energies

Article Effect of Spark Ignition Timing and Water Injection Temperature on the Knock Combustion of a GDI Engine

Aqian Li and Zhaolei Zheng *

Key Laboratory of Low-grade Energy Utilization Technologies and System, Ministry of Education, Chongqing University, Chongqing 400044, China; [email protected] * Correspondence: [email protected]; Tel./Fax: +86-023-6510-2473



Received: 14 August 2020; Accepted: 16 September 2020; Published: 20 September 2020


Abstract: A turbocharged downsizing spark ignition (SI) engine cooperating with an in-cylinder direct injection technology is one of the most effective ways to improve the power and economy of gasoline engines. However, engine knock has limited the application and development of the downsizing of gasoline engines. Water injection technology can effectively suppress the knock. In this study, a method of numerical simulation was used to explore the effect of the water injection temperature on the combustion and suppression of the knock. First of all, the knock of the gasoline engine was induced by advancing the spark timing. Then, when the other conditions were the same, different water injection temperatures were set. The results show that lowering the water injection temperature reduced the knock intensity in the cylinder, but increasing the water injection temperature made the water distribution more uniform, and the peak values of each monitoring point were more consistent. The circulating work power increased with the increase of the water injection temperature. For emissions, as the temperature of the water injection increased, the emissions of soot and unburned hydrocarbons (UHCs) decreased, and NOx slightly increased.

Keywords: water injection; temperature; knock; combustion; emission

1. Introduction At present, improving engine performance and reducing emissions are the main directions of internal combustion engine research [1,2]. A turbocharged downsizing spark ignition (SI) engine cooperating with direct injection technology in the cylinder can effectively improve the performance of a gasoline engine. However, the downsizing technology will increase the maximum temperature and pressure in the cylinder, which easily causes engine knock. The spontaneous combustion of the terminal mixture will cause a violent chemical reaction, and the large amount of heat emitted will rapidly raise the temperature of the mixture in the surrounding area and form a pressure shockwave. The shockwave is continuously reflected and superimposed in the cylinder and is finally transmitted to the piston, connecting rod, cylinder wall, and other components, which ultimately leads to damage to the engine’s mechanical components [3–5]. Therefore, knock has become the main limiting factor for the development of downsizing technology [6–8]. Water injection technology is a direct and simple method to suppress knock and reduce emissions [9–11]. The specific heat capacity of water is large. Liquid water absorbs the heat in the cylinder at a high temperature and then evaporates into water vapor, which can effectively reduce the temperature and pressure. This can effectively reduce the tendency to produce knock [12–14]. In addition, water injection can significantly reduce the temperature and pressure in the cylinder, where water vapor fills the combustion chamber and dilutes the air in the cylinder, and water injection

Energies 2020, 13, 4931; doi:10.3390/en13184931 www.mdpi.com/journal/energies Energies 2020, 13, 4931 2 of 24 can increase the disturbance in the cylinder such that the air and fuel mix more evenly, reducing the possibility of local oxygen enrichment. These are all conducive to reducing NOx emissions [15–17]. If water is injected into the cylinder during the compression stroke, the temperature and pressure can be reduced, thereby reducing the compression work. Water vapor can be used as a working fluid in the expansion stroke to do work externally, thereby improving the thermal efficiency of the gasoline engine. The lower temperature in the cylinder can reduce the heat dissipated from the cylinder wall. The heat radiation ability of water is strong, which can increase the heat transfer rate in the cylinder, improve the energy utilization efficiency, and achieve the effect of energy saving [18–20]. The research on water injection for suppressing knock can be traced back to the research of Harry Ricardo in the early 1930s. Water injection technology has also been applied to some racing cars. Later, the emergence of intercoolers gradually reduced people’s attention to water injection technology. However, the cooling effect of the intercooler cannot meet the needs of the development of downsizing technology. Water injection technology has regained the attention of researchers. With the development of computational combustion dynamics and chemical dynamics, the applied research of water injection technology in internal combustion engines has also gradually deepened [21–24]. There are three main types of water injection technology: fuel–water emulsion, water injection in the intake port, and direct water injection in the cylinder. The fuel–water emulsification method first prepares a stable suspension of gasoline, emulsion, and water, and then the injection system injects the emulsion into the cylinder. However, the emulsification method has the following disadvantages: the emulsification process is very complicated, the stability of the emulsified fuel also requires a lot of work, the price of the emulsifier is expensive, the ratio of water to fuel is fixed, and the quantity of water cannot be adjusted according to the working conditions of the gasoline engine [25–27]. The advantage of water injection in the intake port is that there is basically no need to change the structure of the gasoline engine, and the water quantity can be adjusted. However, liquid water absorbs heat in the intake duct and evaporates into water vapor, which will take up a part of the volume and affect the volumetric efficiency of the gasoline engine. Therefore, water injection in the intake port is limited by the maximum water injection quantity, that is, the water injection volume cannot be too large [19,28,29]. Direct water injection into the cylinder is based on the original injection system but need to add a set of water injection equipment. The water is injected directly into the cylinder and the cooling effect is better than that of water injection in the intake port. In addition, the direct water injection in the cylinder is not limited by the maximum water injection quantity, and the water–fuel ratio can be changed with the change of working conditions. The application of water injection to diesel engines was earlier than that of gasoline engines. The compression ratio and heat load of diesel engines are much larger than those of gasoline engines. Increasing the compression ratio can improve the power performance and fuel economy of diesel engines. However, this also led to a sharp increase in the temperature and pressure in the cylinder, and the trend of knock became more and more obvious. Water injection is a simple and effective method that is used to suppress knock, which has attracted the attention of many scholars. Water injection mainly uses water to absorb the heat in the cylinder and reduce the temperature and pressure in the cylinder, thereby reducing the tendency toward producing knock. Due to the continuous development of the downsizing technology of gasoline engines, the research on the suppression of knock using water injection in gasoline engines has also continued to deepen. The study of water injection into diesel engines also gives many references for the research of water injection into gasoline engines. The characteristics of various water injection methods can be further understood from the research on diesel engine water injection. Abu-Zaid [27] explored the effects of different forms of water–fuel emulsions on the performance of direct-injection diesel engines through experiments. The results show that adding water in the form of an emulsion can improve combustion efficiency. As the quantity of the water in the emulsion increases, the engine torque, power, and brake thermal efficiency also increase. In the range of speeds studied, the efficiency of the brakes using a 20% water emulsion was 3.5% higher than that of diesel without water. Niko et al. [30] conducted experimental and numerical studies on Energies 2020, 13, 4931 3 of 24 some chemical and physical properties of the combustion characteristics of water/oil emulsified fuel (W/OEF). It was found that in diesel engines at several speeds and loads, when using 10% and 15% W/OEF, NOx can be reduced by up to 20%, the concentration of soot is reduced by about 50%, and the specific fuel consumption is hardly increased. Chen et al. [31] studied the influence of inlet water injection on gasoline engine operations through experiments and simulations, where the results showed that as the proportion of water injection increases, the increase in thermal efficiency also increases. Miganakallu et al. [32] used experiments to establish whether a water–methanol mixture was better than pure water or pure methanol in terms of engine performance and found that the water–methanol blend did not show a better performance than pure water or pure methanol. In terms of the combustion time, combustion stability, specific fuel consumption, and exhaust gas temperature, the characteristics of a water–methanol mixture are between pure water and pure methanol. Pure water injection enables an engine to achieve the lowest specific fuel consumption when operating within the controlled knock limit. Chen et al. [33] experimentally explored the effect of spark timing and inlet water injection on natural gas combustion and emissions and found that water injection reduces the combustion speed, resulting in a decrease in the pressure peak, heat release rate, and combustion temperature in the cylinder. As the combustion temperature decreases, the thermal load of the engine decreases. The emission of BSNOx (brake specific nitrogen oxide) decreases with an increase in the quantity of the injected water, while the mass of BSTHC (brake specific total hydrocarbon) and BSCO (brake specific carbon monoxide) increases slightly. Brusca et al. [15] explored the effect of water injection in the inlet on suppressing knock and reducing emissions through experiments. The results show that water injection can effectively improve the anti-knock resistance of fuel, and water injection can reduce the temperature in the cylinder and thus reduce NOx emissions. A gasoline engine can adopt a higher compression ratio after water injection. Valero-Marco et al. [34] studied the potential of direct water injection into a cylinder to expand the operating range toward higher loads via experiments. It was found that water injected into a cylinder can absorb the heat released by the combustion in the cylinder, which can effectively reduce the maximum temperature and pressure, and can also reduce the pressure gradient and the knock tendency of the combustion process. Water injection is an effective strategy for increasing the maximum load. However, this may result in a loss of combustion stability. Zhao et al. [35] used simulation techniques to study the fuel-saving potentials of different water/steam injection layouts. Steam injection into a cylinder can produce up to 10% fuel reduction and steam injection at a turbocharger turbine inlet can reduce the brake specific fuel consumption (BSFC) by 2.3–4.7%. Due to the higher enthalpy introduced in the cylinder, as the steam mass ratio increases, the fuel consumption of the engine is significantly reduced. However, the maximum steam mass ratio is limited by the maximum permitted in-cylinder pressure. Arabaci et al. [36] used experimental methods to explore the effect of direct water injection in the cylinder on the engine performance of a six-stroke engine. The results show that after direct water injection into the cylinder, the specific fuel and exhaust temperatures were reduced by 9% and 7%, respectively. The thermal efficiency increased by about 8.72%. NOx and other emissions were also significantly reduced. However, the power output and fuel consumption increased by 10% and 2% after water injection, respectively. From the studies above, it can be concluded that direct water injection in the cylinder has great potential for suppressing knock trends, improving engine performance and thermal efficiency, and reducing the emission of NOx and other pollutants. Not only can it improve the power and economy of the engine but it can also promote energy saving and emissions reductions. However, there are few studies on the effect of water injection on the knock of gasoline engines under high-speed and high-load conditions. The injection water temperature not only affects the heat absorbed by the water but may also affect the distribution of the water, which in turn affects the combustion and emissions. The physical and chemical effects of water injection on combustion have not yet been unified. This study used a simulation method to explore the influence of advancing the spark timing Energies 2020, 13, 4931 4 of 24 on knock and the influence of water injection temperature on knock suppression, gasoline engine power performance, and emissions under high-speed and high-load gasoline engine conditions.

2. Materials and Methods

2.1. Basic Parameters and Working Conditions In order to investigate the effect of temperature on knock under high-speed and high-load conditions, the speed of the gasoline engine was selected to be 5500 rpm, and the throttle valve was fully opened. The basic parameters and operating conditions of the gasoline direct injection (GDI) engine are shown in Tables1 and2, respectively. The parameters and experimental conditions of this GDI engine and its model are the same as those in a previously published paper exploring the effect of water injection quantity on knock [37].

Table 1. Basic parameters of the gasoline direct injection (GDI) engine [37].

Parameter Numerical Value Number of cylinders 1 Bore 86 mm Stroke 86 mm Compression ratio 9.5 Connecting rod length 142.8 mm Displacement 0.5 L

Table 2. Operating conditions of the GDI engine [37].

Parameter Numerical Value Rotating speed 5500 rev/min Fuel injection time 337.99 CA − ◦ Fuel injection duration 160.908 ◦CA Injection quantity 82.31 mg Spark timing 11 CA − ◦

2.2. Submodels of the Numerical Simulation In the numerical simulation of the engine, each submodel needed to be determined, such as the turbulence model, spray model, and combustion model. The flow situation in the cylinder was very complicated and included complex phenomena, such as fluid flow, chemical reactions, and heat and mass transfer. For the specific introduction of some submodels, please refer to the previous paper [37]. The models of the two papers are the same. The submodels in this paper are shown in Table3.

Table 3. Submodels of the numerical simulation [37].

Model Setup Used Turbulence model k-ε double equation model RT–KH (Kehrin–Helmholz and Fuel broken model Reyleigt–Taylor) broken model Collision model NTC (no time counter collision) model Fuel wall model Wall film model Combustion model Sagemodel NOx model Extended Zeldovich model Soot model Hiroyasu model

The combustion model in this study was the sagemodel, which is unique to the software (Converge, v2.3, Convergent Science, Madison City, WI, USA). The SAGE model can adapt well to various chemical reaction mechanisms, and the calculation is relatively accurate. The chemical reaction mechanism Energies 2020, 13, 4931 5 of 24 is Jia Ming’s one-component mechanism, which includes 41 components and 124 reactions [38]. The mechanism was verified against various experimental data, including shock tube, flow reactor, premixed laminar flame speed, and internal combustion engines over a wide range of temperatures, pressures, and equivalence ratios. The results show that the mechanism was in good agreement with the experimental data. The SAGE model coupled with the single-component mechanism proposed by Jia Ming et al. can reflect the combustion process well. The submodels used in this study are shown in Table3.

2.3. Initial and Boundary Conditions The initial conditions were defined in terms of the temperature, pressure, composition of each substance, etc. For example, at the beginning of the calculation, the temperature, pressure, and turbulent kinetic energy in the cylinder were 1099 K, 2.769 bar, and 1.0 m2/s2, respectively. The boundary conditions required in this paper included the various boundaries of the engine (fixed wall, intake, exhaust), temperature, and pressure. Table4 shows the specific boundary conditions. The starting time of the numerical simulation calculation was 366 CA. At this time, the exhaust − ◦ valve was about to close and the intake valve is about to open.

