sustainability

Article The Effect of Water Injection on the Control of In-Cylinder Pressure and Enhanced Power Output in a Four-Stroke Spark-Ignition Engine

Mingrui Wei 1,2, Thanh Sa Nguyen 1,2,3,*, Richard Fiifi Turkson 1,2,4, Guanlun Guo 1,2 and Jinping Liu 1,2

1 Hubei Key Laboratory of Advanced Technology for Automotive Components, Wuhan University of Technology, Wuhan 430070, China; [email protected] (M.W.); [email protected] (R.F.T.); [email protected] (G.G.); [email protected] (J.L.) 2 Hubei Collaborative Innovation Center for Automotive Components Technology, Wuhan University of Technology, Wuhan 430070, China 3 Faculty of Mechanical Engineering, HoChiMinh University of Transport, HoChiMinh City 760000, Vietnam 4 Mechanical Engineering Department, Ho Polytechnic, P.O. Box HP 217, Ho 036, Ghana * Correspondence: [email protected]; Tel.: +84-906-332-284

Academic Editor: Marc A. Rosen Received: 14 June 2016; Accepted: 24 September 2016; Published: 30 September 2016

Abstract: This paper presents the results for liquid water injection (WI) into a cylinder during the compression and expansion strokes of an internal combustion engine (ICE), with the aim of achieving an optimal in-cylinder pressure and improving power output using CFD simulation. Employing WI during the compression stroke at 80◦ of crank angle (CA) before top dead centre (bTDC) resulted in the reduction of compression work due to a reduction in peak compression pressure by a margin of about 2%. The decreased peak compression pressure also yielded the benefit of a decrease in NOx emission by a margin of 34% as well as the prevention of detonation. Using WI during the expansion stroke (after top dead centre–aTDC) revealed two stages of the in-cylinder pressure: the first stage involved a decrease in pressure by heat absorption, and the second stage involved an increase in the pressure as a result of an increase in the steam volume via expansion. For the case of water addition (WA 3.0%) and a water temperature of 100 ◦C, the percentage decrease of in-cylinder pressure was 2.7% during the first stage and a 2.5% pressure increase during the second stage. Water injection helped in reducing the energy losses resulting from the transfer of heat to the walls and exhaust gases. At 180◦ CA aTDC, the exhaust gas temperature decreased by 42 K, 89 K, and 136 K for WA 1.0, WA 2.0, and WA 3.0, respectively. Increasing the WI temperature to 200 ◦C resulted in a decrease of the in-cylinder pressure by 1.0% during the first stage, with an increase of approximately 4.0% in the second stage. The use of WI in both compression and expansion strokes resulted in a maximum increase of in-cylinder pressure of about 7%, demonstrating the potential of higher power output.

Keywords: CFD simulation; engine efficiency; heat losses; heat transfer; water addition; water injection

1. Introduction In recent years, concerns over environmental pollution and energy balance has resulted in major interest in research regarding the optimal design of internal combustion engines (ICEs), especially automobile engines. There is an increasing trend of the quantity of automobiles sold (including passenger and commercial vehicles) in the world on a year-to-year basis. With the increase in the number of vehicles produced each year, it is accompanied with an increase in the number of engines produced because ICEs currently represent the prime source of power for automobiles. Thus,

Sustainability 2016, 8, 993; doi:10.3390/su8100993 www.mdpi.com/journal/sustainability Sustainability 2016, 8, 993 2 of 22 improving the efficiency of ICEs using different methods leads to considerable benefits accruing from the production of more engine power while utilizing less fuel than would normally be the case. The utilization of liquid WI in spark-ignition (SI) engines has been studied for a number of years by many researchers to reduce NOx emissions [1,2]. In one study [3], hydroxide and hydrogen were formed from the thermal-dissociation process of water at high temperature, which absorbed the heat during combustion. Hence, this method is useful for controlling the peak in-cylinder temperature and for reducing unwanted emissions. Nande et al. [1] concluded that WI is an improved technique for reducing NOx emissions based on an investigation conducted on a hydrogen-fueled SI engine combined with in-cylinder direct WI. In another study, water was injected into the intake manifold to investigate the effects of water on anti-knock ability and the possible reduction in compression work [4]. The study concluded that this WI technology has benefits for anti-knock and higher power output by reducing the work done during the compression stroke. Water was injected into the inlet port up to four times the amount of fuel injected to estimate the effects on combustion and pollutant emission [5]. Many different ratios of water to fuel mass were applied for injecting water into ICEs. Karagöz et al. [6] investigated the addition of hydro-oxygen and water to the intake system of an SI engine using up to 25% of water mass relative to gasoline mass for studying the performance of the engine and emission levels. Using a single cylinder engine, Feng et al. [7] experimented with two kinds of fuel; pure gasoline and 35% volume butanol–gasoline blend +1% H2O addition in different operating modes. The results demonstrated that engine performance, brake specific fuel consumption (BSFC), and CO and HC emissions of the fuel blends were better than those of pure gasoline for the test conditions considered. For a small SI engine driving a generator, Munsin [8] investigated the effects of hydrous ethanol with high water contents up to 40%. Injected water had the effect of reducing NOx emissions by a margin of up to approximately 35% in comparison with pure gasoline. WI technology was also evaluated as an effective method for controlling emission levels and for achieving an enhanced engine performance [5]. The effect of WI on engine performance was also explored in the study conducted by Sari et al. [9] with high water content ethanol mixtures from 5%–40% of mass in 0.668 L of a single-cylinder port injected SI engine. An increased water content enhanced knock avoidance capabilities and aqueous alcohol injection showed promise for higher engine efficiency and lower fuel costs. The conclusion of the study also pointed to the fact that as the WI level supplied to the engine increased, the percentage of useful work increased, while the losses other than unaccounted losses decreased [3]. Based on CFD results, Boretti et al. [10] showed that the brake thermal efficiency was improved by more than 55% with 200% of injected water by mass in comparison with the mass of oxygen and hydrogen mixture operating with a significant amount of exhaust gas recirculation. WI in an ICE is not only useful for enhancing anti-knock capabilities and emission reduction but also yields benefits in the recycling of waste energy from the exhaust gas. The cylinder of an ICE has a limitation in the expansion ratio; that is the burnt air-fuel mixture cannot fully expand in the cylinder. Because the temperature and pressure of exhaust gas are higher than that of ambient conditions, the engine exhaust gas contains pressure energy and thermal energy [11]. Since only about 12%–25% of fuel energy will be used to drive the wheels, some of the thermal energy escapes from engine through the exhaust gases, while the remainder of the energy balance is accounted for by friction, coolant, and other means [12–16]. Based on the heat balance in ICEs (including diesel and gasoline), Wang et al. [17] and Dolz et al. [18] proffered that only about one third of a fuel's chemical energy is converted into useful work and that the remainder of the energy is lost to the operating environment. According to the research conducted by Wang et al. [19], the temperature of the exhaust gas was around 800 K while in another study, a 600–700 K exhaust temperature was reported [20]. In recycling the exhaust gas energy of ICEs, the thermal efficiency is improved leading to significant improvements in power output, fuel economy, and emission levels. Sustainability 2016, 8, 993 3 of 22

Moreover, in the work of Hatazawa et al. [21], Stabler [12], Taylor [13], Yu and Chau [22], and Yang [23], it was stated that 30%–40% of waste heat produced from the fuel burning process was lost to the environment through the exhaust gases. Thus, fuel energy cannot be completely converted into useful work and a significant amount of energy is lost through the transfer of heat to the cylinder walls and to the exhaust gases. If only 6% of the heat contained in exhaust gases is converted into electrical power, this would mean a reduction of fuel consumption by 10% due to the decrease in mechanical losses [24]. Other studies stated that an increase of 20% in fuel efficiency can be easily achieved by converting about 10% of the waste heat into useful work [23,25]. Using WI in combination with the Rankine cycle or Organic Rankine cycle to improve ICE energy efficiency is one method of engine exhaust gas energy recovery. The exhaust pipe is coupled with a Rankine steam cycle system and the high temperature exhaust gas is used to generate steam [26]. This study involved the coupling of a traditional ICE cycle with the steam expansion cycle using steam injection into the expanding cylinder. In this way, the engine thermal efficiency can be improved by 6.3% at 6000 r/min. Based on experimental work by Endo et al. [27], the thermal recovery system showed a maximum thermal cycle efficiency of 13%. In a similar study conducted by Bari and Hossain [28], an additional increase in power of 16% was realized. After system optimization, the additional power increased from 16% to 23.7%.The waste heat from the exhaust gases and cooling water was used in a Homogeneous Charge Compression Ignition (HCCI) engine in the work of Sarabchi [29]. The theoretical thermal efficiency of the system was 5.19% higher than that of the HCCI engine, while the reduction in CO2 emission was 4.067%. The wasted heat recovery technology was also applied on a hydrogen-fueled internal combustion engine (HICE) because more heat is wasted by the HICE, and it produces nearly three times more water than a conventional engine [30]. A waste heat recovery sub-system was used to improve the overall thermal efficiency from 27.2% to 33.6% at engine speeds starting from 1500 to 4500 rpm. In another study [31], water was injected directly into the cylinder to enhance the efficiency of the engine in recovering the wasted heat. Using the waste heat from the exhaust gas and engine coolant system to heat up the injected water, the theoretical thermal efficiency of the ICE reached 53% and 67% when the WI temperature was 120 ◦C and 200 ◦C, respectively. From the reviews on WI in ICEs, WI has a good effect in reducing emissions, especially NO emissions. Injected water has been used as an absorbent for waste heat recovery. However, injected water can affect engine power output during the decrease or increase of in-cylinder pressure which takes place when the injected water absorbs heat and evaporates, respectively. In this study, the injection of liquid water into the intake system of a four-stroke engine equipped with a gasoline direct injection (GDI) system was considered for enhancing the in-cylinder pressure by the use of the AVL Fire CFD simulation code. Water was injected during the compression stroke to control compression pressure and thus reduce compression work. On the other hand, water was injected in the expansion stroke to improve the expansion pressure by taking advantage of the volumetric expansion of steam. The benefits of WI in the ICE were significant by the timed injection of water during the compression and expansion strokes. The effects of WI (for simulation cases) on in-cylinder pressure, heat transfer, NO emission, and other factors were examined and are discussed in this study.

