative velocity of the droplets; r is the radius of the droplets; σ is the gas–liquid surface tensions; µl represents the viscosity of the liquid; CF, Cb, Ck, and Cd are dimensionless constants [23]. The fuel evaporation process involves gas–liquid two-phase flow which is complicated and has a significant effect on the gas formation. The Dukowicz model is adopted as evaporation model due to the fact of its advantage of low calculation cost to describe the mass and heat transfer of fuel evaporation. This model is based on the following assumptions: (1) the droplets are spherical symmetry; (2) quasi steady gas-film around the droplet; (3) uniform droplet temperature along the droplet diameter; (4) uniform physical properties of the surrounding fluid; (5) liquid–vapor thermal equilibrium on the droplet surface. This model assume that the temperature of the droplets is uniform, so the rate of droplets temperature change is determined by the energy balance equation:

dT dm dQ m c d = L d + (2) d pd dt dt dt dQ = αAs(T Ts) (3) dt ∞ − where md means the mass of droplets; cpd is the drag coefficient; Td is the temperature of droplets; α represents the convective heat transfer coefficient through the film around the droplets; As is the area of the droplets surface; T means the far-field temperature; Ts is the temperature of droplets ∞ surface [24]. The Walljet1 model is employed as the wall interaction model due to the fact of its suitability for heated walls, and it ignores the mass exchange with oil film on the cylinder wall [25]. The Schmidt model is used as the particle interaction model which has advantages of high computational efficiency and low grid sensitivity [26]. The selected sub-models in the spray process are listed in Table4.

Table 4. Sub-models of spray process.

Sub-Models Content Breakup Taylor analogy breakup (TAB) Evaporation Dukowicz Wall interaction Walljet1 Particle interaction Schmidt

For the simulation on spark ignition engines, there are several different ignition models. The coherent flame model was applied as the flame model in this paper. The initial flame core of the coherent flame model was spherical. This model had the merit of good adaptability which can be used in combination with multiple combustion models. The in-cylinder combustion process of the engine was actually a complex turbulent combustion process with various chemical reactions. The common combustion model was set to simplify the complicated chemical reaction process and decompose it to corresponding reaction equations to calculate. The extended coherent flame model (ECFM) is suitable for simulating the combustion process of the spark-ignition engine, so it was adopted in the paper [27]. Table5 lists the flame model and parameters used in the paper. Energies 2020, 13, 3735 8 of 16

Table 5. Main parameters of combustion process.

Parameters Content Flame model Coherent flame/Extended coherent flame model (ECFM) Stretch factor 1.6 Initial flame surface density 630

3.3. Validation of Spray Models Choosing suitable spray models and determining their parameters are extremely important for simulating the spray process of the free-piston engine. Therefore, it is signiﬁcant to validate and verify the spray models mentioned in Table4. The validation of the fuel injection model was performed using Jose’s experimental data [28]. The initial calculation parameters of spray process are listed in Table6. Figure6 shows the tested results and simulation results of spray shape at 1.5 bar ambient pressure and 293 K ambient temperature. It is obvious that the simulated spray morphology was in great agreement with the experimental results. Consequently, the above spray models are appropriate for simulating spray process of the free-piston engine.

Table 6. Validated parameters of spray process.

Parameters Content Injection fuel Gasoline Injection time 220 ◦CA Injection mass 6.4 mg Injection ﬂow 12.5 mg/ms Ambient pressure 1.5 bar Ambient temperature 293 K Start velocity 90 m/s Energies 2020, 13, x FOR PEER REVIEWParticle sizes 20 um 9 of 17

237 Figure 6. Comparison of tested and simulated spray morphology.4. Results and Discussions. 238 Figure 6. Comparison of tested and simulated spray morphology.4. Results and Discussions.

239 4. Results and Discussions

240 4.1. Effects of Spray Angle on Spray Process 241 To clearly illustrate the effects of spray angle on spray process, five spray angles θ (30°, 35°, 40°, 242 45°, 50°) were investigated by CFD method in this paper. The initial parameters of the spray process 243 were set according to Table 6. Figure 7 shows the fuel evaporation rate of different spray angles in 244 the spray process. It can be seen that the fuel evaporation rate had all reached above 99% at ignition 245 time (340 °CA), and the highest fuel evaporation rate was 99.8% when the spray angle reached 40°. It 246 is obvious that the spray angle had a slight influence on the fuel evaporation rate. Figure 8 shows the 247 SMD of in-cylinder droplets of different spray angles. It can be found that after being discharged into 248 the cylinder, the fuel breaks into small droplets, and those small droplets come together over time, 249 so the SMD curve first decreased and then increased. At ignition time, the SMD reduced first then 250 increased as the spray angle increased from 30° to 50°. Figure 8 is consistent with the results shown 251 in Figure 7, because smaller SMD results in a higher evaporation rate [29].