Table 4. Boundary conditions [37].

Parameter Numerical Value Temperature at the top of the piston 585.0 K Cylinder liner wall temperature 485.0 K Temperature of combustion chamber wall 550.0 K Temperature of exhaust wall 1100.0 K Exhaust valve wall temperature 1100.0 K Inlet wall temperature 308.0 K Temperature of intake valve wall 800.0 K Spark plug temperature 1050.0 K Static pressure of exhaust port 101,325 Pa

2.4. Model Verification It was necessary to verify the reliability of the model, including the irrelevance of the model and the consistency of the simulation results and experimental results. Encrypting or coarsening the grid to a specified time and space can save calculation time while ensuring accurate calculations. Therefore, the intake and exhaust passages used a thick grid; the cylinder head, intake valve, spark plug, and other parts adopted a fine mesh. Then, adaptive encryption was used for the pressure and temperature of the flow field in the entire cylinder. When the pressure or temperature gradient in the cylinder area exceeded the limit value, the area automatically encrypted the grid. According to the geometric model of the gasoline engine, the basic dimensions of the selected grid were 0.25–8 mm, 0.185–4 mm, and 0.125–2 mm, which were respectively named case1, case2, and case3. The three cases were calculated according to the geometric model and calculation model of the engine. Figure1 shows the average pressure and temperature in the cylinder that was calculated using grids of different sizes. As can be seen from Figure1, case1 was significantly di fferent from the other two cases, while case2 and case3 were not significantly different. Therefore, it can be said that when the basic grid size was 0.185–4 mm, the calculation result was independent of the size of the grid. Considering the calculation accuracy and calculation time, the model used in this study adopts a grid size of 0.185–4 mm as the basic grid size. EnergiesEnergies 2020 2020,, ,13 13,, , 4931x x FOR FOR PEER PEER REVIEW REVIEW 66 of of 25 24 25

5 5 900900 case1 case1 case1 case1 case2 800 case2 4 case2 case2 800 4 case3 case3 case3 case3 700700 33

600600

22 500500

11 400400

0 300

Average pressure in the cylinder (MPa) the cylinder pressureAverage in 0

Average temperature in the cylinder (K) cylinder the in Average temperature 300

Average pressure in the cylinder (MPa) the cylinder pressureAverage in -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 (K) cylinder the in Average temperature -120-120 -100 -100 -80 -80 -60 -60 -40 -40 -20 -20 0 0 20 20 40 40 60 60 80 80 100 100 120 120 Crank angle (° CA) Crank angle (° CA) Crank angle (° CA) Crank angle (° CA) ((aa)) ((bb)) Figure 1. Comparison of the average pressure and temperature in the cylinder (n = 5500 rpm, FigureFigure 1.1. Comparison of the average pressure and temperaturetemperature inin thethe cylindercylinder ((nn == 55005500 rpm,rpm, equivalenceequivalence ratioratio = = 1.1, 1.1, spark spark timing timing = = − −1111 °CA): °CA):CA): ( (aa)) )the thethe average averageaverage pressure pressurepressure in inin the thethe cylinder cylindercylinder and and ( b(b) ) the the − ◦ averageaverage temperaturetemperature in in the the cylinder.cylinder cylinder. .

InIn order orderorder to toto verify verifyverify the thethe accuracy accuracyaccuracy of of theof thethe model, model,model, the thethe numerical numericalnumerical simulation simulationsimulation results resultsresults were werewere compared comparedcompared with withthewith experimental thethe experimentalexperimental results. results.results. Figure 2FigureFigurea shows 2a2a the showshow comparisonss thethe comparisoncomparison between the betweenbetween experimental thethe experimentalexperimental and simulated andand simulatedaveragesimulated pressure average average in pressure thepressure cylinder. in in the the It cancylinder. cylinder. be seen It It fromcan can be be the seen seen figure from from that the the the figure figure simulation that that the the results simulation simulation were basically results results wereconsistentwere basicallybasically with theconsistentconsistent experimental withwith the results.the experimentalexperimental The difference reresults.sults. in the TheThe peak differencedifference value of inin the thethe pressure peakpeak value betweenvalue ofof the the pressuresimulationpressure between between result and the the thesimulation simulation experimental result result result and and the didthe expe expe not exceedrimentalrimental 5%. result result The did ignitiondid not not exceed exceed delay time5%. 5%. The isThe defined ignition ignition as delaythedelay time time time from is is defined defined the moment as as the the oftime time spark from from timing the the moment moment to the moment of of spark spark whentiming timing the to to pressurethe the moment moment in the when when cylinder the the pressure pressure begins intoin deviatethethe cylinder cylinder from begins begins the pure to to deviate compressiondeviate from from the line.the pure pure Figure co compression2mpressionb shows thatline. line. theFigure Figure ignition 2b 2b showsshows delay that timethat the the under ignition ignition the delayexperimentaldelay time time under under conditions the the experimental experimental was 14.1 ◦ CA;conditions conditions the ignition was was 14 delay14.1.1 °CA; °CA; time the the found ignition ignition using delay delay the numericaltime time found found simulation using using the the wasnumericalnumerical 14.0 ◦CA. simulationsimulation Therefore, waswas the 14.014.0 ignition °C°CA.A. delay Therefore,Therefore, time between thethe ignitionignition the experimental deladelayy timetime betweenbetween results and thethe the experimentalexperimental simulation resultsresults was andand only thethe simulationsimulation 0.1 ◦CA. It resultscanresults be said waswas thatonly only the 0.1 0.1 simulation °CA.°CA. ItIt cancan model bebe saidsaid could thatthat e ff thetheectively simulationsimulation simulate modelmodel the actual couldcould effectivelyoperatingeffectively conditionssimulate simulate the the of theactualactual direct operating operating injection condit condit gasolineionsions engine.of of the the direct direct inje injectionction gasoline gasoline engine. engine.

10 10 14 14 Experiment Experiment Simulation Simulation 8 n = 5500 rpm ） 12 8 n = 5500 rpm ） 12

Equivalence ratio = 1.1 CA o

Equivalence ratio = 1.1 CA o Spark timing = -11 CAo o 10 Spark timing = -11 CA 10（ 6 （ 6 8 8

4 4 6 6

4 2 4 2 time delay Ignition Ignition delay time delay Ignition 2 2 Average pressure in the cylinder (MPa) cylinder the in pressure Average

Average pressure in the cylinder (MPa) cylinder the in pressure Average 0 -1000 0 100 200 -100 0 100 200 0 o 0 o Experiment Simulation CrankCrank angle angle ( (CA)CA) Experiment Simulation

(a) (b) (a) (b) Figure 2. ComparisonComparison ofof thethe experimental experimental and and simulated simulated results: results: (a )( thea) the average average pressure pressure results results and (andbFigure) the (b) ignition the2. Comparison ignition delay delay time of time [the37]. experimental[37]. and simulated results: (a) the average pressure results and (b) the ignition delay time [37]. 3. Results 3. ResultsIt can be seen from the relevant literature [4,21,22] [4,21,22] that when the equivalence ratio is from 0.9 to 1.1, thetheIt knockcanknock be trend seentrend from is is more more theobvious. relevant obvious. Theliterature The equivalent equiva [4,21,22]lent ratio ratio that of thewhenof the experimental theexperimental equivalence working working ratio condition is fromcondition 0.9 was to 1.1;was1.1, therefore,the1.1; knocktherefore, the trend equivalent the is moreequivalent ratioobvious. was ratio selectedThe was equiva toselected belent 1.1. ratio to The ofbe spark the1.1. experimental timingThe spark under timingworking the experimental under condition the conditions was 11 CA. The gasoline engine did not knock and its power performance was relatively experimentalwas 1.1; therefore, conditions− ◦ the equivalentwas −11 °CA.ratio Thewas gasolineselected engineto be 1.1.did Thenot sparkknock timing and itsunder power the performanceexperimental was conditions relatively was good −. 11Advancing °CA. The the gasolinespark timing engine can improvedid not theknock power and and its economy power

Energies 2020, 13, x FOR PEER REVIEW 7 of 25

of a gasoline engine but will increase the knock tendency. In this study, by initiating the spark timing, the gasoline engine was induced to knock to better explore the influence of the water Energies 2020, 13, 4931 7 of 24 injection temperature on the knock. Therefore, other conditions, such as the initial and boundary conditions and the fuel injection quantity, were the same as the experimental conditions above, but good.the spark Advancing timing was the sparkadvanced timing from can −11 improve °CA to the −21 power °CA. andThree economy working of conditions a gasoline with engine spark but willtimings increase of −11 the °CA, knock −16 tendency.°CA, and − In21 this°CA study,were selected. by initiating the spark timing, the gasoline engine was inducedFigure 3a to shows knock the to better average explore in-cylinder the influence pressure of theprofiles water for injection different temperature spark timings. on the It can knock. be Therefore,seen that as other the conditions,spark timing such advanced, as the initial the time and boundaryfor the pressure conditions to reach and the fuelpeak injection value decreased quantity, wereand the the peak same value as the increased. experimental This meant conditions that as above, the spark but the timing spark advanced, timing was the advancedrate of the frompressure11 − riseCA also to 21increased.CA. Three If the working rate of conditionsthe pressure with rise spark was too timings large, of it was11 CA,easy to16 formCA, high-pressure and 21 CA ◦ − ◦ − ◦ − ◦ − ◦ wereshockwaves selected. in the cylinder. The high-pressure shock waves propagated in the cylinder and were repeatedlyFigure 3reflected,a shows thewhich average easily in-cylinder induced knock. pressure Conversely, profiles forwhen di ff theerent spark spark timing timings. was Itdelayed, can be seenthe peak that aspressure the spark moved timing away advanced, from the the time top for de thead pressurecenter (TDC), to reach where the peak the value possibility decreased of andafterburning the peak valueincreased, increased. causing This the meant exhaust that temperature as the spark to timing increase advanced, and the theeffective rate of power the pressure of the risegasoline also engine increased. to decrease. If the rate of the pressure rise was too large, it was easy to form high-pressure shockwavesFigure 3b in shows the cylinder. the average The high-pressurein-cylinder temperat shock wavesure profiles propagated for different in the spark cylinder timings. and were The repeatedlychanging trend reflected, of the which temperature easily induced at different knock. spark Conversely, timings was when basically the spark the timing same. wasHowever, delayed, as the peakspark pressure timing advanced, moved away the from peak the value top deadof the center temperature (TDC), whereincreased the possibilityand the time of afterburningto reach the increased,peak value causing decreased. the exhaust In the temperatureprocess of flame to increase propagation, and the the effective temperature power of and the gasolinepressure enginein the tocylinder decrease. increased such that the terminal mixture was subjected to greater heat radiation and greater

3000 14 Spark timing Spark timing -11 °CA 12 2500 -11 °CA -16 °CA -16 °CA 10 -21 °CA -21 °CA 2000 8 1500 6

4 1000

2 500 0 0 Averagepressure the cylinder in (MPa) -20-100 102030405060 (K) cylinder the in temperature Average -20-100 102030405060 Crank angle (°CA) Crank angle (°CA)

(a) (b)

Figure 3.3. AverageAverage pressure pressure and and temperature temperature at diat ffdifferenterent spark spark timings timings (n = (5500n = 5500 rpm, rpm, equivalence equivalence ratio =ratio1.1): = (1.1):a) the (a average) the average pressure pressure in the in cylinder the cylinder and (b and) the ( averageb) the average temperature temperature in the cylinderin the cylinder [37]. [37]. Figure3b shows the average in-cylinder temperature profiles for di fferent spark timings. The changingFigure 4a trend shows of the the instantaneous temperature at heat different release spark rate timings profiles was for basicallydifferent thespark same. timings. However, The astime the corresponding spark timing to advanced, the peak theof the peak instantaneou value of thes heat temperature release rate increased and the time and thecorresponding time to reach to the peak valueof the decreased.pressure were In the rela processtively consistent. of flame propagation, The peak value the of temperature the instantaneous and pressure heat release in the cylinderrate also increasedincreased such with that the the advancement terminal mixture of the was spark subjected timing; to therefore, greater heat it radiationcan be said and that greater the increaseFigure in 4thea shows instantaneous the instantaneous heat release heat rate release caused rate an profiles increase for in di thefferent rate of spark the timings.pressure Therise.time The correspondingincrease in the toheat the release peak ofrate, the especially instantaneous the in heatcrease release in the rate peak and value the time of the corresponding instantaneous to heat the peakrelease of rate, the pressure was due were to the relatively rapid heat consistent. release Theof the peak fuel value over of a the short instantaneous time, which heat resulted release in rate an alsoincreased increased tendency with theto produce advancement knock ofin thethe sparkcylinder. timing; therefore, it can be said that the increase in the instantaneousFigure 4b shows heat releasethe cumulative rate caused heat an increase release in profiles the rate offor the different pressure spark rise. The timings. increase The in thecombustion heat release duration rate, especiallycan be defined the increase as follows: in thethe peakstarting value time of of the the instantaneous combustion is heat the releasecrank angle rate, wascorresponding due to the rapidto 10% heat of the release cumulative of the fuel heat over release, a short and time, the whichending resulted time of incombustion an increased is the tendency crank toangle produce corresponding knock in the to cylinder.90% of the cumulative heat release. The time interval between these two crankFigure angles4b is shows defined the as cumulative the combustion heat release duration. profiles When for the di sparkfferent timings spark timings. were −21 The °CA, combustion −16 °CA, durationand −11 °CA, can be the defined combustion as follows: durations the starting were time 17.7 of °CA, the combustion20.2 °CA, and is the 23.4 crank °CA, angle respectively. corresponding The tocumulative 10% of theheat cumulative released for heat different release, spark and timings the ending was almost time of the combustion same, but as is the the spark crank timing angle corresponding to 90% of the cumulative heat release. The time interval between these two crank angles is defined as the combustion duration. When the spark timings were 21 CA, 16 CA, − ◦ − ◦ Energies 2020, 13, x FOR PEER REVIEW 8 of 25