2. Studied Model A GDI engine model with an axisymmetric cylinder was used for the current study, the parameters of which are presented in Table1. Two injectors were used in the studied engine model: one for controlling fuel injection and the other for water injection, as illustrated in Figure1. An engine model was used with gasoline as a primary reference fuel (fuel comprised of 10% n-heptane and 90% iso-octane). Fuel was injected at a rate of 2.332 × 10−5 kg/cycle and the fuel temperature was fixed at 20 ◦C. The water injector was installed adjacent to the spark plug on the single cylinder engine. SustainabilitySustainability20162016, 8, ,9938, 993 4 of4 22 of 22

Table 1. Engine parameters. Sustainability2016, 8, 993 Table 1. Engine parameters. 4 of 22 Description Details Table 1. Engine parameters. ICE Descriptiontype Single-cylinder Details GDI CompressionDescriptionICE typeratio Single-cylinder Details 13 GDI BoreCompressionICE [mm] type ratio Single-cylinder 80 13 GDI Bore [mm] 80 StrokeCompressionStroke [mm] [mm] ratio 81.4 81.4 13 DisplacementDisplacementBore [mm] [cm 3 [cm] 3] 441441 80 EngineEngineStroke speed speed[mm] [rpm] [rpm] 2000 81.4 2000 Ignition timing 20◦ CA bTDC Displacement [cm3] 441 IgnitionFuelinjection timing timing 20° 60 CA◦ CA bTDC bTDC FuelFuelEngine injection injection speed timing duration [rpm] [ ◦ CA] 60° CA 2000 20bTDC Fuel injectionIgnition duration timing [° CA] 20° CA 20 bTDC Fuel injection timing 60° CA bTDC Fuel injection duration [° CA] 20

FigureFigure 1. Schematic 1. Schematic illustration illustration of ofthe the injector injector position position in inthe the numerical numerical investigation. investigation. Figure 1. Schematic illustration of the injector position in the numerical investigation. As previously stated, this paper used AVL-Fire for performing CFD simulation of the liquid As previously stated, this paper used AVL-Fire for performing CFD simulation of the liquid water water injectionAs previously into the stated, engine this cylinder paper inused an AVL-Fireattempt to for improve performing the in-cylinderCFD simulation pressure. of the The liquid 3D injection into the engine cylinder in an attempt to improve the in-cylinder pressure. The 3D geometrical geometricalwater injection shapes into of the the ICE engine were cylinder built using in an theattempt Pro-Engineer to improve software the in-cylinder (Parametric pressure. Technology The 3D shapes of the ICE were built using the Pro-Engineer software (Parametric Technology Corporation, Corporation,geometrical Needham, shapes of MA, the ICEUS). were The builtcomputational using the Pro-Engineer domain was software divided (Parametric into three regions:Technology the Needham, MA, US). The computational domain was divided into three regions: the manifold intake manifoldCorporation, intake region, Needham, the exhaustMA, US). pipe The region, computational and the piston-chamberdomain was divided region. into It threewas necessary regions: the for region, the exhaust pipe region, and the piston-chamber region. It was necessary for the piston the pistonmanifold and intake valves region, to move the exhaust according pipe toregion, the resolution and the piston-chamber of the crank region.angle andIt was thus, necessary the Fame for and valves to move according to the resolution of the crank angle and thus, the Fame Engine Enginethe Pluspiston module and valves was usedto move for generatingaccording to the the moving resolution mesh. of Figurethe crank 2 presents angle and the thus, engine the modelFame PlusEngine module Plus wasmodule used was for used generating for generating the moving the moving mesh. mesh. Figure Figure2 presents 2 presents the the engine engine model model in in the simulation domain. thein simulation the simulation domain. domain.

FigureFigureFigure 2. The 2.2. TheThe engine engine model model used used used for for for the the the simulation. simulation. simulation.

FameFame Engine Engine Plus Plus was was employed employed for for producing producing 3D 3D hexahedral hexahedral cellscells forfor thethe engine movingmoving Fame Engine Plus was employed for producing 3D hexahedral cells for the engine moving mesh, mesh,mesh, and andinvolved involved the theintake intake port port and and valves, valves, th the cylindere cylinder head, head, the the combustion combustion chamber, andand thethe and involved the intake port and valves, the cylinder head, the combustion chamber, and the exhaust exhaustexhaust port port and and valves. valves. The The hexahedral hexahedral cells cells were were employed employed for for mesh mesh generation generation duedue to thethe betterbetter port and valves. The hexahedral cells were employed for mesh generation due to the better accuracy accuracyaccuracy and andstability stability compared compared to theto the tetrahedral tetrahedral cells cells [32]. [32]. The The number number of of cellscells waswas about 168,498168,498 and stability compared to the tetrahedral cells [32]. The number of cells was about 168,498 at top dead at topat deadtop dead centre centre (TDC) (TDC) position position and and around around 436,286 436,286 cells cells at at bottom bottom dead dead centrecentre (BDC) position; centre (TDC) position and around 436,286 cells at bottom dead centre (BDC) position; about half of aboutabout half halfof the of thecells cells of theof the computational computational mesh mesh around around the the valves valves and and combustion combustion chamber werewere the cells of the computational mesh around the valves and combustion chamber were concentrated concentratedconcentrated to obtain to obtain accurate accurate results. results. The The type type an and dsize size of of simulation simulation mesh mesh havehave an effect onon thethe to obtain accurate results. The type and size of simulation mesh have an effect on the simulation simulationsimulation results results and and calculation calculation time. time. In Inthis this study, study, the the concept concept ofof meshmesh waswas chosen inin thethe Sustainability 2016, 8, 993 5 of 22 Sustainability2016, 8, 993 5 of 22 comparisionresults and calculationof about 43,398 time. cells In this at the study, TDC the position concept and of meshapproximately was chosen 163,110 in the cells comparision at the BDC of positionabout 43,398 [32] in cells a similar at the simulation TDC position of an and engine approximately model. The 163,110 number cells of cells at the was BDC also position compared [32 with] in a 90,000similar cells simulation at TDC of and an engineapproximately model. The 180,000–200,000 number of cells cells was at also BDC compared in a study with regarding 90,000 cells the at combustionTDC and approximately process in a 180,000–200,000compressed natural cells atgas BDC dire inct a injection study regarding engine thewith combustion both simulation process and in a experimentalcompressed naturalstudies gas[33]. direct Figure injection 3 illustrates engine the with numerical both simulation mesh of and the experimental engine model studies with [the33]. pistonFigure at3 illustrates the TDC position. the numerical mesh of the engine model with the piston at the TDC position.

FigureFigure 3. 3. NumericalNumerical mesh mesh for for the the piston piston position position at at TDC. TDC.

The equations governing the dynamics of gases were the expressions for conservation and the The equations governing the dynamics of gases were the expressions for conservation and the thermodynamic laws. Gases were defined as being compressible viscous fluids. The equations thermodynamic laws. Gases were defined as being compressible viscous fluids. The equations governing the in-flow model included the mass conservation equation, momentum conservation, governing the in-flow model included the mass conservation equation, momentum conservation, and and the energy conservation equation. The standard model k- was used for solving the in-cylinder the energy conservation equation. The standard model k-ε was used for solving the in-cylinder flow flow problems. The turbulence model employed was the k- turbulence model for compressible problems. The turbulence model employed was the k-ε turbulence model for compressible fluids and fluids and high Reynolds number flow [34], where k is the turbulence kinetic energy and is the high Reynolds number flow [34], where k is the turbulence kinetic energy and ε is the turbulence turbulence dissipation rate. The SIMPLE algorithm was employed to solve the resulting algebraic dissipation rate. The SIMPLE algorithm was employed to solve the resulting algebraic equations, based equations, based on the pressure-correction method. The pressure-corrections were used to update on the pressure-correction method. The pressure-corrections were used to update the pressure and the pressure and velocity fields so that the velocity components obtained from the solution of the velocity fields so that the velocity components obtained from the solution of the momentum equations momentum equations satisfied the continuity equation. A second order discretization scheme was satisfied the continuity equation. A second order discretization scheme was used for the momentum used for the momentum equations, turbulent kinetic energy, and dissipation rate in combination equations, turbulent kinetic energy, and dissipation rate in combination with the k-ε turbulence model. with the k- turbulence model. One complete engine cycle was simulated at 360◦ CA bTDC at the beginning of the intake stroke One complete engine cycle was simulated at 360° CA bTDC at the beginning of the intake to 360◦ CA aTDC at the end of the exhaust stroke with a calculus step of 1.0◦ CA. For the boundary stroke to 360° CA aTDC at the end of the exhaust stroke with a calculus step of 1.0° CA. For the conditions, an inlet mass was applied at the entry surface of the manifold and a pressure outlet boundary conditions, an inlet mass was applied at the entry surface of the manifold and a pressure was imposed at the exit surface of the exhaust duct. The rest of the geometry was considered as an outlet was imposed at the exit surface of the exhaust duct. The rest of the geometry was considered impenetrable wall and the temperatures were suitably set as given in Table2. In the simulation, intake as an impenetrable wall and the temperatures were suitably set as given in Table 2. In the air was assumed to be an ideal gas and inlet mass flow rate was also assumed as a constant [33]. simulation, intake air was assumed to be an ideal gas and inlet mass flow rate was also assumed as a Its density will therefore be variable. However, the viscosity, thermal conductivity, and specific heat constant [33]. Its density will therefore be variable. However, the viscosity, thermal conductivity, capacity at constant pressure remained constant. and specific heat capacity at constant pressure remained constant. Table 2. Boundary conditions. Table 2. Boundary conditions.