252

253 Figure 7. Transient fuel evaporation rate of different spray angles.

Energies 2020, 13, x FOR PEER REVIEW 9 of 17

237

238 Energies 2020Figure, 13, 3735 6. Comparison of tested and simulated spray morphology.4. Results and Discussions. 9 of 16

239 4. Results and Discussions 4. Results and Discussions 240 4.1. Effects of Spray Angle on Spray Process 4.1. Effects of Spray Angle on Spray Process 241 To clearly illustrate the effects of spray angle on spray process, five spray angles θ (30°, 35°, 40°, To clearly illustrate the effects of spray angle on spray process, five spray angles θ (30◦, 35◦, 40◦, 242 45°, 50°) were investigated by CFD method in this paper. The initial parameters of the spray process 45◦, 50◦) were investigated by CFD method in this paper. The initial parameters of the spray process 243 were set according to Table 6. Figure 7 shows the fuel evaporation rate of different spray angles in were set according to Table6. Figure7 shows the fuel evaporation rate of di fferent spray angles in the 244 the spray process. It can be seen that the fuel evaporation rate had all reached above 99% at ignition spray process. It can be seen that the fuel evaporation rate had all reached above 99% at ignition time 245 time (340 °CA), and the highest fuel evaporation rate was 99.8% when the spray angle reached 40°. It (340 ◦CA), and the highest fuel evaporation rate was 99.8% when the spray angle reached 40◦. It is 246 is obvious that the spray angle had a slight influence on the fuel evaporation rate. Figure 8 shows the obvious that the spray angle had a slight influence on the fuel evaporation rate. Figure8 shows the 247 SMD of in-cylinder droplets of different spray angles. It can be found that after being discharged into SMD of in-cylinder droplets of different spray angles. It can be found that after being discharged into 248 the cylinder, the fuel breaks into small droplets, and those small droplets come together over time, the cylinder, the fuel breaks into small droplets, and those small droplets come together over time, 249 so the SMD curve first decreased and then increased. At ignition time, the SMD reduced first then so the SMD curve first decreased and then increased. At ignition time, the SMD reduced first then 250 increased as the spray angle increased from 30° to 50°. Figure 8 is consistent with the results shown increased as the spray angle increased from 30◦ to 50◦. Figure8 is consistent with the results shown in 251 in Figure 7, because smaller SMD results in a higher evaporation rate [29]. Figure7, because smaller SMD results in a higher evaporation rate [29].

252 Energies 2020, 13, x FOR PEER REVIEW 10 of 17 Figure 7. Transient fuel evaporation rate of diﬀerent spray angles. 253 Figure 7. Transient fuel evaporation rate of different spray angles.

254 Figure 8. Transient sauter mean diameter (SMD) of in-cylinder droplets of different spray angles. 255 Figure 8. Transient sauter mean diameter (SMD) of in-cylinder droplets of different spray angles. The whole spray process mainly includes the fuel injection process and the mixture formation 256 process.The In whole order tospray get betterprocess combustion, mainly includes the fuel the/air ratiofuel injection around the process spark plugand the should mixture be appropriate formation 257 (whichprocess. is aboutIn order 1.2), to and get that better can facilitatecombustion, flame the propagation fuel/air ratio and combustion. around the Ifspark the fuel plug concentration should be 258 appropriate (which is about 1.2), and that can facilitate flame propagation and combustion. If the fuel around the spark plug is too high, a large amount of carbon monoxide (CO) and nitrogen oxide (NOx) 259 areconcentration generated duearound to the the incomplete spark plug combustion. is too high, With a large the leanamount mixture of carbon around monoxide the spark plug,(CO) theand 260 flamenitrogen propagation oxide (NO timex) are is longergenerated and due the combustionto the incomplete speed combustion. decreases. On With account the lean of the mixture spray-guided around 261 directthe spark injection plug, system,the flame the propagation fuel is easier time to is gather longer around and thethe combustion spark plug speed at a decreases. smaller spray On account angle. 262 of the spray-guided direct injection system, the fuel is easier to gather around the spark plug at a 263 smaller spray angle. Figure 9 shows the fuel/air ratio of different spray angles at 340 °CA. It can be 264 found from the figure that as the spray angle was less than 40°, the fuel was mainly concentrated in 265 the lower part of the cylinder, while the fuel mainly accumulated in the upper part of the cylinder as 266 the spray angle was more than 40°. In addition, the fuel concentration near the spark plug became 267 lower when the spray angle increased from 30° to 50°.

268 269 Figure 9. Mixture distribution of different spray angles (340 °CA).

270 4.2. Effects of Spray Angle on Combustion Process 271 The heat release rate (HRR) of different spray angles is shown in Figure 10. It is noticed that the 272 HRR has one peak, and as the spray angle increased, the peak of the HRR appeared earlier and 273 increased first then decreased. It also can be found that the heat release of the smaller spray angle 274 was faster in the early stage of the combustion process. The peak of the HRR achieved the highest 275 value by 14.5 J/°CA when the spray angle was 35°. As the spray angle increased from 35° to 50°, the 276 peak of the HRR decreases, and the heat release process became longer. This phenomenon results 277 from that on one side, the injected fuel of a larger spray angle was more likely to concentrate on

Energies 2020, 13, x FOR PEER REVIEW 10 of 17

254

255 Figure 8. Transient sauter mean diameter (SMD) of in-cylinder droplets of different spray angles.

256 The whole spray process mainly includes the fuel injection process and the mixture formation 257 process. In order to get better combustion, the fuel/air ratio around the spark plug should be 258 appropriate (which is about 1.2), and that can facilitate flame propagation and combustion. If the fuel 259 concentration around the spark plug is too high, a large amount of carbon monoxide (CO) and 260 nitrogen oxide (NOx) are generated due to the incomplete combustion. With the lean mixture around Energies 2020, 13, 3735 10 of 16 261 the spark plug, the flame propagation time is longer and the combustion speed decreases. On account 262 of the spray-guided direct injection system, the fuel is easier to gather around the spark plug at a