Energies 2020, 13, 4931 8 of 24 angle corresponding to 90% of the cumulative heat release. The time interval between these two crank angles is defined as the combustion duration. When the spark timings were −21 °CA, −16 °CA, and −11 °CA,CA, the combustion durationsdurations were 17.7 °CA,CA, 20.2 °CA,CA, andand 23.423.4 °CA,CA, respectively.respectively. TheThe − ◦ ◦ ◦ ◦ cumulative heat releasedreleased for didifferentfferent spark timingstimings waswas almost the same, but as thethe sparkspark timingtiming advanced,advanced, the combustion time was closer to the TDC and the combustion duration was was shorter. shorter. Therefore, properlyproperly increasingincreasing the the spark spark timing timing can can reduce reduce the the combustion combustion duration duration and and increase increase the heatthe heat release release rate, whichrate, which is conducive is conducive to improving to improving the power the andpowe fuelr and economy fuel economy of an engine. of an However, engine. inHowever, the process in the of process the flame of propagatingthe flame propagating from the sparkfrom the plug spark to the plug terminal to the mixture,terminal themixture, terminal the mixtureterminal absorbed mixture moreabsorbed heat more and received heat and a greaterreceived pressure a greater shockwave, pressure whichshockwave, increased which the increased tendency ofthe the tendency terminal of mixture the terminal to produce mixture knock. to produce knock.

240 Spark timing 3000 Spark timing 220 -11 °CA -11 °CA 200 -16 °CA 2500 -16 °CA 180 -21 °CA -21 °CA 160 2000 140 120

1500 100 80 1000 60 40 500 20 Cumulative heat release (J) 0 0 Instantaneousheat release (J/°CA) rate -20 -10 0 10 20 30 40 50 60 -20 -10 0 10 20 30 40 50 60 Crank angle (°CA) Crank angle (°CA)

(a) (b)

Figure 4. InstantaneousInstantaneous heat heat release release rate rate and and cumulative cumulative heat heat release release for fordifferent different spark spark timing timingss (n = (5500n = rpm,5500 equivalence rpm, equivalence ratio = ratio1.1): (a=) i1.1):nstantaneous (a) instantaneous heat release heat rate release and (b) rate cumulative and (b) heat cumulative release. heat release. The degree of knock is characterized by the knock index (KI); the equation for KI is as follows [22]: The degree of knock is characterized by the knock index (KI); the equation for KI is as follows [22]: 푁 1 N KI = 1 ∑X푃푃푚푎푥,푛 KI = 푁 PP N 1 max,n KI represents the strength of the knock, N represents1 the number of monitoring points, PPmax,n represents the difference between the peak pressure at the monitoring point and the peak average KI represents the strength of the knock, N represents the number of monitoring points, PPmax,n representspressure in thethe dicylinder.fference When between KI is the greater peak pressurethan 2, the at engine the monitoring knocks; w pointhen KI and is less the peakthan or average equal pressureto 2, the inengine the cylinder. does not When knock. KI The is greater knock thanoccurs 2, thebecause engine the knocks; time it when takes KI for is the less flame than orfront equal to topropagate 2, the engine to the does terminal not knock.mixture The is longer knock than occurs the because time of thethe timeterminal it takes mixture for the self flame-ignition. front The to propagatearea near the to the wall terminal of the combustion mixture is longer chamber than is the relatively time of thefar away terminal from mixture the spark self-ignition. plug; therefore, The area it nearis prone the wallto knock of the [22 combustion]. To research chamber knock, issome relatively monitoring far away points from were the set spark near plug; the wall therefore, surface it of is pronethe combustion to knock [chamber.22]. To research The monitoring knock, some points monitoring 0–7 were arranged points were anticlockwise set near the and wall were surface evenly of thedistributed combustion along chamber. the wall Theof the monitoring combustion points chamber 0–7 were. Figure arranged 5 specifically anticlockwise shows the and distribution were evenly of distributedthe monitoring along point thes. wall of the combustion chamber. Figure5 specifically shows the distribution of the monitoring points. Figure6 shows the peak-to-peak pressures of di fferent monitoring points for different spark timings. As the spark timing advanced, the peak-to-peak pressure value at each monitoring point became higher and higher. When the spark timing was 21 CA, the peak-to-peak pressure difference − ◦ between the monitoring points was also more obvious. In addition, when the spark timing was 21 CA, the peak-to-peak pressure values of monitoring points 1 and 4 were the largest, especially − ◦ point 4. Using the KI equation, the KI is shown in Figure7. When the spark timing was 21 CA, − ◦ the KI of the gasoline engine was close to 4, which is much greater than 2. According to the equation, when KI is greater than 2, the gasoline engine knocks. In this study, in order to investigate the effect of the water injection temperature on knock, the spark timing was chosen to be 21 CA. − ◦

Energies 2020, 13, x FOR PEER REVIEW 9 of 25 Energies 2020, 13, x FOR PEER REVIEW 9 of 25

Energies 2020, 13, 4931 9 of 24 Energies 2020, 13, x FOR PEER REVIEW 9 of 25

Figure 5. Monitoring point settings [37]. Figure 5. Monitoring point settings [37]. Figure 6 shows the peak-to-peak pressures of different monitoring points for different spark Figure 6 shows the peak-to-peak pressures of different monitoring points for different spark timings. As the spark timing advanced, the peak-to-peak pressure value at each monitoring point timings. As the spark timing advanced, the peak-to-peak pressure value at each monitoring point became higher and higher. When the spark timing was −21 °CA, the peak-to-peak pressure became higher and higher. When the spark timing was −21 °CA, the peak-to-peak pressure difference between the monitoring points was also more obvious. In addition, when the spark difference between the monitoring points was also more obvious. In addition, when the spark timing was −21 °CA, the peak-to-peak pressure values of monitoring points 1 and 4 were the largest, timing was −21 °CA, the peak-to-peak pressure values of monitoring points 1 and 4 were the largest, especially point 4. Using the KI equation, the KI is shown in Figure 7. When the spark timing was especially point 4. Using the KI equation, the KI is shown in Figure 7. When the spark timing was −21 °CA, the KI of the gasoline engine was close to 4, which is much greater than 2. According to the −21 °CA, the KI of the gasoline engine was close to 4, which is much greater than 2. According to the equation, when KI is greater than 2, the gasoline engine knocks. In this study, in order to investigate equation, when KI is greater than 2, the gasoline engine knocks. In this study, in order to investigate the effect of the water injection temperatureFigure 5. Monitoring on knock, point the settings spark [37].timing was chosen to be −21 °CA. the effect of the water injection temperatureFigure 5. Monitoring on knock, pointthe spark settings timing [37]. was chosen to be −21 °CA. 12 12 Spark timing = - 21 °CA Figure 6 shows the peak-to-peak pressures of different Spark Spark timing monitoring timing = - 21 = °CA- 16 °CA points for different spark 10 Spark Spark timing timing = - 16 = °CA- 11 °CA timings. As the spark timing advanced,10 the peak-to-peakn Spark = 5500pressure timing rpm = - 11 °CA value at each monitoring point n = 5500Equivalence rpm = 1.1 became higher and higher. When8 the spark timingEquivalence was = −1.121 °CA, the peak-to-peak pressure 8

difference between the monitoring6 points was also more obvious. In addition, when the spark

6 (MPa) timing was −21 °CA, the peak-to-peak pressure values of monitoring points 1 and 4 were the largest, (MPa) 4 especially point 4. Using the KI4 equation, the KI is shown in Figure 7. When the spark timing was PPmax

−21 °CA, the KI of the gasolinePPmax engine2 was close to 4, which is much greater than 2. According to the 2 equation, when KI is greater than 2, the gasoline engine knocks. In this study, in order to investigate 0 the effect of the water injection temperature0 on knock, the spark timing was chosen to be −21 °CA. -10123456789 -1012345678912 Number of the monitoring Spark point timing = - 21 °CA Number of the monitoring point Spark timing = - 16 °CA 10 Spark timing = - 11 °CA Figure 6. Peak-to-peak pressure at differentn = 5500 monitoring rpm points. Figure 6. Peak-to-peak pressure at differentEquivalence monitoring = 1.1 points. Figure 6. Peak-to-peak8 pressure at different monitoring points.

6 4.0 n = 5500 rpm (MPa)

4.0 n = 5500Equivalence rpm ratio = 1.1 3.5 4 Equivalence ratio = 1.1 3.5 3.0PPmax 3.0 2 2.5 2.5 0

2.0

2.0 -10123456789

Knock intensity Knock 1.5 Number of the monitoring point Knock intensity Knock 1.5 1.0 Figure 6.1.0 Peak-to-peak pressure at different monitoring points. 0.5 0.5 -21 -16 -11 -21 -16o -11 4.0 Spark time( CA)n = 5500 rpm Spark time(oCA) Equivalence ratio = 1.1 3.5 Figure 7. Knock intensity (KI) at different spark timings. 3.0

In order to explore the reasons for the severe knock at monitoring point 4 when the spark timing Figure 7.2.5 Knock intensity (KI) at different spark timings.

was 21 ◦CA, the knockFigure was 7. analyzed Knock intensity in terms (KI) of at the different turbulent spark kinetic timings. energy, temperature, pressure, − 2.0 and fuel/air equivalent ratio. Figure8 shows the distribution of various fields before sparking when

Knock intensity Knock 1.5 the spark timing was 21 ◦CA. Figure8a,b shows that the overall distribution of the pressure and − 1.0 temperature was very uniform, and there was no obvious difference between the monitoring points at this timing. Compared with other0.5 areas, the temperature near the injector (slightly above monitoring point 0) was significantly lower than-21 other areas. This -16 was mainly -11 because the equivalent ratio in Spark time(oCA) the vicinity of the fuel injector was significantly higher than in other areas. The evaporation of the liquid fuel in the cylinder needed to absorb the heat; therefore, the temperature in the area near the injector was significantly lower than elsewhere. It can be seen from Figure8c that when comparing

all the monitoring points,Figure the turbulent 7. Knock intensity kinetic energy(KI) at different on the leftspark and timings. right sides of the combustion

Energies 2020, 13, x FOR PEER REVIEW 10 of 25

In order to explore the reasons for the severe knock at monitoring point 4 when the spark timing was −21 °CA, the knock was analyzed in terms of the turbulent kinetic energy, temperature, pressure, and fuel/air equivalent ratio. Figure 8 shows the distribution of various fields before sparking when the spark timing was −21 °CA. Figure 8a,b shows that the overall distribution of the pressure and temperature was very uniform, and there was no obvious difference between the monitoring points at this timing. Compared with other areas, the temperature near the injector (slightly above monitoring point 0) was significantly lower than other areas. This was mainly because the equivalent ratio in the vicinity of the fuel injector was significantly higher than in other areas. The evaporation of the liquid fuel in the cylinder needed to absorb the heat; therefore, the Energiestemperature2020, 13, 4931 in the area near the injector was significantly lower than elsewhere. It can be seen10 offrom 24 Figure 8c that when comparing all the monitoring points, the turbulent kinetic energy on the left and chamberright sides wall of was the low,combustion and the turbulentchamber wall kinetic was energy low, aboveand the and turbulent below thekinetic combustion energy above chamber and wasbelow high. the The combustion greater the chamber turbulent was kinetic high. energy, The greater the higher the turbulent the intensity kinetic of the energy, turbulence, the higher and the the moreintensity uniform of thethe mixingturbulence, of the and fuel andthe air,more which unifor wasm beneficialthe mixing for theof the flame fuel propagation, and air, which and was was lessbeneficial likely to for knock. the flame This propagation, may have caused and was the less flame likely to to propagate knock. This at dimayfferent have speeds caused in the di ffflameerent to directions.propagate The at different monitoring speeds points in different 0 and 4 located directions at the. The far monitoring left and right points of the 0 and combustion 4 located chamber at the far hadleft small and right turbulent of the kinetic combustion energies. chamber As a result, had smal thesel turbulent monitoring kinetic points energies. displayed As severea result, knock. these Monitoringmonitoring point points 2, locateddisplayed above severe the combustion knock. Moni chamber,toring point was not 2, pronelocated to knockabove duethe tocombustion its large turbulentchamber, kinetic was energy.not prone Figure to 8knockd is the due distribution to its large diagram turbulent of the equivalencekinetic energy. ratio Figure at the moment8d is the ofdistribution sparking. It diagram can be seen of thatthe equivalence the equivalent ratio ratio at in th thee moment vicinity ofof monitoringsparking. It points can be 0, 1,seen and that 4 was the veryequivalent large, even ratio exceeding in the vicinity 1.3. When of monitoring the equivalent points ratio0, 1, wasand 4 small, was very the flame large, propagation even exceeding speed 1.3. increasedWhen the with equivalent the increase ratio in was the small, equivalent the flame ratio. pr Whenopagation the equivalence speed increased ratio reachedwith the 1.1,increase the flame in the propagationequivalent speedratio. wasWhen the the largest, equivalence and then ratio the flamereached combustion 1.1, the flame propagation propagation speed speed decreased was asthe thelargest, equivalence and then ratio the increased. flame combustion Both too rich propag andation too lean speed mixtures decreased were notas the conducive equivalence to flame ratio propagation,increased. Both which too increased rich and the too reaction lean mixtures time of thewere terminal not conducive mixture to and flame increased propagation, the tendency which toincreased produce knock.the reaction Therefore, time of monitoring the terminal points mixt 0,ure 1, and and increased 4 were more the pronetendency to knockto produce than otherknock. monitoringTherefore, points.monitoring points 0, 1, and 4 were more prone to knock than other monitoring points.