BoundaryBoundary Condition Condition Value Value Inlet massInlet flow mass rate flow [g/s] rate [g/s] 0.5 0.5 Outlet pressure [bar] 1.3 Outlet pressureManifold [bar] wall [K] 330 1.3 ManifoldExhaust wall [K] wall [K] 550 330 Exhaust wallPiston [K] [K] 450 550 Intake valve [K] 330 Piston [K] 450 Exhaust valve [K] 550 Intake valveChamber [K] [K] 450 330 Exhaust valveCylinder [K] [K] 450 550 Chamber [K] 450 The initial temperature andCylinder pressure [K] in the engine cylinder 450 needed to be established to provide ◦ the initialThe initial conditions temperature for solving and governingpressure in equations. the engine The cylinder initial needed pressure to at be 360 establishedCA bTDC to was provide fixed the initial conditions for solving governing equations. The initial pressure at 360° CA bTDC was Sustainability 2016, 8, 993 6 of 22 at 1.15Sustainability bar. An2016 initial, 8, 993 temperature of 900 K was set for the residual exhaust gas at the beginning6 of of 22 the intake stroke. The value of the turbulent kinetic energy k was assumed to be spatially uniform and fixed at 1.15 bar. An initial temperature of 900 K was set for the residual exhaust gas at the beginning wasof setthe to intake 5% of stroke. the kinetic The value energy of the of theturbulent mean pistonkinetic speed.energy k was assumed to be spatially uniform andFor was the set combustionto 5% of the kinetic model, energy the Eddy of the Break-upmean piston Model speed. was used; and for NO formation, the OriginalFor the Heywood combustion Model. model, The the Eddy Eddy Break-up Break-up model Model was was used used; as the and combustion for NO formation, model because the it wasOriginal considered Heywood as Model. a typical The example Eddy Break-up of the mixed-is-burnt model was used combustion as the combustion model. model This combustionbecause it modelwas assumesconsidered that as the a typical chemical example reactions of the are completedmixed-is-burnt at the combustion moment of model. mixing, This so thatcombustion turbulent mixingmodel completely assumes that controls the chemical the reaction reactions rate. are comple A multi-componentted at the moment model of mixing, was so employed that turbulent as the evaporationmixing completely model and controls Tab model the reaction for the rate. break-up A multi-component model. For the model effect was of wall employed interaction, as the the Walljet0evaporation was applied. model and Tab model for the break-up model. For the effect of wall interaction, the Walljet0 was applied. 3. Results and Discussion 3. Results and Discussion 3.1. Water Addition During the Compression Stroke 3.1. Water Addition During the Compression Stroke In this section, water was directly injected into the cylinder during the compression stroke in order to examineIn this the section, effects ofwater injected was waterdirectly on injected the in-cylinder into the pressurecylinder andduring average the compression surface temperature stroke in of theorder cylinder to examine walls from the theeffects heat of transfer injected process. water Theon the WI in-cylinder during this pressure period also and had average an effect surface on the combustiontemperature process of the and cylinder emissions. walls from The parameters the heat tran ofsfer WI process. and water The addition WI during (WA) this mass period are also presented had in Tablean effect3. on the combustion process and emissions. The parameters of WI and water addition (WA) mass are presented in Table 3. Table 3. Parameters of WI. Table 3. Parameters of WI. Parameter Value Parameter Value Injection Timing 80◦ CA bTDC Injection Timing 80° CA bTDC Water duration [◦ CA] 10 Water duration [° CA] 10 WaterWater temperature temperature [K] [K] 310 310 5% (WA5% (WA 5); 10% 5); 10% (WA (WA 10); 10); 15% 15% (WA (WA 15); 15); 20% 20% (WA (WA 20); 20); WaterWater mass mass [Kg] [Kg] 25%25% (WA (WA 25) 25)in relation in relation to fuel to fuel mass mass

3.1.1.3.1.1. Effects Effects on on the the In-Cylinder In-Cylinder Pressure Pressure AfterAfter the the direct direct injection injection of of waterwater intointo thethe cylinder, the the liquid liquid water water absorbed absorbed the the heat heat of ofthe the compressedcompressed gas. gas. DuringDuring thethe firstfirst stage,stage, injected water water results results in in the the co coolingoling of of the the compressed compressed charge.charge. The The temperature temperature change change under under thethe effecteffect of WI is indicated indicated in in Figure Figure 4.4 .The The compressed compressed gas gas temperaturetemperature was was decreased, decreased, leading leading to to the the decrease decrease of of the the in-cylinder in-cylinder pressure. pressure. Figure Figure5 illustrates5 illustrates the decreasethe decrease of the of in-cylinder the in-cylinder pressure pressure during during the latterthe latter stages stages of the of the compression compression stroke. stroke.

FigureFigure 4. 4.The The decrease decrease in in compressed compressed chargecharge temperaturetemperature as as a a consequence consequence of of injected injected water. water. Sustainability 2016, 8, 993 7 of 22 Sustainability2016, 8, 993 7 of 22 Sustainability2016, 8, 993 7 of 22

FigureFigure 5. 5.The The decrease decrease of of in-cylinder in-cylinder pressurepressure duringduring the latter stag stageses of of the the compression compression stroke. stroke. Figure 5. The decrease of in-cylinder pressure during the latter stages of the compression stroke. As illustrated in Figure 5, there is a slight decrease of in-cylinder pressure in comparison with As illustrated in Figure5, there is a slight decrease of in-cylinder pressure in comparison with the pureAs illustrated gasoline case. in Figure Thus, 5,the there use ofis aWI slight during decrease the compression of in-cylinder stroke pressure had the in effectcomparison of reducing with the pure gasoline case. Thus, the use of WI during the compression stroke had the effect of reducing thethe compressionpure gasoline work. case. Thus,The reduction the use of of WI compressio during then compressionwork is one ofstroke the techniqueshad the effect for ofimproving reducing the compression work. The reduction of compression work is one of the techniques for improving enginethe compression power. Particularly, work. The thereduction reduction of compressio of compressionn work work is one has of a theconsiderable techniques significance for improving for enginehighengine power.compression power. Particularly, Particularly, ratio engines the the reduction becareductionuse of ofof the compressioncompression high compression work has haspressu a a considerable considerablere at the later significance significance stages of thefor for highcompressionhigh compression compression stroke. ratio ratio engines engines because because ofof thethe highhigh compression pressu pressurere at at the the later later stages stages of ofthe the compressioncompressionAfter stroke.20° stroke. CA bTDC, the combustion was initiated by the introduction of a spark into the ◦ combustionAfterAfter 20 20° chamber.CA CA bTDC, bTDC, The the in-cylinderthe combustion combustion fluid wastemperaturewas initiatedinitiated increased by the introduction introductionquickly, with of ofthe a a sparkinjected spark into intowater the the combustionheatedcombustion to the chamber. chamber. point where The The in-cylinderan in-cylinder overheated fluidfluid steam temperaturetemperature was formed. increased increased The volumetric quickly, quickly, expansion with with the the injected of injected steam water took water heatedplaceheated to during to the the point thepoint combustion where where an an overheated overheatedperiod. Figure steam steam 6 illustrates waswas formed.formed. the increase The The volumetric volumetric in the peak expansion expansion in-cylinder of of steam steampressure took took placeforplace duringthe during volumetric the the combustion combustion expansion period. period.of steam. Figure Figure 6 6illustrates illustrates thethe increaseincrease in the peak peak in-cylinder in-cylinder pressure pressure forfor the the volumetric volumetric expansion expansion of of steam. steam.

Figure 6. The increase of peak in-cylinder pressure for the volumetric expansion of steam. Figure 6. The increase of peak in-cylinder pressure for the volumetric expansion of steam. Figure 6. The increase of peak in-cylinder pressure for the volumetric expansion of steam. As can be seen in Figure 6, 25% of the WI mass in comparison with the fuel mass did not yield theAs highestAs can can be pressurebe seen seen in Figurein because Figure6, 25% 6,of 25%excessive of theof the WI heatWI mass mass ab insorption comparisonin comparison during with thewith the compression the fuel fuel mass mass didstroke. did not not yieldFifteen yield the highestpercentthe highest pressure (15%) pressure of because the WI because of mass excessive yieldedof excessive heat the absorptionhighest heat ab pesorptionak-pressure, during during the compressionrepresenting the compression stroke.about 2%stroke. Fifteen increase Fifteen percent in (15%)thepercent ofpeak the (15%) in-cylinder WI massof the yielded pressure.WI mass the yielded highest the peak-pressure, highest peak-pressure, representing representing about 2% about increase 2% increase in the peakin in-cylinderthe peakWith pressure.in-cylinder the availability pressure. of WI mass, it is possible to realize an increase of in-cylinder pressure duringWithWith the the availabilitycombustion-expansion availability of WIof WI mass, mass, process it is it possible is and possible thus to realize facilitateto realize an increasethe an achievementincrease of in-cylinder of in-cylinder of a pressurehigher pressure power during theduring combustion-expansion the combustion-expansion process andprocess thus and facilitate thus facilitate the achievement the achievement of a higher of a higher power power output, Sustainability 2016, 8, 993 8 of 22

Sustainability2016, 8, 993 8 of 22 as shown in Figure7. It follows, therefore, that the use of WI leads to an improvement in the power Sustainabilityoutput, as2016 shown, 8, 993 in Figure 7. It follows, therefore, that the use of WI leads to an improvement in8 of the 22 outputpower without output the without use of the extra use fuel. of extra fuel. output, as shown in Figure 7. It follows, therefore, that the use of WI leads to an improvement in the power output without the use of extra fuel.