263 Figuresmaller9 sprayshows angle. the fuel Figure/air ratio 9 shows of di theﬀerent fuel/air spray rati angleso of different at 340 ◦CA. spray It can angles be found at 340 from °CA. the It can ﬁgure be 264 thatfound as from the spray the figure angle that was as less the than spray 40 angle◦, the was fuel wasless than mainly 40°, concentrated the fuel was inmainly the lower concentrated part of the in 265 cylinder,the lower while part of the the fuel cylinder, mainly while accumulated the fuel mainly in the upper accumulated part of thein the cylinder upper as part the of spray the cylinder angle was as 266 morethe spray than angle 40◦. In was addition, more than the fuel40°. concentrationIn addition, th neare fuel the concentration spark plug became near the lower spark when plug the became spray 267 anglelower increasedwhen the fromspray 30 angle◦ to 50 increased◦. from 30° to 50°.

268

269 Figure 9. Mixture distribution of diffe differentrent spray anglesangles (340(340 ◦°CA).CA). 4.2. Effects of Spray Angle on Combustion Process 270 4.2. Effects of Spray Angle on Combustion Process The heat release rate (HRR) of different spray angles is shown in Figure 10. It is noticed that 271 The heat release rate (HRR) of different spray angles is shown in Figure 10. It is noticed that the the HRR has one peak, and as the spray angle increased, the peak of the HRR appeared earlier and 272 HRR has one peak, and as the spray angle increased, the peak of the HRR appeared earlier and increased first then decreased. It also can be found that the heat release of the smaller spray angle was 273 increased first then decreased. It also can be found that the heat release of the smaller spray angle faster in the early stage of the combustion process. The peak of the HRR achieved the highest value by 274 was faster in the early stage of the combustion process. The peak of the HRR achieved the highest 14.5 J/ CA when the spray angle was 35 . As the spray angle increased from 35 to 50 , the peak of the 275 value ◦by 14.5 J/°CA when the spray angle◦ was 35°. As the spray angle increased◦ from◦ 35° to 50°, the EnergiesHRR decreases, 2020, 13, x FOR and PEER the heat REVIEW release process became longer. This phenomenon results from that11 on of one 17 276 peak of the HRR decreases, and the heat release process became longer. This phenomenon results side, the injected fuel of a larger spray angle was more likely to concentrate on moving piston when 278277 movingfrom that piston on one when side, the the piston injected moves fuel to ofIDC, a larg therereby spray affecting angle thewas diffusion more likely of spray to concentrate fuel and heat on the piston moves to IDC, thereby affecting the diffusion of spray fuel and heat release process. On the 279 release process. On the other side, the fuel concentration in the upper part of the cylinder increased other side, the fuel concentration in the upper part of the cylinder increased along with the increasing 280 along with the increasing spray angle, which also makes combustion difficult. spray angle, which also makes combustion difficult.

281 Figure 10. Transient heat release rate of different spray angles. 282 Figure 10. Transient heat release rate of different spray angles. Figure 11 depicts the in-cylinder pressures of different spray angles. It can be observed that the 283 Figure 11 depicts the in-cylinder pressures of different spray angles. It can be observed that the peak pressure appeared around a crank angle of 365–370 ◦CA. The peak pressure was delayed along 284 peak pressure appeared around a crank angle of 365–370 °CA. The peak pressure was delayed along with the growing spray angle. When the spray angle was less than 45◦, the spray angle had little effect 285 with the growing spray angle. When the spray angle was less than 45°, the spray angle had little effect 286 on peak pressure, and the highest pressure was 69 bar. When the spray angle was more than 45°, the 287 peak in-cylinder pressure was more sensitive to the spray angle, indicating that the combustion speed 288 had a decrease as shown in Figure 10. 289 Figure 12 presents the in-cylinder fuel distribution of different spray angles at the time of 290 exhaust port open. It is clear that as the spray angle increased, the residual fuel was distributed 291 around the cylinder instead of gathering in the upper part of the cylinder. As the spray angle reached 292 40° or 45°, the area of unburnt fuel in the cylinder was small.

293

294 Figure 11. Transient in-cylinder pressure of different spray angles.

Energies 2020, 13, x FOR PEER REVIEW 11 of 17

278 moving piston when the piston moves to IDC, thereby affecting the diffusion of spray fuel and heat 279 release process. On the other side, the fuel concentration in the upper part of the cylinder increased 280 along with the increasing spray angle, which also makes combustion difficult.