3.00×103 2.50×107 2.33×103 1.93×107 1.65×103 1.35×107 9.75×102 7.75×106 3.00×102 2.00×106 (K) (Pa) (a) (b)

5.00×101 1.50 4.13×101 1.25 3.25×101 1.00 2.38×101 0.75 1.50×101 0.50 (m2/s2) (c) (d)

FigureFigure 8. Distribution 8. Distribution of various of various fields before fields beforesparking sparking (n = 5500 (n rpm,= 5500 equivalence rpm, equivalence ratio = 1.1, ratiospark= timing1.1, = −21spark °CA): timing (a) temperature= 21 CA): field, (a) temperature(b) pressure field,field, (bc)) turbulent pressure field,kinetic (c )energy turbulent distribution, kinetic energyand (d) − ◦ equivalencedistribution, ratio and distribution. (d) equivalence ratio distribution.

When the spark timing was 21 CA, the flame propagation diagram for 12 CA to 15.5 CA − ◦ − ◦ ◦ during the combustion process is shown in Figure9. The red area in the figure is the temperature iso-surface, which is expressed as the front surface during the flame propagation. As can be seen from Figure9, at 13 ◦CA, the flame front was evenly distributed around the spark plug and close to the inner wall of the cylinder, indicating that the speed of the flame propagation from the position of the spark plug to each position in the cylinder was basically the same. Due to the special structure of the combustion chamber, when the piston was in the area near the TDC, the space in the leftmost area of the combustion chamber was relatively narrow, which was not conducive to flame propagation. The flame first reached the intake sidewall surface and then reached the exhaust sidewall surface. The flame spread continuously to the area of monitoring point 4 after 13 ◦CA. At 15 ◦CA, the flame Energies 2020, 13, x FOR PEER REVIEW 11 of 25

When the spark timing was −21 °CA, the flame propagation diagram for −12 °CA to 15.5 °CA during the combustion process is shown in Figure 9. The red area in the figure is the temperature iso-surface, which is expressed as the front surface during the flame propagation. As can be seen from Figure 9, at 13 °CA, the flame front was evenly distributed around the spark plug and close to the inner wall of the cylinder, indicating that the speed of the flame propagation from the position of the spark plug to each position in the cylinder was basically the same. Due to the special structure of the combustion chamber, when the piston was in the area near the TDC, the space in the leftmost area of the combustion chamber was relatively narrow, which was not conducive to flame propagation. The flame first reached the intake sidewall surface and then reached the exhaust Energiessidewall2020 surface., 13, 4931 The flame spread continuously to the area of monitoring point 4 after 13 °CA.11 At of 15 24 °CA, the flame basically disappeared in the area of point 4. Subsequently, at 15.5 °CA, the flames appeared at point 4, indicating that spontaneous ignition occurred at point 4, and then the pressure basically disappeared in the area of point 4. Subsequently, at 15.5 CA, the flames appeared at point 4, and temperature rose sharply. This also shows that monitoring◦ point 4 was the most prone to indicating that spontaneous ignition occurred at point 4, and then the pressure and temperature rose knocking. sharply. This also shows that monitoring point 4 was the most prone to knocking.

−12 °CA −2 °CA 8 °CA

13 °CA 13.5 °CA 14 °CA

14.5 °CA 15 °CA 15.5 °CA

Figure 9. Flame propagation diagram during knock (n = 5500 rpm, equivalence ratio = 1.1, spark timing Figure 9. Flame propagation diagram during knock (n = 5500 rpm, equivalence ratio = 1.1, spark = 21 ◦CA). timing− = −21 °CA). 4. Discussion 4. Discussion 4.1. Effect of the Water Injection Temperature on the Knock Trend 4.1. Effect of the Water Injection Temperature on the Knock Trend In order to investigate whether different water injection temperatures had an impact on knock and the engine performance, this study selected two operating conditions with water injection temperatures of 60 ◦C and 150 ◦C. Under the condition that the other operating parameters of the experimental conditions remained unchanged, only the spark timing of the gasoline engine was advanced from 11 CA to 21 CA to explore the effect of the water injection temperature on the knock combustion. − ◦ − ◦ The parameters of the water injection were as follows: water injection time was 80 CA, the quantity − ◦ of the water injection was 8.231 mg, the water injection pressure was consistent with the fuel injection pressure of 15 MPa, and the water injection temperature was 60 ◦C and 150 ◦C. Energies 2020, 13, x FOR PEER REVIEW 12 of 25

In order to investigate whether different water injection temperatures had an impact on knock and the engine performance, this study selected two operating conditions with water injection temperatures of 60 °C and 150 °C. Under the condition that the other operating parameters of the experimental conditions remained unchanged, only the spark timing of the gasoline engine was advanced from −11 °CA to −21 °CA to explore the effect of the water injection temperature on the knock combustion. The parameters of the water injection were as follows: water injection time was −80 °CA, the quantity of the water injection was 8.231 mg, the water injection pressure was consistent with the fuel injection pressure of 15 MPa, and the water injection temperature was 60 °C and 150 °C.

4.1.1. Influence of the Water Injection Temperature on the Initial Conditions The distribution of the equivalence ratios at different water injection temperatures before the spark timing (−21 °CA) is shown in Figure 10a. The equivalence ratio of most areas in the cylinder was around 1.1. The equivalent ratio near the injector was about 1.5. The equivalence ratios for different water injection temperatures were almost the same. The distribution of the equivalence ratioEnergies before2020, 13the, 4931 spark timing was relatively uniform, except for in the vicinity of the fuel injection12 of 24 nozzle, indicating that the water injection temperature had little effect on the distribution of the equivalence ratio before the spark. Both too rich and too lean mixtures were not conducive to flame 4.1.1. Influence of the Water Injection Temperature on the Initial Conditions propagation, which increased the reaction time of the terminal mixture and increased the tendency to produceThe distribution knock. of the equivalence ratios at different water injection temperatures before the spark timing ( 21 CA) is shown in Figure 10a. The equivalence ratio of most areas in the cylinder Figure 10b− gives◦ the temperature distribution before the spark timing for different water injectionwas around temperatures. 1.1. The equivalent The temperature ratio near distribu the injectortion was was basically about 1.5. the The same. equivalence The temperature ratios for distributiondifferent water was injection uniform temperatures and the temperature were almost wa thes mostly same. distributed The distribution between of the 625 equivalence K and 750 ratio K. Comparingbefore the sparkthe timingtemperature was relatively distribution uniform, at exceptdifferent for in thewater vicinity injection of the fueltemperatures, injection nozzle, the low-temperatureindicating that the area water when injection the water temperature injection had temperature little effect onwas the 150 distribution °C was ofgreater the equivalence than the low-temperatureratio before the spark. area when Both the too water rich and injection too lean temperature mixtures were wasnot 60 °C, conducive where the to flame green propagation, area in the picturewhich increasedrepresents the the reaction low-temperature time of the area terminal mixture and increased the tendency to produce knock.

60 °C 150 °C

(a)

(K) (b)

Figure 10. Equivalent ratio and temperature distributions before the spark timing (n = 5500 rpm, equivalence ratio = 1.1, spark timing = 21 CA): (a) equivalence ratio and (b) temperature. − ◦ Figure 10b gives the temperature distribution before the spark timing for different water injection temperatures. The temperature distribution was basically the same. The temperature distribution was uniform and the temperature was mostly distributed between 625 K and 750 K. Comparing the temperature distribution at different water injection temperatures, the low-temperature area when the water injection temperature was 150 ◦C was greater than the low-temperature area when the water injection temperature was 60 ◦C, where the green area in the picture represents the low-temperature area Figure 11 shows the distribution of water from 21 CA to 10 CA. The distribution area of − ◦ − ◦ water when the water injection temperature was 150 ◦C was wider than that when the water injection temperature was 60 ◦C. This was because the higher the temperature of the water injection, the greater the internal energy of the water. From a microscopic point of view, evaporation is a process in which liquid molecules escape from the liquid’s surface and enter the gas phase space. Liquid molecules need to overcome the attractive force of other molecules on the surface of the interface to do work, and at the same time, they must expand the volume to occupy the gas space. As the temperature of the Energies 2020, 13, x FOR PEER REVIEW 13 of 25

Figure 10. Equivalent ratio and temperature distributions before the spark timing (n = 5500 rpm, equivalence ratio = 1.1, spark timing = −21 °CA): (a) equivalence ratio and (b) temperature.

Figure 11 shows the distribution of water from −21 °CA to −10 °CA. The distribution area of water when the water injection temperature was 150 °C was wider than that when the water injection temperature was 60 °C. This was because the higher the temperature of the water injection, Energiesthe greater2020, 13 the, 4931 internal energy of the water. From a microscopic point of view, evaporation is a 13 of 24 process in which liquid molecules escape from the liquid’s surface and enter the gas phase space. Liquid molecules need to overcome the attractive force of other molecules on the surface of the waterinterface injection to do work, increases, and at the the internal same time, energy they mu ofst the expand liquid the molecules volume to increases occupy the such gas space. that more As water moleculesthe temperature have enough of the water energy injection to overcome increases, thethe internal attractive energy force of ofthe the liquid liquid molecules surface increases molecules and entersuch the that gas more phase water space, molecules resulting have enough in a faster energy macroscopic to overcome evaporation the attractive rate. force Therefore, of the liquid when the surface molecules and enter the gas phase space, resulting in a faster macroscopic evaporation rate. water injection temperature was 150 ◦C, the low-temperature area was wider than that when the Therefore, when the water injection temperature was 150 °C, the low-temperature area was wider water injection temperature was 60 C. Improving the water injection temperature can make the water than that when the water injection◦ temperature was 60 °C. Improving the water injection more uniformly distributed in the cylinder. The more uniform the water distribution, the wider the temperature can make the water more uniformly distributed in the cylinder. The more uniform the low-temperaturewater distribution, area the inwider the cylinder.the low-temperature This helps area to suppress in the cylinder. knock. This helps to suppress knock.

60 °C 150 °C

−21 °CA

−18 °CA

−16 °CA

Energies 2020, 13, x FOR PEER REVIEW 14 of 25

−12 °CA

−10 °CA

FigureFigure 11. 11. WaterWater distribution distribution at different at water different injection water temperatures injection (n temperatures = 5500 rpm, equivalence (n = 5500 rpm, equivalenceratio = 1.1, spark ratio timing= 1.1, = spark −21 °CA). timing = 21 CA). − ◦ 4.1.2. Effect of the Water Injection Temperature on Knock Figure 12 shows the pressure fluctuation graphs of the monitoring point under different water injection temperatures. Compared with the pressure of the monitoring point when there was no water injection, regardless of whether the temperature of the water injection was 150 °C or 60 °C, the direct water injection could effectively decrease the pressure and pressure increase rate of each monitoring point. The pressure fluctuation range of each monitoring point was also reduced, and the combustion was more stable. Liquid water was injected into the cylinder to absorb the heat released by the combustion to become water vapor. As an inert gas, water vapor dilutes the terminal mixture and prolongs the self-ignition time of the mixture, thereby decreasing the knock tendency of the gasoline engine. Compared with the water injection temperature of 60 °C, when the water injection temperature was 150 °C, the pressure at most monitoring points was higher, especially in the range of 18–22 °CA of the crank angle. This was because the temperature of the water injection was increased and the heat absorbed by the water cylinder was reduced; therefore, the pressure was increased a little. However, because the temperature of the injected water was high and the amount of heat absorbed by the water was small, the difference in the peak pressure at each monitoring point under different water injection temperatures was not obvious.