FigureFigure 7. The 7. in-cylinderThe in-cylinder pressure pressure during during the the combustion-expansion combustion-expansion pr processocess with with the the use use of of15% 15% WI WI in

comparisonin comparison with the with pure the gasolinepure gasoline case. case. Figure 7. The in-cylinder pressure during the combustion-expansion process with the use of 15% WI 3.1.2. Effects of WI on the Reduction in Energy Losses via Heat Transfer 3.1.2. Effectsin comparison of WI on with the the Reduction pure gasoline in Energy case. Losses via Heat Transfer During engine operation, there are energy losses via heat transfer to the cylinder walls during During3.1.2.the latter Effects engine stages of WI operation, ofon thethe Reductioncompression there are in energy Energystrokelosses Lossesand the via via combustion-expansion heat Heat transfer Transfer to the cylinder stroke. wallsDuring during the the lattercompression stagesDuring of the engine process, compression operation, the charge stroke there temperature andare energy the combustion-expansion lossesrises viaabove heat the transfer wall stroke.totemperature the cylinder During and walls the thus compression during heat process,thetransfer thelatter chargeoccurs stages temperaturefrom of the the compression in-cylinder rises above gassesstroke the toan wall dthe the temperaturecylinder combustion-expansion walls. and In thusthe combustion-expansion heat stroke. transfer During occurs the from the in-cylindercompressionstroke, the heat gasses process, transfer to thethe rate cylindercharge to the cylindertemperature walls. walls In theri isses the combustion-expansion above highest. the Among wall temperaturethe energy stroke, loss and thethrough thus heat heatheat transfer rate totransfertransfer, the cylinder occursheat fluxes wallsfrom to the isthe the in-cylindercombustion highest. gasses Amongchamber to thewathell energy arecylinder highest loss walls. during through In thethe heat combustion combustion-expansion transfer, process heat fluxesand to the combustionstroke,can reach the asheat chamberhigh transfer as 10 wall MW/mrate to are the2 [35]. highest cylinder Reduction during walls of is thethe the combustionenergyhighest. losses Among processduring the energy these and periods loss can through reach will asresult heat high as 10 MW/mtransfer,in an 2improvement[ 35heat]. Reductionfluxes ofto thethe of enginecombustion the energy efficiency. chamber losses during wall are these highest periods during will the result combustion in an improvementprocess and of The energy losses resulting2 from the transfer of heat to the cylinder walls is mainly through the the enginecan reach efficiency. as high as 10 MW/m [35]. Reduction of the energy losses during these periods will result inpiston an improvement crown, cylinder of the liner, engine comb efficiency.ustion chamber, and valves. A decrease in the surface temperature The energy losses resulting from the transfer of heat to the cylinder walls is mainly through the of theseThe componentsenergy losses will resulting lead to from a reduction the transfer of the of heattransfer to the of cylinderheat to thewalls engine is mainly coolant through and thus the piston crown, cylinder liner, combustion chamber, and valves. A decrease in the surface temperature pistonreduce crown, energy cylinder losses. Figure liner, comb8 illustratesustion chamber,the main andcomponents valves. A in decrease the energy in the loss surface process temperature of the ICE. of theseof these components components will will lead lead to to a a reduction reduction ofof thethe transfer of of heat heat to tothe the engine engine coolant coolant and andthus thus reducereduce energy energy losses. losses. Figure Figure8 illustrates 8 illustrates the the main main components components in in the the energy energy loss loss process process of the of theICE. ICE.

Figure 8. The main components involved in the energy loss process.

During the operationFigure 8. of The an main engine, components heat is transfinvolvederred in the by energy the motion loss process. of fluids and the relative motion betweenFigure fluids 8. andThe mainsolid componentssurfaces. During involved the inlatter the energystages lossof the process. compression stroke, combustion-expansion,During the operation and of anexhaust engine, stroke,heat is transfheat erredis transferred by the motion by convectionof fluids and between the relative the Duringmotionin-cylinder between the gases, operation fluidscylinder ofand head, an solid engine, valves, surfaces. heat and the isDuring transferred cylinder the wall.latter by thestages motion of the of compression fluids and thestroke, relative motioncombustion-expansion, betweenIn steady-flow fluids convectional and and solid exhaust heat surfaces. transfer,stroke, During theheat heat is theflux transferred latter is expressed stages by convectionof by thethe relation: compression between the stroke, combustion-expansion,in-cylinder gases, cylinder and exhaust head, valves, stroke, and heat the iscylinder transferred wall. by convection between the in-cylinder gases, cylinderIn steady-flow head, valves, convectional and the heat cylinder transfer, wall. the heat flux is expressed by the relation: . In steady-flow convectional heat transfer, the heat flux q is expressed by the relation: . q = hc (T − TW ) Sustainability 2016, 8, 993 9 of 22 SustainabilitySustainability20162016,, 88,, 993993 99 ofof 2222

wherewhere isis thethe heat-transferheat-transfer coeffient,coeffient, TT thethe temperaturetemperature ofof flowingflowing fluidfluid stream,stream, andand wherewherehc is the is heat-transferthe heat-transfer coeffient, coeffient,T the T temperature the temperature of flowing of flowing fluid stream, fluid andstream,TW representsand therepresentsrepresents temperature thethe temperature oftemperature the solid surface. ofof thethe solidsolid surface.surface. TheTheThe heat heatheat absorption absorptionabsorption of ofof water waterwater to to formform steamsteam steam leadsleads leads toto to thethe the decreasedecrease decrease inin thethe in theaverageaverage average temperaturetemperature temperature ofof ofthethe workingworking working fluidfluid fluid inin in thethe the cylindercylinder cylinder (Figure(Figure (Figure 5).5).5 ). ThroThro Throughughugh convectionalconvectional convectional heatheat heat transfer,transfer, transfer, thethe thedecreasedecrease decrease inin temperaturetemperature forfor thethe pistonpiston crown,crown, cylindercylinder liner,liner, anandd combustioncombustion chamberchamber amongamong othersothers areare in temperature TW forfor the the piston piston crown, crown, cylinder cylinder liner, liner, an andd combustion combustion chamber chamber among among others others are are illustratedillustratedillustrated in inin Figures FiguresFigures9– 9–12.129–12..

FigureFigureFigure 9. 9.9.The TheThedecrease decreasedecrease in inin average averaverageage surface surface temperaturetemperature T for forfor thethe the pistonpiston piston crown.crown. crown. Figure 9. The decrease in average surface temperature W for the piston crown.

FigureFigure 10.10. TheThe decreasedecrease inin averageaverage surfacesurface temperaturetemperature for for thethe cylindercylinder liner.liner. FigureFigure 10. 10.The The decrease decrease inin averageaverage surface temperature temperature TW forfor the the cylinder cylinder liner. liner.

FigureFigure 11.11. TheThe decreasedecrease inin averageaverage surfacesurface temperaturetemperature for for thethe combustioncombustion chamber.chamber. Figure 11. The decrease in average surface temperature TW for the combustion chamber. Sustainability20162016,, 88,, 993 993 10 of 22 Sustainability2016, 8, 993 10 of 22

Figure 12. The decrease in average surface temperature for the exhaust valves. FigureFigure 12. 12.The The decrease decrease inin averageaverage surface temperature temperature TW forfor the the exhaust exhaust valves. valves.

ForFor Figures Figures 9–12, 9–12, as as the the WI WI mass mass intointo thethe cylinder increases,increases, the the average average surface surface temperature temperature For Figures9–12, as the WI mass into the cylinder increases, the average surface temperature TW of ofthe the piston piston crown, crown, cylinder cylinder liner, liner, combustioncombustion chamber, andand exhaust exhaust valves valves decreases. decreases. For For 25% 25% of the piston crown, cylinder liner, combustion chamber, and exhaust valves decreases. For 25% WI WIWI relative relative to to the the metered metered fuel fuel massmass andand atat 20° CA bTDCbTDC ofof ignitionignition timing, timing, the the values values of ofthe the relative to the metered fuel mass and at 20◦ CA bTDC of ignition timing, the values of the decreasing decreasingdecreasing average average surface surface temperat temperatureure werewere 46 K, 83 K,K, 3535 K,K, andand 20 20 K K for for the the piston piston crown, crown, average surface temperature were 46 K, 83 K, 35 K, and 20 K for the piston crown, cylinder liner, cylindercylinder liner, liner, chamber, chamber, and and exhaustexhaust valves,valves, respectively. TheThe decreasedecrease in in the the average average surface surface chamber, and exhaust valves, respectively. The decrease in the average surface temperature T of the temperaturetemperature of of the the piston piston crown crown andand cylindercylinder liner occursoccurs earlierearlier and and is is lower lower than than that that ofW ofthe the piston crown and cylinder liner occurs earlier and is lower than that of the combustion chamber and combustioncombustion chamber chamber and and exhaust exhaust valvesvalves becausebecause of thethe directiondirection of of the the injected injected water, water, which which is is exhaust valves because of the direction of the injected water, which is directed toward the piston head directeddirected toward toward the the piston piston head head as as indicated indicated inin Figure 13.13. as indicated in Figure 13.