281

282 Figure 10. Transient heat release rate of different spray angles.

283 Figure 11 depicts the in-cylinder pressures of different spray angles. It can be observed that the 284 peak pressure appeared around a crank angle of 365–370 °CA. The peak pressure was delayed along 285 with the growing spray angle. When the spray angle was less than 45°, the spray angle had little effect 286 on peak pressure, and the highest pressure was 69 bar. When the spray angle was more than 45°, the 287 peak in-cylinder pressure was more sensitive to the spray angle, indicating that the combustion speed 288 hadEnergies a decrease2020, 13, 3735 as shown in Figure 10. 11 of 16 289 Figure 12 presents the in-cylinder fuel distribution of different spray angles at the time of 290 exhaust port open. It is clear that as the spray angle increased, the residual fuel was distributed on peak pressure, and the highest pressure was 69 bar. When the spray angle was more than 45 , 291 around the cylinder instead of gathering in the upper part of the cylinder. As the spray angle reached◦ the peak in-cylinder pressure was more sensitive to the spray angle, indicating that the combustion 292 40° or 45°, the area of unburnt fuel in the cylinder was small. speed had a decrease as shown in Figure 10.

293 Figure 11. Transient in-cylinder pressure of diﬀerent spray angles. 294 Figure 11. Transient in-cylinder pressure of different spray angles. Figure 12 presents the in-cylinder fuel distribution of diﬀerent spray angles at the time of exhaust port open. It is clear that as the spray angle increased, the residual fuel was distributed around the cylinder instead of gathering in the upper part of the cylinder. As the spray angle reached 40◦ or 45◦, theEnergies area 2020 of, unburnt13, x FOR PEER fuel inREVIEW the cylinder was small. 12 of 17

295 296 Figure 12. Unburnt fuel distribution of different different spray angles (440 ◦°CA).CA). 4.3. Effects of Spray Angle on Combustion Characteristics 297 4.3. Effects of Spray Angle on Combustion Characteristics For the purpose of describing the effects of spray angle on the engine performance in detail, the 298 For the purpose of describing the effects of spray angle on the engine performance in detail, the main combustion characteristics of different spray angles are shown in Figures 13 and 14. In these 299 main combustion characteristics of different spray angles are shown in Figures 13 and 14. In these figures, CA10, CA50, and CA90 are defined as the position of the crank angle at 10%, 50%, and 90% of 300 figures, CA10, CA50, and CA90 are defined as the position of the crank angle at 10%, 50%, and 90% accumulated heat release which can be explained by Equations (4)–(6); CA10–CA90 represents the 301 of accumulated heat release which can be explained by Equations (4)–(6); CA10–CA90 represents the combustion duration. 302 combustion duration. CA10 = 360 CA (4) − 0.1Qmax 𝐶𝐴10 = 360 −𝐶𝐴. (4) CA50 = CA 360 (5) 0.5Qmax − 𝐶𝐴50 = 𝐶𝐴. − 360 (5) CA90 = CA 360 (6) 0.9Qmax − 𝐶𝐴90 = 𝐶𝐴. − 360 (6) where CA means crank angle; Qmax represents the maximum of the accumulated heat release. 303 where 𝐶𝐴 means crank angle; 𝑄 represents the maximum of the accumulated heat release. 304 As shown in Figure 13, when the spray angle increased CA50 obviously delayed, and when the 305 spray angle was 45°, CA50 was located in the optimal position which was approximately 5 °CA after 306 inner dead center (AIDC). Moreover, with the increase in the spray angle, CA10–CA90 decreased first 307 and then increased. The results are attributed to the following. When the spray angle is less than 40°, 308 the high concentration of the mixture around the spark plug causes incomplete combustion. While 309 the spray angle is more than 45°, most of fuel gathers in the upper part of the cylinder hindering 310 flame propagation and decreasing combustion speed, and some fuel is easier to collide with the 311 moving piston. All these results in longer combustion duration. When the spray angle reaches 40° or 312 45°, the combustion speed is faster resulting in shorter combustion duration and higher efficiency. 313 The combustion efficiency and the indicated thermal efficiency are shown in Figure 14 to present 314 the dynamic performance. This article adopted Equations (7) and (8) to define combustion efficiency 315 and indicated thermal efficiency: 𝜂 = · 100% (7) ·

𝑊 𝜂 = · 100% (8) 𝑄

316 where 𝜂 and 𝜂 represents the combustion efficiency and indicated thermal efficiency respectively; 317 𝑄 as the accumulated heat release; 𝐻 as the low heating value of fuel; 𝑚 is the mass of the fuel 318 injected in the cylinder; 𝑊 represents the work of piston; 𝑄 represents the heat released from 319 fuel burnt. 320 It is obvious that the best combustion efficiency was located at the spray angle of 40°, and the 321 indicated thermal efficiency of that spray angle also achieved the highest value. It can be found that 322 the spray angle had a greater effect on combustion efficiency and indicated thermal efficiency. The 323 exhaustive parameters of Figures 13 and 14 are listed in Table 7.

Energies 2020, 13, x FOR PEER REVIEW 13 of 17 EnergiesEnergies 20202020,, 1313,, x 3735 FOR PEER REVIEW 1312 of of 17 16

324 324 325 Figure 13. Crank Angle 50 and Crank Angle 10–90 of different spray angles. Figure 13. Crank Angle 50 and Crank Angle 10–90 of diﬀerent spray angles. 325 Figure 13. Crank Angle 50 and Crank Angle 10–90 of different spray angles.