18 20 No water injection No water injection Water injection 16 18 Water injection temperature 60 °C Water injection temperature 60 °C 16 14 temperature150 °C Water injection 14 12 temperature 150 °C 12 10 10 8 8 6 6

4 4 2 2 0 No. 0 monitoring point pressure (MPa) pressure point monitoring 0 No. 0 -20-100 102030405060 -20-100 102030405060 No. 1 monitoring point pressure point 1 monitoring No. (MPa) Crank angle (°CA) Crank angle (°CA)

EnergiesEnergies2020, 13 2020, 4931, 13, x FOR PEER REVIEW 14 of 2414 of 25

4.1.2. Effect of the Water Injection Temperature on Knock Figure 12 shows the pressure fluctuation graphs of the monitoring point under different water injection temperatures. Compared with the pressure of the monitoring point when there was no water injection, regardless−12 °CA of whether the temperature of the water injection was 150 ◦C or 60 ◦C, the direct water injection could effectively decrease the pressure and pressure increase rate of each monitoring point. The pressure fluctuation range of each monitoring point was also reduced, and the combustion was more stable. Liquid water was injected into the cylinder to absorb the heat released by the combustion to become water vapor. As an inert gas, water vapor dilutes the terminal mixture and prolongs the self-ignition time of the mixture, thereby decreasing the knock tendency of the gasoline engine. Compared with the water injection temperature of 60 ◦C, when the water injection temperature was 150 ◦C, the pressure at most monitoring points was higher, especially in the range of 18–22 ◦CA of the crank angle. This was because the temperature of the water injection was increased and the heat absorbed by the water−10 cylinder °CA was reduced; therefore, the pressure was increased a little. However, because the temperature of the injected water was high and the amount of heat absorbed by the water was small, the difference in the peak pressure at each monitoring point under different water injection temperatures was not obvious. Figure 13 shows the PPmax at different monitoring points under different water injection temperatures.Figure Compared 11. Water distribution with the no at waterdifferent working water injection condition, temperatures regardless (n = of 5500 whether rpm, equivalence the water injection temperatureratio = 1.1, spark was timing 60 ◦C =or −21 150 °CA).◦C, the PPmax of each monitoring point dropped significantly, which shows that direct water injection in the cylinder could effectively suppress knock. Compared with4.1.2. the water Effect injection of the Water temperature Injection of Temperature 60 ◦C, the PP onmax Knockvalue of each monitoring point was closer when the water injection temperature was 150 C. This can also be explained by noting that an Figure 12 shows the pressure fluctuation◦ graphs of the monitoring point under different water improvement in the temperature of the water injection could make the water distribution more uniform. injection temperatures. Compared with the pressure of the monitoring point when there was no Figure 14 shows the KI values at different water injection temperatures. When the water injection water injection, regardless of whether the temperature of the water injection was 150 °C or 60 °C, the temperature was 60 C, the value of KI was 0.94; when the water injection temperature was 150 C, direct water injection◦ could effectively decrease the pressure and pressure increase rate ◦of each the value of KI was 1.258. The KI values of the engine were much less than 2 at both water injection monitoring point. The pressure fluctuation range of each monitoring point was also reduced, and temperatures, which means that knock did not occur under these two conditions. When the injection the combustion was more stable. Liquid water was injected into the cylinder to absorb the heat temperature was 60 C, the value of KI was less than when the injection temperature was 150 C. released by the combustion◦ to become water vapor. As an inert gas, water vapor dilutes the terminal◦ This was because the lower the temperature of the water, the greater the amount of heat absorbed mixture and prolongs the self-ignition time of the mixture, thereby decreasing the knock tendency of and the lower the temperature and pressure in the cylinder; therefore, the knock suppression effect the gasoline engine. Compared with the water injection temperature of 60 °C, when the water was more obvious. Figure 12 shows that the peak pressure at most of the monitoring points when injection temperature was 150 °C, the pressure at most monitoring points was higher, especially in the water injection temperature was 60 C was lower than when the water injection temperature was the range of 18–22 °CA of the crank◦ angle. This was because the temperature of the water injection 150 C. Therefore, lowering the water injection temperature can reduce the knock strength of the ◦was increased and the heat absorbed by the water cylinder was reduced; therefore, the pressure was gasoline engine. However, improving the water injection temperature can make the water distribution increased a little. However, because the temperature of the injected water was high and the amount in the cylinder more uniform, and the PP of each monitoring point tends to be more consistent. of heat absorbed by the water was small,max the difference in the peak pressure at each monitoring To suppress the knock, the water injection temperature can be appropriately increased. point under different water injection temperatures was not obvious.

18 20 No water injection No water injection Water injection 16 18 Water injection temperature 60 °C Water injection temperature 60 °C 16 14 temperature150 °C Water injection 14 12 temperature 150 °C 12 10 10 8 8 6 6

4 4 2 2 0 No. 0 monitoring point pressure (MPa) pressure point monitoring 0 No. 0 -20-100 102030405060 -20-100 102030405060 No. 1 monitoring point pressure point 1 monitoring No. (MPa) Crank angle (°CA) Crank angle (°CA) Figure 12. Cont.

Energies 2020, 13, 4931 15 of 24 Energies 2020, 13, x FOR PEER REVIEW 15 of 25

16 18 No water injection No water injection 14 Water injection 16 Water injection temperature 60 °C temperature 60 °C 14 Water injection Water injection 12 temperature150 °C temperature 150 °C 12 10 10 8 8 6 6 4 4

2 2

0 0 No.2 monitoring point pressure (MPa) No. 3 monitoring point pressure (MPa) point 3 monitoring No. -20-100 102030405060 -20-100 102030405060 Crank angle (°CA) Crank angle (°CA)

25 16 No water injection No water injiection Water injection 14 Water injection 20 temperature 60 °C temperature 60 °C Water injection 12 Water injection temperature150 °C temperature150 °C 10 15

8

10 6

4 5 2 No. 4No.monitoring point pressure(MPa) 0 No. 5 monitoringpoint pressure (MPa) 0 -20-100 102030405060 -20-100 102030405060 Crank angle (°CA) Crank angle (°CA)

16 No water injection 16 No water injection 14 Water injection Water injection temperature 60 °C 14 temperature 60 °C Water injection 12 Water injection temperature150 °C 12 temperature150 °C

10 10

8 8

6 6

4 4

2 2 No. 7 monitoring point pressure (MPa) pressure point monitoring 7 No. Energies 2020 pressure (MPa) point 6 monitoring No. , 130, x FOR PEER REVIEW 0 16 of 25 -20-100 102030405060 -20-100 102030405060 Crank angle (°CA) Crank angle (°CA) make the water distribution in the cylinder more uniform, and the PPmax of each monitoring point Figure 12. Pressure at each monitoring point under different water injection temperatures (n = 5500 rpm, tends to be Figuremore 12.consistent. Pressure at To each suppress monitoring the point knock, under the different water water injection injection temperature temperatures can (n =be 5500 appropriatelyequivalence increased. ratio = 1.1, spark timing = 21 ◦CA). rpm, equivalence ratio = 1.1, spark− timing = −21 °CA). 12 No water Figure 13 shows the PPmax Waterat injectiondifferent monitoring points under different water injection 10 temperature60 oC temperatures. Compared with the Water no injection water working condition, regardless of whether the water temperature150 oC 8 max injection temperature was 60 °Cn = 5500or 150 rpm °C, the PP of each monitoring point dropped significantly, which shows that direct waterSpark injection timing = -21 in oCA the cylinder could effectively suppress knock. Compared (MPa) 6 Equivalence ratio = 1.1 with the water injection temperature of 60 °C, the PPmax value of each monitoring point was closer

when the water injectionPPmax 4temperature was 150 °C. This can also be explained by noting that an improvement in the temperature of the water injection could make the water distribution more uniform. Figure 14 shows 2the KI values at different water injection temperatures. When the water

injection temperature was 060 °C, the value of KI was 0.94; when the water injection temperature was -1012345678 150 °C, the value of KI was 1.258. TheNumber KI ofvalues the mon itoringof the point engine were much less than 2 at both water injection temperatures, which means that knock did not occur under these two conditions. When the injection temperature wasFigure 60 °C, 13. thePP valuemax of eachof KI monitoring was less than point. when the injection temperature was

150 °C. This was because the lower the temperature of the water, the greater the amount of heat absorbed and the lowerFigure the 13. temperature PPmax of each monitoringand pressure point. in the cylinder; therefore, the knock suppression effect was more obvious. Figure 12 shows that the peak pressure at most of the monitoring points when the water injection temperature was 60 °C was lower than when the water 4.0 n = 5500 rpm injection temperature was 150 °C. Therefore, loweSparkring timing the = -21 water oCA injection temperature can reduce 3.5 the knock strength of the gasoline engine. HoweveEquivalencer, improving ratio = 1.1 the water injection temperature can 3.0

2.5 KI 2.0

1.5

1.0

0.5 o No Water 60 oC 150 C

Figure 14. KI values for different water injection temperatures.

4.2. Influence of the Water Injection Temperature on the Engine Performance The average in-cylinder pressure profile for different water injection temperatures is shown in Figure 15. The water injection significantly reduced the pressure and pressure increase rate. Figure 12 shows that water injection could not only reduce the pressure but also reduced the degree of the pressure fluctuations, making the combustion process more stable. In addition, when the water injection temperature was 60 °C, the pressure was lower than when the water injection temperature was 150 °C. When the water injection temperature was 150 °C, the vaporization rate of the water mist was faster and the enthalpy value was higher; therefore, the pressure rise caused by the vaporization expansion was higher than in the case where the water injection temperature was 60 °C.

Energies 2020, 13, x FOR PEER REVIEW 16 of 25

make the water distribution in the cylinder more uniform, and the PPmax of each monitoring point tends to be more consistent. To suppress the knock, the water injection temperature can be appropriately increased.

12 No water Water injection 10 temperature60 oC Water injection temperature150 oC 8 n = 5500 rpm Spark timing = -21 oCA (MPa) 6 Equivalence ratio = 1.1

PPmax 4

2

0 -1012345678 Number of the monitoring point

Energies 2020, 13, 4931 16 of 24 Figure 13. PPmax of each monitoring point.

4.0 n = 5500 rpm Spark timing = -21 oCA 3.5 Equivalence ratio = 1.1

3.0

2.5 KI 2.0

1.5

1.0

0.5 o No Water 60 oC 150 C

Figure 14. KI values for different water injection temperatures.

4.2. Influence of the Water Injection Temperature on the Engine Performance Figure 14. KI values for different water injection temperatures. The average in-cylinder pressure profile for different water injection temperatures is shown in 4.2.Figure Influence 15. The of waterthe Water injection Injection significantly Temperature reduced on the theEngine pressure Performance and pressure increase rate. Figure 12 shows that water injection could not only reduce the pressure but also reduced the degree of the The average in-cylinder pressure profile for different water injection temperatures is shown in pressure fluctuations, making the combustion process more stable. In addition, when the water Figure 15. The water injection significantly reduced the pressure and pressure increase rate. Figure injection temperature was 60 ◦C, the pressure was lower than when the water injection temperature 12 shows that water injection could not only reduce the pressure but also reduced the degree of the was 150 ◦C. When the water injection temperature was 150 ◦C, the vaporization rate of the water mist pressure fluctuations, making the combustion process more stable. In addition, when the water was faster and the enthalpy value was higher; therefore, the pressure rise caused by the vaporization injection temperature was 60 °C, the pressure was lower than when the water injection temperature Energiesexpansion 2020, 13 was, x FOR higher PEER REVIEW than in the case where the water injection temperature was 60 ◦C. 17 of 25 was 150 °C. When the water injection temperature was 150 °C, the vaporization rate of the water

mist was faster and the enthalpy14 value was higher; therefore, the pressure rise caused by the vaporization expansion was higher than in the case where the water injection temperature was 60 12 °C. 10

8

No water injection 6 Water injection temperature 60 oC Water injection 4 temperature 150 oC n = 5500 rpm o 2 Spark timing = - 21 CA

Average pressure in the cylinder(MPa) in the pressure Average Equivalence ratio = 1.1

-20 0 20 40 60

Crank angle(oCA)

FigureFigure 15. 15.TheThe average average in-cylinder in-cylinder pressures pressures for for different different water water injection injection temperatures. temperatures. The P–V diagrams for different water injection temperatures are shown in Figure 16a. The closed The P–V diagrams for different water injection temperatures are shown in Figure 16a. The curve in Figure 16a was evaluated in the form of an integral to obtain its area, and the work done closed curve in Figure 16a was evaluated in the form of an integral to obtain its area, and the work at different temperatures was obtained. Figure 16b shows the amount of cyclic work done under done at different temperatures was obtained. Figure 16b shows the amount of cyclic work done different working conditions. The circulating work energy per cycle at a water injection temperature under different working conditions. The circulating work energy per cycle at a water injection of 150 C was 1238 J, and the work energy per cycle at a water injection temperature of 60 C was temperature◦ of 150 °C was 1238 J, and the work energy per cycle at a water injection temperature◦ of 1230 J. When the water temperature was higher, the superheated water vaporized in a shorter time, 60 °C was 1230 J. When the water temperature was higher, the superheated water vaporized in a pushing the piston to do work. This made up for the heat absorbed by the evaporation of some liquid shorter time, pushing the piston to do work. This made up for the heat absorbed by the evaporation water into water vapor. Therefore, increasing the water injection temperature improved the cycle of some liquid water into water vapor. Therefore, increasing the water injection temperature work power of the gasoline engine, thereby improving the thermal efficiency of the gasoline engine. improved the cycle work power of the gasoline engine, thereby improving the thermal efficiency of The experimental work energy per cycle was 1184 J. This shows that the combination of the advanced the gasoline engine. The experimental work energy per cycle was 1184 J. This shows that the spark timing and water injection technology can improve the performance of gasoline engines. combination of the advanced spark timing and water injection technology can improve the performance of gasoline engines.