Figure 13. The water spray as directed towards the piston crown. Figure 13. The water spray as directed towards the piston crown. For the average surface temperature of the piston crown and cylinder liner, the quick decrease in the temperature value was realized at 40° CA bTDC, and was earlier than that of the For thethe averageaverage surface surface temperature temperatureT of theof pistonthe piston crown crown and cylinder and cylinder liner, the liner, quick the decrease quick combustion chamber and exhaust valves. WThis is because the fuel sprayed impinges the piston crown decreasein the temperature in the temperature value was value realized was at realized 40◦ CA bTDC,at 40° CA and bTDC, was earlier and was than earlier that of than the combustion that of the and the cylinder liner. Figure 14 illustrates the fuel spray cloud at 40° CA bTDC. combustionchamber and chamber exhaust and valves. exhaust This valves. is because This is the be fuelcause sprayed the fuel impingessprayed impinges the piston the crown piston and crown the The decrease of the average surface temperature of the piston crown, cylinder liner, and the cylinder liner. Figure 14 illustrates the fuel spray cloud◦ at 40° CA bTDC. cylindercombustion liner. Figurechamber, 14 andillustrates exhaust the valves fuel spray leads cloudto a reduction at 40 CA in bTDC. the conductive heat transfer in The decrease of the average surface temperature of the piston crown, cylinder liner, solidsThe and decrease heat losses of the averageto the engine surface coolant. temperature The decreaseTW of thein the piston average crown, surface cylinder temperature liner, combustion of the combustion chamber, and exhaust valves leads to a reduction in the conductive heat transfer in chamber,cylinder and liner exhaust also helps valves to prevent leads to the a reductiondeteriorati inon theof the conductive lubricating heat oil transferfilm. Moreover, in solids a andlower heat solids and heat losses to the engine coolant. The decrease in the average surface temperature of the lossesaverage to the surface engine temperature coolant. Theof the decrease cylinder inwalls the has average a significant surface positive temperature effect on of the the avoidance cylinder linerof cylinderalsoknock helps andliner to pre-ignition prevent also helps the problemsto deterioration prevent which the of deteriorati result the lubricating fromon overheated of oilthe film.lubricating hotspots Moreover, inoil areas film. a lower such Moreover, average as the piston a surface lower averagetemperaturecrown, surface exhaust of temperature the valves, cylinder combustion of walls the cylinder has chamber, a significant walls and hasspark positivea significantplug electrodes. effect positive on the effect avoidance on the of avoidance knock and of pre-ignitionknock and pre-ignition problems whichproblems result which from result overheated from overheated hotspots hotspots in areas in such areas as such the piston as the crown,piston crown,exhaust exhaust valves, valves, combustion combustion chamber, chamber, and spark and plug spark electrodes. plug electrodes. Sustainability 2016, 8, 993 11 of 22 Sustainability2016, 8, 993 11 of 22 Sustainability2016, 8, 993 11 of 22

Figure 14. Amount of fuel spray impinging the piston crown and the cylinder liner.liner. Figure 14. Amount of fuel spray impinging the piston crown and the cylinder liner. 3.1.3. Effects of Water Injection on Combustion 3.1.3. Effects of Water Injection on Combustion 3.1.3. Effects of Water Injection on Combustion Employing WI during the compression stroke affects the combustion process because the heat Employing WI during the compression stroke affects the combustion process because the heat absorptionEmploying of the WI injected during water the compression results in the stroke decre aseaffects of the the in-cylinder combustion pressure process and because working the fluidheat absorption of the injected water results in the decrease of the in-cylinder pressure and working fluid absorptiontemperature. of Thethe injectedburning water conditions results are in changed the decre withase of the the presence in-cylinder of injected pressure water. and working fluid temperature. The burning conditions are changed with the presence of injected water. temperature.The effect The of WIburning on the cond combustionitions are process changed was with evident the presence by the fact of injected that the water. burning time for the The effect of WI on the combustion process was evident by the fact that the burning time for air-fuelThe mixture effect of wasWI on extended. the combustion The injected process water was evidentled to a by decrease the fact inthat the the burning burning rate time and for the the air-fuel mixture was extended. The injected water led to a decrease in the burning rate and the air-fuelmaximum mixture brake wastorque extended. was achieved The injected by the use water of WA led [36].to a decrease in the burning rate and the maximum brake torque was achieved by the use of WA [36]. maximumFigure brake 15 illustrates torque was the achieved maximum by thetemperature use of WA point [36]. for 15% WA in comparison with pure Figure 15 illustrates the maximum temperature point for 15% WA in comparison with pure gasoline.Figure For 15 15% illustrates WA, the the maximum maximum temperature temperature point point moved for around15% WA 3–5° in CAcomparison in comparison with purewith gasoline. For 15% WA, the maximum temperature point moved around 3–5◦ CA in comparison with gasoline.of the use For of pure15% WA,gasoline the maximumwith the poin temperaturet B occurring point after moved TDC around(after 0° 3–5° CA). CA in comparison with of the use of pure gasoline with the point B occurring after TDC (after 0◦ CA). of the use of pure gasoline with the point B occurring after TDC (after 0° CA).

Figure 15. Effect of WI on the maximum temperature point. Figure 15. Effect of WI on the maximum temperature point. 3.1.4. Effect of WI on NO Emission 3.1.4. Effect of WI on NO Emission 3.1.4.A Effect major of benefit WI on NOof using Emission WI in an engine is the reduction of NO emissions. In this study, the reductionA major of benefitbenefitNO emissions of using WIwas in also an engineengineachieved is thethefor reductionreduction WI during ofof NONOthe emissions.emissions.compression In this stroke study, and the is reductionillustrated of ofin NO FigureNO emissions emissions 16. was was also also achieved achieved for WI for during WI during the compression the compression stroke and stroke is illustrated and is inillustrated Figure 16 in. Figure 16. Sustainability 2016, 8, 993 12 of 22 Sustainability2016, 8, 993 12 of 22 Sustainability2016, 8, 993 12 of 22

Figure 16. Effect of WI on reduction ofof thethe NONO emissionemission mass.mass. Figure 16. Effect of WI on reduction of the NO emission mass. As the WI mass supplied to the engine increased, the NO emission mass decreased. For 25% of AsAs the the WI WI mass mass supplied supplied to to the the engine engine increas increased,ed, the the NO NO emission emission mass mass decreased. decreased. For For 25% 25% of WA relative to the gasoline mass, the reduction of NO emission mass was 34%. For the highest WAof WA relative relative to tothe the gasoline gasoline mass, mass, the the reduction reduction of of NO NO emission emission mass mass was was 34%. 34%. For For the the highest peak-pressure case of 15% WA, the reduction of NO emission was about 21%. The main reason for peak-pressure case case of of 15% WA, the reduction of NO emission was about 21%. The The main reason reason for realizing a reduction in NO emissions could be attributed to the fact that the injected water absorbed realizingrealizing a a reduction reduction in NO NO emissions emissions could be be attr attributedibuted to to the the fact fact that that the the injected injected water water absorbed absorbed the heat of the compressed gases leading to reduced combustion temperatures. Figure 17 illustrates thethe heat ofof thethe compressedcompressed gases gases leading leading to to reduced reduced combustion combustion temperatures. temperatures Figure. Figure 17 illustrates17 illustrates the the contours for a high temperature region in the combustion chamber◦ for 730° CA and 15% WA thecontours contours for afor high a high temperature temperature region region in the combustionin the combustion chamber chamber for 730 forCA 730° and 15%CA WAand compared15% WA compared with the use of pure gasoline.0 C and ´ denote high temperature areas of more than 2200 comparedwith the use with of purethe use gasoline. of pureC gasoline.and C denote C and high´ denote temperature high temperature areas of more areas than of more 2200 Kthan for 2200 pure K for pure gasoline and 15% 0WA. The ´ area is smaller than the area marked C and thus implies Kgasoline for pure and gasoline 15% WA. and The 15%C WA.area The is smaller ´ area than is smaller the area than marked the areaC and marked thus implies C and thus that theimplies 15% that the 15% WA case had a lower NO reaction rate than that of the pure gasoline case. thatWA casethe 15% had WA a lower case NOhad reaction a lower rateNO thanreaction that rate of the than pure that gasoline of the pure case. gasoline case.

Figure 17. High temperature region in the combustion chamber at 5° CA bTDC. FigureFigure 17. 17. HighHigh temperature temperature region region in the combustion chamber at 5 °◦ CA bTDC. WI leads to the reduction in the local temperature of the combustion regions within the WI leads to the reduction in the local temperature of the combustion regions within the cylinder.WI leads However, to the reductionthe maximum in the temperature local temperature of the of working the combustion gasses in regions the cylinder within thefor cylinder.the WA cylinder. However, the maximum temperature of the working gasses in the cylinder for the WA However,cases was thehigher maximum than temperaturethat for the ofuse the of working pure gasoline. gasses in Figure the cylinder 18 presents for the WAthe maximum cases was cases was higher than that for the use of pure gasoline. Figure 18 presents the maximum highertemperature than that values for theforuse the of various pure gasoline. WA rates Figure and 18the presents use of thepure maximum gasoline. temperature 15% of WA values gave the for temperature values for the various WA rates and the use of pure gasoline. 15% of WA gave the thehighest various peak-pressure WA rates and and the the use highest of pure average gasoline. temperature. 15% of WA gave the highest peak-pressure and the highest peak-pressure and the highest average temperature. highest average temperature. Sustainability 2016, 8, 993 13 of 22 Sustainability2016, 8, 993 13 of 22