326 326327 FigureFigure 14. CombustionCombustion efficiency efficiency and and indicated indicated ther thermalmal efficiency efficiency of different di fferent spray angles. 327 Figure 14. Combustion efficiency and indicated thermal efficiency of different spray angles. As shown in Figure 13, when the spray angle increased CA50 obviously delayed, and when the 328 Table 7. Performance of the engine. spray angle was 45 , CA50 was located in the optimal position which was approximately 5 CA after 328 ◦ Table 7. Performance of the engine. ◦ inner dead center (AIDC).CA10 Moreover, withCA50 the increase in theCA90 spray angle, CA10–CA90 decreased first Spray Combustion Indicated and then increased. (°CACA10 The Before results are attributed(°CACA50 After to the following. (°CACA90 After When the spray angle is less than 40 , SprayAngle CombustionEfficiency IndicatedEfficiency ◦ the high concentration (°CAInner Before Dead of the mixture(°CAInner around After Dead the spark plugInner(°CA causes AfterDead incomplete combustion. While the Angle(°) Efficiency(%) Efficiency(%) spray angle is moreInnerCenter) than Dead 45 , mostInner ofCenter) fuel Dead gathers in theInnerCenter) upper Dead part of the cylinder hindering flame (°) ◦ (%) (%) propagation30 andCenter) decreasing6.2 combustionCenter) 2.4 speed, and someCenter) 23.0 fuel is easier to95.0 collide with the38.2 moving piston.3035 All these results6.26.0 in longer combustion 2.4 2.4 duration. 23.0 20.5When the spray95.097.0 angle reaches 4038.239.3◦ or 45◦, the combustion3540 speed6.06.2 is faster resulting 2.4 in3.2 shorter combustion 20.5 19.2 duration and97.097.5 higher efficiency.39.339.7 4045The combustion6.25.2 effi ciency and the indicated 3.2 4.8 thermal e 19.2 19.8fficiency are shown97.596.7 in Figure 14 39.7to39.6 present the dynamic4550 performance.5.22.2 This article adopted 4.8 9.8 Equations 19.8 (7)30.0 and (8) to define96.793.3 combustion 39.637.2 efficiency and50 indicated thermal2.2 efficiency: 9.8 30.0 93.3 37.2 329 4.4. Effects of Spray Angle on Emissions Q η = 100% (7) 329 4.4. Effects of Spray Angle on Emissions Hu m f uel · 330 The CO emissions, one of the main factors· to describe the engine performance, are discussed 330331 below.The The CO CO emissions, distribution one of of diffe the rentmain spray factors angles Wtoi describe at 440 °CA the isengine shown performance, in Figure 15. Clearly,are discussed as the ηi = 100% (8) 331332 below.spray angleThe CO increased distribution from of 30°diffe torent 40°, spray the anglesCOQ fwas uel at· 440distributed °CA is sh aroundown in theFigure cylinder, 15. Clearly, and itas wasthe 332333 spraydistributed angle inincreased the upper from part 30°of the to cylinder40°, the asCO the was spray distributed angle continued around to the increase. cylinder, and it was where η and ηi represents the combustion efficiency and indicated thermal efficiency respectively; Q as 333 distributed in the upper part of the cylinder as the spray angle continued to increase. the accumulated heat release; Hu as the low heating value of fuel; m f uel is the mass of the fuel injected in the cylinder; Wi represents the work of piston; Q f uel represents the heat released from fuel burnt. It is obvious that the best combustion efficiency was located at the spray angle of 40◦, and the indicated thermal efficiency of that spray angle also achieved the highest value. It can be found that the spray angle had a greater effect on combustion efficiency and indicated thermal efficiency. The exhaustive parameters of Figures 13 and 14 are listed in Table7.

Energies 2020, 13, 3735 13 of 16

Table 7. Performance of the engine.

CA10 CA50 CA90 Spray Angle Combustion Indicated ( CA Before Inner ( CA After Inner ( CA After Inner ( ) ◦ ◦ ◦ Eﬃciency (%) Eﬃciency (%) ◦ Dead Center) Dead Center) Dead Center) 30 6.2 2.4 23.0 95.0 38.2 35 6.0 2.4 20.5 97.0 39.3 40 6.2 3.2 19.2 97.5 39.7 45 5.2 4.8 19.8 96.7 39.6 50 2.2 9.8 30.0 93.3 37.2

4.4. Eﬀects of Spray Angle on Emissions The CO emissions, one of the main factors to describe the engine performance, are discussed below. The CO distribution of diﬀerent spray angles at 440 ◦CA is shown in Figure 15. Clearly, as the spray angle increased from 30◦ to 40◦, the CO was distributed around the cylinder, and it was distributed in theEnergies upper 2020 part, 13, x of FOR the PEER cylinder REVIEW as the spray angle continued to increase. 14 of 17