14 No water 1200 Water injection 12 temperature 60 oC 1000 Water injection 10 temperature 150 oC 800 8 n = 5500 rpm MPa) Spark timing = - 21 oCA （

6 Equivalence ratio = 1.1 600

4 (J) energy Cycle Pressure 400 2 200 0

0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0 Volume（m3) Experiment 60 ℃ 150 ℃

(a) (b)

Figure 16. P–V diagrams and cycle energyat different water injection temperatures: (a) the P–V diagram cycles and (b) the cycle energies.

The application of water injection into the gasoline engines also has some challenges. The quantity of the water injected into the cylinder needs to be strictly controlled because injecting too much water may dilute the lubricating oil, and the corrosion resistance of various parts of the gasoline engine needs to be strengthened. The water storage and recycling equipment also need further development, and the problem of water freezing should be avoided when the ambient temperature is too low. In the engine working process, the water circulation can be improved as much as possible to save a lot of the trouble of adding water. It is also necessary to develop an effective monitoring system to ensure that the appropriate amount of water enters the cylinder. If there is a problem with the water injection equipment, the engine can start the traditional knock mitigation strategies, such as delaying the spark timing and enriching the mixture. The water

Energies 2020, 13, 4931 17 of 24 Energies 2020, 13, x FOR PEER REVIEW 17 of 25

14 No water 1200 Water injection 12 temperature 60 oC 1000 Water injection 10 temperature 150 oC 800 8 n = 5500 rpm MPa) Spark timing = - 21 oCA （

6 Equivalence ratio = 1.1 600

4 (J) energy Cycle Pressure 400 2 200 0

0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0 Volume（m3) Experiment 60 ℃ 150 ℃

(a) (b)

Figure 16. P–V diagrams and cycle energyat different water injection temperatures: (a) the P–V diagram Figurecycles and16. (P–Vb) the diagrams cycle energies. and cycle energyat different water injection temperatures: (a) the P–V diagram cycles and (b) the cycle energies. The application of water injection into the gasoline engines also has some challenges. The quantity of theThe water application injected intoof water the cylinder injection needs into to the be strictlygasoline controlled engines becausealso has injecting some challenges. too much water The quantitymay dilute of the lubricatingwater injected oil, into and the the cylinder corrosion need resistances to be strictly of various controlled parts of because the gasoline injecting engine too muchneeds towater be strengthened. may dilute the The lubricating water storage oil, and and recycling the corrosion equipment resistance also need of various further development,parts of the gasolineand the problemengine needs of water to be freezing strengthened. should The be avoided water storage when theand ambient recycling temperature equipment isalso too need low. furtherIn the engine development, working and process, the problem the water of circulation water freezing can be should improved be avoided as much when as possible the ambient to save temperaturea lot of the trouble is too oflow. adding In the water. engine It working is also necessary process, tothe develop water circulation an effective can monitoring be improved system as muchto ensure as possible that the to appropriate save a lot amountof the trouble of water of entersadding the water. cylinder. It is Ifalso there necessary is a problem to develop with thean watereffective injection monitoring equipment, system the to ensure engine that can startthe a theppropriate traditional amount knock of mitigation water enters strategies, the cylinder. such asIf theredelaying is a theproblem spark timingwith the and water enriching injection the equipm mixture.ent, The the water engine injection can start pressure, the traditional time, quantity, knock mitigationand other waterstrategies, injection such parameters as delaying have the a greatspark influence timing and on the enriching performance the mixture. of gasoline The engines. water injectionFor example, pressure, if the time, amount quantity, of water and injected other water into the injection cylinder parameters is too much, have the a temperaturegreat influence in theon thecylinder performance will be significantly of gasoline engines. reduced, For which example, will reduce if thethe amount engine’s of water power injected performance. into the Therefore, cylinder isit istoo necessary much, the to furthertemperature explore in thethe influencecylinder will of various be significantly water injection reduced, parameters which will on reduce the engine the engine’ssuch that power the various performance. parameters Therefore, can be coordinated it is necessary to allow to further for to explore the greatest the influence advantages of ofvarious water waterinjection injection technology parameters [37]. on the engine such that the various parameters can be coordinated to allow for to the greatest advantages of water injection technology [37]. 4.3. Effect of the Water Injection Temperature on Emissions 4.3. Effect of the Water Injection Temperature on Emissions 4.3.1. The Influence of Water Injection Temperature on NOx and Unburned Hydrocarbon (UHC) Emissions 4.3.1. The Influence of Water Injection Temperature on NOx and Unburned Hydrocarbon (UHC) EmissionsFigure 17a shows the generated quantity of NOx in the process of combustion. NOx emissions increased first and then decreased, and then tended to remain unchanged. The emission of nitrogen Figure 17a shows the generated quantity of NOx in the process of combustion. NOx emissions oxides in the cylinder was mainly in the form of NO. The formation mechanism of NO is described increased first and then decreased, and then tended to remain unchanged. The emission of nitrogen using the extended Zeldovich model, as shown in the following Equations (1)–(3): oxides in the cylinder was mainly in the form of NO. The formation mechanism of NO is described using the extended Zeldovich model, as shown in the following Equations (1)–(3): O + N2 = NO + N (1)

O + N2 = NO + N (1) N + O2 = NO + O (2) N + O2 = NO + O (2) N + OH = NO + H (3) N + OH = NO + H (3) The reason why NOx first increased to the maximum value and then decreased was that the The reason why NOx first increased to the maximum value and then decreased was that the generation of NOx is a reversible reaction. When the piston moved to the bottom dead center (BDC), generationthe pressure of decreased,NOx is a reversible which promoted reaction. theWhen occurrence the piston of moved a reverse to the reaction, bottom thereby dead center reducing (BDC), the the pressure decreased, which promoted the occurrence of a reverse reaction, thereby reducing the amount of NOx produced. Figure 17b shows the final NOx production quantity. Compared with amount of NOx produced. Figure 17b shows the final NOx production quantity. Compared with the no water injection condition, water injection effectively reduced the quantity of NOx, but the temperature of the water injection has little effect on the NOx emissions. The main factors that

Energies 2020, 13, 4931 18 of 24

the no water injection condition, water injection effectively reduced the quantity of NOx, but the Energies 2020, 13, x FOR PEER REVIEW 18 of 25 temperature of the water injection has little effect on the NOx emissions. The main factors that affected the amount of NOx generated include the following three points: the high temperature included a local affected the amount of NOx generated include the following three points: the high temperature high temperature and oxygen concentration, and the duration of the high temperature. These three included a local high temperature and oxygen concentration, and the duration of the high conditions are indispensable; therefore, reducing the amount of NOx produced can be achieved by temperature. These three conditions are indispensable; therefore, reducing the amount of NOx controllingproduced onecan ofbe them.achieved The by temperature controlling one has of a greatthem. influence The temperature on the amount has a great of NO influencex produced. on the The combination of N and O is a product at high temperatures. When the temperature is higher amount of NOx produced. The combination of N and O is a product at high temperatures. When the thantemperature 1800 ◦C, thisis higher combination than 1800 reaction °C, this becomes combinat obvious.ion reaction As becomes the temperature obvious. rises, As the the temperature reaction rate increases exponentially. At the same time, the amount of NOx generated also increases as the rises, the reaction rate increases exponentially. At the same time, the amount of NOx generated also residenceincreases time as the of residence the combustion time ofproducts the combustion in the high-temperatureproducts in the high-temperature region increases. region In addition, increases. the higher the oxygen concentration, the greater the amount of NOx produced. Liquid water in the In addition, the higher the oxygen concentration, the greater the amount of NOx produced. Liquid cylinderwater canin the absorb cylinder the heat can to absorb effectively the reduceheat to theeffectively temperature reduce and the pressure; temperature therefore, and the pressure; direct injectiontherefore, of water the direct into theinjection cylinder of water can eff intoectively the cyli reducender the can final effectively production reduce of NO thex .final When production the water of injection temperature was 150 ◦C, the NOx emission was slightly higher than when the water injection NOx. When the water injection temperature was 150 °C, the NOx emission was slightly higher than temperaturewhen the waswater 60 injection◦C. The equivalent temperature ratios was under 60 °C. diff erentThe equivalent working conditions ratios under were different all 1.1; therefore, working theconditions main influencing were all factor1.1; therefore, was temperature. the main influencing factor was temperature.

8 5.0

7 No water injection Water injection 4.8 6 temperature 60 °C Water injection 5 temperature150 °C 4.6 4

3 4.4 NOx quantity (mg) quantity NOx

NOx quantity (mg) NOx quantity 2

4.2 1

0 -20-100 102030405060 4.0 Crank angle (°CA) No water 60 ℃ 150 ℃

(a) (b)

FigureFigure 17. 17.The The quantity quantity of of NO NOx underx under di ffdifferenterent working working conditions conditions (n =(n5500 = 5500 rpm, rpm, equivalence equivalence ratio ratio= = 1.1):1.1): (a )(a the) the quantity quantity of of NO NOx withx with di ffdifferenterent crank crank angles angles and and (b )(b the) the NO NOx quantity.x quantity.

FigureFigure 18 18shows shows the temperaturethe temperature field field at diff aterent diff watererent injectionwater injection temperatures temperatures during the during period the ofperiod 10–40 ◦CA.of 10–40 Comparing °CA. Comparing the two operating the two conditions operating with conditions different waterwith different injection temperatures,water injection thetemperatures, temperature the in thetemperature cylinder wasin the higher cylinder and was the higher duration and of the the duration high temperature of the high was temperature longer underwas thelonger condition under ofthe no condition water injection. of no water The directinjection. injection The direct of water injection into the of cylinderwater into could the absorbcylinder thecould heat absorb to effectively the heat reduce to effectively the temperature reduce andthe temperature pressure. Therefore, and pressure. direct waterTherefore, injection direct in water the cylinderinjection could in etheffectively cylinder reduce could NO effectivelyx emissions. reduce When theNO waterx emissions. injection When temperature the water was 150injection◦C, thetemperature NOx emissions was were 150 a°C, little the higher NOx thanemissions when thewere water a little injection higher temperature than when was the 60 water◦C. This injection was becausetemperature improving was the 60 water°C. This injection was temperaturebecause improving slightly reducedthe water the water’sinjection ability temperature to absorb slightly heat, therebyreduced slightly the water’s increasing ability the temperatureto absorb heat, in the ther cylinder.eby slightly However, increasing improving the thetemperature water injection in the temperaturecylinder. However, made the waterimproving more the evenly water distributed, injection whichtemperature helped tomade reduce the thewater NO xmoreemissions. evenly It candistributed, also be seen which from helped Figure to 19reduce that therethe NO wasx emissions. little difference It can inalso the be temperature seen from Figure in the cylinder19 that there at diffwaserent little water difference injection in temperatures the temperature such in that the the cylinde influencer at different of the water water injection injection temperature temperatures on thesuch NOthatx emissions the influence was notof the obvious. water injection temperature on the NOx emissions was not obvious.

Water injection temperature Water injection temperature No water 60 °C 150 °C

Energies 2020, 13, x FOR PEER REVIEW 19 of 25

(a) (b)

Figure 17. The quantity of NOx under different working conditions (n = 5500 rpm, equivalence ratio = 1.1): (a) the quantity of NOx with different crank angles and (b) the NOx quantity.

Figure 18 shows the temperature field at different water injection temperatures during the period of 10–40 °CA. Comparing the two operating conditions with different water injection temperatures, the temperature in the cylinder was higher and the duration of the high temperature was longer under the condition of no water injection. The direct injection of water into the cylinder could absorb the heat to effectively reduce the temperature and pressure. Therefore, direct water injection in the cylinder could effectively reduce NOx emissions. When the water injection temperature was 150 °C, the NOx emissions were a little higher than when the water injection temperature was 60 °C. This was because improving the water injection temperature slightly reduced the water’s ability to absorb heat, thereby slightly increasing the temperature in the cylinder. However, improving the water injection temperature made the water more evenly distributed, which helped to reduce the NOx emissions. It can also be seen from Figure 19 that there Energieswas2020 little, 13, difference 4931 in the temperature in the cylinder at different water injection temperatures such19 of 24 that the influence of the water injection temperature on the NOx emissions was not obvious.

Water injection temperature Water injection temperature No water 60 °C 150 °C

10 °CA

15 °CA

20 °CA

25 °CA

Energies 2020, 13, x FOR PEER REVIEW 20 of 25

30 °CA

40 °CA

Temperature (K) 1800 2050 2300 2550 2800

FigureFigure 18. 18. TheThe temperature temperature field for field different for diinjectionfferent temperatures injection temperatures(n = 5500 rpm, equivalence (n = 5500 ratio rpm, equivalence= 1.1). ratio = 1.1).

Figure 19a shows the generated quantity of UHCs in the process of combustion. The rapid decline of hydrocarbons meant that the fuel was constantly being consumed. The rate of decrease of hydrocarbons was relatively slow and then increased rapidly. This was because the flame center had just been formed near the spark plug during the ignition delay period, where the temperature in the cylinder is not very high, and the consumption of the hydrocarbon fuel was limited. During the acute burning period, the flame spread rapidly from near the spark plug. The contact area between the flame and the combustible gas mixture also increased rapidly, where the hydrocarbon fuel burned rapidly and the mass was greatly reduced. Figure 19b shows the UHC quantity for different water injection temperatures. Compared with no water injection conditions, direct water injection in the cylinder could reduce the quantity of UHCs. This was because water can be decomposed into H2 and O2 at high temperatures according to 2H2O + 2 × 242,000 kJ/kmol = 2H2 + O2. Although the quantity of H2 and O2 was small, it could promote the complete combustion of fuel and improve fuel utilization. Figure 20b shows that when the water injection temperature was 150 °C, the amount of UHCs was significantly lower than with the water injection temperature of 60 °C. The main reasons for the generation of UHCs were incomplete combustion, wall quenching, and oil film adsorption on the wall. Properly improving the temperature of the water injection can make the water more evenly distributed in the cylinder, and it will not be easy to produce flame quenching during the combustion process. This can reduce UHC emissions.