Sustainability2016, 8, 993 13 of 22

Figure 18. Maximal temperature values with various WA rates and the use of pure gasoline. Figure 18. Maximal temperature values with various WA rates and the use of pure gasoline. The effectFigure of 18. WA Maximal on charge temperature gas was values a decrease with various of WAthe averagerates and temperaturethe use of pure (Figuresgasoline. 4 and 5). The effect of WA on charge gas was a decrease of the average temperature (Figures4 and5). The latent heat required to convert the injected water to vapor is absorbed from charge gases’ The latent heat required to convert the injected water to vapor is absorbed from charge gases’ energy. energy.The In effect the combustionof WA on charge process, gas wasthe aresultin decreaseg steamof the averageincreases temperature the in-cylinder (Figures pressure 4 and 5). in In the combustion process, the resulting steam increases the in-cylinder pressure in comparison to comparisonThe latent heatto the required pure gasolineto convert (Figure the injected 6). The water effect to ofvapor water is absorbedinjection fromon the charge decrease gases’ of thein-cylinder pureenergy. gasoline In temperature the (Figure combustion6 has). The been process, effect studied of the water by resultin many injection gresearchers steam on theincreases [4,5,31]. decrease the Figure ofin-cylinder in-cylinder 18 presents pressure temperature maximal in hastemperature beencomparison studied values to by the many with pure various researchers gasoline WA (Figure [rates4,5,31 and ].6). Figure pureThe gaeffect 18soline. presents of Itwater is maximalpossible injection to temperature noticeon the that decrease for values a short of with ◦ variousperiodin-cylinder WA of time rates temperature close and to pure 0° CA,has gasoline. beenthe maximum studied It is possible by temper many to atureresearchers notice in thatthe [4,5,31].chamber for a short Figure was period higher 18 presents of in time the maximal WA close case to 0 CA,thantemperature the in maximum the pure values gasoline temperature with case. various The in theWA higher chamber rates temperature and waspure higher ga forsoline. the in ItWA the is WApossiblecase casewas to thandue notice to in a that the longer purefor aignition short gasoline period of time close to 0° CA, the maximum temperature in the chamber was higher in the WA case case.delay The period. higher The temperature injected gasoline for the WA fuel case had was more due time to a to longer mix ignitionwith the delaysurrounding period. hot The gases, injected than in the pure gasoline case. The higher temperature for the WA case was due to a longer ignition gasolineleading fuel to higher had more pressure time and to mixheat withreleased the during surrounding the premixed hot gases, combustion leading portion to higher of the pressure process. and delay period. The injected gasoline fuel had more time to mix with the surrounding hot gases, heatTherefore, released a duringlocally higher the premixed temperature combustion was found. portion However, of the the process. effect of water Therefore, injection a locally on exhaust higher leading to higher pressure and heat released during the premixed combustion portion of the process. temperaturegas was a wasdecrease found. of However,the in-cylinder the effecttemperature of water in injection relation onto the exhaust use of gas pure was gasoline, a decrease and of is the Therefore, a locally higher temperature was found. However, the effect of water injection on exhaust in-cylinderpresented temperature in the Section in 3.2.3 relation of this to study. the use of pure gasoline, and is presented in the Section 3.2.3 of gas was a decrease of the in-cylinder temperature in relation to the use of pure gasoline, and is this study.presented in the Section 3.2.3 of this study. 3.1.5. Effect of Water Temperature on the In-Cylinder Pressure 3.1.5. Effect of Water Temperature on the In-Cylinder Pressure 3.1.5.In Effectthis section, of Water the Temperat effect ofure the on injected the In-Cylinder water temperature Pressure on the in-cylinder pressure during theIn compression thisIn this section, section, and the the combustion-expansion effect effect of of the the injected injectedwater watestrokesr temperaturetemperature is discussed. on on the15% the in-cylinder in-cylinderof WA and pressure pressurevarious during water during thetemperatures compressionthe compression varying and and combustion-expansion between combustion-expansion 40 °C and 80 °C strokesstrokes were usedis discussed. to study 15% the 15% effectof ofWA WA of and water and various varioustemperature water water temperaturesontemperatures the in-cylinder varying varying pressure. between between 40The 40◦C temperature°C and and 80 80◦C °C were factwereor used useddid to tonot study study show the the any effect effect significant ofof waterwater temperatureeffect on the on thecompression in-cylinderon the in-cylinder pressure.pressure, pressure. Theas illustrated temperature The temperature in factorFigure did 19fact. notTheor showdid curve not any ofshow significantcompression any significant effect pressure on theeffect for compression onvarious the pressure,watercompression temperatures as illustrated pressure, was in Figure asvirtually illustrated 19. the The same.in curve Figure of compression19. The curve pressure of compression for various pressure water for temperatures various waswater virtually temperatures the same. was virtually the same.

Figure 19. The effect of WI temperature on compression pressure. FigureFigure 19. 19.The The effect effect of of WI WI temperaturetemperature on on compression compression pressure. pressure. SustainabilitySustainability2016, 8, 993 1414 ofof 2222

However, during the combustion phase the WI temperature had an effect on the peak-pressure, However, during the combustion phase the WI temperature had an effect on the peak-pressure, as shown in Figure 20. The higher water temperature resulted in a higher peak pressure and thus a as shown in Figure 20. The higher water temperature resulted in a higher peak pressure and thus a higher power output. The higher water temperature resulted in less heat being absorbed from the higher power output. The higher water temperature resulted in less heat being absorbed from the compressed gases and increased the peak pressure once there was volumetric expansion. compressed gases and increased the peak pressure once there was volumetric expansion.

Figure 20. The effect of WI temperature on thethe peak-pressure.peak-pressure.

WIWI hadhad somesome effectseffects onon thethe in-cylinderin-cylinder pressurepressure asas aa resultresult ofof thethe heatheat absorptionabsorption andand thethe steamsteam formation.formation. TheThe possible possible reduction reduction in in energy energy losses losses during during the compressionthe compression stroke, stroke, NO emission NO emission levels, andlevels, prolonged and prolonged combustion combustion were investigated were investigated for the various for the cases various of WI. cases More of WA WI. mass More (25%) WA helped mass to(25%) reduce helped the energy to reduce losses the and energy NO emissions, losses and but NO lowered emissions, the peak-pressure but lowered in the comparison peak-pressure with the in usecomparison of 15% WA with mass. the use of 15% WA mass.

3.2.3.2. Water Addition DuringDuring thethe ExpansionExpansion StrokeStroke WaterWater cancan be be injected injected into into the cylinderthe cylinder during during the expansion the expansion stroke to stroke facilitate to higherfacilitate expansion higher pressure.expansion The pressure. heat absorption The heat absorption from the high from temperature the high temperature of the burnt of gases the burnt facilitated gases the facilitated volumetric the expansionvolumetric of expansion steam, leading of steam, to an leading improvement to an improvement in the work output.in the work The WAoutput. mass The rates WA used mass during rates theused expansion during the stroke, expansion among stroke, other among details, other are given details, in Table are given4. in Table 4.

TableTable 4. 4.WI WI parameters parameters duringduring the expansion stroke. stroke.

Item Value Item Value Injection Timing (° CA) 730 Injection Timing (◦ CA) 730 Water duration (° CA) 10 ◦ WaterWater duration temperature ( CA) [K] 373 10 Water temperature [K]100% (WA 1.0); 200% (WA 373 2.0); 300% (WA 3.0); Water mass [Kg] 100%400% (WA (WA 1.0); 4.0) 200% relative (WA to 2.0); fuel 300%mass (WA 3.0); Water mass [Kg] 400% (WA 4.0) relative to fuel mass 3.2.1. Evaporation of Injected Water in the Cylinder 3.2.1.The Evaporation liquid WI of mass Injected into Water the cylinder in the Cylinder depended on the intake air mass, the temperature of compressedThe liquid air, WI the mass water into temperature, the cylinder and depended the engine on the speed, intake among air mass, other the parameters. temperature The of compressedinjection of excessive air, the water amounts temperature, of liquid and water the into engine the speed,cylinder among could other result parameters. in poor evaporation The injection and ofcould excessive have an amounts effect on of the liquid corrosion water intoresistance the cylinder of the couldengine result leading in poorto a poor evaporation compatibility and could with havethe lubricating an effect onoil. theFigure corrosion 21 presents resistance the evaporation of the engine of injected leading water to a poorfor the compatibility various WI masses with the at lubricatinga water temperature oil. Figure of 21 100 presents °C. the evaporation of injected water for the various WI masses at a water temperature of 100 ◦C. Sustainability 2016, 8, 993 15 of 22 Sustainability2016, 8, 993 15 of 22 Sustainability2016, 8, 993 15 of 22

◦ FigureFigure 21. 21.Relationship Relationship between between the the injectedinjected waterwater mass and the the evaporated evaporated water water mass mass at at100 100 °C. C. Figure 21. Relationship between the injected water mass and the evaporated water mass at 100 °C. ForFor a lower a lower WA WA rate ofrate 3.0, of the 3.0, total the injected total injected water mass water was mass vaporized was vaporized under the under high temperature the high temperatureFor a lower of the WA burning rate gases.of 3.0, A the water total injection injected rate water of 4.0 mass WA wasby mass vaporized was not under suitable the for high the of the burning gases. A water injection rate of 4.0 WA by mass was not suitable for the current engine temperaturecurrent engine of themodel burning because gases. of Aits water poor injection evaporat rateion ofcharacteristics. 4.0 WA by mass Based was onnot the suitable relationship for the model because of its poor evaporation characteristics. Based on the relationship between the injected currentbetween engine the injected model becausewater mass of its and poor the evaporat vaporizedion characteristics. mass, the water Based evaporation on the relationship was not water mass and the vaporized mass, the water evaporation was not instantaneous, as illustrated in betweeninstantaneous, the injected as illustrated water inmass Figure and 21. theIncreasing vaporized the WImass, mass the increased water evaporation the time required was notfor Figure 21. Increasing the WI mass increased the time required for complete evaporation. The time instantaneous,complete evaporation. as illustrated The time in Figure required 21. for Increasing a complete the evaporation WI mass increased of injected the water time atrequired 25, 38, andfor required for a complete evaporation of injected water at 25, 38, and 50◦ CA aTDCis shown in Figure 21 complete50° CA aTDCis evaporation. shown Thein Figure time required21 for the for WA a complete1.0, WA 2.0, evaporation and WA 3.of0 injectedcases, respectively. water at 25, 38, and for50° the CA WA aTDCis 1.0, WA shown 2.0, andin Figure WA 3.0 21 cases,for the respectively. WA 1.0, WA 2.0, and WA 3.0 cases, respectively. 3.2.2. Effect of WI on Expansion Pressure 3.2.2.3.2.2. Effect Effect of of WI WI on on Expansion Expansion Pressure Pressure WI resulted in the decrease of the in-cylinder pressure in the first instance by heat absorption andWI WIa resultedsubsequent resulted in in the increasethe decrease decrease of the ofof thethein-cylinder in-cylinderin-cylinder pressure pres pressuresure via in involumetric the the first first instance instance expansion by by heat in heat theabsorption absorption second andandinstance. a subsequenta subsequent Thus, the increase effectincrease of theinjectedof in-cylinderthe waterin-cylinder on pressure the pressure in-cylinder via volumetricvia pressure volumetric expansion was expansiondivided in theinto in second two the stages second instance. of Thus,instance.pressure the effect Thus,change of the injected in effect comparison of water injected on with thewater in-cylinderthe on pure the ingasoline-cylinder pressure case. pressure was During divided was the divided into first two stageinto stages two of ofstagesWA, pressure the of changepressurein-cylinder in comparisonchange pressure in comparisonwas with lower the than pure with that gasoline the of pure the case. usegasoline of Duringpure case. gasoline the During first with stagethe the first ofopposite WA,stage the effectof in-cylinderWA, taking the pressurein-cylinderplace in was the pressure lower second than wasstage. that lower Figure of thethan 22 use that illustrates of of pure the gasolineusethe ofchange pure with gasolineof thethe oppositein-cylinder with the effect opposite pressure taking effect for place WA taking in3.0 the secondplacecompared stage.in the with Figuresecond the use22 stage. illustrates of pure Figure gasoline. the22 illustrates change of th thee change in-cylinder of the pressure in-cylinder for WApressure 3.0 compared for WA 3.0 with thecompared use of pure with gasoline. the use of pure gasoline.