334 Figure 15. CO distribution of different spray angles (440 CA). 335 Figure 15. CO distribution of different spray angles (440 ◦°CA). Figure 16 displays CO emissions curve in detail. It shows that at 440 CA, the CO emissions 336 Figure 16 displays CO emissions curve in detail. It shows that at 440 ◦°CA, the CO emissions decreased first then increased, as the spray angle increased from 30 to 50 . This phenomenon can 337 decreased first then increased, as the spray angle increased from 30° ◦to 50°. ◦This phenomenon can be be explained as follows. The CO is sensitive to fuel/air ratio and temperature. It can be seen from 338 explained as follows. The CO is sensitive to fuel/air ratio and temperature. It can be seen from Figure Figure9, that as the spray angle increased from 30 to 40 , the mixture around the spark plug became 339 9, that as the spray angle increased from 30° to 40°,◦ the mixture◦ around the spark plug became thinner, thinner, so the CO emissions decreased. But when the spray angle continued to increase, the fuel 340 so the CO emissions decreased. But when the spray angle continued to increase, the fuel mainly mainly accumulated in the upper part of the cylinder. High concentration of the mixture promotes 341 accumulated in the upper part of the cylinder. High concentration of the mixture promotes CO CO production, so the CO emissions increase as the spray angle increases from 40 to 50 . Another 342 production, so the CO emissions increase as the spray angle increases from 40° to 50°.◦ Another◦ reason reason is that when the spray angle increases, the in-cylinder temperature rises first and then decreases, 343 is that when the spray angle increases, the in-cylinder temperature rises first and then decreases, which is shown in Figure 17. The CO is mainly produced by Reaction (9) and consumed by Reaction 344 which is shown in Figure 17. The CO is mainly produced by Reaction (9) and consumed by Reaction (10). In addition, Reaction (10) is dependent on the in-cylinder temperature and requires particularly 345 (10). In addition, Reaction (10) is dependent on the in-cylinder temperature and requires particularly high energy [20]. But at a higher temperature, it is easier to promote NOx formation. It is difficult to 346 high energy [20]. But at a higher temperature, it is easier to promote NOx formation. It is difficult to transform CO to carbon dioxide (CO2) at a lower temperature. It can be seen that the spray angle had 347 transform CO to carbon dioxide (CO2) at a lower temperature. It can be seen that the spray angle had a slight influence on the in-cylinder temperature as the spray angle was less than 50 . Therefore, the 348 a slight influence on the in-cylinder temperature as the spray angle was less than 50°.◦ Therefore, the temperature was not the dominant reason why the CO emissions were high. It is easy to find that the 349 temperature was not the dominant reason why the CO emissions were high. It is easy to find that the CO emissions achieve the lowest value when the spray angle is 40 . 350 CO emissions achieve the lowest value when the spray angle is 40°.◦ + = + HCOHCO + OO2 =CO CO + HO2 (9)(9) CO + OH = CO + H (10) CO + OH = CO2 + H (10)

351

352 Figure 16. CO mass fraction of different spray angles.

Energies 2020, 13, x FOR PEER REVIEW 14 of 17

334 335 Figure 15. CO distribution of different spray angles (440 °CA).

336 Figure 16 displays CO emissions curve in detail. It shows that at 440 °CA, the CO emissions 337 decreased first then increased, as the spray angle increased from 30° to 50°. This phenomenon can be 338 explained as follows. The CO is sensitive to fuel/air ratio and temperature. It can be seen from Figure 339 9, that as the spray angle increased from 30° to 40°, the mixture around the spark plug became thinner, 340 so the CO emissions decreased. But when the spray angle continued to increase, the fuel mainly 341 accumulated in the upper part of the cylinder. High concentration of the mixture promotes CO 342 production, so the CO emissions increase as the spray angle increases from 40° to 50°. Another reason 343 is that when the spray angle increases, the in-cylinder temperature rises first and then decreases, 344 which is shown in Figure 17. The CO is mainly produced by Reaction (9) and consumed by Reaction 345 (10). In addition, Reaction (10) is dependent on the in-cylinder temperature and requires particularly 346 high energy [20]. But at a higher temperature, it is easier to promote NOx formation. It is difficult to 347 transform CO to carbon dioxide (CO2) at a lower temperature. It can be seen that the spray angle had 348 a slight influence on the in-cylinder temperature as the spray angle was less than 50°. Therefore, the 349 temperature was not the dominant reason why the CO emissions were high. It is easy to find that the 350 CO emissions achieve the lowest value when the spray angle is 40°.

HCO + O = CO + HO (9) Energies 2020, 13, 3735 14 of 16 CO + OH = CO + H (10)

351 Energies 2020, 13, x FOR PEER REVIEW 15 of 17 Figure 16. CO mass fraction of diﬀerent spray angles. 352 Figure 16. CO mass fraction of different spray angles.