Energies 2020, 13, x FOR PEER REVIEW 20 of 25

Figure 18. The temperature field for different injection temperatures (n = 5500 rpm, equivalence ratio = 1.1).

Figure 19a shows the generated quantity of UHCs in the process of combustion. The rapid decline of hydrocarbons meant that the fuel was constantly being consumed. The rate of decrease of hydrocarbons was relatively slow and then increased rapidly. This was because the flame center had just been formed near the spark plug during the ignition delay period, where the temperature in the cylinder is not very high, and the consumption of the hydrocarbon fuel was limited. During the acute burning period, the flame spread rapidly from near the spark plug. The contact area between the flame and the combustible gas mixture also increased rapidly, where the hydrocarbon fuel burned rapidly and the mass was greatly reduced. Figure 19b shows the UHC quantity for different water injection temperatures. Compared with no water injection conditions, direct water injection in the cylinder could reduce the quantity of UHCs. This was because water can be decomposed into H2 and O2 at high temperatures according to 2H2O + 2 × 242,000 kJ/kmol = 2H2 + O2. Although the quantity of H2 and O2 was small, it could promote the complete combustion of fuel and improve fuel utilization. Figure 20b shows that when the water injection temperature was 150 °C, the amount of UHCs was significantly lower than with the water injection temperature of 60 °C. The main reasons for the generation of UHCs were incomplete combustion, wall quenching, and oil film adsorption on the wall. Properly improving the temperature of the water injection can make the water more evenly distributedEnergies 2020, 13in, 4931 the cylinder, and it will not be easy to produce flame quenching during20 ofthe 24 combustion process. This can reduce UHC emissions.

0.0025 80 No water injection Water injection 70 temperature 60 ℃ Water injection 0.0020 60 temperature150 ℃

50 0.0015 40

30 0.0010

UHC quantity (mg) quantity UHC 20 UHC quantity (mg) quantity UHC 0.0005 10

0 -20-100 102030405060 0.0000 Crank angle (°CA) No water 60 ℃ 150 ℃

(a) (b)

FigureFigure 19. 19. TheThe quality quality of ofunburned unburned hydrocarbons hydrocarbons (UHCs) (UHCs) under under different different working working conditions conditions (n = 5500(n = 5500rpm, rpm,equivalence equivalence ratio ratio = 1.1):= 1.1): (a) the (a) thequantity quantity of ofHCs HCs at atdifferent different crank crank angles angles and and ( (bb)) the the quantity of UHCs produced with di fferentfferent water injection temperatures.

4.3.2.Figure The Influence 19a shows of the the Water generated Injection quantity Temperature of UHCs on inthe the Soot process and CO of Emissions combustion. The rapid decline of hydrocarbons meant that the fuel was constantly being consumed. The rate of decrease of Figure 20a shows the generated quantity of soot in the process of combustion. The soot first hydrocarbons was relatively slow and then increased rapidly. This was because the flame center had increased continuously, then lowered and finally stabilized. Polycyclic aromatic hydrocarbons just been formed near the spark plug during the ignition delay period, where the temperature in the (PAHs) are the precursors for soot production and the quantity of soot produced is determined by cylinder is not very high, and the consumption of the hydrocarbon fuel was limited. During the acute the two processes of PAH formation and oxidation. The quantity of soot increased first and then burning period, the flame spread rapidly from near the spark plug. The contact area between the decreased because before the spark timing, the temperature was relatively low, and almost no PAHs flame and the combustible gas mixture also increased rapidly, where the hydrocarbon fuel burned were generated. After the spark timing, the temperature rapidly increased to 1500–2000 K, the rapidly and the mass was greatly reduced. Figure 19b shows the UHC quantity for different water temperature was relatively high, the amount of PAHs generated increased rapidly, and the amount injection temperatures. Compared with no water injection conditions, direct water injection in the of soot also increased significantly. Finally, the temperature continued to increase, and some PAHs cylinder could reduce the quantity of UHCs. This was because water can be decomposed into H were oxidized; therefore, the quantity of soot generated decreased. 2 and O at high temperatures according to 2H O + 2 242,000 kJ/kmol = 2H + O . Although the 2 2 × 2 2 quantity of H2 and O2 was small, it could promote the complete combustion of fuel and improve fuel utilization. Figure 20b shows that when the water injection temperature was 150 ◦C, the amount of UHCs was significantly lower than with the water injection temperature of 60 ◦C. The main reasons for the generation of UHCs were incomplete combustion, wall quenching, and oil film adsorption on the wall. Properly improving the temperature of the water injection can make the water more evenly distributed in the cylinder, and it will not be easy to produce flame quenching during the combustion process. This can reduce UHC emissions.

4.3.2. The Influence of the Water Injection Temperature on the Soot and CO Emissions Figure 20a shows the generated quantity of soot in the process of combustion. The soot first increased continuously, then lowered and finally stabilized. Polycyclic aromatic hydrocarbons (PAHs) are the precursors for soot production and the quantity of soot produced is determined by the two processes of PAH formation and oxidation. The quantity of soot increased first and then decreased because before the spark timing, the temperature was relatively low, and almost no PAHs were generated. After the spark timing, the temperature rapidly increased to 1500–2000 K, the temperature was relatively high, the amount of PAHs generated increased rapidly, and the amount of soot also increased significantly. Finally, the temperature continued to increase, and some PAHs were oxidized; therefore, the quantity of soot generated decreased. The quantity of soot produced is shown in Figure 20b. Compared with the no-water condition, water injection significantly reduced the quantity of soot. Under high-temperature and fuel-rich conditions, hydrocarbon fuels in gasoline engines can easily generate soot. Direct water injection in the cylinder can effectively reduce soot emissions, mainly due to the following two aspects. On the one hand, the specific heat capacity of water is large. Water can effectively reduce the temperature Energies 2020, 13, 4931 21 of 24 such that the combustion can be carried out at a low temperature. On the other hand, under the condition of high temperature and a lack of oxygen, the carbon particles produced by combustion can react with water vapor, as shown in Equations (4)–(7). This can effectively reduce soot emissions. Although the intermediate product H2 is of a low quantity, it can increase the combustion rate and improve the combustion status. Figure 20b shows that as the temperature of the water injection increased, the quantity of soot decreased. This is because when the water injection temperature was increased, the water was distributed more evenly in the cylinder. The uneven distribution of water may have caused incomplete combustion. Compared with the water injection temperature of 60 ◦C, when the water injection temperature was 150 ◦C, the water distribution in the cylinder was more uniform, flame quenching was less likely to occur, and the combustion was more stable. The temperature in the cylinder also increased a bit, which was more conducive to flame propagation. Therefore,Energies 2020,properly 13, x FOR PEER increasing REVIEW the temperature of the water injection can reduce soot emissions.21 of 25

0.082 0.12

0.080 0.10

0.08 0.078

0.06 0.076 No water injection 0.04 Water injection temperature 60 ℃ 0.074 Soot quantity (mg) quantity Soot Soot quantity (mg) quantity Soot Water injection 0.02 temperature150 ℃ 0.072 0.00 -20-100 102030405060 Crank angle (°CA) 0.070 No water 60 ℃ 150 ℃

(a) (b)

Figure 20.20.The The quantity quantity of sootof soot generated generated under un diffdererent different working working conditions conditions (n = 5500 rpm,(n = equivalence5500 rpm, equivalenceratio = 1.1): (ratioa) the = soot1.1): quantity(a) the soot at di quantityfferent crank at different angles andcrank (b )angles the soot and quantity (b) the produced soot quantity with produceddifferent water with injectiondifferent temperatures.water injection temperatures.

The quantity of soot produced is shownC + H inO FigureCO 20b.+ H Compared with the no-water condition,(4) 2 → 2 water injection significantly reduced the quantity of soot. Under high-temperature and fuel-rich C + 2H O CO + 2H (5) conditions, hydrocarbon fuels in gasoline engines2 → can easily 2generate soot. Direct water injection in the cylinder can effectively reduce sootCO emissions,+ H O mainlyCO + dueH to the following two aspects. On the(6) 2 → 2 2 one hand, the specific heat capacity of water is large. Water can effectively reduce the temperature 2H + O H O (7) such that the combustion can be carried out2 at a2 low→ temperature.2 On the other hand, under the conditionFigure of 21 high shows temperature the generated and a quantity lack of oxyg of COen, in the the carbon process particles of combustion. produced The by masscombustion of CO wascan react 36 mg with when water there vapor, was as no shown water injection,in Equations and (4 the)–(7). final This production can effectively of CO reduce under soot different emissions. water Althoughinjection temperatures the intermediate did notproduct change H2 significantly,is of a low quantity, where both it can were increase close tothe 31 combustion mg. Water injectionrate and improvecould reduce the thecombustion amountof status. CO produced Figure 20b but theshows temperature that as the of thetemperature water injection of the had water little injection effect on increased,CO emissions. the quantity Figure 21 ofa soot shows decreased. that the quantityThis is because of CO when generated the water first increased injection temperature to the maximum was valueincreased, and thenthe water decreased. was distribute The downwardd more trendevenly of in CO the was cylinder. basically The the uneven same underdistribution different of water mayinjection have temperatures. caused incomplete At this combustion. stage, CO wasCompared mainly with oxidized the water to CO injection2. The eff temperatureect of different of 60 water °C, wheninjection the temperatureswater injection on temperature the oxidation was of 150 CO °C, was th basicallye water distribution the same; therefore,in the cylinder the peak was ofmore the quantityuniform, offlame CO generationquenching determined was less likely its final to production. occur, and The the generation combustion of COwas mainly more includesstable. The the temperaturedirect oxidation in the of hydrocarbon cylinder also fuel increased to form a CO bit, and which CO2 wasto produce more conducive CO through to aflame reduction propagation. reaction. TheTherefore, decomposition properly reactionincreasing of the formaldehyde temperature (CH of the2O) water at high injection temperature can reduce is the soot main emissions. source of CO. CH2O decomposes at high temperature, CH2O and OH interact to form HCO radicals, and HCO C + H2O → CO + H2 (4) radicals further react to form CO. Water injection can reduce the amount of CO generated, mainly due to two aspects. First, direct water injectionC + 2H2 inO the→ cylinderCO + 2H2 reduces the temperature and inhibits(5)

CO + H2O → CO2 + H2 (6)

2H2 + O2 → H2O (7) Figure 21 shows the generated quantity of CO in the process of combustion. The mass of CO was 36 mg when there was no water injection, and the final production of CO under different water injection temperatures did not change significantly, where both were close to 31 mg. Water injection could reduce the amount of CO produced but the temperature of the water injection had little effect on CO emissions. Figure 21a shows that the quantity of CO generated first increased to the maximum value and then decreased. The downward trend of CO was basically the same under different water injection temperatures. At this stage, CO was mainly oxidized to CO2. The effect of different water injection temperatures on the oxidation of CO was basically the same; therefore, the peak of the quantity of CO generation determined its final production. The generation of CO mainly includes the direct oxidation of hydrocarbon fuel to form CO and CO2 to produce CO through a reduction reaction. The decomposition reaction of formaldehyde (CH2O) at high temperature is the main source of CO. CH2O decomposes at high temperature, CH2O and OH interact to form HCO

Energies 2020, 13, x FOR PEER REVIEW 22 of 25 Energies 2020, 13, 4931 22 of 24 radicals, and HCO radicals further react to form CO. Water injection can reduce the amount of CO generated, mainly due to two aspects. First, direct water injection in the cylinder reduces the the conversion of CH2O to CO. The second is that water vapor dilutes the oxygen concentration, temperature and inhibits the conversion of CH2O to CO. The second is that water vapor dilutes the making the mixture evenly distributed, thereby reducing CO emissions. oxygen concentration, making the mixture evenly distributed, thereby reducing CO emissions.

40 50 35

40 30

30 25 No water injection Water injection 20 20 temperature 60 ℃

CO quantity (mg) quantity CO Water injection 15 10 temperature150 ℃ (mg) CO quantity n = 5500 rpm 10 Equivalence ratio = 1.1 0 Spark timing = - 21 oCA 5

-20-100 102030405060 0 Crank angle (°CA) No water 60 ℃ 150 ℃

(a) (b)

Figure 21. The quantity of CO under difffferenterent workingworking conditionsconditions ((nn == 5500rpm, equivalence ratio = 1.1): (a) thethe COCO quality quality at at di differentfferent crank crank angles angles and and (b) ( theb) COthe quantityCO quantity produced produced with diwithfferent different water waterinlet temperatures. inlet temperatures.