Figure 22. The variation of the in-cylinder pressure for WA 3.0 and pure gasoline. FigureFigure 22. 22.The The variation variation ofof thethe in-cylinder pre pressuressure for for WA WA 3.0 3.0 and and pure pure gasoline. gasoline. Sustainability 2016, 8, 993 16 of 22 Sustainability2016, 8, 993 16 of 22

EquationEquation (1)(1) givesgives anan expressionexpression forfor thethe percentagepercentage increaseincrease ofof thethe in-cylinderin-cylinder pressurepressure withwith thethe ◦ useuse ofof WIWI atat 1010° CACA aTDCaTDC duringduring thethe expansionexpansion stroke.stroke.   pw−ppure Percentage[%%] = ×100 100 (1) (1) ppure

where represents the in-cylinder pressure in the WI case with representing the in-cylinder where pw represents the in-cylinder pressure in the WI case with ppure representing the in-cylinder pressurepressure forfor purepure gasoline.gasoline. AsAs thethe WAWA mass mass increased, increased, there there was was a decrease a decrease in the in in-cylinderthe in-cylinder pressure pressure during during the first the stage. first Forstage. WA For 3.0, WA the 3.0, percentage the percentage decrease decrease of the in-cylinderof the in-cylinder pressure pressu wasre 2.7%. was During2.7%. During the second the second stage, thestage, injected the injected water vaporized water vaporized and expanded and expanded in volume. in Thevolume. percentage The percentage increase in increase the in-cylinder in the pressurein-cylinder was pressure positive was during positive this period.during this In Figure period. 23 In, the Figure area 23, of the the positivearea of the percentage positive increasepercentage in theincrease in-cylinder in the pressure in-cylinder was largerpressure than was the arealarger of thethan percentage the area decreaseof the percentage in the in-cylinder decrease pressure. in the Thus,in-cylinder the effect pressure. of WI onThus, improving the effect the of power WI on output improving can be the achieved power by output the increase can be ofachieved the expansion by the pressureincrease duringof the expansion the power pressure stroke. during the power stroke.

FigureFigure 23.23. TheThe percentagepercentage increaseincrease ofof thethe in-cylinderin-cylinder pressure.pressure.

3.2.3.3.2.3. EffectEffect ofof WIWI onon thethe ReductionReduction inin EnergyEnergy LossesLosses throughthrough In-CylinderIn-Cylinder HeatHeat TransferTransfer to to the the ExhaustExhaust GasesGases AsAs mentionedmentioned earlier, earlier, 30%–40% 30%–40% of of fuel fuel energy energy is lostis lost to exhaustto exhaust gases. gases. The The rapid rapid expansion expansion of the of burntthe burnt charge charge inside inside the cylinder the cylinder produces produces turbulent turb fluidulent motions, fluid motions, high temperature high temperature regions, and regions, large heatand transferslarge heat from transfers the fluid from to the the fluid piston to crown,the pist cylinderon crown, wall, cylinder and combustion wall, and combustion chamber. The chamber. rapid successionThe rapid ofsuccession working of cycles working in the cycles cylinder in the creates cylinder an expanding creates an exhaust expanding gas with exhaust high gas pressure with high and temperature,pressure and which temperature, are scavenged which fromare scavenged the cylinder from in preparation the cylinder for in the preparation next cycle whilefor the the next exhaust cycle gaswhile is stillthe expanding.exhaust gas The is still exhaust expanding. gas temperature The exhaus is highert gas temperature than that of is ambient higher andthan contains that of ambient thermal energyand contains which thermal is finally energy lost to thewhich environment. is finally lost Injection to the ofenvironment. liquid water Injection will convert of liquid a portion water of will the heatedconvert gases a portion produced of the from heated the gases combustion produced process from energy the combustion into pressure, process leading energy to aninto increase pressure, in theleading useful to work.an increase The decreasing in the useful temperature work. The at de thecreasing end of thetemperature expansion at process the end will of the conduct expansion heat, leadingprocess towill a decrease conduct inheat, the exhaustleading gasto a temperature decrease in andthe thereforeexhaust gas a reduced temperature magnitude and therefore of wasted a ◦ heatreduced to the magnitude exhaust gases, of wasted as illustrated heat to inthe Figure exhaust 24. Atgases, 180 asCA illustrated aTDC, the in exhaust Figure gas24. temperatureAt 180° CA ◦ decreaseaTDC, the was exhaust 42 K, gas 89 K,temperature and 136 K decrease for WA 1.0,was WA42 K, 2.0, 89 andK, and WA 136 3.0 K at for 100 WAC 1.0, water WA temperature, 2.0, and WA respectively.3.0 at 100 °C Figurewater temperature,25 presents the respectively. decreasing exhaustFigure 25 temperature presents the with decreasing various waterexhaust addition temperature ratios inwith relation various to thewater intake addition mass ratios at the beginningin relation ofto thethe exhaustintake mass process, at the in comparisonbeginning of with the anotherexhaust studyprocess, [25 in]. comparison with another study [25]. Sustainability 2016, 8, 993 17 of 22 Sustainability2016, 8, 993 17 of 22 Sustainability2016, 8, 993 17 of 22

FigureFigure 24. 24.Decrease Decrease in in exhaust exhaust gasgas temperaturetemperature by the the deployment deployment of of WI. WI. Figure 24. Decrease in exhaust gas temperature by the deployment of WI.

Figure 25. Figure 25.Percentage Percentage of of decreasing decreasing exhaust exhaust gasgas temperaturetemperature under various various water water addition addition ratios. ratios. Figure 25. Percentage of decreasing exhaust gas temperature under various water addition ratios. Moreover,Moreover, the the lower lower temperature temperature ofof thethe expandingexpanding gases for for WI WI in in comparison comparison with with the the pure pure Moreover, the lower temperature of the expanding gases for WI in comparison with the pure gasolinegasoline case case showed showed a decreasea decrease in in the the heat heat transfer transfer to to the the cylinder cylinder walls walls in in the the process. process. AccordingAccording to gasoline case showed a decrease in the heat transfer to the cylinder walls in the process. According Heywoodto Heywood [34], [34], for afor Diesel a Diesel engine engine with with a 45%a 45% indicated indicated efficiency, efficiency, 25%25% of the the fuel fuel energy energy is islost lost to to to Heywood [34], for a Diesel engine with a 45% indicated efficiency, 25% of the fuel energy is lost to thethe coolant coolant in in the the cooling cooling system. system. Of Of thisthis 25%,25%, 9%9% isis lost during the the exhaust exhaust stroke, stroke, 8% 8% during during the the the coolant in the cooling system. Of this 25%, 9% is lost during the exhaust stroke, 8% during the combustion stroke, 6% during the expansion stroke, and 2% due to friction. Reducing the surface combustioncombustion stroke, stroke, 6% 6% during during the the expansion expansion stroke,stroke, andand 2%2% due to friction. Reducing Reducing the the surface surface temperature of the piston crown, cylinder liner, and combustion chamber will reduce the heat losses temperaturetemperature of of the the piston piston crown, crown, cylinder cylinder liner, liner, and and combustioncombustion chamber will will reduce reduce the the heat heat losses losses and improve the engine efficiency. Figures 26 and 27 illustrate the reduction of the surface andand improve improve the enginethe engine efficiency. efficiency. Figures Figures 26 and 26 27 anillustrated 27 illustrate the reduction the reduction of the surface of the temperature surface temperature of the piston crown and combustion chamber during the expansion stroke for WA of thetemperature piston crown of the and piston combustion crown and chamber combustion during thechamber expansion during stroke the expansion for WA compared stroke for with WA the compared with the use of pure gasoline. usecompared of pure gasoline. with the use of pure gasoline.