353 Figure 17. Transient in-cylinder temperature of different spray angles. 354 Figure 17. Transient in-cylinder temperature of different spray angles. 5. Conclusions 355 5. Conclusions The effects of injector spray angle on performance of an opposed-piston free-piston engine is 356 investigatedThe effects and of optimized injector byspray using angle CFD on method performanc in thise study.of an opposed-piston Working principle free-piston and spray-guided engine is 357 centralinvestigated combustor and optimized structure by of using the engine CFD method are discussed. in this study. The Working CFD model principle with and moving spray-guided mesh is 358 establishedcentral combustor based on structure piston motion of the of engine the prototype. are discussed. According The to CFD the simulatedmodel with results, moving the e ffmeshects ofis 359 sprayestablished angle onbased spray on process,piston motion combustion of the process,prototype. combustion According characteristics, to the simulated and results, emissions the effects of the 360 free-pistonof spray angle engine on werespray analyzed process, incombustion detail. The proces conclusionss, combustion are as follows: characteristics, and emissions of 361 the free-piston engine were analyzed in detail. The conclusions are as follows: (1) The spray angle had a slight influence on the fuel evaporation rate; 362 (2)(1) WhenThe spray the sprayangle anglehad a increased, slight influe thence fuel on concentration the fuel evaporation near the sparkrate; plug decreased, and that in 363 (2) theWhen upper the partspray of angle the cylinder increased, became the fuel higher; concentration near the spark plug decreased, and that 364 in the upper part of the cylinder became higher; (3) The heat release of smaller spray angle was faster in the early stage of the combustion process; 365 (3) The heat release of smaller spray angle was faster in the early stage of the combustion process; (4) The peak in-cylinder pressure kept minor fluctuations as the spray angle was less than 45 , and it 366 (4) The peak in-cylinder pressure kept minor fluctuations as the spray angle was less than ◦45°, and decreased when the spray angle reached 50 ; 367 it decreased when the spray angle reached ◦50°; (5) As the spray angle reached 40 , the combustion efficiency and indicated thermal efficiency achieve 368 (5) As the spray angle reached◦ 40°, the combustion efficiency and indicated thermal efficiency the highest value of 97.5% and 39.7% compared with other spray angles (30 , 35 , 45 , 50 ); 369 achieve the highest value of 97.5% and 39.7% compared with other spray ◦angles◦ (30°,◦ 35°,◦ 45°, (6) From an emissions perspective, CO emissions decreased first and then increased with increasing 370 50°); spray angle. When the spray angle was 40 , the CO emissions achieved the lowest value. In order 371 (6) From an emissions perspective, CO emissions◦ decreased first and then increased with increasing to obtain a better performance and keep lower CO emissions, the optimal spray angle should be 372 spray angle. When the spray angle was 40°, the CO emissions achieved the lowest value. In order approximately 40 . 373 to obtain a better ◦performance and keep lower CO emissions, the optimal spray angle should be 374 approximately 40°. 375 Further study is still essential to achieve energy savings and higher efficiency of the free-piston 376 engine. In the next stage, the performance of the free-piston engine will be improved by optimizing 377 the intake and exhaust system.

378 Author Contributions: Supervision, Z.X. and L.L.; Investigation, Software, Original draft—editing, Q.Z.; 379 Writing—review, S.L.

380 Funding: This work was supported by the National Natural Science Foundation of China (Grant No. 51875290 381 and No. 51975297).

382 Conflicts of Interest: The authors declare no conflict of interest.

383 References

384 1. Lee, C.H.; Reitz, R.D. CFD simulations of diesel spray tip penetration with multiple injections and with 385 engine compression ratios up to 100:1. Fuel 2013, 111, 289–297, doi:10.1016/j.fuel.2013.04.058. 386 2. Niculae, A.L.; Miron, L.; Chiriac, R. On the possibility to simulate the operation of a SI engine using 387 alternative gaseous fuels. Energy Rep. 2020, 6, 167–176, doi:10.1016/j.egyr.2019.10.035.

Energies 2020, 13, 3735 15 of 16

Further study is still essential to achieve energy savings and higher eﬃciency of the free-piston engine. In the next stage, the performance of the free-piston engine will be improved by optimizing the intake and exhaust system.