5. Conclusions Conclusions InIn this study,study, the the gasoline gasoline engine engine was was induced induced to knockto knock by advancingby advancing the sparkthe spark timing, timing, and thenand thenthe influence the influence of the of water the water injection injection temperature temperat onure the on knock the andknock emissions and emissions was explored. was explored. The specific The specificheat capacity heat capacity of water of is water relatively is relatively large. Water large. entering Water entering the cylinder the cylinder can absorb can the absorb heat releasedthe heat releasedby combustion, by combustion, reducing reducing the temperature the temperature and pressure, and pressure, thereby reducingthereby reducing the tendency the tendency to produce to produceknock. In knock. addition, In addition, water injection water injection will also will have al anso impacthave an on impact emissions. on emissions. Different Different water injection water injectiontemperatures temperatures have diff erenthave different heat absorption heat absorpti capabilitieson capabilities and different and different distributions distributions in the cylinder; in the cylinder;therefore, therefore, different waterdifferent injection water temperaturesinjection temperatures have diff erenthave edifferentffects on effects knock andon knock emissions. and emissions.The conclusions The conclusions drawn from drawn the analysis from the were analysis as follows: were as follows: 1. WhenWhen the the spark spark timing timing was was advanced, advanced, the the maximum maximum pressure pressure and and pressure pressure increase increase rate rate in the cylindercylinder increased increased andand the the position position where where the maximumthe maximum combustion combustion pressure pressure appeared appeared gradually graduallyapproached approached the TDC, whichthe TDC, was which beneficial was forbenefi improvingcial for improving the power andthe economypower and of economy the gasoline of theengine. gasoline However, engine. the However, increase the inthe increase cylinder in the pressure cylinder and pressure rate of pressure and rate increase of pressure increased increase the increasedtendency the of thetendency gasoline of enginethe gasoline to knock. engine to knock. 2. ComparedCompared with with the the temperature temperature of of150 150 °C,◦ C,when when the the water water injection injection temperature temperature was was 60 °C, 60 the◦C, effectthe e offfect reducing of reducing the pressure the pressure in the in cylinde the cylinderr was more was more obvious, obvious, and the and knock the knock intensity intensity was lower.was lower. However, However, the two the twowater water injection injection temp temperatureseratures suppressed suppressed the the occurrence occurrence of of knock. knock. IncreasingIncreasing the the water water injection injection temperature temperature made made the the water water distribution distribution in in the the cylinder cylinder more uniform,uniform, which which helped helped to tosuppress suppress knock. knock. Under Under the condition the condition of no ofknock, no knock, increasing increasing the water the injectionwater injection temperature temperature increased increased the amount the of amount circulating of circulating work. The work.combination The combination of direct water of injectiondirect water in the injection cylinder in and thecylinder advancing and the advancing spark timing the spark improved timing improvedthe powerthe of powerthe gasoline of the engine.gasoline engine. 3. ComparedCompared with with when when water water was was not not injected, injected, the the NO NOxx, ,CO, CO, soot, soot, and and UHC UHC emissions emissions after after the waterwater injection werewere reduced reduced to varyingto varying degrees. degrees. Compared Compared with the waterwith injectionthe water temperature injection temperatureof 60 ◦C, when of 60 the °C, water when injection the water temperature injection temperature was 150 ◦C, was the soot150 °C, emissions the soot and emissions UHCs were and UHCssignificantly were significantly reduced, and reduced, the NO andx emissions the NOx wereemissions slightly were increased. slightly increased. The effect The of the effect water of theinjection water injection temperature temperature on CO emissions on CO emissions was not obvious.was not obvious. Author Contributions: Conceptualization, A.L. and Z.Z.; Methodology, A.L.; Software, A.L.; Validation, A.L. and Z.Z.; Data Curation, A.L.; Writing—original draft preparation, A.L.; writing—review and editing, Z.Z.; Visualization, Z.Z.; Supervision, Z.Z.; Project Administration, Z.Z. All authors have read and agreed to the published version of the manuscript

Energies 2020, 13, 4931 23 of 24

Author Contributions: Conceptualization, A.L. and Z.Z.; Methodology, A.L.; Software, A.L.; Validation, A.L. and Z.Z.; Data Curation, A.L.; Writing—original draft preparation, A.L.; writing—review and editing, Z.Z.; Visualization, Z.Z.; Supervision, Z.Z.; Project Administration, Z.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China Program, grant number 51776024. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Bozza, F.; De Bellis, V.; Teodosio, L. Potentials of cooled EGR and water injection for knock resistance and fuel consumption improvements of gasoline engines. Appl. Energy 2016, 169, 112–125. [CrossRef] 2. Ye, Z.; Li, L. Control options for exhaust gas after treatment and fuel economy of GDI engine systems. In Proceedings of the 42nd IEEE International Conference on Decision and Control (IEEE Cat. No.03CH37475), Maui, HI, USA, 9–12 December 2003; pp. 1783–1788. 3. Hill, M.G. Internal Combustion Engine Fundamentals; McGraw-Hill: New York, NY, USA, 1988. 4. Wang, Z.; Liu, H.; Reitz, R.D. Knocking combustion in spark-ignition engines. Prog. Energy Combust. Sci. 2017, 61, 78–112. [CrossRef] 5. Cho, S.; Song, C.; Kim, N.; Oh, S.; Han, D.; Min, K. Influence of the wall temperatures of the combustion chamber and intake ports on the charge temperature and knock characteristics in a spark-ignited engine. Appl. Therm. Eng. 2020, 116000. [CrossRef] 6. Robert, A.; Richard, S.; Colin, O.; Martinez, L.; De Francqueville, L. LES prediction and analysis of knocking combustion in a spark ignition engine. Proc. Combust. Inst. 2015, 35, 2941–2948. [CrossRef] 7. Kawahara, N.; Tomita, E.; Sakata, Y. Auto-ignited kernels during knocking combustion in a spark-ignition engine. In Proceedings of the Combustion Institute, Heidelberg, Germany, 5–11 August 2006; Volume 31, pp. 2999–3006. 8. Linse, D.; Kleemann, A.; Hasse, C. Probability density function approach coupled with detailed chemical kinetics for the prediction of knock in turbocharged direct injection spark ignition engines. Combust. Flame 2014, 161, 997–1014. [CrossRef] 9. Boretti, A. Water injection in directly injected turbocharged spark ignition engines. Appl. Therm. Eng. 2013, 52, 62–68. [CrossRef] 10. D’Adamo, A.; Berni, F.; Breda, S.; Lugli, M.; Fontanesi, S.; Cantore, G. A Numerical investigation on the potentials of water injection as a fuel efficiency enhancer in highly downsized GDI engines. SAE Tech. Paper Ser. 2015, 1.[CrossRef] 11. Worm, J.; Naber, J.; Duncan, J.; Barros, S.; Atkinson, W. Water injection as an enabler for increased efficiency at high-load in a direct injected, boosted, SI engine. SAE Int. J. Engines 2017, 10, 951–958. [CrossRef] 12. De Bellis, V.; Bozza, F.; Teodosio, L.; Valentino, G. Experimental and numerical study of the water injection to improve the fuel economy of a small size turbocharged SI engine. SAE Int. J. Engines 2017, 10, 550–561. [CrossRef] 13. Iacobacci, A.; Marchitto, L.; Valentino, G. Water injection to enhance performance and emissions of a turbocharged gasoline engine under high load condition. SAE Int. J. Engines 2017, 10, 928–937. [CrossRef] 14. Hoppe, F.; Thewes, M.; Baumgarten, H.; Dohmen, J. Water injection for gasoline engines: Potentials, challenges, and solutions. Int. J. Engine Res. 2015, 17, 86–96. [CrossRef] 15. Brusca, S.; Galvagno, A.; Lanzafame, R.; Mauro, S.; Messina, M. Fuels with low octane number: Water injection as knock control method. Heliyon 2019, 5, e01259. [CrossRef][PubMed] 16. Farokhipour, A.; Hamidpour, E.; Amani, E. A numerical study of NOx reduction by water spray injection in gas turbine combustion chambers. Fuel 2018, 212, 173–186. [CrossRef] 17. Ariemma, G.; Sabia, P.; Sorrentino, G.; Bozza, P.; De Joannon, M.; Ragucci, R. Influence of Water Addition on MILD Ammonia Combustion Performances and Emissions; Elsevier: Amsterdam, The Netherlands, 2020. 18. Wang, J.; Duan, X.; Liu, Y.; Wang, W.; Liu, J.; Lai, M.-C.; Li, Y.; Guo, G. Numerical investigation of water injection quantity and water injection timing on the thermodynamics, combustion and emissions in a hydrogen enriched lean-burn natural gas SI engine. Int. J. Hydrogen Energy 2020, 45, 17935–17952. [CrossRef] Energies 2020, 13, 4931 24 of 24

19. Merola, S.S.; Irimescu, A.; Vaglieco, B.M. Influence of water injection on combustion identified through spectroscopy in an optical direct injection spark ignition engine. Fuel 2020, 273, 117729. [CrossRef] 20. Thewes, M.; Baumgarten, H.; Scharf, J.; Birmes, G.; Hoppe, F. Water injection—High power and high efficiency combined. In Proceedings of the 25th Aachen Colloquium Automobile and Engine Technology, Aachen, Germany, 10–12 October 2016. 21. Zhuang, Y.; Sun, Y.; Teng, Q.; He, B.; Chen, W.; Qian, Y. Investigation of water injection benefits on downsized boosted direct injection spark ignition engine. Fuel 2020, 264, 116765. [CrossRef] 22. Zhen, X.; Wang, Y.; Xu, S.; Zhu, Y. Study of knock in a high compression ratio spark-ignition methanol engine by multi-dimensional simulation. Energy 2013, 50, 150–159. [CrossRef] 23. Ghazal, O.H. Combustion analysis of hydrogen-diesel dual fuel engine with water injection technique. Case Stud. Therm. Eng. 2019, 13, 100380. [CrossRef] 24. Serrano, J.; Jiménez-Espadafor, F.; López, A. Analysis of the effect of the hydrogen as main fuel on the performance of a modified compression ignition engine with water injection. Energy 2019, 173, 911–925. [CrossRef] 25. Bratkov, A.A.; Azev, V.S.; Radchenko, E.D.; Gladkikh, V.A.; Livshits, S.M. Addition of water to motor fuels? One of the means for conservation and expansion of fuel resources. Chem. Technol. Fuels Oils 1980, 16, 731–735. [CrossRef] 26. Wu, Y.-Y.; Chen, B.-C.; Hwang, J.-J.; Chen, C.Y. Performance and emissions of motorcycle engines using water-fuel emulsions. Int. J. Veh. Des. 2009, 49, 91. [CrossRef] 27. Abu-Zaid, M. Performance of single cylinder, direct injection diesel engine using water fuel emulsions. Energy Convers. Manag. 2004, 45, 697–705. [CrossRef] 28. Serrano, J.; Jiménez-Espadafor, F.; López, A. Analysis of the effect of different hydrogen/diesel ratios on the performance and emissions of a modified compression ignition engine under dual-fuel mode with water injection. Hydrogen-diesel dual-fuel mode. Energy 2019, 172, 702–711. [CrossRef] 29. Bernal, J.L.L.; Ferreira, J.V. Model of water injection process during closed phase of spark ignition engine. Energy 2019, 174, 1121–1132. [CrossRef] 30. Samec, N. Numerical and experimental study of water/oil emulsified fuel combustion in a diesel engine. Fuel 2002, 81, 2035–2044. [CrossRef] 31. Chen, B.; Zhang, L.; Han, J.; Chen, X. Investigating the effect of increasing specific heat and the influence of charge cooling of water injection in a TGDI engine. Appl. Therm. Eng. 2019, 149, 1105–1113. [CrossRef] 32. Miganakallu, N.; Yang, Z.; Rogó˙z,R.; Yang, Z.; Kapusta, Ł.J.; Christensen, C.; Barros, S.; Naber, J. Effect of water–methanol blends on engine performance at borderline knock conditions in gasoline direct injection engines. Appl. Energy 2020, 264, 114750. [CrossRef] 33. Chen, Z.; He, J.; Chen, H.; Wang, L.; Geng, L. Experimental study of the effects of spark timing and water injection on combustion and emissions of a heavy-duty natural gas engine. Fuel 2020, 276, 118025. [CrossRef] 34. Valero-Marco, J.; Lehrheuer, B.; López, J.J.; Pischinger, S. Potential of water direct injection in a CAI/HCCI gasoline engine to extend the operating range towards higher loads. Fuel 2018, 231, 317–327. [CrossRef] 35. Zhao, R.; Zhang, Z.; Zhuge, W.; Zhang, Y.; Yin, Y. Comparative study on different water/steam injection layouts for fuel reduction in a turbocompound diesel engine. Energy Convers. Manag. 2018, 171, 1487–1501. [CrossRef] 36. Arabaci, E.; Içingür, Y.; Solmaz, H.; Uyumaz, A.; Yilmaz, E.; Içıngür, Y.; Yılmaz, E. Experimental investigation of the effects of direct water injection parameters on engine performance in a six-stroke engine. Energy Convers. Manag. 2015, 98, 89–97. [CrossRef] 37. Li, A.; Zheng, Z.-L.; Peng, T. Effect of water injection on the knock, combustion, and emissions of a direct injection gasoline engine. Fuel 2020, 268, 117376. [CrossRef] 38. Liu, Y.-D.; Jia, M.; Xie, M.-Z.; Pang, B. Enhancement on a skeletal kinetic model for primary reference fuel oxidation by using a semidecoupling methodology. Energy Fuels 2012, 26, 7069–7083. [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/).