Figure 26. Decrease in the combustion chamber average temperature during the expansion process. FigureFigure 26. 26.Decrease Decrease in in the the combustion combustion chamber chamber averageaverage temperature during during the the expansion expansion process. process. Sustainability 2016, 8, 993 18 of 22 Sustainability2016, 8, 993 18 of 22 Sustainability2016, 8, 993 18 of 22

FigureFigure 27. 27.Decrease Decrease in in the the piston piston crown crown averageaverage temperaturetemperature during during the the expansion expansion process. process. Figure 27. Decrease in the piston crown average temperature during the expansion process. TheThe effect effect of of WI WI temperature temperature on on the the percentage percentage increaseincrease of the the in-cylinder in-cylinder pressure pressure is isgiven given in in The effect of WI temperature on the percentage increase of the in-cylinder pressure is given in FigureFigure 28 .28. The The water water temperature temperature was was raised raised toto 200200 ◦°CC for direct injection injection into into the the cylinder. cylinder. A Awater water Figure 28. The water temperature was raised to 200 °C for direct injection into the cylinder. A water temperaturetemperature of 200of 200◦C °C was was achieved achieved by usingby using the heatthe heat from from the coolingthe cooling system system to heat to heat the injectedthe injected water temperature of 200 °C was achieved by using the heat from the cooling system to heat the injected water in the first instance and the heat of the exhaust gases in the second instance. The water in thewater first in instance the first and instance the heat and of thethe exhaustheat of gasesthe exhaust in the secondgases in instance. the second The waterinstance. temperature The water had temperature had a positive and significant effect on the increase of the in-cylinder pressure. As the a positivetemperature and significanthad a positive effect and on significant the increase effect of theon the in-cylinder increase pressure.of the in-cylinder As thewater pressure. temperature As the water temperature increased, the percentage decrease in pressure was lower. More than 4% of the increased,water temperature the percentage increased, decrease the in percentage pressure wasdecreas lower.e in More pressure than was 4% oflower. the increasedMore than pressure 4% of the was increased pressure was realized during the second stage, where the volumetric expansion of steam realizedincreased during pressure the second was realized stage, whereduring the the volumetric second stage, expansion where the of volumetric steam for using expansion a WA of of steam 3.0 at a for using a WA of◦ 3.0 at a temperature of 200 °C took place. On the other hand, the percentage temperaturefor using a of WA 200 ofC 3.0 took at place.a temperature On the otherof 200 hand, °C took the percentageplace. On the decrease other hand, in pressure the percentage during the decrease in pressure during the first stage was only 1%. firstdecrease stage was in pressure only 1%. during the first stage was only 1%.

Figure 28. The effect of water temperature on the percentage increase of the in-cylinder pressure. FigureFigure 28. 28.The The effect effect of of water water temperature temperature onon thethe percentagepercentage increase increase of of the the in-cylinder in-cylinder pressure. pressure. WI also showed a potential benefit of improved engine efficiency via waste heat recovery from WI also showed a potential benefit of improved engine efficiency via waste heat recovery from theWI large also amount showed of aenergy potential from benefit the stream of improved of exhaust engine gases.efficiency via waste heat recovery from the large amount of energy from the stream of exhaust gases. the large amount of energy from the stream of exhaust gases. 3.3. Water Addition in Both Compression and Expansion Strokes 3.3. Water Addition in Both Compression and Expansion Strokes 3.3. WaterWI Additionduring the in Both compression Compression stroke and had Expansion the bene Strokesfits of reducing compression work and NO WI during the compression stroke had the benefits of reducing compression work and NO emission, and increasing the peak-pressure. Employing WI during the expansion stroke affected the emission,WI during and increasing the compression the peak-pressure. stroke had Employ the benefitsing WI ofduring reducing the expansion compression stroke work affected and the NO expansion pressure in two stages: a decrease in pressure in the first stage of heat absorption and an emission,expansion and pressure increasing in two the stages: peak-pressure. a decrease Employing in pressure WI in duringthe first thestage expansion of heat absorption stroke affected and an the increase in pressure at the second stage for the volumetric expansion of the steam formed. The useful expansionincrease in pressure pressure in at two the second stages: stage a decrease for the volumetric in pressure expansion in the first of the stage steam of heatformed. absorption The useful and advantages of WI cases can therefore be harnessed by using a sequence of timed WI during an anadvantages increase in of pressure WI cases at thecan secondtherefore stage be harnesse for the volumetricd by using expansiona sequence ofof thetimed steam WI formed.during an The engine cycle. For example, WI takes place for the first time during the compression stroke at 80° CA usefulengine advantages cycle. For of example, WI cases WI can takes therefore place for be the harnessed first time by during using the a sequence compression of timed stroke WI at during 80° CAan engine cycle. For example, WI takes place for the first time during the compression stroke at 80◦ CA Sustainability 2016, 8, 993 19 of 22

Sustainability2016, 8, 993 19 of 22 bTDC with the second injection of water taking place during the expansion stroke at 10◦ CA aTDC. bTDC with the second injection of water taking place during the expansion stroke at 10° CA aTDC. The parameters of WI in this section are presented in Table5. The parameters of WI in this section are presented in Table 5. Table 5. WI parameters during the compression and expansion strokes. Table 5. WI parameters during the compression and expansion strokes.

ItemItem Value Value 1st1st WI WI Injection Timing Timing 80◦ 80°CA CA bTDC bTDC ◦ Water duration [° [ CA]CA] 10 10 Water temperature [K] [K] 310 310 Water mass [Kg] [Kg] WA WA 15 15 2nd2nd WI WI ◦ Injection Timing Timing 10 10°CA CA aTDC aTDC ◦ Water duration [° [ CA]CA] 10 10 Water temperature [K] [K] 200 200 Water mass [Kg] 3.0 WA Water mass [Kg] 3.0 WA

InIn thisthis example,example, therethere waswas nono percentagepercentage decreasedecrease inin pressurepressure becausebecause ofof thethe improvedimproved effecteffect ofof thethe firstfirst WIWI onon thethe in-cylinderin-cylinder pressure.pressure. InIn anan internalinternal combustioncombustion engine,engine, asas thethe expansionexpansion pressurepressure increases,increases, thethe workwork producedproduced isis higher.higher. Figure 29 illustratesillustrates thethe timetime periodsperiods forfor thethe percentagepercentage increaseincrease inin thethe in-cylinderin-cylinder pressurepressure byby usingusing twotwo waterwater injections.injections.

Figure 29. The percentage increase in thethe in-cylinderin-cylinder pressurepressure duringduring thethe twotwo WIWI stages.stages.

T1: theT1: percentage the percentage increase increase in the in in-cylinder the in-cylinder pressure pressure because because of the of first the WI.first WI. T2: theT2: period the period for the for second the WIsecond and itsWI effect and on its the effect in-cylinder on the pressure in-cylinder due topressure heat absorption. due to heat absorption. T3: the percentage increase in the in-cylinder pressure because of evaporation during the second WI. T3: the percentage increase in the in-cylinder pressure because of evaporation during the second T4: the percentage increase in the in-cylinder pressure due to the combined effect of the two waterWI. injections. T4: the percentage increase in the in-cylinder pressure due to the combined effect of the two Inwater this injections. study, two temperatures of injected water were used to demonstrate the combined benefits of two WI timings described in Sections 3.1 and 3.2. The maximal percentage increase in pressure In this study, two temperatures of injected water were used to demonstrate the combined was around 7% with no decrease in the in-cylinder pressure during the expansion stroke. The same benefits of two WI timings described in Sections 3.1 and 3.2. The maximal percentage increase in injected water temperature can be used to increase the expansion pressure, leading to a reduction in pressure was around 7% with no decrease in the in-cylinder pressure during the expansion stroke. NO emissions. The same injected water temperature can be used to increase the expansion pressure, leading to a On the whole, injected water presented opportunities for reducing NO emissions and for reduction in NO emissions. improving engine efficiency due to a reduction in heat losses. The heat absorption of injected water On the whole, injected water presented opportunities for reducing NO emissions and for led to a decrease in the local temperature of the combustion areas and could decrease NO emissions. improving engine efficiency due to a reduction in heat losses. The heat absorption of injected water led to a decrease in the local temperature of the combustion areas and could decrease NO emissions. The phase change process from liquid to gas increased the expansion pressure and improved the Sustainability 2016, 8, 993 20 of 22

The phase change process from liquid to gas increased the expansion pressure and improved the engine power output. The decrease in the average surface temperature of the cylinder liner and other hotspots helped to prevent deterioration of the lubricating oil film. This also enhanced the anti-knock resistance, and prolonged the engine lifespan. However, excessive water injection by mass could negatively affect the engine, in terms of ignition delay and cylinder corrosion. The disadvantages of the liquid WI method in ICEs need to be evaluated in detail before it is applied.

4. Conclusions The results of a CFD simulation were carried out to investigate the effect of WA on the in-cylinder pressure and facilitate higher power output. Different timings of WI were used in this study and the following conclusions were drawn:

(1) WI in the compression stroke facilitated a reduction in compression work, a 34% reduction in NO emission for the WA 25 case, and an increase in the peak pressure by a margin of about 2% for WA 15. (2) Employing WI during the expansion stroke indicated two stages of in-cylinder pressure change. For WA 3.0 and a water temperature of 100 ◦C, the percentage decrease in the in-cylinder pressure was 2.7% during the first stage with a percentage increase of about 2.5% during the second stage. WI helped to reduce the energy losses via heat transfer to the cylinder walls and the exhaust gases. At 180◦ CA aTDC, the exhaust gas temperature decreased by 42 K, 89 K, and 136 K for WA 1.0, WA 2.0, and WA 3.0, respectively. Increasing the WI temperature to 200 ◦C, the percentage decrease in the in-cylinder pressure was 1.0% during the first stage and more than 4.0% in the second stage. (3) Two timed water injections could be used to decrease the compression work and increase the pressure of the expanding burnt gases during the expansion stroke. The maximum percentage increase of the in-cylinder pressure was about 7% which showed the potential for achieving higher engine power output.

Acknowledgments: The authors would like to express their gratitude to the China Scholarship Council for providing financial support for this study in the form of CSC grant numbers 2012704029 and 2013GXZ993. The authors also wish to thank the National Research Project Fund, China (Grant Number 51276132) for additional financial support for this study. The input of various anonymous reviewers regarding how the original manuscript could be improved was greatly appreciated. Author Contributions: This is to confirm that all authors have contributed in diverse ways from the design of the research to the final manuscript preparation. Conflicts of Interest: The authors declare no potential conflicts of interest.

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