Author Contributions: Supervision, Z.X. and L.L.; Investigation, Software, Original draft—editing, Q.Z.; Writing—review, S.L. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (Grant No. 51875290 and No. 51975297). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Lee, C.H.; Reitz, R.D. CFD simulations of diesel spray tip penetration with multiple injections and with engine compression ratios up to 100:1. Fuel 2013, 111, 289–297. [CrossRef] 2. Niculae, A.L.; Miron, L.; Chiriac, R. On the possibility to simulate the operation of a SI engine using alternative gaseous fuels. Energy Rep. 2020, 6, 167–176. [CrossRef] 3. Wang, Q.; Wu, F.; Zhao, Y.; Bai, J.; Huang, R. Study on combustion characteristics and ignition limits extending of micro free-piston engines. Energy 2019, 179, 805–814. [CrossRef] 4. Yuan, C.; Han, C.; Xu, J. Numerical evaluation of pilot-ignition technology used for a hydrogen fuelled free piston engine generator. Int. J. Hydrogen Energy 2017, 42, 28599–28611. [CrossRef] 5. Sofianopoulos, A.; Zhou, Y.; Lawler, B.; Mamalis, S. Gas exchange processes of a small HCCI free piston engine—A computational study. Appl. Therm. Eng. 2017, 127, 1582–1597. [CrossRef] 6. Feng, H.; Guo, C.; Yuan, C.; Guo, Y.; Zuo, Z.; Roskilly, A.P.; Jia, B. Research on combustion process of a free piston diesel linear generator. Appl. Energy 2016, 161, 395–403. [CrossRef] 7. Geng, H.; Wang, Y.; Xi, B.; Li, Z.; Zhen, X.; Fu, C.; Hu, Y. Study on HCCI combustion improvement by using dual assisted compression ignition (DACI) on a hydraulic free piston engine fueled with methanol fuel. Appl. Therm. Eng. 2020, 167, 114782. [CrossRef] 8. Duronio, F.; De Vita, A.; Montanaro, A.; Villante, C. Gasoline direct injection engines—A review of latest technologies and trends. Part 2. Fuel 2020, 265, 116947. [CrossRef] 9. Raju, V.R.K.; Rao, S.S. Effect of Fuel Injection Pressure and Spray Cone Angle in DI Diesel Engine Using CONVERGETM CFD Code. Procedia Eng. 2015, 127, 295–300. [CrossRef] 10. Yuan, C.; Liu, Y.; Han, C.; He, Y. An investigation of mixture formation characteristics of a free-piston gasoline engine with direct-injection. Energy 2019, 173, 626–636. [CrossRef] 11. Guo, C.; Song, Y.; Feng, H.; Zuo, Z.; Jia, B.; Zhang, Z.; Roskilly, A. Effect of fuel injection characteristics on the performance of a free-piston diesel engine linear generator: CFD simulation and experimental results. Energy Convers. Manag. 2018, 160, 302–312. [CrossRef] 12. Huang, H.; Guo, X.; Huang, R.; Li, J.; Pan, M.; Chen, Y.; Pan, X. Assessment of n-pentanol additive and EGR rates effects on spray characteristics, energy distribution and engine performance. Energy Convers. Manag. 2019, 202, 112210. [CrossRef] 13. Shrivastava, P.; Verma, T.N. Effect of fuel injection pressure on the characteristics of CI engine fuelled with biodiesel from Roselle oil. Fuel 2020, 265, 117005. [CrossRef] 14. Shi, Z.; Lee, C.-F.; Wu, H.; Li, H.; Wu, Y.; Zhang, L.; Bo, Y.; Liu, F. Effect of injection pressure on the impinging spray and ignition characteristics of the heavy-duty diesel engine under low-temperature conditions. Appl. Energy 2020, 262, 114552. [CrossRef] 15. Shi, L.; Ji, C.; Wang, S.; Su, T.; Cong, X.; Wang, D.; Tang, C. Effects of second injection timing on combustion characteristics of the spark ignition direct injection gasoline engines with dimethyl ether enrichment in the intake port. Energy 2019, 180, 10–18. [CrossRef] 16. Han, T.; Singh, R.; Lavoie, G.; Woolridge, M.S.; Boehman, A.L. Multiple injection for improving knock, gaseous and particulate matter emissions in direct injection SI engines. Appl. Energy 2020, 262, 114578. [CrossRef] 17. Shu, J.; Fu, J.; Liu, J.; Ma, Y.; Wang, S.; Deng, B.; Zeng, D. Effects of injector spray angle on combustion and emissions characteristics of a natural gas (NG)-diesel dual fuel engine based on CFD coupled with reduced chemical kinetic model. Appl. Energy 2019, 234, 182–195. [CrossRef] Energies 2020, 13, 3735 16 of 16

18. Shmelev, V. An internal combustion alternator with both free piston and cylinder. Eng. Sci. Technol. Int. J. 2019, 22, 947–955. [CrossRef] 19. Jia, B.; Mikalsen, R.; Smallbone, A.; Roskilly, A.P. A study and comparison of frictional losses in free-piston engine and crankshaft engines. Appl. Therm. Eng. 2018, 140, 217–224. [CrossRef] 20. Yu, H.; Xu, Z.; Zhang, Q.; Liu, L.; Hua, R. Two-Stroke Thermodynamic Cycle Optimization of a Single-Cylinder Free-Piston Engine Generator. Adv. Mater. Sci. Eng. 2019, 2019, 1–11. [CrossRef] 21. Ciampolini, M.; Bigalli, S.; Balduzzi, F.; Bianchini, A.; Romani, L.; Ferrara, G. CFD Analysis of the Fuel–Air Mixture Formation Process in Passive Prechambers for Use in a High-Pressure Direct Injection (HPDI) Two-Stroke Engine. Energies 2020, 13, 2846. [CrossRef] 22. Saaidia, R.; Jemni, M.A.; Abid, M.S. Simulation and Empirical Studies of the Commercial SI Engine Performance and Its Emission Levels When Running on a CNG and Hydrogen Blend. Energies 2017, 11, 29. [CrossRef] 23. O’Rourke, P.J.; Amsden, A.A. The Tab Method for Numerical Calculation of Spray Droplet Break-Up; SAE Paper 872089; SAE: Warrendale, PA, USA, 1987; Volume 87, pp. 1–10. 24. Dukowicz, J.K. A particle-fluid numerical model for liquid sprays. J. Comput. Phys. 1980, 35, 229–253. [CrossRef] 25. Naber, J.; Reitz, R.D. Modeling Engine Spray/Wall Impingement. SAE Tech. Pap. Ser. 1988, 88, 118–140. [CrossRef] 26. O’Rourke, P.J. Statistical properties and numerical implementation of a model for droplet dispersion in a turbulent gas. J. Comput. Phys. 1989, 83, 345–360. [CrossRef] 27. Nishida, K.; Hiroyasu, H. Simplified Three-Dimensional Modeling of Mixture Formation and Combustion in a D.I. Diesel Engine. SAE Tech. Pap. Ser. 1989, 1, 276–293. [CrossRef] 28. Jose, R.I.; Cervantes, T. Direct Injection System for a Two Stroke Engine. Master’s Thesis, Chalmers University of Technology, Gothenburg, Sweden, 2011. 29. Guo, H.; Ma, X.; Li, Y.; Liang, S.; Wang, Z.; Xu, H.; Wang, J. Effect of flash boiling on microscopic and macroscopic spray characteristics in optical GDI engine. Fuel 2017, 190, 79–89. [CrossRef]

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